Use of an electrophysiological technique for stepwise detection of trace agonist constituents of Hochuekkito in Xenopus oocytes injected with serotonin 2C receptor mRNA

An electrophysiological bioassay was used to isolate the active compound from Hochuekkito (HET), which the current authors previously described as having potent agonist action against serotonin 2C receptors (5-HT2CR). Synthetic 5-HT2CR mRNA was injected into Xenopus oocytes to specifically express these receptors. Crude extracts and purified products were subjected to an electrophysiological bioassay using the voltage clamp method. HET stimulated a 5-HT2CR-induced current response, whereas Juzentaohoto (JTT), which has anti-depressive action similar to that of HET, did not. Current responses were not observed with an extract mixed with five types of herbal medicines common to HET and JTT but were detected with an extract with the five types of herbal medicines found in HET alone (Hoc5). When the responses to each of the five types of Hoc5 were examined, current responses were noted with Cimicifugae rhizoma (CR) and Citrus unshiu Markovich extracts. Since efficacy and the EC50 value were higher for CR, its constituents were separated using three-dimensional high-performance liquid chromatography and the current response at each of the isolated peaks was examined. One constituent displayed a strong response and was identified as a single substance with a molecular weight of 283.1393 based on liquid chromatography/mass spectrometry. These results will contribute to the isolation of 5-HT2CR-stimulating constituents in HET and the identification of trace constituents with agonist action.