Solitary chemosensory cells — taste,common chemical sense or what?

Summary Secondary solitary chemosensory cells (SCCs) occur scattered within the epidermis of lampreys, teleosts and ranid tadpoles. Counts in representative telost species revealed that SCC's outnumber chemosensory cells organized in taste buds. Therefore, SCCs may be considered the structural substrate of a basic and probably important vertebrate chemosense. However, detailed information on structure, innervation and function is only available from specialized fins in a few teleost species, where SCCs are sufficiently concentrated. The foremost research model has been the anterior dorsal fin (ADF) in rocklings, which contains millions of SCCs but no other specialized chemosensory elements. It has been shown that these ADF-SCCs are innervated from the recurrent facial nerve. Electrophysiological recordings revealed that there is virtually no overlap in stimulus spectrum between the ADF-SCCs and pelvic fin taste buds; SCC responses could only be triggered by dilutions of heterospecific fish body mucus. Results of behavioural experiments indicate that fish mucus is indeed a relevant stimulus. Therefore it is hypothesized that the biological role of the ADF-SCCs is predator avoidance rather than search for food. Whether these findings are valid for rockings only, or can be generalized for the scattered SCC systems in more than 20000 species of fish and in some amphibians, remains an open question. Further investigations on the function and biological roles of the SCC chemosense will be crucially important to improve our understanding of sensory perception and its evolution in aquatic vertebrates.     

Cell-to-Cell Communication in the Taste Bud: ATP and Acetylcholine as Primary Mediators

It was shown that physiological processes in taste buds (peripheral sensory gustatory organs in vertebrates) are realized with the involvement of several signal systems. In these structures, a number of “classical” neurotransmitters, including glutamate, serotonin, GABA, ATP, noradrenaline, and others, as well as receptors to these agents, were identified. The physiological roles of the above systems (separate ones and all as a whole) remain, however, far from final elucidation. We studied purinergic and cholinergic systems in the taste buds. Based on the data obtained in behavioral experiments using knockout animals, which indicated that ATP is an afferent neurotransmitter, we found stimulation-induced secretion of ATP by type-II cells. The release of ATP does not require the entry of external calcium and is mediated by ion channels permeable for ATP. The obtained data allowed us to explain the fact that classical synaptic structures are absent in the type-II cells. The type-I cells coat other elements including type-II cells; they provide formation of compartments in the intercellular space of the taste buds (this limits ATP diffusion). We showed that taste cells of just type I mostly generate calcium signals in response to the action of ATP and acetylcholine. These cell responses are generated with the involvement of metabotropic purine receptors (isoforms P2Y1, P2Y2, and P2Y4) and muscarinic receptors (isoforms M1, M3, and M5), respectively. Functioning of these receptors is combined with a phosphoinositide cascade, mobilization of intracellular Ca²⁺, and subsequent activation of calcium-activated Cl^– channels. It seems probable that purinergic and cholinergic signal systems in type-I cells are elements of negative feedback in the taste buds, which promote the process of adaptation to the action of gustatory stimuli.     

Sensory structures at the surface of fish skin I. Putative chemoreceptors

Fish skin contains solitary epidermal sensory cells which, on evidence from their cytology, are believed to be chemosensory. The external appearance of the apical sensory processes of these cells, as seen by scanning electron microscopy, is shown in four species of ostariophysan teleosts, and is compared with the morphology of the pores of external taste buds. The apical processes of the gustatory cells are simple in form in all cases so far investigated in gnathostome fishes, but in some cases the solitary sensory cells have apical processes divided distally into a number of smaller processes. In the dipnoan fish Protopterus amphibius , external taste buds have simple blunt gustatory processes protruding through a cap of mucus that covers the taste bud pore. Solitary sensory cells in this species have a bulbous undivided apical process. In the lampreys, the 'end buds' have an apical morphology different from the taste bud pores of teleost fish. Lamprey epidermis has numerous solitary sensory cells each bearing a number of microvilli.     

