Environmentally-induced analgesia: age-related differences in a hormonally-mediated, nonopioid system.

In order to study the age-related differences in analgesia produced by environmental stimuli, 3-month-old, 11-month-old, and 23-month-old rats were immersed in 6 degrees C water for 5 min. Prior to cold-water exposure, different groups of rats were injected (i.p.) with 7 mg/kg of naltrexone or saline or had adrenalectomies performed. Following cold-water immersion, pain sensitivity was measured for 2 hr. In the saline-treated groups, there was a progressive increase in the analgesia displayed after cold-water exposure as the age of the group increased. For each age group, naltrexone did not attenuate the analgesia produced by cold water whereas adrenalectomy potentiated the analgesia. The effects of naltrexone and adrenalectomy confirm that cold-water exposure produces a hormonally-mediated, nonopioid analgesic response. Thus, these results demonstrate that aged animals display an enhanced analgesic response to cold-water stress that is similar to other neuroendocrine indexes of increased hormonal response to stress in aged organisms.