Taste Receptor Sites Research Articles

Sugar receptor response of the food-canal taste sensilla in a nectar-feeding swallowtail butterfly, Papilio xuthus

The feeding behavior in nectar-feeding insects is triggered by a sugar-receptor response in contact chemosensilla. The contact chemosensilla are distributed not only on tarsi and the outside of the proboscis but also on the inside of the food canal in Lepidoptera. Although the chemosensilla inside the food canal are assumed to detect sweet taste during the passage of nectar through the food canal, their electrophysiological function has received little attention. In the nectar-feeding Asian swallowtail butterfly, Papilio xuthus (Lepidoptera: Papilionidae), we found 15- to 30-mum-long sensilla neatly lined up along the inside galea wall, which forms the food canal in the proboscis. The receptor neurons of these sensilla responded to sucrose. We hypothesized that starch and sucrose compete with each other for a taste receptor site on the sensilla. When we added starch and sucrose to the food-canal sensilla, the electrophysiological responses of food-canal sensilla were inhibited in parallel with the food-sucking behavior of the butterflies. These results suggest that the food-canal sensilla are involved in the behavioral control of nectar-sucking in this butterfly species.

Modeling of the Psychophysical Response Curves Using the Grand Canonical Ensemble in Statistical Physics

This paper aims to investigate the peripheral mechanism of taste perception by the use of the grand canonical ensemble in statistical physics. It allows a better understanding of this process and its interpretation at a microscopic level. The experimental part allowed us to obtain psychophysical curves relative to four nutritive sweeteners (sucrose, fructose, glucose, and maltitol). These curves represent the intensity of sweetness as a function of sugar concentration in water. A Sensory Measuring Unit for Recording Flux (SMURF) device is used to obtain intensity–time curves for each of the studied sweeteners. To model these curves we have established the expressions of some models using grand canonical ensemble formalism in statistical physics and applying some simplification approaches. The fitting of the psychophysical data with statistical models by numerical simulation demonstrates a good correlation between the models and the experimental curves. Some physicochemical parameters interfere in the expression of the established models. These parameters are classified in two categories: on one hand, the steric aspect, which is manifested by the density of taste receptor site, and the number of molecules per receptor site and on the other hand, an energetic aspect illustrated by the concentration at half saturation, which gives indirectly the sweet molecule-receptor site energy of interaction.

Role of saliva in the maintenance of taste sensitivity.

Saliva is the principal fluid component of the external environment of the taste receptor cells and, as such, could play a role in taste sensitivity. Its main role includes transport of taste substances to and protection of the taste receptor. In the initial process of taste perception, saliva acts as a solvent for taste substances; salivary water dissolves taste substances, and the latter diffuse to the taste receptor sites. During this process, some salivary constituents chemically interact with taste substances. For example, salivary buffers (e.g., bicarbonate ions) decrease the concentration of free hydrogen ions (sour taste), and there are some salivary proteins which may bind with bitter taste substances. Another effect of saliva on taste transduction is that some salivary constituents can continuously stimulate the taste receptor, resulting in an alteration of taste sensitivity. For example, the taste detection threshold for NaCl is slightly above the salivary sodium concentrations with which the taste receptor is continuously stimulated. In contrast, saliva protects the taste receptor from damage brought about by dryness and bacterial infection, and from disuse atrophy via a decrease in transport of taste stimuli to the receptor sites. This is a long-term effect of saliva that may be related to taste disorders. These various effects of saliva on the taste perception differ depending on the anatomical relationship between the taste buds and oral openings of the ducts of the salivary glands. Many taste buds are localized in the trenches of the foliate and circumvallate papillae, where the lingual minor salivary glands (von Ebner's glands) secrete saliva. Taste buds situated at the surface of the anterior part of the tongue and soft palate are bathed with the mixed saliva secreted mainly by the three major salivary glands.

Facial taste responses of the channel catfish to binary mixtures of amino acids.

