Thirst is one of the most basic human signals that allows for our survival. For decades, research on the subject presumed that the brain only knows thirst has been quenched once the water enters the bloodstream. However, research on mice, published in Nature, has found a set of thirst-predicting neurons.
“There are many things you experience every day when you become thirsty or drink water that can’t be explained by [the old] model,” Zachary A. Knight, coauthor of the study and assistant professor in the department of physiology at University of California, San Francisco, tells mental_floss.
He explains that if you’re thirsty and you drink a glass of water, your thirst feels quenched within seconds, “but it takes up to 20 minutes for that water going down your throat to actually reach your bloodstream and change anything.” Likewise, Knight says, it’s a common experience to feel thirsty within seconds of eating something salty—and that response also poses a question: “When that food is still in your mouth or going down your throat, there’s been no change in your blood," he notes. "How does your brain know that there’s going to be this change in the blood that occurs 10 to 20 minutes into the future?”
Knight and his lab team suspected the answers to these questions resided in a deep brain structure known as the subfornical organ (SFO). Thanks to advances in optogenetics—using tiny fiber-optic probes in the brain to stimulate neurons—and methods of encoding fluorescent proteins into individual neurons, researchers were able to view real-time neural activity in the brains of thirsty mice.
When they stimulated neurons in the SFO of the mice, the animals drank water. Similarly, giving the mice salt stimulated these “thirst” neurons. When they blocked the neurons altogether, the mice didn’t drink water at all—even when their physiology suggested they should. “What was very surprising, and also explains a lot … is that these neurons that people had thought for a long time just sensed the signals in the blood actually receive a second class of signals from the mouth and the oral cavity,” Knight says.
But that’s not the most exciting part, he says. The signals that come from the mouth and oral cavity “basically tell these neurons, on a rapid timescale, about food and water that’s going down the throat in a way that allows these neurons to essentially predict how that food or water is going to change the osmolarity of the blood 20 minutes in the future, when it’s absorbed.”
So how do these thirst neurons know that it’s water going down a person’s throat? Knight says the physical sensation or viscosity of the liquid in your mouth might be picked up on by your nerves, which then send the information to the brain. Another important component—which Knight calls “the most surprising result”—appears to be temperature, “because when you drink water, you’re cooling down your throat” in most cases. “The amount that your throat has been cooled probably loosely correlates with how much water you drank in the very recent term,” Knight says.
Thus, it may explain why people often crave cold water when thirsty or feel quenched by drinking a cold beverage, even if it’s not water. “It’s one of the cues these thirst neurons use to figure out how much water you’ve just been drinking,” he suggests.
Indeed, in one experiment, they found that simply applying a piece of cold metal to a mouse’s tongue would activate the thirst neurons, and the activity would decline when the metal was removed. Even more compelling, they found mice will often lick a piece of cold metal if they are thirsty.
Knight finds this temperature-dependent function a “bizarre phenomenon” that he could see one day being used in creating drinks that are warm but can manipulate your thirst neurons into turning on, so you’d perceive the beverages as tasting cold.
“You could just eliminate refrigerators and ice,” he says.