Copper at low levels impairs memory of adult zebrafish (Danio rerio) and affects swimming performance of larvae

Molecular variations to the proteome of zebrafish larvae induced by environmentally relevant copper concentrations

Contaminants are increasingly accumulating in aquatic environments and biota, with potential adverse effects on individual organisms, communities and ecosystems. However, studies that explore the molecular changes in fish caused by environmentally relevant concentrations of metals, such as copper (Cu), are limited. This study uses embryos of the model organism zebrafish (Danio rerio) to investigate effect of Cu on the proteome and amino acid (AA) composition of fish. Wild-type embryos at 24 h post-fertilisation were exposed to Cu (2 µg L-1 to 120 µg L-1) for 96 h and the number of healthy larvae were determined based on larvae that had hatched and did not display loss of equilibrium (LOE). The effect concentrations where Cu caused a 10 % (EC10) or 50 % (EC50) decrease in the number of healthy larvae were calculated as 3.7 µg L-1 and 10.9 µg L-1, respectively. Proteomics analysis of embryos exposed to the EC10 and EC50 concentrations of Cu revealed the proteome to differ more strongly after 48 h than 96 h, suggesting the acclimatisation of some larvae. Exposure to excess Cu caused differentially expressed proteins (DEPs) involved in oxidative stress, mitochondrial respiration, and neural transduction as well as the modulation of the AAs (Proline, Glycine and Alanine). This is the first study to suggest that LOE displayed by Cu-stressed fish may involve the disruption to GABAergic proteins and the calcium-dependent inhibitory neurotransmitter GABA. Moreover, this study highlights that proteomics and AA analysis can be used to identify potential biomarkers for environmental monitoring.

Assessing the disparity: comparative toxicity of Copper in zebrafish larvae exposes alarming consequences of permissible concentrations in Brazil

Copper (Cu) is a naturally occurring metal with essential micronutrient properties. However, this metal might also pose increased adverse environmental and health risks due to industrial and agricultural activities. In Brazil, the maximum allowable concentration of Cu in drinking water is 2 mg/L. Despite this standard, the impact of such concentrations on aquatic organisms remains unexplored. This study aimed to evaluate the toxicity of CuSO4 using larval zebrafish at environmentally relevant concentrations. Zebrafish (Danio rerio) larvae at 72 hr post-fertilization (hpf) were exposed to nominal CuSO4 concentrations ranging from 0.16 to 48 mg/L to determine the median lethal concentration (LC50), established at 8.4 mg/L. Subsequently, non-lethal concentrations of 0.16, 0.32, or 1.6 mg/L were selected for assessing CuSO4 -induced toxicity. Morphological parameters, including body length, yolk sac area, and swim bladder area, were adversely affected by CuSO4 exposure, particularly at 1.6 mg/L (3.31 mm ±0.1, 0.192 mm2 ±0.01, and 0.01 mm2 ±0.05, respectively). In contrast, the control group exhibited values of 3.62 mm ±0.09, 0.136 mm2 ±0.013, and 0.3 mm2 ±0.06, respectively. Behavioral assays demonstrated impairments in escape response and swimming capacity, accompanied by increased levels of reactive oxygen species (ROS) and lipid peroxidation. In addition, decreased levels of non-protein thiols and reduced cellular viability were noted. Data demonstrated that exposure to CuSO4 at similar concentrations as those permitted in Brazil for Cu adversely altered morphological, biochemical, and behavioral endpoints in zebrafish larvae. This study suggests that the permissible Cu concentrations in Brazil need to be reevaluated, given the potential enhanced adverse health risks of exposure to environmental metal contamination.

