We've known for a while now that your mental health and your immune system are closely tied. But the connection is still full of mystery. In a new study, researchers looked at how inflammation may change the way one neurotransmitter works in your brain — and how that change may be involved in depression. For the study, published online today in Molecular Psychiatry, researchers took blood, cerebrospinal fluid, and plasma samples from 50 participants with depression to look for markers of inflammation. All participants also had brain scans done to look for any changes in areas of the brain associated with glutamate, a neurotransmitter which has been previously linked to mood disorders. The researchers found that those participants with markers of inflammation also had higher than normal levels of glutamate in those areas of the brain. Glutamate is necessary for your neurons to communicate with each other, so this might sound like a good thing. But, it actually throws that carefully balanced system out of whack, and may be messing with the way this circuit works. This finding suggests that inflammation may exacerbate depression by changing glutamate functioning. However, this research also doesn't prove that inflammation is always a part of depression all of the time. Instead, it suggests that looking at inflammation may be the key to sussing out which patients will do best on certain treatments. In particular, the results point us in the direction of ketamine, a drug that has recently gained a lot of attention as a fast-acting treatment for severe depression, which acts on glutamate receptors in the brain. Other research has clearly shown that people with depression often feel physical symptoms (e.g. headaches and backaches). And there's evidence to suggest depression is a risk factor for conditions like heart disease and cancer. Although we can't draw any sweeping conclusions based on this latest study, it's more than worth the effort to figure out how these major forces in our bodies are linked.