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How the brain knows when eating must stop.

At some point during a meal, the brain instructs the hand to put down the fork: The diner has had enough. The brain sends this message before all the food has left the stomach and entered the bloodstream. In probing what signals satiety, psychologists have identified a novel method by which the brain evaluates the contents of the gut.

The vagus nerve, which carries two-way communication between the gut and the brain, transmits distinctly different patterns of electric signals in response to carbohydrates and to protein in the gut, finds Gary J. Schwartz of Johns Hopkins Medical Institutions in Baltimore. These signals largely reflect gut wall contractions-mechanical motions that mix and grind up food, he reported at last week's Society for Neuroscience meeting in Washington, D.C.

There's another component of the vagus nerve's reaction to protein. Schwartz suspects that hormonelike peptides produced by the gut in response to food are responsible for amplifying the signals triggered by the motions of the stomach and small intestine.

Most scientists had assumed that nerves sample gut contents through receptors that, like taste buds on the tongue, directly discriminate specific classes of chemicals being eaten, Schwartz notes. His data from anesthetized rats now indicate that this gut-level "tasting" may rely instead on indirect cues from nonspecific features of the digestive system, such as gut wall motility. Because other stimuli can also elicit both these motions and peptide production, the brain apparently decodes what's eaten-and how much-from the distinctive pattern of the vagus nerve's response to each nutrient, together with other information the brain receives from the gut.

Most nutrient absorption occurs in the stomach and duodenum-the upper segment of the small intestine. As Schwartz and his team infused glucose, a sugar, or peptone, a protein, directly into the duodenum, they recorded both gut wall movements and the corresponding signals to the brain.

In terms of the intensity and timing of contractions, and the corresponding intensity and duration of the vagus nerve's electric firing, "2 calories of the protein produced about a 30 to 40 percent larger [and longer] response than did 2 calories of glucose," Schwartz found.

He suspects that part of the effect may be mediated by the mast cells of the immune system. Present throughout the gut, these cells "are almost the perfect, ubiquitous transducer," he explains. "They can change a mechanical, thermal, or chemical stimulus into a neural signal." Moreover, he notes, vagus nerve fibers winding throughout the fingerlike villi lining the duodenum "are in a perfect position to taste what's happening to those mast cells."

"These are wonderfully coherent, novel, and interesting results," says Gerard P. Smith of Cornell Medical Center in White Plains, N.Y. While "we knew the gut talks to the brain over these [vagus nerve] fibers, what no one before has shown us is how the fibers respond to nutrients by changing their neural firing."

Adds Jaak Panksepp of Bowling Green (Ohio) State University, this information may prove "very important in learning what terminates a meal." However, he notes that over a day or so, the body adjusts its calorie consumption based on past meals and current needs. Therefore, Schwartz's new findings, he says, may have small consequences for how animals adjust their 24-hour caloric intake.
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Title Annotation:vagus nerve responds differently to each nutrient to decode what and how much has been eaten
Author:Raloff, Janet
Publication:Science News
Article Type:Brief Article
Date:Nov 30, 1996
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