We investigated the neural processing of binary gustatory mixtures of amino acids by the facial taste system of the channel catfish, Ictalurus punctatus. In vivo electrophysiological recordings indicated that the magnitude of both integrated and single-unit facial taste responses to binary mixtures of amino acids was greatest if the components bound to independent receptor sites. Facial taste responses were obtained from 32 multiunit and 55 single taste fiber preparations to binary mixtures of amino acids whose components bind to independent taste receptor sites (group I) or to the same or highly cross-reactive taste receptor sites (group II). All component stimuli were adjusted in concentration to provide approximately equal response magnitude as determined by either the height of the integrated multiunit taste response or by the number of action potentials generated/3 s of response time/single taste fiber. The mixture discrimination index (MDI), defined as the response to the mixture divided by the average of the responses to the component stimuli, was calculated for each test of a binary mixture. MDIs of group I binary mixtures for both the integrated multiunit and single fiber data were significantly greater than those for either the control or group II binary mixtures. In a subset of multiunit recordings, the MDIs of a group I binary mixture across three log units of stimulus concentration were similar and significantly greater than those of a group II binary mixture. Analysis of the single fiber data also indicated that the MDIs of group I binary mixtures were significantly larger than those of group II binary mixtures for both alanine-best and arginine-best taste fibers; however, the MDIs of group I binary mixtures calculated from recordings from arginine-best taste fibers were significantly greater than those recorded from alanine-best taste fibers.

The Arginine Taste Receptor: Physiology, Biochemistry, and Immunohistochemistrya

The amino acid, L-arginine (L-Arg), is a potent taste stimulus for the channel catfish, Ictalurus punctatus. Receptor binding studies demonstrated a high-affinity binding of L-Arg to putative taste receptor sites. This binding could be inhibited by preincubation of the tissue in the lectins Phaseolus vulgaris agglutinin (PHA) and Ricinus communis agglutinin I (RCA I). Neurophysiological studies demonstrated that the L-Arg receptor is a stimulus-gated ion channel type receptor whose conductance was stimulated by L-Arg and inhibited by D-arginine (D-Arg). To purify the receptor we subjected CHAPS solubilized partial membrane preparation from barbel epithelium to RCA I lectin affinity chromatography. The bound proteins were eluted with D-galactose. When these proteins were reconstituted into lipid bilayers, L-Arg activated single channel currents with conductances between 45 and 85 pS. Sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) of the eluted protein showed a distinct band at approximately 83 kDa. Polyclonal antibodies raised against this 83-kDa band in guinea pigs reacted with numerous small (approximately 1 micron) sites within the taste pore of every taste bud when applied to fixed nonpermeabilized barbels. This observation suggests that the antibodies recognize an externally-facing epitope of the putative Arg receptor. The antibodies also inhibited L-Arg-stimulated currents in reconstitution studies. Sephacryl S-300 HR chromatography of the eluant from the affinity column showed a high molecular weight peak (> 700 kDa) which was recognized by the antibodies. Reconstitution of the protein from this peak into a lipid bilayer resulted in L-Arg-stimulated channels that could be inhibited by D-Arg. This high molecular weight component may be aggregates of the arginine taste receptor.

Multiple human taste receptor sites: a molecular modeling approach.

Numerous experimental data on the human peripheral taste system suggest the existence of multiple low-affinity and low-specificity receptor sites which are responsible for the detection and the complete discrimination of a very large number of organic molecules. According to this hypothesis, a given molecule interacts with numerous taste receptors and vice versa. Statistical analysis of taste intensities estimated by 58 human subjects for various molecules enables the calculation of taste intermolecular distances. For the present modeling study, we hypothesized that a short taste distance (i.e. taste similarity) between two distinct molecules indicates that they bind with similar distributions of affinities to the taste receptors, and hence display similar binding motifs. In order to find common molecular binding motifs among 14 selected organic tastants, hydrogen-bonding and hydrophobic interaction properties were mapped onto their molecular surfaces. The 14 surfaces were then cut in 240 fragments, most of which were made up of 2-4 potentially interacting zones. A correspondence index was defined to measure the analogy between two optimally superimposed fragments. The 75 most representative fragments were all matched pairwise. Twelve distinct clusters of fragments were isolated from the 2775 calculated comparisons. These 12 fragment types were used to calculate structural similarity distances. We then performed a combinatorial analysis to identify which fragment combination best reconciled structural and taste distances. We finally identified an optimal subset of seven fragment types out of the 12, which significantly and best accounted for the 91 pairwise taste distances between all 14 modeled tastants. These seven validated fragment types are therefore presented as good candidates to be recognized by the same number of distinct taste receptor sites. Potential applications of these identified binding motifs to tastant design are suggested.

