Discriminative stimulus effects of ethanol in mice lacking the γ-aminobutyric acid type A receptor δ subunit

Background: Genetically altered mice have been used to examine gene contributions to ethanol phenotypes. Recently, mice with a targeted deletion of the δ subunit of the γ-aminobutyric acid (GABA)_A receptor have been generated. These mice display decreased sensitivity to neuroactive steroids and altered responses to some behavioral effects of ethanol. Given the application of drug discrimination to characterize receptor-mediated stimulus effects of ethanol and given the data showing altered ethanol responses in mice lacking the δ subunit of the GABA_A receptor, these mice were characterized in an ethanol-discrimination procedure. It has been shown that neurosteroids will substitute for the discriminative stimulus effects of ethanol, and this study aimed to determine whether the substitution patterns of neuroactive steroids or other GABA_A-positive modulators would be altered in these mice. Methods: Twelve adult δ^+/+ and δ^-/- mice were trained to discriminate between ethanol 1.5 g/kg and saline in daily 15-min food-reinforced operant sessions. Once the discrimination was trained, substitution tests with ethanol, pentobarbital, midazolam, androsterone, alphaxalone, pregnanolone, morphine, zolpidem, and MK-801 were conducted. Results: Both δ^+/+ and δ^-/- mice acquired ethanol discrimination in a similar number of days. Ethanol, midazolam, alphaxalone, pregnanolone, and MK-801 fully substituted (>80%) for ethanol in both δ^+/+ and δ^-/- mice. Pentobarbital fully substituted for ethanol in δ^-/- mice but only partially substituted (74%) for ethanol in δ^+/+ mice. Androsterone, zolpidem, and morphine did not substitute for ethanol in either δ^+/+ or δ^-/- mice. There were no significant differences in the response rate-suppressing effects of any of the compounds between δ^+/+ and δ^-/- mice. Conclusions: The training dose of ethanol resulted in substitution of five GABA_A receptor ligands, indicating a robust GABA_A mediation of ethanol's discriminative stimulus effects. Deletion of the δ subunit of the GABA_A receptor does not alter the acquisition of an ethanol/saline discrimination or the substitution patterns of GABA_A-positive modulators. Therefore, the δ subunit is not necessary in the mediation of ethanol-like effects of any of the GABA _A ligands tested, including sensitivity to ethanol, barbiturate, benzodiazepine, and neurosteroid discriminative stimulus effects.