Continuous social defeat induces an increase of endogenous opioids in discrete brain areas of the mongolian gerbil

Dyads of a victor and a loser of mongolian gerbils (Meriones unguiculatus) coexisted for seven days; isolated animals served as a further experimental group. beta-Endorphin, Met-enkephalin and dynorphin were measured in several brain areas and in the anterior and neurointermediate pituitary. beta-Endorphin and Met-enkephalin were increased in the amygdala of defeated as compared to victorious animals. Met-enkephalin in the hypothalamus and in the striatum were lower in isolated than in coexisting gerbils. Coexistence decreased beta-endorphin in the amygdala and in the hypothalamus as compared to isolation. The results provide biochemical evidence for the role of central endogenous opioid-peptide systems in the physiology of victory and defeat. Dynorphin showed no variation with social conflict and social status.