Neurotrophin receptors in taste buds of adult zebrafish (Danio rerio)

Vimentin Localization in the Zebrafish Oral Cavity: A Potential Role in Taste Buds Regeneration

The morphology of the oral cavity of fish is related to their feeding habits. In this context, taste buds are studied for their ability to catch chemical stimuli and their cell renewal capacity. Vimentin RV202 is a protein employed as a marker for mesenchymal cells that can differentiate along different lineages and to self-renew, while Calretinin N-18 is employed as a marker of sensory cells, and ubiquitin is a protein crucial for guiding the fate of stem cells throughout development. In this study, a surface morphology investigation and an immunohistochemical analysis have been conducted. The results of the present study reveal, for the first time, the presence of Vimentin RV202 in a taste bud cell population of zebrafish. Some taste bud cells are just Vimentin RV202-immunoreactive, while in other cells Vimentin RV202 and Calretinin N-18 colocalize. Some taste buds are just reactive to Calretinin N-18. Vimentin RV202-immunoreactive cells have been observed in the connective layer and in the basal portion of the taste buds. The immunoreactivity of ubiquitin was restricted to sensory cells. Further studies are needed to elucidate the role of Vimentin RV202 in the maturation of taste bud cells, its potential involvement in the regeneration of these chemosensory organs, and its eventual synergic work with ubiquitin.

Transcription Pattern of Neurotrophic Factors and Their Receptors in Adult Zebrafish Spinal Cord

In vertebrates, neurotrophins and their receptors play a fundamental role in the central and peripheral nervous systems. Several studies reported that each neurotrophin/receptor signalling pathway can perform various functions during axon development, neuronal growth, and plasticity. Previous investigations in some fish species have identified neurotrophins and their receptors in the spinal cord under physiological conditions and after injuries, highlighting their potential role during regeneration. In our study, for the first time, we used an excellent animal model, the zebrafish (Danio rerio), to compare the mRNA localization patterns of neurotrophins and receptors in the spinal cord. We quantified the levels of mRNA using qPCR, and identified the transcription pattern of each neurotrophin/receptor pathway via in situ hybridization. Our data show that ngf/trka are the most transcribed members in the adult zebrafish spinal cord.

Potential Neuroprotective Role of Calretinin-N18 and Calbindin-D28k in the Retina of Adult Zebrafish Exposed to Different Wavelength Lights

The incidence rates of light-induced retinopathies have increased significantly in the last decades because of continuous exposure to light from different electronic devices. Recent studies showed that exposure to blue light had been related to the pathogenesis of light-induced retinopathies. However, the pathophysiological mechanisms underlying changes induced by light exposure are not fully known yet. In the present study, the effects of exposure to light at different wavelengths with emission peaks in the blue light range (400-500 nm) on the localization of Calretinin-N18 (CaR-N18) and Calbindin-D28K (CaB-D28K) in adult zebrafish retina are studied using double immunofluorescence with confocal laser microscopy. CaB-D28K and CaR-N18 are two homologous cytosolic calcium-binding proteins (CaBPs) implicated in essential process regulation in central and peripheral nervous systems. CaB-D28K and CaR-N18 distributions are investigated to elucidate their potential role in maintaining retinal homeostasis under distinct light conditions and darkness. The results showed that light influences CaB-D28K and CaR-N18 distribution in the retina of adult zebrafish, suggesting that these CaBPs could be involved in the pathophysiology of retinal damage induced by the short-wavelength visible light spectrum.

Neurotrophins in Zebrafish Taste Buds

Simple Summary

Zebrafish is a powerful vertebrate model organism, whose similarities with mammals are fundamental to validate its use for experimental purposes. In this study, the authors demonstrate the presence of neurotrophic factors, namely neurotrophins, in numerous taste bud cells of this fish. The reported results suggest an essential role of these factors in taste bud function. Interestingly, the results described in this study are in accordance with those reported in some mammalian species. Therefore, despite the different anatomical characteristics of the anterior digestive tract in mammals and fish, the taste buds maintain similarities in both shape and functional mechanisms in the two classes.

Abstract

The neurotrophin family is composed of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), Neurotrophin 3 (NT3) and NT4. These neurotrophins regulate several crucial functions through the activation of two types of transmembrane receptors, namely p75, which binds all neurotrophins with a similar affinity, and tyrosine kinase (Trk) receptors. Neurotrophins, besides their well-known pivotal role in the development and maintenance of the nervous system, also display the ability to regulate the development of taste buds in mammals. Therefore, the aim of this study is to investigate if NGF, BDNF, NT3 and NT4 are also present in the taste buds of zebrafish (Danio rerio), a powerful vertebrate model organism. Morphological analyses carried out on adult zebrafish showed the presence of neurotrophins in taste bud cells of the oropharyngeal cavity, also suggesting that BDNF positive cells are the prevalent cell population in the posterior part of the oropharyngeal region. In conclusion, by suggesting that all tested neurotrophins are present in zebrafish sensory cells, our results lead to the assumption that taste bud cells in this fish species contain the same homologous neurotrophins reported in mammals, further confirming the high impact of the zebrafish model in translational research.

