Afferent and efferent connections of the cerebellum of a salmonid, the rainbow trout (Oncorhynchus mykiss): A tract-tracing study

Neural connections of the torus semicircularis in the adult Zebrafish

The torus semicircularis (TS) of teleosts is a key midbrain center of the lateral line and acoustic sensory systems. To characterize the TS in adult zebrafish, we studied their connections using the carbocyanine tracers applied to the TS and to other related nuclei and tracts. Two main TS nuclei, central and ventrolateral, were differentiable by their afferent connections. From central TS, (TSc) numerous toropetal cells were labeled bilaterally in several primary octaval nuclei (anterior, magnocellular, descending, and posterior octaval nuclei), in the secondary octaval nucleus, in the caudal octavolateralis nucleus, and in the perilemniscular region. In the midbrain, numerous toropetal cells were labeled in the contralateral TSc. In the diencephalon, toropetal cells labeled from the TSc were observed ipsilaterally in the medial prethalamic nucleus and the periventricular posterior tubercle nucleus. TSc toropetal neurons were also labeled bilaterally in the hypothalamic anterior tuberal nucleus (ATN) and ipsilaterally in the parvicellular preoptic nucleus but not in the telencephalon. Tracer application to the medial octavolateralis nucleus revealed contralateral projections to the ventrolateral TS (TSvl), whereas tracer application to the secondary octaval nucleus labeled fibers bilaterally in TSc and neurons in rostral TSc. The TSc sends ascending fibers to the ipsilateral lateral preglomerular region that, in turn, projects to the pallium. Application of DiI to the optic tectum labeled cells and fibers in the TSvl, whereas application of DiI to the ATN labeled cells and fibers in the TSc. These results reveal that the TSvl and TSc are mainly related with the mechanosensory lateral line and acoustic centers, respectively, and that they show different higher order connections.

Ascending Visual Pathways to the Telencephalon in Teleosts with Special Focus on Forebrain Visual Centers, Associated Neural Circuitries, and Evolution

Visual pathways to the telencephalon in teleost fishes have been studied in detail only in a few species, and their evolutionary history remained unclear. On the basis of our recent studies we propose that there were two visual pathways in the common ancestor of teleosts, while one of them became lost in acanthopterygian fishes that emerged relatively recently. Our in-depth analyses on the connections of visual centers also revealed that there are connections shared with those of mammals, and retinotopic organization of the ascending connections is maintained at least to the level of the diencephalon in the yellowfin goby. The major visual telencephalic center, or the lateral part of the dorsal telencephalon (Dl), shows considerable species differences in the number of regions and cytoarchitecture. In particular, four highly specialized compartments are noted in the Dl of gobies, and we analyzed about 100 species of teleosts to investigate the evolution of the compartments in the Dl, which indicated that four compartments emerged only in Gobiiformes, while there are fewer specialized compartments in some other percomorph lineages. We also discuss the connections of forebrain visual centers with the cerebellum and other lower brain centers and infer possible functions of the circuitries.

Histological and Molecular Characterization of the Inferior Olivary Nucleus and Climbing Fibers in the Goldfish, Carassius auratus

The cerebellum receives inputs via the climbing fibers originating from the inferior olivary nucleus in the ventral medulla. In mammals, the climbing fibers entwine and terminate onto both major and peripheral branches of dendrites of the Purkinje cells. In this study, the inferior olivary nucleus and climbing fiber in the goldfish were investigated with several histological techniques. By neural tracer application to the hemisphere of the cerebellum, labeled inferior olivary neurons were found in the ventral edge of the contralateral medulla. Kainate stimulated Co ^{+ +} uptake and gephyrin immunoreactivities were found in inferior olivary neurons, indicating, respectively, that they receive both excitatory (glutamatergic) and inhibitory (GABAergic or glycinergic) inputs. Inferior olivary neurons express vglut2.1 transcripts, suggesting they are glutamatergic. Around 85% of inferior olivary neurons were labeled with anti-calretinin antiserum. Calretinin immunoreactive (ir) climbing fiber terminal-like structures were distributed near the Purkinje cells and in the molecular layer. Double labeling immunofluorescence with anti-calretinin and zebrin II antisera revealed that the calretinin-ir climbing fibers run along and made synaptic-like contacts on the major dendrites of the zebrin II-ir Purkinje cells. In teleost fish, cerebellar efferent neurons, eurydendroid cells, also lie near the Purkinje cells and extend dendrites outward to intermingle with dendrites of the Purkinje cells within the molecular layer. Here we found no contacts between the climbing fiber terminals and the eurydendroid cell dendrites. These results support the idea that Purkinje cells, but not eurydendroid cells, receive strong inputs via the climbing fibers, similar to the mammalian situation.

