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

Neuron mapping in the Molly fish optic tectum: An emphasis on the adult neurogenesis process

Teleost fish exhibit the most pronounced and widespread adult neurogenesis. Recently, functional development and the fate of newborn neurons have been reported in the optic tectum (OT) of fish. To determine the role of neurogenesis in the OT, this study used histological, immunohistochemical, and electron microscopic investigations on 18 adult Molly fish specimens (Poecilia sphenops). The OT of the Molly fish was a bilateral lobed structure located in the dorsal part of the mesencephalon. It exhibited a laminated structure made up of alternating fiber and cellular layers, which were organized into six main layers. The stratum opticum (SO) was supplied by optic nerve fibers, in which the neuropil was the main component. Radial bipolar neurons that possessed bundles of microtubules were observed in the stratum fibrosum et griseum superficiale (SFGS). Furthermore, oligodendrocytes with their processes wrapped around the nerve fibers could be observed. The stratum album centrale (SAC) consisted mainly of the axons of the stratum griseum centrale (SGC) and the large tectal, pyriform, and horizontal neurons. The neuronal cells of the SO and large tectal cells of the SAC expressed autophagy-related protein-5 (APG5). Interleukin-1β (IL-1β) was expressed in both neurons and glia cells of SGC. Additionally, inducible nitric oxide synthase (iNOS) was expressed in the neuropil of the SAC synaptic layer and granule cells of the stratum periventriculare (SPV). Also, transforming growth factor beta (TGF-β), SRY-box transcription factor 9 (SOX9), and myostatin were clearly expressed in the proliferative neurons. In all strata, S100 protein and Oligodendrocyte Lineage Transcription Factor 2 (Olig2) were expressed by microglia, oligodendrocytes, and astrocytes. In conclusion, it was possible to identify different varieties of neurons in the optic tectum, each with a distinct role. The existence of astrocytes, proliferative neurons, and stem cells highlights the regenerative capacity of OT. RESEARCH HIGHLIGHTS: The OT of the Molly fish exhibited a laminated structure made up of alternating fiber and cellular layers, which were organized into six main layers. Radial bipolar neurons that possessed bundles of microtubules were observed in the stratum fibrosum et griseum superficiale (SFGS). The stratum album central (SAC) consisted mainly of the axons of the stratum griseum centrale (SGC) and the large tectal, pyriform, and horizontal neurons. Inducible nitric oxide synthase (iNOS) was expressed in the neuropil of the SAC synaptic layer and granule cells of the stratum periventricular (SPV). Also, transforming growth factor beta (TGF-β), SRY-box transcription factor 9 (SOX9), and myostatin were clearly expressed in the proliferative neurons. The existence of astrocytes, proliferative neurons, and stem cells highlights the regenerative capacity of OT.

Exposure to flutolanil at environmentally relevant concentrations can induce image and non-image-forming failure of zebrafish larvae through neuro and visual disruptions

Numerous pesticides pose a threat to aquatic ecosystems, jeopardizing aquatic animal species and impacting human health. While the contamination of aquatic environment by flutolanil and its adverse effects on animal in the treatment of rich sheath blight have been reported, the neuro-visual effects of flutolanil at environmentally relevant concentrations remain unknown. In this study, we administered flutolanil to zebrafish embryos (0, 0.125, 0.50 and 2.0 mg/L) for 4 days to investigate its impact on the neuro and visual system. The results revealed that flutolanil induced abnormal behavior in larvae, affecting locomotor activity, stimuli response and phototactic response. Additionally, it led to defective brain and ocular development and differentiation. The disruption extended to the neurological system and visual phototransduction of larvae, evidenced by significant disturbances in genes and proteins related to neurodevelopment, neurotransmission, eye development, and visual function. Untargeted metabolomics analysis revealed that the GABAergic signaling pathway and increased levels of glutamine, glutamate, andγ-aminobutyric acid were implicated in the response to neuro and visual system injury induced by flutolanil, contributing to aberrant development, behavioral issues, and endocrine disruption. This study highlights the neuro-visual injury caused by flutolanil in aquatic environment, offering fresh insights into the mechanisms underlying image and non-image effects.

ASS1 deficiency is associated with impaired neuronal differentiation in zebrafish larvae

Citrullinemia type 1 (CTLN1) is a rare autosomal recessive urea cycle disorder caused by deficiency of the cytosolic enzyme argininosuccinate synthetase 1 (ASS1) due to pathogenic variants in the ASS1 gene located on chromosome 9q34.11. Even though hyperammenomia is considered the major pathomechanistic factor for neurological impairment and cognitive dysfunction, a relevant subset of individuals presents with a neurodegenerative course in the absence of hyperammonemic decompensations. Here we show, that ASS1 deficiency induced by antisense-mediated knockdown of the zebrafish ASS1 homologue is associated with defective neuronal differentiation ultimately causing neuronal cell loss and consecutively decreased brain size in zebrafish larvae in vivo. Whereas ASS1-deficient zebrafish larvae are characterized by markedly elevated concentrations of citrulline - the biochemical hallmark of CTLN1, accumulation of L-citrulline, hyperammonemia or therewith associated secondary metabolic alterations did not account for the observed phenotype. Intriguingly, coinjection of the human ASS1 mRNA not only normalized citrulline concentration but also reversed the morphological cerebral phenotype and restored brain size, confirming conserved functional properties of ASS1 across species. The results of the present study imply a novel, potentially non-enzymatic (moonlighting) function of the ASS1 protein in neurodevelopment.

