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

Negative impacts of social isolation on behavior and neuronal functions are recovered after short-term social reintroduction in zebrafish

Recently, social isolation measures were crucial to prevent the spread of the coronavirus pandemic. However, the lack of social interactions affected the population mental health and may have long-term consequences on behavior and brain functions. Here, we evaluated the behavioral, physiological, and molecular effects of a social isolation (SI) in adult zebrafish, and whether the animals recover such changes after their reintroduction to the social environment. Fish were submitted to 12 days of SI, and then reintroduced to social context (SR). Behavioral analyses to evaluate locomotion, anxiety-like and social-related behaviors were performed after SI protocol, and 3 and 6 days after SR. Cortisol and transcript levels from genes involved in neuronal homeostasis (c-fos, egr, bdnf), and serotonergic (5-HT) and dopaminergic (DA) neurotransmission (thp, th) were also measured. SI altered social behaviors in zebrafish such as aggression, social preference, and shoaling. Fish submitted to SI also presented changes in the transcript levels of genes related to neural activity, and 5-HT/DA signaling. Interestingly, most of the behavioral and molecular changes induced by SI were not found again 6 days after SR. Thus, we highlight that SR of zebrafish to their conspecifics played a positive role in social behaviors and in the expression of genes involved in different neuronal signaling pathways that were altered after 12 days of SI. This study brings unprecedented data on the effects of SR in the recovery from SI neurobehavioral alterations, and reinforces the role of zebrafish as a translational model for understanding the neurobiological mechanisms adjacent to SI and resocialization.

Multiple faces of stress in the zebrafish (Danio rerio) brain

The changing expressions of certain genes as a consequence of exposure to stressors has not been studied in detail in the fish brain. Therefore, a stress trial with zebrafish was conducted, aiming at identifying relevant gene regulation pathways in different regions of the brain. As acute stressors within this trial, feed rewarding, feed restriction, and air exposure have been used. The gene expression data from the experimental fish brains have been analyzed by means of principal component analyses (PCAs), whereby the individual genes have been compiled according to the regulation pathways in the brain. The results did not indicate a mutual response across the treatment and gender groups. To evaluate whether a similar sample structure belonging to a large sample size would have allowed the classification of the gene expression patterns according to the treatments, the data have been bootstrapped and used for building random forest models. These revealed a high accuracy of the classifications, but different genes in the female and male zebrafish were found to have contributed to the classification algorithms the most. These analyses showed that less than eight genes are, in most cases, sufficient for an accurate classification. Moreover, mainly genes belonging to the stress axis, to the isotocin regulation pathways, or to the serotonergic pathways had the strongest influence on the outcome of the classification models.

Cerebellar Purkinje Cells Control Posture in Larval Zebrafish (Danio rerio)

Cerebellar dysfunction leads to postural instability. Recent work in freely moving rodents has transformed investigations of cerebellar contributions to posture. However, the combined complexity of terrestrial locomotion and the rodent cerebellum motivate development of new approaches to perturb cerebellar function in simpler vertebrates. Here, we used a powerful chemogenetic tool (TRPV1/capsaicin) to define the role of Purkinje cells - the output neurons of the cerebellar cortex - as larval zebrafish swam freely in depth. We achieved both bidirectional control (activation and ablation) of Purkinje cells while performing quantitative high-throughput assessment of posture and locomotion. Activation disrupted postural control in the pitch (nose-up/nose-down) axis. Similarly, ablations disrupted pitch-axis posture and fin-body coordination responsible for climbs. Postural disruption was more widespread in older larvae, offering a window into emergent roles for the developing cerebellum in the control of posture. Finally, we found that activity in Purkinje cells could individually and collectively encode tilt direction, a key feature of postural control neurons. Our findings delineate an expected role for the cerebellum in postural control and vestibular sensation in larval zebrafish, establishing the validity of TRPV1/capsaicin-mediated perturbations in a simple, genetically-tractable vertebrate. Moreover, by comparing the contributions of Purkinje cell ablations to posture in time, we uncover signatures of emerging cerebellar control of posture across early development. This work takes a major step towards understanding an ancestral role of the cerebellum in regulating postural maturation.

Structural and Functional Organization of Visual Responses in the Inferior Olive of Larval Zebrafish

The olivo-cerebellar system plays an important role in vertebrate sensorimotor control. Here, we investigate sensory representations in the inferior olive (IO) of larval zebrafish and their spatial organization. Using single-cell labeling of genetically identified IO neurons, we find that they can be divided into at least two distinct groups based on their spatial location, dendritic morphology, and axonal projection patterns. In the same genetically targeted population, we recorded calcium activity in response to a set of visual stimuli using two-photon imaging. We found that most IO neurons showed direction-selective and binocular responses to visual stimuli and that the functional properties were spatially organized within the IO. Light-sheet functional imaging that allowed for simultaneous activity recordings at the soma and axonal level revealed tight coupling between functional properties, soma location, and axonal projection patterns of IO neurons. Taken together, our results suggest that anatomically defined classes of IO neurons correspond to distinct functional types, and that topographic connections between IO and cerebellum contribute to organization of the cerebellum into distinct functional zones.

A synaptic corollary discharge signal suppresses midbrain visual processing during saccade-like locomotion

In motor control, the brain not only sends motor commands to the periphery, but also generates concurrent internal signals known as corollary discharge (CD) that influence sensory information processing around the time of movement. CD signals are important for identifying sensory input arising from self-motion and to compensate for it, but the underlying mechanisms remain unclear. Using whole-cell patch clamp recordings from neurons in the zebrafish optic tectum, we discovered an inhibitory synaptic signal, temporally locked to spontaneous and visually driven locomotion. This motor-related inhibition was appropriately timed to counteract visually driven excitatory input arising from the fish's own motion, and transiently suppressed tectal spiking activity. High-resolution calcium imaging revealed localized motor-related signals in the tectal neuropil and the upstream torus longitudinalis, suggesting that CD enters the tectum via this pathway. Together, our results show how visual processing is suppressed during self-motion by motor-related phasic inhibition. This may help explain perceptual saccadic suppression observed in many species.

Zebrafish Models of Rare Neurological Diseases like Spinocerebellar Ataxias (SCAs): Advantages and Limitations

Spinocerebellar ataxia (SCA) is a heterogeneous group of rare familial neurodegenerative disorders that share the key feature of cerebellar ataxia. Clinical heterogeneity, diverse gene mutations and complex neuropathology pose significant challenges for developing effective disease-modifying therapies in SCAs. Without a deep understanding of the molecular mechanisms involved for each SCA, we cannot succeed in developing targeted therapies. Animal models are our best tool to address these issues and several have been generated to study the pathological conditions of SCAs. Among them, zebrafish (Danio rerio) models are emerging as a powerful tool for in vivo study of SCAs, as well as rapid drug screens. In this review, we will summarize recent progress in using zebrafish to study the pathology of SCAs. We will discuss recent advancements on how zebrafish models can further clarify underlying genetic, neuroanatomical, and behavioral pathogenic mechanisms of disease. We highlight their usefulness in rapid drug discovery and large screens. Finally, we will discuss the advantages and limitations of this in vivo model to develop tailored therapeutic strategies for SCA.

