Branched chorda tympani neurons and interactions among taste receptors

Understanding responses to chemical mixtures: looking forward from the past

Our goal in this article is to provide a perspective on how to understand the nature of responses to chemical mixtures. In studying responses to mixtures, researchers often identify "mixture interactions"-responses to mixtures that are not accurately predicted from the responses to the mixture's individual components. Critical in these studies is how to predict responses to mixtures and thus to identify a mixture interaction. We explore this issue with a focus on olfaction and on the first level of neural processing-olfactory sensory neurons-although we use examples from taste systems as well and we consider responses beyond sensory neurons, including behavior and psychophysics. We provide a broadly comparative perspective that includes examples from vertebrates and invertebrates, from genetic and nongenetic animal models, and from literature old and new. In the end, we attempt to recommend how to approach these problems, including possible future research directions.

Physiology of Taste Processing in the Tongue, Gut, and Brain

The gustatory system detects and informs us about the nature of various chemicals we put in our mouth. Some of these have nutritive value (sugars, amino acids, salts, and fats) and are appetitive and avidly ingested, whereas others (atropine, quinine, nicotine) are aversive and rapidly rejected. However, the gustatory system is mainly responsible for evoking the perception of a limited number of qualities that humans taste as sweet, umami, bitter, sour, salty, and perhaps fat [free fatty acids (FFA)] and starch (malto-oligosaccharides). The complex flavors and mouthfeel that we experience while eating food result from the integration of taste, odor, texture, pungency, and temperature. The latter three arise primarily from the somatosensory (trigeminal) system. The sensory organs used for detecting and transducing many chemicals are found in taste buds (TBs) located throughout the tongue, soft palate esophagus, and epiglottis. In parallel with the taste system, the trigeminal nerve innervates the peri-gemmal epithelium to transmit temperature, mechanical stimuli, and painful or cooling sensations such as those produced by changes in temperature as well as from chemicals like capsaicin and menthol, respectively. This article gives an overview of the current knowledge about these TB cells' anatomy and physiology and their trigeminal induced sensations. We then discuss how taste is represented across gustatory cortices using an intermingled and spatially distributed population code. Finally, we review postingestion processing (interoception) and central integration of the tongue-gut-brain interaction, ultimately determining our sensations as well as preferences toward the wholesomeness of nutritious foods. © 2021 American Physiological Society. Compr Physiol 11:1-35, 2021.

Identification of the Lingual Papillae in the sugar glider (Petaurus breviceps) by scanning electron microscopy and light microscopy

A morphological description of the sugar glider's (Petaurus breviceps) tongue was obtained from five individual sugar gliders and analysed under a scanning electron microscope and a light microscope using four methods of staining. The tongue has a noticeably elongated structure with three regions: apex, corpus and radix. Transition to the corpus is indicated by the sulcus medianus linguae and the frenulum on its ventral surface. The dorsal tongue displayed a raised mucous membrane manifesting as a type of papillae. Filiform papillae were differentiated into two subtypes: brush-like papillae, with a short base splitting into 5-6 processes; and flower bud filiform papillae that are shorter and wider than the former, and divided into a single main process and 4-5 secondary processes. Fungiform papillae with 1-2 taste buds are interspersed between filiform papillae and absent on the radix. Conical papillae cover the lateral margin of the corpus and increase in size when closer to the radix. Vallate papillae-three in number-are arranged as a triangular structure, with the middle papillae pointing towards the root of the tongue. A continuous groove wrapped around each papilla with several taste buds is found on its lateral surface. Two of the vallate papillae are located symmetrically on each side and are much smaller than the most caudal middle vallate that is directed posteriorly. A positive reaction using separate staining with periodic acid-Schiff and Alcian blue (pH 2.5) showed Weber's glands and von Ebner's glands appearing beneath the papillae, mainly on the muscle layer of the radix. Different reactions to the stains indicated that Weber's gland contains sialomucin and/or sulfomucin, whereas von Ebner's gland consists of serous cells secreting glycogen and/or neutral mucin. The collagen structure was identified using Masson's trichrome stain in the two bottom layers of the tongue from the apex, decreasing gradually towards the radix.

The Functional and Neurobiological Properties of Bad Taste

The gustatory system serves as a critical line of defense against ingesting harmful substances. Technological advances have fostered the characterization of peripheral receptors and have created opportunities for more selective manipulations of the nervous system, yet the neurobiological mechanisms underlying taste-based avoidance and aversion remain poorly understood. One conceptual obstacle stems from a lack of recognition that taste signals subserve several behavioral and physiological functions which likely engage partially segregated neural circuits. Moreover, although the gustatory system evolved to respond expediently to broad classes of biologically relevant chemicals, innate repertoires are often not in register with the actual consequences of a food. The mammalian brain exhibits tremendous flexibility; responses to taste can be modified in a specific manner according to bodily needs and the learned consequences of ingestion. Therefore, experimental strategies that distinguish between the functional properties of various taste-guided behaviors and link them to specific neural circuits need to be applied. Given the close relationship between the gustatory and visceroceptive systems, a full reckoning of the neural architecture of bad taste requires an understanding of how these respective sensory signals are integrated in the brain.

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.

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.

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

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

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.

