Download GABA and GABA receptors

Presynaptic terminal
Glial
Postsynaptic neuron
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GABA and GABA receptors
Lecture 1. GABAA receptors
Lecture 2. GABAB receptors
Lecture 3. GABA homeostasis
Lecture 4. Modulation of GABAergic
synaptic transmission
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What is GABA?
γ-aminobutyric acid
• Inhibitory neurotransmitter.
• ~1/3 of synaptic transmission in the brain is mediated by
GABA.
• Neurons that synthesize and release GABA is called
GABAergic neurons.
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GABA receptors
• GABAA receptors
– Ligand-gated ion channels
– Fast synaptic inhibition
• GABAB receptors
– GTP-binding protein coupled receptors
– Slow synaptic inhibition
• GABAC receptors
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Lecture 1. GABAA receptors
•
•
•
•
•
molecular structures
location and function
single channel recording
pharmacology
two types of inhibition
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Benzodiazepines
Barbiturates
Neurosteroids
Anesthetics
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GABAAR is ligand-gated ion channel
Receptor
Transmitter
Pore
Channel
Extracellular side
Gate
Cytoplasmic side
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Molecular structure of GABAA receptors
ρ1-ρ3
Moss & Smart
2001
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ρ subunits are part of GABAA receptor family
ρ1-ρ3
Moss & Smart
2001
•structure
•function
•pharmacology
Dendrogram of the deduced amino
acid sequences of GABAAR subunits.
from Cherubini and Conti 2001
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Distribution of GABAAR α-subunit
mRNA in rat brain
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Molecular structure of GABAA receptors
• Multiple subunits: α1−6, β1−3, γ1−3, δ, ε, π, θ,
ρ1−3
• Each subunit contains 4 putative
transmembrane domains, TM2 is believed to
form the lining of the channel.
• Hetero- or homo-oligomeric proteins.
• Pentamer with α:β:γ at a ratio of 2:2:1
• subunit composition determines functional
properties and pharmacology.
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Location
Presynaptic terminal
Glial
Synaptic cleft
GABAAR
Cl
-
Postsynaptic neuron
IPSP
Cl- and HCO312
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Nernst equation
-
Cl
10 mM
Cl125 mM
RT ln [Cl]o
zF [Cl]i
[Cl] o
ECl = -60 log
[Cl] i
ECl = -66 mV
ECl =
z = charge of diffusible ion (Cl- = -1)
R = universal gas constant
T = absolute temperature
F = Faraday's Constant
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Current
passing
recording
Current
passing
recording
Inhibitory
interneuron
Motor
neuron
IPSP
AP
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Reversal potential of IPSP
Postsynaptic
potential
-
Cl
flux
Postsynaptic
current
-35
-55
ECl-
-74
-99
(mV)
Current clamp
Voltage clamp
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IPSP reduces cell excitability
• Membrane
hyperpolarization
– drive membrane potential
away from the threshold
potential.
• Reduction in membrane
resistance
– reduce the excitatory input.
This is known as “shunting
inhibition”.
Threshold potential
ECl-
Threshold potential
ECl16
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Single channel recording
Open
GABA
Closed
Bound
Closed
Open
Bound
Open
Bound
Closed
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ligand binding sites on GABAA receptors
benzodiazepine
picrotoxin
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• Agonists
– GABA, muscimol
Response (%)
GABAA receptor pharmacology
• Antagonists
Response (%)
Zn2+
Neurosteroids
Benzodiazepines
Anesthetics
Barbituates
Alcohol
1
10
KD
1000
Dose
100
affinity
50
0
Response (%)
• Modulators
–
–
–
–
–
–
50
0
– Bicuculline, picrotoxin,
gabazine
Maximum Response
100
affinity
1
10
100
Dose
efficacy
100
50
0
1000
efficacy
1
10
100
1000
Dose
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Phasic and tonic inhibitions
50 pA
Bicuculline
20 pA
40 ms
– high agonist dose ~1
mM
– quantal release
• Action potentialdependent IPSPs
• Action potentialindependent mIPSPs
– synaptic receptors
– sensitive to gabazine
1 min
– low agonist dose ~1µM
– unknown mechanisms
•
•
•
•
reverse uptake
spill over
extracellular matrix
channel spontaneous
open
– extrasynaptic receptors
– insensitive to gabazine
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GABAAR and disease
• GABAAR is a major target for developing
therapeutics.
–
–
–
–
–
pain
epilepsy
anxiety
depression
sleeping disorders
• Mutations in GABAARs are found to be linked to
epilepsy.
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GABAAR Summary
• GABA-gated anion channels.
• The primary inhibitory receptors in the
mammalian brain.
• Pre-, post-synaptic and extrasynaptic area,
mediating inhibitory postsynaptic potentials
(IPSPs) and tonic inhibition.
• Important targets for therapeutic agents.
• Mutations in the genes encoding GABAA
receptor subunit correlate with certain type of
epilepsy.
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Questions
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Lecture 2. GABAB receptors
• molecular structures
• location and function
• modulation
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GABABRs are G-protein coupled receptors
Transmitter
Receptor
Channel
G protein
βγ
Gate
α
GTP
NH2
P
P
P
P
Extracellular side
Cytoplasmic side
COOH
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Molecular structure of GABAB receptors
• Heterodimer linked
by coiled-coil
domain
• GABAB1a-f
• GABAB2, 35%
homology with
GABAB1
• coupled to Gi/Go
Marshall, FH et al, 1999
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Function of GABABRs
2+
+
Ca
K
GABABR
βγ
βγ
βγ
α
GTP PP
P
Adenylyl cyclase
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GABABRs - postsynaptic
GABABR
fast IPSP
ECl
-
slow IPSP
EK
+
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GABABRs - presynaptic
Glutamatergic
GABAergic
GABAARs
GABABR
GluRs
GABABR
IPSP
Threshold
EPSP
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GABABR function
• opening K+ channels in the postsynaptic
membrane.
• closing Ca2+ channels in the presynaptic
terminal.
– GABAergic: autoreceptor
– glutamatergic: heteroreceptor
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GABABR pharmacology
• Agonists
– GABA, (-)baclofen, APPA
• Antagonists
– saclofen, phaclofen, CGP35348, CGP55845A
• Modulators
– CGP7930
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GABABR and disease
• GABABR agonist
– antispasticity
– antinociceptive
– suppression of drug craving
• GABABR antagonist
– suppress absence seizure in animal models
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GABABR Summary
• G-protein coupled receptors.
• Heterodimer with GABAB1 and GABAB2.
• Mediate slow IPSP via opening K+ channel at
postsynaptic membrane.
• Decrease synaptic release via inhibit Ca2+
channels in the presynaptic terminal.
• targets for therapeutic agents.
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Function of GABAARs and GABABRs
• Under several conditions GABARmediated response can be excitatory.
– GABAARs
• EIPSC higher than the threshold (Cl- or HCO3-).
• Disinhibition
– GABABRs
• Presynaptic inhibition on inhibitory neurons
• Activating K+ channel may recruit T-type Ca2+
channel to induce oscillation in thalamus.
• Disinhibition
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Reference books
• Principles of
neuroscience
4th Edition
Eric R. Kandel
Jame H. Schwartz
Thomas M. Jessell
•
From neuron to
brain
4th Edition
John G. Nicholls
Robert Martin
Bruce G. Wallace
Paul A. Fuchs
New York: Elsevier
Sunderland: Sinauer
Associates
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