Zebrafish larvae show negative phototaxis to near-infrared light

Protocol to perform ex vivo electroretinography on adult zebrafish

Ex vivo electroretinography (ERG) provides insight into the health and functionality of retinal cells, the integrity of phototransduction, and the visual cycle and allows for the direct application of pharmaceuticals to the retinal tissues. Here, we present a protocol for performing ex vivo ERG on adult zebrafish. We describe steps for zebrafish tissue dissection, mounting tissues, and assembling and connecting cassettes. We then detail procedures for running the Diagnosys software and processing and analyzing the resulting data.

Extremely low frequency magnetic field distracts zebrafish from a visual cognitive task

Electromagnetic fields emitted from overhead power lines and subsea cables are widely regarded to be a disruptive factor for animals using the natural magnetic field as orientation cue for guiding their directed movements. However, it is not known if anthropogenic electromagnetic fields also have the potential to disturb animals attending to information from other sensory modalities. To find out, we trained adult zebrafish (Danio rerio) individually to perform avoidance behavior in response to a visual signal (green LED light spot), which in the exposure group was presented simultaneously with a sinusoidally changing magnetic field (0.3 Hz, group A: 0.015 mT, group B: 0.06 mT). Despite the salience of the visual signal, which was both sufficient and necessary to elicit conditioned avoidance responses, the 0.06 mT magnetic condition had a negative impact on learning performance and response behavior. This suggests that extremely low frequency technical magnetic fields of Earth strength amplitude can act as cross-modal distractor that diverts the attention of animals away from environmentally relevant cues based on nonmagnetic sensory modalities. Our research highlights the need to study the role of anthropogenic magnetic fields as sensory pollutant beyond the scope of magnetic orientation behavior.

Is the diel cycle of routine metabolic rate in mummichog (Fundulus heteroclitus) affected by near-infrared lighting used for visualizing behavior of fishes at night?

The metabolic rate of a freely moving fish (routine metabolic rate) is tightly coupled with volitional movement (spontaneous activity), both of which commonly show strong daily cycles linked to the species-specific diel activity pattern. Mummichog (Fundulus heteroclitus), an important estuarine fish in the north western Atlantic Ocean, are historically reported as diurnal (i.e., more active during daylight). Our recent laboratory studies on a Bay of Fundy population, however, showed a free-running (i.e., similarly active daytime and night-time) or even nocturnal (i.e., more active at night-time) diel activity pattern. In the laboratory, near-infrared (NIR) illumination is commonly used with a NIR-sensitive camera to visualize fish activity across the light-dark periods of the day. Because NIR light is close to the visible light spectrum and certain fishes show sensitivity to NIR, the use of NIR with mummichog possibly could disturb the animals and obscure the identification of their true diel activity pattern. We aimed to determine if NIR illumination (940 nm wavelength) influences the diel activity pattern of mummichog. We used measurements of routine metabolic rate (oxygen consumption rate, MO2) as a proxy for activity, as evaluating the effect of NIR requires treatments where NIR lights are off, which precludes visualization and direct assessment of fish activity at night-time. We measured routine MO2 of mummichogs over 6 days, exposed to either NIR off-on-off (2 days for each off or on period) or the opposite sequence of NIR on-off-on (to control for time-dependent effects). NIR lights did not influence the diel cycle of routine MO2, and activity by proxy, in mummichog. Thus, NIR illumination is a suitable method to visualize mummichog during light-dark diel cycles. Routine MO2, and presumably activity, was similar or higher during night-time periods compared to daytime periods, confirming a free-running or nocturnal activity pattern for at least certain populations of mummichog.

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.

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.

