Agonist function of the recombinant α4β3δ GABAA receptor is dependent on the human and rat variants of the α4‐subunit

1. It is known that the α 4 -subunit is likely to occur in the brain predominantly in α 4 β 3 δ receptors at extrasynaptic sites. Recent studies have revealed that the α 1 -, α 4 -, γ 2 - and δ-subunits may colocalize extrasynaptically in dentate granule cells of the hippocampus. In the present study, we characterized a series of recombinant GABA A receptors containing human (H) and rat (R) α 1 /α 4 -, β 2 /β 3 - and γ 2s /δ-subunits in Xenopus oocytes using the two-electrode voltage-clamp technique. 2. Both Hα 1 β 3 δ and Hα 4 β 3 γ 2s receptors were sensitive to activation by GABA and pentobarbital. Contrary to earlier findings that the α 4 β 3 δ combination was more sensitive to agonist action than the α 4 β 3 γ 2s receptor, we observed extremely small GABA-and pentobarbital-activated currents at the wild-type Hα 4 β 3 δ receptor. However, GABA and pentobarbital activated the wild-type Rα 4 β 3 δ receptor with high potency (EC 50 = 0.5 ± 0.7 and 294 ± 5 μmol/L, respectively). 3. Substituting the Hα 4 subunit with Rα 4 conferred a significant increase in activation on the GABA and pentobarbital site in terms of reduced EC 50 and increased I max . When the Hα 4 subunit was combined with the Rβ 3 and Rδ subunit in a hetero-pentameric form, the amplitude of GABA- and pentobarbital-activated currents increased significantly compared with the wild-type Hα 4 β 3 δ receptor. 4. Thus, the results indicate that the Rα 4 β 3 δ, Hα 4 β 3 δ and Hα 4 β 3 γ 2s combinations may contribute to functions of extrasynaptic GABA A receptors. The presence of the Rα 4 subunit at recombinant GABA A receptors containing the δ-subunit is a strong determinant of agonist action. The recombinant Hα 4 β 3 δ receptor is a less sensitive subunit composition in terms of agonist activation.