Effects of chorda-lingual nerve injury and repair on human taste

Citric acid detection threshold and magnitude response weremeasured on the anterior tongue in 10 patients with unilateralchorda-lingual nerve transections before and after repair. Fungiformtaste buds were analysed by videomicroscopy. Preliminary datasuggests that humans can regenerate fungiform taste buds andrecover some taste sensitivity after repair.     

Taste profiles from single circumvallate papillae: comparison with fungiform profiles

Two experiments were conducted to investigate the psychophysicalresponse characteristics of single circumvallate papillae. InExperiment 1, 12 circumvallate papillae in four subjects werechemically stimulated to assess identification of taste qualities.Single circumvallate papillae were found to mediate multipletaste qualities, and the taste profiles obtained from differentpapillae were similar within the same subject. Moreover, sucrose,quinine monohydrochloride and citric acid elicited unitary andcharacteristic quality responding in these papillae from allsubjects, whereas NaCl elicited predominantly sour and/or bitterresponses from three of the four subjects. Experiment 2 directly compared responses obtained from singlecircumvallate papillae with those obtained from fungiform regionsof the tongue. Data for 10 subjects showed significantly greatersour responses to citric acid and NaCl in circumvallate papillaeand significantly greater salty responses to these compoundson the anterior tongue. In addition, the taste profiles forcitric acid and NaCl were distinct for circumvallate papillae,while those from the anterior tongue were similar. These datasuggest that the bitterness and sweetness of quinine and sugar,respectively, can be identified on the basis of sensory informationarising from either circumvallate or fungiform regions, butthat differentiation of the tastes of salts and acids may dependon a comparison of the input from both regions and/or additionalinformation arising from foliate regions.     

Expression and characterization of delayed rectifying K+ channels in anterior rat taste buds

Delayed rectifying K⁺ (DRK) channels in taste cells have been implicated in the regulation of cell excitability and as potential targets for direct and indirect modulation by taste stimuli. In the present study, we have used patch-clamp recording to determine the biophysical properties and pharmacological sensitivity of DRK channels in isolated rat fungiform taste buds. Molecular biological assays at the taste bud and single-cell levels are consistent with the interpretation that taste cells express a variety of DRK channels, including members from each of the three major subfamilies: KCNA, KCNB, and KCNC. Real-time PCR assays were used to quantify expression of the nine DRK channel subtypes. While taste cells express a number of DRK channels, the electrophysiological and molecular biological assays indicate that the Shaker Kv1.5 channel (KCNA5) is the major functional DRK channel expressed in the anterior rat tongue. transduction     

The distribution and origin of keratin 20-containing taste buds in rat and human

Sections of tissues containing lingual and extra-lingual taste buds were evaluated with monoclonal antibodies against cytokeratins. In the caudal third of the rat's tongue, keratin 20 immunoreactivity was restricted to taste buds, whereas keratins 7, 8, 18, and 19 were expressed in vallate and foliate taste buds and in cells of salivary ducts that merge with these taste epithelia. Hence, antibodies against keratin 20 most clearly distinguished differentiated taste cells from all other cells. In rat epiglottis, taste buds and isolated bipolar cells were keratin-20-positive. In rat nasopalatine papilla and palate, antibodies against keratin 20 identified Merkel cells, none of which was near to the keratin-20-negative taste buds. Nor were Merkel cells present at epiglottal taste buds or the keratin-20-negative fungiform taste buds or elsewhere in rat tongue. Hence, Merkel cells make no contribution to rat fungiform, epiglottal, nasopalatine, or palatal taste buds. Human and rat keratin-20-positive tissues are reported to be endodermal derivatives with the exception of Merkel cells and luminal urothelial cells. In rats the distribution of keratin-20-positive taste buds was in full agreement with the classical view that the posterior third of the tongue is derived from endoderm (keratin-20-positive taste buds), whereas the anterior two-thirds of the tongue is derived from stomadeal ectoderm (keratin-20-negative taste buds). The equally intense keratin 20 immunoreactivity of human fungiform and vallate taste buds violates this traditional rostro-caudal segregation and suggests that endodermally derived tissues may be present in the tip of the human tongue.     

Development of anterior gustatory epithelia in the palate and tongue requires epidermal growth factor receptor.

