Good heavens! Finally a landslide analysis of basal ganglia circuitry in teleosts

Tanimoto et al.1 report essential information on teleostean basal ganglia circuitry. This analysis opens gateways into studying neurophysiology, neuropharmacology, and behavior in zebrafish, guided by this complex functional neural system common to all vertebrates.

Genoarchitectonics of the larval zebrafish diencephalon

The brain is spatially organized into subdivisions, nuclei and areas, which often correspond to functional and developmental units. A segmentation of brain regions in the form of a consensus atlas facilitates mechanistic studies and is a prerequisite for sharing information among neuroanatomists. Gene expression patterns objectively delineate boundaries between brain regions and provide information about their developmental and evolutionary histories. To generate a detailed molecular map of the larval zebrafish diencephalon, we took advantage of the Max Planck Zebrafish Brain (mapzebrain) atlas, which aligns hundreds of transcript and transgene expression patterns in a shared coordinate system. Inspection and co-visualization of close to 50 marker genes have allowed us to resolve the tripartite prosomeric scaffold of the diencephalon at unprecedented resolution. This approach clarified the genoarchitectonic partitioning of the alar diencephalon into pretectum (alar part of prosomere P1), thalamus (alar part of prosomere P2, with habenula and pineal complex), and prethalamus (alar part of prosomere P3). We further identified the region of the nucleus of the medial longitudinal fasciculus, as well as the posterior and anterior parts of the posterior tuberculum, as molecularly distinct basal parts of prosomeres 1, 2, and 3, respectively. Some of the markers examined allowed us to locate glutamatergic, GABAergic, dopaminergic, serotoninergic, and various neuropeptidergic domains in the larval zebrafish diencephalon. Our molecular neuroanatomical approach has thus (1) yielded an objective and internally consistent interpretation of the prosomere boundaries within the zebrafish forebrain; has (2) produced a list of markers, which in sparse combinations label the subdivisions of the diencephalon; and is (3) setting the stage for further functional and developmental studies in this vertebrate brain.

The Neuromeric/Prosomeric Model in Teleost Fish Neurobiology

The neuromeric/prosomeric model has been rejuvenated by Puelles and Rubenstein [Trends Neurosci. 1993;16(11):472-9]. Here, its application to the (teleostean) fish brain is detailed, beginning with a historical account. The second part addresses three main issues with particular interest for fish neuroanatomy and looks at the impact of the neuromeric model on their understanding. The first one is the occurrence of four early migrating forebrain areas (M1 through M4) in teleosts and their comparative interpretation. The second issue addresses the complex development and neuroanatomy of the teleostean alar and basal hypothalamus. The third topic is the vertebrate dopaminergic system, with the focus on some teleostean peculiarities. Most of the information will be coming from zebrafish studies, although the general ductus is a comparative one. Throughout the manuscript, comparative developmental and organizational aspects of the teleostean amygdala are discussed. One particular focus is cellular migration streams into the medial amygdala.

A versatile transcription factor: Multiple roles of orthopedia a (otpa) beyond its restricted localization in dopaminergic systems of developing and adult zebrafish (Danio rerio) brains

Many transcription factors boost neural development and differentiation in specific directions and serve for identifying similar or homologous structures across species. The expression of Orthopedia (Otp) is critical for the development of certain cell groups along the vertebrate neuraxis, for example, the medial amygdala or hypothalamic neurosecretory neurons. Therefore, the primary focus of the present study is the distribution of Orthopedia a (Otpa) in the larval and adult zebrafish (Danio rerio) brain. Since Otpa is also critical for the development of zebrafish basal diencephalic dopaminergic cells, colocalization of Otpa with the catecholamine synthesizing enzyme tyrosine hydroxylase (TH) is studied. Cellular colocalization of Otpa and dopamine is only seen in magnocellular neurons of the periventricular posterior tubercular nucleus and in the posterior tuberal nucleus. Otpa-positive cells occur in many additional structures along the zebrafish neuraxis, from the secondary prosencephalon down to the hindbrain. Furthermore, Otpa expression is studied in shh-GFP and islet1-GFP transgenic zebrafish. Otpa-positive cells only express shh in dopaminergic magnocellular periventricular posterior tubercular cells, and only colocalize with islet1-GFP in the ventral zone and prerecess caudal periventricular hypothalamic zone and the perilemniscal nucleus. The scarcity of cellular colocalization of Otpa in islet1-GFP cells indicates that the Shh-islet1 neurogenetic pathway is not active in most Otpa-expressing domains. Our analysis reveals detailed correspondences between mouse and zebrafish forebrain territories including the zebrafish intermediate nucleus of the ventral telencephalon and the mouse medial amygdala. The zebrafish preoptic Otpa-positive domain represents the neuropeptidergic supraopto-paraventricular region of all tetrapods. Otpa domains in the zebrafish basal plate hypothalamus suggest that the ventral periventricular hypothalamic zone corresponds to the otp-expressing basal hypothalamic tuberal field in the mouse. Furthermore, the mouse otp domain in the mammillary hypothalamus compares partly to our Otpa-positive domain in the prerecess caudal periventricular hypothalamic zone (Hc-a).

Anatomy and function of retinorecipient arborization fields in zebrafish

In 1994, Burrill and Easter described the retinal projections in embryonic and larval zebrafish, introducing the term "arborization fields" (AFs) for the retinorecipient areas. AFs were numbered from 1 to 10 according to their positions along the optic tract. With the exception of AF10 (neuropil of the optic tectum), annotations of AFs remained tentative. Here we offer an update on the likely identities and functions of zebrafish AFs after successfully matching classical neuroanatomy to the digital Max Planck Zebrafish Brain Atlas. In our system, individual AFs are neuropil areas associated with the following nuclei: AF1 with the suprachiasmatic nucleus; AF2 with the posterior parvocellular preoptic nucleus; AF3 and AF4 with the ventrolateral thalamic nucleus; AF4 with the anterior and intermediate thalamic nuclei; AF5 with the dorsal accessory optic nucleus; AF7 with the parvocellular superficial pretectal nucleus; AF8 with the central pretectal nucleus; and AF9d and AF9v with the dorsal and ventral periventricular pretectal nuclei. AF6 is probably part of the accessory optic system. Imaging, ablation, and activation experiments showed contributions of AF5 and potentially AF6 to optokinetic and optomotor reflexes, AF4 to phototaxis, and AF7 to prey detection. AF6, AF8 and AF9v respond to dimming, and AF4 and AF9d to brightening. While few annotations remain tentative, it is apparent that the larval zebrafish visual system is anatomically and functionally continuous with its adult successor and fits the general cyprinid pattern. This study illustrates the synergy created by merging classical neuroanatomy with a cellular-resolution digital brain atlas resource and functional imaging in larval zebrafish.

