The effects of aging on mu and delta opioid receptors in the spinal cord of Fischer-344 rats.

Previous research has demonstrated that the antinociceptive efficacy of opioids decreases with advancing age. This study utilized radioligand binding techniques to determine if this decline is due to a change in the receptor density (Bmax) and/or affinity (measured as Kd) of the mu (mu) and/or delta (delta) opioid receptors in the spinal cord with advancing age. Saturation binding analysis with [3H][d-Ala2,N-methyl-Phe4,Gly5-ol]enkephalin (DAMGO: a mu-opioid selective agonist) and [3H]naltrindole (a delta-opioid selective antagonist) revealed no age-related changes in Bmax for either the mu or delta-opioid receptors. The Kd value for naltrindole was likewise unaffected by age. The Kd value for DAMGO however, was significantly higher in the aged group as compared with the young and mature groups, indicating a decreased affinity of spinal mu-opioid receptors for DAMGO. Copyright 1998 Elsevier Science B.V.