We characterized the gustatory phenotypes of neonatal mice having null mutations for epidermal growth factor receptor (egfr(-/-)), brain-derived neurotrophic factor (bdnf(-/-)), or both. We counted the number and diameter of fungiform taste buds, the prevalence of poorly differentiated or missing taste cells, and the incidence of ectopic filiform-like spines, each as a function of postnatal age and anterior/posterior location. Egfr(-/-) mice and bdnf(-/-) mice had similar reductions in the total number of taste buds on the anterior portions of the tongue and palate. Nonetheless, there were significant differences in their gustatory phenotypes. EGFR deficiency selectively impaired the development of anterior gustatory epithelia in the mouth. Only bdnf(-/-) mice had numerous taste buds missing from the foliate, vallate, and posterior fungiform papillae. Only egfr(-/-) fungiform taste papillae had robust gustatory innervation, markedly reduced cytokeratin 8 expression in taste cells, and a high incidence of a filiform-like spine. Egfr/bdnf double-null mutant mice had a higher frequency of failed fungiform taste bud differentiation. In bdnf(-/-) mice taste cell development failed because of sparse gustatory innervation. In contrast, in young egfr(-/-) mice the abundance of axons innervating fungiform papillae and the normal numbers of geniculate ganglion neurons implicate gustatory epithelial defects rather than neural defects.     

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.     

Chemosensitive responses from the cat epiglottis

Responses were recorded from single and few-fiber preparationsof the cat superior laryngeal nerve during stimulation of theepiglottis with 0.5 M KCl, NH⁴Cl, NaCl, and LiCl; distilledwater, 0.005 M citric acid, 0.01 N HCl; and light touch. Epiglottalreceptive fields were mapped. The order of effective stimuliis KCl > HCl > NH⁴Cl > distilled water > citricacid; NaCl and LiCl are generally not effective. Since (a) similarstimuli elicit responses from the cat chorda tympani nerve duringstimulation of tongue taste buds, and (b) receptive fields ofsuperior laryngeal nerve fibers compare well with a map of epiglottaltaste buds, we conclude that the chemosensitive responses fromthe epiglottis are probably mediated by taste buds.     

Labeling afferent nerves in the tongue by peripheral transganglionic transport of HRP and WGA-HRP in the rat

Peripheral transganglionic transport of horseradish pcroxidase(HRP) and wheat germ agglutinin–horseradish peroxidaseconjugate (WGA–HRP) was used to label afferent fibersin the taste buds and lingual epithelium of the rat. Microinjectionsof the tracer were made in the brain stem central projectionarea of the afferent nerves to the tongue. Optimal labelingof nerve endings in the tongue was obtained when 2 µlof 20% HRP was injected into the brain stem and postinjectionsurvival times of 24–36 h were used. The distributionof single nerves was studied by using this tracing procedurein combination with strategic transections of the various afferentnerves supplying the tongue. Labeled nerve fibers from the combinedchorda tympani–lingual nerve were found in the epitheliumand in taste buds in the fungiform and anterior foliate papillaeof the anterior 3/4 of the tongue. Labeled nerve fibers in theepithelium of the anterior 2/3 of the tongue but none in tastebuds were found when the lingual nerve alone was studied, althoughnumerous perigeminal fibers were found. The glossopharyngealnerve was found to innervate die posterior 1/4 of the tongueepithelium including the taste buds of the circumvallate papillae.The glossopharyngeal nerve on one side was found to innervatethe taste buds on both sides of the midline. The results showthat this tracing procedure can be a useful supplement to othermethods for studying afferent nerves in the tongue.     

Location of taste buds in intact taste papillae by a selective staining method.

Taste buds were found to stain strongly and selectively in intact papillae with highly acidic dyes such as ponceau S. In intact tongues the taste buds in the fungiform, circumvallate and foliate papillae of the cynomolgus monkey and in the fungiform papillae of the rat as well as the taste discs in the fungiform papillae of the frog could be visualized. This method enables a rapid location and counting of taste buds in taste papillae without preparing histological sections. In cynomolgus tongue material fixed in formalin, the dyes penetrate into the buds. In fresh tongues only the taste pore region of the buds stains, which suggests that in vivo taste buds are impenetrable underneath the pore.     

Location of taste buds in intact taste papillae by a selective staining method

Summary Taste buds were found to stain strongly and selectively in intact papillae with highly acidic dyes such as ponceau S. In intact tongues the taste buds in the fungiform, circumvallate and foliate papillae of the cynomolgus monkey and in the fungiform papillae of the rat as well as the taste discs in the fungiform papillae of the frog could be visualized. This method enables a rapid location and counting of taste buds in taste papillae without preparing histological sections. In cynomolgus tongue material fixed in formalin, the dyes penetrate into the buds. In fresh tongues only the taste pore region of the buds stains, which suggests that in vivo taste buds are impenetrable underneath the pore.     

