Review Article Transendothelial Transport and Its Role in...

Review ArticleTransendothelial Transport and Its Role in Therapeutics

Ravi Kant Upadhyay

Department of Zoology DDU Gorakhpur University Gorakhpur 273009 UP India

Correspondence should be addressed to Ravi Kant Upadhyay rkupadhyayahoocom

Received 15 April 2014 Revised 13 June 2014 Accepted 18 June 2014 Published 27 August 2014

Academic Editor Athanasios K Petridis

Copyright copy 2014 Ravi Kant UpadhyayThis is an open access article distributed under the Creative CommonsAttribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Present review paper highlights role of BBB in endothelial transport of various substances into the brain More specificallypermeability functions of BBB in transendothelial transport of various substances such as metabolic fuels ethanol amino acidsproteins peptides lipids vitamins neurotransmitters monocarbxylic acids gases water andminerals in the peripheral circulationand into the brain have been widely explained In addition roles of various receptors ATP powered pumps channels andtransporters in transport of vital molecules in maintenance of homeostasis and normal body functions have been described indetail Major role of integral membrane proteins carriers or transporters in drug transport is highlighted Both diffusion andcarrier mediated transport mechanisms which facilitate molecular trafficking through transcellular route to maintain influx andoutflux of important nutrients and metabolic substances are elucidated Present review paper aims to emphasize role of importanttransport systemswith their recent advancements inCNSprotectionmainly for providing a rapid clinical aid to patientsThis reviewalso suggests requirement of new well-designed therapeutic strategies mainly potential techniques appropriate drug formulationsand new transport systems for quick easy and safe delivery of drugs across blood brain barrier to save the life of tumor and virusinfected patients

1 Introduction

Blood brain barrier (BBB) is a vasculature of the centralnervous system (CNS) that is formed by capillary endothelialcells This is not a fixed structure but its function dependson the complex interplay between the different cell typessuch as the endothelial cells astrocytes pericytes and theextracellular matrix of the brain and blood flow maintainedin the microvessels or brain capillaries BBB is physicallylocated in endothelium of blood vessels (capillaries) and actsas a ldquophysical barrierrdquo due to formation of complex tightjunctions between adjacent endothelial cells Both luminaland abluminal membranes contain specific transport systemsand regulate transcellular traffic Thus BBB facilitates mostmolecular traffic to transcellular route and maintain influxand outflux of important nutrients andmetabolic substancesIt acts as a rate limiting structure which obstructs transcapil-larymovement of substrates in the peripheral circulation intothe brainThis is a dynamic barrier which acts as a firewall ofa computer which permits transient flow of nutrients gasesand smaller molecules into the brain and keeps out harmfulmetabolites such as drugs ions toxins which potentially

circulate in the blood stream It also protects the brain fromsudden rising blood pressure and transcapillary movementof substrates in the peripheral circulation into the brainThus association of astrocytes with the brain endothelial cellsprovides a modular organization that allows precise controlover the substances that enter or leave the brain

The blood brain barrier actually consists of several com-ponents It includes endothelial cells astrocytes and bloodcapillaries in the brain which form the major structuralcomponent of the blood brain barrier These are quitedifferent from other capillaries found in the body as theirendothelial wall possesses tight junctions which obstructtransport between cells More often only small moleculesthat can diffuse through these cells can cross the barrierMorespecifically the endothelial cells also possess transporterswhich show permeability characteristics and allow transportof oxygen and CO

2across the BBB bu these selectively

prevent other substances from crossing [1] In addition bloodcapillaries in the brain are enclosed by the flattened ldquoend-feetrdquo of astrocytic cells which also act as a partial activebarrier These are found in large numbers and maintain thefunctional integrity of BBBThese are highly specialized glial

Hindawi Publishing CorporationInternational Scholarly Research NoticesVolume 2014 Article ID 309404 39 pageshttpdxdoiorg1011552014309404

2 International Scholarly Research Notices

bull It regulates influx and outfluxes of important ions and maintains immediate microenvironment of brain cells Management of reliable neuronal signaling and regulates transport of essential molecules

Transportation of nutrients and other essential biomolecules

bull It also protects the brain from action of metals toxicants poisons

and safeguards the integrity of the brain

Protection of neuronal tissues and cells

mononeuropathy polyneuropathy myopathy cerebral tumors cerebro-vascular accidents such as thrombosis embolism haemorrhage and vasculitis

Drug delivery for brain tumors and physical injuries

hormones and neurotransmitters BBB acts as a barrier to toxic agents

For treatment of neurological disease and disorders epilepsy seizurestrauma Parkinsonrsquos disease multiple sclerosis dementia Alzheimersrsquos disease

Figure 1 Showing important applications of transendothelial transport through BBB

cells which have star like shape due to presence of longbranches These respond to CNS in a reactive process that isknown as astrogliosis which is recognized as a pathologicalmarker of neuropathologies and CNS disorders These cellswrap themselves around the capillaries like insulation ona wire and display graded changes that result in specificsignaling events These cells show vast molecular arsenalsat the disposal and show reactive astrogliosis and glial scarformationThere are two different types of astrocytes that isprotoplasmic astrocytes and fibrous astrocytes First type ofastrocytes occurs in gray matter or specially areas rich in cellbodies while second type occurs in white matter that is areaformedmainly of axonsThese cells also possess long dendritelike processes that overlap all the brain vessels and surroundmany synapses These cells play primary role in synaptictransmission and information exchanges [2 3] by operatingthrough certain gradations mechanisms and transcellularfunctions both in healthy tissues [4 5] and in the state ofpathologies like ischemic injury [6ndash9] Astrocytes influencepolarity of blood brain barrier [10] Transmitters and mod-ulators released by neurons astrocytes and endotheliumallow complex signaling between cells in the neurovascularunit and many features of the BBB phenotype are subject tomodulation under physiological or pathological conditionsFor example opening of the BBBrsquos tight junctions may occurunder normal conditions to allow the passage of growthfactors and antibodies into the brain and in inflammation andcan contribute to brain oedema

The blood brain barrier provides a layer of protection forthe brain from harmful or foreign substances that may injurethe brain [11] It also protects the brain from action of metalstoxicants poisons hormones neurotransmitters drugs andother foreign or xenobiotic substances [12] mainly noxiousagents [13] It regulates the movement of ions and molecules[14] between the blood and CNS (Figure 1) The barrier is

also crucial and provides appropriate environment for properneural functions as well as for protection of CNS from injuryand diseases Therefore before targeting the neurovascularunit protection of the BBB is essential instead of a classicalneuron-centric approach that only roads development ofneuroprotective drugs that may result in improved clinicaloutcomes after neural diseases [15] (Figure 1) Hence forhealthy functioning of the brain easy passage of moleculesand ions pass through brain barrier (BBB) is highly needfulMore often all essential vital metabolites are to be passedthrough the BBB to protect the neurocompartments frompotential disruptive and damaging xenobiotic agents [16]Moreover role of BBB is more important for maintainingnormal physiology of brain and for safety of neuronaltissues from toxic substances by restricting its entry into theneurocompartment (Figure 1) [16]

2 Functions

BBB acts as a transport barrier that allows passage of selectivemolecules into the brain and mediating solute flux It alsoacts as a dynamic interface which separates the brain fromthe circulatory system It limits the entry of biomoleculesserves as a ldquometabolic barrierrdquo and allows passage of intra-cellular and extracellular enzymes and neutral amino acidsIt facilitates the entry of metabolically required nutrientsinto the brain and excludes or effluxes potentially harmfulcompounds or lipid-soluble molecules mainly toxicants [14]It functions as a perfect barricade to check the permeability ofunspecified and unwantedmolecules from entering the brainBut it allows transport of most essential molecules acrossthe membrane and maintains the homeostasis in the mostvital organs of the human body Normally it allows transportof water carbon dioxide oxygen and most lipid solublesubstances such as alcohol aesthetics but it is impermeable

International Scholarly Research Notices 3

Transport of molecules across membrane and BBB

Neurotransmitters

Ethanol

Peptides

MineralsAmino acids

LipidsVitamins

Metals

Electrolytes Water

Drugs

Monocarboxylicacids

PorphyrinsProteins

Metabolic fuels glucose

Gases CO2 O2NO and ammonia

Figure 2 Showing transport of various biomolecules and ions to central nervous system

to plasma proteins and non-lipid soluble substances in bothcerebrospinal fluid and parenchyma of brain (Figure 2) Thebarrier shows slight permeability to the electrolytes suchas sodium chloride and potassium Capillary wall possessesthree classes of specialized ldquoefflux pumpsrdquo which bind tothree broad classes of molecules that is water-soluble lipidsoluble and gaseous compounds Inversely BBB transportsback essential molecules into the blood and transports themto the brain Therefore it is highly important and essentialthat water-soluble compounds including the fuel moleculessuch as glucose for energy production and amino acids forprotein synthesis must cross the BBB (Figure 1)

The blood brain barrier (BBB) and blood cerebrospinalfluid barrier (BCSF) possess a variety of carrier-mediatedtransfer systems to support and protect brain function[9] But for major function of homeostasis brain vesselshave special carriers on both sides of the cells formingthe capillary walls which transport these substances fromblood to brain Same carriers also remove waste productsand other unwanted molecules in the opposite directionIn addition low pore size disallows entry of biomoleculesinto the brain (Figure 3) More often endothelial capillariesin the brain lack endothelial transport and disallow largemolecules to pass but smaller molecules can pass through itOnly glucose and other low molecular fat-soluble moleculesare also disallowed but few lipid soluble molecules suchas barbiturate drugs rapidly cross through into the brainMore specifically endothelial light junctions obstruct themovement of polar molecules but most of the lipophilic

substances such as oxygen carbon dioxide diffuse acrossthe brain or molecules having smaller size or by passivediffusionThus endothelial cells directly or indirectly protectthe brain by disallowing the harmful toxic substances andcatabolize harmful drugs by using drugmetabolizing enzymesystem BBB accomplishes vital functions by maintainingin- and outflow of certain biomolecules (Figure 4) TheBBB is semipermeable and allows only few molecules tocross but prevents xenobiotics from entry into the brainand protects it from noxious agents [13] Thus BBB playsimportant role inmanagement of endogenous and xenobioticcompounds by the brain and provides a layer of protectionfor the brain from harmful or foreign substances that mayinjure the brain It also protects the brain from action ofmetals toxicants poisons hormones and neurotransmitters(Figure 1) Meanwhile excess or entry of harmful compoundis denied by special mechanisms and structures possiblythese can diffuse straight through capillary walls into thebrainThese are stopped to move through the tight junctionsfound between membranes of adjacent endothelial cells

More specifically three different barrier layers limit andregulate molecular exchange at the interfaces between theblood and the neural tissue or its fluid spaces the BBBbetween the blood and brain interstitial fluid the choroidplexus epithelium between the blood and ventricular cere-brospinal fluid (CSF) and the arachnoid epithelium betweenthe blood and subarachnoid CSF These are used for sub-stances with a high lipid solubility and may move across theblood brain barrier by simple diffusion More specifically


Active transport based systemsP-glycoproteins or trans-

membranous proteins ATP-dependent pumps transporters and permeabilitizers in transportation of various nutrients and other essential

elements are well definedVitaminsacidsproteinspeptides

and solid lipid microparticles

Diffusion based systems

Particlesnutrientsgases and drugs

Mechanical operation systemBBB loosening by physical disruption

Drugstransport of niosomesproniosomesmicelles

microemulsions andnanoemulsions

Smaller size biodegradablenon-biodegradable

Carrier and noncarrier based transportation systems

Figure 3 Showing various transport systems used for transportation of molecules and ions

movement of molecules with high electrical charge is sloweddown Morphologically the polar distribution of transportproteins mediates amino acid homeostasis in the brain Fur-thermore endothelial cells that line cerebralmicrovessels alsomaintain microenvironment for reliable neuronal signalingand regulate transport of essential molecules BBB sepa-rates the pools of neurotransmitters and neuroactive agentswhich act centrally and peripherally It also regulates influxand outfluxes of important ions and maintains immediatemicroenvironment of brain cells (Figure 1)

In brain capillary endothelium contains tight junctionswhich are tighter and more complex in comparison toendothelium found in other tissues It is polarized intoluminal and brain facing plasma membrane and transducessignals from the vascular system and from the brain Thisallows molecules to pass through the cell membranes ofendothelial cells and reach the brain [17] More specifi-cally lipid soluble molecules or micelles could pass throughthis varying structure while lipid insoluble molecules arerestricted from transport More exceptionally few moleculeslike glucose oxygen and carbon dioxide also actively trans-ported across the barrier These molecules perform variouscellular functions mainly cell signaling and communicationin the brain In this mechanism important contributionis made by the transmembrane proteins occludin and theclaudins

BBB also limits the transfer of virus from blood tobrain and protects the invasion of virus Thus it acts as ananatomical barrier and avoids invasion of various pathogenicinfections toxic effects of drugs and harmful wastes Indi-rectly BBB assists in maintaining immune responses andprotective functions BBB greatly limits the efficacy of manyneuroprotective drugs [18] but it allows the permeability of

arsenicals molybdate and methyl mercury [19] and toxicmetals BBB also limits the usefulness of the most effectivechemotherapeutic agents [20] It is only possible due topresence of an elaborate and dense network of capillaryvessels that feeds the brain and removes waste products [2122] Nevertheless several disorders and diseases can affect thebrain leading to some loss of BBB integrity [23]Therefore ina state of neuropathological diseases or virus pathogenesisneural protection can be made by altered BBB functions fordrug deliveryMoreover slight changes by loosening the tightjunction could provide good delivery of drugs Hence inpresent review article endothelial transport and its importantrole in drug delivery for therapeutics and clinical care of CNSrelated diseases have been widely elucidated in detail

3 Endothelial Transport

Due to selective and controlled transport of moleculesthere occurs a concentration difference in several importantconstituents in cerebrospinal fluid from ECF (extracellularfluid) that occurs in the body partsThis is due to few cellularassociations glued together to form physical obstructionswhich separate the blood components from ECF and ISFNormally concentration difference on both sides is physicallycontrolled by certain physical barriers which prohibit largemolecules from coming across the endothelial capillariesor from blood into the cerebrospinal fluid or into theinterstitial fluid occuring in the brain These barriers areblood cerebrospinal fluid barrier and the blood brain barrierthat exist between blood and the cerebrospinal fluid and brainfluid respectively But all these components occur in theinterstitial fluids of the body Meanwhile in the capillary


Delivery of various biomolecules and drugs to CNS and nerve cells

Nutrients Drugs

Transendothelialtransport

Fuels and gases

Nutrients

Drugs

Water

Pool of variousmolecules

Figure 4 Showing diagrammatic representation of BBB and its therapeutic applications

beds of most of the organs rapid passage of molecules takesplace from the blood through the endothelial wall of thecapillaries into the interstitial fluid It plays important rolein maintaining composition of interstitial fluid much likethat of blood Further permeability of important moleculesalso keeps plasma membrane receptors and transporters ofthe cells bathed into the interstitial fluid and allows directinteractions with amino acids vitamins hormones proteinsor other compounds of the blood Under normal conditionsthe BBB acts as a barrier to toxic agents and safeguards theintegrity of the brain It is highly selective for transport ofnutrients gases few peptide neurotransmitters and other

ions of physiological use and is less toxic or nonharmful tothe brain Most molecules cannot diffuse across the BBB orthrough a pure phospholipid bilayer at rates sufficient tomeetcellular needs except two gases like CO

2and O

2 and small

hydrophobic moleculesBBB functions as a semipermeable membrane and disal-

lows passage of large molecular substances from the bloodinto the cerebrospinal fluid or into the interstitial fluids ofthe brain even though these same substances are readilytransfered into the interstitial fluids of the body Hencebarriers exist both at the choroid plexus and at the tissuecapillary membranes essentially in all areas of the brain


parenchyma except in some areas of the hypothalamuspineal gland and area postrema where substances diffusewith ease into the tissue space More specifically simplediffusion occurs in these areas that are quite importantbecause most of sensory receptors are attached to theseregions which quickly respond to specific changes in thebody fluids mainly changes occur in osmolality and glucoseconcentration Further these responses provide the signalsfor nervous and hormonal feedback regulation of each of thefactors In general the blood cerebrospinal fluid and bloodbrain barriers are highly permeable to water carbon dioxideoxygen andmost lipid soluble substances such as alcohol andaesthetics These are slightly permeable to the electrolytessuch as sodium chloride and potassium and impermeableto plasma proteins and most nonlipid soluble large organicmolecules Therefore the blood cerebrospinal fluid andblood brain barriers often make it impossible to achieveeffective concentrations of therapeutic drugs such as proteinantibodies and nonlipid soluble drugs in the cerebrospinalfluid or parenchyma of brainThese also remove out harmfulblood toxicants and drug metabolites out of the brain Morespecifically all essential molecules are transported across thebarrier while harmful molecules are denied entry and BBBperforms a very effective job of maintaining homeostasis forthe most vital organ of the human body [24]

In addition there are few drugscompounds whichincrease the permeability of the blood brain barrier [25]temporarily by increasing the osmotic pressure in the bloodwhich loosens the tight junctions between the endothelialcells By loosening the tight junctions restrictiveness ofthe barrier could decrease and makes it easier to allow amolecule to pass through it But this should be done in a verycontrolled environment because of the risk associated withthese drugs First the brain can be flooded with moleculesthat are floating through the blood stream usually blockedby the barrier Secondly when the tight junctions loosen thehomeostasis of the brain can also be thrown off which canresult in seizures and the compromised function of the brain[25] This method is applicable for diffusion for psychoactivedrugs But rate of entry of compounds that diffuse into thebrain depends on their lipid solubility Moreover substanceswith high lipid solubility may move across the blood brainbarrier by simple diffusion while lipid insoluble compoundsare denied For example the permeability of lipid-solublecompounds such as ethanol nicotine iodoantipyrine anddiazepam is very high hence their uptake by the brain islimited only by blood flow In contrast polar molecules suchas glycine and catecholamines enter the brain very slowlythereby isolating the brain from neurotransmitters in theplasma

31 Transport of Molecules and Ions Across BiomembraneMore specifically transport proteins are transmembrane pro-teins which contain multiple membrane spanning segmentsand are generally 120572 helixThese proteins lined the membraneand allow movement of hydrophilic substances withoutcoming into contact with the hydrophobic interior of themembrane Oppositely small hydrophobic molecules passthrough the membrane by simple diffusion Meanwhile few

gases like O2and CO

2and small unchanged polar molecules

such as urea and ethanol can move by simple diffusion acrossthemembrane (Figure 3) Nometabolic energy is required fordiffusion because these molecules simply move from a highto a low concentration in a gradient manner Thus plasmamembrane regulates the transport of molecules into and outof the cell by simple diffusion and active transport (Figure 4)

Brain capillary endothelial cells contain three classesof transmembrane proteins which mediate transport ofmolecules ions sugars amino acids and other metabolitesacross cell membranes Most of these transport proteins areintegral membrane proteins which remain embedded in theplasmamembrane and othermembranesThese containmul-tiple transmembrane domains which permit the controlledand selective transport of molecules and ions across themembraneThese transmembrane proteins couple the energyreleased by hydrolysis of ATP with the energy required fortransport of substances against their concentration gradientDifferent classes of pumps belong to transmembrane proteinfamily exhibit characteristic structural and functional prop-erties and act as ATP powered pumps or cassettes channelsand transporters These also work as ATP-dependent effluxpump and are member of intrinsic membrane proteinslike P-glycoprotein (P-gp) [26 27] This pump also occursin the luminal plasma membrane of BMEC and preventsthe intracellular accumulation of an extensive variety ofchemotherapeutic agents and hydrophobic compounds

Cytoplasmic matrix contains various kinds of ions whichassist in maintaining osmotic pressure and acid base balancein the cells Retentions of ions in the matrix produce anincrease in osmotic pressure that allows entrance of water inthe cell The concentration of ions in the intracellular fluiddiffers from interstitial fluid Hence a high order of differenceexists between intracellular K+ and extracellular Na+ in nerveand muscle cells Therefore free calcium ions may occurin cells or circulating blood while free ions of phosphate(HPO

4

minus and H2PO4

minus) occur in the matrix and blood Theseions maintain buffering system and tend to stabilize pHof blood and cellular fluids Thus electrolytes play a vitalrole in the maintenance of osmotic pressure and acid baseequilibrium in thematrixMg2+ ions phosphate and so forthare good examples of the electrolytes Electrolytes like Na+and Clminus move across by specific channels and transport pro-teins (Table 1) More exceptionally few important mineralsoccuring in matrix in nonionizing state are Na K Ca CuI Fe Mn Mo Cl Zn Co Ni and so forth because of theirmuch essential physiological need Thus the compositionof interstitial fluid resembles that of blood and specificreceptors or transporters in the plasmamembrane of the cellsbeing bathed by the interstitial fluidmay interact directlywithamino acids hormones or other compounds from the blood(Table 1) More often barrier limits the accessibility of blood-borne toxins and other potentially harmful compounds tothe neurons of the CNS because of restriction imposedby transcapillary movement of substrates in the peripheralcirculation into the brain

32 Transport of Fuels Glucose is main primary energysubstrate or metabolic fuel of the brainThis maintains major


Table 1 Cellular functions of various kinds of metal and nonmetal ions inside cell and its transport mechanisms

Element Ionic form Transport mechanism Functions References

Molybdenum MoO42 Ion transporters

Cofactor or activator of certainenzymes (eg nitrogen fixationnucleic acid metabolism andaldehyde oxidation)

Cobalt Co2+ Ion transporters DMT1 Constituent of vitamins B12

Copper Cu+ Cu2+ ATP 7A catalyzesCu-transporting ATPase

Constituent of plastocyanin andcofactor of respiratory enzymes [28ndash32]

Iodine (heaviesttrace element) I Diffusion and ion

transporters

Constituent of thyroxintriiodothyronine and otherthyroid hormones

Boron BO33minus B4O

minus2 Ion transporters Activates arabinose isomerase

Zinc Zn2+ Enzyme and proteincarriers

Cofactor of certain enzymes (egcarbonic anhydrasecarboxypeptidase)

