INFLUENCES OF AGE AND NOCICEPTIVE REACTIVITY ON THE ANALGESIC ACTIONS OF NALTREXONE IN THE RAT

The actions of naltrexone on the tail avoidance reflex (TAR) was studied in female rats of differing ages (7-40 weeks). Naltrexone produced dose-related analgesia in younger (7-15 weeks) but not in older (19-40 weeks) rats. The age-related decrease in analgesic action was most demonstrable at higher doses of naltrexone. Naltrexone produced transient increases in tail surface temperatures in younger rats which may have diminished the observed analgesic actions. Negative correlations between thermal stimulus intensity and TAR latency were determined for all ages of rats studied. The slopes of these stimulus-response relationships became steeper with increasing age. These data suggest that the analgesic actions of naltrexone decrease with age and are associated with less nociceptive reactivity,