Ethanol potently and competitively inhibits binding of the alcohol antagonist Ro15-4513 to α4/6β3δ GABAA receptors

Although GABA(A) receptors have long been implicated in mediating ethanol (EtOH) actions, receptors containing the nonsynaptic delta subunit only recently have been shown to be uniquely sensitive to EtOH. Here, we show that delta subunit-containing receptors bind the imidazo-benzodiazepines (BZs) flumazenil and Ro15-4513 with high affinity (Kd < 10 nM), contrary to the widely held belief that these receptors are insensitive to BZs. In immunopurified native cerebellar and recombinant 6 subunit-containing receptors, binding of the alcohol antagonist [H-3]Ro15-4513 is inhibited by low concentrations of EtOH (K-i approximate to 8 mM). Also, Ro15-45113 binding is inhibited by BZ-site ligands that have been shown to reverse the behavioral alcohol antagonism of Ro15-45113 (i.e., flumazenil, beta-carbolinecarboxylate ethyl ester (beta-CCE), and N-methyl-beta-carboline-3-carboxamide (FG7142), but not including any classical BZ agonists like diazepam). Experiments that were designed to distinguish between a competitive and allosteric mechanism suggest that EtOH and Ro15-4513 occupy a mutually exclusive binding site. The fact that only Ro15-4513, but not flumazenil, can inhibit the EtOH effect, and that Ro15-4513 differs from flumazenil by only a single group in the molecule (an azido group at the V position of the BZ ring) suggest that this azido group in Ro15-45113 might be the area that overlaps with the alcohol-binding site. Our findings, combined with previous observations that Ro15-4513 is a behavioral alcohol antagonist, suggest that many of the behavioral effects of EtOH at relevant physiological concentrations are mediated by EtOH/Ro15-4513-sensitive GABA(A) receptors.