Structural studies on sweet taste inhibitors: lactisole, dl-2(4-methoxyphenoxy)-propanoic acid

Lactisole, dl -2-(4-methoxyphenoxy)-propanoic acid (HPMP) has the formula C 10 O 4 H 12 , M r = 196.20, and is monoclinic, C 2 /c. a = 34.944(5), b = 5.2146(14), c = 11.201(2) Å, β = 101.495(13)°, V = 2000.1(7) Å 3 , Z = 8, D x = 1.3031(5) mg m −3 , λ(Mo Kα) = 0.71073 Å, μ = 0.9 cm −1 , F (000) = 832, R = 0.0392 for 1468 unique observed diffractometer data ( I ⩾ 2.5σ( I )). In the molecule two planar fragments, the acetic acid group and the phenyl ring, are almost perpendicular (interplanar angle 80.4(1)°). The crystal structure is characterized by cyclic dimers formed by hydrogen bonds between carboxyl groups across centers of inversion. The sodium salt of lactisole, NaPMP, is also a selective inhibitor of the sweetness of sucrose and was studied in aqueous solution in order to elucidate the mechanism of sweet taste inhibition. Solution properties, FT-IR spectra and the effect of NaPMP on the structure of water as determined by Raman spectra in the region of the OH stretching vibration were investigated. The hydrophobicity of NaPMP together with the steric hindrance caused by this molecule at the entrance of the sweet taste receptor site are probably at the origin of its inhibitory effect.

Selective interactions of lectins with amino acid taste receptor sites in the channel catfish

Selective interactions of lectins with amino acid taste receptor sites in the channel catfish

Are umami taste receptor sites structurally related to glutamate CNS receptor sites?

Umami tasting substances, MSG (monosodium glutamate), HG (glutamic acid), LGDE (l-glutamic acid diethyl ester), DLHCA (dl-homocysteic acid), DLAAA (dl-aminoadipic acid) and 5′GMP, were tested on the hamster and the human. Ten mM MSG was routinely used in the hamster as it elicited strong chorda tympani responses. Similar response amplitudes were found for MSG, HG, LGDE, DLAAA 10 mM, DLHCA 8 mM and sucrose 100 mM. A 5 μM concentration of 5′GMP eventually was an efficient stimulus on a few preparations. Such a low concentration is very seldom efficient as a taste stimulus in rodents, indicating a higher specificity of receptor mechanisms than what is usually found for sweet taste, for example. The synergy between MSG and 5′GMP was found in the hamster CT only for concentrations lower than those of the literature, i.e., a mixture of 12 μM 5′GMP and 2.5 mM MSG showed a reinforcement of 50% in response amplitude equivalent to a 100% increase in concentration. We take this as an evidence of an umami component in the hamster CT response to glutamate; in accordance with literature data, we could not find reinforcement for higher concentrations which were in fact near saturation. Responses to MSG, HG, LGDE, DLAAA and DLHCA, among 38 other organic stimuli, were studied in 42 hamster chorda tympani. Responses to HG, LGDE and 5′GMP, among 51 other organic stimuli, were recorded in another run of 60 hamster CT. Results show that several receptors are involved in the chemoreception of these compounds used as umami tasting stimuli. The correlations between the CT response profiles obtained for these compounds indicate that the hamster tongue receptors classify the different agonists of the so-called NMDA- and non-NMDA receptors in the same way as brain receptors do. The question arises then as whether they could simply be the same in the brain and on the tongue.

