Effect of ramosetron on conditioned emotional stress-induced colonic dysfunction as a model of irritable bowel syndrome in rats

The aim of this study was to establish a pathophysiologic model of irritable bowel syndrome, and then to evaluate the pharmaceutical efficacy of ramosetron, a potent serotonin 3 (5-HT3) receptor antagonist, and other anti-irritable bowel syndrome agents in this model. Rats stressed by a conditioned stress procedure exhibited marked prolongation of freezing time, an index of fear level, and an increase in the frequency of defecation (P < 0.01). A corticotropin-releasing factor (CRF) antagonist, α-helical CRF, inhibited both defecation and freezing behavior, while the antidiarrheal loperamide inhibited defecation only. The 5-HT3 receptor antagonists ramosetron, cilansetron and alosetron also inhibited defecation (ED50 values: 0.012, 0.094, 0.078 mg/kg p.o., respectively) without affecting freezing behavior. Ramosetron showed longer-lasting effect on defecation than cilansetron. Stress also resulted in increases in both proximal and distal colonic transit rates. Ramosetron and other 5-HT3 receptor antagonists at doses inhibiting stress-induced defecation also ameliorated both stress-stimulated colonic transit rates. These results suggest that ramosetron, as well as agents used for the treatment of irritable bowel syndrome with diarrhea, has beneficial effects against emotional stress-induced colonic dysfunction. Furthermore, this emotional stress model may be useful in evaluation of drugs to treat irritable bowel syndrome presenting with diarrhea.