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PubMed2006

Human taste: peripheral anatomy, taste transduction, and coding.

The anatomy, physiology and psychology of taste provide a glimpse into a uniquely heterogeneous sensory world; a world that is robust in its importance to flavor, redundant in its transductive heterogeneity and complexity, requisite in that feeding and hence life usually depend upon taste input, regenerative in that taste cells constantly turn over and regrow after tissue damage, and resistant to disease, loss of neural innervation and epithelial destruction. This chapter considers our current state of knowledge in anatomy, taste bud physiology, molecular biology of bitter, sweet, sour, savory and salty tastes, afferent signaling and quality coding, human perception, and pathophysiology and senescence of taste. We highlight some of the advances made in molecular biology of taste and point out areas where further research is needed ranging from taste bud development and regeneration, to within-taste bud processing, to central/perceptual coding networks for taste. Our hope is that this chapter will provide a background for greater understanding of taste physiology, perception, disease, and future sensory research.

Brain↗
PubMed1986

Functional scanning electron microscopical observations on the oral roof in an air-breathing teleost, Clarias batrachus (LINN.).

Scanning Electron Microscopical (SEM) as well as light microscopical features of the oral roof including barbels of the air-breathing teleost, Clarias batrachus (LINN.) have been described in relation to its feeding habit. 3 distinct types of taste buds have been recorded for the first time in the barbels, lip, and in and around the teeth bearing suprapharyngeal teeth pad of the fish studied. The taste buds in the barbels and lip are elevated over the epithelium and confer wart like appearance which probably help in mechanoreception in addition to chemoreception. The taste buds in the suprapharyngeal teeth pad slightly rise above the epithelium whereas those in the borders of it do not. They also possess numerous minute papillae in the gustatory pores. These taste buds seem to act as chemoreceptors. SEM and light microscopical observations on the structure and distribution of taste buds in the oral roof clearly reveal that the fish is purely a taste-feeder and scans and gropes the food materials from the bottom of the pond.

Animals↗
PubMed2002

Expression and physiological actions of cholecystokinin in rat taste receptor cells.

Gustatory perception arises not only from intracellular transduction cascades within taste receptor cells but also from cell-to-cell communication among the cells of the taste bud. This study presents novel data demonstrating that the brain-gut peptide cholecystokinin (CCK) is expressed in subsets of taste receptor cells, and that it may play a signaling role unknown previously within the taste bud. Immunocytochemistry revealed positively stained subsets of cells within taste buds throughout the oral cavity. These cells typically displayed round nuclei with full processes, similar to those classified as light cells. Peptide expression was verified using nested PCR on template cDNA derived from mRNA extracted from isolated posterior taste buds. Multiple physiological actions of cholecystokinin on taste receptor cells were observed. An outward potassium current, recorded with the patch-clamp technique, was inhibited by exogenous application of sulfated cholecystokinin octapeptide in a reversible and concentration-dependent manner. Pharmacological analysis suggests that this inhibition is mediated by CCK-A receptors and involves PKC phosphorylation. An inwardly rectifying potassium current, typically invariant to stimulation, was also inhibited by cholecystokinin. Additionally, exogenous cholecystokinin was effective in elevating intracellular calcium as measured by ratiometric techniques with the calcium-sensitive dye fura-2. Pharmacology similarly demonstrated that these calcium elevations were mediated by CCK-A receptors and were dependent on intracellular calcium stores. Collectively, these observations suggest a newly discovered role for peptide neuromodulation in the peripheral processing of taste information.

Animals↗
PubMed1995

Glycoconjugates and keratin 18 define subsets of taste cells.

Sections of neonatal, normal adult and denervated adult rat tongue were examined with lectin histochemistry. Attention was focused upon intragemmal cells (cells within the taste bud) and the surrounding perigemmal cells. Informative staining patterns were observed with four of 12 lectins: Ulex europaeus (UEA-I), Bauhinia purpurea (BPA), Helix pomatia (HPA) and Lotus tetragonolobus (LTA) agglutinins. In normal adult tongues, BPA bound to those lingual epithelial cells lacking contact with the basal lamina. After they formed, vallate taste buds were laterally surrounded by distinctive BPA-positive cells. HPA reacted selectively with 28% and LTA with 23% of the intragemmal cells in vallate/foliate taste buds. In double-stained taste buds there was a statistically significant overlap of LTA-positive cells and keratin 18-positive cells. The overlap between HPA binding and keratin 18 was more marked: double-stained cells comprized 67% of all stained cells. During taste bud development in neonates keratin 18 synthesis preceded HPA binding. In contrast, during the replacement of adult taste cells, keratin 18 synthesis and HPA binding were generally concurrent. Keratin 18 and HPA probably identify the same subset of older taste receptor cells. HPA may bind to glycoconjugates on the surface of keratin 18-positive cells. In denervated adult tongue the loss of all UEA-I-positive or BPA-positive perigemmal cells suggests that perigemmal as well as intragemmal cells are nerve-dependent.

