Source: Olichel/Pixabay

How does the brain know if thirst has been quenched? This may sound like a simple question. However, the behind quenching thirst has been a mystery until recently. For centuries, scientists have been trying to figure out how the body, brain, and mind regulate thirst and how humans and animals know when thirst has been quenched.

Finally, after three years of intense investigation, neuroscientists at the University of California, San Francisco have identified that signals from the gut communicate with specific brain cells that control thirst via the . These findings, “A Gut-To-Brain Signal of Fluid Osmolarity Controls Thirst Satiation,” were published online today in the journal Nature.

This -edge UCSF study shows that salt concentrations in the gut are measured within the GI tract and relayed to the brain via the gut-brain axis. In the paper's abstract, the authors sum up the significance of their findings:

"Here we show that the water and salt content of the gastrointestinal tract are precisely measured and then rapidly communicated to the brain to control drinking behaviour in mice. We demonstrate that this osmosensory signal is necessary and sufficient for satiation during normal drinking, involves the vagus nerve and is transmitted to key forebrain neurons that control thirst and vasopressin secretion."

"We've discovered a new way that the gut talks to the brain," UCSF graduate student and first author of this paper, Christopher Zimmerman of the Knight Lab, said in a statement.

Madison Avenue advertisers have long known that tapping into the cycle of going from insatiable thirst to the “Ahhh!” feeling of thirst-quenching satisfaction is a universal experience that can be used to sell refreshing beverages to thirsty consumers. (e. ., "Take the Plunge," "Freedom from Thirst!," or the globally-recognized “Pop ‘n' Pour” sound signature.)

Although an ice-cold beverage feels like it quenches thirst as soon as it hits your lips, it actually takes 10 minutes or longer to be absorbed by the body in ways that improve hydration levels.

Source: Knight lab/UCSF

Thirst can elicit primal urges that are even stronger than hunger. Anecdotally, there’s a general belief that humans can live longer without food than without water, although the scientific evidence behind such claims is practically impossible to verify. That said, until now neuroscientists have been perplexed by how thirst signals are picked up in the body and how the urge to quench thirst—or when thirst has been satiated—are represented in the brain.

Using novel techniques, the researchers in Knight's lab at UCSF were able to watch in real-time as thirsty mice quenched their thirst. Through a series of complex experiments that began in 2016, Zimmerman and colleagues finally discovered that the GI tract has built-in salt sensors that convey varying degrees of thirst directly to the brain.

According to Zimmerman et al., there are so-called “thirst neurons" in the median preoptic nucleus of the hypothalamus (seen in green and red in the image above) that calculate whether or not an animal is thirsty. "No one had ever observed this happening in a single cell before," Zimmerman said.

Although these findings are potentially groundbreaking, senior author, Zachary Knight points out that the body's thirst-sensing system appears to be relatively simple. Nevertheless, working out the details of how thirst fits into more complicated systems such as regulating body temperature and feeding will require more research. Knight plans to continue using his lab's state-of-the-art methods of gauging how the gut communicates with the brain in upcoming research. "This is a prototype of the kind of science we're going to be doing in my lab in the years to come," he concluded.