Antagonism of the ethanol-like discriminative stimulus effects of ethanol, pentobarbital, and midazolam in cynomolgus monkeys reveals involvement of specific GABA_A receptor subtypes

The γ-aminobutyric acid (GABA)_A receptors mediating the discriminative stimulus effects of ethanol were studied by comparing the potency of ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4Himidazol( 1,5-a)benzodiazepine-3- carboxylate (Ro15-4513) and ethyl 8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H- imidazol(1,5-a)-benzodiazepine-3-carboxylate (flumazenil, Ro15-1788) to antagonize ethanol, pentobarbital (PB), and midazolam substitution for ethanol. Ro15-4513 has high affinity for receptors containing α_4/6 and α₅ subunits and lower affinity for α₁, α₂, and α₃ subunits. Flumazenil is nonselective for GABA_A receptors containing α₁, α₂, α₃, and α₅ subunits and has low affinity for α_4/6-containing receptors. Male (n = 9) and female (n = 8) cynomolgus monkeys (Macaca fascicularis) were trained to discriminate ethanol (1.0 or 2.0 g/kg i.g., 30-min pretreatment) from water. Ethanol, PB, and midazolam dose-dependently substituted for ethanol (80% ethanol-appropriate responding). Ro15-4513 (0.003-0.56 mg/kg i.m., 5-min pretreatment) shifted the ethanol, PB, and midazolam doseresponse functions rightward in a vast majority of monkeys tested (15/15, 16/17, and 11/12, respectively). In contrast, flumazenil (0.30-10.0 mg/kg i.m., 5-min pretreatment) shifted the ethanol, PB, and midazolam dose-response functions rightward in 9 of 16, 12 of 16, and 7 of 9 monkeys tested, respectively. In the monkeys showing antagonism with both Ro15-4513 and flumazenil, ethanol and PB substitution were antagonized more potently by Ro15-4513 than by flumazenil, whereas midazolam substitution was antagonized with similar potency. There were no sex or training dose differences, with the exception that flumazenil failed to antagonize ethanol substitution in males trained to discriminate 2.0 g/kg ethanol. GABA_A receptors with high affinity for Ro15-4513 (i.e., containing α_4/6 and α₅ subunits) may be particularly important mediators of the multiple discriminative stimulus effects of ethanol through GABA_A receptor systems.