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.

The Everted Amygdala of Ray-Finned Fish: Zebrafish Makes a Case

The amygdala, a complex array of nuclei in the forebrain, controls emotions and emotion-related behaviors in vertebrates. Current research aims to understand the amygdala's evolution in ray-finned fish such as zebrafish because of the region's relevance for social behavior and human psychiatric disorders. Clear-cut molecular definitions of the amygdala and its evolutionary-developmental relationship to the one of mammals are critical for zebrafish models of affective disorders and autism. In this review, I argue that the prosomeric model and a focus on the olfactory system's organization provide ideal tools for discovering deep ancestral relationships between the emotional systems of zebrafish and mammals. The review's focus is on the "extended amygdala," which refers to subpallial amygdaloid territories including the central (autonomic) and the medial (olfactory) amygdala required for reproductive and social behaviors. Amphibians, sauropsids, and lungfish share many characteristics with the basic amygdala ground plan of mammals, as molecular and hodological studies have shown. Further exploration of the evolution of the amygdala in basally derived fish vertebrates requires researchers to test these "tetrapod-based" concepts. Historically, this has been a daunting task because the forebrains of basally derived fish vertebrates look very different from those of more familiar tetrapod ones. An extreme case are ray-finned fish (Actinopterygii) like zebrafish because their telencephalon develops through a distinct outward-growing process called eversion. To this day, scientists have struggled to determine how the everted telencephalon compares to non-actinopterygian vertebrates. Using the teleost zebrafish as a genetic model, comparative neurologists began to establish quantifiable molecular definitions that allow direct comparisons between ray-finned fish and tetrapods. In this review, I discuss how the most recent discovery of the zebrafish amygdala ground plan offers an opportunity to identify the developmental constraints of amygdala evolution and function. In addition, I explain how the zebrafish prethalamic eminence (PThE) topologically relates to the medial amygdala proper and the nucleus of the lateral olfactory tract (nLOT). In fact, I consider these previously misinterpreted olfactory structures the most critical missing evolutionary links between actinopterygian and tetrapod amygdalae. In this context, I will also explain why recognizing both the PThE and the nLOT is crucial to understanding the telencephalon eversion. Recognizing these anatomical hallmarks allows direct comparisons of the amygdalae of zebrafish and mammals. Ultimately, the new concepts of the zebrafish amygdala will overcome current dogmas and reach a holistic understanding of amygdala circuits of cognition and emotion in actinopterygians.

Severe hypoxia exposure inhibits larval brain development but does not affect the capacity to mount a cortisol stress response in zebrafish

Fish nursery habitats are increasingly hypoxic and the brain is recognized as highly hypoxia-sensitive, yet there is a lack of information on the effects of hypoxia on the development and function of the larval fish brain. Here, we tested the hypothesis that by inhibiting brain development, larval exposure to severe hypoxia has persistent functional effects on the cortisol stress response in zebrafish (Danio rerio). Exposing 5 days post-fertilization (dpf) larvae to 10% dissolved O2 (DO) for 16 h only marginally reduced survival, but it decreased forebrain neural proliferation by 55%, and reduced the expression of neurod1, gfap, and mbpa, markers of determined neurons, glia, and oligodendrocytes, respectively. The 5 dpf hypoxic exposure also elicited transient increases in whole body cortisol and in crf, uts1, and hsd20b2 expression, key regulators of the endocrine stress response. Hypoxia exposure at 5 dpf also inhibited the cortisol stress response to hypoxia in 10 dpf larvae and increased hypoxia tolerance. However, 10% DO exposure at 5 dpf for 16h did not affect the cortisol stress response to a novel stressor in 10 dpf larvae or the cortisol stress response to hypoxia in adult fish. Therefore, while larval exposure to severe hypoxia can inhibit brain development, it also increases hypoxia tolerance. These effects may transiently reduce the impact of hypoxia on the cortisol stress response but not its functional capacity to respond to novel stressors. We conclude that the larval cortisol stress response in zebrafish has a high capacity to cope with severe hypoxia-induced neurogenic impairment.

Behavioral effects of early-life exposure to perfluorooctanoic acid might synthetically link to multiple aspects of dopaminergic neuron development and dopamine functions in zebrafish larvae

Perfluorooctanoic acid (PFOA) is known as an environmental endocrine disruptor and has developmental neurotoxicity that could be associated with behavior changes in human and animal studies. Previous studies have shown that PFOA might affect the dopaminergic nervous system. However, the mode of action underlying the effects of PFOA remains poorly understood. Our study used zebrafish as an animal model to investigate the effects of early-life PFOA exposure on dopaminergic neuron development and dopamine functions in zebrafish larvae. Zebrafish fertilized eggs were exposed to different concentrations of PFOA (0, 10, 100, 1000 μg/L). After exposure to PFOA for 7 days, the locomotor activity of zebrafish was decreased; the mRNA levels of nuclear receptor subfamily 4 group a member 2b (nr4a2b), paired box 2 and 5 (pax2, pax5), tyrosine hydroxylase 1/2 (th1/th2) and dopamine transporter (dat) were increased; mRNA and protein level of mesencephalic astrocyte-derived neurotrophic factor (manf) were decreased. Neural cell proliferation in the preoptic area of hypothalamus was increased. In conclusion, dopaminergic neuron development might be one of the targets of early-life PFOA exposure. The neurobehavior changes induced by PFOA exposure might link to multiple aspects of dopaminergic neuron development and dopamine functions in zebrafish larvae.

