Search strategy is regulated by somatostatin signaling and deep brain photoreceptors in zebrafish

A large-scale CRISPR screen reveals context-specific genetic regulation of retinal ganglion cell regeneration

Many genes are known to regulate retinal regeneration after widespread tissue damage. Conversely, genes controlling regeneration after limited cell loss, as per degenerative diseases, are undefined. As stem/progenitor cell responses scale to injury levels, understanding how the extent and specificity of cell loss impact regenerative processes is important. Here, transgenic zebrafish enabling selective retinal ganglion cell (RGC) ablation were used to identify genes that regulate RGC regeneration. A single cell multiomics-informed screen of 100 genes identified seven knockouts that inhibited and 11 that promoted RGC regeneration. Surprisingly, 35 out of 36 genes known and/or implicated as being required for regeneration after widespread retinal damage were not required for RGC regeneration. The loss of seven even enhanced regeneration kinetics, including the proneural factors neurog1, olig2 and ascl1a. Mechanistic analyses revealed that ascl1a disruption increased the propensity of progenitor cells to produce RGCs, i.e. increased 'fate bias'. These data demonstrate plasticity in the mechanism through which Müller glia convert to a stem-like state and context specificity in how genes function during regeneration. Increased understanding of how the regeneration of disease-relevant cell types is specifically controlled will support the development of disease-tailored regenerative therapeutics.

Light wavelength modulates search behavior performance in zebrafish

Visual systems have evolved to discriminate between different wavelengths of light. The ability to perceive color, or specific light wavelengths, is important as color conveys crucial information about both biotic and abiotic features in the environment. Indeed, different wavelengths of light can drive distinct patterns of activity in the vertebrate brain, yet what remains incompletely understood is whether distinct wavelengths can invoke etiologically relevant behavioral changes. To address how specific wavelengths in the visible spectrum modulate behavioral performance, we use larval zebrafish and a stereotypic light-search behavior. Prior work has shown that the cessation of light triggers a transitional light-search behavior, which we use to interrogate wavelength-dependent behavioral modulation. Using 8 narrow spectrum light sources in the visible range, we demonstrate that all wavelengths induce motor parameters consistent with search behavior, yet the magnitude of search behavior is spectrum sensitive and the underlying motor parameters are modulated in distinct patterns across short, medium, and long wavelengths. However, our data also establishes that not all motor features of search are impacted by wavelength. To define how wavelength modulates search performance, we performed additional assays with alternative wavelengths, dual wavelengths, and variable intensity. Last, we also tested blind larvae to resolve which components of wavelength dependent behavioral changes potentially include signaling from non-retinal photoreception. These findings have important implications as organisms can be exposed to varying wavelengths in laboratory and natural settings and therefore impose unique behavioral outputs.

Context-dependent hyperactivity in syngap1a and syngap1b zebrafish models of SYNGAP1-related disorder

Background and aims

SYNGAP1-related disorder (SYNGAP1-RD) is a prevalent genetic form of Autism Spectrum Disorder and Intellectual Disability (ASD/ID) and is caused by de novo or inherited mutations in one copy of the SYNGAP1 gene. In addition to ASD/ID, SYNGAP1 disorder is associated with comorbid symptoms including treatment-resistant-epilepsy, sleep disturbances, and gastrointestinal distress. Mechanistic links between these diverse symptoms and SYNGAP1 variants remain obscure, therefore, our goal was to generate a zebrafish model in which this range of symptoms can be studied.

Methods

We used CRISPR/Cas9 to introduce frameshift mutations in the syngap1a and syngap1b zebrafish duplicates (syngap1ab) and validated these stable models for Syngap1 loss-of-function. Because SYNGAP1 is extensively spliced, we mapped splice variants to the two zebrafish syngap1a and b genes and identified mammalian-like isoforms. We then quantified locomotory behaviors in zebrafish syngap1ab larvae under three conditions that normally evoke different arousal states in wild-type larvae: aversive, high-arousal acoustic, medium-arousal dark, and low-arousal light stimuli.

Results

We show that CRISPR/Cas9 indels in zebrafish syngap1a and syngap1b produced loss-of-function alleles at RNA and protein levels. Our analyses of zebrafish Syngap1 isoforms showed that, as in mammals, zebrafish Syngap1 N- and C-termini are extensively spliced. We identified a zebrafish syngap1 α1-like variant that maps exclusively to the syngap1b gene. Quantifying locomotor behaviors showed that syngap1ab mutant larvae are hyperactive compared to wild-type but to differing degrees depending on the stimulus. Hyperactivity was most pronounced in low arousal settings, and hyperactivity was proportional to the number of mutant syngap1 alleles.

Limitations

Syngap1 loss-of-function mutations produce relatively subtle phenotypes in zebrafish compared to mammals. For example, while mouse Syngap1 homozygotes die at birth, zebrafish syngap1ab-/- survive to adulthood and are fertile, thus some aspects of symptoms in people with SYNGAP1-Related Disorder are not likely to be reflected in zebrafish.

Conclusion

Our data support mutations in zebrafish syngap1ab as causal for hyperactivity associated with elevated arousal that is especially pronounced in low-arousal environments.

