It appears that little is known regarding how water is detected in mammalian mouth and throat. However, a recent study published in Nature Neuroscience(1) showed that acid-sensing taste receptor cells (TRC) participate in taste responses to water.
First, the team screened knock-out mice in order to identify which TRC were involved:
They used genetic knockout mice to look for the cells, silencing different types of TRCs, then flushing the rodents’ mouths with water to see which cells responded. “The most surprising part of the project” was that the well-known, acid-sensing, sour TRCs fired vigorously when exposed to water, Oka says. When given the option of drinking either water or a clear, tasteless, synthetic silicone oil, rodents lacking sour TRCs took longer to choose water, suggesting the cells help to distinguish water from other fluids.(2)
Then, the acid-sensing TRCs thus identified were artificially activated using optogenetics techniques:
They bred mice to express light-sensitive proteins in their acid-sensing TRCs, which make the cells fire in response to light from a laser. After training the mice to drink water from a spout, the team replaced the water with an optic fiber that shone blue light on their tongues. When the mice “drank” the blue light, they acted as though they were tasting water, Oka says.(2)
(1) Zocchi, D., Wennemuth, G., Oka, Y., The cellular mechanism for water detection in the mammalian taste system, Nature Neuroscience 20, 927–933 (2017) doi:10.1038/nn.4575
(2) Scientists discover a sixth sense on the tongue—for water, Science