Specific l-arginine taste receptor sites in the catfish, Ictalurus punctatus: biochemical and neurophysiological characterization

We report here the characterization of the arginine binding site(s) and corroborative neurophysiological studies. Binding of l-[ 3H]arginine to Fraction P2 from taste epithelium was measured by a modification of the method of Krueger and Cagan 20. Parameters for measuring maximal binding activity were established for both duration of incubation and pH of medium. At pH 7.8, the apparent single rate constant for association ( k obs) at 4°C was 4.72 × 10 +5·M −1·min −1. Dissociation was more complex, yielding two rate constants of 1.77·min −1 and 8.34 × 10 −3·min −1. These data suggest the presence of two affinity states for l-arganine. The K D values as calculated from the ratio k −1 /k +1 were 1.3 × 10 −6 M and 1.8 × 10 −8 M. Homologous inhibition studies of l-arginine bindig were not fit by a simple mass action relationship (Hill Coefficient 0.79), but were best fit by a two-site model with IC 50 values of 1.6 × 10 −6 M for the high affinity state and 9 × 10 −4 M for the low affinity state. Multiunit neural recordings examined the stimulatory effectiveness of a number of guanidinium-containing compounds. Compared with l-arginine, only l-arginine methyl ester and l-α-amino-β-guanidino propionic acid ( l-AGPA) were effective stimuli. Cross-adaptation experiments demonstrated that at 10 −4 M l-arginine methyl ester, l-AGPA and, to a lesser extent, d-arginine were effective cross-adapting stimuli to 10 −6 M l-arginine. In competition binding studies l-arginine methyl ester, l-AGPA and d-arginine also inhibited binding of l-[ 3H]arginine (10 −6 M), but each recognized only one affinity state. Inhibition by the poorly cross-adapting stimuli l-glutamate, glycine and l-alanine occurred only above 10 −3 M, indicating that the binding sites for l-arginine are selective. These studies suggest that there are at least two affinity states of l-arginine binding, that the binding sites are specific, and that effective agonists of l-arginine receptors must contain a guanidinium group and an unblocked l-α-amino group.

Beta‐Adrenergic Control of von Ebner's Glands in the Rata

Von Ebner’s lingual salivary glands secrete through multiple ducts into the clefts of the circumvallate and foliate papillae. At the microscopic level, gland acini are filled with secretory granules.’ Since 80% of the lingual taste buds are contained in the foliate and circumvallate papillae in the rat: and since taste receptor sites on the microvilli are bathed in saliva: these glands provide a microenvironment important in taste transduction in posterior tongue taste buds. Little is known concerning the neural control of von Ebner’s glands. Recently we have shown that the source of their parasympathetic control is via cells in the inferior salivatory nucleus in the medulla? We now have applied the morphometric methodology used by Getchell and Getchell’ in a study of Bowman’s glands to characterize the extent of /3-adrenergic control of the von Ebner’s gland acini.

Enantiomeric Specificity of Alanine Taste Receptor Sites in Catfisha

Enantiomeric Specificity of Alanine Taste Receptor Sites in Catfish^a

Biochemical studies of taste sensation—XII. Specificity of binding of taste ligands to a sedimentable fraction from catfish taste tissue

1. 1. The specificity of amino acid binding sites in a sedimentable fraction prepared from catfish taste epithelium was examined. Using seven 3H-labeled amino acids as ligands and the unlabeled amino acids in binding competition assays, the presence of possibly three classes of amino acid binding sites was deduced. 2. 2. Site 1 binds l-thr, l-ser, l-ala and possibly d-ala and β-ALA, Site 2 binds l-ser, l-ala, GLY, d-ALA, and β-ALA and Site 3 binds l-arg and l-lys. 3. 3. Additional evidence supporting the specificity of Site 2 was obtained from the specificity of enhancement of l-ALA binding. 4. 4. The results demonstrate the presence of some major classes of taste receptor sites, and provide a basis for understanding taste receptor specificity at the biochemical level.