Manganese Mn2+ (Mn2+)[4] DMT1Cofactor of certain enzymes (egseveral kinases isocitricdecarboxylase)

Iron Fe2+ Fe3+ DMT1 Constituent of haemoglobinmyoglobin and cytochromes

Magnesium Mg2+ Ion transporters Constituent of chlorophyllactivates ATPase enzyme

Sulphur SO42 Ion transporters Constituent of coenzyme A

biotin thiamine and proteins

Phosphorus PO43minus H2PO4 Ion transporters

Constituent of lipids proteinsnucleic acids sugar phosphatesand nucleoside phosphates

Calcium Ca2+ Ion transportersConstituent of plant cell wallsmatrix component of bone tissuecofactor of coagulation enzymes

Potassium K+ Diffusion and ionchannels

Cofactor for pyruvate kinase andK+-stimulated ATPase

Sodium Na+ Diffusion and ionchannels

Perform nerve functions acidbase balance and homeostasis

Cadmium Cd2+ DMT1Zinc2+ Zn ZIP6 (LIV-1) Zinc transporters Metabolic cellular functions [33]

Aluminium Al Transferrin-receptormediated endocytosis

Metal ions Fe Cu and Ag

Multiple transportproteins Na+K+ pumpK+ channel Na+ lysinesymporter channel andsodium-lysinetransporter

Work as electrolyte in plasma [34]

Metal ions Fe Cu and AgMembrane transporterproteins such as ABCtransport proteins

Enzymatic cellular functions [35]

Free metal ion andcomplexes

Fe Cu Ag oxidesand sulphates

Transferrin carries metalspecies such as the freemetal ion and complexesof the metal with anamino acid or protein

Transmigration of proteins [36 37]

Divalent metals Fe Cu Ag andzinc

(DMT1 DCT1 andNramp2) transportsferroprotein

[36 38 39]


Table 1 Continued


Divalent metals

Fe3+ and cobalt(Co2+) Ca++Fe++ Mn++Cd++ Cu++

Ni++ Pb++ Zn ++

Divalent metaltransporter 1 divalentcation transporter 1(DCT1)

Iron-overload disorders [40 41]

Trivalent metals Al and Mn Diferric transferrin [42]

Methyl mercury Transport by theL-cysteine system [43]

metabolic functions and physiology of neuronal networkpresent inside brain and its regular supply is essentiallyrequired Glucose is a polar substrate whose combustionor catabolism needs large oxygen consumption Interest-ingly glucose transport occurs through both endothelialcells by facilitated diffusion and by GLUT 1 and GLUT3 transporters [44 45] present on the surface of neuronsand endothelial cells Normally brain capillary endothelialcells possess insulin-independent GLUT-1 glucose trans-porters which mediate the facilitated diffusion of glucosethrough the blood brain barrier [44 46] Moreover rate ofglucose transport through endothelium depends on energymetabolism occurring inside brain or depends on glucoseutilization Thus glucose itself functions as a rate limitingligand or metabolite and it is transferred from ECF toneurons when its concentration falls in blood lower thanthe normal glucose level When cellular concentration ofglucose becomes high its transport is obstructed automati-cally Further transporters found on the surface of neuronaland endothelial cells differ in their activity In fact sametransporters also occur on other cell membranes whichtransport two to three times more glucose than normally itis metabolized by the brainThus glucose utilization dependson concentration resides with the cell that also determines itstransport There are several glucose transporter syndromesknown But among all most common syndromes glucosetransporter type 1 deficiency syndrome is important becauseit more severely effect glucose transport in children Thusimpairment of GLUT-1 results in a low glucose concentrationin the CSF and causes hypoglycorrhachia with clinical symp-toms like seizures mental retardation and compromisedbrain development in children Moreover most mammaliancells express GLUT1 uniporter transporter that transportsblood glucose to fulfill cellular energy needs This uniporter(GLUT1) transporter traverses between two conformationalsites or posses two phases one a glucose binding site faces theoutside of membrane while glucose binding site faces inside(Table 2) during the unidirectional transport of glucose fromthe cell exterior inward to the cytosol Hence in a state whenglucose concentration becomes higher inside the cellularoutside GLUT1 catalyze the net export of glucose from thecytosol to the extracellular medium More specifically it isthe stereo specificity of the glucose transport system thatpermits transport of d-glucose rather than l-glucose to enterthe brain Similarly hexoses such as mannose and maltoseare rapidly transported into the brain while the galactose

uptake takes place intermediately and fructose uptake occursvery slowly Interestingly uptake of 2-deoxyglucose occursso fast that it competitively inhibits the transport of glucoseFurther soon after glucose uptake and its transport into thecells it is rapidly phosphorylated to form glucose 6 phosphatewhich cannot be transported out of the cell This stepcompletes more rapidly and at a constant rate Further evenif intracellular glucose level is low then glucose is importedfrom the medium Sometime the rate of glucose transportinto the ECF normally exceeds the rate required for energymetabolism by the brain Thus glucose transport becomesrate limiting if blood glucose levels fall lower than the normalrange In such hypoglycemic condition glucose level lowersapproximately to 60mgd which also reduced the 119870m valuesof the GLUT-1 transporters found in the endothelial cells(Table 2) Moreover monosaccharide transport proteins playimportant role in carbohydrate assimilation distributionmetabolism and homeostasis [46] (Figures 3 and 4)

33 Transport of Monocarboxylic Acids Monocarboxylatetransporters play important role in the maintenance of theglycolytic metabolism through proton linked transport ofmonocarboxylic acid [60] (Figure 2) These are transportedby a separate stereospecific system [61] that is slower thanthe transport system for glucose In the cerebrovascularendothelium monocarboxylic acid transporter 1 (Mct1) con-trols blood-brain transport of short chain monocarboxylicacids such as L-lactate acetate pyruvate ketone bodiesacetoacetate and 120573-hydroxybutyrate monocarboxylic andketoacids to support energy metabolism and play potentialrole in treating brain diseases (Table 2) [66] Monocarboxy-late transporter (MCT) isoforms 1ndash4 catalyze the proton-linked transport of monocarboxylates such as L-lactateacross the plasma membrane whereas MCT8 and MCT10transporters transport thyroid hormone and aromatic aminoacids [61] Furthermore MCTs 1ndash4 also play importantmetabolic roles including energy metabolism in the brainskeletal muscle heart tumor cells T-lymphocyte activationgluconeogenesis in the liver and kidney spermatogenesisbowel metabolism of short-chain fatty acids and drug trans-port These transporters showed their distinct propertiesexpression profile and subcellular localization matching theparticularmetabolic needs of a tissue (Table 2)Mct1 functionis acutely decreased in rat brain cerebrovascular endothelialcells by 120573-adrenergic signaling through cyclic adenosine


Table 2 Transport mechanisms of gases solvents and substances across blood brain barrier

Type of substancegas Transporter Location Functions ReferencesSmall inorganicmolecules that is O2CO2 NO and H2O

Diffusion Ionic poresreceptors Influx or efflux [47]

Oxygen Diffusion Ionic poresreceptors Vitality homeostasisand cellular metabolism [48]

Ammonia Passive diffusion Ionic poresreceptors Irritation toxic [49]Nitric oxide Passive diffusion Ionic poresreceptors Toxic [50]

Water Diffusion and byaquaporins 1ndash5

Membrane surfaceaquaporins

Controls water relationsregulators oftranscellular water flow

[51ndash54]

Ethanol By filtration movingthrough water spaces Ionic pores

Alcohol-inducedoxidativenitrosativestress alters brainmitochondrialmembrane propertiesand alters basalextracellular glutamateconcentrations andclearance in themesolimbic system

[55ndash57]

Solutes Paracellular ortranscellular diffusion Luminal surface Influx or efflux [58 59]

Glucose hexose(GLUT-1)

Facilitated diffusion byGLUT 1 and GLUT 3Transporters

Glucosemorphine-6-120573-Dglucuronide

Influx and effluxmonosaccharidetransport Proteins playimportant role incarbohydrateassimilationdistributionmetabolism andhomeostasis

[44ndash46]

Monocarboxylate Proton linked transport Maintenance ofglycolytic metabolism

Proliferation ofT-lymphocytes [60 61]

Monocarboxylate(MCTI) Facilitated diffusion

Benzoic lacticlovastatin and pyruvicacid

Influx and effluxpromisingpharmacological targetsincluding for cancerchemotherapy

[61 62]

Anion exchange (band 3) Facilitated diffusion ClminusHCO3minus lactate and

metal-anion com- Influx and efflux

Tf-FMMstransferring-conjugatedfluorescein loadedmagnetic nanoparticles

Receptor mediatedtranscytosis Nanoparticles Influx drug delivery [63]

Polycationic proteinsand lectins

Absorptive-mediatedtranscytosis Membrane receptors Influx therapeutic [64]

Peptide

Saturable or facilitatedtransport andtransmembrane diffusionor passive diffusion

Membrane receptors Influx therapeutic [64]

Saturated fatty acids Facilitated diffusion Octanoate InfluxUrea Facilitated diffusion EffluxXenobiotics Xenobiotics transporters Ionic poresreceptors Out flux toxicity [65ndash67]


Table 2 Continued

Type of substancegas Transporter Location Functions References

Xenobiotics 1-methyl-4-phenylpyridinium(MPP+)

Uptake2(Oct1-3Slc22a1-3PmatSlc29a4) Net andMate1Slc47a1transporters

ATP-driven xenobioticefflux transporters [35 68 69]

monophosphate (cAMP) It constitutively cycles throughclathrin vesicles and recycling endosomes in a pathway thatis not dependent on cAMP signaling in endothelial cellsThus regulated and unregulated vesicular trafficking of Mct1in cerebrovascular endothelial cells are highly significant forunderstanding normal brain energy metabolism etiologyand potential of therapeutic approaches to treating brainstrokesdiseases [66] Normally during fasting or starva-tion level of ketone bodies in the blood gets elevatedand MCT1 transporter gets upregulated Moreover in adultsand neonates during prolonged starvation ketone bodiesserve as important fuels for the brain This results in anelevation rate and capacity of monocarboxylic acid trans-port substantially in suckling neonates Therefore due toincreased metabolic requirements of the developing brain inadults higher concentrations of monocarboxylic acids aresupplied in breast milk that fulfill energy needs of neonatesIn addition metabolism of short-chain fatty acids (SCFA)mainly acetate appears to occur mainly in astrocytes and itstransport in the brain is performed by monocarboxylic acidtransporter family (Table 2) [62] It is inhibited by phloretina high-affinity inhibitor that binds to MCT1 and slows downphysiological activities [62] More often potent and specificMCT1 inhibitors can prevent proliferation of T-lymphocytesthat may help to achieve promising pharmacological targetsincluding cancer chemotherapy [46]

34 Transport of Substances andGases Brain and other tissueorgans possess various structural barriers that are made up ofcellular vasculaturemonolayer of endothelial cellsThese cellsform elaborate tight junction complexes that efficiently limitthe paracellular transport of solutes The BBB has a numberof highly selective mechanisms for transport of nutrients intothe brain But there occur four basic mechanisms by whichsolute molecules move across membranes First mechanismis simple diffusion which proceeds from low to high con-centrations Second is facilitated diffusion a form of carrier-mediated endocytosis in which solute molecules bind to spe-cific membrane protein carriers This also controls from lowto high concentrationThird is simple diffusion which occursthrough an aqueous channel formed within the membraneFourth is active transport through a protein carrier with aspecific binding site that undergoes a change in affinity Activetransport requires ATP hydrolysis and conducts movementagainst the concentration gradient Moreover movementbetween cells is paracellular diffusion [58] while diffusionacross the cells is called transcellular diffusion Both types ofdiffusionmechanism are found in the brain both ofwhich arenonsaturable and noncompetitive It is a spontaneous process

depending on random movement of solutes It occurs due tofree-energy change of a solute diffusing across a membranethat directly depends on the magnitude of the concentrationgradient Moreover para cellular diffusion does not occur toany great extent at the BBB due to presence of tight junctionsor structural barriers Moreover para cellular diffusion doesnot occur to any great extent at the BBB due to presenceof tight junctions or structural barriers but in transcellulardiffusion the general rule is followed that is higher thelipophilicity of a substance greater will be its diffusioninto the brain [59] More specifically biogenic amines aretransported through BBB by diffusion and it never occursby carrier-mediated transport Contrary to this for uptakeof other substances uptake1 carrier involves that occuringat the luminal surface of BBB More specifically if twosubstances are similar but vary in molecular weight thesmaller substance will penetrate more rapidly consequentlysmall inorganic molecules (ie O

2 CO2 NO and H

2O) are

highly permeable Additionally hydrogen bond reductionof a compound will enhance its membrane permeabilityRemoval or masking of hydrogen bonding donor group froma compoundwill effectively decrease the transfer energy fromwater into the cell membrane [47]

More specifically capillary endothelial cells also possesswide range of transporters that selectively transport solutesand xenobiotics [65] These transporters are found at theluminal (blood) side of membranes in vivo and maintainspecific transport mechanisms for transport of various bio-materials [66] This is the main reason that the capillarybeds most of organs allow rapid passage of molecules fromblood through the endothelial wall of the capillaries intothe interstitial fluid Therefore diffusion or transport ofbiomolecules across the membrane is operated with thehelp of integral and transmembrane proteins and receptorsMeanwhile transported molecules and ions interact to spe-cific receptors or transporters in the plasma membrane ofthe cells These receptors remain embedded or bathed bythe interstitial fluid and may interact directly with aminoacids hormones or other compounds from the transportedor transferred from blood Due to semipermeable natureof BBB many nonpolar substances such as drugs andinert gases directly diffuse through the endothelial cellmembranes while a large number of other compounds aretransported through the endothelial capillaries by facilitativetransport Contrary to this nonessential fatty acids cannotcome across the blood brain barrier but essential fatty acidsare transported across the barrier Further metabolic fuelsmonocarboxylic acids ethanol vitamins and neurotrans-mitters are carried by carrier-mediated transport because


low lipid soluble biomolecules can traverse the blood brainbarrier But water and gases like O

2 CO2 and NO

2diffuse

directly across the endothelial wall (Table 2) Diffusionplays a crucial role in brain function The spaces betweencells can be likened to the water phase of foam and manysubstances move within this complicated region Diffusionin this interstitial space quantified frommeasurements basedon novel microtechniques Besides delivering glucose andoxygen from the vascular system to brain cells diffusionalso moves informational substances between cells a processknown as volume transmission Therefore diffusion is alsofound essential to many therapies that deliver drugs to thebrain The diffusion-generated concentration distributionsof well-chosen molecules also reveal or image the structureof brain tissue These concepts and methods are highlyapplicable for making therapeutic measures for improvementof neurodegenerative diseases [67]

In state of any infection concentration of both gases dif-fers largely Therefore patients under different physiologicaland pathological conditions show nonrespiratory acidosisand a decrease in paO2 induced an increase of CBF whenthe oxygen tension in cerebral venous blood fell below 35ndash40mm Hg At the same time the cerebral glucose uptakethe cerebral lactate output and the lactate-pyruvate ratio incerebral venous blood increased Critical conditions occurredfor the oxygen supply of the brain when cerebral venousp1198742

fell below approximately 30mm Hg In edematous brainareas of patients rCBF decreased with increasing brain watercontent Disease state and invasion of infection largely affecthematocrit vascular diameter blood viscosity blood flowmetabolic rate nonlinear oxygen dissociation curve arterialPO2 P50

(oxygen tension at 50 hemoglobin saturationwith O

2) and carbon monoxide concentration Finally the

various types of hypoxia such as hypoxic anemic and carbonmonoxide hypoxia arise [70] It also affects oxygen transportin brain microcirculation [70] More often systemic andmyocardial oxygen transport shows responses to brain deathin pigs [48] Both SVR and EO(2) decreased after brain death(119875 lt 01) and remained low while lactate level remainsunchanged Moreover profound metabolic alterations occurin condition of brain death and major complications arerelated to oxygen transport and its supply [48]

It is clear that capillary blood flow played an importantrole in the transport of gases and extra vascular vasomotoraction The exchange of gases such as CO

2 O2 N2O and Xe

and volatile anesthetics in cerebral tissues takes place rapidlyby simple diffusion (Table 2)This depends on cerebral bloodflow and cerebral metabolic rate of oxygen consumption inthe cortical capillary levels [71] Direct external applicationof oxygen to cerebral cortex causes cerebral vasoconstrictionwhile carbon dioxide resulted in dilation of cerebral vessels[72] Further concentration of these gases in the brain comesinto equilibrium with the plasma which is primarily limitedby the cerebral blood flow rate In addition gaseous exchangebrain also depends on pure ventilation and neurologicalphysiological and pathological state [73] Moreover endtidal carbon dioxide acts as a marker of stress response inpatients [73] while inert gases like N

2O and Xe can be used

as markers to measure cerebral blood flow An interesting

contrast is observed between CO2and H+ with regard to

their effects on brain pH As the permeability of CO2greatly

exceeds in BBB than the H+ the pH of the brain interstitialfluid reflects blood pCO

2rather than blood pH Interestingly

central respiratory chemoreceptors sense the changes thatoccur after alteration in CO

2concentration and pH in

the brain tissue [74] Consequently patients which facingmetabolic acidosis show compensatory respiratory alkalosisand their brain contain high pH and become alkaloticBesides these gases ammonia is believed to play key role inthe development of hepatic encephalopathy with increasedformation of glutamine playing a central role Ammoniapasses through BBB by passive diffusion [49] while nitricoxide donor induces long-term survival in rats [50]

35 Diffusion of Water Water rapidly enters the brain pass-ing through capillary membranes via the intercellular gapsbetween or pores in the endothelial cells or special areaswhere the cytoplasm is very thin Normally due to highpermeability water moves freely into or out of the brainas the osmolality of the plasma changes In additioncellmembrane possesses specific channels or pores throughwhich diffusion of water occurs in association of certainproteins These proteins are named aquaporins that belongto family of membranous proteins These proteins allowwater and few other small uncharged molecules such asglycerol to cross biomembrane These proteins are found inall living organisms and at least 6 different water channelproteins have been identified in various cell membranes inhumansThese aquaporin proteins form complexes that spanthe membrane and water moves through these channelspassively in response to osmotic gradients (Table 2) Thesechannel proteins are present in highest concentrations intissues where rapid transmembrane water movement occursfor example in renal tubulesMoreover aquaporin 0 found inthe lens in the eye has a role inmaintaining lens clarityWhileaquaporin 1 occurs on the red cell membrane and proximalconvoluted tubule thin descending limb of the Loop ofHenlein the kidney choroid plexus smooth muscle unfenestratedcapillary endothelium exocrine sweat glands hepatic bileducts and gallbladder epithelium where it perform waterdiffusion Moreover aquaporin 2 is ADH-responsive waterchannel specifically found in the collecting duct in the innermedulla Furthermore insertion of the channel into the apicalmembrane also occurs followingADH stimulation Aquapor-ins 3 and 4 normally are found in the basolateral membranein the collecting duct but they are not altered by ADH levelsRecently aquaporin 4 has also been identified in the ADH-secreting neurons of the supraoptic and paraventricularnuclei in the hypothalamus that functions as a hypothalamicosmoreceptor and regulates body water balance Aquaporin5 occurs in lacrimal and salivary glands and in the lungsThus aquaporins increase the water permeability of cellmembranes while other membranes of aquaporins familytransport hydroxyl-containing molecules such as glycerolrather than water Few structural mechanisms involved ingating induced regulation of aquaporins [75] Aquaporinsor water channel proteins are also found in insects thewestern tarnished plant bugLygus hesperus (Table 2) [49] and


plants These proteins control water relations [54] and act asregulators of transcellular water flow [52]

Water also easily enters the lymphatic capillaries viagaps between the lymphatic endothelial cells These gapsfunction as flap valves and promote forward lymphflowwhenthe capillaries are compressed In other areas of the bodythe water permeability of capillary membranes is quite lowbecause pores or slits are lacking in the blood brain barrierand it greatly limits water movement by the intercellularpathway Hence water transport occurs by dissolving in themembrane phase This refers to water crossing the lipidbilayer of the cell membrane by diffusion But lipid compo-sition of different cell membranes varies which determinesrate of fluid flow across cell membranes that greatly varies Insome membranes the water flux is very high and cannot beaccounted for water diffusion across lipid barriersThereforeit is possible and operative that membranes possess someproteins that form aqueous channel through which water canpass This is inferred in artificial lipid bilayers water doesnot cross membrane easily and the same process occurs innaturalmembranes In addition substances which are water-soluble typically do not cross lipid membranes easily unlessspecific transport mechanisms are present Neverthelessparadoxically water crosses nearly all the membranes in thebody with ease Moreover water permeability and transportare regulated by the capillary endothelium and rate ofcerebral blood flow In fact in normal condition permeabilityconstant of the cerebral capillary wall to the diffusion ofwater is about the same as that required for its diffusionacross lipid membranes Thus osmotic pressure allows waterto move across membranes and its unavailability producesdeficiencies known as fenestrations and diffusion across thelipid cell membranes of the endothelial cells Fenestrationsare found only in capillaries in special areas where very highwater permeability is necessary for the function of these areasThese fenestrated capillaries and vascular sinusoids permitrapid exchange of water and solute between the plasma andextracellular fluid and supply the choroid plexus (Table 2)[53] High water permeability is maintained in glomerularcapillaries found in kidney than in muscle capillaries Otherareaswith fenestrations are the capillaries in the intestinal villiand in ductless glands

