GABA 1 Flashcards
Who first discovered GABA?
Roberts and Frankel in 1950
How was GABA discovered?
Paper chromatography of brain extracts
Florey 1953
Showed that brain extracts have inhibitory action on crayfish stretch receptor organ
Kuffler 1958
Applied GABA mimicked inhibitory action on crayfish stretch receptor organ
Roberts: 1950s and 60s
Elucidated GABA metabolism through radioactive precursors
Obata 1967
Iontophoretic gather application mimics inhibitory action of Purkinje cells and released into ventricles on Purkinje stimulation
GABA levels in the brain
2500 nmol/gram
Is there more inhibition or excitation in the brain?
Inhibition
How is GABA distributed?
Throughout the brain (30% of all nerve endings)
inhibitory interneurons everywhere
long axon tracts: cortex-substantia nigra; cerebellar Purkinje cells (and many others)
How was the distribution of GABA determined?
Distribution of GABA receptors (Promoter:fusion/mRNA/Protein) Distribution of metabolising enzymes (GAD, GABA-T) Distribution of GABA and/or vesicular GABA transporter Uptake of 3H GABA Electrophysiology (gold standard)
Recurrent inhibition
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Afferent inhibition
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The GABA shunt
draw diagram
GABA transaminase (GABA-T)
Catalyses the conversion of Alpha-ketoglutarate to glutamate
Glutamic acid decarboxylase (GAD)
Catalyses the conversion of glutamate to GABA
Vesicular transport of GABA
VGAT
Reuptake
2Na+/1Cl- per GABA 4 Distinct Genes:GAT-1,2,3 & BGT-3 ( GABA/betaine transporter) Nerve Terminals and Glial membranes
Which of the GABA transporters have established blockers?
GAT etc…
VGAT doesn’t
Coupling of synthesis and transport
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What is GABA synthesis coupled to?
GABA synthesis is functionally coupled to vesicular transport into synaptic vesicles
How many variants of GAD are there in humans?
There are two variants: 65 and 67
Why is GABA synthesized by SV-associated GAD is preferentially transported into the SV by vesicular GABA transporters (VGATs)?
VGAT forms a protein complex with GAD on the SV
Succinic semialdehyde dehydrogenase deficiency
First documented inborn error of GABA metabolism
symptoms include mental retardation, ataxia, motor deficiencies
Disorders related to aberrant GABA transmission
Epilepsy, Huntington’s disease, Parkinson’s disease, tardive dyskinesia, alcoholism, sleep disorders
Physiology of GABAergic transmission
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When is GABA transmission excitatory?
In invertebrates and developing neurons
GABAergic transmission in. embryonic neurons
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GABAergic transmission in some adult neurons
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GABA receptors
Historically there were three classes but now there are technically two, as C is now a subtype of A
GABAA, and GABAC
Classic Ligand-gated ion channels analogous to nicotinic receptor - postsynaptic
GABAB
7TM G-Coupled receptors – presynaptic (coupled to K+ channels)
Identification of the GABA receptors
Hans Mohler – 1980’s
Photolabelling
Eric Barnard & Anne Stephenson – 1980’s
Purified and reconstituted protein
Eric Barnard’s & Peter Seeburg’s groups – 1986-7
Sequenced via cDNA based on peptides
Compare GABAA and GABAC receptors
Draw the table
GABAA receptors
19 different subunits α1-6, β1-4, γ1-3,δ,ε,θ
8 families alternative splicing exists
most common form: α12,β22,γ21
Mechanism of activation
Two agonist molecules needed to open channel
Agonist binding is very rapid, low affinity (Kd 100 µmol)
rapid confirmation change opens channel (milliseconds)
Desensitisation involves transition to high affinity state (Kd > 1 µmol)
Which proteins are GABAA receptors usually clustered with?
GABARAP
Neuroligin-2
Gephyrin
Collybistin
How does clustering affect GABAA receptor kinetics?
Clustering decreases affinity and decreases the rate of desensitisation
Clusters have properties consistent with those of native synaptic transmission
Extrasynaptic GABAA receptors
Lower EC50 thus more sensitive
Tonic inhibition in clustered and diffuse GABAA receptors
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IPSP in clustered but not diffuse
