Sweet-sensitive protein from bovine taste buds: isolation and assay

A Pharmacological Perspective on the Study of Taste

The study of taste has been guided throughout much of its history by the conceptual framework of psychophysics, where the focus was on quantification of the subjective experience of the taste sensations. By the mid-20th century, data from physiologic studies had accumulated sufficiently to assemble a model for the function of receptors that must mediate the initial stimulus of tastant molecules in contact with the tongue. But the study of taste as a receptor-mediated event did not gain momentum until decades later when the actual receptor proteins and attendant signaling mechanisms were identified and localized to the highly specialized taste-responsive cells of the tongue. With those discoveries a new opportunity to examine taste as a function of receptor activity has come into focus. Pharmacology is the science designed specifically for the experimental interrogation and quantitative characterization of receptor function at all levels of inquiry from molecules to behavior. This review covers the history of some of the major concepts that have shaped thinking and experimental approaches to taste, the seminal discoveries that have led to elucidation of receptors for taste, and how applying principles of receptor pharmacology can enhance understanding of the mechanisms of taste physiology and perception.

Taste sensitivity to trehalose and its alteration by gene dosage in Drosophila melanogaster

The taste sensitivity to the disaccharide trehalose of Drosophila melanogaster is under the genetic control by the Tre gene on the X chromosome. The gene is genetically dimorphic for high and low sensitivity and is likely to be functioning in the primary step of chemoreception. We have determined the cytological localization of the Tre gene to be between 5A10 and 5B1-3 by analyzing the sensitivity to trehalose in flies which are segmentally aneuploid bearing either deficiencies or duplicated fragments of T(X;Y) translocations. We also constructed flies which are aneuploidy and thus carry different dosage of Tre and/or Tre(+) alleles in order to examine the gene dosage effect on trehalose sensitivity and to deduce the nature of the gene's action. Trehalose sensitivity decreased in females carrying half the normal dosage of a given Tre allele, but a proportional increase in sensitivity was not observed in flies bearing a duplication of the Tre alleles. The changes in sensitivity in various aneuploid flies suggest that there is an upper limit to the number of molecules that can be incorporated into the receptor membrane. Genetic evidence strongly suggests that Tre is the structural gene for the trehalose receptor. We present a model to account for the mechanism of genetical control on the sensitivity to trehalose.

Differential changes in taste perception induced by benzoic acid prickling

Benzoic acid (Bz) is a prickling compound used to preserve foods. However, its effects on taste are unknown. This work examines Bz-taste interaction using psychophysical methods [magnitude estimation (ME) and paired comparison (PC)] to measure taste intensity in aqueous solutions of pure tastants (T) and their respective mixtures with 10 mM Bz (Mix). Prototypical tastants induced basic taste qualities (mM): sucrose [90-1440, sweetness (Sw)], citric acid [1-64, sourness (So)], NaCl [15-960, saltiness (Sa)], quinine [0.01-0.64, bitterness (Bitt)], KCl (12.5-400, Sa and Bitt). MEs were analysed using Steven's and Beidler's equations. Bz increased Sw (all concentrations) and ionic tastes (low concentrations) and Bz effects were reduced by concentration increase according with quality and tastant Bz reduced Bitt(Quinine) (high concentrations). Bz reduced taste slopes (percentage decrease): Sw 45% (P<.02), So 34% (P<.01), Sa 35% or 41% (NaCl or KCl, P<.03), Bitt 33% or 60% (quinine P<.01 or KCl P<.04). Bz reduced K(diss) (affinity(-1)) (percentage reduction): Sw 79% (P<.0002), So 40% (P<.03), Sa(NaCl) 63% (P<.005), Sa(KCl) 48% (P<.04), Bitt(KCl) 64% (P<.04). Bz reduced ME(max) (percentage reduction): Sw 31% (P<.004), Bitt(Quinine) 29% (P<.03). PCs confirmed taste increases by Bz (percentage of 'Mix(intensity)>T(intensity)' answers/total answers): Sw 79-69% (90-1440 mM sucrose), So 75% (1 mM citric acid) and 71% (2 mM citric acid), Sa 75-71% (15-120 mM NaCl). Negative concentration dependence of taste increases by Bz suggests different levels of interaction. Biophysical and neurophysiological changes are discussed in relation with Bz properties and mechanism of interaction with taste.

