Does Inflammation Cause Depression?

Some researchers think it plays a role:

Studies have shown that people with depression or bipolar disorder, both those who had a physical illness and those who were medically healthy, had higher levels of inflammation. And as the depression faded, so, too, did the evidence of inflammation. Similarly, a 2009 study showed that mice that with chronic inflammation showed depressive symptoms, but blocking a key inflammatory enzyme alleviated the downer behavior in the mice.

The big discovery has been that depressed patients who have proven most resistant to traditional treatments (such as therapy or selective serotonin re-uptake inhibitor drugs) seem to have particularly high rates of inflammation. And in studies from the last few years, inhibiting inflammatory cytokines (signaling cells found in both the immune and nervous systems) seems to help alleviate depressive symptoms. Miller said that these results suggest that "cytokines might have an effect on fundamental dopamine synthesis," an important chemical process that, if thrown out of whack, can have big impacts on mood, energy and motivation.

I have also encountered speculation that the enormous colonies of bacteria we carry around may influence our mood and behavior. The irrational states of mind that trouble us may one day be explained, and even controlled.