Although rikkosan is a Kampo medicine effective for orofacial pain, mechanism of action and pharmacokinetics of rikkosan is still unclear. Furthermore, the quantitative analysis of the analgesic effect of rikkosan has not been reported. To optimize the administration conditions, the analgesic effect of rikkosan was evaluated by pseudonocieceptive responses (Writhing syndrome and Tail pinch). Twenty minutes pre-treatment with rikkosan (100 or 200mg/kg) significantly reduced the acetic acid-induced stretch movements (Writhing syndrome) in mice, however, the analgesic effect was impaired at 400mg/kg. There were no significant differences in the analgesic effect between rikkosan (100 and 200mg/kg), acetaminophen (300mg/kg), aspirin (300mg/kg) and morphine (10mg/kg), and extension of pre-treatment period (40-90 min) potentiated the analgesic effect of 400mg/kg rikkosan. On the other hand, all analgesics, except morphine, failed to inhibit biting reflex induced by Tail pinch. These results suggest that more than 40 min prior administration of less than 200mg/kg rikkosan is required to obtain the maximum analgesic effect. Moreover, the different mechanism of action of rikkosan from morphine is also suggested.
