Many people would say pure water tastes like nothing. But if water has no flavor, how do we know what we’re drinking is water? Our tongues do have a way to detect water, a new study shows. They do it not by tasting the water itself, but by sensing acid — which we usually call sour.
All mammals need water to survive. That means they should be able to tell whether they’re putting water in their mouths. Our sense of taste has evolved to detect other important substances, such as sugar and salt. So detecting water would make sense too, Yuki Oka says. He studies the brain at the California Institute of Technology in Pasadena.
Oka and his colleagues had already found that a brain area called the hypothalamus (Hy-poh-THAAL-uh-mus) can control thirst. But the brain alone can’t taste. It has to receive a signal from the mouth to know what we’re tasting. “There has to be a sensor that senses water, so we choose the right fluid,” Oka says. If you couldn’t sense water, you might drink another liquid by accident. And if that liquid is toxic, that could be a fatal mistake.
To hunt for this water sensor, Oka and his group studied mice. They dripped onto the animals’ tongues liquids with different flavors: sweet, sour and savory. They also dripped pure water. At the same time, the researchers recorded the electrical signals from the nerve cells attached to the taste buds. As expected, the scientists saw strong nerve responses to all the flavors. But they saw a similar strong response to water. Somehow, the taste buds were detecting water.
The mouth is a wet place. It’s filled with saliva — a mixture of enzymes and other molecules. They include bicarbonate ions — tiny molecules with negative charge. The bicarbonate makes saliva, and your mouth, a little bit basic. Basic substances have a higher pH than pure water. They are the opposite of acidic substances, which have a lower pH than water.
When water pours into your mouth it washes away that basic saliva. An enzyme in your mouth instantly kicks in to replace those ions. It combines carbon dioxide and water to make bicarbonate. As a side effect, it also produces protons.
The bicarbonate is basic, but the protons are acidic — and some taste buds have a receptor that senses acid. These receptors to detect the flavor we call “sour” — like in lemons. When the newly made protons hit acid-sensing receptors, the receptors send a signal to the taste bud nerve. And the taste bud nerve fires — not because it detected water, but because it detected acid.
To confirm this, Oka and his group used a technique called optogenetics. With this method, scientists insert a light-sensitive molecule into a cell. When light shines on the cell, the molecule triggers an electrical impulse.
Oka’s team added a light-sensitive molecule to the sour-sensing taste bud cells of mice. Then they shined light on the animals’ tongues. Their taste buds reacted and the animals licked, thinking they sensed water. If the light was attached to a water spout, the animals would lick it — even though the spout was dry.
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The team also knocked out the sour-sensing molecule in other mice. That means they blocked the genetic instructions for making this molecule. Without it, those mice couldn’t tell if what they were drinking was water. They’d even drink a thin oil instead! Oka and his group published their results May 29 in the journal Nature Neuroscience.
“This provides a starting point for how the detection for water is processed in the brain,” says Scott Sternson. He works at a Howard Hughes Medical Institute research center in Ashburn, Va. He studies how the brain controls behavior but was not part of this study. Sternson says it’s crucial to learn how we sense simple but vital things, such as water. “It’s important for the basic understanding of how our bodies work,” he says. The study was in mice, but their taste systems are similar to those of other mammals, including humans.
Just because acid-sensing molecules sense water doesn’t mean that water “tastes” sour. It doesn’t mean water has a flavor at all. Flavor is a complex interaction between taste and smell. Acid-sensing cells detect sour, and they detect water. But water detection, Oka notes, “is not water taste perception.” So water may still taste like nothing. But to our tongues, it’s definitely something.
acidic An adjective for materials that contain acid. These materials often are capable of eating away at some minerals such as carbonate, or preventing their formation in the first place.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
base (in chemistry) A chemical that produces hydroxide ions (OH - ) in a solution. Basic solutions are also referred to as alkaline. (in genetics) A shortened version of the term nucleobase. These bases are building blocks of DNA and RNA molecules.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
colleague Someone who works with another; a co-worker or team member.
enzymes Molecules made by living things to speed up chemical reactions.
gene (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.
hypothalamus A region of the brain that controls bodily functions by releasing hormones. The hypothalamus is involved in regulating appetite through release of appetite-suppressing hormones.
ion (adj. ionized) An atom or molecule with an electric charge due to the loss or gain of one or more electrons. An ionized gas, or plasma, is where all of the electrons have been separated from their parent atoms.
journal (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.
liquid A material that flows freely but keeps a constant volume, like water or oil.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.
mechanism The steps or process by which something happens or “works.” It may be the spring that pops something from one hole into another. It could be the squeezing of the heart muscle that pumps blood throughout the body. It could be the friction (with the road and air) that slows down the speed of a coasting car. Researchers often look for the mechanism behind actions and reactions to understand how something functions.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
nerve A long, delicate fiber that transmits signals across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold or pain.
neuroscience The field of science that deals with the structure or function of the brain and other parts of the nervous system. Researchers in this field are known as neuroscientists.
optogenetics A technique that uses light to better understand genes and cells in the nervous system, especially the brain. Recent research is using the technology to study other types of cells and tissues too.
perception The state of being aware of something — or the process of becoming aware of something — through use of the senses.
proton A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.
receptor (in biology) A molecule in cells that serves as a docking station for another molecule. That second molecule can turn on some special activity by the cell.
sensor A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly. (in biology) The structure that an organism uses to sense attributes of its environment, such as heat, winds, chemicals, moisture, trauma or an attack by predators.
taste One of the basic properties the body uses to sense its environment, especially foods, using receptors (taste buds) on the tongue (and some other organs).
taste buds A collection of 50 to 100 or so taste receptors, they’re found on the tongues of land animals. When certain chemicals in food or other materials trigger a response in these receptors, the brain detects one or more flavors — sweet, sour, salty, bitter or umami.
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.