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.

Cfdp1 controls the cell cycle and neural differentiation in the zebrafish cerebellum and retina

BACKGROUND

Although the cell cycle and cell differentiation should be coordinately regulated to generate a variety of neurons in the brain, the molecules that are involved in this coordination still remain largely unknown. In this study, we analyzed the roles of a nuclear protein Cfdp1, which is thought to be involved in chromatin remodeling, in zebrafish neurogenesis.

RESULTS

Zebrafish cfdp1 mutants maintained the progenitors of granule cells (GCs) in the cerebellum, but showed defects in their differentiation to GCs. cfdp1 mutants showed an increase in phospho-histone 3 (pH 3)-positive cells and apoptotic cells, as well as a delayed cell cycle transition from the G2 to the M phase in the cerebellum. The inhibition of tp53 prevented apoptosis but not GC differentiation in the cfdp1 mutant cerebellum. A similar increase in apoptotic cells and pH 3-positive cells, and defective cell differentiation, were observed in the cfdp1 mutant retina. Although mitotic spindles formed, mitosis was blocked before anaphase in both the cerebellum and retina of cfdp1 mutant larvae. Furthermore, expression of the G2/mitotic-specific cyclin B1 gene increased in the cfdp1 mutant cerebellum.

CONCLUSIONS

Our findings suggest that Cfdp1 regulates the cell cycle of neural progenitors, thereby promoting neural differentiation in the brain.

An Optical Illusion Pinpoints an Essential Circuit Node for Global Motion Processing

Direction-selective (DS) neurons compute the direction of motion in a visual scene. Brain-wide imaging in larval zebrafish has revealed hundreds of DS neurons scattered throughout the brain. However, the exact population that causally drives motion-dependent behaviors-e.g., compensatory eye and body movements-remains largely unknown. To identify the behaviorally relevant population of DS neurons, here we employ the motion aftereffect (MAE), which causes the well-known "waterfall illusion." Together with region-specific optogenetic manipulations and cellular-resolution functional imaging, we found that MAE-responsive neurons represent merely a fraction of the entire population of DS cells in larval zebrafish. They are spatially clustered in a nucleus in the ventral lateral pretectal area and are necessary and sufficient to steer the entire cycle of optokinetic eye movements. Thus, our illusion-based behavioral paradigm, combined with optical imaging and optogenetics, identified key circuit elements of global motion processing in the vertebrate brain.

Non-thalamic origin of zebrafish sensory nuclei implies convergent evolution of visual pathways in amniotes and teleosts

Ascending visual projections similar to the mammalian thalamocortical pathway are found in a wide range of vertebrate species, but their homology is debated. To get better insights into their evolutionary origin, we examined the developmental origin of a thalamic-like sensory structure of teleosts, the preglomerular complex (PG), focusing on the visual projection neurons. Similarly to the tectofugal thalamic nuclei in amniotes, the lateral nucleus of PG receives tectal information and projects to the pallium. However, our cell lineage study in zebrafish reveals that the majority of PG cells are derived from the midbrain, unlike the amniote thalamus. We also demonstrate that the PG projection neurons develop gradually until late juvenile stages. Our data suggest that teleost PG, as a whole, is not homologous to the amniote thalamus. Thus, the thalamocortical-like projections evolved from a non-forebrain cell population, which indicates a surprising degree of variation in the vertebrate sensory systems.

QDPR homologues in Danio rerio regulate melanin synthesis, early gliogenesis, and glutamine homeostasis

Dihydropteridine reductase (QDPR) catalyzes the recycling of tetrahydrobiopterin (BH₄), a cofactor in dopamine, serotonin, and phenylalanine metabolism. QDPR-deficient patients develop neurological symptoms including hypokinesia, truncal hypotonia, intellectual disability and seizures. The underlying pathomechanisms are poorly understood. We established a zebrafish model for QDPR deficiency and analyzed the expression as well as function of all zebrafish QDPR homologues during embryonic development. The homologues qdpra is essential for pigmentation and phenylalanine metabolism. Qdprb1 is expressed in the proliferative zones of the optic tectum and eye. Knockdown of qdprb1 leads to up-regulation of pro-proliferative genes and increased number of phospho-histone3 positive mitotic cells. Expression of neuronal and astroglial marker genes is concomitantly decreased. Qdprb1 hypomorphic embryos develop microcephaly and reduced eye size indicating a role for qdprb1 in the transition from cell proliferation to differentiation. Glutamine accumulation biochemically accompanies the developmental changes. Our findings provide novel insights into the neuropathogenesis of QDPR deficiency.

Expression of RNA-binding protein Rbfox1l demarcates a restricted population of dorsal telencephalic neurons within the adult zebrafish brain

Rbfox RNA-binding proteins are expressed in the adult mammalian brain and are required for proper brain development and function. Studies in mice and humans have implicated Rbfox1/RBFOX1 in autism, neuronal excitation and epilepsy, and Rbfox2/RBFOX2 in cerebellar development. The zebrafish has emerged as a prominent model system for brain study, possessing neuroanatomical conservation with mammals and an extensive capacity for adult neurogenesis and plasticity. In this study, we characterize Rbfox1l and Rbfox2 expression in the adult zebrafish brain. While Rbfox2 is expressed broadly, Rbfox1l is expressed in restricted populations of neurons in the dorsal telencephalon and cerebellum. In the dorsal telencephalon, Rbfox1l is expressed in a specific population of neurons spanning Dm and Dc regions. In the cerebellum, Rbfox1l and Rbfox2 are expressed in the Purkinje cell layer, reminiscent of Rbfox1 and Rbfox2 expression in the mammalian cerebellum. Our findings motivate future studies of Rbfox function in the zebrafish brain.

