As animals eat, brain activity tells them when to stop. In a recent study, researchers looked at the role two different types of brain cells play in controlling how mice consume food. The results, they say, might tell us more about our own appetites.
Scientists found that signals from both the mouth and stomach triggered neuron activity, which dictated how quickly and how much the mice ate, according to the study published in November in the journal Nature.
Researchers already knew that a part of the brain called the caudal nucleus of the solitary tract (cNTS) is related to regulating eating. Previous studies of anesthetized animals and brain slices revealed groups of neurons that are important for satisfaction and for stopping animals from eating too much.
In the past, scientists have anesthetized animals and either inflated balloons in their stomachs or infused their stomachs with food to tangentially study the eating process, according to Science’s Catherine Offord. But when mice are anesthetized, they don’t show the same motor and sensory feedback that is created when they are awake and eating. Prior to the new experiment, the activity of the cNTS during eating had not been studied in awake animals, because it’s located too deep in the brainstem to easily access.
To get around this problem, the researchers developed a new strategy to monitor the cNTS when a mouse is awake. They implanted a light sensor in each rodent’s brain so that the neurons would release a fluorescent signal when they were triggered, writes Nature News. Scientists found that stimulating a group of brain cells called prolactin-releasing hormone neurons, or PRLH, limited food but not water consumption, confirming they are involved in eating.
When the team infused food directly into the stomachs of the mice—as scientists had done in past research—signals from the gastrointestinal tract activated the PRLH neurons, with activity ramping up during the infusion and peaking after the infusion ended. But when the mice ate food naturally, something different happened: The signals controlling the PRLH neurons came from the mouth instead, and the cells’ activity pattern changed, according to a statement from the University of California, San Francisco (UCSF).
Stimulating the PRLH neurons limited how much the mice ate, but only if they were actively eating, suggesting that these cells use signals from the mouth to regulate how quickly the mice eat, Zachary Knight, a co-author of the study and a neurobiologist at UCSF, tells Science.
“It was a total surprise that these cells were activated by the perception of taste,” says lead author Truong Ly, a neuroscientist at UCSF, in the statement.
Meanwhile, other brain cells called GCG neurons responded to signals from the gut over a longer timescale. These were activated even when the team infused food into the mice’s stomachs, and when the researchers stimulated these cells, the mice acted as though they were full and showed less interest in food.
These results show that signals from the mouth and gut activate different circuits and thus affect eating in different ways—PRLH cells may control eating speed, and GCG cells may monitor how much has been consumed.
“Together, these two sets of neurons create a feed-forward, feed-back loop,” Knight says in the statement. “One is using taste to slow things down and anticipate what’s coming. The other is using a gut signal to say, ‘This is how much I really ate. OK, I’m full now!’”
Knight and other researchers point out that additional brain regions may be involved in regulating food consumption, per Science.
The PRLH response may be helpful for preventing gastrointestinal distress from eating too quickly, the study authors write. Next, the researchers want to better understand how signals from taste and the gut interact.