カエル坐骨神経の複合活動電位に及ぼす漢方薬の作用

抄録

　Traditional Japanese medicine (Kampo medicine) is known to have a variety of pharmacological actions including antinociception. We have recently revealed that transient receptor potential (TRP) channel agonists such as capsaicin, zingerone [each TRP vanilloid-1 (TRPV1) agonist] and (-)-menthol [TRP melastatin-8 (TRPM8) agonist], which are contained in capsicum, ginger and peppermint, respectively, have an inhibitory action on nerve conduction without TRP channel activation. Taking into consideration that Kampo medicine contains many plant-derived chemicals, it is possible that this inhibits nerve conduction. The present study examined how several kinds of Kampo medicine and also its related chemicals affect compound action potentials (CAPs) recorded from the frog sciatic nerve by using the air-gap method. Daikenchuto, rikkosan, kikyoto and rikkunshito reduced the peak amplitude of the CAP in a concentration-dependent manner; daikenchuto had an IC₅₀ value of 1.1 mg/ml. When examined at a concentration of 2 mg/ml, the extents of the reductions by daikenchuto, rikkosan, kikyoto and rikkunshito were 70, 30, 25 and 15%, respectively. Daikenchuto being the most effective in inhibiting CAPs is composed of three kinds of extract powder, ginseng, Japanese pepper and processed ginger, in which are contained not only TRPV1 but also TRP ankyrin-1 (TRPA1) agonists. When the actions of daikenchuto-related chemicals on frog CAPs were examined, a TRPV1 agonist piperine (in black pepper) at 70 µM reduced CAP peak amplitude by 20%, and TRPA1 agonists, allyl isothiocyanate (in wasabi) and cinnamaldehyde (in cinnamon), reduced the amplitude with the IC₅₀ values of 1.4 mM and 1.2 mM, respectively. These results indicate that Kampo medicine has an ability to inhibit nerve conduction. It is suggested that this action of Kampo medicine, particularly daikenchuto, may be partly due to nerve conduction inhibition by plant-derived TRP agonists contained in Kampo medicine.