Taste transduction. A bitter-sweet beginning

TARGETED EXPRESSION OF IP3SPONGE AND IP3DSRNA IMPAIRES SUGAR TASTE SENSATION INDROSOPHILA

We evaluated the role of IP(3) in sugar taste reception in Drosophila melanogaster by inactivating the IP(3) signaling using genetic tools. We used the "IP(3) sponge," composed of the modified ligand-binding domain from the mouse IP(3) receptor, which was designed to absorb IP(3) in competition with native IP(3) receptors. Another tool was a transgene that generates double-stranded RNA against IP(3) receptor mRNA. Both inhibitors diminished the sensitivity of flies to trehalose and sucrose, as estimated by behavioral assays and electrophysiological recordings from the sugar receptor cells. The result indicates that IP(3) signaling is indispensable for sugar reception in Drosophila.

Qualitative and quantitative differences between taste buds of the rat and mouse

Background

Numerous electrophysiological, ultrastructural, and immunocytochemical studies on rodent taste buds have been carried out on rat taste buds. In recent years, however, the mouse has become the species of choice for molecular and other studies on sensory transduction in taste buds. Do rat and mouse taste buds have the same cell types, sensory transduction markers and synaptic proteins? In the present study we have used antisera directed against PLCβ2, α-gustducin, serotonin (5-HT), PGP 9.5 and synaptobrevin-2 to determine the percentages of taste cells expressing these markers in taste buds in both rodent species. We also determined the numbers of taste cells in the taste buds as well as taste bud volume.

Results

There are significant differences (p < 0.05) between mouse and rat taste buds in the percentages of taste cells displaying immunoreactivity for all five markers. Rat taste buds display significantly more immunoreactivity than mice for PLCβ2 (31.8% vs 19.6%), α-gustducin (18% vs 14.6%), and synaptobrevin-2 (31.2% vs 26.3%). Mice, however, have more cells that display immunoreactivity to 5-HT (15.9% vs 13.7%) and PGP 9.5 (14.3% vs 9.4%). Mouse taste buds contain an average of 85.8 taste cells vs 68.4 taste cells in rat taste buds. The average volume of a mouse taste bud (42,000 μm³) is smaller than a rat taste bud (64,200 μm³). The numerical density of taste cells in mouse circumvallate taste buds (2.1 cells/1000 μm³) is significantly higher than that in the rat (1.2 cells/1000 μm³).

Conclusion

These results suggest that rats and mice differ significantly in the percentages of taste cells expressing signaling molecules. We speculate that these observed dissimilarities may reflect differences in their gustatory processing.

The Quantitative Prediction of Bitterness-Suppressing Effect of Sweeteners on the Bitterness of Famotidine by Sweetness-Responsive Sensor

The purpose of the present study was the quantitative prediction of the bitterness-suppressing effect of sweeteners (sucrose or sugar alcohols) on the bitterness of famotidine (or quinine sulfate as control) solutions using an artificial taste sensor. Firstly, we examined the response characteristics of the sensor response to sweetness. The sensor membrane is charged negatively in the presence of sweeteners, which tend to receive protons from one of the components of the sensor membrane. The magnitude of the sensor response was shown to increase in direct proportion to the concentration of the sweetener. Secondly, we used direct or indirect methods to evaluate and predict the bitterness-suppressing effect of sweeteners on 1 mg/ml famotidine and 81.4 microM quinine sulfate solutions. In direct method, a regression between the sensor output of the sweetness-responsive sensor and the bitterness intensity obtained in human gustatory tests of famotidine solutions containing sweeteners at various concentrations, was performed. As a result, we were able to predict directly the bitterness intensity of the mixed solution. Finally, we also evaluated the bitterness intensity of the dissolution media of commercially available, orally disintegrating tablets containing famotidine by the combined usage of bitterness- and sweetness-responsive sensor. We found that the sugar alcohols in the tablet seem to be effective in the bitterness-suppression of famotidine from these tablets, especially in the initial phase (within 30 s) of the disintegration process.

Gustation: intersensory experience par excellence

On the face of it, basic tactile sensation might seem the only essential sensory requirement for the delivery of foods and beverages to the digestive system. In practice, however, the appropriate delivery of raw materials for the maintenance and repair of the body requires complex sensory and cognitive processes, such that flavour sensation arguably constitutes the pre-eminent example of an integrated multicomponent perceptual experience. To raise the profile of the chemical senses amongst researchers in other perceptual domains, I review here the contribution of various sense modalities to the flavour of foods and beverages. Further, in the light of these multisensory inputs, the physiological and psychophysical research summarised in this paper invites optimism that novel ways will be found to intervene when nutritional status is compromised either by specific dietary restraints, or by taste and smell disorders.

