Loss of zolpidem efficacy in the hippocampus of mice with the GABAA receptor γ2 F77I point mutation

Zolpidem is a hypnotic benzodiazepine site agonist with some gamma-aminobutyric acid (GABA)(A) receptor subtype selectivity. Here, we have tested the effects of zolpidem on the hippocampus of gamma 2 subunit (gamma 2F77I) point mutant mice. Analysis of forebrain GABA(A) receptor expression with immunocytochemistry, quantitative [H-3]muscimol and [S-35] t-butylbicyclophosphorothionate (TBPS) autoradiography, membrane binding with [H-3]flunitrazepam and [H-3]muscimol, and comparison of miniature inhibitory postsynaptic current (mIPSC) parameters did not reveal any differences between homozygous gamma 2I77/I77 and gamma 2F77/F77 mice. However, quantitative immunoblot analysis of gamma 2I77/I77 hippocampi showed some increased levels of gamma 2, alpha 1, alpha 4 and delta subunits, suggesting that differences between strains may exist in unassembled subunit levels, but not in assembled receptors. Zolpidem (1 mu m) enhanced the decay of mIPSCs in CA1 pyramidal cells of control (C57BL/6J, gamma 2F77/F77) mice by similar to 60%, and peak amplitude by similar to 20% at 33-34 degrees C in vitro. The actions of zolpidem (100 nm or 1 mu m) were substantially reduced in gamma 2I77/I77 mice, although residual effects included a 9% increase in decay and 5% decrease in peak amplitude. Similar results were observed in CA1 stratum oriens/alveus interneurons. At network level, the effect of zolpidem (10 mu m) on carbachol-induced oscillations in the CA3 area of gamma 2I77/I77 mice was significantly different compared with controls. Thus, the gamma 2F77I point mutation virtually abolished the actions of zolpidem on GABA(A) receptors in the hippocampus. However, some residual effects of zolpidem may involve receptors that do not contain the gamma 2 subunit.