FURTHER EVIDENCE FOR THE IMPLICATION OF A KAPPA-OPIOID RECEPTOR MECHANISM IN THE PRODUCTION OF PSYCHOLOGICAL STRESS-INDUCED ANALGESIA

The analgesic effect induced by exposure to psychological stress, using a communication box (psychological stress-induced analgesia, PSY-SIA), was completely antagonized by 10 min pretreatment with 0.5, 1 and 2 mg/kg of nor-binaltor-phimine and with 0.5 and 1 mg/kg of Mr2266, selective K-opioid receptor antagonists, in the tail pinch method. Neither footshock (FS)-nor forced swimming (SW)-SIA was affected by these antagonists. The selective ʴ-opioid receptor antagonist naltrindole, at doses up to 20 mg/kg, had no appreciable effect on PSY-SIA. Daily morphine treatment, 10 mg/kg, s.c., resulted in tolerance to the analgesic effect, and concurrent exposure to PSY-stress suppressed the development of morphine tolerance. The substitution of treatment with U-50,488H for PSY-stress still resulted in analgesia on the initial day; and likewise, the suppression by U-50.488H of the development of morphine tolerance was replicated by PSY-stress. Pretreatment with nor-binaltorphimine antagonized the suppressive effect of PSY-stress on the development of morphine tolerance without affecting the analgesic effect of morphine per se. These results provide further evidence that PSY-SIA involves the mediation by K-opioid receptor mechanisms. © 1990, The Japanese Pharmacological Society. All rights reserved.