Roles of chemical mediators in the taste system

Recent advances in neural mechanisms of taste are reviewed with special reference to neuroactive substances. In the first section, taste transduction mechanisms of basic tastes are explained in two groups, whether taste stimuli directly activate ion channels in the taste cell membrane or they bind to cell surface receptors coupled to intracellular signaling pathways. In the second section, putative transmitters and modulators from taste cells to afferent nerves are summarized. The candidates include acetylcholine, catecholamines, serotonin, amino acids and peptides. Studies favor serotonin as a possible neuromodulator in the taste bud. In the third section, the role of neuroactive substances in the central gustatory pathways is introduced. Excitatory and inhibitory amino acids (e.g., glutamate and GABA) and peptides (substance P and calcitonin gene-related peptide) are proved to play roles in transmission of taste information in both the brainstem relay and cortical gustatory area. In the fourth section, conditioned taste aversion is introduced as a model to study gustatory learning and memory. Pharmacobehavioral studies to examine the effects of glutamate receptor antagonists and protein kinase C inhibitors on the formation of conditioned taste aversion show that both glutamate and protein kinase C in the amygdala and cortical gustatory area play essential roles in taste aversion learning. Recent molecular and genetic approaches to disclose biological mechanisms of gustatory learning are also introduced. In the last section, behavioral and pharmacological approaches to elucidate palatability, taste pleasure, are described. Dopamine, benzodiazepine derivatives and opioid substances may play some roles in evaluation of palatability and motivation to ingest palatable edibles.

New perspectives in a gustatory physiology: transduction, development, and plasticity

Major advances in the understanding of mammalian gustatory transduction mechanisms have occurred in the past decade. Recent research has revealed that a remarkable diversity of cellular mechanisms are involved in taste stimulus reception. These mechanisms range from G protein-and second messenger-linked receptor systems to stimulus-gated and stimulus-admitting ion channels. Contrary to widely held ideas, new data show that some taste stimuli interact with receptive sites that are localized on both the apical and basolateral membranes of taste cells. Studies of taste system development in several species indicate that the transduction pathways for some stimuli are modulated significantly during the early postnatal period. In addition, recent investigations of adult peripheral gustatory system plasticity strongly suggest that the function of the Na+ sensing system can be modulated by circulating hormones, growth factors, or cytokines.

Sweeteners: state of knowledge review

Sweeteners are widely used in the food and pharmaceutical industry. The purpose of this paper is to review our current knowledge of sweet taste from chemical, biochemical, electrophysiological, psychophysical, and psychological points of view. The most common sweetners likely to be used in food and pharmaceuticals will be examined in detail. First, the chemical structures of sweet compounds including saccharides, diterpene glycosides, polyols, amino acids, dipeptides, and other nonsugars will be discussed. Second, biochemical approaches to understanding sweetner receptors will be reviewed. Third, electrophysiological and behavioral approaches to understanding sweetner receptors will be discussed. Fourth, psychophysical studies in humans will be shown to be consistent with biochemical and neurophysiological data. In addition, the basic mechanisms of sweet taste revealed by psychophysical studies will be given, including the role of multiple receptor sites, hydrogen bonding, and sodium transport. Finally, the factors that affect preference for sweet taste including the psychological and physiological variables associated with sweet preference will be explored.

