Regeneration of fungiform taste buds: temporal and spatial characteristics

EGR4 is critical for cell-fate determination and phenotypic maintenance of geniculate ganglion neurons underlying sweet and umami taste

The sense of taste starts with activation of receptor cells in taste buds by chemical stimuli which then communicate this signal via innervating oral sensory neurons to the CNS. The cell bodies of oral sensory neurons reside in the geniculate ganglion (GG) and nodose/petrosal/jugular ganglion. The geniculate ganglion contains two main neuronal populations: BRN3A+ somatosensory neurons that innervate the pinna and PHOX2B+ sensory neurons that innervate the oral cavity. While much is known about the different taste bud cell subtypes, considerably less is known about the molecular identities of PHOX2B+ sensory subpopulations. In the GG, as many as 12 different subpopulations have been predicted from electrophysiological studies, while transcriptional identities exist for only 3 to 6. Importantly, the cell fate pathways that diversify PHOX2B+ oral sensory neurons into these subpopulations are unknown. The transcription factor EGR4 was identified as being highly expressed in GG neurons. EGR4 deletion causes GG oral sensory neurons to lose their expression of PHOX2B and other oral sensory genes and up-regulate BRN3A. This is followed by a loss of chemosensory innervation of taste buds, a loss of type II taste cells responsive to bitter, sweet, and umami stimuli, and a concomitant increase in type I glial-like taste bud cells. These deficits culminate in a loss of nerve responses to sweet and umami taste qualities. Taken together, we identify a critical role of EGR4 in cell fate specification and maintenance of subpopulations of GG neurons, which in turn maintain the appropriate sweet and umami taste receptor cells.

Interleukin (IL)-1 Receptor Signaling Is Required for Complete Taste Bud Regeneration and the Recovery of Neural Taste Responses following Axotomy

Experimental or traumatic nerve injury causes the degeneration of associated taste buds. Unlike most sensory systems, the sectioned nerve and associated taste buds can then regenerate, restoring neural responses to tastants. It was previously unknown whether injury-induced immune factors mediate this process. The proinflammatory cytokines, interleukin (IL)-1α and IL-1β, and their requisite receptor are strongly expressed by anterior taste buds innervated by the chorda tympani nerve. We tested taste bud regeneration and functional recovery in mice lacking the IL-1 receptor. After axotomy, the chorda tympani nerve regenerated but was initially unresponsive to tastants in both WT and Il1r KO mice. In the absence of Il1r signaling, however, neural taste responses remained minimal even >8 weeks after injury in both male and female mice, whereas normal taste function recovered by 3 weeks in WT mice. Failed recovery was because of a 57.8% decrease in regenerated taste buds in Il1r KO compared with WT axotomized mice. Il1a gene expression was chronically dysregulated, and the subset of regenerated taste buds were reinnervated more slowly and never reached full volume as progenitor cell proliferation lagged in KO mice. Il1r signaling is thus required for complete taste bud regeneration and the recovery of normal taste transmission, likely by impairing taste progenitor cell proliferation. This is the first identification of a cytokine response that promotes taste recovery. The remarkable plasticity of the taste system makes it ideal for identifying injury-induced mechanisms mediating successful regeneration and recovery.SIGNIFICANCE STATEMENT Taste plays a critical role in nutrition and quality of life. The adult taste system is highly plastic and able to regenerate following the disappearance of most taste buds after experimental nerve injury. Several growth factors needed for taste bud regeneration have been identified, but we demonstrate the first cytokine pathway required for the recovery of taste function. In the absence of IL-1 cytokine signaling, taste bud regeneration is incomplete, preventing the transmission of taste activity to the brain. These results open a new direction in revealing injury-specific mechanisms that could be harnessed to promote the recovery of taste perception after trauma or disease.

Single-cell transcriptomics uncovers the differentiation of a subset of murine esophageal progenitors into taste buds in vivo

Mouse esophagus is lined with a stratified epithelium, which is maintained by the constant renewal of unipotent progenitors. In this study, we profiled mouse esophagus by single-cell RNA sequencing and found taste buds specifically in the cervical segment of the esophagus. These taste buds have the same cellular composition as the ones from the tongue but express fewer taste receptor types. State-of-the-art transcriptional regulatory network analysis allowed the identification of specific transcription factors associated to the differentiation of immature progenitors into the three different taste bud cell types. Lineage tracing experiments revealed that esophageal taste buds arise from squamous bipotent progenitor, thus demonstrating that all esophageal progenitors are not unipotent. Our cell resolution characterization of cervical esophagus epithelium will enable a better understanding of esophageal progenitor potency and insights into the mechanisms involved in the development of taste buds.

RNF43/ZNRF3 negatively regulates taste tissue homeostasis and positively regulates dorsal lingual epithelial tissue homeostasis

Taste bud cells are renewed throughout life in a process requiring innervation. Recently, we reported that R-spondin substitutes for neuronal input for taste cell regeneration. R-spondin amplifies WNT signaling by interacting with stem-cell-expressed E3 ubiquitin ligases RNF43/ZNRF3 (negative regulators of WNT signaling) and G-protein-coupled receptors LGR4/5/6 (positive regulators of WNT signaling). Therefore, we hypothesized that RNF43/ZNRF3 may serve as a brake, controlled by gustatory neuron-produced R-spondin, for regulating taste tissue homeostasis. Here, we show that mice deficient for Rnf43/Znrf3 in KRT5-expressing epithelial stem/progenitor cells (RZ dKO) exhibited taste cell hyperplasia; in stark contrast, epithelial tissue on the tongue degenerated. WNT signaling blockade substantially reversed all these effects in RZ dKO mice. Furthermore, innervation becomes dispensable for taste cell renewal in RZ dKO mice. We thus demonstrate important but distinct functions of RNF43/ZNRF3 in regulating taste versus lingual epithelial tissue homeostasis.

