Candidate taste receptors in Drosophila.
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Little is known about the molecular mechanisms of taste perception in animals, particularly the initial events of taste signaling. A large and diverse family of seven transmembrane domain proteins was identified from the Drosophila genome database with a computer algorithm that identifies proteins on the basis of structure. Eighteen of 19 genes examined were expressed in the Drosophila labellum, a gustatory organ of the proboscis. Expression was not detected in a variety of other tissues. The genes were not expressed in the labellum of a Drosophila mutant, pox-neuro70, in which taste neurons are eliminated. Tissue specificity of expression of these genes, along with their structural similarity, supports the possibility that the family encodes a large and divergent family of taste receptors. (+info)
How do inositol and glucose modulate feeding in Manduca sexta caterpillars?
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Many species of caterpillar possess taste cells that respond exceptionally vigorously to the sugar alcohol myo-inositol. We examined the functional significance of these inositol-sensitive taste cells in Manduca sexta caterpillars through an integrated series of electrophysiological and behavioral studies. Neural recordings from all the gustatory chemosensilla revealed that M. sexta have only two pairs of inositol-sensitive taste cells, which respond strongly and selectively to myo-inositol, and two pairs of sugar-sensitive taste cells, which respond relatively weakly to sugars (glucose and sucrose). Behavioral studies established that myo-inositol incites feeding and counteracts the inhibitory effects of aversive taste stimuli (e.g. caffeine) on feeding, but does not promote increased consumption once feeding has been initiated. In contrast, glucose and sucrose did not produce any robust effects on feeding. We failed to obtain any evidence of sensory inhibition between taste cells that responded to myo-inositol and caffeine, indicating that myo-inositol counteracts the inhibitory effects of caffeine on feeding through a central gustatory mechanism. We conclude that sensory input from the inositol-sensitive taste cells, but not the sugar-sensitive taste cells, plays an important role in regulating feeding in M. sexta. (+info)
Transduction ion channels directly gated by sugars on the insect taste cell.
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Insects detect sugars and amino acids by a specialized taste cell, the sugar receptor cell, in the taste hairs located on their labela and tarsi. We patch-clamped sensory processes of taste cells regenerated from the cut end of the taste hairs on the labelum of the flashfly isolated from the pupa approximately 20 h before emergence. We recorded both single channel and ensemble currents of novel ion channels located on the distal membrane of the sensory process of the sugar receptor cell. In the stable outside-out patch membrane excised from the sensory processes, we could repeatedly record sucrose-induced currents for tens of minutes without appreciable decrease. An inhibitor of G-protein activation, GDP-beta-S, did not significantly decrease the sucrose response. These results strongly suggested that the channel is an ionotropic receptor (a receptor/channel complex), activated directly by sucrose without mediation by second messengers or G protein. The channel was shown to be a nonselective cation channel. Analyses of single channel currents showed that the sucrose-gated channel has a single channel conductance of approximately 30 pS and has a very short mean open time of approximately 0.23 ms. It is inhibited by external Ca(2+) and the dose-current amplitude relation could be described by a Michaelis-Menten curve with an apparent dissociation constant of approximately 270 mM. We also report transduction ion channels of the receptor/channel complex type directly gated by fructose and those gated by L-valine located on the sensory process. (+info)
Glutamic acid, twenty years later.
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This review examines progress in understanding the physiologic functions of glutamic acid in the body since the first symposium on glutamic acid physiology and biochemistry was held at the Mario Negri Institute in Milan in 1978. The topics reviewed, although not exhaustive, include the metabolism of glutamic acid, umami taste, the role of glutamic acid as a neurotransmitter, glutamate safety and the development of new drugs resulting from the knowledge of the neurodegeneration induced by high doses of glutamic acid. (+info)
Glutamate and the flavor of foods.
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Investigations of the effects of glutamic acid or its salts on the flavor, hedonic characteristics or acceptance of foods have emphasized a sodium salt of L-glutamic acid, monosodium glutamate (MSG). Food palatability increased with appropriate concentrations of MSG, and could be represented f(MSG and NaCl) as points on the surface of an elliptic paraboloid. Hedonic effects differed between foods, were a function of concentrations of MSG and other ingredients and did not necessarily become positive when only MSG was added. For example, with boiled or fried rice, ratings were neutral or negative with MSG alone, positive for fried rice with MSG and NaCl, but positive for boiled rice only if soy sauce was also added. A one-dish meal, Chinese noodle, became positive with an appropriate concentration of NaCl plus a MSG-5'ribonucleotide mixture, but not with NaCl alone. Flavor of meat, fish or vegetable stocks containing natural glutamates differed from that of quinine, sucrose, NaCl or acid solutions. Glutamates and free amino acids plus nucleotides were necessary for full boiled potato flavor. Overall, the effects of MSG on foods were different from those of NaCl but often interacted with NaCl, and positive effects were facilitated by low concentrations of 5'-ribonucleotides. (+info)
Function and importance of glutamate for savory foods.
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Flavoring systems are of vital importance in savory food manufacturing. Many industrially prepared foods are particularly attractive to potential consumers primarily because of their typical flavors. Therefore, it is no surprise that the food industry dealing with these product segments shows great interest in the use of food or food ingredients carrying the typical umami taste and flavor enhancement systems. Figures are provided showing the importance of glutamate in traditional cuisines and also in meals prepared by industrial manufacturing. It is also interesting to see how food intake patterns of glutamate differ from one cultural group to another. The ever-growing importance of balanced food formulations (carbohydrates, fats, proteins and minerals) brings special challenges to the use of different ingredients, requiring development of appropriate flavor delivery systems. Again flavor enhancement is of great importance. Questions about the addition of glutamate or the total glutamate content of foods are of little importance, from a scientific point of view. However, in a given legal framework, important business opportunities can be realized. One of the main concerns of manufacturers of savory food is how to provide the consumer with tasty foods while complying with increasingly severe local legal constraints concerning the use of many potent flavoring systems. (+info)
Umami and food palatability.
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Umami is the term that identifies the taste of substances such as L-glutamate salts, which were discovered by Ikeda in 1908. Umami is an important taste element in natural foods; it is the main taste in the Japanese stock "dashi," and in bouillon and other stocks in the West. The umami taste has characteristic qualities that differentiate it from other tastes, including a taste-enhancing synergism between two umami compounds, L-glutamate and 5'-ribonucleotides, and a prolonged aftertaste. The key qualitative and quantitative features of umami are reviewed in this paper. The continued study of the umami taste will help to further our general understanding of the taste process and improve our knowledge of how the taste properties of foods contribute to appropriate food selection and good nutrition. (+info)
Intensification of sensory properties of foods for the elderly.
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Taste and smell losses in the elderly can reduce appetite and lead to inadequate dietary intake. Although these chemosensory deficits are generally not reversible, sensory interventions including intensification of taste and odor can compensate for perceptual losses. One method for "treatment" of chemosensory losses involves sensory enhancement of foods with flavors and monosodium glutamate (MSG). Amplification of flavor and taste can improve food palatability and acceptance, increase salivary flow and immunity, and reduce oral complaints in both sick and healthy elderly. (+info)