Implication of endogenous opioid mechanism in the production of the antinociceptive effect induced by psychological stress in mice.

Psychological (PSY) stress using the communication box produced a short-lasting antinociceptive effect which was less potent than that induced by physical stress such as footshock (FS) and forced swimming (SW) in mice. Naloxone completely antagonized PSY-stress induced analgesia (SIA) when the analgesia was measured by the tail pinch (TP) method; however, the antagonist did not reverse the effect in the tail flick (TF) assay. On the other hand, FS-SIA was antagonized by naloxone in both methods, while naloxone failed to reverse SW-SIA in either TF or TP assessment. Daily exposure to psychological stress developed tolerance to the analgesia. One-way cross-tolerance between PSY-SIA and morphine and the naloxone antagonism of PSY-SIA by the tail pinch method lead to the suggestion that an endogenous opioid system may be involved in the underlying mechanism for its production. On the contrary, from the findings of cross-tolerance between SW- or FS-SIA and the lack of naloxone antagonism in the TF method, the involvement of a more complicated mechanism is suggested in PSY-SIA. In both tests, U-50488H, a selective kappa-agonist, produced profound analgesia; however, no appreciable antagonism of naloxone was found in the TF test, whereas the effect was completely blocked by naloxone in the TP test. From the similarity in naloxone antagonism of PSY-stress and U-50488H induced analgesia, the participation of a common mechanism which may be mediated by kappa-opioid receptors, is suggested in the production of PSY-SIA.