Have you ever wondered why water is tasteless but we can still distinguish water from many other liquids?
When viewed under a microscope, our tongue resembles the surface of an alien planet, rough with taste buds.
The tongue perceives five basic tastes: salty, sour, sweet, bitter, and umami (meat sweetness). But perhaps the taste buds in mammals also have the ability to sense a sixth taste – the taste of water , thereby adding a voice to the centuries-old debate: whether water has a taste of its own. Or is it simply an environment for other tastes?
Insects and amphibians have nerve cells that sense water. Patricia Di Lorenxo, a behavioral neuroscientist at Binghamton University (a part of the State University of New York system), says evidence is gradually being established that there are similar cells in mammals. breast.
In particular, several recent brain-scanning studies suggest that an area of the human cerebral cortex has a special ability to respond to water.
The cells contain sour taste receptors that are thought to help us perceive the taste of water – (Image Istockphoto).
In previous studies, Zachary Knight, a neuroscientist at the University of California, San Francisco, and colleagues found that a specific amount of neurons in the hypothalamus produce the sensation of thirst, while also signaling signal when the animal starts or stops drinking water.
However, the brain must have received information about the presence of water from the mouth and tongue, because when the animal stops drinking for a long time, the signal from the intestines or from the blood tells the brain that the body thirst is gone.
In an effort to solve the question of whether the human tongue can sense the taste of water, Yuki Oka – a neuroscientist at the California Institute of Technology in Pasadena – and his colleagues looked for cells containing water receptors. in the tongue of the mouse.
They used mutant mice, then turned off certain types of cells containing taste receptors in turn, then put water in the mice’s mouths to see which cells would respond.
One surprise was that the sour taste receptor was strongly reflected when the rats drank water. In addition, when mice were given a choice between water and an odorless, colorless artificial silicone oil, mice lacking sour taste receptors took longer to choose a glass of water. That suggests these cells help distinguish water from other liquids.
The team then tested the cells containing the sour taste receptors to see if this made the mice want to drink water. They bred multiple mice that had a light-sensitive protein located in the cells containing its sour-tasting receptor, making the cells active in response to laser light.
After training the mice to drink water from a faucet, the team replaced it with an optical tube that emitted blue light. When rats “drink” this blue light, they acted as if they were drinking real water. Some have even licked the light 2,000 times in 10 minutes to quench their thirst.
The rats didn’t know that the light was an artificial source of water, but they drank for longer than usual. This suggests that although the sour taste receptor in the tongue may make people thirsty, it does not play a role in telling the brain when to stop.
More research is needed to determine exactly how the cells containing the sour taste receptors respond to water, and specifically what those mice perceive when drinking the water. But Yuki Oka guessed that the water pushed away saliva – a mucus that tastes sour and salty – and changed the pH of the sour-tasting cells, making them more active.
The results of this study are published in the journal Nature Neuroscience.