Zebrafish as model organisms for toxicological evaluations in the field of food science

Food safety has long been an area of concern. The selection of stable and efficient model organisms is particularly important for food toxicology studies. Zebrafish (Danio rerio) are small model vertebrates, and 70% of human genes have at least one zebrafish ortholog. Zebrafish have advantages as model organisms due to their short life cycle, strong reproductive ability, easy rearing, and low cost. Zebrafish embryos have the advantage of being sensitive to the breeding environment and thus have been used as biosensors. Zebrafish and their embryos have been widely used for food toxicology assessments. This review provides a systematic and comprehensive summary of food toxicology studies using zebrafish as model organisms. First, we briefly introduce the multidimensional mechanisms and structure-activity relationship studies of food toxicological assessment. Second, we categorize these studies according to eight types of hazards in foods, including mycotoxins, pesticides, antibiotics, heavy metals, endocrine disruptors, food additives, nanoparticles, and other food-related ingredients. Finally, we list the applications of zebrafish in food toxicology studies in line with future research prospects, aiming to provide a valuable reference for researchers in the field of food science.

Additive effects of microplastics on accumulation and toxicity of cadmium in male zebrafish

Microplastics (MPs) have emerged as contaminants of concern because of their ubiquitous presence in almost all aquatic environments. The ecological effects of MPs are complex and depend on multiple factors including their age, size and the ecological matrix. There is an urgent need for multifactorial studies to elucidate their impacts. We measured the effects of virgin and naturally aged MPs, alone, pretreated with cadmium (Cd), or in combination with ionic Cd, on the bioaccumulation of Cd, metallothionein expression, behavior, and histopathology of adult zebrafish (Danio rerio). Zebrafish were exposed to virgin or aged polyethylene MPs (0.1% MPs enriched diets, w/w) or waterborne Cd (50 μg/L) or a combination of the two for 21 days. There was an additive interaction between water-borne Cd and MPs on bioaccumulation in males but not in females, Cd accumulation increased by twofold when water-borne Cd and MPs were combined. Water-borne Cd significantly induced higher levels of metallothionein compared to MPs pre-exposed to Cd. However, Cd-treated MPs caused greater damage to the intestine and liver compared to untreated MPs suggesting that bound Cd could be released or modulate MPs toxicity. We also showed that co-exposure to water-borne Cd and MPs increased anxiety in the zebrafish, compared with water-borne Cd alone, suggesting using microplastics as a vector may increase toxicity. This study demonstrates that MPs can enhance the toxicity of Cd, but further study is needed to elucidate the mechanism.

Brain metabolite profiles provide insight into mechanisms for behavior sexual dimorphisms in zebrafish (Danio rerio)

The zebrafish (Danio rerio) has historically been a useful model for research in genetics, ecology, biology, toxicology, and neurobehavior. Zebrafish have been demonstrated to have brain sexual dimorphism. However, the sexual dimorphism of zebrafish behavior demands our attention, particularly. To evaluate the behavior and brain sexual dimorphisms in zebrafish, this study assessed sex differences in adult D. rerio in four behavioral domains, including aggression, fear, anxiety, and shoaling, and further compared with metabolites in the brain tissue of females and males. Our findings showed that aggression, fear, anxiety and shoaling behaviors were significantly sexually dimorphic. Interestingly, we also show through a novel data analysis method, that the female zebrafish exhibited significantly increased shoaling behavior when shoaled with male zebrafish groups and, for the first time, we offer evidence that male shoals are beneficial in dramatically alleviating anxiety in zebrafish. In addition, there were significant changes in metabolites in zebrafish brain tissue between the sexes. Furthermore, zebrafish behavioral sexual dimorphism may be associated with brain sexual dimorphism, with significant differences in brain metabolites. Therefore, to prevent the influence or even bias of behavioral sex differences on results, it is suggested that behavioral studies or behavioral-based other relevant investigations consider sexual dimorphism of behavior and brain.

