A slow trickle of dopamine that starts in the gut and lands in a small patch of the amygdala is what lets the brain learn to like a flavor, according to a study published Nov. 4 in Nature. University of California, San Francisco researchers traced the signal in mice and then found a matching hotspot in human brain data. Zachary Knight, a UCSF professor of physiology and the paper’s senior author, reaches for black coffee when he explains why the finding matters.
Save more, waste less: The Cool Down’s newsletter sends smart, simple changes that help you and the planet in your everyday life.
We may earn a commission if you sign up.
The experiments themselves used sugar water and a milkshake-like liquid, not coffee or caffeine. Knight’s coffee example is his reading of the mechanism, and the paper did not test it directly.
Dopamine that builds over minutes rather than milliseconds
Most dopamine research has concentrated on fast bursts set off by sights, smells and tastes. The Nature paper describes a different tempo: after nutrients reach the stomach, dopamine release climbs over several minutes and can persist for a long while. Knight said this slow system is what drives people to learn to like certain foods over time. In mice the release landed mainly in the anterior basolateral amygdala, a region tied to emotion and learning, rather than in the striatal targets that dominate older dopamine studies.
James Grove, a former UCSF graduate student and the study’s first author, and colleagues delivered nutrients through a catheter straight into the stomach so that taste could not contribute. A preprint version of the work from the Knight lab reports that the gut sensors involved match those already known to drive flavor-nutrient learning: SGLT1 for sugars, and GPR40, GPR120 and CD36 for fat. Dopamine neurons in the ventral tegmental area that make cholecystokinin turned out to be needed for sugar learning, though not for learning to avoid a flavor paired with illness.
Mice that could no longer link a flavor to calories
Two manipulations carry the causal argument. When the researchers interrupted the pathway, eating stayed normal, but mice could no longer learn that a new flavor predicted calories. In the reverse experiment they activated dopamine fibers in the amygdala just after mice tasted a calorie-free flavor, and the animals came to prefer it, as if the stimulation had stood in for food. The ScienceDaily account of the UCSF release presents both results as evidence that the pathway teaches rather than drives appetite. The preprint adds that the stimulation changed neither total intake nor avoidance behavior.
The conditioning protocol was long and deliberate. According to the preprint, mice drank a flavored solution during one-hour sessions across nine days, and each lick triggered an infusion of 8% glucose or water into the stomach. Activity was tracked with fiber photometry and miniature microscopes that read out dopamine and calcium signals. Neurons in the anterior amygdala that carry the D1 dopamine receptor were found to combine sweet taste with the nutrient signal, and silencing them prevented the flavor learning altogether.
A left-sided hotspot in 13 men and 38 adults
The human evidence came from collaborators rather than from new recordings in Knight’s lab. Dana Small, a Yale psychologist, reanalyzed an earlier PET study of 13 men who drank either a milkshake or a drink without calories and found dopamine release concentrated in the left basolateral amygdala. Marc Tittgemeyer and colleagues ran a separate fMRI study of 38 adults and saw more activity in the same region after they consumed glucose. The UCSF news release distributed through EurekAlert describes both datasets.
In both species the nutrient response was considerably stronger on the left than on the right, a lopsidedness the researchers did not expect because circuits for basic functions such as feeding are usually assumed to be symmetrical. The preprint reports that cutting the vagus nerve on one side reduced responses by about half on either side without producing the asymmetry, which points away from the vagus as the source. Why the left side is favored remains unexplained.
The human data carry limits worth stating. The PET reanalysis involved 13 men, and the fMRI cohort of 38 adults was a separate experiment, so no single group of people was followed from nutrient delivery to a learned preference. The mouse experiments supply the causal tests, while the human datasets show that the same left-sided region responds when nutrients arrive. Matching locations across species is suggestive, and it falls short of showing that people come to like coffee through this circuit.
Coffee, caffeine and an untested GLP-1 idea
Knight’s coffee illustration runs like this: bitterness is an evolutionary warning, but once the brain links a particular brew with the lift from caffeine, the drink starts to taste a lot better. Neuroscience News quotes him on that point, and the logic matches the mice, where a neutral flavor became attractive after it was paired with nutrients. Caffeine was not one of the delivered stimuli, so the bitter-coffee case remains an extrapolation from calorie learning.
Bitterness is a useful test case because the brain treats it as a warning. Knight’s point, relayed in the UCSF account, is that experience can override that warning when the flavor reliably arrives with something the body values, whether calories or a caffeine lift. The same logic could explain why people acquire a taste for strong cheese, dark beer or bitter greens, though none of those foods were part of the experiments.
Knight also speculated that GLP-1 weight-loss drugs might work partly by dampening these gut-triggered dopamine signals, which could make foods less rewarding over time, according to a News-Medical reprint of the UCSF account. His team did not test any GLP-1 drug. The work was supported by National Institutes of Health grants, including R01-DK106399 and R01-DK138127, and the open question for the next round of experiments is whether the same slow signal can be shown to shape a human preference for a bitter drink.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
Save more, waste less: Subscribe to The Cool Down →
We may earn a commission if you sign up.
More from Morning Overview
- A handful of car transmissions are so tough that mechanics say they almost never die
- Melting permafrost is setting off a newly identified kind of catastrophic landslide, scientists say
- Cadillac cut the price of its cheapest EV, which goes up to 317 miles on a charge
- Winter weather advisories cover I-80, I-70 and the Bighorns with up to 15 inches of snow
