Neuroscience
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
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
GluR2
Na+
Na+
Na+
GluR2
GluR2
Tetrameric structure
Dimers of GluR2 and GluR1, GluR3 or GluR4
Cation channel
closed
Allosteric change
opens Na+ channel
Ligand gated anion channels e.g. The GABAA
receptor complex
Cl-
Cl











GABA

GABA
GABA
Pentamer
Chloride channel
Allosteric change
opens Cl- channel

GABA


Other modulators affect
GABAA function
Neurosteroids
Z hypnotics
Ligand gated ion channels
Glycine binding site
Na+
BDZ

GABA

Cl-

Na+
Ca2+
GABA

Na+

GABAA
AMPA
NMDA
The NMDA receptor
admits Ca2+ as well as Na+.
It is blocked by Mg2+ at
low potentials. Glycine is
a co-agonist.
Ligand gated ion channels
Na+
5-HT3A

Na+
Ca2+
5-HT3B-E



5-HT3B-E

5-HT3B-E
δ
5-HT3B-E
5-HT3
K+
The 5-HT3 receptor is a pentamer.
The open receptor is permeable to
Na+ and K+

Nicotinic ACh
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
G-protein linked receptors are a single
protein chain
binding site
There is a binding site outside and
a G-protein binding site inside
out
in
α
Intracellular loops
α
γ
β
G protein
GDP GTP
GTP
-ve
AC
When activated, the G-protein
hydrolyses GDP to GTP
The GTP activated α subunit interacts
with the enzyme adenylate cyclase
Bi-directional regulation of adenylate cyclase
Regulation of adenylate
cyclase is bi-directional
binding site
Some receptors inhibit
adenylate cyclase
out
in
α
Intracellular loops
α
γ
-ve
AC
+ve
α
GTP
cAMP activates protein
kinase A
β
G protein
GDP GTP
ATP
Some receptors activate
adenylate cyclase
cAMP
G protein linked receptors-phosphatidyl
inositol
Some G protein linked receptors
stimulate phosphatidyl inositol
turnover
out
in
Phosphatidylinositol 4,5-biphosphate
(PIP2) is cleaved into inositol (1,4,5)
trisphosphate (IP3) and diacylglycerol
(DAG).
α
Intracellular loops
α
γ
GTP
β
G protein
GDP GTP
PIP2
IP3 & DAG
IP3 causes releases Ca2+ from the ER
DAG and Ca2+ activate protein kinase
C and other kinases.
G protein linked receptors- ion channels
Some G protein linked receptors are
coupled to ion channels
Activation of the receptor opens the
K+ channel
out
K+
in
α
GTP
Intracellular loops
K+
α
γ
β
G protein
K+
GDP GTP
K+ leaves the cell causing
hyperpolarization
The 5-HT1A autoreceptor is coupled to
a K+ channel
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 resting membrane potential
AA-
AK+
K+
Cl-
A-
K+
K+
A-
The cell membrane is impermeable
to Na+ but permeable to K+
Na+/K+ ATPase pumps 3Na+ out and
2K+ in
Large anions are fixed to cellular
components
Extracellular Cl- ions balance the
large anions
There are concentration gradients
and an electrochemical gradient
K+
+
K
K+
K+
A-
K+
Na+
Cl-
Na+
Na+
Na+
Na+ Cl Na+
+
Na+ + Na
Na ClNa+
Na+
K+
A-
K+
Cl-
K+
Cl-
Cl-
Na+
The resting membrane potential (RMP)
Inside
Outside
mM
mM
K+
100
K+
5
Na+
15
Na+
150
Cl-
13
Cl-
150
A-
385
A-
0
The unequal distribution of ions leads to a negative charge inside the cell
RMP ≈70 mV
Ligand gated (cat)ion channels
K+
AA-
AK+
K+
Cl-
A-
K+
K+
A-
When a ligand gated cation channel is
activated
Na+ channels in the membrane open
K+
K+
K+
A-
K+
Cl-
Cl-
Na+
Na+
K+
A-
K+
Na+
Cl- Na+
+
Na+ + Na
Na ClNa+
Cl-
Cl-
Na+