Developmental toxicity and neurobehavioral effects of sodium selenite and selenium nanoparticles on zebrafish embryos

Selenium, a trace mineral, is essential for several physiological processes in humans and animals. It is an antioxidant vital for the immunological response, DNA synthesis, thyroid hormone metabolism, and antioxidant defense enzymes. Zebrafish embryos and larvae were exposed to different concentrations of sodium selenite (SodSe) and selenium nanoparticles (SeNs) at various developmental stages. The study evaluated the impact of SodSe and SeNs on larvae survival, hatching rate, and morphological abnormalities. Also, acridine orange staining was used to analyze the apoptotic cell death, and behavioral tests were conducted to assess anxiety-like behaviors. The results showed that both SodSe and SeNs influence the development and neurobehavior of zebrafish larvae in a concentration-dependent manner. SodSe at high concentration causes low survival rates, delayed hatching, and increased morphological defects in zebrafish larvae. In addition, exposure to SodSe resulted in elevated apoptosis in different larval tissues. Zebrafish larvae treated with SodSe and SeNs exhibited anxiety-like behaviour, increased thigmotaxis, less exploratory behaviour, and less swimming patterns. The nerve conductions and stimuli responses evaluated through acetylcholine esterase (AChE) and cortisol assays, revealed a decrease in the activity in a dose-dependent manner of SodSe and SeNs. Interestingly, the effects of SeNs were lower even at higher concentrations when compared with SodSe at lower concentrations on zebrafish embryos. This shows that SeNs synthesized through biological methods may be less toxic and may have lower effect on the development and neurobehavior of zebrafish larvae. Thus, our study confirms the cytotoxic and neurobehavioral effects of SodSe and suggests the use of SeNs at lower concentration to provide insights into better understanding of developmental stages and metabolic pathways in zebrafish larvae.

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.

Influence of Methylene Blue or Dimethyl Sulfoxide on Larval Zebrafish Development and Behavior

The use of larval zebrafish developmental testing and assessment, specifically larval zebrafish locomotor activity, has been recognized as a higher throughput testing strategy to identify developmentally toxic and neurotoxic chemicals. There are, however, no standardized protocols for this type of assay, which could result in confounding variables being overlooked. Two chemicals commonly employed during early-life stage zebrafish assays, methylene blue (antifungal agent) and dimethyl sulfoxide (DMSO, a commonly used vehicle) have been reported to affect the morphology and behavior of freshwater fish. In this study, we conducted developmental toxicity (morphology) and neurotoxicity (behavior) assessments of commonly employed concentrations for both chemicals (0.6-10.0 μM methylene blue; 0.3%-1.0% v/v DMSO). A light-dark transition behavioral testing paradigm was applied to morphologically normal, 6 days postfertilization (dpf) zebrafish larvae kept at 26°C. Additionally, an acute DMSO challenge was administered based on early-life stage zebrafish assays typically used in this research area. Results from developmental toxicity screens were similar between both chemicals with no morphological abnormalities detected at any of the concentrations tested. However, neurodevelopmental results were mixed between the two chemicals of interest. Methylene blue resulted in no behavioral changes up to the highest concentration tested, 10.0 μM. By contrast, DMSO altered larval behavior following developmental exposure at concentrations as low as 0.5% (v/v) and exhibited differential concentration-response patterns in the light and dark photoperiods. These results indicate that developmental DMSO exposure can affect larval zebrafish locomotor activity at routinely used concentrations in developmental neurotoxicity assessments, whereas methylene blue does not appear to be developmentally or neurodevelopmentally toxic to larval zebrafish at routinely used concentrations. These results also highlight the importance of understanding the influence of experimental conditions on larval zebrafish locomotor activity that may ultimately confound the interpretation of results.

Non-invasive measurement of circadian clock activity in the turquoise killifish

Non-invasive measurement of circadian rhythms is important for longitudinal assays of the rhythmic swimming behavior of the turquoise killifish (Nothobranchius furzeri). Here, we introduce a custom-built, video-based system for non-invasive circadian rhythm measurement. We describe imaging tank setup, video recording and editing, and fish movement tracking. We then detail circadian rhythm analysis. This protocol can be used for repetitive and longitudinal analysis of circadian rhythms in the same fish with minimal stress and can be applied to other fish species. For complete details on the use and execution of this protocol, please refer to Lee et al..¹.

