Merkel-like basal cells in Necturus taste buds contain serotonin

Diversity and evolution of serotonergic cells in taste buds of elasmobranchs and ancestral actinopterygian fish

A subset of gustatory cells are serotonin immunoreactive (ir) in the mammalian taste bud. In the taste bud of lamprey, elongated gustatory-like cells are also serotonin-ir. In contrast, flattened serotonin-ir cells are located only in the basal region of the taste buds in the teleosts and amphibians. These serotonin-ir cells are termed as basal cells. To evaluate the evolution and diversity of serotonergic cells in the taste bud of amniote animals, we explored the distribution and morphology of serotonin-ir cells in the taste buds of ancestral actinopterygian fish (spotted gar, sturgeon, Polypterus senegalus) and elasmobranch (stingray). In all examined animals, the taste buds contained serotonin-ir cells in their basal part. The number of serotonin-ir basal cells in each taste bud was different between these fish species. They were highest in the stingray and decreased in the order of the Polypterus, sturgeon, and gar. While serotonin immunoreactivity was observed only in the basal cells in the taste buds of the ancestral actinopterygian fish, some elongated cells were also serotonin-ir in addition to the basal cells in the stingray taste buds. mRNA of tryptophan hydroxylase 1 (tph1), a rate-limiting enzyme of the serotonin synthesis, is expressed in both the elongated and basal cells of stingray taste buds, indicating that these cells synthesize the serotonin by themselves. These results suggest that the serotonin-ir basal cells arose from the ancestor of the cartilaginous fish, and serotonin-ir cells in the elasmobranch taste bud exhibit an intermediate aspect between the lamprey and actinopterygian fish.

Synergistic Interaction of 5-HT1B and 5-HT2B Receptors in Cytoplasmic Ca2+ Regulation in Human Umbilical Vein Endothelial Cells: Possible Involvement in Pathologies

The aim of this work was to explore the involvement of 5-HT1B and 5-HT2B receptors (5-HT1BR and 5-HT2BR) in the regulation of free cytoplasmic calcium concentration ([Ca2+]i) in human umbilical vein endothelial cells (HUVEC). We have shown by quantitative PCR analysis, that 5-HT1BR and 5-HT2BR mRNAs levels are almost equal in HUVEC. Immunofluorescent staining demonstrated, that 5-HT1BR and 5-HT2BR are expressed both in plasma membrane and inside the cells. Intracellular 5-HT1BR are localized mainly in the nuclear region, whereas 5-HT2BR receptors are almost evenly distributed in HUVEC. 5-HT, 5-HT1BR agonist CGS12066B, or 5-HT2BR agonist BW723C86 added to HUVEC caused a slight increase in [Ca2+]i, which was much lower than that of histamine, ATP, or SFLLRN, an agonist of protease-activated receptors (PAR1). However, activation of 5-HT1BR with CGS12066B followed by activation of 5-HT2BR with BW723C86 manifested a synergism of response, since several-fold higher rise in [Ca2+]i occurred. CGS12066B caused more than a 5-fold increase in [Ca2+]i rise in HUVEC in response to 5-HT. This 5-HT induced [Ca2+]i rise was abolished by 5-HT2BR antagonist RS127445, indicating that extracellular 5-HT acts through 5-HT2BR. Synergistic [Ca2+]i rise in response to activation of 5-HT1BR and 5-HT2BR persisted in a calcium-free medium. It was suppressed by the phospholipase C inhibitor U73122 and was not inhibited by the ryanodine and NAADP receptors antagonists dantrolene and NED-19. [Ca2+]i measurements in single cells demonstrated that activation of 5-HT2BR alone by BW723C86 caused single asynchronous [Ca2+]i oscillations in 19.8 ± 4.2% (n = 3) of HUVEC that occur with a long delay (66.1 ± 4.3 s, n = 71). On the contrary, histamine causes a simultaneous and almost immediate increase in [Ca2+]i in all the cells. Pre-activation of 5-HT1BR by CGS12066B led to a 3-4 fold increase in the number of HUVEC responding to BW723C86, to synchronization of their responses with a delay shortening, and to the bursts of [Ca2+]i oscillations in addition to single oscillations. In conclusion, to get a full rise of [Ca2+]i in HUVEC in response to 5-HT, simultaneous activation of 5-HT1BR and 5-HT2BR is required. 5-HT causes an increase in [Ca2+]i via 5-HT2BR while 5-HT1BR could be activated by the membrane-permeable agonist CGS12066B. We hypothesized that CGS12066B acts via intracellular 5-HT1BR inaccessible to extracellular 5-HT. Intracellular 5-HT1BR might be activated by 5-HT which could be accumulated in EC under certain pathological conditions.

Merkel-like basal cells in the nasal septal island of dromedaries: Ultrastructure and possible functions

Unlike other Merkel cell types, the morphology and functions of the Merkel-like basal cells remain unclear. The aim of the present study was to investigate the ultrastructural features of Merkel-like basal cells in the nasal septal island (NSI) of dromedaries (Camelus dromedarius) using transmission electron microscopy and to speculate their potential functions. Ten pairs of nasal septal islands obtained from ten heads of dromedary camels were used for the current study. Interestingly, these cells have been identified in the basal layer of the neuroepithelium of the dromedary nasal septal island near the sensory nerve endings. These cells were ovoid to elliptical in shape and rested on the basal lamina. Their surface had spine like cytoplasmic processes which interwined with the adjacent basal cells. Their nuclei were large lobulated with 2-3 deep notches. Moreover, numerous dense-core granules surrounded by electron-lucent halo were aggregated in the basal portion of the cells close to the nerve ending as well as melanin pigments in the apical portion. The ultrastructural characteristics of the Merkel-like basal cells of NSI were typical to those of Merkel cells, but with some morphological differences, including their location, cellular attachments, and connections to other structures. The potential functions were discussed in the light of the cellular context and architecture. The Merkel-like basal cells of the NSI neuroepithelium might play a role in nociception and magnetoreception in dromedaries.

