Summary: Researchers at Caltech report that sour-sensing taste cells on the tongue are essential for detecting water.
Source: CalTech.
Sour-sensing taste pathway also mediates water detection in the mammalian tongue
New research from the California Institute of Technology shows that taste cells tuned to sour stimuli also play a key role in sensing water on the tongue. The findings, published in the May 29 issue of Nature Neuroscience, were produced in the laboratory of Yuki Oka, assistant professor of biology.
“The tongue detects a variety of key chemical cues called tastants—such as salt, sugar, amino acids, and acids—through specialized taste cells,” Oka explains. “What remained unclear was how animals detect plain water in the mouth. Because many insects can ‘taste’ water, we hypothesized that mammals might use part of their taste system to sense water as well.”
How the study was conducted
Graduate student Dhruv Zocchi led experiments measuring electrical activity in taste nerves of mice in response to different tastants and to pure water. As expected, taste nerves produced characteristic responses to the five canonical tastes—sweet, sour, bitter, salty, and umami—but they also showed activation when water alone was presented. This observation suggested that certain taste cells are capable of detecting water.
Taste perception relies on distinct subsets of taste receptor cells (TRCs), each tuned to specific taste qualities. To determine which cell types responded to water, the team used genetic and pharmacological methods to silence individual TRC populations. For example, blocking salt-sensing receptors abolished nerve responses to salty stimuli without affecting other tastes. When the researchers silenced sour-sensing TRCs, however, the neural responses to water were eliminated entirely. This result indicated that water detection depends on the same acid-sensitive TRCs previously linked to sour taste.
Optogenetics confirms sour cells signal water
To directly test whether sour-sensing cells alone could drive water-seeking behavior, the researchers applied optogenetics: they engineered mice so that activating these cells with blue light would mimic their natural activity. The experiment removed access to actual water and arranged for the water spout to emit blue light when touched. Thirsty engineered mice approached the spout and licked, responding to the light as if it were water. Although the animals were not rehydrated, they continued to lick the light source, demonstrating that stimulation of acid-sensing TRCs provides a sensory cue interpreted as water.
Notably, classical sour taste is often perceived as aversive—animals typically avoid strongly acidic fluids such as lemon juice. Yet optogenetic activation of sour cells did not provoke avoidance in these mice. That observation raises new questions about how the brain interprets signals from the same cellular population under different conditions.
Internal state shapes the value of water signals
The team found that the effect of activating sour-sensing TRCs depended on the animal’s internal state. Optogenetic self-stimulation reliably produced drinking-like behavior only when animals were water-deprived; stimulation did not trigger the same response when animals were simply food- or salt-deprived. Conversely, thirsty animals that lacked functional acid-sensing TRCs had difficulty distinguishing water from nonaqueous fluids. Together, these findings indicate that acid-sensitive TRCs supply a cue for external water and that the hedonic value—or pleasantness—of that cue is strongly modulated by the animal’s thirst state.
“It’s important to emphasize that stimulating these cells does not rehydrate animals,” Oka says. “Rather, it shows how peripheral taste signals are interpreted by the brain to guide drinking behavior depending on internal need. Our next step is to investigate how brain circuits assign hedonic value to these sensory inputs and how that value changes with physiological state.”
Funding: Research was supported by startup funds from Caltech leadership and the Division of Biology and Biological Engineering, the Searle Scholars Program, the Edward Mallinckrodt, Jr. Foundation, the Okawa Foundation, the McKnight Foundation, and the Klingenstein-Simons Fellowship Award.
Reporting: Lori Dajose, CalTech. Image credit: Oka Laboratory/Caltech.
The cellular mechanism for water detection in the mammalian taste system: Initiation of drinking depends on both internal state and peripheral detection of water. While neural circuits that regulate thirst centrally are well studied, the peripheral recognition of external water was unclear. This study demonstrates that acid-sensing taste receptor cells (previously associated with sour taste) mediate taste nerve responses to water. Genetic silencing of these cells abolishes water-evoked neural responses. Optogenetic stimulation of acid-sensing TRCs in thirsty animals provokes robust drinking behavior toward light even in the absence of water; this behavior occurs only under water deprivation, indicating that the hedonic value of water cues is state-dependent. Thirsty animals lacking functional acid-sensing TRCs show impaired discrimination between water and nonaqueous fluids. These results reveal that acid-sensing TRCs provide a peripheral cue for external water.
This summary is based on research conducted by Dhruv Zocchi, Gunther Wennemuth, and Yuki Oka, and published in Nature Neuroscience.