Behavioral Pharmacogenetic Analysis on the Role of the α4 GABAA Receptor Subunit in the Ethanol-Mediated Impairment of Hippocampus-Dependent Contextual Learning

Background: A major effect of low-dose ethanol is impairment of hippocampus-dependent cognitive function. alpha 4/delta -containing GABA(A)Rs are highly expressed within the dentate gyrus region of the hippocampus where they mediate a tonic inhibitory current that is sensitive to the enhancement by low ethanol concentrations. These receptors are also powerful modulators of learning and memory, suggesting that they could play an important role in ethanol's cognitive impairing effects. The goal of this study was to develop a high-throughput cognitive ethanol assay, amenable to use in genetically modified mice that could be used to test this hypothesis. Methods: We developed a procedure where preexposure to a conditioning chamber is used to rescue the "immediate shock deficit.'' Using this task, ethanol can be specifically targeted at the hippocampus-dependent process of contextual learning without interfering with pain sensitivity or behavioral performance. Results: Validation of this task in C57BL/6 mice indicated that 1.0 g/kg ethanol and 10 mg/kg allopregnanolone disrupt contextual learning. Ro15-4513 reversed the effects of ethanol but not allopregnanolone, whereas it produced an impairment when given alone. The high-throughput nature of this task allowed for its application in a large cohort of alpha 4 GABA(A)R KO mice. Loss of the alpha 4 GABA(A)R subunit produced an enhanced sensitivity to the cognitive impairing effects of ethanol. This is consistent with the enhanced ethanol sensitivity of synaptic GABA(A)Rs that has been previously observed in the dentate gyrus in these mice, but inconsistent with the reduced ethanol sensitivity of extrasynaptic GABA(A)Rs observed in the same cells. Conclusions: Overall, these findings are consistent with our hypothesis that ethanol acts directly at GABA(A) receptors to impair hippocampus-dependent cognitive function. Furthermore, validation of this high-throughput assay will allow for future studies to use anatomically and temporally restricted genetic manipulations to probe more deeply into the neural mechanisms of ethanol action on learning and memory circuits.