Environmentally induced analgesia: age-related decline in a neurally mediated, nonopioid system.

In order to examine the function of an endogenous system of pain inhibition during aging, rats (3, 14, and 24 months old) were exposed to 90 s of hind-paw shock. To investigate the pharmacology and anatomy involved in the production of hind-paw shock induced analgesia, the effects of naltrexone (7 mg/kg), scopolamine (5 mg/kg), and adrenalectomy were examined. Results revealed that there was an age-related reduction in the degree of analgesia produced by hind-paw shock. Naltrexone and adrenalectomy did not alter the analgesia elicited by hind-paw shock. Scopolamine reduced the analgesia produced by hind-paw shock, and the effectiveness of scopolamine blockage declined with age. The results of a second experiment demonstrated that the effect of scopolamine was specific to the analgesia induced by hind-paw shock because scopolamine was ineffective in modifying the analgesia produced by a different stressor (cold water). These results suggest that the decline in hind-paw shock induced analgesia is the result of an alteration in the function of the cholinergic system.