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.

Muscle Regeneration of the Tongue: A review of current clinical and regenerative research strategies

Various abnormalities of the tongue, including cancers, commonly require surgical removal to sequester growth and metastasis. However, even minor resections can affect functional outcomes such as speech and swallowing, thereby reducing quality of life. Surgical resections alone create volumetric muscle loss whereby muscle tissue cannot self-regenerate within the tongue. In these cases, the tongue is reconstructed typically in the form of autologous skin flaps. However, flap reconstruction has many limitations and unfortunately is the primary option for oral and reconstructive surgeons to treat tongue defects. The alternative, but yet undeveloped strategy for tongue reconstruction is regenerative medicine, which widely focuses on building new organs with stem cells. Regenerative medicine has successfully treated many tissues, but research has inadequately addressed the tongue as a vital organ in need of tissue engineering. In this review, we address the current standard for tongue reconstruction, the cellular mechanisms of muscle cell development, and the stem cell studies that have attempted muscle engineering within the tongue. Until now, no review has focused on engineering the tongue with regenerative medicine, which could guide innovative strategies for tongue reconstruction.

R-spondin substitutes for neuronal input for taste cell regeneration in adult mice

Taste bud cells regenerate throughout life. Taste bud maintenance depends on continuous replacement of senescent taste cells with new ones generated by adult taste stem cells. More than a century ago it was shown that taste buds degenerate after their innervating nerves are transected and that they are not restored until after reinnervation by distant gustatory ganglion neurons. Thus, neuronal input, likely via neuron-supplied factors, is required for generation of differentiated taste cells and taste bud maintenance. However, the identity of such a neuron-supplied niche factor(s) remains unclear. Here, by mining a published RNA-sequencing dataset of geniculate ganglion neurons and by in situ hybridization, we demonstrate that R-spondin-2, the ligand of Lgr5 and its homologs Lgr4/6 and stem-cell-expressed E3 ligases Rnf43/Znrf3, is expressed in nodose-petrosal and geniculate ganglion neurons. Using the glossopharyngeal nerve transection model, we show that systemic delivery of R-spondin via adenovirus can promote generation of differentiated taste cells despite denervation. Thus, exogenous R-spondin can substitute for neuronal input for taste bud cell replenishment and taste bud maintenance. Using taste organoid cultures, we show that R-spondin is required for generation of differentiated taste cells and that, in the absence of R-spondin in culture medium, taste bud cells are not generated ex vivo. Thus, we propose that R-spondin-2 may be the long-sought neuronal factor that acts on taste stem cells for maintaining taste tissue homeostasis.

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.

Taste bud formation depends on taste nerves

It has been known for more than a century that, in adult vertebrates, the maintenance of taste buds depends on their afferent nerves. However, the initial formation of taste buds is proposed to be nerve-independent in amphibians, and evidence to the contrary in mammals has been endlessly debated, mostly due to indirect and incomplete means to impede innervation during the protracted perinatal period of taste bud differentiation. Here, by genetically ablating, in mice, all somatic (i.e. touch) or visceral (i.e. taste) neurons for the oral cavity, we show that the latter but not the former are absolutely required for the proper formation of their target organs, the taste buds.

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.

Maintenance of Mouse Gustatory Terminal Field Organization Is Dependent on BDNF at Adulthood

The rodent peripheral gustatory system is especially plastic during early postnatal development and maintains significant anatomical plasticity into adulthood. Thus, taste information carried from the tongue to the brain is built and maintained on a background of anatomical circuits that have the capacity to change throughout the animal's lifespan. Recently, the neurotrophin brain-derived neurotrophic factor (BDNF) was shown to be required in the tongue to maintain normal levels of innervation in taste buds at adulthood, indicating that BDNF is a key molecule in the maintenance of nerve/target matching in taste buds. Here, we tested whether maintenance of the central process of these gustatory nerves at adulthood also relies on BDNF by using male and female transgenic mice with inducible CreERT2 under the control of the keratin 14 promoter or under control of the ubiquitin promoter to remove Bdnf from the tongue or from all tissues, respectively. We found that the terminal fields of gustatory nerves in the nucleus of the solitary tract were expanded when Bdnf was removed from the tongue at adulthood and with even larger and more widespread changes in mice where Bdnf was removed from all tissues. Removal of Bdnf did not affect numbers of ganglion cells that made up the nerves and did not affect peripheral, whole-nerve taste responses. We conclude that normal expression of Bdnf in gustatory structures is required to maintain normal levels of innervation at adulthood and that the central effects of Bdnf removal are opposite of those in the tongue.SIGNIFICANCE STATEMENT BDNF plays a major role in the development and maintenance of proper innervation of taste buds. However, the importance of BDNF in maintaining innervation patterns of gustatory nerves into central targets has not been assessed. Here, we tested whether Bdnf removal from the tongue or from all structures in adult mice impacts the maintenance of how taste nerves project to the first central relay. Deletion of Bdnf from the tongue and from all tissues led to a progressively greater expansion of terminal fields. This demonstrates, for the first time, that BDNF is necessary for the normal maintenance of central gustatory circuits at adulthood and further highlights a level of plasticity not seen in other sensory system subcortical circuits.

