July 19, 2005
Tuesday

Things are always bittersweet for a select group of taste buds that researchers have found do double duty in discerning what's sweet and what's bitter.

The secret lies in two different chemical messengers that send the brain different signals depending on what kind of substance lands on the tongue.

The same taste buds in the tongue that detect sweet also pick up on bitter, and researchers think they tell the brain the difference by using two different chemical messengers that pass along information to the nearest nerve connection.

In a report published online Monday by The Proceedings of the National Academy of Sciences, researchers from Ohio State University said it appears that two small proteins, called NPY (neuropeptide Y) and CCK (cholecystokin), have different effects on the same taste buds.

Taste buds are actually clusters of 50 to 100 cells in the tongue. Nerve fibers connect each bud to the brain, but only a few cells in each taste bud touch those nerves. It had been supposed that the cells that weren't on the trunk line still have some way of sending a signal to that fiber, but researchers hadn't been sure how that happened.

Understanding how distant cells still manage to get information to the brain advances both biology and potentially the ability to treat various brain disorders that disrupt those signals.

"We knew that many taste bud cells that have receptors for bitter stimuli lacked this connection to the brain, but we couldn't see how a cell could tell the brain when it was stimulated by a bitter taste," said Scott Herness, the study's lead author and a professor of oral biology and neuroscience at Ohio State.

Herness and his team several years ago had found that CCK is active in some taste bud cells, and seemed to be involved in sending signals to the brain. They now think that the CCK protein tells the brain that something bitter is on the tongue, while NPY sends a message that something sweet is being eaten.

"We were surprised to see that NPY had the exact opposite action of CCK,'' Herness said. "But this would ensure that the brain gets a clear message of what kind of taste is on the tongue."

For the current study, sponsored by the National Institutes of Health, the researchers experimented with taste bud cells taken from the rear of the tongues of rats - the back of the tongue has the highest concentration of taste buds.

After isolating single cells from individual taste buds, they attached very small, fine electrodes to these single cells in petri dishes to record the electrical activity in each cell. They also applied NPY to those cells and compared the resulting electrical signals given off compared to their earlier work with CCK.

"NPY activated a completely different signal than CCK did, suggesting that the peptides trigger completely different responses in individual cells," Herness said.

The researchers didn't consider how either taste affected the individual cells, although they plan to do such tests soon. But Herness speculates that CCK may override NPY when a bitter sensation is present, while NPY dominates when there's a sweet taste.

On the Net:

www.pnas.org

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