Chemical and electrical synaptic interactions among taste bud cells

Chemical synapses between taste cells were first proposed based on electron microscopy of fish taste buds. Subsequently, researchers found considerable evidence for electrical coupling in fish, amphibian, and possibly mammalian taste buds. The development lingual slice and isolated cell preparations allowed detailed investigations of cell-cell interactions, both chemical and electrical, in taste buds. The identification of serotonin and ATP as taste neurotransmitters focused attention onto chemical synaptic interactions between taste cells and research on electrical coupling faded. Findings from Ca²⁺ imaging, electrophysiology, and molecular biology indicate that several neurotransmitters, including ATP, serotonin, GABA, acetylcholine, and norepinephrine, are secreted by taste cells and exert paracrine interactions in taste buds. Most work has been done on interactions between Type II and Type III taste cells. This brief review follows the trail of studies on cell-cell interactions in taste buds, from the initial ultrastructural observations to the most recent optogenetic manipulations.

FGF- and SHH-based molecular signals regulate barbel and craniofacial development in catfish

BACKGROUND

Catfish (Siluriformes) are characterized by unique morphologies, including enlarged jaws with movable barbels and taste buds covering the entire body surface. Evolution of these characteristics was a crucial step in their adaptive radiation to freshwater environments. However, the developmental processes of the catfish craniofacial region and taste buds remain to be elucidated; moreover, little is known about the molecular mechanisms underlying the morphogenesis of these structures.

RESULTS

In Amur catfish (Silurus asotus), three pairs of barbel primordia are formed by 2 days post-fertilization (dpf). Innervation of the peripheral nerves and formation of muscle precursors are also established during early development. Taste buds from the oral region to the body trunk are formed by 4 dpf. We then isolated catfish cognates Shh (SaShh) and Fgf8 (SaFgf8), which are expressed in maxillary barbel primordium at 1-2 dpf. Further, SHH signal inhibition induces reduction of mandibular barbels with abnormal morphology of skeletal elements, whereas it causes no apparent abnormality in the trigeminal and facial nerve morphology. We also found that mandibular barbel lengths and number of taste buds are reduced by FGF inhibition, as seen in SHH signal inhibition. However, unlike with SHH inhibition, the abnormal morphology of the trigeminal and facial nerves was observed in FGF signal-inhibited embryos.

CONCLUSION

The developmental processes of Amur catfish are consistent with those reported for other catfish species. Thus, developmental aspects of craniofacial structures and taste buds may be conserved in Siluriformes. Our findings also suggest that SHH signaling plays a crucial role in the formation of barbels and taste buds, without affecting nerve projection, while FGF signaling is required for the development of barbels, taste buds, and branchial nerves. Thus, SHH and FGF signaling plays key roles in the ontogenesis and evolution of some catfish-specific characteristics.

Developing a sense of taste

Taste buds are found in a distributed array on the tongue surface, and are innervated by cranial nerves that convey taste information to the brain. For nearly a century, taste buds were thought to be induced by nerves late in embryonic development. However, this view has shifted dramatically. A host of studies now indicate that taste bud development is initiated and proceeds via processes that are nerve-independent, occur long before birth, and governed by cellular and molecular mechanisms intrinsic to the developing tongue. Here we review the state of our understanding of the molecular and cellular regulation of taste bud development, incorporating important new data obtained through the use of two powerful genetic systems, mouse and zebrafish.

Confocal imaging of Merkel-like basal cells in the taste buds of zebrafish

The oropharyngeal cavity in fish supports a range of sensory modalities, including detection of chemical and mechanical stimuli. Taste buds are found throughout this tissue and may participate in both processes. We used confocal microscopy and immunohistochemistry to characterize the morphology of Merkel-like cells and their association with other cell types and nerve fibers of the taste bud in the vertebrate model, the zebrafish. In addition, we document procedures for the observation of these structures in whole-tissue preparations from larvae and adults using zebrafish-specific and monoclonal antibodies. A single microvillus Merkel-like cell was found in each taste bud regardless of age or location. Merkel-like cells were neurosecretory, as indicated by labelling with the styryl dye, FM1-43, and the synaptic vesicle marker, SV2. Merkel-like cells were associated with SV2- and calretinin-positive taste receptor cells, received innervation from discoid aggregations of nerve fibers, and retained serotonin-filled synaptic vesicles oriented within the cytoplasm toward adjacent innervation. Moreover, a ring-like formation of nerve endings was identified with the neuronal marker, zn-12 that circumscribed the taste receptor area, surrounding calretinin-immunoreactive taste cell microvilli, and appeared to associate with the nerve plexus adjacent to Merkel-like cells. We suggest that these nerve fibers are somatosensory, perhaps associated with mechanoreception or the common chemical sense.

Fgf signaling controls pharyngeal taste bud formation through miR-200 and Delta-Notch activity

Taste buds, the taste sensory organs, are conserved in vertebrates and composed of distinct cell types, including taste receptor, basal/presynaptic and support cells. Here, we characterize zebrafish taste bud development and show that compromised Fgf signaling in the larva results in taste bud reduction and disorganization. We determine that Fgf activity is required within pharyngeal endoderm for formation of Calb2b(+) cells and reveal miR-200 and Delta-Notch signaling as key factors in this process. miR-200 knock down shows that miR-200 activity is required for taste bud formation and in particular for Calb2b(+) cell formation. Compromised delta activity in mib(-/-) dramatically reduces the number of Calb2b(+) cells and increases the number of 5HT(+) cells. Conversely, larvae with increased Notch activity and ascl1a(-/-) mutants are devoid of 5HT(+) cells, but have maintained and increased Calb2b(+) cells, respectively. These results show that Delta-Notch signaling is required for intact taste bud organ formation. Consistent with this, Notch activity restores Calb2b(+) cell formation in pharyngeal endoderm with compromised Fgf signaling, but fails to restore the formation of these cells after miR-200 knock down. Altogether, this study provides genetic evidence that supports a novel model where Fgf regulates Delta-Notch signaling, and subsequently miR-200 activity, in order to promote taste bud cell type differentiation.

