Dr Sasha Gartisde Institute of Neuroscience Newcastle University

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Dr Sasha Gartisde Institute of Neuroscience Newcastle University Neuroscience

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Dr Sasha Gartisde Institute of Neuroscience Newcastle University. Neuroscience. Drugs, receptors, and transporters. Most psychoactive drugs interfere with neurotransmission The main targets are enzymes, transporters and receptors. Drugs, receptors, and transporters. Enzymes - PowerPoint PPT Presentation

Transcript of Dr Sasha Gartisde Institute of Neuroscience Newcastle University

Page 1: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Dr Sasha GartisdeInstitute of Neuroscience

Newcastle University

Neuroscience

Page 2: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Drugs, receptors, and transporters

• Most psychoactive drugs interfere with neurotransmission

• The main targets are enzymes, transporters and receptors

Page 3: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Drugs, receptors, and transporters

• Enzymes– monoamine oxidase inhibitors, L-DOPA,

anticholinesterases • Transporters-– SSRIs antidepressants, cocaine, buproprion

• Receptors – antipsychotics, anxiolytics (BDZs),

Page 4: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Neurotransmitter receptors

• Specialized proteins• Embedded in the cell membrane• Bind neurotransmitter (or drug)• Induce intracellular response in response to

extracellular event

Page 5: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Neurotransmitter receptors

• 2 types– Ligand gated ion channel (ionotropic)– G-protein linked (metabotropic)

Page 6: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Ligand gated cation channels: e.g. The glutamate AMPA receptor

Tetrameric structureDimers of GluR2 and GluR1, GluR3 or GluR4

GluR2

GluR2GluR2

Cation channelclosed Allosteric change

opens Na+ channel

Na+

Na+Na+

Page 7: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Ligand gated anion channels e.g. The GABAA receptor complex

PentamerChloride channel Allosteric change

opens Cl- channel

GABA

GABA

Cl-Cl-

Other modulators affect GABAA functionNeurosteroidsZ hypnotics

BDZ

BARBS

GABAGABA

Page 8: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Ligand gated ion channels

AMPAGABAA

BDZ

BARBS

GABAGABA

Na+

Cl-

NMDA

Na+

Glu

Glu Glu

GluGluR2

GluR2

GluR

1

GluR1

GluN1

GluN1

GluN

2

GluN2

Glycine binding site

Ca2+

The NMDA receptor admits Ca2+ as well as Na+. It is blocked by Mg2+ at low potentials. Glycine is a co-agonist.

Na+

Page 9: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Ligand gated ion channels

5-HT3A 5-HT3B-E

5-HT3B-E

5-HT3B-E

5-HT3B-E

5-HT3

Na+

K+

δ

Na+

K+

Nicotinic ACh

ACh

Ca2+

The 5-HT3 receptor is a pentamer.The open receptor is permeable to Na+ and K+

The nicotinic acetyl choline receptor is a pentamer.The open receptor is permeable to Na+ and K+. Some forms are also Ca2+ permeable.

Page 10: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

G protein linked receptors-adenylate cyclase

7-transmembrane structure

binding site

Intracellular loops αγβ

G protein

α

AC-ve

GDP GTP

GTP

out

in

G-protein linked receptors are a single protein chain

There is a binding site outside and a G-protein binding site inside

When activated, the G-protein hydrolyses GDP to GTP

The GTP activated α subunit interacts with the enzyme adenylate cyclase

Page 11: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

binding site

Intracellular loops αγβ

G protein

α

AC

ATP cAMP

-veα

+ve

GDP GTP

GTP

out

in

Regulation of adenylate cyclase is bi-directional

Some receptors inhibit adenylate cyclase

Some receptors activate adenylate cyclase

cAMP activates protein kinase A

Bi-directional regulation of adenylate cyclase

Page 12: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

G protein linked receptors-phosphatidyl inositol

Intracellular loops αγβ

G protein

α

PIP2 IP3 & DAGGDP GTP

GTP

out

in

Some G protein linked receptors stimulate phosphatidyl inositol turnover

Phosphatidylinositol 4,5-biphosphate (PIP2) is cleaved into inositol (1,4,5) trisphosphate (IP3) and diacylglycerol (DAG).

IP3 causes releases Ca2+ from the ER DAG and Ca2+ activate protein kinase C and other kinases.

Page 13: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

G protein linked receptors- ion channels

Intracellular loops αγβ

G protein GDP GTP

out

in

Some G protein linked receptors are coupled to ion channels

Activation of the receptor opens the K+ channel

K+ leaves the cell causing hyperpolarization

The 5-HT1A autoreceptor is coupled to a K+ channel

α

GTP

K+

K+

K+

Page 14: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Summary: receptors

• Neurotransmitter receptors are membrane bound

• Ligand gated ion channel or G-protein linked• Multiple subtypes/ isoforms

Page 15: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

What do neurotransmitter receptors do?

