Morphometric and immunocytochemical assessment of fungiform taste buds after interruption of the chorda-lingual nerve

Immune responses in the injured olfactory and gustatory systems: a role in olfactory receptor neuron and taste bud regeneration?

Sensory cells that specialize in transducing olfactory and gustatory stimuli are renewed throughout life and can regenerate after injury unlike their counterparts in the mammalian retina and auditory epithelium. This uncommon capacity for regeneration offers an opportunity to understand mechanisms that promote the recovery of sensory function after taste and smell loss. Immune responses appear to influence degeneration and later regeneration of olfactory sensory neurons and taste receptor cells. Here we review surgical, chemical, and inflammatory injury models and evidence that immune responses promote or deter chemosensory cell regeneration. Macrophage and neutrophil responses to chemosensory receptor injury have been the most widely studied without consensus on their net effects on regeneration. We discuss possible technical and biological reasons for the discrepancy, such as the difference between peripheral and central structures, and suggest directions for progress in understanding immune regulation of chemosensory regeneration. Our mechanistic understanding of immune-chemosensory cell interactions must be expanded before therapies can be developed for recovering the sensation of taste and smell after head injury from traumatic nerve damage and infection. Chemosensory loss leads to decreased quality of life, depression, nutritional challenges, and exposure to environmental dangers highlighting the need for further studies in this area.

ENaC-Dependent Sodium Chloride Taste Responses in the Regenerated Rat Chorda Tympani Nerve After Lingual Gustatory Deafferentation Depend on the Taste Bud Field Reinnervated

The chorda tympani (CT) nerve is exceptionally responsive to NaCl. Amiloride, an epithelial Na+ channel (ENaC) blocker, consistently and significantly decreases the NaCl responsiveness of the CT but not the glossopharyngeal (GL) nerve in the rat. Here, we examined whether amiloride would suppress the NaCl responsiveness of the CT when it cross-reinnervated the posterior tongue (PT). Whole-nerve electrophysiological recording was performed to investigate the response properties of the intact (CTsham), regenerated (CTr), and cross-regenerated (CT-PT) CT in male rats to NaCl mixed with and without amiloride and common taste stimuli. The intact (GLsham) and regenerated (GLr) GL were also examined. The CT responses of the CT-PT group did not differ from those of the GLr and GLsham groups, but did differ from those of the CTr and CTsham groups for some stimuli. Importantly, the responsiveness of the cross-regenerated CT to a series of NaCl concentrations was not suppressed by amiloride treatment, which significantly decreased the response to NaCl in the CTr and CTsham groups and had no effect in the GLr and GLsham groups. This suggests that the cross-regenerated CT adopts the taste response properties of the GL as opposed to those of the regenerated CT or intact CT. This work replicates the 5 decade-old findings of Oakley and importantly extends them by providing compelling evidence that the presence of functional ENaCs, essential for sodium taste recognition in regenerated taste receptor cells, depends on the reinnervated lingual region and not on the reinnervating gustatory nerve, at least in the rat.

Epidermal growth factor attenuates lingual papillae lesions in a rat model of sialoadenectomy

Salivary epidermal growth factor (EGF) plays an important role in the maintenance of the oral and gastro-esophageal mucosa. Sialoadenectomy delays healing of oral wounds and affects lingual papillae. In this work, we aimed to determine the effect of EGF deficiency induced by sialoadenectomy and evaluate the effect of exogenous EGF administration on the lingual papillae and taste buds in rats. Thirty male adult Wistar albino rats were equally divided into 3 groups; sham-operated control group, sialoadenectomy group and group of sialoadenectomy + EGF. EGF was given 8 weeks after sialoadenectomy in a dose of 1 μg /ml/day in drinking water for 2 weeks. The anterior two-thirds of the tongue was dissected and cut longitudinally into two halves; one half for light microscope and the other for electron microscope examinations. Saliva and blood were collected to determine salivary and plasma EGF. Our results revealed that sialoadenectomy significantly reduced plasma and saliva levels of EGF which resulted in severe disruption of the architecture of lingual papillae. These changes were effectively improved by the exogenous EGF administration. In conclusion, EGF supplementation reversed the effects of sialoadenectomy and restored almost normal architecture of lingual papillae and taste buds.

Expression of Oncofetal Antigen 5T4 in Murine Taste Papillae

Background: Multicellular taste buds located within taste papillae on the tongue mediate taste sensation. In taste papillae, taste bud cells (TBCs), such as taste receptor cells and taste precursor cells, and the surrounding lingual epithelium including epithelial progenitors (also called taste stem/progenitor cells) are maintained by continuous cell turnover throughout life. However, it remains unknown how the cells constituting taste buds proliferate and differentiate to maintain taste bud tissue. Based on in situ hybridization (ISH) screening, we demonstrated that the oncofetal antigen 5T4 (also known as trophoblast glycoprotein: TPBG) gene is expressed in the adult mouse tongue. Results: In immunohistochemistry of coronal tongue sections, 5T4 protein was detected at a low level exclusively in the basal part of the lingual epithelium in developing and adult mice, and at a high level particularly in foliate papillae and circumvallate papillae (CVPs). Furthermore, immunohistochemistry of the basal part of CVPs indicated that the proliferation marker PCNA (proliferating cell nuclear antigen) co-localized with 5T4. 5T4 was strongly expressed in Krt5+ epithelial progenitors and Shh+ taste precursor cells, but weakly in mature taste receptor cells. The number of proliferating cells in the CVP was higher in 5T4-knockout mice than in wild-type (WT) mice, while neither cell differentiation nor the size of taste buds differed between these two groups of mice. Notably, X-ray irradiation enhanced cell proliferation more in 5T4-knockout mice than in WT mice. Conclusion: Our results suggest that 5T4, expressed in epithelial progenitors (taste stem/progenitor cells), and taste precursor cells, may influence the maintenance of taste papillae under both normal and injury conditions.

Hedgehog Signaling Regulates Taste Organs and Oral Sensation: Distinctive Roles in the Epithelium, Stroma, and Innervation

The Hedgehog (Hh) pathway has regulatory roles in maintaining and restoring lingual taste organs, the papillae and taste buds, and taste sensation. Taste buds and taste nerve responses are eliminated if Hh signaling is genetically suppressed or pharmacologically inhibited, but regeneration can occur if signaling is reactivated within the lingual epithelium. Whereas Hh pathway disruption alters taste sensation, tactile and cold responses remain intact, indicating that Hh signaling is modality-specific in regulation of tongue sensation. However, although Hh regulation is essential in taste, the basic biology of pathway controls is not fully understood. With recent demonstrations that sonic hedgehog (Shh) is within both taste buds and the innervating ganglion neurons/nerve fibers, it is compelling to consider Hh signaling throughout the tongue and taste organ cell and tissue compartments. Distinctive signaling centers and niches are reviewed in taste papilla epithelium, taste buds, basal lamina, fibroblasts and lamellipodia, lingual nerves, and sensory ganglia. Several new roles for the innervation in lingual Hh signaling are proposed. Hh signaling within the lingual epithelium and an intact innervation each is necessary, but only together are sufficient to sustain and restore taste buds. Importantly, patients who use Hh pathway inhibiting drugs confront an altered chemosensory world with loss of taste buds and taste responses, intact lingual touch and cold sensation, and taste recovery after drug discontinuation.

