Presence of a pair-mate regulates the behavioral and physiological effects of opioid manipulation in the monogamous titi monkey (Callicebus cupreus)

The role of opioid receptors in infant-mother attachment has been well established. Morphine, a preferential μ opioid receptor (MOR) agonist, attenuates separation distress vocalizations and decreases physical contact between infant and mother. However, there is little research on how opioid receptors are involved in adult attachment. The present study used the monogamous titi monkey (Callicebus cupreus) to explore the role of opioid receptors in the behavioral and physiological components of pair-bonding. In Experiment 1, paired male titi monkeys (N = 8) received morphine (0.1, 0.5, or 1.0 mg/kg), the opioid antagonist naloxone (1.0 mg/kg), vehicle, or a disturbance control and were filmed with their pair-mate for 1 h. In Experiment 2, the same eight males received morphine (0.25 mg/kg), naloxone (1.0 mg/kg), vehicle, or a disturbance control and were filmed for an hour without their pair-mates. All video sessions were scored for social and non-social behaviors. Blood was sampled immediately prior to drug administration and at the end of the hour session. Plasma was assayed for cortisol, oxytocin, and vasopressin. In Experiment 1, opioid manipulation had no effect on affiliative behaviors; however, morphine dose-dependently decreased locomotor behavior and increased scratching. In Experiment 2 in which males were separated from their pair-mates, naloxone increased locomotion. Morphine dose-dependently attenuated the rise in cortisol, while naloxone potentiated the increase of cortisol. The cortisol increase following naloxone administration was greater when a male was alone compared to when the male was with his pair-mate. Naloxone increased vasopressin but only when the male was tested without his pair-mate. The present study found that the absence of a pair-mate magnified naloxone's effects on stress-related hormones and behaviors, suggesting that the presence of a pair-mate can act as a social buffer against the stress-inducing effects of naloxone.