Involvement of different mechanisms, opioid and non-opioid forms, in the analgesia induced by footshock (FS) and immobilized-water immersion (IW) stress.

It has been recognized that various stressful procedures such as foot shock (1-3), environ mental heat (4), immobilization (4, 5) and fighting (6) produce an analgesic effect in experimental animals. Akil et al. (1) and Chesher and Chan (7) showed that the analgesic effect induced by foot shock stress is antagonized by naloxone, a potent opioid receptor blocker, and suggested the involve ment of the endogenous opioid system in the underlying mechanisms of stress-induced analgesia (SIA). On the other hand, Lal et al. (8) described a lack of the antagonistic effect of naloxone on the analgesia induced by forced swimming stress. Thus, the differ ence and the characteristic nature of various SIA has not been definitely clarified and comparative studies were made on the underlying mechanisms in the production of the analgesia induced by morphine and stresses. Male ddN mice (23-26 g) were used throughout. Foot shock (FS), 90V, 1 sec duration, 0.2 Hz, was applied through the grid floor for 30 min. As the immobilized water immersion (IW) stress, animals were confined in a plastic box and immersed in water, 25°C, for 30 min. Analgesic effect was assessed by the modified Haffner's method (9). Morphine-HCI and naloxone-HCI were dissolved in saline and administered intra peritoneally in a volume of 0.1 ml/10 g of body weight. Both FS and IW-stress produced short lasting analgesia as shown in Fig. 1. The intensity of the analgesia was maximal immediately after the termination of stress exposure and was nearly equipotent to the analgesic effect of 5 to 10 mg/kg of morphine. The analgesic effect was antagonized by 2 mg/kg i.p. of naloxone, completely in FS