SOX2 regulation by hedgehog signaling controls adult lingual epithelium homeostasis

Adult tongue epithelium is continuously renewed from epithelial progenitor cells, a process that requires hedgehog (HH) signaling. In mice, pharmacological inhibition of the HH pathway causes taste bud loss within a few weeks. Previously, we demonstrated that sonic hedgehog (SHH) overexpression in lingual progenitors induces ectopic taste buds with locally increased SOX2 expression, suggesting that taste bud differentiation depends on SOX2 downstream of HH. To test this, we inhibited HH signaling in mice and observed a rapid decline in Sox2 and SOX2-GFP expression in taste epithelium. Upon conditional deletion of Sox2, differentiation of both taste and non-taste epithelial cells was blocked, and progenitor cell number increased. In contrast to basally restricted proliferation in controls, dividing cells were overabundant and spread to suprabasal epithelial layers in mutants. SOX2 loss in progenitors also led non-cell-autonomously to taste cell apoptosis, dramatically shortening taste cell lifespans. Finally, in tongues with conditional Sox2 deletion and SHH overexpression, ectopic and endogenous taste buds were not detectable; instead, progenitor hyperproliferation expanded throughout the lingual epithelium. In summary, we show that SOX2 functions downstream of HH signaling to regulate lingual epithelium homeostasis.

Neuronal delivery of Hedgehog directs spatial patterning of taste organ regeneration

How organs maintain and restore functional integrity during ordinary tissue turnover or following injury represents a central biological problem. The maintenance of taste sensory organs in the tongue was shown 140 years ago to depend on innervation from distant ganglion neurons, but the underlying mechanism has remained unknown. Here, we show that Sonic hedgehog (Shh), which encodes a secreted protein signal, is expressed in these sensory neurons, and that experimental ablation of neuronal Shh expression causes loss of taste receptor cells (TRCs). TRCs are also lost upon pharmacologic blockade of Hedgehog pathway response, accounting for the loss of taste sensation experienced by cancer patients undergoing Hedgehog inhibitor treatment. We find that TRC regeneration following such pharmacologic ablation requires neuronal expression of Shh and can be substantially enhanced by pharmacologic activation of Hedgehog response. Such pharmacologic enhancement of Hedgehog response, however, results in additional TRC formation at many ectopic sites, unlike the site-restricted regeneration specified by the projection pattern of Shh-expressing neurons. Stable regeneration of TRCs thus requires neuronal Shh, illustrating the principle that neuronal delivery of cues such as the Shh signal can pattern distant cellular responses to assure functional integrity during tissue maintenance and regeneration.

Sonic hedgehog from both nerves and epithelium is a key trophic factor for taste bud maintenance

The integrity of taste buds is intimately dependent on an intact gustatory innervation, yet the molecular nature of this dependency is unknown. Here, we show that differentiation of new taste bud cells, but not progenitor proliferation, is interrupted in mice treated with a hedgehog (Hh) pathway inhibitor (HPI), and that gustatory nerves are a source of sonic hedgehog (Shh) for taste bud renewal. Additionally, epithelial taste precursor cells express Shh transiently, and provide a local supply of Hh ligand that supports taste cell renewal. Taste buds are minimally affected when Shh is lost from either tissue source. However, when both the epithelial and neural supply of Shh are removed, taste buds largely disappear. We conclude Shh supplied by taste nerves and local taste epithelium act in concert to support continued taste bud differentiation. However, although neurally derived Shh is in part responsible for the dependence of taste cell renewal on gustatory innervation, neurotrophic support of taste buds likely involves a complex set of factors.

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.

Shrinkage of ipsilateral taste buds and hyperplasia of contralateral taste buds following chorda tympani nerve transection

The morphological changes that occur in the taste buds after denervation are not well understood in rats, especially in the contralateral tongue epithelium. In this study, we investigated the time course of morphological changes in the taste buds following unilateral nerve transection. The role of the trigeminal component of the lingual nerve in maintaining the structural integrity of the taste buds was also examined. Twenty-four Sprague-Dawley rats were randomly divided into three groups: control, unilateral chorda tympani nerve transection and unilateral chorda tympani nerve transection + lingual nerve transection. Rats were allowed up to 42 days of recovery before being euthanized. The taste buds were visualized using a cytokeratin 8 antibody. Taste bud counts, volumes and taste receptor cell numbers were quantified and compared among groups. No significant difference was detected between the chorda tympani nerve transection and chorda tympani nerve transection + lingual nerve transection groups. Taste bud counts, volumes and taste receptor cell numbers on the ipsilateral side all decreased significantly compared with control. On the contralateral side, the number of taste buds remained unchanged over time, but they were larger, and taste receptor cells were more numerous postoperatively. There was no evidence for a role of the trigeminal branch of the lingual nerve in maintaining the structural integrity of the anterior taste buds.

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.

Expanded terminal fields of gustatory nerves accompany embryonic BDNF overexpression in mouse oral epithelia

Brain-derived neurotrophic factor (BDNF) is expressed in gustatory epithelia and is required for gustatory neurons to locate and innervate their correct target during development. When BDNF is overexpressed throughout the lingual epithelium, beginning embryonically, chorda tympani fibers are misdirected and innervate inappropriate targets, leading to a loss of taste buds. The remaining taste buds are hyperinnervated, demonstrating a disruption of nerve/target matching in the tongue. We tested the hypothesis here that overexpression of BDNF peripherally leads to a disrupted terminal field organization of nerves that carry taste information to the brainstem. The chorda tympani, greater superficial petrosal, and glossopharyngeal nerves were labeled in adult wild-type (WT) mice and in adult mice in which BDNF was overexpressed (OE) to examine the volume and density of their central projections in the nucleus of the solitary tract. We found that the terminal fields of the chorda tympani and greater superficial petrosal nerves and overlapping fields that included these nerves in OE mice were at least 80% greater than the respective field volumes in WT mice. The shapes of terminal fields were similar between the two groups; however, the density and spread of labels were greater in OE mice. Unexpectedly, there were also group-related differences in chorda tympani nerve function, with OE mice showing a greater relative taste response to a concentration series of sucrose. Overall, our results show that disruption in peripheral innervation patterns of sensory neurons have significant effects on peripheral nerve function and central organization of their terminal fields.

