Differential effects of quercetin glycosides on GABAC receptor channel activity

Quercetin, a representative flavonoid, is a compound of low molecular weight found in various colored plants and vegetables. Quercetin shows a wide range of neuropharmacological activities. In fact, quercetin naturally exists as monomer-(quercetin-3-O-rhamnoside) (Rham1), dimer-(Rutin), or trimer-glycosides [quercetin-3-(2(G)-rhamnosylrutinoside)] (Rham2) at carbon-3 in fruits and vegetables. The carbohydrate components are removed after ingestion into gastrointestinal systems. The role of the glycosides attached to quercetin in the regulation of gamma-aminobutyric acid class C (GABA(C)) receptor channel activity has not been determined. In the present study, we examined the effects of quercetin glycosides on GABA(C) receptor channel activity by expressing human GABA(C) alone in Xenopus oocytes using a two-electrode voltage clamp technique and also compared the effects of quercetin glycosides with quercetin. We found that GABA-induced inward current (I (GABA) ) was inhibited by quercetin or quercetin glycosides. The inhibitory effects of quercetin and its glycosides on I (GABA) were concentration-dependent and reversible in the order of Rutin a parts per thousand quercetin a parts per thousand Rham 1 > Rham 2. The inhibitory effects of quercetin and its glycosides on I (GABA) were noncompetitive and membrane voltage-insensitive. These results indicate that quercetin and its glycosides regulate GABA(C) receptor channel activity through interaction with a different site from that of GABA, and that the number of carbohydrate attached to quercetin might play an important role in the regulation of GABA(C) receptor channel activity.