GABA and Glycine Flashcards
What type of NT is GABA?
Amino acid.
What type of NT is glycine?
Amino acid.
How is GABA synthesised?
Glutamic acid decarboxylase (GAD) removes a carboxyl group from glutamate to convert it into GABA.
Why can the presence of GAD be used as a marker to identify GABAergic neurons?
GAD is only found in neurons that synthesise GABA.
Where in the brain is GABA located?
In highly diverse inhibitory interneurons and projection neurons throughout the brain.
What is the role of GAT-1 in the first pathway for the inactivation of GABAergic transmission?
It transports GABA back into GABAergic terminals after it was released by exocytosis.
What is the role of vGAT in the first pathway for the inactivation of GABAergic transmission?
It pumps GABA back into vesicles in the GABAergic terminals.
What is the role of GABA transaminase (GABA-T) in the first pathway for the inactivation of GABAergic transmission?
It degrades GABA into succinic semialdehyde (SSA).
Describe what happens to SSA in the first pathway for the inactivation of GABAergic transmission.
It is converted to succinate by succinate semialdehyde dehydrogenase (SSADH).
Describe what happens to succinate in the first pathway for the inactivation of GABAergic transmission.
It cascades through the citric acid cycle to become alpha-ketoglutarate, which can be converted into glutamate.
How is GABAc pharmacologically distinct from GABAa?
GABAc can be activated by cis-4-aminobut-2-enoate (CACA) but GABAa cannot.
Classic GABAa agonists, e.g. isoguvacine, have no effect on GABAc.
Why might GABAc just be a variant of GABAa?
GABAc is a homomeric complex of “rho” subunits sharing a considerable amino acid sequence homology with the remaining GABAa subunits.
How is the net flow of GABA from GABAergic neurons to astrocytes offset in the second pathway for the inactivation of GABAergic transmission?
There is a steady flow of astrocytic glutamine (GABA precursor) into GABAergic terminals.
Describe how glutamine is converted into GABA in the second pathway for the inactivation of GABAergic transmission?
Glutaminase converts glutamine into glutamate.
GAD converts glutamate into GABA.
Which GABA receptor(s) are ionotropic?
GABAa and GABAc.
Which GABA receptor(s) are metabotropic?
GABAb.
Are GABA receptors located presynaptically or postsynaptically?
Both.
How do GABAb receptors cause presynaptic inhibition?
They suppress calcium influx and reduce NT release.
How do GABAb receptors cause postsynaptic inhibition?
They activate potassium currents that hyperpolarise the cell.
How do GABAa receptors cause inhibition?
The inward chloride currents that go through GABAa receptors contribute to membrane inhibition via IPSCs evoked in presynaptic terminals or in postsynaptic cells.
Describe the composition of GABAa receptors.
It is a pentameric complex comprised of possibly more than 2000 different subunit combinations.
How many subunits and subunit families do GABAa receptors have?
19 different subunits in 8 families.
Which is the most prevalent GABAa subunit?
Alpha-1 (a1).
Which is the most prevalent GABAa subtype?
Alpha-1, beta-2, gamma-2 (a1B2y2).
Where is the a2 GABAa subunit most abundant?
Regions where a1 is absent.
Where is the a3 subunit expressed?
Regions complementary to a1.
Where is the a6 subunit expressed?
Almost exclusively in the cerebellum.
Why do receptors have different subunit compositions?
To have different functional characteristics.
What is the function of alpha GABAa subunits?
Recognition of benzodiazepines.
What is the function of beta GABAa subunits?
Recognition of GABA (endogenous agonist).
What is the function of gamma GABAa subunits?
It is a structural subunit that interacts with the PSD to keep the receptor anchored in the membrane.
Which ion is the channel pore of GABAa receptors specific to?
Chloride ions.
What does GABAa activation produce at inhibitory synapses?
IPSP (inhibitory postsynaptic potential).
What are the two agonists of GABAa receptors?
GABA.
Muscimol.
What is the antagonist of GABAa receptors?
Bicuculline.
What is the channel blocker for GABAa receptors?
Picrotoxin.
Briefly describe the process of IPSP production.
NT binds to GABA receptor.
GABA receptor undergoes conformational change.
Ion channel pore opens.
Influx of chloride ions into the postsynaptic cell.
Membrane potential is repolarised towards RMP or hyperpolarised.
Name all 8 subunit families for GABAa and how many subunits each contains.
Alpha (a) = 6.
Beta (B) = 3.
Gamma (y) = 3.
Rho (p) = 3.
Delta (S) = 1.
Epsilon (E) = 1.
Theta (0) = 1.
Pi = 1.
Which GABAa subunits can form heteromeric complexes?
Alpha, beta, gamma, delta and epsilon.
Describe the GABAa/c receptor’s formation and pharmacological properties.
Homomeric complex of rho-subunits.
Expressed in the retina.
Resistant to bicuculline and baclofen.
Where in the body is the pi-subfamily expressed?
Reproductive organs.
Where do inhibitory interneurons often make axosomatic synpases?
Close to the axon hillock.