Neural pathways of olfactory kin imprinting and kin recognition in zebrafish

Teleost fish exhibit extraordinary cognitive skills that are comparable to those of mammals and birds. Kin recognition based on olfactory and visual imprinting requires neuronal circuits that were assumed to be necessarily dependent on the interaction of mammalian amygdala, hippocampus, and isocortex, the latter being a structure that teleost fish are lacking. We show that teleosts—beyond having a hippocampus and pallial amygdala homolog—also have subpallial amygdalar structures. In particular, we identify the medial amygdala and neural olfactory central circuits related to kin imprinting and kin recognition corresponding to an accessory olfactory system despite the absence of a separate vomeronasal organ.

Sonic hedgehog expression in zebrafish forebrain identifies the teleostean pallidal signaling center and shows preglomerular complex and posterior tubercular dopamine cells to arise from shh cells

Ventralization, a major patterning process in the developing vertebrate neural tube (central nervous system, CNS), depends on Sonic hedgehog (SHH) as a main signaling morphogen. We studied the CNS of late larval and young adult zebrafish in a transgenic shh-GFP line revealing increased neuroanatomical detail due to the progressed differentiation state compared to earlier stages. Some major findings emerge from the present study. (a) shh -GFP is still expressed along the adult zebrafish CNS neuraxis in most locations seen in larvae. (b) We newly identify a ventroposterior shh pallidal domain representing the basal telencephalic signaling center important for basal ganglia development known in other vertebrates (i.e., the anterior entopeduncular area-basal medial ganglionic eminence of mammals). (c) We further show late-emerging shh-GFP positive radial glia cells in the medial zone of the dorsal telencephalon (i.e., the teleostan pallial amygdala). (d) Immunostains for tyrosine hydroxylase demonstrate that there is selective colocalization in adult dopamine cells with shh-GFP in the posterior tuberculum, including in projection cells to striatum, which represents a striking parallel to amniote mesodiencephalic dopamine cell origin from shh expressing floor plate cells. (e) There is no colocalization of shh and islet1 as shown by respective shh-GFP and islet1-GFP lines. (f) The only radially far migrated shh-GFP cells are located in the preglomerular area. (g) There are no adult cerebellar and tectal shh-GFP cells confirming their exclusive role during early development as previously reported by our laboratory.

Serotonin systems in three socially communicating teleost species, the grunting toadfish (Allenbatrachus grunniens), a South American marine catfish (Ariopsis seemanni), and the upside-down catfish (Synodontis nigriventris)

We investigated immunohistochemically the distribution of serotonergic cell populations in three teleost species (one toadfish, Allenbatrachus grunniens, and two catfishes, Synodontis nigriventris and Ariopsis seemanni). All three species exhibited large populations of 5-HT positive neurons in the paraventricular organ (PVO) and the dorsal (Hd) and caudal (Hc) periventricular hypothalamic zones, plus a smaller one in the periventricular pretectum, a few cells in the pineal stalk, and - only in catfishes - in the preoptic region. Furthermore, the rhombencephalic superior and inferior raphe always contained ample serotonergic cells. In each species, a neuronal mass extended into the hypothalamic lateral recess. Only in the toadfish, did this intraventricular structure contain serotonergic cells and arise from Hd, whereas in the catfishes it emerged from medially and represents the dorsal tuberal nucleus seen in other catfishes as well. Serotonergic cells in PVO, Hd and Hc were liquor-contacting. Those of the PVO extended into the midline area of the periventricular posterior tubercular nucleus in both catfishes. Dopaminergic, liquor-contacting neurons were additionally investigated using an antibody against tyrosine hydroxylase (TH) in S. nigriventris showing that TH was never co-localized with serotonin. Because TH antibodies are known to reveal mostly or only the TH1 enzyme, we hypothesize that th1-expressing dopamine cells (unlike th2-expressing ones) do not co-localize with serotonin. Since the three investigated species engage in social communication using swim bladder associated musculature, we investigated the serotonergic innervation of the hindbrain vocal or electromotor nuclei initiating the social signal. We found in all three species serotonergic fibers seemingly originating from close-by serotonergic neurons of inferior raphe or anterior spinal cord. Minor differences appear to be rather species-specific than dependent on the type of social communication.

Adult islet1 Expression Outlines Ventralized Derivatives Along Zebrafish Neuraxis

Signals issued by dorsal roof and ventral floor plates, respectively, underlie the major patterning process of dorsalization and ventralization during vertebrate neural tube development. The ventrally produced morphogen Sonic hedgehog (SHH) is crucial for vertebrate hindbrain and spinal motor neuron development. One diagnostic gene for motor neurons is the LIM/homeodomain gene islet1, which has additional ventral expression domains extending into mid- and forebrain. In order to corroborate motor neuron development and, in particular, to improve on the identification of poorly documented zebrafish forebrain islet1 populations, we studied adult brains of transgenic islet1-GFP zebrafish (3 and 6 months). This molecular neuroanatomical analysis was supported by immunostaining these brains for tyrosine hydroxylase (TH) or choline acetyltransferase (ChAT), respectively, revealing zebrafish catecholaminergic and cholinergic neurons. The present analysis of ChAT and islet1-GFP label confirms ongoing adult expression of islet1 in zebrafish (basal plate) midbrain, hindbrain, and spinal motor neurons. In contrast, non-motor cholinergic systems lack islet1 expression. Additional presumed basal plate islet1 positive systems are described in detail, aided by TH staining which is particularly informative in the diencephalon. Finally, alar plate zebrafish forebrain systems with islet1 expression are described (i.e., thalamus, preoptic region, and subpallium). We conclude that adult zebrafish continue to express islet1 in the same brain systems as in the larva. Further, pending functional confirmation we hypothesize that the larval expression of sonic hedgehog (shh) might causally underlie much of adult islet1 expression because it explains findings beyond ventrally located systems, for example regarding shh expression in the zona limitans intrathalamica and correlated islet1-GFP expression in the thalamus.

The Mormyrid Optic Tectum Is a Topographic Interface for Active Electrolocation and Visual Sensing

The African weakly electric fish Gnathonemus petersii is capable of cross-modal object recognition using its electric sense or vision. Thus, object features stored in the brain are accessible by multiple senses, either through connections between unisensory brain regions or because of multimodal representations in multisensory areas. Primary electrosensory information is processed in the medullary electrosensory lateral line lobe, which projects topographically to the lateral nucleus of the torus semicircularis (NL). Visual information reaches the optic tectum (TeO), which projects to various other brain regions. We investigated the neuroanatomical connections of these two major midbrain visual and electrosensory brain areas, focusing on the topographical relationship of interconnections between the two structures. Thus, the neural tracer DiI was injected systematically into different tectal quadrants, as well as into the NL. Tectal tracer injections revealed topographically organized retrograde and anterograde label in the NL. Rostral and caudal tectal regions were interconnected with rostral and caudal areas of the NL, respectively. However, dorsal and ventral tectal regions were represented in a roughly inverted fashion in NL, as dorsal tectal injections labeled ventral areas in NL and vice versa. In addition, tracer injections into TeO or NL revealed extensive inputs to both structures from ipsilateral (NL also contralateral) efferent basal cells in the valvula cerebelli; the NL furthermore projected back to the valvula. Additional tectal and NL connections were largely confirmatory to earlier studies. For example, the TeO received ipsilateral inputs from the central zone of the dorsal telencephalon, torus longitudinalis, nucleus isthmi, various tegmental, thalamic and pretectal nuclei, as well as other nuclei of the torus semicircularis. Also, the TeO projected to the dorsal preglomerular and dorsal posterior thalamic nuclei as well as to nuclei in the torus semicircularis and nucleus isthmi. Beyond the clear topographical relationship of NL and TeO interconnections established here, the known neurosensory upstream circuitry was used to suggest a model of how a defined spot in the peripheral sensory world comes to be represented in a common associated neural locus both in the NL and the TeO, thereby providing the neural substrate for cross-modal object recognition.

