Conditioned Defensive Responses and Hypoalgesia in Rats Exposed to Defeat and Alpha-Colony Odors

Abstract

This article describes the results of recent studies that examine the effects, in rats, of defeat by a dominant conspecific on subsequent defensive responses and hypoalgesia. In Experiment 1, male subjects were exposed to either aggressive (alpha) or nonaggressive conspecific colonies and tested 24 hr later, with or without alpha-colony odors, for freezing and burying of a wall prod that had been the source of a single electric shock. Although prior exposure to defeat and the presence of alpha-colony odors during testing had no effects, per se, the combination of these factors significantly suppressed prod burying and increased freezing. In Experiment 2, a 12-hr exposure session to the alpha-colony odors, for previously defeated intruders, resulted in the extinction of the fear-mediated suppressions in burying and increments in freezing during later prod-shock tests with alpha-colony odors present. In Experiment 3, male rats were either defeated as colony intruders 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 4, an injection of naltrexone prior to a defeat session increased freezing during defeat and during formalin testing 24 hr later, whereas injecting naltrexone prior to testing did not affect freezing but significantly reduced conditioned hypoalgesia. In Experiment 5, a 12-hr pretest exposure session to alpha-colony odors extinguished this conditioned hypoalgesic response for previously defeated rats. These fear-induced alterations in defensive behavior and pain sensitivity are discussed in terms of associative, opioid/nonopioid, and adaptive evolutionary processes.