Development, circuitry, and function of the zebrafish cerebellum

The cerebellum represents a brain compartment that first appeared in gnathostomes (jawed vertebrates). Besides the addition of cell numbers, its development, cytoarchitecture, circuitry, physiology, and function have been highly conserved throughout avian and mammalian species. While cerebellar research in avian and mammals is extensive, systematic investigations on this brain compartment in zebrafish as a teleostian model organism started only about two decades ago, but has provided considerable insight into cerebellar development, physiology, and function since then. Zebrafish are genetically tractable with nearly transparent small-sized embryos, in which cerebellar development occurs within a few days. Therefore, genetic investigations accompanied with non-invasive high-resolution in vivo time-lapse imaging represents a powerful combination for interrogating the behavior and function of cerebellar cells in their complex native environment.

Zebrafish as a Potential Model for Neurodegenerative Diseases: A Focus on Toxic Metals Implications

In the last century, industrial activities increased and caused multiple health problems for humans and animals. At this moment, heavy metals are considered the most harmful substances for their effects on organisms and humans. The impact of these toxic metals, which have no biological role, poses a considerable threat and is associated with several health problems. Heavy metals can interfere with metabolic processes and can sometimes act as pseudo-elements. The zebrafish is an animal model progressively used to expose the toxic effects of diverse compounds and to find treatments for different devastating diseases that human beings are currently facing. This review aims to analyse and discuss the value of zebrafish as animal models used in neurological conditions, such as Alzheimer's disease (AD), and Parkinson's disease (PD), particularly in terms of the benefits of animal models and the limitations that exist.

From perception to behavior: The neural circuits underlying prey hunting in larval zebrafish

A key challenge for neural systems is to extract relevant information from the environment and make appropriate behavioral responses. The larval zebrafish offers an exciting opportunity for studying these sensing processes and sensory-motor transformations. Prey hunting is an instinctual behavior of zebrafish that requires the brain to extract and combine different attributes of the sensory input and form appropriate motor outputs. Due to its small size and transparency the larval zebrafish brain allows optical recording of whole-brain activity to reveal the neural mechanisms involved in prey hunting and capture. In this review we discuss how the larval zebrafish brain processes visual information to identify and locate prey, the neural circuits governing the generation of motor commands in response to prey, how hunting behavior can be modulated by internal states and experience, and some outstanding questions for the field.

Diversity of cellular physiology and morphology of Purkinje cells in the adult zebrafish cerebellum

This study was designed to explore the functional circuitry of the adult zebrafish cerebellum, focusing on its Purkinje cells and using whole-cell patch recordings and single cell labeling in slice preparations. Following physiological characterizations, the recorded single cells were labeled for morphological identification. It was found that the zebrafish Purkinje cells are surprisingly diverse. Based on their physiology and morphology, they can be classified into at least three subtypes: Type I, a narrow spike cell, which fires only narrow Na⁺ spikes (<3 ms in duration), and has a single primary dendrite with an arbor restricted to the distal molecular layer; Type II, a broad spike cell, which fires broad Ca²⁺ spikes (5-7 ms in duration) and has a primary dendrite with limited branching in the inner molecular layer and then further radiates throughout the molecular layer; and Type III, a very broad spike cell, which fires very broad Ca²⁺ spikes (≥10 ms in duration) and has a dense proximal dendritic arbor that is either restricted to the inner molecular layer (Type IIIa), or radiates throughout the entire molecular layer (Type IIIb). The graded paired-pulse facilitation of these Purkinje cells' responses to parallel fiber activations and the all-or-none, paired-pulse depression of climbing fiber activation are largely similar to those reported for mammals. The labeled axon terminals of these Purkinje cells end locally, as reported for larval zebrafish. The present study provides evidence that the corresponding functional circuitry and information processing differ from what has been well-established in the mammalian cerebellum.

Notch-mediated re-specification of neuronal identity during central nervous system development

Neuronal identity has long been thought of as immutable, so that once a cell acquires a specific fate, it is maintained for life.¹ Studies using the overexpression of potent transcription factors to experimentally reprogram neuronal fate in the mouse neocortex^2,3 and retina^4,5 have challenged this notion by revealing that post-mitotic neurons can switch their identity. Whether fate reprogramming is part of normal development in the central nervous system (CNS) is unclear. While there are some reports of physiological cell fate reprogramming in invertebrates,^6,7 and in the vertebrate peripheral nervous system,⁸ endogenous fate reprogramming in the vertebrate CNS has not been documented. Here, we demonstrate spontaneous fate re-specification in an interneuron lineage in the zebrafish retina. We show that the visual system homeobox 1 (vsx1)-expressing lineage, which has been associated exclusively with excitatory bipolar cell (BC) interneurons,^9-12 also generates inhibitory amacrine cells (ACs). We identify a role for Notch signaling in conferring plasticity to nascent vsx1 BCs, allowing suitable transcription factor programs to re-specify them to an AC fate. Overstimulating Notch signaling enhances this physiological phenotype so that both daughters of a vsx1 progenitor differentiate into ACs and partially differentiated vsx1 BCs can be converted into ACs. Furthermore, this physiological re-specification can be mimicked to allow experimental induction of an entirely distinct fate, that of retinal projection neurons, from the vsx1 lineage. Our observations reveal unanticipated plasticity of cell fate during retinal development.

From whole organism to ultrastructure: progress in axonal imaging for decoding circuit development

Since the pioneering work of Ramón y Cajal, scientists have sought to unravel the complexities of axon development underlying neural circuit formation. Micrometer-scale axonal growth cones navigate to targets that are often centimeters away. To reach their targets, growth cones react to dynamic environmental cues that change in the order of seconds to days. Proper axon growth and guidance are essential to circuit formation, and progress in imaging has been integral to studying these processes. In particular, advances in high- and super-resolution microscopy provide the spatial and temporal resolution required for studying developing axons. In this Review, we describe how improved microscopy has revolutionized our understanding of axonal development. We discuss how novel technologies, specifically light-sheet and super-resolution microscopy, led to new discoveries at the cellular scale by imaging axon outgrowth and circuit wiring with extreme precision. We next examine how advanced microscopy broadened our understanding of the subcellular dynamics driving axon growth and guidance. We finally assess the current challenges that the field of axonal biology still faces for imaging axons, and examine how future technology could meet these needs.

Functional brain imaging in larval zebrafish for characterising the effects of seizurogenic compounds acting via a range of pharmacological mechanisms

BACKGROUND AND PURPOSE

Functional brain imaging using genetically encoded Ca2+ sensors in larval zebrafish is being developed for studying seizures and epilepsy as a more ethical alternative to rodent models. Despite this, few data have been generated on pharmacological mechanisms of action other than GABAA antagonism. Assessing larval responsiveness across multiple mechanisms is vital to test the translational power of this approach, as well as assessing its validity for detecting unwanted drug-induced seizures and testing antiepileptic drug efficacy.

EXPERIMENTAL APPROACH

Using light-sheet imaging, we systematically analysed the responsiveness of 4 days post fertilisation (dpf; which are not considered protected under European animal experiment legislation) transgenic larval zebrafish to treatment with 57 compounds spanning more than 12 drug classes with a link to seizure generation in mammals, alongside eight compounds with no such link.

