Our ability to distinguish between sweet and bitter tastes may reside in the same taste bud cells, say US scientists.
The tongue can distinguish five main tastes
Cells in our tongues sense four basic tastes - sweet, salty, sour and bitter - plus salts of certain acids.
But scientists did not understand the mechanism used by these cells to convert stimulation into taste.
Chemical messengers are key, the Ohio State University team told Proceedings of the National Academy of Sciences.
By studying rats they found two chemical messengers - one for sweet and one for bitter tastes - present in the same taste buds found at the back of the tongue.
Scientists found the chemical for bitter taste, called CCK, a few years ago.
This is the first time that the chemical for sweet taste, called neuropeptide Y (NPY) has been found in the tongue's taste buds.
How we taste
Taste buds - the tiny bumps on our tongues - are clusters of 50 to 100 cells.
Nerve fibres connect each bud to the brain to send signals about taste, but only a few of the cells in each bud touch these fibres.
Therefore, scientists had thought that cells that did not have a connection to a nerve fibre must have some other way of sending a signal.
Dr Scott Herness and his team believe the chemical messengers CCK and NPY are the answer.
CCK may tell neighbouring cells that are attached to the nerve fibres that a bitter taste is on the tongue, while NPY will tell them that the taste is sweet, they said.
In their study, they isolated taste bud cells from the tongues of rats and attached very small electrodes to these single cells.
They then compared the resulting electrical signals given off when NPY was applied with those given off with exposure to CCK.
NPY activated a completely different signal than CCK did, suggesting that they trigger completely different responses in the same individual cells.
Dr Herness said: "We were surprised to see that NPY had the exact opposite action of CCK.
"But this would ensure that the brain gets a clear message of what kind of taste is on the tongue."
When they stained the cells to show up whether they contained NPY or CCK, they found some contained both.
Dr Herness said: "It may be that these cells release both peptides when something is sweet or bitter 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."
He said their next step would be to see how either taste directly affects individual cells.
Professor Tim Jacob, an expert in smell and taste at Cardiff University, said the work was very interesting.
"CCK and NPY are acting as a push-pull mechanisms for sweet-bitter tastes."
He said these chemicals might be released by the same cell into the outside spaces surrounding taste receptor cells rather than into the narrow gap that cells normally use to communicate, which is called the synaptic cleft.
"Classical synaptic transmission may have to be revisited," he said.