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Few pleasures compare to a long, cold drink on a hot day. Your brain follows as a glass of water or other tasty beverage makes its way through your digestive system – but how? Scientists have known for some time that thirst is controlled by neurons that send a stimulus to let go of the glass when the right amount is swallowed. What exactly tells them the time has come is a bit of a mystery, though.
In a previous study, a team of researchers found that the act of swallowing a liquid—from truly water to oil—is enough to trigger a temporary shunt of thirst. But they knew that swallowing wasn’t their only source of satisfaction. There were signals from the depths of the body that quenched the thirst.
In Article published Wednesday in NatureScientists from the same lab report tracking the signals down the neck, through one of the body’s most important nerves, to the intestines, and eventually to an unexpected place for this trigger: a series of small vessels in the liver.
The act of swallowing can provide a quick way for the body to monitor fluid intake. However, everything you swallow quickly reaches the stomach and intestines, and then its identity becomes clear in your body, such as whether the body can meet its water needs. Water changes the concentration of nutrients in your blood, and researchers believe this is what triggers true satiety.
“There’s a mechanism to make sure what you’re drinking is water, nothing more,” said Yuki Oka, a professor at Caltech and author of both studies. Dr. To find out where the body senses changes in the concentration of your blood, Oka and colleagues first poured water into the intestines of mice and watched the behavior of the nerves that connect the brain to the intestinal region, which is believed to work similarly in humans. . One of the main nerves, the vagus nerve, is fired most recently when water reaches the intestines, suggesting that this is the route information takes as it travels to the brain.
Then, one by one, the researchers went and cut off the connections of each of the nerves to different areas in the gut. Surprisingly, nothing changed when they cut contact with the guts.
Instead, it was the liver’s portal veins — the vessels that carried this blood from the gut to the filtering organ — whose isolation returned messages to the brain.
Dr. Oka said these vessels carry nutrients and fluid to the liver, so they can be a monitoring center for thirst. The team found that just water flowing through the portal veins was not enough to ignite the nerve. Something about the arrival of water had to trigger another part of the body’s hydration Rube Goldberg machine.
Researchers narrowed it down to a hormone called vasoactive gut peptide, or VIP. As the water reaches the portal veins, the VIP levels rise and it is the VIP rather than the water itself that causes the vagus to fire and stimulate the brain.
As intriguing as this is, scientists don’t know how water is causing this rise. They hope to continue to follow the signals and determine exactly which cells and molecules are connecting these humble vessels and peptide with the big abbreviation.
Dr. “This is the most important thing that we are in a good position to tackle next,” Oka said.
And there is probably more to learn. While VIP causes the vagus nerve to make noise, the signal isn’t as strong as researchers expected when it works alone. Water is so important to the functioning of the body that Dr. Oka and her team think that our brain probably has more than one redundant way to track it. With every glass of water you drink, you are advancing this system step by step.
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