GABA receptor

Member of a family of receptors for neurotransmitters that includes those for glycine and the nicotinic acetylcholine receptor. Opened by g -amino butyric acid (GABA). There are two main classes; GABA A and GABAC receptors are ionotropic, GABAB receptor is metabotropic. 1. GABAA receptor (ca 250 kD) is a ligand-gated chloride channel specifically blocked by bicuculline and picrotoxin, a hetero-oligomer with (probably) five subunits, generally two pairs of a and b subunits and a g subunit, but there are multiple isoform and splicing variants of these and some additional tissue-specific isoforms. The a chains (53 kD) are needed for binding of benzodiazepine, though the site is probably shared with the g subunit, and the bchains (58 kD) bind GABA. The subunits are thought to form a tight group with the chloride channel in the centre. There is considerable protein sequence similarity between GABAA receptor and the nicotinic acetylcholine receptor. Properties of the receptor can be modified by phosphorylation. Insect GABA receptor resemble vertebrate GABAA but do not bind bicuculline and have significantly different pharmacological profiles. 2. The GABAB receptor (80 kD) is a G-protein-coupled receptor found in the brain and differs from the GABAA receptor both in agonist specificity (baclofenis a specific agonist) and its effects on cells. It is negatively coupled to adenylate cyclase through a G o-protein and thus acts indirectly on N-type calcium channels. Inhibitory effects mediated through this receptor are due to a reduction in catecholamine release. Has sequence similarity with metabotropic glutamate receptors. 3. The GABAC receptor resembles GABAA but is restricted to the retina.