Cellular-Resolution Imaging of Vestibular Processing across the Larval Zebrafish Brain

The vestibular system, which reports on motion and gravity, is essential to postural control, balance, and egocentric representations of movement and space. The motion needed to stimulate the vestibular system complicates studying its circuitry, so we previously developed a method for fictive vestibular stimulation in zebrafish, using optical trapping to apply physical forces to the otoliths. Here, we combine this approach with whole-brain calcium imaging at cellular resolution, delivering a comprehensive map of the brain regions and cellular responses involved in basic vestibular processing. We find responses broadly distributed across the brain, with unique profiles of cellular responses and topography in each region. The most widespread and abundant responses involve excitation that is graded to the stimulus strength. Other responses, localized to the telencephalon and habenulae, show excitation that is only weakly correlated to stimulus strength and that is sensitive to weak stimuli. Finally, numerous brain regions contain neurons that are inhibited by vestibular stimuli, and these neurons are often tightly localized spatially within their regions. By exerting separate control over the left and right otoliths, we explore the laterality of brain-wide vestibular processing, distinguishing between neurons with unilateral and bilateral vestibular sensitivity and revealing patterns whereby conflicting signals from the ears mutually cancel. Our results confirm previously identified vestibular responses in specific regions of the larval zebrafish brain while revealing a broader and more extensive network of vestibular responsive neurons than has previously been described. This provides a departure point for more targeted studies of the underlying functional circuits.

Ablation of a Neuronal Population Using a Two-photon Laser and Its Assessment Using Calcium Imaging and Behavioral Recording in Zebrafish Larvae

To identify the role of a subpopulation of neurons in behavior, it is essential to test the consequences of blocking its activity in living animals. Laser ablation of neurons is an effective method for this purpose when neurons are selectively labeled with fluorescent probes. In the present study, protocols for laser ablating a subpopulation of neurons using a two-photon microscope and testing of its functional and behavioral consequences are described. In this study, prey capture behavior in zebrafish larvae is used as a study model. The pretecto-hypothalamic circuit is known to underlie this visually-driven prey catching behavior. Zebrafish pretectum were laser-ablated, and neuronal activity in the inferior lobe of the hypothalamus (ILH; the target of the pretectal projection) was examined. Prey capture behavior after pretectal ablation was also tested.

Venlafaxine in Embryos Stimulates Neurogenesis and Disrupts Larval Behavior in Zebrafish

Venlafaxine, a widely prescribed antidepressant, is a selective serotonin and norepinephrine reuptake inhibitor in humans, and this drug is prevalent in municipal wastewater effluents. While studies have shown that this drug affects juvenile fish behavior, little is known about the developmental impact on nontarget aquatic animals. We tested the hypothesis that venlafaxine deposition in the egg, mimicking maternal transfer of this antidepressant, disrupts developmental programming using zebrafish (Danio rerio) as a model. Embryos (1-4 cell stage) were microinjected with either 1 or 10 ng venlafaxine, which led to a rapid reduction (90%) of this drug in the embryo at hatch. There was a concomitant increase in the concentration of the major metabolite o-desmethylvenlafaxine during the same period. Embryonic exposure to venlafaxine accelerated early development, increased hatching rate and produced larger larvae at 5 days post fertilization. Also, there was an increase in neuronal birth in the hypothalamus, dorsal thalamus, posterior tuberculum, and the preoptic region, and this corresponded with a higher spatial expression of nrd4, a key marker of neurogenesis. The venlafaxine-exposed larvae were less active and covered shorter distance in a light and dark behavioral test compared to the controls. Overall, zygotic exposure to venlafaxine disrupts early development, including brain function, and compromises larval behavior, suggesting impact of this drug on developmental programming in zebrafish.

Dominance behaviour in a non-aggressive flatfish, Senegalese sole (Solea senegalensis) and brain mRNA abundance of selected transcripts

Dominance is defined as the preferential access to limited resources. The present study aimed to characterise dominance in a non-aggressive flatfish species, the Senegalese sole (Solea senegalensis) by 1) identifying dominance categories and associated behaviours and 2) linking dominance categories (dominant and subordinate) with the abundance of selected mRNA transcripts in the brain. Early juveniles (n = 74, 37 pairs) were subjected to a dyadic dominance test, related to feeding, and once behavioural phenotypes had been described the abundance of ten selected mRNAs related to dominance and aggressiveness was measured in the brain. Late juveniles were subjected to two dyadic dominance tests (n = 34, 17 pairs), related to feeding and territoriality and one group test (n = 24, 4 groups of 6 fish). Sole feeding first were categorized as dominant and sole feeding second or not feeding as subordinate. Three social behaviours (i. "Resting the head" on another fish, ii. "Approaching" another fish, iii. "Swimming above another" fish) were associated with dominance of feeding. Two other variables (i. Total time occupying the preferred area during the last 2 hours of the 24 h test, ii. Organisms occupying the preferred area when the test ended) were representative of dominance in the place preference test. In all tests, dominant fish compared to subordinate fish displayed a significantly higher number of the behaviours "Rest the head" and "Approaches". Moreover, dominant sole dominated the sand at the end of the test, and in the group test dominated the area close to the feed delivery point before feed was delivered. The mRNA abundance of the selected mRNAs related to neurogenesis (nrd2) and neuroplasticity (c-fos) in dominant sole compared to subordinate were significantly different. This is the first study to characterise dominance categories with associated behaviours and mRNA abundance in Senegalese sole and provides tools to study dominance related problems in feeding and reproduction in aquaculture.

