Low concentrations of ethanol do not affect radioligand binding to the δ-subunit-containing GABA_A receptors in the rat brain

In the present study, we investigated the co-localization pattern of the δ subunit with other subunits of GABA_A receptors in the rat brain using immunoprecipitation and Western blotting techniques. Furthermore, we investigated whether low concentrations of ethanol affect the δ-subunit-containing GABA_A receptor assemblies in the rat brain using radioligand binding to the rat brain membrane homogenates as well as to the immunoprecipitated receptor assemblies. Our results revealed that δ subunit is not co-localized with γ₂ subunit but it is associated with the α₁, α₄ or α₆, β₂ and/or β₃ subunit(s) of GABA_A receptors in the rat brain. Ethanol (1-50 mM) neither affected [³H]muscimol (3 nM) binding nor diazepam-insensitive [³H]Ro 15-4513 (2 nM) binding in the rat cerebellum and cerebral cortex membranes. However, a higher concentration of ethanol (500 mM) inhibited the binding of these radioligands to the GABA_A receptors partially in the rat cerebellum and cerebral cortex. Similarly, ethanol (up to 50 mM) did not affect [³H]muscimol (15 nM) binding to the immunoprecipitated δ-subunit-containing GABA_A receptor assemblies in the rat cerebellum and hippocampus but it inhibited the binding partially at a higher concentration (500 mM). These results suggest that the native δ-subunit-containing GABA_A receptors do not play a major role in the pharmacology of clinically relevant low concentrations of ethanol.