TrkB expression and dependence divides gustatory neurons into three subpopulations

Background

During development, gustatory (taste) neurons likely undergo numerous changes in morphology and expression prior to differentiation into maturity, but little is known this process or the factors that regulate it. Neuron differentiation is likely regulated by a combination of transcription and growth factors. Embryonically, most geniculate neuron development is regulated by the growth factor brain derived neurotrophic factor (BDNF). Postnatally, however, BDNF expression becomes restricted to subpopulations of taste receptor cells with specific functions. We hypothesized that during development, the receptor for BDNF, tropomyosin kinase B receptor (TrkB), may also become developmentally restricted to a subset of taste neurons and could be one factor that is differentially expressed across taste neuron subsets.

Methods

We used transgenic mouse models to label both geniculate neurons innervating the oral cavity (Phox2b+), which are primarily taste, from those projecting to the outer ear (auricular neurons) to label TrkB expressing neurons (TrkB^GFP). We also compared neuron number, taste bud number, and taste receptor cell types in wild-type animals and conditional TrkB knockouts.

Results

Between E15.5-E17.5, TrkB receptor expression becomes restricted to half of the Phox2b + neurons. This TrkB downregulation was specific to oral cavity projecting neurons, since TrkB expression remained constant throughout development in the auricular geniculate neurons (Phox2b-). Conditional TrkB removal from oral sensory neurons (Phox2b+) reduced this population to 92% of control levels, indicating that only 8% of these neurons do not depend on TrkB for survival during development. The remaining neurons failed to innervate any remaining taste buds, 14% of which remained despite the complete loss of innervation. Finally, some types of taste receptor cells (Car4+) were more dependent on innervation than others (PLCβ2+).

Conclusions

Together, these findings indicate that TrkB expression and dependence divides gustatory neurons into three subpopulations: 1) neurons that always express TrkB and are TrkB-dependent during development (50%), 2) neurons dependent on TrkB during development but that downregulate TrkB expression between E15.5 and E17.5 (41%), and 3) neurons that never express or depend on TrkB (9%). These TrkB-independent neurons are likely non-gustatory, as they do not innervate taste buds.

BDNF is required for taste axon regeneration following unilateral chorda tympani nerve section

Taste nerves readily regenerate to reinnervate denervated taste buds; however, factors required for regeneration have not yet been identified. When the chorda tympani nerve is sectioned, expression of brain-derived neurotrophic factor (BDNF) remains high in the geniculate ganglion and lingual epithelium, despite the loss of taste buds. These observations suggest that BDNF is present in the taste system after nerve section and may support taste nerve regeneration. To test this hypothesis, we inducibly deleted Bdnf during adulthood in mice. Shortly after Bdnf gene recombination, the chorda tympani nerve was unilaterally sectioned causing a loss of both taste buds and neurons, irrespective of BDNF levels. Eight weeks after nerve section, however, regeneration was differentially affected by Bdnf deletion. In control mice, there was regeneration of the chorda tympani nerve and taste buds reappeared with innervation. In contrast, few taste buds were reinnervated in mice lacking normal Bdnf expression such that taste bud number remained low. In all genotypes, taste buds that were reinnervated were normal-sized, but non-innervated taste buds remained small and atrophic. On the side of the tongue contralateral to the nerve section, taste buds for some genotypes became larger and all taste buds remained innervated. Our findings suggest that BDNF is required for nerve regeneration following gustatory nerve section.

Taste Bud-Derived BDNF Is Required to Maintain Normal Amounts of Innervation to Adult Taste Buds

Gustatory neurons transmit chemical information from taste receptor cells, which reside in taste buds in the oral cavity, to the brain. As adult taste receptor cells are renewed at a constant rate, nerve fibers must reconnect with new taste receptor cells as they arise. Therefore, the maintenance of gustatory innervation to the taste bud is an active process. Understanding how this process is regulated is a fundamental concern of gustatory system biology. We speculated that because brain-derived neurotrophic factor (BDNF) is required for taste bud innervation during development, it might function to maintain innervation during adulthood. If so, taste buds should lose innervation when Bdnf is deleted in adult mice. To test this idea, we first removed Bdnf from all cells in adulthood using transgenic mice with inducible CreERT2 under the control of the Ubiquitin promoter. When Bdnf was removed, approximately one-half of the innervation to taste buds was lost, and taste buds became smaller because of the loss of taste bud cells. Individual taste buds varied in the amount of innervation each lost, and those that lost the most innervation also lost the most taste bud cells. We then tested the idea that that the taste bud was the source of this BDNF by reducing Bdnf levels specifically in the lingual epithelium and taste buds. Taste buds were confirmed as the source of BDNF regulating innervation. We conclude that BDNF expressed in taste receptor cells is required to maintain normal levels of innervation in adulthood.

Influences of age, tongue region, and chorda tympani nerve sectioning on signal detection measures of lingual taste sensitivity

Although the ability to taste is critical for ingestion, nutrition, and quality of life, a clear understanding of the influences of age, sex, and chorda tympani (CT) resection on taste function in different regions of the anterior tongue is generally lacking. In this study we employed criterion-free signal detection analysis to assess electric and chemical taste function on multiple tongue regions in normal individuals varying in age and sex and in patients with unilateral CT resections. The subjects were 33 healthy volunteers, ranging from 18 to 87 years of age, and 9 persons, 27 to 77 years of age, with unilateral CT lesions. The influences of age, sex, tongue region, and chorda tympani resections on signal detection sensitivity (d') and response bias (β) measures was assessed in 16 tongue regions to weak electric currents and solutions of sucrose, sodium chloride, and caffeine. Significant age-related decrements in d' were found for sucrose (p=0.012), sodium chloride (p=0.002), caffeine (p=0.006), and electric current (EC) (p=0.0001). Significant posterior to anterior, and medial to lateral, gradients of increasing performance were present for most stimuli. β was larger on the anterior than the posterior tongue for the electrical stimulus in the youngest subjects, whereas the opposite was true for sucrose in the oldest subjects. No sex differences were apparent. d' was depressed ipsilateral to the CT lesion side to varying degrees in all tongue regions, with the weakest influences occurring on the medial and anterior tongue. CT did not meaningfully influence β. This study is the first to employ signal detection analysis to assess the regional sensitivity of the tongue to chemical and electrical stimuli. It clearly demonstrates that tongue regions differ from one another in terms of their age-related sensitivity and their susceptibility to CT lesions.