Afferent and efferent connections of the nucleus posterior tuberis in the firemouth cichlid, Thorichthys meeki

The nucleus posterior tuberis (NPT) in teleost fishes, also called posterior tuberal nucleus, is situated in the posterior tuberculum of the diencephalon. It is fused across the midline and densely packed with small cells, but little is known about its connections. In this study, the afferent and efferent connections of the NPT were examined by means of tracer applications of the carbocyanine dye DiI in the firemouth cichlid, Thorichthys meeki. Retrogradely labeled cell bodies were found in the corpus mamillare and nucleus periventricularis of the inferior lobe; and anterogradely labeled terminal fibers were detected in the medial zone of the dorsal telencephalon, medial part of the nucleus lateralis tuberis, dorsal posterior thalamic nucleus, torus lateralis, medial part of the nucleus diffusus of the inferior lobe, and tectum opticum. All these connections show an ipsilateral tendency. The NPT is apparently a significant relay nucleus in the diencephalon of T. meeki, and possibly involved in a variety of feedback circuits. It seems also to be part of a tecto-hypothalamo-telencephalic pathway in cichlids.

Transduction of Brain Neurons in Juvenile Chum Salmon (Oncorhynchus keta) with Recombinant Adeno-Associated Hippocampal Virus Injected into the Cerebellum during Long-Term Monitoring

Corpus cerebelli in juvenile chum salmon is a multiprojective region of the brain connected via afferent and efferent projections with the higher regions of the brainstem and synencephalon, as well as with multiprojection regions of the medulla oblongata and spinal cord. During the postembryonic development of the cerebellum in chum salmon, Oncorhynchus keta, the lateral part of the juvenile cerebellum gives rise to the caudomedial part of the definitive cerebellum, which is consistent with the data reported for zebrafish and mouse cerebellum. Thus, the topographic organization of the cerebellum and its efferents are similar between fish (chum salmon and zebrafish) and mammals, including mice and humans. The distributions of recombinant adeno-associated viral vectors (rAAVs) after an injection of the base vector into the cerebellum have shown highly specific patterns of transgene expression in bipolar neurons in the latero-caudal lobe of the juvenile chum tectum opticum. The distribution of rAAVs in the dorsal thalamus, epithalamus, nucleus rotundus, and pretectal complex indicates the targeted distribution of the transgene via the thalamo-cerebellar projections. The detection of GFP expression in the cells of the epiphysis and posterior tubercle of juvenile chum salmon is associated with the transgene’s distribution and with the cerebrospinal fluid flow, the brain ventricles and its outer surface. The direct delivery of the rAAV into the central nervous system by intracerebroventricular administration allows it to spread widely in the brain. Thus, the presence of special projection areas in the juvenile chum salmon cerebellum, as well as outside it, and the identification of the transgene’s expression in them confirm the potential ability of rAAVs to distribute in both intracerebellar and afferent and efferent extracerebellar projections of the cerebellum.

Patterns of Organization of Cerebellum and Spinal Cord of the Red-Tail Shark (Epalzeorhynchos bicolor): Histological, Morphometrical, and Immunohistochemical Studies

Teleosts exhibit enormous heterogeneity in brain morphology, especially in the patterns of the organization of cerebellum. The cerebellum of a red-tail shark that we analyzed was well-developed and included three main divisions: the valvula cerebelli, the corpus cerebelli, and the vestibulolateral lobe. Characteristically, the cerebellar cortex contained three well-distinct layers: an outer molecular, intermediate ganglionic, and inner granular layer. The ganglionic layer possessed irregularly arranged Purkinje cells and eurydendroid cells that extended their processes into the molecular layer. Both Purkinje cells and eurydendroid cells showed immunoreactivity for iNOS2. Moreover, astrocytes in the cerebellum showed the expression of glial fibrillary acidic protein. The most striking observation in the cerebellum of shark was the lack of deep cerebellar nuclei and a well-identified white matter. On the other hand, the gray substance in the spinal cord displays a characteristic pattern in its organization, in which the dorsal horns lie quite close together, giving the gray substance the shape of an inverted Y and possessing large neurons. Notably, the white matter possessed myelinated nerve fibers. The current study provides the first report on the organization of layers and neurons in the cerebellum and spinal cord of red-tail shark. This research will contribute to the neuroanatomy and evolutionary studies of the brain of Cyprinidae.

