Quick—of the four (or five, or seven) basic tastes, which one is the one confounding scientists regarding how it works?

It’s SALTY!

Surprised?
Scientists have traced the tastes of sweetness, umami, and bitter to specific receptors on the cells of the tongue, and are close to finding the receptor for sour. How we perceive saltiness, however, still remains elusive. Scientists believe the trouble stems from the fact that humans are so sensitive to sodium: we need it to function, but too much will kill us.

Because we need sodium in pretty much every important bodily function, from nerve firing to blood pressure control to cell water saturation control, molecules that are sensitive to sodium are embedded in cell membranes all over the body.

Why is this a problem? Because the process for locating the other taste receptors involved finding a likely gene for the receptor, destroying it in mice embryos, and testing whether the mice could still experience the taste. If the genes for salt reception are destroyed, the mice won’t live long enough to even be born.

To get around this, two research groups worked to develop a drug method of temporarily blocking a certain type of sodium channel in the mice’s mouths.

So what did they find out when they did this? When the treated animals were deprived of salt, they were unable to distinguish between salty and plain water. This is what they expected if the blocked sodium channel, ENaC, was required to taste salt.

But they found something surprising. Even when the ENaC was blocked, the nerve that carries salty sensations from the tongue to the brain was still showing some activity. They also found that the ENaC-less mice didn’t actively seek out salt and avoided water with very high concentrations of it. So even though they couldn’t taste the salt, the drug treated mice avoided bowls of salty water like normal mice.

This suggested to the researchers that our perception of salt involves two different mechanisms: one involving something that makes salt attractive/tasty, and another that makes high concentrations of it something to avoid. This makes sense biologically: lack of salt can kill us, as can too much salt.

So the search is currently on for the second receptor, the one that makes us reject salt. Come on, taste scientists, you can do it!