Prevalence between different α subunits performing the benzodiazepine binding sites in native heterologous GABAA receptors containing the α2 subunit

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Abstract
The presence of two heterologous α subunits and a single benzodiazepine binding site in the GABAA receptor implicates the existence of pharmacologically active and inactive α subunits. This fact raises the question of whether a particular α subtype could predominate performing the benzodiazepine binding site. The hippocampal formation expresses high levels of α subunits with different benzodiazepine binding properties (α1, α2 and α5). Thus, we first demonstrated the existence of α2–α1 (36.3 ± 5.2% of the α2 population) and α2–α5 (20.2 ± 2.1%) heterologous receptors. A similar α2–α1 association was observed in cortex. This association allows the direct comparison of the pharmacological properties of heterologous native GABAA receptors containing a common (α2) and a different (α1 or α5) α subunit. The α2 subunit pharmacologically prevailed over the α1 subunit in both cortex and hippocampus (there was an absence of high-affinity binding sites for Cl218,872, zolpidem and [3H]zolpidem). This prevalence was directly probed by zolpidem displacement experiments in α2–α1 double immunopurified receptors (Ki = 295 ± 56 nm and 200 ± 8 nm in hippocampus and cortex, respectively). On the contrary, the α5 subunit pharmacologically prevailed over the α2 subunit (low- and high-affinity binding sites for zolpidem and [3H]L-655,708, respectively). This prevalence was probed in α2–α5 double immunopurified receptors. Zolpidem displayed a single low-affinity binding site (Ki = 1.73 ± 0.54 µm). These results demonstrated the existence of a differential dominance between the different α subunits performing the benzodiazepine binding sites in the native GABAA receptors.