Panksepp J, H. B. C. R. B. P. S. J. 1978. The biology of social attachments: opiates alleviate separation distress. Biological Psychiatry 13(5), 607-18.

The possibility that brain opiate systems participate in the control of social affect was assessed by determining capacity of low doses of exogenous opiates (0.125-0.50 mg/kg oxymorphone, and 0.10-0.50 mg/kg morphine sulfate) to reduce distress vocalizations of socially isolated puppies. Low doses of opiates were capable of profoundly reducing crying as well as the motor agitation they exhibit during brief periods of social isolation. Since reductions in crying could be obtained with morphine in the absence of any gross behavioral disturbances, the possibility is entertained that brain opiates may function to control the intensity of emotions arising from social separation. Possible parallels between the biological nature of narcotic addiction and the formation of social bonds are discussed.