Inconsistencies in variable reporting and methods in larval zebrafish behavioral assays

New approaches in developmental neurotoxicity (DNT) screening are needed due to the tens of thousands of chemicals requiring hazard assessments. Zebrafish (Danio rerio) are an alternative vertebrate model for DNT testing, but without a standardized protocol for larval behavioral assays, comparison of results among laboratories is challenging. To evaluate the congruence of protocols across laboratories, we conducted a literature review of DNT studies focusing on larval zebrafish behavior assays and cataloged experimental design consistencies. Our review focused on 51 unique method variables in publications where chemical exposure occurred in early development and subsequent larval locomotor evaluation focused on assays that included a light/dark photoperiod transition. We initially identified 94 publications, but only 31 exclusively met our inclusion criteria which focused on parameters that are important to an assay employed by our laboratory. No publication reported 100% of the targeted variables; only 51 to 86% of those variables were reported in the reviewed publications, with some aspects of the experimental design consistent among laboratories. However, no protocol was exactly the same for any two publications. Many of these variables had more than one parameter/design reported, highlighting the inconsistencies among methods. Overall, there is not only a strong need for the development of a standardized testing protocol for larval zebrafish locomotor assays, but there is also a need for a standardized protocol for reporting experimental variables in the literature. Here we include an extensive guideline checklist for conducting larval zebrafish developmental behavior assays.

Zebrafish (Danio rerio) shoaling in light and dark conditions involves a complex interplay between vision and lateral line

We know little about how - or even if in some species - fish shoal in darkness. We hypothesized that 'dark shoaling' occurs in zebrafish and therefore must depend upon lateral line sensory input. Shoaling in groups of five adult zebrafish was analyzed with motion tracking software. We measured average inter-individual distance, time near the arena wall (thigmotaxis zone) and total distance traveled under normal room light, and in near-complete darkness (infrared light at 850 nm). These observations were repeated in fish treated with cobalt chloride (CoCl₂), which ablates lateral line function. In untreated controls, dark shoaling was reduced compared to in light, but nonetheless still present. Elimination of lateral line sensory input by CoCl₂ treatment similarly reduced, but did not eliminate, shoaling under both light and dark. Our findings indicate that normal zebrafish shoaling in light or dark requires both visual and lateral line inputs, with neither alone sufficient for normal shoaling.

Evaluation of the developmental effects of a glyphosate-based herbicide complexed with copper, zinc, and manganese metals in zebrafish

The use of glyphosate-based herbicides (GBH) has increased dramatically, being currently the most used herbicides worldwide. Glyphosate acts as a chelating agent, capable of chelate metals. The synergistic effects of metals and agrochemicals may pose an environmental problem as they have been shown to induce neurological abnormalities and behavioural changes in aquatic species. However, as their ecotoxicity effects are poorly understood, evaluating the impacts of GBH complexed with metals is an ecological priority. The main objective of the study was to evaluate the potentially toxic effects caused by exposure to a GBH (1 μg a.i. mL^-1), alone or complexed with metals (Copper, Manganese, and Zinc (100 μg L^-1)), at environmentally relevant concentrations, during the early period of zebrafish (Danio rerio) embryo development (96 h post-fertilization), a promising model for in vivo developmental studies. To clarify the mechanisms of toxicity involved, lethal and sublethal development endpoints were assessed. At the end of the exposure, biochemical and cell death parameters were evaluated and, 24 h later, different behavioural responses were assessed. The results showed that metals induced higher levels of toxicity. Copper caused high mortality, low hatching, malformations, and changes in biochemical parameters, such as decreased Catalase (CAT) activity, increased Glutathione Peroxidase (GPx), Glutathione S-Transferase (GST), reduced Glutathione (GSH) and decreased Acetylcholinesterase (AChE) activity, also inducing apoptosis and changes in larval behaviour. Manganese increased the activity of SODs enzymes. Zinc increased mortality, reactive oxygen species (ROS) levels, superoxide dismutase activity (SODs) and caused a decrease in AChE activity. Embryos/larvae exposed to the combination of GBH/Metal also showed teratogenic effects during their development but in smaller proportions than the metal alone. Although more studies are needed, the results suggest that GBH may interfere with the mechanisms of metal toxicity at the biochemical, physiological, and behavioural levels of zebrafish.

