Stress-induced analgesia: Adaptation following chronic cold water swims

Rats exposed to novel stressful events subsequently display increased nociceptive thresholds for up to 2 h. The present study investigated whether the analgesia induced by one such stressor, a brief, forced, cold water swim would show adaptation with repeated exposures in the same manner that other stress-induced physiological responses adapt. Acutely exposed rats displayed profound postswim elevations in flinch-jump thresholds, but rats that were chronically exposed to 14 daily cold water swims displayed thresholds similar to unstressed rats when tested 30 min after the final swim condition, indicating adaptation of pain thresholds to continued stress. Moreover, peripheral and core hypothermia could not account for analgesic effectiveness, since both the nonanalgesic chronic and analgesic acute groups displayed significantly lower rectal and skin temperatures throughout the testing period.