According to new research by California Institute of Technology, mammalian taste receptors that respond to sour tastants are also capable of detecting water, which as we all know is tasteless. Recently published in the Nature Neuroscience journal, the study has found that sour taste buds in mice get stimulated when they come in contact with pure water. Speaking about the findings, Yuki Oka, a professor of biology at the university, said:
The tongue can detect various key nutrient factors, called tastants— such as sodium, sugar, and amino acids—through taste. 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.
Mammalian tongue contains taste cells, more technically known as gustatory cells, that are responsible for detecting the five basic tastes: sweet, sour, salty, bitter and umami (as in the taste of tomatoes). Depending on the tastant, the receptors send information in the form of neurological signals to specific gustatory regions in the brain via what are called taste nerves.
As part of the research, the team recorded the activity of these nerves in murine subjects in response to different tastants as well as water. While the gustatory cells and nerves detected the five tastes in an expected way, something entirely new and bizarre came to light during the experiment. The sour-sensing taste cells, according to the scientists, showed discernible activity upon contact with pure water. Dhruv Zocchi, the paper’s first author, said:
This was exciting because it implied that some taste cells are capable of detecting water.
To test the accuracy of their findings, the researchers used drugs and genetic engineering to individually ‘turn off’ specific taste buds on the tongue. Consequently, when the sweet-sensitive taste cells were blocked, the mice failed to detect sweetness of any kind. Oka explained:
To our surprise, when we silenced sour taste cells, water responses were also completely blocked. The results suggested that water is sensed through sour taste cells.
To further substantiate the findings, the team set up an optogenetic experiment in which they used a bright beam of blue light to stimulate the animals’ sour receptors. This in turn caused the mice to try and ‘drink’ the light, thinking that it was actually water. The researchers added:
These results raise the question: What information about taste are sour cells really relaying to the brain? Maybe sour cells are not directly linked to the unpleasant sourness that we perceive, but instead they may induce a different type of taste, like water, when stimulated… It’s important to note that stimulation of these cells does not alleviate thirst. But this finding helps us understand how the brain interprets water signals under normal and thirsty states. Next, we would like to tackle the mechanisms by which the hedonic value or ‘pleasantness’ of sensory inputs are regulated by brain activity.