Ethanol Tolerance and Withdrawal Responses in GABAA Receptor Alpha 6 Subunit Null Allele Mice and in Inbred C57BL/6J and Strain 129/SvJ Mice

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Abstract
We have been using a genetic strategy to define the contribution of specific candidate genes, such as those encoding subunits of the γ‐aminobutyric acid type A receptor, to various ethanol sensitive responses. We have used the gene knockout approach in mouse embryonic stem cells to create mice in which the gene encoding the α6 subunit of the γ‐aminobutyric acid type A receptor is rendered nonfunctional. In the present report, we provide a detailed characterization of several behavioral responses to ethanol in these null allele mice. In a separate series of experiments, behavioral response to ethanol was compared between two inbred strains of mice that are commonly used as background stock in knockout experiments, namely C57BL/6J and Strain 129/SvJ. Wild type (α6+/+) and homozygous null allele (α6−/−) mice did not differ to the ataxic effects of ethanol on acute functional tolerance (95.8 ± 8.7 vs. 98.8 ± 5.7 mg/dl ± SEM, respectively). Withdrawal hyperexcitability was assessed following chronic exposure to ethanol vapor (EtOH) or air (CONT) in inhalation chambers in a multiple withdrawal treatment paradigm. At the end of the last treatment cycle, mice were scored for handling induced convulsions (HIC). After adjusting for differences in blood ethanol concentration between genotypes at the end of the final treatment cycle, we observed a greater area under the 24‐hr HIC curves in mice treated with ethanol (p < 0.0001) but did not detect an effect of genotype (α6+/+/CONT 3.1 ± 2.0; α6−/−/CONT 5.5 ± 2.5; α6+/+/EtOH 30.1 ± 6.2; α6−/−/EtOH 33.0 ± 5.8 mean units ± SEM). We also examined these mice for differences in protracted tolerance; at ∼26 hr into the final withdrawal cycle, each mouse was injected with ethanol (3.5 mg/g body weight) and sleep time was measured. We detected a significant effect of treatment (p +/+/CONT 57.4 ± 7.6; α6‐/‐/CONT 59.0 ± 7.6; 6+/+/EtOH 34.8 ± 7.4; α6−/−/EtOH 30.8 ± 5.6 min ± SEM). From these data we conclude that the α6 subunit of the GABAA‐R exerts little if any influence on acute functional tolerance, withdrawal hyperexcitability, or protracted tolerance. Strain 129/SvJ and C57BL/6J mice were also compared for acute functional tolerance and were found not to differ (96.3 ± 4.4 vs. 94.8 ±11.3 mg/dl ± SEM, respectively). Withdrawal hyperexcitability was assessed by comparing the area under the 24 hr HIC curves. Strain 129/SvJ mice displayed a much greater basal HIC response compared to C57BL/6J mice (19.8 ± 4.3 vs. 0.2 ± 0.2 mean units ± SEM, respectively); after adjusting for differences in blood ethanol concentration between strains at the end of the final ethanol treatment cycle, the HIC response was markedly enhanced by ethanol treatment in Strain 129/SvJ mice but not in C57BL/6J mice (50.4 ± 3.1 vs. 9.5 ± 5.4 mean units ± SEM, respectively). The effects of treatment (p p p < 0.01) were significant. Since many gene knockout mice are maintained on a mixed genetic background of Strain 129/SvJ and C57BL/6J, we conclude that significant differences in tests of withdrawal hyperexcitability may be confounded by the influence of genes that cosegregate with the gene targeted allele.