Activation of the hypothalamic feeding centre upon visual prey detection

The visual system plays a major role in food/prey recognition in diurnal animals, and food intake is regulated by the hypothalamus. However, whether and how visual information about prey is conveyed to the hypothalamic feeding centre is largely unknown. Here we perform real-time imaging of neuronal activity in freely behaving or constrained zebrafish larvae and demonstrate that prey or prey-like visual stimuli activate the hypothalamic feeding centre. Furthermore, we identify prey detector neurons in the pretectal area that project to the hypothalamic feeding centre. Ablation of the pretectum completely abolishes prey capture behaviour and neurotoxin expression in the hypothalamic area also reduces feeding. Taken together, these results suggest that the pretecto-hypothalamic pathway plays a crucial role in conveying visual information to the feeding centre. Thus, this pathway possibly converts visual food detection into feeding motivation in zebrafish.

Regeneration of Zebrafish CNS: Adult Neurogenesis

Regeneration in the animal kingdom is one of the most fascinating problems that have allowed scientists to address many issues of fundamental importance in basic biology. However, we came to know that the regenerative capability may vary across different species. Among vertebrates, fish and amphibians are capable of regenerating a variety of complex organs through epimorphosis. Zebrafish is an excellent animal model, which can repair several organs like damaged retina, severed spinal cord, injured brain and heart, and amputated fins. The focus of the present paper is on spinal cord regeneration in adult zebrafish. We intend to discuss our current understanding of the cellular and molecular mechanism(s) that allows formation of proliferating progenitors and controls neurogenesis, which involve changes in epigenetic and transcription programs. Unlike mammals, zebrafish retains radial glia, a nonneuronal cell type in their adult central nervous system. Injury induced proliferation involves radial glia which proliferate, transcribe embryonic genes, and can give rise to new neurons. Recent technological development of exquisite molecular tools in zebrafish, such as cell ablation, lineage analysis, and novel and substantial microarray, together with advancement in stem cell biology, allowed us to investigate how progenitor cells contribute to the generation of appropriate structures and various underlying mechanisms like reprogramming.

Motivated state control in larval zebrafish: behavioral paradigms and anatomical substrates

Over the course of each day, animals prioritize different objectives. Immediate goals may reflect fluctuating internal homeostatic demands, prompting individuals to seek out energy supplies or warmth. At other times, the environment may present temporary challenges or opportunities. Homeostatic demands and environmental signals often elicit persistent changes in an animal's behavior to meet needs and challenges over extended periods of time. These changes reflect the underlying motivational state of the animal. The larval zebrafish has been established as an effective genetically tractable vertebrate system to study neural circuits for sensory-motor reflexes. Fewer studies have exploited zebrafish to study brain circuits that control motivated behavior. In part this is because appropriate conceptual frameworks, anatomical knowledge, and behavioral paradigms are not yet well established. This review sketches a general conceptual framework for studying motivated state control in animal models, how this applies to larval zebrafish, and the current knowledge on neuroanatomical substrates for state control in this model.

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.

Afferent Connectivity of the Zebrafish Habenulae

The habenulae are bilateral nuclei located in the dorsal diencephalon that are conserved across vertebrates. Here we describe the main afferents to the habenulae in larval and adult zebrafish. We observe afferents from the subpallium, nucleus rostrolateralis, posterior tuberculum, posterior hypothalamic lobe, median raphe; we also see asymmetric afferents from olfactory bulb to the right habenula, and from the parapineal to the left habenula. In addition, we find afferents from a ventrolateral telencephalic nucleus that neurochemical and hodological data identify as the ventral entopeduncular nucleus (vENT), confirming and extending observations of Amo et al. (2014). Fate map and marker studies suggest that vENT originates from the diencephalic prethalamic eminence and extends into the lateral telencephalon from 48 to 120 hour post-fertilization (hpf). No afferents to the habenula were observed from the dorsal entopeduncular nucleus (dENT). Consequently, we confirm that the vENT (and not the dENT) should be considered as the entopeduncular nucleus "proper" in zebrafish. Furthermore, comparison with data in other vertebrates suggests that the vENT is a conserved basal ganglia nucleus, being homologous to the entopeduncular nucleus of mammals (internal segment of the globus pallidus of primates) by both embryonic origin and projections, as previously suggested by Amo et al. (2014).

