Role of the nucleus raphe magnus in opiate analgesia as studied by the microinjection technique in the rat.

The analgesic effects of morphine (5 microgram, 0.2 microliter) microinjected into the nucleus raphé magnus (NRM) and the surrounding reticular formation of the rat were tested using vocalization after electric shock to the tail as the test for analgesia. Only sites in the NRM produced powerful analgesic effects, strongest analgesia being equivalent to 3 mg/kg i.v. morphine. The analgesia produced by the microinjection was reversed by systemic naloxone. Pretreatment with systemic cinanserin, a blocker of serotonergic receptors, led to a pronounced diminution of the analgesic effects of the morphine. The effects of microinjections of naloxone (5 microgram 0.2 microliter) were studied for their effect on analgesia produced by systemic morphine. The analgesia following 3 mg/kg i.v. morphine was diminished by the microinjection of naloxone but the naloxone almost completely reversed the analgesic effects of 1.5 mg/kg i.v. morphine. These results further substantiate the role of the NRM in analgesic mechanisms.