KEY RESULTS

We show 4dpf zebrafish are responsive to a wide range of mechanisms implicated in seizure generation, with cerebellar circuitry activated regardless of the initiating pharmacology. Analysis of functional connectivity revealed compounds targeting cholinergic and monoaminergic reuptake, in particular, showed phenotypic consistency broadly mapping onto what is known about neurotransmitter-specific circuitry in the larval zebrafish brain. Many seizure-associated compounds also exhibited altered whole brain functional connectivity compared with controls.

CONCLUSIONS AND IMPLICATIONS

This work represents a significant step forward in understanding the translational power of 4dpf larval zebrafish for use in neuropharmacological studies and for studying the events driving transition from small-scale pharmacological activation of local circuits, to the large network-wide abnormal synchronous activity associated with seizures.

Natural and anthropogenic sources of habitat variation influence exploration behaviour, stress response, and brain morphology in a coastal fish

Evolutionary ecology aims to better understand how ecologically important traits respond to environmental heterogeneity. Environments vary both naturally and as a result of human activities, and investigations that simultaneously consider how natural and human-induced environmental variation affect diverse trait types grow increasingly important as human activities drive species endangerment. Here, we examined how habitat fragmentation and structural habitat complexity affect disparate trait types in Bahamas mosquitofish Gambusia hubbsi inhabiting tidal creeks. We tested a priori predictions for how these factors might influence exploratory behaviour, stress reactivity and brain anatomy. We examined approximately 350 adult Bahamas mosquitofish from seven tidal-creek populations across Andros Island, The Bahamas that varied in both human-caused fragmentation (three fragmented and four unfragmented) and natural habitat complexity (e.g. fivefold variation in rock habitat). Populations that had experienced severe human-induced fragmentation, and thus restriction of tidal exchange from the ocean, exhibited greater exploration of a novel environment, stronger physiological stress responses to a mildly stressful event and smaller telencephala (relative to body size). These changes matched adaptive predictions based mostly on (a) reduced chronic predation risk and (b) decreased demands for navigating tidally dynamic habitats. Populations from sites with greater structural habitat complexity showed a higher propensity for exploration and a relatively larger optic tectum and cerebellum. These patterns matched adaptive predictions related to increased demands for navigating complex environments. Our findings demonstrate environmental variation, including recent anthropogenic impacts (<50 years), can significantly affect complex, ecologically important traits. Yet trait-specific patterns may not be easily predicted, as we found strong support for only six of 12 predictions. Our results further highlight the utility of simultaneously quantifying multiple environmental factors-for example had we failed to account for habitat complexity, we would not have detected the effects of fragmentation on exploratory behaviours. These responses, and their ecological consequences, may be complex: rapid and adaptive phenotypic responses to anthropogenic impacts can facilitate persistence in human-altered environments, but may come at a cost of population vulnerability if ecological restoration was to occur without consideration of the altered traits.

The tectum/superior colliculus as the vertebrate solution for spatial sensory integration and action

The superior colliculus, or tectum in the case of non-mammalian vertebrates, is a part of the brain that registers events in the surrounding space, often through vision and hearing, but also through electrosensation, infrared detection, and other sensory modalities in diverse vertebrate lineages. This information is used to form maps of the surrounding space and the positions of different salient stimuli in relation to the individual. The sensory maps are arranged in layers with visual input in the uppermost layer, other senses in deeper positions, and a spatially aligned motor map in the deepest layer. Here, we will review the organization and intrinsic function of the tectum/superior colliculus and the information that is processed within tectal circuits. We will also discuss tectal/superior colliculus outputs that are conveyed directly to downstream motor circuits or via the thalamus to cortical areas to control various aspects of behavior. The tectum/superior colliculus is evolutionarily conserved among all vertebrates, but tailored to the sensory specialties of each lineage, and its roles have shifted with the emergence of the cerebral cortex in mammals. We will illustrate both the conserved and divergent properties of the tectum/superior colliculus through vertebrate evolution by comparing tectal processing in lampreys belonging to the oldest group of extant vertebrates, larval zebrafish, rodents, and other vertebrates including primates.

Reconstruction scheme for excitatory and inhibitory dynamics with quenched disorder: application to zebrafish imaging

An inverse procedure is developed and tested to recover functional and structural information from global signals of brains activity. The method assumes a leaky-integrate and fire model with excitatory and inhibitory neurons, coupled via a directed network. Neurons are endowed with a heterogenous current value, which sets their associated dynamical regime. By making use of a heterogenous mean-field approximation, the method seeks to reconstructing from global activity patterns the distribution of in-coming degrees, for both excitatory and inhibitory neurons, as well as the distribution of the assigned currents. The proposed inverse scheme is first validated against synthetic data. Then, time-lapse acquisitions of a zebrafish larva recorded with a two-photon light sheet microscope are used as an input to the reconstruction algorithm. A power law distribution of the in-coming connectivity of the excitatory neurons is found. Local degree distributions are also computed by segmenting the whole brain in sub-regions traced from annotated atlas.

Pyramidal Neurons of the Zebrafish Tectum Receive Highly Convergent Input From Torus Longitudinalis

The torus longitudinalis (TL) is a midbrain structure unique to ray finned fish. Although previously implicated in orienting behaviors elicited by changes in ambient lighting, the role of TL in visual processing is not well-understood. TL is reciprocally connected to tectum and is the only known source of synaptic input to the stratum marginalis (SM) layer of tectal neuropil. Conversely, tectal pyramidal neurons (PyrNs) are the only identified tectal neuron population that forms a dendrite in SM. In this study we describe a zebrafish gal4 transgenic that labels TL neurons that project to SM. We demonstrate that the axonal TL projection to SM in zebrafish is glutamatergic. Consistent with these axons synapsing directly onto PyrNs, SM-targeted dendrites of PyrNs contain punctate enrichments of the glutamatergic post-synaptic marker protein PSD95. Sparse genetic labeling of individual TL axons and PyrN dendrites enabled quantitative morphometric analysis that revealed (1) large, sparsely branched TL axons in SM and (2) small, densely innervated PyrN dendrites in SM. Together this unique combination of morphologies support a wiring diagram in which TL inputs to PyrNs exhibit a high degree of convergence. We propose that this convergence functions to generate large, compound visual receptive fields in PyrNs. This quantitative anatomical data will instruct future functional studies aimed at identifying the precise contribution of TL-PyrN circuitry to visual behavior.

Using zebrafish (Danio rerio) models to understand the critical role of social interactions in mental health and wellbeing

Social behavior represents a beneficial interaction between conspecifics that is critical for maintaining health and wellbeing. Dysfunctional or poor social interaction are associated with increased risk of physical (e.g., vascular) and psychiatric disorders (e.g., anxiety, depression, and substance abuse). Although the impact of negative and positive social interactions is well-studied, their underlying mechanisms remain poorly understood. Zebrafish have well-characterized social behavior phenotypes, high genetic homology with humans, relative experimental simplicity and the potential for high-throughput screens. Here, we discuss the use of zebrafish as a candidate model organism for studying the fundamental mechanisms underlying social interactions, as well as potential impacts of social isolation on human health and wellbeing. Overall, the growing utility of zebrafish models may improve our understanding of how the presence and absence of social interactions can differentially modulate various molecular and physiological biomarkers, as well as a wide range of other behaviors.

