Water may not be a tasteless liquid, say scientists who have found that our tongue is able to detect a unique taste of water using its sour-sensing cells. Taste cells relay information about tastants to the brain via nerves called the taste nerves. Researchers measured the electrical responses from taste nerves in mice to various tastants as well as to water. The nerves responded in predictable ways to different basic tastes – sweet, sour, bitter, salty and umami – but they were also stimulated by pure water. “This was exciting because it implied that some taste cells are capable of detecting water,” said Yuki Oka, assistant professor at the California Institute of Technology in the US.
Each basic taste is mediated by distinct subsets of taste cells. In order to test which taste cells respond to water, the team genetically and pharmacologically blocked the function of individual cell populations. For example, when the salt taste receptor was blocked, salt no longer triggered activity in taste nerves, but responses to other tastes were not affected. “When we silenced sour taste cells, water responses were also completely blocked. The results suggested that water is sensed through sour taste cells,” Oka said.
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To prove that the sour cells indeed contribute to water detection, the team used a technique called optogenetics that allowed them to stimulate sour cells with light instead of water. The researchers removed water from the animals’ water bottle and made it so that the bottle’s spout emitted a blue light when the animals touched it.They discovered that thirsty genetically engineered mice would go to the spout for water, encounter the light and “drink” it. Though the mice were not rehydrated, they kept licking the water source because the light created a sensory cue of water.
A sour taste is often associated with an unpleasant taste quality that reduces animals’ preference toward fluid – for example, mice avoid drinking lemon juice. When the team stimulated sour cells with light, they did not observe that kind of aversive behaviour in the engineered mice. “Maybe sour cells are not directly linked to the unpleasant sourness that we perceive, but instead they may induce a different type of taste, like water, when stimulated,” said Oka. The study was published in the journal Nature Neuroscience.