188. Identification of a neural pathway mediating psychological stress-induced defecation in rats　 —Activation of the hypothalamus–raphe magnus–spinal defecation center axis induces defecation —

It is known that psychological stress causes abdominal pain, diarrhea and constipation; however, the neural pathways responsible for evoking these symptoms have not been fully elucidated. In this study, we identified the neural circuit involved in psychological stress-induced defecation in rats.

We found that stimulation of neurons in the paraventricular hypothalamic nucleus (PVH) and the dorsomedial hypothalamus (DMH) enhance colorectal motility. The colorectal motor response was abolished by blockade of serotonin receptor in the spinal defecation center and/or by transection of the pelvic nerve. These findings indicate that enhanced colorectal motility due to activation of the hypothalamic nuclei is mediated by descending serotonergic neurons projecting from the raphe magnus nucleus to the spinal defecation center, followed by activation of the parasympathetic pelvic nerve. In addition, we found that oxytocin receptor antagonist administered to the raphe magnus suppressed the motility response, suggesting involvement of oxytocin in synaptic transmission between neurons of the hypothalamus and the raphe magnus nucleus.

We also showed that selective inhibition of the hypothalamus→raphe pathway by using a chemogenetic approach reduced defecation during water avoidance stress in freely behaving rats. This suggests that the neural pathway from the PVH and DMH to the raphe magnus would be related to stress-induced defecation.

In summary, this study provides evidence that the hypothalamus‒raphe‒spinal defecation center axis mediates stress-induced defecation in rats.　

Involvement of the hypothalamus-raphe magnus-spinal defecation center axis in stress-induced defecation in rats. Yuki N, Sawamura T, Mori A, Yamaguchi H, Horii Y, Shiina T, Shimizu Y. Communication Biology 9 (1): 411, 2026.

＜Figure Legends＞

Schematic illustration of the neural circuit involved in psychological stress-induced defecation in rats. Psychological stress signals are processed in hypothalamic regions including the paraventricular hypothalamic nucleus (PVH), the dorsomedial hypothalamus (DMH) and dorsal hypothalamic area (DA). Oxytocinergic input from these hypothalamic regions is then transmitted to the raphe magnus nucleus. This input activates descending serotonergic neurons in the raphe magnus nucleus, which in turn activate the parasympathetic pelvic nerve, thereby promoting defecation.

Department of Basic Veterinary Science, Laboratory of Physiology, Joint Graduate School of Veterinary Sciences, Gifu University, Japan.