Developmental time course of peripheral cross-modal sensory interaction of the trigeminal and gustatory systems

Few sensory modalities appear to engage in cross-modal interactions within the peripheral nervous system, making the integrated relationship between the peripheral gustatory and trigeminal systems an ideal model for investigating cross-sensory support. The present study examined taste system anatomy following unilateral transection of the trigeminal lingual nerve (LX) while leaving the gustatory chorda tympani intact. At 10, 25, or 65 days of age, rats underwent LX with outcomes assessed following various survival times. Fungiform papillae were classified by morphological feature using surface analysis. Taste bud volumes were calculated from histological sections of the anterior tongue. Differences in papillae morphology were evident by 2 days post-transection of P10 rats and by 8 days post in P25 rats. When transected at P65, animals never exhibited statistically significant morphological changes. After LX at P10, fewer taste buds were present on the transected side following 16 and 24 days survival time and remaining taste buds were smaller than on the intact side. In P25 and P65 animals, taste bud volumes were reduced on the denervated side by 8 and 16 days postsurgery, respectively. By 50 days post-transection, taste buds of P10 animals had not recovered in size; however, all observed changes in papillae morphology and taste buds subsided in P25 and P65 rats. Results indicate that LX impacts taste receptor cells and alters epithelial morphology of fungiform papillae, particularly during early development. These findings highlight dual roles for the lingual nerve in the maintenance of both gustatory and non-gustatory tissues on the anterior tongue.

Role of neurotrophin in the taste system following gustatory nerve injury

Taste system is a perfect system to study degeneration and regeneration after nerve injury because the taste system is highly plastic and the regeneration is robust. Besides, degeneration and regeneration can be easily measured since taste buds arise in discrete locations, and nerves that innervate them can be accurately quantified. Neurotrophins are a family of proteins that regulate neural survival, function, and plasticity after nerve injury. Recent studies have shown that neurotrophins play an important role in the developmental and mature taste system, indicating neurtrophin might also regulate taste system following gustatory nerve injury. This review will summarize how taste system degenerates and regenerates after gustatory nerve cut and conclude potential roles of neurotrophin in regulating the process.

Degeneration of fungiform and circumvallate papillae following molar extraction in rats

OBJECTIVE

Proper occlusion facilitates food intake and gustatory function is indispensable for the enjoyment of food. Although an interaction between dentoalveolar and gustatory afferent neurons has been suggested by previous studies, the relationship between occlusion and gustation remains unclear. This study investigated the effect of upper molar extraction which diminished occlusal support on peripheral gustatory receptors in rats.

MATERIALS AND METHODS

Thirty-six 7-week-old male Wistar rats were randomly assigned to either the experimental or the control group. All maxillary molars were extracted from rats in the experimental group under anesthesia, while a sham operation was conducted in the control group. The rats were euthanized 7, 14 or 28 days after the procedure. The morphology of the circumvallate papillae and taste buds using immunohistochemical methods and the fungiform papillae were visualized with 1% methylene blue.

RESULTS

Defects in the gustatory epithelium were observed after maxillary molar extraction. Rats in the experimental group had significantly fewer fungiform papillae, narrower circumvallate papillae, shallower trench depth, smaller trench area, smaller taste bud area, lower ratios of taste bud area to trench area and fewer taste buds than those in the control group.

CONCLUSIONS

The findings indicate that molar extraction would affect peripheral gustatory receptors. This is the first study to characterize changes in rat fungiform and circumvallate papillae after maxillary molar extraction. This study suggests a possible synergic relationship between dentoalveolar perception and gustatory function, which has clinical implications that occlusion is closely correlated with gustatory perception.

[Changes in the innervation of the taste buds in diabetic rats]

INTRODUCTION

Abnormal sensations such as pain and impairment of taste are symptoms of approximately 10% of patients having diabetes mellitus.

AIM

The aim of the study was to investigate and quantify the different neuropeptide containing nerve fibres in the vallate papilla of the diabetic rat.

METHODS

Immunohistochemical methods were used to study the changes of the number of different neuropeptide containing nerve terminals located in the vallate papillae in diabetic rats. Diabetes was induced in the rats with streptozotocin.

RESULTS

Two weeks after streptozotocin treatment the number of the substance P, galanin, vasoactive intestinal polypeptide and neuropeptide Y immunoreactive nerve terminals was significantly increased (p<0.05) in the tunica mucosa of the tongue. The number of the lymphocytes and mast cells was also increased significantly. Some of the immunoreactive nerve terminals were located in the lingual epithelium both intragemmally and extragemmally and were seen to comprise dense bundles in the lamina propria just beneath the epithelium. No taste cells were immunoreactive for any of the investigated peptides. Vasoactive intestinal polypeptide and neuropeptide Y immunoreactive nerve fibres were not detected in the taste buds. For weeks after streptozotocin administration the number of the substance P, calcitonin gene related peptide and galanin immunoreactive nerve terminals was decreased both intragemmally and intergemmally. In case of immediate insulin treatment, the number of the immunoreactive nerve terminals was similar to that of the controls, however, insulin treatment given 1 week later to diabetic rats produced a decreased number of nerve fibers. Morphometry revealed no significant difference in papilla size between the control and diabetic groups, but there were fewer taste buds (per papilla).

CONCLUSIONS

Increased number of immunoreactive nerve terminals and mast cells 2 weeks after the development of diabetes was the consequence of neurogenic inflammation which might cause vasoconstriction and lesions of the oral mucosa. Taste impairment, which developed 4 weeks after streptozotocin treatment could be caused by neuropathic defects and degeneration or morphological changes in the taste buds and nerve fibres.

