Evaluation of native GABA A receptors containing an α5 subunit

Abstract

The type A receptor for γ-aminobutyric acid (GABA), or GABA A receptor, is a pentamer of highly variable quaternary structure. It includes two α subunits, drawn from a pool of six genes, which largely determine benzodiazepine pharmacology of the receptor. In brain sections, both [ 3H]RY-80 (ethyl-8-acetylene-5,6-dihydro-5-methyl-6-oxo-4 H-imidazo[1,5 a][1,4]benzodiazepine-3-carboxylate) and [ 3H]L-655,708 (ethyl ( S)-11,12,13,13 a-tetrahydro-7-methoxy-9-oxo-9 H-imidazo[1,5- a]pyrrolo[2,1- c][1,4]benzodiazepine-1-carboxylate), which are selective for the benzodiazepine site of α5 subunit-containing receptors, showed high-affinity, specific binding, but to fewer regions than did the nonselective benzodiazepine, [ 3H]flunitrazepam. The pattern mirrored α5 mRNA distribution, and was similar to that previously reported for [ 3H]L-655,708 binding. Displacement of [ 3H]RY-80 bound to hippocampal homogenates, and of [ 3H]flunitrazepam bound to cerebellar and hippocampal homogenates showed comparable displacement by flumazenil ( K i's 5–7 nM). However, the K i's for diazepam and for clobazam to displace [ 3H]RY-80 binding in hippocampus were about fourfold higher than for [ 3H]flunitrazepam, and the K i for clonazepam was sixfold larger, suggesting that these benzodiazepine receptor agonists bind with relatively lower affinity at hippocampal α5-containing receptors.