Localization of BDNF and Calretinin in Olfactory Epithelium and Taste Buds of Zebrafish (Danio rerio)

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family and it is involved in several fundamental functions in the central and peripheral nervous systems, and in sensory organs. BDNF regulates the chemosensory systems of mammals and is consistently expressed in those organs. In zebrafish, the key role of BDNF in the biology of the hair cells of the inner ear and lateral line system has recently been demonstrated. However, only some information is available about its occurrence in the olfactory epithelium, taste buds, and cutaneous isolated chemosensory cells. Therefore, this study was undertaken to analyze the involvement of BDNF in the chemosensory organs of zebrafish during the larval and adult stages. To identify cells displaying BDNF, we compared the cellular pattern of BDNF-displaying cells with those immunoreactive for calretinin and S100 protein. Our results demonstrate the localization of BDNF in the sensory part of the olfactory epithelium, mainly in the ciliated olfactory sensory neurons in larvae and adult zebrafish. Intense immunoreaction for BDNF was also observed in the chemosensory cells of oral and cutaneous taste buds. Moreover, a subpopulation of olfactory sensory neurons and chemosensory cells of olfactory rosette and taste bud, respectively, showed marked immunopositivity for calcium-binding protein S100 and calretinin. These results demonstrate the possible role of BDNF in the development and maintenance of olfactory sensory neurons and sensory cells in the olfactory epithelium and taste organs of zebrafish during all stages of development.

The BDNF/TrkB Neurotrophin System in the Sensory Organs of Zebrafish

The brain-derived neurotrophic factor (BDNF) was discovered in the last century, and identified as a member of the neurotrophin family. BDNF shares approximately 50% of its amino acid with other neurotrophins such as NGF, NT-3 and NT-4/5, and its linear amino acid sequences in zebrafish (Danio rerio) and human are 91% identical. BDNF functions can be mediated by two categories of receptors: p75NTR and Trk. Intriguingly, BDNF receptors were highly conserved in the process of evolution, as were the other NTs’ receptors. In this review, we update current knowledge about the distribution and functions of the BDNF-TrkB system in the sensory organs of zebrafish. In fish, particularly in zebrafish, the distribution and functions of BDNF and TrkB in the brain have been widely studied. Both components of the system, associated or segregated, are also present outside the central nervous system, especially in sensory organs including the inner ear, lateral line system, retina, taste buds and olfactory epithelium.

Localization of Neurotrophin Specific Trk Receptors in Mechanosensory Systems of Killifish (Nothobranchius guentheri)

Neurotrophins (NTs) and their signal-transducing Trk receptors play a crucial role in the development and maintenance of specific neuronal subpopulations in nervous and sensory systems. NTs are supposed to regulate two sensory systems in fish, the inner ear and the lateral line system (LLS). The latter is one of the major mechanosensory systems in fish. Considering that annual fishes of the genus Nothobranchius, with their short life expectancy, have become a suitable model for aging studies and that the occurrence and distribution of neurotrophin Trk receptors have never been investigated in the inner ear and LLS of killifish (Nothobranchius guentheri), our study aimed to investigate the localization of neurotrophin-specific Trk receptors in mechanosensory systems of N. guentheri. For histological and immunohistochemical analysis, adult specimens of N. guentheri were processed using antibodies against Trk receptors and S100 protein. An intense immunoreaction for TrkA and TrkC was found in the sensory cells of the inner ear as well as in the hair cells of LLS. Moreover, also the neurons localized in the acoustic ganglia displayed a specific immunoreaction for all Trk receptors (TrkA, B, and C) analyzed. Taken together, our results demonstrate, for the first time, that neurotrophins and their specific receptors could play a pivotal role in the biology of the sensory cells of the inner ear and LLS of N. guentheri and might also be involved in the hair cells regeneration process in normal and aged conditions.

Expression of ghrelin and leptin in the chemosensory system of adult zebrafish

Numerous data show that the chemosensory system seems to be modulated by changes in the circulating levels of different molecules such as ghrelin, orexin, leptin, NPY, CCK. The chemosensory system of the zebrafish is represented by the taste buds (skin, oral and oropharyngeal), the olfactory rosette and the solitary chemosensorial cells (SCCs). The purpose of our study was to analyze the distribution of two peripheral hormones such as ghrelin and leptin in the chemosensory organs of the zebrafish. Our results demonstrated the presence of immunoreaction for all antibodies used in the zebrafish chemosensory organs even if with different distribution. In particular, IR was observed for ghrelin in the olfactory rosette while IR for leptin was found in the olfactory rosette, in the skin and oropharyngeal taste buds and in the gills. Both these hormones were detected in the intestine, used as a control.