Impact of chorda tympani nerve injury on cell survival, axon maintenance, and morphology of the chorda tympani nerve terminal field in the nucleus of the solitary tract

Chorda tympani nerve transection (CTX) has been useful to study the relationship between nerve and taste buds in fungiform papillae. This work demonstrated that the morphological integrity of taste buds depends on their innervation. Considerable research focused on the effects of CTX on peripheral gustatory structures, but much less research has focused on the central effects. Here, we explored how CTX affects ganglion cell survival, maintenance of injured peripheral axons, and the chorda tympani nerve terminal field organization in the nucleus of the solitary tract (NTS). After CTX in adult rats, the chorda tympani nerve was labeled with biotinylated dextran amine at 3, 7, 14, 30, and 60 days post-CTX to allow visualization of the terminal field associated with peripheral processes. There was a significant and persistent reduction of the labeled chorda tympani nerve terminal field volume and density in the NTS following CTX. Compared with controls, the volume of the labeled terminal field was not altered at 3 or 7 days post-CTX; however, it was significantly reduced by 44% and by 63% at 30 and 60 days post-CTX, respectively. Changes in the density of labeled terminal field in the NTS paralleled the terminal field volume results. The dramatic decrease in labeled terminal field size post-CTX cannot be explained by a loss of geniculate ganglion neurons or degeneration of central axons. Instead, the function and/or maintenance of the peripheral axonal process appear to be affected. These new results have implications for long-term functional and behavioral alterations.

Chorda tympani nerve terminal field maturation and maintenance is severely altered following changes to gustatory nerve input to the nucleus of the solitary tract

Neural competition among multiple inputs can affect the refinement and maintenance of terminal fields in sensory systems. In the rat gustatory system, the chorda tympani, greater superficial petrosal, and glossopharyngeal nerves have distinct but overlapping terminal fields in the first central relay, the nucleus of the solitary tract. This overlap is largest at early postnatal ages followed by a significant refinement and pruning of the fields over a 3 week period, suggesting that competitive mechanisms underlie the pruning. Here, we manipulated the putative competitive interactions among the three nerves by sectioning the greater superficial petrosal and glossopharyngeal nerves at postnatal day 15 (P15), P25, or at adulthood, while leaving the chorda tympani nerve intact. The terminal field of the chorda tympani nerve was assessed 35 d following nerve sections, a period before the sectioned nerves functionally regenerated. Regardless of the age when the nerves were cut, the chorda tympani nerve terminal field expanded to a volume four times larger than sham controls. Terminal field density measurements revealed that the expanded terminal field was similar to P15 control rats. Thus, it appears that the chorda tympani nerve terminal field defaults to its early postnatal field size and shape when the nerves with overlapping fields are cut, and this anatomical plasticity is retained into adulthood. These findings not only demonstrate the dramatic and lifelong plasticity in the central gustatory system, but also suggest that corresponding changes in functional and taste-related behaviors will accompany injury-induced changes in brainstem circuits.

Expression of Six1 and Six4 in mouse taste buds

Members of the Six gene family are expressed in various tissues including sensory organs, such as the inner ear and olfactory epithelium. We examined the expression of Six1 and Six4 mRNAs in mouse taste buds by using in situ hybridization. Six1 was detected immunohistochemically in the nuclei of taste bud cells, in a subset of type-II cells, as shown by double-immunolabeling with anti-Six1 together with anti-PLCβ2 or anti-IP(3)R3 antibodies. Six1-immunoreactive (IR) nuclei appeared at embryonic day 17.5 in the dorsal epithelium, and in the trench wall epithelium of circumvallate papillae at postnatal day 5. At this stage, Six1-IR nuclei were observed in all newly-formed type-II cells. During postnatal development, type-II cells increased in number, but those with Six1-IR nuclei showed no apparent increase. After transection of the bilateral glossopharyngeal nerve, type-II cells gradually disappeared; but some of them remained in the epithelium even at 11-17 days post-transection. The remaining type-II cells showed Six1-immunoreactivity. At 24 days after nerve transection, regenerating type-II cells appeared; and strong Six1-immunoreactivity was observed in them. Also, enhanced green fluorescent protein-immunoreactivity and β-galactosidase-immunoreactivity, which were indicators for Six1 transcripts and Six4 transcripts, respectively, overlapped. These results suggest that Six1 and Six4 genes are expressed in the taste bud cells, in newly formed or surviving type-II cells.

Fungiform taste bud degeneration in C57BL/6J mice following chorda-lingual nerve transection

Taste buds are dependent on innervation for normal morphology and function. Fungiform taste bud degeneration after chorda tympani nerve injury has been well documented in rats, hamsters, and gerbils. The current study examines fungiform taste bud distribution and structure in adult C57BL/6J mice from both intact taste systems and after unilateral chorda-lingual nerve transection. Fungiform taste buds were visualized and measured with the aid of cytokeratin 8. In control mice, taste buds were smaller and more abundant on the anterior tip (<1 mm) of the tongue. By 5 days after nerve transection taste buds were smaller and fewer on the side of the tongue ipsilateral to the transection and continued to decrease in both size and number until 15 days posttransection. Degenerating fungiform taste buds were smaller due to a loss of taste bud cells rather than changes in taste bud morphology. While almost all taste buds disappeared in more posterior fungiform papillae by 15 days posttransection, the anterior tip of the tongue retained nearly half of its taste buds compared to intact mice. Surviving taste buds could not be explained by an apparent innervation from the remaining intact nerves. Contralateral effects of nerve transection were also observed; taste buds were larger due to an increase in the number of taste bud cells. These data are the first to characterize adult mouse fungiform taste buds and subsequent degeneration after unilateral nerve transection. They provide the basis for more mechanistic studies in which genetically engineered mice can be used.

Effects of streptozotocin-induced diabetes on taste buds in rat vallate papillae

Some studies have documented taste changes in patients with diabetes mellitus (DM). In order to understand the relationships between taste disorders caused by DM and the innervation and morphologic changes in the taste buds, we studied the vallate papillae and their taste buds in rats with DM. DM was induced in these rats with streptozotocin (STZ), which causes the death of beta cells of the pancreas. The rats were sacrificed and the vallate papillae were dissected for morphometric and quantitative immunohistochemical analyses. The innervations of the vallate papillae and taste buds in diabetic and control rats were detected using immunohistochemistry employing antibodies directed against protein gene product 9.5 (PGP 9.5) and calcitonin gene-related peptide (CGRP). The results showed that PGP 9.5- and CGRP-immunoreactive nerve fibers in the trench wall of diabetic vallate papillae, as well as taste cells in the taste buds, gradually decreased both intragemmally and intergemmally. The morphometry revealed no significant difference in papilla size between the control and diabetic groups, but there were fewer taste buds per papilla (per animal). The quantification of innervation in taste buds of the diabetic rats supported the visual assessment of immunohistochemical labeling, that the innervation of taste cells was significantly reduced in diabetic animals. These findings suggest that taste impairment in diabetic subjects may be caused by neuropathy defects and/or morphological changes in the taste buds.