Spacing patterns on tongue surface-gustatory papilla

The dorsal surface of the mammalian tongue is covered with four kinds of papillae, fungiform, circumvallate, foliate and filiform papillae. With the exception of the filiform papillae, these types of papillae contain taste buds and are known as the gustatory papillae. The gustatory papillae are distributed over the tongue surface in a distinct spatial pattern. The circumvallate and foliate papillae are positioned in the central and lateral regions respectively and the fungiform papillae are distributed on the anterior part of the tongue in a stereotyped array. The patterned distribution and developmental processes of the fungiform papillae indicate some similarity between the fungiform papillae and the other epithelial appendages, including the teeth, feathers and hair. This is because 1) prior to the morphological changes, the signaling molecules are expressed in the fungiform papillae forming area with a stereotyped pattern; 2) the morphogenesis of the fungiform papillae showed specific structures in early development, such as epithelial thickening and mesenchymal condensation and 3) the fungiform papillae develop through reciprocal interactions between the epithelium and mesenchymal tissue. These results led us to examine whether or not the early organogenesis of the fungiform papillae is a good model system for understanding both the spacing pattern and the epithelial-mesenchymal interaction during embryogenesis.     

Morphometry and cellular dynamics of denervated fungiform taste buds in the hamster

For most species and gustatory papillae denervation resultsin a virtual disappearance of taste buds. This is not the casefor hamster fungiform papillae, which contain taste buds thatsurvive denervation. To characterize these taste buds, in thisstudy, counts and measurements were made of all buds on theanterior 3 mm of the hamster tongue at 36 or 91 days after resectingthe chorda/lingual nerve. Taste bud numbers were, at both timeperiods, unaffected by denervation. However, bud dimensionswere affected with denervated buds 25–30% smaller thancontrol ones. Counts of taste bud cells indicated that decreasesin bud size may result from shrinkage, but not a loss of cells.Tritiated thymidine autoradiography was used to evaluate whetherdenervation influences the mitotic activity or the migratorypattern of bud cells. For every animal, the average number oflabelled cells per bud was slightly lower on the denervatedthan the control side of the tongue. However, when labelledcell positions were evaluated at 0.25, 3 and 6 days after thymidine,the distances from the sides of the bud increased at increasingtimes after injection for both the innervated and the denervatedbuds. Stem cells were located laterally or basally in the bud.Labelled cells that migrated into the centers of the buds werefew and seen only at 6 days post-injection time in both controland experimental buds. The moderate effects of denervation ontaste bud sizes and mitotic activities may indicate a generalizedatrophy. Remarkably intact were taste bud numbers and the migratorypatterns of cells, features of anterior tongue taste buds inthe hamster that are relatively invulnerable to resection ofthe chorda /lingual nerve.     

Effect of sensory and sympathetic denervation of the frog tongue on the catecholamine containing cells of the taste buds]

The structure of catecholamine-containing dumb-bell shaped cells of the taste buds was studied by luminescent microscopy in the epithelial layer of the frog's tongue (Rana temporaria). On the unilateral section of the lingual nerve, a maintained adrenergic innervation of vessels and of the epithelium was observed, a decreased number of dumb-bell shaped cells in the taste bud, and their significant enlargement, and increased cathecholamine luminescence. With desympathization, no adrenergic nerves were observed on the vessels and the epithelium of the tongue. The size of the taste buds in desympathized cells of the tongue is sharply decreased and their number is increased. There is a tendency to grouping of the dumbbell shaped cells into 3--4 taste buds in one fungiform papillina. The experiments with sensory and sympathetic denervation of the frog tongue distinctly showed the trophic action of sensory and sympathetic nerves on the taste organ of the frog.     

Taste bud density in circumvallate and fungiform papillae of the bovine tongue

Taste bud quantitation may provide useful parameters for interspecies comparisons of the gustatory system. The present study is a morphometric analysis of bovine taste papillae. Circumvallate and fungiform papillae from six bovine tongues were serially sectioned and, following staining, analyzed. Circumvallate papillae were found to have a mean volume of 3.66 +/- 2.82 mm3, a mean number of taste buds per papilla of 445 +/- 279, and a mean taste bud density of 155 +/- 112 buds/mm3. Values for lateral fungiform papillae for the same three parameters were 0.384 +/- 0.184 mm3, 13.2 +/- 13.4, and 40.8 +/- 46.6 buds/mm3, respectively. Values for dorsal fungiform papillae were 0.438 +/- 0.246 mm3, 4.39 +/- 4.78, and 14.0 +/- 17.1 buds/mm3, respectively. Circumvallate papillae were found to have a significantly greater volume, number of taste buds per papilla, and taste bud density than either type of fungiform papilla. These data should serve as background for biochemical, endocrinological, or neurological studies involving the bovine tongue.     

