Organization of geniculate and trigeminal ganglion cells innervating single fungiform taste papillae: a study with tetramethylrhodamine dextran amine labeling

The Fungiform Papilla Is a Complex, Multimodal, Oral Sensory Organ

When solid or liquid stimuli contact the tongue tip during eating, the sensations of taste, touch and temperature are immediately evoked, and tongue function relies on these simultaneous multimodal responses. We focus on the fungiform papilla of the anterior tongue as a complex organ for taste, tactile and thermal modalities, all via chorda tympani nerve innervation from the geniculate ganglion. Rather than a review, our aim is to revise the classic archetype of the fungiform as predominantly a taste bud residence only and instead emphasize an amended concept of the papilla as a multimodal organ. Neurophysiological maps of fungiform papillae in functional receptive fields demonstrate responses to chemical, stroking and cold lingual stimuli. Roles are predicted for elaborate extragemmal nerve endings in tactile and temperature sensations, and potential functions for keratinocytes in noncanonical sensory signaling. The fungiform papilla is presented as a polymodal lingual organ, not solely a gustatory papilla.

Neurotoxicity and Chemoreception: A Systematic Review of Neurotoxicity Effects on Smell and Taste

Several different types of exposure have the potential to produce olfactory and gustatory deficits related to neurotoxicity. Although the literature contains relatively few studies of such chemoreceptive dysfunction in the context of toxic exposure, this review explores the strength of such published associations. Several studies collectively demonstrated moderately strong evidence for an association between manganese dust exposure and olfactory deficits. Evidence of associations between individual chemicals, therapeutics, and composites, such as World Trade Center debris, and olfactory and gustatory deficits remains limited or mixed. Further need for controlled studies for clinical management, exposure limits, and policy development is identified.

Variations in the amplitude and duration of hyolaryngeal elevation during swallow: Effect of sour and carbonated liquid bolus

BACKGROUND

Altering sensory properties of food is relevant to the management of swallowing disorders. The current investigation aimed to examine the influence of sour and carbonated stimuli on measures of hyolaryngeal elevation.

METHOD

Sixty healthy adults in the age range of 18 to 35 years were assessed using Digital Accelerometry Swallowing Imaging (DASI) while swallowing 5 ml of neutral, sour and carbonated liquid bolus.

RESULTS

Sour stimulus was significantly different with higher amplitude and longer durational measures of hyolaryngeal elevation compared to the two other stimuli.

CONCLUSION

Sour stimulus facilitates better hyolaryngeal elevation, which implicates its usage in dysphagia management.

Trigeminal trophic syndrome: an updated review

Trigeminal trophic syndrome (TTS) is a rare disease process that is thought to occur after insult to the trigeminal nerve. The earliest descriptions of this condition were provided in the early 20th century, yet it remains relatively unknown, with approximately 200 cases since described. Most commonly seen in older women, TTS characteristically involves persistent facial ulceration with loss of sensation and paresthesia along the distribution of the trigeminal dermatome. Ulceration often occurs in the alar region, following self-manipulation in response to paresthesias. Time of onset of TTS after trigeminal insult may vary from weeks to decades, and emergence of ulceration may be associated with psychiatric disorders. Diagnosis is clinical and made by exclusion of similarly presenting conditions. Histology is nonspecific yet necessary to exclude other causes of facial ulceration. Although there is not yet a standard management strategy, a number of successful approaches have been reported including pharmaceutical and surgical interventions, installation of a protector, and transcutaneous nerve stimulation. However, because of the self-inflicted manifestations of this disorder, behavioral modifications remain of the utmost importance. This review serves to address the history, epidemiology, pathogenesis, clinical presentation, histology, diagnosis, differential diagnosis, and management options for TTS.

Geniculate Ganglion Neurons are Multimodal and Variable in Receptive Field Characteristics