Expression of tryptophan hydroxylase in developing mouse taste papillae

Gustatory papillae and associated taste buds receive and process chemical information from the environment. In mammals, their development takes place during the late phase of embryogenesis. However, the cellular factors that regulate the differentiation of taste papillae remain largely unknown. Here, we show by quantitative real time RT-PCR that both isoforms of tryptophan hydroxylase (TPH1 and TPH2), the first and rate limiting enzyme of serotonin (5-HT) synthesis, are expressed in developing circumvallate papillae. Immuno-staining experiments further indicated that TPH is localized both in gustatory fibers and in differentiated taste receptor cells. These results point to the synthesis of 5-HT in gustatory papillae, and allow one to hypothesize that the development of taste buds might be modulated by serotonin.

Contact Endoscopic Comparison of Morphology of Human Fungiform Papillae of Healthy Subjects and Patients with Transected Chorda Tympani Nerve

BACKGROUND

The chorda tympani nerve (CTN) carries gustatory fibers from taste buds of fungiform papillae (fPap) of the anterior portion of the tongue. Accordingly, middle ear surgery with transection of the CTN may result in gustatory impairment. With use of contact endoscopy, the present study aimed to compare number and shape of fPap and subepithelial vessel formation in patients after CTN transection with that of healthy controls.

MATERIAL AND METHODS

The number of fPap per square centimeter was obtained in addition to measures of gustatory sensitivity using electrogustometry and intensity ratings of natural taste stimuli. Data from 32 healthy subjects (mean age 40 yr; 16 female, 16 male) were compared with those from 14 patients whose CTN had been cut and partly resected during middle ear surgery because of cholesteatoma (mean age 38 yr; 7 female, 7 male). Middle ear surgery was performed by two of the authors.

RESULTS

In healthy subjects, a higher fPap density was found on the tongue's tip compared with its edge; younger subjects had higher densities than older subjects. No sex-related differences were observed. Patients with transected CTN exhibited a significant decrease of taste function as measured with both natural and electric stimuli. In patients, we found significantly fewer and flatter fPap on the side of the tongue where surgery had been performed in comparison with the contralateral side. In addition, fPap density at the ipsilateral side was significantly lower compared with fPap density on the respective side in healthy controls.

CONCLUSION

As established through contact endoscopy, the number of fPap decreases in relation to deafferentation and also in relation to the subjects' age, both of which are accompanied by a decreased gustatory sensitivity.

Characterization and long-term maintenance of rat taste cells in culture

Taste cells have a limited life span and are replaced from a basal cell population, although the specific factors involved in this process are not well known. Short- and long-term cultures of other sensory cells have facilitated efforts to understand the signals involved in proliferation, differentiation, and senescence, yet few studies have reported successful primary culture protocols for taste cells. Furthermore, no studies have demonstrated both proliferation and differentiation in vitro. In this study, we have developed an in vitro culture system to maintain and utilize rat primary taste cells for more than 2 months without losing key molecular and biochemical features. Gustducin, phospholipase C-beta2 (PLC-beta2), T1R3, and T2R5 mRNA were detected in the cultured cells by reverse transcriptase-polymerase chain reaction. Western blot analysis demonstrated gustducin and PLC-beta2 expression in the same samples, which was confirmed by immunocytochemistry. Labeling with bromo-2-deoxyuridine (BrdU) demonstrated proliferation, and a subset of BrdU-labeled cells were also immunoreactive for either gustducin or PLC-beta2, indicating differentiation of newly generated cells in vitro. Cultured cells also exhibited increases in intracellular calcium in response to several taste stimuli. These results indicate that taste cells from adult rats can be generated and maintained under the described conditions for at least 2 months. This system will enable further studies of the processes involved in proliferation, differentiation, and function of mammalian taste receptor cells in an in vitro preparation.

Building sensory receptors on the tongue

Neurotrophins, neurotrophin receptors and sensory neurons are required for the development of lingual sense organs. For example, neurotrophin 3 sustains lingual somatosensory neurons. In the traditional view, sensory axons will terminate where neurotrophin expression is most pronounced. Yet, lingual somatosensory axons characteristically terminate in each filiform papilla and in each somatosensory prominence within a cluster of cells expressing the p75 neurotrophin receptor (p75NTR), rather than terminating among the adjacent cells that secrete neurotrophin 3. The p75NTR on special specialized clusters of epithelial cells may promote axonal arborization in vivo since its over-expression by fibroblasts enhances neurite outgrowth from overlying somatosensory neurons in vitro. Two classical observations have implicated gustatory neurons in the development and maintenance of mammalian taste buds--the early arrival times of embryonic innervation and the loss of taste buds after their denervation in adults. In the modern era more than a dozen experimental studies have used early denervation or neurotrophin gene mutations to evaluate mammalian gustatory organ development. Necessary for taste organ development, brain-derived neurotrophic factor sustains developing gustatory neurons. The cardinal conclusion is readily summarized: taste buds in the palate and tongue are induced by innervation. Taste buds are unstable: the death and birth of taste receptor cells relentlessly remodels synaptic connections. As receptor cells turn over, the sensory code for taste quality is probably stabilized by selective synapse formation between each type of gustatory axon and its matching taste receptor cell. We anticipate important new discoveries of molecular interactions among the epithelium, the underlying mesenchyme and gustatory innervation that build the gustatory papillae, their specialized epithelial cells, and the resulting taste buds.

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.

Expression of amiloride-sensitive epithelial sodium channels in mouse taste cells after chorda tympani nerve crush

Our previous electrophysiological study demonstrated that amiloride-sensitive (AS) and -insensitive (AI) components of NaCl responses recovered differentially after the mouse chorda tympani (CT) was crushed. AI responses reappeared earlier (at 3 weeks after the nerve crush) than did AS ones (at 4 weeks). This and other results suggested that two salt-responsive systems were differentially and independently reformed after nerve crush. To investigate the molecular mechanisms of formation of the salt responsive systems, we examined expression patterns of three subunits (alpha, beta and gamma) of the amiloride-sensitive epithelial Na(+) channel (ENaC) in mouse taste cells after CT nerve crush by using in situ hybridization (ISH) analysis. The results showed that all three ENaC subunits, as well as alpha-gustducin, a marker of differentiated taste cells, were expressed in a subset of taste bud cells from an early stage (1-2 weeks) after nerve crush, although these taste buds were smaller and fewer in number than for control mice. At 3 weeks, the mean number of each ENaC subunit and alpha-gustducin mRNA-positive cells per taste bud reached the control level. Also, the size of taste buds became similar to those of the control mice at this time. Our previous electrophysiological study demonstrated that at 2 weeks no significant response of the nerve to chemical stimuli was observed. Thus ENaC subunits appear to be expressed prior to the reappearance of AI and AS neural responses after CT nerve crush. These results support the view that differentiation of taste cells into AS or AI cells is initiated prior to synapse formation.