Functional Regeneration of the Sensory Root via Axonal Invasion

Regeneration following spinal root avulsion is broadly unsuccessful despite the regenerative capacity of other PNS-located nerves. By combining focal laser lesioning to model root avulsion in zebrafish, time-lapse imaging, and transgenesis, we identify that regenerating DRG neurons fail to recapitulate developmental paradigms of actin-based invasion after injury. We demonstrate that inducing actin reorganization into invasive components via pharmacological and genetic approaches in the regenerating axon can rescue sensory axon spinal cord entry. Cell-autonomous induction of invasion components using constitutively active Src induces DRG axon regeneration, suggesting an intrinsic mechanism can be activated to drive regeneration. Furthermore, analyses of neuronal activity and animal behavior show restoration of sensory circuit activity and behavior upon stimulating axons to re-enter the spinal cord via invasion. Altogether, our data identify induction of invasive components as sufficient for functional sensory root regeneration after injury.

Dorsal root ganglion (DRG) sensory axons are unable to regenerate into the spinal cord after injury. Nichols and Smith demonstrate in zebrafish that injured DRG axons do not initiate actin-based invasion components during re-entry into the spinal cord. Pharmacological and cell-autonomous genetic manipulations that promote actin-mediated cell invasion to restore sensory behavior.

Retinotectal circuitry of larval zebrafish is adapted to detection and pursuit of prey

Retinal axon projections form a map of the visual environment in the tectum. A zebrafish larva typically detects a prey object in its peripheral visual field. As it turns and swims towards the prey, the stimulus enters the central, binocular area, and seemingly expands in size. By volumetric calcium imaging, we show that posterior tectal neurons, which serve to detect prey at a distance, tend to respond to small objects and intrinsically compute their direction of movement. Neurons in anterior tectum, where the prey image is represented shortly before the capture strike, are tuned to larger object sizes and are frequently not direction-selective, indicating that mainly interocular comparisons serve to compute an object's movement at close range. The tectal feature map originates from a linear combination of diverse, functionally specialized, lamina-specific, and topographically ordered retinal ganglion cell synaptic inputs. We conclude that local cell-type composition and connectivity across the tectum are adapted to the processing of location-dependent, behaviorally relevant object features.

Experience, circuit dynamics, and forebrain recruitment in larval zebrafish prey capture

Experience influences behavior, but little is known about how experience is encoded in the brain, and how changes in neural activity are implemented at a network level to improve performance. Here we investigate how differences in experience impact brain circuitry and behavior in larval zebrafish prey capture. We find that experience of live prey compared to inert food increases capture success by boosting capture initiation. In response to live prey, animals with and without prior experience of live prey show activity in visual areas (pretectum and optic tectum) and motor areas (cerebellum and hindbrain), with similar visual area retinotopic maps of prey position. However, prey-experienced animals more readily initiate capture in response to visual area activity and have greater visually-evoked activity in two forebrain areas: the telencephalon and habenula. Consequently, disruption of habenular neurons reduces capture performance in prey-experienced fish. Together, our results suggest that experience of prey strengthens prey-associated visual drive to the forebrain, and that this lowers the threshold for prey-associated visual activity to trigger activity in motor areas, thereby improving capture performance.

Anatomy and Connectivity of the Torus Longitudinalis of the Adult Zebrafish

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

Integration of Swimming-Related Synaptic Excitation and Inhibition by olig2+ Eurydendroid Neurons in Larval Zebrafish Cerebellum

The cerebellum influences motor control through Purkinje target neurons, which transmit cerebellar output. Such output is required, for instance, for larval zebrafish to learn conditioned fictive swimming. The output cells, called eurydendroid neurons (ENs) in teleost fish, are inhibited by Purkinje cells and excited by parallel fibers. Here, we investigated the electrophysiological properties of glutamatergic ENs labeled by the transcription factor olig2. Action potential firing and synaptic responses were recorded in current clamp and voltage clamp from olig2⁺ neurons in immobilized larval zebrafish (before sexual differentiation) and were correlated with motor behavior by simultaneous recording of fictive swimming. In the absence of swimming, olig2⁺ ENs had basal firing rates near 8 spikes/s, and EPSCs and IPSCs were evident. Comparing Purkinje firing rates and eurydendroid IPSC rates indicated that 1-3 Purkinje cells converge onto each EN. Optogenetically suppressing Purkinje simple spikes, while preserving complex spikes, suggested that eurydendroid IPSC size depended on presynaptic spike duration rather than amplitude. During swimming, EPSC and IPSC rates increased. Total excitatory and inhibitory currents during sensory-evoked swimming were both more than double those during spontaneous swimming. During both spontaneous and sensory-evoked swimming, the total inhibitory current was more than threefold larger than the excitatory current. Firing rates of ENs nevertheless increased, suggesting that the relative timing of IPSCs and EPSCs may permit excitation to drive additional eurydendroid spikes. The data indicate that olig2⁺ cells are ENs whose activity is modulated with locomotion, suiting them to participate in sensorimotor integration associated with cerebellum-dependent learning.SIGNIFICANCE STATEMENT The cerebellum contributes to movements through signals generated by cerebellar output neurons, called eurydendroid neurons (ENs) in fish (cerebellar nuclei in mammals). ENs receive sensory and motor signals from excitatory parallel fibers and inhibitory Purkinje cells. Here, we report electrophysiological recordings from ENs of larval zebrafish that directly illustrate how synaptic inhibition and excitation are integrated by cerebellar output neurons in association with motor behavior. The results demonstrate that inhibitory and excitatory drive both increase during fictive swimming, but inhibition greatly exceeds excitation. Firing rates nevertheless increase, providing evidence that synaptic integration promotes cerebellar output during locomotion. The data offer a basis for comparing aspects of cerebellar coding that are conserved and that diverge across vertebrates.

Neuron types in the zebrafish optic tectum labeled by an id2b transgene

The larval zebrafish optic tectum has emerged as a prominent model for understanding how neural circuits control visually guided behaviors. Further advances in this area will require tools to monitor and manipulate tectal neurons with cell type specificity. Here, we characterize the morphology and neurotransmitter phenotype of tectal neurons labeled by an id2b:gal4 transgene. Whole-brain imaging of stable transgenic id2b:gal4 larvae revealed labeling in a subset of neurons in optic tectum, cerebellum, and hindbrain. Genetic mosaic labeling of single neurons within the id2b:gal4 expression pattern enabled us to characterize three tectal neuron types with distinct morphologies and connectivities. The first is a neuron type previously identified in the optic tectum of other teleost fish: the tectal pyramidal neuron (PyrN). PyrNs are local interneurons that form two stratified dendritic arbors and one stratified axonal arbor in the tectal neuropil. The second tectal neuron type labeled by the id2b:gal4 transgene is a projection neuron that forms a stratified dendritic arbor in the tectal neuropil and an axon that exits tectum to form a topographic projection to torus longitudinalis (TL). A third neuron type labeled is a projection neuron with a nonstratified dendritic arbor and a descending axonal projection to tegmentum. These findings establish the id2b:gal4 transgenic as a useful tool for future studies aimed at elucidating the functional role of tectum, TL, and tegmentum in visually guided behaviors.

