It's probably not news to you that stress is bad for your body in all kinds of ways, but there's a new one to add to the list: a new study highlights how stress can cause inflammation of the brain, and that leads to problems with memory loss and depression, researchers say.
To figure this out, neuroscientists from Ohio State University taught mice to navigate their way out of a maze - a task they initially remembered how to do from previous lessons. But as a more aggressive mouse was introduced to the group, causing stress for the other animals, they gradually forgot which direction was out. "The stressed mice didn't recall it," says lead researcher Jonathan Godbout. "The mice that weren't stressed, they really remembered it."
Scans showed signs of inflammation in the brains of the stressed mice, brought on by the immune system's response to the outside pressure. The presence of immune cells called macrophages suggests that the animals' immune systems were attacking their own brains, causing the inflammation and preventing new brain cells from growing.
The effect was long-lasting too: for four weeks after the experiment, the stressed-out mice were found to be cowering in corners, the equivalent of social avoidance in humans and a major symptom of depression.
When the researchers introduced an anti-inflammatory drug to the affected animals, the brain cell deficit and social avoidance behaviour remained, although macrophage levels dropped and the memories of the mice returned to normal, pointing to a direct link between the inflammation and memory issues.
The study is particularly concerned with the impacts of long-term stress on the hippocampus area of the brain, which plays a key role in emotional response and memory retention.
"This is chronic stress," says Godbout. "It's not just the stress of giving a talk or meeting someone new… It's possible we could identify targets that we can treat pharmacologically or behaviourally."
While plenty of previous studies have covered similar ground – the way that stress, anxiety, depression and memory are related – this is believed to be the first one to associate short-term memory loss with brain inflammation, and the brain inflammation itself on a reaction from the body's immune system.
The next step is to see if the same reactions are present in the human brain: if so, it could open up the possibility of new treatments for stress and depression, if doctors can find ways of preventing or limiting the inflammation that occurs in the hippocampus.
The study has been published in The Journal of Neuroscience.