Uncovering multiscale structure in the variability of larval zebrafish navigation

Animals chain movements into long-lived motor strategies, exhibiting variability across scales that reflects the interplay between internal states and environmental cues. To reveal structure in such variability, we build Markov models of movement sequences that bridges across time scales and enables a quantitative comparison of behavioral phenotypes among individuals. Applied to larval zebrafish responding to diverse sensory cues, we uncover a hierarchy of long-lived motor strategies, dominated by changes in orientation distinguishing cruising versus wandering strategies. Environmental cues induce preferences along these modes at the population level: while fish cruise in the light, they wander in response to aversive stimuli, or in search for appetitive prey. As our method encodes the behavioral dynamics of each individual fish in the transitions among coarse-grained motor strategies, we use it to uncover a hierarchical structure in the phenotypic variability that reflects exploration-exploitation trade-offs. Across a wide range of sensory cues, a major source of variation among fish is driven by prior and/or immediate exposure to prey that induces exploitation phenotypes. A large degree of variability that is not explained by environmental cues unravels motivational states that override the sensory context to induce contrasting exploration-exploitation phenotypes. Altogether, by extracting the timescales of motor strategies deployed during navigation, our approach exposes structure among individuals and reveals internal states tuned by prior experience.

Key HPI axis receptors facilitate light adaptive behavior in larval zebrafish

The vertebrate stress response (SR) is mediated by the hypothalamic-pituitary-adrenal (HPA) axis and contributes to generating context appropriate physiological and behavioral changes. Although the HPA axis plays vital roles both in stressful and basal conditions, research has focused on the response under stress. To understand broader roles of the HPA axis in a changing environment, we characterized an adaptive behavior of larval zebrafish during ambient illumination changes. Genetic abrogation of glucocorticoid receptor (nr3c1) decreased basal locomotor activity in light and darkness. Some key HPI axis receptors (mc2r [ACTH receptor], nr3c1), but not nr3c2 (mineralocorticoid receptor), were required to adapt to light more efficiently but became dispensable when longer illumination was provided. Such light adaptation was more efficient in dimmer light. Our findings show that the HPI axis contributes to the SR, facilitating the phasic response and maintaining an adapted basal state, and that certain adaptations occur without HPI axis activity.

Transcriptional control of visual neural circuit development by GS homeobox 1

As essential components of gene expression networks, transcription factors regulate neural circuit assembly. The homeobox transcription factor encoding gene, gs homeobox 1 (gsx1), is expressed in the developing visual system; however, no studies have examined its role in visual system formation. In zebrafish, retinal ganglion cell (RGC) axons that transmit visual information to the brain terminate in ten arborization fields (AFs) in the optic tectum (TeO), pretectum (Pr), and thalamus. Pretectal AFs (AF1-AF9) mediate distinct visual behaviors, yet we understand less about their development compared to AF10 in the TeO. Using gsx1 zebrafish mutants, immunohistochemistry, and transgenic lines, we observed that gsx1 is required for vesicular glutamate transporter, Tg(slc17a6b:DsRed), expression in the Pr, but not overall neuron number. gsx1 mutants have normal eye morphology, yet they exhibit impaired visual ability during prey capture. RGC axon volume in the gsx1 mutant Pr and TeO is reduced, and AF7 that is active during feeding is missing which is consistent with reduced hunting performance. Timed laser ablation of Tg(slc17a6b:DsRed)-positive cells reveals that they are necessary for AF7 formation. This work is the first to implicate gsx1 in establishing cell identity and functional neural circuits in the visual system.

Protocol for controlling visual experience during zebrafish development and modulation of motor behavior

Sensory experience instructs neurodevelopment and refines sensory processing. Here, we describe a minimally invasive protocol to immobilize zebrafish during early development to control visual experience. We describe how to prepare larvae for embedding in agarose at two separate timepoints in development. Then we describe how to build a behavior rig and use software to track zebrafish behaviors. Finally, we detail analyzing behavioral data to validate the protocol and determine outcomes of sensory dependent plasticity. For complete details on the use and execution of this protocol, please refer to Hageter et al. (2023).1.

Drugs prescribed for Phelan-McDermid syndrome differentially impact sensory behaviors in shank3 zebrafish models

Background: Altered sensory processing is a pervasive symptom in individuals with Autism Spectrum Disorders (ASD); people with Phelan McDermid syndrome (PMS), in particular, show reduced responses to sensory stimuli. PMS is caused by deletions of the terminal end of chromosome 22 or point mutations in Shank3. People with PMS can present with an array of symptoms including ASD, epilepsy, gastrointestinal distress, and reduced responses to sensory stimuli. People with PMS are often medicated to manage behaviors like aggression and/or self-harm and/or epilepsy, and it remains unclear how these medications might impact perception/sensory processing. Here we test this using zebrafish mutant shank3ab PMS models that likewise show reduced sensory responses in a visual motor response (VMR) assay, in which increased locomotion is triggered by light to dark transitions. Methods: We screened three medications, risperidone, lithium chloride (LiCl), and carbamazepine (CBZ), prescribed to people with PMS and one drug, 2-methyl-6-(phenylethynyl) pyridine (MPEP) tested in rodent models of PMS, for their effects on a sensory-induced behavior in two zebrafish PMS models with frameshift mutations in either the N- or C- termini. To test how pharmacological treatments affect the VMR, we exposed larvae to selected drugs for 24 hours and then quantified their locomotion during four ten-minute cycles of lights on-to-off stimuli. Results: We found that risperidone normalized the VMR in shank3 models. LiCl and CBZ had no effect on the VMR in any of the three genotypes. MPEP reduced the VMR in wildtype (WT) to levels seen in shank3 models but caused no changes in either shank3 model. Finally, shank3 mutants showed resistance to the seizure-inducing drug pentylenetetrazol (PTZ), at a dosage that results in hyperactive swimming in WT zebrafish. Conclusions: Our work shows that the effects of drugs on sensory processing are varied in ways that can be highly genotype- and drug-dependent.

