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People who suffer from severe clinical depression tend to have brains that are 30 percent more inflamed than healthy brains, according to a new study by the Centre for Addiction and Mental Health (CAMH) in Canada. The findings may lead to new depression treatments that target brain inflammation.

“This finding provides the most compelling evidence to date of brain inflammation during a major depressive episode,” said senior author Dr. Jeffrey Meyer of CAMH’s Campbell Family Mental Health Research Institute. “Previous studies have looked at markers of inflammation in blood, but this is the first definitive evidence found in the brain.”

Specifically, the researchers were able to measure the activation of immune cells, known as microglia, that play a key role in the brain’s inflammatory response. They used a brain imaging technique called positron emission tomography (PET) to conduct brain scans on 20 patients with depression (but otherwise healthy) as well as 20 healthy control participants.

Results showed a significant elevation of brain inflammation in participants with depression; levels of inflammation were highest among those with the most severe depression.

Although the process of inflammation is one way in which the brain protects itself (similar to the inflammation of a sprained ankle), too much inflammation can be damaging. A mounting body of evidence suggests that brain inflammation may generate the symptoms of depression, such as low mood, loss of appetite, and inability to sleep.

But what was previously unclear was whether inflammation played a role in clinical depression independent of any other physical illness.

“This discovery has important implications for developing new treatments for a significant group of people who suffer from depression,” says Meyer, who also holds a Canada Research Chair in the neurochemistry of major depression. “It provides a potential new target to either reverse the brain inflammation or shift to a more positive repair role, with the idea that it would alleviate symptoms.”

The desire to find new ways to treat depression is driven by the reality that more than half of people with severe depression do not respond to antidepressant treatments and four percent of the general population is in the midst of a clinical episode. Current treatments do not target inflammation, and treating depression with anti-inflammatories is one avenue for future research, said Meyer.

“Depression is a complex illness and we know that it takes more than one biological change to tip someone into an episode,” said Meyer. “But we now believe that inflammation in the brain is one of these changes and that’s an important step forward.”

The findings are published in the journal JAMA Psychiatry.

Source: Centre for Addiction and Mental Health