Afferent chorda tympani (CT) fibers innervating anterior tongue fungiform papillae have neuron cell bodies in the geniculate ganglion (GG). To characterize electrophysiological and receptive field properties, we recorded extracellular responses from single GG neurons to lingual application with chemical, thermal and mechanical stimuli. Receptive field size was mapped by electrical stimulation of individual fungiform papillae. Responses of GG neurons to room temperature chemical stimuli representing five taste qualities, and distilled water at 4 °C and mechanical stimulation were used. Based on responses to these stimuli, GG neurons were divided into CHEMICAL, CHEMICAL/THERMAL, THERMAL and TACTILE groups. Neurons in the CHEMICAL group responded to taste stimuli but not to either cold water or stroking stimuli. CHEMICAL/THERMAL neurons responded to both taste stimuli and cold water. THERMAL neurons responded only to cold water and TACTILE neurons responded only to light stroking stimuli. The receptive field sizes for CHEMICAL, and CHEMICAL/THERMAL neurons averaged five papillae exceeding the field size of THERMAL and TACTILE neurons which averaged about two papillae. Detailed analysis of the receptive field of CHEMICAL/THERMAL neurons revealed that within one field only a subset of the fungiform papillae making up the receptive field responded to the cold stimuli, whereas the other papillae responded only to chemical stimuli. These finding demonstrate that fungiform papilla are complex sensory organs with a multisensory function suggesting a unique role in detecting and sampling food components prior to ingestion.

The transcription factor Phox2b distinguishes between oral and non-oral sensory neurons in the geniculate ganglion

Many basic characteristics of gustatory neurons remain unknown, partly due to the absence of specific markers. Some neurons in the geniculate ganglion project to taste regions in the oral cavity, whereas others innervate the outer ear. We hypothesized that the transcription factor Phox2b would identify oral cavity-projecting neurons in the geniculate ganglion. To test this possibility, we characterized mice in which Phox2b-Cre mediated gene recombination labeled neurons with tdTomato. Nerve labeling revealed that all taste neurons projecting through the chorda tympani (27%) and greater superficial petrosal nerves (15%) expressed Phox2b during development, whereas non-oral somatosensory neurons (58%) in the geniculate ganglion did not. We found tdTomato-positive innervation within all taste buds. Most (57%) of the fungiform papillae had labeled innervation only in taste buds, whereas 43% of the fungiform papillae also had additional labeled innervation to the papilla epithelium. Chorda tympani nerve transection eliminated all labeled innervation to taste buds, but most of the additional innervation in the fungiform papillae remained. Some of these additional fibers also expressed tyrosine hydroxylase, suggesting a sympathetic origin. Consistent with this, both sympathetic and parasympathetic fibers innervating blood vessels and salivary glands contained tdTomato labeling. Phox2b-tdTomato labels nerve fascicles in the tongue of the developing embryo and demonstrates a similar stereotyped branching pattern DiI-labeling.

Sour taste increases swallowing and prolongs hemodynamic responses in the cortical swallowing network

Sour stimuli have been shown to upregulate swallowing in patients and in healthy volunteers. However, such changes may be dependent on taste-induced increases in salivary flow. Other mechanisms include genetic taster status (Bartoshuk LM, Duffy VB, Green BG, Hoffman HJ, Ko CW, Lucchina LA, Weiffenbach JM. Physiol Behav 82: 109-114, 2004) and differences between sour and other tastes. We investigated the effects of taste on swallowing frequency and cortical activation in the swallowing network and whether taster status affected responses. Three-milliliter boluses of sour, sour with slow infusion, sweet, water, and water with infusion were compared on swallowing frequency and hemodynamic responses. The sour conditions increased swallowing frequency, whereas sweet and water did not. Changes in cortical oxygenated hemoglobin (hemodynamic responses) measured by functional near-infrared spectroscopy were averaged over 30 trials for each condition per participant in the right and left motor cortex, S1 and supplementary motor area for 30 s following bolus onset. Motion artifact in the hemodynamic response occurred 0-2 s after bolus onset, when the majority of swallows occurred. The peak hemodynamic response 2-7 s after bolus onset did not differ by taste, hemisphere, or cortical location. The mean hemodynamic response 17-22 s after bolus onset was highest in the motor regions of both hemispheres, and greater in the sour and infusion condition than in the water condition. Genetic taster status did not alter changes in swallowing frequency or hemodynamic response. As sour taste significantly increased swallowing and cortical activation equally with and without slow infusion, increases in the cortical swallowing were due to sour taste.

