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

Two experiments, with 160 10-day-old Sprague-Dawley rat pups, examined the behavioral characteristics of the neonatal opioid system during distressful situations, using a modification of the hot-plate paw-lick test. Ss were analgesic to heat following intraperitoneal morphine (0.5 mg/kg). Subcutaneous naloxone (0.5 mg/kg) prevented the analgesia. Morphine analgesia was significantly greater in Ss group-isolated from the dam. Saline controls group-isolated from the dam exhibited longer latencies than their nest-housed siblings. Individual isolation for 5 min markedly increased paw-withdrawal latency, and this effect was naltrexone reversible. Analgesia was not seen when Ss were tested directly from the nest or when grouped with others for 5 min. It is suggested that the opioid systems for stress and pain are functional in 10-day-old rats and that short-term isolation from the dam is a probable natural stressor modulated by endogenous opioid release. (51 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)