Anti-nociceptive activity of narcotic agonists and partial agonists in mice given biogenic amines by intracerebroventricular injection.

Using a tail immersion technique in mice, the anti-nociceptive activity of some narcotic agonists (diamorphine, etorphine, morphine and pethidine), partial agonists (cyclazocine, nalorphine and pentazocine) and naloxone have been determined alone, and after the intracerbroventricular (ICV) injection of 5-hydroxytryptamine (5-HT) or noradrenaline (NA). In common with morphine, the anti-nociceptive effects of each agonist and partial agonist studied were significantly increased by ICV 5-HT, and significantly attenuated by ICV NA. The marginal anti-nociceptive activity of naloxone was not significantly affected by the ICV injection of either amine. It is concluded that the anti-nociceptive effects of all narcotic agonist and partial agonist agents are determined by the balance within the brain of the activities of the two amines, 5-HT and NA.