Lingual and palatal gustatory afferents each depend on both BDNF and NT-4, but the dependence is greater for lingual than palatal afferents

Neurons of the geniculate ganglion innervate taste buds located in two spatially distinct targets, the tongue and palate. About 50% of these neurons die in Bdnf(-/-) mice and Ntf4/5(-/-) mice. Bdnf(-/-)/Ntf4/5(-/-) double mutants lose 90-95% of geniculate ganglion neurons. To determine whether different subpopulations are differentially influenced by neurotrophins, we quantified neurons from two ganglion subpopulations separately and remaining taste buds at birth within each target field in wild-type, Bdnf(-/-), Ntf4/5(-/-), and Bdnf(-/-)/Ntf4/5(-/-) mice. In wild-type mice the same number of neurons innervated the anterior tongue and soft palate and each target contained the same number of taste buds. Compared to wild-type mice, Bdnf(-/-) mice showed a 50% reduction in geniculate neurons innervating the tongue and a 28% loss in neurons innervating the soft palate. Ntf4/5(-/-) mice lost 58% of the neurons innervating the tongue and 41% of the neurons innervating the soft palate. Taste bud loss was not as profound in the NT-4 null mice compared to BDNF-null mice. Tongues of Bdnf(-/-)/Ntf4/5(-/-) mice were innervated by 0 to 4 gustatory neurons and contained 3 to 16 taste buds at birth, indicating that some taste buds remain even when all innervation is lost. Thus, gustatory neurons are equally dependent on BDNF and NT-4 expression for survival, regardless of what peripheral target they innervate. However, taste buds are more sensitive to BDNF than NT-4 removal.

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.

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.

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.

The effect of β-bungarotoxin, or geniculate ganglion lesion on taste bud development in the chick embryo

Chick taste bud (gemmal) primordia normally appear on embryonic day (E) 16 and incipient immature, spherical-shaped buds at E17. In ovo injection of beta-bungarotoxin at E12 resulted in a complete absence of taste buds in lower beak and palatal epithelium at developmental ages E17 and E21. However, putative gemmal primordia (solitary clear cells; small, cell groupings) remained, lying adjacent to salivary gland duct openings as seen in normal chick gemmal development. Oral epithelium was immunonegative to neural cell adhesion molecule (NCAM) suggesting gemmal primordia are nerve-independent. Some NCAM immunoreactivity was evident in autonomic ganglion-like cells and nerve fibers in connective tissue. After unilateral geniculate ganglion/otocyst excision on E2.5, at developmental ages E18 and posthatching day 1, approximately 12% of surviving ipsilateral geniculate ganglion cells sustained approximately 54% of the unoperated gemmal counts. After E18, proportional stages of differentiation in surviving developing buds probably reflect their degree of innervation, as well as rate of differentiation. Irrespective of the degree of geniculate ganglion damage, the proportion of surviving buds can be sustained at the same differentiated bud stage as on the unoperated side, or may differentiate to a later bud stage, consistent with the thesis that bud maturation, maintenance, and survival are nerve-dependent.

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

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

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.

Long-term recordings from afferent taste fibers

The receptor cells of taste buds have a life span of about 10 days but it is not known if response characteristics of these receptors alter during the turnover cycle. To examine taste cell responses over time, a micromachined polyimide sieve electrode array was implanted between the cut ends of the rat chorda tympani nerve, which then regenerated through the electrode array. Long-term stable recordings from regenerated single afferent fibers innervating taste buds were possible using this technique for up to 21 days. Responses to taste stimuli recorded from the same fiber changed with time. The changes occurred in both the magnitude of response and the relative response profiles to four chemical stimuli, NaCl, sucrose, HCl, and quinine HCl. These changes in response characteristics were hypothesized to result from changes in the taste receptor cells as the receptor cells turnover in the taste buds.

Distinctive neurophysiological properties of embryonic trigeminal and geniculate neurons in culture

Neurons in trigeminal and geniculate ganglia extend neurites that share contiguous target tissue fields in the fungiform papillae and taste buds of the mammalian tongue and thereby have principal roles in lingual somatosensation and gustation. Although functional differentiation of these neurons is central to formation of lingual sensory circuits, there is little known about electrophysiological properties of developing trigeminal and geniculate ganglia or the extrinsic factors that might regulate neural development. We used whole cell recordings from embryonic day 16 rat ganglia, maintained in culture as explants for 3-10 days with neurotrophin support to characterize basic properties of trigeminal and geniculate neurons over time in vitro and in comparison to each other. Each ganglion was cultured with the neurotrophin that supports maximal neuron survival and that would be encountered by growing neurites at highest concentration in target fields. Resting membrane potential and time constant did not alter over days in culture, whereas membrane resistance decreased and capacitance increased in association with small increases in trigeminal and geniculate soma size. Small gradual differences in action potential properties were observed for both ganglion types, including an increase in threshold current to elicit an action potential and a decrease in duration and increase in rise and fall slopes so that action potentials became shorter and sharper with time in culture. Using a period of 5-8 days in culture when neural properties are generally stable, we compared trigeminal and geniculate ganglia and revealed major differences between these embryonic ganglia in passive membrane and action potential characteristics. Geniculate neurons had lower resting membrane potential and higher input resistance and smaller, shorter, and sharper action potentials with lower thresholds than trigeminal neurons. Whereas all trigeminal neurons produced a single action potential at threshold depolarization, 35% of geniculate neurons fired repetitively. Furthermore, all trigeminal neurons produced TTX-resistant action potentials, but geniculate action potentials were abolished in the presence of low concentrations of TTX. Both trigeminal and geniculate neurons had inflections on the falling phase of the action potential that were reduced in the presence of various pharmacological blockers of calcium channel activation. Use of nifedipine, omega-conotoxin-MVIIA and GVIA, and omega-agatoxin-TK indicated that currents through L-, N-, and P/Q- type calcium channels participate in the action potential inflection in embryonic trigeminal and geniculate neurons. The data on passive membrane, action potential, and ion channel characteristics demonstrate clear differences between trigeminal and geniculate ganglion neurons at an embryonic stage when target tissues are innervated but receptor organs have not developed or are still immature. Therefore these electrophysiological distinctions between embryonic ganglia are present before neural activity from differentiated receptive fields can influence functional phenotype.