The Brain of the Archerfish Toxotes chatareus: A Nissl-Based Neuroanatomical Atlas and Catecholaminergic/Cholinergic Systems

Over recent years, the seven-spot archerfish (Toxotes chatareus) has emerged as a new model for studies in visual and behavioral neuroscience thanks to its unique hunting strategy. Its natural ability to spit at insects outside of water can be used in the laboratory for well controlled behavioral experiments where the fish is trained to aim at targets on a screen. The need for a documentation of the neuroanatomy of this animal became critical as more research groups use it as a model. Here we present an atlas of adult T. chatareus specimens caught in the wild in South East Asia. The atlas shows representative sections of the brain and specific structures revealed by a classic Nissl staining as well as corresponding schematic drawings. Additional immunostainings for catecholaminergic and cholinergic systems were conducted to corroborate the identification of certain nuclei and the data of a whole brain scanner is available online. We describe the general features of the archerfish brain as well as its specificities, especially for the visual system and compare the neuroanatomy of the archerfish with other teleosts. This atlas of the archerfish brain shows all levels of the neuraxis and intends to provide a solid basis for further neuroscientific research on T. chatareus, in particular electrophysiological studies.

Should we redefine the classic lateral pallium?

The quadripartite model of the telencephalic pallium of amniotes offered by the Puelles school includes a medial, dorsal, lateral, and ventral pallium. Watson and Puelles ([2016] J. Comp. Neurol. this issue) now newly propose that the mammalian ventral pallium gives rise not only to all of the pallial amygdala but also to the olfactory cortex, which hitherto was considered to arise from the lateral pallium. Thus, the region of the lateral pallium was misidentified in the quadripartite model, as the designated histogenetic unit gives rise to the insular cortex/claustrum and should therefore be considered a most ventrolateral part of the dorsal pallium (its ventrolateral subdivision). The mesopallium of birds then is the homologue of this ventrolateral dorsal pallial part, not of the classic lateral pallium. The region designated as the ventral pallium in the initial quadripartite model should therefore be divided in the new Watson/Puelles model into a smaller ventral pallium and a lateral pallium. J. Comp. Neurol. 525:1509-1513, 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.

Molecular neuroanatomy and chemoarchitecture of the neurosecretory preoptic-hypothalamic area in zebrafish larvae

The paraventricular nucleus (PVN) in mammals is the main hypothalamic nucleus controlling hormone release in the pituitary and plays pivotal roles in homeostasis. While the location of a PVN-homologous region has been described in adult fish as the neurosecretory preoptic area (NPO), this region has not been clearly defined in larval zebrafish due to the difficulty in defining cytoarchitectonic nuclear boundaries in the larval brain. Here we identify the precise location of the larval zebrafish NPO using conserved transcription factor and neuropeptide gene expressions. Our results identify the dorsal half of the preoptic-hypothalamic orthopedia a (otpa) domain as the larval NPO and the homologous region to the mammalian PVN. Further, by reconstructing the locations of cells producing zebrafish neuropeptides found in the mammalian PVN (CCK, CRH, ENK, NTS, SS, VIP, OXT, AVP), we provide the first 3D arrangement map of NPO neuropeptides in the larval zebrafish brain. Our results show striking conservation of transcription factor expression (otp, arx, dlx5a, isl1) in and around the NPO/PVN together with neuropeptide expression within it. Finally, we describe the exact anatomical location of cells producing Oxt and Avp in the adult zebrafish. Thus, our results identify the definitive borders and extent of the PVN homologous region in larval zebrafish and serve as an important basis for cross-species comparisons of PVN/NPO structure and function.

Basal ganglia: insights into origins from lamprey brains

The lamprey brain has now been shown to have basal ganglia circuitry, with an output that acts tonically on midbrain and brainstem motor centers and is modulated by ascending dopaminergic input. This condition was believed to represent the tetrapod condition, but now appears to be far more ancient.

Secondary neurogenesis and telencephalic organization in zebrafish and mice: a brief review

Most zebrafish neurodevelopmental studies have focused on the embryo, which is characterized by primary neurogenesis of mostly transient neurons. Secondary neurogenesis becomes dominant in the hatching larva, when major brain parts are established and begin to differentiate. This developmental period allows for a comparative analysis of zebrafish brain organization with amniotes at equivalent stages of neurogenesis. Within a particular time window, the early forebrains of mice (Embyronic stage [E] 12.5/13.5 days [d]) and zebrafish (3 d) reveal highly comparable expression patterns of genes involved in neurogenesis, for example proneural and other transcription factors (Neurogenin1, NeuroD, Mash1/Zashla and Pax6). Further topological correspondences are seen in the expression of LIM and homeobox genes, such as Lhx6/7, Tbr2 and Dlx2a. When this analysis is extended to gamma-aminobutyric acid/glutamic acid decarboxylase (GABA/GAD) cell patterns during this critical time window, an astonishing degree of similarity between the two species is again seen, for example regarding the presence of GABA/GAD cells in the subpallium, with the pallium only starting to be invaded by such cells from the subpallium. Furthermore, the expression of proneural and other genes correlates with GABA cell patterns (e.g. Mash1/Zash1a gene expression in GABA-positive and Neurogenin1/NeuroD in GABA-negative telencephalic regions) in mice and zebrafish. Data from additional vertebrates, such as Xenopus, are also highly consistent with this analysis. Therefore, the vertebrate forebrain appears to undergo a phylotypic stage of secondary neurogenesis, characterized by regionally separated GABAergic (inhibitory) versus glutamatergic (excitatory) cell production sites, which are obscured later in development by tangential migration. This period is highly advantageous for molecular neuroanatomical cross-species comparisons.

