Dominant mice show much lower concentrations of methionine-enkephalin in brain tissue than subordinates: cause or effect?

Possible relationships between dominance rank and brain methionine-enkephalin content were studied by using a method of consecutive rank determination in groups of mice reassembled according to their initial ordinal dominance rank. Thus, 10 groups of 3 Balb/c AnN mice of identical ordinal rank were formed with the consecutive dominants of 3 groups of 10 mice. Dominant, subdominant, subordinate and non-aggressive mice were recognized in these groups. Rank order was strongly correlated with brain Met-enkephalin content. Dominant mice contain less than a fourth Met-enkephalin immunologic reactivity in the brainstem and about half the forebrain concentration than subordinates. Brainstem Met-enkephalin correlated with the final ordinal rank and, accordingly, it was found to decrease exponentially after mice acquired alpha-dominance status. Different concentrations in the forebrain correlated with the initial but not the final ordinal rank and, therefore, probably preceded dominance status.