Effect of yokukansan, a traditional Japanese medicine, on social and aggressive behaviour of para-chloroamphetamine-injected rats

Objectives Yokukansan, a traditional Japanese medicine, has been approved by the Ministry of Health, Labour, and Welfare of Japan as a remedy for neurosis, insomnia or night crying and irritability in children. It has recently been reported to improve behavioural and psychological symptoms of dementia, such as hallucinations, agitation, and aggressiveness in patients with some forms of senile dementia. Little is known about the mechanism underlying the effectiveness of yokukansan. Our aim was to clarify the involvement of yokukansan in serotonergic function in para‐chloroamphetamine (PCA)‐induced aggressive behaviour in rats. Methods The effect of yokukansan on social interactions, including social and aggressive behaviour, was examined in PCA‐injected rats. Concentration and release level of serotonin (5‐HT) in the hypothalamus were measured. Key findings PCA reduced not only the 5‐HT concentration but also the high K+ ‐induced 5‐HT release in the rat hypothalamus. Social interaction tests showed a significant decrease in social behaviour and a significant increase in aggressive behaviour in the PCA‐treated rats. The decrease in social behaviour was ameliorated by the 5‐HT1A agonist buspirone and further decreased by a 5‐HT1A antagonist, N‐[2‐[4‐(2‐methoxyphenyl)‐1‐piperazinyl]ethyl]‐N‐(2‐pyridinyl)cyclo‐hexanecarboxamide trihydrochloride (WAY‐100635), whereas it was further decreased by the 5‐HT2A agonist, 2,5‐dimethoxy‐4‐iodoamphetamine (DOI), and ameliorated by the 5‐HT2A antagonist ketanserin. On the other hand, the increase in aggressive behaviour was ameliorated by buspirone but not affected by WAY‐100635, whereas it was enhanced by DOI and ameliorated by ketanserin. A single injection of yokukansan ameliorated the PCA‐induced decrease in social behaviour but not aggressive behaviour. Chronic treatment for 14 days with yokukansan ameliorated PCA‐induced abnormal behaviour, decreased social behaviour and increased aggressive behaviour, but it did not ameliorate PCA‐induced decreases in the cerebral 5‐HT concentration and 5‐HT release. The ameliorative effects of chronic yokukansan on behaviour were counteracted by co‐administration of WAY‐100635. Conclusions These results suggest that yokukansan might have two different effects: an acute effect on social behaviour and a chronic effect on aggressive behaviour. One of the mechanisms of these effects of yokukansan may be related to the agonistic effect on 5‐HT1A receptors.