Rat gustatory neurons in the geniculate ganglion express glutamate receptor subunits

Taste receptor cells are innervated by primary gustatory neurons that relay sensory information to the central nervous system. The transmitter(s) at synapses between taste receptor cells and primary afferent fibers is (are) not yet known. By analogy with other sensory organs, glutamate might a transmitter in taste buds. We examined the presence of AMPA and NMDA receptor subunits in rat gustatory primary neurons in the ganglion that innervates the anterior tongue (geniculate ganglion). AMPA and NMDA type subunits were immunohistochemically detected with antibodies against GluR1, GluR2, GluR2/3, GluR4 and NR1 subunits. Gustatory neurons were specifically identified by retrograde tracing with fluorogold from injections made into the anterior portion of the tongue. Most gustatory neurons in the geniculate ganglion were strongly immunoreactive for GluR2/3 (68%), GluR4 (78%) or NR1 (71%). GluR1 was seen in few cells (16%). We further examined if glutamate receptors were present in the peripheral terminals of primary gustatory neurons in taste buds. Many axonal varicosities in fungiform and vallate taste buds were immunoreactive for GluR2/3 but not for NR1. We conclude that gustatory neurons express glutamate receptors and that glutamate receptors of the AMPA type are likely targeted to synapses within taste buds.     

Ecomorphology of solitary chemosensory cell systems in fish: a review

Synopsis Solitary chemosensory cells (SCCs) are present in the skin of a wide spectrum of lower vertebrates, such as lampreys, elasmobranchs, teleost fishes and some amphibians (Kotrschal 1991, Whitear 1992). However, due to the difficulties studying them, virtually all our present knowledge on SCCs stems from the anterior dorsal fin of two species of rocklings (Gadidae). This fin is a peculiar chemosensory organ, carrying approximately 5 million SCCs (Kotrschal et al. 1984, Kotrschal & Whitear 1988). The evidence derived from this model on the structure of SCCs, on their innervation and brain representation, on the flow dynamics at the receptors, on their electrophysiological responses and behavioral relevance indicates that this fin is actively sampling for substances leaked from other fish, such as body mucus and bile components. Possibly, the rockling anterior dorsal fin aids in predators avoidance. To generate hypotheses on the functions and biological roles of the generalized., scattered SCC systems present in most fishes, their structural parameters are put in perspective to taste bud structure and function and to the rockling results. Ecomorphological reasoning serves to establish testable hypotheses: in essence, SCC systems spread over the body surface may be designed as general water samplers, but not for the exact localization of a stimulus source. If the function of the latter is equally dependent on water flow, as the rockling fin organ, fish would have to rely either on the ambient water flow, or speed up their own swimming to optimize SCC input. If SCCs are indeed evolved in the context of predator avoidance, a comparison between life history intervals and between species should reveal, that the system varies in accordance with predation pressure. It is concluded, that in fish, SCCs are certainly an important source of environmental information. If we do not understand functions and biological roles of SCCs, it will not be possible to explain fish behavior and ecology. Evidently, further investigations are urgently needed.     

Taste buds as peripheral chemosensory processors

Taste buds are peripheral chemosensory organs situated in the oral cavity. Each taste bud consists of a community of 50–100 cells that interact synaptically during gustatory stimulation. At least three distinct cell types are found in mammalian taste buds – Type I cells, Receptor (Type II) cells, and Presynaptic (Type III) cells. Type I cells appear to be glial-like cells. Receptor cells express G protein-coupled taste receptors for sweet, bitter, or umami compounds. Presynaptic cells transduce acid stimuli (sour taste). Cells that sense salt (NaCl) taste have not yet been confidently identified in terms of these cell types. During gustatory stimulation, taste bud cells secrete synaptic, autocrine, and paracrine transmitters. These transmitters include ATP, acetylcholine (ACh), serotonin (5-HT), norepinephrine (NE), and GABA. Glutamate is an efferent transmitter that stimulates Presynaptic cells to release 5-HT. This chapter discusses these transmitters, which cells release them, the postsynaptic targets for the transmitters, and how cell–cell communication shapes taste bud signaling via these transmitters.     