Receptive field size, chemical and thermal responses, and fiber conduction velocity of rat chorda tympani geniculate ganglion neurons

Afferent chorda tympani (CT) fibers innervating taste and somatosensory receptors in fungiform papillae have neuron cell bodies in the geniculate ganglion (GG). The GG/CT fibers branch in the tongue to innervate taste buds in several fungiform papillae. To investigate receptive field characteristics of GG/CT neurons, we recorded extracellular responses from GG cells to application of chemical and thermal stimuli. Receptive field size was mapped by electrical stimulation of individual fungiform papillae. Response latency to electrical stimulation was used to determine fiber conduction velocity. Responses of GG neurons to lingual application of stimuli representing four taste qualities, and water at 4°C, were used to classify neuron response properties. Neurons classified as SALT, responding only to NaCl and NH4Cl, had a mean receptive field size of six papillae. Neurons classified as OTHER responded to salts and other chemical stimuli and had smaller mean receptive fields of four papillae. Neurons that responded to salts and cold stimuli, classified as SALT/THERMAL, and neurons responding to salts, other chemical stimuli and cold, classified as OTHER/THERMAL, had mean receptive field sizes of six and five papillae, respectively. Neurons responding only to cold stimuli, categorized as THERMAL, had receptive fields of one to two papillae located at the tongue tip. Based on conduction velocity most of the neurons were classified as C fibers. Neurons with large receptive fields had higher conduction velocities than neurons with small receptive fields. These results demonstrate that GG neurons can be distinguished by receptive field size, response properties and afferent fiber conduction velocity derived from convergent input of multiple taste organs.

TRPs in taste and chemesthesis

TRP channels are expressed in taste buds, nerve fibers, and keratinocytes in the oronasal cavity. These channels play integral roles in transducing chemical stimuli, giving rise to sensations of taste, irritation, warmth, coolness, and pungency. Specifically, TRPM5 acts downstream of taste receptors in the taste transduction pathway. TRPM5 channels convert taste-evoked intracellular Ca(2+) release into membrane depolarization to trigger taste transmitter secretion. PKD2L1 is expressed in acid-sensitive (sour) taste bud cells but is unlikely to be the transducer for sour taste. TRPV1 is a receptor for pungent chemical stimuli such as capsaicin and for several irritants (chemesthesis). It is controversial whether TRPV1 is present in the taste buds and plays a direct role in taste. Instead, TRPV1 is expressed in non-gustatory sensory afferent fibers and in keratinocytes of the oronasal cavity. In many sensory fibers and epithelial cells lining the oronasal cavity, TRPA1 is also co-expressed with TRPV1. As with TRPV1, TRPA1 transduces a wide variety of irritants and, in combination with TRPV1, assures that there is a broad response to noxious chemical stimuli. Other TRP channels, including TRPM8, TRPV3, and TRPV4, play less prominent roles in chemesthesis and no known role in taste, per se. The pungency of foods and beverages is likely highly influenced by the temperature at which they are consumed, their acidity, and, for beverages, their carbonation. All these factors modulate the activity of TRP channels in taste buds and in the oronasal mucosa.

Modulation of central gustatory coding by temperature

Changes in oral temperature can influence taste perception, indicating overlap among mechanisms for taste and oral somesthesis. Medullary gustatory neurons can show cosensitivity to temperature, albeit how these cells process combined taste and thermal input is poorly understood. Here, we electrophysiologically recorded orosensory responses (spikes) from 39 taste-sensitive neurons in the nucleus tractus solitarii of anesthetized mice during oral delivery of tastants adjusted to innocuous cool (16 and 18°C), room (22°C, baseline), and warm (30 and 37°C) oral temperatures. Stimuli included (in mM) 100 sucrose, 30 NaCl, 3 HCl, 3 quinine, an umami mixture, and water. Although cooled water excited few cells, water warmed to 30 and 37°C significantly excited 33% and 64% of neurons, respectively. Warmth induced responses of comparable magnitude to room temperature tastants. Furthermore, warming taste solutions influenced the distribution of gustatory responses among neurons and increased (P < 0.05) neuronal breadth of tuning across taste qualities. The influence of warmth on response magnitude was stimulus specific. Across neurons, warming facilitated responses to sucrose and umami in a superadditive manner, as these responses exceeded (P < 0.05) the arithmetic sum of activity to warming alone and the taste stimulus tested at room temperature. Superadditive increases (P < 0.05) in responding were also noted in some cells for warmed HCl. Yet warming induced only simple additive or subtractive effects on responses to quinine and NaCl. Data show temperature is a parameter of gustatory processing, like taste quality and concentration, in medullary circuits for taste.

