Inflammation and dimensions of reward processing following exposure to the influenza vaccine

Abstract

Background: Alterations in reward processing are a central feature of depression and may be influenced by inflammation. Indeed, inflammation is associated with deficits in reward-related processes in animal models and with dysregulation in reward-related neural circuitry in humans. However, the downstream behavioral manifestations of such impairments are rarely examined in humans.

Methods: The influenza vaccination was used to elicit a mild inflammatory response in 41 healthy young adults (age range: 18-22, 30 female). Participants provided blood samples and completed behavioral measures of three key aspects of reward-reward motivation, reward learning, and reward sensitivity-before and 1 day after receiving the influenza vaccine.

Results: The influenza vaccine led to mild but significant increases in circulating levels of the pro-inflammatory cytokine interleukin-6 (IL-6) (p < .001). Consistent with hypotheses, increases in IL-6 predicted lower reward motivation (p = .029). However, contrary to hypotheses, increases in IL-6 predicted increased performance on a reward learning task (p = .043) and were not associated with changes in reward sensitivity (p's > .288).

Conclusions: These findings contribute to an emerging literature on the nuanced associations between inflammation and reward and demonstrate that even mild alterations in inflammation are associated with multiple facets of reward processing.

Keywords: Anhedonia; Depression; Inflammation; Reward learning; Reward motivation; Sickness behavior.

Copyright © 2018 Elsevier Ltd. All rights reserved.

Figures

Figure 1.

Results from generalized estimating equations for the Effort Expenditure for Rewards Task with 95% confidence interval error bars. Greater increases in IL-6 from pre- to post-influenza vaccine were significantly associated with fewer hard trial choice over and above the effects of time (pre-vs. post-vaccine), task specific variables (i.e., probability, expected value, reward magnitude, trial number), sex, BMI and cohort (b = −0.65, p = .029).

Figure 2.

Greater increases in IL-6 were significantly associated with greater increases in total response bias (i.e., reward responsiveness) from pre- to post-vaccine on the PRT (r = .40, p = .019) (Panel A). Analyses remain significant when removing the outlier on changes in total response bias (i.e., the participant with the highest values on the y axis in Panel A). The relationship between change in IL-6 and change in reward learning did not reach significance (r = .07, p = .710) (Panel B) nor did the association between change in IL-6 and change in reward sensitivity on the PRT (r = .28, p = .121) (Panel C). Note that reward sensitivity (logβ) and learning rate (log(ε1−ε)) parameters in the transformed space were used to prevent issues with non-Gaussianity.