Genetic and pharmacological studies have demonstrated that a2- and a4-containing GABA(A) receptors mediate the anxiolytic effects of a number of agents. Flavonoids are a class of ligands that act at GABA(A) receptors and possess anxiolytic effects in vivo. Here we demonstrate that the synthetic flavonoid, 3-hydroxy-2'-methoxy-6-methylflavone (3-OH-2'MeO6MF) potentiates GABA-induced currents at recombinant a1/2ß2, a1/2/4/6ß1-3¿2L but not a3/5ß1-3¿2L receptors expressed in Xenopus oocytes. The enhancement was evident at micromolar concentrations (EC(50) values between 38 and 106µM) and occurred in a flumazenil-insensitive manner. 3-OH-2'MeO6MF displayed preference for ß2/3- over ß1-containing receptors with the highest efficacy observed at a2ß2/3¿2L, displaying a 4-11-fold increase in efficacy over a2ß1¿2L and a1/4/6-containing subtypes. In contrast, 3-OH-2'MeO6MF acted as a potent bicuculline-sensitive activator, devoid of potentiation effects at extrasynaptic a4ß2/3d receptors expressed in oocytes. The affinity of 3-OH-2'MeO6MF for a4ß2/3d receptors (EC(50) values between 1.4 and 2.5µM) was 10-fold higher than at a4ß1d GABA(A) receptors. 3-OH-2'MeO6MF acted as a full agonist at a4ß2/3d (105% of the maximal GABA response) but as a partial agonist at a4ß1d (61% of the maximum GABA response) receptors. In mice, 3-OH-2'MeO6MF (1-100mg/kg i.p.) induced anxiolytic-like effects in two unconditioned models of anxiety: the elevated plus maze and light/dark paradigms. No sedative or myorelaxant effects were detected using holeboard, actimeter and horizontal wire tests and only weak barbiturate potentiating effects on the loss of righting reflex test. Taken together, these data suggest that 3-OH-2'MeO6MF is an anxiolytic without sedative and myorelaxant effects acting through positive allosteric modulation of the a2ß2/3¿2L and direct activation of a4ß2/3d GABA(A) receptor subtypes.
Keywords: GABA(A) receptors, flavonoids, anxiolytics, allosteric modulation, allosteric activation