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.

Acid-sensing ion channel 2 (ASIC2) in the intestine of adult zebrafish.

Acid-sensing ion channels (ASICs) in mammals monitor acid sensing and mechanoreception. They have a widespread expression in the central and peripheral nervous system, including the gut. The distribution of ASICs in zebrafish is known only in larvae and at the mRNA level. Here we have investigated the expression and cell distribution of ASIC2 in the gut of adult zebrafish using PCR, Western blot and immunohistochemistry. ASIC2 mRNA was detected in the gut, and a protein consistent with predicted ASIC2 (64kDa molecular mass) was detected by Western blot. ASIC2 positivity was found in a subpopulation of myenteric neurons in the enteric nervous system, as well in enteroendocrine epithelial cells. These data demonstrate for the first time the occurrence of ASIC2 in the gut of adult zebrafish where it presumably acts as a chemosensor and a mechanosensor.

The lingual dorsal surface of the blue-tongue skink (Tiliqua scincoides).

The blue-tongue lizard (Tiliqua scincoides) is a variety of large skink common throughout Australia. There are seven species of Tiliqua and all of them have long bodies, short limbs and short and robust tails. T. scincoides occurs in a wide range of habitats; its diet is omnivorous. When threatened, it opens the mouth and protrudes its characteristic large fleshy cobalt blue tongue. It is currently found as a popular species and also as a pet animal in the European countries. No data are available in literature about the morphology of the tongue of T. scincoides; therefore, the aim of the present study was to investigate by means of scanning electron microscopy and light microscopy, the anatomy of the dorsal lingual surface. Our results demonstrate the presence of a tongue tip with a smooth surface without papillae. The foretongue was characterized by a stratified epithelium with foliate-like papillae and deep inter-papillar spaces in the middle part and cylindrical papillae with a flat surface in the lateral parts. All the posterior area of the tongue was characterized by more compacted papillae and the inter-papillar spaces were very narrow. Light microscopy showed the presence of melanin throughout the tongue. No taste buds were recognized on the lingual dorsal surface. Therefore, the papillae probably have a mechanical function showing an important role in the swallowing phase. The morphology of the tongue surface can be correlated to the diet and, different roles, as in other examined species, can be hypothesized for different areas.

Enteric glial cells express full-length TrkB and depend on TrkB expression for normal development.

The embryonic development of the enteric nervous system (ENS) from neural crest precursor cells requires neurotrophic signaling. Neurotrophins (NTs) are a family of growth factors that bind Trk receptors to signal diverse functions, including development and maintenance of different cell populations in the peripheral nervous system. In this study we investigated the expression and cell localization of TrkB, the high affinity receptor for brain-derived neurotrophic factor and NT-4, in the murine ENS using Western blot and immunohistochemistry. The results demonstrate that enteric glial cells within the ENS express full-length TrkB at all stages tested. The ENS of TrkB deficient mice have reduced expression of glial cell markers, and a disarrangement of glial cells and the plexular neuropil. These results strongly suggest TrkB has essential roles in the normal development and maintenance of glial cells in the ENS.

Scanning electron microscopical study of the lingual epithelium of green iguana (Iguana iguana).

During the last few years, green iguanas (Iguana iguana) have turned out to be one of the most popular pets. They are omnivorous. In their way of feeding, this crucial function is performed by capturing of the preys and mostly, this is carried out by the tongue. The role of the tongue is also fundamental during the intra-oral transport and during the swallowing of food. This has been reported in several studies about chameleons, agamids and iguanids, nevertheless published data about the mechanisms of capturing and swallowing the prey, and the morphological descriptions about the tongue epithelium, are scarce. Therefore, the aim of this present study was to analyse the morphology of the lingual epithelium in green iguanas by scanning electron microscopy. Three different areas were demonstrated on the tongue surface: the tongue tip, characterized by a smooth epithelium without papillae, a foretongue, completely covered by numerous closely packed cylindriform papillae, and a hindtongue with conical-like papillae. Some taste buds were recognized on the middle and the posterior parts of the tongue. Different functional roles could be hypothesized for the three tongue areas: the tongue tip could have a role related to the movements of the prey immediately after the capturing, while the middle papillae and the hindtongue could have an important role concerning the swallowing phase.

Calretinin in the peripheral nervous system of the adult zebrafish.

Calretinin is a calcium-binding protein found widely distributed in the central nervous system and chemosensory cells of the teleosts, but its presence in the peripheral nervous system of fishes is unknown. In this study we used Western blot analysis and immunohistochemistry to investigate the occurrence and distribution of calretinin in the cranial nerve ganglia, dorsal root ganglia, sympathetic ganglia, and enteric nervous system of the adult zebrafish. By Western blotting a unique and specific protein band with an estimated molecular weight of around 30 kDa was detected, and it was identified as calretinin. Immunohistochemistry revealed that calretinin is selectively present in the cytoplasm of the neurons and never in the satellite glial cells. In both sensory and sympathetic ganglia the density of neurons that were immunolabelled, their size and morphology, as well as the intensity of immunostaining developed within the cytoplasm, were heterogeneous. In the enteric nervous system calretinin immunoreactivity was detected in a subset of enteric neurons as well as in a nerve fibre plexus localized inside the muscular layers. The present results demonstrate that in addition to the central nervous system, calretinin is also present in the peripheral nervous system of zebrafish, and contribute to completing the map of the distribution of this protein in the nervous system of teleosts.

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.

Effects of increased nerve growth factor plasma levels on the expression of TrkA and p75 in rat testicles.