The long adventurous journey of rhombic lip cells in jawed vertebrates: a comparative developmental analysis

This review summarizes vertebrate rhombic lip and early cerebellar development covering classic approaches up to modern developmental genetics which identifies the relevant differential gene expression domains and their progeny. Most of this information is derived from amniotes. However, progress in anamniotes, particularly in the zebrafish, has recently been made. The current picture suggests that rhombic lip and cerebellar development in jawed vertebrates (gnathostomes) share many characteristics. Regarding cerebellar development, these include a ptf1a expressing ventral cerebellar proliferation (VCP) giving rise to Purkinje cells and other inhibitory cerebellar cell types, and an atoh1 expressing upper rhombic lip giving rise to an external granular layer (EGL, i.e., excitatory granule cells) and an early ventral migration into the anterior rhombencephalon (cholinergic nuclei). As for the lower rhombic lip (LRL), gnathostome commonalities likely include the formation of precerebellar nuclei (mossy fiber origins) and partially primary auditory nuclei (likely convergently evolved) from the atoh1 expressing dorsal zone. The fate of the ptf1a expressing ventral LRL zone which gives rise to (excitatory cells of) the inferior olive (climbing fiber origin) and (inhibitory cells of ) cochlear nuclei in amniotes, has not been determined in anamniotes. Special for the zebrafish in comparison to amniotes is the predominant origin of anamniote excitatory deep cerebellar nuclei homologs (i.e., eurydendroid cells) from ptf1a expressing VCP cells, the sequential activity of various atoh1 paralogs and the incomplete coverage of the subpial cerebellar plate with proliferative EGL cells. Nevertheless, the conclusion that a rhombic lip and its major derivatives evolved with gnathostome vertebrates only and are thus not an ancestral craniate character complex is supported by the absence of a cerebellum (and likely absence of its afferent and efferent nuclei) in jawless fishes

Differential expression of dopaminergic cell markers in the adult zebrafish forebrain

Although the simultaneous presence of tyrosine hydroxylase (TH), aromatic amino acid decarboxylase (AADC), dopamine transporter (DAT), and vesicular monoamine transporter 2 (VMAT2) is considered as a phenotypic signature of dopamine (DA) neurons, it has been suggested that they are not uniformly expressed in all dopaminergic brain nuclei. Moreover, in nonmammalian vertebrates, two tyrosine hydroxylase genes (TH1 and TH2) are found, and they exhibit different expression patterns in zebrafish brains. Here we present a detailed description of the distribution of TH1, TH2, AADC, DAT, and VMAT2 transcripts, in relation to TH and DA immunoreactivity to better characterize dopaminergic nuclei in the adult zebrafish forebrain. TH2-positive cells in the hypothalamus are strongly DA immunoreactive (DAir), providing direct evidence that they are dopaminergic. DAir cells are also found in most TH1-positive or TH-immunoreactive (THir) nuclei. However, the DAir signal was weaker than THir in the olfactory bulb, telencephalon, ventral thalamus, pretectum, and some posterior tubercular and preoptic nuclei. These cell populations also exhibited low levels of VMAT2 transcripts, suggesting that low DA is due to a lower vesicular DA accumulation. In contrast, cell populations with low levels of AADC did not always have low levels of DA. DAT transcripts were abundantly expressed in most of the TH1- or TH2-positive territories. In addition, DAT and/or VMAT2 transcripts were found in some periventricular cell populations such as in the telencephalon without TH1 or TH2 expression. Thus, expression patterns of dopaminergic cell markers are not homogeneous, suggesting that the gene regulatory logic determining the dopaminergic phenotype is unexpectedly complex.

The zebrafish cerebellar upper rhombic lip generates tegmental hindbrain nuclei by long-distance migration in an evolutionary conserved manner

The upper rhombic lip (URL) of the developing mammalian cerebellum produces different neuronal cell types in a temporal sequence. The first neuronal populations arising from this proliferation zone include the progenitors of the parabrachial, parabigeminal, and laterodorsal-pedunculopontine tegmental hindbrain nuclei. By means of expression analysis, histology, and retrograde neuronal tracing, we have identified the zebrafish homologues of these nuclei, namely, the secondary gustatory/viscerosensory nucleus, the nucleus isthmi, and the superior reticular nucleus, respectively, in the embryonic and larval brain of a stable transgenic wnt1:Gal4-VP16-14 x UAS:GFP zebrafish strain. Combining time-lapse confocal imaging with individual cell tracing, we characterize the migratory behavior of these neuronal precursor populations in detail by revealing their migration path, velocity, and directionality. In addition, we identify neuronal progenitors of the secondary gustatory/viscerosensory nucleus and nucleus isthmi/superior reticular nucleus as belonging to the polysialic acid (PSA)-expressing cell population in the cerebellar plate that migrates in a PSA-dependent manner. Finally, we reveal that circuitries involved in the processing of sensory information (visual, gustatory, general viscerosensory) are already established in the zebrafish larva at day 4 of development. Also the wnt1-expressing pretectal neuronal precursors (not originating from the URL) sending mossy fiber-like projections into the cerebellar corpus are established at that time. In sum, our results show that the origin of neurons of some tegmental hindbrain nuclei, namely, nucleus isthmi/superior reticular nucleus and secondary gustatory/viscerosensory nucleus is in the URL, and that the temporal order of cell types produced by the URL and their developmental program are conserved among vertebrate species.

Phylotypic expression of the bHLH genes Neurogenin2, Neurod, and Mash1 in the mouse embryonic forebrain

In the anamniote model animals, zebrafish and Xenopus laevis, highly comparable early forebrain expression patterns of proneural basic helix-loop-helix (bHLH) genes relevant for neurogenesis (atonal homologs, i.e., neurogenins/NeuroD and achaete-scute homologs, i.e., Ascl/ash) were previously revealed during a particular period of development (zebrafish: 3 days; frog: stage 48). Neurogenins/NeuroD on the one hand and Ascl1/ash1 on the other hand exhibit essentially mutually exclusive spatial patterns, probably reflecting different positional information received within the neural tube, and appear to underlie glutamatergic versus GABAergic neuronal differentiation, respectively. Significant data suggest that similar complementary localizations of these proneural genes and corresponding differentiation pathways also exist in the mouse, the prominent mammalian model. The present article reports on detailed mouse brain bHLH gene expression patterns to fill existing gaps in the identification of expression domains, especially outside the telencephalon. Clearly, there are strong similarities in the complementarity of territories expressing Ascl1/Mash 1 versus neurogenins/NeuroD in the entire mouse forebrain, except for the pretectal alar plate and basal plate of prosomeres 1-3. The analysis substantiates localization of neurogenins/NeuroD in the pallium, eminentia thalami, and dorsal thalamus, and expression of Ascl1/Mash 1 in the striatal and septal subpallium, preoptic region, ventral thalamus, and hypothalamus, which is highly similar to the situation described in Xenopus and zebrafish. Thus, all three vertebrate model species display a "phylotypic stage or period" corresponding to a temporally and spatially defined control of neurogenesis during forebrain development, ultimately resulting in the differentiation of distinct populations of glutamatergic versus GABAergic neurons.

