New research has linked COVID-19 infection with injury to the dopamine-releasing neurons in the brain. Researchers said this connection might explain long COVID symptoms like fatigue and brain fog.
The study was published Friday in the eBioMedicine journal and authored by scientists from the Center for Addiction and Mental Health in Canada. According to the center, it provides the strongest evidence thus far that COVID is associated with injury to these neurons.
Long COVID, a condition triggered by the viral infection that lingers for months or even years, was identified as an issue before the pandemic was even over. As of 2023, an estimated 18 million people in the U.S. were dealing with long COVID and the U.S. Centers for Disease Control and prevention continues to monitor the condition. Overall, more than 103 million people in the U.S. have been diagnosed with COVID since the start of the pandemic, based Johns Hopkins data.
According to the Center for Addiction and Mental Health, long COVID is estimated to affect 5% of the world population. Symptoms can be both persistent and sometimes debilitating, including memory problems and low mood.
“Despite its prevalence, no evidence-based treatments currently exist, largely due to limited understanding of the underlying brain pathology,” the center noted.
To study long COVID’s impact on the brain, the study authors used positron emission tomography (PET) brain imaging. With the PET imaging, they were able to “measure a well-established marker of dopamine neuron integrity in people with long COVID and healthy individuals.”
Dopamine is a neurotransmitter that we make in our brains, per the Cleveland Clinic. It basically works as a chemical messenger, communicating between cells in the brain and the rest of the body, and it also acts as a hormone. Body functions like movement, memory, pleasurable rewards, motivation, behavior and cognition, attention, sleep, arousal, mood, listening and lactation are associated with dopamine.
As a hormone, it plays a role in the fight-or-flight response and dopamine is also involved with vasodilation and vasoconstriction, insulin and urine removal, gastrointestinal movement and lymphocyte activity.
“Dopamine is known as the ‘feel-good’ hormone,” the Cleveland Clinic explained. “It gives you a sense of pleasure. It also gives you the motivation to do something when you’re feeling pleasure.”
The research team found significantly lower levels of the imaging marker across all major regions of the striatum structure in the brains of people with long COVID compared to healthy individuals. This structure plays a central role in motivation, movement and thinking. Lower levels of the imaging tracker indicate reduced dopamine nerve terminal density, and they seemed to form a symptom map.
In the ventral striatum – known as part of the brain’s “reward center” – lower imaging marks were associated with greater loss of motivation. In the dorsal putamen, an input-receiving structure involved neural circuits related to reward processing and procedural learning, lower markers were associated with slowed movement speed. In the caudate putamen, a centrally-location portion of the brain, marker loss was associated with memory difficulties.
“Our findings provide compelling evidence that long COVID involves the loss of dopamine-releasing neurons,” says Dr. Jeffrey Meyer, senior scientist at the Brain Health Imaging Centre, Canada Research Chair, and senior author of the study. “This kind of injury is well known to produce symptoms like lack of motivation and motor slowing, and may contribute to memory difficulties in other neurological conditions. Our results suggest a similar process is occurring in long COVID.”
These findings build on research the team previously conducted. That work “showed that people with long COVID have elevated levels of inflammation in the brain, especially in regions that are rich in dopamine-releasing neurons,” said the CAMH.
“We know that inflammation can injure dopamine neurons. While our earlier research showed high-levels of inflammation in those regions, this study provides direct evidence that the dopamine neuron marker is reduced in the same regions – and that this loss correlates with patients’ symptoms,” Meyer.
Going forward, this new understanding of long COVID opens up new options for treatment. Meyer said that the results indicate the condition is at least partly a disorder of the brain’s dopamine system, and medications that augment the function of dopamine-releasing neurons might be useful.
“For five years I have been seeking answers on what happened to me after I contracted COVID in 2021,” said Susan Deuville, lived experience research advisor to Dr. Meyer. “It was a crushing loss of the life I had and the person I was before. The research of Dr. Meyer brings hope. It also validates what long COVID sufferers have always known – long COVID is real and the effects are devastating.”
Next, Meyer’s team plans to launch a clinical trial focused on dopamine function. They hope to help with memory, motivation, and fatigue in people with long COVID in collaboration with University Health Network (UHN).
