Think back to the last glass of water you drank. Did you actually taste the water? Before you answer, keep in mind that taste and feel involve two different mechanisms. For example, if you coated your tongue with a thin plastic film, you wouldn’t be able to taste the sweetness of a jelly bean, but you would be able to feel its presence in your mouth. In much the same way, we can feel the water we drink, it's not clear whether we actually taste anything beyond trace elements in the water.

Water is essential for animals, but surprisingly little is known about how different animals taste water as they are drinking it. There is virtually no research on humans, and there is only a smattering of studies that suggests cats and rats can taste water. There is, however, a general agreement among taste and behavior scientists that insects can taste water.

Peter Cameron, a cell biologist at the University of California, Berkeley, explained that "insects have a unique set of neurons, including water sensing ones, but the actual water taste receptor was unknown." Thus, Cameron set out to identify one for his PhD thesis project, done under the guidance of Professor Kristin Scott. His work is published in a recent issue of Nature.

Cameron began in 2005 by examining genes responsible for taste in Drosophila, a favored insect in biological research. Specifically, he compared the genes between two groups of flies: a control group and one with a genetic mutation that knocked out all of the taste neurons. He found a gene, dubbed pickpocket 28 (ppk28), that expresses a protein (PPK28) in taste neurons. That is the protein responsible for recognizing water.

When Cameron and co-workers stimulated PPK28 neurons with different taste solutions, the neurons showed more activity in plain water than solutions of salts, sugars, acids, and bitter chemicals. They also found that increasing concentrations of these chemicals correlated to a further decrease in the activity of the neurons. "This makes sense, because when you have higher concentrations of a salt, for example, you have less relative water in the solution (higher osmolarity)," Cameron explained.

To further correlate the function of PPK28 with water taste, Cameron compared the behavior of control flies to that of mutant flies that lacked the ppk28 gene. On average, the control flies drank water for about 10 seconds, but the mutant flies drank for only 3 seconds (they reported water consumption in drinking time because the volume changes caused by flies were too small for reliable measurements). The difference in behavior was limited to water, as both population of flies ingested sucrose for equal durations.

In separate experiments, the neurons lacking the ppk28 gene showed virtually no response to water. They averaged an activity of less than 1 spike per second, while the control flies typically exhibited around 12 spikes per second.

The results indicate that PPK28 is a unique water taste receptor. Cameron states, “No other water taste receptor has been found before. PPK28 is the only known one in Drosophila, but it’s possible that other mechanisms exist.” For example, mechanical receptors could assist in sensing water during drinking. He also stressed, “evidence strongly suggests that PPK28 alone serves as the water sensor; it doesn’t seem to sense anything else.”

Cameron suggests that “their results and experimental procedures could provide a framework for investigating other animals.” The field is relatively young. Outside of insects, it has not even been established that other animals can taste water.

Insects' sensory systems are quite different than ours, allowing them to detect things that we cannot. For example, many insects can sense infrared and/or UV light. So, although it can be difficult to imagine tasting water, for an insect it's part of a normal sensory experience, as natural as detecting green light.

Nature, 2010. DOI: 10.1038/nature09011

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