Hyperthermia and antinociceptive activity of thyrotropin-releasing hormone and morphine following central administration in rats.

This is an assessment of thyrotropin-releasing hormone (TRH) and morphine effects on nociceptive activity and temperature reaction in male Wistar rats following introduction of the substances into the periaqueductal gray matter or preoptic anterior hypothalamic nuclei via before hand cannulation. Morphine (10 micrograms) had marked antinociceptive activity, as shown by the tail flick-test, hot plate-test and mechanical pressure according to Randall-Selitto. TRH (5 micrograms) showed antinociceptive activity upon introduction into the periaqueductal gray matter, better expressed in the mechanical pressure test. Morphine (10 micrograms) and TRH (5 micrograms) gave rise to a hyperthermic reaction. The antinociceptive activity of morphine reduced upon preliminary administration of TRH, while its hyperthermic effect remained unchanged. These data are in support of the hypothesis that TRH may act as a functional opiate antagonist in the central nervous system.