Copy number variants in patients with intellectual disability affect the regulation of ARX transcription factor gene

Protein-coding mutations in the transcription factor-encoding gene ARX cause various forms of intellectual disability (ID) and epilepsy. In contrast, variations in surrounding non-coding sequences are correlated with milder forms of non-syndromic ID and autism and had suggested the importance of ARX gene regulation in the etiology of these disorders. We compile data on several novel and some already identified patients with or without ID that carry duplications of ARX genomic region and consider likely genetic mechanisms underlying the neurodevelopmental defects. We establish the long-range regulatory domain of ARX and identify its brain region-specific autoregulation. We conclude that neurodevelopmental disturbances in the patients may not simply arise from increased dosage due to ARX duplication. This is further exemplified by a small duplication involving a non-functional ARX copy, but with duplicated enhancers. ARX enhancers are located within a 504-kb region and regulate expression specifically in the forebrain in developing and adult zebrafish. Transgenic enhancer-reporter lines were used as in vivo tools to delineate a brain region-specific negative and positive autoregulation of ARX. We find autorepression of ARX in the telencephalon and autoactivation in the ventral thalamus. Fluorescently labeled brain regions in the transgenic lines facilitated the identification of neuronal outgrowth and pathfinding disturbances in the ventral thalamus and telencephalon that occur when arxa dosage is diminished. In summary, we have established a model for how breakpoints in long-range gene regulation alter the expression levels of a target gene brain region-specifically, and how this can cause subtle neuronal phenotypes relating to the etiology of associated neuropsychiatric disease.

Voluntary exercise induces neurogenesis in the hypothalamus and ependymal lining of the third ventricle

In the adult hypothalamus and ependymal lining of the third ventricle, tanycytes function as multipotential progenitor cells that enable continuous neurogenesis, suggesting that tanycytes may be able to mediate the restoration of homeostatic function after stroke. Voluntary wheel running has been shown to alter neurochemistry and neuronal function and to increase neurogenesis in rodents. In the present study, we found that voluntary exercise improved the survival rate and energy balance of stroke-prone spontaneously hypertensive rats (SHRSP/Kpo). We also investigated the effect of exercise on the proliferation and differentiation of hypothalamic cells using immunoreactivity for tanycytes and neural markers. The proliferation of elongated cells, which may be the tanycytes, was enhanced in exercising SHRSP compared to sedentary rats before and after stroke. In addition, the proliferation of cells was correlated with the induction of fibroblast growth factor-2 in the subependymal cells of the third ventricle and in the cerebrospinal fluid. Some of the newborn cells of exercising SHRSP showed differentiation into mature neurons after stroke. Our results suggest that voluntary exercise correlates with hypothalamic neurogenesis, leading to recovery of homeostatic functions in the adult brain after stroke.

Neurodevelopmental origin and adult neurogenesis of the neuroendocrine hypothalamus

The adult hypothalamus regulates many physiological functions and homeostatic loops, including growth, feeding and reproduction. In mammals, the hypothalamus derives from the ventral diencephalon where two distinct ventricular proliferative zones have been described. Although a set of transcription factors regulating the hypothalamic development has been identified, the exact molecular mechanisms that drive the differentiation of hypothalamic neural precursor cells (NPCs) toward specific neuroendocrine neuronal subtypes is yet not fully disclosed. Neurogenesis has been also reported in the adult hypothalamus at the level of specific niches located in the ventrolateral region of ventricle wall, where NPCs have been identified as radial glia-like tanycytes. Here we review the molecular and cellular systems proposed to support the neurogenic potential of developing and adult hypothalamic NPCs. We also report new insights on the mechanisms by which adult hypothalamic neurogenesis modulates key functions of this brain region. Finally, we discuss how environmental factors may modulate the adult hypothalamic neurogenic cascade.

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.

β-asarone reverses chronic unpredictable mild stress-induced depression-like behavior and promotes hippocampal neurogenesis in rats

In this study, we investigated the influence of β-asarone, the major ingredient of Acorus tatarinowii Schott, on depressive-like behavior induced by the chronic unpredictable mild stresses (CUMS) paradigm and to clarify the underlying mechanisms. The results show that β-asarone treatment partially reversed the CUMS-induced depression-like behaviors in both the forced swim and sucrose preference tests. The behavioral effects were associated with increased hippocampal neurogenesis indicated by bromodeoxyuridine (BrdU) immunoreactivity. β-Asarone treatment significantly increased the expression of brain-derived neurotrophic factor (BDNF) at levels of transcription and translation. Moreover, CUMS caused significant reduction in ERK1/2 and CREB phosphorylation, both of which were partially attenuated by β-asarone administration. It is important to note that β-asarone treatment had no effect on total levels or phosphorylation state of any of the proteins examined in ERK1/2-CREB pathway in no stress rats, suggesting that β-asarone acts in a stress-dependent manner to block ERK1/2-CREB signaling. We did not observe a complete reversal of depression-like behaviors to control levels by β-asarone. To our knowledge, the present study is the first to demonstrate that adult neurogenesis is involved in the antidepressant-like behavioral effects of β-asarone, suggesting that β-asarone is a promising candidate for the treatment of depression.

