Negative emotional states and physical pain are intimately connected. Numerous people who suffer from chronic, persistent pain also deal with negative emotions and loss of motivation. Some even become clinically depressed eventually, and doctors sometimes prescribe antidepressants to treat chronic pain, even though the pathways that link pain and mood are poorly understood.
Now, studying the brains and behavior of rats, a team of researchers at Washington University School of Medicine in St. Louis has identified circuitry in the brain that appears to link pain to negative emotional states, specifically involving inhibited motivation and anhedonia. Anhedonia is defined as the inability to experience pleasure from rewards; inhibited motivation refers to a lack of interest in earning rewards. The researchers measured these behaviors in the rodents, monitoring their sucrose consumption and their willingness to press a lever to earn a sucrose tablet.
The findings, published online Oct. 18 in the journal Nature Neuroscience, could lead to treatments targeting the emotion-related aspects of pain. Such developments ideally would allow chronic pain sufferers to experience a better quality of life, even when it’s not possible to completely eliminate pain itself.
“Pain involves more than just unpleasant physical sensations; it has an emotional component, too,” said Jose Moron-Concepcion, PhD, one of the study’s co-senior investigators. “Most research has sought to understand and eliminate pain’s sensory component, while its emotional component and associated co-morbidities such as depression, anxiety and the inability to feel pleasure largely have been ignored. By understanding the whole picture, we might make it easier for people to live and cope with pain.”
He explained that there is a sensory component to pain, which is the unpleasant physical sensation people feel, and there is an affective component to pain, which is the negative emotional component of pain, particularly persistent pain. Anhedonia is a frequent aspect of negative emotional pain and can, for example, increase a person’s vulnerability to opioid use disorder.
Moron-Concepcion, the Henry E. Mallinckrodt Professor of Anesthesiology, led the study at the Washington University Pain Center with co-senior investigator, Meaghan Creed, PhD, assistant professor of anesthesiology, of psychiatry, of neuroscience and of biomedical engineering. Their goal was to uncover the brain circuitry involved in the relationship between pain and related changes in one’s motivational state.
Their team was able to identify specific dopamine neurons in a brain region called the ventral tegmental area that decrease their activity in the presence of pain. Alterations in dopamine activity in this brain region also are associated with motivation. And the brain area where these neurons are located is part of a pathway linked to reward-motivated behavior, suggesting that changes in the dopamine neurons along this pathway may be important in mediating the negative emotional states associated with pain.