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toms among patients with IBS; in fact, transit and contractile abnormalities have been observed in a subset of patients with IBS. The primary alteration of mucosal absorption and secretion has been suggested as non-contributory to the frequent stools with diarrhea in patients with IBS5).Several previous studies have shown a relation-ship between the accelerated transit of colonic contents with IBS and diarrhea. In animal models, previous studies have shown that stress loading increases stool frequency and causes diarrhea6). Mönnikes et al. reported a significant acceleration of colonic transit in rats with restraint stress compared to that in non-restrained rats7).Furthermore, visceral sensation hypersensitivity from the autonomic nervous system associated with the brain-gut interaction is thought to be the pathophysiology of IBS8). Corticotropin-releasing hormone (CRH) is an important factor in explaining the pathophysiology of brain-gut interactions. CRH stimulates the pituitary adrenocorticotropic hormone and increases intestinal motility in the human body9). Recently, there has been a growing interest in serotonin (5-Hydroxytryptamine; 5-HT) because of its possible involvement in IBS. 5-HT released by enterochromaffin cells (EC cells) within the mucosa via intraluminal distension or irritation stimulates 5-HT3 receptors located on the primary afferent neurons of both splanchnic and vagal fibers, thereby modulating a sensory response10).Although increased stool water in diarrhea is thought to result from diminished contact time of the luminal contents with the colonic mucosa, it is well known that approximately 80% of ingested water is absorbed from the small intestinal mucosa11). Therefore, small intestine dysfunction induces diarrhea in many diseases; however, the correlation between diarrhea and motility alter-ation in the small intestine among patients with IBS, especially childhood and adolescent age, has not been well evaluated. In this study, we aimed to assess the small intestinal transit in adolescent IBS rat models with restraint stress and to determine the role of small intestinal motility in the patho-physiology of IBS.Materials and MethodsAnimal modelsThe experiments were performed using adoles-272cent male Sprague-Dawley rats aged 5-6 weeks as adolescent, weighing 160-250 g and were housed in cages in a standardized environment with a temperature of 24 °C, relative humidity of 55 % ± 15 %, and a 12-hour/12-hour light-dark cycle for a day. We chose male rats because we wanted to avoid female hormonal effects. They were allowed to access food and water freely. The animal care and experimental protocols were approved by the Institutional Review Board of Juntendo University (No. 310185).The animals were randomly assigned to two groups: a restraint group and a control group. The restraint group was isolated in the individual compartments of stress cages (Natsume Seisakusho Co. Ltd. Tokyo, Japan; KN-325-C-3) for 1 hour before dissection. Rats in the control group assumed an hour of isolation in the cages without restriction. The rats in both groups did not have free access to food and water during isolation.All methods were performed in accordance with relevant guidelines and regulations, and this study was conducted in compliance with the ARRIVE guidelines.Fecal pellet output and water contentsThe number of fecal pellet outputs during the one-hour isolation was counted. The stool first excreted during isolation was collected, as well as the stool located in the most distal side of the gastrointestinal tract after isolation was collected for comparison. These fecal pellets were stored at a temperature of -80 °C and were freeze-dried overnight after weighing. The freeze-dried fecal weight was measured on the next day to calculate the water content (%), which was as follows: (fecal weight before drying - fecal weight after drying) / fecal weight before drying × 100 %.Small intestinal transitWe examined the intestinal propulsion of powdered carbon to evaluate small intestinal motility under restraint stress. The rats received 0.5 mL of a powdered carbon suspension in saline per 100 g of their weight (5 % W/V) intragastri-cally through an oral sonde (Primetech Co. Ltd. Tokyo, Japan; FTP-15-78-50). After administra-tion, the rats in the restraint group were immedi-ately exposed to stress, as described above. Rats in