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Taste sensation is originated once a substance reacts chemically with taste receptor cells (TRCs) placed on the taste buds within the oral cavity, mostly on the tongue. Taste is one among the five senses that belong to the sensory system. Taste perception is a key sensory input of all organisms. A taste receptor is a type of receptor which facilitates the taste sensation. Taste receptors are located on the taste receptor cell membranes. The gustatory system of mammals senses five basic taste qualities such as bitter, sweet, salty, sour and umami, the taste of glutamate.
In the common language, the word “taste” is often used to describe sensations arising from the oral cavity. However, the biological definition of taste, or gustation, is narrower and includes only sensations mediated by a specialized anatomically and physiologically defined chemosensory gustatory system. Along with taste sensations, food usually simultaneously evokes other sensations, e.g., odor, touch, temperature, and irritation. Although it is not always easy to separate all these sensations perceptually, the non-gustatory components are sensed by different systems, olfaction, and somatosensation.
Taste receptors provide animals with valuable sensory information for evaluation of food. The sense of taste evokes responses that range from innate behavioral actions such as aversion and attraction to food sources to the pleasure of food consumption. Notably, a single taste receptor cell expresses a large repertoire of taste receptor proteins.
This suggests that each cell is capable of recognizing multiple tastants. Taste perception also plays a major role in interoceptive (hunger, safety, and specialized appetites) and exteroceptive signals (vision, olfaction, and somatosensation) and to generate behavioral responses to taste stimuli.
Taste receptors are evolved to protect that organism against ingestion of poisonous food compounds. Also, recent reports suggest that these proteins may have additional functions apart from the detection of taste. Human taste receptors are also found in human airway smooth muscle tissue and the effect of tastants on the function of human bronchi is currently being studied.
The gustatory system in mammals includes taste receptor cells organized in taste buds located within gustatory papillae. Most of the taste papillae belong to three types— fungiform, foliate, and vallate—and are located in the tongue. There is also a substantial number of non-lingual taste papillae in the palate, oropharynx, larynx, epiglottis, and the upper esophagus. Apical ends of the TRCs are exposed to the oral cavity and interact with taste stimuli, usually water-soluble chemicals. This interaction generates signals that are transmitted to the brain via branches of three cranial nerves, VII (facial), IX (glossopharyngeal), and X (vagus).
In the past several years, tremendous progress has been achieved with the discovery and characterization of vertebrate taste receptors from the T1R and T2R families, which are involved in recognition of bitter, sweet, and umami taste stimuli. Individual differences in taste, at least in some cases, can be attributed to allelic variants of the T1R and T2R genes.
The survival of all animals depends on the consumption of nutrients. However, sources of nutrients often also contain toxic substances. Taste helps animals to decide whether food is beneficial for them and should be consumed or whether it is dangerous for them and should be rejected. Probably, taste evolved to ensure animals choose food appropriate for body needs.
The current consensus is that human taste sensations can be divided into five qualities: bitter, sour, salty, sweet, and umami (savory; the prototypical stimulus being the amino acid glutamate). Aversive bitter taste often indicates the presence of toxins in food. Bitter and sour tastes may also signal spoiled food. The main salty taste stimuli are sodium salts, but some non-sodium salts also have a salty taste component.
This suggests that salty taste signals the presence of either sodium or minerals in general. The most common natural sweet taste stimuli are sugars, which indicate the presence of carbohydrates in food. The most common umami taste stimulus is L-glutamate, which may indicate the presence of protein. Other important nutrients include lipids, calcium, and water, but the existence of taste qualities corresponding to them is debatable.
The existence of several different taste qualities implies that each taste quality has a specific coding mechanism mediated by specialized taste receptors. Current data support this hypothesis. Reception of taste qualities that humans describe as sweet, umami, and bitter involves proteins from the T1R and T2R families. Candidate receptors have been proposed for salty and sour tastes.
There is substantial interest in developing novel taste stimuli and taste modifiers for humans and other animals. For humans, areas of interest include making food and drinks healthier without sacrificing their palatability and making oral medications more acceptable to patients. A substantial demand exists for artificial sweet and umami compounds, enhancers of salty, sweet, and umami taste, blockers of bitter taste, and pharmaceutical compounds with improved sensory properties.
There is also a demand for improvement in the taste quality of food for companion and farm animals and for developing nonlethal repellents of wild animals, e.g., non-toxic chemicals with aversive taste. Development of such products has been hampered by lack of knowledge of the molecular identity of the taste receptors. Discovery of taste receptors, characterization of their active sites involved in interactions with agonists and antagonists, and development of high-throughput techniques for in vitro screening of taste stimuli will facilitate the design of novel taste-active compounds.
Allelic variation of human taste receptors can affect food perception, choice, and consumption. As a result, it can influence nutrition and potentially predispose individuals to certain diseases. Thus, some taste receptor alleles can be disease risk factors. Genotypes of these receptors may be useful as biological markers to identify predispositions to some diseases and to suggest interventions for disease prevention. Available data provide some examples of the role of taste receptor variation in human nutrition and health. Sensitive alleles of human TAS2R38 receptor respond to PTC, PROP, and related compounds that contain a thiourea (N – C = S) moiety.
Some plants consumed by humans contain glucosinolates, compounds that also contain the thiourea moiety. A recent study has shown that TAS2R38 genotype affects the perception of the bitterness of glucosinolate-containing plants, such as broccoli, turnip, and horseradish. Allelic variation of TAS2R38 may have even more widespread effects on food choice, as it was shown to be associated with preferences for sucrose and sweet-tasting beverages and foods in children (but not adults) (Mennella, Pepino, & Reed, 2005). Taste receptor variation may be a biomarker of predisposition to alcoholism.
Ethanol flavor has bitter and sweet taste components. Variation in bitter and sweet taste responsiveness is associated with the perception of ethanol flavor and consumption of alcoholic beverages (Bachmanov et al., 2003). In mice, allelic variation of the Tas1r3 sweet taste receptor gene is associated with voluntary ethanol consumption (Bachmanov et al., 2002). Although hedonic responses to sweet taste are considered as one of the biomarkers of predisposition to alcoholism in humans (Kampov-Polevoy, Eick, Boland, Khalitov, & Crews, 2004), genes responsible for this association are still unknown. Higher sensitivity to ethanol bitterness may protect against excessive alcohol consumption.
Taste receptors function as one of the interfaces between internal and external milieus. Tremendous progress has been achieved in the past few years with the discovery of the T1R and T2R receptors and the understanding of their function.
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