Academic Half-Day

The Chemical Basis for Neuronal Communication

Marie-Pierre Thibeault-Eybalin, R4

November 5th, 2008

Introduction 100 billion (1011) neurons in the brain Up to 100,000 terminal contacts / neuron

1016 connections between neurons / brain Connections = Synapses

Chemical messenger is released at pre-synaptic membrane of axon or dendrite terminal

It travels across synaptic cleft It binds onto its receptor on post-synaptic

membrane of other neuron It activates effector system

Chemical messenger may be released at non-synaptic locations to influence distant neurons

Criteria to define chemical messenger as neurotransmitter1. Localization: A putative neurotransmitter must be

localized to the presynaptic elements of an identified synapse and must be present also within the neuron from which the presynaptic terminal arises.

2. Release: The substance must be shown to be released from the presynaptic element upon activation of that terminal and simultaneously with depolarization of the parent neuron.

3. Identity: Application of the putative neurotransmitter to the target cells must be shown to produce the same effects as those produced by stimulation of the neurons in question.

Examples of neurotransmitters

Synaptic transmission Variable synaptic delay from pre-synaptic

neurotransmitter release to excitation or inhibition of post-synaptic neuron

Synaptic delay depends on complexity of transduction mechanisms at post-synaptic membrane

Synaptic delay Receptor Neurotransmitter Effector system

Fast Few msec Ligand-gated ion channel

Small molecule Flux of ions to generate transmembrane electrical potential (EPSP, IPSP ± AP if reach threshold)

Slow Hundreds of msec G-protein-coupled

Neuropeptide Indirect effect on ion channel Enzyme activation to produce 2nd chemical messenger (intracellular)

Sequence of events

Regulatory mechanisms

To regulate amount of neurotransmitter release Pre-synaptic receptor-mediated autoregulation

Neurotransmitter in synaptic cleft binds to pre-synaptic receptor

Inhibitory feedback mechanism Retrograde transmission

2nd chemical messenger diffuses from post-synaptic to pre-synaptic membranes, e.g. NO

Secretory vesicles1. Small = Synaptic vesicles

For small molecules Synthesized within vesicles, e.g. NE or uploaded by high-affinity ATP-

proton-coupled transporters in terminals, e.g. ACh Recycled 50 nm diameter Cluster in active zones

2. Large dense-cored vesicles For neuropeptides, "built-in" in neuronal soma ± co-stored

small molecule Not-recycled 75-150 nm diameter Found in intraneuronal locations + terminals, less numerous

3. Neurosecretory vesicles Hypothalamic neuron terminals in neurohypophysis For neurohormones 150-200 nm diameter

Secretory vesicles

Exocytic release

Docking complex

Fusion pore

Signal transduction Most receptors are transmembrane glycoproteins Binding of neurotransmitter to receptor induces conformational

change 4 transduction mechanisms

Ligand-gated ion channels G-protein-coupled receptors Enzymes e.g. tyrosine kinase Ligand-dependent regulators of nuclear transcription e.g. testosterone

Receptors often named after family of neurotransmitters they bind e.g. cholinergic and adrenergic receptors

Multiple subtypes based on response Nicotinic ACh receptors usually excitatory Muscarinic ACh receptors usually inhibitory

Individual neurotransmitter family members of have different potency Rank order of potency according to EC50%

Concentration of individual neurotransmitter required to reach 50% of maximal response expected

The same neurotransmitter may have excitatory or inhibitory responses depending on receptor type

Structure of neurotransmitter receptorsLigand-gated ion channels Multiple subunits =

transmembrane glycoproteins connected via intra-and extra-cellular loops

Cylindrical Binding site in transmembrane

portion Conformation changes opens

gate inside channel Selectively pass small ions 2 genetic families based on AA

sequence homology Nicotinic ACh, serotonin,

GABA, glycine Glutamate

      

            

     

G-protein-coupled receptors Glycoprotein chains with

multiple transmembrane loops -helices β-pleated sheets

Binding site in transmembrane or extra-cellular portion

3 components Receptor GTP-binding heterotrimer Effector protein (enzyme or

ion channel) Examples

Rhodopsin Odorants Biogenic amines Bioactive peptides β2-adrenergic receptor

G-protein action

Examples of effector proteinsEnzyme 2nd intracellular messenger

Adenylyl cyclase cAMP

Guanylate cyclase cGMP

Phospholipase C IP3 and DAG

Phospholipase A2 Members of the eicosanoid family

Receptor regulation

Desensitization Reduction in receptor agonist-induced response after

seconds to minutes of stimulation mediated by conformational changes

Homologous Heterologous

Phosphorylation of intracellular portion of receptor altering its binding affinity

Downregulation of receptor number at post-synaptic membrane

Internalization of receptor by invagination of post-synaptic membrane

Maintenance of synaptic environment To reduce or eliminate neurotransmitters in synaptic cleft Enzymatic degradation

ACh cleaved by acetylcholinesterase Neuropeptides degraded by peptidases

Transporter-mediated reuptake of small molecules (not neuropeptides) by pre-and post-synaptic neuron or glia (extraneuronal monoamine transport; EMT) NET for norepinephrine DAT for dopamine SERT for serotonin

After reuptake, neurotrasmitter either recycled or degraded by mitochondria (MAO) COMT for norepinephrine

Pharmacologic modification of synaptic transmission

Drugs may affect: Neurotransmitter synthesis Vesicular uptake and storage Depolarization-induced exocytosis Neurotransmitter receptor binding Termination of neurotransmitter action Post-synaptic effector system

Metyrosine for pheochromocytoma

VAMT Reserpine

-Methyldopa

Guanethidine

Propranolol

Yohimbine

Cocaine

MAO inhibitors

Synopsis of clinical points Many drugs function by altering chemical transmission at the

synaptic cleft. Neuropeptides play a role in the body's response to stress. Some drugs must traverse the plasma membrane to access

receptors. Epinephrine is used in cardiopulmonary resuscitation and to treat

anaphylactic reactions. The excess production of catecholamines, seen in tumors such as

pheochromocytoma, can be treated by the drug metyrosine. Reserpine is sometimes used to treat hypertension. Reserpine may precipitate Parkinson-like symptoms or

galactorrhea, or worsen clinical depression. α-Methyldopa is effective for managing hypertension during

pregnancy. The side effects of guanethidine include reduced heart rate, nasal

congestion, and orthostatic hypotension. Propranolol is used in the management of angina pectoris,

hypertension, and congestive heart failure. Yohimbine may be effective in treating male impotence of vascular

or psychogenic origin. Amphetamines enhance motor performance and relieve fatigue;

these are habit-forming if used inappropriately.