The Japanese herbal medicine 'saiko-keishi-to' activates GABAA receptors of rat sensory neurons in culture.

The mechanism underlying the anticonvulsant action of 'Saiko-Keishi-To' (SK), the Japanese herbal medicine, was examined using whole-cell patch-clamp recording from cultured rat dorsal root ganglion cells. Neurons were held at -60 mV and perfused with an internal solution containing a high concentration of Cl-. Under these circumstances, SK produced an inward current which reversed at +8 mV, and was identical to the gamma-aminobutyric acid (GABA)-induced chloride current (IC). The SK-induced current was completely blocked by bicuculline. SK depressed a high voltage-activated calcium current (HVA-ICa), but this depression could not be antagonized by either phaclofen or saclofen. Of nine crude herbal drugs constituting SK, only Bupleuri radix, Ginseng radix, Zingiberis rhizoma and Paeoniae radix produced the inward currents blocked by bicuculline. These results suggest that SK, and four of its crude herbal drugs, activate the ICl mediated by the GABAA receptor. In addition, SK depresses a HVA-ICa through some mechanism other than an activation of GABAB receptors.