Postnatal development of chorda tympani axons in the rat nucleus of the solitary tract

The chorda tympani nerve (CT), one of three nerves that convey gustatory information to the nucleus of the solitary tract (NTS), displays terminal field reorganization after postnatal day 15 in the rat. Aiming to gain insight into mechanisms of this phenomenon, CT axon projection field and terminal morphology in NTS subdivisions were examined using tract tracing, light microscopy, and immunoelectron microscopy at four postnatal ages: P15, P25, P35, and adult. The CT axons that innervated NTS rostrolateral subdivision both in the adult and in P15 rats were morphologically distinct from those that innervated the rostrocentral, gustatory subdivision. In both subdivisions, CT terminals reached morphological maturity before P15. Rostrolateral, but not rostrocentral axons, went through substantial axonal branch elimination after P15. Rostrocentral CT synapses, however, redistribute onto postsynaptic targets in the following weeks. CT terminal preference for GABAergic postsynaptic targets was drastically reduced after P15. Furthermore, CT synapses became a smaller component of the total synaptic input to the rostrocentral NTS after P35. The results underlined that CT axons in rostrocentral and rostrolateral subdivisions represent two distinct populations of CT input, displaying different morphological properties and structural reorganization mechanisms during postnatal development.

The cell biology of taste

Taste buds are aggregates of 50–100 polarized neuroepithelial cells that detect nutrients and other compounds. Combined analyses of gene expression and cellular function reveal an elegant cellular organization within the taste bud. This review discusses the functional classes of taste cells, their cell biology, and current thinking on how taste information is transmitted to the brain.

Responses of the hamster chorda tympani nerve to sucrose+acid and sucrose+citrate taste mixtures

Studies of taste receptor cells, chorda tympani (CT) neurons, and brainstem neurons show stimulus interactions in the form of inhibition or enhancement of the effectiveness of sucrose when mixed with acids or citrate salts, respectively. To investigate further the effects of acids and the trivalent citrate anion on sucrose responses in hamsters (Mesocricetus auratus), we recorded multifiber CT responses to 100 mM sucrose; a concentration series of HCl, citric acid, acetic acid, sodium citrate (with and without amiloride added), potassium citrate, and all binary combinations of acids and salts with 100 mM sucrose. Compared with response additivity, sucrose responses were increasingly suppressed in acid + sucrose mixtures with increases in titratable acidity, but HCl and citric acid were more effective suppressors than acetic acid. Citrate salts suppressed sucrose responses and baseline CT neural activity to a similar degree. Citrate salts also elicited prolonged, concentration-dependent, water-rinse responses. The specific loss in sucrose effectiveness as a CT stimulus with increasing titratable acidity was confirmed; however, no increase in sucrose effectiveness was found with the addition of citrate. Further study is needed to define the chemical basis for effects of acids and salts in taste mixtures.

Transsynaptic transport of wheat germ agglutinin expressed in a subset of type II taste cells of transgenic mice

Background

Anatomical tracing of neural circuits originating from specific subsets of taste receptor cells may shed light on interactions between taste cells within the taste bud and taste cell-to nerve interactions. It is unclear for example, if activation of type II cells leads to direct activation of the gustatory nerves, or whether the information is relayed through type III cells. To determine how WGA produced in T1r3-expressing taste cells is transported into gustatory neurons, transgenic mice expressing WGA-IRES-GFP driven by the T1r3 promoter were generated.

Results

Immunohistochemistry showed co-expression of WGA, GFP and endogenous T1r3 in the taste bud cells of transgenic mice: the only taste cells immunoreactive for WGA were the T1r3-expressing cells. The WGA antibody also stained intragemmal nerves. WGA, but not GFP immunoreactivity was found in the geniculate and petrosal ganglia of transgenic mice, indicating that WGA was transported across synapses. WGA immunoreactivity was also found in the trigeminal ganglion, suggesting that T1r3-expressing cells make synapses with trigeminal neurons. In the medulla, WGA was detected in the nucleus of the solitary tract but also in the nucleus ambiguus, the vestibular nucleus, the trigeminal nucleus and in the gigantocellular reticular nucleus. WGA was not detected in the parabrachial nucleus, or the gustatory cortex.

Conclusion

These results show the usefulness of genetically encoded WGA as a tracer for the first and second order neurons that innervate a subset of taste cells, but not for higher order neurons, and demonstrate that the main route of output from type II taste cells is the gustatory neuron, not the type III cells.

