Even swallowing quenches the first thirst – and the salty taste the desire for salt.

Brain Science

We thirst for water, and we demand salt, sodium chloride. What its effect and taste are the sodium ions (Na +), their concentration in the body is important – and that this is not the same everywhere. Outside the cells there is more Na + than in the cells where there is more K + (potassium), this gradient is vital, pumps in the cell membranes are constantly busy maintaining it. The kidneys, however, take care that the sodium content in the blood is not too large and not too small.

But the brain must help to control the desire. To satisfy it, the salty taste on the tongue is enough for now. This is what neurologists around Yuki Oka describe at the California Institute of Technology in Nature: They found a small population of neurons in the brain stem of mice that play a central role in regulating the craving for salt. When these neurons were stimulated with light, the mice licked a salt crystal even though they had enough sodium in the body. And once they had the salt on their tongue, the activity of these neurons declined. Salt infusion directly into the stomach did not do that.

Also, the thirst for water apparently undergoes the first breastfeeding, when the drink wets the lips, and a second, when it is swallowed. The responsible sensors record how much volume is swallowed, and they react the stronger the colder the drink is, which is probably the reason why advertising likes to show icy images. Christoph Zimmerman, a physiologist at the University of California at San Francisco, has already demonstrated these effects in 2016. The question remained open: How does the brain know how much a drink really quenches the thirst? Saltwater does not do that, no matter how cool it is.

Zimmerman and colleagues, in a recent paper (Nature, March 27), had their lab mice drink salty water while watching the thirsty neurons in the hypothalamus, the central physiological control center in the brain. In fact, their drinking activity immediately receded, but then rose again, as if another sensor had reported: too salty, stay thirsty!

To test whether such signals come from the stomach, the researchers led the liquid directly into the stomachs of thirsty mice. And it really depended on how salty the water was. Saltwater did not reduce the activity of thirst neurons in the brain. Apparently, signals from the lips and throat may be revised and corrected by later signals from the stomach or duodenum. When the water is too salty, the thirst neurons start to jump again, so to speak.

With optogenetic methods – which activate neurons with light – Californian researchers have also been able to detect a set of neurons that integrate all of these signals – plus information about saline levels in the blood.

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