Rapid-onset, short-duration induction of colorectal contractions in anesthetized, adult, male rats.

Substantial clinical and preclinical evidence indicates that transient receptor potential vanilloid 1 (TRPV1) receptors are expressed on terminals of colorectal chemoreceptors and mechanoreceptors and are involved in various rectal hypersensitivity disorders with common features of colorectal overactivity. These stimulatory properties of TRPV1 receptors on colorectal function suggested that brief stimulation of TRPV1 might provide a means of pharmacologically activating the colorectum to induce defecation in patients with an "unresponsive" colorectum. The current studies explored the basic features of TRPV1 receptor-induced contractions of the colorectum in anesthetized rats with and without acute spinal cord injury (aSCI). Cumulative concentration-response curves to intrarectal (IR) capsaicin (CAP) solutions (0.003% to 3.0%) were performed in anesthetized aSCI and spinal intact rats. CAP produced an "inverted U", cumulative concentration-response curve with a threshold for inducing colorectal contractions at 0.01% and a peak response at 0.1% and slight decreases in responses up to 3%. Decreases in responses with concentrations > 0.1% are due to a rapid desensitization (i.e. < 30 min) of TRPV1 receptors to each successive dose. Desensitization appeared fully recovered within 24 hours in spinal intact rats. Colorectal contractions were completely blocked by atropine, indicating a reflexogenic activation of parasympathetic neurons, and responses were completely unaffected by a neurokinin 2 receptor antagonist, indicating that release of neurokinin A (NKA) from afferent terminals and subsequent direct contractions of the smooth muscle was not involved. IR administration of 3 other TRPV1 receptor agonists produced similar results as CAP. Significance Statement Individuals with spinal cord injury often lose control of defecation. Time consuming and inconvenient bowel programs using digital stimulation of the rectum and manual extraction of stool are used to empty the bowel. We show that intrarectal administration of the TRPV1 receptor agonist, capsaicin, can induce rapid onset, short duration colorectal contractions capable of inducing defecation in spinal cord injured and intact rats. Therefore, TRPV1 agonists show promise as a potential therapeutics to induce defecation in individuals with neurogenic bowel.