A randomized, double-blind, placebo-controlled study found that a brief inflammatory response can temporarily reduce pleasure and the desire to socialize in healthy women. The findings were published in Psychoneuroendocrinology and may help clarify how immune activation is linked to some reward-related symptoms associated with depression.
The study focused on anhedonia, the reduced ability to experience pleasure or interest in activities that were previously enjoyable. Researchers also examined social motivation and sensitivity to positive social cues, such as smiling faces.
The trial included 40 healthy women between 25 and 44 years old. Eighteen received a low dose of endotoxin through an intravenous injection, while 22 received saline as a placebo. Endotoxin, a component of certain bacteria, produces a short-lived inflammatory response. Neither the participants nor the researchers working directly with them knew which injection each participant had received.
Before the injection and during the following hours, the participants provided blood samples and reported symptoms such as fatigue and feeling sick. They also completed tasks involving emotional faces and rated their desire to take part in activities including chatting with another person, playing a puzzle game, watching short film clips or sitting quietly alone.
Two hours after the injection, the women who received endotoxin reported more anhedonia and a lower desire to chat than those in the placebo group. The reduction in motivation appeared specific to social activity: endotoxin did not significantly change interest in the puzzle, film clips or quiet time. These effects remained after the researchers accounted for physical sickness symptoms and fatigue.
The increase in anhedonia was temporary. Scores returned to their initial levels four hours after the injection, even though inflammatory markers were still elevated. Among participants who received endotoxin, larger increases in the inflammatory protein TNF-α during the first hour were associated with greater increases in anhedonia and larger decreases in the desire to chat.
The effects on positive social cues were less pronounced. Participants in the placebo group became slightly more accurate at recognizing happy faces during the second test, while those who received endotoxin did not improve. However, the placebo group had been somewhat less accurate at the beginning, so the researchers urged caution in interpreting this result. Endotoxin did not affect the attention task involving emotional and neutral faces.
The authors said the results suggest that acute inflammation can induce anhedonia and reduce social motivation, with more limited effects on sensitivity to positive social signals. These changes could represent specific reward-related pathways through which immune activation contributes to inflammation-associated depression.
The researchers noted several limitations. The sample was small and included only women aged 25 to 44, so the findings may not apply to men or other age groups. Endotoxin causes a brief and intense inflammatory response, which differs from the persistent, lower-level inflammation observed in some people with depression. The study also measured participants’ stated desire to engage in activities rather than observing whether they actually sought social contact. Reduced social motivation during acute illness may additionally serve as an adaptive way to conserve energy.
Sources: PsyPost.