• Receptors transfer the external signal (neurotransmitter) to the target cell

• Ligand gated ion channels – have direct effects on membrane excitability

• G-protein linked receptors– have indirect effects on membrane excitability– mediate other intracellular responses– modulate responses to ligand-gated ion channels

Page 16: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

The cell membrane is impermeable to Na+ but permeable to K+

Na+/K+ ATPase pumps 3Na+ out and 2K+ inLarge anions are fixed to cellular componentsExtracellular Cl- ions balance the large anionsThere are concentration gradients and an electrochemical gradient

K+

K+

K+K+

K+

K+

Na+

Na+

Na+

Na+

Na+ Na+

Na+

Na+

Na+

K+K+

The resting membrane potential

Na+ Na+

Na+

A-

A-

A-

A-

A-

A-

A-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

K+

K+K+

K+

ATPase

Page 17: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

mM

K+ 5

Na+ 150

Cl- 150

A- 0

mM

K+ 100

Na+ 15

Cl- 13

A- 385

Inside Outside

The resting membrane potential (RMP)

The unequal distribution of ions leads to a negative charge inside the cell RMP ≈70 mV

Page 18: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Ligand gated (cat)ion channels

When a ligand gated cation channel is activatedNa+ channels in the membrane open

K+

K+

K+

K+

K+

K+

Na+

Na+

Na+ Na+

Na+

Na+

K+

Na+

Na+

Na+

A-

A-

A-

A-

A-

A-

A-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

K+

K+K+

K+

ATPase

Page 19: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Ligand gated (cat)ion channels

• Na+ rushes in down its concentration gradient

• The Na+ carries positive charge• This increases the membrane

potential to a more positive value

K+

K+

K+

K+

K+

K+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

K+K+

Na+

Na+

Na+

A-

A-

A-

A-

A-

A-

A-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

K+

K+

K+

K+

ATPase

Na+

Na+

Page 20: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Membrane depolarization

• If the membrane potential reaches -55mV

• Voltage-gated Na+ channels open

• Huge quantities of Na+ are allowed to enter the cell and an action potential occurs

Mem

bran

e po

tenti

al (m

V)

RMP

time

-70

-15

+30

Small positive deflections in the membrane potential caused by receptor activation and cation influx induce an action potential

-55

Page 21: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

When a ligand gated anion channel is activatedCl- channels in the cell membrane open

K+

K+

K+

K+

K+

K+

Na+

Na+

Na+

Na+ Na+

Na+

Na+

Na+

K+K+

Na+

Na+

Na+

A-

A-

A-

A-

A-

A-

A-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

K+

K+K+

K+

ATPase

Ligand gated anion channels

Cl-

Cl-Cl-

Cl-

Page 22: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Ligand gated anion channels

• Some Cl- moves in down its concentration gradient

• Cl- carries negative charge• The membrane potential is

decreased to a more negative value

K+

K+

K+

K+

K+

Na+

Na+

K+K+

Na+

A-

A-

A-

A-

A-

A-

A-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

K+

K+

K+

K+

ATPase

Cl-

Cl-

Cl-

Cl-

Cl-

Na+

Na+ Na+

Na+

Na+

Na+

Na+

Na+

Cl-

Cl-

Page 23: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Membrane hyperpolarization

• Negative deflections offset any excitatory potentials

• The cell is less likely to fire an action potential

Mem

bran

e po

tenti

al (m

V)

RMP

time

-70

-15

+30

Small negative deflections in the membrane potential caused by receptor activation and chloride ion influx reduce the probability of an action potential

-55

Page 24: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

G-protein linked K+channels

• Some GPCRs open K+ channels• K+ moves out down its

concentration gradient• The membrane potential is

decreased to a more negative value

K+

K+

K+

K+

K+

Na+

K+

K+

A-

A-

A-

A-

A-

A-

A-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

K+

K+

K+

ATPase

Cl-

Na+

Na+ Na+

Na+

Na+

Na+

Na+

Na+

K+

K+

K+

Page 25: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Membrane hyperpolarization

• Negative deflections offset any excitatory potentials

• The cell is less likely to fire an action potential

Mem

bran

e po

tenti

al (m

V)

RMP

time

-70

-15

+30

Small negative deflections in the membrane potential caused by receptor activation and K+ efflux reduce the probability of an action potential

-55

Page 26: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Neurotransmitter receptors

• All neurotransmitters interact with multiple receptor subtypes

• Subtypes mediate different effects and have different distributions

• Drugs (but not the neurotransmitter) can distinguish between them

Page 27: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

GABA and Glutamate receptors

• GABAA ligand gated Cl- ion channel (complex)

• GABAB G-protein linked ↓AC , opens K+ channel

• NMDA• AMPA ligand gated cation channel• Kainate• mGluR1-5 –metabotropic (G protein linked)

Page 28: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Monoamine receptorsDA -all G-protein linked