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

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

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

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

Gustatory neuron types in the periphery: a functional perspective

Robert P. Erickson's research and writings formed the intellectual backdrop and guiding force for much of the major research on sensory coding in taste. As articulated best by Erickson, consideration focused on the relative merits of labeled-line and across-fiber pattern theory. The present article focuses primarily on a review of the electrophysiological and behavioral studies on salt taste and salt taste-mediated behavior in rodents. The evidence clearly shows that the peripheral gustatory system consists of a few neuron types/groups with well-defined physiological response characteristics. Electrophysiological studies of the chorda tympani nerve define a physiological group of narrowly tuned neurons selectively responsive to NaCl stimuli. It appears that this is a sodium-sensing module that functions primarily in the detection, recognition, and ingestion of NaCl.

A novel family of mammalian taste receptors

In mammals, taste perception is a major mode of sensory input. We have identified a novel family of 40-80 human and rodent G protein-coupled receptors expressed in subsets of taste receptor cells of the tongue and palate epithelia. These candidate taste receptors (T2Rs) are organized in the genome in clusters and are genetically linked to loci that influence bitter perception in mice and humans. Notably, a single taste receptor cell expresses a large repertoire of T2Rs, suggesting that each cell may be capable of recognizing multiple tastants. T2Rs are exclusively expressed in taste receptor cells that contain the G protein alpha subunit gustducin, implying that they function as gustducin-linked receptors. In the accompanying paper, we demonstrate that T2Rs couple to gustducin in vitro, and respond to bitter tastants in a functional expression assay.

The neural code for taste in the nucleus of the solitary tract of the rat: effects of adaptation

Adaptation of the tongue to NaCl, HCl, quinine or sucrose was used as a tool to study the stability and organization of response profiles in the nucleus of the solitary tract (NTS). Taste responses in the NTS were recorded in anesthetized rats before and after adaptation of the tongue to NaCl, HCl, sucrose or quinine. Results showed that the magnitude of response to test stimuli following adaptation was a function of the context, i.e., adaptation condition, in which the stimuli were presented. Over half of all taste responses were either attenuated or enhanced following the adaptation procedure: NaCl adaptation produced the most widespread, non-stimulus-selective cross-adaptation and sucrose adaptation produced the least frequent cross-adaptation and the most frequent enhancement of taste responses. Adaptation to quinine cross-adapted to sucrose and adaptation to HCl cross-adapted to quinine in over half of the units tested. The adaptation procedure sometimes unmasked taste responses where none were present beforehand and sometimes altered taste responses to test stimuli even though the adapting stimulus did not itself produce a response. These effects demonstrated a form of context-dependency of taste responsiveness in the NTS and further suggest a broad potentiality in the sensitivity of NTS units across taste stimuli. Across unit patterns of response remained distinct from each other under all adaptation conditions. Discriminability of these patterns may provide a neurophysiological basis for residual psychophysical abilities following adaptation.

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

Single gustatory nerve fibers branch and innervate several taste buds. In turn, individual taste buds may receive innervation from numerous gustatory nerve fibers. To evaluate the pattern of sensory innervation of fungiform papilla-bearing taste buds, we used iontophoretic fluorescent injection to retrogradely label the fibers that innervate single taste papillae in the hamster. For each animal, a single taste papilla was injected through the gemmal pore with 3.3% tetramethylrhodamine dextran amine. Fungiform papillae either at the tongue tip (0.5-1.5 mm from the tip) or more posteriorly (1.5-3.0 mm from the tip) were injected. After one to seven days survival, the geniculate and trigeminal ganglia and the tongue were sectioned and examined for labeled cells and fibers, respectively. Analysis of the number and topographic distribution of geniculate cells innervating single taste papillae, shows that: (i) 15 +/- 4 (S.D.) ganglion cells converge to innervate a single fungiform taste bud; (ii) more ganglion cells innervate anterior- (range: 13-35 cells) than posterior-lying buds (range: five to 12 cells), which, in part, may be related to bud volume (microm3); and (iii) ganglion somata innervating a single taste bud are scattered widely within the geniculate ganglion. Analysis of labeled fibers in the tongue demonstrated that two to eight taste buds located within 2 mm of the injected taste bud share collateral innervation with the injected taste bud. Since all buds with labeled fibers were located in close proximity (within a 2-mm radius), widely dispersed geniculate ganglion cells converge to innervate closely spaced fungiform taste buds. Trigeminal ganglion (mandibular division) cells were also labeled in every case and, as with the geniculate ganglion, a dispersed cell body location and collateralization pattern among papillae were observed. This study shows that iontophoresis of tetramethylrhodamine dextran amine, selectively applied to individual peripheral receptor end-organs, effectively locates sensory ganglion cells in two different ganglia that project to these sites. Moreover, the marker demonstrates collateral branches of sensory afferents associated with the labeled fibers and the nearby receptor areas innervated by these collaterals. The labeling of single or clusters of receptor cells, as well as identified sensory afferents, affords future possibilities for combining this technique with immunocytochemistry to establish the relationships of innervation patterns with neurotransmitters and neurotropic substances within identified cells.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Responses of single lingual nerve fibers to thermal and chemical stimulation

