Partial agonistic effect of yokukansan on human recombinant serotonin 1A receptors expressed in the membranes of Chinese hamster ovary cells

Yokukansan (YKS), which is a traditional Japanese medicine, showed a partial agonistic effect on serotonin (5-HT)1A receptors (Emax = 55.1% vs. 5-HT). Furthermore, we found that a 5-HT1A receptor partial agonistic effect of YKS was mainly attributed to Uncaria hook, of the seven constituent herbs. Yokukansan (YKS) is a traditional Japanese medicine consisted of seven medicinal herbs and has been used for treatment of neurosis, insomnia, and behavioral and psychological symptoms of dementia (BPSD) in Japan. The aim of the present study is to clarify the intrinsic activity of YKS on serotonin (5-HT)1A and 5-HT2A receptors and also to determine the constituent herbs which are responsible for the effect of YKS. The dry powdered extracts of YKS, seven constituent herbs, and YKS-analogues which were produced by eliminating one of the constituent herbs from YKS in the manufacturing process, were used for the evaluation. Competitive binding assays for 5-HT receptors and [ 35S]GTPγS binding assays for the evaluation of agonistic/antagonistic activity were performed using Chinese hamster ovary cell membranes stably expressing human recombinant 5-HT1A or 5-HT2A receptors. YKS (6.25–400 μg/ml) concentration-dependently inhibited the binding of [ 3H]8-OH-DPAT to 5-HT1A receptors. The IC 50 value was estimated to be 61.2 μg/ml. In contrast, YKS failed to inhibit the binding of [ 3H]ketanserin to 5-HT2A receptors. Only Uncaria hook (3.13–50 μg/ml), of the seven constituent herbal extracts, inhibited the [ 3H]8-OH-DPAT binding to 5-HT1A receptors in a concentration-dependent manner, and the IC 50 value was estimated to be 7.42 μg/ml. The extracts of YKS or Uncaria hook increased [ 35S]GTPγS binding to 5-HT1A receptors to approximately 50% of that of a full agonist, 5-HT. Both the competitive binding and [ 35S]GTPγS binding of YKS to 5-HT1A receptors were remarkably attenuated by eliminating Uncaria hook from YKS, but it was almost unchanged when one of the other constituent herbs was eliminated from YKS. These results suggest that YKS has a partial agonistic effect on 5-HT1A receptors, which is mainly attributed to Uncaria hook.