Panksepp, J., Herman, B. H., Vilberg, T. et al. 1980. Endogenous opioids and social behavior. Neuroscience and Biobehavioral Reviews 4(4), 473-487.

Evidence for the hypothesis that brain-opioids mediate social affect and social attachments is summarized. Opiates and opioids are very effective in reducing social separation-induced distress vocalizations (DVs), in puppies, young guinea pigs and chicks, while opiate antagonists can increase DVs. In studies of specific social behaviors in rodents, morphine (at doses 1 mg/kg and below) decreases proximity maintenance time in socially housed animals, increased play, decreases maternal aggression but has no effect on pup retrieval. Naloxone reduces play and disrupts pup-retrieval, but has no consistent effect on proximity maintenance time of socially housed animals. In young rats tested in social learning situations, morphine delays and naloxone hasten extinction. These data are consistent with the proposition that brain opioids modulate social emotions and behaviors.