Binding proteins for sweet compounds from gustatory papillae of the cow, pig and rat

The intensely sweet proteins thaumatin and monellin were covalently attached to affinity column supports. Lingual tissue extracts were incubated with the affinity columns which were then eluted with glycine-HCl pH 3.4, the sweet peptide aspartame, or gymnemic acid, which is a sweet taste modifier. SDS-PAGE analysis of eluates from the columns showed that 156 kDa and 47 kDa proteins were the main components from cow fungiform papillae which were specifically bound to thaumatin and monellin. These proteins could be displaced from the column with 0.5 mM aspartame or 0.5 mg/ml gymnemic acid. With circumvallate papillae small amounts of 47 kDa protein were also found. The 47 kDa protein was also the major component bound to a gymnemic acid affinity column and could be displaced from the column with 0.5 mg/ml gymnemic acid. Control experiments with other lingual tissue components indicated that these proteins are localised in the gustatory papillae. Similar protein patterns were also found in extracts of pig fungiform papillae and rat lingual preparations.

Development of chorda tympani nerve taste responses in the hamster

To determine whether changes in salt and sugar responses occur during development in the hamster, multifiber responses were recorded from the chorda tympani nerve while stimulating the anterior tongue of preweanling, early postweanling, and adult hamsters. Gustatory stimuli included 0.1 and 0.5 M solutions of NH4Cl, NaCl, LiCl, and KCl, and concentration series (0.01-1.0 M) of glucose, fructose, sucrose, maltose, lactose, and (0.0005-0.01 M) saccharin. Dramatic alterations in hamster peripheral gustatory sensitivities occurred with age, with the direction and magnitude of change dependent on the specific stimulus. Response magnitudes to 0.1 M solutions of NaCl and LiCl decreased with age compared to the NH4Cl response, whereas responses to all other salt stimuli remained constant during development. Responses to all sugars and saccharin compared to the NH4Cl response increased during development across a large concentration range; however, the age at which mature responses were achieved depended on the specific "sweet" stimulus. Whereas these findings demonstrate that the hamster peripheral gustatory system is dynamic during postnatal development, the hamster has a unique developmental pattern of salt taste development compared to other species. Specifically, the effectiveness of NaCl and LiCl decrease during development compared to NH4Cl in the hamster, but increase dramatically in the rat and sheep. Thus, the developmental patterns are opposite in direction for the hamster compared to the rat and sheep and may relate to the environmental pressures imposed upon each species.

Photoaffinity labeling of thaumatin-binding protein in monkey circumvallate papillae

Thaumatin I is an intensely sweet-tasting protein. It was photo-crosslinked with taste papillae of crab-eating monkey by using a conjugated photo-affinity reagent [3H]azidobenzoylthaumatin I. Serial sections of SDS-polyacrylamide gel electrophoresis of the 0.1 M sodium phosphate buffer-soluble fraction from taste papillae had a large peak of radioactivity at the Mr region of approx. 70,000; fractions from non-taste papillae did not. Excess unlabeled thaumatin I reduced the photo-crosslinking at the 70 kDa region; acetylated thaumatin I (which is not sweet) did not. The results show that taste papillae of the monkey contain a protein of Mr approx. 50,000, which binds to thaumatin I (Mr 22,209) but not to completely acetylated thaumatin I. The possibility that the thaumatin-binding protein is a sweet receptor protein is discussed.

Binding of [35S]saccharin to a protein fraction of rat tongue epithelia

The binding of [35S]saccharin to ammonium sulfate fractions from homogenates of rat tongue epithelia was measured by equilibrium dialysis. The 40--60% saturated ammonium sulfate fraction from the buffer-soluble fraction had the highest saccharin-binding activity. Binding of [35S]saccharin to the 40--60% ammonium sulfate fraction was inhibited by unlabeled saccharin sodium salt. The inhibition increased with increasing unlabeled saccharin concentration and was nearly complete above 10 mM. [35S]Saccharin binding to the 40--60% ammonium sulfate fraction extracted from the tongue epithelia was inhibited by glucose, lactose and sucrose, while binding to similar fractions from tongue muscle was not affected by these sugars. The inhibition of binding of labeled saccharin to the epithelial fraction increased with increasing glucose concentrations. About 35% of the binding was inhibited by 1 M glucose. No significant difference in the amount of inhibition was seen among the three sugars at 0.1 M. The 40--60% ammonium sulfate fraction from tongue epithelium devoid of taste buds bound much less [35S]saccharin than did a similar fraction from epithelium with taste buds. Binding of [35S]saccharin by the preparation from epithelium devoid of taste buds was not inhibited by glucose. The results provide evidence that the 40--60% ammonium sulfate fraction from tongue epithelia with taste buds contains a protein which binds saccharin and sugars. We hypothesize that it is a sweet taste receptor protein.