Animals↗
PubMed2001

The distribution of PGP9. 5, BDNF and NGF in the vallate papilla of adult and developing mice.

The development and innervation of vallate papillae and taste buds in mice were studied using antibodies against the neuronal marker, protein gene product 9.5 (PGP 9.5), and against nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). PGP 9.5 immunohistochemical studies revealed that the earliest sign of median vallate papilla formation was an epithelial bulge at embryonic day 13 (E13), and at E14, a dense nerve plexus was found within the connective tissue core of the papilla. Thin nerve fibers penetrated the apical and medial trench wall epithelium of the papilla at E16 and a few of these began to invade the lateral trench wall epithelium at E17. At postnatal day 1 (P1), the newly formed taste buds were recognizable and a small number of PGP 9.5-immunoreactive (IR) cells appeared on the medial trench wall epithelium. The number of PGP 9.5-IR taste bud cells then increased gradually and reached the adult level at postnatal week 2. PGP 9.5 immunoreactivity increased systematically with age. NGF and BDNF immunoreactivity was first seen at the boundary between the columnar cells in the apical epithelium of the developing vallate papilla at E13, then in the medial and lateral trench walls at E15 (BDNF) or E18 (NGF). At P1, BDNF immunoreactivity was exclusively present in the newly formed taste buds of the medial trench wall. The number of BDNF-IR taste bud cells then increased gradually, reaching the adult level at P7. Similar degrees of NGF and BDNF immunoreactivity were seen in the developing vallate papilla. In the present study, we found that the vallate papilla was formed prior to its innervation, and we propose that initiation of papilla formation does not require any direct influence from the specific gustatory nerve. We also suggest that neurotrophins in the early developing vallate papillae might act as local tropic factors for the embryonic growth of nerve fibers to induce differentiation of the taste buds.

Animals↗
PubMed1995

Keratin 18 is associated with a subset of older taste cells in the rat.

All or nearly all intragemmal (elongated) cells of rat taste buds were immunopositive for keratins 7, 8, and 19. In contrast, keratin 18 was detected in 19 +/- 5 cells per taste bud (mean +/- sd), or about 25% of the intragemmal cells. During taste bud development keratins 7, 8, and 19 were evident initially in polygonal cells and later in elongated taste cells. Keratin 8 appeared in vallate taste cells at P0 (postnatal day 0), followed by keratins 7 and 19 at P1, and keratin 18 at P2-P3. Keratin 18 was always limited to elongated cells. The assemblage of elongated taste cells comprising a taste bud began with a single elongated cell, rather than with the synchronous elongation of a cluster of cells. Developmental errors were observed at P2-P3, e.g., some vallate taste cells had a misoriented axis. In order to study the pace of keratin differentiation during cell turnover we injected bromodeoxyuridine (BrdU) into adult rats to monitor taste cell age. Keratin-19-positive intragemmal cells differentiated within 1 day. In contrast, keratin 18 was first detected in cells aged 3 days. Hence, both in taste cell development and replacement, keratin 18 was restricted to the older cells; it was the last taste cell keratin to become expressed during differentiation.

Animals↗
PubMed1977

Histological changes following unilateral re-innervation of the circumvallate papilla of rat.

