t’s August. It’s hot. You reach for water.
Thirst is our signal that our bodies’ equilibrium is off. We get a signal, we drink, we’re happy. Yet, our brains tell us to start and stop drinking long before water reaches our blood to restore our equilibrium. What gives?
Research published Wednesday in Nature points to a group of neurons deep in our brains that responds to signals from our mouths and our blood, prompting us to drink and getting us to stop. These neurons, the research team said, anticipate the need for water, long before we physiologically need it.
“The conventional view is that the body signals its needs as they come up,” said researcher Zachary Knight, an assistant professor of physiology at the University of California, San Francisco, and lead researcher of the study. “But the emerging view is that the system is more anticipatory than reactionary.”
Knight and his team focused on the subfornical organ, part of a region of the brain that maintains fluid balance in our bodies. Stimulating neurons within the SFO of mice caused them to drink water, even when they shouldn’t have been thirsty. Giving them salt seemed to stimulate the neurons.
Then they gave mice water after restricting them overnight. The team saw that SFO neurons started firing, but started declining the second the mice started lapping up water. The mice stopped drinking when the neurons hit their baseline level of activity and blocking the neurons from working kept the mice from drinking, even when they physiologically needed to.
“Until recently, it wasn’t clear which neurons were the ‘thirst neurons,'” he said. “But now that we can image these neurons in a living animal, we’re able to learn more.”
But what about the feeling of quenched thirst?
That, he said, is partly explained by the effect of simply having a cool sensation in your mouth.
“Cool water causes the thirst neurons to go down. Your blood doesn’t care if the water is cool or not, but we prefer cold water,” he said.
This is why patients who can’t have food or drink before a procedure get ice chips — the coolness helps them feel less thirsty, he said.
And the researchers also saw that eating, when we drink the most water, triggered the neurons to fire.
The next step would be to repeat these experiments in people, said Lawrence Armstrong, a professor and director of the Human Performance Laboratory at the University of Connecticut.
“There is information here that may be applicable to humans,” said Armstrong, who was not involved with the study. “But it’s not analogous.”
He added, however, that the study is useful in a number of respects.
“The idea of homeostatic and regulatory control of drinking has been around since the 1960s,” he said. “But this is taking us many steps forward in understanding how the brain predicts changes in fluid balance, with the goal of reducing overconsumption or underconsumption.”