Neuron/target plasticity in the peripheral gustatory system

Taste bud volume on the anterior tongue in adult rats is matched by an appropriate number of innervating geniculate ganglion cells. The larger the taste bud, the more geniculate ganglion cells that innervate it. To determine if such a match is perturbed in the regenerated gustatory system under different dietary conditions, taste bud volumes and numbers of innervating neurons were quantified in adult rats after unilateral axotomy of the chorda tympani nerve and/or maintenance on a sodium-restricted diet. The relationship between taste bud size and innervation was eliminated in rats merely fed a sodium-restricted diet; individual taste bud volumes were smaller than predicted by the corresponding number of innervating neurons. Surprisingly, the relationship was disrupted in a similar way on the intact side of the tongue in unilaterally sectioned rats, with no diet-related differences. The mismatch in these groups was due to a decrease in average taste bud volumes and not to a change in numbers of innervating ganglion cells. In contrast, individual taste bud volumes were larger than predicted by the corresponding number of innervating neurons on the regenerated side of the tongue; again, with no diet-related differences. However, the primary variable responsible for disrupting the function on the regenerated side was an approximate 20% decrease in geniculate ganglion cells available to innervate taste buds. Therefore, the neuron/target match in the peripheral gustatory system is susceptible to surgical and/or dietary manipulations that act through multiple mechanisms. This system is ideally suited to model sensory plasticity in adults.

Sensitive periods for the effect of dietary sodium restriction on intact and denervated taste receptor cells

Unilateral chorda tympani nerve (CT) section combined with dietary sodium restriction leads to striking alterations in sodium taste function. The regenerated rat CT exhibits deficits in sodium sensitivity, and surprisingly, there are also functional alterations in the intact, contralateral nerve. The studies presented here describe the functional "sensitive periods" for these aberrations and the number of taste buds present during corresponding stages. The regenerated CT is sensitive to dietary sodium restriction during the first 2 wk after denervation, whereas the intact CT is sensitive to dietary manipulation during the first week postsection. Therefore, distinct mechanisms are responsible for the effects of sodium restriction combined with denervation, because separate sensitive periods exist for the regenerated and intact CT nerves. Identification of mature taste buds with an antibody directed at anti-keratin 19 revealed that there is a loss of ~85% of taste buds on the denervated side of the tongue under control and low-sodium diets within the first week postsection. Thus, sodium restriction does not differentially affect the loss of taste buds following denervation.

Time course of morphological alterations of fungiform papillae and taste buds following chorda tympani transection in neonatal rats

The time course of structural changes in fungiform papillae was analyzed in rats that received unilateral chorda tympani nerve transection at 10 days of age. Morphological differences between intact and denervated sides of the tongue were first observed at 8 days postsection, with an increase in the number of fungiform papillae that did not have a pore. In addition, the first papilla with a filiform-like appearance was noted on the denervated side at 8 days postsectioning. By 11 days after surgery, the total number of papillae and the number of papillae with a pore were significantly lower on the transected side of the tongue as compared to the intact side. At 50 days postsection, there was an average of 70.5 fungiform papillae on the intact side and a mean of only 20.8 fungiform papillae the denervated side. Of those few remaining papillae on the cut side, an average of 13.5 papillae were categorized as filiform-like, while no filiform-like papillae occurred on the intact side. Significant reduction in taste bud volume was noted at 4 days posttransection and further decrements in taste bud volume were noted at 8 and 30 days postsection. Electron microscopy of the lingual branch of the trigeminal nerve from adult rats that received neonatal chorda tympani transection showed normal numbers of both myelinated and unmyelinated fibers. Thus, in addition to the well-characterized dependence of taste bud maintenance on the chorda tympani nerve, the present study shows an additional role of the chorda tympani nerve in papilla maintenance during early postnatal development.

Incidence of regeneration of the chorda tympani nerve after middle ear surgery

We retrospectively reviewed 52 patients who underwent middle ear surgery during which the chorda tympani nerves were severed and who then underwent secondary surgery 1 to 5 years later. In 22 patients (42.3%), regenerated chorda tympani nerves (entire length of the tympanic segment) were detected in the submucosal layer of the reconstructed eardrum during the secondary surgery. Before the secondary surgery, 16 patients (30.8%) showed threshold recovery on electrogustometry. When 5 regenerated nerves were observed by transmission electron microscopy, myelinated nerve fibers were detected in a small fascicle or connective tissue, but the number of myelinated axons was significantly decreased (7.4% to 84.6%; p = .01) compared with that in normal subjects (1,911 +/- 324; n = 4). There was a significant difference in the incidence of regeneration between the group with end-to-end anastomosis (5/5 or 100%) and that with nerve gap defects (17/47 or 36.2%; p <.05); this finding suggests that repair of the sectioned nerve produces a better incidence of regeneration than leaving the nerve unrepaired.

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.

Morphological and functional study of regenerated chorda tympani nerves in humans

It is still unclear whether the chorda tympani nerves in humans regenerate after being severed during middle ear surgery, although functional studies have demonstrated recovery of taste 1 to 2 years after surgery. To date, 12 cases of regenerated chorda tympani nerves have been found in our series of patients during secondary surgery. The regenerated nerves of 3 cases of the 12 were removed as samples during secondary surgery to detect regenerated myelinated axons. All regenerated nerves were in the submucosal connective tissue layer of the reconstructed eardrum. In the regenerated nerves, myelinated nerve fibers existed in a small fascicle or in connective tissue, but the number of myelinated axons was low compared with that in normal subjects ( 1.752 +/- 78; n = 3), and the distribution was sparse. The total number of regenerated myelinated axons varied from 141 (8.3%) to 979 (55.9%). From a functional study using electrogustometry, incomplete recovery of electrogustation was observed in all 3 cases before secondary surgery, suggesting that chorda tympani nerves actually regenerate in the middle ear and do function.

Neonatal chorda tympani transection permanently disrupts fungiform taste bud and papilla structure in the rat

The present report examined the morphology of fungiform papillae in adult rats that received bilateral chorda tympani transection at 10 days of age. Tongue tissue was examined using surface-structure analysis. Counts were made of fungiform papillae with a pore, fungiform papillae with no pore and fungiform papillae with a keratinized conical surface; a feature referred to as "filiform-like. " Neonatal chorda tympani nerve transection resulted not only in a loss of taste buds but also in a permanent loss in numbers of fungiform papillae. Compared with an average of 152 fungiform papillae in sham-operated control rats, there was an average of only 54 fungiform papillae after neonatal chorda tympani transection. Nearly 80% of these fungiform papillae in neonatal chorda tympani transected rats were filiform-like. No filiform-like papillae were noted in sham-operated rats. These results suggest that the chorda tympani nerve is necessary during an early postnatal period of development to maintain normal fungiform papillae morphology.

