The effects of rewarding ventral tegmental area stimulation and environmental enrichment on lipopolysaccharide-induced sickness behavior and cytokine expression in female rats

Abstract

Responding for rewarding brain stimulation has been used to track hedonic status in animals. In addition to neurochemical alterations, stimulation of the lateral hypothalamus or ventral tegmentum has been shown to influence immunological processes, including elevation of peripheral natural killer cell activity. In the present study, we examined whether ventral tegmental area (VTA) stimulation or environmental enrichment altered the severity of lipopolysaccharide (LPS)-induced sickness behaviors and the provocation of cytokine expression induced by the endotoxin. Accordingly, rats received either trials of brain stimulation reward or exposure to an enriched environment and subsequently challenged with 150 ug/kg i.p. of LPS. Groups receiving LPS and saline injections without further manipulation were also included. Using the real-time RT-PCR and a multiplex bead assay, mRNA and protein levels for several cytokines and their receptors were determined to evaluate how these may vary as a consequence of reward. Both brain stimulation and environmental enrichment similarly diminished sickness behaviors associated with the endotoxin. Receptor gene levels were generally stable across groups. Levels of IL-6 within the VTA were increased in the group receiving LPS challenge alone and environmental enrichment was associated with modestly reduced IL-6 levels within this brain region. Taken together, these data suggest that rewarding brain stimulation and environmental enrichment buffer against malaise provoked by endotoxin challenge. Moreover, IL-6 expression within the VTA may influence the development of sickness behavior following inflammatory stimuli.