Two tyrosine hydroxylase genes in vertebrates New dopaminergic territories revealed in the zebrafish brain

Tyrosine hydroxylase (TH) is the rate limiting enzyme for dopamine synthesis, catalyzing transformation of l-tyrosine to l-DOPA. Two TH genes (TH1 and TH2) have been reported to exist in the genome of some teleost fishes, TH1 being orthologous to the mammalian TH gene (Candy and Collet, 2005). Here we show that two TH genes are commonly found in genomes of jawed vertebrates. Our analyses of molecular phylogeny and gene synteny strongly suggest that the two TH genes emerged as a consequence of a whole genome duplication before the divergence of jawed vertebrates, and that TH2 was secondarily lost in eutherians (placental mammals). The distribution of TH1 and TH2 transcripts revealed that TH1 and TH2 are differentially expressed in the zebrafish adult brain, as often observed for duplicated genes. In particular we found that TH2 transcripts were much more abundant than TH1 in the hypothalamus, and that the TH2 cells along the periventricular zone are devoid of TH immunoreactivity, due to the lack of affinity of the available anti-TH antibodies. Although these neurons have been considered to be dopamine-uptaking cells in previous studies, the expression of other monoaminergic markers such as aromatic amino acid decarboxylase (AADC), dopamine transporter (DAT), and vesicular monoamine transporter 2 (VMAT2) suggests that these TH2 cells are dopamine-synthesizing neurons.

An evolutionary interpretation of teleostean forebrain anatomy

During the past few years, our investigations of the forebrain in the zebrafish (a teleost fish) have shown that its molecular anatomy and expression patterns of genes involved in the regulation of neuronal transmitter phenotypes, such as gamma-aminobutyric acid- (GABA-)ergic neurons, are very similar to those seen in mammalian model organisms such as mouse and rat. For example, we have been able to identify previously undiscovered homologies, such as subpallial regions in the zebrafish that are homologous to the medial and lateral ganglionic eminences in mammals, as well as regions homologous to the larval eminentia thalami and its adult derivative, the bed nucleus of the stria medullaris. Furthermore, in what we term the partial eversion model of the telencephalon in teleosts, we propose homologies to all four mammalian pallial areas and conclude that the posterior zone of the dorsal telencephalic area in teleosts is homologous to the piriform cortex and is formed by a migratory stream of cells originating in a dorsomedial zone of the pallium (the primordial medial zone of area dorsalis telencephali). In this review we critically discuss and justify these findings in the context of forebrain evolution in fishes.

Optimized Gal4 genetics for permanent gene expression mapping in zebrafish

Combinatorial genetics for conditional transgene activation allows studying gene function with temporal and tissue specific control like the Gal4-UAS system, which has enabled sophisticated genetic studies in Drosophila. Recently this system was adapted for zebrafish and promising applications have been introduced. Here, we report a systematic optimization of zebrafish Gal4-UAS genetics by establishing an optimized Gal4-activator (KalTA4). We provide quantitative data for KalTA4-mediated transgene activation in dependence of UAS copy numbers to allow for studying dosage effects of transgene expression. Employing a Tol2 transposon-mediated KalTA4 enhancer trap screen biased for central nervous system expression, we present a collection of self-reporting red fluorescent KalTA4 activator strains. These strains reliably transactivate UAS-dependent transgenes and can be rendered homozygous. Furthermore, we have characterized the transactivation kinetics of tissue-specific KalTA4 activation, which led to the development of a self-maintaining effector strain "Kaloop." This strain relates transient KalTA4 expression during embryogenesis via a KalTA4-mediated autoregulatory mechanism to live adult structures. We demonstrate its use by showing that the secondary octaval nucleus in the adult hindbrain is likely derived from egr2b-expressing cells in rhombomere 5 during stages of early embryogenesis. These data demonstrate prolonged and maintained expression by Kalooping, a technique that can be used for permanent spatiotemporal genetic fate mapping and targeted transgene expression in zebrafish.

Axonal projections originating from raphe serotonergic neurons in the developing and adult zebrafish, Danio rerio, using transgenics to visualize raphe-specific pet1 expression

Serotonin is a major central nervous modulator of physiology and behavior and plays fundamental roles during development and plasticity of the vertebrate central nervous system (CNS). Understanding the developmental control and functions of serotonergic neurons is therefore an important task. In all vertebrates, prominent serotonergic neurons are found in the superior and inferior raphe nuclei in the hindbrain innervating most CNS regions. In addition, all vertebrates except for mammals harbor other serotonergic centers, including several populations in the diencephalon. This, in combination with the intricate and wide distribution of serotonergic fibers, makes it difficult to sort out serotonergic innervation originating from the raphe from that of other serotonergic cell populations. To resolve this issue, we isolated the regulatory elements of the zebrafish raphe-specific gene pet1 and used them to drive expression of an eGFP transgene in the raphe population of serotonergic neurons. With this approach together with retrograde tracing we 1) describe in detail the development, anatomical organization, and projection pattern of zebrafish pet1-positive neurons compared with their mammalian counterparts, 2) identify a new serotonergic population in the ventrolateral zebrafish hindbrain, and 3) reveal some extent of functional subdivisions within the zebrafish superior raphe complex. Together, our results reveal for the first time the specific innervation pattern of the zebrafish raphe and, thus, provide a new model and various tools to investigate further the role of raphe serotonergic neurons in vertebrates.

Secondary neurogenesis in the brain of the African clawed frog, Xenopus laevis, as revealed by PCNA, Delta-1, Neurogenin-related-1, and NeuroD expression

After primary neurogenesis in the Xenopus laevis embryo, a massive new surge of neurogenesis and related neurogenic and proneural gene expression occurs in the spinal cord at the beginning of the larval period (starting at Stage 46), which corresponds to well-documented secondary neurogenesis in larval zebrafish central nervous system development. Here, we document related neural proliferation and gene expression patterns in the brain of Xenopus, in various embryonic and larval stages, showing the distribution of proliferative cells (immunostaining of cells containing the proliferating cell nuclear antigen; the auxiliary protein of DNA polymerase delta; PCNA), and the activity of some critical genes expressed during neurogenesis (i.e., Delta-1, Neurogenin-related-1, NeuroD). This study reveals that the early larval stage in Xenopus (Stage 48) displays patterns of proliferation (PCNA), as well as of neurogenic (Delta-1) and proneural (Ngnr-1; NeuroD) gene expression that are qualitatively almost identical to those seen in the 3-day postembryonic zebrafish or the 12.5/13.5-day embryonic mouse. Furthermore, a comparable bauplan of early proliferation zones (including their neuromeric organization) as described in the postembryonic zebrafish apparently exists in tetrapods (Xenopus). Altogether, the data presented suggest a common brain bauplan on the level of early proliferation patterns and neurogenic/proneural gene activity in anamniotes, if not vertebrates.