Molecular characterization of prosomeric and intraprosomeric subdivisions of the embryonic zebrafish diencephalon

During development of the early neural tube, positional information provided by signaling gradients is translated into a grid of transverse and longitudinal transcription factor expression domains. Transcription factor specification codes defining distinct histogenetic domains within this grid are evolutionarily conserved across vertebrates and may reflect an underlying common vertebrate bauplan. When compared to the rich body of comparative gene expression studies of tetrapods, there is considerably less comparative data available for teleost fish. We used sensitive multicolor fluorescent in situ hybridization to generate a detailed map of regulatory gene expression domains in the embryonic zebrafish diencephalon. The high resolution of this technique allowed us to resolve abutting and overlapping gene expression of different transcripts. We found that the relative topography of gene expression patterns in zebrafish was highly similar to those of orthologous genes in tetrapods and consistent with a three-prosomere organization of the alar and basal diencephalon. Our analysis further demonstrated a conservation of intraprosomeric subdivisions within prosomeres 1, 2, and 3 (p1, p2, and p3). A tripartition of zebrafish p1 was identified reminiscent of precommissural (PcP), juxtacommissural (JcP), and commissural (CoP) pretectal domains of tetrapods. The constructed detailed diencephalic transcription factor gene expression map further identified molecularly distinct thalamic and prethalamic rostral and caudal domains and a prethalamic eminence histogenetic domain in zebrafish. Our comparative gene expression analysis conformed with the idea of a common bauplan for the diencephalon of anamniote and amniote vertebrates from fish to mammals.

Control of a specific motor program by a small brain area in zebrafish

Complex motor behaviors are thought to be coordinated by networks of brain nuclei that may control different elementary motor programs. Transparent zebrafish larvae offer the opportunity to analyze the functional organization of motor control networks by optical manipulations of neuronal activity during behavior. We examined motor behavior in transgenic larvae expressing channelrhodopsin-2 throughout many neurons in the brain. Wide-field optical stimulation triggered backward and rotating movements caused by the repeated execution of J-turns, a specific motor program that normally occurs during prey capture. Although optically-evoked activity was widespread, behavioral responses were highly coordinated and lateralized. 3-D mapping of behavioral responses to local optical stimuli revealed that J-turns can be triggered specifically in the anterior-ventral optic tectum (avOT) and/or the adjacent pretectum. These results suggest that the execution of J-turns is controlled by a small group of neurons in the midbrain that may act as a command center. The identification of a brain area controlling a defined motor program involved in prey capture is a step toward a comprehensive analysis of neuronal circuits mediating sensorimotor behaviors of zebrafish.

Neurogenesis in zebrafish - from embryo to adult

Neurogenesis in the developing central nervous system consists of the induction and proliferation of neural progenitor cells and their subsequent differentiation into mature neurons. External as well as internal cues orchestrate neurogenesis in a precise temporal and spatial way. In the last 20 years, the zebrafish has proven to be an excellent model organism to study neurogenesis in the embryo. Recently, this vertebrate has also become a model for the investigation of adult neurogenesis and neural regeneration. Here, we summarize the contributions of zebrafish in neural development and adult neurogenesis.

Aluminum exposure impacts brain plasticity and behavior in Atlantic salmon (Salmo salar)

Aluminum (Al) toxicity occurs frequently in natural aquatic ecosystems as a result of acid deposition and natural weathering processes. Detrimental effects of Al toxicity on aquatic organisms are well known and can have consequences for survival. Fish exposed to Al in low pH waters will experience physiological and neuroendocrine changes that disrupt homeostasis and alter behavior. To investigate the effects of Al exposure to both brain and behavior, Atlantic salmon (Salmo salar) kept in water treated with Al (pH 5.7, 0.37±0.04 µmol 1-1 of Al) for 2 weeks were compared to fish kept in a control condition (pH 6.7, <0.04 µmol 1-1 of Al). Fish exposed to Al and acidic conditions had increased Al accumulation in the gills and decreased gill Na+, K+-ATPase activity, which impaired osmoreguatory capacity and caused physiological stress, indicated by elevated plasma cortisol and glucose levels. Here we show for the first time that exposure to Al in acidic conditions also impaired learning performance in a maze task. Al toxicity reduced the expression of NeuroD1 transcript levels in the forebrain of exposed fish. As in mammals, these data show that exposure to chronic stress, such as acidified Al, can reduce neural plasticity during behavioral challenges in salmon, and may impair coping ability to new environments.

Requirement for zebrafish ataxin-7 in differentiation of photoreceptors and cerebellar neurons

The expansion of a polyglutamine (polyQ) tract in the N-terminal region of ataxin-7 (atxn7) is the causative event in spinocerebellar ataxia type 7 (SCA7), an autosomal dominant neurodegenerative disorder mainly characterized by progressive, selective loss of rod-cone photoreceptors and cerebellar Purkinje and granule cells. The molecular and cellular processes underlying this restricted neuronal vulnerability, which contrasts with the broad expression pattern of atxn7, remains one of the most enigmatic features of SCA7, and more generally of all polyQ disorders. To gain insight into this specific neuronal vulnerability and achieve a better understanding of atxn7 function, we carried out a functional analysis of this protein in the teleost fish Danio rerio. We characterized the zebrafish atxn7 gene and its transcription pattern, and by making use of morpholino-oligonucleotide-mediated gene inactivation, we analysed the phenotypes induced following mild or severe zebrafish atxn7 depletion. Severe or nearly complete zebrafish atxn7 loss-of-function markedly impaired embryonic development, leading to both early embryonic lethality and severely deformed embryos. More importantly, in relation to SCA7, moderate depletion of the protein specifically, albeit partially, prevented the differentiation of both retina photoreceptors and cerebellar Purkinje and granule cells. In addition, [1–232] human atxn7 fragment rescued these phenotypes showing strong function conservation of this protein through evolution. The specific requirement for zebrafish atxn7 in the proper differentiation of cerebellar neurons provides, to our knowledge, the first in vivo evidence of a direct functional relationship between atxn7 and the differentiation of Purkinje and granule cells, the most crucial neurons affected in SCA7 and most other polyQ-mediated SCAs. These findings further suggest that altered protein function may play a role in the pathophysiology of the disease, an important step toward the development of future therapeutic strategies.

