Quenched your Thirst yet? Are you Sure?

What is the source of thirst? Where does the satisfaction of a glass of cold water on a hot summer day originate from? How does the relationship between the brain and the body regulate the body’s need for water?

Until recently, the hypothalamus region of the brain was thought to be responsible for reminding us to drink water when the water levels in the blood are low. (The hypothalamus region regulates hormones and basic body functions such as thirst, blood pressure, and body temperature.) But scientists think that there is more to the story than this because the feeling of thirst fades away almost once the water we drink passes down our throat, despite the fact that it takes at least 10 minutes for this water to enter the bloodstream and rejuvenate our system.

Neuroscientists Zachary Knight and Christopher Zimmerman from the University of California in San Francisco discovered in 2016 that as soon as a fluid passes down the throat, the “thirst neurons” in the brain’s hypothalamus region became silent. These sensors in the mouth and throat seemed to make an estimate of the amount and temperature of the liquid consumed, with the swallowing movement.

How about when we drink seawater? Wondering this, the team gave mice salty water and examined their brain activity by using flexible optical fibers attached near the hypothalamus region. As soon as the mice drank saltwater, thirst signals in the brain shut down, but in a few minutes, it revived and told the body, “This is too salty, keep drinking water!” So, at another point in the digestive system, there should be a mechanism that communicates with the brain, controlling the body’s water demand for a second time.

Thus, scientists continued their experiments by injecting fluids directly into the stomach of mice to study how the brain would react. Thirst signals shut down when they injected the stomach with fresh water, but the signals restarted after a while when they injected salty water. More interestingly, the saltier the water, the stronger the signal was in the brain. A place in the stomach, possibly at the beginning of the small intestine, was clearly measuring the salt concentration of the consumed fluid and reporting it to the brain. In fact, not just the salt concentration, but anything affecting blood osmolarity was being diagnosed this way.

However, in order for saltwater to trigger thirst signals, the body must already be dehydrated. In other words, if there is enough water in the body, the salty liquid injection to the stomach does not trigger the feeling of thirst. This means that in order to develop a sense of thirst, the body must first remain dehydrated.

“This is the first time we’ve been able to watch in real-time as single neurons integrate signals from different parts of the body to control a behaviour like drinking,” Knight says, and adds, “This opens the door to studying how all these signals interact, such as how stress or body temperature influences thirst and appetite.”

These studies further complicate what we already know about the regulation of water levels in the body. Furthermore, although the experiments are performed on mice, the brain structure of humans and mice have significant similarities, especially in the hypothalamus region. Therefore, the applicability of findings to humans is on the agenda.