Afferent and efferent connections of the nucleus prethalamicus in the yellowfin goby Acanthogobius flavimanus

The nucleus prethalamicus (PTh) receives fibers from the optic tectum and then projects to the dorsal telencephalon in the yellowfin goby Acanthogobius flavimanus. However, it remained unclear whether the PTh is a visual relay nucleus, because the optic tectum receives not only visual but also other sensory modalities. Furthermore, precise telencephalic regions receiving prethalamic input remained unknown in the goby. We therefore investigated the full set of afferent and efferent connections of the PTh by direct tracer injections into the nucleus. Injections into the PTh labeled cells in the optic tectum, ventromedial thalamic nucleus, central and medial parts of the dorsal telencephalon, and caudal lobe of the cerebellum. We found that the somata of most tecto-prethalamic neurons are present in the stratum periventriculare. Their dendrites ascend to reach the major retinorecipient layers of the tectum. The PTh is composed of two subnuclei (medial and lateral) and topographic organization was appreciated only for tectal projections to the lateral subnucleus (PTh-l), which also receives sparse retinal projections. In contrast, the medial subnucleus receives fibers only from the medial tectum. We found that the PTh projects to nine subregions in the dorsal telencephalon and four in the ventral telencephalon. Furthermore, cerebellar injections revealed that cerebello-prethalamic fibers cross the midline twice to innervate the PTh-l on both sides. The present study is the first detailed report on the full set of the connections of PTh, which suggests that the PTh relays visual information from the optic tectum to the telencephalon.

Anatomy and Connectivity of the Torus Longitudinalis of the Adult Zebrafish

This study describes the cytoarchitecture of the torus longitudinalis (TL) in adult zebrafish by using light and electron microscopy, as well as its main connections as revealed by DiI tract tracing. In addition, by using high resolution confocal imaging followed by digital tracing, we describe the morphology of tectal pyramidal cells (type I cells) that are GFP positive in the transgenic line Tg(1.4dlx5a-dlx6a:GFP)^ot1. The TL consists of numerous small and medium-sized neurons located in a longitudinal eminence attached to the medial optic tectum. A small proportion of these neurons are GABAergic. The neuropil shows three types of synaptic terminals and numerous dendrites. Tracing experiments revealed that the main efference of the TL is formed of parallel-like fibers that course within the marginal layer of the optic tectum. A toral projection to the thalamic nucleus rostrolateralis is also observed. Afferents to the TL come from visual and cerebellum-related nuclei in the pretectum, namely the central, intercalated and the paracommissural pretectal nuclei, as well as from the subvalvular nucleus in the isthmus. Additional afferents to the TL may come from the cerebellum but their origins could not be confirmed. The tectal afferent projection to the TL originates from cells similar to the type X cells described in other cyprinids. Tectal pyramidal neurons show round or piriform cell bodies, with spiny apical dendritic trees in the marginal layer. This anatomical study provides a basis for future functional and developmental studies focused on this cerebellum-like circuit in zebrafish.

Differential expression of five prosomatostatin genes in the central nervous system of the catshark Scyliorhinus canicula

Five prosomatostatin genes (PSST1, PSST2, PSST3, PSST5, and PSST6) have been recently identified in elasmobranchs (Tostivint et al., General and Comparative Endocrinology, 2019, 279, 139-147). In order to gain insight into the contribution of each somatostatin to specific nervous systems circuits and behaviors in this important jawed vertebrate group, we studied the distribution of neurons expressing PSST mRNAs in the central nervous system (CNS) of Scyliorhinus canicula using in situ hybridization. Additionally, we combined in situ hybridization with tyrosine hydroxylase (TH) immunochemistry for better characterization of PSST1 and PSST6 expressing populations. We observed differential expression of PSST1 and PSST6, which are the most widely expressed PSST transcripts, in cell populations of many CNS regions, including the pallium, subpallium, hypothalamus, diencephalon, optic tectum, midbrain tegmentum, and rhombencephalon. Interestingly, numerous small pallial neurons express PSST1 and PSST6, although in different populations judging from the colocalization of TH immunoreactivity and PSST6 expression but not with PSST1. We observed expression of PSST1 in cerebrospinal fluid-contacting (CSF-c) neurons of the hypothalamic paraventricular organ and the central canal of the spinal cord. Unlike PSST1 and PSST6, PSST2, and PSST3 are only expressed in cells of the hypothalamus and in some hindbrain lateral reticular neurons, and PSST5 in cells of the region of the entopeduncular nucleus. Comparative data of brain expression of PSST genes indicate that the somatostatinergic system of sharks is the most complex reported in any fish.

