Diversity of underlying mechanisms in the production of analgesic and pentobarbital-hypnosis prolonging effects of various analgesic drugs and stresses.

Stressful stimuli, electric footshock (FS), immobilized-water immersion (IW), and cold-water swimming (CWS), produced analgesia and prolonged the pentobarbital hypnosis as well as morphine and clonidine. Naloxone completely antagonized the analgesic effects of morphine and FS and partially that of IW; however, that of clonidine and CWS were not reversed by naloxone. Naloxone eliminated the hypnosis prolonging effect of morphine and FS, but failed to reverse the effect of clonidine, IW and CWS. Differences in the analgesic and hypnosis prolonging effects and also the respective naloxone sensitivity of each drug and stress suggest the diversity of the underlying mechanisms.