Straub tail, the deprivation effect and addiction to aggression

Background. It has been observed that male mice who are consistently winning fights with conspecifics in the settings of the sensory contact model can for no apparent reason 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; in which case the subject is motivated to repeatedly display. To check this hypothesis, we exposed the mice who had won 20 fights in succession with conspecifics to a behavioral sensitization procedure. Material/Methods. Male mice who engage in aggression against conspecifics and win were identified using the sensory contact model. The effects of the mu-opioid receptor agonist morphine (10 mg/kg, i.p.) on their behavior were examined in an open-field test before and after 5and 14-day deprivation of aggression. Additionally, aggression levels (latency to the first attack, the number of attacks and the total attack duration) were measured before and after deprivation during a 10-min agonistic interaction and then compared. Results. 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. Latency to raise the tail after morphine injection was significantly shorter in the winners than in the controls. The aggression level in the winners was higher after than before deprivation. Conclusion. It has been concluded that, in the winners, the mu-opioid receptors became tolerant to the effects of the endogenous mu-opioids due to the activation of opioidergic systems in the brain 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 proposed by Robinson and Berridge (2003).