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Scientists tackle questions of taste

By CHRISTINE DELL'AMORE, UPI Consumer Health Correspondent

SAN FRANCISCO, Feb. 16 (UPI) -- Why do some of us abhor broccoli or adore garlic? Scientists may be getting closer to figuring out what controls the human mechanism of taste.

It could be that taste bud cells, which communicate directly with nerve fibers and neighboring nerve cells in the gut, in turn dictate the sensitivity of our palates, researchers reported Friday.

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A type of neurotransmitter, called ATP, may be this link between taste and the nerves in the digestive system, Susan Kinnamon, a professor at Colorado State University in Fort Collins, told a panel at the American Association for the Advancement of Science meeting.

Taste buds are sensory, onion-shaped structures containing about 50 to 100 cells, with three types of cells present in each bud. The surface of the tongue is covered with these buds, which respond to bitter, sweet, salty and sour. Yet decoding the way taste buds work has long been a "big puzzle" to scientists, who are confused why taste cells don't communicate with the nervous system like every other sensory system, Kinnamon said. Other body processes employ glutamate -- a common neurotransmitter -- to interact with the nervous system, Kinnamon said.

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To test if the ATP transmitter is indeed the mysterious player, Kinnamon and colleagues stripped lab mice of their ATP and serotonin, a central nervous system neurotransmitter. The results, which were published in a 2005 issue of Science, found that mice without ATP receptors were not able to taste, but mice lacking serotonin receptors were. This leads Kinnamon to believe serotonin may act as an signaling aide for ATP as it talks with the nervous system.

Kinnamon also noted depressed people sometimes experience a depressed ability to taste. Because a lack of serotonin in the body accompanies depression, this finding poses an intriguing hypothesis: Serotonin may have a meaningful say in how the body perceives taste.

With that in mind, drugs that treat depression by increasing serotonin in the body -- SSRIs, for instance -- could have the potential to enhance taste sensitivity in depressed patients, Kinnamon said.

However, such research is still in its nascent stages, and it's too early to make any conclusions, Kinnamon said.

Even so, "this suggests that maybe the taste bud, the entrance to the gut, is actually involved in the sensitivity of taste," Kinnamon told United Press International.

Other research presented on taste research at the meeting suggest the ability to taste bitter foods may have shaped human evolution. For instance, scientists discovered in the 1930s that a person's ability to taste a bitter compound called phenylthiocarbamide, or PTC, was controlled by the presence of a gene.

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It's possible these taste perceptions could aid in regulating intake of certain foods, or even warn a person to avoid a food entirely, said Stephen P. Wooding, an assistant professor at the University of Texas Southwestern Medical Center in Dallas. For instance, early human ancestors who tasted bitter-tasting plants in turn avoided eating toxic foods.

Wooding has done research on why people who lack the gene for bitter taste are still around, as theoretically they would have been weeded out during natural selection. Yet his research found that even these "non-tasters," as he calls them, still have active PTC receptors, suggesting they do serve an unknown purpose.

And as for the "broccoli question" -- or why some people develop a likeness for bitter tastes -- Wooding said nothing is certain, although it could be the body is sensing the compound is beneficial.

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