A pharmacological characterization of GABA, THIP and DS2 at binary α4β3 and β38 receptors : GABA activates β38 receptors via the β3(+)8(-) interface

There is growing evidence that GABA (γ-aminobutyric acid) can activate GABAA receptors (GABAARs) in the absence of an α subunit. In this study, we compared the pharmacology of homomeric and binary α4, β3 or δ subunits with ternary α4β3δ to identify subunit interfaces that contribute to the pharmacology of GABA, THIP, and DS2, and the antagonists, Zn2þ, gabazine and bicuculline. β3δ receptors form functional GABA-gated channels when expressed in Xenopus oocytes with a pharmacology that differs to homomeric β3, binary α4β3 and ternary α4β3δ receptors. GABA had similar potency at α4β3 and β3δ receptors (25 mM and 26 mM, respectively) but differed at α4β3δ receptors where GABA exhibited a biphasic concentration-response (EC50 (1)¼12.6 nM; EC50 (2)¼6.3 μM). THIP activated β3δ receptors (EC50¼456 μM) but was a more potent activator of α4β3 (EC50¼27 μM) and α4β3δ receptors (EC50 (1)¼ 27.5 nM; EC50 (2)¼29.5 μΜ), indicating that the α4 subunit significantly contribute to its potency. The δpreferring modulator, DS2 had marginal or no effect at β3δ and α4β3 receptors, indicating a role for both the α4 and δ subunits for its potency. Gabazine inhibited GABA-elicited currents at β3δ receptors whereas bicuculline activated these receptors. Mutational analysis verified that GABA binds to the β3(þ) δ() interface formed by the β3 and δ subunits. In conclusion, evaluating agents against binary GABAARs such as β3δ and α4β3 receptors enables identification of interfaces that may contribute to the pharmacology of the more complex ternary α4β3δ receptors.