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When you are thirsty, a small amount of fresh water provides instant relief. But swallow saltwater and you will always feel dry.
That's because your brain is trying to maintain salt concentration in your body in a very narrow range, says Zachary Knight, an badociate professor of physiology at the University of California at San Francisco and a researcher at Howard Hughes Medical. Institute.
"If you meet, for example, a change of 10%, you will be very sick," he says. "A change of 20% and you could die."
Knight and a team of researchers wanted to know how the brain prevents this from happening. They report the results of their research in an article published Wednesday in the journal Nature.
"There has to be a mechanism for the brain to determine how salty the solutions you're drinking are and use them to refine your thirst," says Knight. "But the mechanism was unknown."
The Knight team has therefore begun to study brain cells called neurons of thirst.
First, the team injected fresh water directly into the throats of thirsty mice.
"In the space of a minute or two, infusing water into the stomach quickly extinguishes these thirsty neurons in the brain," says graduate student Chris Zimmerman. from the Knight Lab who conducted the experiment. "And not only that," says Zimmerman, "if we give [the mouse] access to the water, he does not drink at all. "
The team then repeated the experiment using salt water. And this time, the thirsty neurons stayed on and the animals kept looking for fresh water that would reduce the salt concentration in their bodies.
Further study revealed the functioning of the system. The cells in the intestine constantly measure salinity and communicate this information to thirsty brain neurons.
"What's really exciting about this, is not only that we have discovered this new signal from the digestive tract to the brain, but also that we have discovered that it plays a very specific role in the control of our behavior, "says Zimmerman.
A second study in Nature examines a different system that also affects salt intake.
"We wanted to know how appetite for sodium is regulated by the brain," says Yuki Oka, an badistant professor of biology at Caltech and author of the study.
The Oka team began by using a technique called optogenetics to activate sodium appetite neurons.
The effect on the mice was immediate. "They pick up a piece of salt and start eating it," says Oka.
When the team turned off the neurons of sodium appetite, the animals stopped eating salt.
But how does this system work when there is no scientist who topples?
Previous research has shown that part of the response involved cells that measure salt concentrations in the blood.
But the Oka team understood that this was probably not the solution because the animals needed only a small amount of sodium in their diet. They must stop eating salt long before blood levels begin to rise.
The scientists thought that there must be a second "stop button" somewhere – a switch that could be reversed sooner.
They found it in the taste buds of animals.
"When you put sodium salt on the tongue and then taste it, it's enough to suppress neurons from the sodium appetite," says Oka. This is so that we know how to stop consuming salt before having consumed a harmful dose.
And sports drinks, say UCSF scientists, contain exactly the same concentration of sodium found in our body; it is so that drinks replace sodium without triggering the brain's reaction to "stop eating salt".
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