Defeat-induced hypoalgesia in the rat: effects of conditioned odors, naltrexone, and extinction.

In Experiment 1, male rats were either defeated as a colony intruder by alpha conspecifics or had no defeat experience, and 24 hr later they were given a paw injection of formalin prior to observational tests with or without alpha-colony odors. The combination of defeat and tests with these odors produced conditioned hypoalgesia (i.e., a suppression in paw licking) and freezing. In Experiment 2, defeated rats were given either an injection of naltrexone or saline prior to defeat and 24 hr later prior to testing. An injection of naltrexone prior to defeat increased freezing during defeat and later testing. In contrast, naltrexone during testing did not affect freezing but significantly reduced hypoalgesia. In Experiment 3, a 12-hr exposure session with alpha-colony odors extinguished hypoalgesia in previously defeated rats. These findings are discussed in terms of associative, opioid/nonopioid, and adaptive evolutionary processes.