Contribution of “diazepam-insensitive” GABA A receptors to the alcohol antagonist properties of Ro 15–4513 and related imidazobenzodiazepines

Abstract

Both in vivo and in vitro studies have shown that Ro 15–4513 can antagonize many of the pharmacologic actions of ethanol. In contrast to many benzodiazepine receptor (BzR) ligands, Ro 15–4513 binds with high affinity to a novel GABA A receptor subtype, referred to as “diazepam-insensitive” (DI). This study examined the contribution of DI GABA A receptors to the modulation of ethanol-induced sleep time by Ro 15–4513 and related imidazobenzodiazepines [e.g., Ro 19–4603, Ro16–6028, and ZG-63 ( t-butyl-8-chloro-5,6-dihydro-5-methyl-6-oxo-imidazo[1,5,a] [1,4] benzodiazepine-3-carboxylate)] that possess high affinities for this GABA A receptor subtype. Ro 15–4513 (0.6–5 mg/kg) significantly reduced ethanol (3.5 g/kg, IP) sleep time in mice ( p < 0.001, analysis of variance). This effect was not blocked by BzR antagonists ZK 93426 (5 mg/kg) and Ro 14-7437 (5 mg/kg), which possess low affinities for DI but bind with high affinities to other “diazepam-sensitive” (DS) GABA A receptor isoforms. Although Ro 19–4603 (2.5 mg/kg) also reduced ethanol sleep time ( p < 0.01), this effect was attenuated by coadministration of ZK 93426 (2.5 mg/kg). Ro 16-6028 (2.5 mg/kg) prolonged ( p < 0.01) ethanol sleep time. However, in the presence of either Ro 14-7437 (5 mg/kg) or ZK 93426 (2.5 mg/kg) ethanol-induced sleep time was reduced to values approaching those obtained with ethanol in the presence of Ro 15–4513. A low dose (2.5 mg/ kg) of ZG-63 did not significantly affect alcohol sleep time. However, in the presence of ZK 93426, ZG-63 increased sleep time ( p < 0.01). In contrast, a higher dose of ZG-63 (7.5 mg/kg) prolonged sleep time, an effect that was attenuated by either ZK 93426 or Ro 14-7437. These findings indicate that although antagonism of ethanol-induced sleep time by Ro 15–4513 is mediated, at least in part, through DI, the contribution of this GABA A receptor isoform to the modulation of ethanol-induced sleep time by structurally related imidazobenzodiazepines is more variable. The synthesis of compounds with higher selectivity and a range of intrinsic efficacies at DI GABA A receptors is required to better define the role of this receptor isoform in ethanol-induced sleep time.