36 Transport of Ethanol Alcohol consumption causes intox-ication of cells as it is transported into the brain Oncealcohol is consumed it leaves the gastrointestinal (GI) tractand enters into the bloodstream through blood capillariesIt is carried to the heart by streaming blood from wherealcohol is sent to the lungs and also come back to the heartwith the oxygenated blood Alcohol is pumped through thearterial system and distributed in all organs of the bodyEthanol travels within the arteries that lie between the skulland the brain itself These arteries branch out into capillariesand dive deep into the brain tissue Ethanol passes throughthese capillaries so as to reach the neurons in the brainUnfortunately in the brain there is no barrier for ethanoland it crosses the blood brain barrier very easily It is a polarsolvent lipophilic in nature that mixes easily with the fat in

the membrane It makes it easy for ethanol to cross the bloodbrain barrier In the case of other biological membranesethanol moves across by filtration moving through waterspaces because it dissolves in water where it is absorbed bypassive diffusion and moves with the concentration gradientthrough the membrane Similarly in the brain capillariesdue to lipophilic property ethanol allows moving by passivediffusion across the endothelial cell membrane and throughthe astrocyte layers Similarly small lipophilic drugs diffusepassively across the blood brain barrier including nicotinemarijuana and heroin that cause intoxication in the brain(Table 2) Contrary to this water-soluble nutrients such asglucose and large water-soluble molecules such as vitaminsneed specific transporters to be transported across the BBBThis process requires energy and is known as active trans-port of biomolecules Increased uptake of alcohol increasesneurobiological effects [55] and alters basal extracellularglutamate concentrations and clearance in the mesolimbicsystem of alcohol-preferring [56] Alcohol-induced oxida-tivenitrosative stress alters brain mitochondrial membraneproperties in experimental animals (Table 2) [57]

37 Transport of Vitamins Vitamins are indispensabledietary compounds which require in small amounts fewmicrograms to milligrams per day to be present in ourfood These do not serve as an energy supply They are lowmolecular weight compounds and their insufficient intakecauses malnutrition and multiple metabolic defects anddiseases Vitamin deficiency causes certain physiological andbiochemical abnormalities which affect body metabolismgrowth and development Vitamins cannot be synthesizedinside body and are required in small amounts to supportnormal metabolism They are needed for normal functiongrowth and maintenance of body tissues and for regulationof chemical reactions in the body Vitamin D affectsbrain development and function and its low levels causeneuropsychiatric diseases like autistic spectrum disorder andschizophrenia [76] Vitamin D deficiency in early life affectsneuronal differentiation axonal connectivity dopamineontogeny and brain structure and function (Table 3)[76] Deficiency of vitamin E may cause neurologicaldysfunction myopathies and diminished erythrocyte lifespan Chemically vitamins belong to diverse classes ofcompounds and are classified according to their solubilityinto water and fat Since vitamins cannot be synthesizedby the brain they must be obtained from the blood Thusspecific transport systems are required for carrying vitaminsacross blood brain barrier (Table 3) Generally thesetransport systems possess a low capacity since the brainrequires only small amounts of the vitamins and efficienthomeostatic mechanisms preserve brain vitamin contentwithout the need for a rapid influx from the blood Howvitamins are transferred across the mammalian blood brainbarrier and choroid plexus into brain and CSF and howvitamin homeostasis in brain is achieved is an importantissue (Table 3) [77]

The majority of vitamins are water-soluble but a few vitalvitamins such as A D E and K are fat-soluble They areabsorbed in the lymph and are transported in the blood


Table 3 Vitamins and their transport across the blood brain barrier (BBB) mechanisms and functions

Fat-soluble Transport mechanism Physiological functions Deficiency disease References

Vitamin A (retinol andretinoic acid)

Vitamins show specifichigh-affinity to proteinswhich transport themThese are alsotransported after beingmixed with lipid micellesformed with the aid ofbile salts lysolecithinlower glycerides andcholesterol

Stored in liver maintainsgeneral health and vigorof epithelial cells

Xerophthalmia [59 78]

Vitamin D (calciferol)

Vitamin D and itshydroxylatedmetabolites aretransported in the bloodbound to a transportprotein DBP which isalso very important inthe placental transfer of25-hydroxy-vitamin Dl

Involved in intestinalabsorption of calcium andphosphorus and incalcium metabolism andbone formation

Rickets andosteomalaciait also affects braindevelopment functionand its low levels causeneuropsychiatricdiseases like autisticspectrum disorder andschizophrenia Itsdeficiency in early lifeaffects neuronaldifferentiation axonalconnectivity dopamineontogeny and brainstructure and function

[76 79]

Vitamin E(Tocopherol)

Intestinal absorptionand transport occur withchylomicronsAlpha-tocopherol ismostly transferred toparenchymal cells of theliver where it is stored

Inhibit catabolism (ieoxidation) of certain fattyacids of cellularmembranes

Hemolytic anemiaaffects brain functionsmainly related tomitochondrial andperoxisomes

[80]

Vitamin K(naphthoquinone)

Vitamin K is a quinonecompound in the humanbody in a storage form asmenaquinone (MK)distribution includesregulated amounts inmitochondrialmembranes The humanbrain which has lowamounts of typicalvitamin K dependentfunction (eg gammacarboxylase) hasrelatively high levels ofMK and differentregions of brain havedifferent amounts

Required for theformation of prothrombin(an essential componentof blood clotting)Coenzyme Q is a quinonesynthesized de novo andthe levels of synthesisdecline with age Thelevels of MK aredependent on dietaryintake and generallyincrease with age

Sever bleeding duringdelivery delayed bloodclotting MK has acharacterized role in thetransfer of electrons tofumarate in prokaryotesA newly recognizedfumarate cycle has beenidentified in brainastrocytes

[81]

Water Soluble VitaminsVitamin B (thiamine) By diffusion

Essential for synthesis ofacetylcholinerapidly destroyed by heatcarbohydrate metabolism

Beriberi polyneuritisaffect impulsetransmission thoughtsand intelligence

Vitamin B2 (riboflavin) By diffusion

Forms the coenzyme FADwhich is involved inmetabolism ofcarbohydrates andproteins

Dermatitis blurredvision


Table 3 Continued


Niacin (nicotinic acid) By diffusion

Forms the coenzymeNAD which is involved inenergy-releasingreactions In lipidmetabolism it inhibitsproduction of cholesteroland helps in fat breakdown

Pellagra

Vitamin B6 (pyridoxine)

Transported across brainand choroid plexus innonphosphorylated Bform and has separatecarriersThe vitamin B6derivative pyridoxalphosphate is a cofactorin the synthesis ofGABA

Forms coenzymesinvolved in amino acidmetabolism in brain andis involved in fatmetabolism it imposesinhibitory effects on thebrain neurons that showprotective role duringhypoxia or ischemia

Convulsion bothcofactor andnoncofactor vitamin(eg of B(1) B(3) B(6)and E) have potentialrole in the therapy ofbrain disorders

[77]

Folic acid By diffusion and proteintransporters

Essential for synthesis ofDNAovercooking destroys it

Macrocytic anaemia

Biotin (vitamin H) By diffusion and proteintransporters

Acts as coenzyme inmetabolism ofcarbohydrates fatty acidsand nucleic acid

Mental depressionmuscular paindermatitis and nausea

Vitamin B12(cyanocobalamin)

Vitamin B12 permeaseABC proteintransporter it transportsprotein containing twotransmembrane domains(T) and two cytosolicATP binding domain

Acts as coenzymenecessary for DNAsynthesis red blood cellfor motion growth andnerve function

Malfunctioning of CNS

Vitamin C (ascorbic acid)

Absorbs throughdiffusion by glucoseGLUT1 transporterglucose GLUT1 alsotransportsdehydroascorbic acidinto the brain and itstransport is inhibited byd-glucoseSlc23a1 is ascorbic-acidtransporter that is aNa+-dependent system

Promotes proteinsynthesis (collagen)wound healing and ironabsorption protects bodyagainst infectionsrapidly destroyed by heat

Scurvyvitamin C crosses theplacenta and is anessential requirement inthe perinatal period

[82 83]

with carrier proteins from which these are delivered tothe brain Both fat-soluble and water-soluble vitamins havedifferent mechanism of absorption and transport The water-soluble vitamins are B and C which are absorbed easilythrough diffusion (Table 3) The B vitamins include thiaminriboflavin niacin folate pyridoxine and B12 The water-soluble vitamins are easily dissolved and can be excretedin the urine Moreover vitamin deficiency would only becorrected by ingesting that vitamin through diet Never-theless dietary deficiency of some vitamins can produceneurological disease Transport and metabolism of vitaminB6 forms are transported across brain and choroid plexus in

nonphosphorylated B form [84] and have separate carriers(Table 3) [77] Moreover after transport vitamins are accu-mulated by brain cells by separate specialized systems Moreoften both cofactor and noncofactor vitamins (eg of B(1)B(3) B(6) and E) have potential role in the therapy of braindisorders [77] In bacteria E coli vitamin B12 permease isABC protein transporter that imparts a variety of moleculesform the environment These transport proteins contain twotransmembrane domains (T) and two cytosolic ATP bindingdomains In mammals nearly 50 different ABC transporterproteins are known these are mostly expressed in the liverintestine kidney and brain sites where natural toxic and


waste products are removed by the bodyThemain substratesof these ABC transporter proteins are sugar amino acidscholesterol bile acids phospholipids peptides proteinstoxins and foreign substances (Table 3) More specificallysome ABC proteins flip phospholipids and other lipid solublesubstrates from one membrane leaflet to the opposite leaflet(Table 3)

Vitamin C crosses the blood brain barrier in the oxidizedform through the glucose transporters [82] In contrast theoxidized form of vitamin C dehydroascorbic acid (oxidizedascorbic acid) readily enters the brain and is retained in thebrain tissue in the form of ascorbic acid More specificallyglucose GLUT1 also transports dehydroascorbic acid into thebrain and its transport is inhibited by d-glucose but not by l-glucose (Table 3)The facilitative glucose transporter GLUT1is expressed on endothelial cells at the blood brain barrier andis responsible for glucose entry into the brain [82] It is animportantmechanism bywhich the brain acquires vitaminCFurther oxidation of ascorbic acid is an important regulatorystep in accumulation of the vitamin by the brain [82] VitaminC concentrations in the brain exceed those in blood by 10-fold In both tissues the vitamin is present primarily in thereduced form ascorbic acid

Fat-soluble vitamins such as A D E and K are absorbedin the lymph and are transported in the blood with the helpof specific carrier proteins from which these are deliveredto the brain Transport of vitamins A (retinol retinoic acid)and D (125-dihydroxyvitamin D3 [125-(OH)2D3] and 25-hydroxyvitamin D3 [25-(OH)D3]) derivatives through therat brain capillary endothelial wall that is the blood brainbarrier (BBB) occurs in two forms (Table 3) first afterbinding to albumin and a plasma transport protein to whichthese vitamins bind with specific high-affinity Moreovervitamin D and its hydroxylated metabolites are transportedin the blood bound to a transport protein DBP [79] that isalso very important in the placental transfer of 25-hydroxy-vitamin Dl Moreover during absorption from the intestinevitamin A and 120573-carotene become finely dispersed in theintestinal fluid in mixed lipid micelles formed with the aid ofbile salts lysolecithin lower glycerides and cholesterolThesefacilitate the transfer of the vitamin and other lipid compo-nents across the mucosal cell membrane [78] Interestinglyester is transported in association with 120573-carotene in the 120573-lipoprotein fraction More specifically physiologically activevitamin A alcohol on the other hand is attached to a specificprotein carrier with properties very similar to ceruloplasminin the 120572

2-globulin fraction (Table 3) [78]

Vitamin E includes eight naturally occurring fat-solublenutrients called tocopherols and dietary intake of vitaminE activity is essential in many species (Table 3) [80] 120572-Tocopherol is absorbed via the lymphatic pathway and trans-ported in association with chylomicrons In plasma alpha-tocopherol is found in all lipoprotein fractions but mostlyassociated with apo-B-containing lipoproteins in man Inrats approximately 50 of alpha-tocopherol is bound tohigh density lipoproteins (HDL) After intestinal absorptionand transport with chylomicrons alpha-tocopherol is mostlytransferred to parenchymal cells of the liver wheremost of thefat-soluble vitamin is stored Very small amount of vitamin

E is stored in the nonparenchymal cells (endothelial stellateand Kupffer cells) 120572-Tocopherol is secreted in associationwith very low density lipoprotein (VLDL) from the liver [80]In the rat about 90 of total body mass of alpha-tocopherolis recovered in the liver skeletal muscle and adipose tissueMost 120572-tocopherol is located in the mitochondrial fractionsand in the endoplasmic reticulum whereas little is found incytosol and peroxisomes (Table 3) [80]

Normally vitamin E derived from food is absorbed in theintestine and then transported into the liver on moleculescalled chylomicrons After a meal chylomicrons are formedto transport fat-soluble vitamins (such as vitamin E) dietaryfats and cholesterol from the intestine to the liver Oncein the liver 120572TTP transfers vitamin E from chylomicronsto very low-density lipoproteins (VLDLs) which carry fatfat-soluble vitamins and cholesterol from the liver to othertissues throughout the body The VLDLs are then releasedinto the bloodstream so the accompanying vitamin E canbe used in the body The 120572TTP protein is also thought totransport vitamin E to nerve cells (neurons) in the brainVitamins or their derivatives perform important roles ascatalysts of enzymatic reactions hence they are also knownas coenzymes Biotin pyridoxal-phosphate and riboflavincan be covalently bound to the apoenzymes and are calledprosthetic groups Other coenzymes in contrast serve asmobile carriers of reactive chemical compounds Most reac-tions of primary metabolism require the help of vitaminsThe TTPA gene provides instructions for making the 120572-tocopherol transfer protein (120572TTP) which is found in theliver and brain This protein controls the distribution ofvitamin E obtained from the diet (also called 120572-tocopherol)to cells and tissues throughout the body Vitamin E is anantioxidant that protects cells in the body from the damagingeffects of unstable molecules called free radicals (Table 3)

Quinone compounds act as membrane resident carriersof electrons between components of the electron transportchain in the periplasmic space of prokaryotes and in themito-chondria of eukaryotes Vitamin K is a quinone compoundin the human body in a storage form as menaquinone (MK)distribution includes regulated amounts in mitochondrialmembranes The human brain which has low amounts oftypical vitamin K dependent function (eg gamma carboxy-lase) has relatively high levels of MK and different regions ofbrain have different amounts [81] Coenzyme Q is a quinonewhich synthesizes de novo and its levels of synthesis declinewith age The levels of MK are dependent on dietary intakeand generally increase with age MK has a characterized rolein the transfer of electrons to fumarate in prokaryotes Anewly recognized fumarate cycle has been identified in brainastrocytes (Table 3) [81]

Slc23a1 is ascorbic-acid transporter that is essential forvitamin C transport into the brain many tissues (Table 3)Vitamin C crosses the placenta and is essentially requiredin the perinatal period (Sotiriou et al 2002) [83] It is usedto prevent scurvy and works as a cofactor for hydroxylasesrequired for posttranslational modifications that stabilizecollagen In addition vitamin C is also essential for per-forming many enzymatic reactions Vitamin C also acts as afree radical scavengerMoreover few specific nonoverlapping


transport proteinsmediate the transport of the oxidized formof vitamin C dehydroascorbic acid and the reduced form L-ascorbic acid across biological membranes Vitamin C alsocrosses the blood-brain barrier in the oxidized form throughthe glucose transporters [82] (Table 3) Dehydroascorbic aciduptake occurs by facilitated-diffusion glucose transportersGLUT 1 3 and 4 but under physiological conditions Thesetransporters are unlikely to play a major role in the uptakeof vitamin C due to the high concentrations of glucose thateffectively block its influx In addition L-ascorbic acid enterscells via Na+-dependent systems that have two isoformsSVCT1 and SVCT2v transporters Transport by both isoformsis stereospecific with a pH optimumof approximately 75 anda Na+ and ascorbic acid stoichiometry of 2 1 SVCT2 mayexhibit a higher affinity for ascorbic acid than SVCT1 but witha lower maximum velocity [83] The two isoforms also differin their tissue distribution SVCT1 is present in epithelialtissues whereas SVCT2 is vitamin C transport systems ofmammalian cells occuring in most tissues with the exceptionof lung and skeletal muscle and mammalian cells (Table 3)[85]

38 Transport of Neurotransmitters Neurotransmitters aresmall water-soluble molecules which are diffused across themembrane of presynaptic and post synaptic neuron fiberThere are three main categories of substances that act asneurotransmitters First category is amino acids such asglutamic acid GABA aspartic acid and glycine secondcategory is of peptides such as vasopressin somatostatinand neurotensin third category is of monoamines like nore-pinephrine dopamine serotonin and acetylcholine Amongall neurotransmitters glutamic acid (=glutamate) and GABAare major neurotransmitters of the brain but monoaminesand acetylcholine perform specialized modulating functionsoften confined to specific structures The peptides neuro-transmitters perform specialized functions in the hypotha-lamus and act as cofactors elsewhere in the brain Based onchemical nature and receptor binding brain neurotransmit-ters are different and perform varied functions For exampleglutamate performs excitatory function whereas others (likeGABA) are primarily inhibitory (Table 4) Dopamine inter-acts with receptors and shows either excitatory or inhibitoryeffect These are receptors which determine whether a trans-mitter acts rapidly by direct action on an ion channel (egnicotinic acetylcholine receptors) or slowly by a second-messenger system that allows for synaptic plasticity (egmuscarinic acetylcholine receptors) Interestingly both speedand mechanism of transmitter inactivation after the signalalso act as a factor In addition different neurotransmitterssuch as acetylcholine serotonin and catecholamine aresynthesized released transported inactivate and signal indifferent manner These widely differ in delivery in neuronalcircuits located in the brain (Table 4) [86]

More often CNS contains many neurotransmitters butperipheral nervous system contains acetylcholine and nore-pinephrine neurotransmitters Acetylcholine is a major neu-rotransmitter that occurs in the peripheral nervous systemIt is usually but not always an excitatory neurotransmitterin contrast to the monoamine neurotransmitters which are

nearly always with a few exceptions inhibitory Acetylcholineis synthesized in the brain from acetyl-CoA and formed inglucose metabolism and is in association with choline whichis actively transported across the blood brain barrier Theacetyl-CoA and choline are independently synthesized inthe neuron cell body (motor neurons) Most dietary cholinecomes from phosphatidyl choline (Table 4) the major phos-pholipid in the membranes of plants and animals but it doesnot occur in bacteria Acetylcholine is independently trans-ported along the axon to the synapse where they are conju-gated into acetylcholine Acetylcholine receptors are locatedon the surface of post synaptic cell membrane of nerve cells ofbrain as well as outside the brain onmuscle cell surface whereit controls excitation Acetylcholine is transported throughreceptor-mediated transport either by ligand gated channelsor by G protein coupled receptor receptors Similarly cholineis also transported carrier-mediated process Acetylcholineis largely inhibited by certain molecules such as dimethylaminoethanol hemicholinium and tetraethyl ammoniumchloride As it is true that the brain and its blood brainbarrier cannot synthesize choline its transport occurs bytransporters which regulate the formation of acetylcholine inthe central nervous system (Table 4) [87] Moreover bindingof neurotransmitters to a G protein coupled receptor inducesthe opening and closing of a separate ion channel proteinover a period of seconds or minutes More exceptionallyneurotransmitters mainly opioids are directly released fromthe brain to the blood through saturable glycoproteins (Pgp)transport system [88] It is also true that all neurotransmittersare released by exocytosis which involves certain molecularevents and protein complexes [86] Moreover bisazaaromaticquaternary ammonium salts act as ligands for the bloodbrain barrier choline transporter [89] by exploiting nutrienttransporters at the blood brain barrier and can improve braindistribution of small molecules [90] while stress inducescholine transport across blood barrier (Table 4) [91]

381 AminoAcidNeurotransmitters Glycine is a neurotrans-mitter that is only found in vertebrate animals After synthe-sis it is released into a synapse and binds to a receptor whichmakes the postsynaptic membrane more permeable to Clminusions Thus glycine hyperpolarizes the membrane and makesit less likely to depolarize It is an inhibitory neurotransmitter(Table 4) Aspartate is neurotransmitter that is primarilylocalized to the ventral spinal cord and opens an ion channelIt is inactivated by reabsorption into the presynaptic mem-brane Aspartate is an excitatory neurotransmitter whichincreases the likelihood of depolarization in the postsynapticmembrane Moreover both glutamic acid and aspartic acidare the two acidic amino acids found in proteins which pos-sess 2 carboxyl groups These amino acids destroy neuronswhen released in excessive amounts Interestingly both ofthese form an excitatoryinhibitory pair in the ventral spinalcord comparable to the excitatoryinhibitory pair formed byglutamate and GABA in the brain Glutamate is the mostcommonneurotransmitter in the brain It is always excitatorybecause of simple receptors that increase the flow of positiveions by opening ion channels Glutamate stimulation isterminated by a (chloride-independent)membrane transport


Table 4 Major neurotransmitter transporters occur at blood brain barrier

Neurotransmitters Transportermechanism Location Influx or effluxfunction References

Neurotransmitters DiffusionMembrane of presynapticand postsynaptic neuronfiber

Influx [86]

Choline Carrier-mediatedprocess

Membrane of presynapticand postsynaptic neuronfiber

Influx and efflux regulatethe formation ofacetylcholine in thecentral nervous system

[87]

AcetylcholineFacilitated diffusionsaturable glycoproteins(Pgp) transport system

Neuronal circuits of brainmembrane of presynapticand postsynaptic neuronfiber

Influx Synaptictransmission occurs inperipheral nervoussystem

[86 88ndash90]

Glycine Receptor mediated Membrane receptorsMakes the postsynapticmembrane morepermeable to Cl-ions

Aspartate Aspartate ion channel Membrane receptors

Glutamate

Glutamatechloride-independentmembrane transportsystem

Membrane receptors

Glutamateasparate ATP-driven Na+-K+-ase Membrane receptors

Biogenic aminesnorepinephrine

Norepinephrinetransporter uptake1 anduptake2carrier-mediatedtransporters Net(Slc6a2) Dat (Slc6a3)and Sert (Slc6a4)

Norepinephrine

Norepinephrinetransporter Alzheimerrsquosdisease and attentiondeficient hyperactivitydisorder

[68 92 93][94ndash96] [97]

Melatonin Membrane receptors

Melatonin receptors inthe area postremalocated outside theblood-brain barrier

[98 99]

Peptides Membrane receptors [100]Cholecystokinin-1receptor agonist

Cholecystokinin-1receptor agonist Membrane receptors [101 102]

GI hormone Membrane receptors [103 104]Insulin transferrininsulin-like growthfactors and vasopressin

Receptor-mediatedtranscytosis Membrane receptors [105]

Serotonin Serotonin

GABA (GAT2 BGT1) Amino acid transporter GABA

GABA imposesinhibitory effects on thebrain neurons that showprotective role duringhypoxia or ischemiaactive transport into theastrocyte glial cells

