Flavan-3-ol esters: new agents for exploring modulatory sites on GABAA receptors

BACKGROUND AND PURPOSE Enhancement of GABAergic function is the primary mechanism of important therapeutic agents such as benzodiazepines, barbiturates, neurosteroids, general anaesthetics and some anticonvulsants. Despite their chemical diversity, many studies have postulated that these agents may bind at a common or overlapping binding site, or share an activation domain. Similarly, we found that flavan-3-ol esters act as positive modulators of GABAA receptors, and noted that this action resembled the in vitro profile of general anaesthetics. In this study we further investigated the interactions between these agents. EXPERIMENTAL APPROACH Using two-electrode voltage clamp electrophysiological recordings on receptors of known subunit composition expressed in Xenopus oocytes, we evaluated positive modulation by etomidate, loreclezole, diazepam, thiopentone, 5 alpha-pregnan-3 alpha-ol-20-one (THP) and the flavan-3-ol ester 2S, 3R-trans 3-acetoxy-4'-methoxyflavan (Fa131) on wild-type and mutated GABA(A) receptors. KEY RESULTS The newly identified flavan, 2S, 3S-cis 3-acetoxy-3',4'-dimethoxyflavan (Fa173), antagonized the potentiating actions of Fa131, etomidate and loreclezole at alpha 1 beta 2 and alpha 1 beta 2 gamma 2L GABA(A) receptors. Furthermore, Fa173 blocked the potentiation of GABA responses by high, but not low, concentrations of diazepam, but did not block the potentiation induced by propofol, the neurosteroid THP or the barbiturate thiopental. Mutational studies on 'anaesthetic-influencing' residues showed that, compared with wild-type GABA(A) receptors, alpha 1M236W beta 2 gamma 2L and alpha 1 beta 2N265S gamma 2L receptors are resistant to potentiation by etomidate, loreclezole and Fa131. CONCLUSIONS AND IMPLICATIONS Fa173 is a selective antagonist that can be used for allosteric modulation of GABA(A) receptors. Flavan-3-ol derivatives are potential ligands for etomidate/loreclezole-related binding sites at GABA(A) receptors and the low-affinity effects of diazepam are mediated via the same site.