Structure and Development of the Notochord ‘Elastica Externa’ and Nearby Components of the Elastic Fibre System of Agnathans

Between 15 days and 3 months in age, the ‘elastica externa’ of the notochord sheath of larval lampreys develops from patches of moderately dense and amorphous material into a thick, continuous and electron-dense layer. In both lampreys and hagfish, this layer stains strongly with Verhoeff's elastic stain and aldehyde fuchsin and is penetrated by collagen fibrils on both its outer and inner boundaries. Peroxidase labelling using an antibody raised against human elastin specifically labels both the notochord ‘elastica externa’ and the elastic fibre system of lampreys. The diameters of the microfibrils (10–13 nm) of the oxytalan, elaunin and elastic fibres of lampreys and hagfish are the same as those of higher vertebrates. The connective tissue immediately dorsal and ventral to the notochord of lampreys contains mainly oxytalan fibres in very young ammocoetes, a combination of oxytalan, elaunin and elastic fibres in older ammocoetes, and predominantly elastic fibres in adult lampreys. While the region of the endomeninx at the base of the spinal cord contains almost exclusively oxytalan fibres in young ammocoetes, it also possesses numerous elastic fibres in adult lampreys. These findings indicate that, as in higher vertebrates, the elastic fibres of lampreys develop from oxytalan fibres via elaunin fibres.     

Neuromodulation of Transduction and Signal Processing in the End Organs of Taste

Chemical synapses transmit gustatory signals from taste receptorcells to sensory afferent axons. Chemical (and electrical) synapsesalso provide a lateral pathway for cells within the taste budto communicate. Lateral synaptic pathways may represent someform of signal processing in the peripheral end organs of taste.Efferent synaptic input may also regulate sensory transductionin taste buds. To date, the synaptic neurotransmitter(s) orneuromodulator(s) released at chemical synapses in taste budshave not been identified unambiguously. This paper summarizesthe attempts that have been made over the past 40 years to identifythe neuroactive substances acting at taste bud synapses. Wereview the four traditional criteria for identifying chemicaltransmitters elsewhere in the nervous system—localization,uptake/degradation, release and physiological actions—andapply these criteria to neuroactive substances in taste buds.The most complete evidence to date implicates serotonin as aneuromodulator of taste transduction in the end organs. However,studies also suggest that adrenergic, cholinergic and peptidergicneurotransmission may be involved in taste buds. Chem. Senses21: 353–365, 1996.     

Gustatory neurons derived from epibranchial placodes are attracted to, and trophically supported by, taste bud-bearing endoderm in vitro

Taste buds are multicellular receptor organs innervated by the VIIth, IXth, and Xth cranial nerves. In most vertebrates, taste buds differentiate after nerve fibers have reached the lingual epithelium, suggesting that nerves induce taste buds. However, under experimental conditions, taste buds of amphibians develop independently of innervation. Thus, rather than being induced by nerves, the developing taste periphery likely regulates ingrowing nerve fibers. To test this idea, we devised a culture approach using axolotl embryos. Gustatory neurons were generated from cultured epibranchial placodes, and when cultured alone, axon outgrowth was random over 4 days, a time period coincident with axon growth to the periphery in vivo. In contrast, cocultures of placodal neurons with oropharyngeal endoderm (OPE), the normal taste bud-containing target for these neurons, resulted in neurite growth toward the target tissue. Unexpectedly, placodal neurons also grew toward flank ectoderm (FE), which these neurons do not encounter in vivo. To compare further the impact of OPE and FE explants on gustatory neurons, cocultures were extended and examined at 6, 8, and 10 days, when, in vivo, placodal fibers have innervated the epithelium but prior to taste bud formation, when taste buds have differentiated and are innervated, and when the mouth has opened and larvae have begun to feed, respectively. The behavior of placodal axons with respect to target type did not differ between OPE and FE cocultures at 6 days. However, by 8 days, differences in axonal outgrowth were observed with respect to target type, and these differences were enhanced by 10 days in vitro. Most clearly, exuberant placodal fibers grew in 10-day OPE cocultures, and numerous neurites had invaded OPE explants by this time, whereas gustatory neurites were sparse in FE cocultures, and rarely approached and almost never contacted FE explants. Thus, embryonic endoderm destined to give rise to taste buds specifically attracts its innervation early in development, as placodal neurons send out axons. Later, when gustatory axons synapse with differentiated taste buds in vivo, the OPE provides trophic support for cultured gustatory neurons.     

