Environmentally induced analgesia: ontogeny of a nonopioid system

Previous research has demonstrated that exposure to 90 sec of hind paw shock activates an endogenous pain control system that involves cholinergic sites. The purpose of the present investigation was to examine the development of function of this nonopioid analgesic system. Research on the ontogeny of the cholinergic system suggests that this receptor system exhibits an extended period of postnatal development, with various neurochemical indexes reaching maturity between 30 to 50 days of age. Results revealed that exposure to hind paw shock produced very low levels of analgesia in 10- and 28-day-old rats. The analgesic response was more evident in 2-month-old rats, but the degree of hind paw shock-induced analgesia was not at its maximum until 3 months of age. Systemic injection of naltrexone had no effect on the degree of analgesia induced by hind paw shock, while a systemic injection of scopolamine significantly attenuated the analgesia displayed by the 28-day-old and 2- and 3-month-old rats. Thus, the neurochemical indexes of cholinergic development over-estimated the degree of functional maturity of the endogenous analgesia system activated by hind paw shock.