Long-term stress can cause memory loss and inflammation in the brain - and the immune system is to blame.

By studying mice exposed to bullying by an aggressive intruder, researchers found the animals were unable to remember the location of an escape hole in a maze they had previously mastered.

The effect was only seen after repeat visits by the bullying 'alpha mouse', leading the researchers to conclude that one-off stressful situations do not trigger forgetfulness.

Long-term stress (illustrated by a stock image) can cause memory loss and inflammation in the brain - and the immune system is to blame, according to a study in mice

'This is chronic stress. It's not just the stress of giving a talk or meeting someone new,' said lead researcher Jonathan Godbout, associate professor of neuroscience at Ohio State.

The stressed-out rodents had changes in their brains, including inflammation brought on by their own immune system.

Short-term memory loss was caused by inflammation in the brain, itself the result of the appearance of immune cells called macrophages.

The mice in the study were exposed to what psychologists call 'repeated social defeat' – in other words they were bullied by a dominant alpha mouse.

By studying mice exposed to bullying by an aggressive intruder, researchers found the animals were unable to remember the location of an escape hole in a maze they had previously mastered (stock image)

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This mimics the chronic psychosocial stress experienced by humans including soldiers, people in abusive relationships, or those who report to bullying bosses.

Writing in the Journal of Neuroscience, the scientists describe their work on the hippocampus area of the brain which controls memory and emotional response.

They found that the bullied mice had trouble with spatial memory and avoided social contact for up to four weeks, indicating depressive-type behaviour.

When the mice were given a drug to reduce inflammation, the depressive symptoms remained even though the memory loss and inflammatory macrophages disappeared.

This led to the conclusion that post-stress memory trouble is directly linked to inflammation, and the immune system, rather than brain damage.

The researchers studied the hippocampus region of the mice's brains (shown in purple in this illustration of the human brain). They found that the bullied mice had trouble with spatial memory and avoided social contact for up to four weeks, indicating depressive-type behaviour

The good news is that the mice recovered naturally within 28 days of the end of the aggressive bullying.

The impact on memory and confirmation that brain inflammation is caused by the immune system are important new discoveries, said Professor Godbout, and could pave the way for immune-based treatments.

'It's possible we could identify targets that we can treat pharmacologically or behaviourally,' he said.