Stress Prompts Anxiety Because Of Immune Cells Recruited To Brain, Animal Study Shows

Why do we feel anxious when we're under stress? A new study in mice might have the answer.

Researchers from The Ohio State University found that when mice were exposed to prolonged stress, immune cells called monocytes are "called up" to the brain, prompting feelings of anxiety.

"In the absence of tissue damage, we have cells migrating to the brain in response to the region of the brain that is activated by the stressor," study researcher John Sheridan, professor of oral biology and associate director of Ohio State’s Institute for Behavioral Medicine Research, said in a statement. "In this case, the cells are recruited to the brain by signals generated by the animal's interpretation of social defeat as stressful."

The study, published in the Journal of Neuroscience, involved subjecting mice to the kind of chronic stress a human might experience. In mouse terms, this meant letting a group of male mice live together long enough so that they could establish a "hierarchy," and then adding an aggressive male mouse to the mix. This new mouse prompted feelings of "fight or flight" among the other male mice.

Researchers subjected the mice to this scenario for zero, one, three or six cycles, in order to test for levels of anxiety. They found that the more cycles the mice went through, the higher their levels of anxiety. They also identified an association between growing levels of anxiety and higher monocyte levels in the mice's brains, particularly the brain regions of the amygdala, hippocampus and prefrontal cortex.

"These findings indicate that monocyte recruitment to the brain in response to social stress represents a novel cellular mechanism that contributes to the development of anxiety," the researchers wrote in the study.

Earlier this year, researchers from Northwestern University's Feinberg School of Medicine found that the "love hormone" oxytocin could also play a role in triggering anxiety. Their study, conducted in mice, shows that oxytocin not only prompts the recall of past stressful situations, but also boosts anxiety and fear in response to the idea of future stress.