Tracing of Afferent Connections in the Zebrafish Cerebellum Using Recombinant Rabies Virus

The cerebellum is involved in some forms of motor coordination and learning, and in cognitive and emotional functions. To elucidate the functions of the cerebellum, it is important to unravel the detailed connections of the cerebellar neurons. Although the cerebellar neural circuit structure is generally conserved among vertebrates, it is not clear whether the cerebellum receives and processes the same or similar information in different vertebrate species. Here, we performed monosynaptic retrograde tracing with recombinant rabies viruses (RV) to identify the afferent connections of the zebrafish cerebellar neurons. We used a G-deleted RV that expressed GFP. The virus was also pseudotyped with EnvA, an envelope protein of avian sarcoma and leucosis virus (ALSV-A). For the specific infection of cerebellar neurons, we expressed the RV glycoprotein (G) gene and the envelope protein TVA, which is the receptor for EnvA, in Purkinje cells (PCs) or granule cells (GCs), using the promoter for aldolase Ca (aldoca) or cerebellin 12 (cbln12), respectively. When the virus infected PCs in the aldoca line, GFP was detected in the PCs’ presynaptic neurons, including GCs and neurons in the inferior olivary nuclei (IOs), which send climbing fibers (CFs). These observations validated the RV tracing method in zebrafish. When the virus infected GCs in the cbln12 line, GFP was again detected in their presynaptic neurons, including neurons in the pretectal nuclei, the nucleus lateralis valvulae (NLV), the central gray (CG), the medial octavolateralis nucleus (MON), and the descending octaval nucleus (DON). GFP was not observed in these neurons when the virus infected PCs in the aldoca line. These precerebellar neurons generally agree with those reported for other teleost species and are at least partly conserved with those in mammals. Our results demonstrate that the RV system can be used for connectome analyses in zebrafish, and provide fundamental information about the cerebellar neural circuits, which will be valuable for elucidating the functions of cerebellar neural circuits in zebrafish.

Neural connections of the pretectum in zebrafish (Danio rerio)

The pretectum is a complex region of the caudal diencephalon which in adult zebrafish comprises both retinorecipient (parvocellular superficial, central, intercalated, paracommissural, and periventricular) and non-retinorecipient (magnocellular superficial, posterior, and accessory) pretectal nuclei distributed from periventricular to superficial regions. We conducted a comprehensive study of the connections of pretectal nuclei by using neuronal tracing with fluorescent carbocyanine dyes. This study reveals specialization of efferent connections of the various pretectal nuclei, with nuclei projecting to the optic tectum (paracommissural, central, and periventricular pretectal nuclei), the torus longitudinalis and the cerebellar corpus (paracommissural, central, and intercalated pretectal nuclei), the lateral hypothalamus (magnocellular superficial, posterior, and central pretectal nuclei), and the tegmental regions (accessory and superficial pretectal nuclei). With regard to major central afferents to the pretectum, we observed projections from the telencephalon to the paracommissural and central pretectal nuclei, from the optic tectum to the paracommissural, central, accessory and parvocellular superficial pretectal nuclei, from the cerebellum to the paracommissural and periventricular pretectal nuclei and from the nucleus isthmi to the parvocellular superficial and accessory pretectal nuclei. The parvocellular superficial pretectal nucleus sends conspicuous projections to the contralateral magnocellular superficial pretectal nucleus. The composite figure of results reveals large differences in connections of neighbor pretectal nuclei, indicating high degree of nuclear specialization. Our results will have important bearings in functional studies that analyze the relationship between specific circuits and behaviors in zebrafish. Comparison with results available in other species also reveals differences in the organization and connections of the pretectum in vertebrates.