A phylotypic stage in vertebrate brain development: GABA cell patterns in zebrafish compared with mouse

A recent comparison of early forebrain gene expression in mouse and zebrafish revealed highly comparable expression patterns of developmentally relevant genes, for example, of proneural (Neurogenin1, NeuroD, Mash1/Zash1a) genes involved in neurogenesis at a particular time window (mouse: embryonic day 12.5/13.5; zebrafish: 3 days). Here we extend this analysis to the description of gamma-aminobutyric acid (GABA) cell patterns in the early postembryonic zebrafish brain (i.e., during early secondary neurogenesis). We find again an astonishing degree of correspondences of GABA cell patterns between zebrafish and mouse during this previously established critical time window, for example, regarding absence of GABA cells in certain forebrain regions (pallium, dorsal thalamus, eminentia thalami) or with respect to the spatiotemporal occurrence of GABA cells (e.g., late cerebellar GABA cells). Furthermore, there is perfect correlation with previously established proneural gene expression patterns (i.e., absence of Mash1/Zash1a gene expression in GABA-cell-free forebrain regions) between mouse and zebrafish. The available information in additional vertebrate species, especially in Xenopus, is also highly consistent with our analysis here and suggests that a "phylotypic stage" of neurogenesis during vertebrate brain development may be present.

The adult central nervous cholinergic system of a neurogenetic model animal, the zebrafish Danio rerio

The central nervous cholinergic system of the zebrafish (Danio rerio), a model animal for neurogenetics, is documented here using immunohistochemical methods for visualizing choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme. Neuronal cell bodies containing ChAT are present in the telencephalon (lateral nucleus of ventral telencephalic area), preoptic region (anterior/posterior parvocellular and magnocellular preoptic nuclei), diencephalon (habenula, dorsal thalamus, posterior tuberculum), mesencephalon (Edinger-Westphal (EW) nucleus, oculomotor nerve nucleus, rostral tegmental nucleus, tectal type XIV neurons), isthmic region (nucleus lateralis valvulae, secondary gustatory-viscerosensory nucleus, nucleus isthmi (NI), perilemniscal nucleus, superior reticular nucleus (SRN)) and rhombencephalon (trochlear, trigeminal, abducens, facial, glossopharyngeal-vagal motor nerve nuclei, rostral and caudal populations of octavolateralis efferent neurons). In addition, some ChAT positive neurons are present in the rhombencephalic reticular formation, the central gray, and in cells accompanying the descending trigeminal tract. Obvious ChAT positive terminal fields are present in the supracommissural nucleus of area ventralis telencephali and the medial zone of area dorsalis telencephali, parvocellular superficial pretectal nucleus, torus semicircularis, medial octavolateralis nucleus, facial, glossopharyngeal, and vagal lobes, and in the inferior lobe (around the periventricular nucleus of the lateral recess and in the diffuse nucleus). The identification of all central nervous cholinergic systems provided here in this model system is pivotal for future detailed studies of their development and maintenance, e.g., with regard to the zebrafish ventral telencephalic and isthmic superior reticular neuronal populations, likely representing the homologues of at least part of the cholinergic basal forebrain and pedunculopontine/laterodorsal tegmental ascending activating systems of mammals, respectively.

Connections of the ventral telencephalon (subpallium) in the zebrafish (Danio rerio)

Connections of the medial precommissural subpallial ventral telencephalon, i.e., dorsal (Vd, interpreted as part of striatum) and ventral (Vv, interpreted as part of septum) nuclei of area ventralis telencephali, were studied in the zebrafish (Danio rerio) using two tracer substances (DiI or biocytin). The following major afferent nuclei to Vd/Vv were identified: medial and posterior pallial zones of dorsal telencephalic area, and the subpallial supracommissural and postcommissural nuclei of the ventral telencephalic area, the olfactory bulb, dorsal entopeduncular, anterior and posterior parvocellular preoptic and suprachiasmatic nuclei, anterior, dorsal and central posterior dorsal thalamic, as well as rostrolateral nuclei, periventricular nucleus of the posterior tuberculum, posterior tuberal nucleus, various tuberal hypothalamic nuclei, dorsal tegmental nucleus, superior reticular nucleus, locus coeruleus, and superior raphe nucleus. Efferent projections of the ventral telencephalon terminate in the supracommissural nucleus of area ventralis telencephali, the posterior zone of area dorsalis telencephali, habenula, periventricular pretectum, paracommissural nucleus, posterior dorsal thalamus, preoptic region, midline posterior tuberculum (especially the area dorsal to the posterior tuberal nucleus), tuberal (midline) hypothalamus and interpeduncular nucleus. Strong reciprocal interconnections likely exist between septum and preoptic region/midline hypothalamus and between striatum and dorsal thalamus (dopaminergic) posterior tuberculum. Regarding ascending activating/modulatory systems, the pallium shares with the subpallium inputs from the (noradrenergic) locus coeruleus, and the (serotoninergic) superior raphe, while the subpallium additionally receives such inputs from the (dopaminergic) posterior tuberculum, the (putative cholinergic) superior reticular nucleus, and the (putative histaminergic) caudal hypothamalic zone.

Identification and morphogenesis of the eminentia thalami in the zebrafish

This study documents early zebrafish brain expression patterns (2-5 days postfertilization) of proliferating neural (PCNA) as well as early-determined (Pax6, Zash-1a, Zash-1b, neurogenin1, neuroD) and differentiating (Hu-proteins) neuronal cells. These patterns are used to outline the spatiotemporal local dynamics of secondary neurogenesis as well as neuronal migration and differentiation in the region of the eminentia thalami. The analysis presented not only allows identification for the first time of the eminentia thalami in the zebrafish model system (because it forms a neurogenin1/neuroD-guided locus of neurogenesis in contrast to adjacent preoptic region and ventral thalamus) but furthermore shows that the entopeduncular complex is a derivative of the embryonic zebrafish eminentia thalami, which has never been reported for a teleost before. An analysis of the relevant literature shows that the mammalian entopeduncular nucleus/avian paleostriatum primitivum/reptilian globus pallidus clearly are part of the basal ganglia (i.e., the pallidum). In amniote embryos, an anterior entopeduncular area is recognized at the base of the medial ganglionic eminence (i.e., the future pallidum; part of alar plate of prosomere 5), separate from the more posterior eminentia thalami (alar prosomere 4). There is a comparable periventricular eminentia thalami in (young and adult) amphibians and teleosts. However, the migrated anterior entopeduncular nucleus of anuran amphibians likely is homologous to part of the pallidum of other vertebrates and has no developmental relationship to the eminentia thalami. In contrast, the migrated teleostean entopeduncular complex does not correspond to a pallidal division but is indeed the adult derivative of the early-recognized eminentia thalami as shown in this study.