Acid-sensing ion channels and transient-receptor potential ion channels in zebrafish taste buds

Sensory information from the environment is required for life and survival, and it is detected by specialized cells which together make up the sensory system. The fish sensory system includes specialized organs that are able to detect mechanical and chemical stimuli. In particular, taste buds are small organs located on the tongue in terrestrial vertebrates that function in the perception of taste. In fish, taste buds occur on the lips, the flanks, and the caudal (tail) fins of some species and on the barbels of others. In fish taste receptor cells, different classes of ion channels have been detected which, like in mammals, presumably participate in the detection and/or transduction of chemical gustatory signals. However, since some of these ion channels are involved in the detection of additional sensory modalities, it can be hypothesized that taste cells sense stimuli other than those specific for taste. This mini-review summarizes current knowledge on the presence of transient-receptor potential (TRP) and acid-sensing (ASIC) ion channels in the taste buds of teleosts, especially adult zebrafish. Up to now ASIC4, TRPC2, TRPA1, TRPV1 and TRPV4 ion channels have been found in the sensory cells, while ASIC2 was detected in the nerves supplying the taste buds.

Neurotrophins and their Trk-receptors in the cerebellum of zebrafish

Neurotrophins (NTs) and their specific Trk-receptors are key molecules involved in the regulation of survival, proliferation, and differentiation of central nervous system during development and adulthood in vertebrates. In the present survey, we studied the expression and localization of neurotrophins and their Trk-receptors in the cerebellum of teleost fish Danio rerio (zebrafish). Teleostean cerebellum is composed of a valvula, body and vestibulolateral lobe. Valvula and body show the same three-layer structure as cerebellar cortex in mammals. The expression of NTs and Trk-receptors in the whole brain of zebrafish has been studied by Western blotting analysis. By immunohistochemistry, the localization of NTs has been observed mainly in Purkinje cells; TrkA and TrkB-receptors in cells and fibers of granular and molecular layers. TrkC was faintly detected. The occurrence of NTs and Trk-receptors suggests that they could have a synergistic action in the cerebellum of zebrafish. J. Morphol. 277:725-736, 2016. © 2016 Wiley Periodicals, Inc.

Morphological differences in adipose tissue and changes in BDNF/Trkb expression in brain and gut of a diet induced obese zebrafish model

Obesity is a multifactorial disease generated by an alteration in balance between energy intake and expenditure, also dependent on genetic and non-genetic factors. Moreover, various nuclei of the hypothalamus receive and process peripheral stimuli from the gastrointestinal tract, controlling food intake and therefore energy balance. Among anorexigenic molecules, brain-derived neurotrophic factor (BDNF) acts through the tyrosine-kinase receptor TrkB. Numerous data demonstrate that the BDNF/TrkB system has a fundamental role in the control of food intake and body weight. Quantitative PCR and immunohistochemistry for both BDNF and TrkB were used to determine changes in levels in the brain and gastro-intestinal tract of an experimental zebrafish model of diet-induced obesity. Overfed animals showed increased weight and body mass index as well as accumulation of adipose tissue in the visceral, subcutaneous and hepatic areas. These changes were concomitant with decreased levels of BDNF mRNA in the gastro-intestinal tract and increased expression of TrkB mRNA in the brain. Overfeeding did not change the density of cells displaying immunoreactivity for BDNF or TrkB in the brain although both were significantly diminished in the gastro-intestinal tract. These results suggest an involvement of the BDNF/TrkB system in the regulation of food intake and energy balance in zebrafish, as in mammals.

Brain-derived neurotrophic factor: mRNA expression and protein distribution in the brain of the teleost Nothobranchius furzeri

BDNF (brain-derived neurotrophic factor) is a member of the neurotrophin family and it is implicated in regulating brain development and function. The BDNF gene organization and coding sequence are conserved in all vertebrates. The present survey was conducted in a teleost fish, Nothobranchius furzeri, because it is an emerging model of aging studies due to its short lifespan and shows the high rate of adult neurogenesis typical of anamniotes. The present survey reports: 1) the identification and characterization of the cDNA fragment encoding BDNF protein, and 2) the localization of BDNF in the whole brain. BDNF mRNA expression was assessed by in situ hybridization, by employing an antisense RNA probe; BDNF protein was detected by employing a sensitive immunohistochemical technique, along with highly specific affinity-purified antibodies to BDNF. Both BDNF mRNA and protein were detected in neurons and glial cells of all regions of the brain of N. furzeri. Interestingly, BDNF was localized also in brain areas involved in adult neurogenic activities, suggesting a specific role for this neurotrophic factor in controlling cell proliferation. These results provide baseline information for future studies concerning BDNF involvement in the aging processes of the teleost brain.