The Zebrafish Visual System: From Circuits to Behavior

Visual stimuli can evoke complex behavioral responses, but the underlying streams of neural activity in mammalian brains are difficult to follow because of their size. Here, I review the visual system of zebrafish larvae, highlighting where recent experimental evidence has localized the functional steps of visuomotor transformations to specific brain areas. The retina of a larva encodes behaviorally relevant visual information in neural activity distributed across feature-selective ganglion cells such that signals representing distinct stimulus properties arrive in different areas or layers of the brain. Motor centers in the hindbrain encode motor variables that are precisely tuned to behavioral needs within a given stimulus setting. Owing to rapid technological progress, larval zebrafish provide unique opportunities for obtaining a comprehensive understanding of the intermediate processing steps occurring between visual and motor centers, revealing how visuomotor transformations are implemented in a vertebrate brain.

Oligodendrocytes express synaptic proteins that modulate myelin sheath formation

Vesicular release from neurons promotes myelin sheath growth on axons. Oligodendrocytes express proteins that allow dendrites to respond to vesicular release at synapses, suggesting that axon-myelin contacts use similar communication mechanisms as synapses to form myelin sheaths. To test this, we used fusion proteins to track synaptic vesicle localization and membrane fusion in zebrafish during developmental myelination and investigated expression and localization of PSD95, a dendritic post-synaptic protein, within oligodendrocytes. Synaptic vesicles accumulate and exocytose at ensheathment sites with variable patterning and most sheaths localize PSD95 with patterning similar to exocytosis site location. Disruption of candidate PDZ-binding transsynaptic adhesion proteins in oligodendrocytes cause variable effects on sheath length and number. One candidate, Cadm1b, localizes to myelin sheaths where both PDZ binding and extracellular adhesion to axons mediate sheath growth. Our work raises the possibility that axon-glial communication contributes to myelin plasticity, providing new targets for mechanistic unraveling of developmental myelination.

Oligodendrocyte processes can detect and respond to axonal vesicular release. The authors here show in zebrafish that transsynaptic adhesion molecules, molecules that promote synapse formation and maturation in neurons, are expressed by oligodendrocytes and required for myelin sheath growth.

The zebrafish subcortical social brain as a model for studying social behavior disorders

Social behaviors are essential for the survival and reproduction of social species. Many, if not most, neuropsychiatric disorders in humans are either associated with underlying social deficits or are accompanied by social dysfunctions. Traditionally, rodent models have been used to model these behavioral impairments. However, rodent assays are often difficult to scale up and adapt to high-throughput formats, which severely limits their use for systems-level science. In recent years, an increasing number of studies have used zebrafish (Danio rerio) as a model system to study social behavior. These studies have demonstrated clear potential in overcoming some of the limitations of rodent models. In this Review, we explore the evolutionary conservation of a subcortical social brain between teleosts and mammals as the biological basis for using zebrafish to model human social behavior disorders, while summarizing relevant experimental tools and assays. We then discuss the recent advances gleaned from zebrafish social behavior assays, the applications of these assays to studying related disorders, and the opportunities and challenges that lie ahead.

Neuronal Architecture of a Visual Center that Processes Optic Flow

Animals use global image motion cues to actively stabilize their position by compensatory movements. Neurons in the zebrafish pretectum distinguish different optic flow patterns, e.g., rotation and translation, to drive appropriate behaviors. Combining functional imaging and morphological reconstruction of single cells, we revealed critical neuroanatomical features of this sensorimotor transformation. Terminals of direction-selective retinal ganglion cells (DS-RGCs) are located within the pretectal retinal arborization field 5 (AF5), where they meet dendrites of pretectal neurons with simple tuning to monocular optic flow. Translation-selective neurons, which respond selectively to optic flow in the same direction for both eyes, are intermingled with these simple cells but do not receive inputs from DS-RGCs. Mutually exclusive populations of pretectal projection neurons innervate either the reticular formation or the cerebellum, which in turn control motor responses. We posit that local computations in a defined pretectal circuit transform optic flow signals into neural commands driving optomotor behavior. VIDEO ABSTRACT.

STIM1 Is Required for Remodeling of the Endoplasmic Reticulum and Microtubule Cytoskeleton in Steering Growth Cones

The spatial and temporal regulation of calcium signaling in neuronal growth cones is essential for axon guidance. In growth cones, the endoplasmic reticulum (ER) is a significant source of calcium signals. However, it is not clear whether the ER is remodeled during motile events to localize calcium signals in steering growth cones. The expression of the ER-calcium sensor, stromal interacting molecule 1 (STIM1) is necessary for growth cone steering toward the calcium-dependent guidance cue BDNF, with STIM1 functioning to sustain calcium signals through store-operated calcium entry. However, STIM1 is also required for growth cone steering away from semaphorin-3a, a guidance cue that does not activate ER-calcium release, suggesting multiple functions of STIM1 within growth cones (Mitchell et al., 2012). STIM1 also interacts with microtubule plus-end binding proteins EB1/EB3 (Grigoriev et al., 2008). Here, we show that STIM1 associates with EB1/EB3 in growth cones and that STIM1 expression is critical for microtubule recruitment and subsequent ER remodeling to the motile side of steering growth cones. Furthermore, we extend our data in vivo, demonstrating that zSTIM1 is required for axon guidance in actively navigating zebrafish motor neurons, regulating calcium signaling and filopodial formation. These data demonstrate that, in response to multiple guidance cues, STIM1 couples microtubule organization and ER-derived calcium signals, thereby providing a mechanism where STIM1-mediated ER remodeling, particularly in filopodia, regulates spatiotemporal calcium signals during axon guidance.SIGNIFICANCE STATEMENT Defects in both axon guidance and endoplasmic reticulum (ER) function are implicated in a range of developmental disorders. During neuronal circuit development, the spatial localization of calcium signals controls the growth cone cytoskeleton to direct motility. We demonstrate a novel role for stromal interacting molecule 1 (STIM1) in regulating microtubule and subsequent ER remodeling in navigating growth cones. We show that STIM1, an activator of store-operated calcium entry, regulates the dynamics of microtubule-binding proteins EB1/EB3, coupling ER to microtubules, within filopodia, thereby steering growth cones. The STIM1-microtubule-ER interaction provides a new model for spatial localization of calcium signals in navigating growth cones in the nascent nervous system.