Aluminium exposure leads to neurodegeneration and alters the expression of marker genes involved to parkinsonism in zebrafish brain

Aluminium, despite being extremely widespread in the world, is a non-essential metal to human metabolism. This metal is known to have toxic effects on a variety of organs including the brain and is considered an etiological factor in neurodegenerative diseases. However, the molecular mechanisms by which aluminium exerts neurotoxic effects are not yet completely understood. Zebrafish is an animal model also used to study neurodegenerative diseases since the overall anatomical organization of the central nervous system is relatively conserved and similar to mammals. Adult zebrafish were exposed to 11 mg/L of Al for 10, 15, and 20 days and the neurotoxic effects of aluminium were analysed by histological, biochemical, and molecular evaluations. Histological stainings allowed to evaluation of the morphology of the brain parenchyma, the alteration of myelin and the activation of neurodegenerative processes. The expression of the Glial Fibrillary Acidic Protein, a marker of glial cells, was evaluated to observe the quantitative alteration of this important protein for the nervous system. In addition, the poly(ADP-ribose) polymerase activity was measured to verify a possible oxidative DNA damage caused by exposure to this metal. Finally, the evaluation of the markers involved in Parkinsonism was assessed by Real-Time PCR to better understand the role of aluminium in the regulation of genes related to Parkinson's neurodegenerative disease. Data showed that aluminium significantly affected the histology of cerebral tissue especially in the first periods of exposure, 10 and 15 days. This trend was also followed by the expression of GFAP. At longer exposure times, there was an improvement/stabilization of the overall neurological conditions and decrease in PARP activity. In addition, aluminium is involved in the deregulation of the expression of genes closely related to Parkinsonism. Overall, the data confirm the neurotoxicity induced by aluminium and shed a light on its involvement in neurodegenerative processes.

Zebrafish: A Pharmacological Model for Learning and Memory Research

Learning and memory are essential to organism survival and are conserved across various species, especially vertebrates. Cognitive studies involving learning and memory require using appropriate model organisms to translate relevant findings to humans. Zebrafish are becoming increasingly popular as one of the animal models for neurodegenerative diseases due to their low maintenance cost, prolific nature and amenability to genetic manipulation. More importantly, zebrafish exhibit a repertoire of neurobehaviors comparable to humans. In this review, we discuss the forms of learning and memory abilities in zebrafish and the tests used to evaluate the neurobehaviors in this species. In addition, the pharmacological studies that used zebrafish as models to screen for the effects of neuroprotective and neurotoxic compounds on cognitive performance will be summarized here. Lastly, we discuss the challenges and perspectives in establishing zebrafish as a robust model for cognitive research involving learning and memory. Zebrafish are becoming an indispensable model in learning and memory research for screening neuroprotective agents against cognitive impairment.

Histological alterations and oxidative stress in adult zebrafish muscle after aluminium exposure

Aluminium (Al) is among the most abundant metals in nature, and its presence in the environment is further increasing by anthropogenic activities. In water bodies, the Al concentrations ranged between 0.001 and 50 mg/L, raising concerns about the health of aquatic organisms. For this reason, zebrafish was chosen as the model, since it is well suited for ecotoxicological studies. Adult specimens were exposed to 11 mg/L of Al for 10, 15 and 20 days to assess both the morphology and the oxidative state of muscle tissue. Considering the involvement of ROS, the activity of the main antioxidant enzymes, metallothioneins contents, but also oxidative damage and enzymes involved in energy consumption and neuromuscular transmission were assessed. Collected data showed an increase in the thickness of the endomysium and resorbed myofibrils in the organisms exposed to Al for 10 days, and an increase of myotomes' size in the organisms exposed to Al for 15 days. Moreover, the organisms exposed for less time to Al, it was evident an activation of anaerobic metabolism and the increased activity of antioxidant enzymes such as superoxide dismutase, glutathione peroxidase and glutathione S-transferases. However, these effects stabilized with increasing exposure time. In addition, only after 20 days of treatment did the oxidative damage to the proteins and the activity of acetylcholinesterase increase while the levels of metallothioneins and the lipid peroxidation were lower for all treated animals when compared to the control group. Overall, the biochemical and histological changes induced by aluminium exposure in the muscular tissue represent a relevant contribution to understanding the environmental risk due to the diffusion of this metal within the aquatic compartment.