Taste bud development and patterning in sighted and blind morphs of Astyanax mexicanus

In the blind cave-dwelling morph of A. mexicanus, the eye degenerates while other sensory systems, such as gustation, are expanded compared to their sighted (surface-dwelling) ancestor. This study compares the development of taste buds along the jaws of each morph. To determine whether cavefish have an altered onset or rate of taste bud development, we fluorescently labeled basal and receptor cells within taste buds over a developmental series. Our results show that taste bud number increases during development in both morphs. The rate of development is, however, accelerated in cavefish; a small difference in taste bud number exists at 5 dpf reaching threefold by 22 dpf. The expansion of taste buds in cavefish is, therefore, detectable after the onset of eye degeneration. This study provides important insights into the timing of taste bud expansion in cavefish as well as enhances our understanding of taste bud development in teleosts in general.

Neurochemical characterization of sea lamprey taste buds and afferent gustatory fibers: presence of serotonin, calretinin, and CGRP immunoreactivity in taste bud bi-ciliated cells of the earliest vertebrates

Neuroactive substances such as serotonin and other monoamines have been suggested to be involved in the transmission of gustatory signals from taste bud cells to afferent fibers. Lampreys are the earliest vertebrates that possess taste buds, although these differ in structure from taste buds in jawed vertebrates, and their neurochemistry remains unknown. We used immunofluorescence methods with antibodies raised against serotonin, tyrosine hydroxylase (TH), gamma-aminobutyric acid (GABA), glutamate, calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), calretinin, and acetylated alpha-tubulin to characterize the neurochemistry and innervation of taste buds in the sea lamprey, Petromyzon marinus L. For localization of proliferative cells in taste buds we used bromodeoxyuridine labeling and proliferating cell nuclear antigen immunohistochemistry. Results with both markers indicate that proliferating cells are restricted to a few basal cells and that almost all cells in taste buds are nonproliferating. A large number of serotonin-, calretinin-, and CGRP-immunoreactive bi-ciliated cells were revealed in lamprey taste buds. This suggests that serotonin participates in the transmission of gustatory signals and indicates that this substance appeared early on in vertebrate evolution. The basal surface of the bi-ciliated taste bud cells was contacted by tubulin-immunoreactive fibers. Some of the fibers surrounding the taste bud were calretinin immunoreactive. Lamprey taste bud cells or afferent fibers did not exhibit TH, GABA, glutamate, or NPY immunoreactivity, which suggests that expression of these substances evolved in taste buds of some gnathostomes lines after the separation of gnathostomes and lampreys.

Cellular mechanisms in taste buds

In the soft palate, tongue, pharynx and larynx surrounding the oral region, taste buds are present, allowing the sensation of taste. On the tongue surface, 3 kinds of papillae are present: fungiform, foliate, and circumvallate. Approximately 5,000 taste buds cover the surface of the human tongue, with about 30% fungiform, 30% foliate and 40% circumvallate papillae. Each taste bud comprises 4 kinds of cells, namely high dark (type I), low light (type II), and intermediate (type III) cells in electron density and Merkel-like taste basal cells (type IV) located at a distance from taste pores. Type II cells sense taste stimuli and type III cells transmit taste signals to sensory afferent nerve fibers. However, type I and type IV cells are not considered to possess obvious taste functions. Synaptic interactions that mediate communication in taste cells provide signal outputs to primary afferent fibers. In the study of taste bud cells, molecular functional techniques using single cells have recently been applied. Serotonin (5-HT) plays a role in cell-to-cell transmission of taste signals. ATP fills the criterion of a neurotransmitter that activates receptors of taste nerve fibers. Findings on 5-HT and ATP suggest that various different transmitters and receptors are present in taste buds. However, no firm evidence for taste-evoked release from type III cells has been identified, except for 5-HT and ATP. These results suggest that different transmitters and receptors may not be present in taste buds. Accordingly, an understanding of how transmitters might function remains elusive.

Cell communication in taste buds

Taste bud cells communicate with sensory afferent fibers and may also exchange information with adjacent cells. Indeed, communication between taste cells via conventional and/or novel synaptic interactions may occur prior to signal output to primary afferent fibers. This review discusses synaptic processing in taste buds and summarizes results showing that it is now possible to measure real-time release of synaptic transmitters during taste stimulation using cellular biosensors. There is strong evidence that serotonin and ATP play a role in cell-to-cell signaling and sensory output in the gustatory end organs.

Mouse taste buds use serotonin as a neurotransmitter

Synapses between gustatory receptor cells and primary sensory afferent fibers transmit the output signal from taste buds to the CNS. Several transmitter candidates have been proposed for these synapses, including serotonin (5-HT), glutamate, acetylcholine, ATP, peptides, and others, but, to date, none has been unambiguously identified. We used Chinese hamster ovary cells stably expressing 5-HT2C receptors as biodetectors to monitor 5-HT release from taste buds. When taste buds were depolarized with KCl or stimulated with bitter, sweet, or sour (acid) tastants, serotonin was released. KCl- and acid-induced 5-HT release, but not release attributable to sweet or bitter stimulation, required Ca2+ influx. In contrast, 5-HT release evoked by sweet and bitter stimulation seemed to be triggered by intracellular Ca2+ release. These experiments strongly implicate serotonin as a taste bud neurotransmitter and reveal unexpected transmitter release mechanisms.