Localization of BDNF expression in the developing brain of zebrafish

The brain-derived neurotrophic factor (BDNF) gene is expressed in differentiating and post-mitotic neurons of the zebrafish embryo, where it has been implicated in Huntington's disease. Little is known, however, about the full complement of neuronal cell types that express BDNF in this important vertebrate model. Here, we further explored the transcriptional profiles during the first week of development using real-time quantitative polymerase chain reaction (RT-qPCR) and whole-mount in situ hybridization (WISH). RT-qPCR results revealed a high level of maternal contribution followed by a steady increase of zygotic transcription, consistent with the notion of a prominent role of BDNF in neuronal maturation and maintenance. Based on WISH, we demonstrate for the first time that BDNF expression in the developing brain of zebrafish is structure specific. Anatomical criteria and co-staining with genetic markers (shh, pax2a, emx1, krox20, lhx2b and lhx9) visualized major topological domains of BDNF-positive cells in the pallium, hypothalamus, posterior tuberculum and optic tectum. Moreover, the relative timing of BDNF transcription in the eye and tectum may illustrate a mechanism for coordinated development of the retinotectal system. Taken together, our results are compatible with a local delivery and early role of BDNF in the developing brain of zebrafish, adding basic knowledge to the study of neurotrophin functions in neural development and disease.

Expression and anatomical distribution of TrkB in the encephalon of the adult zebrafish (Danio rerio)

Neurotrophins are a family of growth factor primarily acting in the nervous system, throughout two categories of membrane receptors on the basis of their high (Trk receptors) or low (p75NTR) affinity. Both neurotrophins and Trk receptors are phylogenetically conserved and are expressed not only in the central and peripheral nervous system but also in non-nervous tissues of vertebrates and some invertebrates. The brain-derived neurotrophic factor (BDNF)/TrkB system plays an important role in the development, phenotypic maintenance and plasticity of specific neuronal populations. Considering that this system is poorly characterized in the central nervous system of teleosts, the expression and anatomical distribution of TrkB in the brain of the adult zebrafish using reverse transcriptase-polymerase chain reaction (RT-PCR), Western-blot and immunohistochemistry were analysed. Both the riboprobe and the antibody used were designed to map within the catalytic domain of TrkB. RT-PCR detected specific TrkB mRNA in brain homogenates, while Western-blot identified one unique protein band with an estimated molecular weight of 145kDa, thus corresponding with the TrkB full-length isiform of the receptor. Immunohistochemistry showed specific TrkB immunoreactivity in restricted areas of the encephalon, i.e. the hypothalamus and a specific neuronal subpopulation of the reticular formation. The present results demonstrate, for the first time, that, as in mammals, the encephalon of adult zebrafish expresses TrkB in specific zones related to food intake, behaviour or motor activity.

Zebrafish crypt neurons project to a single, identified mediodorsal glomerulus

Crypt neurons are a third type of olfactory receptor neurons with a highly unusual "one cell type--one receptor" mode of expression, the same receptor being expressed by the entire population of crypt neurons. Attempts to identify the target region(s) of crypt neurons have been inconclusive so far. We report that TrkA-like immunoreactivity specifically labeled somata, axons, and terminals of zebrafish crypt neurons and reveal a single glomerulus, mdg2 of the dorsomedial group, as target glomerulus of crypt neurons. Injection of a fluorescent tracing dye into the mdg2 glomerulus retrogradely labeled mostly crypt neurons, as assessed by quantitative morphometry, whereas no crypt neurons were found after injections in neighboring glomeruli. Our data provide strong evidence that crypt neurons converge onto a single glomerulus, and thus form a labeled line consisting of a single sensory cell type, a single olfactory receptor and a single target glomerulus.

Morphology of the European sea bass (Dicentrarchus labrax) tongue

The European sea bass, a member of the Moronidae family, is a food fish, considered one of the first models for the intensive breeding in salt water. It has nowadays an important and increasing presence in the international fishing markets. Sea basses are carnivorous, feeding on little fishes and invertebrates. Considering the important role of the tongue during the intraoral transport and the swallowing of food, scarce data are present in literature about its morphology. The aim of this study was to analyze the morphology of the tongue by means of scanning electron and light microscopy. Adult sea basses were obtained from the aquarium of the Sicilian Center of Experimental Ichthyiopathology of the University of Messina. The fishes were anaesthetized with MS 222 and the heads were then quickly removed and processed for the paraffin embedding and SEM processing. Three different tongue regions could be distinguished: an apex, a body, and a root. Scanning electron and light microscopy showed the presence of numerous canine-like teeth, surrounded by taste buds and numerous fungiform and conical papillae. The teeth were curved and their tips were posteriorly oriented. The results confirm, in teleosts too, the fundamental role of the tongue in the mechanics of food ingestion. Moreover, the presence of taste buds demonstrates the interaction of food processing and taste. These data could be a potential source to identify new and better methods of nutrition in the breeding of this fish.

