<P> The TAS1R1 + TAS1R3 heterodimer receptor functions as the savory 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 . </P> <P> The TAS1R2 + TAS1R3 heterodimer receptor functions as the sweet receptor by binding to a wide variety of sugars and sugar substitutes . TAS1R2 + 3 expressing cells are found in circumvallate papillae and foliate papillae near the back of the tongue and palate taste receptor cells in the roof of the mouth . These cells are shown to synapse upon the chorda tympani and glossopharyngeal nerves to send their signals to the brain . The TAS1R3 homodimer also functions as a sweet receptor in much the same way as TAS1R2 + 3 but has decreased sensitivity to sweet substances . Natural sugars are more easily detected by the TAS1R3 receptor than sugar substitutes . This may help explain why sugar and artificial sweeteners have different tastes . </P> <P> The TAS2R proteins function as bitter taste receptors . There are 43 human TAS2R genes, each of which (excluding the five pseudogenes) lacks introns and codes for a GPCR protein . These proteins, as opposed to TAS1R proteins, have short extracellular domains and are located in circumvallate papillae, palate, foliate papillae, and epiglottis taste buds, with reduced expression in fungiform papillae . Though it is certain that multiple TAS2Rs are expressed in one taste receptor cell, it is still debated whether mammals can distinguish between the tastes of different bitter ligands . Some overlap must occur, however, as there are far more bitter compounds than there are TAS2R genes . Common bitter ligands include cycloheximide, denatonium, PROP (6 - n - propyl - 2 - thiouracil), PTC (phenylthiocarbamide), and β - glucopyranosides . </P> <P> Signal transduction of bitter stimuli is accomplished via the α - subunit of gustducin . This G protein subunit activates a taste phosphodiesterase and decreases cyclic nucleotide levels . Further steps in the transduction pathway are still unknown . The βγ - subunit of gustducin also mediates taste by activating IP (inositol triphosphate) and DAG (diglyceride). These second messengers may open gated ion channels or may cause release of internal calcium . Though all TAS2Rs are located in gustducin - containing cells, knockout of gustducin does not completely abolish sensitivity to bitter compounds, suggesting a redundant mechanism for bitter tasting (unsurprising given that a bitter taste generally signals the presence of a toxin). One proposed mechanism for gustducin - independent bitter tasting is via ion channel interaction by specific bitter ligands, similar to the ion channel interaction which occurs in the tasting of sour and salty stimuli . </P>

The receptors for bitter are located in the
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