Taste cell function. Structural and biochemical implications

Maintenance of constant relations between receptor cell types and branching from a single gustatory nerve fiber during normal cell turnover and regeneration requires cell-cell recognition likely mediated by timed expression of molecules at surfaces of taste bud cells, nerve endings, and in extracellular matrix. These processes assure stability of gustatory quality representation during intragemmal remodeling. Coincidentally, features of gemmal cell lifespan, including elongation, differentiation, and migration prior to apoptosis, must also be orchestrated by molecular signals. This article reviews the potential roles played by a variety of molecular markers for some relevant classes of proteins, peptides, and enzymes, which were presumed to be important for carrying out these gustatory cellular functions.

Functional status of the regenerated chorda tympani nerve as assessed in a salt taste discrimination task

We tested whether the recovered ability of rats to discriminate NaCl from KCl after chorda tympani nerve transection (CTX) is causally linked to nerve regeneration or some other compensatory process. Rats were presurgically trained in an operant NaCl vs. KCl discrimination task. Rats with regenerated nerves, histologically confirmed by anterior tongue taste pore counts and tested 62 days after CTX (CTX-62R; n = 5), performed as well as those tested 62 days after sham surgery (Sham-62; n = 5), but both of these groups initially performed slightly worse than animals tested 7 days after sham surgery (Sham-7; n = 4). Performance of rats tested either 7 (CTX-7P; n = 5) or 62 (CTX-62P; n = 4) days after CTX in which nerve regeneration was prevented was severely disrupted. Adulteration of the stimuli with amiloride, an epithelial sodium channel blocker, impaired discrimination performance in a similar dose-dependent manner in the Sham-7 (n = 2), Sham-62 (n = 5), and CTX-62R (n = 5) groups, suggesting that the functional status of the amiloride-sensitive transduction pathway returns to normal in rats with regenerated chorda tympani nerves. Performance of CTX rats without regenerated nerves (CTX-7P, n = 2; CTX-62P, n = 4) was further degraded by amiloride treatment, suggesting that taste receptors innervated by other nerves are sensitive to amiloride. In conclusion, nerve regeneration is an essential component underlying full recovery of salt discrimination function after CTX.

An in vitro model for the study of the role of innervation in circumvallate papillae morphogenesis

The following study was done to demonstrate the reliability of an in vitro model for use in the study of early events and the role of innervation in mouse circumvallate papillae development. Gestational day (gd)-11 fetuses were partially dissected to produce explants that included the mandibular, hyoid, third and fourth branchial arches and their ganglia. In ganglionectomized explants, the nodose ganglia and either the geniculate, petrosal or both ganglia were removed. Explants were cultivated in roller tube culture for 24, 48, 72, and 96 h of culture and examined for the presence of papillary structures. Innervation was verified by immunostaining for neural cell adhesion molecule (NCAM). In all control explants, circumvallate papillae had formed by 72 h in culture. These papillae were innervated by fibers originating in petrosal or nodose ganglia, although, in a small number, fibers from the geniculate also contributed. Circumvallate papillae also formed in some explants in which either the geniculate or petrosal ganglia had been removed. However, placodal structures failed to mature into papillary structures even by 96 h in explants in which both ganglia had been removed. Our results demonstrate that an in vitro model using branchial arch explants supports the morphogenesis of an epithelial placode through the formation of a definite papillary structure, the circumvallate papilla, with an integrated nerve. Our results also indicate that, whereas the initial stages in gustatory papillae formation, the formation of a placode, are nerve-independent, the maturation of the placodal structure to form a papilla requires the presence of an intact nerve.

Cellular expression of alpha-gustducin and the A blood group antigen in rat fungiform taste buds cross-reinnervated by the IXth nerve

Although taste buds are trophically dependent on their innervation, cross-reinnervation experiments have shown that their gustatory sensitivities are determined by the local epithelium. Both the gustatory G-protein, alpha-gustducin, and the cell-surface carbohydrate, the A blood group antigen, are expressed by significantly fewer fungiform than vallate taste cells in the rat. In these experiments, one side of the anterior portion of the tongue was cross-reinnervated by the IXth nerve in order to determine whether the molecular expression of taste bud cells is determined by the epithelium from which they arise or by the nerve on which they are trophically dependent. The proximal portion of the IXth nerve was anastomosed to the distal portion of the chorda tympani (CT) nerve using fibrin glue (IX-CT rats). Control animals had the CT cut and reanastomosed using the same technique (CT-CT rats), or had the CT avulsed from the bulla and resected to prevent regeneration (CTX rats). The animals survived for 12 weeks postoperatively, and the tongues were removed, stained with methylene blue, and the fungiform taste pores counted on both sides. Tissue from the anterior 5 mm of the tongue was cut into 50-microm sections, which were incubated with antibodies against alpha-gustducin and the human blood group A antigen. In both CT-CT and IX-CT rats, there was regeneration of fungiform taste buds, although in both groups there were significantly fewer taste buds on the operated side of the tongue. The normal vallate papilla had a mean of 8.37 alpha-gustducin-expressing cells and 5.22 A-expressing cells per taste bud, whereas the fungiform papillae contained 3.06 and 0.23 cells per taste bud, respectively. In both CT-CT and IX-CT rats there was a normal number of cells expressing alpha-gustducin or the A antigen in regenerated taste buds; in the CTX animals there was a significant decrease in the expression of these markers. These results demonstrate that the molecular phenotype of taste bud cells is determined by the local epithelium from which they arise and not by properties of the innervating nerve.

Taste neurons have multiple inductive roles in mammalian gustatory development

The embryonic loss of brain-derived neurotropic factor (BDNF)-dependent taste axons in bdnf null mutant mice secondary impairs the development of gustatory epithelia and taste buds. In normal mice gustatory development continues for at least two weeks postnatally as axons promote taste bud formation. We conclude that taste axons in the fungiform, foliate, vallate and nasopalate papillae: i) promote papilla development, and ii) establish competent gustatory cells and iii) mature taste buds. Hence, gustatory innervation contributes critically to at least three of the multiple inductive interactions controlling the development of mammalian gustatory structures.

