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Keita SAKAGUCHI, Tamaki IKUSE, Keisuke JIMBO, Yoshikazu OHTSUKA, Toshiaki SHIMIZUObjectives: The correlation between altered small intestinal motility and irritable bowel syndrome is not well evaluated. This study aimed to assess the small intestinal and colonic transits in an adolescent irritable bowel syndrome rat model with restraint stress and determine the role of small intestinal motility in the irritable bowel syndrome pathophysiology.Materials: Restraint stress was utilized to prepare adolescent irritable bowel syndrome rat models that were evaluated for clinical signs, including stool frequency and diarrhea. The small intestinal motility and transit rate were also evaluated.Methods: The amounts of mRNA encoding corticotropin-releasing hormone, mast cell, and serotonin (5-Hydroxytryptamine) receptor 3a were quantified using real-time polymerase chain reaction; the 5-Hydroxytryptamine expression was evaluated using immunostaining.Results: Restraint stress significantly increased the number of fecal pellet outputs, stool water content, and small intestinal motility in the adolescent irritable bowel syndrome rat models. There was no difference in real-time polymerase chain reaction results; however, immunostaining analysis revealed that 5-Hydroxytryptamine expression in the small intestine was significantly increased in the adolescent irritable bowel syndrome rat models.Conclusions: In the rat model of adolescent irritable bowel syndrome with restraint stress, we observed an increase in small intestinal and colonic motility. In the small intestine, enhanced 5-Hydroxytryptamine secretion in the distal portion may be involved in increasing the small intestinal motility. Although the present study focused on 5-Hydroxytryptamine, further investigation of other factors that regulate intestinal peristalsis may lead to the establishment of more effective treatment methods for adolescent irritable bowel syndrome.Key words: adolescent, enterochromaffin cells, gastrointestinal motility, irritable bowel syndrome, serotonin 3 receptorJuntendo Medical Journal2022. 68(3), 271-281Original ArticlesEvaluating Small Intestinal Motility in a Rat Model of Adolescent Irritable Bowel SyndromeIntroductionIrritable bowel syndrome (IBS) is a common functional gastrointestinal disorder characterized by the symptoms of abdominal pain and altered bowel habits, such as diarrhea or constipation, in the absence of any organic diseases. Although the pathophysiology of this disease remains uncertain, sufficient compelling evidence has accumulated to indicate that IBS is a multifactorial syndrome resulting from interactions among gastrointestinal Department of Pediatrics and Adolescent Medicine, Juntendo University Graduate School of Medicine, Tokyo, JapanCorresponding author: Takahiro Kudo (ORCID: 0000-0003-4708-8049)Department of Pediatrics, Juntendo University Faculty of Medicine2-1-1 Hongo, Bunkyo-ku, Tokyo, 113-8421, JapanTEL: +81-3-3813-3111　FAX: +81-3-5800-0216　E-mail: t-kudo@juntendo.ac.jp〔Received　Dec. 7, 2021〕〔Accepted　Feb. 10, 2022〕J-STAGE Advance published date: Jun. 9, 2022Copyright © 2022 The Juntendo Medical Society. This is an open access article distributed under the terms of Creative Commons Attribution License (CC BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original source is properly credited. doi: 10.14789/jmj.JMJ21-0050-OAmotility, visceral hypersensitivity, intestinal inflam-mation, gastrointestinal infection, fecal flora alter-ations, bacterial overgrowth, food sensitivity, genetic factors, and psychosocial dysfunction1). The prevalence of IBS is high in children, with a global prevalence of 2-20%, and IBS has a significant impact on daily activities, school life, friendship and health related quality of life of affected children2, 3). Therefore, the Rome IV criteria established diag-nostic criteria for IBS in children and adolescents4).Colonic transit disorders may contribute to symp-271Masamichi SATO, Takahiro KUDO, Nobuyasu ARAI, Reiko KYODO, Kenji HOSOI,