Panksepp J., Bean, N. J., Bishop, P. et al. 1980. Opioid blockade and social comfort in chicks. Pharmacology, Biochemistry, and Behavior 13(5), 673-83.

When young animals are separated from their normal social environment in groups they distress vocalize (DV) less than when isolated alone. Opioid blockade with naloxone (1 mg/kg peripherally, and 1 microgram centrally) increased crying more in group tested chicks than individually tested ones. The serotonin receptor blocking agent methysergide and the cholinergic blocking agent atropine sulfate produced similar effects. The testing of birds in mirrored environments also produced a reduction of DV's, and all three receptor blocking agents augmented DV's more in mirrored environments than in mirrorless ones. However, in studies evaluating changes in this comfort response by facilitation of opiate, serotonin (5 HT), and acetylcholine (ACh) activity (with morphine, quipazine and pilocarpine, respectively), only morphine was found to magnify the comforting effect of mirrors. The effects of naloxone on contact comfort, and the acquisition and expression of imprinting were further evaluated, and opioid blockade reduced all these measures of social comfort. It is concluded that endogenous opioid activity (and to a lesser extent, 5 HT and ACh) contribute to the comfort which animals derive from their social environment.