A slow dopamine signal from the gut teaches mice to like foods

Researchers at the University of California, San Francisco, report that a slow dopamine signal running from the gut to the brain teaches animals to like foods that make them feel good. Sci.News is a secondary report of the study by J.C.R. Grove and colleagues. Sci.News

The proposed mechanism starts in the digestive system. Sensors there detect nutrients such as sugar and fat and send signals toward the brain. The resulting dopamine signal builds after food reaches the gut and can persist for many minutes. Professor Zachary Knight, the study's senior author, told Sci.News that scientists know a great deal about the fast-acting dopamine pathways. He said the slower side of dopamine is much more of a mystery, and that people are much less conscious of it in daily life.

The mouse experiments were designed to separate nutrient effects from taste. The researchers delivered sugar water or a milkshake-like liquid directly into the animals' stomachs, bypassing taste. The nutrients set off a sustained release of dopamine in the basolateral amygdala, a part of the brain.

Two manipulations tested whether this signal does the teaching. When the scientists blocked it, the mice still ate normally but failed to learn that a new flavor meant calories. When they artificially stimulated the same dopamine fibers after the mice tasted a calorie-free flavor, the animals came to prefer that flavor.

Knight said the work shows that this slow dopamine system drives learning to like certain foods over time, and that the team can see where and how it happens. He offered coffee as an example. Once the brain associates the flavor of a favorite brew with the uplift from caffeine, he said, coffee starts to taste a lot better.

The team also looked at people. Sci.News reports that a reanalysis of an earlier PET study of 13 men, along with a new fMRI study of 38 adults, found nutrient-linked activity in the same region. In both mice and humans, the response was stronger on the left side of the brain, which surprised the authors.

Knight said the findings raise the possibility that food preferences could one day be adjusted. By understanding the steps between nutrient detection in the gut and learning in the brain, he said, it might be possible to fine-tune them to subtly change preferences for different foods.

He also raised GLP-1 drugs, which mimic a hormone produced in the gut and can dramatically change eating behavior. He said the team did not study them and that whether they affect the newly identified pathway is unknown. He suggested such drugs could potentially reduce nutrient-triggered dopamine signaling and make food less reinforcing over time.

The blocking and stimulating experiments were done only in mice, according to Sci.News's report of the Grove study, and the human data show activity in the same region rather than the same manipulation.