What tastes good to you right now? A sweet carrot? A bitter beer? Or maybe some salty pretzels? Whatever it is, how it tastes to you is determined by your cells. So how do we taste?
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We separate tastes for humans (and probably most mammals) into five categories: bitter, sour, sweet, salty, and umami (the taste generated when you consume certain amino acids, such as MSG). Our taste preferences when we are born are genetically encoded. We prefer sweet tastes as babies, because “sweet” is associated with healthy foods that contain proteins and energy. We avoid bitter and sour things, because these are associated with toxins and acidic foods, like spoiled fruits or harmful plants. Humans (and mice) generally prefer salty foods when our bodies are low in sodium with our tastes fitting our physiological need. As we mature (or most of us anyway), we acquire tastes for more bitter and sour things, such as coffee and lemons, and may prefer salty and sweet foods even though we really don’t need to build up our salt or sugar supply.
From studies in mice, it is thought that the five tastes are determined by separate taste-receptor cells (TRCs) in the mouth each tuned to a specific taste. TRCs are organized into taste buds, composed of 50-100 cells, and these taste buds are housed within papillae (the bumps on your tongue). Taste buds from all regions of the mouth contain cells that respond to the five tastes and are connected to nerves that carry taste information to your brain stem and into the primary gustatory cortex (where your response to a food and your perception of flavor is dictated). Contrary to popular belief, there is no “taste map” on the tongue.
Recently published online in Nature, Jayaram Chandrashekar and colleagues (including scientists from the National Institute of Health in Bethesda) illustrate how salty taste occurs in sodium sensing in mice. Each TRC that determines a salty taste contains a sodium channel (or sodium transporter). This channel is present in many cells within your body, including your kidneys, lungs, and sweat glands, and is important in salt transport. Its role is so important that if this channel is made completely nonfunctional in mice, death occurs soon after birth. In order to study the importance of this channel in taste, scientists used a clever technique (attaching the channel mutation-causing agent to a known TRC-specific gene) to generate mice that were missing the sodium channel only in their taste buds. Unlike their ‘normal’ peers, these mice displayed a complete loss of salt attraction and sodium taste response while responding normally to the other four tastes. And most importantly, all of the TRC cells that carried the mutated salt channel did not carry any markers for the other four taste sensors, showing again that individual TRC cells are only able to determine one taste.
Whether or not this specific sodium channel is as important to salty tastes in humans as it is in mice is unknown. Due to our molecular similarities, the authors note that it is likely. But unlike mice, our innate responses to salty taste may be overridden by our high-salt diets.
Now, pass me those fries. All this talk of salt has made me hungry.