NucleosidetransportersEquilibrativenucleoside transporters(ENTes amp ENTei)

Nucleoside transporters NucleosidePurine and pyrimidinenucleosides purine andpyrimidine nucleosides

Ion transportersNaK-ATPase Active transport Potassium Nerve conductionCa-ATPase Active transport Calcium Bone marrowSodium Active transport Sodium Nerve conduction


Table 4 Continued


Potassium chloride Active transport Potassiumchloridecotransporter Nerve conduction

ATP-binding cassette(ABC) transporters

P-glycoprotein (P-gp)multidrug resistancetransporter(mouseMdr1a [Mdr3] Mdr1a[Mdr3]Mdr1b [Mdr1] andMdr2 human MDR1 amp2)

Active transport

Organic hydrophobicand amphipathic cations120573-adrenergic blockersanthracyclinescyclosporine A digoxinetoposideglucocorticoidsloperamidemorphine phenytoinvecuronium verapamiland vinca alkaloids

Inhibition of drugtransport

Multidrug resistanceassociated protein(MRP1)

Active transport

Amphipathic anions andglutathione glucuronideand sulfate conjugatesbenzylpenicillindoxorubicin etoposidemethotrexate vinblastineandvincristine

Efflux

MRP2 (c-MOAT) Active transport Organic anions Efflux

MRP3 (MOAT-D) Active transport Methotrexate anionicconjugates Efflux

MRP4 (MOATB) Active transport Nucleoside-basedantivirals Efflux

MRP5 (MOAT-C) Active transportCyclic nucleotidesglutathione conjugatesand organic anions

Efflux

MRP6 (MOAT-E) Active transport Peptides Efflux

Breast cancer resistanceprotein BCRP) Active transport

Similar to P-gp substratescamptothecin derivativesdaunorubicindoxorubicinmitoxantrone andtopotecan

Efflux

OCT1-3 Active transport

Monoamineneurotransmitterscationic neurotoxinsamantadine memantinedesipramine dopamineand serotonin

Influx and efflux

OCTN1 Active transportAntiarrhythmic drugstetraethylammoniumverapamil

Influx and efflux

OCTN2120573-lactam antibioticsL-carnitine andtetraethylammonium

Influx and efflux

Organic ion transportingpolypeptide 1 (oatp1OATP1)

Active transport

Bulky organic cationsglucuronide conjugatessteroid hormones estronesulfate indinavirnelfinavir opioid agonistsand ouabain

Influx and efflux


Table 4 Continued


Oatp2 OATP2 Active transport

Amphipathic anions bileacids cholate digoxinestrogen conjugatesopioid agonists andouabain taurocholatedehydroepiandrosteronesulfate andestrone-1-sulfate efflux[26]-enkephalin353-triiodo-L-thyronine

Influx and efflux

Oatp3 Active transportDigoxin [26]-enkephalinfexofenadine353-triiodo-L-thyronine

Influx and efflux

BBB-specific aniontransporter type 1 (Bsat1oatp14)

Active transport T4 Influx and efflux

OATP-A OAT1 OAT3 Active transport

Amphipathic anions bileacids deltorphinII estrone-1-sulfatefexofenadineopioid peptides andbenzylpenicillinCimetidinedehydroepiandrosteronesulfate estrone sulfateindoxyl sulfate and PAH

Influx and efflux

system that is only used for reabsorbing glutamate andaspartate across the presynaptic membrane Glutamate andaspartate reenter the cell by a transporter driven by the highextracellular concentrations of Na+ and the high intracellularconcentrations of K+ Sodium enters the cell along withthe amino acids while potassium leaves the cell in similarproportion Thus entry of both glutamate and asparate isindirectly powered by the ATP-driven Na+-K+-ase (sodiumpump) which creates the high ion concentration gradients(Table 4) As aspartate is an essential amino acid that doesnot synthesize in biological systems Hence it is eithersupplemented in the diet as synthetic chemical or suppliedby any exogenous source

Aspartate binds specifically to the NMDA glutamatereceptor which is regulated both by a glutamatemolecule andby voltage NMDA receptors interact with ligand repeatedlyand develop capacity for an activity-dependent increase insynaptic efficiency that work as long-term potentiation orLTP which plays important role in learning and memoryNMDA receptors are found densely concentrated in the cere-bral cortex mainly in hippocampus (CA1 region) amygdalaand basal ganglia These are highly vulnerable to glutamicacid that makes damaging effects due to excessive excitatoryneurotransmitter release Similarly excitotoxicity due to glu-tamic acid is a major destructive process seen in stokes andother forms of brain ischemia Moreover granule cells of thedentate gyrus of the hippocampus are rich in nitric oxide syn-thetase Glutamate stimulation of NMDA receptors resultsin nitric oxide synthesis and enhancing neurotransmitter

release from adjacent synapses Here nitric oxide works asneuromodulator (Table 4) Similarly monosodium glutamate(MSG) occurs as a major component of soya sauce anddestroys nerve cells when fed to young animals Normallyit does not cross the blood-brain barrier but functionsas a neurotransmitter is an important question Increasedalertness (or anxiety) due to caffeine may be mainly dueto blockage of adenosine receptors which normally inhibitglutamate release In turn glutamate released into synapses iseither reabsorbed directly into neurons by the ion-exchangetransport system or is soaked up by astrocytes or glial cellswhich convert the glutamate into glutamine amoleculewhichcannot cause excitotoxicity The glutamine can then be safelytransported back to neurons for reconversion into glutamateOne of the damaging effects of mercury poisoning is swellingof astrocytes which are rendered unable to soak up glutaminefrom synapses contributing to excitotoxicity (Table 4)

382 Gamma Amino Butyric Acid (GABA) GABA is themajor inhibitory neurotransmitter of the brain occurringin 30ndash40 of all synapses Its concentration in the brainis 200ndash1000 times greater than that of the monoamines oracetylcholine It occurs highly concentrated in the substantianigra and globus pallidus nuclei of the basal ganglia followedby the hypothalamus the periaqueductal grey matter andthe hippocampus GABA receptor is connected to a chlorideion channel that allows more chloride ions to enter insidethe cell and make the membrane less likely to depolarizeGABA is synthesized in the brain from the Krebs citric


acid molecule that is 120572 keto glutarate (Table 4) GABAis commonly inactivated after release into the synapse byactive transport into the astrocyte glial cells that are closelyassociatedwith synapses GABA is synthesized fromglutamicacid and is catabolized back into the citric acid cycle Thevitamin B6 derivative pyridoxal phosphate is a cofactor inthe synthesis of GABA It imposes inhibitory effects on thebrain neurons that show protective role during hypoxia orischemia Both alcohol and barbiturates show similar effectson the GABA receptors In fact potentiation of chlorideinflux into neurons is a major mechanism in the effect ofethanol on the brain (Table 4)

383 Primary Monoamine Neurotransmitters Dopaminenorepinephrine and serotonin are primary monoamineneurotransmitters Biochemically both dopamine andnorepinephrine are catecholamines while serotonin is anindolamine Both dopamine and epinephrine are primarilyinhibitory neurotransmitters and make postsynaptic cellsinhibitory for catecholamines There are 3-4 times moredopaminergic cells in the CNS than adrenergic cellsDopamine in the caudate nucleus facilitates posture whereasdopamine in the nucleus is found associated with animalrsquosspeed and pleasure There are two primary dopaminereceptor types D

1(stimulatory) and D

2(inhibitory) both

act through G-proteins D2receptors (Table 4) More often

these occur on the dopaminergic neurons and provide partialnegative feedback These are autoreceptors that can inhibitboth dopamine synthesis and release Moreover amino acidtyrosine is converted into dihydroxyphenylalanine (DOPA)by the tyrosine hydroxylase enzyme using oxygen ironand tetrahydrobiopterin (THB) that act as cofactors Highconcentrations of dopamine inhibit tyrosine hydroxylaseactivity through an influence on the THB cofactor There arefour main dopaminergic tracts in the brain the nigrostriataltract tuberoinfundibular tract mesolimbic tract andmesocortical tract Dopamine cells project topographicallyto the areas they innervate Overstimulation of D

2receptors

in the mesolimbic and mesocortical systems evokesschizophrenia Dopamine is also responsible for inductionof vomiting by stimulation of D

2cells in the chemoreceptor

trigger zone stimulation of growth hormone release byD2receptors and increased exploration and locomotion

Interestingly in the males sexual behavior is increased bydopamine agonists whereas sexual behavior in females isincreased by dopamine antagonists Moreover for treatmentof Parkinsonian patients DOPA treatment is given but itdevelops psychotic symptoms resembling schizophrenia(Table 4)

Norepinephrine Norepinephrine is a major neurotransmitterthat occurs in the peripheral nervous system It is synthe-sized from dopamine means of the enzyme dopamine beta-hydroxylase (DBH) with oxygen copper and vitamin Cas cofactors inside adrenal medulla Dopamine synthesisoccurs in the cytoplasm but norepinephrine is synthesizedin the neurotransmitter storage vesicles Cells which utilizenorepinephrine for formation of epinephrine use s-adenyl

methionine (SAMe) as a methyl group donor an elevatedlevel of cortisol in the medulla induce phenylethanolamineN methyltransferase (PNMT) an enzyme which catalyzesthe conversion of norepinephrine to epinephrine Themost prominent norepinephrine-containing (noradrenergic)nucleus identified is locus ceruleus located in the pons whichaccounts for over 40 of noradrenergic neurons in the ratbrain Other noradrenergic neurons are clustered in lateraltegmental area neocortex hippocampus and cerebellumthat receive noradrenergic stimulation exclusively from thelocus ceruleus (Table 4) Most of the dopaminergic innerva-tion of the hypothalamus comes from the lateral tegmentalnuclei

Epinephrine plays important role in short-term andlong-term regulator of stress and development of illness[106] It occurs in much lesser amount than norepinephrineDesipramine inhibits norepinephrine reuptake with littleeffect on dopamine Similarly imipramine and amitriptylineare inhibitors of norepinephrine and serotonin reuptake bythe presynaptic terminals but are more potent for sero-tonin Beta-noradrenergic receptors also apparently inhibitfeeding whereas alpha-receptors seem to stimulate feedingMoreover MAO inhibitors reduce metabolism of all cate-cholamines it is believed that the antidepressant effect ismore related to norepinephrine than to dopamine Morespecifically tricyclic antidepressants such as amitriptyline 3-ring structure increase appetite that is considered as a sideeffect that is induced in weight gain Contrary to this bothcocaine and amphetamine reduce appetite But cocaine actsas a potent inhibitor of catecholamine reuptake but it doesnot act as an antidepressant However excessive cortisolsecretion causes depression and is associatedwith diminishednoradrenergic inhibition of corticotropin-releasing hormonesecretion in the hypothalamus which induces anxiety inexperimental animals Similarly triple reuptake inhibitors(TRIs) inhibit serotonin-norepinephrine-dopamine reup-take enhance monoaminergic neurotransmission by block-ing the action of the monoamine transporters and raiseextracellular concentrations of these neurotransmitters inbaboons and humans (Table 4) [92]

Similarly it is also well confirmed that catecholaminergicand serotonergic neurotransmitter systems are implicated inthe pathophysiology of attention-deficithyperactivitydisorder (ADHD) The amino acid tyrosine is theprecursor for synthesis of the catecholamines dopamineand norepinephrine while tryptophan is the precursor ofserotonin Similarly a disturbed transport of tyrosine aswell as other amino acids generates number of psychiatricdisorders such as schizophrenia and bipolar disordersMoreover an altered tryptophan and alanine transport infibroblasts causes attention-deficithyperactivity disorderin growing juveniles (ADHD [107] In addition naturalantioxidant compounds which pass through BBB are betterneuroprotective agents and show novel approach towardsexcitotoxicity protection and oxidative stress associatedwith excess 120573 amyloid (A120573) preservation in AD This isrepresented by selective glutamatergic antagonists thatpossess antioxidant capabilities Both GSH and (R)-120572-lipoic acid (LA) get covalently linked with the NMDA


receptor antagonists memantine (MEM) and are used intreatments of Alzheimerrsquos disease (AD) It is a symptomaticapproach based on the use of cholinesterase inhibitors orN-methyl-D-aspartate (NMDA) receptor antagonists [108]Norepinephrine transporter (NET) plays important rolein pathophysiology of many neurodegenerative diseasessuch as Alzheimerrsquos disease and hyperactivity disorders[93] Dopamine norepinephrine and serotonin excessand their respective amino acid precursors have also beenassociated with brain dysfunction [109] Furthermoredecreased availability of acetylcholine during critical illnessleads to decreased counter-regulatory activity in response toinflammatory disease states that causes additional injury andneurotransmitter imbalances (Table 4)

Serotonin Serotonin is a neurotransmitter which is inde-pendently synthesized from tryptophan transported acrossthe blood brain barrier Normally its 1-2 concentration isfound in the brain but highest concentration occurs in thepineal body Serotonin neurotransmitter neurons are locatedin the raphe nuclei Vitamin D hormone regulates serotoninsynthesis [110] Serotonin synthesis is a 2-step process thefirst step of which requires the enzyme tryptophan hydrox-ylase with oxygen iron and THB as cofactors Neither theenzyme nor the cofactors are rate limiting for either stepof these reactions Serotonin concentration in the brainis far more sensitive to the effects of diet than any othermonoamine neurotransmitter Serotonin concentration canbe increased up to 10-fold by dietary supplementation inlaboratory animals Hence consumption of a meal rich incarbohydrate branch-chained amino acids and tryptophanhas a particularly dramatic effect because both glucose fromcarbohydrate and branch-chained amino acids (especiallyleucine) increase insulin secretion Insulin facilitates thetransport of the branch-chained amino acids into musclecells thereby reducing the competition tryptophan facesfor the large neutral amino acid transporter that takes itacross the blood brain barrier In mammals noradrenergicneurons near the optic nerve are inhibited by light Indarkness norepinephrine stimulation of pineal cells causesthe release of cyclic AMP second-messenger which activates(phosphorylates) the N-acetyl transferase enzyme whichcatalyzes acetylation of serotonin (Table 4)

Melatonin Melatonin (MEL) is an endogenous neurohor-mone that performs many biological functions and showspowerful antioxidant effect [98] It is synthesized fromserotonin in a 2-step process that takes an acetyl groupfrom acetyl-CoA and a methyl group from by SAMe(sminusadenosyl methionine) Melatonin induces pigment light-ening in cells and acts as a neurotransmitter that regu-lates diurnal (circadian) and seasonal behavior and physi-ology in mammals Melatonin ameliorates neural functionby promoting endogenous neurogenesis through the MT2melatonin receptor in ischemic-stroke mice [111] Melatoninshows protective effects against ischemic stroke and reducesoxidativeinflammatory stress This results in preservationof BBB integrity and enhances endogenous neurogenesis byupregulating neurodevelopmental geneprotein expression

[111] Circulatingmelatonin canmodulate baroreceptor reflexcontrol of HR thus resetting it toward lower HR valuesThe modulatory effects of melatonin may be mediated viamelatonin receptors in the area postrema located outside theblood brain barrier (Table 4) [8] MEL protects the braintissue from the oxidative stress induced hypobaric hypoxia(HH) and prohibit loss of pericellular haloes shrunken neu-rons with scanty cytoplasm and hyperchromatic pyknotic orabsent nuclei reactive gliosis and edema Melatonin counter-acts the deleterious effects of oxidative stress and defies neu-ronal death reactive astrogliosis memory impairment andcognitive dysfunctions Therefore all dietary supplementscontaining melatonin are proved highly useful because oftheir neuroprotective activities for the therapy of hypoxia-induced consequences Melatonin in humans acts as aninhibitor of sexual activity and also stimulates productionof brown adipose tissue a special form of fat which (whenburned) only produces heat not ATP This is especiallyimportant for hibernating animals (Table 4)

Similar to melatonin insulin in the brain also regulatesthe metabolism molecular composition and cognitive per-formance of microcircuits and reduces food intake cerebralinsulin levels are altered in diabetes aging obesity andAlzheimerrsquos disease [112] Insulin level in the brain is severalfolds higher than blood plasma levels Local applicationof glucose or glibenclamide to neurogliaform cells mimicsthe excitation suppressing effect of external insulin on localmicrocircuits via insulin receptors Thus neurogliaform cellsmight link GABAergic and insulinergic action in corticalmicrocircuits [112] There is an increase in vascular disorderssuch as hypertension and Alzheimerrsquos disease (AD) Thereis association between hypertension and an increased riskof developing AD in population and antihypertensive med-ications in the management of cognitive disorders whichimprove independent blood pressure lowering effects andlead to better treatment andor prevention options for AD[113] (Table 4)

384 Peptide Neurotransmitters Peptides are the most com-mon neurotransmitters in the hypothalamus Their complexstructure can allow for high receptor specificity They are allsynthesized on ribosomes and are inactivated by hydrolysisat the synapse rather than by reuptake Peptides are farmore potent than other neurotransmitters requiring onlyvery small amounts to produce a profound effect Evenvery minute amounts of somatostatin can inhibit growthhormone release Opioid peptides include the endorphinsenkephalins and dynorphins Enkephalins are frequentlyfound in presynaptic (axoaxonic) synapses Opiates andenkephalins (or endorphins) inhibit the firing of locusceruleus neuronsThe highest concentration of opioid recep-tors is found in the sensory limbic and hypothalamic regionsof the brain and is particularly high in the amygdala andperiaqueductal grey area Opioids tend to be released asslower-acting cotransmitters which modulate the action ofthe associated neurotransmitter (such as glutamate) whichis being released from the same synapse Although opioidsare generally inhibitory they have an excitatory effect on


hippocampal pyramidal neurons mediated by inhibition ofGABA release (Table 4)

Neuropeptides play a crucial role in the normal func-tion of the central nervous system and peptide receptorshold great promise as therapeutic targets for the treatmentof several CNS disorders[104] But it is very difficult toharness drug-like properties of peptides to transform theminto marketable therapeutic molecules because of their poorstability solubility and incompatibility Moreover recenttechnical advances have broken these limitations and somanytherapeutic peptide drugs are developed to treat a wide rangeof diseases such as diabetes cancer and pain Further thesame is favored and practiced by clinicians for potentialutility of agonists at central neurotensin cholecystokininneuropeptide Y and oxytocin receptors [104] Moreoversuccessful approaches have been developed to increase thestability and longevity of peptides in vivo (Table 4) It leadsto the improvement in delivery because of easy penetrationacross the blood brain barrier [104] Further therapeuticpotential of peptide agonists is also harnessed for the treat-ment of major CNS disorders such as schizophrenia anxietydepression and autism

Neuropeptides are signaling molecules which participatein the modulation of synaptic transmission and are stored indense core synaptic vesiclesThese are released after profoundexcitation [100] Only in the extracellular space neuropep-tides act on G-protein coupled receptors to exert a relativelyslow action both pre- and postsynaptically [100]Thereforeneuropeptide modulators are considered ideal candidates toinfluence epileptic tissue overexcited during seizures Manyof these peptides are considered to participate in endogenousneuroprotective actions because they bind to their own recep-tors that implicate them in epilepsy and other CNS disordersMainly neuropeptide receptors occur in the hippocampusand are widely concerned to temporal lobe epilepsy Sim-ilarly receptors of other neuropeptides like somatostatinneuropeptide Y galanin dynorphin enkephalin substance Pcholecystokinin vasoactive intestinal polypeptide hormonessuch as ghrelin angiotensins corticotropin-releasing hor-mone adrenocorticotropin thyrotropin-releasing hormoneoxytocin and vasopressin involved in epilepsy [100] There-fore activation and inhibition of receptors by oral applicationof peptides as drugs are typically not efficient because of lowbioavailability rapid degradation and insufficient penetra-tion of peptides through the blood brain barrier Furtherdevelopment of nonpeptide agonists and antagonists of neu-ropeptide receptors as well as gene therapeutic approachesleads to the local production of agonists and antagonistswithin the central nervous system (Table 4) [100]

Similarly neuropeptide substance P (SP) has been impli-cated in inflammation pain depression and breast cancercell (BCC) growth SP plays role in trafficking of BCCs(human MDA-MB-231 and MDA-MB-231BrM2 cells) acrossthe blood brain barrier (BBB) and brain microvascularendothelial cells (BMECs) using in vitro and in vivo models[114] The proinflammatory peptide substance P promotesblood brain barrier breaching by breast cancer cells throughchanges in microvascular endothelial cell tight junctionsSP secreted from BCCs induces transmigration of BCCs

across the BBB leading to activation of BMECs and secretionof TNF-120572 and Ang-2 resulting in BBB impairment andcolonization of tumor cells in brain (Table 4) Thereforetherapies based on SP inhibition in combination with othertherapies may prevent breaching of the BBB by BCCs andtheir colonization in brain (Table 4) [114]

Similarly gastrointestinal hormones act in the centralregulation of energy metabolism and show potential sensoryroles for the circumventricular organs [103] These gener-ate a variety of circulating signals which provide essentialinformation to the central nervous system (CNS) regardingnutritional statusThe gastrointestinal system producesmanyof such molecules that show profound effects on feedingbehavior and the control of metabolism as a consequenceof their ability to regulate the neural circuitry involvedin metabolic homeostasis Many of these substances showlipophobic characteristics which could be used for therapeu-tics diseases neurosensory organs [103] More often sensorycircumventricular organs (CVOs) found in the brain are notprotected by the normal blood brain barrier These possessreceptors for and functional actions of gastrointestinal hor-mones such as amylin cholecystokinin ghrelin and peptideYY in the area postrema and subfornical organ These playsignificant roles for the sensory CVOs in the regulation ofenergy balance (Table 4) [103]

