INVERSE BUT NOT FULL BENZODIAZEPINE AGONISTS MODULATE RECOMBINANT ALPHA-6-BETA-2-GAMMA-2 GABA-A RECEPTORS IN TRANSFECTED HUMAN EMBRYONIC KIDNEY-CELLS

We compared the modulation of GABA (gamma-aminobutyric acid)-activated currents by benzodiazepines in recombinant GABA(A) receptors containing either one of two alpha-subunits, alpha-1 or alpha-6. Luddens et al. (Nature, 346 (1990) 648-651) have previously demonstrated that the alpha-6 subunit is part of a cerebellar receptor subtype which selectively binds Ro15-4513, an antagonist of alcohol-induced motor ataxia. Here we report that the imidazobenzodiazepine Ro15-4513 (ethyl 8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo-(1,5-a)(1,4)benzodiazepine-3-carboxylate) reduced GABA-activated currents in recombinant alpha-6-beta-2-gamma-2 and alpha-1-beta-2-gamma-2 receptors, thus acting consistently as an inverse agonist. Moreover, another well characterized negative modulator, DMCM (methyl-4-ethyl-6,7-dimethoxy-beta-carboline-3-carboxylate), also reduces GABA activated-currents in both receptors. In contrast, flunitrazepam (FNZM), a benzodiazepine agonist, increases GABA-activated currents in alpha-1-beta-2-gamma-2 receptors, but not in alpha-6-beta-2-gamma-2 receptors. This study lends further support to the hypothesis that the binding sites of full and partial inverse agonists are different.