Morphological and biochemical heterogeneity in facial and vagal nerve innervated taste buds of the channel catfish, Ictalurus punctatus

In catfish, the facial nerve innervates taste buds distributed over the entire body including the barbels, while the glossopharyngeal and vagal nerves innervate oropharyngeal taste buds. Facial nerve innervated taste buds (FITBs) are thought to be involved in food detection and localization, while glossopharyngeal and vagal nerve innervated taste buds (VITBs) evaluate the palatability of food prior to ingestion. Physiological studies indicate that both oral and extra-oral taste buds detect sapid substances such as amino acids and nucleotides, but the facial taste system is more sensitive to some of these substances. The anatomical, molecular, and/or physiological mechanisms underlying the functional differences in these two gustatory pathways remain to be identified. In the current investigation we compare the basic morphological features of FITBs and VITBs and the distribution of the following metabolites: gamma-aminobutyric acid (GABA), glutamate, aspartate, alanine, taurine, and glutathione. Vagal innervated taste buds are significantly longer and narrower than FITBs, with fewer taste cells and a smaller nerve plexus. Each of the metabolites examined was heterogeneously distributed in taste cells with notably more GABA positive cells present in the VITBs. Patterns of metabolite colocalization suggest the presence of several taste cell subtypes. The morphological and metabolite differences noted between FITBs and VITBs provide a potential anatomical basis for the previously noted differences in physiological sensitivity.

A paracrine signaling role for serotonin in rat taste buds: expression and localization of serotonin receptor subtypes

Recent advances in peripheral taste physiology now suggest that the classic linear view of information processing within the taste bud is inadequate and that paracrine processing, although undemonstrated, may be an essential feature of peripheral gustatory transduction. Taste receptor cells (TRCs) express multiple neurotransmitters of unknown function that could potentially participate in a paracrine role. Serotonin is expressed in a subset of TRCs with afferent synapses; additionally, TRCs respond physiologically to serotonin. This study explored the expression and cellular localization of serotonin receptor subtypes in TRCs as a possible route of paracrine communication. RT-PCR was performed on RNA extracted from rat posterior taste buds with 14 primer sets representing 5-HT1 through 5-HT7 receptor subtype families. Data suggest that 5-HT1A and 5-HT3 receptors are expressed in taste buds. Immunocytochemistry with a 5-HT1A-specific antibody demonstrated that subsets of TRCs were immunopositive for 5-HT1A. With the use of double-labeling, serotonin- and 5-HT1A-immunopositive cells were observed exclusively in nonoverlapping populations. On the other hand, 5-HT3-immunopositive taste receptor cells were not observed. This observation, combined with other data, suggests 5-HT3 is expressed in postsynaptic neural elements within the bud. We hypothesize that 5-HT release from TRCs activates postsynaptic 5-HT3 receptors on afferent nerve fibers and, via a paracrine route, inhibits neighboring TRCs via 5-HT1A receptors. The role of the 5-HT1A-expressing TRC within the taste bud remains to be explored.

Serotonin differentially modulates the electrical properties of different subsets of taste receptor cells in bullfrog

Serotonin (5-hydroxytryptamin, 5-HT) is localized in taste bud cells of vertebrates. Effects of the external application of 5-HT on the membrane currents of frog taste receptor cells (TRCs) were investigated using patch-clamp technique in whole-cell configuration. The 5-HT (0.1-1 micro m) and 5-HT1A receptor agonist (+/-)-8-OH-2-(D1-n-propyl-amino)tetralin (8-OH-DPAT) (1-20 micro m) inhibited both voltage-gated sodium current (INa) and voltage-gated potassium current (IK) in 50% of TRCs, but potentiated IK without any significant effect on INa in another subset of 18% of TRCs. Voltage-gated currents in the residual TRCs were not affected by 5-HT or 8-OH-DPAT. External application of 10 micro m forskolin and 300 micro m 8-cpt cAMP [8-(4-chlorophenylthio)adenosine 3':5'-cyclic monophosphate] mimicked the inhibitory effect of 5-HT and 8-OH-DPAT on IK and INa while internal dialysis with 50 micro m protein kinase A inhibitor prevented the 5-HT-mediated inhibitory effects on IK and INa in TRCs. Internal dialysis of TRCs with high Ca2+-pipette solution (1 micro m) increased the IK in 58% of TRCs. The 5-HT reversibly increased the [Ca2+]i in 17% of TRCs when measured by Ca2+-imaging using a Ca2+-sensitive dye (fura-2 AM). These results suggest that 5-HT differentially modulates the voltage-gated membrane currents in different subsets of TRCs.

Recent progress in studies on Merkel cell biology

Merkel cells ubiquitously distribute in the skin of vertebrates, from cyclostomes to mammals. It is well known that mammalian Merkel cells coupled with axon terminals of type I sensory nerve fibers form slowly adapting mechanoreceptors, Merkel endings, within the epidermis. However, there are still many unresolved problems in the biology of Merkel cells. We reviewed recently acquired knowledge about the histochemical nature of Merkel cell granules, the morphological heterogeneity of Merkel cells and the roles of neurotrophins and their receptors for the development and survival of the cells. We discuss the functional significance of Merkel cell granules and the heterogeneity of Merkel cell populations.

