GABA[subscript A] receptor subtype selectivity underlying selective behavioral profiles of neuroactive flavonoids

Although many flavonoids have been reported to exert their neuroactivities through the benzodiazepine (BZ) site of GABAA receptors, the underlying mechanisms for the behavioral effects of flavonoids are less understood. Baicalin, a naturally occurring flavonoid, was previously reported to induce anxiolytic-like effect, devoid of sedation and myorelaxation, in mice through GABAA receptors. The present study further expanded the behavioral pharmacology profile of baicalin, and subtype selectivity was explored as a possible mechanism underlying its in vivo effects. In the picrotoxin-induced seizure, step-through passive avoidance and rotarod tests, anticonvulsant, amnesic and motor incoordination effects commonly associated with classical BZs were not observed with baicalin at effective anxiolytic doses, demonstrating a separation of the anticonvulsant, amnesic and motor incoordination effects from anxiolytic-like effect. Moreover, baicalin showed significant preference to α2- and α3- containing subtypes compared with α1- and α5- containing subtypes in whole-cell patch clamp studies. Its subtype selectivity indicated that baicalin exerted anxiolytic-like effect mainly through the α2- and α3- containing subtypes, suggesting α2- and α3- containing subtypes were important drug targets for flavonoid-based anxiolytics. In addition, 6,2’-dihydroxyflavone (DHF) was characterized for potential inverse agonistic activity, and its mechanism of action was explored for GABAA receptor subtype selectivity. In electrophysiological studies on neuroblastoma IMR-32 cells, DHF decreased GABA-currents, which could be blocked by a BZ site antagonist. In mice behavioral models, DHF elicited significant anxiogenic-like effects in the elevated plus-maze test, and enhanced cognitive performance in the step-through passive avoidance test, while not exhibiting any proconvulsant effects. On recombinant GABAA receptors, DHF decreased GABA-currents in α1β3γ2, α2β3γ2, or α5β3γ2, but not α3β3γ2 subtypes. The results demonstrated DHF as a partial inverse agonist of GABAA receptors with selectivity in receptor subtypes as well as behavioral effects, extending the efficacy of flavonoids from agonists and antagonists to inverse agonists of GABAA receptors. Moreover, the selective profile displayed in mice behavioral models supported DHF as a useful lead compound for the development of cognition-enhancing agents devoid of convulsion side effect