Salty much? These brain cells decide when tasty becomes blech
If this year's turkey seems over brined, blame your brain.
The question of when salty becomes too salty is decided by a special set of neurons in the front of the brain, researchers report in the journal Cell.
A separate set of neurons in the back of the brain adjusts your appetite for salt, the researchers showed in a series of experiments on mice.
"Sodium craving and sodium tolerance are controlled by completely different types of neurons," says Yuki Oka, an author of the study and a professor of biology at Caltech.
The finding could have health implications because salt ingestion is a "major issue" in many countries, including the United States, says Nirupa Chaudhari, a professor of physiology and biology at the University of Miami's Miller School of Medicine.
Too much salt can cause high blood pressure and raise the risk for heart disease and stroke, says Chaudhari, who was not involved in the study.
Craving, to a point
The study sought to explain the complicated relationship that people and animals have with salt, also known as sodium chloride.
We are happy to drink sodas, sports drinks, and even tap water that contain a little salt, Oka says. "But if you imagine a very high concentration of sodium like ocean water, you really hate it."
This aversion to super salty foods and beverages holds unless your body is really low on salt, something that's pretty rare in people these days. But experiments with mice found that when salt levels plummet, the tolerance for salty water goes up.
"Animals start liking ocean water," Oka says.
The reason for this change involves at least two different interactions between the body and brain, Oka's team found.
When the concentration of sodium in the bloodstream begins to fall below healthy levels, a set of neurons in the back of the brain respond by dialing up an animal's craving for salt.
"If you stimulate these neurons, then animals run to a sodium source and start eating," Oka says.
Meanwhile, a different set of neurons in the front of the brain monitors the saltiness of any food or water the mice are consuming. And usually, these neurons will set an upper limit on saltiness.
But when salt levels get extremely low, the body sends a signal that overrides these salt-limiting neurons. That allows mice to tolerate the saltiness of sea water.
The scientists were able to mimic this phenomenon in the lab by stimulating these neurons.
Connecting body and brain
The finding adds to scientists' understanding of interoception, which involves sensations like hunger, pain, and thirst and tells the brain what's going on inside the body. It's a relatively unexplored form of sensory information, unlike the sensory information coming from the eyes, ears, nose, tongue and skin.
"The brain receives tons of sensory information from the heart, the lungs, the stomach, the intestine," says Stephen Liberles, a professor and Howard Hughes Medical Institute investigator at Harvard Medical School. "And how these work has remained more mysterious."
The new study found evidence that the brain cells involved in salt tolerance respond to hormone-like substances called prostaglandins. These substances, which circulate in the bloodstream, are best known for their role in causing inflammation, fever, and pain.
Now it's becoming increasingly clear they also play a role in altering salt tolerance.
"The question is: How is the same chemical, the same prostaglandin molecule ... reused in different contexts?" Liberles says.
Answering that question might make it possible to develop a prostaglandin drug to discourage people from eating too much salt.
Salt overconsumption has become a worldwide problem because humans evolved in times when salt was scarce, says Chaudhari.
"Wars were fought over salt just a few centuries ago," she says. "We think of sodium chloride, table salt, as so plentiful in our diet and our environment, but it wasn't always."
Understanding how the brain processes saltiness might help food companies develop a palatable salt substitute, she says.
At least one previous effort failed badly, she says, for a simple reason: "It tasted really foul."
So finding a better option may require more than just research on how the brain monitors salt intake, she says. Scientists also need to understand how that substitute will interact with our taste buds.