23. Exploring Taste Sensation in Mice Pleasing tastes are an evolutionary adapta

Question

23. Exploring Taste Sensation in Mice Pleasing tastes are an evolutionary adaptation to encourage animals to consume nutritious foods. Zhao and coauthors (2003) examined the two major pleasurable taste sensations: sweet and umami. Umami is a "distinct savory taste" triggered by amino acids, especially aspartate and glutamate, and probably encourages animals to consume protein-rich foods. Monosodium glutamate (MSG) is a flavor enhancer that exploits this sensitivity. At the time the article was published, specific taste recep- tor proteins for sweet and umami had been tentatively char- acterized. Three such proteins were known-T1R1, T1R2, and T1R3-which function as heterodimeric receptor complexes: T1R1-T1R3 was tentatively identified as the umami receptor, and T1R2-T1R3 as the sweet receptor. It was not clear how taste sensation was encoded and sent to the brain, and two possible models had been suggested. In the cell-based model individual taste-sensing cells express only one kind of recep- tor; that is, there are "sweet cells," "bitter cells," "umami cells and so on, and each type of cell sends its information to the brain via a different nerve. The brain "knows" which taste is detected by the identity of the nerve fiber that transmits the message. In the receptor-based model, individual taste-sensing cells have several kinds of receptors and send different mes sages along the same nerve fiber to the brain, the message depending on which receptor is activated. Also unclear at the time was whether there was any interaction between the dif ferent taste sensations, or whether parts of one taste-sensing system were required for other taste sensations 03 (a) Previous work had shown that different taste receptor proteins are expressed in non-overlapping sets of taste receptor cells. Which model does this support? Explain your reasoning

Explanation / Answer

The umami and sweet taste receptors are heteromeric, G-protein-coupled receptors. T1R1-T1R3 is activated by glutamate and aspartate, as well as certain 5?- ribonucleotides, such as inosine and guanosine. MSG and glutamyl peptides inhibits the responses of sweet receptors. Ligands that are inhibited by umami compounds have a common activation site, the ECD of T1R2. High-potency sweeteners (HPS), which bind and activate T1Rs to stimulate sweet taste, present a low- or no-calorie alternative to sugar consumption. T1Rs facilitate the identification and assimilation of nutrients. T1Rs are important receptors in the transduction of sweet and umami tastes, which help to ensure the consumption of sugars and amino acids.T1R3 is expressed in both anterior and posterior fields in TRCs whose morphology is consistent with type II cells. It is coexpressed with either T1R1 or T1R2, although a fraction of T1R3-expressing TRCs coexpress neither. T1R3 requires coexpression of T1R2 to be fully responsive to a wide variety of sweet substances such as simple sugars, artificial sweeteners, d-amino acids, and sweet proteins. all four domains of the T1R2/T1R3 dimer — the two N-terminal domains and the two transmembrane domains — have been implicated in binding of ligands, each with distinct affinities to its corresponding ligands. There is still debate as to whether the T1R1/T1R3 dimer is the only functional glutamate receptor in TRCs.

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