The goals of this study were to characterize the responses of: (1) thermally-sensitive fibers of the lingual branch of the trigeminal nerve to cooling from 35 degrees to 10 degrees C at a rate of 1 degrees C/s; and (2) these neurons to a mid-range concentration of NaCl (150 mM), glucose (150 mM), citric acid (0.3 mM), and quinine-HCl (3 mM) at 35 degrees and 25 degrees C. A cluster analysis of 47 neurons' responses to cooling revealed two major groups and one minor group. Group 1 neurons (n=19) had a shorter latency, exhibited faster time-to-peak activity, and responded over a smaller range of temperature compared to Group 2 neurons (n=22). Group 3 neurons (n=6) exhibited the longest response latency and responded over a wider cooler range of temperature. Twenty-five out of thirty-one thermally-sensitive, non-tactile lingual neurons responded weakly to at least one chemical stimulus, with some neurons responding to 2, 3, or all 4 chemical stimuli. Group 1 neurons responded to more chemical stimuli at 35 degrees C, while Group 2 neurons responded more at 25 degrees C. Under their optimal temperature conditions, Group 1 and Group 2 neurons responded most often to citric acid and least often to glucose, with NaCl and Q-HCl eliciting an intermediate number of responses. As a whole, the responses of thermally-sensitive fibers to chemical stimulation were modest at best with an absence of chemical specificity. There was no evidence of a 'best' stimulus, although there was a suggestion of temporal coding.

Embryonic and early fetal development of human taste buds: a transmission electron microscopical study

BACKGROUND

Taste buds are assemblies of slender epithelial cells that receive chemical stimuli from the outer (oral) environment. In contrast to the large and well documented information on the morphology of taste buds in adult humans and animals, there are only a few reports on fetal ones, and ultrastructural studies of prenatal human taste buds are lacking completely. Therefore, the present investigation has been carried out to study the taste bud primordium, its morphological changes including synaptogenesis, cell differentiation, and taste pore formation from the time of the onset of taste bud formation around the 8th week until the 15th postovulatory week.

METHODS

Taste bud primordia of 42 human embryonic/fetal tongues have been examined by means of transmission electron microscopy.

RESULTS

Nerve fibers approach the lingual epithelium between the 6th and 7th postovulatory week. They penetrate the basal lamina during the 8th week and form synapses with poorly differentiated, elongated, epithelial cells. By the 12th week, more differentiated cell types are seen: 1) electron-dense cells resembling type III cells of the adult taste bud containing large numbers of dense-cored vesicles (80-150 nm in diameter); 2) electron-dark cells with well developed endoplasmic reticulum and many apical mitochondria, being candidates for type II cells. Basally, these cells have foot-like processes containing dense-cored vesicles (120-200 nm in diameter), but they do not synapse to nerve fibers. Type I cells, characterized by apically located dense secretory granules, are not observed. First shallow grooves above the taste bud primordium are found around the 10th week. Untypically differentiated apical cellular processes extend onto the surface. Most of the taste pores develop around the 14th to 15th week. In the taste pit, mucous material is not present during the first 15 weeks of gestation. Synapses between cells and afferent nerve fibers were found by the 8th week, reaching a maximum around the 12th to 13th week.

CONCLUSIONS

The early presence of taste bud cells containing dense-cored vesicles suggests an at least dual function of embryonic/ fetal taste buds: First, from the 8th until the 14th week, non-gustatory, paracrine functions should be considered. After the 14th week of gestation, when typical taste pores are present, the taste buds possibly start their gustatory function. Differentiated marginal cells are possibly involved in the formation of the taste pore. The lack of type I cells producing the mucous material in the taste pit indicates that the taste bud has not achieved a fully developed function until the 15th week of gestation.