Cholecystokinin (CCK) seems to function in the pro-duction of satiety administration of small quantities of thispeptide into the ventricles or the paraventricular nucleusinhibits feeding It is associated with dopamine synapses insome limbic areas and appears tomodulate dopamine releaseSuch peptide synergy with other transmitters is commonThese neuropeptides show synergy as GABA is found asso-ciated with somatostatin and serotonin with substance PSimilarly low doses of the peptide vasopressin can enhancelearning in laboratory animals but humans with vasopressindeficiency show no signs of memory impairment It increasesblood pressure and should be approached with cautionSafer analogues may yet be found Similarly leptin regulatesenergy expenditure and body weight by acting both on thehypothalamus and on peripheral targets [101] and centralactions of leptin are enhanced by cholecystokinin (CCK)The interaction between leptin and CCK causes physiologicalalterations in animals experimental after infusion of moreleptin into the CNS [101] Similarly apolipoprotein AIV (apo-AIV) and cholecystokinin (CCK) concentration generategastrointestinal satiation signals that are stimulated by fatconsumption [102]Therefore peripheral apo-AIV requires anintact CCK system and vagal afferents to activate neurons inthe hindbrain to reduce food intake [102]

385 Biogenic Amine Neurotransmitters The trafficking ofneurotransmitters within the brain depends greatly on theso-called uptake1 and uptake2 carrier-mediated processeswhich maintain the balance between neurotransmitters inthe intracellular and extracellular fluids of the nervousparenchyma [68] These transporters also make the extracel-lular clearance of biogenic amine neurotransmitters at synap-tic clefts Both uptake systems regulate the neurotransmitterconcentration and modulateterminate receptor-mediated


effects within the neurovascular unit (NVU) Uptake2 (Oct1-3Slc22a1-3 PmatSlc29a4) and Mate1Slc47a1 transportersare also involved in the transport of xenobiotics Morespecifically functional Net and Oct1 transporters occur at theluminal BRB surface These heterogeneous transport proper-ties of the brain and retina NVUs suggest that the BBB helpsprotect the brain against biogenic amine neurotransmitters inthe plasma while the BRB hasmore of ametabolicendocrinerole (Table 4) [68]

More specifically both transporters uptake1 and uptake2play important role in the clearance of dopamine sero-tonin and norepinephrine at the synaptic cleft [94 97]These are classified based on their affinitycapacity for sub-stratesinhibitors and sensitivity to ions More specificallyuptake1 transporter binds with high-affinity to Na+Clminus-dependent unidirectional symporters such as Net (Slc6a2)Dat (Slc6a3) and Sert (Slc6a4) while uptake2 transportersshow lower affinityUptake1 is polyspecific and shows bindingto Na+Clminus-independent symporters Uptake1 transporterassists in transportation of some biogenic amine neurotrans-mitters and xenobiotics in both directions that depends ontheir concentration gradient Among all symporters bestknown are Oct1-3 (Slc22a1-3) and Pmat [95 96 115] It isalso true that trafficking of biogenic amine neurotransmittersis not restricted to neurons but it might also occur at theblood-nerve barriers and in some peripheral tissues [116ndash118] Therefore it is much possible that both uptake12transporters could modulate the local availability of biogenicamine neurotransmitters by luminal (blood) andor ablumi-nal (nerve side) carrier systems More exceptionally uptake2carriers could also transport many drugs and xenobiotics like1-methyl-4-phenylpyridinium (MPP+) (Table 4) [69]

39 Exchange of Metal Ions between Plasma and BrainCapillaries in the brain possess an impermeable barrierto some solutes and regulate the movement of substancesbetween blood and brain These brain capillary endothelialcells facilitate the transcapillary exchange of some saltsMoreover for sodium and potassium transport endothelialcell contains distinct types of ion transport systems on the twosides of the capillary wall that is the luminal and antiluminalmembranes of the endothelial cell (Table 5) [119] Thereforehighly specific solutes can be pumped across the capillaryagainst an electrochemical gradient These transport systemsplay important role in the active secretion of fluid from bloodto brain and in maintaining a constant concentration of ionsin the brainrsquos interstitial fluid [120] Further to performmajorphysiological functions exchange of important metal ionssuch as K+ and Na+ takes place very fast from blood totissues but it shows slow rate when entered into the brain[41 121] More specifically Na+ exchange across the bloodbrain barrier takes place by carrier-mediated transport [120]mainly by brain capillary Na+ K+ATPase system [122] Thisis located primarily on the antiluminal membrane of theendothelial cell that may also mediate removal of interstitialfluid K+ from the brain and maintain a constant brain K+concentration in the face of fluctuating plasma concentra-tions Thus antiluminal location of Na+ K+-ATPase may

have important role in secretion of interstitial fluid anextrachoroidal source of CSF (Table 5) [119]

More specifically plasma membrane of metazoan cellspossesses multiple transport proteins Na+K+ pumpK+channel Na+ lysine symporter channel and sodium-lysine transporter which transport metal ions and aminoacids from the extracellular medium into the cell Thesetransporter proteins facilitate active movement of smallmolecules or ions [34] There occur three different types oftransport proteins first type is the uniport transport passa single type of molecule down its concentration gradientwhile symporters are cotransport proteins which allow themovement of both types of ions in the same direction outsideto inside An antiporter allows movement of one moleculeagainst its concentration gradient and never allows othermolecules to go inside More specifically pumps utilizethe energy released by ATP hydrolyzed by Na+K+ATPasefor active transport of specific ions or small moleculesagainst their chemical gradient Contrarily these channelspermit movement of specific ions or water down theirelectrochemical gradient Third type of ionic movement isperformed by membrane transporter proteins such as ABCtransport proteins (Table 5) [35]

Further it is also true that brain lacks some metaltransporters and blood brain barrier stops excess transportof metallic ions BBB restricts entry of neurotoxic metalsinto the central nervous system (CNS) that may causeneurodegenerative diseases Though it is a hard fact thatexcess ofmetals (Cu FeMn andZn) contributes to neurode-generative diseases but in trace amount these are essentialfor human health [33] Pb tin and Hg requirement is notvery clear hence its transport occurs very scarely Moreoverplasma membrane of rat brain neuronal cells also containszinc transporters (Table 5) [33]

But aluminium uptake takes place by transferrin-receptor-mediated endocytosis [36] Few hydrophilic pro-teins like transferrin carry metal species such as the freemetal ion and complexes of the metal with an amino acidor protein But these would not be expected to be able todistribute metal ions across the blood brain barrier (BBB)at a rate that is sufficient to meet the requirements of thebrain as BBB limits the diffusion of non-lipophilic substancesinto brain An example of metal species specific transportat the BBB involves Me Hg which forms a complex withL-cysteine that is transported into the brain and perhapsinto and out of astrocytes Cu is delivered to the CNSbut it must be transported across either the blood brainbarrier or the CSF barrier and it accumulates within braincapillaries of experimental mice [28ndash30] ATP 7A catalyzesthe stepwise transport of Cu across brain endothelial cellsand astrocytes to supply neurons with a regulated supply ofCu [31 32] that is quite similar of brain Fe metabolism [130]Fe uptake occurs across the cerebelar vascular endotheliumafter which it associates with glial cells and neurons for take-up Choroid plexus actively produces and secretes CSF andhas pivotal role in maintaining the homeostasis but it mayalso provide a pathway for metal transport mainly for Cu[131] Androstane causes receptor-mediated upregulation ofATP-driven xenobiotic efflux transporters at the blood brain


Table 5 Major amino acid transporters occur at blood brain barrier

Type of amino acid Transporter Location Influx or efflux References

Influx

Amino acid carrier systemor alanine preferringtransporter and 1-dihydroxy-phenylalanine(1-DOPA)

Antiluminal surface ofthe brain capillary [123 124]

Alanine glycine prolineand 120574-aminobutyric acid

A-(alanine-preferring-)system carrier

Antiluminal surface ofthe brain capillary Influx [64 125]

Small neutral aminoacids (system A)L-alanineL-asparaginineL-cysteine glutamineglycine L-histidineL-proline and L-serine

A-system carriersecondary active transport

Antiluminal surface ofthe brain capillary Influx efflux [126]

Basic acidic and120573-amino acidsglutamate aspartate and120573-amino acid taurine

Simple carrier system Membrane receptors Influx [127 128]

L-proline Na+-gradient-dependenttransport Membrane receptors [129]

Large neutral amino acid(system L) (LAT1 andLAT2)

Facilitated diffusion

Branched or aromaticside chain aminoacidsL-DOPA120572-methyl-DOPAgabapentin L-leucinemelphalanL-phenylalanineL-tryptophan andL-tyrosine

Influx

Na+-LNAA neutralamino Acid


Alanine glycinehistidine isoleucineleucine methioninephenylalaninethreonine tryptophantyrosine and valine

Efflux

Neutral amino acidsystem ASC) (ASCT1and ASCT2)


L-alanine L-aspartateL-cysteineL-glutamate glycineL-isoleucine L-leucineL-methionine L-serineL-threonine andL-valine

Efflux

System B0+ Simple carrier system Basic and neutral aminoacids Efflux

System 120573 Simple carrier system 120573-alanine taurineBasic amino acid Simple carrier system Lysine InfluxAcidic amino acid(system xminus) Facilitated diffusion L-aspartate L-glutamate

System Xminus Simple carrier system L-aspartate L-glutamate Efflux

N-systemA-system carrierSecondary activetransport

L-asparaginineglutamine glutamateL-histidine and L-serine

Excitatory aminoacid (EAAT1-3) Simple carrier system Aspartate glutamate Influx


Table 5 Continued


Amine (cationsystem y+) Active transport

L-arginine cholineL-cysteine lysine andornithine

Influx

System T Simple carrier system Thyroid hormones T3and T4

barrier [35] while blood brain barrier flux of aluminummanganese iron and other metals is suspected to contributeto metal-induced neurodegeneration (Tables 1 and 5) [36]

Certain metal transporters maintain metal flux acrossthe blood brain barrier choroid plexuses as well as insensory nerves and this metal uptake occurs from the nasalcavity [37] Thus for maintaining homeostasis and normalbody health physiological amounts of useful metals aretransported by protein transporters But blood brain barrierflux of aluminum manganese iron and other metals issuspected to contribute to metal-induced neurodegenerationand causes dyshomeostasis [36] Aluminium uptake takesplace by transferrin-receptor-mediated endocytosis and ofaluminum citrate by system Xc and an organic anion trans-porter [132] Contrary to this manganese uptake takes placeby transferrin- and non-transferrin-dependent mechanismswhich may include store-operated calcium channels andthe lack of transporter-mediated manganese brain effluxIron uptake takes place by both transferrin-dependent andindependent mechanisms of brain while copper is deliveredto the brain by copper transporters ATP7A and ATP7BMembrane transporters are often quite substrate specificFor example the divalent metal transporter (DMT1 DCT1and Nramp2) transports divalent metals but not metals inother valence states Another example of substrate specificityis greater binding affinity of transferrin for Fe+3 than Fe+2[36 38 39] (Tables 1 and 5) As a result transferrin-receptor-mediated endocytosis (TfR-ME) for Fe+3 occurs more thanfor Fe+2 [36 38 39] Furthermore the affinity of the trans-ferring receptor for halo transferrin (diferric transferrin) isconsiderably greater than for monoferric transferring [42]TfR-ME is also believed to play a role in the transport of othertrivalent metals into the brain such as Al and Mn Similarlythere seems to be a putative role of zinc transporters ZnTand Zip in regulating brain zinc concentration by uptakemechanisms Zinc uptake is facilitated by L- and D-histidineBrain uptake of metals may involve a non-energy-dependentprocess store-operated calcium channels andor an ATP-dependent calcium pump [36] Methyl mercury can form acomplex with L-cysteine that mimics methionine enablingits transport by the L system (Tables 1 and 5) [43]

The transport of essential metals and other nutrientsacross tight membrane barriers such as the gastrointestinaltract and blood brain barrier ismediated by specific transportmechanisms [133] These transporters take up metals at theapical surface and export them at the basolateral surface andare involved in their intracellular distribution Transportersfor each of the major essential metals calcium iron and zincare separate and are divalent metal transporter 1 that mediate

the transport of nonessential metals across tight membranebarriers For example the intestinal iron transporter divalentmetal transporter 1mediates the uptake of lead and cadmiumThus levels of essential metals are strictly regulated bytransporters [133] When dietary levels of essential metalsbecome low levels of the corresponding transporters increasein the intestine after which there is a greater potential forincreased transport of toxic metals In the brain the strictregulation of metals prevents injury that would potentiallyresult from oxidative damage induced by the essential metalsiron copper and zinc Indeed the oxidative damage foundin neurodegenerative diseases is likely to be due to higherlevels of these metals [133] Involvement of intracellulartransporters raises the levels of iron zinc and copper thatmight be due to a disruption in the activity of transportersin Alzheimerrsquos disease Similarly exposure to toxicants alsoaffects the activity of transporters that potentially couldcontribute to the aetiologyprogression of neurodegenerativediseases (Tables 1 and 5) [133]

The regulation of metal ion transport [120] within neu-rons is critical for normal brain function [40] Moreover forregulation of redox metals such as iron become essential(Fe) because excess levels of these metals can contributeto oxidative stress and protein aggregation leading to neu-ronal death More importantly divalent metal transporter 1(DMT1) plays a central role in the regulation of Fe as wellas other metals hence failure of DMT1 regulation is linkedto human brain pathology DMT1 is regulated by Ndfip1(Nedd4 family-interacting protein 1) an adaptor proteinthat recruits E3 ligases to ubiquitinate target proteins Inhuman neurons Ndfip1 is upregulated and binds to DMT1 inresponse to Fe and cobalt (Co) exposure [40]This interactionresults in the ubiquitination and degradation of DMT1resulting in reduced metal entry while induction of Ndfip1expression protects neurons frommetal toxicity and removalof Ndfip1 by shRNAi results in hypersensitivity to metalsMore specifically Nedd4-2 as an E3 ligase is recruited byNdfip1 for the ubiquitination ofDMT1within humanneuronsand Ndfip1(minusminus) brains accumulate Fe within neurons andsuggest a critical role for Ndfip1 in regulating metal transportin human neurons (Tables 1 and 5) [40]

DMT1 possesses four or more isoforms that transporteight different metals for multiple purposes [134] TwomRNA isoforms differ in the 31015840 UTR +IRE DMT1 hasIRE (iron responsive element) but minusIRE DMT1 lacks thisfeature The +minusIRE proteins differ in the distal 18 or 25amino acid residues after shared identity for the proxi-mal 543 residues The +minusIRE proteins as the apical irontransporter in the lumen specifically minusIRE form participate


in metal detoxification and remove out metal toxicity Itplays important role in management of toxic challenges andmetal homeostasis in body tissues [134] and is involved inendosomal exit of ironNormally airways represent a gatewaytissue for metal entry

Furthermore divalent metal-ion transporter 1 (DMT1)is iron-preferring membrane transport protein that playsindispensable roles in intestinal nonheme-iron absorptionand iron acquisition by erythroid precursor cells Raremutations in human DMT1 result in severe microcytic-hypochromic anemia An iron responsive element (IRE) inthe mRNA 31015840-untranslated region permits the regulationof some isoforms by iron status [135] More specificallynatural resistance associated with macrophage protein 1(NRAMP1) is the only member of the mammalian SLC11gene family which contributes to antimicrobial function byextruding from the phagolysosome divalent metal ions (egMn2+) SLC11 gene also encodes proteins like divalent cationtransporter 1 (DCT1) and solute carrier family 11 [136 137]Possibly it may be essential cofactors for bacteria-derivedenzymes or required for bacterial growth An intestinalnonheme-iron transporter DMT1 is a validated therapeutictarget in hereditary hemochromatosis (HHC) and other iron-overload disorders [135] DMT1 represents a large family oforthologous metal ion transporter proteins that are highlyconserved from bacteria to humans [138] As its namesuggests DMT1 binds a variety of divalent metals includingcadmium (Cd2+) and copper (Cu2+) DMT1 is best known forits role in transporting ferrous iron (Fe2+) and its expressionis regulated by body iron stores to maintain iron homeostasis(Tables 1 and 5) DMT1 is also important in the absorptionand transport of manganese (Mn2+) [139] More specificallyduring brain injury the homeostasis of transition metal ions(eg Fe2+ Co

2

+) is grossly unbalancedThis renders the brainenvironment toxic due to unwelcomed and uncontrolledmetal entry into stressed neurons precipitating in theirpremature death Ndfip1 protects neurons against excesstransition metal exposure In the presence of excess Co

2

+

or Fe2+ ions Ndfip1 is upregulated Moreover Ndfip1 bindsto DMT1 leading to DMT1 ubiquitination and degradationinhibits metal entry and allows neuronal recovery Ndfip1has larger relation to brain diseases (eg Parkinsonrsquos disease)arising from metal accumulation Ndfip1 protects humanneurons from death by ubiquitination and degradation ofmetal transporterDMT1DMT1may be themajor transporterof manganese across the blood brain barrier and expressionof this protein in the nasal epithelium provides a routefor direct absorption of metals into the brain [140] DMT1expression is found to be increased in the substantia nigraof Parkinsonrsquos patients and in the ventral mesencephalon ofanimal models Its expression in the brain increases withthe age [141] that also increases the susceptibility to metalinduced pathologies Moreover the DMT1 encoding geneSLC11A2 occurs on the long arm of chromosome 12 (12q13)close to susceptibility regions for Alzheimerrsquos disease [142]and restless legs syndrome Similarly C allele of SNP rs407135on the DMT1 encoding gene SLC11A2 is associated withshorter disease duration in cases of spinal onset amyotrophic

lateral sclerosis [143] DMT1 is also found implicated inAlzheimerrsquos disease onset in males as well [142] MoreoverCC haplotype for SNPs 1254TC IVS34+44CA is associatedwith Parkinsonrsquos disease susceptibility [144] while variantalleles on several SLC11A2 SNPs are associated with ironanemia (Tables 1 and 5) These are also found as a riskfactor for manganese intoxication and restless legs syndrome[145] Therefore it is true that the lack or overexpression oftransporters causes toxic accumulation of divalent metalsespecially iron andor manganese It generates metal defi-ciency and enhances cellular responses in affected persons[134] Moreover lack of metal transporters works as impor-tant aetiological factors in a variety of neurodegenerativediseases including Alzheimerrsquos disease Parkinsonrsquos diseaseamyotrophic lateral sclerosis and multiple sclerosis

310 Transport of Amino Acids Brain capillary endothelialcells are connected by extensive tight junctions and arepolarized into luminal (blood-facing) and abluminal (brain-facing) plasma membrane domains This affects polar dis-tribution of transport proteins and mediates amino acid(AA) homeostasis in the brain The existence of two facili-tative transporters for neutral amino acids (NAAs) on bothmembranes provides the brain access to essential AAs Morespecifically there occurs a common carriertransport systemfor large amino acids across the blood brain barrier whichregulates concentration of amino acid and takes out excess orelevated level of amino acids More often carriertransportsystem can lower down the transport of amino acids bycompetitive inhibition or may convert into the requiredamino acids But distinct transport systems facilitate theuptake of basic acidic and beta amino acids In case ofphenylketonuria and maple syrup urine disease few aminoacids like phenylalanine accumulated in excess in the bloodstop uptake of other essential amino acids Further excessof amino acids affects the LNAA carrier and allows transferof more amino acids beyond damaging level into the CNSThus large amino acids like phenylalanine leucine tyrosineisoleucine valine tryptophan methionine and histidineenter the cerebrospinal fluid rapidly like glucose through thecommon carrier system or a single amino acid transportersystem Possibly it may be a leucine preferring transporterOn the other hand entry of small amino acid such asalanine glycine proline and 120574-aminobutyric acid (GABA)is restricted or controlled because higher influx of theseamino acids may enhance the control of neurotransmittersThese amino acids are required for syntheses of neuro-transmitters in the brain and are selectively transportedout of the CNS into the blood through amino acid carriersystem or alanine-preferring transporter and 1-dihydroxy-phenylalanine (1-DOPA) In addition few smaller neutralamino acids synthesized in the brain and several of them actas putative neurotransmitters But all these neutral L-aminoacids show variable transport into the brain by differenttransport systems [123 124] Neutral L-amino acids transportdepends on amino acid concentration and its utilizationbecause excess amount of it may stop the transport Morespecifically amino acid that cannot be synthesized by brainand are metabolically essential require its regular supply


It is maintained by protein breakdown and dietary supplyof amino acids Contrarily both transport and uptake oflarge amino acids into the brain are inhibited by the syn-thesis of 2 aminonorbornane-2-carboxylic acids but not by2-(methyl-isobutyric acid (MeAIB) [123 124] This is themain reason that supply of neutral amino acids such asalanine glycine proline and 120574-aminobutyric acid (GABA)is restricted into the brain by these inhibitors [125] Someof these neutral large amino acids are transported by A-(alanine-preferring)-system carrier [64] This carrier systemis not found on the luminal surface of the blood brainbarrier but may occur on antiluminal surface of the braincapillary [126] There is a contrast in its transportationbecause distinct transport systems facilitate the brain uptakeof basic acidic and 120573-amino acids Lysine and arginine areessential amino acids which are provided from the bloodafter dietary assimilation The acidic amino acids glutamateand aspartate are both important metabolic intermediates aswell as neurotransmitters [127 128] while the brain contentof these amino acids is maintained primarily by de novosynthesis They also can be transported into the brain ata slow rate across the blood brain barrier The 120573-aminoacid taurine is present at high concentrations in the brainand is involved in volume regulation Absorption surface inGI tract contains brush boarder membrane that possess anovel system which mediate Na+neutral amino acids [146]and show Na+-gradient-dependent transport of L-proline[129]More specifically transport of cationic and zwitterionicamino acids [147] and methyl mercury transport across theblood brain barrier occurs by an amino acid carrier [148]Both leucine and alanine are taken up by cultured cerebralcapillary endothelial cells [149]

