Flavonoid myricetin modulates GABAA receptor activity through activation of Ca²⁺ channels and CaMK-II pathway

The flavonoid myricetin is found in several sedative herbs, for example, the St. John's Wort, but its influence on sedation and its possible mechanism of action are unknown. Using patch-clamp technique on a brain slice preparation, the present study found that myricetin promoted GABAergic activity in the neurons of hypothalamic paraventricular nucleus (PVN) by increasing the decay time and frequency of the inhibitory currents mediated by GABAA receptor. This effect of myricetin was not blocked by the GABAA receptor benzodiazepine- (BZ-) binding site antagonist flumazenil, but by KN-62, a specific inhibitor of the Ca²⁺/calmodulin-stimulated protein kinase II (CaMK-II). Patch clamp and live Ca²⁺ imaging studies found that myricetin could increase Ca²⁺ current and intracellular Ca²⁺ concentration, respectively, via T- and L-type Ca²⁺ channels in rat PVN neurons and hypothalamic primary culture neurons. Immunofluorescence staining showed increased phosphorylation of CaMK-II after myricetin incubation in primary culture of rat hypothalamic neurons, and the myricetin-induced CaMK-II phosphorylation was further confirmed by Western blotting in PC-12 cells. The present results suggest that myricetin enhances GABAA receptor activity via calcium channel/CaMK-II dependent mechanism, which is distinctively different from that of most existing BZ-binding site agonists of GABAA receptor. © 2012 Xiao Hu Zhang et al.