In addition to their well-known roles within the nervous system, the neurotrophins and their receptors regulate some functions in the reproductive system. In this study we used combined morphological and immunohistochemical techniques to investigate the presence and cellular localization in the rat testicle of the two receptors of nerve growth factor (NGF), i.e. TrkA and p75(NTR). Furthermore, to evaluate whether increased plasma levels of NGF affect the ageing process, 4-methylcathechol (4-MC), an inductor of NGF synthesis, was administered. Both TrkA and p75(NTR) were expressed in rat testicles, but the pattern and intensity of immunoreaction were marginally different between them. In adult rats TrkA was expressed in spermatozoa and spermatids, and p75 was expressed in spermatogonia. In newborn rats TrkA immunoreactivity was found in the Leydig cells, whereas p75 was detected in a cellular layer that surrounds the seminiferous tubules. In adult treated animals the immunoreaction for TrkA and p75(NTR) was also localized in the spermatocytes, whereas in newborn treated rats no changes in the pattern of immunoreaction was observed. The present findings suggest a role of the NGF/TrkA/p75 system in the physiology of reproduction, but the practical relevance of this remains to be established.

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.

TrkA and p75NTR in the ovary of adult cow and pig.

Neurotrophins play a critical role in the development of the mammalian ovary, oogenesis and folliculogenesis. In this study we investigated the cell localization of the two main receptors for nerve growth factor (NGF), TrkA and p75NTR, in the cow and pig ovary, using immunohistochemistry. Specific immunoreactivity for TrkA and p75NTR was detected in the ovary of both species, but the pattern and intensity of immunostaining were marginally different between them. The follicular cells regularly expressed immunoreactivity for both receptors. Immunoreactivity was also detected in the oocytes, independently of the maturational stage of the follicles, with the exception of primordial and primary follicles of the pig which did not display p75NTR. Taken together, these results suggest a possible direct role of NGF on oocytes expressing TrkA and p75NTR, in addition to the well-known roles in other ovary functions. The practical relevance of these data remains to be clarified.

Immunohistochemical detection of [corrected] TrkB in the enteric nervous system of the small intestine in pigeon (Columba livia).

The presence and cell localization of TrkB, the main receptor for the neurotrophins (NTs), was investigated immunohistochemically in the small intestine of adult pigeons, with special reference to the enteric nervous system (ENS). Several neuronal (neurofilament proteins and PGP 9.5) and glial cell (S100 protein) markers were studied in parallel. TrkB immunoreactivity (TrkB-IR) was found to be restricted to immunohistochemically-identified glial cells present in the enteric plexuses, and to Schwann cells forming the perivascular plexus. Also, TrkB-IR was detected in enterochromaffin cells and in unidentified dendritic cells within the gut-associated lymphoid tissue. The present results demonstrate that as for mammals, TrkB in the ENS is restricted to the glial cells. The possible function of the TrkB ligands, however, remains to be established.

Neurotrophin receptor-like proteins in the bovine (Bos taurus) lymphoid organs, with special reference to thymus and spleen.

Increasing evidence suggests that neurotrophins could regulate immune functions acting directly or indirectly on immunocompetent cells. The indirect pathway involves stromal cells of the primary and secondary lymphoid organs. In the present study the occurrence of Trk proteins (TrkA, TrkB and TrkC), regarded as the high-affinity signal-transducing receptors for neurotrophins, was investigated in cow lymphoid organs using immunohistochemistry. The thymus and spleen of both fetal and adult animals, and the palatine tonsils, lymph nodes and Peyer's patches of adult animals, were analysed. Unidentified cells displaying TrkA-like immunoreactivity were found in the fetal thymus, whereas those expressing this protein in the adult gland were identified as epithelial cells. In the spleen, immunoreactive TrkA was observed in cells of the white pulp. TrkB immunoreactivity in both fetal and adult thymus and spleen was localized in monocyte/macrophage cells. As a rule, TrkC was absent from the thymus and the spleen independent of the animal's age. Different types of stromal cells, but never the lymphocytes themselves, displayed TrkA, TrkB, or TrkC immunoreactivity in the other lymphoid organs analysed. As in other vertebrate species, Trk proteins in the lymphoid organs of the cow were localized in the stromal, non-lymphoid cells, thus suggesting that neurotrophins might regulate the immune function acting indirectly on lymphocytes.

Distribution of neurotrophin receptors in human palatine tonsils: an immunohistochemical study.

Increasing evidence indicates that nerve growth factor (NGF) exerts effects on cells of the immune system, but the possible immunomodulatory effect of other neurotrophins (brain-derived neurotrophic factor, BDNF; neurotrophin-3, NT-3; and NT-4/5) has not been studied. Neurotrophins act on responsive cells by binding a low-affinity pan-neurotrophin receptor (p75), and more specific high-affinity receptors (gp140trkA, gp145trkB and gp145trkC considered as preferred signaling transduction receptors for NGF, BDNF and NT-3, respectively). The expression of neurotrophin receptor proteins may be considered, therefore, as a potential indication of neurotrophin activity. In the present study we investigated the distribution of both types of neurotrophin receptors in the human palatine tonsils using immunohistochemical methods. In the follicular germinal centers both lymphocytes and follicular dendritic cells (FDC) displayed gp75 IR, but not IR for trk neurotrophin receptor proteins. gp140trkA-like IR and gp145trkC-like IR were encountered on paracortical interdigitating cells (PIC), and in the high endothelial venule cells. gp145trkB-like IR was found in a cell subpopulation which probably represented macrophages. Present results suggest that NGF, NT-3 and NT-4/5 may act in PIC and indirectly in lymphocytes, whereas BDNF and NT-4/5 could control macrophages. The role of p75 on lymphocytes and FDC and whether trk neurotrophin receptor proteins present in lymphoid tissues are functional receptors for neurotrophins remains to be elucidated.