Rikkunshito ameliorates colonic dysfunction and hormonal imbalance in an IBS mouse model with modulation of transient receptor potential channel activity

Purpose Rikkunshito, a traditional Japanese herbal medicine, is used to improve gastrointestinal (GI) function. However, its mechanisms of action in irritable bowel syndrome (IBS) remain unclear. This study aimed to investigate the effects of Rikkunshito on colonic pacemaker activity, inflammation, GI hormones, pain-related behaviors, and transient receptor potential (TRP) channel activity in a zymosan-induced IBS mouse model. Methods Rikkunshito composition was analyzed by UPLC. Pacemaker potentials were recorded in colonic interstitial cells of Cajal (ICCs) to examine ghrelin receptor–mediated Ca 2+ signaling pathways. Zymosan-induced IBS was established in male C57BL/6 mice treated with Rikkunshito, sulfasalazine, or amitriptyline. Colonic pathology, inflammation, stool consistency, body weight, pain-related behaviors, gastrointestinal hormones, and epithelial barrier–related gene expression were evaluated. TRPV1, TRPV4, and TRPA1 channel activities were analyzed using whole-cell patch-clamp recordings in TRP-overexpressing HEK293T cells. Results Rikkunshito depolarized colonic ICCs membranes and markedly suppressed pacemaker potential amplitude. These effects were mediated through ghrelin receptor–coupled G protein signaling and required extracellular Ca 2+ influx and intracellular Ca 2+ release via the PLC–IP 3 pathway. In addition, MAPK signaling was involved, whereas PKC and PKA signaling were not. In vivo , Rikkunshito ameliorated colonic shortening, inflammation, diarrhea, and pain-related behaviors in zymosan-induced IBS mice. TNF-α expression and VIP levels were significantly reduced, and NPY levels were restored, while 5-HT showed a downward trend. Furthermore, Rikkunshito reinstated the expression of aquaporins and tight junction–related genes, indicating reinforcement of mucosal barrier integrity. In HEK293T cells, Rikkunshito inhibited TRPV1, slightly enhanced TRPV4, and robustly activated TRPA1 currents. Conclusion Rikkunshito exerts protective effects in IBS by coordinately regulating colonic pacemaker activity via ghrelin receptor–dependent Ca 2+ signaling, modulating neuroendocrine balance, improving epithelial barrier function, and differentially regulating TRP channel activity. These findings support the potential of Rikkunshito as a complementary therapeutic agent for IBS through integrated modulation of gut motility, inflammation, and visceral sensory pathways.