Straub tail, the deprivation effect and addiction to aggression
  It has been observed that male mice who are consistently winning fights with conspecifics can raise their tail, which is very similar to a morphine-induced Straub tail response. Since this response is a typical index of opiate activation, it has been proposed that the opioidergic systems of such mice are chronically activated. This activation appeared to be a potent factor, which leads to addiction to aggression. To check this hypothesis, we exposed the mice who had won 20 fights in succession with conspecifics to a behavioral sensitization procedure. The effects of the mu-opioid receptor agonist morphine (10 mg/kg, i.p.) on the behavior of winners were examined in an open-field test before and after 5- and 14-day deprivation of aggression. Morphine had a much stronger stimulating effect on the open-field behavior of 60 % of the winners deprived of aggression for 14 days than on that of the control mice. Morphine did not stimulate behavioral activity in the winners before or after deprivation for 5 days. The aggression level in the winners was higher after than before deprivation. It has been concluded that, in the winners, the mu-opioid receptors became tolerant to the effects of morphine and became sensitized after long aggression deprivation. The development of addiction to aggression due to repeated victories is discussed in the light of the theory of addiction (Robinson,Berridge, 2003). 
Natalia N. Kudryavtseva