Mechanisms of Reversible GABA A Receptor Plasticity after Ethanol Intoxication

The time-dependent effects of ethanol (EtOH) intoxication on GABA A receptor (GABA A R) composition and function were studied in rats. A cross-linking assay and Western blot analysis of microdissected CA1 area of hippocampal slices obtained 1 h after EtOH intoxication (5 g/kg, gavage), revealed decreases in the cell-surface fraction of α4 and δ, but not α1, α5, or γ2 GABA A R subunits, without changes in their total content. This was accompanied (in CA1 neuron recordings) by decreased magnitude of the picrotoxin-sensitive tonic current ( I tonic ), but not miniature IPSCs (mIPSCs), and by reduced enhancement of I tonic by EtOH, but not by diazepam. By 48 h after EtOH dosing, cell-surface α4 (80%) and γ2 (82%) subunit content increased, and cell-surface α1 (−50%) and δ (−79%) and overall content were decreased. This was paralleled by faster decay of mIPSCs, decreased diazepam enhancement of both mIPSCs and I tonic , and paradoxically increased mIPSC responsiveness to EtOH (10–100 m m ). Sensitivity to isoflurane- or diazepam-induced loss of righting reflex was decreased at 12 and 24 h after EtOH intoxication, respectively, suggesting functional GABA A R tolerance. The plastic GABA A R changes were gradually and fully reversible by 2 weeks after single EtOH dosing, but unexplainably persisted long after withdrawal from chronic intermittent ethanol treatment, which leads to signs of alcohol dependence. Our data suggest that early tolerance to EtOH may result from excessive activation and subsequent internalization of α4βδ extrasynaptic GABA A Rs. This leads to transcriptionally regulated increases in α4 and γ2 and decreases in α1 subunits, with preferential insertion of the newly formed α4βγ2 GABA A Rs at synapses.