Neuropeptides and social behavior of rats tested in dyadic encounters

The effects of various neuropeptides on social behavior was studied in a test procedure in which 7-day isolated animals were tested together with non-isolated partners in dyadic encounters. The short-term isolation procedure increased the frequency and duration of social activities of the rats, but hardly affected non-social explorative behaviors of the animals. Systemic injection of certain neuropeptides, i.c. prolyl-leucyl-glycinamide (PLG), thyrotropin releasing hormone (TRH) and the ACTH 4-9 analog ORG 2766, reversed the isolation-induced increase in social activity, similarly as previously observed with antidepressant drugs. Subcutaneous treatment with beta-endorphin, alpha-endorphin and des-Tyr-gamma-endorphin increased social interactions in 7-day isolated animals. beta-Endorphin enhanced social behavior of non-isolated rats as well, whereas gamma-MSH decreased the social interactions of these animals. Both peptides affected especially social contact behavior. The potent action of beta-endorphin suggests that this peptide and opioid systems may play a physiological role in social behavior. It is proposed that a possible functional antagonism between ACTH-like peptides, especially gamma-MSH, and beta-endorphin may operate in social behavior. The action of the peptides may be rather specific for social behavior, since none of the neuropeptides affected non-social explorative behaviors of the rats during the social interaction test.