Study links brain inflammation triggered by chronic pain to anxiety and depression -- ScienceDaily

In work with rodents, Catherine Cahill, associate professor of
anesthesiology & perioperative care at UCI, Christopher Evans of
UCLA's Brain Research Institute, and colleagues discovered that
pain-derived brain inflammation causes the accelerated growth and
activation of immune cells called microglia. These cells trigger
chemical signals within neurons that restrict the release of dopamine, a
neurotransmitter that helps control the brain's reward and pleasure
centers.