Blockade by naloxone and naltrexone of the TRH-induced stimulation of colonic transit in the rabbit.

Thyrotropin releasing hormone (TRH), administered intracerebroventricularly (i.c.v.) in microgram quantities to anesthetized rabbits, produces marked stimulation of colonic motility, transit, fluid accumulation, and accompanied by a portal hyperserotonemia. In this study we found that pretreatment of rabbits with naloxone (2.5 mg/kg) or naltrexone (1.0 mg/kg) attenuated or blocked the TRH-induced colonic transit and increase in luminal fluid, but not the hypermotility nor the hyperserotonemia. In this respect the narcotic antagonist effects resemble those produced by the antiserotonin compounds or opiate agonists.