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 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.