Differential effects of lipopolysaccharide in the social behavior of dominant and submissive mice.

Acutely infected animals show a set of non-specific behavioral changes known as sickness behavior. Recent studies have shown that occurrence of sickness behavior is regulated according to a motivational perspective. Thus, the display of sickness behavior may compete with display of other behaviors. In this work, we sought to determine the effects of lipopolysaccharide (LPS) administration (15 microg/mouse i.p.) in the social behavior of dominant and submissive mice. Results showed that social hierarchy influences the expression of sickness behavior. While dominant mice treated with LPS showed an expected reduction in total frequency of behaviors displayed, such decrease did not happen following the same treatment to submissive mice. Similar results occurred regarding social and aggressive behavior. The use of a motivational perspective provides the assumption that, due to their high social ranking, dominant mice were able to prioritize recuperative behavior. Submissive mice, on the other hand, even though treated with LPS, seemed to essentially focus on social defensive behaviors since they remained in the presence of the dominant individuals. Effects of sickness on the hierarchical organization of mice remain to be further investigated.