Environmentally induced analgesia: an age-related decline in an endogenous opioid system.

The purpose of the present study was to examine in rats the relationship between the age-related decline in an opioid receptor system and the function of the endogenous opioid pain-inhibitory system activated by front-paw shock. Results revealed that the analgesia displayed following front-paw shock declined between 5- to 7-month-old, 15- to 17-month-old, and 22- to 24-month-old age groups. The administration of naloxone significantly attenuated the shock-induced analgesia. Thus, the endogenous opioid pain modulatory system activated by front-paw shock (the nucleus raphe alatus and a descending pathway lying within the dorsolateral funiculus of the spinal cord) declines in function with increasing age. This research also confirms that there is a parallel between the age-related decline in the neurochemical indexes of the opioid receptor system and the function of these receptors in producing analgesia in response to aversive stimulation.