Overexpression of artemin in the tongue increases expression of TRPV1 and TRPA1 in trigeminal afferents and causes oral sensitivity to capsaicin and mustard oil

Artemin, a member of the glial cell line-derived neurotrophic factor (GDNF) family, supports a subpopulation of trigeminal sensory neurons through activation of the Ret/GFRalpha3 receptor tyrosine kinase complex. In a previous study we showed that artemin is increased in inflamed skin of wildtype mice and that transgenic overexpression of artemin in skin increases TRPV1 and TRPA1 expression in dorsal root ganglia neurons. In this study we examined how transgenic overexpression of artemin in tongue epithelium affects the anatomy, gene expression and calcium handling properties of trigeminal sensory afferents. At the RNA level, trigeminal ganglia of artemin overexpresser mice (ART-OEs) had an 81% increase in GFRalpha3, a 190% increase in TRPV1 and a 403% increase in TRPA1 compared to wildtype (WT) controls. Myelinated and unmyelinated fibers of the lingual nerve were increased in diameter, as was the density of GFRalpha3 and TRPV1-positive innervation to the dorsal anterior tongue and fungiform papilla. Retrograde labeling of trigeminal afferents by WGA injection into the tip of the tongue showed an increased percentage of GFRalpha3, TRPV1 and isolectin B4 afferents in ART-OE mice. ART-OE afferents had larger calcium transients in response to ligands of TRPV1 (capsaicin) and TRPA1 (mustard oil). Behavioral sensitivity was also exhibited by ART-OE mice to capsaicin and mustard oil, measured using a two-choice drinking test. These results suggest a potential role for artemin-responsive GFRalpha3/TRPV1/TRPA1 sensory afferents in mediating sensitivity associated with tissue injury, chemical sensitivity or disease states such as burning mouth syndrome.

Regional differences in taste bud distribution and alpha-gustducin expression patterns in the mouse fungiform papilla

The regional differences between distribution patterns and alpha-gustducin expression patterns of the fungiform (FF) taste buds were investigated in the adult mouse, using hematoxylin-eosin staining and immunofluorescence histochemistry on the most anterior region of the tongue (the first millimeter) through the intermediate region of the tongue (the last 1-4 mm). Paraffin sections were prepared from the tip to posterior regions (anterior and intermediate region containing the FF taste buds) of the adult mouse tongue. Results indicate that there were significant regional differences in size and density of taste buds, the cell counts of the single taste bud, and the alpha-gustducin-immunoreactive taste buds between the 2 regions. The taste bud had a characteristic onion-like appearance, and the alpha-gustducin-immunoreactive cell was spindle shaped with elongated processes extending from the base to the pore of the taste buds. These results provide a detailed insight to better understand regional descriptions of mouse taste bud density and size and alpha-gustducin expression with the mouse model.

Exuberant neuronal convergence onto reduced taste bud targets with preservation of neural specificity in mice overexpressing neurotrophin in the tongue epithelium

A mouse fungiform taste bud is innervated by only four to five geniculate ganglion neurons; their peripheral fibers do not branch to other buds. We examined whether the degree or specificity of this exclusive innervation pattern is influenced by brain-derived neurotrophic factor (BDNF), a prominent lingual neurotrophin implicated in taste receptoneural development. Labeled ganglion cells were counted after injecting single buds with different color markers in BDNF-lingual-overexpressing (OE) mice. To evaluate the end-organs, taste buds and a class of putative taste receptor cells were counted from progeny of BDNF-OE mice crossbred with green fluorescent protein (GFP) (gustducin) transgenic mice. Fungiform bud numbers in BDNF-OE mice are 35%, yet geniculate neuron numbers are 195%, of wild-type mice. Neurons labeled by single-bud injections in BDNF-OE animals were increased fourfold versus controls. Injecting three buds, each with different color markers, resulted in predominantly single-labeled ganglion cells, a discrete innervation pattern similar to controls. Thus, hyper-innervation of BDNF-OE buds involves many neurons innervating single buds, not increased fiber branching. Therefore, both wild-type and BDNF-OE mice exhibit, in fungiform buds, the same, "discrete" receptoneural pattern, this despite dramatic neurotrophin overexpression-related decreases in bud numbers and increases in innervation density. Hyperinnervation did not affect GFP positive cell numbers; proportions of GFP cells in BDNF-OE buds were the same as in wild-type mice. Total numbers of ganglion cells innervating buds in transgenic mice are similar to controls; the density of taste input to the brain appears maintained despite dramatically reduced receptor organs and increased ganglion 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.

Trigeminal trophic syndrome

Trigeminal trophic syndrome (TTS) is a rare cause of facial ulceration, which is believed to develop after insult to the trigeminal ganglia or other parts of the peripheral/central nervous system in the trigeminal pathway. The pathogenesis of TTS is poorly understood. Developing a better understanding of TTS will allow early recognition and improved treatment. Although the ulcers develop predominantly on the ala nasi, the literature on ulcer locations is limited. In this article, we review the epidemiologic aspects of TTS, expand on our knowledge of the anatomic location of the ulcers, and discuss current theories for its aetiology and briefly review the approaches to its management.