Induction of ectopic taste buds by SHH reveals the competency and plasticity of adult lingual epithelium

Taste buds are assemblies of elongated epithelial cells, which are innervated by gustatory nerves that transmit taste information to the brain stem. Taste cells are continuously renewed throughout life via proliferation of epithelial progenitors, but the molecular regulation of this process remains unknown. During embryogenesis, sonic hedgehog (SHH) negatively regulates taste bud patterning, such that inhibition of SHH causes the formation of more and larger taste bud primordia, including in regions of the tongue normally devoid of taste buds. Here, using a Cre-lox system to drive constitutive expression of SHH, we identify the effects of SHH on the lingual epithelium of adult mice. We show that misexpression of SHH transforms lingual epithelial cell fate, such that daughter cells of lingual epithelial progenitors form cell type-replete, onion-shaped taste buds, rather than non-taste, pseudostratified epithelium. These SHH-induced ectopic taste buds are found in regions of the adult tongue previously thought incapable of generating taste organs. The ectopic buds are composed of all taste cell types, including support cells and detectors of sweet, bitter, umami, salt and sour, and recapitulate the molecular differentiation process of endogenous taste buds. In contrast to the well-established nerve dependence of endogenous taste buds, however, ectopic taste buds form independently of both gustatory and somatosensory innervation. As innervation is required for SHH expression by endogenous taste buds, our data suggest that SHH can replace the need for innervation to drive the entire program of taste bud differentiation.

Ingestion of bacterial lipopolysaccharide inhibits peripheral taste responses to sucrose in mice

A fundamental role of the taste system is to discriminate between nutritive and toxic foods. However, it is unknown whether bacterial pathogens that might contaminate food and water modulate the transmission of taste input to the brain. We hypothesized that exogenous, bacterially-derived lipopolysaccharide (LPS), modulates neural responses to taste stimuli. Neurophysiological responses from the chorda tympani nerve, which innervates taste cells on the anterior tongue, were unchanged by acute exposure to LPS. Instead, neural responses to sucrose were selectively inhibited in mice that drank LPS during a single overnight period. Decreased sucrose sensitivity appeared 7days after LPS ingestion, in parallel with decreased lingual expression of Tas1r2 and Tas1r3 transcripts, which are translated to T1R2+T1R3 subunits forming the sweet taste receptor. Tas1r2 and Tas1r3 mRNA expression levels and neural responses to sucrose were restored by 14 days after LPS consumption. Ingestion of LPS, rather than contact with taste receptor cells, appears to be necessary to suppress sucrose responses. Furthermore, mice lacking the Toll-like receptor (TLR) 4 for LPS were resistant to neurophysiological changes following LPS consumption. These findings demonstrate that ingestion of LPS during a single period specifically and transiently inhibits neural responses to sucrose. We suggest that LPS drinking initiates TLR4-dependent hormonal signals that downregulate sweet taste receptor genes in taste buds. Delayed inhibition of sweet taste signaling may influence food selection and the complex interplay between gastrointestinal bacteria and obesity.

Glial responses after chorda tympani nerve injury

The chorda tympani (CT) nerve innervates lingual taste buds and is susceptible to damage during dental and inner ear procedures. Interruption of the CT results in a disappearance of taste buds, which can be accompanied by taste disturbances. Because the CT usually regenerates to reinnervate taste buds successfully within a few weeks, a persistence of taste disturbances may indicate alterations in central nervous function. Peripheral injury to other sensory nerves leads to glial responses at central terminals, which actively contribute to abnormal sensations arising from nerve damage. Therefore, the current study examined microglial and astrocytic responses in the first central gustatory relay, the nucleus of the solitary tract (nTS), after transection of the CT. Damage to the CT resulted in significant microglial responses in terms of morphological reactivity and an increased density of microglial cells from 2 to 20 days after injury. This increased microglial population resulted primarily from microglial proliferation from 1.5 to 3 days, which was supplemented by microglial migration within subdivisions of the nTS between days 2 and 3. Unlike other nerve injuries, CT injury did not result in recruitment of bone marrow-derived precursors. Astrocytes also reacted in the nTS with increased levels of glial fibrillary acidic protein (GFAP) by 3 days, although none showed evidence of cell division. GFAP levels remained increased at 30 days, by which time microglial responses had resolved. These results show that nerve damage to the CT results in central glial responses, which may participate in long-lasting taste alterations following CT lesion.

Reactive microglia after taste nerve injury: comparison to nerve injury models of chronic pain

The chorda tympani (CT), which innervates taste buds on the anterior portion of the tongue, is susceptible to damage during inner ear surgeries. Injury to the CT causes a disappearance of taste buds, which is concurrent with significant microglial responses at central nerve terminals in the nucleus of the solitary tract (nTS). The resulting taste disturbances that can occur may persist for months or years, long after the nerve and taste buds have regenerated. These persistent changes in taste sensation suggest alterations in central functioning and may be related to the microglial responses. This is reminiscent of nerve injuries that result in chronic pain, where microglial reactivity is essential in maintaining the altered sensation (i.e., pain). In these models, methods that diminish microglial responses also diminish the corresponding pain behavior. Although the CT nerve does not contain nociceptive pain fibers, the microglial reactivity after CT damage is similar to that described in pain models. Therefore, methods that decrease microglial responses in pain models were used here to test if they could also affect microglial reactivity after CT injury. Treatment with minocycline, an antibiotic that dampens pain responsive microglia, was largely ineffective in diminishing microglial responses after CT injury. In addition, signaling through the toll-like 4 receptor (TLR4) does not seem to be required after CT injury as blocking or deleting TLR4 had no effect on microglial reactivity. These results suggest that microglial responses following CT injury rely on different signaling mechanisms than those described in nerve injuries resulting in chronic pain.

