Tongue Tug-of-War: Taste Buds Battle to Decide if Salty Foods are Yummy

Our tongues are great at telling us when something tastes good or when something tastes awful, signaling to us when food is spoiled or rotten. Yet some things that taste good to us, like salt and sugar, can cross the line to "bad" when too much is used. Until now, though, scientists have been unsure how our brains can tell when saltiness crosses the line from yummy to yucky.

In order to find out how we sense when there's too much of a good thing on our tongue, researchers examined flies. More specifically, they looked at the equivalent of a fly's tongue--its long, curly proboscis. They zoomed in on the proboscis' sensilla, hair-like structures that serve as taste buds.

Like our own taste buds, there are several different types of sensilla. Some attract flies to a taste while others repel them. In order to examine how different flavors affect the sensilla, the researchers loaded an electrode with a mixture of water and different concentrations of salt. They then touched each type of sensilla with the mixture, detecting the electrical signals fired in response to the salt.

The scientists found that, up to a point, increasing salt concentrations would produce increasingly strong electrical signals in the attractive sensilla. After that point, though, the electrical signals dropped off as the concentration continued to rise. The repellent sensilla then began to take over, giving off stronger and stronger signals in turn as the salt became stronger.

The scientists weren't done yet, though. They further tested the flies by mutating a gene called Ir76b that codes for a protein that they suspected was involved in the action of the attractive sensilla. In turned out that loss of Ir76b function caused flies to avoid the otherwise attractive low-salt food. The mutated gene actually only impaired the attractive sensilla, which left the repellent "taste buds" to react and spark an electrical signal, winning the day.

"It's an unusual setup, but it makes sense because the local sodium concentration outside tatse receptor cells appears to be a lot lower than that surrounding most cells," said Yali Zhang, the lead researcher, in a news release. "The taste receptor cells don't need to keep the gate closed to protect themselves from that excess sodium."

The findings have implications not only for flies, though. Salt intake is an issue that all animals had to deal with in the past, including humans. Although these taste buds have different makeups in various species, it's likely that this "tug-of-war" between attractive and repellent sensors is probably the same.

The findings are published in the journal Science.