Disruptions of circadian rhythms, sleep, and stress responses in zebrafish: New infrared-based activity monitoring assays for toxicity assessment

Behavioural disruptions are sensitive indicators of alterations to normal animal physiology and can be used for toxicity assessment. The small vertebrate zebrafish is a leading model organism for toxicological studies. The ability to continuously monitor the toxicity of drugs, pollutants, or environmental changes over several days in zebrafish can have high practical application. Although video-recordings can be used to monitor short-term zebrafish behaviour, it is challenging to videorecord prolonged experiments (e.g. circadian behaviour over several days) because of the darkness periods (nights) and the heavy data storage and image processing requirements. Alternatively, infrared-based activity monitors, widely used in invertebrate models such as drosophila, generate simple and low-storage data and could optimize large-scale prolonged behavioural experiments in zebrafish, thus favouring the implementation of high-throughput testing strategies. Here, we validate the use of a Locomotor Activity Monitor (LAM) to study the behaviour of zebrafish larvae, and we characterize the behavioural phenotypes induced by abnormal light conditions and by the Parkinsonian toxin MPP⁺. When zebrafish were deprived from daily light-cycle synchronization, the LAM detected various circadian disruptions, such as increased activity period, phase shifts, and decreased inter-daily stability. Zebrafish exposed to MPP⁺ (10, 100, 500 μM) showed a concentration-dependent decrease in activity, sleep disruptions, impaired habituation to repetitive startles (visual-motor responses), and a slower recovery to normal activity after the startle-associated stress. These phenotypes evidence the feasibility of using infrared-based LAM to assess multi-parameter behavioural disruptions in zebrafish. The procedures in this study have wide applicability and may yield standard methods for toxicity testing.

Behavioural impairment and oxidative stress by acute exposure of zebrafish to a commercial formulation of tebuconazole

Tebuconazole is a systemic follicular fungicide known to cause diverse problems in non-target organisms namely associated to the pure active ingredient. As such, the objective of this work was to evaluate developmental changes induced by a tebuconazole commercial formulation to a non-target animal model. Zebrafish embryos at ± 2 h post-fertilization were exposed to tebuconazole wettable powder concentrations (0.05, 0.5 and 5 mg L^-1) for 96 h with developmental toxicity assessed throughout the exposure period and biochemical parameters evaluated at the end of the exposure. Behavioural assessment (spatial exploration and response to stimuli) was conducted 24 h after the end of the exposure. While no developmental and physiological alterations were observed, exposure to tebuconazole resulted in an increased generation of reactive oxidative species at the 0.05 and 0.5 mg L^-1 concentrations and a decreased GPx activity at the 0.5 mg L^-1 concentration suggesting a potential protection mechanism. There was also a change in the avoidance-escape behaviour supporting an anxiolytic effect suggesting possible alterations in the central nervous system development demanding further studies.

24-Epibrassinolide modulates the neurodevelopmental outcomes of high caffeine exposure in zebrafish (Danio rerio) embryos

Previous embryonic fish data have shown caffeine to induce potential teratogenic and long-term neurodevelopmental outcomes through oxidative stress-mediated apoptosis. In this context, antioxidants may have the potential to counteract the caffeine-induced effects. Therefore, the present study aimed to investigate the potential protective role of 24-epibrassinolide (24-EPI), a natural brassinosteroid with proven antioxidant properties, against caffeine-induced teratogenic effects during early zebrafish development. Embryos (~2 h post-fertilization - hpf) were exposed to 0.5 mM caffeine, co-exposed to 24-EPI (0.01, 0.1 and 1 μM) and to 24-EPI alone (1 μM) for 96 h. During exposure, lethal and sublethal developmental parameters were evaluated. At the end of the exposure, biochemical evaluations were made, and 24 h after, different behavioural paradigms were assessed. An increased number of animals showing oedema and malformations were observed after caffeine exposure, while these were reduced after co-exposure to 24-EPI concentration, namely the tail curvature. The results showed oxidative stress and related parameters similar among treatments. Yet, caffeine exposure resulted in locomotor deficits (decreased speed and distance) and disrupted anxiety-like and avoidance responses. The co-exposure to caffeine and to the highest 24-EPI concentrations resulted in less pronounced behavioural deficits. Overall, there was an absence of effects in the embryo/larvae exposed solely to 24-EPI, while caffeine caused developmental and neurotoxic effects. Although further studies are needed, the results showed promising protective effects of the highest 24-EPI concentration tested against the toxicity induced by caffeine in zebrafish.