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.

Aging profoundly delays functional recovery from gustatory nerve injury

The peripheral taste system remains plastic during adulthood. Sectioning the chorda tympani (CT) nerve, which sends sensory information from the anterior tongue to the central nervous system, causes degeneration of distal fibers and target taste buds. However, taste function is restored after about 40 days in young adult rodents. We tested whether aging impacts the reappearance of neural responses after unilateral CT nerve injury. Taste bud regeneration was minimal at day 50-65 after denervation, and most aged animals died before functional recovery could be assessed. A subset (n=3/5) of old rats exhibited normal CT responses at day 85 postsectioning, suggesting the potential for efficient recovery. The aged taste system is fairly resilient to sensory receptor loss and major functional changes in normal aging. However, injury to the taste system reveals a surprising vulnerability in old rodents. The gustatory system provides an excellent model to study mechanisms underlying delayed recovery from peripheral nerve injury. Strategies to accelerate recovery and restore normal function will be of interest, as the elderly population continues to grow.

Light and Electron Microscopic Observation of Regenerated Fungiform Taste Buds in Patients with Recovered Taste Function after Severing Chorda Tympani Nerve

Objectives:

The aim of this study was to evaluate the mean number of regenerated fungiform taste buds per papilla and perform light and electron microscopic observation of taste buds in patients with recovered taste function after severing the chorda tympani nerve during middle ear surgery.

Methods:

We performed a biopsy on the fungiform papillae (FP) in the midlateral region of the dorsal surface of the tongue from 5 control volunteers (33 total FP) and from 7 and 5 patients with and without taste recovery (34 and 29 FP, respectively) 3 years 6 months to 18 years after surgery. The specimens were observed by light and transmission electron microscopy. The taste function was evaluated by electrogustometry.

Results:

The mean number of taste buds in the FP of patients with completely recovered taste function was significantly smaller (1.9 ± 1.4 per papilla; p < 0.01) than that of the control subjects (3.8 ± 2.2 per papilla). By transmission electron microscopy, 4 distinct types of cell (type I, II, III, and basal cells) were identified in the regenerated taste buds. Nerve fibers and nerve terminals were also found in the taste buds.

Conclusions:

It was clarified that taste buds containing taste cells and nerve endings do regenerate in the FP of patients with recovered taste function.

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.

Developmental expression of Bdnf, Ntf4/5, and TrkB in the mouse peripheral taste system

Brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT4), and their TrkB receptor regulate taste system development. To determine where and when gustatory neurons come in contact with these important factors, temporospatial expression patterns of Bdnf, Ntf4/5, and TrkB in the peripheral taste system were examined using RT-PCR. In the lingual epithelium, Ntf4/5 mRNA expression was higher than that of Bdnf at embryonic day 12.5 (E12.5), and the expression of both factors decreased afterwards. However, Ntf4/5 expression decreased at an earlier age than Bdnf. Bdnf and Ntf4/5 are expressed in equal amounts at E12.5 in geniculate ganglion, but Bdnf expression increased from E14.5 to birth, whereas Ntf4/5 expression decreased. These findings indicate that NT4 functions at early embryonic stages and is derived from different sources than Bdnf. TrkB expression in the geniculate ganglion is robust throughout development and is not a limiting factor for neurotrophin function in this system.

Effects of sensory or motor nerve deafferentation on oromotor function in mice

OBJECTIVE

To investigate the effect of sensory or motor nerve damage to the tongue using a mouse model.

STUDY DESIGN

Animal study.

SETTING

Research laboratory.

SUBJECTS AND METHODS

Adult male and female mice from inbred strains B6 (n = 19) and D2 (n = 25). Following lick training, bilateral lingual-chorda tympani nerve cuts (LX) (n = 6 B6, n = 7 D2), unilateral hypoglossal nerve cuts (HX) (n = 7 B6, n = 9 D2), or sham surgery (n = 6 B6, n = 9 D2) was performed. Mice were lick tested postsurgically with both water and sucrose (4 days total). Following testing, post mortem dissections and microscopic analysis of tongue papillae were performed.

RESULTS

In both strains, HX and LX mice demonstrated a significant reduction in volume per lick (VPL) in the surgical groups relative to shams. Neither motor nor sensory nerve transection affected local lick rate. In most LX mice in both strains, taste papillae were reduced compared with HX or sham mice.

CONCLUSION

Mice of either strain with either a sensory or a motor nerve injury have a significant loss of VPL during ingestion of either a neutral (water) or preferred (sucrose) stimulus. This reduction in VPL reflects a deficit in licking. Lick rate was not affected by deafferentation. A reduction in fungiform papillae following LX but not HX mice was noted.

Attenuation of peripheral salt taste responses and local immune function contralateral to gustatory nerve injury: effects of aldosterone

Dietary sodium restriction coupled with axotomy of the rat chorda tympani nerve (CTX) results in selectively attenuated taste responses to sodium salts in the contralateral, intact chorda tympani nerve. Converging evidence indicates that sodium deficiency also diminishes the activated macrophage response to injury on both the sectioned and contralateral, intact sides of the tongue. Because a sodium-restricted diet causes a robust increase in circulating aldosterone, we tested the hypothesis that changes in neurophysiological and immune responses contralateral to the CTX could be mimicked by aldosterone administration instead of the low-sodium diet. Taste responses in rats with CTX and supplemental aldosterone for 4-6 days were similar to rats with CTX and dietary sodium restriction. Responses to sodium salts were as much as 50% lower compared with sham-operated and vehicle-supplemented rats. The group-related functional differences were eliminated with lingual application of amiloride, suggesting that a major transduction pathway affected was through epithelial sodium channels. Consistent with the functional results, few macrophages were observed on either side of the tongue in rats with CTX and aldosterone. In contrast, macrophages were elevated on both sides of the tongue in rats with CTX and the vehicle. These results show that sodium deficiency or administration of aldosterone suppresses the immune response to neural injury, resulting in attenuation of peripheral gustatory function. They also show a potential key link among downstream consequences of sodium imbalance, taste function, and immune activity.