Chorda tympani nerve transection at different developmental ages produces differential effects on taste bud volume and papillae morphology in the rat

Chorda tympani nerve transection (CTX) results in morphological changes to fungiform papillae and associated taste buds. When transection occurs during neonatal development in the rat, the effects on fungiform taste bud and papillae structure are markedly more severe than observed following a comparable surgery in the adult rat. The present study examined the potential "sensitive period" for morphological modifications to tongue epithelium following CTX. Rats received unilateral transection at 65, 30, 25, 20, 15, 10, or 5 days of age. With each descending age at the time of transection, the effects on the structural integrity of fungiform papillae were more severe. Significant losses in total number of taste buds and filiform-like papillae were observed when transection occurred 5-30 days of age. Significant reduction in the number of taste pores was indicated at every age of transection. Another group of rats received chorda tympani transection at 10, 25, or 65 days of age to determine if the time course of taste bud degeneration differed depending on the age of the rat at the time of transection. Taste bud volumes differed significantly from intact sides of the tongue at 2, 8, and 50 days post-transection after CTX at 65 days of age. Volume measurements did not differ 2 days post-transection after CTX at 10 or 25 days of age, but were significantly reduced at the other time points. Findings demonstrate a transitional period throughout development wherein fungiform papillae are highly dependent upon the chorda tympani for maintenance of morphological integrity.     

Influence of physical exercise on human preferences for various taste solutions

The effects of physical exercise on preference for various sapid solutions was studied in 58 healthy university students. After 30 min of exercise using a bicycle ergometer at 50% VO2max (maximal oxygen uptake) intensity, a rating scale test on taste hedonic tone and the triangle test for taste absolute threshold were done. The test solutions were sucrose, NaCl, citric acid, caffeine and monosodium glutamate (MSG). Preference scale values for sucrose and citric acid increased after exercise, whereas the values for NaCl, caffeine and MSG were not changed. The absolute thresholds for all the sapid solutions did not differ for pre- and post-exercise. These findings indicate that in humans preference for sucrose and citric acid increase after physical exercise.      

Are human taste thresholds similar on the right and left sides of the tongue?

The human tongue is a relatively symmetrical anatomical structure and is generally assumed functionally equivalent on both sides. Experimental evaluation of this assumption is complicated by the fact that psychophysical measurements tend to vary considerably across testing sessions. To address functional laterality, we determined the detection thresholds of six right-handed and six left-handed subjects for Na saccharin, NaCl, citric acid and quinine HCl. Five pairs of interwoven, left and right unilateral thresholds were obtained for each taste stimulus in 12 subjects (n = 480 separate thresholds). In most cases mean sensitivity based on multiple measurements was found to be laterally symmetrical, however, we observed a few cases of lateral asymmetry of both general and compound-specific sensitivity. Threshold values were found to vary considerably across sessions, consistent with the test-retest variability previously reported for whole mouth thresholds. We conclude that taste threshold sensitivity is equivalent on the left and right anterior tongue for most individuals. Given the occasional exceptions to this rule, however, it is advisable to employ a counterbalanced design for any experimental or clinical testing protocol in which treatments are applied asymmetrically to the tongue.     

Age-related alteration of taste bud distribution in the common marmoset

Alteration in the number of taste buds on the soft palate (SP), fungiform (FF), foliate (FL) and circumvallate (CV) papillae in the common marmoset at different postnatal ages was examined histologically. After paraffin embedding, complete serial sections at 10 microm thickness were made and stained by HE. Digitized images for each section were examined carefully. The number of FF taste buds at day 1 was 334. While only 20% of all the taste buds at birth possessed a taste pore, 39% of 174 SP taste buds at day 1 possessed a taste pore. The number of taste buds with pores at day 1 was small for the center CV (19 of 59), one side CV (7 of 25), and one side FL (2 of 16). These results suggest that the functional maturation of SP taste buds may precede maturation in other areas of the tongue. The total number of taste buds increased with increasing age, reached a maximum at 2 months of age: FF, 1069; SP, 609; CV-center, 530; CV-side, 390; FL, 201, and decreased thereafter. Almost all taste buds possessed a taste pore after 2 months of age. The decrease in the number of taste buds in the oral cavity with increase in age may change taste sensitivity.     

Quantification of fungiform papillae and taste pores in living human subjects

A method developed to quantify taste buds in living human subjectsto study the relationship between taste sensitivity and tastebud distribution was used to count the taste buds in 10 humansubjects; fungiform papillae were mapped in 12 subjects. Tastebuds were identified by staining taste pores with methyleneblue, and images of the papillae and their taste pores wereobtained with videomicroscopy and an image processor. Fungiformpapillae showed a 3.3-fold range in density, from 22.1 to 73.6papillae/cm² with an average of 41.1 ± 16.8/cm² (s.d.,n = 2). There was a 14-fold range in taste pore density, from36 to 511 pores/cm² among subjects, with an average of 193 ±133/cm² (s.d., n = 10). Fungiform papillae contained from 0to 22 taste pores, with an average per subject of 3.75 ±1.4 taste pores/papilla (s.d., n = 10). We hypothesize thatsome differences in human taste sensitivity may be related tothese variations in taste bud density.