Developmental aspects of nociception.

Research has documented the existence of multiple, endogenous systems that modulate nociception. Based on the effects of opioid antagonists and endocrine lesions, endogenous analgesia systems have been organized into four classes: neural-opioid, neural-nonopioid; hormonal-opioid; hormonal-nonopioid. Developmental research on the ontogeny of endogenous analgesic function has revealed differential rates of maturation. Front-paw shock, a stimulus that activates a neural-opioid analgesic response, has been shown to be functionally mature by 28 days of age in the rat. Similarly, hind-paw shock, a stimulus that elicits a neural-nonopioid analgesic response, reaches maturity after two months of age. However, the hormonal-opioid analgesic system activated by cold-water immersion reaches adult levels by 10 days of age. Food deprivation produces a hormonal-opioid analgesic response in adult rats, and food deprivation/isolation of rat pups has been found to elicit an analgesic response in 6-day-old rats. From these data it seems that the rate of development of the different endogenous analgesic systems is related to the activation of neural or hormonal components. Whether the differential rates of development and the neural-hormonal distinction are related to the ecological validity of the activating stimulus remains to be determined.