Cellular signal transduction of sweetener-induced taste

Artificial sweeteners and salts producing a metallic taste sensation activate TRPV1 receptors

Throughout the world many people use artificial sweeteners (AS) for the purpose of reducing caloric intake. The most prominently used of these molecules include saccharin, aspartame (Nutrasweet), acesulfame-K, and cyclamate. Despite the caloric advantage they provide, one key concern in their use is their aversive aftertaste that has been characterized on a sensory level as bitter and/or metallic. Recently, it has been shown that the activation of particular T2R bitter taste receptors is partially involved with the bitter aftertaste sensation of saccharin and acesulfame-K. To more fully understand the biology behind these phenomena we have addressed the question of whether AS could stimulate transient receptor potential vanilloid-1 (TRPV1) receptors, as these receptors are activated by a large range of structurally different chemicals. Moreover, TRPV1 receptors and/or their variants are found in taste receptor cells and in nerve terminals throughout the oral cavity. Hence, TRPV1 activation could be involved in the AS aftertaste or even contribute to the poorly understood metallic taste sensation. Using Ca(2+) imaging on TRPV1 receptors heterologously expressed in the human embryonic kidney (HEK) 293 cells and on dissociated primary sensory neurons, we find that in both systems, AS activate TRPV1 receptors, and, moreover, they sensitize these channels to acid and heat. We also found that TRPV1 receptors are activated by CuSO(4), ZnSO(4), and FeSO(4), three salts known to produce a metallic taste sensation. In summary, our results identify a novel group of compounds that activate TRPV1 and, consequently, provide a molecular mechanism that may account for off tastes of sweeteners and metallic tasting salts.

Sweetener preference of C57BL/6ByJ and 129P3/J mice

Previous studies have shown large differences in taste responses to several sweeteners between mice of the C57BL/6ByJ (B6) and 129P3/J (129) inbred strains. The goal of this study was to compare behavioral responses of B6 and 129 mice to a wider variety of sweeteners. Seventeen sweeteners were tested using two-bottle preference tests with water. Three main patterns of strain differences were evident. First, sucrose, maltose, saccharin, acesulfame-K, sucralose and SC-45647 were preferred by both strains, but the B6 mice had lower preference thresholds and higher solution intakes. Second, the amino acids D-phenylalanine, D-tryptophan, L-proline and glycine were highly preferred by B6 mice, but not by 129 mice. Third, glycyrrhizic acid, neohesperidin dihydrochalcone, thaumatin and cyclamate did not evoke strong preferences in either strain. Aspartame was neutral to all 129 and some B6 mice, but other B6 mice strongly preferred it. Thus, compared with the 129 mice the B6 mice had higher preferences for sugars, sweet tasting amino acids and several but not all non-caloric sweeteners. Glycyrrhizic acid, neohesperidin, thaumatin and cyclamate are not palatable to B6 or 129 mice.

New hypotheses for the GPCR 3D arrangement based on a molecular model of the human sweet-taste receptor

A molecular model of the human sweet-taste receptor has been inferred from superpositions of 3D maps of sweetener interaction sites (themselves previously deduced from extensive structure-activity relationship studies on highly potent sweeteners) onto three well-known G protein-coupled receptors (GPCRs)-rhodopsin, beta(2)- and alpha(2A)-adrenergic receptors-assumed to be linked by common evolutionary origins. The model gives new answers to old questions on the GPCR 3D structure, such as on the orientation and arrangement of the binding helices, their interaxial distances, radial orientations and relative heights. The model should be useful as a new approach to the rational design of drugs.