311 Transport of Proteins Delivery of therapeutic peptidesand proteins to the central nervous system is the biggestchallenge for development of more effective neuropharma-ceuticals [105] Many plasma proteins are not able to crossthe endothelial tissue barrier because of their large size andhydrophilicity This is the main reason why concentrationsof certain proteins such as insulin and transferrin varywith the change in plasma concentrations Consequentlytheir concentrations in the brain become very low exceptfew exceptions Due to controlled uptake of some proteinsbrain becomes saturable and their further uptake is stoppedReceptor-mediated transcytosis across the blood brain bar-rier may be useful way to transport protein and peptide ther-apeutics into the brain [150] Moreover proteins like insulintransferrin insulin-like growth factors and vasopressin crossthe BBB by receptor-mediated transcytosis (Table 4) [123]Interestingly protein receptors occur in outnumber in braincapillary endothelial cells which bind to transporting proteinand form a receptor complex This complex is endocytosedinto the endothelial cell to form a vesicle which assists ineventual release of intact protein on the other side of theendothelial cells that is still not fully known Contrary tothis blood brain barrier is impermeable to most molecules[105] and only amphiphilic derivative of a peptide couldmake it possible to deliver into the brain For this purposea drug peptide is suitable and designed to self-assemble into

nanofiber and its active epitope is tightly wrapped around thenanofiber core [105]Thus a novel drug delivery system couldmaintain high brain permeability through receptor-mediatedendocytosis (Table 5)

Moreover mechanism of nanoparticle mediated drugtransport across the BBB appears to be receptor-medi-ated endocytosis [151] Thus transport of nanoparti-clesnanomolecules across the blood brain barrier requiresboth specific and nonspecific interactions with proteinsexpressed on the luminal and or abluminal surface ofthe brain endothelium cells [152] More specifically a high-affinity iron chelator is conjugated to lactoferrinmolecules bycarbon dioxide mediated coupling reaction [153] SimilarlyTf-FMMs transferring-conjugated fluorescein loadedmagnetic nanoparticles are also delivered by receptor-mediated transcytosis [63] while antitransferrin receptorantibodies with a Fab cargo are delivered across the BBB byreceptor-mediated endocytosis [154] In another methodpolycationic proteins and lectins are allowed to cross theblood brain barrier by a similar but nonspecific processcalled absorptive-mediated transcytosis In this processpeptide binding to specific receptors in the membraneis not mandatory and proteins are directly absorbed tothe endothelial cell membrane based on charge or affinityfor sugar moieties of membrane glycoproteins Rests ofthe subsequent transcytotic events are followed similar toreceptor-mediated transcytosis It is also important thattransport capacity of absorptive-mediated transcytosis isgreater because the number of receptors present in themembrane does not limit it In addition cationizationmay provide a mechanism for enhancing brain uptake ofalmost any protein but both endocytosis and transcytosismechanisms play important role in the distribution ofmacromolecules [152] The degree of passage of smallpeptides across the blood brain barrier significantly exceedsthat of the vascular markers while peptide transport acrossthe membrane barriers shows either saturable or facilitatedtransport and transmembrane diffusion or passive diffusionMore specifically low molecular weight small peptides aretransported in and out of the central nervous system bycarrier-mediated transcytosis across the blood brain barrierwhich acts as a saturable mechanism (Table 4)

312 Transport of Drugs BBB is impermeable to proteinsdrugs and other biomolecules as these are not able tocross the blood brain barrier because of their size andhydrophilicity But few peptide hormones which regulatebody metabolism and normal functions are transportedeasily Similarly both insulin and transferring across BBBand their concentration vary in plasma because its uptakein the brain is greater due to their size and lipid sol-ubility These are carried to the brain by specific trans-port processes mainly membrane bound efflux pumps andchannels The major transport mechanism which carriesimportant proteins and hormones is receptor-mediated tran-scytosis Contrary to this BBB restricts entry of most of thebiomolecules mainly proteins peptides carbohydrates andvaccinesHence delivery of therapeutic peptides andproteinsto the central nervous system is the biggest challenge for


development of more effective neuropharmaceuticals [105]More often therapeutic agents may reach the target sites atintracellular locations The brain capillary endothelial cell ishighly enriched in receptors for these proteins and followingbinding of protein to the receptor a portion of themembranecontaining the protein-receptor complex is endocytosed intothe endothelial cell to forma vesicle Although the subsequentroute of passage of the protein through the endothelial cellis not known there is eventual release of intact proteinon the other side of the endothelial cell The blood brainbarrier is impermeable to most molecules [105] but duringtrafficking of biomolecules a portion of compound is lostdue to ineffective partitioning across the membrane Thispartitioning across the membrane is widely concerned topolarity lipophilicity of molecules that attribute easy passageacross the membrane Therefore amphiphilic derivatives ofa peptide are easily delivered into the brain It was designedto self-assemble into nanofiber which in the active peptideepitope is tightly wrapped around the nanofiber core [105]

Recently several neuroprotective proteins and peptidesof potential therapeutic value have been designed and usedto fulfill the crucial need for effective and safe transcapillarydelivery methods to the brain Hence most promising drugdelivery through brain capillaries is possible by augmenta-tion of pinocytotic vesicles This is a cellular mechanismin which large molecules of neurotherapeutic potential areconjugated to peptidomimetic ligands for delivery Later onthese molecules bind to selected peptide receptors whichinternalize and transported it in small vesicles across thecytoplasmic brain capillary barrier These conjugates werefound functionally active and effective in animal modelsof neurological disease Similarly neurotrophin a brain-derived neurotrophic factor easily passes through BBB andhas great therapeutic value Interestingly all short peptideswith hydrophilic nature showed favorable safety profiles inbrain after coming across the BBB found neuroprotectiveExogenous recombinant human erythropoietin was provedbeneficial in treating global and focal cerebral ischemiaand reducing nervous system inflammation in experimentalanimals Moreover other than neuroprotective compoundsmonoclonal antibodies are also used to pass through BBB byreceptor-mediated endocytosis mechanism Similarly metal-lothioneins a superfamily of highly conserved amino acidscontains low molecular weight polypeptides play significantrole in the regulation of essential metals which are alsointernalized by receptormediated endocytosisThus variableefficiencies of endocytosis mechanisms such as intracellulartrafficking aiding in the release of therapeutic agents intothe cytoplasm are important aspects in drug delivery andtherapeutic potencyThere are possibilities that after diffusionand translocation of the therapeutic agents these remainsusceptible targets of certain catalytic enzymes or physicallypartition into the nucleus or in any other suborganelles thatmay also alter its actual activity Further excess delivery oftherapeutic agents may create competitive problem to someother biomolecules that may hinder normal functions ofcells cellular organelles enzymes and signaling moleculesas well In addition metabolic wastes may also overburdenthe cell cytoplasm that inhibit so many normal cellular

functions and give rise to drug induced adverse effects Useof nanoparticles may solve this problem due to controlledrelease of drugs in required quantity These can easily cutdown concentration of metabolic waste materials bymaskingthe therapeutic agents from their biological environmentNanoparticles allow controlled (sustained) drug release fromthe matrix determine required bioavailability and showreduction of the dosing frequency These are proved themost successful drug carriers due to their high stabilityhigh carrier capacity and feasibility of incorporation of bothhydrophilic and hydrophobic substances into brain or insidecells These also show feasibility to deliver drugs by followingvariable routes of administration including oral applicationand inhalation

Normally twomechanisms are employed to ascertain thatthe internalization of biomolecules mainly liquids is liquidsare seep in by pinocytosis and solids by phagocytosis Butcarrier-mediated delivery of drugs by nanoparticles is alsodeveloped in which nanoparticles are ingested by cells fromthe medium or from anymicroenvironment surrounding thecell More often nanoparticles are pouring in by receptor-mediated endocytosis that operates by membrane manip-ulation to envelope and allows materials to absorb insideMoreover nanoparticles get inside the cells by three differentmechanisms that is phagocytosis pinocytosis and receptor-mediated endocytosis Phagocytosis is associated with fewcells types such as macrophages neutrophils and dendriticcells that can absorb materials of micrometer in size thatis 10 120583m in diameter Similarly pinocytosis is a universalmechanism which occurs in all cell types and it delivers dif-ferent types of liquids having a submicron size and substancesin solution inside the cell More specifically larger sizednanoparticles are taken up by the cell by phagocytosis whilesmaller ones are absorbed by pinocytosis and almost ingestedby all cell types Therefore both types of nanoparticles haveimportant but separate advantages Furthermore there isanother mechanism which is known as absorptive-mediatedtranscytosis that is especially used to traverse polycationicproteins and lectins This is a nonspecific process in whichbiomaterials rather than binding to specific receptors in themembrane proteins absorb the endothelial cell membranebased on charge or affinity for sugar moieties of mem-brane glycoproteins Its subsequent transcytotic events areprobably similar to receptor-mediated transcytosis Henceoverall capacity of absorptive-mediated transcytosis is fargreater than that of receptor-mediated endocytosis becausethe number of receptors present in the membrane does notlimit it Thus cationization may provide a mechanism forenhancing brain uptake of almost any protein

In case of facilitated diffusion (a form of carrier-mediatedendocytosis) binding of a ligand occur to a transporter on oneside of the membrane that triggers a conformational changein the protein receptor Facilitated diffusion is passive (ieenergy independent) and contributes to transport at the BBBof substances such as monocarboxylates hexoses aminesamino acids nucleoside glutathione and small peptide[155] Carrier-mediated transport can also be divided into anumber of differentmechanismsdependent on energy andorcotransport of another substance Cotransport may be in


the same direction (symport) or in the opposite direction(antiport) This process proceeds from a region of highconcentration to a region of low concentration Endocytosiscan be segregated into bulk-phase also known as fluidphase endocytosis and mediated endocytosis (receptor andabsorptive-mediated) Bulk-phase endocytosis (pinocytosis)is the nonspecific uptake of extracellular fluids and occurs ata constitutive level within the cell via mechanisms which areindependent of ligand binding [156] Bulk-phase endocytosisis temperature and energy dependent noncompetitive andnonsaturable Bulk-phase endocytosis occurs to a very lim-ited degree in the endothelial cells of the cerebral microvas-culature [157]

Receptor-mediated endocytosis (RME) provides a meansfor selective uptake of macromolecules Cells have receptorsfor the uptake of many different types of ligands includinghormones growth factors enzymes and plasma proteinsRME occurs at the brain for substances such as transferrin[158] insulin [159] leptin [160] and IGF-I and IGF-II [161]and is a highly specific type of energy dependent transportSubstances that enter a cell by means of RME becomebound to receptors that collect in specialized areas of theplasma membrane known as coated pits The coated pitscontain the electron dense clathrin protein and other proteins[162] When these bound to ligand pits invaginate into thecytoplasm and then pinch free of the plasma membraneform coated vesicles The clathrin vesicle coat is rapidlyremoved to form smooth-coated endosomes that form acompartment of uncoupling receptor and ligand (CURL)[163] The endosomal membrane contains proton ATPasesthat result in acidification of the endosome interior anddissociation of the ligand from the receptor within the CURL

Absorptive-mediated transport (AME) is triggered byan electrostatic interaction between a positively chargedsubstance usually a charge moiety of a peptide and thenegatively charged plasma membrane surface (ie glycoca-lyx) [164] AME has a lower affinity and higher capacitythan receptor-mediated endocytosis The development ofmany new drug delivery technologies focuses on AME[165] Another significant transport mechanism at the BBBis carrier-mediated efflux This mechanism is involved inextruding drugs from the brain and is a major obstacle formany pharmacological agents ABC (ATP binding cassette)transporter P-glycoprotein is the principle efflux mechanismof these agents [166] There also exist efflux transportersfor organic anions via multidrug resistance associated pro-tein (MRP) [167] and anionic and cationic cyclic peptide[168] Additionally the peptide transport system- (PTS-)1 shows efflux transport of synthetic opioid peptide Tyr-MIF-1 [169] Peptides are the most common neurotrans-mitters in the hypothalamus Their complex structure canallow for high receptor specificity They are all synthesizedon ribosomes and are all inactivated by hydrolysis at thesynapse (rather than by reuptake) Peptides are far morepotent than other neurotransmitters requiring only verysmall amounts to produce a profound effect Even veryminute amounts of somatostatin can inhibit growth hormonerelease

3121 CNS Protection Drug delivery to the brain is a chal-lenging task because presence of blood brain barrier (BBB)complicates the delivery of drugs to the brain [64] It is aphysiological barrierwhich imposesmajor obstacle for a largenumber of drugs including antibiotics antineoplastic agentsand neuropeptides to pass through the endothelial capillariesto brain There are some other reasons that is drug maybind to nontransporters in larger amount that render the drugineffective or seem theoretically active But it is true that drugmight show inability to pass through the barrier with theadhered protein Besides enzyme action may also make thedrug inactive or in a nontherapeutic intermediate compoundAll these circumstances with the drug and its delivery mustbe accounted for making effective drug formulations to treatthe CNS disease [27] Though several methods ways andstrategies have been developed for drug delivery to the brainbut most of them are proved invasive and lack the targetspecificity Therefore for active transfer of drug to the brainsafer disruption of BBB or its loosening is highly importantFurther for successful delivery of drugs to the CNS bloodbrain barrier disruption or opening with intra-arterial infu-sion therapy allows both the chemotherapeutic agents andantibodies to enter through blood brain barrier [20] ThusBBB dysfunction could be of great therapeutic value in condi-tions in which neuronal damage is secondary or exacerbatedby BBB damage More exceptionally for delivery of drugs orany therapeutic agent BBB is disrupted by ultrasonic soundwaves or it may forcibly break down by sudden raised highblood pressure due to hypertension trauma ischemia neuralinflammation and very high concentration of substancesmicrowave radiation and exposure to infectious agents thatcan open the BBB In addition reducing halting or reversingthe structure and function of BBB might open new ways todeliver chemotherapeutic agents in case of brain tumor Butforced opening or structural damage to the BBB allows theuncontrolled passage of drugs [13] Further there are severalareas of the brain where BBB is very thin or supposed tobe loose or weak from here drug can allow pass to thebrain This also allows important metabolic substances tocross into the brain somewhat freely These areas of brain areidentified as circumventricular regions these are pineal bodyneurohypophysis and area postrema

Further drug delivery strategies have been improvedfor safe delivery of different types of drugs to CNSThese improved strategies include liposomes colloidal drugcarriers micelles chimeric peptide technology intranasaland olfactory route of administration and nanotechnol-ogy Moreover disruption or damage of endothelium couldallow expression of endothelial receptors which are normallydownregulated opening new communication loops betweenendothelium pericytes astrocytes and microglia These alsoplay important role in barrier repairing Moreover infectiouspathogenic viruses affect different tissue and organs butCNS diseases are severely caused by virus invade variousregions of brainTherefore drug delivery methods that couldtarget virus present in these areas may play important rolein modern drug development mainly for CNS protectionFurthermore nanoenabled delivery systems have been madethat offer more promising solution to enhance the targeted


delivery of the drugs into the brain Therefore drugs whichare loaded with nanocarriers can easily target virus multipli-cation inside brain because they could easily pass throughBBB In addition few drug carriers are proteins and itsdeficiency (P-glycoprotein) at the BBB inhibits the effluxactivity of certain biomolecules at the blood brain barrier[170]

More often most of the drug delivery methods are basedon structure activity relationships drug-to-drug receptorinteractions and structure transport relationships mainlymembrane permeation In addition drugs which are selectedbased on throughput screening may show higher bindingin silico but in reality in vivo systems invariably show poormembrane permeation because of low binding by targetreceptor Therefore it seems very difficult to pass on drugmore simply through the blood barrier in theoreticallyrelevant concentration and underlying experimental aspectof drug targeting [27] More often drugs undergo insignifi-cant transport through the brain capillary endothelium thatmakes up the blood brain barrier (BBB) in vitro But invivo system drug must bind to some specific receptor ifbinding occurs partially it will affect drug delivery Furtherin vivo condition absorption of drug in neuronal and otherbrain cells seems to be difficult after drug delivery becauseits route is obstructed in many ways Second drug mayconvert into noninteracting metabolite or a portion of itmight show higher binding to other ligands mainly proteinsThough therapeutically drug seems to be more appropriateby biological systembut it becomes ineffective or attains somehighly reactive active molecular when it reaches inside braincells If drug becomes able to pass through the barrier itmight adhere to the unwanted protein [171] Another problemis created by presence of some catabolic enzymes in the braintissues which could change the native or active form of thedrug or cleave it into an inactive molecule or slow actingpharmaceutical that may be deconstructed once it is insidethe brain tissue rendering it useless Therefore accumulatedtherapeutic biomaterials in the brain play a crucial role inthe pathogenesis of neuronal diseases [170] Hence an activepenetration of drug through blood brain barrier is highlyneedful for its delivery at the target area in the brain fortreatment of CNS disorders and diseases as well

BBB is nonselective to pass drugs by diffusion or byactive transport and is major hurdle for successful CNSdrug development But it is true that molecules like glucoseand fatlipid soluble drugs can rapidly cross into the brainContrary to this delivery of many of the drug types is verydifficult to carry them into the brain because of fat insolubleTherefore such drugs cannot be made available to the brainbecause of nondelivery across the blood brain barrier Thereare so many factors which influence the drug delivery orits ability to traverse the blood brain barrier It is truethat the membrane barrier disallows larger molecules whilesmaller molecules are carrying over to the brain Similarlycharged molecules rapidly get into the brain Thereforelipophilicity does not seem to be necessary or the only factorthat may assist the drug for safe passage to brain Thereseems to be a role of multiple factors or complex molecularproperties that make drug able to pass through the BBB

More exceptionally barrier permeability is also related tomembrane or luminal surface of brain capillary compositionof CSF or ISF functional groups change on molecularand ionic surfaces or presence of charged residues of themolecules In addition surface activity of the moleculestheir relative size and specific binding of transporter proteinsenergy driven cassettes and opening and closing of channelsdue to ionic concentration are noted as important factors[172]

Further it is true that all traditional drug delivery meth-ods are based on trial-and-errorsThese are applied invariablyfor few selected drugs that had appropriate structure-activityrelationships or drug-receptor interactions and structure-transport relationships are intact It allows drugs for suitableand successful membrane permeation Moreover all modernmethods concerned with drug development are based onrational drug design and almost all new drugs require useof receptor-based high throughput screening methods tofind appropriateness of the drug among thousands of newcompounds But it is not true that drugs that are selectedwithhigh throughput screening solely on the basis of structure-activity relationships may show target receptor binding inanimal system because they behave differently Drug mightshow invariably poor membrane permeation properties invivo Such drugs will undergo insignificant transport throughthe brain capillary endothelium which makes up the bloodbrain barrier (BBB) in vivo However several approaches fordirect drug delivery or direct convection enhanced deliveryare used to inject the drug into brain or cerebrospinal fluidor intranasal delivery These techniques are highly unsafelocal invasive metabolizable or short lasting Contrary tothis drugs that are delivered through vascular route throughBBB in physiologically required amount first infused andspread in larger portion of the brainThere are three differenttypes of strategies available for drug delivery These areneurologicaldirect injections drug delivery methods Morespecifically neurosurgical pharmacological and physiologi-cal strategies include BBB disruption by osmotic imbalanceor by using vasoactive compounds intraventricular druginfusion and intracerebral implants In pharmacologicalmethods lipid carrier or liposomes are used for drug deliveryPhysiological strategies are followed by applying endoge-nous transport mechanisms by using either carrier-mediatedtransport of nutrients or receptor-mediated transport ofpeptides Fromclinical investigations physiological strategiesare proved better and potential delivery methods because ofwider safety cover provided by drug transport

313 Cancer and Tumor Therapy Similar to blood brainbarrier blood tumor barrier also imposes so many obstaclesto drug delivery in the CNS More often brain tumormicrovesselscapillaries limit drug delivery to tumors byforming a blood brain barrier [173] Even though the bloodbrain tumor barrier is more permeable than the blood brainbarrier [173 174] it significantly restricts the delivery ofanticancer drugs and limits systematic chemotherapeutics ofbrain tumors This causes failure of drug target and madethe delivery process extremely difficult to treat solid tumorsin the brain This is the main casue of clinical failures of


many potential antitumor drugs that is usually not due tolack of potency but rather it occurs due to nondelivery ofdrug to the brain and into the tumors [175] In additionpharmaceuticals used in tumor specific therapies are foundinsufficient to check aberrant signaling pathways in braintumors [176] It makes the chemotherapeutic treatment ofbrain tumors ineffective and required amount of drug couldnot be delivered into the brain [177] Further due to very hightoxicity of antitumor chemotherapeutic drugs these cannotbe administered in sufficient concentration by conventionaldelivery methods Therefore such methods are not muchhelpful to ascertain long-term survival of the patients withbrain tumors and most of clinical cases of brain tumors areproving fatal [177] However new well-designed therapeuticstrategies that could make an easy and successful drugdelivery should be developed to save the life of patientsDrug delivery should be more responsive for delivering anappropriate therapeutic concentration of antitumor drug byapplying safer drug delivery systems or methods by beachingany physical and physiological obstacles [178]

Therefore potential techniques are to be developedfor treatment of brain tumors [175] There are commonapproaches which have been followed in the past by clin-icians that is high dose intravenous chemotherapy intra-arterial drug delivery local drug delivery via implanted poly-mers or catheters and disruption of BBB and biochemicalmodulation of drug [178] Few other drug delivery methodsare intracerebro ventricular convection enhanced deliveryBBBBTB disruption and BTB permeability modulation[179] Further to enhance the BTB permeability acceleratedtherapeutic molecules are allowed to pass through it bycellular vasomodulator-mediated transportationmechanismBTB targeting specific proteins are used to increase anti-neoplastic drug delivery to brain tumors [179] In addi-tion K (Ca) channels are potential target for biochemicalmodulation of BTB permeability to increase antineoplasticdrug delivery selectively to brain tumors [173] Further BTBpermeability is also enhanced with accelerated formationof pinocytic vesicles which could transport drugs acrossthe BTB with the help of some channel activators [174]For example infused minoxidil sulphate (MS) is a selectiveK (ATP) channel activator that comes across the BTBreaches the brain tumor and facilitates delivery of certainmacromolecules mainly Her-2 antibody adenoviral-greenflorescent protein and carboplatin to brain tumors [173]This antibody significantly increases the survival in ratshaving brain tumors Hence rat brain tumor models aredesigned to enhance drug delivery to brain tumor followingintracarotid infusion of bradykinin (Bk) nitric oxide (NO)donors or agonists of soluble guanylate cyclase (SGC) andcalcium dependent potassiumK (Ca) channels [174] Furthermodulation of these channels by specific agonists and agentsthat produce NO and cGMP in situ is essentially requiredContrary to this water-soluble compounds are limited bythe surface areapermeability of the tumor capillaries [180]Therefore in new methods BBB manipulations are beingperformed for safe delivery of drug to the brain but theseshould be noninvasive physiological and pharmacologicalNo doubt drug delivery into solid tumors can assist in

development of novel targeted molecular based therapies fortreatment of patients It could also be possible by selectiveopening of blood tumor barrier by a nitric oxide donor andinfusion required amount of drug into it Certainly safe drugdelivery will increase survival and life expectancy in tumorpatients [50 181]