MANF regulates dopaminergic neuron development in larval zebrafish

Mesencephalic astrocyte derived neurotrophic factor (MANF) is recognized as a dopaminergic neurotrophic factor, which can protect dopaminergic neurons from neurotoxic damage. However, little is known about the function of MANF during the vertebrate development. Here, we report that MANF expression is widespread during embryonic development and in adult organs analyzed by qPCR and in situ hybridization in zebrafish. Knockdown of MANF expression with antisense splice-blocking morpholino oligonucleotides resulted in no apparent abnormal phenotype. Nevertheless, the dopamine level of MANF morphants was lower than that of the wild type larvae, the expression levels of the two tyrosine hydroxylase gene transcripts were decreased and a decrease in neuron number in certain groups of th1 and th2 cells in the diencephalon region in MANF morphants was observed. These defects were rescued by injection of exogenous manf mRNA. Strikingly, manf mRNA could partly restore the decrease of th1 positive cells in Nr4a2-deficient larvae. These results suggest that MANF is involved in the regulation of the development of dopaminergic system in zebrafish.

Intralineage directional Notch signaling regulates self-renewal and differentiation of asymmetrically dividing radial glia

Asymmetric division of progenitor/stem cells generates both self-renewing and differentiating progeny and is fundamental to development and regeneration. How this process is regulated in the vertebrate brain remains incompletely understood. Here, we use time-lapse imaging to track radial glia progenitor behavior in the developing zebrafish brain. We find that asymmetric division invariably generates a basal self-renewing daughter and an apical differentiating sibling. Gene expression and genetic mosaic analysis further show that the apical daughter is the source of Notch ligand that is essential to maintain higher Notch activity in the basal daughter. Notably, establishment of this intralineage and directional Notch signaling requires the intrinsic polarity regulator Partitioning defective protein-3 (Par-3), which segregates the fate determinant Mind bomb unequally to the apical daughter, thereby restricting the self-renewal potential to the basal daughter. These findings reveal with single-cell resolution how self-renewal and differentiation become precisely segregated within asymmetrically dividing neural progenitor/stem lineages.

What is the Thalamus in Zebrafish?

Current research on the thalamus and related structures in the zebrafish diencephalon identifies an increasing number of both neurological structures and ontogenetic processes as evolutionary conserved between teleosts and mammals. The patterning processes, for example, which during the embryonic development of zebrafish form the thalamus proper appear largely conserved. Yet also striking differences between zebrafish and other vertebrates have been observed, particularly when we look at mature and histologically differentiated brains. A case in point is the migrated preglomerular complex of zebrafish which evolved only within the lineage of ray-finned fish and has no counterpart in mammals or tetrapod vertebrates. Based on its function as a sensory relay station with projections to pallial zones, the preglomerular complex has been compared to specific thalamic nuclei in mammals. However, no thalamic projections to the zebrafish dorsal pallium, which corresponds topologically to the mammalian isocortex, have been identified. Merely one teleostean thalamic nucleus proper, the auditory nucleus, projects to a part of the dorsal telencephalon, the pallial amygdala. Studies on patterning mechanisms identify a rostral and caudal domain in the embryonic thalamus proper. In both, teleosts and mammals, the rostral domain gives rise to GABAergic neurons, whereas glutamatergic neurons originate in the caudal domain of the zebrafish thalamus. The distribution of GABAergic derivatives in the adult zebrafish brain, furthermore, revealed previously overlooked thalamic nuclei and redefined already established ones. These findings require some reconsideration regarding the topological origin of these adult structures. In what follows, I discuss how evolutionary conserved and newly acquired features of the developing and adult zebrafish thalamus can be compared to the mammalian situation.

Neuronal regeneration in a zebrafish model of adult brain injury

Neural stem cells in the subventricular zone (SVZ) of the adult mammalian forebrain are a potential source of neurons for neural tissue repair after brain insults such as ischemic stroke and traumatic brain injury (TBI). Recent studies show that neurogenesis in the ventricular zone (VZ) of the adult zebrafish telencephalon has features in common with neurogenesis in the adult mammalian SVZ. Here, we established a zebrafish model to study injury-induced neurogenesis in the adult brain. We show that the adult zebrafish brain possesses a remarkable capacity for neuronal regeneration. Telencephalon injury prompted the proliferation of neuronal precursor cells (NPCs) in the VZ of the injured hemisphere, compared with in the contralateral hemisphere. The distribution of NPCs, viewed by BrdU labeling and ngn1-promoter-driven GFP, suggested that they migrated laterally and reached the injury site via the subpallium and pallium. The number of NPCs reaching the injury site significantly decreased when the fish were treated with an inhibitor of γ-secretase, a component of the Notch signaling pathway, suggesting that injury-induced neurogenesis mechanisms are at least partly conserved between fish and mammals. The injury-induced NPCs differentiated into mature neurons in the regions surrounding the injury site within a week after the injury. Most of these cells expressed T-box brain protein (Tbr1), suggesting they had adopted the normal neuronal fate in this region. These results suggest that the telencephalic VZ contributes to neural tissue recovery following telencephalic injury in the adult zebrafish, and that the adult zebrafish is a useful model for regenerative medicine.

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