Serious brain injuries have been known to increase the risks of depression, even years after a concussion. Now researchers think they know why.
 

Ohio State University scientists have determined that a head injury can trigger immune-system brain cells to go on "high alert" and overreact to a second injury, infection, or other stresses to the body by becoming excessively inflammatory — a condition linked with depressive complications.
 

The findings, published online in the journal Biological Psychiatry, could help explain some of the midlife mental-health issues suffered by individuals who experienced multiple concussions as young adults. The researchers also noted those depressive symptoms are likely inflammation-related, which means they may not respond to common antidepressants.
 

Since aging already increases brain inflammation, people who have had a brain injury may experience more serious symptoms of depression, according to the OSU research, which involved studies of mice.
 

"A lot of people with a history of head injury don't develop mental-health problems until they're in their 40s, 50s or 60s," said Jonathan Godbout, an associate professor of neuroscience at OSU a researcher in the Institute for Behavioral Medicine Research who led the new study. "That suggests there are other factors involved, and that's why we're looking at this two-hit idea — the brain injury being the first and then an immune challenge. It's as if one plus one plus one equals 15. There can be a multiplier effect."
 

This work applies to concussive brain injuries that result in a diffuse trauma to the brain, from which people recover fairly quickly, typically showing no problems with thinking or moving about a week after the injury to the brain.
 

But in the new study, researchers found immune cells in the brains of injured mice remained on high alert weeks after the injury and were "primed" to respond to a second challenge to the body with an exaggerated response.
 

"These results tell us the TBI mice are having an amplified and prolonged activation of [brain immune cells], and that was associated with development of depressive symptoms in the mice," Godbout said.