Immunohistochemical, electrophysiological, and electron microscopical study of rat fungiform taste buds after regeneration of chorda tympani through the non-gustatory lingual nerve

The sensory innervation of fungiform papillae on the rat dorsal tongue is derived from branches of two cranial nerves: the lingual branch of the trigeminal nerve which provides somatosensory innervation and the chorda tympani (CT) branch of the facial nerve, which provides innervation to the taste buds. Removal of the CT results in degeneration of the taste buds. Removal of both nerves results in reduction in size of fungiform papillae and an altered pattern of keratinization in its epithelium. Regeneration of nerves to the epithelium restores the pre-operative condition. Thus, in addition to their sensory functions, both the CT and lingual seem to exert trophic effects on the phenotypic expression of epithelial cells in the fungiform papillae. We severed both the CT and lingual nerves in rats and sutured the proximal stump of the CT to the distal stump of the lingual to promote regeneration of the CT along the lingual nerve pathway. At the same time, we prevented the proximal stump of the lingual from regenerating into the tongue. Our purpose was to determine whether and how the innervation pattern of the regenerated taste bud might be different from normal under these experimental conditions. We found that reinnervation by the CT through the lingual nerve occurs, that this restores the anatomical and functional integrity of the fungiform taste buds and papillae, and that some papillae, but not all, were richly innervated with subgemmal, extragemmal, and perigemmal neuron-specific enolase, calcitonin gene-related peptide, substance P, and neurokinin A-positive fibers. Moreover, responses to taste stimuli were recorded electrophysiologically from the CT.

Ipsi- and bilateral interactions in taste

In two experiments, the integration over spatial extent in taste was investigated for threshold sensitivity to NaCl and for suprathreshold intensity perception of saltiness. The area of stimulation was doubled by adding either an ipsilateral or a bilateral stimulus. The two stimuli could be of equal or unequal intensity. The data showed that at threshold level a probability summation model applied to all bilateral and most of the ipsilateral stimulus combinations. Probability summation failed to predict detection probability when two stimuli with different intensities were presented at the same tongue side. For suprathreshold stimuli, the magnitude of the saltiness sensation as estimated by a line-length method depended on the level of stimulation. The possible peripheral interaction mechanisms and central factors contributing to the taste response were discussed.

Development of fungiform papillae, taste buds, and their innervation in the hamster

Fungiform taste buds in mature hamsters are less subject to neurotrophic influences than those of other species. This study evaluates taste-bud neurotrophism during development in hamsters by examining the relation between growing nerves and differentiating fungiform papillae. Chorda tympani (CT) or lingual (trigeminal) nerve (LN) fibers were labelled with Lucifer Yellow as they grew into (CT fibers) or around (LN fibers) developing taste buds. Developing fungiform papillae and taste pores were counted with the aid of a topical tongue stain. The tongue forms on embryonic days (E) 10.5-11 and contains deeply placed CT and LN fibers but no papillae. By E12, the tongue epithelium develops scattered elevations. These "eminences" selectively become innervated by LN fibers that grow to the epithelium earlier and in larger numbers than CT fibers. Definitive fungiform papillae form rapidly during E13-14 and become heavily innervated by LN fibers. Intraepithelial CT fibers, rare at E13, invariably innervate fungiform papillae containing nascent taste buds at E14. During E14-15 (birth = E15-16), most papillae contain taste buds with pores, extensive perigemmal LN innervation, and extensive intragemmal CT innervation. At birth, numbers of fungiform papillae and taste pores are adultlike. The results show that fungiform eminences begin forming in the absence of innervation. The subsequent differentiation of definitive fungiform papillae and their innervation by LN fibers occur synchronously, prior to the differentiation of taste buds and their CT innervation. The hamster is precocious (e.g., compared to rat) in terms of LN development and the structural maturity of the anterior tongue at birth.

The taste system of the channel catfish: from biophysics to behavior

Catfish, described as 'swimming tongues', are unique experimental models for studies of taste reception because of the extensive distribution of taste buds over their external body surface and within their oropharyngeal cavity. Both the extraordinary numbers of taste buds and their high sensitivity to amino acids have made it possible to perform in the same species: biochemical and biophysical studies of stimulus recognition and signal transduction; electrophysiological recordings of taste activity from receptor cells, afferent nerve fibers and CNS relays; and behavioral studies of taste-controlled food search, biting and mastication. The close correspondence of results obtained with these diverse experimental approaches has provided critical information concerning vertebrate gustation.

Transcellular labeling by DiI demonstrates the glossopharyngeal innervation of taste buds in the lingual epithelium of the axolotl

Innervation of the axolotl lingual epithelium by the glossopharyngeal nerve was examined to reveal its sensory target cells. The carbocyanine dye diI was applied to the nerve stump in the tongue fixed with paraformaldehyde. After a diffusion period of several months, the tongues were examined with a conventional epifluorescence microscope and a confocal laser scanning microscope (LSM) in wholemounts or preparations sectioned with a vibratome. Beneath the epithelium the labeled nerve fibers spread horizontally to form a meshwork of fibers, from which fascicles of fibers extended upward perpendicularly to the epithelium to innervate taste buds. Numerous taste buds were labeled by possible transcellular diffusion of diI. At the base of the taste bud, the nerve fibers branched and formed a basal plexus of fine fibers, on which numerous varicosities were seen. One or at most several taste cells were labeled in a taste bud. In the basal part of taste buds, the cell without an apical process, the basal cell, was also labeled. In the epithelium, between the taste buds, a few solitary cells were labeled. In some cases, a single fascicle of fibers innervating these cells was clearly shown by the LSM. In addition, fine fibers apparently formed free nerve endings in the epithelial cell layer. The results showed that the IX nerve innervated not only taste cells, but also presumed mechanosensory basal cells in the taste bud and the solitary cells of unknown function in the non-taste lingual epithelium. Afferent nerve responses to mechanical stimulation of the tongue may be explained by these non-taste cellular elements in the epithelium.

