Brain inflammation induces anhedonia in women but not men

For several years, researchers have insisted that inflammation in the brain was tied to depression and anxiety, but research published this week casts some doubt on this generalized conclusion. In fact, there seems to be a difference between the genders in the effect of inflammation and this is an important finding.

Inflammation reduces the brain’s response to rewards in women, but not in men, according to the new study in Biological Psychiatry: Cognitive Neuroscience and Neuroimaging. Reduced activity in the brain’s reward center is the signature of anhedonia, a core feature of depression that reflects a loss of enjoyment in things or activities—including the ability to find meaning in work and relationships. Women are two-to-three times more likely to be diagnosed with depression and the new findings pinpoint a key difference in men and women that could contribute to the lopsided rates of the disorder.

What the researchers say: “Our study is the first to show that there are sex differences in neural sensitivity to reward in response to inflammation, which has important implications,” said the senior author. “This may suggest one reason women experience depression at a far greater rate than men, particularly for the kinds of depression that may be inflammatory in nature.”

In the study, healthy men and women received a substance to increase inflammation. Dr. Eisenberger and colleagues measured activity in one of the main reward regions of the brain, the ventral striatum, while the participants played a game to receive a monetary reward. Women with greater inflammatory responses showed less brain response in anticipation of potential rewards, but the relationship was not present in men.

“This suggests that women with chronic inflammatory disorders may be particularly vulnerable to developing depression through decreases in sensitivity to reward. Clinicians who treat female patients with inflammatory disorders may want to pay close attention to these patients for possible onset of depressive symptoms,” said the researchers.

“This study highlights the important gender differences that exist in the human brain and suggests a mechanism that might help explain the greater prevalence of depression in women compared to men,” they concluded.

Because the increased inflammation had no effect on how men’s brains responded to reward, inflammation-induced anhedonia may be an important contributor to the increased rates of depression in women.

So, what? One of the problems in looking at the differences between men and women when studying depression and other mood disorders is that the sexes exhibit different symptoms. For example, depressed men tend to exhibit anger, loss of concentration, infidelity, bullying, loss of interest in work, family and hobbies. They do not necessarily show anhedonia or some of the other “traditional” symptoms of depression such as helplessness or hopelessness or self-loathing.

Therefore, it was thought that women were more depressed than men. More recent research has indicated that both genders have more equal rates of depression. I think more research needs to be directed into the issue of inflammation and depression. I am sure inflammation is one of the causal issues of depression in women; this may well be true of men, however depressed males may show the depressive effects of inflammation differently.