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

Background

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

Results

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

Conclusion

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

Discrete innervation of murine taste buds by peripheral taste neurons

The peripheral taste system likely maintains a specific relationship between ganglion cells that signal a particular taste quality and taste bud cells responsive to that quality. We have explored a measure of the receptoneural relationship in the mouse. By injecting single fungiform taste buds with lipophilic retrograde neuroanatomical markers, the number of labeled geniculate ganglion cells innervating single buds on the tongue were identified. We found that three to five ganglion cells innervate a single bud. Injecting neighboring buds with different color markers showed that the buds are primarily innervated by separate populations of geniculate cells (i.e., multiply labeled ganglion cells are rare). In other words, each taste bud is innervated by a population of neurons that only connects with that bud. Palate bud injections revealed a similar, relatively exclusive receptoneural relationship. Injecting buds in different regions of the tongue did not reveal a topographic representation of buds in the geniculate ganglion, despite a stereotyped patterned arrangement of fungiform buds as rows and columns on the tongue. However, ganglion cells innervating the tongue and palate were differentially concentrated in lateral and rostral regions of the ganglion, respectively. The principal finding that small groups of ganglion cells send sensory fibers that converge selectively on a single bud is a new-found measure of specific matching between the two principal cellular elements of the mouse peripheral taste system. Repetition of the experiments in the hamster showed a more divergent innervation of buds in this species. The results indicate that whatever taste quality is signaled by a murine geniculate ganglion neuron, that signal reflects the activity of cells in a single taste bud.

Temperature modulates taste responsiveness and stimulates gustatory neurons in the rat geniculate ganglion

In humans, temperature influences taste intensity and quality perception, and thermal stimulation itself may elicit taste sensations. However, peripheral coding mechanisms of taste have generally been examined independently of the influence of temperature. In anesthetized rats, we characterized the single-cell responses of geniculate ganglion neurons to 0.5 M sucrose, 0.1 M NaCl, 0.01 M citric acid, and 0.02 M quinine hydrochloride at a steady, baseline temperature (adapted) of 10, 25, and 40 degrees C; gradual cooling and warming (1 degrees C/s change in water temperature >5 s) from an adapted tongue temperature of 25 degrees C; gradual cooling from an adapted temperature of 40 degrees C; and gradual warming from an adapted temperature of 10 degrees C. Hierarchical cluster analysis of the taste responses at 25 degrees C divided 50 neurons into two major categories of narrowly tuned (Sucrose-specialists, NaCl-specialists) and broadly tuned (NaCl-generalists(I), NaCl- generalists(II), Acid-generalists, and QHCl-generalists) groups. NaCl specialists were excited by cooling from 25 to 10 degrees C and inhibited by warming from 10 to 25 degrees C. Acid-generalists were excited by cooling from 40 to 25 degrees C but not from 25 to 10 degrees C. In general, the taste responses of broadly tuned neurons decreased systematically to all stimuli with decreasing adapted temperatures. The response selectivity of Sucrose-specialists for sucrose and NaCl-specialists for NaCl was unaffected by adapted temperature. However, Sucrose-specialists were unresponsive to sucrose at 10 degrees C, whereas NaCl-specialists responded equally to NaCl at all adapted temperatures. In conclusion, we have shown that temperature modulates taste responsiveness and is itself a stimulus for activation in specific types of peripheral gustatory neurons.

Elucidation of mammalian bitter taste

A family of approximately 30 TAS2R bitter taste receptors has been identified in mammals. Their genes evolved through adaptive diversification and are linked to chromosomal loci known to influence bitter taste in mice and humans. The agonists for various TAS2Rs have been identified and all of them were established as bitter tastants. TAS2Rs are broadly tuned to detect multiple bitter substances, explaining, in part, how mammals can recognize numerous bitter compounds with a limited set of receptors. The TAS2Rs are expressed in a subset of taste receptor cells, which are distinct from those mediating responses to other taste qualities. However, cells devoted to the detection of sweet, umami, and bitter stimuli share common signal transduction components. Transgenic expression of a human TAS2R in sweet or bitter taste receptor-expressing cells of mice induced either strong attraction or aversion to the receptor's cognate bitter tastant. Thus, dedicated taste receptor cells appear to function as broadly tuned detectors for bitter substances and are wired to elicit aversive behavior.