Follistatin modulates a BMP autoregulatory loop to control the size and patterning of sensory domains in the developing tongue

The regenerative capacity of many placode-derived epithelial structures makes them of interest for understanding the molecular control of epithelial stem cells and their niches. Here, we investigate the interaction between the developing epithelium and its surrounding mesenchyme in one such system, the taste papillae and sensory taste buds of the mouse tongue. We identify follistatin (FST) as a mesenchymal factor that controls size, patterning and gustatory cell differentiation in developing taste papillae. FST limits expansion and differentiation of Sox2-expressing taste progenitor cells and negatively regulates the development of taste papillae in the lingual epithelium: in Fst(-/-) tongue, there is both ectopic development of Sox2-expressing taste progenitors and accelerated differentiation of gustatory cells. Loss of Fst leads to elevated activity and increased expression of epithelial Bmp7; the latter effect is consistent with BMP7 positive autoregulation, a phenomenon we demonstrate directly. We show that FST and BMP7 influence the activity and expression of other signaling systems that play important roles in the development of taste papillae and taste buds. In addition, using computational modeling, we show how aberrations in taste papillae patterning in Fst(-/-) mice could result from disruption of an FST-BMP7 regulatory circuit that normally suppresses noise in a process based on diffusion-driven instability. Because inactivation of Bmp7 rescues many of the defects observed in Fst(-/-) tongue, we conclude that interactions between mesenchyme-derived FST and epithelial BMP7 play a central role in the morphogenesis, innervation and maintenance of taste buds and their stem/progenitor cells.

Culture of endodermal stem/progenitor cells of the mouse tongue

The tongue represents a very accessible source of tissue-specific epithelial stem cells of endodermal origin. However, little is known about the properties of these cells and the mechanisms regulating their proliferation and differentiation. Foxa2, an endodermal marker, is expressed throughout the tongue epithelium during embryonic development but becomes confined to a minority of basal cells and some taste bud sensory cells in the adult tongue. Using a previously described line of transgenic mice in which enhanced green fluorescent protein (eGFP) is expressed under the control of a human keratin 5 promoter region (Krt5-eGFP), we have isolated a subpopulation of cells in the basal epithelial layer of the mouse tongue with a high efficiency of generating holoclones of undifferentiated cells in culture with a feeder layer. Krt5-GFP(hi) cells can both self renew and give rise to differentiated stratified keratinized epithelial cells when cultured on an air-liquid interface.

Experimentally cross-wired lingual taste nerves can restore normal unconditioned gaping behavior in response to quinine stimulation

Studies examining the effects of transection and regeneration of the glossopharyngeal (GL) and chorda tympani (CT) nerves on various taste-elicited behaviors in rats have demonstrated that the GL (but not the CT) nerve is essential for the maintenance of both an unconditioned protective reflex (gaping) and the neural activity observed in central gustatory structures in response to lingual application of a bitter substance. An unresolved issue, however, is whether recovery depends more on the taste nerve and the central circuits that it supplies and/or on the tongue receptor cell field being innervated. To address this question, we experimentally cross-wired these taste nerves, which, remarkably, can regenerate into parts of the tongue they normally do not innervate. We report that quinine-stimulated gaping behavior was fully restored, and neuronal activity, as assessed by Fos immunohistochemistry in the nucleus of the solitary tract and the parabrachial nucleus, was partially restored only if the posterior tongue (PT) taste receptor cell field was reinnervated; the particular taste nerve supplying the input was inconsequential to the recovery of function. Thus, PT taste receptor cells appear to play a privileged role in triggering unconditioned gaping to bitter tasting stimuli, regardless of which lingual gustatory nerve innervates them. Our findings demonstrate that even when a lingual gustatory nerve (the CT) forms connections with taste cells in a non-native receptor field (the PT), unconditioned taste rejection reflexes to quinine can be maintained. These findings underscore the extraordinary ability of the gustatory system to adapt to peripherally reorganized input for particular behaviors.

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.

Neuronal-epithelial interactions in mammalian gustatory epithelium

There are two neurally controlled morphogenetic programmes in the fungiform papillae of the rodent tongue. When gustatory axons are present they not only promote the expression of taste receptor cells but also simultaneously suppress the outgrowth of an ectopic filiform spine. Innervation during development is essential for the initial formation of taste buds. Consequently, denervation during development irreversibly prevents the establishment of a permanent line of stem cells for taste buds, in contrast to the reliable re-emergence of taste buds after the reinnervation of denervated adult gustatory epithelium. Monoclonal antibodies to keratins selectively recognize cells in particular differentiated states and may contribute to an understanding of the cell lineages and cell-cell interactions that lead to taste buds and filiform spines. Regeneration may recapitulate the late steps in taste development but not the early steps, which are slower and subject to irreversible alteration with denervation. Arguments are set forth that the nerve dependence of taste buds and some other cutaneous receptor cells provides an important selective advantage. Nerve dependence ensures that a more protective integumental covering will replace any superficial, secondary sensory cells rendered useless by denervation.

