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Mechanisms of Taste Transduction.docx

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Steve Risavy

Mechanisms of Taste Transduction The process by which an environmental stimulus causes an electrical response in a sensory receptor cell is called transduction. Taste transduction involves several different processes and each basic taste uses one or more of these mechanisms:  Directly pass through ion channels (salt and sour)  Bind and block ion channels (sour)  Bind to G-protein-coupled receptors in the membrane that activate second messenger systems Saltiness The taste of salt is mostly the taste of the cation Na and its concentration must be quite high in + order to taste it (at least 10 nM). Salt-sensitive taste cells have a special Na -selective channel that is blocked by the drug amiloride (see Fig. 8.5). This channel is insensitive to voltage and is + always open. When +ou sip chick soup, for example, the Na concentration rises outside the receptor cell. Na then diffuses down its concentration gradient which results in an inward current and depolarization (receptor potential) of the membrane. The receptor potential causes the voltage-gated sodium and calcium channels to open and trigger the release of neurotransmitter onto the gustatory afferent axon. Sourness Protons are the causative agents of acidity and sourness. They are known to affect sensitive taste receptors in at least two ways (see Fig. 8.5): + +  First, H can permeate the amiloride-sensitive sodium channel and cause an inward H current and depolarize the cell. + +  Second, hydrogen ions can bind to and block K -selective channels. When the K permeability of a membrane is decreased, it depolarizes. Bitterness There are two families of taste receptor genes (T1R and T2R) which encode for a variety of G- protein-coupled taste receptors. Bitter substances are detected by the 30 or so different types of T2R receptors, however, animals are not very good at telling different bitter tastants apart because each bitter taste cell expresses many, and perhaps all, of the 30 bitter receptor proteins. Because each taste cell can send only one
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