Neuron and stimulus typologies in the rat gustatory system

Although gustatory neurons may be categorized in terms of one or a few characteristics (e.g. 'best stimulus'), such typologies are essentialistic and inconsistent with modern taxonomic methods. If olythetic taxonomic criteria are used and the variability among neuronal responses is closely analyzed, neuronal 'types' are found to disappear, at least within the acid-salt range. This applies to both the primary nerve level (chorda tympani nerve) and secondary level (nucleus tractus solitarius) of the taste system in the rat. In the same context, taste stimuli may fall into different groups if several very similar stimuli are used (e.g. sodium and lithium salts). This is not surprising, and may depend on the choice of stimulus arrays rather than a differentiation of a few stimulus types by the taste system. Finally, it should be noted that the arguments regarding neuron and stimulus typologies presented here for the taste system are also valid for other sensory systems, although the conclusions may be different.

Hydrophobicity of biosurfaces as shown by chemoreceptive thresholds in Tetrahymena, Physarum and Nitella

Responses (chemotaxis and changes in membrane potential) of Tetrahymena, Physarum, and Nitella against aqueous solution of homologous series of n-alcohols, n-aldehydes and n-fatty acids were studied for clarifying the hydrophobic character of chemoreceptive membranes. Results were: (1) All organisms studied responded to homologous compounds examined when the concentration of these chemicals exceeded their respective threshold, Cth, and the response, R, were expressed approximately as R=alpha log (C/Cth) for C greater than Cth. (2) Increase of the length of hydrocarbon chain in homologues decreased Cth. Plots of log Cth against the number of carbon atoms, n, in n-alcohols, n-aldehydes and n-fatty acids showed linear relationships as represented by long Cth=-An+B. A and B are positive constants for respective functional end groups of the chemicals and biological membranes used. The above empirical equation was interpreted in terms of the partition equilibrium of methylene groups between bulk solution and membrane phase. Parameter A was shown to be a measure of hydrophobicity of the membrane, and B represented the sensitivity of chemoreception of the membrane. (3) Thresholds, Cth, for various hydrophobic reagents were compared with those of human olfactory reception, T. Plots of log T against log Cth fell on straight lines for respective organisms with different slopes which were proportional to parameter A.

Structural relationships of sugars to taste

Chemical modification of sugars and their simple analogues indicates that these types of compound are almost always sweet, bitter, or bitter/sweet; hence, the two basic tastes may be intimately associated features of the same molecule. Stepwise modification at each chiral center around the sugar ring allows the sapid functions in these molecules to be mapped and leads to the inescapable conclusion that sugar molecules may be "polarized" on taste bud receptors, so that one end of the molecule elicits sweetness and the other bitterness. However, more extensive chemical modification evidently causes the molecule to realign itself in entirely different ways on the receptor. In most oligosaccharides only one sugar residue is likely to bind to the taste receptor, and this is probably a nonreducing end group, because the anomeric center of glucopyranose types of structure does not appear to affect sweetness. Sweetness depresses bitterness and bitterness depresses sweetness. Hence, it is not possible to make structural comparisons between analogues without correcting for these effects. However, some semiquantitative studies have established the value of current hydrogen bond theories of sweetness and the ideal oxygen-oxygen interorbital spacings for sweetness criteria in sugar molecules.

Stimulation of the gerbil's gustatory receptors by disaccharides

The gustatory responses from the chorda tympani nerve of the Mongolian gerbil, Meriones unguiculatus, were treated with 13 disaccharides. Sucrose was the most stimulatory sugar. The ability of fructosyl glycosides to stimulate may depend upon the linkage between fructose and the glycoside. Disaccharides possessing 1 leads to 3, 1 leads to 4, or 1 leads to 6 linkages were poor stimuli compared to sucrose which has a 1 leads to 2 linkage. Glucopyranosyl disaccharides with an alpha-linkage were better stimuli than the beta-anomers, while galactopyranosyl disaccharides possessing a beta-linkage were better than their alpha-amoners.

