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in 5-HTR3a in the small intestine; however, the distal small intestine expressed more 5-HTR3a than the proximal small intestine in both the restraint and control groups. Therefore, the distal small intestine may be mainly involved in small intestinal motility. The high expression of EC cells in the small intestine may result in increased intes-tinal motility and may be involved in developing diarrheal symptoms in IBS. This suggests that the small intestine is more susceptible to stress than the colon. In our study, real-time PCR showed no significant difference in mast cells between the restraint and control groups. However, there are some studies on an increased number of mast cells in patients with IBS. Chadwick et al.20) reported that the number of mast cells increased in the colonic mucosa of patients with IBS. We found that 5HTR3a was not significantly increased by stress loading, but its expression was significantly higher in the distal small intestine. In addition, 5HT was increased by stress loading, but the increase was more prominent in the small intestine than in the colon. Furthermore, when compared within the small intestine, the increase in the distal small intestine was significantly greater than that in the proximal. Therefore, we believe that the increase of 5HT in the distal small intestine, where the expression of 5HTR3a is originally high, is strongly involved in the enhancement of small intestinal peristalsis. In the present study, we used stress loading, so the involvement of 5HT is likely to be large, but we believe that the site of high expres-sion of 5HTR3a is also important. Weston et al.21) reported that the number of these cells increased in the ileal mucosa of patients with IBS. It is possible that the reason the mast cells did not increase in our study was because the stress introduced was acute and not chronic. However, it remains unclear whether stress and IBS are correlated with an increase in the number of mast cells. The number of mast cells can also be increased by inflammation. Based on these facts, it can be reported that the adolescent IBS rat model created in our study had no inflammation and that the IBS rat model could be accurately created.Nevertheless, our study had some limitations. First, in the creation of adolescent IBS model rats, the stress was acute, not chronic. While it might have been better if we chose chronic stress because 280IBS is a chronic syndrome, there was the possi-bility of ulcer formation due to chronic stress, which we wanted to avoid. We decided that chronic stress causing intestinal ulceration should not be a part of the evaluation, since this study dealt with functional intestinal disorders. Second, in the present study, only diarrhea-type IBS model rats were included. The pathogenesis of IBS includes diarrhea, constipation, and mixed types. Therefore, it is suggested that the results may differ depending on the pathology. Third, we were unable to eval-uate increased CRH release. We tried to evaluate the increase in CRH using RT-PCR, but it could not be detected. Therefore, we determined whether the adolescent IBS rat model could be created based on clinical symptoms. Fourth, 5-HTR3a was evaluated using immunostaining; however, the difference in expression levels could not be quanti-fied. RT-PCR did not show a significant increase in 5-HTR3a expression in both the small intestine and colon. If the expression level had been quanti-fied using immunostaining, the evaluation would be more accurate. In the present study, the evaluation focused on 5-HT. However, there are other factors known to be involved in intestinal motility. We would like to evaluate these factors in future studies.IBS model rats could be created by applying restraint stress. We found that the small intestine was prominently involved in enhancing intestinal peristalsis, which causes diarrhea. In particular, the distal small intestine of adolescent IBS model rats may be significantly involved in enhancing small intestine movement due to acute stress loading.The authors thank Ms. Yumiko Sakurai from the Department of Pediatric and Adolescent Medicine, Juntendo University Graduate School of Medicine and the members of the Laboratory of Morphology and Image Analysis, Research Support Center, Juntendo University Graduate School of Medicine for providing technical assistance with microscopy. We would like to thank Editage (www.editage.com) for English language editing.The authors received no financial support for the research.AcknowledgmentsFunding