Electrophysiology of Necturus taste cells

Taste buds are sensory end organs that detect chemical substances occurring in foodstuffs and relay the relative information to the brain. The mechanisms by which the chemical stimuli are converted into biological signals represent a central issue in taste research. Our understanding of how taste buds accomplish this operation relies on the detailed knowledge of the biological properties of taste bud cells-the taste cells-and of the functional processes occurring in these cells during chemostimulation. The amphibian Necturus maculosus (mudpuppy) has proven to be a very useful model for studying basic cellular processes of vertebrate taste reception, some of which are still awaiting to be explored in mammals. The main advantages offered by Necturus are the large size of its taste cells and the relative accessibility of its taste buds, which can therefore be handled easily for experimental manipulations. In this review, I summarize the functional properties of Necturus taste cells studied with electrophysiological techniques (intracellular recordings and patch-clamp recordings). My focus is on ion channels in taste cells and on their role in signal transduction, as well as on the functional relationships among the cells inside Necturus taste buds. This information has revealed to be well suited to outline some of the general physiological processes occurring during taste reception in vertebrates, including mammals, and may represent a useful framework for understanding how taste buds work.

Serotonin inhibits voltage-gated sodium current by cyclic adenosine monophosphate-dependent mechanism in bullfrog taste receptor cells

We have investigated the effect of 5-hydroxytryptamine (serotonin) (5-HT) on the membrane properties of bullfrog taste receptor cells (TRCs) using patch-clamp technique. External application of 5-HT reversibly suppressed the voltage-gated Na(+) current (I(Na)) in about half of the TRCs sampled. The magnitude of suppression of peak I(Na) was dependent on the holding potential of the cell. Forskolin and cyclic adenosine monophosphate (cAMP) mimicked the suppressive effect of 5-HT on I(Na), but an internal protein kinase A-inhibitor potentiated I(Na). These results suggest that 5-HT suppresses I(Na) of bullfrog TRCs via protein kinase A-dependent phosphorylation, resulting in suppression of the excitability of bullfrog TRCs.

Cellular mechanisms of taste transduction

Taste receptor cells respond to gustatory stimuli using a complex arrangement of receptor molecules, signaling cascades, and ion channels. When stimulated, these cells produce action potentials that result in the release of neurotransmitter onto an afferent nerve fiber that in turn relays the identity and intensity of the gustatory stimuli to the brain. A variety of mechanisms are used in transducing the four primary tastes. Direct interaction of the stimuli with ion channels appears to be of particular importance in transducing stimuli reported as salty or sour, whereas the second messenger systems cyclic AMP and inositol trisphosphate are important in transducing bitter and sweet stimuli. In addition to the four basic tastes, specific mechanisms exist for the amino acid glutamate, which is sometimes termed the fifth primary taste, and for fatty acids, a so-called nonconventional taste stimulus. The emerging picture is that not only do individual taste qualities use more than one mechanism, but multiple pathways are available for individual tastants as well.

Merkel cells are responsible for the initiation of taste organ morphogenesis in the frog

Taste organs in the frog have a distinctive cell type located exclusively in the basal portion. In the same fashion as type III cells in mammalian taste buds, these basal cells show immunoreactivity for serotonin antibody. Further, these cells are morphologically similar to epidermal Merkel cells. To determine the significance of these serotonergic basal cells, we examined the early development of taste organs during metamorphosis of the frog by focusing on the origin and possible roles of serotonergic basal cells. For convenience of description, five stages of development (metamorphic stage to climax stages A-D) are defined. In the metamorphic stage, a few noninnervated Merkel cells appear at the upper layer of the lingual epithelium. No neuronal elements are seen in the epithelium at this stage. At climax stages A-B, immature fungiform papillae become discernible in the dorsal surface of the tongue, where the Merkel cells are located. Merkel cells then move downward and extend their cytoplasmic processes toward the basal lamina. These cells are identified by their intense immunoreactivity for serotonin. During the later stages, many nerve fibers in the subepithelial connective tissue approach the epithelium containing Merkel cells. At climax stages C-D, Merkel cells extend cytoplasmic processes along the basal lamina toward the center of the newly forming fungiform papillae. The morphology of these Merkel cells exactly coincides with that of serotonergic basal cells in adult taste organs. Profuse exocytotic release of dense-cored granules of Merkel cells toward the nerve fibers through the basal lamina is frequently seen in these stages. The present study indicates that serotonergic basal cells are derived from intraepithelial Merkel cells, which act as target sites for growing nerves and may be responsible for the initiation of taste organ morphogenesis.

Serotonin transporter messenger RNA expression in neural crest-derived structures and sensory pathways of the developing rat embryo