Postnatal development of gustatory recipient zones within the nucleus of the solitary tract

Previous studies have examined pre- and postsynaptic development of the first-order central gustatory relay, located in the rostral nucleus of the solitary tract (NST). This region of the NST is innervated by primary gustatory axons arising from the facial-intermediate nerve. However, a large portion of the gustatory NST is innervated by axons arising from the glossopharyngeal nerve, and although the time course for development of N.VII recipient zones has been defined development of glossopharyngeal afferent terminal fields has not been examined. Moreover, the time course for development of projection neurons located postsynaptic to gustatory afferent axons has not been examined in any portion of the NST. The objectives of the present study were to 1) define the time course for development of N.VII and N.IX terminal fields and 2) examine temporal relationships between development of afferent terminal fields and development of projection neurons located postsynaptic to gustatory afferent axons. To this end, triple fluorescent labeling procedures were used to simultaneously visualize developing axons and projection neurons. Results show that afferent terminal fields develop along the rostrocaudal axis of the NST. Axons of the N.VII terminal field are present in the rostral NST at P1 and develop to approximately P25. Axons and terminal endings of N.IX do not enter the NST until approximately P9-P10, and these terminal fields develop within the intermediate NST until approximately P45. Many NST neurons destined to project axons to the second-order central gustatory relay, located in the caudal parabrachial nucleus (PBN), do not possess axonal connections with the PBN during the first 2-3 weeks of postnatal life. As afferent terminal fields develop, these neurons establish connections with the PBN between the ages of approximately P7 and P45-P60. The delay between afferent terminal field development and development of PBN projection neurons in the N.VII terminal field is approximately 3 weeks. The delay between pre- and postsynaptic development in the N.IX terminal field is approximately 1 week. Potential relationships between pre- and postsynaptic development are discussed, in addition to relationships between anatomical development in the NST and the emergence of taste-guided behaviors.

Effects of early postnatal receptor damage on dendritic development in gustatory recipient zones of the rostral nucleus of the solitary tract

The rostral gustatory zone of the nucleus of the solitary tract (NST) exhibits extensive anatomical development during the first 3 weeks of postnatal life, and this development requires the presence of intact gustatory receptors during a critical period. We have previously shown that unilateral damage induced to fungiform papillae of the anterior tongue at postnatal day 2 (P2) alters normal migration and ramification of chorda tympani (CT) axons in the rostral NST. In addition to alterations of axonal development, P2 receptor damage decreases the intraneuronal distance between neurons that project axons to the second-order central gustatory relay, located in the caudal parabrachial nucleus (PBN). This observation suggested that P2 receptor damage may alter both axonal development and dendritic development in the rostral gustatory NST. The present study evaluated potential changes in dendritic development of PBN projection neurons following either P2 or P10 receptor damage. Morphological studies were first conducted to quantitatively define somatic characteristics of neurons that project axons to the PBN. Independent experiments used fluorescent labeling combined with subsequent Golgi-impregnation to study dendritic architecture of identified PBN projection neurons. Results confirmed that P2 receptor damage alters dendritic development of PBN projection neurons located in CT terminal fields. Anterior tongue receptor damage at P2 (1) reduces planar length of first- and second-order dendritic branches, (2) reduces the mean number of second-order branches per neuron, and (3) reduces the density of spine processes on second-order dendritic branches. A critical period exists for these effects, similar to that reported for axonal development, insofar as P2 receptor damage alters dendritic development of PBN projection neurons, whereas P10 receptor damage does not. Dendrites of identified PBN projection neurons located in regions of the NST that receive primary afferent axons from the glossopharyngeal nerve are not affected by anterior tongue damage at P2. These results show that early postnatal receptor damage influences both pre- and postsynaptic development in the rostral gustatory NST. These anatomical changes are undoubtedly related to alterations in taste-guided behaviors that are observed following P2 receptor damage.

Early development and innervation of taste bud-bearing papillae on the rat tongue

Early development of fungiform papillae on the fetal rat tongue was examined: (1) to determine whether morphogenesis of the taste bud-bearing fungiform papillae is induced by nerve and (2) to study the growth pattern of the two sensory nerves that innervate the papilla. The papillae first appear on the 15th day of gestation (E15; E1 is the day when the dam is sperm positive) in rows parallel to the midline sulcus. There appears to be a medial-lateral and an anterior-posterior gradient in the sequence of papilla differentiation. The epithelium of the early papilla resembles a multilayered placode topped by a flattened surface periderm. Close examination of the peridermal cells at the apex of the papillae reveals that the cells have fewer surface microvilli and their cytoplasm is more electron opaque than that of similar cells in interpapillary regions. The basal cells in the placode-like epithelium differ from those in interpapillary regions in that they are postmitotic and have more mitochondria. At later stages, the papilla acquires a mesenchymal core and nerves grow into the core. Results from organ culture experiments of tongue fragments taken from E14 fetuses indicate that morphogenesis of fungiform papillae is initiated in the absence of sensory nerve influence, but the nerve exerts a trophic effect on their maintenance. The two sensory nerves of the tongue, the chorda tympani and the lingual branch of the trigeminal nerve, enter the tongue mesenchyme at E14 and grow toward the epithelium. By E15 the chorda tympani branches have reached the developing fungiform papillae, by E16 many have entered the papilla, and by E17 they have penetrated the epithelium at the papilla apex. Their fibers are associated exclusively with the cells at the papilla apex, where the taste bud will develop. The trigeminal nerve ramifies beneath the surface of the entire epithelium by E15. Later, it, too, sends branches into fungiform papillae; these ascend along the trunk of the chorda tympani and at E17 terminate in the connective tissue core around the chorda tympani field. The results are compatible with the notion that the tongue epithelium exerts a general tropic effect on growing axons of both sensory nerves, and the epithelial cells of the fungiform papilla apex exert a similar effect to which only the chorda tympani axons are responsive.

