The tongue's ability to differentiate between sweet and bitter tastes may actually reside in the same taste bud, report scientists.
The study by researchers at Ohio State University explains the discovery of a chemical messenger called neuropeptide Y (NPY) in taste bud cells.
Although researchers have long known that NPY is active in the brain and gut, this is the first study to show that it is also active in taste bud cells.
Taste buds (or lingual papillae) are small structures on the upper surface of the tongue that provide information about the taste of food being eaten.
The human tongue has about 10,000 taste buds and it is known that there are five taste sensations: sweet, bitter, and umami, which work with a signal through a G-protein coupled receptor. Salty and sour which work with ion channels.
Contrary to popular understanding, taste is not experienced on different parts of the tongue. Though there are small differences in sensation, which can be measured with highly specific instruments, all taste buds, essentially clusters of 50 to 100 cells, can respond to all types of taste.
"This finding gives scientists a deeper understanding of how the human brain may distinguish between different types of tastes," says Scott Herness, the study's lead author and a professor of oral biology and neuroscience at Ohio State University .
The current study builds on previous work by Hernessand his colleagues. A few years ago, the team found that another chemical messenger, cholecystokinin (CCK), is active in some taste bud cells.
They think that these two peptides - small proteins that enable cells to talk to one another - have different effects in the same cells.
CK may send opposite signals to the brain, depending on what kind of substance is on the tongue.
Given the current findings, Herness thinks that CCK 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," he said. "But this would ensure that the brain gets a clear message of what kind of taste is on the tongue."
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, adds Herness.
The relationship between NPY and CCK may provide the answer. Herness' laboratory was the first to find CCK in taste bud cells. These results suggested that CCK may tell other cells - those attached to nerve fibers that transmit messages to the brain - that a bitter taste was on the tongue.
In the current study, the researchers conducted their experiments on taste bud cells taken from the rear of the tongues of rats - the back of the tongue has the highest concentration of taste buds.
They isolated single cells from individual taste buds and attached very small, fine electrodes to these single cells in Petri dishes in order to record the electrical activity of each cell.
The scientists also applied NPY to these cells. Cells are like tiny batteries, as each has its own electrical charge.
They compared the resulting electrical signal given off by NPY to the findings from the earlier work on CCK.
"NPY activated a completely different signal than CCK did, suggesting that the peptides trigger completely different responses in individual cells," reports Herness.
The researchers also stained some of the cells in order to see whether or not both peptides were present. This procedure uses fluorescent light to let researchers actually see the peptides under a microscope.
They initially found that NPY is expressed in only a subset of taste bud cells. Yet every cell that expressed NPY also expressed CCK.
"It may be that these cells release both peptides when something sweet or bitter is on the tongue. CCK might excite the bitter taste and at the same time inhibit the sweet taste, so the bitter message gets to the brain," say the scientists.
In short, CCK may override NPY during a bitter sensation, while NPY may override CCK during a sweet sensation.
Sour and salty - the two other dominant tastes - seem to work in totally different ways than sweet and bitter, claim the researchers.
Future research will focus on applying bitter and sweet stimuli to taste bud cells that contain both NPY and CCK, and see how each peptide reacts.
Full findings are published in this week's online edition of the Proceedings of the National Academy of Sciences.