Dopamine system, cerebellum, and nucleus ruber in fish and mammals

Small teleost fish including zebrafish and medaka have been used as animal models for research because of their small body size, vast amounts of eggs produced, their rapid development, low husbandry costs, and transparency during embryogenesis. Although the body size and appearance seem different, fish and mammals including human still possess anatomical and functional similarities in their brains. This review summarizes the similarities of brain structures and functions between teleost fish and mammalian brains, focusing on the dopamine system, functional regionalization of the cerebellum, and presence of the nucleus ruber.

Nucleus Ruber of Actinopterygians

Nucleus ruber is known as an important supraspinal center that controls forelimb movements in tetrapods, and the rubral homologue may serve similar functions in fishes (motor control of pectoral fin). However, two apparently different structures have been identified as 'nucleus ruber' in actinopterygians. One is nucleus ruber of Goldstein (1905) (NRg), and the other nucleus ruber of Nieuwenhuys and Pouwels (1983) (NRnp). It remains unclear whether one of these nuclei (or perhaps both) is homologous to tetrapod nucleus ruber. To resolve this issue from a phylogenetic point of view, we have investigated the distribution of tegmental neurons retrogradely labeled from the spinal cord in eight actinopterygian species. We also investigated the presence/absence of the two nuclei with Nissl- or Bodian-stained brain section series of an additional 28 actinopterygian species by comparing the morphological features of candidate rubral neurons with those of neurons revealed by the tracer studies. Based on these analyses, the NRg was identified in all actinopterygians investigated in the present study, while the NRnp appears to be absent in basal actinopterygians. The phylogenetic distribution pattern indicates that the NRg is the more likely homologue of nucleus ruber, and the NRnp may be a derived nucleus that emerged during the course of actinopterygian evolution.

Gustatory and general visceral centers and their connections in the brain of adult zebrafish: a carbocyanine dye tract-tracing study

The central connections of the gustatory/general visceral system of the adult zebrafish (Danio rerio) were examined by means of carbocyanine dye tracing. Main primary gustatory centers (facial and vagal lobes) received sensory projections from the facial and vagal nerves, respectively. The vagal nerve also projects to the commissural nucleus of Cajal, a general visceral sensory center. These primary centers mainly project on a prominent secondary gustatory and general visceral nucleus (SGN/V) located in the isthmic region. Secondary projections on the SGN/V were topographically organized, those of the facial lobe mainly ending medially to those of the vagal lobe, and those from the commissural nucleus ventrolaterally. Descending facial lobe projections to the medial funicular nucleus were also noted. Ascending fibers originating from the SGN/V mainly projected to the posterior thalamic nucleus and the lateral hypothalamus (lateral torus, lateral recess nucleus, hypothalamic inferior lobe diffuse nucleus) and an intermediate cell- and fiber-rich region termed here the tertiary gustatory nucleus proper, but not to a nucleus formerly considered as the zebrafish tertiary gustatory nucleus. The posterior thalamic nucleus, tertiary gustatory nucleus proper, and nucleus of the lateral recess gave rise to descending projections to the SGN/V and the vagal lobe. The connectivity between diencephalic gustatory centers and the telencephalon was also investigated. The present results showed that the gustatory connections of the adult zebrafish are rather similar to those reported in other cyprinids, excepting the tertiary gustatory nucleus. Similarities between the gustatory systems of zebrafish and other fishes are also discussed. J. Comp. Neurol. 525:333-362, 2017. © 2016 Wiley Periodicals, Inc.

Eppur Si Muove: Evidence for an External Granular Layer and Possibly Transit Amplification in the Teleostean Cerebellum

The secreted signaling factor Sonic Hedgehog (Shh) acts in the floor plate of the developing vertebrate CNS to promote motoneuron development. In addition, shh has dorsal expression domains in the amniote alar plate (i.e., in isocortex, superior colliculus, and cerebellum). For example, shh expressing Purkinje cells act in transit amplification of external granular layer (EGL) cells of the developing cerebellum. Our previous studies had indicated the presence of an EGL in anamniote zebrafish, but a possible role of shh in the zebrafish cerebellar plate remained elusive. Therefore, we used an existing zebrafish transgenic line Tg(2.4shha-ABC-GFP)sb15; Shkumatava et al., 2004) to show this gene activity and its cellular localization in the larval zebrafish brain. Clearly, GFP expressing cells occur in larval alar zebrafish brain domains, i.e., optic tectum and cerebellum. Analysis of critical cerebellar cell markers on this transgenic background and a PH3 assay for mitotic cells reveals that Purkinje cells and eurydendroid cells are completely non-overlapping postmitotic cell populations. Furthermore, shh-GFP cells never express Zebrin II or parvalbumin, nor calretinin. They are thus neither Purkinje cells nor calretinin positive migrating rhombic lip derived cells. The shh-GFP cells also never correspond to PH3 positive cells of the ventral cerebellar proliferative zone or the upper rhombic lip-derived EGL. From this marker analysis and the location of shh-GFP cells sandwiched between calretinin positive rhombic lip derived cells and parvalbumin positive Purkinje cells, we conclude that shh-GFP expressing cells qualify as previously reported olig2 positive eurydendroid cells, which are homologous to the amniote deep cerebellar nuclei. We confirm this using double transgenic progeny of shh-GFP and olig2-dsRed zebrafish. Thus, these zebrafish eurydendroid cells may have the same role in transit amplification as Purkinje cells do in amniotes.

