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the colonic transit time in diarrhea-type IBS was shorter, whereas no difference in transit time was observed in the small intestine. Therefore, there was no influence on the small intestine movement in IBS. However, in the present study, small intes-tinal motility was significantly elevated, as observed in the small intestinal transit rate. Furthermore, colon motility was increased, as observed in the fecal pellet output and water content examination. Our IBS rat model had a shortened transit time in the entire intestine. In general, it is known that water is absorbed in the large intestine. However, in reality, 80% of the water we ingest is absorbed in the small intestine. Therefore, it is suggested that increased peristalsis of the small intestine may play a significant role in the appearance of diarrhea symptoms.In this study, 5-HT, 5-HTR3a, and mast cells were evaluated in the colon and small intestine as factors that regulate the movement of the intes-tinal tract. Most 5-HT are synthesized and stored in EC cells, and 5-HT receptors are located Figure 8　The expression of 5-Hydroxytryptamine Receptor 3A (5-HTR3a) in the small intestine and colon. Bar = 100 μm. A: Photographs of the small intestine tissue. The expression of 5-HTR3a between the restraint and control groups was compared. B: Photographs of the colon tissue. The expression of 5-HTR3a between the restraint and control groups was compared.throughout the intestinal tract. 5-HTR3 antago-nists reportedly reduce visceral hypersensitivity and pain in patients with IBS19). Therefore, 5-HT and 5-HTR3 play important roles in the gastroin-testinal tract motility. However, their expression in the small intestine remains unclear. In our study, the motility of the small intestine in the restraint group as adolescent IBS model rats was signifi-cantly enhanced. Therefore, an increase in 5-HT and 5-HTR3 levels in the small intestine was expected. In the restraint group, EC cell expression in the small intestine was significantly increased, but not 5-HTR3a. This result suggests that 5-HT strongly affects the enhancement of small intestinal motility under acute stress. When the increase in EC cell expression in the small intestine among rats in the restraint group was evaluated with respect to the segment of the small intestine, we observed that the expression was significantly increased in the distal small intestine compared to that in the proximal small intestine. Furthermore, real-time PCR did not show a stress-related increase 279