Anatomy of neurogenesis in the early zebrafish brain

The detailed architecture of postembryonic (i.e. secondary, as opposed to primary) neurogenesis in the zebrafish brain at 2 days postfertilization was investigated by studying expression domains of various proneural basic helix-loop-helix genes (i.e. neuroD=nrd, neurogenin1=ngn1, Zash-1b) and neurogenic genes (i.e. Notch-1a, deltaA) on the level of in situ-hybridized sectioned material and compared with brain sections of the same age immunostained for PCNA (a proliferation marker) or for Hu-proteins (marker for early neuronal differentiation). Whereas both Notch-1a and deltaA domains are present in all proliferative zones of the brain, only the more diffuse and scattered deltaA expression appears to extend additionally into the adjacent postmitotic gray matter. Zash-1b is the first achaete-scute orthologue shown here to be expressed exclusively in all proliferative central nervous zones (except for the eye). The ngn1 and nrd genes are typically both expressed in overlapping fashion in many-but not all-brain regions, with ngn1 being more restricted towards the ventricular proliferative zones and nrd extending more laterally. This fact as well as comparisons with PCNA- and Hu-immunostains indicate that a great proportion of ngn1-positive cells are mitotic, but some appear to extend into the postmitotic gray matter where the nrd-domains lie. A comparison of the relative extents of PCNA- (proliferative), nrd- (freshly determined) and Hu-positive (differentiating) cell populations allows to determine the relative maturation state of a given brain part. Similar to findings in late embryonic amniote brains, expression of nrd is absent (from 2 to 5 days) in the zebrafish subpallium, ventral preoptic region, ventral thalamus and hypothalamus. These four regions are also free of ngn1 expression (with the exception of an unusual peripheral ngn1 domain in the preoptic region), indicating that a neurogenetic network not involving nrd and ngn1 is at work there. Our characterization of locally distinct patterns and dynamics of secondary neurogenesis in the entire early (2 dpf) postembryonic zebrafish brain delivers the blueprint for more specialized functional studies.

Expression of Zash-1a in the postembryonic zebrafish brain allows comparison to mouse Mash1 domains

Four areas in the late embryonic murine forebrain, i.e. the subpallium (striatum), the preoptic region, the ventral thalamus, and the hypothalamus, have been described to express the basic helix-loop-helix (bHLH) gene mammalian achaete-scute homolog Mash1 (Ascl1, Mouse Genome Informatics) in a complementary fashion to another bHLH gene, neurogenin1 (ngn1) (Neurod3, Mouse Genome Informatics), which is expressed in directly adjacent forebrain regions. We report here that the four regions previously identified as subpallium, preoptic region, ventral thalamus and hypothalamus (i.e. ventral inferior lobe) in the postembryonic zebrafish brain show Zash-1a expression at 3 days postfertilization (dpf), whereas none of those areas express the bHLH gene neuroD (nrd) between 2 and 5 dpf. This indicates that two well established alternative genetic pathways involved in neurogenesis in the amniote (mammalian) brain are present in homologous phenotypic locations in the anamniote (zebrafish) brain as well and that these pathways possibly act similarly in the generation of different neuronal phenotypes (e.g. subpallial GABAergic interneurons versus pallial glutamatergic projection neurons, or dopaminergic neurons versus other neurotransmitter phenotypes). Furthermore, previous initial identification of early postembryonic brain subdivisions in the zebrafish is strongly corroborated by these expression patterns.

BrdU-, neuroD (nrd)- and Hu-studies reveal unusual non-ventricular neurogenesis in the postembryonic zebrafish forebrain

In the postembryonic zebrafish forebrain, subpial locations of neurogenesis do exist in the early cerebellar external granular layer, and--unusually among vertebrates--in the primordial pretectal (M1) and preglomerular (M2) Anlagen as shown here with 5-bromo-2'-deoxyuridine (BrdU)/Hu-immunocytochemistry and in situ hybridization of neuroD. An intermediate BrdU incubation time of 12-16 h reveals in addition to proliferative ventricularly located cells those in M1 and M2. This BrdU saturation-labeling shows--in conjunction with a Hu-assay demonstrating earliest neuronal differentiation--that proliferating cells in M1 and M2 represent neuronal progenitors. This is demonstrated by single BrdU-labeled and double BrdU-/Hu-labeled cells in these aggregates. Further, expression of NeuroD--a marker for freshly determined neuronal cells--confirms this unusual subpial postembryonic forebrain neurogenesis.

Development of the catecholaminergic system in the early zebrafish brain: an immunohistochemical study

Tyrosine hydroxylase-containing cells (TH cells) were investigated immunohistochemically in early and late postembryonic zebrafish brain sections (at 2 and 5 days postfertilization [dpf]) yielding an improved neuroanatomical resolution of spatiotemporal developmental dynamics of the catecholaminergic system. Additionally, double-immunolabel preparations for visualizing TH cells and cells containing the proliferating cell nuclear antigen (PCNA cells) were carried out allowing for a prosomeric interpretation of early forebrain TH cell clusters. Many TH cell populations recently described in the adult zebrafish brain could be identified in the present study by location and cell type already in the 5 dpf (e.g. eight of 12 adult diencephalic TH cell populations) and 2 dpf (e.g. five of 12 adult TH cell populations) zebrafish brain. Early and adult diencephalic TH cells are restricted to the pretectum (P1) and ventral thalamus (P3) in the alar plate, and to various TH groups in the basal plate posterior tuberculum (P3), as well as to various populations in the hypothalamus (secondary prosencephalon). The alar plate ventral thalamic and most anterodorsal posterior tubercular TH cell populations range among the earliest detectable ones. There was no indication of migration of TH cells from the midbrain-hindbrain boundary or anterior neural ridge into the diencephalon.

Hypothalamic inferior lobe and lateral torus connections in a percomorph teleost, the red cichlid (Hemichromis lifalili)

The neuroanatomic connections of the inferior lobe and the lateral torus of the percomorph Hemichromis lifalili were investigated by 1,1', dioctadecyl-3,3,3',3'-tetramethylindo-carbocyanine perchlorate (DiI) tracing. The inferior lobe and the lateral torus both receive afferents from the secondary gustatory nucleus. Additional afferents reach the inferior lobe from the nucleus glomerulosus, nucleus suprachiasmaticus, dorsal and central posterior thalamic nucleus, nucleus lateralis valvulae, magnocellular part of the magnocellular nucleus of the preoptic region, caudal nucleus of the preglomerular region, posterior tuberal nucleus, area dorsalis of the telencephalon, and a tegmental nucleus (T2). Efferents from the inferior lobe and the lateral torus terminate in the dorsal hypothalamic neuropil and corpus mamillare. Furthermore, the inferior lobe projects to the medial nucleus of the lateral tuberal hypothalamus and perhaps makes axo-axonal synapses in the tractus tectobulbaris rectus. The inferior lobe and the torus lateralis have reciprocal connections with the preglomerular tertiary gustatory nucleus and posterior thalamic nucleus and are also mutually interconnected. The inferior lobe is also reciprocally connected with the medial nucleus of the preglomerular region, reticular formation and sparsely with the anterior dorsal thalamic and the ventromedial thalamic nuclei. Thus, whereas the lateral torus is exclusively connected with the gustatory system, the inferior lobe is of a multisensory nature. In comparison with the goldfish (Carassius auratus), the connectivity pattern of the inferior lobe of Hemichromis lifalili reflects its specialization with respect to the visual system, as it receives qualitative (i.e., dorsal posterior, anterior, and ventromedial thalamic nuclei) as well as quantitative (i.e., nucleus glomerulosus) additional visual input.