Generating intravital super-resolution movies with conventional microscopy reveals actin dynamics that construct pioneer axons

Super-resolution microscopy is broadening our in-depth understanding of cellular structure. However, super-resolution approaches are limited, for numerous reasons, from utilization in longer-term intravital imaging. We devised a combinatorial imaging technique that combines deconvolution with stepwise optical saturation microscopy (DeSOS) to circumvent this issue and image cells in their native physiological environment. Other than a traditional confocal or two-photon microscope, this approach requires no additional hardware. Here, we provide an open-access application to obtain DeSOS images from conventional microscope images obtained at low excitation powers. We show that DeSOS can be used in time-lapse imaging to generate super-resolution movies in zebrafish. DeSOS was also validated in live mice. These movies uncover that actin structures dynamically remodel to produce a single pioneer axon in a ‘top-down’ scaffolding event. Further, we identify an F-actin population – stable base clusters – that orchestrate that scaffolding event. We then identify that activation of Rac1 in pioneer axons destabilizes stable base clusters and disrupts pioneer axon formation. The ease of acquisition and processing with this approach provides a universal technique for biologists to answer questions in living animals.

Summary: Actin dynamics are examined in zebrafish axons using DeSOS, a new super-resolution technique combining deconvolution with stepwise optical saturation microscopy that allows detailed intravital imaging of cells in their native environments.

Chronology-based architecture of descending circuits that underlie the development of locomotor repertoire after birth

The emergence of new and increasingly sophisticated behaviors after birth is accompanied by dramatic increase of newly established synaptic connections in the nervous system. Little is known, however, of how nascent connections are organized to support such new behaviors alongside existing ones. To understand this, in the larval zebrafish we examined the development of spinal pathways from hindbrain V2a neurons and the role of these pathways in the development of locomotion. We found that new projections are continually layered laterally to existing neuropil, and give rise to distinct pathways that function in parallel to existing pathways. Across these chronologically layered pathways, the connectivity patterns and biophysical properties vary systematically to support a behavioral repertoire with a wide range of kinematics and dynamics. Such layering of new parallel circuits equipped with systematically changing properties may be central to the postnatal diversification and increasing sophistication of an animal’s behavioral repertoire.

Newborn babies have limited abilities. Indeed, most of our actions shortly after birth are the result of reflexes that serve our most basic need: to stay alive. As we get older, however, our behaviour gradually becomes more sophisticated. During this time, the billions of cells in our brain form new connections to build intricate ‘circuits’ of neurons that allow for more complicated thoughts and actions.

It is clear that the brain circuits that support new behaviours must develop in a way that does not interfere with the existing circuits that are vital for survival. However, the challenge has been to find a way to peer into a brain as it develops to see how these new circuits form.

In recent years, zebrafish have revolutionised research into neuronal circuits in animals. Developing over the course of a few days, these small transparent fish provide a window into the brain during the earliest stages of development. Indeed, the circuits of neurons that descend from the brain and connect to the spinal cord have already been mapped in these animals. Now, Pujala and Koyama have begun to follow the careful development of these ‘descending’ neurons, and relate it to the appearance of new behaviours in young zebrafish.

Time-lapse imaging with a fluorescent protein that is active only in specific descending neurons revealed that new circuits are laid down over existing ones, like the growth rings in a tree. Next, at different timepoints in zebrafish development, Pujala and Koyama traced these neurons backwards from the spine to the brain to identify which connections formed first. This showed that the spinal connections develop one after the other, in the same order that the neurons mature.

Next, Pujala and Koyama asked how the activity of neurons that mature early or late in development relates to specific behaviours in young zebrafish. Early-born circuits connect to neurons that produce powerful, reflex-driven, whole-body movements such as an escape response. The later circuits connect to different neurons through slower, less direct pathways; the late-born neurons also generate the refined movements that are acquired later in a zebrafish’s development and help the fish to explore its environment.

These findings show that descending circuits in zebrafish run parallel to each other, but with distinct connections and properties that allow them to control different kinds of movements. While this study was conducted using an animal model, a better understanding of how such circuits develop and the movements they control may one day aid the treatment of patients with neurodegenerative diseases or injuries where connections have been lost.

Combining near-infrared fluorescence with Brainbow to visualize expression of specific genes within a multicolor context

Fluorescent proteins are a powerful experimental tool, allowing the visualization of gene expression and cellular behaviors in a variety of systems. Multicolor combinations of fluorescent proteins, such as Brainbow, have expanded the range of possible research questions and are useful for distinguishing and tracking cells. The addition of a separately driven color, however, would allow researchers to report expression of a manipulated gene within the multicolor context to investigate mechanistic effects. A far-red or near-infrared protein could be particularly suitable in this context, as these can be distinguished spectrally from Brainbow. We investigated five far-red/near-infrared proteins in zebrafish: TagRFP657, mCardinal, miRFP670, iRFP670, and mIFP. Our results show that both mCardinal and iRFP670 are useful fluorescent proteins for zebrafish expression. We also introduce a new transgenic zebrafish line that expresses Brainbow under the control of the neuroD promoter. We demonstrate that mCardinal can be used to track the expression of a manipulated bone morphogenetic protein receptor within the Brainbow context. The overlay of near-infrared fluorescence onto a Brainbow background defines a clear strategy for future research questions that aim to manipulate or track the effects of specific genes within a population of cells that are delineated using multicolor approaches.

Pioneer axons employ Cajal's battering ram to enter the spinal cord

Sensory axons must traverse a spinal cord glia limitans to connect the brain with the periphery. The fundamental mechanism of how these axons enter the spinal cord is still debatable; both Ramon y Cajal’s battering ram hypothesis and a boundary cap model have been proposed. To distinguish between these hypotheses, we visualized the entry of pioneer axons into the dorsal root entry zone (DREZ) with time-lapse imaging in zebrafish. Here, we identify that DRG pioneer axons enter the DREZ before the arrival of neural crest cells at the DREZ. Instead, actin-rich invadopodia in the pioneer axon are necessary and sufficient for DREZ entry. Using photoactivable Rac1, we demonstrate cell-autonomous functioning of invasive structures in pioneer axon spinal entry. Together these data support the model that actin-rich invasion structures dynamically drive pioneer axon entry into the spinal cord, indicating that distinct pioneer and secondary events occur at the DREZ.

The fundamental mechanism of how sensory axons traverse a spinal cord glia limitans remains debatable, with some suggesting a role for boundary cap cells at the dorsal root entry zone (DREZ). Here, authors use time-lapse imaging of DRG axons at the DREZ to show that pioneer axons enter the DREZ before the presence of boundary cap cells, and that this entry is critically dependent on the development of actin-rich invasion structures reminiscent of invadopodia.

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.

Whole-Brain Calcium Imaging during Physiological Vestibular Stimulation in Larval Zebrafish

The vestibular apparatus provides animals with postural and movement-related information that is essential to adequately execute numerous sensorimotor tasks. In order to activate this sensory system in a physiological manner, one needs to macroscopically rotate or translate the animal's head, which in turn renders simultaneous neural recordings highly challenging. Here we report on a novel miniaturized, light-sheet microscope that can be dynamically co-rotated with a head-restrained zebrafish larva, enabling controlled vestibular stimulation. The mechanical rigidity of the microscope allows one to perform whole-brain functional imaging with state-of-the-art resolution and signal-to-noise ratio while imposing up to 25° in angular position and 6,000°/s² in rotational acceleration. We illustrate the potential of this novel setup by producing the first whole-brain response maps to sinusoidal and stepwise vestibular stimulation. The responsive population spans multiple brain areas and displays bilateral symmetry, and its organization is highly stereotypic across individuals. Using Fourier and regression analysis, we identified three major functional clusters that exhibit well-defined phasic and tonic response patterns to vestibular stimulation. Our rotatable light-sheet microscope provides a unique tool for systematically studying vestibular processing in the vertebrate brain and extends the potential of virtual-reality systems to explore complex multisensory and motor integration during simulated 3D navigation.

