Our tongues might contain yet another sense that we have failed to recognize until now – and it's for water. The new sense could explain how animals distinguish between water and other liquids, and may settle a long-standing debate about whether Adam’s ale actually has a taste.
“The tongue can detect various key nutrient factors, called tastants – such as sodium, sugar, and amino acids – through taste,” explains Yuki Oka, who led the study published in Nature Neuroscience, in a statement. “However, how we sense water in the mouth was unknown. Many insect species are known to 'taste' water, so we imagined that mammals also might have a machinery in the taste system for water detection.”
The team tested mice to see how they respond to the taste of pure water. Taste cells on the tongue transmit information about what is in the mouth to the brain via specific nerves, which fire upon stimulation. The researchers simply recorded which nerves were responding when the mice tried different tastants.
The tastes we consider as basic – sweet, sour, salty, bitter, and umami – gave the expected results, but curiously the nerves also responded to pure water. This implies that water does in fact have a taste. How our brains figure out how something tastes depends on which taste cells are stimulated. So to figure out what was happening when the rodents drank water, they created various strains of mice that had different taste cells knocked out.
This revealed that while the water continued to be sensed when the mice could not taste sweet, salty, bitter, or umami, when the sour taste cells were silenced, the rodents also ceased to taste water. To test this further, they then employed a bit of optogenetics.
The team engineered mice so that the sour taste cells were sensitive to light, and then trained the mice to drink water from a spout, before replacing the water with a blue laser. The mice continued to “drink” from the laser when they felt thirsty, as the light was stimulating the sour taste cells and thus tricking the brain into thinking that they were actually drinking water.
Interestingly, the tongue may be key in sensing when you are drinking water, but it is not involved with telling your brain when to stop. The researchers found that the two processes must be separate. “It's important to note that stimulation of these cells does not alleviate thirst,” says Oka. Even though the mice were licking the blue light, which was tricking the brain into thinking they were drinking, they didn’t stop.
Since this is true for mice, the researchers suspect that all mammals – including us – might also have a sense of taste for water.