Upregulation of intracellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1 after unilateral nerve injury in the peripheral taste system

In the peripheral taste system, activated macrophages are recruited to both sides of the tongue after unilateral sectioning of the chorda tympani nerve (CT). Neural degeneration elicits macrophage entry in other systems by upregulating vascular adhesion molecules. We hypothesized that CT sectioning leads to a bilateral increase in intracellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1 expression on lingual vessels. To test this hypothesis, rats were euthanized at time points from 6 hr to 7 days post-sectioning. Frozen sections of tongue were processed for immunohistochemical staining for ICAM-1 and VCAM-1. Tongue homogenates from additional rats were analyzed with ELISA. ICAM-1 expression increases first on the denervated side of the tongue at 24 hr post-section and then on the uninjured side at 48 hr post-section. ICAM-1 remains elevated through Day 7 post-sectioning on both sides of the tongue. Dietary sodium restriction, which prevents the macrophage response to nerve sectioning, had no effect on ICAM-1 levels. VCAM-1+ vessels are increased on the denervated side of the tongue at 24-48 hr post-section in control-fed rats. However, dietary sodium restriction prevents the increase. These results indicate that vascular adhesion molecules are differentially regulated by CT sectioning. We suggest that macrophage entry, migration, and modulation of taste function are downstream of dynamic expression of adhesion molecules.

Upregulation of the chemokine monocyte chemoattractant protein-1 following unilateral nerve injury in the peripheral taste system

Macrophages are recruited to both sides of the tongue following unilateral chorda tympani (CT) nerve injury. The mechanisms responsible for recruiting these macrophages to the peripheral taste system are unknown. Neural degeneration in other systems leads to the upregulation of small molecules that function as chemoattractant cytokines, or chemokines. The chemokines monocyte chemoattractant protein (MCP)-1 and macrophage inflammatory protein (MIP)-1alpha are important regulators of macrophage recruitment to sites of infection and injury. We hypothesized that CT nerve sectioning leads to a bilateral upregulation of MCP-1 and MIP-1alpha. We examined lingual protein levels of MCP-1 and MIP-1alpha by enzyme-linked immunosorbent assays (ELISA)s at several time points after unilateral CT section in rats. MCP-1 was significantly upregulated on the intact side of the tongue at 12 h after sectioning, and on the injured side at 24-48 h post-injury. However, MIP-1alpha expression did not significantly change following CT nerve sectioning. These data indicate that chemokines are differentially regulated following neural injury, and that MCP-1 may contribute to the bilateral macrophage response to neural injury. Furthermore, the increase in MCP-1 occurs even in uninjured, distant sites, and may be upstream from the deficits in neural responses from the contralateral CT after sectioning.

Membrane excitability of wing and rod cells in frog taste discs following denervation

The frog tongue has a disc-shaped taste organ (taste disc) on the top of fungiform papillae. The taste disc contains two types of cells, wing cells with a sheet-like apical process and rod cells with a rod-like apical process. Both wing and rod cells can produce action potentials. Unlike the taste buds of mammals, frog taste discs do not degenerate over a long period after denervation. Here we report that the shapes of wing and rod cells isolated from taste discs in the bullfrog (Rana catesbeiana) remained unchanged 1 month after cutting bilateral glossopharyngeal nerves. By applying the whole cell patch-clamp technique to isolated wing and rod cells, we found voltage-dependent inward currents and outward currents and action potentials in denervated frogs as seen in normal frogs. These results suggest that the maintenance of morphological integrity and electrical excitability of taste cells does not require a nerve supply in frogs.

Taste function after section of chorda tympani nerve in middle ear surgery

OBJECTIVE

To investigate prospectively the clinical manifestations and the functional recovery of taste after section of chorda tympani nerve (CTN) during middle ear surgery, the subjective and objective study on the patients was performed.

METHODS

Thirty-five patients underwent surgery with unilateral (n=32) or bilateral (n=3) section of CTN between January 2000 and April 2002. The patients were asked about taste symptoms before surgery and 2 weeks and 2 years after surgery. The CTN function was also measured with electrogustometry (EGM) at the same time points.

RESULTS

In unilateral section of CTN, 19/32 (59.4%) complained of taste disorder and 11/32 (34.4%) tongue numbness. Most of these taste symptoms disappeared within 2 years although the EGM threshold did not recover. 16/17 patients (94.1%) who used to cook everyday had little difficulty in flavoring dishes. In bilateral section of CTN, the patients had no problem of taste at 2 years after surgery, either.

CONCLUSION

These findings help explain the potential complications to the patients before surgery, although the number of cases was small in this series.

Lipopolysaccharide-induced up-regulation of activated macrophages in the degenerating taste system

Unilateral chorda tympani (CT) nerve section and maintenance on a sodium-restricted diet leads to a rapid decrease in neurophysiological taste responses to sodium in the contralateral, intact CT nerve. Up-regulation of immune function with lipopolysaccharide (LPS; 100 microg i.p.) induces a recovery of normal sodium taste responses, suggesting that the sodium-deficient diet is immunosuppressive. In fact, there is a bilateral increase in the number of lingual, activated macrophages in control-fed rats receiving CT nerve section that does not occur in sodium-deficient rats after sectioning. In the current study, we hypothesized that the LPS-induced recovery of normal taste function in sodium-deficient rats is based on an increase in the activated macrophage response to denervation. Rats receiving a unilateral CT nerve section, a sodium-restricted diet, and/or an injection of LPS (100 microg; i.p.) were overdosed with pentobarbital at day 2 postsectioning, and tongues were rapidly dissected and frozen. Cryosections were then immunohistochemically stained to determine the percentage of ED1 staining for activated macrophages or the number of alphabeta or gammadelta T cells. Activated macrophage levels were significantly increased in sodium-restricted rats that received LPS following unilateral CT nerve section, supporting our hypothesis. These novel findings suggest that LPS overcomes the immunosuppression induced by the sodium-restricted diet and also indicate that the immune system plays a role in regulating taste function after neural injury.