Somatosensory mechanisms in zebrafish lacking dorsal root ganglia

Dorsal root ganglion (DRG) sensory neurons transmit all somatosensory information from the trunk region of the body. erbb3 mutant zebrafish do not form DRG neurons because the neural crest cells that generate them migrate aberrantly. Here we report that homozygous erbb3 mutants appear to swim and feed normally, and that they survive through adulthood, despite never forming DRG neurons. The source of sensory compensation in adult erbb3 mutants remains unknown, although it may be from lateral line ganglion neuromasts which are reduced, but present, in erbb3 mutants. We also provide new information about the development of DRG neurons in wild-type juvenile zebrafish.

Expression and cell localization of brain-derived neurotrophic factor and TrkB during zebrafish retinal development

Brain-derived neurotrophic factor (BDNF) signaling through TrkB regulates different aspects of neuronal development, including survival, axonal and dendritic growth, and synapse formation. Despite recent advances in our understanding of the functional significance of BDNF and TrkB in the retina, the cell types in the retina that express BDNF and TrkB, and the variations in their levels of expression during development, remain poorly defined. The goal of the present study is to determine the age-dependent changes in the levels of expression and localization of BDNF and TrkB in the zebrafish retina. Zebrafish retinas from 10 days post-fertilization (dpf) to 180 dpf were used to perform PCR, Western blot and immunohistochemistry. Both BDNF and TrkB mRNAs, and BDNF and full-length TrkB proteins were detected at all ages sampled. The localization of these proteins in the retina was very similar at all time points studied. BDNF immunoreactivity was found in the outer nuclear layer, the outer plexiform layer and the inner plexiform layer, whereas TrkB immunoreactivity was observed in the inner plexiform layer and, to a lesser extent, in the ganglion cell layer. These results demonstrate that the pattern of expression of BDNF and TrkB in the retina of zebrafish remains unchanged during postembryonic development and adult life. Because TrkB expression in retina did not change with age, cells expressing TrkB may potentially be able to respond during the entire lifespan of zebrafish to BDNF either exogenously administered or endogenously produced, acting through paracrine mechanisms.

Morphology of the lingual dorsal surface and oral taste buds in Italian lizard (Podarcis sicula)

The Italian lizard (Podarcis sicula) is the most diffused reptile in Italy, but it is also present in other European countries. This lizard belongs to the Lacertidae family, lives near walls, slants and along the borders of the paths; its diet includes bugs and aracnids. No data are so far available in literature about the three-dimensional morphology of the tongue of Podarcis sicula, therefore the aim of the present paper was to study by scanning electron and light microscopy the three-dimensional characteristics of the dorsal lingual surface and moreover the presence of chemosensory receptors like the taste buds in the oral cavity. Our results demonstrate that the Podarcis sicula tongue is a triangular muscular membranous organ, dorsoventrally flattened and that three different areas can be observed: a bifid apex, a body and a root. No papillae were observed in the apex, characterized by a flattened mucosa and by two deep median pouches. In the body cylindrical papillae with a flat surface are present, aborally gradually substituted by imbricated papillae. Foliate-like papillae were observed in the lateral parts of the tongue body. No sensory structures were showed on the lingual dorsal surface, while they were numerous in the oral cavity, particularly on the gingival epithelium. The light microscopy shows, on the dorsal surface, a stratified pavimentous not keratinized epithelium, conversely keratinized along the ventral surface. Many caliciform cells on the lateral parts of the papillae, deputed to the secretion of mucus, were also observed. Therefore, the results obtained in this paper could give a contribution to the knowledge of the tongue anatomy in a species widely diffused in different European countries and could be of help for clinical purposes in reptiles.

Expression of brain-derived neurotrophic factor and TrkB in the lateral line system of zebrafish during development

The neuromasts of the lateral line system are regarded as a model to study the mechanisms of hearing, deafness, and ototoxicity. The neurotrophins (NTs), especially brain-derived neurotrophic factor (BDNF), and its signaling receptor TrkB are involved in the development and maintenance of neuromasts. To know the period in which the BDNF/TrkB complex has more effects in the neuromast biology, the age-related changes were studied. Normal zebrafish from 10 to 180 days post-fertilization (dpf), as well as transgenic ET4 zebrafish 10 and 20 dpf, was analyzed using qRT-PCR, western blot, and immunohistochemistry. BDNF and TrkB mRNAs followed a parallel course, peaking at 20 dpf, and thereafter progressively decreased. Specific immunoreactivity for BDNF and TrkB was found co-localized in all hairy cells of neuromasts in 20 and 30 dpf; then, the number of immunoreactive cells decreased, and by 180 dpf BDNF remains restricted to a subpopulation of hairy cells, and TrkB to a few number of sensory and non-sensory cells. At all ages examined, TrkB immunoreactivity was detected in sensory ganglia innervating the neuromasts. The present results demonstrate that there is a parallel time-related decline in the expression of BDNF and TrkB in zebrafish. Also, the patterns of cell expression suggest that autocrine/paracrine mechanisms for this NT system might occur within the neuromasts. Because TrkB in lateral line ganglia did not vary with age, their neurons are potentially capable to respond to BDNF during the entire lifespan of zebrafish.

