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GABAergic
Synaptic transmissin
Question | Answer |
---|---|
GABAergic synapse | 1. Synthesis 2. Storage 3. Metabolism - uptake and breakdown 4. Action - GABA receptors |
GABA synthesis | From glutamate via glutamic acid decarboxylase (GAD) in cytosol. |
GAD | Two forms: 67 and 65 preferentially located in nerve terminal 67: constitutively active saturated with PLP 65: apoenzyme with no PLP bound |
GABA synthesis - drugs | 1. Hydrazides - react with PLP (i.e. eliminate) 2. ATP - inhibits activation of GAD by PLP 3. iP - promotes activation of GAD by PLP 4. Glu competitive inhibitors, e.g. allylglycine |
GABA storage | From cytosol to vesicle via VGAT, proton driven transport. |
GABA metabolism | 1. Uptake by GABA uptake T (GAT) - glial and neuronal 2. Breakdown by GABA transaminase (GABA-T) in mitochondria |
GABA metabolism - drugs | 1. glial GAT inhibitor: B-alanine 2. neuronal GAT inhibitor: ACHC |
GABAergic synapse | 1. Synthesis 2. Storage 3. Metabolism - uptake and breakdown 4. Action - GABA receptors |
GABA synthesis | From glutamate via glutamic acid decarboxylase (GAD) in cytosol. |
GAD | Two forms: 67 and 65 preferentially located in nerve terminal 67: constitutively active saturated with PLP 65: apoenzyme with no PLP bound |
GABA synthesis - drugs | 1. Hydrazides - react with PLP (i.e. eliminate) 2. ATP - inhibits activation of GAD by PLP 3. iP - promotes activation of GAD by PLP 4. Glu competitive inhibitors, e.g. allylglycine |
GABA storage | From cytosol to vesicle via VGAT, proton driven transport. |
GABA metabolism | 1. Uptake by GABA uptake T (GAT) - glial and neuronal 2. Breakdown by GABA transaminase (GABA-T) in mitochondria |
GABA metabolism - drugs | 1. glial GAT inhibitor: B-alanine 2. neuronal GAT inhibitor: ACHC 3. GABA-T inhibitor: sodium valproate and gamma-vinyl GABA (anticonvulsant) |
GABA actions - GABA receptors | 1. GABAa - ionotropic (- bicuculline) 2. GABAb - metabotopic, baclofen (-2-OH-saclofen) |
GABAa actions | - primary mediators of fast inhibitory synaptic transmission in the CNS, e.g. modify spike timing - permit chloride and bicarbonate ions - hyperpolarising or increasing cell conductance - depolarising in immature neurons |
GABAa R modulation sites | 1. Benzodazepine 2. Barbiturate 3. Anaesthetics - volatile and intravenous 4. Picrotoxin 5. Steroid 6. GABA |
GABAa R structure | - Cyc-loop superfamily - Pentameric ligand-gated channel - 4 membrane spanning regions (M1-M4) - M2 form pores |
GABAa 7 subunits - subunit composition determines the functional properties and localisation (phasic and tonic) | 1) GABA affinity 2) activation and deactivation kinetics (duration of conductance change) 3) onset and recovery from desensitisation 4) single channel conductance (magnitude of conductance change) |
GABA subunits e.g. A1B2Y2 (major: anticonvulsant and sedative actions of BZDs) e.g. A2B3Y2 (minor: anxiolytic action of BZDs) | A and B form functional receptors +Y is required for BZD pharmacology A determines the type D associates with A4 and A6 : high affinity with (-)BZD E related to Y, with ABY form insensitive to BZD Pie related to B, (-)steroid Theta to B1, (-)GABA |
GABAa receptor composition | (A1-6) (B1-3) (Y1-3) and others 2A and 2B subunits and a fifth subunit belongs to any of the other classes |
GABAa - phasic inhibition | - in synaptic cleft, producing IPSCs - IPSC shape: activated rapidly, decayed slowly, and biphasically depending on intrinsic properties of GABAa-R (subunits) |
GABAa - tonic inhibition | - non-synaptic communication (extrasynaptic) - spillover neurotransmitters (ambient GABA) - mostly ABD composition (Delta subunit), ideal for low concentration for sustained time period |