The strange link between depression, motivation, and inflammation just became a little less hazy. New research from scientists at Emory University has found that low-grade inflammation appears to have a direct effect on the brain’s feel-good HQ, the dopamine system.

Reported in the journal Trends in Cognitive Sciences, the findings suggest that inflammation and its response in the immune system could underlie the sense of lack of motivation often experienced among people with depression, schizophrenia, and other behavioral disorders.

If you're sick or injured, your body has developed a mechanism to help you conserve energy for recovery, hence why you feel drained of energy and everyday life is a bit of struggle if you catch a cold. This appears to be done by recalibrating your dopamine system and reducing levels of the “feel good" neurotransmitter to dampen your motivation.

"When your body is fighting an infection or healing a wound, your brain needs a mechanism to recalibrate your motivation to do other things so you don't use up too much of your energy," corresponding author Michael Treadway, an associate professor in Emory's Department of Psychology, said in a statement. "We now have strong evidence suggesting that the immune system disrupts the dopamine system to help the brain perform this recalibration."

Based on a theoretical framework and computational models, the researchers on this project theorize that chronic inflammation sparks this response in the body. They argue that the immune system’s cytokines, proteins used to signal the location of the inflammation, also have this effect on the dopamine system, and subsequently influence psychological motivation.

Effectively, you experience the “down in the dumps” feeling you get when you fall ill, without the underlying illness.

It's thought this mechanism developed among our ancestors when our environments were more immediately dangerous, rife with parasites and potential predators. However, in the 21st century, this mechanism can also be provoked by low-grade inflammation from lifestyle factors, such as chronic stress, obesity, or a lack of exercise. 

"If our theory is correct, then it could have a tremendous impact on treating cases of depression and other behavioral disorders that may be driven by inflammation," added Andrew Miller, William P. Timmie Professor of Psychiatry and Behavioral Sciences in Emory's School of Medicine. "It would open up opportunities for the development of therapies that target energy utilization by immune cells, which would be something completely new in our field."