Copper II oxide nanoparticles (CuONPs) alter metabolic markers and swimming activity in zebra-fish (Danio rerio)

The present study aimed to compare the metabolic effects caused by using copper oxide nanoparticles with two distinct morphologies nanorods and nanosphere. The CuONPs in the form of nanorods were characterized in the order of 500 nm, on a scale of 20, 100, and 500 nm. Meanwhile, the nanosphere CuONPs were characterized in the order of 5 nm, on a 30 nm scale. The analysis of metabolic rate was performed using the closed respirometry technique, specific ammonia excretion, and swimming ability as biomarkers, the physiological effects on Danio rerio were investigated. For the experiments, 88 fish were used, exposed for 24 h at concentrations of 0, 50, 100, and 200 μg/L of copper oxide nanoparticles in the form of nanospheres and nanorods, respectively. The tests carried out with the nanorods demonstrated metabolic alterations in fish, with an increase of 294% and 321% in the metabolic rate at concentrations of 100 μg/L and 200 μg/L, respectively. Furthermore, there was a decrease in specific ammonia excretion by 34% and 83% and in swimming capacity by 34% and 55% at concentrations of 100 and 200 μg/L, respectively, when compared to the control. The tests performed with nanospheres did not show significant changes compared to the control. These experiments showed that different morphological structures of the same copper oxide nanoparticle caused different effects on fish metabolism. It is concluded that the characterization of nanoparticles is essential to understand their effects on fish, since their structural forms can cause different toxic effects on D. rerio.

Microplastics- and copper-induced changes in neurogenesis and DNA methyltransferases in the early life stages of zebrafish

In this study, zebrafish embryos were exposed to microplastics (MPs, 2 mg/L) and copper (Cu, 60 and 125 μg/L), alone or combined, for 14 days, and the development of motor neurons was assessed through gene expression and immunohistochemistry. DNA methyltransferases (DNMTs) genes expression was also evaluated. The results showed a downregulation of neuronal proliferation (sox2, pcna), neurogenesis (neuroD, olig2), and motor neurons development (islet) related genes, implying potential deficits in the neurogenesis of the exposed zebrafish early life stages. Downregulation of the maintenance and de novo DNMTs expression was also found, indicating that the DNA methylation patterns could be modulated by MPs and Cu. A high relative volume of proliferating cell nuclear antigen (PCNA)-positive cells was found in the fish retina from the MPs exposed group, suggesting that MPs increased the rate of cellular division. In contrast, a significant decrease of PCNA-positive cells, and therefore a lower cell proliferation, was found in the retina and brain of zebrafish exposed to Cu and Cu + MPs, which could lead to cognitive and behavioral functions impairment. No alterations were found in the relative volume of ISL1&2-positive cells. This study contributes to the knowledge of the mechanisms by which MPs and Cu cause neurotoxicity, fundamental for a comprehensive and realistic ecological risk assessment in aquatic populations.

Response of Olfactory Sensory Neurons to Mercury Ions in Zebrafish: An Immunohistochemical Study

Olfactory sensory neurons (OSNs) of fish belong to three main types: ciliated olfactory sensory neurons (cOSNs), microvillous olfactory sensory neurons (mOSNs), and crypt cells. Mercury is a toxic metal harmful for olfaction. We exposed the olfactory epithelium of zebrafish to three sublethal Hg2+ concentrations. Molecular markers specific for the different types of OSNs were immunohistochemically detected. Image analysis of treated sections enabled counting of marked cells and measurement of staining optical density indicative of the response of OSNs to Hg2+ exposure. The three types of OSNs reacted to mercury in a different way. Image analysis revealed that mOSNs are more susceptible to Hg2+ exposure than cOSNs and crypt cell density decreases. Moreover, while the ratio between sensory/nonsensory epithelium areas is unchanged, epithelium thickness drops, and dividing cells increase in the basal layer of the olfactory epithelium. Cell death but also reduction of apical processes and marker expression could account for changes in OSN immunostaining. Also, the differential results between dorsal and ventral halves of the olfactory rosette could derive from different water flows inside the olfactory chamber or different subpopulations in OSNs.