Bovine circumvallate taste buds: taste cell structure and immunoreactivity to alpha-gustducin

The taste buds of bovine circumvallate papillae were investigated under light and electron microscopy both by histological and immunohistochemical methods. Taste buds existed in the inner epithelium of the trench of the papillae. Under electron microscopy, two types of taste cells, type I and type II, could be classified according to the existence of dense-cored vesicles and cytoplasmic density. Type I had electron-lucent cytoplasm and possessed many electron-dense cored vesicles in the apical cytoplasm. It was considered that the electron-dense materials of the vesicles were released and constituted the pore substance. This type of cell possessed long and thick apical processes in the taste pore. Type II had denser electron cytoplasm compared with that of type I and possessed many electron-lucent vesicles in the apical cytoplasm. This type of cell possessed microvilli in the taste pore. To know the immunoreactivity to alpha-gustducin in bovine circumvallate taste buds, we used the immunoblotting method and the immunohistochemical method. The alpha-gustducin reaction band at 40 kDa was displayed in the specimen of Western blots. The immunohistochemical property of the antiserum to alpha-gustducin was investigated by using the avidin-biotin complex (ABC) method and the 1.4-nm gold and silver enhancement methods. A subset of taste cells showed the immunoreactivity under light microscopy. The electron microscopic specimens with the 1.4-nm gold and silver enhancement method revealed that only type II cells exhibited the alpha-gustducin immunoreactivity.

Ultrastructural localization of gustducin immunoreactivity in microvilli of type II taste cells in the rat

Gustducin is a transducin-like G protein (guanine nucleotide-binding protein) that is expressed in taste bud cells. Gustducin is believed to be involved in bitter and possibly sweet taste transduction. In the present study, we demonstrate that a subset of type II cells displays immunoreactivity to antisera directed against gustducin in taste buds of rat circumvallate papilla. Immunogold particles are present both in the microvilli and cytoplasm of the immunoreactive cells. Quantitative analysis of the data suggests that the number of colloidal gold particles (P<0.001) and nanogold particles (P<0.01) in the immunoreactive type II cells are much greater than in type I cells. There are also approximately 2.5 times (P<0.05) as many colloidal gold particles associated with the microvilli versus the cytoplasm in the immunoreactive type II cells. The ultrastructural distribution of gustducin immunoreactivity is consistent with its proposed role in the initial events of sensory transduction by gustatory receptor cells.

Identification of a phospholipase C beta subtype in rat taste cells

From rat circumvallate papillae a novel phospholipase C (PLC) subtype has been cloned and identified as most closely related to human PLC beta2. The corresponding mRNA was only detected in sensory lingual tissue but not in non-taste lingual tissue or any other tissues examined by Northern blot analysis. In situ hybridization revealed that a subset of taste receptor cells of circumvallate papillae was specifically labeled. A functional involvement of this PLC beta subtype in taste signal transduction emerged from biochemical analysis monitoring the second messenger response in circumvallate preparations induced by denatonium benzoate. This bitter agent elicited a rapid and transient increase of the inositol 1,4,5-trisphosphate level; this response was blocked by U73122, a potent inhibitor of PLC, and by PLC beta2-specific antibodies. These data indicate that a phospholipase C beta2 isoform mediates a denatonium benzoate-induced second messenger response of taste sensory cells in the circumvallate papillae.

Expression of the putative membrane fatty acid transporter (FAT) in taste buds of the circumvallate papillae in rats

The putative membrane fatty acid transporter (FAT) protein and its mRNA, originally expressed in adipose tissue, were found in the tongue of rats. Northern blot analysis showed a significant expression of FAT mRNA in the epithelial layer of circumvallate papillae. Immunohistochemical staining revealed that immunoreactivity for FAT is specifically localized in the apical part of taste bud cells, possibly gustatory cells, in the circumvallate papillae.

Differential expression of alpha-gustducin in taste bud populations of the rat and hamster

The G-protein subunit alpha-gustducin, which is similar to rod transducin, has been implicated in the transduction of both sweet- and bitter-tasting substances. In rodents, there are differences in sensitivity to sweet and bitter stimuli in different populations of taste buds. Rat fungiform taste buds are more responsive to salts than to sweet stimuli, whereas those on the palate respond predominantly to sweet substances. In contrast, hamster fungiform taste buds are more sensitive to sweet-tasting stimuli. Taste buds in the vallate and foliate papillae of both species are sensitive to bitter compounds. These differences in sensitivity should be reflected in the numbers of gustducin-containing cells in different taste bud populations. We examined taste buds in the rat and hamster for immunoreactivity to an antibody against alpha-gustducin. Immunofluorescence of labeled taste cells was examined by confocal microscopy, and the cells were counted. Gustducin-positive cells were seen in all taste bud regions; they were spindle-shaped, with circular cross-sections and apical processes that extended to the taste pore. Cells with this characteristic shape in rat vallate taste buds are Type II (light) cells. In the rat, taste buds of the fungiform papillae had fewer gustducin-positive cells (3.1/taste bud) than those of other regions, including the posterior tongue and palate (>8.9/taste bud). Hamster fungiform taste buds contained twice as many gustducin-expressing cells (6.8/taste bud) as those of the rat. These data support the hypothesis that alpha-gustducin is involved in the transduction of both sweet- and bitter-tasting stimuli by mammalian taste receptor cells.

Receptor-regulated ion channels

Recent advances in the study of receptor-regulated ion channels include the cloning of the genes encoding three types of potassium channel that are favorite targets of receptors for transmitters and hormones. Studies of these channels have also provided a strong indication that G-protein betagamma subunits may gate ion channels via direct protein-protein interactions. Similarities between channel regulation by natriuretic peptides and channel regulation by secreted peptide products of the Alzheimer's beta-amyloid precursor protein offer hints for the existence of a receptor for the latter. There are also other novel examples of channel regulation in excitable and nonexcitable cells, including liver cells and blood cells.