Expression and distribution of S100 protein in the nervous system of the adult zebrafish (Danio rerio)

S100 proteins are EF-hand calcium-binding protein highly preserved during evolution present in both neuronal and non-neuronal tissues of the higher vertebrates. Data about the expression of S100 protein in fishes are scarce, and no data are available on zebrafish, a common model used in biology to study development but also human diseases. In this study, we have investigated the expression of S100 protein in the central nervous system of adult zebrafish using PCR, Western blot, and immunohistochemistry. The central nervous system of the adult zebrafish express S100 protein mRNA, and contain a protein of approximately 10 kDa identified as S100 protein. S100 protein immunoreactivity was detected widespread distributed in the central nervous system, labeling the cytoplasm of both neuronal and non-neuronal cells. In fact, S100 protein immunoreactivity was primarily found in glial and ependymal cells, whereas the only neurons displaying S100 immunoreactivity were the Purkinje's neurons of the cerebellar cortex and those forming the deep cerebellar nuclei. Outside the central nervous system, S100 protein immunoreactivity was observed in a subpopulation of sensory and sympathetic neurons, and it was absent from the enteric nervous system. The functional role of S100 protein in both neurons and non-neuronal cells of the zebrafish central nervous system remains to be elucidated, but present results might serve as baseline for future experimental studies using this teleost as a model.

Differential distribution of S100 protein and calretinin in mechanosensory and chemosensory cells of adult zebrafish (Danio rerio)

Calcium-binding proteins play a critical role in vertebrate sensory cells, and some of them have been detected in mechanosensory and chemosensory cells of bony and cartilaginous fishes. In this study immunohistochemistry and Western blot were used to investigate the occurrence and the distribution of S100 protein and calretinin in mechanosensory (neuromasts of the lateral line system; maculae and cristae ampullaris of the inner ear) as well as chemosensory (superficial and oral taste buds; olfactory epithelium) cells in adult zebrafish (Danio rerio). Specific protein bands with an estimated molecular weight of around 10 kDa and 30 kDa were detected by Western blot and were identified with S100 protein and calretinin, respectively. S100 protein and calretinin were observed segregated in mechanosensory and chemosensory cells, and the presence of S100 protein in a cell excluded that of calretinin, and vice versa. As a rule, the mechanosensory cells were S100 protein positive, whereas the chemosensory ones displayed calretinin immunoreactivity. Calretinin was also detected in nerve fibers supplying some of the investigated organs. In the olfactory epithelium, S100 protein immunoreactivity was present in the crypt olfactory sensory neurons, whereas calretinin immunoreactivity was widespread in olfactory sensory neurons and probably other olfactory cells. In this localization the co-expression of S100 protein and calretinin cannot be excluded. These results demonstrate the cell segregation of two specific calcium-binding proteins, and they enable to selectively label these cells by using easily reproducible immunohistochemical techniques associated to well-known antibodies.

Neurotrophin and Trk neurotrophin receptors in the inner ear of Salmo salar and Salmo trutta

Neurotrophins (NTs) and their signal transducing Trk receptors play a critical role in the development and maintenance of specific neuronal populations in the nervous system of higher vertebrates. They are responsible for the innervation of the inner ear cochlear and vestibular sensory epithelia. Neurotrophins and Trks are also present in teleosts but their distribution in the inner ear is unknown. Thus, in the present study, we used Western-blot analysis and immunohistochemistry to investigate the expression and cell localization of both NTs and Trk receptors in the inner ear of alevins of Salmo salar and Salmo trutta. Western-blot analysis revealed the occurrence of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), but not nerve growth factor (NGF), as well as all three Trk receptors, i.e. TrkA, TrkB and TrkC, the estimated molecular weights of which were similar to those expected for mammals. Specific immunoreactivity for neurotrophins was detected mainly in the sensory epithelia. In particular, BDNF immunoreactivity was found in the maculae of the utricle and saccule, whereas NT-3 immunoreactivity was present in the sensory epithelium of the cristae ampullaris. As a rule the sensory epithelia of the inner ear lacked immunoreactivity for Trks, thus excluding possible mechanisms of autocrinia and/or paracrinia. By contrast, overlapping subpopulations of neurons in the statoacoustic ganglion expressed TrkA (about 15%), TrkB (about 65%) and TrkC (about 45%). The present results demonstrate that, as in mammals and birds, the inner ear of teleosts expresses the components of the neurotrophin-Trk system, but their roles remain to be elucidated.

