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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 analgesic response to heat (48 degrees C) relative to nonisolated siblings. Morphine administration markedly increased heat escape latencies and decreased distress vocalizations during isolation. Naltrexone, an 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.

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