ABSTRACT
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, ß3 or δ subunits with ternary α4ß3δ to identify subunit interfaces that contribute to the pharmacology of GABA, THIP, and DS2, and the antagonists, Zn(2+), gabazine and bicuculline. ß3δ receptors form functional GABA-gated channels when expressed in Xenopus oocytes with a pharmacology that differs to homomeric ß3, binary α4ß3 and ternary α4ß3δ receptors. GABA had similar potency at α4ß3 and ß3δ receptors (25µM and 26µM, respectively) but differed at α4ß3δ receptors where GABA exhibited a biphasic concentration-response (EC50 (1)=12.6nM; EC50 (2)=6.3µM). THIP activated ß3δ receptors (EC50=456µM) but was a more potent activator of α4ß3 (EC50=27µM) and α4ß3δ receptors (EC50 (1)=27.5nM; EC50 (2)=29.5µΜ), indicating that the α4 subunit significantly contribute to its potency. The δ-preferring modulator, DS2 had marginal or no effect at ß3δ and α4ß3 receptors, indicating a role for both the α4 and δ subunits for its potency. Gabazine inhibited GABA-elicited currents at ß3δ receptors whereas bicuculline activated these receptors. Mutational analysis verified that GABA binds to the ß3(+)δ(-) interface formed by the ß3 and δ subunits. In conclusion, evaluating agents against binary GABAARs such as ß3δ and α4ß3 receptors enables identification of interfaces that may contribute to the pharmacology of the more complex ternary α4ß3δ receptors.