The Oral Cavity of the Adult Zebrafish (Danio rerio)

The zebrafish is a common model for developmental studies including those regarding tooth, palate and tongue. Nevertheless, little information is available about the morphology of the oral cavity in this teleost, especially in adult animals. In this study we used light, scanning and transmission electron microscopy to describe in detail the morphology of the oral cavity of adult zebrafish. The oral cavity could be divided into three different zones: the outer containing the lips, the intermediate corresponding to the internal valves and the internal that corresponds to the tongue. In the upper and lower intermediate zones, there were semilunar shaped valves, more prominent in the upper part. The internal lower zones correspond to the palate and the tongue, which is an individualized structure filled with numerous transversal ridges. Both the intermediate and internal zones were covered by a stratified epithelium containing numerous mucous and rodlet cells. Present data provide the first description of the morphology and structure of the oral cavity in the adult zebrafish and might serve as a baseline for developmental studies of the oral cavity using this teleost as a model.

Comparative analysis of neurotrophin receptors and ligands in vertebrate neurons: tools for evolutionary stability or changes in neural circuits?

To better understand the role of multiple neurotrophin ligands and their receptors in vertebrate brain evolution, we examined the distribution of trk neurotrophin receptors in representatives of several vertebrate classes. Trk receptors are largely expressed in homologous neuronal populations among different species/classes of vertebrates. In many neurons, trkB and trkC receptors are co-expressed. TrkB and trkC receptors are primarily found in neurons with more restricted, specialized dendritic and axonal fields that are thought to be involved in discriminative or 'analytical' functions. The neurotrophin receptor trkA is expressed predominantly in neurons with larger, overlapping dendritic fields with more heterogeneous connections ('integrative' or 'modulatory' systems) such as nociceptive and sympathetic autonomic nervous system, locus coeruleus and cholinergic basal forebrain. Surveys of trk receptor expression and function in the peripheral nervous system of different vertebrate classes reveal trends ranging from dependency on a single neurotrophin to a more complex dependency on increasing numbers of neurotrophins and their receptors, for example, in taste and inner ear innervation. Gene deletion studies in mice provide evidence for a complex regulation of neuronal survival of sensory ganglion cells by different neurotrophins. Although expression of neurotrophins and their receptors is predominantly conserved in most circuits, increasing diversity of neurotrophin ligands and their receptors and a more complex dependency of neurons on neurotrophins might have facilitated the formation of at least some new neuronal entities.

Epidermal growth factor (EGF) expression in lateral line system and in taste buds of adult zebrafish (Brachidanio rerio)

The mechano and chemosensory organs of adult teleosts undergoes a continuous cell renewal and turnover which is regulated in part by growth factors. Here, we investigated the occurrence and the cell localization of epidermal growth factor (EGF) in the lateral line system and taste bud of adult zebrafish, using Western blot and immunohistochemistry associated to a polyclonal antibody against mammalian EGF. Furthermore, the distribution of S100 protein was studied in parallel to label hair sensory cells in the lateral line system. Western blot revealed one unique protein band with an estimated molecular weight of about 13 kDa, equivalent to the EGF of mammals. Specific immunoreactivity for EGF was observed in the epithelial basal and/or supporting cells of the neuromasts of the lateral line system and taste buds. Conversely, the sensory cells in both sensory structures were devoid of immunostaining. Present results demonstrate the occurrence of EGF in mechano and sensory system of adult zebrafish, suggesting a role for this molecule in the cell renewal and turnover of these structures.

Immunohistochemical detection of neurotrophin-3 and -4, and their receptors in mouse taste bud cells

Neurotrophin-3 (NT3) and neurotrophin-4 (NT4) affect the survival and maintenance of central and peripheral neurons. Using an immunohistochemical method, we examined whether the taste bud cells in the circumvallate papillae of normal mice expressed NT3, NT4, and their respective receptors TrkC and TrkB, and if so, what type of cells in the taste buds expressed them. Double immunostaining for either of them and PGP 9.5, NCAM, or gustducin was used to determine which cell types expressed which neurotrophins and receptors. Normal taste bud cells expressed NT3, NT4, and the TrkB receptor, but not TrkC. The percentage of NT3-immunoreactive cells among all taste bud cells was 89.0%, that of NT4-immunoreactive cells, 58.6%, and that of TrkB-immunoreactive cells, 80.8%. Almost none of the NT4-immunoreactive cells were reactive with anti-PGP 9.5 or the anti-NCAM antibody, but they could be stained with anti-gustducin, revealing that NT4-immunoreactive cells were contained only in the type-II--and possibly type-I--cell population. On the other hand, NT3-, and TrkB-immunoreactive cells included type-III cells, together with type-II, -I, and basal cells, because they were positive for PGP 9.5 and gustducin. We conclude that NT4 may exert trophic actions on all types of taste bud cells by binding to their TrkB receptors, and NT3 may also have a similar, though negligible role.