Modulation of taste peripheral signal through interpapillar inhibition in hamsters

Single taste buds from fungiform papillae were iontophoretically stimulated with chemicals filling glass microelectrodes while a single unit was recorded in the taste pore of a neighbor papilla. High signal-to-noise ratio responses were observed in the recorded papilla as antidromic action potentials. These responses were possibly modulated by the simultaneous stimulation of another adjacent papilla. A decrease in the frequency of firing and/or both decrementing spikes were observed during such dual papillae stimulations. These inhibitory effects were not modified by the section of the chordo-lingual nerve, suggesting the tongue is able to process the gustatory information thanks to interpapillar negative feedback, prior to transmitting the signal to the central nervous system. Branched chorda tympani fibers can account for responses observed for single papillae stimulations; inhibitions and decrementing spikes may suggest the contribution of another mechanism of interaction between two different single fibers.

Trigeminal modulation of gustatory neurons in the nucleus of the solitary tract

Electrophysiological methods were used to investigate the effects of trigeminal nerve stimulation or transection on responses of single gustatory neurons in the nucleus of the solitary tract (NTS) to tastants (NaCl, sucrose, citric acid, monosodium glutamate) in pentobarbital-anesthetized rats. Unilateral transection of the lingual nerve, or the mandibular branch of the trigeminal nerve, resulted in significant reductions (by 21 and 29%, respectively; P<0.01) in tastant-evoked responses, with no further effect following bilateral transection. Electrical stimulation of the central cut end of the mandibular nerve directly excited nine of 14 gustatory NTS units. For these units, central mandibular stimulation facilitated the tastant-evoked responses in six, depressed responses in three, and had no effect in five. Facilitation of tastant-evoked responses peaked 4 min after mandibular stimulation and recovered within 8 min. Electrical stimulation of the peripheral cut end of the mandibular nerve significantly reduced tastant-evoked responses in nine other NTS units, with a maximal reduction at 4 min post-stimulation followed by recovery. Stimulation of the superior cervical sympathetic ganglion did not affect NTS tastant-evoked responses. These results suggest the presence of complex central modulation of NTS neurons by trigeminal afferents, as well as a peripheral depressant effect on gustatory processing possibly mediated via neuropeptide release from trigeminal nerve endings in the tongue.

Expression of activating transcription factor 3 and growth-associated protein 43 in the rat geniculate ganglion neurons after chorda tympani injury

The purpose of this study was to evaluate the degree of damage in the geniculate ganglion and its target organ as a result of chorda tympani (CT) injury. We performed unilateral transection of the rat CT and examined expression of the activating transcription factor 3 (ATF3), a neuronal injury marker, and the growth-associated protein 43 (GAP-43), a regeneration-associated molecule. The mean proportion of ATF3-immunoreactive (ir) neurons in the geniculate ganglion was approximately 32% at 3 days after CT injury, but these neurons were never detected in the naive ganglion. Using in situ hybridization, the mean percentage of GAP-43 mRNA-labeled neurons (signal : noise ratio > or = 10) was observed to have increased significantly to approximately 60% for 1-7 days after CT injury, while that in the naive ganglion was < 15%. The results of morphological studies using scanning electron microscopy and immunohistochemistry indicated that atrophic change and reduction of protein gene-product 9.5-ir fibers in the denervated papillae, mainly in the intragemmal region, were observed after CT injury. Increase in GAP-43 mRNA, suggesting CT axonal regeneration, may have a role in recovery from taste disorders. However, this regenerative process may be involved in abnormal activity in the axotomized neurons or the adjacent intact neurons and so one must not disregard the existence of injured geniculate ganglions when considering the treatment of diseases that cause CT injury.

Relationship of papillae number to bitter intensity of quinine and PROP within and between individuals

Subjects were asked to assess the bitterness of one 6-n-propyl-2-thiouracil (PROP) and two quinine HCl (QHCl) concentrations presented via filter papers of varying sizes. The number of taste papillae stimulated by these filter papers was counted in each individual. Whole mouth sensitivity to PROP was determined in a separate session. In support of other demonstrations of spatial summation, these data indicated that perceived bitterness intensity increased as a function of area of stimulation within subjects. Between subjects, there was a significant trend for the perceived bitterness of PROP to increase with the lingual density of fungiform papillae, although this trend was highly variable and was only demonstrable among those who showed at least moderate sensitivity to PROP. On the other hand, the number of stimulated fungiform papillae failed to account for individual differences in perceived bitterness of QHCl.