Microplastics alone or co-exposed with copper induce neurotoxicity and behavioral alterations on zebrafish larvae after a subchronic exposure

Microplastics (MPs, <5 mm) have been frequently detected in aquatic ecosystems, representing both health and ecological concerns. However data about the combined effects of MPs and other contaminants is still limited. This study aimed to evaluate the impact of MPs and the heavy metal copper (Cu) on zebrafish (Danio rerio) larvae development and behavior. Zebrafish embryos were subchronically exposed to MPs (2 mg/L), two sub-lethal concentrations of Cu (60 and 125 µg/L) and binary mixtures of MPs and Cu using the same concentrations, from 2-h post fertilization until 14 days post fertilization. Lethal and sub-lethal responses (mortality, hatching, body length) were evaluated during the embryogenesis period, and locomotor, avoidance, anxiety and shoaling behaviors, and acetylcholinesterase (AChE) activity were measured at 14 dpf. The results showed that survival of larvae was reduced in groups exposed to MPs, Cu and Cu+MPs. Regarding the behavioral patterns, the higher Cu concentration and mixtures decreased significantly the mean speed, the total distance traveled and the absolute turn angle, demonstrating an adverse effect on swimming competence of zebrafish larvae. Exposure to MPs and Cu, alone or combined, also affected avoidance behavior of zebrafish, with larvae not reacting to the aversive stimulus. There was a significant inhibition of AChE activity in larvae exposed to all experimental groups, compared to the control group. Moreover, a higher inhibition of AChE was noticed in larvae exposed to MPs and both Cu+MPs groups, comparatively to the Cu alone groups. Our findings demonstrate the adverse effects of MPs, alone or co-exposed with Cu, on fish early life stages behavior. This study highlights that MPs and heavy metals may have significant impacts on fish population fitness by disrupting locomotor and avoidance behaviors.

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.

Exposure to Artificial Light at Night and the Consequences for Flora, Fauna, and Ecosystems

The present review draws together wide-ranging studies performed over the last decades that catalogue the effects of artificial-light-at-night (ALAN) upon living species and their environment. We provide an overview of the tremendous variety of light-detection strategies which have evolved in living organisms - unicellular, plants and animals, covering chloroplasts (plants), and the plethora of ocular and extra-ocular organs (animals). We describe the visual pigments which permit photo-detection, paying attention to their spectral characteristics, which extend from the ultraviolet into infrared. We discuss how organisms use light information in a way crucial for their development, growth and survival: phototropism, phototaxis, photoperiodism, and synchronization of circadian clocks. These aspects are treated in depth, as their perturbation underlies much of the disruptive effects of ALAN. The review goes into detail on circadian networks in living organisms, since these fundamental features are of critical importance in regulating the interface between environment and body. Especially, hormonal synthesis and secretion are often under circadian and circannual control, hence perturbation of the clock will lead to hormonal imbalance. The review addresses how the ubiquitous introduction of light-emitting diode technology may exacerbate, or in some cases reduce, the generalized ever-increasing light pollution. Numerous examples are given of how widespread exposure to ALAN is perturbing many aspects of plant and animal behaviour and survival: foraging, orientation, migration, seasonal reproduction, colonization and more. We examine the potential problems at the level of individual species and populations and extend the debate to the consequences for ecosystems. We stress, through a few examples, the synergistic harmful effects resulting from the impacts of ALAN combined with other anthropogenic pressures, which often impact the neuroendocrine loops in vertebrates. The article concludes by debating how these anthropogenic changes could be mitigated by more reasonable use of available technology - for example by restricting illumination to more essential areas and hours, directing lighting to avoid wasteful radiation and selecting spectral emissions, to reduce impact on circadian clocks. We end by discussing how society should take into account the potentially major consequences that ALAN has on the natural world and the repercussions for ongoing human health and welfare.