Inhibitory interactions among rodent taste axons

The left side of the tongue of the Mongolian gerbil, Meriones unguiculatus, was experimentally innervated with both chorda tympani nerves. While this dual innervation did not increase the number or volume of fungiform taste buds on the left side, at least half of the taste buds were dually innervated since they could be neurotrophically maintained by either chorda tympani nerve. Impulse discharges occurred simultaneously in the native (left) and foreign (right) chorda tympani nerves when the taste stimulus was restricted to the left side of the tongue. The marked attenuation of the phasic or tonic portions of some taste responses suggested that dual innervation had enhanced inhibition, especially of foreign chorda tympani responses. This was confirmed when electrical stimulation of the native chorda tympani reduced the peak summated action potential discharges of the foreign chorda tympani to NaCl or sucrose by an average of 52 and 41%, respectively. Inhibition began within seconds and continued with an 11.5-min half-life. The inhibition was unaffected by acutely disconnecting either chorda tympani nerve from the brain. We propose that dual chorda tympani innervation accentuated lateral inhibitory connections that may function normally to reduce spurious sensory signals in taste axons.

Effects of early postnatal receptor damage on development of gustatory recipient zones within the nucleus of the solitary tract

The temporal correspondence between neuroanatomical and neurophysiological development of peripheral and central gustatory neurons has suggested that morphological development of the first-order central gustatory relay, located in the rostral nucleus of the solitary tract (NST), may be dependent on afferent input from peripheral gustatory pathways. The objective of the present study was to determine the effects of perinatal receptor damage on development of gustatory recipient zones within the rostral and intermediate NST. Results show that damage induced to fungiform receptors of the anterior tongue at postnatal day 2 (P2) alters normal development of NST terminal fields associated with the chorda tympani nerve (CT) and greater superficial nerve (GSP), and that alterations in the CT/GSP terminal field persist in adulthood after peripheral gustatory receptors have regenerated. Damage induced to fungiform receptors at P2 does not alter the normal development of glossopharyngeal terminal fields in the intermediate NST. Receptor damage produced at P10 and P20 is without effect on normal development of the CT/GSP terminal field. Thus, fungiform receptor damage at P2 produces specific alterations in the development of NST terminal fields that receive projections from the facial-intermediate nerve, and receptor damage effects are only obtained during a critical period of postnatal development. P2 receptor damage has the overall effect of eliminating caudally directed migration of CT/GSP axons to additional projection neurons that establish connections with the second-order central gustatory relay located in the parabrachial nucleus (PBN). Behavioral studies were conducted to determine the functional consequences of early receptor damage. Results from behavioral studies show that bilateral damage to fungiform papillae at P2 alters normal adult preferences to low and intermediate concentrations of NaCl and sucrose tastes, yet aversions to citric acid and quinine HCl are not obviously affected. Therefore, anatomical alterations in the CT/GSP terminal field produced by P2 receptor damage are accompanied by specific changes in adult taste preference responses.

Receptive fields and gustatory responsiveness of frog glossopharyngeal nerve. A single fiber analysis

Receptive fields and responsiveness of single fibers of the glossopharyngeal (IXth) nerve were investigated using electrical, gustatory (NaCl, quinine HCl, acetic acid, water, sucrose, and CaCl2), thermal, and mechanical stimulation of the single fungiform papillae distributed on the dorsal tongue surface in frogs. 172 single fibers were isolated. 58% of these fibers (99/172) were responsive to at least one of the gustatory stimuli (taste fibers), and the remaining 42% (73/172) were responsive only to touch (touch fibers). The number of papillae innervated by a single fiber (receptive field) was between 1 and 17 for taste fibers and between 1 and 10 for touch fibers. The mean receptive field of taste fibers (X = 6.6, n = 99) was significantly larger than that of touch fibers (X = 3.6, n = 73) (two-tailed t test, P less than 0.001). In experiments with natural stimulation of single fungiform papillae, it was found that every branch of a single fiber has a similar responsiveness. Taste fibers were classified into 14 types (Type N, Q, A, NA, NCa, NCaA, NCaW, NCaAW, NCaWS, NQ, NQA, NQAS, NQWarm, Multiple) on the basis of their responses to gustatory and thermal stimuli. The time course of the response in taste fibers was found to be characteristic of their types. For example, the fibers belonging to Type NQA showed phasic responses, those in Type NCa showed tonic responses, etc. These results indicate that there are several groups of fibers in the frog IXth nerve and that every branch of an individual fiber has a similar responsiveness to the parent fiber.