4 Conclusion

The blood brain barrier provides a layer of protection forthe brain from harmful or foreign substances that mayinjure the brain It also protects the brain from action ofmetals toxicants poisons hormones and neurotransmittersThe blood brain barrier (BBB) and blood cerebrospinalfluid barrier (BCSF) possess a variety of carrier-mediatedtransport systems to support and protect brain functionsThese transport mechanisms are used for management oflevel of various substances including xenobiotic compoundsby the brain Substances with high lipid solubility maymove across the blood brain barrier by simple diffusion butnon lipid soluble substances are prohibited from diffusionBBB is a very tightly packed endothelial construction thatdisallows large molecules to pass through it but allowssmaller molecules to pass through tight junctions Moreoften glucose and other low molecular fat-soluble moleculesare also disallowed but few lipid soluble molecules suchas barbiturate drugs rapidly cross the BBB and reach thebrain More specifically movement of molecules with highelectrical charge is slowed down Morphologically the polardistribution of transport proteinsmediates amino acid home-ostasis in the brain Furthermore endothelial cells that linecerebral microvessels also maintain microenvironment forreliable neuronal signaling and regulate transport of essentialmolecules Similarly BBB exerts the greatest control over theimmediate microenvironment of brain cells

No doubt BBB like other barriers protects organs fromharmful substances (eg xenobiotics) including drugs andforeign compounds and from xenobiotics and noxiousagents It regulates the movement of ions molecules andcells between the blood and CNS It plays a crucial role inmaintaining the appropriate environment for proper neuralfunction as well as protecting the brain from injury anddisease BBB separates the pools of neurotransmitters andneuroactive agents that act centrally and peripherally andregulates the ionicmicroenvironment present inside neuronsBBB assists in healthy functioning of the brain and makespossible easy and uninterrupted supply of various moleculesand ions These molecules and ions maintain life supportsystem and physiologically protect the neurocompartmentsfrom potential disruptive and damaging effects of xenobioticagents Moreover BBB protects the brain from toxic sub-stances and restricts its entry into the neurocompartmentThus most of the neuroprotective drugs should improve thebrain function only by toning up BBB and CNS rather thantargeting neurovascular units These drugs may open theBBBrsquos tight junctions and allow the passage of growth factorsand antibodies into the brain and reduce inflammationwhich causes oedema Therefore for therapeutic purposesand protection of CNS integrity of blood brain barrier is


highly important for having better clinical outcomes in caseof neural diseases

Further molecular events and protein complexesinvolved in release of neurotransmitters by exocytosis shouldbe known because involvement of neurotransmitters mayalso induce modulatory functions in many neuronal circuitsFurthermore physiological stress response to gases suchas CO

2and clustered nursing intervention can minimize

chances of strokes and brain ischemia More specificallyneuronal cells require both electrolytes and nonelectrolytesfor performing nerve transmission but electrolytes play a vitalrole in the maintenance of osmotic pressure and acid baseequilibrium in matrix Mainly Mg2+ ions and phosphatesmaintain this property Contrary to this nonelectrolytesnormally occuring in ionizing state are Na K Mg Cu IFe Mn Fl Mo Cl Zn Co Ni and so forth These performvarious cellular functions and act as important cofactorsfor certain catabolic enzymes or associating apoenzymaticparts of many vitamins Metal ions are also component partof some hormones lipids proteins nucleic acids sugarphosphates and nucleoside phosphates The iron as animportant metal ion occurs in the hemoglobin ferritincytochromes and some enzymes as catalase and cytochromeoxidase while calcium ions occur in the blood matrix andbone The copper (Cu) manganese (Mn) molybdenum(Mo) and zinc (Zn) are useful as cofactors for enzymeactions while iodine and fluorine are essential for thethyroid and the enamel metabolism Both cell cytoplasm andmatrix are an enlarged depository of various kinds of ionsThese are highly essential for maintaining osmotic pressureand acid base balance in the cells However retention of ionsin the matrix produces an increase in osmotic pressure andallows the entrance of water in the cell

Further monocarboxylate transporters (MCTs) play amajor role in the maintenance of the glycolytic metabolismthrough the proton-linked transmembrane transport of lac-tate [60] In addition carrier-mediated transfer systemsthat is ATP powered pumps (ATP) cassettes channels andmolecular drug transporters are used to support transportof drugs and nourishments to protect and maintain vitalbrain function Moreover role of P-glycoproteins or trans-membranous proteins ATP-dependent pumps transportersand permeablitizers in transportation of various nutrientsand other essential elements is well defined Hence ATP-dependent efflux drug transporter such as Bcrp (breast cancerresistance protein) with diverse spectrum of hydrophilic andhydrophobic substrates ranging from anticancer antiviraland antihypertensive drugs should be used Further ther-apeutic impact of MCT transporters and its expression inplasma membrane of glioblastomas should be confirmedbecause monocarboxylic acid transporters control the trans-port of short chainmonocarboxylic and keto acids includingpyruvate and lactate to support brain energy metabolism Italso regulates vesicular trafficking of MCTs in cerebrovascu-lar endothelial cells Therefore for developing novel thera-peutic approaches for treating brain diseases such as strokeneuronal functions both MCT and its inhibitors should beknown These MCT inhibitors have a role in prevention

of proliferation of T lymphocytes that confirms and workas promising pharmacological targets including cancertherapy

Further alternative methods such as disruption or loos-ening of BBB may assist in diffusion of most essentialnutrients and biomaterials into the brain Similarly deliv-ery of drugs and other substances could be possible bydifferent carriers and drug transporters but seems to bea challenging task in clinical research Moreover K (Ca)channel modulation also assists in enhanced permeabilityof macromolecules into the brain These are consideredas promising target for biochemical modulation of BTBpermeability and increase the antineoplastic drug deliveryselectively to brain tumors Moreover K (Ca) channel arealso used to deliver Her-2 monoclonal antibody and greenfluorescent protein-adenoviral vectors selectively to brainfor curing neoplastic neurogenerative and viral diseasesFurther for an enhanced drug delivery to the brain tumorintracarotid infusion of bradykinin (BK) nitric oxide (NO)donors or agonists of soluble guanylate cyclase (sGC) andcalcium-dependent potassium (K (Ca)) channels were foundmore effective and safer Interestingly nitric oxide (NO) isimplicated in the regulation of vascular permeability andblood flow Bcrp is an integral protein in cancer and normalcells are a major component of the BBB located on theendothelial cells near tight junctions Similarly multidrugresistant protein 4 shows low permeability across the BBBresulting in low distribution to the brain Therefore brainto blood efflux transport systems also play important role inthe cerebral clearance of endogenous neurotoxic compoundssuch as prostaglandins and beta amyloid that can reducethe physiological effects of CNS related disorders Furtherby management of endogenous and xenobiotic compoundsby the brain permeability functions of BBB are to be reex-plored Further regulation of oxygen metabolism plasmino-gen activator thrombolytic drugs after stroke peroxisome-proliferator-activated receptors and inhibitors of kinasepathways could also assist in drug delivery In addition afteruse of drugs or other metabolites or CNS formed neurotox-icants vesicles play important role in its release Interestinglyvesicles protect cells from accumulation of wastes or drugsand show many clinical applications ranging from biomark-ers to anticancer therapy Therefore alternate methods ofdrugs delivery and substance transport should be practiced tofind an appropriate solution of CNS related pathogenicity andneurodegenerating diseases Hence to fulfill drug deliverytasks better understanding is required among clinicians andimmunologists for starting new research initiatives to makelandmark innovations in the field of pharmacology drugdelivery and clinical therapeutics of CNS related diseases

In addition molecular interactions with biological bar-riers should be properly investigated in experimental ani-mals as well as in vitro systems to develop highly efficientdrug delivery systems Moreover advanced methods areessentially required for easy delivery of healers peptidesproteins growth factors vaccines and antibodies for neu-roprotection against CNS diseases Therefore collaborativeresearch efforts should be made by clinicians and molecular


biologists to developexplore new innovative methods ofdrug delivery Further there seems an instant need of newsmaller pharmaceuticals having target specific designs toprotect the CNS from various pathological impairmentsHence use of natural transporters such as ABC cassettesaquaporins cationic pumps and substance specific ionicchannels should favor to support transport of drugs andnourishments to maintain vital brain functions Moreoverdrugs that can enhance permeability functions of nerve cellplasma membrane should be promoted It will not only helpto deliver the pharmaceuticals but also assist in finding newsignaling pathways for transport of substances mainly fuelmaterials and drugs for successful treatment of infectiousviral diseases Further normal supply of substances couldmaintain integration of CNS functions such as learningmemory sleep cycle movement hormone regulation visionhearing and many other functions More often after explor-ing transcellular signaling pathways generated by beneficialdrug candidates its action and authenticating behavior couldbe assessed much easily Hence strong recommendationsare being made to upgrade pharmaceutical technologies bycombining them with the recent advances in membranebiology and clinical sciences to extend the therapeutic use ofpharmaceuticals for treatment of neurological diseases andCNS impairments Further biomedical researchers shouldincrease the spectrum of pharmaceuticals by carrying themto targeted locations Thus by developing new endothelialtransporterscarriers and new drug candidates a rapid solu-tion of neurodegenerative and neuropathological disease andCNS protection could be achieved Further in vitro highthroughput screens and computational BBB tools togetherwith understanding of brain penetration may enable thedrug discovery community to minimize the access of drugcandidate into the CNS compartmentTherefore compoundswhich can reduce apoptosis oxidative stress induce neuritisoutgrowth synaptic plasticity and memory loss due to detri-mental effects imposed by xenobiotics and pathogens shouldbe of greater use No doubt new upgraded drug delivery sys-tems and therapeutic candidates will provide novel treatmentfor different types of CNS and neurodegenerative diseases[182]

Abbreviations

BBB Blood brain barrierCNS Central nervous systemCSF Cerebrospinal fluidGLUT Glucose transporterGABA Gama amino butyric acidMSG Monosodium glutamateNVU Neurovascular unitDMT Divalent metal transporterCURL Uncoupling receptor and ligandRME Receptor-mediated endocytosisBk BradykininNO Nitric oxide donorsSGC Soluble guanylate cyclaseMCT Monocarboxylate transportersDOPA Dihydroxy-phenylalanine

AME Absorptive-mediatedTf-FMMs Transferring-conjugated fluorescein loaded

magnetic nanoparticles

Conflict of Interests

Author has no conflict of interests The author alone isresponsible for the content and writing of the paper

References

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[2] B A Barres ldquoWhat is a glial cellrdquo GLIA vol 43 no 1 pp 4ndash52003

[3] B A Barres ldquoThe mystery and magic of glia a perspective ontheir roles in health and diseaserdquoNeuron vol 60 no 3 pp 430ndash440 2008

[4] J De Keyser J P Mostert and M W Koch ldquoDysfunctionalastrocytes as key players in the pathogenesis of central nervoussystem disordersrdquo Journal of the Neurological Sciences vol 267no 1-2 pp 3ndash16 2008

[5] G Seifert K Schilling and C Steinhauser ldquoAstrocyte dysfunc-tion in neurological disorders a molecular perspectiverdquoNatureReviews Neuroscience vol 7 no 3 pp 194ndash206 2006

[6] M V Sofroniew ldquoAstrocyte failure as a cause of CNS dysfunc-tionrdquoMolecular Psychiatry vol 5 no 3 pp 230ndash232 2000

[7] M V Sofroniew ldquoReactive astrocytes in neural repair andprotectionrdquo Neuroscientist vol 11 no 5 pp 400ndash407 2005

[8] M V Sofroniew ldquoMolecular dissection of reactive astrogliosisand glial scar formationrdquo Trends in Neurosciences vol 32 no12 pp 638ndash647 2009

[9] T Takano N A Oberheim M L Cotrina and M NedergaardldquoAstrocytes and ischemic injuryrdquo Stroke vol 40 no 3 pp S8ndashS12 2009

[10] D W Beck H V Vinters M N Hart and P A Cancilla ldquoGlialcells influence polarity of the blood-brain barrierrdquo Journal ofNeuropathology and Experimental Neurology vol 43 no 3 pp219ndash224 1984

[11] K M Mok P P Lie D D Mruk et al ldquoThe apical ectoplasmicspecialization-blood-testis barrier functional axis is a novel tar-get for male contraceptionrdquoAdvances in Experimental Medicineand Biology vol 763 pp 334ndash355 2012

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the glucose transportersrdquo Journal of Clinical Investigation vol100 no 11 pp 2842ndash2848 1997

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[89] G Zheng Z Zhang P R Lockman et al ldquoBis-azaaromaticquaternary ammonium salts as ligands for the blood-brain bar-rier choline transporterrdquo Bioorganic and Medicinal ChemistryLetters vol 20 no 11 pp 3208ndash3210 2010

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[91] R K Young and A R Villalobos ldquoStress-induced stimulationof choline transport in cultured choroid plexus epitheliumexposed to low concentrations of cadmiumrdquo The AmericanJournal of Physiology Regulatory Integrative and ComparativePhysiology vol 306 no 5 pp R291ndashR303 2014

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[93] C Mark B Bornatowicz M Mitterhauser et al ldquoDevelopmentand automation of a novel NET-PET tracer [11C]MeAPPIrdquoNuclear Medicine and Biology vol 40 no 2 pp 295ndash303 2013

[94] L C Daws ldquoUnfaithful neurotransmitter transporters focus onserotonin uptake and implications for antidepressant efficacyrdquoPharmacology andTherapeutics vol 121 no 1 pp 89ndash99 2009

[95] G Eisenhofer ldquoThe role of neuronal and extraneuronal plasmamembrane transporters in the inactivation of peripheral cate-cholaminesrdquoPharmacologyampTherapeutics vol 91 no 1 pp 35ndash62 2001

[96] H Duan and J Wang ldquoSelective transport of monoamineneurotransmitters by human plasma membrane monoaminetransporter and organic cation transporter 3rdquo Journal of Phar-macology and Experimental Therapeutics vol 335 no 3 pp743ndash753 2010

[97] A S Kristensen J Andersen T N Joslashrgensen et al ldquoSLC6neurotransm it ter transporters structure function and regula-tionrdquo Pharmacological Reviews vol 63 no 3 pp 585ndash640 2011

[98] C Vornicescu B Bosca D Crisan et al ldquoNeuroprotective effectof melatonin in experimentally induced hypobaric hypoxiardquo

Romanian Journal of Morphology and Embryology vol 54 no4 pp 1097ndash1106 2013

[99] L A Campos J Cipolla-Neto and L C Michelini ldquoMelatoninmodulates baroreflex control via area postremardquo Brain andBehavior vol 3 no 2 pp 171ndash177 2013

[100] A Dobolyi K A Kekesi G Juhasz A D Szekely G Lovasand Z Kovacs ldquoReceptors of peptides as therapeutic targets inepilepsy researchrdquo Current Medicinal Chemistry vol 21 no 6pp 764ndash787 2014

[101] V Cano B Merino L Ezquerra B Somoza and M Ruiz-Gayo ldquoA cholecystokinin-1 receptor agonist (CCK-8) mediatesincreased permeability of brain barriers to leptinrdquo BritishJournal of Pharmacology vol 154 no 5 pp 1009ndash1015 2008

[102] C C Lo W Langhans M Georgievsky et al ldquoApolipoproteinAIV requires cholecystokinin and vagal nerves to suppress foodintakerdquo Endocrinology vol 153 no 12 pp 5857ndash5865 2012

[103] T D Hoyda P M Smith and A V Ferguson ldquoGastroin-testinal hormone actions in the central regulation of energymetabolism potential sensory roles for the circumventricularorgansrdquo International Journal of Obesity vol 33 supplement 1pp S16ndashS21 2009

[104] P McGonigle ldquoPeptide therapeutics for CNS indicationsrdquoBiochemical Pharmacology vol 83 no 5 pp 559ndash566 2011

[105] M Mazza R Notman J Anwar et al ldquoNanofiber-baseddelivery of therapeutic peptides to the brainrdquo ACS Nano vol7 no 2 pp 1016ndash1026 2013

[106] D L Wong T C Tai D C Wong-Faull et al ldquoEpinephrinea short- and long-term regulator of stress and development ofillness a potential new role for epinephrine in stressrdquo Cellularand Molecular Neurobiology vol 32 no 5 pp 737ndash748 2012

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[108] P Sozio L S Cerasa S Laserra et al ldquoMemantine-sulfurcontaining antioxidant conjugates as potential prodrugs toimprove the treatment ofAlzheimerrsquos diseaserdquoEuropean Journalof Pharmaceutical Sciences vol 49 no 2 pp 187ndash198 2013

[109] C G HughesM B Patel and P P Pandharipande ldquoPathophys-iology of acute brain dysfunction whatrsquos the cause of all thisconfusionrdquo Current Opinion in Critical Care vol 18 no 5 pp518ndash526 2012

[110] R P Patrick and B N Ames ldquoVitamin D hormone regulatesserotonin synthesis Part 1 relevance for autismrdquo The FASEBJournal vol 28 no 6 pp 2398ndash2413 2014

[111] C M Chern J F Liao Y H Wang and Y C Shen ldquoMelatoninameliorates neural function by promoting endogenous neuro-genesis through theMT2melatonin receptor in ischemic-strokemicerdquo Free Radical Biology andMedicine vol 52 no 9 pp 1634ndash1647 2012

[112] G Molnar N Farago A K Kocsis et al ldquoGABAergic neurogli-aform cells represent local sources of insulin in the cerebralcortexrdquoThe Journal of Neuroscience vol 34 no 4 pp 1133ndash11372014

[113] L Nelson N Tabet C Richardson and P Gard ldquoAntihyper-tensives angiotensin glucose and Alzheimerrsquos diseaserdquo ExpertReview of Neurotherapeutics vol 13 no 5 pp 477ndash482 2013

[114] P L Rodriguez S Jiang Y Fu S Avraham andH K AvrahamldquoThe proinflammatory peptide substance P promotes blood-brain barrier breaching by breast cancer cells through changesin microvascular endothelial cell tight junctionsrdquo InternationalJournal of Cancer vol 134 no 5 pp 1034ndash1044 2014


[115] J D Catravas and C N Gillis ldquoSingle-pass removal of [14C]-5-hydroxytryptamine and [3H]norepinephrine by rabbit lungin vivo kinetics and sites of removalrdquo Journal of Pharmacologyand Experimental Therapeutics vol 224 no 1 pp 28ndash33 1983

[116] N V Chemuturi and M D Donovan ldquoRole of organic cationtransporters in dopamine uptake across olfactory and nasalrespiratory tissuesrdquo Molecular Pharmaceutics vol 4 no 6 pp936ndash942 2007

[117] G Eisenhofer ldquoThe role of neuronal and extraneuronal plasmamembrane transporters in the inactivation of peripheral cate-cholaminesrdquo Pharmacology and Therapeutics vol 91 no 1 pp35ndash62 2001

[118] S Ramamoorthy P D Prasad P Kulanthaivel F H LeibachR D Blakely and V Ganapathy ldquoExpression of a cocaine-sensitive norepinephrine transporter in the human placentalsyncytiotrophoblastrdquo Biochemistry vol 32 no 5 pp 1346ndash13531993

[119] M M Sanchez del Pino R A Hawkins and D R PetersonldquoBiochemical discrimination between luminal and abluminalenzyme and transport activities of the blood-brain barrierrdquoJournal of Biological Chemistry vol 270 no 25 pp 14907ndash149121995

[120] A L Betz ldquoTransport of ions across the blood-brain barrierrdquoFederation Proceedings vol 45 no 7 pp 2050ndash2054 1986

[121] S Nag ldquoUltracytochemical localisation of Na+K+-ATPase incerebral endothelium in acute hypertensionrdquo Acta Neuropatho-logica vol 80 no 1 pp 7ndash11 1990

[122] L M Mir H Banoun and C Paoletti ldquoIntroduction of definiteamounts of nonpermeant molecules into living cells after elec-tropermeabilization direct assess to the cytosolrdquo ExperimentalCell Research vol 175 no 1 pp 15ndash25 1988

[123] W M Pardridge ldquoBrain metabolism a perspective from theblood-brain barrierrdquo Physiological Reviews vol 63 no 4 pp1481ndash1535 1983

[124] Q R Smith S Momma M Aoyagi and S I RapoportldquoKinetics of neutral amino acid transport across the blood-brainbarrierrdquo Journal of Neurochemistry vol 49 no 5 pp 1651ndash16581987

[125] M Kayakabe T Kakizaki R Kaneko et al ldquoMotor dysfunctionin cerebellar Purkinje cell-specific vesicular GABA transporterknockout micerdquo Frontiers in Cellular Neuroscience vol 7 article286 2014

[126] A L Betz and G W Goldstein ldquoPolarity of the blood-brain barrier neutral amino acid transport into isolated braincapillariesrdquo Science vol 202 no 4364 pp 225ndash227 1978

[127] R P Shank and G Campbell LeM ldquoGlutamine and alpha-ketoglutarate uptake and metabolism by nerve terminalenriched material from mouse cerebellumrdquo NeurochemicalResearch vol 7 no 5 pp 601ndash616 1982

[128] C B Divito and S M Underhill ldquoExcitatory amino acidtransporters roles in glutamatergic neurotransmissionrdquoNeuro-chemistry International vol 73 pp 172ndash180 2014

[129] K Hayashi S Yamamoto K Ohe A Miyoshi and T KawasakildquoNa+-gradient-dependent transport of l-proline and analysis ofits carrier system in brush-border membrane vesicles of theguinea-pig ileumrdquo Biochimica et Biophysica Acta vol 601 no3 pp 654ndash663 1980

[130] M W B Bradbury ldquoThe blood-brain barrier transport acrossthe cerebral endotheliumrdquo Circulation Research vol 57 no 2pp 213ndash222 1985