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+
AA-
A-
A-
A-
K+
K+
Na+
K+
K+
K+
AA-
K+
K+
Na+
Na+
K+
Na+
Cl-
Na+
Cl-
Na+
Na+
Cl-
Na+
Cl-
Na+
Cl-
K+
Na+
K+
Cl-
Cl-
Na+
Membrane depolarization
+30
Membrane potential (mV)
• 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
-15
-55
-70
RMP
time
Small positive deflections in the membrane potential caused by receptor activation and
cation influx induce an action potential
When a ligand gated anion channel
is activated
Cl- channels in the cell membrane
open
K+
AA-
A-
K+
K+
Cl-
Cl-
K+
A-
K+
K+
A-
K+
K+
A-
K+
Na+
ClCl-
Cl-
Cl-
Na+
Na+
Na+
Cl- Na+
+
Na+ + Na
Na ClNa+
Na+
K+
A-
K+
K+
Cl-
Cl-
Na+
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+
AA-
A-
A-
A-
K+
K+
K+
K+
A- Na+
K+
A-
Cl-
Cl-
Cl-
K+
K+
K+
Na+
Cl-
Cl-
Cl-
Cl-
Na+
Na+
Cl- Na+
+
Na+ + Na
Na ClNa+
Na+
K+
Cl-
K+
Cl-
Cl-
Na+
Membrane hyperpolarization
+30
Membrane potential (mV)
• Negative deflections
offset any excitatory
potentials
• The cell is less likely to
fire an action potential
-15
-55
-70
RMP
time
Small negative deflections in the membrane potential caused by receptor activation and
chloride ion influx reduce the probability of an action potential
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+
A-
K+
K+
A-
A-
A-
K+
K+
K+
K+
Cl-
Na+
Cl-
Na+
AK+
A-
Na+
K+
K+
A-
K+
Cl-
Na+
Na+
Na+
Cl-
Na+
Na+
Cl-
ClCl-
Na+
Cl-
Membrane hyperpolarization
+30
Membrane potential (mV)
• Negative deflections
offset any excitatory
potentials
• The cell is less likely to
fire an action potential
-15
-55
-70
RMP
time
Small negative deflections in the membrane potential caused by receptor activation and
K+ efflux reduce the probability of an action potential
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 receptors
DA -all G-protein linked
D2- like inhibit AC, open K+ channels
• D1- like stimulate AC
NA –all G protein linked
1 -stimulate PI cycle
2 -inhibit AC, open K+ channels
 -stimulate AC
5-HT -mixed
5-HT1 - inhibit AC, open K+ channels
5-HT2 - stimulate PI cycle
5-HT3 - ligand gated ion channel
Cholinergic receptors
Muscarinic –G protein linked
M1 – stimulates PI cycle
Nicotinic –ligand gated ion channel
Neuronal –α7 homomer /αβ heteromers
Ganglionic
NMJ
Receptor families
5-HT1A
5-HT1B
5-HT1
5-HT1D
α1
5-HT1E
5-HT1F
α
5-HT2A
5-HT
5-HT2
5-HT2B
5-HT3
5-HT2C
NA
5-HT4
5-HT5
5-HT6
5-HT7
β
α2
α3
β1
β2
β3
α2A
α2B
α1D
α2A
α2B
α2C
• 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 responsiveness
3 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
out
out
in
α
α
γ
β
G protein
GDP GTP
GTP
-ve
AC
in
α
P
β-arrestin
-ve
AC
GTP
α
γ
β
Phosphorylated receptor binds β-arrestin
G 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
P
β-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
Monoamine reuptake transporters
12 transmembrane spanning
protein
Out
In
Transport driven by
concentration gradients of Na+
and Cl-
DAT, NAT, and SERT (5HTT) have
high sequence homology
Many drugs have poor
transporter selectivity
Monoamine
Na+ ClOut
In
Monoamine reuptake transporters
Neurotransmitters
Releasing agents (amphetamines)
bind and are transported
Out
Antidepressants (TCAs, SSRIs, NARIs)
Cocaine
Bupropion
bind and block transport
In
Monoamine
ClNa+
Out
In