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Dr Sasha GartisdeInstitute 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– monoamine oxidase inhibitors, L-DOPA,
anticholinesterases • Transporters-– SSRIs antidepressants, cocaine, buproprion
• Receptors – antipsychotics, anxiolytics (BDZs),
Neurotransmitter receptors
• Specialized proteins• Embedded in the cell membrane• Bind neurotransmitter (or drug)• Induce intracellular response in response to
extracellular event
Neurotransmitter receptors
• 2 types– Ligand gated ion channel (ionotropic)– G-protein linked (metabotropic)
Ligand gated cation channels: e.g. The glutamate AMPA receptor
Tetrameric structureDimers of GluR2 and GluR1, GluR3 or GluR4
GluR2
GluR2
GluR2
Cation channelclosed Allosteric change
opens Na+ channel
Na+
Na+Na+
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
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+
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.
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
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
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.
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+
Summary: receptors
• Neurotransmitter receptors are membrane bound
• Ligand gated ion channel or G-protein linked• Multiple subtypes/ isoforms
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
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
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
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
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+
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
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-
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-
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
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+
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
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
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)
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
Cholinergic receptors
Muscarinic –G protein linkedM1 – stimulates PI cycle
Nicotinic –ligand gated ion channelNeuronal –α7 homomer /αβ heteromersGanglionicNMJ
Receptor familiesNA
α
α1
α2A
α2B
α1D
α2
α2A
α2B
α2C
α3
β
β1
β2
β3
• All GABA receptors are inhibitory• Other neurotransmitters have mixture of
inhibitory and excitatory receptors
Receptor localization
• Receptors are found at postsynaptic, presynaptic and somatodendritic sites.
• Some are also found extrasynaptically
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
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
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
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
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.
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
Receptor desensitization/down regulation
3. ‘Down regulation’: reduction of receptor number in the membrane. – receptor internalization – enhanced receptor degradation – reduced receptor synthesis
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
Internalization• Receptors which are bound to β-arrestin are subject to internalization
Pβ-arrestin
P
β-arrestin
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
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
Receptor sensitization/upregulation
Denervation supersensitivity
NB. 5-HT2 receptors desensitize in response to both agonist and antagonist stimulation
Summary
• Receptor types• Membrane & intracellular effects• Locations & roles• Receptor subtypes• Receptor adaptation
Drugs, receptors, and transporters
• Most psychoactive drugs interfere with neurotransmission
• The main targets are enzymes, transporters and receptors
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
Monoamine
Na+ Cl-
In
Out
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
