Sexually dimorphic response of colorectal motility to noxious stimuli in the colorectum in rats.

KEY POINTS: This study showed a remarkable sex difference in responses of colorectal motility to noxious stimuli in the colorectum in rats: colorectal motility was enhanced in response to intracolonic administration of a noxious stimulant, capsaicin, in male rats but not in female rats. The difference in descending neurons from the brain to spinal cord operating after noxious stimulation could be responsible for the sex difference. In male rats, serotoninergic and dopaminergic neurons are dominantly activated, both of which activate the spinal defaecation centre. In female rats, GABAergic neurons in addition to serotoninergic neurons are activated. GABA may compete for facilitative action of 5-HT in the spinal defaecation centre, and thereby colorectal motility is not enhanced in response to intracolonic administration of capsaicin. The findings provide a novel insight into pathophysiological mechanisms of sex differences in functional defaecation disorders such as irritable bowel syndrome. ABSTRACT: We previously demonstrated that noxious stimuli in the colorectum enhance colorectal motility through activation of descending pain inhibitory pathways in male rats. It can be expected that the regulatory mechanisms of colorectal motility differ in males and females owing to remarkable sex differences in descending pain inhibitory pathways. Thus, we aimed to clarify sex differences in responses of colorectal motility to noxious stimuli in rats. Colorectal motility was measured in vivo in anaesthetized rats. Administration of a noxious stimulant, capsaicin, into the colorectal lumen enhanced colorectal motility in male rats but not in female rats. Quantitative PCR and immunohistochemistry showed that TRPV1 expression levels in the dorsal root ganglia and in the colorectal mucosa were comparable in male and female rats. When a GABAA receptor inhibitor was intrathecally administered to the L6-S1 level of the spinal cord, colorectal motility was facilitated in response to intracolonic capsaicin even in female rats. The capsaicin-induced response in the presence of the GABA blocker in female rats was inhibited by intrathecal administration of 5-HT2 and -3 receptor antagonists but not by a D2-like dopamine receptor antagonist. Our findings demonstrate that intracolonic noxious stimulation activates GABAergic and serotoninergic descending neurons in female rats, whereas serotoninergic and dopaminergic neurons are dominantly activated in male rats. Thus, the difference in the descending neurons operating after noxious stimulation would be responsible for the sexually dimorphic responses of colorectal motility. Our findings provide a novel insight into pathophysiological mechanisms of sex differences in functional defaecation disorders such as irritable bowel syndrome.