Flavan-3-ol esters: new agents for exploring modulatory sites on GABA(A) receptors |
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Authors: | Fernandez Sebastian P Karim Nasiara Mewett Kenneth N Chebib Mary Johnston Graham Ar Hanrahan Jane R |
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Affiliation: | Department of Pharmacology Faculty of Pharmacy, The University of Sydney, Sydney, New South Wales, Australia. |
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Abstract: | BACKGROUND AND PURPOSEEnhancement 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 APPROACHUsing 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, 5α-pregnan-3α-ol-20-one (THP) and the flavan-3-ol ester 2S,3R-trans 3-acetoxy-4′-methoxyflavan (Fa131) on wild-type and mutated GABAA receptors.KEY RESULTSThe newly identified flavan, 2S,3S-cis 3-acetoxy-3′,4′-dimethoxyflavan (Fa173), antagonized the potentiating actions of Fa131, etomidate and loreclezole at α1β2 and α1β2γ2L GABAA 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 GABAA receptors, α1M236Wβ2γ2L and α1β2N265Sγ2L receptors are resistant to potentiation by etomidate, loreclezole and Fa131.CONCLUSIONS AND IMPLICATIONSFa173 is a selective antagonist that can be used for allosteric modulation of GABAA receptors. Flavan-3-ol derivatives are potential ligands for etomidate/loreclezole-related binding sites at GABAA receptors and the low-affinity effects of diazepam are mediated via the same site. |
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Keywords: | GABAA receptors Xenopus oocytes GABA (γ-aminobutyric acid) flavonoids flavans anaesthetic agents sedative effects α1-containing GABAA receptors |
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