Evaluation of native GABA(A) receptors containing an alpha 5 subunit

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