14 DAY TRIAL // ATP is a molecule typically linked to processing energy in cells and more recently found to boost muscular growth. That's why researchers were surprised to find that this molecule is behind the way we feel the taste of food.
When food molecules touch the taste buds on tongue, mouth cavity and pharynx, the bud cells trigger signals transmitted to nearby taste nerves that, in turn, send the information to the brain, which differentiates between sweet, salty, sour, bitter, or umami (protein taste). But in this circuit, the neurotransmitter that passed information from taste buds to the nerves was not known. Most of the candidate molecules, like norepinephrine and serotonin, failed the trials.
A team at Colorado State University in Fort Collins, led by Sue Kinnamon, based their trial on the fact that ATP is a neurotransmitter in other body processes, like passing blood-oxygen levels data from carotid bodies to nerves.
"Because both carotid bodies and taste buds detect chemicals, the team wondered whether ATP might be the mystery neurotransmitter," said co-author Leslie Stone-Roy.
First, the team tested taste buds removed from normal mice: when stimulated with flavored solutions, the sensory cells released ATP. Next, the team engineered mice that lacked receptors in cells containing ATP.
Electrodes attached to the mice' gustatory nerves showed reactions to touch and various flavors. The nerves in the engineered mice reacted normally to touch, but were insensitive to tastes. Of course, in normal mice the buds were sensitive to both touch and tastes.
In another experiment, the mice were placed in cages equipped with two water bottles. One bottle contained drinking water, the other water with various flavors. Normal mice preferred the flavored water; those lacking ATP in the taste buds did not, drinking similarly from both the flavored and unflavored bottles.
"From the 1960s to the present, scattered papers have suggested a number of different candidates for a taste neurotransmitter, but nobody was talking about ATP. It wasn't even on the radar screen," said Scott Herness, a taste researcher at Ohio State University in Columbus.
But he warns that many other neurotransmitters have been discovered in the taste buds.
"The continuing question will be to determine what are these neurotransmitters' roles. They may convey information among the cells that make up each bud."