The human tongue is like an alien landscape with thousands of tiny taste-buds that sense five basic tastes: sweet, salty, sour, bitter, and umami (a savoury taste sensation produced by glutamates).

Scientists have now discovered though that mammalian taste buds may have an additional sixth sense – for water. This could explain how animals can tell water from other fluids, and adds fuel to the debate regarding whether water has a taste of its own or is merely a vehicle for other flavours.

Philosophers have always claimed that water has no flavour; even Aristotle referred to it as “tasteless” around 330 B.C.E. amphibians and insects however, are known to have water-sensing nerve cells, and now there is evidence that mammals may have similar cells.

Patricia Di Lorenzo, a behavioural neuroscientist at the State University of New York in Binghamton says that some new recent brain scan studies suggest that a region of human cortex responds specifically to water. Critics disagree, saying that any perceived flavour is just the after-effect of whatever was tasted earlier, such as the sweetness of water after we eat salty food.

The study, which was published in Nature Neuroscience, posits that although signals from TRCs in the tongue can trigger drinking, they don’t play a role in telling the brain when to stop.

According to Zachary Knight, a neuroscientist at the University of California, San Francisco, virtually nothing is known about the molecular and cellular mechanism by which water is detected in the mouth and throat, and the neural pathway by which that signal is transmitted to the brain.

Knight and other researchers found distinct populations of neurons that can trigger thirst and signal when an animal should start and stop drinking, within an area of the brain called the hypothalamus. This means that the information about water must go from the mouth and tongue to the brain because animals stop drinking long before the blood or gut could tell them to.

Results of the study conducted on mice suggest that although signals from water-sensing taste receptor cells (TRCs) in the tongue can trigger drinking, they don’t play a role in telling the brain when to stop.

More research is needed, but Di Lorenzo says, “When you find a counterexample to the dominant view that there are only five basic taste groups, it tells you you need to go back to the drawing board.”

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