Zebrafish and Medaka: Important Animal Models for Human Neurodegenerative Diseases

Animal models of human neurodegenerative disease have been investigated for several decades. In recent years, zebrafish (Danio rerio) and medaka (Oryzias latipes) have become popular in pathogenic and therapeutic studies about human neurodegenerative diseases due to their small size, the optical clarity of embryos, their fast development, and their suitability to large-scale therapeutic screening. Following the emergence of a new generation of molecular biological technologies such as reverse and forward genetics, morpholino, transgenesis, and gene knockout, many human neurodegenerative disease models, such as Parkinson's, Huntington's, and Alzheimer's, were constructed in zebrafish and medaka. These studies proved that zebrafish and medaka genes are functionally conserved in relation to their human homologues, so they exhibit similar neurodegenerative phenotypes to human beings. Therefore, fish are a suitable model for the investigation of pathologic mechanisms of neurodegenerative diseases and for the large-scale screening of drugs for potential therapy. In this review, we summarize the studies in modelling human neurodegenerative diseases in zebrafish and medaka in recent years.

A Systematic Analysis of Metal and Metalloid Concentrations in Eight Zebrafish Recirculating Water Systems

Metals and metalloids are integral to biological processes and play key roles in physiology and metabolism. Nonetheless, overexposure to some metals or lack of others can lead to serious health consequences. In this study, eight zebrafish facilities collaborated to generate a multielement analysis of their centralized recirculating water systems. We report a first set of average concentrations for 46 elements detected in zebrafish facilities. Our results help to establish an initial baseline for trouble-shooting purposes, and in general for safe ranges of metal concentrations in recirculating water systems, supporting reproducible scientific research outcomes with zebrafish.

Swimming in the maze: An overview of maze apparatuses and protocols to assess zebrafish behavior

Most preclinical behavioral assays use rodents as model animals, leaving room for species-specific biases that could be avoided by an expanded cross-species approach. In this context, zebrafish emerges as an alternative model organism to study neurobiological mechanisms of anxiety, preference, learning, and memory, as well as other phenotypes with relevance to neuropsychiatric disorders. In recent years, several zebrafish studies using different types of mazes have been published. However, the protocols and apparatuses' shapes and dimensions vary widely in the literature. This variation may puzzle researchers attempting to implement maze behavioral assays and challenges the reproducibility across institutions. This review aims to provide an overview of the behavioral paradigms assessed in different types of mazes in zebrafish reported in the last couple of decades. Also, this review aims to contribute to a better characterization of multi-behavioral assessment 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.

Zebrafish as a Promising Tool for Modeling Neurotoxin-Induced Alzheimer's Disease

Drug discovery and development for Alzheimer's disease (AD) are complex and challenging due to the higher failure rate in the drug development process. The overproduction and deposition of Aβ senile plaque and intracellular neurofibrillary tangle (NFT) formation are well-recognized diagnostic hallmarks of AD. Numerous transgenic models of Alzheimer's disease have restrictions on cost-effectiveness and time in the preclinical setup. Zebrafish has emerged as an excellent complementary model for neurodegenerative research due to simpler organisms with robust, clearly visible behavior forms. Glutaminergic and cholinergic pathways responsible for learning and memory are present in zebrafish and actively participate in the transmission process. Therefore, it is imperative to study neurotoxic agents' mechanisms that induce dysfunction of memory, learning, and neurons in the zebrafish. This review illustrates the in-depth molecular mechanism of several neurotoxic agents such as okadaic acid, cigarette smoke extract, and metals to produce cognitive deficits or neurodegeneration similar to mammals. These updates would determine an ideal and effective neurotoxic agent for producing AD pathophysiology in the zebrafish brain for preclinical screening.