Neural plasticity in the gustatory system

Sensory systems adapt to changing environmental influences by coordinated alterations in structure and function. These alterations are referred to as plastic changes. The gustatory system displays numerous plastic changes even in receptor cells. This review focuses on the plasticity of gustatory structures through the first synaptic relay in the brain. Unlike other sensory systems, there is a remarkable amount of environmentally induced changes in these peripheral-most neural structures. The most consistent and largest changes occur to stimuli that also impact on homeostatic systems, especially when the environmental manipulation is instituted during early development.

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.

Taste bud cell dynamics during normal and sodium-restricted development

Taste bud volume increases over the postnatal period to match the number of neurons providing innervation. To clarify age-related changes in fungiform taste bud volume, the current study investigated developmental changes in taste bud cell number, proliferation rate, and life span. Taste bud growth can largely be accounted for by addition of cytokeratin-19-positive taste bud cells. Examination of taste bud cell kinetics with 3H-thymidine autoradiography revealed that cell life span and turnover periods were not altered during normal development but that cells were produced more rapidly in young rats, a prominent modification that could lead to increased taste bud size. By comparison, dietary sodium restriction instituted during pre- and postnatal development results in small taste buds at adulthood as a result of fewer cytokeratin-19-positive cells. The dietary manipulation also had profound influences on taste bud growth kinetics, including an increased latency for cells to enter the taste bud and longer life span and turnover periods. These studies provide fundamental, new information about taste bud development under normal conditions and after environmental manipulations that impact nerve/target matching.

Contribution of free nerve endings in the laryngeal epithelium to CO2 reception in rats

The superior laryngeal nerve (SLN) contains CO2-sensitive fibers. In the laryngeal epithelium, two candidates for CO2 reception have been identified, namely the intraepithelial free nerve endings and the taste buds. To elucidate the contribution of free nerve endings to CO2 reception, electrophysiological activities were recorded during various stages of regeneration of nerve endings following SLN-crush in rats. The left SLN was crushed surgically and maintained from 4 to 40 days for regeneration of nerve endings. Laryngeal sections were processed for immunohistochemical staining of protein gene product 9.5 to observe regeneration of free nerve endings and taste buds in the epithelium. By day 4 after SLN-crush, both the free nerve endings and taste buds had disappeared. Regeneration of the free nerve endings was recognized from day 8, while that of the taste buds started at day 16. On day 40, the number of taste buds on SLN-crush side was similar to that on the untreated side. Electrophysiological recording of SLN throughout the regeneration period (excluding day 4), showed response to intralaryngeal 9% CO2 (stimulation or inhibition) whether or not taste buds were present. Our results showed intralaryngeal CO2 reception without taste bud involvement, indicating that the free nerve endings in the laryngeal epithelium are receptive to intralaryngeal CO2.

Up-regulation of activated macrophages in response to degeneration in the taste system: Effects of dietary sodium restriction

Dietary sodium restriction combined with unilateral chorda tympani nerve section leads to a rapid and specific decrease in neurophysiological taste responses to sodium in the contralateral, intact chorda tympani (Hill and Phillips [1994] J. Neurosci. 14:2904-2910). Previous work demonstrated that dietary sodium restriction may induce these early functional deficits by inhibiting immune activity after denervation (Phillips and Hill [1996] Am. J. Physiol. 271:R857-R862). However, little is known about the leukocyte response to denervation of taste buds in fungiform papillae. In the current study, it was hypothesized that T cells and macrophages are increased in the tongue after unilateral denervation in control-fed but not sodium-restricted animals. Adult, specified pathogen-free rats received unilateral chorda tympani nerve section or sham section followed by dietary sodium restriction or maintenance on control diet. At day 1, 2, 5, 7, or 50 postsectioning, immunostaining was used to detect the percentage of staining for activated macrophages, the number of alpha beta T cells, and the number of delta gamma epithelial T cells in the tongue. The number of lingual T cells did not significantly differ between treatment groups following denervation. However, there was a dramatic bilateral increase in ED1(+) staining for activated macrophages in control-fed rats that peaked at day 2 postsectioning. In contrast, sodium-restricted rats did not show an increase in activated macrophages above baseline at any time postsectioning. Further analysis of extralingual macrophages indicated that the deficit in immune activity in sodium-restricted rats is localized to the tongue and is not widespread. A model for immune modulation of taste receptor cell function is proposed based on these novel findings.

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.

Injury-induced functional plasticity in the peripheral gustatory system

Combining unilateral denervation of anterior tongue taste buds with a low-sodium diet in rats results in a rapid, dramatic, and selective attenuation of neurophysiological sodium taste responses from the intact side of the tongue. The transduction pathway responsible for the attenuated response is through the epithelial sodium channel (Hill and Phillips, 1994). Current experiments extend these findings by detailing the effects of experimentally induced injury on taste responses from anterior tongue taste receptors in sodium-restricted rats. Experiments focused on functional salt taste responses from the intact chorda tympani nerve in sodium-restricted rats in which a gustatory nerve was sectioned that innervates the anterior tongue (chorda tympani), the posterior tongue (glossopharyngeal), or palatal taste receptors (greater superficial petrosal) or in which a nongustatory nerve was sectioned that also has its target in the anterior tongue (trigeminal). An additional group was studied that received thermal injury to the anteroventral tongue. Substantial and selective suppression of sodium salt responses occurred in a graded manner generally related to the distance from the target field of the injury to anterior tongue taste buds. The order of effectiveness was: chorda tympani section > trigeminal section > thermal injury = glossopharyngeal section > greater superficial petrosal section. These results support the hypothesis that local, diffusible factors liberated from immune-derived cells as a result of neural and/or epithelial damage are involved in regulating the transduction pathway responsible for sodium salt sensation, and that these factors may become evident through dietary sodium restriction.