Biochemical studies of taste sensation XI. Isolation, characterization and taste ligand binding activity of plasma membranes from catfish taste tissue

Plasma membranes were isolated from taste receptor-containing epithelium of the channel catfish, Ictalurus punctatus. The membranes were prepared by ultracentrifugation of a sedimentable fraction in sucrose, using either a discontinous density gradient or a continous linear density gradient. The plasma membranes were characterized by their increased content of 5′-nucleotidase and by electron microscopy, as well as by a greatly diminished content of NADH-cytochrome c reductase and succinate-cytochrome c reductase. The recovery of binding activity for taste ligands was low, because of the long time-period required for ultracentrifugation, but of the recovered activity 80% occurred in the plasma-membrane preparation. Binding of seven chemostimulatory amino acids was demonstrated and found to correspond reasonably well with earlier binding data obtained using a less pure sedimentable fraction. The data provide direct evidence supporting the long-standing hypothesis that taste receptor sites are localized to the plasma membranes.

Two different receptor sites for Ca 2+ and Na + in frog taste responses

Distilled water, 1 mM CaCl2 and 500 mM NaCl (pH 4.5) are effective stimuli which excite chemoreceptors of the frog tongue. To learn whether or not these taste stimuli react with different taste receptor sites, a proteolytic enzyme was topically applied to the tongue dorsum. Responses were recorded from the frog glossopharyngeal nerve during taste stimulation. After application of 0.1% pronase E to the dorsal tongue surface, the magnitude of the NaCl response remained unchanged, but the magnitude of the water and CaCl2 responses was markedly decreased. The selective suppression by the pronase E treatment indicates that there are two different receptor sites for Ca2+ and Na+ in the frog taste receptor cell and the receptor sites responsible for the generation of the water and the Ca2+ response may be composed of a protein.

Three-dimensional mapping of the bitter taste receptor site

Three-dimensional mapping of the bitter taste receptor site

Biochemical studies of taste sensation—VIII. Partial characterization of alanine-binding taste receptor sites of catfish Ictalurus punctatus using mercurials, sulfhydryl reagents, trypsin and phospholipase C

1. 1. Taste receptors for l-alanine in the channel catfish Ictalurus punctatus have been partially characterized. The binding activity, which is localized to a sedimentable fraction (Fraction P2), was assayed with l-[ 3H]alanine as the ligand. 2. 2. Addition of HgCl 2 or p- mercuribenzoate to the assay at 0.1–1 mM markedly inhibited binding. The effect was not reversible and was unaffected by increased l-alanine in the binding assay. 3. 3. The sulfhydryl reagents iodoacetate, 5,5′-dithiobis(2-nitrobenzoic acid), arsenite, and N- ethylmaleimide did not show appreciable inhibition of binding. The results suggest that the inhibitory effect of mercurials is not on specific sulfhydryl groups at alanine-binding sites. 4. 4. Treatment of Fraction P2 with phospholipase C decreased binding activity and treatment with trypsin led to increased binding activity.

Taste responses in sheep medulla: changes during development.

Response characteristics of taste neurons in the sheep solitary tract and nuclei alter during development. Solitary tract cells in younger fetuses respond to stimulation of the tongue with fewer salts and acids than do cells in older fetuses, lambs, and adults. Further, responses to specific salts and acids develop in a particular sequence, not randomly. These changes may relate to maturation of taste receptor sites.

Three-dimensional mapping of the sweet taste receptor site.

The active sites of the receptors for sweet and bitter tastants are shown to be related by a simple symmetry operation. This relationship, in turn, allows the identification of the critical geometrical features of both receptor sites. The model proposed for the sweet site is shown to be consistent with a large number of (conformationally rigid) sweet molecules.

Structural functions of taste in the sugar series: Binding characteristics of disaccharides

AbstractThe previous finding that only one half of a disaccharide molecule binds to the taste receptor site is fully substantiated but the linkage and the sugar residues of the disaccharide are important. The study is extended to the field of bitterness and it is observed that binding on to the bitter receptor is analogous to that on to the sweet receptor. With reducing disaccharides, it is the non‐reducing glycosyl unit that binds to the taste bud protein. Therefore, oligosaccharides constitute ideal models for this study despite their free anomeric centres.