Context-dependent hyperactivity in syngap1a and syngap1b zebrafish autism models

Background and Aims

SYNGAP1 disorder is a prevalent genetic form of Autism Spectrum Disorder and Intellectual Disability (ASD/ID) and is caused by de novo or inherited mutations in one copy of the SYNGAP1 gene. In addition to ASD/ID, SYNGAP1 disorder is associated with comorbid symptoms including treatment-resistant-epilepsy, sleep disturbances, and gastrointestinal distress. Mechanistic links between these diverse symptoms and SYNGAP1 variants remain obscure, therefore, our goal was to generate a zebrafish model in which this range of symptoms can be studied.

Methods

We used CRISPR/Cas9 to introduce frameshift mutations in the syngap1a and syngap1b zebrafish duplicates (syngap1ab) and validated these stable models for Syngap1 loss-of-function. Because SYNGAP1 is extensively spliced, we mapped splice variants to the two zebrafish syngap1a and b genes and identified mammalian-like isoforms. We then quantified locomotory behaviors in zebrafish syngap1ab larvae under three conditions that normally evoke different arousal states in wild type larvae: aversive, high-arousal acoustic, medium-arousal dark, and low-arousal light stimuli.

Results

We show that CRISPR/Cas9 indels in zebrafish syngap1a and syngap1b produced loss-of-function alleles at RNA and protein levels. Our analyses of zebrafish Syngap1 isoforms showed that, as in mammals, zebrafish Syngap1 N- and C-termini are extensively spliced. We identified a zebrafish syngap1 α1-like variant that maps exclusively to the syngap1b gene. Quantifying locomotor behaviors showed that syngap1ab larvae are hyperactive compared to wild type but to differing degrees depending on the stimulus. Hyperactivity was most pronounced in low arousal settings, with overall movement increasing with the number of mutant syngap1 alleles.

Conclusions

Our data support mutations in zebrafish syngap1ab as causal for hyperactivity associated with elevated arousal that is especially pronounced in low-arousal environments.

The canonical HPA axis facilitates and maintains light adaptive behavior

The vertebrate stress response (SR) is mediated by the hypothalamic-pituitary-adrenal (HPA) axis and contributes to generating context appropriate physiological and behavioral changes. Although the HPA axis plays vital roles both in stressful and basal conditions, research has focused on the response under stress. To understand broader roles of the HPA axis in a changing environment, we characterized an adaptive behavior of larval zebrafish during ambient illumination changes. The glucocorticoid receptor (nr3c1) was necessary to maintain basal locomotor activity in light and darkness. The HPA axis was required to adapt to light more efficiently but became dispensable when longer illumination was provided. Light adaptation was more efficient in dimmer light and did not require the mineralocorticoid receptor (nr3c2). Our findings show that the HPA axis contributes to the SR at various stages, facilitating the phasic response and maintaining an adapted basal state, and that certain adaptations occur without HPA axis activity.

Optimizing assays of zebrafish larvae swimming performance for drug discovery

INTRODUCTION

Zebrafish larvae are one of the few vertebrates amenable to large-scale drug discovery screens. Larval swimming behavior is often used as an outcome variable and many fields of study have developed assays for evaluating swimming performance. An unintended consequence of this wide interest is that details related to assay methodology and interpretation become scattered across the literature. The aim of this review is to consolidate this information, particularly as it relates to high-throughput approaches.

AREAS COVERED

The authors describe larval swimming behaviors as this forms the basis for understanding their experimentally evoked swimming or spontaneous activity. Next, they detail how swimming activity can serve as an outcome variable, particularly in the multi-well formats used in large-scale screening studies. They also highlight biological and technical factors that can impact the sensitivity and variability of these measurements.

EXPERT OPINION

Careful attention to animal husbandry, experimental design, data acquisition, and interpretation of results can improve screen outcomes by maximizing swimming activity while minimizing intra- and inter-larval variability. The development of more sensitive, quantitative methods of assessing swimming performance that can be incorporated into high-throughput workflows will be important in order to take full advantage of the zebrafish model.

Thalamic regulation of a visual critical period and motor behavior

During the visual critical period (CP), sensory experience refines the structure and function of visual circuits. The basis of this plasticity was long thought to be limited to cortical circuits, but recently described thalamic plasticity challenges this dogma and demonstrates greater complexity underlying visual plasticity. Yet how visual experience modulates thalamic neurons or how the thalamus modulates CP timing is incompletely understood. Using a larval zebrafish, thalamus-centric ocular dominance model, we show functional changes in the thalamus and a role of inhibitory signaling to establish CP timing using a combination of functional imaging, optogenetics, and pharmacology. Hemisphere-specific changes in genetically defined thalamic neurons correlate with changes in visuomotor behavior, establishing a role of thalamic plasticity in modulating motor performance. Our work demonstrates that visual plasticity is broadly conserved and that visual experience leads to neuron-level functional changes in the thalamus that require inhibitory signaling to establish critical period timing.