Biochemical studies of taste sensation. III. Preparation of a suspension of bovine taste bud cells and their labeling with a fluorescent probe

A method to prepare suspensions of taste bud cells is described. Bovine circumvallate papillae, which contain most of the taste buds in this animal, are incubated in collagenase-containing medium and the epidermal sidewall tissue is then dissected from the inner gelatinous dermis. The sidewall tissue, which contains the taste buds, is gently homogenized by manual operation of an all-glass homogenizer with a loose-fitting pestle. The suspended material is separated on a discontinous Ficoll gradient (2%, 8%, 10%, 12% w/w). The material banding at the 8-2% interface is greatly enriched in spindle-shaped cells that are morphologically similar to taste bud cells as they appear in situ. These cells are not seen when the procedure is done with tissues devoid of taste buds, namely the upper surface of the circumvallate papilla or epithelium from the intermolar eminence. Fluorescence analysis indicates that the hydrophobic probe, 8-anilino-1-naphthalenesulfonate (ANS), binds to relatively nonpolar sites in the suspension. It is postulated that the probe is adsorbing onto the surface membrane of the cell. These preparations may be useful in studying specificity and transduction in taste sensation.

Membrane resistance change of the frog taste cells in response to water and Nacl

The electrical properties of the frog taste cells during gustatory stimulations with distilled water and varying concentrations of NaCl were studied with intracellular microelectrodes. Under the Ringer adaptation of the tongue, two types of taste cells were distinguished by the gustatory stimuli. One type, termed NaCl-sensitive (NS) cells, responded to water with hyperpolarizations and responded to concentrated NaCl with depolarizations. In contrast, the other type of cells, termed water-sensitive (WS) cells, responded to water depolarizations and responded to concentrated NaCl with hyperpolarizations. The membrane resistance of both taste cell types increased during the hyperpolarizing receptor potentials and decreased during the depolarizing receptor potentials, Reversal potentials for the depolarizing and hyperpolarizing responses in each cell type were a few millivolts positive above the zero membrane potential. When the tongue was adapted with Na-free Ringer solution for 30 min, the amplitude of the depolarizing responses in the NS cells reduced to 50% of the control value under normal Ringer adaptation. On the basis of the present results, it is concluded (a) that the depolarizing responses of the NS and WS cells under the Ringer adaptation are produced by the permeability increase in some ions, mainly Na+ ions across the taste cell membranes, and (b) that the hyperpolarizing responses of both types of taste cells are produced by a decrease in the cell membrane permeability to some ions, probably Na+ ions, which is slightly enhanced during the Ringer adaptation.

Disc electrophoresis of extracts from the taste buds located in circumvallate papillae of rat tongues

The epithelium of the circumvallate papillae of rat tongues was stripped off by treatment with 0.005% elastase in a state when many taste buds were present. The taste buds were isolated from the stripped epithelium by further treatment with 0.005% elastase and 0.08% trypsin. A protein which was thought to be characteristic of taste buds was found from semimicro disc polyacrylamide gel electrophoretic studies of the stripped epithelia with and without taste buds. This result was supported by micro disc polyacrylamide gel electrophoretic studies of isolated taste buds.

Do unique proteins exist in taste buds?

Proteins in papillae on the bovine tongue were analyzed by semi-micro, polyacrylamide gel electrophoresis. All the proteins in the papillae with taste buds were observed to be common to proteins in the surrounding epithelium without taste buds. The protein band which was reported to form a weak complex with compounds called sweet by man was also found in all parts of the tongue epithelium. The receptor molecules for chemical stimuli may be distributed in all the cells of the tongue epithelium or the content of receptor molecules in taste bud papillae may be extremely low.