Researchers are taking a closer look at evidence directly connecting inflammation with a lack of motivation.
A study from 2017, for instance, which measured immune response markers in the brains of 14 people with moderate or severe depression, suggested that individuals with suicidal thoughts also had clear signs of brain inflammation.
New research conducted by investigators at Emory University, in Atlanta, GA, now puts forward the idea that low-grade inflammation in the body may lead to a lack of motivation by curtailing dopamine, a hormone and neurotransmitter that plays an important role in the brain's reward circuit, which drives motivation-related behaviors.
In their study paper — featured in the journal Trends in Cognitive Sciences — the researchers suggest that inflammatory mechanisms prompt the release of less dopamine as a way of getting the brain to save energy and focus it toward healing the wound or infection that set off the inflammation.
"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."
Study co-author Michael Treadway, Ph.D.
"We now have strong evidence suggesting that the immune system disrupts the dopamine system to help the brain perform this recalibration," says Treadway, who is an associate professor in the Department of Psychology at Emory.
A theory with clinical implications
In their paper, the researchers review the evidence suggesting a direct link between inflammation and a lack of motivation, and they also propose a computational method of assessing the impact of chronic, low-grade inflammation on the amount of energy that the brain expends on behaviors driven by self-motivation.
The research is based on previous studies, which showed that some pro-inflammatory molecules (cytokines) can affect the way in which the dopamine system in the brain functions.
At present, the researchers explain, many people may experience chronic, low-grade inflammation, due to aging, stress, metabolic conditions, or unhealthful lifestyle habits. These factors may leave their marks on the adaptive mechanism that allows the brain to save energy in order to target the source of inflammation, with possible consequences for mental health.
"If our theory [about the link between inflammation and motivational impairment] is correct, then it could have a tremendous impact on treating cases of depression and other behavioral disorders that may be driven by inflammation," says study co-author Dr. Andrew Miller.
"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," Dr. Miller suggests.
However, "We're not proposing that inflammation causes these [mental health] disorders," Treadway clarifies. "The idea is that a subset of people with these disorders may have a particular sensitivity to the effects of the immune system, and this sensitivity could contribute to the motivational impairments they are experiencing," he explains.
Currently, the research team is at work on a clinical trial on depression, looking to verify the theory that they have suggested in their study paper.