6,3′-dinitroflavone is a low efficacy modulator of GABAA receptors

6,3'-Dinitroflavone (6,3'-DNF) is a synthetic flavone derivative that exerts anxiolytic effects in the elevated plus maze. Based on the finding that this effect is blocked by Ro15-1788 (ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate) which is a specific antagonist at the benzodiazepine binding site of GABA(A) receptors we investigated the interaction of 6,3'-DNF with several recombinant GABAA receptor subtypes. Inhibition of [H-3]flunitrazepam binding to recombinant GABAA receptors in transiently transfected HEK293 cells indicated that 6,3'-DNF exhibited the highest affinity for GABAA receptors composed of alpha 1 beta 2 gamma 2 subunits and a 2-20 fold lower affinity for homologous receptors containing alpha 2, alpha 3, or alpha 5 subunits. Two-electrode voltage-clamp experiments in Xenopus oocytes indicated that 6,3'-DNF does not induce chloride flux in the absence of GABA, but exerts low efficacy inverse agonistic modulatory effects on GABA-elicited currents in the GABA(A) receptor subtypes alpha 1 beta 2 gamma 2 and alpha 5 beta 2 gamma 2. In the subtypes alpha 2 beta 2 gamma 2, alpha 3 beta 2 gamma 2, alpha 4 beta 2 gamma 2, alpha 6 beta 2 gamma 2 or alpha 4 beta 2 gamma delta and alpha 4 beta 3 delta, 6,3'-DNF exerts either none or very low efficacy positive modulatory effects. In contrast, 100 nM Ro15-1788 exhibited weak to moderate partial agonistic effects on each receptor investigated. These data indicate that Ro-15-1788 only can antagonize the weak inverse agonist effects of 6,3'-DNF on alpha 1 beta 2 gamma 2 and alpha 5 beta 2 gamma 2 receptors, but will enhance the weak agonistic effects on the other receptor subtypes investigated. The possible mechanism of the Ro15-1788 sensitive anxiolytic effect of 6,3'-DNF is discussed. (C) 2008 Elsevier B.V. All rights reserved.