Identification of the zebrafish red nucleus using Wheat Germ Agglutinin transneuronal tracing

The red nucleus is located in the rostral midbrain of the vertebrate brain and controls motor coordination during locomotion. It receives input from the cerebellum and sends its output to the spinal cord. The presence of the red nucleus is well established in tetrapods, and its existence has also been suggested in teleosts but its presence and position has still been under discussion. By using wheat germ agglutinin (WGA) as a genetically encoded anterograde tracer, we recently identified contralateral projections from the cerebellum to a putative red nucleus in the zebrafish midbrain tegmentum. In this report we further revealed red nucleus derived from this contralateral afferent from the cerebellum using WGA and contralateral projections to the hindbrain-spinal cord junction site using DiI-mediated retrograde tracing. Thus the structure that we have identified by anterograde and retrograde tracing fulfills the anatomical demands for the red nucleus: the location in the midbrain tegmentum, contralateral afferent from the cerebellum (cerebello-ruber projection) and contralateral efferent to the spinal cord (rubro-spinal projection).

Establishment of Gal4 transgenic zebrafish lines for analysis of development of cerebellar neural circuitry

The cerebellum is involved in some forms of motor coordination and motor learning. Here we isolated transgenic (Tg) zebrafish lines that express a modified version of Gal4-VP16 (GFF) in the cerebellar neural circuits: granule, Purkinje, or eurydendroid cells, Bergmann glia, or the neurons in the inferior olive nuclei (IO) which send climbing fibers to Purkinje cells, with the transposon Tol2 system. By combining GFF lines with Tg lines carrying a reporter gene located downstream of Gal4 binding sequences (upstream activating sequence: UAS), we investigated the anatomy and developmental processes of the cerebellar neural circuitry. Combining an IO-specific Gal4 line with a UAS reporter line expressing the photoconvertible fluorescent protein Kaede demonstrated the contralateral projections of climbing fibers. Combining a granule cell-specific Gal4 line with a UAS reporter line expressing wheat germ agglutinin (WGA) confirmed direct and/or indirect connections of granule cells with Purkinje cells, eurydendroid cells, and IO neurons in zebrafish. Time-lapse analysis of a granule cell-specific Gal4 line revealed initial random movements and ventral migration of granule cell nuclei. Transgenesis of a reporter gene with another transposon Tol1 system visualized neuronal structure at a single cell resolution. Our findings indicate the usefulness of these zebrafish Gal4 Tg lines for studying the development and function of cerebellar neural circuits.

Immunohistochemical distribution of calretinin and calbindin (D-28k) in the brain of the cladistian Polypterus senegalus

Polypteriform fishes are believed to be basal to other living ray-finned bony fishes, and they may be useful for providing information of the neural organization that existed in the brain of the earliest ray-finned fishes. The calcium-binding proteins calretinin (CR) and calbindin-D28k (CB) have been widely used to characterize neuronal populations in vertebrate brains. Here, the distribution of the immunoreactivity against CR and CB was investigated in the olfactory organ and brain of Polypterus senegalus and compared to the distribution of these molecules in other ray-finned fishes. In general, CB-immunoreactive (ir) neurons were less abundant than CR-ir cells. CR immunohistochemistry revealed segregation of CR-ir olfactory receptor neurons in the olfactory mucosa and their bulbar projections. Our results confirmed important differences between pallial regions in terms of CR immunoreactivity of cell populations and afferent fibers. In the habenula, these calcium-binding proteins revealed right-left asymmetry of habenular subpopulations and segregation of their interpeduncular projections. CR immunohistochemistry distinguished among some thalamic, pretectal, and posterior tubercle-derived populations. Abundant CR-ir populations were observed in the midbrain, including the tectum. CR immunoreactivity was also useful for characterizing a putative secondary gustatory/visceral nucleus in the isthmus, and for distinguishing territories in the primary viscerosensory column and octavolateral region. Comparison of the data obtained within a segmental neuromeric context indicates that some CB-ir and CR-ir populations in polypteriform fishes are shared with other ray-finned fishes, but other positive structures appear to have evolved following the separation between polypterids and other ray-finned fishes.