Murine strain differences in taste responsivity and organization of the rostral nucleus of the solitary tract

Taste responsivity and organization of fungiform papillae, geniculate ganglion neurons and gustatory recipient zones of the nucleus of the solitary tract (NST) were examined in C57BL/6NCrlBR (C57) mice, BALB/c6NCrlBR (BALB/c) mice and CB6F1/CrlBr (CB6) mice, an F1 hybrid cross between BALB/c and C57 mice. Results from behavioral studies confirm that C57 and CB6 mice exhibit higher preferences to sucrose and lower preferences to NaCl, as compared to BALB/c mice. No strain differences were confirmed for aversion responses to citric acid or quinine HCl taste stimuli. Anatomical analyses show that the number and organization of fungiform papillae do not reliably differ between C57, BALB/c, and CB6 mice, nor do volumes of glossopharyngeal terminal fields in the NST. However, strain-specific differences exist in the number of neurons contained in the geniculate ganglion, volume of chorda tympani (CT) terminal fields in the rostral NST, and number of NST neurons contained in CT terminal fields. BALB/c and CB6 mice possess a greater number of geniculate ganglion neurons and larger CT terminal fields, as compared to C57 mice. However, strain differences in the number of geniculate ganglion neurons and terminal field volume are not obviously correlated with strain differences in gustatory responsivity. The only reliable relationship confirmed between taste responsivity and neuroanatomical organization of the rostral NST relates to the absolute number of neurons contained in CT terminal fields, and corresponding neuronal density within CT terminal fields. Chorda tympani terminal fields of C57 and CB6 mice contain an average of 379 neurons, whereas CT terminal fields of BALB/c mice contain an average of 531 neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Collateral reinnervation of taste buds after chronic sensory denervation: a morphological study

Peripheral transganglionic transport of horseradish peroxidase (HRP) was used to label afferent fibers in the taste buds and lingual epithelium 2-12 weeks after chronic chorda tympani or combined chorda tympani-lingual nerve lesions. From 4-12 weeks after a chronic chorda tympani lesion, taste buds could be found. These were innervated by fibers from the ipsilateral lingual nerve. From 8-12 weeks after a chronic chorda tympani-lingual nerve lesion, nerve fibers from the contralateral lingual nerve could be found in a few taste buds on the denervated side of the tongue. Thus, collateral sprouting took place over the midline in this instance. These findings indicate that intact gustatory axons do not sprout into denervated taste buds, but trigeminal fibers in the lingual nerve do have this ability.

Ultrastructure of mouse vallate taste buds: III. Patterns of synaptic connectivity

We have used serial high voltage electron micrographs and computer-generated, three-dimensional reconstructions to study morphological relationships and patterns of synaptic connectivity in taste buds from the circumvallate papillae of the mouse. The intragemmal arborizations of 40 sensory nerve fibers were examined from 7 taste buds that were sectioned serially. We identified the synaptic connections from taste cells onto the reconstructed nerve fibers and classified the presynaptic taste cells based on previously established ultrastructural criteria. From these data we were able to extract the following information for the reconstructed nerve fibers: (1) the morphology of intragemmal nerve fibers and their arborizations within the taste bud, (2) the total number of synaptic connections from taste bud cells onto the nerve fibers, and (3) the taste cell types associated with each of the synapses. Fifty-six synapses were studied. Synapses were often found to be located at either the branch points or terminations of nerve fiber processes. The maximum number of taste cells observed to synapse onto a single nerve fiber was 5. Several nerve fibers had no apparent synapses. Dark cells (type I), intermediate cells, and light cells (type II) all formed synaptic connections with sensory nerve fibers. In no cases did dark cells and light cells synapse onto the same sensory nerve fiber. Our observation that any given nerve fiber receives its synaptic input from morphologically similar taste cells provides evidence for specificity in taste bud synaptic connections. We speculate that the observed pattern of synaptic connections is related to taste bud function. Since all of the synapses onto a given nerve fiber are from morphologically similar taste cells, we postulate that there is a correlation between taste cell morphology and sensory responsiveness. Intracellular electrophysiological studies on taste cells, in which responses to focally applied chemical stimuli are followed by characterization of the ultrastructural features of the same taste cells, will prove or disprove this hypothesis.

Clinical evaluation of taste

Partial loss of taste function can take a variety of forms. Losses can be specific to one taste quality or to one tongue locus. In addition, the shapes of psychophysical functions can be altered so that taste intensity no longer grows normally with concentration. Magnitude matching, an efficient psychophysical scaling method (based on magnitude estimation of stimuli from two sensory continua), can provide a relatively quick assessment of a patient's ability to taste the four taste qualities--sweet, salty, sour, and bitter. When taste intensity and loudness are scaled in the same session, a person with normal hearing who has taste loss will match taste intensities to abnormally weak sounds. Spatial losses are detected by placing pieces of filter paper soaked in taste solutions on specific tongue loci. Dysgeusia, the presence of a chronic taste in the mouth, can result from abnormal substances in the mouth (e.g., via saliva or from poor oral hygiene) or can reflect disorders of the central nervous system.