Thalamic neurons drive distinct forms of motor asymmetry that are conserved in teleost and dependent on visual evolution

Brain laterality is a prominent feature in Bilateria, where neural functions are favored in a single brain hemisphere. These hemispheric specializations are thought to improve behavioral performance and are commonly observed as sensory or motor asymmetries, such as handedness in humans. Despite its prevalence, our understanding of the neural and molecular substrates instructing functional lateralization is limited. Moreover, how functional lateralization is selected for or modulated throughout evolution is poorly understood. While comparative approaches offer a powerful tool for addressing this question, a major obstacle has been the lack of a conserved asymmetric behavior in genetically tractable organisms. Previously, we described a robust motor asymmetry in larval zebrafish. Following the loss of illumination, individuals show a persistent turning bias that is associated with search pattern behavior with underlying functional lateralization in the thalamus. This behavior permits a simple yet robust assay that can be used to address fundamental principles underlying lateralization in the brain across taxa. Here, we take a comparative approach and show that motor asymmetry is conserved across diverse larval teleost species, which have diverged over the past 200 million years. Using a combination of transgenic tools, ablation, and enucleation, we show that teleosts exhibit two distinct forms of motor asymmetry, vision-dependent and - independent. These asymmetries are directionally uncorrelated, yet dependent on the same subset of thalamic neurons. Lastly, we leverage Astyanax sighted and blind morphs, which show that fish with evolutionarily derived blindness lack both retinal-dependent and -independent motor asymmetries, while their sighted surface conspecifics retained both forms. Our data implicate that overlapping sensory systems and neuronal substrates drive functional lateralization in a vertebrate brain that are likely targets for selective modulation during evolution.

A tapt1 knock-out zebrafish line with aberrant lens development and impaired vision models human early-onset cataract

Bi-allelic mutations in the gene coding for human trans-membrane anterior-posterior transformation protein 1 (TAPT1) result in a broad phenotypic spectrum, ranging from syndromic disease with severe skeletal and congenital abnormalities to isolated early-onset cataract. We present here the first patient with a frameshift mutation in the TAPT1 gene, resulting in both bilateral early-onset cataract and skeletal abnormalities, in addition to several dysmorphic features, in this way further expanding the phenotypic spectrum associated with TAPT1 mutations. A tapt1a/tapt1b double knock-out (KO) zebrafish model generated by CRISPR/Cas9 gene editing revealed an early larval phenotype with eye malformations, loss of vision, increased photokinetics and hyperpigmentation, without visible skeletal involvement. Ultrastructural analysis of the eyes showed a smaller condensed lens, loss of integrity of the lens capsule with formation of a secondary lens and hyperplasia of the cells in the ganglion and inner plexiform layers of the retina. Transcriptomic analysis pointed to an impaired lens development with aberrant expression of many of the crystallin and other lens-specific genes. Furthermore, the phototransduction and visual perception pathways were found to be significantly disturbed. Differences in light perception are likely the cause of the increased dark photokinetics and generalized hyperpigmentation observed in this zebrafish model. In conclusion, this study validates TAPT1 as a new gene for early-onset cataract and sheds light on its ultrastructural and molecular characteristics.

A Critical Review of Zebrafish Neurological Disease Models-2. Application: Functional and Neuroanatomical Phenotyping Strategies and Chemical Screens

Extensive phylogenetic conservation of molecular pathways and neuroanatomical structures, associated with efficient methods for genetic modification, have been exploited increasingly to generate zebrafish models of human disease. A range of powerful approaches can be deployed to analyze these models with the ultimate goal of elucidating pathogenic mechanisms and accelerating efforts to find effective treatments. Unbiased neurobehavioral assays can provide readouts that parallel clinical abnormalities found in patients, although some of the most useful assays quantify responses that are not routinely evaluated clinically, and differences between zebrafish and human brains preclude expression of the full range of neurobehavioral abnormalities seen in disease. Imaging approaches that use fluorescent reporters and standardized brain atlases coupled with quantitative measurements of brain structure offer an unbiased means to link experimental manipulations to changes in neural architecture. Together, quantitative structural and functional analyses allow dissection of the cellular and physiological basis underlying neurological phenotypes. These approaches can be used as outputs in chemical modifier screens, which provide a major opportunity to exploit zebrafish models to identify small molecule modulators of pathophysiology that may be informative for understanding disease mechanisms and possible therapeutic approaches.

Zebrafish behavior as a gateway to nervous system assembly and plasticity

Nervous system assembly relies on a diversity of cellular processes ranging from dramatic tissue reorganization to local, subcellular changes all driven by precise molecular programs. Combined, these processes culminate in an animal's ability to plan and execute behaviors. Animal behavior can, therefore, serve as a functional readout of nervous system development. Benefitting from an expansive and growing set of molecular and imaging tools paired with an ever-growing number of assays of diverse behaviors, the zebrafish system has emerged as an outstanding platform at the intersection of nervous system assembly, plasticity and behavior. Here, we summarize recent advancements in the field, including how developing neural circuits are refined to shape complex behaviors and plasticity.