A growing body of evidence suggests that serotonin plays an important role in the early development of both neural and non-neural tissues from vertebrate and invertebrate species. Serotonin is removed from the extracellular space by the cocaine- and antidepressant-sensitive serotonin transporter, thereby limiting its action on receptors. In situ hybridization histochemistry was used to delineate serotonin transporter messenger RNA expression during rat embryonic development. Serotonin transporter messenger RNA was widely expressed beginning prior to organogenesis and throughout the second half of gestation. Strikingly, serotonin transporter messenger RNA was detected in neural crest cells, some of which respond to serotonin in vitro, and neural crest-derived tissues, such as autonomic ganglia, tooth primordia, adrenal medulla, chondrocytes and neuroepithelial cells, in the skin, heart, intestine and lung. Within the peripheral sensory pathways, two major cells types were serotonin transporter messenger RNA-positive: (i) sensory ganglionic neurons and (ii) neuroepithelial cells which serve as targets for the outgrowing sensory neurons. Several sensory organs (cochlear and retinal ganglionic cells, taste buds, whisker and hair follicles) contained serotonin transporter messenger RNA by late gestation. The expression of serotonin transporter messenger RNA throughout the sensory pathways from central nervous system relay stations [Hansson S. R. et al. (1997) Neuroscience 83, 1185-1201; Lebrand C. et al. (1996) Neuron 17, 823-835] to sensory nerves and target organs as shown in this study suggests that serotonin may regulate peripheral synaptogenesis, and thereby influence later processing of sensory stimuli. If the early detection of serotonin transporter messenger RNA in skin and gastrointestinal and airway epithelia correlates with protein activity, it may permit establishment of a serotonin concentration gradient across epithelia, either from serotonin in the amniotic fluid or from neuronal enteric serotonin, as a developmental cue. Our results demonstrating serotonin transporter messenger RNA in the craniofacial and cardiac areas identify this gene product as the transporter most likely responsible for the previously identified accumulation of serotonin in skin and tooth germ [Lauder J. M. and Zimmerman E. F. (1988) J. craniofac. Genet. devl Biol. 8, 265-276], and the fluoxetine-sensitive effects on craniofacial [Lauder J. M. et al. (1988) Development 102, 709-720; Shuey D. L. et al. (1992) Teratology 46, 367-378; Shuey D. L. et al. (1993) Anat. Embryol., Berlin 187, 75-85] and cardiac [Kirby M. L. and Waldo K. L. (1995) Circulation Res. 77, 211-215; Yavarone M. S. et al. (1993) Teratology 47, 573-584] malformations. Serotonin transporter messenger RNA was detected in several neural crest cell lineages and may be useful as an early marker for the sensory lineage in particular. The distribution of serotonin transporter messenger RNA in early development supports the hypothesis that serotonin may play a role in neural crest cell migration and differentiation [Lauder J. M. (1993) Trends Neurosci. 16, 233-240], and that the morphogenetic actions of serotonin may be regulated by transport. The striking pattern of serotonin transporter messenger RNA throughout developing sensory pathways suggests that serotonin may play a role in establishing patterns of connectivity critical to processing sensory stimuli. As a target for drugs, such as cocaine, amphetamine derivatives and antidepressants, expression of serotonin transporter during development may reflect critical periods of vulnerability for fetal drug exposure. The widespread distribution of serotonin transporter messenger RNA during ontogeny suggests a previously unappreciated role of serotonin in diverse physiological systems during embryonic development.

Possible role of serotonin in Merkel-like basal cells of the taste buds of the frog, Rana nigromaculata

Merkel-like basal cells in the taste buds of the frog were examined by fluorescence histochemistry, immunohistochemistry and electron microscopy. There were about 16-20 basal cells arranged in a radial fashion at the base of each taste bud. These cells were strongly immunopositive for serotonin antiserum. They were characterised by the presence of numerous dense-cored granules in the cytoplasm ranging from 80 to 120 nm in diameter, and of microvilli protruding from the cell surface. For 4 mo after sensory denervation by cutting the gustatory nerves, all cell types of the taste bud were well preserved and maintained their fine structure. Even at 4 mo after denervation, the basal cells exhibited a strong immunoreaction with serotonin antiserum. To investigate the function of serotonin in the basal cells in taste bud function, serotonin deficiency was induced by administration of p-chlorophenylalanine (PCPA), an inhibitor of tryptophan hydroxylase, and of p-chloroamphetamine (PCA), a depletor of serotonin. After administration of these agents to normal and denervated frogs for 2 wk, a marked decrease, or complete absence, of immunoreactivity for serotonin was observed in the basal cells. Ultrastructurally, degenerative changes were observed in both types of frog; numerous lysosome-like myelin bodies were found in all cell types of the taste buds. The number of dense-cored granules in the basal cells also was greatly decreased by treatment with these drugs. Serotonin in Merkel-like basal cells appears to have a trophic role in maintenance of the morphological integrity of frog taste bud cells.

Roles of chemical mediators in the taste system

Recent advances in neural mechanisms of taste are reviewed with special reference to neuroactive substances. In the first section, taste transduction mechanisms of basic tastes are explained in two groups, whether taste stimuli directly activate ion channels in the taste cell membrane or they bind to cell surface receptors coupled to intracellular signaling pathways. In the second section, putative transmitters and modulators from taste cells to afferent nerves are summarized. The candidates include acetylcholine, catecholamines, serotonin, amino acids and peptides. Studies favor serotonin as a possible neuromodulator in the taste bud. In the third section, the role of neuroactive substances in the central gustatory pathways is introduced. Excitatory and inhibitory amino acids (e.g., glutamate and GABA) and peptides (substance P and calcitonin gene-related peptide) are proved to play roles in transmission of taste information in both the brainstem relay and cortical gustatory area. In the fourth section, conditioned taste aversion is introduced as a model to study gustatory learning and memory. Pharmacobehavioral studies to examine the effects of glutamate receptor antagonists and protein kinase C inhibitors on the formation of conditioned taste aversion show that both glutamate and protein kinase C in the amygdala and cortical gustatory area play essential roles in taste aversion learning. Recent molecular and genetic approaches to disclose biological mechanisms of gustatory learning are also introduced. In the last section, behavioral and pharmacological approaches to elucidate palatability, taste pleasure, are described. Dopamine, benzodiazepine derivatives and opioid substances may play some roles in evaluation of palatability and motivation to ingest palatable edibles.

Serotonin inhibits calcium-activated K+ current in rat taste receptor cells

Little is definitively known of the identity or actions of neurotransmitters utilized within mammalian taste buds. Serotonin has been immunocytochemically localized to taste cells of several species but its physiological actions are unknown. Using whole-cell patch clamp recordings on dissociated posterior rat taste cells, data are presented to suggest that exogenously applied serotonin inhibits a calcium-activated potassium current by up to 50%. This current, best visualized at depolarized holding potentials, is both apamin- and charybdotoxin-sensitive. Approximately 60% of the tested taste cells were serotonin sensitive. This inhibition was mimicked by N-(3-trifluoromethylphenyl)piperazine (TFMPP), a general serotonin receptor agonist, by 8-hydroxy-dipropylaminotetralin (8-OH-DPAT), a selective 5-HT1A receptor agonist, but not by phenylbiguanide, a 5-HT3 receptor agonist. These are the first data to establish a physiological effect of serotonin on mammalian taste cells.