Aquatic Models: Water Quality and Stability and Other Environmental Factors

The use of aquatic animals in ecotoxicology, genetic, and biomedical research has grown immensely in recent years, especially due to the increased use of zebrafish in the laboratory setting. Because water is the primary environment of most aquatic species, the composition and management of this water is paramount to ensuring their health and welfare. In this publication, we will describe the important variables in water quality that can influence animal health and research results, using the zebrafish model for detailed specifics of optimal conditions. Wherever possible, recommendations are provided to reduce the potential impact of poor or highly variable water quality, and standards are given which can be used as institutional goals to maximize animal health and welfare and reduce research variability. It is increasingly important that authors of publications describing work done using aquatic models characterize water quality and other environmental conditions of the animal environment so that the work can be repeated and understood in context of these important factors. It is clear that there are a great many extrinsic factors which may influence research outcomes in the aquatics model laboratory setting, and consequently, an increased level of funding will be essential to support continued research exploring these and other important husbandry conditions. References from a large body of literature on this subject are provided.

Learning steers the ontogeny of an efficient hunting sequence in zebrafish larvae

Goal-directed behaviors may be poorly coordinated in young animals but, with age and experience, behavior progressively adapts to efficiently exploit the animal’s ecological niche. How experience impinges on the developing neural circuits of behavior is an open question. We have conducted a detailed study of the effects of experience on the ontogeny of hunting behavior in larval zebrafish. We report that larvae with prior experience of live prey consume considerably more prey than naive larvae. This is mainly due to increased capture success and a modest increase in hunt rate. We demonstrate that the initial turn to prey and the final capture manoeuvre of the hunting sequence were jointly modified by experience and that modification of these components predicted capture success. Our findings establish an ethologically relevant paradigm in zebrafish for studying how the brain is shaped by experience to drive the ontogeny of efficient behavior.

Behavioural toxicity of environmental relevant concentrations of a glyphosate commercial formulation - RoundUp® UltraMax - During zebrafish embryogenesis

The use of herbicides with glyphosate as an active ingredient (a.i.) has increased dramatically in recent years, with its residues often being found in either soil or water. Nevertheless, concerns have arisen about its harmful side effects for both ecosystems and wildlife health. Therefore, the objective of this work was to assess the effects of a commercial formulation of glyphosate (RoundUp® UltraMax), at environmentally relevant concentrations on zebrafish embryos through a set of behavioural patterns. Zebrafish embryos were exposed to 0, 1, 2 and 5 μg a.i. mL-1 concentrations of the glyphosate formulation for 72 h (from 2.5 to 75 h post-fertilization (hpf)). After exposure, larvae were washed and allowed to develop until 144 hpf. At this point, the larvae behaviour was evaluated using a battery of tests to assess the general exploratory motility, escape-like responses, anxiety-related behaviours and social interactions. In addition, cortisol levels were assessed. No significant changes were observed relative to the exploratory behaviour in the standard open field. The anxiety-related behaviours were similar among groups, and no social interference was observed following exposure to these glyphosate concentrations. Likewise, cortisol levels remained similar among treatments. Still, the larvae exposed to 5 μg a.i. mL-1 did not react to the presence of an aversive stimulus, supporting glyphosate-induced changes in the sensory-motor coordination during development. In general, these results indicate a possible neurotoxic effect of this glyphosate-based formulation that should be further evaluated. In addition, the results obtained could impose a risk for wildlife sensitive species that should not be neglected.

24-Epibrassinolide protects against ethanol-induced behavioural teratogenesis in zebrafish embryo

Embryonic studies have demonstrated the neurotoxic, teratogenic, and neurobehavioral toxicity of ethanol (EtOH). Although multiple mechanisms may contribute to these effects, oxidative stress has been described as the major damage pathway. In this regard, natural antioxidants have the potential to counteract oxidative stress-induced cellular damage. Therefore, the present study aimed to investigate the potential protective role of 24-epibrassinolide (24-EPI), a natural brassinosteroid with proved antioxidant properties, in EtOH-induced teratogenic effects during early zebrafish development. Embryos (~2 h post-fertilization - hpf) were exposed to 1 % EtOH, co-exposed to 24-EPI (0.01, 0.1 and 1 μM) and to 24-EPI alone (1 μM) for 24 h. Following exposure, biochemical evaluations were made at 26 hpf, developmental analysis was made throughout the embryo-larval period, and behavioural responses were evaluated at 120 hpf. Exposure to 1 % EtOH caused an increase in the number of malformations, which were diminished by 24-EPI. In addition, EtOH induced an accumulation of GSSG and consequent reduction of GSH:GSSG ratio, indicating the involvement of oxidative mechanisms in the EtOH-induced effects. These were reverted by 24-EPI as proved by the GSSG levels and GSH:GSSG ratio that returned to control values. Furthermore, exposure to EtOH resulted in behavioural deficits at 120 hpf as observed by the disrupted response to an aversive stimulus, suggesting the involvement of neurotoxic mechanisms. 24-EPI restored the behavioural deficits observed in a dose-dependent manner. The absence of effects in the embryos exposed solely to 24-EPI showed its safety during the exposure period. In conclusion, EtOH caused developmental teratogenicity and behavioural toxicity by inducing glutathione changes, which were prevented by 24-EPI.