The present work was designed to study the histological changes of the rat's circumvallate papilla following unilateral glossopharyngeal re-innervation. Unilateral re-innervation was achieved by transection of the 9th nerve and avulsion of its central segment on the right side and crushing that of the left side. The latter was thus allowed to regenerate and to re-innervate the papilla. On the first 2 post-operative weeks, the nerve bundles at the base of the papilla and the subgemmal nerve plexus showed signs of Wallerian degeneration and concomitant disappearance of taste buds. On the 3rd post-operative week, the papilla was invaded by regenerating nerves and the primordial taste buds started to appear. From the 4th week on wards, the number of regenerating taste buds on the left outer, left inner and right inner trench walls was not significantly different from the sham-operated controls. The only buds which failed to reappear were those of the upper two thirds of the right outer trench walls. The newly formed buds were smaller in size than those of the sham-operated controls with a relative diminution of 30% of the number of intragemmal cells per section of taste bud.

Animals↗
PubMed2004

Acid-sensing ion channel-2 is not necessary for sour taste in mice.

The acid-sensitive cation channel acid-sensing ion channel-2 (ASIC2) is widely believed to be a receptor for acid (sour) taste in mammals on the basis of its physiological properties and expression in rat taste bud cells. Using reverse transcriptase-PCR, we detected expression of ASIC1 and ASIC3, but not ASIC4, in mouse and rat taste buds and nonsensory lingual epithelium. Surprisingly, we did not detect mRNA for ASIC2 in mouse taste buds, although we readily observed its expression in rat taste buds. Furthermore, in Ca2+ imaging experiments, ASIC2 knock-out mice exhibited normal physiological responses (increases in intracellular Ca2+ concentrations) to acid taste stimuli. Our results indicate that ASIC2 is not required for acid taste in mice, and that if a universal mammalian acid taste transduction mechanism exists, it likely uses other acid-sensitive receptors or ion channels.

Acid Sensing Ion Channels↗
PubMed1982

The nature of the substance P-containing nerve fibres in taste papillae of the rat tongue.

The nature of the association of substance P (SP) with taste buds in the rat tongue was investigated by immunohistochemical and radioimmunoassay techniques. Both the circumvallate and fungiform papillae were found to receive a rich innervation by substance P-containing fibres. Although these fibres were closely associated with the taste buds in these structures, they assumed a perigemmal rather than an intragemmal location. Bilateral lesions of the glossopharyngeal nerve resulted in the depletion of taste buds from the vallate papilla and a large reduction in substance P immunoreactive fibres in this area. Lesions of the chorda tympani, which led to the degeneration of taste buds in fungiform papillae, had no effect on the immunohistochemical appearance of substance P in these papilla or on the substance P levels in the anterior part of the tongue. Lesions of the mandibular division of the trigeminal nerve or neonatal capsaicin treatment had no effect on the structural integrity of taste buds in fungiform papillae but led to the depletion of substance P-immunoreactive fibres from these papillae. Both of these procedures caused a 71% reduction in the substance P content of the anterior tongue, ipsilaterally after the nerve lesion and bilaterally after capsaicin treatment. The results are discussed in relation to the possible functional role of substance P-containing fibres within nerves supplying taste structures of the tongue.

Animals↗
PubMed2000

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.

Animals↗
PubMed1998

Functional redundancy and gustatory development in bdnf null mutant mice

In the mouse nasopalate papilla and in the trenches of the foliate and vallate papillae, taste buds accumulated primarily during the first 2 weeks after birth. Null mutation for brain-derived neurotrophic factor caused extensive death of embryonic taste neurons, with the secondary outcome that most taste buds failed to form. However not all taste neurons died; functional redundancy rescued a variable number. The primary research objective was to identify the likely site of the taste neuron rescue factor that substituted for BDNF. In this quest taste bud abundance served as a useful gauge of taste neuron abundance. The proportion of taste buds that developed was variable and uncorrelated among the nasopalate, vallate, and foliate gustatory papillae within each bdnf null mutant mouse. Thus, in spite of shared IXth nerve innervation, the vallate and foliate papillae independently varied in residual gustatory innervation. This variation rules against the rescue of gustatory neurons by system-wide factors or by factors acting on the IXth ganglion or nerve trunk. Therefore it is likely that surviving BDNF-deprived taste neurons were stochastically rescued by a redundant neurotrophic factor at the level of the local gustatory epithelium. These findings broaden the classic expectation that target tissue supplies only a single neurotrophic factor that can sustain sensory (taste) neurons.

Journal Article↗
PubMed2005

Two families of candidate taste receptors in fishes.

