If you think about being thirsty at all, it seems like a fairly simple thought process: Find water. Drink it. Move on. But in fact there is something rather profound going on as you take that long, refreshing drink after a run or a hot day in the garden.
As you become dehydrated, there is less water in your blood, and neurons in your brain send out the word that it’s time to look for water.
Then, once you take a drink, you feel almost instantly satisfied. But if that is obvious, it is also mysterious. You aren’t pouring water directly into your bloodstream, after all. It will take at least 10 or 15 minutes, maybe longer, for the water in your stomach to make its way into the blood. And yet somehow, the brain knows.
Sometimes that process isn’t as straightforward as it should be: People with a syndrome called polydipsia feel excessive thirst and drink enormous quantities of water. That can be dangerous, because if the blood is diluted too much, a person can die — a victim of water intoxication.
As neuroscientists ponder how and why we thirst, a group of researchers at the California Institute of Technology has shed light on one small corner of the problem. Interested in how the brain keeps track of what the body is drinking, they have identified a set of neurons that receive messages as thirsty mice gulp down water. Passed around in the brain’s thirst centers, these messages seem to be behind the sensation of swift satisfaction that comes after a drink, and also suggest that it’s not just what is drunk, but how it is slurped down, that affects the brain. If the circuits work the same way in people, it may be key to understanding the neuroscience of what happens as we feel thirsty.
In the last few years, biologists have been mapping the neurons within an area in the brain that regulates thirst, said Yuki Oka, a professor at Caltech and senior author of the new paper, which was published Wednesday in Nature. Cells in this region had been observed going quiet after an animal had water, but it was not clear exactly why.
A staining image of the thirst integration center in a mouse's brain. The cells stained red are triggered by thirst. Oka Lab/Caltech
A graduate student in Dr. Oka’s laboratory, Vineet Augustine, did a series of experiments with mice that had been genetically modified to make tracing the connections between their neurons easier. In these experiments, when a neuron caused another neuron to turn off, it got tagged, leaving a trail of bread crumbs through the brain.
What Mr. Augustine found was that certain neurons in a region called the median preoptic nucleus were responsible for telling other cells in the thirst center that drinking was occurring. Further experiments showed that mice without functioning versions of these neurons drank twice as much as normal mice. In the opposite scenario, when the cells were artificially activated, even dehydrated mice did not experience thirst.
Intriguingly, what these cells are responding to is not the presence of water itself, Mr. Augustine said. The researchers discovered that letting a mouse take big gulps of water would spur the neurons into action. But giving it water in a gel form, which had to be chewed before it could be swallowed, did not. Neither did providing water in tiny, two-second-long sips, even when the animals consumed the same total amount of water. In fact, giving the mice oil to drink had just the same effect on the neurons as gulping water.
“That indicated to us that it’s probably the speed — the speed of this ingestion — to which these neurons are responding,” Mr. Augustine said.
Apparently, a series of quick swallows is an evolutionarily acceptable shorthand for drinking water — reliable enough for the body to use it as a way to signal when enough has been consumed.
Aside from the specter of water intoxication, there are good reasons to drink only the minimum amount necessary. When an animal lowers its head to drink, Dr. Oka speculated, it’s in quite a vulnerable position. “If you double the time of ingestion, that should double the risk of being prey,” he said.
The researchers are now hoping to investigate other ways the body monitors water intake. For instance, they want to see whether there are sensors in the gut that keep the brain apprised of the arrival of water. It seems likely, Mr. Augustine said, that the brain gets some notice, and adjusts accordingly.