Quercetin antagonism of GABAAρ1 receptors is prevented by ascorbic acid through a redox-independent mechanism

Abstract

Quercetin is a natural flavonoid widely distributed in plants that acts as a neuroprotective agent and modulates the activity of different synaptic receptors and ion channels, including the ionotropic GABA receptors. GABAArho1 receptors were shown to be antagonized by quercetin, but the mechanisms underlying these antagonistic actions are still unknown. We have analyzed here if the antagonistic action produced by quercetin on GABAArho1 receptors was related to its redox activity or due to alternative mechanism/s. Homomeric GABAArho1 receptors were expressed in frog oocytes and GABA-evoked responses electrophysiologically recorded. Quercetin effects on GABAArho1 receptors were examined in the absence or presence of ascorbic acid. Chemical protection of cysteines by selective sulfhydryl reagents and site directed mutagenesis experiments were also used to determine ρ₁ subunit residues involved in quercetin actions. Quercetin antagonized GABAArho1 receptor responses in a dose-dependent, fast and reversible manner. Quercetin inhibition was prevented in the presence of ascorbic acid, but not by thiol reagents that modify the extracellular Cys-loop of these receptors. H141, an aminoacidic residue located near to the rho1 subunit GABA binding site, was involved in the allosteric modulation of GABAArho1 receptors by several agents including ascorbic acid. Quercetin similarly antagonized GABA-evoked responses mediated by mutant (H141D)GABAArho1 and wild-type receptors, but prevention exerted by ascorbic acid on quercetin effects was impaired in mutant receptors. Taken together the present results suggest that quercetin antagonistic actions on GABAArho1 receptors are mediated through a redox-independent allosteric mechanism.