A taste receptor is a type of cellular receptor that facilitates the sensation of taste. When food or other substances enter the mouth, molecules interact with saliva and are bound to taste receptors in the oral cavity and other locations. Molecules which give a sensation of taste are considered "sapid". Vertebrate taste receptors are divided into two families:
Type 1, sweet, first characterized in 1999: TAS1R2 – TAS1R3 Type 2, bitter, first characterized in 2000: In humans there are 25 known different bitter receptors, in cats there are 12, in chickens there are three, and in mice there are 35 known different bitter receptors. Visual, olfactive, "sapictive" (the perception of tastes), trigeminal (hot, cool), mechanical, all contribute to the perception of taste. Of these, transient receptor potential cation channel subfamily V member 1 (TRPV1) vanilloid receptors are responsible for the perception of heat from some molecules such as capsaicin, and a CMR1 receptor is responsible for the perception of cold from molecules such as menthol, eucalyptol, and icilin.
Tissue distribution The gustatory system consists of taste receptor cells in taste buds. Taste buds, in turn, are contained in structures called papillae. There are three types of papillae involved in taste: fungiform papillae, foliate papillae, and circumvallate papillae. (The fourth type - filiform papillae do not contain taste buds). Beyond the papillae, taste receptors are also in the palate and early parts of the digestive system like the larynx and upper esophagus. There are three cranial nerves that innervate the tongue; the vagus nerve, glossopharyngeal nerve, and the facial nerve. The glossopharyngeal nerve and the chorda tympani branch of the facial nerve innervate the TAS1R and TAS2R taste receptors. Next to the taste receptors in on the tongue, the gut epithelium is also equipped with a subtle chemosensory system that communicates the sensory information to several effector systems involved in the regulation of appetite, immune responses, and gastrointestinal motility. In 2010, researchers found bitter receptors in lung tissue, which cause airways to relax when a bitter substance is encountered. They believe this mechanism is evolutionarily adaptive because it helps clear lung infections, but could also be exploited to treat asthma and chronic obstructive pulmonary disease. The sweet taste receptor (T1R2/T1R3) can be found in various extra-oral organs throughout the human body such as the brain, heart, kidney, bladder, nasal respiratory epithelium and more. In most of the organs, the receptor function is unclear. The sweet taste receptor found in the gut and in the pancreas was found to play an important role in the metabolic regulation of the gut carbohydrate-sensing process and in insulin secretion. This receptor is also found in the bladder, suggesting that consumption of artificial sweeteners which activates this receptor might cause excessive bladder contraction.
Function Taste helps to identify toxins, maintain nutrition, and regulate appetite, immune responses, and gastrointestinal motility. Five basic tastes are recognized today: salty, sweet, bitter, sour, and umami. Salty and sour taste sensations are both detected through ion channels. Sweet, bitter, umami, and fat taste, however, are detected by way of G protein-coupled taste receptors. In addition, some agents can function as taste modifiers, as miraculin or curculin for sweet or sterubin to mask bitter.
Mechanism of action The standard bitter, sweet, or umami taste receptor is a G protein-coupled receptor with seven transmembrane domains. Ligand binding at the taste receptors activate second messenger cascades to depolarize the taste cell. Gustducin is the most common taste Gα subunit, having a major role in TAS2R bitter taste reception. Gustducin is a homologue for transducin, a G-protein involved in vision transduction. Additionally, taste receptors share the use of the TRPM5 ion channel, as well as a phospholipase PLCβ2.
Savory or glutamates (Umami) The TAS1R1+TAS1R3 heterodimer receptor functions as an umami receptor, responding to L-amino acid binding, especially L-glutamate. The umami taste is most frequently associated with the food additive monosodium glutamate (MSG) and can be enhanced through the binding of inosine monophosphate (IMP) and guanosine monophosphate (GMP) molecules. TAS1R1+3 expressing cells are found mostly in the fungiform papillae at the tip and edges of the tongue and palate taste receptor cells in the roof of the mouth. These cells are shown to synapse upon the chorda tympani nerves to send their signals to the brain, although some activation of the glossopharyngeal nerve has been found. Alternative candidate umami taste receptors include splice variants of metabotropic glutamate receptors, mGluR4 and mGluR1, and the NMDA receptor. Not all animals use T1R1+T1R3 as the umami receptor; shifts in what a receptor detects often happen with evolution. For examples, sweet-sensing songbirds have repurposed their umami receptor into a sweet receptor. Almost every major vertebrate lineage uses a TAS1R to detect amino acids, a signal of the nutritional quality of food. However, glutamate is not among the more commonly-detected amino acids; instead, early vertebrates may have focused on basic and branched-chain amino acids, as this was the shared preference among bichir and elephant shark, representative of the bony and cartilaginous fishes respectively.
Sweet
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