Opioid‐mediation of separation distress in 10‐day‐old rats: Reversal of stress with maternal stimuli

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Abstract
A relationship between distress vocalizations, response to nociception and their opioid mediation in 10-day-old maternally isolated rat pups was established. The comforting effects of several classes of biological stimuli were examined. Short-term (5 min) isolation from mother, siblings and nest caused a significant analagesic repsonse to heat (48°C) relative to nonisolated siblings. Morphine administration markedly increased heat escape latencies and decreased distress vocalizations during isolation. Naltrexone, and opioid antagonist, had the opposite effect; escape latencies were halved and distress vocalizations doubled. Contact with an anesthetized female, dam or virgin, immediately reduced both analgesia and vocalizations. Home-bedding was only effective after 5 min exposure, whereas clean bedding did not reduce isolation-induced behaviors. These results are discussed in terms of infant learning and motivation under natural circumstances.