We don’t think about the taste of water very much, despite the fact that we’d have been dead long ago without a way to sense the substance that makes up 50 to 60 percent of our bodies. There is something, somewhere, in the mouth that tells us we are drinking it. Mouse research has previously indicated that drinking water triggers the firing of nerves that ferry taste information from the mouth to the brain. And now, scientists publishing in Nature Neuroscience have pinpointed a specific set of taste cells in mice that cause these nerves to fire. Surprisingly, they are cells we are already quite familiar with.
The researchers began by testing mice that each lacked the ability to sense one of the five basic tastes: sweet, bitter, umami, salty, and sour. Some mice had been genetically engineered to lack a protein important to their missing taste; others were normal but had been treated with a drug that kept certain taste cells from functioning, or exposed to a toxin that destroyed their function. The idea, says Yuki Oka, a professor at the California Institute of Technology who led the study, was that if any of the hampered taste cells were responsible for sensing water, the taste nerves would no longer fire when researchers gave the mice a drink. But they didn’t actually expect that to happen: They had hypothesized that there would turn out to be another, as-yet undiscovered group of cells doing the job.
When Oka and his graduate student saw the the silent nerves of mice who could not taste sour, they thought it was some mistake. Their sour taste cells had been impaired, yes, but why that would make their nerves not register the arrival of water was puzzling. The team devised a new experiment to look deeper, engineering mice whose sour cells could be activated by blue light. They replaced the normal water bottles in the mouse cages with waterless bottles with lasers that would shine on the tongues of the mice when they tried to drink, and settled down to watch what happened.
First, the mice were deprived of water for a bit so they would seek it out. Then, when they had access to the laser-shooting bottles, they went over to get a drink. When the blue light hit their sour cells, their taste nerves lit up and looked for all the world as if the mice were really drinking water. And their thirst, sure enough, kept driving them back to get a little more blue light. The effect was real.
Here’s why this works, the researchers believe: The cells we know as sour taste cells are probably more accurately thought of as sensors of pH. The acid pucker of lemons sets them off, but so too does the arrival of water. It turns out, the team found, that there is an enzyme attached to the outer surface of sour taste cells that slices up water molecules, producing a sudden rush of hydrogen ions and a drop in pH. The drop activates a pH-sensitive channel in the membrane that causes the cell to send a message to the brain, announcing, “Water’s in the mouth.”
So why does sour taste sour and water taste like water, if they’re using the same cells to communicate to the brain? Perception involves far more than just a button being pushed in the mouth, says Oka. Case in point: The mice who drank light, though it looked like they were getting water as far as their taste cells were concerned, still drank incessantly, as something in their brains told them, no, you are not really hydrated yet. There are inputs other than the activation of taste cells.
In an earlier set of experiments, Oka and colleagues identified a set of neurons in the mouse brain that, when triggered, make mice want to drink water. “Now the next question is how do they interact with each other,” he says, speaking of the sensors in the mouth and the neurons in the brain. That interaction may be key to understanding why it is we thirst, and how we know we’re getting the hydration we need.
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