3-Hydroxy-2′-methoxy-6-methylflavone: A potent anxiolytic with a unique selectivity profile at GABAA receptor subtypes

Genetic and pharmacological studies have demonstrated that alpha 2- and alpha 4-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 alpha 1/2 beta 2, alpha 1/2/4/6 beta 1-3 gamma 2L but not alpha 3/5 beta 1-3 gamma 2L receptors expressed in Xenopus oocytes. The enhancement was evident at micromolar concentrations (EC50 values between 38 and 106 mu M) and occurred in a flumazenil-insensitive manner. 3-OH-2'MeO6MF displayed preference for beta 2/3- over beta 1-containing receptors with the highest efficacy observed at alpha 2 beta 2/3 gamma 2L, displaying a 4-11-fold increase in efficacy over alpha 2 beta 1 gamma 2L and alpha 1/4/6-containing subtypes. In contrast, 3-OH-2'MeO6MF acted as a potent bicuculline-sensitive activator, devoid of potentiation effects at extrasynaptic alpha 4 beta 2/3 delta receptors expressed in oocytes. The affinity of 3-OH-2'MeO6MF for alpha 4 beta 2/3 delta receptors (EC50 values between 1.4 and 2.5 mu M) was 10-fold higher than at alpha 4 beta 1 delta GABA(A) receptors. 3-OH-2'MeO6MF acted as a full agonist at alpha 4 beta 2/3 delta (105% of the maximal GABA response) but as a partial agonist at alpha 4 beta 1 delta (61% of the maximum GABA response) receptors. In mice, 3-OH-2'MeO6MF (1-100 mg/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 alpha 2 beta 2/3 gamma 2L and direct activation of alpha 4 beta 2/3 delta GABA(A) receptor subtypes. (C) 2011 Elsevier Inc. All rights reserved.