Vertebrates receive tastants, such as sugars, amino acids, and nucleotides, via taste bud cells in epithelial tissues. In mammals, two families of G protein-coupled receptors for tastants are expressed in taste bud cells-T1Rs for sweet tastants and umami tastants (l-amino acids) and T2Rs for bitter tastants. Here, we report two families of candidate taste receptors in fish species, fish T1Rs and T2Rs, which show significant identity to mammalian T1Rs and T2Rs, respectively. Fish T1Rs consist of three types: fish T1R1 and T1R3 that show the highest degrees of identity to mammalian T1R1 and T1R3, respectively, and fish T1R2 that shows almost equivalent identity to both mammalian T1R1 and T1R2. Unlike mammalian T1R2, fish T1R2 consists of two or three members in each species. We also identified two fish T2Rs that show low degrees of identity to mammalian T2Rs. In situ hybridization experiments revealed that fish T1R and T2R genes were expressed specifically in taste bud cells, but not in olfactory receptor cells. Fish T1R1 and T1R2 genes were expressed in different subsets of taste bud cells, and fish T1R3 gene was co-expressed with either fish T1R1 or T1R2 gene as in the case of mammals. There were also a significant number of cells expressing fish T1R2 genes only. Fish T2R genes were expressed in different cells from those expressing fish T1R genes. These results suggest that vertebrates commonly have two kinds of taste signaling pathways that are defined by the types of taste receptors expressed in taste receptor cells.

Amino Acid Sequence↗
PubMed1988

Ultrastructure of taste cells and synapses in the mudpuppy Necturus maculosus.

Taste buds in the mudpuppy Necturus maculosus were examined with electron microscopy. Three cell types (dark, light, and basal) were identified and reconstructed from serial thick sections. Dark and light cells extend from the basal lamina to the surface of the tongue. The apical process of the dark cells was usually quite lamellar when viewed in cross section, in contrast to light cells, whose apical process appeared more cylindrical. Basal cells are situated at the base of the bud and do not extend processes to the surface of the tongue. The cytoplasm of basal cells contains numerous clear and dense-cored vesicles. Small, spinelike processes (2-3 microns in length) project outward from the basal cells into the cytoplasm of the surrounding tast receptor cells. Morphologically, basal cells in mudpuppy taste buds resemble Merkel cells. Unmyelinated afferent nerve fibers enter the taste bud at the base and course through the lower portion of the bud. Synapses were found between taste receptor cells and nerve fibers, between basal cells and nerve fibers, and between basal cells and taste receptor cells. Over 65% of the synapses observed in the mudpuppy taste bud involved the basal cell. These findings suggest that basal cells play some role in chemosensory signal processing or integration of the taste response.

Animals↗
PubMed2005

Morphology of the pharyngeal cavity, especially the surface ultrastructure of gill arches and gill rakers in relation to the feeding ecology of the catfish Rita rita (Siluriformes, Bagridae).

Gill arches and the gill rakers of a sluggish, carnivorous catfish, Rita rita, show significant differences of their surface ultrastructure, which are recognized adaptive modifications in relation to food and feeding ecology of fish. Gill rakers on the first and second pairs of gill arches are borne on the oral side and are long and stout at the epi-ceratobranchial union. Gill rakers on the third and fourth pairs of gill arches, in contrast, are borne on the oral and aboral sides and are relatively delicate and short. Long and stout gill rakers on the first and second pairs of gill arches are considered primarily to prevent entry of undesirably large food items into the pharynx. Two types of taste buds, Type I and Type II, occur on the gill arches and the gill rakers. The raised taste buds, located at the apical ends of the gill rakers on the third, fourth, and the fifth pairs of gill arches could increase gustatory efficiency in the pharynx. Differences in the distribution of taste buds on the pharyngeal sides of different gill arches indicate that the posterior part of the pharynx plays a more crucial role in gustation than does the anterior part. Co-occurrence of teeth and taste buds on the epi- and hypopharyngeal bones denotes that food processing and gustation occur simultaneously in the pharynx. Villiform and caniform teeth on the epi- and hypopharyngeal bones are associated with a complex food-processing cycle. Mucous secretions, oozing through mucous cell openings, provide lubrication facilitating smooth passage of food through the pharynx. The angle of curvature at the epi-ceratobranchial union of the first to fourth pairs of gill arches could assist the ventral drag of ceratobranchials in lowering of the pharyngeal floor, thus resulting in a great expansion of the pharynx, as needed to accommodate the large quantities of food captured.

