Thirst is one of those things you probably never think about. You just feel thirsty and fix it by getting a glass of water. When you quench your thirst, you stop drinking and pay little attention to why the thirsty feeling stopped. But it’s taken researchers years to figure out exactly where in the brain this is happening. A team finally pinpointed it in mice, and the location is likely to be similar or the same in humans.
Where is the thirst center in the brain?
The reason the thirst center has been so hard to find is that it sits in a small control center in the brain called the hypothalamus. The hypothalamus is a key relay center between your brain and the rest of your body. Your brain receives signals from your system about how organs are running and determines if the settings need to be tweaked. For a number of systems, this tweaking happens through the hypothalamus. Once the brain finds a system that needs adjusting, it sends a message to the hypothalamus for relay to the rest of the body.
The hypothalamus is uniquely positioned to do this because it’s not protected from the blood the way the rest of the brain is. The rest of the brain has a seal around all of its blood vessels to make doubly sure that nothing potentially harmful disrupts brain activity. But that also means it can’t send signals back out to the body through the blood. Instead, it offloads most sensing of the blood and messages that need to go out through the blood to the hypothalamus, which has no seal around its blood vessels.
How does thirst work?
The hypothalamus monitors the blood and sends signals to the brain about whether more water is needed. The brain also receives chemical signals from other parts of the body about whether enough blood is circulating. When the brain finds the water levels are too low, it sends a signal back to the hypothalamus, which then releases a signal that makes you thirsty. When the body’s water level returns to normal, the brain senses it and shuts down the thirst center in the hypothalamus.
What did these researchers do differently?
The hypothalamus is a tiny part of the brain about the size of an almond in humans. In mice, it’s many times smaller. In the past, researchers tried to understand how thirst turns on and off by putting a small electrical stimulator into different areas of the hypothalamus in mice. Unfortunately, even the smallest stimulators were too big for these tiny brain regions and they would end up setting off many different areas of the hypothalamus. This research team used a recently developed laser technique to get around that hurdle.
They programmed the nerve cells of their mice to carry activators that responded to a focused beam of light. This light is more precise than an electrical stimulator, which meant the team could hit small groups of nerve cells and see how the mice responded. They found two areas responsible for feeling thirsty: an “on” switch and an “off” switch. When they lit up the “on” switch, the mice ran to the water. When they lit up the “off” switch, the mice stopped drinking regardless of how thirsty they were.
What does this mean for you?
Many diseases can affect how thirsty a person feels. Some cause a person to feel too thirsty, while others lead to a person not drinking enough water. Now that researchers have a sense of how the brain is controlling thirst, they can make drugs that target those parts of the brain. And since the hypothalamus doesn’t have the usual brain seal around it, drugs can hit their target a lot more easily.