The Direct Actions of GABA, 2'-Methoxy-6-Methylflavone and General Anaesthetics at ß3γ2L GABAA Receptors: Evidence for Receptors with Different Subunit Stoichiometries.

ABSTRACT

2'-Methoxy-6-methylflavone (2'MeO6MF) is an anxiolytic flavonoid which has been shown to display GABAA receptor (GABAAR) ß2/3-subunit selectivity, a pharmacological profile similar to that of the general anaesthetic etomidate. Electrophysiological studies suggest that the full agonist action of 2'MeO6MF at α2ß3γ2L GABAARs may mediate the flavonoid's in vivo effects. However, we found variations in the relative efficacy of 2'MeO6MF (2'MeO6MF-elicited current responses normalised to the maximal GABA response) at α2ß3γ2L GABAARs due to the presence of mixed receptor populations. To understand which receptor subpopulation(s) underlie the variations observed, we conducted a systematic investigation of 2'MeO6MF activity at all receptor combinations that could theoretically form (α2, ß3, γ2L, α2ß3, α2γ2L, ß3γ2L and α2ß3γ2L) in Xenopus oocytes using the two-electrode voltage clamp technique. We found that 2'MeO6MF activated non-α-containing ß3γ2L receptors. In an attempt to establish the optimal conditions to express a uniform population of these receptors, we found that varying the relative amounts of ß3γ2L subunit mRNAs resulted in differences in the level of constitutive activity, the GABA concentration-response relationships, and the relative efficacy of 2'MeO6MF activation. Like 2'MeO6MF, general anaesthetics such as etomidate and propofol also showed distinct levels of relative efficacy across different injection ratios. Based on these results, we infer that ß3γ2L receptors may form with different subunit stoichiometries, resulting in the complex pharmacology observed across different injection ratios. Moreover, the discovery that GABA and etomidate have direct actions at the α-lacking ß3γ2L receptors raises questions about the structural requirements for their respective binding sites at GABAARs.