Distribution patterns of the paraneuronal endocrine cells in the skin, gills and the airways of fishes as determined by immunohistochemical and histological methods

Summary The neuro-endocrine cells of fish skin and respiratory surfaces, and their bioactive secretion as far as is known, are reviewed, and compared with similar elements in tetrapods, particularly amphibians. In the skin of teleost fish, immunohistochemistry has shown that Merkel cells react for serotonin, neuron-specific enolase and enkephalins. The pharmacology is not established in dipnoans or lampreys. In some teleosts, neuromasts react for substance P and leu-enkephalins; substance P is also reported from some ampullary organs (electroreceptors). Taste buds of teleosts may react for enkephalin and substance P. Basal cells of taste buds react for serotonin and neuron-specific enolase. Some unicellular skin glands of teleosts express bioactive compounds, including serotonin and some peptides; this ectopic expression is paralleled in amphibian skin glands. The dipnoan Protopterus has innervated pulmonary neuro-endocrine cells in the pneumatic duct region with dense-cored vesicles. In Polypterus and Amia the lungs have serotonin-positive neuro-endocrine cells that are apparently not innervated. In fish gills, a closed type of neuro-endocrine cell reacts for serotonin, an open type for enkephalins and some calcium-binding proteins (calbindin, calmodulin and S-100 protein). The functions of neuro-endocrine cells in fishes await investigation, but it is assumed they are regulatory.     

Solitary chemosensory cells in the developing chemoreceptorial epithelium of the vallate papilla

Summary The solitary chemosensory cells are considered typical of aquatic vertebrates and have never been described in the oral cavity of terrestrial vertebrates. We describe elements with ultrastructural characteristics of the solitary chemosensory cell in the gustatory epithelium of rats in the first week of extrauterine life. These elements appeared isolated, located among keratinocytes, and wrapped by glial-like elements. They showed a bipolar shape with a round cell body, a thin apical process, and a thicker basal one. Nerve fibers contacted the cell body and the processes. The cells showed small dense granules mainly located near nerve contacts. Small bundles of tonofilaments were visible in the perinuclear cytoplasm. Similar elements were not found in rats after the first week of extrauterine life. The present study demonstrates in a mammal that the development of taste buds is preceded by the presence of epithelial elements with ultrastructural characteristics of the solitary chemosensory cells described in lower vertebrates. This finding suggests that the oral chemoreception in the suckling rats may be mediated by three different patways (i.e., gustatory system, common chemical sense, and solitary chemosensory cell system).     

Shared and unique G alpha proteins in the zebrafish versus mammalian senses of taste and smell

Chemosensory systems in vertebrates employ G protein-coupled receptors as sensors. In mammals, several families of olfactory and gustatory receptors as well as specific G alpha proteins coupling to them have been identified, for example, gustducin for taste. Orthologous receptor families have been characterized in fish, but the corresponding G alpha genes have not been well investigated so far. We have performed a comprehensive search of several lower vertebrate genomes to establish the G alpha protein family in these taxa and to identify those genes that may be involved in chemosensory signal transduction in fish. We report that gustducin is absent from the genomes of all teleost and amphibian species analyzed, presumably due to independent gene losses in these lineages. However, 2 other G alpha genes, Gi1b and G14a, are expressed in zebrafish taste buds and 4 G proteins, Go1, Go2, Gi1b, and Golf2, were detected in the olfactory epithelium. Golf2, Gi1b, and G14a are expressed already shortly after hatching, consistent with the physiological and behavioral responses of larvae to odorants and tastants. Our results show general similarity to the mammalian situation but also clear-cut differences and as such are essential for using the zebrafish model system to study chemosensory perception.     

Building sensory receptors on the tongue

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

Solitary chemosensory cells: why do primary aquatic vertebrates need another taste system?

The taste-like system of solitary chemosensory cells (SCCs) has almost eluded scientific attention. This is particularly remarkable, since recent surveys have revealed that this system of epidermal cells is widespread and abundant among the anamniotic aquatic vertebrates. In the rocklings (Gadidae, Teleostei), high densities of SCCs occur at a specialized dorsal fin. Recent evidence from this model indicates that SCCs are narrowly tuned to dilutions of fish body mucus and bile. Thus, SCCs may sample the ambient water for the upstream presence of potential competitors or predators. However, in sea robins (Triglidae, Teleostei), SCCs seem to be involved in finding food. Information from many more species is needed to explain why SCCs and taste buds have been maintained in parallel for such a long evolutionary period of time - from the age of the agnathans to that of the most advanced teleost fishes.     