Luminance Changes Drive Directional Startle through a Thalamic Pathway

Looming visual stimuli result in escape responses that are conserved from insects to humans. Despite their importance for survival, the circuits mediating visual startle have only recently been explored in vertebrates. Here we show that the zebrafish thalamus is a luminance detector critical to visual escape. Thalamic projection neurons deliver dim-specific information to the optic tectum, and ablations of these projections disrupt normal tectal responses to looms. Without this information, larvae are less likely to escape from dark looming stimuli and lose the ability to escape away from the source of the loom. Remarkably, when paired with an isoluminant loom stimulus to the opposite eye, dimming is sufficient to increase startle probability and to reverse the direction of the escape so that it is toward the loom. We suggest that bilateral comparisons of luminance, relayed from the thalamus to the tectum, facilitate escape responses and are essential for their directionality.

Hypothalamic Projections to the Optic Tectum in Larval Zebrafish

The optic tectum of larval zebrafish is an important model for understanding visual processing in vertebrates. The tectum has been traditionally viewed as dominantly visual, with a majority of studies focusing on the processes by which tectal circuits receive and process retinally-derived visual information. Recently, a handful of studies have shown a much more complex role for the optic tectum in larval zebrafish, and anatomical and functional data from these studies suggest that this role extends beyond the visual system, and beyond the processing of exclusively retinal inputs. Consistent with this evolving view of the tectum, we have used a Gal4 enhancer trap line to identify direct projections from rostral hypothalamus (RH) to the tectal neuropil of larval zebrafish. These projections ramify within the deepest laminae of the tectal neuropil, the stratum album centrale (SAC)/stratum griseum periventriculare (SPV), and also innervate strata distinct from those innervated by retinal projections. Using optogenetic stimulation of the hypothalamic projection neurons paired with calcium imaging in the tectum, we find rebound firing in tectal neurons consistent with hypothalamic inhibitory input. Our results suggest that tectal processing in larval zebrafish is modulated by hypothalamic inhibitory inputs to the deep tectal neuropil.

CNS-resident progenitors direct the vascularization of neighboring tissues

Organ growth requires the coordinated invasion and expansion of blood vessel networks directed by tissue-resident cells and morphogenetic cues. A striking example of this intercellular communication is the vascularization of the central nervous system (CNS), which is driven by neuronal progenitors, including neuroepithelial cells and radial glia. Although the importance of neuronal progenitors in vascular development within the CNS is well recognized, how these progenitors regulate the vasculature outside the CNS remains largely unknown. Here we show that CNS-resident radial glia direct the vascularization of neighboring tissues during development. We find that genetic ablation of radial glia in zebrafish larvae leads to a complete loss of the bilateral vertebral arteries (VTAs) that extend along the ventrolateral sides of the spinal cord. Importantly, VTA formation is not affected by ablation of other CNS cell types, and radial glia ablation also compromises the subsequent formation of the peri-neural vascular plexus (PNVP), a vascular network that surrounds the CNS and is critical for CNS angiogenesis. Mechanistically, we find that radial glia control these processes via Vegfab/Vegfr2 signaling: vegfab is expressed by radial glia, and genetic or pharmacological inhibition of Vegfab/Vegfr2 signaling blocks the formation of the VTAs and subsequently of the PNVP. Moreover, mosaic overexpression of Vegfab in radial glia is sufficient to partially rescue the VTA formation defect in vegfab mutants. Thus, our findings identify a critical function for CNS-resident progenitors in the regulation of vascularization outside the CNS, serving as a paradigm for cross-tissue coordination of vascular morphogenesis and growth.

Development of the cerebellum in turbot (Psetta maxima): Analysis of cell proliferation and distribution of calcium binding proteins

The morphogenesis, cell proliferation and neuronal differentiation of the turbot (Psetta maxima) cerebellum has been studied using conventional histological techniques and immunohistochemical methods for proliferating cell nuclear antigen and calcium binding proteins. As in other vertebrates, the cerebellar anlage emerges as proliferative plates of neural tissue during the embryonic period. The anlage of the cerebellum persists without morphological changes until the end of the larval life when the mantle zone is differentiated. The major ontogenetic changesthat drive the formation of the cerebellar subdivisions begin in late premetamorphic larvae when cerebellar plates growth and merge medially. This transformation is accomplished by the reorganization of proliferative zones as well as by the onset of cell differentiation. The cerebellum becomes fully differentiated during metamorphosis when parvalbumin and calretinin were detected in Purkinje and eurydendroid cells. Sustained proliferation is maintained in all subdivisions of the cerebellum and this support the robust growth of this part of the brain that takes place during the metamorphic and juvenile periods.The location and histological organization of the proliferative activity in the turbot mature cerebellum are described and their functional significance was analyzed in light of the information available for other teleosts.

Postnatal Migration of Cerebellar Interneurons

Due to its continuing development after birth, the cerebellum represents a unique model for studying the postnatal orchestration of interneuron migration. The combination of fluorescent labeling and ex/in vivo imaging revealed a cellular highway network within cerebellar cortical layers (the external granular layer, the molecular layer, the Purkinje cell layer, and the internal granular layer). During the first two postnatal weeks, saltatory movements, transient stop phases, cell-cell interaction/contact, and degradation of the extracellular matrix mark out the route of cerebellar interneurons, notably granule cells and basket/stellate cells, to their final location. In addition, cortical-layer specific regulatory factors such as neuropeptides (pituitary adenylate cyclase-activating polypeptide (PACAP), somatostatin) or proteins (tissue-type plasminogen activator (tPA), insulin growth factor-1 (IGF-1)) have been shown to inhibit or stimulate the migratory process of interneurons. These factors show further complexity because somatostatin, PACAP, or tPA have opposite or no effect on interneuron migration depending on which layer or cell type they act upon. External factors originating from environmental conditions (light stimuli, pollutants), nutrients or drug of abuse (alcohol) also alter normal cell migration, leading to cerebellar disorders.

Distinct responses of Purkinje neurons and roles of simple spikes during associative motor learning in larval zebrafish

To study cerebellar activity during learning, we made whole-cell recordings from larval zebrafish Purkinje cells while monitoring fictive swimming during associative conditioning. Fish learned to swim in response to visual stimulation preceding tactile stimulation of the tail. Learning was abolished by cerebellar ablation. All Purkinje cells showed task-related activity. Based on how many complex spikes emerged during learned swimming, they were classified as multiple, single, or zero complex spike (MCS, SCS, ZCS) cells. With learning, MCS and ZCS cells developed increased climbing fiber (MCS) or parallel fiber (ZCS) input during visual stimulation; SCS cells fired complex spikes associated with learned swimming episodes. The categories correlated with location. Optogenetically suppressing simple spikes only during visual stimulation demonstrated that simple spikes are required for acquisition and early stages of expression of learned responses, but not their maintenance, consistent with a transient, instructive role for simple spikes during cerebellar learning in larval zebrafish.