Animals↗
PubMed1991

[Immunocytochemistry of keratins in the tongue mucosa of rats].

C12 and E2 monoclonal antibodies to keratins stained taste bud cells of foliate papillae as well as the cells of the associated glands. H4 monoclonal antibody to keratin reacted with the surrounding epithelial cells and did not react with the taste bud cells. The results show that the keratin subtype differs between taste bud and surrounding epithelial cells. The similar keratin composition of taste bud and associated gland cells confirm the view on the existence of the same type of cells in these two structures.

Animals↗
PubMed1990

[Immunocytochemistry of keratins in the mucous membrane of the rat tongue].

C12 and E2 monoclonal antibodies to keratins stained taste bud cells of foliate papillae as well as the cells of the associated glands. H4 monoclonal antibody to keratin reacted with the surrounding epithelial cells and did not react with the taste bud cells. The results show that the keratin subtype differs between taste bud and surrounding epithelial cells. The similar keratin composition of taste bud and associated gland cells confirms the view about existence of the same type cells in these two structures.

Animals↗
PubMed2001

Epithelial overexpression of BDNF or NT4 disrupts targeting of taste neurons that innervate the anterior tongue.

Brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4) are essential for the survival of geniculate ganglion neurons, which provide the sensory afferents for taste buds of the anterior tongue and palate. To determine how these target-derived growth factors regulate gustatory development, the taste system was examined in transgenic mice that overexpress BDNF (BDNF-OE) or NT4 (NT4-OE) in basal epithelial cells of the tongue. Overexpression of BDNF or NT4 caused a 93 and 140% increase, respectively, in the number of geniculate ganglion neurons. Surprisingly, both transgenic lines had severe reduction in fungiform papillae and taste bud number, primarily in the dorsal midregion and ventral tip of the tongue. No alterations were observed in taste buds of circumvallate or incisal papillae. Fungiform papillae were initially present on tongues of newborn BDNF-OE animals, but many were small, poorly innervated, and lost postnatally. To explain the loss of nerve innervation to fungiform papillae, the facial nerve of developing animals was labeled with the lipophilic tracer DiI. In contrast to control mice, in which taste neurons innervated only fungiform papillae, taste neurons in BDNF-OE and NT4-OE mice innervated few fungiform papillae. Instead, some fibers approached but did not penetrate the epithelium and aberrant innervation to filiform papillae was observed. In addition, some papillae that formed in transgenic mice had two taste buds (instead of one) and were frequently arranged in clusters of two or three papillae. These results indicate that target-derived BDNF and NT4 are not only survival factors for geniculate ganglion neurons, but also have important roles in regulating the development and spatial patterning of fungiform papilla and targeting of taste neurons to these sensory structures.

Animals↗
PubMed1998

Behavioral discrimination between quinine and KCl is dependent on input from the seventh cranial nerve: implications for the functional roles of the gustatory nerves in rats.

The rat glossopharyngeal nerve (GL), which innervates posterior tongue taste buds, contains several physiologically defined taste fiber types; at least one type is primarily responsive to certain alkaloids (such as quinine), and another is primarily responsive to acids and salts. In contrast, the chorda tympani (CT), which innervates anterior tongue taste buds, does not appear to contain fibers that differentially respond to quinine relative to salts and acids. It was therefore predicted that GL transection should disrupt behavioral discriminations between quinine and either acids or salts. Water-restricted rats were trained to press one of two levers if a sampled taste stimulus was quinine (0.1-1.0 mM) and the second lever if the sampled stimulus was KCl (0.1-1.0 M). Sham surgery, GL transection, and sublingual and submaxillary salivary gland extirpation were found to have no effect relative to presurgical performance. Both CT transection and combined GL and CT transection caused a substantial and approximately equal decrement in discrimination performance. Removal of the gustatory branches of the seventh cranial nerve [CT and greater superficial petrosal (GSP)] nearly eliminated the discrimination of the taste stimuli, and combined transection of the CT, GL, and GSP unequivocally reduced performance to chance levels. Although these findings were not presaged by the known electrophysiology, they nonetheless compare favorably with other studies reporting little effect of GL transection on behavioral responses to quinine. These results, in the context of other discrimination studies reported in the literature, suggest that, in rats, the neural coding of taste quality depends primarily on the input of the facial nerve.

Alkaloids↗