哺乳动物舌面味蕾的发育

根据近年来有关大鼠、小鼠味觉发育方面的大量研究,对哺乳动物味蕾(taste buds)发育的情况进行了综述和讨论.哺乳动物舌面上的味蕾分布在菌状乳头(fungiform papillae,FF)、叶状乳头(foliate papillae,FL)、轮廓状乳头(circumvallate papillae,CV)之中,味蕾细胞(taste bud cells)不断地进行着周期性的更新,味蕾的形态、数量和功能随动物随年龄而变化.有关味孔头的研究表明,味乳头(gustatory papillae)在味蕾形成和维持味蕾存在及正常发育方面有着独特的功能.味乳头和味蕾的发育过程与细胞信号分子(signaling molecules)、味觉神经(gustatory nerve fibers)等许多因素有着密切的关系,其中有些作用机理至今尚无定论.     

Peptide regulators of peripheral taste function

The peripheral sensory organ of the gustatory system, the taste bud, contains a heterogeneous collection of sensory cells. These taste cells can differ in the stimuli to which they respond and the receptors and other signaling molecules they employ to transduce and encode those stimuli. This molecular diversity extends to the expression of a varied repertoire of bioactive peptides that appear to play important functional roles in signaling taste information between the taste cells and afferent sensory nerves and/or in processing sensory signals within the taste bud itself. Here, we review studies that examine the expression of bioactive peptides in the taste bud and the impact of those peptides on taste functions. Many of these peptides produced in taste buds are known to affect appetite, satiety or metabolism through their actions in the brain, pancreas and other organs, suggesting a functional link between the gustatory system and the neural and endocrine systems that regulate feeding and nutrient utilization.     

The mitochondrial DNA molecule of the hagfish (myxine glutinosa) and vertebrate phylogeny

The vertebrates are traditionally classified into two distinct groups, Agnatha (jawless vertebrates) and Gnathostomata (jawed vertebrates). Extant agnathans are represented by hagfishes (Myxiniformes) and lampreys (Petromyzontiformes), frequently grouped together within the Cyclostomata. Whereas the recognition of the Gnathostomata as a clade is commonly acknowledged, a consensus has not been reached regarding whether or not Cyclostomata represents a clade. In the present study we have used newly established sequences of the protein-coding genes of the mitochondrial DNA molecule of the hagfish to explore agnathan and gnathostome relationships. The phylogenetic analysis of Pisces, using echinoderms as outgroup, placed the hagfish as a sister group of Vertebrata sensu stricto, i.e., the lamprey and the gnathostomes. The phylogenetic analysis of the Gnathostomata identified a basal divergence between gnathostome fishes and a branch leading to birds and mammals, i.e., between ``Anamnia' and Amniota. The lungfish has a basal position among gnathostome fishes with the teleosts as the most recently evolving lineage. The findings portray a hitherto unrecognized polarity in the evolution of bony fishes. The presently established relationships are incompatible with previous molecular studies. Received: 15 August 1997 / Accepted: 1 October 1997     

Laryngeal chemosensory clusters

The expression of molecules involved in the transductory cascade of the sense of taste (TRs, alpha-gustducin, PLCbeta2, IP3R3) has been described in lingual taste buds or in solitary chemoreceptor cells located in different organs. At the laryngeal inlet, immunocytochemical staining at the light and electron microscope levels revealed that alpha-gustducin and PLCbeta2 are mainly localized in chemosensory clusters (CCs), which are multicellular organizations differing from taste buds, being generally composed of two or three chemoreceptor cells. Compared with lingual taste buds, CCs are lower in height and smaller in diameter. In laryngeal CCs, immunocytochemistry using the two antibodies identified a similar cell type which appears rather unlike the alpha-gustducin-immunoreactive (IR) and PLCbeta2-IR cells visible in lingual taste buds. The laryngeal IR cells are shorter than the lingual ones, with poorly developed basal processes and their apical process is shorter and thicker. Some cells show a flask-like shape due to the presence of a large body and the absence of basal processes. CCs lack pores and their delimitation from the surrounding epithelium is poorly evident. The demonstration of the existence of CCs strengthens the hypothesis of a phylogenetic link between gustatory and solitary chemosensory cells.