Morphine antagonizes TRH-induced intestinal transit and hyperserotonemia in rabbits

Thyrotropin releasing hormone (TRH), administered intracerebroventricularly (i.c.v.) in microgram quantities to anesthetized rabbits, produces marked stimulation of colonic motility, transit, and fluid accumulation, and is accompanied by a portal hyperserotonemia. In this study we found that pretreatment of rabbits with morphine (10 mg/kg i.v.) completely blocked the TRH-induced colonic transit and increase in luminal fluid, as well as the hyperserotonemia. A small dose of morphine (200 micrograms) given i.c.v., but not i.v., also inhibited the intestinal transit and portal hyperserotonemia. These results suggest that morphine inhibits TRH-induced intestinal transit and fluid accumulation, at least in part, by inhibition of a neural pathway mediating intestinal serotonin release.