"When animals are thirsty, they start drinking," he says. "But they [also] have to stop drinking, otherwise they are going to just drink a huge amount of water, which is not healthy."
And they do stop, of course. But what's surprising is that animals stop drinking long before their bodies have a chance to absorb the liquid they're consuming.
"Hydration of the body takes 10 to 15 minutes," Oka says. "But normally, animals stop drinking within one minute."
Somehow the brain figures out that liquid is on the way and sends a signal that quenches thirst, even though the body is still dehydrated.
Oka wondered, "Where is this signal coming from? How does the brain know that we are drinking water?"
To find out, he had his team study mice using techniques including optogenetics, which lets researchers turn brain circuits on and off in living mice. This allowed the team to identify specialized cells in the animal's thirst center that would fire when a mouse was drinking, but stayed quiet if the mouse was eating.
The ability is remarkable because both activities use the same muscles in the throat, Oka says. He suspects that the cells can tell the difference between the fast muscle movements of gulping and the slower pace of chewing and swallowing.
The team also showed that these specialized brain cells were part of a circuit that could switch off the brain's thirsty signal. "If we stimulate those neurons in [a] thirsty animal, you can completely stop [it from] drinking water," he says.
The finding in mice could explain a rare disorder that occurs in humans, says Walter Koroshetz, director of the National Institute of Neurological Disorders and Stroke, which helped fund the study.
It's called psychogenic polydipsia, and it causes people to drink dangerous amounts of water. "You become really sick," Koroshetz says.
It's possible, he says, that the disorder is caused by some sort of fault in the brain circuit that usually tells us when to stop drinking.
The research shows how new tools are allowing scientists to tweak specific brain circuits in ways they never could before, Koroshetz says. These tools are partly the result of a federal effort launched during the Obama administration called the BRAIN Initiative that is aimed at deepening our understanding of how our brains work.
"I'm in awe of what's been coming out," Koroshetz says. "No one could imagine doing any of this four or five years ago."
Scientists are starting with brain circuits that control basic functions, like drinking, Koroshetz says. But a long-term goal, he says, is to learn how to manipulate the circuits involved in complex psychiatric disorders like depression and schizophrenia.