A direct primary afferent projection of the trigeminal nerve to the valvula cerebelli in the spiny eel Macrognathus zebrinus

This study is a re-examination of the direct primary sensory input to the valvula cerebelli in spiny eel. The valvula in Macrognathus zebrinus receives a primary afferent projection from the trigeminal nerve as revealed by injections of biotinylated dextran amines into the rostrum. The descending trigeminal nucleus and nucleus of the tractus solitarius are innervated as well. Injections with DiI into the valvula labeled fibers in the descending trigeminal nucleus. The projection of tactile information from the rostrum to the valvula may be an adaptation of food search for spiny eels in regard to their highly mobile rostrum.

Cerebellar output in zebrafish: an analysis of spatial patterns and topography in eurydendroid cell projections

The cerebellum is a brain region responsible for motor coordination and for refining motor programs. While a great deal is known about the structure and connectivity of the mammalian cerebellum, fundamental questions regarding its function in behavior remain unanswered. Recently, the zebrafish has emerged as a useful model organism for cerebellar studies, owing in part to the similarity in cerebellar circuits between zebrafish and mammals. While the cell types composing their cerebellar cortical circuits are generally conserved with mammals, zebrafish lack deep cerebellar nuclei, and instead a majority of cerebellar output comes from a single type of neuron: the eurydendroid cell. To describe spatial patterns of cerebellar output in zebrafish, we have used genetic techniques to label and trace eurydendroid cells individually and en masse. We have found that cerebellar output targets the thalamus and optic tectum, and have confirmed the presence of pre-synaptic terminals from eurydendroid cells in these structures using a synaptically targeted GFP. By observing individual eurydendroid cells, we have shown that different medial-lateral regions of the cerebellum have eurydendroid cells projecting to different targets. Finally, we found topographic organization in the connectivity between the cerebellum and the optic tectum, where more medial eurydendroid cells project to the rostral tectum while lateral cells project to the caudal tectum. These findings indicate that there is spatial logic underpinning cerebellar output in zebrafish with likely implications for cerebellar function.

Anatomy of zebrafish cerebellum and screen for mutations affecting its development

The cerebellum is important for the integration of sensory perception and motor control, but its structure has mostly been studied in mammals. Here, we describe the cell types and neural tracts of the adult zebrafish cerebellum using molecular markers and transgenic lines. Cerebellar neurons are categorized to two major groups: GABAergic and glutamatergic neurons. The Purkinje cells, which are GABAergic neurons, express parvalbumin7, carbonic anhydrase 8, and aldolase C like (zebrin II). The glutamatergic neurons are vglut1(+) granule cells and vglut2(high) cells, which receive Purkinje cell inputs; some vglut2(high) cells are eurydendroid cells, which are equivalent to the mammalian deep cerebellar nuclei. We found olig2(+) neurons in the adult cerebellum and ascertained that at least some of them are eurydendroid cells. We identified markers for climbing and mossy afferent fibers, efferent fibers, and parallel fibers from granule cells. Furthermore, we found that the cerebellum-like structures in the optic tectum and antero-dorsal hindbrain show similar Parvalbumin7 and Vglut1 expression profiles as the cerebellum. The differentiation of GABAergic and glutamatergic neurons begins 3 days post-fertilization (dpf), and layers are first detectable 5 dpf. Using anti-Parvalbumin7 and Vglut1 antibodies to label Purkinje cells and granule cell axons, respectively, we screened for mutations affecting cerebellar neuronal development and the formation of neural tracts. Our data provide a platform for future studies of zebrafish cerebellar development.