Mutations that affect the survival of selected amacrine cell subpopulations define a new class of genetic defects in the vertebrate retina

Amacrine neurons are among the most diverse cell classes in the vertebrate retina. To gain insight into mechanisms vital to the production and survival of amacrine cell types, we investigated a group of mutations in three zebrafish loci: kleks (kle), chiorny (chy), and bergmann (bgm). Mutants of all three genes display a severe loss of selected amacrine cell subpopulations. The numbers of GABA-expressing amacrine interneurons are sharply reduced in all three mutants, while cell loss in other amacrine cell subpopulations varies and some cells are not affected at all. To investigate how amacrine cell loss affects retinal function, we performed electroretinograms on mutant animals. While the kle mutation mostly influences the function of the inner nuclear layer, unexpectedly the chy mutant phenotype also involves a loss of photoreceptor cell activity. The precise ration and arrangement of amacrine cell subpopulations suggest that cell-cell interactions are involved in the differentiation of this cell class. To test whether defects of such interactions may be, at least in part, responsible for mutant phenotypes, we performed mosaic analysis and demonstrated that the loss of parvalbumin-positive amacrine cells in chy mutants is due to extrinsic (cell-nonautonomous) causes. The phenotype of another amacrine cell subpopulation, the GABA-positive cells, does not display a clear cell-nonautonomy in chy animals. These results indicate that environmental factors, possibly interactions among different subpopulations of amacrine neurons, are involved in the development of the amacrine cell class.

Effects of glossopharyngeal nerve section on the expression of neurotrophins and their receptors in lingual taste buds of adult mice

The expression of neurotrophins and neurotrophin receptors is essential for the proper establishment and function of many sensory systems. To determine which neurotrophins and neurotrophin receptors are expressed in taste buds, and in taste buds of mice following denervation, antibodies directed against the neurotrophins and their receptors were applied to adult mouse gustatory tissue. Immunohistochemistry reveals that nerve growth factor (NGF)-like immunoreactive (LIR), tyrosine kinase (trk) A-LIR, trkB-LIR, and p75-LIR elongated, differentiated taste cells are present within all lingual taste buds, whereas neither neurotrophin (NT)-3- nor trkC-LIR was detected in taste cells. Double-label immunohistochemistry using markers of different taste cell types in brain-derived neurotrophic factor (BDNF)LacZ mice reveals that BDNF (beta-gal) and trkB colocalize, mainly in type III taste cells. NGF, pro-NGF, and trkA coexist in type II taste cells, i.e., those expressing phospholipase Cbeta2 (PLCbeta2). p75-LIR also is present in both BDNF and NGF taste cell populations. To determine the neural dependence of neurotrophin expression in adult taste buds, glossopharyngeal nerves were cut unilaterally. During the period of denervation (10 days to 3 weeks), taste buds largely disappear, and few neurotrophin-expressing cells are present. Three weeks after nerve transection, nerve fascicles on the operated side of the tongue exhibit BDNF-LIR, NGF-LIR, and ubiquitin carboxyl terminal hydrolase (PGP 9.5)-LIR. However, BDNF-LIR staining intensity but not NGF-LIR or PGP 9.5-LIR is increased in nerve fascicles on the operated compared with the unoperated side. Five weeks following nerve transection, NT and NT receptor expression resumes and appears normal in taste buds and nerves. These results indicate that neurotrophin expression in taste buds is dependent on gustatory innervation, but expression in nerves is not dependent on contact with taste buds.

S100 protein is a useful and specific marker for hair cells of the lateral line system in postembryonic zebrafish

The neuromast of the lateral line system of zebrafish has become an ideal model for the study of both developmental genetics and the vertebrate auditory system. Interestingly, the hair cells of this system have been found to selectively display immunoreactivity for S100 protein in some teleosts. In order to provide a selective marker for the sensory cells of the lateral line system, we have analyzed immunohistochemically the expression of S100 protein in zebrafish from the larval to the adult stage. In larval and adult animals S100 protein immunoreactivity was detected restricted to the hair cells of both superficial and canal neuromasts. Apparently the expression of S100 protein by hair cells was independent of the age, but it was expressed heterogeneously in the hair cells of canal neuromasts. The results of this work provide a feasible method to easily identify sensory cells in the neuromasts, and may be of interest in studies regarding development, differentiation or turnover of hair cells.