The human tongue can detect five basic tastes: sweet, sour, bitter, salty, and umami. Now, researchers from California Institute of Technology have found a sixth sense that can detect water.
"The tongue can detect various key nutrient factors, called tastants — such as sodium, sugar, and amino acids — through taste," says lead researcher Yuki Oka, from the California Institute of Technology. "However, how we sense water in the mouth was unknown. Many insect species are known to 'taste' water, so we imagined that mammals also might have a machinery in the taste system for water detection."
Oka measured the electrical responses from taste receptor cells on the tongues of mice when in contact with pure water and other tastes. The study, published by Nature Neuroscience, found that the taste nerves responded to the five basic tastes and also to water.
"This was exciting because it implied that some taste cells are capable of detecting water," says team member Dhruv Zocchi in a statement.
Then the researchers went in to see how mice responded to various flavors when different taste receptor cells were inactivated. They found that when the salty taste receptor was blocked, salt activity was diminished while other activity was fine. But when they blocked all five taste receptor cell types, the acid-sensing sour cells were activated in response to water.
"To our surprise, when we silenced sour taste cells, water responses were also completely blocked," Oka says. "The results suggested that water is sensed through sour taste cells."
Oka's team them used a technique called optogenetics to activate cells. The goal was to see if they could get mice to drink water without having any real water. They were successful.
The "why" is yet to be determined, but Oka believes the pH levels within cells is altered when water swishes around with saliva and this causes the cells to fire.