Immunohistochemical localization of glucokinase in rainbow trout brain

In mammals, glucosensing neurons reside in brain areas known to play a critical regulatory role in energy homeostasis and neuroendocrine function. In fish, although no glucosensing neurons have been characterized to date, glucokinase (GCK; the main glucosensing marker in mammals) activity and expression were found in hypothalamus and hindbrain of rainbow trout where they related to food intake regulation and glucose homeostasis. However, there are no available studies in literature regarding GCK distribution in brain areas as well as the cell types expressing that protein. In the present study we hypothesize that, as occurs in mammals, GCK would be localized where glucosensing areas have been described. In this sense, we have found GCK immunoreactivity in several areas of trout hypothalamus, of which some of them are related to glucose metabolism, energy homeostasis and food intake, including the lateral hypothalamus, anterior tuberal nucleus, posterior tuberal nucleus and lateral tuberal nucleus. On the other hand, GCKimmunoreactivity was also observed in other areas where the glucosensor system is probably functional,such as the preoptic area and the oculomotor nucleus. Therefore, in this study using immunoreactive techniques, we have demonstrated in those specific areas of the rainbow trout brain previously described as glucosensor the presence of GCK in different cell types.

Effects of algal-produced neurotoxins on metabolic activity in telencephalon, optic tectum and cerebellum of Atlantic salmon (Salmo salar)

Neurotoxins from algal blooms have been reported to cause mortality in a variety of species, including sea birds, sea mammals and fish. Farmed fish cannot escape harmful algal blooms and their potential toxins, thus they are more vulnerable for exposure than wild stocks. Sublethal doses of the toxins are likely to affect fish behaviour and may impair cognitive abilities. In the present study, changes in the metabolic activity in different parts of the Atlantic salmon (Salmo salar) brain involved in central integration and cognition were investigated after exposure to sublethal doses of three algal-produced neurotoxins; saxitoxin (STX), brevetoxin (BTX) and domoic acid (DA). Fish were randomly selected to four groups for i.p. injection of saline (control) or one of the neurotoxins STX (10 microg STX/kg bw), BTX (68 microg BTX/kg bw) or DA (6 mg DA/kg bw). In addition, 14C-2-deoxyglucose was i.m. injected to measure brain metabolic activity by autoradiography. The three regions investigated were telencephalon (Tel), optic tectum (OT) and cerebellum (Ce). There were no differences in the metabolic activity after STX and BTX exposure compared to the control in these regions. However, a clear increase was observed after DA exposure. When the subregions with the highest metabolic rate were pseudocoloured in the three brain regions, the three toxins caused distinct differences in the respective patterns of metabolic activation. Fish exposed to STX displayed similar patterns as the control fish, whereas fish exposed to BTX and DA showed highest metabolic activity in subregions different from the control group. All three neurotoxins affected subregions that are believed to be involved in cognitive abilities in fish.

Organization of the torus longitudinalis in the rainbow trout (Oncorhynchus mykiss): an immunohistochemical study of the GABAergic system and a DiI tract-tracing study

The torus longitudinalis (TL) is a tectum-associated structure of actinopterygian fishes. The organization of the TL of rainbow trout was studied with Nissl staining, Golgi methods, immunocytochemistry with antibodies to gamma-aminobutyric acid (GABA), glutamic acid decarboxylase (GAD), and the GABA(A) receptor subunits delta and beta2/beta 3, and with tract tracing methods. Two types of neuron were characterized: medium-sized GABAergic neurons and small GABA-negative granule cells. GABA(A) receptor subunit delta-like immunoreactivity delineated two different TL regions, ventrolateral and central. Small GABAergic cells were also observed in marginal and periventricular strata of the optic tectum. These results indicate the presence of local GABAergic inhibitory circuits in the TL system. For tract-tracing, a lipophilic dye (DiI) was applied to the TL and to presumed toropetal nuclei or toral targets. Toropetal neurons were observed in the optic tectum, in pretectal (central, intermediate, and paracommissural) nuclei, in the subvalvular nucleus, and associated with the pretectocerebellar tract. Torofugal fibers were numerous in the stratum marginale of the optic tectum. Toropetal pretectal nuclei also project to the cerebellum, and a few TL cells project to the cerebellar corpus. The pyramidal cells of the trout tectum were also studied by Golgi methods and local DiI labeling. The connections of trout TL revealed here were more similar to those recently reported in carp and holocentrids (Ito et al. [2003] J. Comp. Neurol. 457:202-211; Xue et al. [2003] J. Comp. Neurol. 462:194-212), than to those reported in earlier studies. However, important differences in organization of toropetal nuclei were noted between salmonids and these other teleosts.