Molecular mechanisms of bitter and sweet taste transduction.

2 Abbreviations AC adenylyl cyclase ATD amino-terminal domain cNMP cyclic nucleotide monophosphate dpa D-phenylalanine taste sensitivity locus G protein guanine nucleotide binding regulatory protein GPCR G protein coupled receptor IP 3 inositol trisphosphate NCBI National Center for Biotechnology Information PDE phosphodiesterase PLC phospholipase C PROP 6-n-propyl-2-thiouracil sac saccharin taste sensitivity locus TEA tetraethylammonium TRC taste receptor cell The sense of taste plays a critical role in the life and nutritional status of humans and other organisms. Human taste perception may be categorized according to four well-known and widely accepted descriptors, sweet, bitter, salty and sour (corresponding to particular taste qualities or modalities), and two more controversial qualities: fat and amino acid taste. The ability to identify sweet-tasting foodstuffs is particularly important as it provides us and other vertebrates with a means to seek out needed carbohydrates with high nutritive value. The perception of bitter, on the other hand, is essential for its protective value, enabling humans to avoid potentially deadly plant alkaloids and other environmental toxins. The focus of this review is on recent advances in our understanding of the transduction elements and signaling mechanisms underlying bitter and sweet taste transduction (see Figure 1). The sensations of bitter and sweet tastes are initiated by the interaction of sapid molecules ('tastants') with G protein coupled receptors (GPCRs) in the apical membranes of taste receptor cells (TRCs). TRCs are specialized epithelial cells with many neuronal properties including the ability to depolarize and form synapses. TRCs are typically clustered in groups of ~100 within taste buds. The apical surface of TRCs, which makes contact with the oral cavity, is rich in convoluted microvilli containing GPCRs, ion channels and other transduction elements. The basolateral aspect of TRCs contains ion channels and synapses with afferent taste nerves.