Aggression and the three opioid families (endorphins, enkephalins, and dynorphins) in mice

Previous studies suggest that brain opioid activity decreases aggression in animal models. The main objective of the current study was to examine the possible genetic relationship between intermale aggression and brain levels of enkephalins, endorphins, and dynorphins in 11 inbred strains of mice. Pursuit, rattling, and attack behaviors were observed in a dyadic encounter with a standard opponent. It appeared that, as expected, enkephalins and endorphins were always negatively correlated with aggression scores. The findings indicate that brain Met⁵-enkephalin levels were significantly and highly positively correlated with attack latency. Brain adrenocorticotrophic hormone (ACTH) and β-endorphin levels were significantly and negatively correlated with the number of rattlings, which is consistent with the hypothesis that rattling is a stress-related behavior. In contrast with Met5-enkephalin, ACTH and β-endorphin, the correlations between dynorphin A and aggression scores were nonsignificant and very low. These preliminary results suggest that common genetic sources of variation contribute to differences between the 11 inbred strains in both endogenous opioidergic systems and intermale aggression. Further studies are required to confirm the genetic relationship between offensive aggression and brain enkephalins and endorphins and to better understand the mechanisms underlying the role of endogenous opioids in offensive aggression with regard to opioid receptor activity.