Isoliquiritigenin is a Novel NMDA Receptor Antagonist in Kampo Medicine Yokukansan

Effects of a traditional Japanese medicine, yokukansan, which is composed of seven medicinal herbs, on glutamate-induced cell death were examined using primary cultured rat cortical neurons. Yokukansan (10-300 mu g/ml) inhibited the 100 mu M glutamate-induced neuronal death in a concentration-dependent manner. Among seven constituent herbs, higher potency of protection was found in Uncaria thorn (UT) and Glycyrrhiza root (GR). A similar neuroprotective effect was found in four components (geissoschizine methyl ether, hirsuteine, hirsutine, and rhynchophylline) in UT and four components (glycycoumarin, isoliquiritigenin, liquiritin, and 18 beta-glycyrrhetinic acid) in GR. In the NMDA receptor binding and receptor-linked Ca2+ influx assays, only isoliquiritigenin bound to NMDA receptors and inhibited the glutamate-induced increase in Ca2+ influx. Glycycoumarin and 18 beta-glycyrrhetinic acid bound to NMDA receptors, but did not inhibit the Ca2+ influx. The four UT-derived components did not bind to NMDA receptors. The present results suggest that neuroprotective components (isoliquiritigenin, glycycoumarin, liquiritin, and 18 beta-glycyrrhetinic acid in GR and geissoschizine methyl ether, hirsuteine, hirsutine, and rhynchophylline in UT) are contained in yokukansan, and isoliquiritigenin, which is one of them, is a novel NMDA receptor antagonist.