Taste receptor cells that discriminate between bitter stimuli

Recent studies showing that single taste bud cells express multiple bitter taste receptors have reignited a long-standing controversy over whether single gustatory receptor cells respond selectively or broadly to tastants. We examined calcium responses of rat taste receptor cells in situ to a panel of bitter compounds to determine whether individual cells distinguish between bitter stimuli. Most bitter-responsive taste cells were activated by only one out of five compounds tested. In taste cells that responded to multiple stimuli, there were no significant associations between any two stimuli. Bitter sensation does not appear to occur through the activation of a homogeneous population of broadly tuned bitter-sensitive taste cells. Instead, different bitter stimuli may activate different subpopulations of bitter-sensitive taste cells.

In situ Ca2+ imaging reveals neurotransmitter receptors for glutamate in taste receptor cells

The neurotransmitters at synapses in taste buds are not yet known with confidence. Here we report a new calcium-imaging technique for taste buds that allowed us to test for the presence of glutamate receptors (GluRs) in living isolated tissue preparations. Taste cells of rat foliate papillae were loaded with calcium green dextran (CaGD). Lingual slices containing CaGD-labeled taste cells were imaged with a scanning confocal microscope and superfused with glutamate (30 micromter to 1 mm), kainate (30 and 100 micrometer), AMPA (30 and 100 micrometer), or NMDA (100 micrometer). Responses were observed in 26% of the cells that were tested with 300 micrometer glutamate. Responses to glutamate were localized to the basal processes and cell bodies, which are synaptic regions of taste cells. Glutamate responses were dose-dependent and were induced by concentrations as low as 30 microm. The non-NMDA receptor antagonists CNQX and GYKI 52466 reversibly blocked responses to glutamate. Kainate, but not AMPA, also elicited Ca(2+) responses. NMDA stimulated increases in [Ca(2+)](i) when the bath medium was modified to optimize for NMDA receptor activation. The subset of cells that responded to glutamate was either NMDA-unresponsive (54%) or NMDA-responsive (46%), suggesting that there are presumably two populations of glutamate-sensitive taste cells-one with NMDA receptors and the other without NMDA receptors. The function of GluRs in taste buds is not yet known, but the data suggest that glutamate is a neurotransmitter there. GluRs in taste cells might be presynaptic autoreceptors or postsynaptic receptors at afferent or efferent synapses.

Adaptation to capsaicin burn: effects of concentration and individual differences

Human subjects rated the time course of the burn produced by three concentrations of capsaicin applied to the tongue via filter papers. Data were fit to a dynamic model composed of a level detector, a change detector, and a double integrator. These three processes responded differently to concentration. 6-n-Propyl-2-thiouracil (PROP) taster status correlated positively with the integrator process. Although a minority of subjects showed evidence of the integrator process, any subject with an integrator process at a given concentration also showed it at any higher concentration.

Experimental microneurosurgery of the trigeminal ganglion and ophthalmic-maxillary nerve in the rat: subtemporal fossa approach

In the researches of the innervation relationship between trigeminal ganglion and a particular structure in the head, it is usually necessary to apply neural tracers into the ganglion for anterograde nerve tracing study or perform bilateral trigeminal nerve transecting for degeneration study. A common surgical approach for exposing these structures in the rat was to remove a piece of skull and a portion of brain. While investigating the innervation of rat's pineal gland from its trigeminal ganglion, we used a subtemporal fossa approach, whereby the mortality of the animal was remarkably reduced and the contamination of the intracranial structures by the tracer was proved to be least. This is the first description of bilateral surgeries of the trigeminal ganglion and trigeminal nerve in rat using extracranial approach. Detailed surgical procedures were presented and their advantages discussed.

Thermal stimulation of taste

The first electrophysiological recordings from animal and human taste nerves gave clear evidence of thermal sensitivity, and studies have shown that as many as half of the neurons in mammalian taste pathways respond to temperature. Because temperature has never been shown to induce sensations of taste, it has been assumed that thermal stimulation in the gustatory system is somehow nulled. Here we show that heating or cooling small areas of the tongue can in fact cause sensations of taste: warming the anterior edge of the tongue (chorda tympani nerve) from a cold temperature can evoke sweetness, whereas cooling can evoke sourness and/or saltiness. Thermal taste also occurs on the rear of the tongue (glossopharyngeal nerve), but the relationship between temperature and taste is different there than on the front of the tongue. These observations indicate the human gustatory system contains several different types of thermally sensitive neurons that normally contribute to the sensory code for taste.