The tongue's ability to differentiate between sweet and bitter tastes may reside in the same taste bud cells, a new study reports.
The study explains the discovery of a chemical messenger called neuropeptide Y (NPY) in taste bud cells. Though 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.
That finding gives scientists a deeper understanding of how the human brain may distinguish between different types of tastes, said 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 Herness and 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 let cells talk to one another - have different effects in the same cells.
The researchers report their findings in this week's online edition of the Proceedings of the National Academy of Sciences.
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."
Taste buds are really clusters of 50 to 100 cells. Nerve fibers connect each bud to the brain, but only a few of the cells in each taste bud touch these fibers. The prevailing thought was that cells that don't have a connection to a nerve fiber must have some way of sending a signal to that fiber. But researchers weren't sure how that happened.
"We knew that many taste bud cells that have receptors for bitter stimuli lacked this connection to the brain," Herness said. "But we couldn't see how a cell could tell the brain when it was stimulated by a bitter taste."
The relationship between NPY and CCK may provide the answer. A few years ago, 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. They attached very small, fine electrodes to these single cells in Petri dishes in order to record the electrical activity of each cell. They 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 what they had found in 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," Herness said.
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.
"That surprised us, too," Herness said. "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."
Although the researchers did not examine how either taste affected individual cells (they plan to do that next), Herness thinks that 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, Herness said.
The researchers would eventually like to figure how these work, too, but for now their next step is to apply bitter and sweet stimuli to taste bud cells that contain both NPY and CCK and see how each peptide reacts.
Herness conducted the study with Fang-li Zhao, a postdoctoral researcher in dentistry at Ohio State; Yu Cao, a graduate student in neuroscience also at Ohio State; and Tiansheng Shen and Namik Kaya, both with the University of Maryland.