Phenotypic chemical and mutant screening of zebrafish larvae using an on-demand response to electric stimulation

Behavioral responses of zebrafish larvae to environmental cues are important functional readouts that should be evoked on-demand and studied phenotypically in behavioral, genetical and developmental investigations. Very recently, it was shown that zebrafish larvae execute a voluntary and oriented movement toward the positive electrode of an electric field along a microchannel. Phenotypic characterization of this response was not feasible due to larva's rapid movement along the channel. To overcome this challenge, a microfluidic device was introduced to partially immobilize the larva's head while leaving its mid-body and tail unrestrained in a chamber to image motor behaviors in response to electric stimulation, hence achieving quantitative phenotyping of the electrically evoked movement in zebrafish larvae. The effect of electric current on the tail-beat frequency and response duration of 5-7 days postfertilization zebrafish larvae was studied. Investigations were also performed on zebrafish exposed to neurotoxin 6-hydroxydopamine and larvae carrying a pannexin1a (panx1a) gene knockout, as a proof of principle applications to demonstrate on-demand movement behavior screening in chemical and mutant assays. We demonstrated for the first time that 6-hydroxydopamine leads to electric response impairment, levodopa treatment rescues the response and panx1a is involved in the electrically evoked movement of zebrafish larvae. We envision that our technique is broadly applicable as a screening tool to quantitatively examine zebrafish larvae's movements in response to physical and chemical stimulations in investigations of Parkinson's and other neurodegenerative diseases, and as a tool to combine recent advances in genome engineering of model organisms to uncover the biology of electric response.

Toxic effects of a mancozeb-containing commercial formulation at environmental relevant concentrations on zebrafish embryonic development

The toxicological knowledge of mancozeb (MZ)-containing commercial formulations on non-target species is scarce and limited. Therefore, the objective of this work was to represent a realistic application scenario by evaluating the toxicity of environmental relevant and higher concentrations of a commercial formulation of MZ using zebrafish embryos. Following determination of the 96-h LC₅₀ value, the embryos at the blastula stage (~ 2 h post-fertilisation, hpf) were exposed to 0.5, 5, and 50 μg L^-1 of the active ingredient (~ 40× lower than the 96-h LC₅₀). During the exposure period (96 h), lethal, sublethal, and teratogenic parameters, as well as behaviour analysis, at 120 hpf, were assayed. Biochemical parameters such as oxidative stress-linked enzymes (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and glutathione reductase (GR)), reactive oxygen species (ROS) levels, and glutathione levels (GSH and GSSG), as well as the activity of degradation (glutathione S-transferase (GST) and carboxylesterase (CarE)), neurotransmission (acetylcholinesterase (AChE)), and anaerobic respiration (lactate dehydrogenase (LDH))-related enzymes, were analysed at the end of the exposure period. Exposed embryos showed a marked decrease in the hatching rate and many malformations (cardiac and yolk sac oedema and spinal torsions), with a higher prevalence at the highest concentration. A dose-dependent decreased locomotor activity and a response to an aversive stimulus, as well as a light-dark transition decline, were observed at environmental relevant concentrations. Furthermore, the activities of SOD and GR increased while the activity of GST, AChE, and MDA contents decreased. Taken together, the involvement of mancozeb metabolites and the generation of ROS are suggested as responsible for the developmental phenotypes. While further studies are needed to fully support the hypothesis presented, the potential cumulative effects of mancozeb-containing formulations and its metabolites could represent an environmental risk which should not be disregarded.