THYROTROPIN-RELEASING-HORMONE (TRH) AND CNS REGULATION OF ANORECTAL MOTILITY IN THE RAT

The effect of thyrotropin-releasing hormone (TRH) upon anorectal motility was investigated in acute male rat preparations. Micromolar doses of TRH were intrathecally (i.t.) infused at the L6 spinal level at a rate of 1 mu l/min over 8 min. TRH infusions in 1.0-1000 mu M concentrations elicited biphasic, dose-dependent anorectal contractions as measured by a rectal manometer. The 100 mu M dose yielded the most significant increase in contractions over the greatest period of time. Atropine, administered as a pretreatment (100 mu g s.c.), blocked contractions normally produced by i.t. infusion of TRH (1000 mu M). Intravenous infusions of atropine (10 mu g) through a jugular catheter immediately blocked anorectal contractions produced by i.t. infusion of 100 mu M TRH, Sectioning of the hypogastric nerve, which supplies sympathetic innervation to the colon and internal anal sphincter, did not significantly affect contractions induced by 100 mu M TRH applied intrathecally. Disruption of the major pelvic ganglion fibers, however, completely abolished the contractions induced by 100 mu M TRH, either through the interruption of preganglionic parasympathetic fibers in the pelvic nerve, or by disrupting postganglionic fibers. These findings extend the role of TRH in the regulation of defecatory behaviors.