Impacts of Cetylpyridinium Chloride on the Survival, Development, Behavior, and Oxidative Stress of Early-Life-Stage Zebrafish (Danio rerio)

Cetylpyridinium chloride (CPC) is a widely used surfactant that has been detected in various water ecosystems. However, knowledge on the toxicity of CPC to fish remains scarce. Here, we examined the survival, development, behavior, and oxidative stress in the early life stages of zebrafish exposed to CPC (0, 4, 40, 400, and 1200 μg/L) until 120 h post-fertilization (hpf). Results showed that CPC induced significant mortality at 400 and 1200 μg/L, with a 120 h-EC₅₀ value of 175.9 μg/L. CPC significantly decreased the heart rate of embryos (48 hpf; 4–400 μg/L) and larvae (72 hpf; 40 and 400 μg/L). At 120 hpf, CPC exhibited a dual effect on the locomotion activity (decreased at 400 μg/L and increased at 4 and 40 μg/L) and elevated the reactive oxygen species, superoxide dismutase, and glutathione levels in zebrafish larvae at 400 µg/L. In addition, a correlation analysis revealed that CPC-induced oxidative stress might play a critical role in mediating the cardiac and behavioral toxicity of CPC to zebrafish larvae. Our findings suggest that CPC may disturb the fish’s development, behavior, and oxidative status at environmentally relevant concentrations, which should not be ignored when assessing its potential risks to aquatic ecosystems.

Environmental and Molecular Modulation of Motor Individuality in Larval Zebrafish

Innate behavioral biases such as human handedness are a ubiquitous form of inter-individual variation that are not strictly hardwired into the genome and are influenced by diverse internal and external cues. Yet, genetic and environmental factors modulating behavioral variation remain poorly understood, especially in vertebrates. To identify genetic and environmental factors that influence behavioral variation, we take advantage of larval zebrafish light-search behavior. During light-search, individuals preferentially turn in leftward or rightward loops, in which directional bias is sustained and non-heritable. Our previous work has shown that bias is maintained by a habenula-rostral PT circuit and genes associated with Notch signaling. Here we use a medium-throughput recording strategy and unbiased analysis to show that significant individual to individual variation exists in wildtype larval zebrafish turning preference. We classify stable left, right, and unbiased turning types, with most individuals exhibiting a directional preference. We show unbiased behavior is not due to a loss of photo-responsiveness but reduced persistence in same-direction turning. Raising larvae at elevated temperature selectively reduces the leftward turning type and impacts rostral PT neurons, specifically. Exposure to conspecifics, variable salinity, environmental enrichment, and physical disturbance does not significantly impact inter-individual turning bias. Pharmacological manipulation of Notch signaling disrupts habenula development and turn bias individuality in a dose dependent manner, establishing a direct role of Notch signaling. Last, a mutant allele of a known Notch pathway affecter gene, gsx2, disrupts turn bias individuality, implicating that brain regions independent of the previously established habenula-rostral PT likely contribute to inter-individual variation. These results establish that larval zebrafish is a powerful vertebrate model for inter-individual variation with established neural targets showing sensitivity to specific environmental and gene signaling disruptions. Our results provide new insight into how variation is generated in the vertebrate nervous system.

Algorithms underlying flexible phototaxis in larval zebrafish

To thrive, organisms must maintain physiological and environmental variables in suitable ranges. Given that these variables undergo constant fluctuations over varying time scales, how do biological control systems maintain control over these values? We explored this question in the context of phototactic behavior in larval zebrafish. We demonstrate that larval zebrafish use phototaxis to maintain environmental luminance at a set point, that the value of this set point fluctuates on a time scale of seconds when environmental luminance changes, and that it is determined by calculating the mean input across both sides of the visual field. These results expand on previous studies of flexible phototaxis in larval zebrafish; they suggest that larval zebrafish exert homeostatic control over the luminance of their surroundings, and that feedback from the surroundings drives allostatic changes to the luminance set point. As such, we describe a novel behavioral algorithm with which larval zebrafish exert control over a sensory variable.

Approaches to Test the Neurotoxicity of Environmental Contaminants in the Zebrafish Model: From Behavior to Molecular Mechanisms

The occurrence of neuroactive chemicals in the aquatic environment is on the rise and poses a potential threat to aquatic biota of currently unpredictable outcome. In particular, subtle changes caused by these chemicals to an organism's sensation or behavior are difficult to tackle with current test systems that focus on rodents or with in vitro test systems that omit whole-animal responses. In recent years, the zebrafish (Danio rerio) has become a popular model organism for toxicological studies and testing strategies, such as the standardized use of zebrafish early life stages in the Organisation for Economic Co-operation and Development's guideline 236. In terms of neurotoxicity, the zebrafish provides a powerful model to investigate changes to the nervous system from several different angles, offering the ability to tackle the mechanisms of action of chemicals in detail. The mechanistic understanding gained through the analysis of this model species provides a good basic knowledge of how neuroactive chemicals might interact with a teleost nervous system. Such information can help infer potential effects occurring to other species exposed to neuroactive chemicals in their aquatic environment and predicting potential risks of a chemical for the aquatic ecosystem. In the present article, we highlight approaches ranging from behavioral to structural, functional, and molecular analysis of the larval zebrafish nervous system, providing a holistic view of potential neurotoxic outcomes. Environ Toxicol Chem 2021;40:989-1006. © 2020 SETAC.