How can IPSPs attenuate the excitatory effects of EPSPs?
A hyperpolarising IPSP makes it more difficult for excitatory inputs to bring the membrane potential to threshold.
GABAa receptors can be targeted by which four classes of drugs?
Benzodiazepines.
Barbiturates.
Neurosteroids.
General anaesthetics.
How do drugs interact with GABAa receptors?
Via their own unique allosteric sites on the receptor.
Name four symptoms that drugs that facilitate agonist-induced receptor activation may produce.
Sedation.
Anticonvulsant activity.
Muscle relaxation.
Anterograde amnesia.
Benzodiazepines, barbiturates and steroids are positive allosteric modulators of GABAa receptors. How do they potentiate the actions of GABAa?
By shifting the dose-response curve to the left.
How do benzodiazepines, barbiturates and steroids affect GABAa receptor channel opening?
Benzodiazepines = increase channel opening frequency.
Barbiturates = increase channel opening duration.
Steroids = increase channel opening frequency and duration.
Are GABAb receptors ionotropic or metabotropic?
Metabotropic.
Describe the structure of GABAb receptors.
7 transmembrane spanning domains.
Coupled to G-proteins and an array of second messenger systems.
Amino acid sequence homology of around 35%.
GABAb receptors are the only GPCR currently known that functions as a what?
Heterodimer.
How do subunits associate in GABAb receptors and what are the implications of this?
Associate via their C-terminal regions.
Via these interactions, B(R1) subunit can stimulate the B(R2) subunit to activate the G-protein.
Why must the B(R1) and B(R2) subunits form a heterodimeric complex in order to function?
B(R1) binds to GABA, which B(R2) does not.
B(R2) is required to stimulate activation of the G-protein.
Postsynaptic GABAb receptors couple to potassium channels. What does this lead to?
Hyperpolarisation.
Presynaptic GABAb receptors couple to calcium channels. What does this lead to?
Decrease in calcium conductance.
Inhibition NT release.
Describe the three ways in which postsynaptic GABAb receptors gate potassium channels.
They activate slow inhibitory postsynaptic potentials via activation of GIRKs.
They function as autoreceptors presynaptically via actions on voltage-gated calcium channels.
They negatively couple to adenylyl cyclase through which they influence downstream molecular signalling pathways.
What are GIRKs?
G-protein coupled inwardly-rectifying potassium channels.
Name the five possible roles of GABAb receptors.
Control of epilepsy.
Regulation of HPA axis.
Enhancement of antidepressant drug action.
Suppression of cocaine withdrawal symptoms.
Alleviation of pain.
What does activation of GABAa and GABAb receptors under baseline conditions lead to?
Reduces postsynaptic depolarisation.
Prevents NMDAR conductance.
How do GABAa and GABAb receptors interact during high frequency activation?
There is a decreased release of GABA due to the activation of presynaptic GABAb receptors.
This reduces postsynaptic GABAa activation.
What does activation of GABAa and GABAb receptors during high frequency activation lead to?
Increases postsynaptic depolarisation.
Increases NMDAR conductance.
(This is one element of synaptic plasticity.)
Is glycine an inhibitory or excitatory NT?
Inhibitory.
What is the evidence that glycine acts as a co-agonist at NMDARs?
It potentiates the NMDA response in cultured mouse brain neurons.
Which amino acid is involved in synthesising glycine from glucose?
Serine.
Which enzyme is involved in synthesising glycine from serine?
Serine hyrdroxymethyltransferase (SHMT).
What type of enzyme is SHMT?
Pyridoxal phosphate-dependent enzyme.
Which vesicular transporter packages glycine into vesicles?
VIAAT (vesicular inhibitory amino acid transporter).
How is glycine removed from the synaptic cleft?
By uptake transporters (GLYT1 and GLYT2) on astrocytes and presynaptic terminals.
Where is GLYT1 expressed?
Astrocytes.
Neurons.
Where is GLYT2 expressed?
Axons.
Presynaptic terminals.
In which areas of the brain are GLTY1 and GLYT2 expressed?
Caudal areas.
How and where is glycine degraded?
By glycine cleavage systems (GCS) in mitochondria.
Some vesicles contain both glycine and GABA. Their co-release activates what?
Dedicated postsynaptic receptors.
Why has it been suggested that other vesicular glycine transporters exist other than VIAATs?
In some areas of the CNS that are rich with GABA and/or glycine, the VIAAT is missing.
Where are glycine receptors (GlyR) primarily found?
In the ventral spinal cord.
What are the two roles of GlyR in the spinal cord?
Motor coordination.
Neuronal inhibition.
How does strychnine act as a GlyR antagonist?
Upon binding, it prevents the opening of chloride channels.
Which subunits is a GlyR typically comprised of?
3 alpha(1) and two beta subunits.
How are GlyRs stabilised in the PSD and what is gephyrin’s role in this?
They are stabilised in the PSD by a subsynaptic scaffold.
Gephyrin acts as the core protein of the scaffold.
Where is the gephyrin binding site found on GlyRs?
On the beta subunit.