Imaging of viral neuroinvasion in the zebrafish reveals that Sindbis and chikungunya viruses favour different entry routes

Alphaviruses, such as chikungunya virus (CHIKV) and Sindbis virus (SINV), are vector-borne pathogens that cause acute illnesses in humans and are sometimes associated with neuropathies, especially in infants and elderly patients. Little is known about their mechanism of entry into the central nervous system (CNS), even for SINV, which has been used extensively as a model for viral encephalopathies. We previously established a CHIKV infection model in the optically transparent zebrafish larva; here we describe a new SINV infection model in this host. We imaged in vivo the onset and progression of the infection caused by intravenous SINV inoculation. Similar to that described for CHIKV, infection in the periphery was detected early and was transient, whereas CNS infection started at later time points and was persistent or progressive. We then tested the possible mechanisms of neuroinvasion by CHIKV and SINV. Neither virus relied on macrophage-mediated transport to access the CNS. CHIKV, but not SINV, always infects endothelial cells of the brain vasculature. By contrast, axonal transport was much more efficient with SINV than CHIKV, both from the periphery to the CNS and between neural tissues. Thus, the preferred mechanisms of neuroinvasion by these two related viruses are distinct, providing a powerful imaging-friendly system to compare mechanisms and prevention methods of encephalopathies.

Responses of cerebellar Purkinje cells during fictive optomotor behavior in larval zebrafish

Although most studies of the cerebellum have been conducted in mammals, cerebellar circuitry is highly conserved across vertebrates, suggesting that studies of simpler systems may be useful for understanding cerebellar function. The larval zebrafish is particularly promising in this regard because of its accessibility to optical monitoring and manipulations of neural activity. Although several studies suggest that the cerebellum plays a role in behavior at larval stages, little is known about the signals conveyed by particular classes of cerebellar neurons. Here we use electrophysiological recordings to characterize subthreshold, simple spike, and climbing fiber responses in larval zebrafish Purkinje cells in the context of the fictive optomotor response (OMR)-a paradigm in which fish adjust motor output to stabilize their virtual position relative to a visual stimulus. Although visual responses were prominent in Purkinje cells, they lacked the direction or velocity sensitivity that would be expected for controlling the OMR. On the other hand, Purkinje cells exhibited strong responses during fictive swim bouts. Temporal characteristics of these responses are suggestive of a general role for the larval zebrafish cerebellum in controlling swimming. Climbing fibers encoded both visual and motor signals but did not appear to encode signals that could be used to adjust OMR gain, such as retinal slip. Finally, the observation of diverse relationships between simple spikes and climbing fiber responses in individual Purkinje cells highlights the importance of distinguishing between these two types of activity in calcium imaging experiments.

Functional Profiles of Visual-, Auditory-, and Water Flow-Responsive Neurons in the Zebrafish Tectum

The tectum has long been known as a hub of visual processing, and recent studies have elucidated many of the circuit-level mechanisms by which tectal neurons filter visual information. Here, we use population-scale imaging of tectal neurons expressing a genetically encoded calcium indicator to characterize tectal responses to non-visual stimuli in zebrafish. We identify ensembles of neurons responsive to stimuli for each of three sensory modalities: vision, audition, and water flow sensation. These ensembles display consistently represented response profiles to our stimuli, and each has a preferred stimulus and salient feature to which it is most responsive. Each sensory modality drives a unique spatial profile of activity in the tectal neuropil, suggesting that the neuropil's laminar structure functionally subserves multiple modalities. The positions of the responsive neurons in the periventricular layer are also distinct across modalities, and very few neurons are responsive to multiple modalities. The cells contributing to each ensemble are highly variable from trial to trial, but ensembles contain "cores" of reliably responsive cells, suggesting a mechanism whereby they could maintain consistency in reporting salient stimulus features while retaining flexibility to report on similar stimuli. Finally, we find that co-presentation of auditory or water flow stimuli suppress visual responses in the tectum.

AMPA receptor mediated synaptic excitation drives state-dependent bursting in Purkinje neurons of zebrafish larvae

Purkinje neurons are central to cerebellar function and show membrane bistability when recorded in vitro or in vivo under anesthesia. The existence of bistability in vivo in awake animals is disputed. Here, by recording intracellularly from Purkinje neurons in unanesthetized larval zebrafish (Danio rerio), we unequivocally demonstrate bistability in these neurons. Tonic firing was seen in depolarized regimes and bursting at hyperpolarized membrane potentials. In addition, Purkinje neurons could switch from one state to another spontaneously or with current injection. While GABAAR or NMDAR were not required for bursting, activation of AMPARs by climbing fibers (CFs) was sufficient to trigger bursts. Further, by recording Purkinje neuron membrane potential intracellularly, and motor neuron spikes extracellularly, we show that initiation of motor neuron spiking is correlated with increased incidence of CF EPSPs and membrane depolarization. Developmentally, bistability was observed soon after Purkinje neuron specification and persists at least until late larval stages.

Scattering of Sculpted Light in Intact Brain Tissue, with implications for Optogenetics

Optogenetics uses light to control and observe the activity of neurons, often using a focused laser beam. As brain tissue is a scattering medium, beams are distorted and spread with propagation through neural tissue, and the beam's degradation has important implications in optogenetic experiments. To address this, we present an analysis of scattering and loss of intensity of focused laser beams at different depths within the brains of zebrafish larvae. Our experimental set-up uses a 488 nm laser and a spatial light modulator to focus a diffraction-limited spot of light within the brain. We use a combination of experimental measurements of back-scattered light in live larvae and computational modelling of the scattering to determine the spatial distribution of light. Modelling is performed using the Monte Carlo method, supported by generalised Lorenz-Mie theory in the single-scattering approximation. Scattering in areas rich in cell bodies is compared to that of regions of neuropil to identify the distinct and dramatic contributions that cell nuclei make to scattering. We demonstrate the feasibility of illuminating individual neurons, even in nucleus-rich areas, at depths beyond 100 μm using a spatial light modulator in combination with a standard laser and microscope optics.

Neuronal activity biases axon selection for myelination in vivo

An essential feature of vertebrate neural development is ensheathment of axons with myelin, an insulating membrane formed by oligodendrocytes. Not all axons are myelinated, but mechanisms directing myelination of specific axons are unknown. Using zebrafish, we found that activity-dependent secretion stabilized myelin sheath formation on select axons. When VAMP2-dependent exocytosis was silenced in single axons, oligodendrocytes preferentially ensheathed neighboring axons. Nascent sheaths formed on silenced axons were shorter in length, but when activity of neighboring axons was also suppressed, inhibition of sheath growth was relieved. Using in vivo time-lapse microscopy, we found that only 25% of oligodendrocyte processes that initiated axon wrapping were stabilized during normal development and that initiation did not require activity. Instead, oligodendrocyte processes wrapping silenced axons retracted more frequently. We propose that axon selection for myelination results from excessive and indiscriminate initiation of wrapping followed by refinement that is biased by activity-dependent secretion from axons.