[131] YKuo J Gitschier and S Packman ldquoDevelopmental expressionof the mouse mottled and toxic milk genes suggests distinctfunctions for the Menkes and Wilson disease copper trans-portersrdquoHumanMolecular Genetics vol 6 no 7 pp 1043ndash10491997

[132] N Vanduyn R Settivari J Levora S Zhou J Unrineand R Nass ldquoThe metal transporter SMF-3DMT-1 mediatesaluminum-induced dopamine neuron degenerationrdquo Journal ofNeurochemistry vol 124 no 1 pp 147ndash157 2013

[133] J P Bressler ldquoMetal transporters in intestine and brain theirinvolvement in metal-associated neurotoxicitiesrdquo Human ampExperimental Toxicology vol 26 no 3 pp 221ndash229 2007

[134] M D Garrick K G Dolan C Horbinski et al ldquoDMT1 amammalian transporter for multiple metalsrdquo BioMetals vol 16no 1 pp 41ndash54 2003

[135] A Shawki P B Knight B D Maliken E J Niespodzany andB MacKenzie ldquoH+-coupled divalent metal-ion transporter-1 functional properties physiological roles and therapeuticsrdquoCurrent Topics in Membranes vol 70 pp 169ndash214 2012

[136] ldquoSolute carrier family 11 (proton-coupled divalent metal iontransporters) member 2rdquo GeneCards 2011

[137] S Vidal A M Belouchi M Cellier B Beatty and P GrosldquoCloning and characterization of a second human NRAMPgene on chromosome 12q13rdquo Mammalian Genome vol 6 no4 pp 224ndash230 1995

[138] C Au A Benedetto and M Aschner ldquoManganese transport ineukaryotes the role of DMT1rdquo Neurotoxicology vol 29 no 4pp 569ndash576 2008

[139] H Gunshin B Mackenzie U V Berger et al ldquoCloning andcharacterization of a mammalian proton-coupled metal-iontransporterrdquo Nature vol 388 no 6641 pp 482ndash488 1997

[140] MAschner ldquoThe transport ofmanganese across the bloodbrainbarrierrdquo Neurotoxicology vol 27 no 3 pp 311ndash314 2006

[141] Y Ke Y Z Chang X L Duan et al ldquoAge-dependent andiron-independent expression of twomRNA isoforms of divalentmetal transporter 1 in rat brainrdquo Neurobiology of Aging vol 26no 5 pp 739ndash748 2006

[142] S E Jamieson J K White J M M Howson et al ldquoCandidategene association study of solute carrier family 11a members1 (SLC11A1) and 2 (SLC11A2) genes in Alzheimerrsquos diseaserdquoNeuroscience Letters vol 374 no 2 pp 124ndash128 2005

[143] H Blasco P Vourcrsquoh Y Nadjar et al ldquoAssociation betweendivalentmetal transport 1 encoding gene (SLC11A2) and diseaseduration in amyotrophic lateral sclerosisrdquo Journal of the Neuro-logical Sciences vol 303 no 1-2 pp 124ndash127 2011

[144] Q He T Du X Yu et al ldquoDMT1 polymorphism and risk ofParkinsonrsquos diseaserdquo Neuroscience Letters vol 501 no 3 pp128ndash131 2011

[145] L Xiong P Dion J Montplaisir et al ldquoMolecular geneticstudies of DMT1 on 12q in French-Canadian restless legssyndrome patients and familiesrdquo American Journal of MedicalGenetics B Neuropsychiatric Genetics vol 144 no 7 pp 911ndash9172007

[146] D Yao B Mackenzie H Ming et al ldquoA novel system A isoformmediating Na+neutral amino acid cotransportrdquo Journal ofBiological Chemistry vol 275 no 30 pp 22790ndash22797 2000

[147] L J van Winkle P M Iannaccone A L Campione and RL Garton ldquoTransport of cationic and zwitterionic amino acidsin preimplantation at conceptusesrdquo Developmental Biology vol142 no 1 pp 184ndash193 1990


[148] L E Kerper N Ballatori and T W Clarkson ldquoMethylmercurytransport across the blood-brain barrier by an amino acidcarrierrdquo The American Journal of PhysiologymdashRegulatory Inte-grative and Comparative Physiology vol 262 no 5 pp R761ndashR765 1992

[149] C CWHughes andP L Lantos ldquoUptake of leucine and alanineby cultured cerebral capillary endothelial cellsrdquo Brain Researchvol 480 no 1-2 pp 126ndash132 1989

[150] D T Wiley P Webster A Gale and M E Davis ldquoTranscy-tosis and brain uptake of transferrin-containing nanoparticlesby tuning avidity to transferrin receptorrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 110 no 21 pp 8662ndash8667 2013

[151] J Kreuter ldquoDrug delivery to the central nervous system bypolymeric nanoparticles what do we knowrdquo Advanced DrugDelivery Reviews vol 71 pp 2ndash14 2014

[152] G Xiao and L S Gan ldquoReceptor-mediated endocytosis andbrain delivery of therapeutic biologicsrdquo International Journal ofCell Biology vol 2013 Article ID 703545 14 pages 2013

[153] G Kamalinia F Khodagholi F Atyabi et al ldquoEnhanced braindelivery of deferasirox-lactoferrin conjugates for iron chelationtherapy in neurodegenerative disorders in vitro and in vivostudiesrdquoMol Pharm vol 10 no 12 pp 4418ndash4431 2013

[154] G Manich I Cabezon J del Valle et al ldquoStudy of thetranscytosis of an anti-transferrin receptor antibody with a Fab1015840cargo across the blood-brain barrier in micerdquo European Journalof Pharmaceutical Sciences vol 49 no 4 pp 556ndash564 2013

[155] A Tsuji and I I Tamai ldquoCarrier-mediated or specializedtransport of drugs across the blood-brain barrierrdquo AdvancedDrug Delivery Reviews vol 36 no 2-3 pp 277ndash290 1999

[156] M Simionescu L Ghitescu A Fixman and N SimionesculdquoHow plasma macromolecules cross the endotheliumrdquo News inPhsyiological Sciences vol 2 pp 97ndash100 1987

[157] WM Pardridge Y S Kang J Yang and J L Buciak ldquoEnhancedcellular uptake and in vivo biodistribution of a monoclonalantibody following cationizationrdquo Journal of PharmaceuticalSciences vol 84 no 8 pp 943ndash948 1995

[158] J B Fishman J B Rubin J V Handrahan J R Connor and RE Fine ldquoReceptor-mediated transcytosis of transferrin acrossthe blood-brain barrierrdquo Journal of Neuroscience Research vol18 no 2 pp 299ndash304 1987

[159] K R Duffy andWM Pardridge ldquoBlood-brain barrier transcy-tosis of insulin in developing rabbitsrdquo Brain Research vol 420no 1 pp 32ndash38 1987

[160] W A Banks and A J Kastin ldquoReversible association of thecytokinesMIP-1120572 andMIP-1120573with the endothelia of the blood-brain barrierrdquoNeuroscience Letters vol 205 no 3 pp 202ndash2061996

[161] K R Duffy W M Pardridge and R G Rosenfeld ldquoHumanblood-brain barrier insulin-like growth factor receptorrdquoMetabolism Clinical and Experimental vol 37 no 2 pp136ndash140 1988

[162] M SMoore D TMahaffey FM Brodsky and R G AndersonldquoAssembly of clathrin-coated pits onto purified plasma mem-branesrdquo Science vol 236 no 4801 pp 558ndash563 1987

[163] P Stahl and A L Schwartz ldquoReceptor-mediated endocytosisrdquoThe Journal of Clinical Investigation vol 77 no 3 pp 657ndash6621986

[164] N K Gonatas A Stieber W F Hickey and S H HerbertldquoEndosomes and Golgi vesicles in adsorptive and fluid phaseendocytosisrdquo Journal of Cell Biology vol 99 no 4 pp 1379ndash1390 1984

[165] W M Pardridge ldquoVector-mediated drug delivery to the brainrdquoAdvanced Drug Delivery Reviews vol 36 no 2-3 pp 299ndash3211999

[166] C Cordon-Cardo J P OrsquoBrien D Casals et al ldquoMultidrug-resistance gene (P-glycoprotein) is expressed by endothelialcells at blood-brain barrier sitesrdquo Proceedings of the NationalAcademy of Sciences of the United States of America vol 86 no2 pp 695ndash698 1989

[167] H Kusuhara H Suzuki and Y Sugiyama ldquoThe role of P-glycoprotein and canalicular multispecific organic anion trans-porter in the hepatobiliary excretion of drugsrdquo Journal ofPharmaceutical Sciences vol 87 no 9 pp 1025ndash1040 1998

[168] I Tamai T Ogihara H Takanaga H Maeda and A TsujildquoAnion antiport mechanism is involved in transport of lac-tic acid across intestinal epithelial brush-border membranerdquoBiochimica et Biophysica Acta Biomembranes vol 1468 no 1-2 pp 285ndash292 2000

[169] W A Banks A J Kastin L M Maness W Huang and J BJaspan ldquoPermeability of the blood-brain barrier to amylinrdquo LifeSciences vol 57 no 22 pp 1993ndash2001 1995

[170] J Bian-Sheng J Cen L Liu and L He ldquoIn vitro and in vivostudy of dodichyl phosphate on the efflux of P-glycoproteinat the blood brainrdquo International Journal of DevelopmentalNeuroscience 2013

[171] I Tamai and A Tsuji ldquoDrug delivery through the blood-brainbarrierrdquoAdvanced Drug Delivery Reviews vol 19 no 3 pp 401ndash424 1996

[172] A Seelig R Gottschlich and R M Devant ldquoA method todetermine the ability of drugs to diffuse through the blood-brain barrierrdquo Proceedings of the National Academy of Sciencesof the United States of America vol 91 no 1 pp 68ndash72 1994

[173] N S Ningaraj M K Rao and K L Black ldquoAdenosine51015840-triphosphate-sensitive potassium channel-mediated blood-brain tumor barrier permeability increase in a rat Brain tumormodelrdquo Cancer Research vol 63 no 24 pp 8899ndash8911 2003

[174] N S Ningaraj M Rao K Hashizume K Asotra and K LBlack ldquoRegulation of blood-brain tumor barrier permeabilityby calcium-activated potassium channelsrdquo Journal of Pharma-cology and Experimental Therapeutics vol 301 no 3 pp 838ndash851 2002

[175] J Rautio and P J Chikhale ldquoDrug delivery systems for braintumor therapyrdquo Current Pharmaceutical Design vol 10 no 12pp 1341ndash1353 2004

[176] A B Etame R J Diaz C A Smith T G Mainprize KHynynen and J T Rutka ldquoFocused ultrasound disruption ofthe blood-brain barrier a new frontier for therapeutic deliveryin molecular neurooncologyrdquo Neurosurgical Focus vol 32 no1 article E3 2012

[177] H-L Liu H-W Yang M-Y Hua and K-C Wei ldquoEnhancedtherapeutic agent delivery through magnetic resonanceimaging-monitored focused ultrasound blood-brain barrierdisruption for brain tumor treatment an overview of thecurrent preclinical statusrdquo Neurosurgical Focus vol 32 no 1 pE4 2012

[178] J Blakeley ldquoDrug delivery to brain tumorsrdquo Current Neurologyand Neuroscience Reports vol 8 no 3 pp 235ndash241 2008

[179] K L Black and N S Ningaraj ldquoModulation of brain tumorcapillaries for enhanced drug delivery selectively to braintumorrdquo Cancer Control vol 11 no 3 pp 165ndash173 2004

[180] R D Fross P C Warnke and D R Groothuis ldquoBlood flowand blood-to-tissue transport in 9L gliosarcomas the role of the


brain tumormodel in drug delivery researchrdquo Journal of Neuro-Oncology vol 11 no 3 pp 185ndash197 1991

[181] A Weyerbrock S Walbridge J E Saavedra L K Keefer andE H Oldfield ldquoDifferential effects of nitric oxide on blood-brain barrier integrity and cerebral blood flow in intracerebralC6 gliomasrdquo Neuro-Oncology vol 13 no 2 pp 203ndash211 2011

[182] R K Upadhyay ldquoDrug delivery systems CNS protection andthe blood brain barrierrdquo BioMed Research International vol2014 Article ID 869269 2014

Submit your manuscripts athttpwwwhindawicom

Neurology Research International

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Alzheimerrsquos DiseaseHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


BioMed Research International


Research and TreatmentSchizophrenia

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


Neural Plasticity


Parkinsonrsquos Disease


Research and TreatmentAutism

Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Neuroscience Journal

Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Psychiatry Journal


Computational and Mathematical Methods in Medicine

Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Brain ScienceInternational Journal of

StrokeResearch and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Neurodegenerative Diseases


Journal of

Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


bull It regulates influx and outfluxes of important ions and maintains immediate microenvironment of brain cells Management of reliable neuronal signaling and regulates transport of essential molecules

Transportation of nutrients and other essential biomolecules

bull It also protects the brain from action of metals toxicants poisons

and safeguards the integrity of the brain

Protection of neuronal tissues and cells

mononeuropathy polyneuropathy myopathy cerebral tumors cerebro-vascular accidents such as thrombosis embolism haemorrhage and vasculitis

Drug delivery for brain tumors and physical injuries

hormones and neurotransmitters BBB acts as a barrier to toxic agents

For treatment of neurological disease and disorders epilepsy seizurestrauma Parkinsonrsquos disease multiple sclerosis dementia Alzheimersrsquos disease

Figure 1 Showing important applications of transendothelial transport through BBB

cells which have star like shape due to presence of longbranches These respond to CNS in a reactive process that isknown as astrogliosis which is recognized as a pathologicalmarker of neuropathologies and CNS disorders These cellswrap themselves around the capillaries like insulation ona wire and display graded changes that result in specificsignaling events These cells show vast molecular arsenalsat the disposal and show reactive astrogliosis and glial scarformationThere are two different types of astrocytes that isprotoplasmic astrocytes and fibrous astrocytes First type ofastrocytes occurs in gray matter or specially areas rich in cellbodies while second type occurs in white matter that is areaformedmainly of axonsThese cells also possess long dendritelike processes that overlap all the brain vessels and surroundmany synapses These cells play primary role in synaptictransmission and information exchanges [2 3] by operatingthrough certain gradations mechanisms and transcellularfunctions both in healthy tissues [4 5] and in the state ofpathologies like ischemic injury [6ndash9] Astrocytes influencepolarity of blood brain barrier [10] Transmitters and mod-ulators released by neurons astrocytes and endotheliumallow complex signaling between cells in the neurovascularunit and many features of the BBB phenotype are subject tomodulation under physiological or pathological conditionsFor example opening of the BBBrsquos tight junctions may occurunder normal conditions to allow the passage of growthfactors and antibodies into the brain and in inflammation andcan contribute to brain oedema

The blood brain barrier provides a layer of protection forthe brain from harmful or foreign substances that may injurethe brain [11] It also protects the brain from action of metalstoxicants poisons hormones neurotransmitters drugs andother foreign or xenobiotic substances [12] mainly noxiousagents [13] It regulates the movement of ions and molecules[14] between the blood and CNS (Figure 1) The barrier is

also crucial and provides appropriate environment for properneural functions as well as for protection of CNS from injuryand diseases Therefore before targeting the neurovascularunit protection of the BBB is essential instead of a classicalneuron-centric approach that only roads development ofneuroprotective drugs that may result in improved clinicaloutcomes after neural diseases [15] (Figure 1) Hence forhealthy functioning of the brain easy passage of moleculesand ions pass through brain barrier (BBB) is highly needfulMore often all essential vital metabolites are to be passedthrough the BBB to protect the neurocompartments frompotential disruptive and damaging xenobiotic agents [16]Moreover role of BBB is more important for maintainingnormal physiology of brain and for safety of neuronaltissues from toxic substances by restricting its entry into theneurocompartment (Figure 1) [16]

2 Functions

BBB acts as a transport barrier that allows passage of selectivemolecules into the brain and mediating solute flux It alsoacts as a dynamic interface which separates the brain fromthe circulatory system It limits the entry of biomoleculesserves as a ldquometabolic barrierrdquo and allows passage of intra-cellular and extracellular enzymes and neutral amino acidsIt facilitates the entry of metabolically required nutrientsinto the brain and excludes or effluxes potentially harmfulcompounds or lipid-soluble molecules mainly toxicants [14]It functions as a perfect barricade to check the permeability ofunspecified and unwantedmolecules from entering the brainBut it allows transport of most essential molecules acrossthe membrane and maintains the homeostasis in the mostvital organs of the human body Normally it allows transportof water carbon dioxide oxygen and most lipid solublesubstances such as alcohol aesthetics but it is impermeable



Neurotransmitters

Ethanol

Peptides

MineralsAmino acids

LipidsVitamins

Metals

Electrolytes Water

Drugs

Monocarboxylicacids

PorphyrinsProteins





























Nutrients Drugs


Fuels and gases

Nutrients

Drugs

Water





2and O

2 and small







gases like O2and CO





4

minus and H2PO4












transporters


Boron BO33minus B4O





























[36 38 39]


Table 1 Continued


Divalent metals


Ni++ Pb++ Zn ++

















[51ndash54]



[55ndash57]






[44ndash46]






[61 62]







Peptide





Table 2 Continued








2 CO2 NO and H

2O) are





2 CO2 and NO

2diffuse








2 O2 N2O and Xe






























[76 79]





[80]





[81]








Table 3 Continued




Pellagra





[77]



Macrocytic anaemia












[82 83]























Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of




Neurotransmitters

Ethanol

Peptides

MineralsAmino acids

LipidsVitamins

Metals

Electrolytes Water

Drugs

Monocarboxylicacids

PorphyrinsProteins





























Nutrients Drugs


Fuels and gases

Nutrients

Drugs

Water





2and O

2 and small







gases like O2and CO





4

minus and H2PO4












transporters


Boron BO33minus B4O





























[36 38 39]


Table 1 Continued


Divalent metals


Ni++ Pb++ Zn ++

















[51ndash54]



[55ndash57]






[44ndash46]






[61 62]







Peptide





Table 2 Continued








2 CO2 NO and H

2O) are





2 CO2 and NO

2diffuse








2 O2 N2O and Xe






























[76 79]





[80]





[81]








Table 3 Continued




Pellagra





[77]



Macrocytic anaemia












[82 83]























Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of























Nutrients Drugs


Fuels and gases

Nutrients

Drugs

Water





2and O

2 and small







gases like O2and CO





4

minus and H2PO4












transporters


Boron BO33minus B4O





























[36 38 39]


Table 1 Continued


Divalent metals


Ni++ Pb++ Zn ++

















[51ndash54]



[55ndash57]






[44ndash46]






[61 62]







Peptide





Table 2 Continued








2 CO2 NO and H

2O) are





2 CO2 and NO

2diffuse








2 O2 N2O and Xe






























[76 79]





[80]





[81]








Table 3 Continued




Pellagra





[77]



Macrocytic anaemia












[82 83]























Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of






gases like O2and CO





4

minus and H2PO4












transporters


Boron BO33minus B4O





























[36 38 39]


Table 1 Continued


Divalent metals


Ni++ Pb++ Zn ++

















[51ndash54]



[55ndash57]






[44ndash46]






[61 62]







Peptide





Table 2 Continued








2 CO2 NO and H

2O) are





2 CO2 and NO

2diffuse








2 O2 N2O and Xe






























[76 79]





[80]





[81]








Table 3 Continued




Pellagra





[77]



Macrocytic anaemia












[82 83]























Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of











transporters


Boron BO33minus B4O





























[36 38 39]


Table 1 Continued


Divalent metals


Ni++ Pb++ Zn ++

















[51ndash54]



[55ndash57]






[44ndash46]






[61 62]







Peptide





Table 2 Continued








2 CO2 NO and H

2O) are





2 CO2 and NO

2diffuse








2 O2 N2O and Xe






























[76 79]





[80]





[81]








Table 3 Continued




Pellagra





[77]



Macrocytic anaemia












[82 83]























Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of



Table 1 Continued


Divalent metals


Ni++ Pb++ Zn ++

















[51ndash54]



[55ndash57]






[44ndash46]






[61 62]







Peptide





Table 2 Continued








2 CO2 NO and H

2O) are





2 CO2 and NO

2diffuse








2 O2 N2O and Xe






























[76 79]





[80]





[81]








Table 3 Continued




Pellagra





[77]



Macrocytic anaemia












[82 83]























Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of











[51ndash54]



[55ndash57]






[44ndash46]






[61 62]







Peptide





Table 2 Continued








2 CO2 NO and H

2O) are





2 CO2 and NO

2diffuse








2 O2 N2O and Xe






























[76 79]





[80]





[81]








Table 3 Continued




Pellagra





[77]



Macrocytic anaemia












[82 83]























Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of



Table 2 Continued








2 CO2 NO and H

2O) are





2 CO2 and NO

2diffuse








2 O2 N2O and Xe






























[76 79]





[80]





[81]








Table 3 Continued




Pellagra





[77]



Macrocytic anaemia












[82 83]























Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of




2 CO2 and NO

2diffuse








2 O2 N2O and Xe






























[76 79]





[80]





[81]








Table 3 Continued




Pellagra





[77]



Macrocytic anaemia












[82 83]























Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of




















[76 79]





[80]





[81]








Table 3 Continued




Pellagra





[77]



Macrocytic anaemia












[82 83]























Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of



Table 3 Continued




Pellagra





[77]



Macrocytic anaemia












[82 83]























Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of






















Influx [86]




[87]




[86 88ndash90]



Glutamate


Membrane receptors




Norepinephrine


[68 92 93][94ndash96] [97]



[98 99]





Serotonin Serotonin







Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of



Table 4 Continued





Active transport




Active transport


Efflux





Efflux




Efflux



Influx and efflux


Influx and efflux


Influx and efflux


Active transport


Influx and efflux


Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of



Table 4 Continued




Influx and efflux


Influx and efflux





Influx and efflux









2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of






2(inhibitory) both



2receptors




































Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of





























Influx















Influx




Efflux




Efflux








Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of



Table 5 Continued




Influx











2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of





2


2

+

































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of































4 Conclusion














Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of













Abbreviations






References
















































































































































































































Neural Plasticity










Psychiatry Journal









Journal of

































































































































































































Neural Plasticity










Psychiatry Journal









Journal of


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