Scientists at UC San Francisco have identified a dopamine pathway connecting the digestive system to the brain that explains how people learn to enjoy initially unpleasant flavors like bitter coffee through repeated exposure.
The research, published November 4 in Nature, focuses on a slower dopamine system that responds to information from inside the body after food is consumed. This differs from the rapid dopamine signals that occur within seconds of seeing something rewarding.
When food enters the digestive tract, specialized sensors detect nutrients including sugar and fat. Information about those nutrients travels to the brain and produces a dopamine response that develops gradually over several minutes, according to Zachary Knight, PhD, professor of Physiology and senior author of the study.
Knight and graduate student James Grove introduced nutrients directly into the stomachs of mice while monitoring dopamine activity. They used either sugar water or a liquid resembling a milkshake, allowing them to separate nutrient effects from the experience of tasting food. The results revealed a strong, prolonged dopamine response in a brain region called the anterior basolateral amygdala, or BLA.
When researchers interrupted this pathway in mice, the animals lost the ability to learn that an unfamiliar flavor was associated with receiving calories. When they artificially activated dopamine fibers in the BLA after mice tasted a flavor containing no calories, the animals developed a preference for that flavor despite receiving no nutritional reward.
Brain imaging studies in humans revealed a similar pattern. Dana Small, PhD, a psychologist at Yale University, analyzed data from a positron emission tomography study of 13 men and found dopamine release was especially concentrated in the left side of the BLA after nutrient delivery.
A separate fMRI study led by Marc Tittgemeyer, PhD, examining 38 adults after they consumed glucose found increased activity in the same brain region. Both mouse and human studies showed the dopamine response was considerably stronger in the left BLA than the right, surprising researchers because brain networks for essential functions like feeding are typically symmetrical.
Knight suggested the findings could eventually help change which foods people find appealing. He noted that the dopamine pathway discovered might explain some effects of GLP-1 medications, which mimic a hormone naturally released by the gut and can alter appetite and eating behavior. However, Knight's team did not investigate whether GLP-1 drugs influence the identified dopamine pathway in their study.
"By understanding the steps between nutrient detection in the gut and learning in the brain, it might be possible to fine-tune them to subtly change our preferences for different foods," Knight said.