Opiate modulation of separation-induced distress in non-human primates

Infant rhesus monkeys respond to separation from their mothers with a dramatic increase in vocalizations and activation of autonomic and pituitary-adrenal systems. Using the mother-infant separation paradigm in rhesus monkeys, we focused on the role of opiate systems in modulating the behavioral and neuroendocrine consequences of a brief, naturally occurring stressor. In the first experiment, morphine 0.1 mg/kg significantly decreased separation-induced vocalizations without affecting activity levels. In the second experiment, naloxone 1.0 mg/kg increased distress vocalizations but lower doses had no effect. In the third experiment we blocked the effect of morphine 0.1 mg/kg with naloxone 0.1 mg/kg, a dose of naloxone that had no intrinsic effects of its own. This suggests that the reduction of separation-induced vocalizations by morphine is mediated by opiate receptors. The last experiment demonstrated that separation-induced increases in pituitary-adrenal hormones can also be modulated by opiate agonists and antagonists. These findings are consistent with work in non-primate species and support the hypothesis that opiate receptors are specifically involved in mediating separation-induced vocalizations and pituitary-adrenal activation in primates.