Mechanisms of Reversible GABA_AReceptor Plasticity after Ethanol Intoxication

Abstract

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 ␣4 and ␦, but not ␣1, ␣5, or ␥2 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 ␣4 (80%) and ␥2 (82%) subunit content increased, and cell-surface ␣1 (Ϫ50%) and ␦ (Ϫ79%) 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 mM). 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 ␣4␤␦ extrasynaptic GABA A Rs. This leads to transcriptionally regulated increases in ␣4 and ␥2 and decreases in ␣1 subunits, with preferential insertion of the newly formed ␣4␤␥2 GABA A Rs at synapses.

Oxytocin prevents ethanol actions at δ subunit-containing GABA A receptors and attenuates ethanol-induced motor impairment in rats

“…The δ-GABA A Rs, sometimes called the "one-glass-of-wine" receptors (24), are located in several brain regions, including the thalamus, cerebellum, hippocampus, and striatum (25,26), and show high sensitivity to relatively low doses of ethanol (2)(3)(4)(5)(6)(7)(8)27). A number of previous studies demonstrate the involvement of δ-GABA A Rs in a range of ethanolinduced functional effects, including ataxia, sedation, and reward (2-8).…”

“…Further, knockdown of δ subunit expression in the medial shell of the nucleus accumbens reduces alcohol intake in rats (6). Finally, overstimulation and subsequent internalization of α4βδ extrasynaptic GABA A Rs after persistent stimulation by ethanol seem to underlie the development of rapid tolerance to the ataxic effects of ethanol (4).…”

“…The δ-GABA A Rs also represent one of the major targets of ethanol in the CNS, particularly at relatively low ethanol concentrations, and mediate some of the rewarding and ataxic effects of ethanol (2)(3)(4)(5)(6)(7)(8). For instance, a genetic polymorphism that exaggerates the actions of ethanol at δ-GABA A Rs also potentiates ethanolinduced motor impairment (2).…”

“…Given the role of α4βδ extrasynaptic GABA A Rs in mediating tolerance to the ataxic effects of ethanol (4), and the modulation of ethanol tolerance by OT (14), we hypothesized that OT might act to prevent ethanol actions directly at δ-GABA A Rs. Such an action would be expected to reduce alcohol-induced motor impairment.…”

Early developmental alterations in GABAergic protein expression in fragile X knockout mice

“…Biochemical studies have demonstrated that the α4 and δ subunits are often present in the same receptor complex (Sur et al, 1999). Furthermore, α4βδ receptor expression has been shown to be very plastic in the CNS, with α4 protein levels subject to rapid regulation after administration of GABAergic drugs (Birzniece et al, 2006;Liang et al, 2007).…”