Vertebrates GABA receptor subunits

LCCH3 has 47% identity to vertebrate GABA receptor / subunits, but is unlikely to be a subunit of native insect GABA receptors. Although it can form functional heteromultimers with Rdl subunits, these are unlike any known native insect GABA receptors in that they are bicucculine-sensitive, PTX-insensitive and undergo a slow desensitization [26, 49). Furthermore, the distributions of LCCH3 and Rdl in the Drosophila nervous system do not overlap [50]. 

Nineteen different ionotropic GABA receptor subunits, named ai 6, P 3, 7i 3, S, e, 71 and d and 3, have been cloned from vertebrates. Experiments using recombinant expression and immunoprecipitation indicate that native vertebrate GABAa receptors contain at least a, fi and y subunits, sometimes also with <5, s, K ox 0 subunits, while GABAc receptors are believed to be composed only of p subunits. 

Electron microscopy studies have revealed that, like acetylcholine receptors, the ion channel of the GABA receptor is formed by the pentameric assembly of hetero-oligomeric subunits (129) each subunit has four trans-membrane spanning domains and all five sub-uinits are arranged so that their second transmembrane domains comprise the ion channel wall. Cloning of the subunits from vertebrates has resulted in nearly 20 cDNAs, which have... 

ACh interacts with the nicotinic ACh receptor to initiate an end-plate potential (EPP) in muscle or an excitatory postsynaptic potential (EPSP) in peripheral ganglia (Chapter 6). The nicotinic receptor of vertebrate skeletal muscle is a pentamer composed of 4 distinct subunits a, /3, y, and S) in the stoichiometric ratio of 2 1 1 1, respectively. In mature, innervated muscle end plates, the y subunit is replaced by the closely related e subunit. The nicotinic receptor is prototypical of other pen-tameric ligand-gated ion channels, which include the receptors for the inhibitory amino acids (y-aminobutyric acid [GABA] and glycine) and S-HT serotonin receptors (Figure 9-1). 

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