Neurotrophic factors in the tongue: expression patterns, biological activity, relation to innervation and studies of neurotrophin knockout mice

How taste buds develop and how they become innervated has been a matter of debate for a long time. Brain-derived neurotropic factor (BDNF) and neurotrophin-3 (NT3) mRNA expression patterns suggested a possible involvement in lingual gustatory and somatosensory innervation. Studies of null-mutated mice showed that BDNF-/- mice had few abnormal taste buds and were unable to discriminate between primary tastes. NT3-/- mice had a severe loss of lingual somatosensory innervation. These novel findings may have clinical implications in rare human conditions such as familial dysautonomia and/or in more common cases of problems with loss of taste and sensation in the mouth such as those seen after injury to the nerves, either by accident or following oral/facial surgery. Knowledge about which proteins that are required to stimulate nerve fibers to grow into mucous membranes of the oral cavity during development suggests that these same proteins might become helpful in stimulating regeneration of injured nerves in patients, perhaps helping them to regain lost taste and sensory functions. Here, the presence of glial cell-derived neurotrophic factor (GDNF) families of neurotrophic factors and receptors in the tongue is also discussed. Further, a model for the development and innervation of taste buds in mammals is proposed.

Scanning electron microscopy of denervated taste buds in hamster: morphology of fungiform taste pores

BACKGROUND

Taste pores of fungiform papillae are critical for taste function. Taste nerve injury affects the pore, rendering it refractory to staining with vital dyes. Whether pores of denervated fungiform papillae disappear or undergo more modest structural changes to account for diminished staining was the subject of the present study.

METHODS

The chorda tympani in the hamster was served unilaterally and the anterior tongue prepared for scanning electron microscopy after 31 days of survival.

RESULTS

Taste pores were found on 92% of control fungiform papillae. They were round openings formed by the free margins of keratinocytes, and centered in hillock-shaped elevations of the papillary surface. Hillocks were encircled by an indentation which, in turn, was surrounded by a circular epithelial rim. These structures associated with fungiform pores distinguish pores on the anterior tongue from those on the posterior tongue. The pores led to a channel that penetrated into the papilla. The experimental side of the tongue had markedly fewer pores. Definitive pores were present on only 53% of denervated papillae. The papillae that lacked pores either exhibited a small hillock and a subtle depression in place of the pore, or had entirely flat apical surfaces. The denervated papillae that retained pores exhibited structural changes. The pores had smaller diameters and led to shallower channels than control pores. Moreover, these persistent pores were associated with hillocks, indentations and rims that were more variable and less distinct than those of control papillae.

CONCLUSIONS

Pores of fungiform papillae in hamster are associated with specialized surfaces features of the papillary epithelium. Denervation results in changes that range from disappearance of the pores to their shrinkage and the atrophy of pore-associated epithelial structures.

Neural cell adhesion molecule, neuron-specific enolase and calcitonin gene-related peptide immunoreactivity in hamster taste buds after chorda tympani/lingual nerve denervation

Hamster fungiform papilla taste buds persist in an atrophic form following sensory denervation. While atrophic and innervated taste buds are morphologically similar, it is not known whether their gemmal cells have similar molecular characteristics. Three neurochemicals, neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide have been implicated in trophic phenomena, synaptogenesis and cell recognition in neurons and sensory neuroepithelia. The present study uses immunocytochemical localization of these molecular markers to characterize normal and denervated fungiform taste buds following unilateral chorda tympani/lingual nerve denervation in hamsters. In normal taste buds, immunoreactivity to neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide was present in a group of cells located centrally in the bud as well as in fungiform nerve fibres and endings. After denervation, gemmal cell immunoreactivity to all three markers was reduced and often confined to a single or a few bud cell(s). Also, fibre staining was absent except for sparse calcitonin gene-related peptide-immunoreactive fibres associated with blood vessels and within the fungiform papillae. These remaining fibres may be autonomic or somatomotor in origin. These results indicate that sensory denervation of hamster taste buds reduces, but does not wholly eliminate the immunoreactivity of surviving gemmal cells to neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide. While the number of taste bud cells expressing the markers appears to be nerve-dependent, immunoreactivity in sensory-denervated bud cells of hamster may reflect the influence of local tissue factors.

Mice with a targeted disruption of the neurotrophin receptor trkB lose their gustatory ganglion cells early but do develop taste buds

The alleged ability of taste afferents to induce taste buds in developing animals is investigated using a mouse model with a targeted deletion of the tyrosine kinase receptor trkB for the neurotrophin BDNF. This neurotrophin was recently shown to be expressed in developing taste buds and the receptor trkB has been shown to be expressed in the developing ganglion cells that innervate the taste buds. Our data show a reduction of geniculate ganglion cells to about 5% of control animals in neonates. Degeneration of ganglion cells starts when processes reach the central target (solitary tract) but before they reach the peripheral target (taste buds). Degeneration of ganglion cells is almost completed in trkB knockout mice before taste afferents reach in control animals the developing fungiform papillae. Four days later the first taste buds can be identified in fungiform papillae of both control and trkB knockout mice in about equal number and density. Many taste buds undergo a normal maturation compared to control animals. However, the more lateral and caudal fungiform papillae grow less in size and become less conspicuous in older trkB knockout mice. No intragemmal innervation can be found in trkB knockout taste buds but a few extragemmal fibers enter the apex and end between taste had cells without forming specialized synapses. Taste buds of trkB knockout mice appear less well organized than those of control mice, but some cells show similar vesicle accumulations as control taste bud cells in their base but no synaptic contact to an afferent. These data strongly suggest that the initial-development of many fungiform papillae and taste buds is independent of the specific taste innervation. It remains to be shown why others appear to be more dependent on proper innervation.

Differential expression of brain-derived neurotrophic factor and neurotrophin 3 mRNA in lingual papillae and taste buds indicates roles in gustatory and somatosensory innervation

Although many studies have demonstrated the dependency of taste bud function and/or survival on intact innervation, relatively few have dealt with the development of taste bud innervation. Using in situ hybridization histochemistry, we show that brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT3) mRNA are expressed in a specific pattern in the taste buds, tongue papillae, and lingual epithelium during development and that expression persists into adulthood. BDNF mRNA is expressed in a fraction of the taste cells of the developing and adult taste buds in rats, showing different labeling intensities among the labeled cells. NT3 and mRNA seems to be located in areas other than those where BDNF mRNA is expressed, mainly in the superior epithelial surfaces of circumvallate papillae, the outer surface epithelium of foliate papilae, the superior surface and the lateral epithelium of the fungiform papillae, and the epithelium of the filiform papillae. NT3 mRNA labeling is also observed among muscle and connective tissue of the tongue. The morphological appearance, expression of NT3 mRNA, and ramification of nerve fibers in defined epithelial structures in the posterior wall of the anterior filiform papillae suggest the existence of a mechanosensory apparatus in these papillae. Nerve growth factor and neurotrophin 4 probes did not give rise to selective labeling in tongue, although their presence cannot be totally excluded. Based on present and prior studies, we suggest that BDNF is needed during initiation and for maintenance of gustatory innervation of taste buds and gustatory papillae and that NT3 is mainly needed for somatosensory innervation of the tongue.