Effects of acute inflammatory challenge on anhedonia and social reward processing: A randomized controlled trial of endotoxin.

Anhedonia is a core feature of depression that reflects reward system dysfunction. Acute inflammatory challenge has been shown to induce transient anhedonia and reward processing deficits, but its effects on distinct components of social reward processes remain poorly understood. This randomized, double-blind, placebo-controlled study evaluated anhedonia and social reward responsivity following acute inflammatory challenge. Low-dose endotoxin (0.8 ng/kg body weight; n = 18) or placebo (same volume of 0.9% saline; n = 22) was administered to female adults (ages 25-44). Anhedonia (Snaith-Hamilton Pleasure Questionnaire) and blood samples for cytokine assessment were collected pre-infusion and 2 and 4 h post-infusion. Social reward motivation (wanting to engage in a social task) was assessed at a baseline eligibility visit and 2 h post-infusion. Sensitivity to positive social cues (attentional bias toward and recognition of happy faces) was assessed pre-infusion and 2 h post-infusion. Results showed that endotoxin increased anhedonic symptoms (p = .005) and decreased social motivation by 2 h post-infusion (p = .001); within the endotoxin condition, changes in both outcomes were associated with increases in TNF-α (p's < .04). A significant condition-by-time interaction for happy-face recognition (p = .011) was driven by increased accuracy in the placebo (p = .04) but not endotoxin (p = .43) condition. Endotoxin administration did not affect attentional bias toward happy faces. Findings suggest that acute inflammatory challenge induces anhedonia and reduces social motivation, with more limited effects on sensitivity to positive social cues, highlighting specific reward-related pathways through which immune activation may contribute to inflammation-associated depression. Future work could examine whether immune-related changes in social motivation contribute to motivational deficits in depression.