Ontogeny of an endogenous, nonopioid and hormonally mediated analgesic system.

Rats of different ages (10-day, 28-day, and 3-month-old) were exposed to cold-water stress in order to activate an endogenous analgesic system. The effects of naltrexone (7 mg/kg) and dexamethasone (.4 mg/kg) were also studied to examine the role of the opioid and hormonal systems in cold-water-induced analgesia. Following cold-water exposure, nociception was measured with the tail-flick procedure for 2 hr. Results revealed that cold water produced a significant level of analgesia in the 10-day, 28-day, and 3-month-old age groups with no differences between age groups. In addition, in each age group naltrexone did not block the analgesia while dexamethasone attenuated the analgesia produced by cold water. The effects of naltrexone and dexamethasone confirm that cold-water immersion activates a nonopioid, hormonally mediated analgesic system in each age group. Thus, this experiment found that the endogenous, nonopioid, and hormonally mediated analgesic system activated by cold water is functional early in the development of the rat. The early development of this hormonally mediated analgesic system is in contrast to the slower development of endogenous analgesia systems that are mediated by the central nervous system.