GABA_A receptor function and regional analysis of subunit mRNAs in long-sleep and short-sleep mouse brain

The greater sensitivity of long-sleep (LS), as compared with short-sleep (SS), mice to ethanol is due in part to differences in GABA_A receptor function in specific brain regions. To determine if differences in subunit composition of GABA_A receptors contribute to this differential sensitivity, we measured α₁ and γ₂ subunit mRNAs with Northern analysis and in situ hybridization and γ_2S, γ_2L and α₆ subunit mRNAs with polymerase chain reaction (PCR) amplification. No differences in mRNAs in whole brain were apparent by Northern analysis. In situ hybridization revealed that α₁ and γ₂ subunit mRNAs were co-localized in many brain regions but that they still had distinct patterns of hybridization. However, the few differences observed between LS and SS mice in the levels of hybridization for these subunits did not show a regional distribution consistent with ethanol sensitivity differences. Similar ratios of γ_2L and γ_2S subunit mRNAs were found in LS and SS mouse cerebral cortex and hippocampus, and both mouse lines expressed essentially only γ_2L subunit mRNA in cerebellum, mRNA for the α₆ subunit was detected only in cerebellum and also was qualitatively similar between LS and SS mice. Studies of muscimol-stimulated ³⁶Cl^- uptake by cortical membrane vesicles confirmed earlier findings that ethanol does not enhance function of GABA_A receptors in SS mice when assayed at 30°C. However, at 34°C ethanol did increase this function in SS mice although the enhancement remained greater in LS mice. These functional results, together with the results showing similar levels of α₁, γ_2S, γ_2L and α₆ subunits in LS and SS mice, suggest that the ethanol-insensitivity of SS mouse GABA_A receptors cannot be due solely to lack of subunits required for ethanol action and further suggest that differences in catalytic mechanisms affecting post-translational processing may account for some genetic differences in ethanol sensitivity of GABA_A receptors.