Endotoxin elicits ambivalent social behaviors.

The acute phase response to infection is reliably accompanied by decreases in social investigation; however, social behavior is commonly assayed in inescapable environments using unfamiliar social stimuli. In this experiment, male Wistar rats were raised from weaning with 2 familiar, same-sex conspecifics. In adulthood, rats were implanted with radiotelemetry devices that permitted localization in space, and were challenged with LPS treatments (150 μg/kg, i.p.) in a novel, semi-natural arena which afforded the treated (Focal) animal exclusive control of social exposure, and the ability to avoid social interactions. LPS reliably elicited thermoregulatory responses (transient hypothermia and fever) during the scotophase following injection, but did not yield changes in the proportion of time spent engaged in social interactions: both LPS- and saline-treated rats spent approximately 10% of the night with their familiar cagemates. Injection treatments markedly altered the spatial distribution of activity: LPS-treated rats exhibited significant increases in the amount of time spent as far as possible from their cagemates. The data suggest that sickness responses to LPS may give rise to a transient state of social ambivalence-characterized by a persistent motivation to engage in social contact, but also by increased avoidance of social environments. Selective maintenance of social motivation illustrates plasticity in the expression of sickness behaviors and may be adaptive in social species.