Thirst, though, represents only one side of maintaining homeostasis. Salt—more specifically, sodium—is one of the most important things an animal consumes. Animals need sodium for neurons to fire action potentials, for proteins to keep their shape and function, and for chemical reactions to occur inside cells—the everyday business of life. We need to maintain a concentration of sodium ions in the fluids of our bodies to enable all those functions. It’s the other side of thirst.
“There are only a couple of things that are so important for your body that there’s a completely innate drive to get it if you fall into deficiency,” Knight said. “Oxygen, food, water, and sodium.”
However, animals like us do not experience salt desire as a powerful, controlling drive as we do with oxygen, food, and water. Sensors signal salt levels to the brain; in addition to the OVLT and SFO, sensors in the heart detect the stretching of atria and ventricles. But there is no analogous salt pang when we need it, the way a stomach churns for food or a scratchy throat cries out for water. Instead, the need to consume salt is mediated by taste and the brain’s reward pathways. “The taste of salt is bimodal,” Knight said. “It tastes good at low doses; at high doses it tastes disgusting, like drinking seawater.”
Imagine the urge to eat a big bag of potato chips. If the body needs salt, those chips will cause a surge of pleasurable dopamine to flood the brain. If the body doesn’t need salt, that dopamine drip disappears. “It’s pretty much reinforcement learning,” said Yuki Oka, a neurobiologist at the California Institute of Technology who studies how the body maintains homeostasis. “More dopamine means a repeated behavior.”
Everyone Thirsts Differently
Scientists monitoring a river collect data and then have a choice about whether to act on their findings. Similarly, just because the brain measures the blood’s sodium levels doesn’t mean it has to act on that information.
Take Elena Gracheva’s thirteen-lined ground squirrels. Gracheva, a neurophysiologist at the Yale School of Medicine, studies these rodents, native to North American grasslands, to understand how specific brain regions control thirst. The thirteen-lined ground squirrel is an ideal model for this, she said, because it hibernates for more than half the year, without eating or drinking. “They’re like monks,” Gracheva said. “They don’t go outside for eight months. They don’t have water in their underground burrow.” How do they not get thirsty?
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It isn’t that the squirrels don’t need water. They do. Their bodies cry out for it. But according to Gracheva’s research, during hibernation their brain ignores the body’s signals.
In mammals, a drop in blood water levels (which means a simultaneous rise in salt concentration, all things being equal) triggers two coupled processes. The hypothalamus pumps out the hormone vasopressin, which tells the kidneys to retain water rather than let it out as urine, and the SFO kicks off the thirst drive to direct the animal to drink. However, while ground squirrels are hibernating, their vasopressin levels jump, but the animal still doesn’t drink. “The circuit for vasopressin was normal, but thirst neurons were downregulated,” Gracheva said. “These two pathways are uncoupled.” The body is trying to retain the water it has but does not act to consume more.
