The effects of aging on opioid modulation of feeding in rats.

Feeding responses to naloxone and butorphanol tartrate were measured in Fisher-344 rats with ages of 2, 12, 22 and 28 months. The two younger groups were 10-100 times more sensitive than the older groups to the suppressive effects of naloxone on feeding. Additionally, the older rats were less responsive to the feeding enhancement following butorphanol injections. These results are consistent with reports of age-related changes in endogenous opioid systems.