Brain and Gut Communicate About Salt Content to Control Thirst

How your brain knows when you’re thirsty and when you are satiated has been a mystery for more than a century. Some evidence has pointed to signals from the body controlling feelings of thirst, but exactly where these signals come from has been a source of debate. In a new study, researchers from the University of California, San Francisco (UCSF) present evidence that these signals come from the gut. Their work, reported in Nature, reveals how the gastrointestinal tract measures the salt concentration in the intestine and relay the info to the brain.

In a previous study, the UCSF team had investigated where in the brain thirst signals in the body registered. Using an optical fiber threaded into the brain, the researchers watched a set of neurons rapidly switch off when thirsty mice took a sip of water and the liquid hit their throat.

This study showed that a thirst signal exists in the throat, but also hinted at some other biological mechanism at play. When mice consumed salt water, the thirst neurons turned on temporarily, but re-activated shortly after. This led the researchers to investigate the gut.

In the current study, the researchers performed a series of experiments to determine what happens in the brain when mice consume salt water. Plain water delivered directly into the gut shut off neurons for thirst, but a salt water infusion kept the neurons active.

The researchers next looked to where in the brain these signals are being evaluated; they did this by mounting miniature microscopes onto the heads of the mice. Inside the hypothalamus, they identified single neurons that take input from the gut, throat, and blood and circulate thirst information.

Not only does this research shed new light on the mechanism the brain uses to communicate thirst, the techniques used for this study could help scientists work out other systems in the future, including regulation of feeding and body temperature.

Image: Neurons that control thirst in the mouse brain are switched on (green and red) when the gut senses salty fluid. Image courtesy of Knight Lab/UCSF/HHMI.