Morphine inhibits TRH-induced intestinal transit increases

The current study examined the effects of intraperitoneal (IP) and intracisternal (IC) administration of the opiate agonist, morphine, and an opioid, central beta-endorphin, on thyrotropin releasing hormone (TRH)-induced small intestinal transit increases. Anesthetized rats, 14-day and older, were studied to determine age-related differences. Results showed that in all age groups IP morphine (2 mg/kg) blocked TRH (15 μg)-induced increases in transit of a charcoal bolus. Morphine 1 μg and beta-endorphin 1 μg administered IC in 0.6 μl failed to block TRH (10 μg)-induced increases in intestinal transit in 14-day-old rats. However, both morphine and beta-endorphin 1 μg IC blocked TRH-induced increases in adult rats. Dose-response studies demonstrated that higher doses (> 1 μg) of morphine IC were required to block TRH-induced increases in preweaning rats.