Taste Receptors and Transduction

Eating serves a basic biological role—a key reason we eat is to acquire nutrients necessary for our health and survival. The sense of taste influences what we eat by helping us to judge a food's quality before we swallow it. Sweet and savory tastes suggest the presence of carbohydrates and proteins, important nutrient sources. Salty taste indicates that foods contain sodium chloride or other salts necessary for proper cellular function. Bitter taste is often the sign of toxins; sour taste may indicate spoilage. Of course, how we use this taste information depends on our experience, our preferences, and our motivations. For example, we may avoid sweet-tasting foods because we want to lose weight, and we can learn to enjoy bitter-tasting drinks like beer and coffee. Before we can make any of these judgments, we must first detect the taste stimuli present in the foods we eat. Humans and other mammals use a small number of receptor proteins, present on the surface of sensory cells in the oral cavity, to recognize taste stimuli (called tastants). Upon interaction with these tastants, the activated taste receptors trigger a cascade of molecular interactions within the sensory cell to transduce, or change, the chemical signal into a cellular signal that can be communicated to peripheral nerves. These nerves then carry taste information to the brain, where tastes are perceived.

The gustatory (i.e., taste) system is composed of three principal anatomical divisions: (1) taste buds within the gustatory epithelium of the tongue and the soft palate, which contain the taste sensory cells; (2) cranial nerves that connect these taste buds to gustatory areas of the brain stem; and (3) higher gustatory areas, such as the ventroposteriomedial nucleus of the thalamus and the gustatory cortex. The olfactory regions of the brain and brain stem are critical for processing taste information received from the periphery, helping to make sense of the signals the mouth detects. The taste buds, in contrast, are primarily concerned with the detection of taste stimuli. When tastants enter the mouth, they gain access to taste receptors on taste sensory cells within the taste buds, thus initiating the process of gustatory transduction.

Thousands of food compounds and other molecules have a taste. But the perceptual qualities elicited by those stimuli appear limited to just five: sweet, sour, bitter, salty, and umami (the savory taste of glutamate found in foods like meats and cheeses). For example, table sugar tastes sweet, but so do various no-calorie sweeteners, some amino acids and proteins, and even some heavy metals, such as lead. Why do such varied chemicals all taste sweet? The answer lies in the way taste stimuli are recognized. The gustatory system uses a few distinct populations of sensory cells in the oral cavity to detect tastants. Each sensory cell population is dedicated to the encoding of a different taste quality (i.e., sweet, bitter, etc.), and each of these sensory cell population expresses a different taste receptor or family of receptors.

The association of specific taste receptor types with distinct sensory cell populations dedicated to the coding of particular taste qualities dictates that all stimuli that can activate a particular taste receptor will elicit the same taste quality. In other words, activation of a “sweet” taste cell leads to the perception of sweetness, and activation of a “bitter” cell elicits bitterness. Which compounds can evoke these perceptions is dictated by the taste receptor(s) contained in different sensory cell types: The selectivity of these taste receptor(s) dictates that only certain tastants can elicit a particular taste quality. For example, the single taste receptor type expressed in “sweet” taste cells responds to numerous sugars and other natural and artificial sweeteners. Because all of these chemically diverse compounds can activate the taste receptor specifically expressed on “sweet” taste cells, all are perceived as sweet. Similarly, “bitter” taste cells employ a small number of taste receptors that together can recognize many hundreds of bitter-tasting compounds. Indeed, if your sweet taste receptor were expressed in “bitter” taste cells, table sugar would taste bitter to you. This entry covers the salt, sour, sweet, umami, and bitter tastes as well as the complexity of taste transduction.

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