GABAergic neurotransmission in rat taste buds: immunocytochemical study for GABA and GABA transporter subtypes

Gamma-aminobutyric acid (GABA) is known to be a candidate for the neurotransmitter involved in the sense of taste. We hereby studied GABA and its termination system, GABA transporters, in rat taste buds by immunocytochemical approaches. Immunoblot analysis of three GABA transporter subtypes (GAT1, GAT2 and GAT3) revealed that the immunoreactive bands of GAT2 and GAT3, but not GAT1, were detected in the tongue. GAT3-immunoreactive band was recognized only in the circumvallate papilla containing a large number of taste buds while GAT2-immunoreactive bands were seen in all areas of the tongue. GAT2 immunoreactivity appeared to be specifically in the nerve fibers beneath the lingual epithelium. Both GAT3 and GABA immunoreactivities were detected only in taste buds. A few GAT3-immunoreactive cells were found in a cross-section of each taste bud but most GAT3-immunoreactive cells were localized in the margin of the taste bud. GAT3 was predominantly concentrated in the distal portion of the GAT3-immunoreactive cells. In contrast, GABA-immunoreactive cells were seen more frequently within each taste bud and the immunoreactivity was distributed throughout the perikarya of the cells. These results suggest that the GABA-uptake system is present in the taste buds and the GABAergic neurotransmission involved in the sensation of taste is terminated by the uptake of GABA into certain taste cells via GAT3.

Serotonin modulates voltage-dependent calcium current in Necturus taste cells

Necturus taste buds contain two primary cell types: taste receptor cells and basal cells. Merkel-like basal cells are a subset of basal cells that form chemical synapses with taste receptor cells and with innervating nerve fibers. Although Merkel-like basal cells cannot interact directly with taste stimuli, recent studies have shown that Merkel-like basal cells contain serotonin (5-HT), which may be released onto taste receptor cells in response to taste stimulation. With the use of whole cell voltage clamp, we examined whether focal applications of 5-HT to isolated taste receptor cells affected voltage-activated calcium current (I(Ca)). Two different effects were observed. 5-HT at 100 microM increased I(Ca) in 33% of taste receptor cells, whereas it decreased I(Ca) in 67%. Both responses used a 5-HT receptor subtype with a pharmacological profile similar to that of the 5-HT1A receptor, but the potentiation and inhibition of I(Ca) by 5-HT were mediated by two different second-messenger cascades. The results indicate that functional subtypes of taste receptor cells, earlier defined only by their sensitivity to taste stimuli, may also be defined by their response to the neurotransmitter 5-HT and suggest that 5-HT released by Merkel-like basal cells could modulate taste receptor function.

Taste buds develop autonomously from endoderm without induction by cephalic neural crest or paraxial mesoderm

Although it had long been believed that embryonic taste buds in vertebrates were induced to differentiate by ingrowing nerve fibers, we and others have recently shown that embryonic taste buds can develop normally in the complete absence of innervation. This leads to the question of which tissues, if any, induce the formation of taste buds in oropharyngeal endoderm. We proposed that taste buds, like many specialized epithelial cells, might arise via an inductive interaction between the endodermal epithelial cells that line the oropharynx and the adjacent mesenchyme that is derived from both cephalic neural crest and paraxial mesoderm. Using complementary grafting and explant culture techniques, however, we have now found that well-differentiated taste buds will develop in tissue completely devoid of neural crest and paraxial mesoderm derivatives. When the presumptive oropharyngeal region was removed from salamander embryos prior to the onset of cephalic neural crest migration, taste buds developed in grafts and explants coincident with their appearance in intact control embryos. Similarly, explants from neurulae in which movement of paraxial mesoderm had not yet begun also developed taste buds after 9–12 days in vitro. We conclude that neither cranial neural crest nor paraxial mesoderm is responsible for the induction of embryonic taste buds. Surprisingly, the ability to develop taste buds late in embryonic development seems to be an intrinsic feature of the oropharyngeal endoderm that is determined by the completion of gastrulation.

Measurement of membrane potential and [Ca2+]i in cell ensembles: application to the study of glutamate taste in mice

We have studied the spectral properties of the voltage-sensitive dye, 1-(3-sulfonatopropyl)-4-[beta [2-(di-n-octylamino)-6-naphtyl]vinyl] pyridinium betaine (di-8-ANEPPS), and the Ca(2+)-sensitive dye, fura-2, in azolectin liposomes and in isolated taste buds from mouse. We find that the fluorescence excitation spectra of di-8-ANEPPS and fura-2 are largely nonoverlapping, allowing alternate ratio measurements of membrane potential and intracellular calcium ([Ca2+]i). There is a small spillover of di-8-ANEPPS fluorescence at the excitation wavelengths used for fura-2 (340 and 360 nm). However, voltage-induced changes in the fluorescence of di-8-ANEPPS, excited at the fura-2 wavelengths, are small. In addition, di-8-ANEPPS fluorescence is localized to the membrane, whereas fura-2 fluorescence is distributed throughout the cytoplasm. Because of this, the effect of spillover of di-8-ANEPPS fluorescence in the [Ca2+]i estimate is < 1%, under the appropriate conditions. We have applied this method to study of the responses of multiple taste cells within isolated taste buds. We show that membrane potential and [Ca2+]i can be measured alternately in isolated taste buds from mouse. Stimulation with glutamate and glutamate analogs indicates that taste cells express both metabotropic and ionotropic receptors. The data suggest that the receptors responding to 2-amino-4-phosphonobutyrate (L-AP4), presumably metabotropic L-glutamate receptors, do not mediate excitatory glutamate taste responses.

