OPIOID-LIKE ANALGESIA IN DEFEATED MICE

Mice exposed to repeated attacks by other mice showed decreased nociception in response to radiant heat focused on their tails. This form of analgesia was blocked by centrally acting opiate antagonists and was not observed in morphine-tolerant mice; mice repeatedly subjected to defeat showed much less analgesia after receiving morphine than mice that were not subjected to defeat. Mice of the CXBK strain, which respond weakly to morphine displayed only moderate analgesia following defeat. Apparently, endogenous opioid-mediated analgesic mechanisms are readily activated by situations involving biologically significant forms of stress, such as defeat.