TMT-Opsins differentially modulate medaka brain function in a context-dependent manner

Vertebrate behavior is strongly influenced by light. Light receptors, encoded by functional opsin proteins, are present inside the vertebrate brain and peripheral tissues. This expression feature is present from fishes to human and appears to be particularly prominent in diurnal vertebrates. Despite their conserved widespread occurrence, the nonvisual functions of opsins are still largely enigmatic. This is even more apparent when considering the high number of opsins. Teleosts possess around 40 opsin genes, present from young developmental stages to adulthood. Many of these opsins have been shown to function as light receptors. This raises the question of whether this large number might mainly reflect functional redundancy or rather maximally enables teleosts to optimally use the complex light information present under water. We focus on tmt-opsin1b and tmt-opsin2, c-opsins with ancestral-type sequence features, conserved across several vertebrate phyla, expressed with partly similar expression in non-rod, non-cone, non-retinal-ganglion-cell brain tissues and with a similar spectral sensitivity. The characterization of the single mutants revealed age- and light-dependent behavioral changes, as well as an impact on the levels of the preprohormone sst1b and the voltage-gated sodium channel subunit scn12aa. The amount of daytime rest is affected independently of the eyes, pineal organ, and circadian clock in tmt-opsin1b mutants. We further focused on daytime behavior and the molecular changes in tmt-opsin1b/2 double mutants, and found that-despite their similar expression and spectral features-these opsins interact in part nonadditively. Specifically, double mutants complement molecular and behavioral phenotypes observed in single mutants in a partly age-dependent fashion. Our work provides a starting point to disentangle the highly complex interactions of vertebrate nonvisual opsins, suggesting that tmt-opsin-expressing cells together with other visual and nonvisual opsins provide detailed light information to the organism for behavioral fine-tuning. This work also provides a stepping stone to unravel how vertebrate species with conserved opsins, but living in different ecological niches, respond to similar light cues and how human-generated artificial light might impact on behavioral processes in natural environments.

Hypothalamic Pomc Neurons Innervate the Spinal Cord and Modulate the Excitability of Premotor Circuits

Locomotion requires energy, yet animals need to increase locomotion in order to find and consume food in energy-deprived states. While such energy homeostatic coordination suggests brain origin, whether the central melanocortin 4 receptor (Mc4r) system directly modulates locomotion through motor circuits is unknown. Here, we report that hypothalamic Pomc neurons in zebrafish and mice have long-range projections into spinal cord regions harboring Mc4r-expressing V2a interneurons, crucial components of the premotor networks. Furthermore, in zebrafish, Mc4r activation decreases the excitability of spinal V2a neurons as well as swimming and foraging, while systemic or V2a neuron-specific blockage of Mc4r promotes locomotion. In contrast, in mice, electrophysiological recordings revealed that two-thirds of V2a neurons in lamina X are excited by the Mc4r agonist α-MSH, and acute inhibition of Mc4r signaling reduces locomotor activity. In addition, we found other Mc4r neurons in spinal lamina X that are inhibited by α-MSH, which is in line with previous studies in rodents where Mc4r agonists reduced locomotor activity. Collectively, our studies identify spinal V2a interneurons as evolutionary conserved second-order neurons of the central Mc4r system, providing a direct anatomical and functional link between energy homeostasis and locomotor control systems. The net effects of this modulatory system on locomotor activity can vary between different vertebrate species and, possibly, even within one species. We discuss the biological sense of this phenomenon in light of the ambiguity of locomotion on energy balance and the different living conditions of the different species.

Somatostatin 1.1 contributes to the innate exploration of zebrafish larva

Pharmacological experiments indicate that neuropeptides can effectively tune neuronal activity and modulate locomotor output patterns. However, their functions in shaping innate locomotion often remain elusive. For example, somatostatin has been previously shown to induce locomotion when injected in the brain ventricles but to inhibit fictive locomotion when bath-applied in the spinal cord in vitro. Here, we investigated the role of somatostatin in innate locomotion through a genetic approach by knocking out somatostatin 1.1 (sst1.1) in zebrafish. We automated and carefully analyzed the kinematics of locomotion over a hundred of thousand bouts from hundreds of mutant and control sibling larvae. We found that the deletion of sst1.1 did not impact acousto-vestibular escape responses but led to abnormal exploration. sst1.1 mutant larvae swam over larger distance, at higher speed and performed larger tail bends, indicating that Somatostatin 1.1 inhibits spontaneous locomotion. Altogether our study demonstrates that Somatostatin 1.1 innately contributes to slowing down spontaneous locomotion.

PiVR: An affordable and versatile closed-loop platform to study unrestrained sensorimotor behavior

Tools enabling closed-loop experiments are crucial to delineate causal relationships between the activity of genetically labeled neurons and specific behaviors. We developed the Raspberry Pi Virtual Reality (PiVR) system to conduct closed-loop optogenetic stimulation of neural functions in unrestrained animals. PiVR is an experimental platform that operates at high temporal resolution (70 Hz) with low latencies (<30 milliseconds), while being affordable ( Collapse

Key Words Collapse

MESH Headings

• Animals
• Behavior, Animal/physiology
• Chemotaxis
• Drosophila melanogaster/physiology
• Larva/physiology
• Light
• Locomotion
• Male
• Neurons/physiology
• Odorants
• Optogenetics
• Sensation/physiology
• Sensorimotor Cortex/physiology
• Software
• Taste/physiology
• Virtual Reality
• Zebrafish

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Grants

• R01 NS113048 NINDS NIH HHS
• R25 GM067110 NIGMS NIH HHS
• National Science Foundation
• National Institute of Health
• Gordon and Betty Moore Foundation

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Affiliation(s)

David Tadres Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara, Santa Barbara, California, United States of America Neuroscience Research Institute, University of California, Santa Barbara, Santa Barbara, California, United States of America Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland

Matthieu Louis Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara, Santa Barbara, California, United States of America Neuroscience Research Institute, University of California, Santa Barbara, Santa Barbara, California, United States of America Department of Physics, University of California, Santa Barbara, Santa Barbara, California, United States of America