Embryonic taste buds develop in the absence of innervation

It has been hypothesized that taste buds are induced by contact with developing cranial nerve fibers late in embryonic development, since descriptive studies indicate that during embryonic development taste cell differentiation occurs concomitantly with or slightly following the advent of innervation. However, experimental evidence delineating the role of innervation in taste bud development is sparse and equivocal. Using two complementary experimental approaches, we demonstrate that taste cells differentiate fully in the complete absence of innervation. When the presumptive oropharyngeal region was taken from a donor axolotl embryo, prior to its innervation and development of taste buds, and grafted ectopically on to the trunk of a host embryo, the graft developed well-differentiated taste buds. Although grafts were invaded by branches of local spinal nerves, these neurites were rarely found near ectopic taste cells. When the oropharyngeal region was raised in culture, numerous taste buds were generated in the complete absence of neural elements. Taste buds in grafts and in explants were identical to those found in situ both in terms of their morphology and their expression of calretinin and serotonin immunoreactivity. Our findings indicate that innervation is not necessary for complete differentiation of taste receptor cells. We propose that taste buds are either induced in response to signals from other tissues, such as the neural crest, or arise independently through intrinsic patterning of the local epithelium.

Development of voltage-dependent currents in taste receptor cells

Taste buds, the specialized end organs of gustation, comprise a renewing sensory epithelium. Undifferentiated basal cells become taste receptor cells by elongating and extending processes apically toward the taste pore. Mature taste cells are electrically excitable and express voltage-dependent Na+ Ca2+, and K+ currents, whereas basal stem cells exhibit only slowly activating K+ currents. The question we have addressed in the present study is whether contact with the taste pore is required for expression of voltage-dependent inward currents in Necturus taste cells. Mature taste cells were distinguished from developing cells by labeling the apical surface of the cells with fluorescein-isothiocyanate-conjugated wheat germ agglutinin (FITC-WGA), while the tissue was still intact. Elongate cells lacking FITC-WGA staining were interpreted as developing taste cells that had not yet reached the taste pore. Giga-seal whole-cell recording revealed that most developing taste cells lacked inward currents. Although some developing cells expressed inward currents, they were much smaller than those of mature cells, and the activation kinetics of the K+ currents were slower than in mature cells. Electron microscopy confirmed the identity of labeled and unlabeled cells. All FITC-WGA-labeled cells exhibited the ultrastructural characteristics of mature taste receptor cells, whereas most unlabeled taste cells had a characteristic morphology that was markedly different from mature taste receptor cells or basal stem cells. These data suggest that contact with the taste pore is required for the development of inward currents in taste cells.

The frog taste disc: a prototype of the vertebrate gustatory organ

The frog taste disc (TD) is apparently the largest gustatory organ found in vertebrates and seems to differentiate into a specialized variety of the prototypic scheme of the taste bud. An explanation for this unusual organization is lacking although it is possible to speculate the existence of environmental and nutritional requirements. Up to the present time, the most common model of the TD was based on two main cell types (sensory and sustentacular). This model may oversimplify the morphology of this structure since more numerous cell types have been described. We now propose a new model of the TD, based on comprehensive data on the ultrastructure of the organ obtained in the last 20 years. The main conclusions are the following: (1) the TD is a pluristratified epithelium with a general organization similar to that of the olfactory and vomeronasal epithelium; (2) it has skeleton composed of three different types of epithelial cells; (3) the chemoreceptorial surface is covered by different microenvironments; (4) three different types of neuro-epithelial systems are present; the type II is an 'open' sensory cell with axonal contacts devoid of vesicles; the type III is an 'open' sensory cell with synaptic-like junctions; the type i.v. is a 'closed' sensory cell with a 'Merkel-neurite complex'; (5) the nerve fibers in the basal plexus are mostly cholinergic while the peridiscal nerve fibers are mostly peptidergic. The presence of several cell types in the TD must be considered using these large receptors in electrophysiological studies or as a source of isolated cells, and their complexity must induce caution in the interpretation of the data. Text books of histology usually describe the peripheral structures associated with taste as very simple: an idea that probably must be revised. A taste organ is a highly complex structure composed of several sensory systems and a comparative approach can aid comprehension of its general organization. The study of the 'large taste organs' present in some species of amphibians can provide useful data for knowledge of the gustatory system of vertebrates.

Localization of serotonin in taste buds: a comparative study in four vertebrates

To investigate monoaminergic synaptic mechanisms in taste buds, we examined taste buds of mice, rats, rabbits, and mudpuppies for the presence of the neurotransmitter candidate, serotonin. Immunocytochemistry revealed serotonin-like immunostaining in cells in mammalian taste buds and Merkel-like basal cells in taste buds of mudpuppies. In untreated mudpuppies and in mammals injected with the precursor to serotonin, L-tryptophan, certain taste cells showed serotonin-like immunoreactivity, although in mammalian taste buds the immunostaining was relatively weak. After pretreating mammals with 5-hydroxytryptophan (5-HTP), the intermediate precursor between L-tryptophan and serotonin, several taste cells showed strong immunoreactivity for serotonin. These findings indicate that mammalian taste cells normally contain serotonin and that taste cells can take up 5-HTP and convert it to serotonin. Immunocytochemistry on wholemount preparations demonstrated that serotonergic cells of mudpuppies (i.e., Merkel-like basal cells) were disposed in a ring at the periphery of taste buds. Similarly, serotonergic cells in mammalian taste buds tended to be located at the periphery of taste buds. Based on the position of serotonergic cells in the taste bud and on recent physiological studies on the actions of serotonin in taste buds, we postulate that serotonin functions as a neuromodulator or neurotransmitter in vertebrate taste buds.