Connections of the ventral telencephalon and tyrosine hydroxylase distribution in the zebrafish brain (Danio rerio) lead to identification of an ascending dopaminergic system in a teleost

We studied the connections and catecholaminergic organization of the subpallium in the zebrafish, in particular to demonstrate the origin of the ascending dopaminergic system of teleosts, by using the tracers DiI or biocytin in combination with tyrosine hydroxylase (TH) immunohistochemistry. Retrogradely labeled cells were found in the olfactory bulb, the area dorsalis telencephali, the preoptic region, the dorsal and ventral thalamus, the posterior tubercle, the preglomerular region, and the medulla oblongata. Moreover, the zebrafish subpallium has strong reciprocal connections with the tuberal hypothalamus. Double-labeled cells (for TH and tracer) were identified in two locations of the rostral posterior tubercle: small round neurons in its periventricular nucleus and large pear-shaped cells adjacent to it. These double-labeled cells of the posterior tubercle presumably represent the teleostean dopaminergic system ascending to the striatum.

Expression domains of neuroD (nrd) in the early postembryonic zebrafish brain

NeuroD acts as one of the earliest transcription factors promoting differentiation in newborn neurons immediately after determination by neurogenin1 (ngn1). Here, we show the expression of neuroD in the 2 days postfertilization (dpf) zebrafish brain by in situ hybridization on sections. Generally, neuroD expressing cells lie one to several cell rows away from the ventricular surface, i.e., they seem to flank proliferative cells. However, most cells of the gray matter lie even more peripheral and are unstained. Highly specific local patterns of neuroD expressing cells are observed along the entire rostrocaudal zebrafish brain axis at that age with characteristic neuroD expression free zones (e.g., preoptic region and hypothalamus). Interestingly, these regions where neuroD expression is absent display a higher proliferative activity compared to neuroD-positive areas. To date it is an open question, whether this negative correlation of neuroD-expression and proliferation has a causal background.

The teleostean forebrain: a comparative and developmental view based on early proliferation, Pax6 activity and catecholaminergic organization

An improved comparative interpretation of the teleostean forebrain suggests that the dorsal tier (Vd,Vc) and ventral tier (Vv,Vl) nuclei of the ventral telencephalic area (subpallium) represent the striatum and septum, respectively. Among other arguments, a dopaminergic innervation originating in the diencephalic posterior tubercle reaches Vd and dense efferents of Vv project to the midline hypothalamus in the adult zebrafish subpallium. The adult area dorsalis telencephali represents the teleostean pallium. Regulatory genes typically expressed in the early amniote subpallium (e.g., Dlx-1) are also restricted to the presumptive zebrafish ventral telencephalic area. Further, early Pax6 protein distribution in the zebrafish telencephalon corresponds to the migrating stream noted at the pallial-subpallial boundary in amniotes, but a ventricular, radial glia-based expression in the pallium is absent. The peripherally migrated, adult diencephalic preglomerular complex of the basal plate posterior tubercle (early: M2) provides sensory inputs to the pallium. Early Pax6 protein distribution indicates that at least part of M2 may directly originate from alar plate ventral thalamic Pax6-expressing cells. Dopaminergic cells of the basal plate posterior zebrafish forebrain (P1-P3) are restricted to the ventral thalamic prosomere (P3), including those forming the adult ascending dopaminergic system. Moreover, the latter likely depend developmentally on the dorsally adjacent alar plate Pax6-expressing cells.

Detailed immunohistology of Pax6 protein and tyrosine hydroxylase in the early zebrafish brain suggests role of Pax6 gene in development of dopaminergic diencephalic neurons

Spatiotemporal developmental dynamics of Pax6 protein containing (i.e., Pax6) cells were investigated immunohistochemically in embryonic and postembryonic zebrafish brain sections (especially at 2 and 5 day), allowing for a neuroanatomically detailed resolution previously only reported for the mouse. Besides strikingly close correspondences of early Pax6 domains - including many spatiotemporal changes - in mouse and zebrafish brains, some critical differences were noted. There is no pallial (i.e., cortical) Pax6 expression domain in the ventricular proliferative layer in the zebrafish as in the mouse. The main pallial Pax6 domain in the zebrafish is comparable to the migrating stream of Pax6 cells at the pallial-subpallial boundary. This indicates that some developmental functions of Pax6 (i.e., inhibition of subpallial cell migration into pallium by Pax6 migrating stream) might be shared with the mouse and maybe all vertebrates, while others (i.e., control of intrapallial neuronal radial migration via Pax6 expressing radial glia cells) may be special for mammals. Another prominent feature in the early zebrafish forebrain is that the alar plate ventral thalamic Pax6 domain extends far caudolaterally into the periphery of the basal plate posterior tuberculum and hypothalamic inferior lobe. This indicates that the alar plate ventral thalamus invades the forebrain basal plate and contributes to the development of basal forebrain structures. The close spatiotemporal association of Pax6 cells and TH cells of the ventral thalamus indicates a local role of Pax6 in the development of ventral thalamic (as recently demonstrated in the mouse) and, maybe, posterior tubercular TH cells. However, our confocal microscopical analysis of zebrafish brain sections double-immunostained for Pax6 and TH did not reveal cells double-labeled for these two proteins in this location, but rather indicates an inductive interaction of Pax6 cells onto TH cells.

The teleostean (zebrafish) dopaminergic system ascending to the subpallium (striatum) is located in the basal diencephalon (posterior tuberculum)

Tyrosine hydroxylase immunohistochemistry is used to demonstrate catecholaminergic neuronal populations in the fore- and midbrain of adult zebrafish (Danio rerio). While no catecholaminergic neurons are found in the midbrain, various immunoreactive populations were found in the diencephalon (hypothalamus, posterior tuberculum, ventral thalamus, pretectum) and telencephalon (preoptic region, subpallium, olfactory bulb). The posterior tubercular catecholaminergic cells include three cytological types (small round, large pear-shaped, and bipolar liquor-contacting cells). Furthermore, the retrograde neuronal tracers DiI or biocytin were applied to demonstrate ascending projections to the basal telencephalon (incl. the striatum). A double-label approach was used - together with tyrosine hydroxylase immunohistochemistry - in order to visualize neurons positive for tyrosine hydroxylase and a retrograde tracer. Double-labeled cells were identified in two locations in the posterior tuberculum (i.e, small round neurons in the periventricular nucleus of the posterior tuberculum and large pear-shaped cells adjacent to it). They are interpreted as the teleostean dopaminergic system ascending to the striatum, since previous work [16] established that no noradrenergic neurons exist in the forebrain of the adult zebrafish.