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A Model to Study NMDA Receptors in Early Nervous System Development

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels that play critical roles in neuronal development and nervous system function. Here, we developed a model to study NMDARs in early development in zebrafish, by generating CRISPR-mediated lesions in the NMDAR genes, grin1a and grin1b, which encode the obligatory GluN1 subunits. While receptors containing grin1a or grin1b show high Ca²⁺ permeability, like their mammalian counterpart, grin1a is expressed earlier and more broadly in development than grin1b Both grin1a ^-/- and grin1b ^-/- zebrafish are viable. Unlike in rodents, where the grin1 knockout is embryonic lethal, grin1 double-mutant fish (grin1a ^-/- ; grin1b ^-/-), which lack all NMDAR-mediated synaptic transmission, survive until ∼10 d dpf (days post fertilization), providing a unique opportunity to explore NMDAR function during development and in generating behaviors. Many behavioral defects in the grin1 double-mutant larvae, including abnormal evoked responses to light and acoustic stimuli, prey-capture deficits, and a failure to habituate to acoustic stimuli, are replicated by short-term treatment with the NMDAR antagonist MK-801, suggesting that they arise from acute effects of compromised NMDAR-mediated transmission. Other defects, however, such as periods of hyperactivity and alterations in place preference, are not phenocopied by MK-801, suggesting a developmental origin. Together, we have developed a unique model to study NMDARs in the developing vertebrate nervous system.SIGNIFICANCE STATEMENT Rapid communication between cells in the nervous system depends on ion channels that are directly activated by chemical neurotransmitters. One such ligand-gated ion channel, the NMDAR, impacts nearly all forms of nervous system function. It has been challenging, however, to study the prolonged absence of NMDARs in vertebrates, and hence their role in nervous system development, due to experimental limitations. Here, we demonstrate that zebrafish lacking all NMDAR transmission are viable through early development and are capable of a wide range of stereotypic behaviors. As such, this zebrafish model provides a unique opportunity to study the role of NMDAR in the development of the early vertebrate nervous system.

Molecular and cellular determinants of motor asymmetry in zebrafish

Asymmetries in motor behavior, such as human hand preference, are observed throughout bilateria. However, neural substrates and developmental signaling pathways that impose underlying functional lateralization on a broadly symmetric nervous system are unknown. Here we report that in the absence of over-riding visual information, zebrafish larvae show intrinsic lateralized motor behavior that is mediated by a cluster of 60 posterior tuberculum (PT) neurons in the forebrain. PT neurons impose motor bias via a projection through the habenular commissure. Acquisition of left/right identity is disrupted by heterozygous mutations in mosaic eyes and mindbomb, genes that regulate Notch signaling. These results define the neuronal substrate for motor asymmetry in a vertebrate and support the idea that haploinsufficiency for genes in a core developmental pathway destabilizes left/right identity.

Many animals show individual left/right biases in motor behaviour, but underlying neural substrates have proven elusive. Here the authors describe neurons that maintain individual, context-dependent lateralisation of swimming behaviour in zebrafish.

From behavior to circuit modeling of light-seeking navigation in zebrafish larvae

Bridging brain-scale circuit dynamics and organism-scale behavior is a central challenge in neuroscience. It requires the concurrent development of minimal behavioral and neural circuit models that can quantitatively capture basic sensorimotor operations. Here, we focus on light-seeking navigation in zebrafish larvae. Using a virtual reality assay, we first characterize how motor and visual stimulation sequences govern the selection of discrete swim-bout events that subserve the fish navigation in the presence of a distant light source. These mechanisms are combined into a comprehensive Markov-chain model of navigation that quantitatively predicts the stationary distribution of the fish’s body orientation under any given illumination profile. We then map this behavioral description onto a neuronal model of the ARTR, a small neural circuit involved in the orientation-selection of swim bouts. We demonstrate that this visually-biased decision-making circuit can capture the statistics of both spontaneous and contrast-driven navigation.

All animals with the ability to move use sensory signals to help them navigate towards areas that seem better than their current location. Such areas might contain desirable things like food and mates, or they might allow an animal to escape from threats such as predators. But how the brain gives rise to this navigation behavior is unclear.

Karpenko et al. have now obtained insights into the underlying mechanism by studying a behavior in zebrafish larvae called phototaxis. Phototaxis is the tendency to move in response to light. The advantage of using zebrafish larvae to study this behavior is that their brains are small and semi-transparent. This makes it possible to record the activity of almost every neuron. As a result, an individual’s brain activity can be mapped on to their behavior more precisely than in most other species.

To probe how visual cues influence fish behavior, Karpenko et al. exposed individual fish to a carefully controlled virtual light source and then tracked their movements with a camera. The fish used two strategies to move towards the light. They selected their next movement based partly on the difference in the amount of light reaching each of their eyes, and partly on the change in overall brightness with each swim movement. Karpenko et al. used this information to build a numerical model of fish phototaxis, and to show how a simple brain circuit could generate this behavior.

Species whose brains differ in size and structure may nevertheless develop similar strategies to perform similar tasks. By quantifying a generic behavior in a simple animal model, this study could provide insights into comparable behaviors in other species. In addition, the study suggests a simple mechanism for how animals select actions on the basis of sensory signals, which may also be relevant to other species and other tasks.

