Behaviorally functional opioid systems in infant rats: II. Evidence for pharmacological, physiological, and psychological mediation of pain and stress.

In order to determine the behavioral characteristics of the neonatal opioid system during distressful situations, a modification of the hot-plate paw-lick test used on adults was developed for infant rats. Ten-day-old pups were analgesic to heat following morphine administration. Pretreatment with an opioid antagonist prevented the analgesia. Morphine analgesia was significantly greater in pups group isolated from the dam. Saline control pups group isolated from the dam exhibited longer latencies than their nest-housed siblings. Individual isolation for 5 min increased paw-withdrawal latency markedly. This was also naltrexone reversible. This analgesia was not seen when pups were tested directly from the nest or when grouped with other pups for the 5 min. It is suggested that the opioid system(s) for stress and pain are functional in Day 10 rats and short-term isolation from the dam is a probable natural stressor that is modulated by endogenous opioid release.