D2- like inhibit AC, open K+ channels• D1- like stimulate AC

NA –all G protein linked1 -stimulate PI cycle2 -inhibit AC, open K+ channels -stimulate AC

5-HT -mixed5-HT1 - inhibit AC, open K+ channels5-HT2 - stimulate PI cycle5-HT3 - ligand gated ion channel

Page 29: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Cholinergic receptorsMuscarinic –G protein linked

M1 – stimulates PI cycle

Nicotinic –ligand gated ion channelNeuronal –α7 homomer /αβ heteromersGanglionicNMJ

Page 30: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Receptor familiesNA

α

α1

α2 A

α2 B

α1 D

α2

α2 A

α2 B

α2 C

α3

β

β1

β2

β3

Page 31: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

• All GABA receptors are inhibitory• Other neurotransmitters have mixture of

inhibitory and excitatory receptors

Page 32: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Receptor localization

• Receptors are found at postsynaptic, presynaptic and somatodendritic sites.

• Some are also found extrasynaptically

Page 33: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Postsynaptic receptors

• Postsynaptic receptors can be excitatory or inhibitory• Sometimes both are found on the same cell

e.g. 5-HT2A, α1, D1&2, nACh, NMDA

Page 34: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Presynaptic receptors

• Presynaptic receptors are always inhibitory

• They inhibit neurotransmitter release by inhibiting voltage-gated Ca2+ channels or enhancing K+-channel activation.

• They can also decrease release by modulating intracellular Ca2+.

e.g. 5-HT1B, α2, D2, mACh2, GABAB

Page 35: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Somatodendritic receptors

• Somatodendritic receptors are on the cell body (soma) and dendrites.

• They respond to local levels of transmitter• Somatodendritic autoreceptors inhibit firing • Most activate GPRC- K+ channels

e.g. 5-HT1A, α2, D2

Page 36: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Receptor adaptation

Continuous exposure of cells to agonists causes loss of responsiveness3 phases.1. Reduction in receptor affinity 2. Reduction in receptor function 3. Reduction of receptor number

Page 37: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Receptor desensitization and down regulation

1. Reduction in receptor affinity. Rapid and reversible. G-protein binding affects the receptor affinity.

2. ‘Homologous desensitization’:- change of receptor coupling. Phosphorylation of GPCRs allows interaction with arrestins which prevents G protein coupling.

Page 38: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Desensitization/uncoupling• G-protein coupled receptors must be coupled to their

intracellular G-protein

αγβ

G protein

α

AC-ve

GDP GTP

GTP

out

in

αγβ

α

AC-ve

GTP

out

in

P

β-arrestin

Phosphorylated receptor binds β-arrestinG protein cannot bind

Page 39: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Receptor desensitization/down regulation

3. ‘Down regulation’: reduction of receptor number in the membrane. – receptor internalization – enhanced receptor degradation – reduced receptor synthesis

Page 40: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Receptor down regulation

– There is a constant turn over of receptors– Receptors are synthesised in the nucleus, trafficked to the

membrane, inserted in the membrane, internalized and degraded

Page 41: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Internalization• Receptors which are bound to β-arrestin are subject to internalization

Pβ-arrestin

P

β-arrestin

Page 42: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Receptor adaptation in psychopharmacology

• Adaptation in response to increased agonist concentration

• E.g.1 Increased somatodendritic 5-HT levels in response to SSRI down regulate somatodendritic 5HT1A autoreceptors

• E.g.2 Increased synaptic DA levels in response antipsychotics down regulate D1 receptors

Page 43: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Receptor sensitization/upregulation

Reduced exposure to agonists and continuous exposure to antagonists causes increased responsiveness

1. Increase in receptor affinity. Rapid and reversible. When G-protein is bound receptor affinity is greater.

2. ‘Up regulation’: increase in receptor number in the membrane. – Receptor trafficking – Enhanced receptor synthesis – Reduced receptor degradation

Page 44: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Receptor sensitization/upregulation

Denervation supersensitivity

NB. 5-HT2 receptors desensitize in response to both agonist and antagonist stimulation

Page 45: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Summary

• Receptor types• Membrane & intracellular effects• Locations & roles• Receptor subtypes• Receptor adaptation

Page 46: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

Drugs, receptors, and transporters

• Most psychoactive drugs interfere with neurotransmission

• The main targets are enzymes, transporters and receptors

Page 47: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

12 transmembrane spanning protein

Transport driven by concentration gradients of Na+ and Cl-

DAT, NAT, and SERT (5 HTT) have high sequence homology

Many drugs have poor transporter selectivity

Monoamine reuptake transporters

In

Out

MonoamineNa+ Cl-

In

Out

Page 48: Dr Sasha  Gartisde Institute of Neuroscience Newcastle University

NeurotransmittersReleasing agents (amphetamines)

bind and are transported

Antidepressants (TCAs, SSRIs, NARIs)CocaineBupropion bind and block transport

Monoamine reuptake transporters

In

Out

MonoamineNa+ Cl-

In

Out