Emergence of consistent intra-individual locomotor patterns during zebrafish development

The analysis of larval zebrafish locomotor behavior has emerged as a powerful indicator of perturbations in the nervous system and is used in many fields of research, including neuroscience, toxicology and drug discovery. The behavior of larval zebrafish however, is highly variable, resulting in the use of large numbers of animals and the inability to detect small effects. In this study, we analyzed whether individual locomotor behavior is stable over development and whether behavioral parameters correlate with physiological and morphological features, with the aim of better understanding the variability and predictability of larval locomotor behavior. Our results reveal that locomotor activity of an individual larva remains consistent throughout a given day and is predictable throughout larval development, especially during dark phases, under which larvae demonstrate light-searching behaviors and increased activity. The larvae’s response to startle-stimuli was found to be unpredictable, with no correlation found between response strength and locomotor activity. Furthermore, locomotor activity was not associated with physiological or morphological features of a larva (resting heart rate, body length, size of the swim bladder). Overall, our findings highlight the areas of intra-individual consistency, which could be used to improve the sensitivity of assays using zebrafish locomotor activity as an endpoint.

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.

Melatonin synthesis and clock gene regulation in the pineal organ of teleost fish compared to mammals: Similarities and differences

The pineal organ of all vertebrates synthesizes and secretes melatonin in a rhythmic manner due to the circadian rhythm in the activity of arylalkylamine N-acetyltransferase (AANAT) - the rate-limiting enzyme in melatonin synthesis pathway. Nighttime increase in AANAT activity and melatonin synthesis depends on increased expression of aanat gene (a clock-controlled gene) and/or post-translation modification of AANAT protein. In mammalian and avian species, only one aanat gene is expressed. However, three aanat genes (aanat1a, aanat1b, and aanat2) are reported in fish species. While aanat1a and aanat1b genes are expressed in the fish retina, the nervous system and other peripheral tissues, aanat2 gene is expressed exclusively in the fish pineal organ. Clock genes form molecular components of the clockwork, which regulates clock-controlled genes like aanat gene. All core clock genes (i.e., clock, bmal1, per1, per2, per3, cry1 and cry2) and aanat2 gene (a clock-controlled gene) are expressed in the pineal organ of several fish species. There is a large body of information on regulation of clock genes, aanat gene and melatonin synthesis in the mammalian pineal gland. However, the information available on clock genes, aanat genes and melatonin synthesis in photoreceptive pineal organ of teleosts is fragmentary and not well documented. Therefore, we have reviewed published information on rhythmic expression of clock genes, aanat genes as well as synthesis of melatonin, and their regulation by photoperiod and temperature in teleostean pineal organ as compared to mammalian pineal gland. A critical analysis of the literature suggests that in contrast to the mammalian pineal gland, the pineal organ of teleosts (except salmonids) possesses a well developed indigenous clock composed of clock genes for regulation of rhythmic expression of aanat2 gene and melatonin synthesis. Further, the fish pineal organ also possesses essential molecular components for responding to light and temperature directly. The fish pineal organ seems to act as a potential master biological clock in most of the teleosts.

Parallel Multimodal Circuits Control an Innate Foraging Behavior

Foraging strategies emerge from genetically encoded programs that are similar across animal species. Here, we examine circuits that control a conserved foraging state, local search behavior after food removal, in Caenorhabditis elegans. We show that local search is triggered by two parallel groups of chemosensory and mechanosensory glutamatergic neurons that detect food-related cues. Each group of sensory neurons suppresses distinct integrating neurons through a G protein-coupled metabotropic glutamate receptor, MGL-1, to release local search. The chemosensory and mechanosensory modules are separate and redundant; glutamate release from either module can drive the full behavior. A transition from local search to global search over several minutes after food removal is associated with two changes in circuit function. First, the spontaneous activity of sensory neurons falls. Second, the motor pattern generator for local search becomes less responsive to sensory input. This multimodal, distributed short-term food memory provides robust control of an innate behavior.

Abnormal repetitive behaviors in zebrafish and their relevance to human brain disorders

Abnormal repetitive behaviors (ARBs) are a prominent symptom of numerous human brain disorders and are commonly seen in rodent models as well. While rodent studies of ARBs continue to dominate the field, mounting evidence suggests that zebrafish (Danio rerio) also display ARB-like phenotypes and may therefore be a novel model organism for ARB research. In addition to clear practical research advantages as a model species, zebrafish share high genetic and physiological homology to humans and rodents, including multiple ARB-related genes and robust behaviors relevant to ARB. Here, we discuss a wide spectrum of stereotypic repetitive behaviors in zebrafish, data on their genetic and pharmacological modulation, and the overall translational relevance of fish ARBs to modeling human brain disorders. Overall, the zebrafish is rapidly emerging as a new promising model to study ARBs and their underlying mechanisms.

Masking of a circadian behavior in larval zebrafish involves the thalamo-habenula pathway

Changes in illumination can rapidly influence behavior that is normally controlled by the circadian clock. This effect is termed masking. In mice, masking requires melanopsin-expressing retinal ganglion cells that detect blue light and project to the thalamus. It is not known whether masking is wavelength-dependent in other vertebrates, nor is it known whether the thalamus is also involved or how it influences masking. Here, we address these questions in zebrafish. We find that diel vertical migration, a circadian behavior in larval zebrafish, is effectively triggered by blue, but not by red light. Two-photon calcium imaging reveals that a thalamic nucleus and a downstream structure, the habenula, have a sustained response to blue but not to red light. Lesioning the habenula reduces light-evoked climbing. These data suggest that the thalamo-habenula pathway is involved in the ability of blue light to influence a circadian behavior.