Water and suspended sediment runoff from vineyard watersheds affecting the behavior and physiology of zebrafish

Viticulture plays an important role in generating income for small farms globally. Historically, vineyards use large quantities of phytosanitary products, such as Bordeaux mixture [Ca(OH)₂ + CuSO₄], to control plant diseases. These products result in the accumulation of copper (Cu) in the soil and increases the risk of transfer to water bodies. Thus, it is important to evaluate whether the presence of Cu-bearing particles in water is toxic to aquatic fauna. This study conducted chemical, mineralogical, and particle size evaluations on water samples and sediments collected from a watershed predominantly cultivated with old vineyards. The proportion of Cu-rich nanoparticles (<10 nm) in the sediment was ~27%. We exposed zebrafish to different dilutions of water and sediment samples that collected directly from the study site (downstream river) under laboratory conditions. Then, we evaluated their exploratory behavior and the stress-related endocrine parameter, whole-body cortisol. We also carried out two experiments in which zebrafish were exposed to Cu. First, we determined the median lethal concentration (LC_{50-96 h}) of Cu and then assessed whether Cu exposure results in effects similar to those associated with exposure to the water and sediment samples collected from the study site. The water and sediment samples directly impacted the exploratory behavior of zebrafish, showing clear anxiety-like behavioral phenotype and stress in terms of cortisol increase (during the second rain event). The Cu exposure did not mimic the same behavioral changes triggered by the water and sediment samples, although it had caused similar stress in the fish. Our results highlight that even at low concentrations, the water and sediment samples from vineyard watershed runoff were able to induce behavioral and endocrine changes that may harm the ecological balance of an aquatic environment.

Toxicological effects induced on early life stages of zebrafish (Danio rerio) after an acute exposure to microplastics alone or co-exposed with copper

Data about the toxicological interactions of MPs and heavy metals in biota is limited, particularly in fish early life stages. This study aimed to evaluate the toxicological effects of MPs and copper (Cu), alone or combined, in zebrafish early life stages. Embryos were exposed from 2 until 96-h post-fertilization (hpf) to MPs (2 mg/L), three sub-lethal concentrations of Cu (15, 60 and 125 μg/L) and binary mixtures containing Cu and MPs (Cu15+MPs, Cu60+MPs, Cu125+MPs). Lethal and sub-lethal parameters, histopathological changes, biochemical biomarkers, gene expression and behavior were assessed. Our findings showed that Cu and Cu + MPs decreased embryos survival and hatching rate. Increased ROS levels were observed in larvae exposed to the two lowest Cu and Cu + MPs groups, suggesting an induction of oxidative stress. An increased CAT and GPx activities were observed in Cu and Cu + MPs, implying a response of the antioxidant defense system to overcome the metal and MPs stress. The sod1 expression was downregulated in all Cu groups and in the two highest Cu + MPs exposed groups. AChE was significantly inhibited in Cu and Cu + MPs groups, indicating neurotoxicity. A disruption of avoidance and social behaviors were also noticed in the Cu125 and Cu125+MPs exposed larvae. Evidences of Cu-toxicity modulation by MPs were observed in some endpoints. Overall, the findings of this study highlight that Cu alone or co-exposed with MPs lead to oxidative stress, neurotoxicity and ultimately behavioral alterations in early life stages of zebrafish, while MPs alone do not produce significant effects on zebrafish larvae.

The synergistic toxicity of Cd(II) and Cu(II) to zebrafish (Danio rerio): Effect of water hardness

We have evaluated the interactive toxicity of Cu(II) and Cd(II) in water with different hardness levels using adult zebrafish (Danio rerio). Zebrafish were exposed to Cd(II) (0.2-22 μM) or Cu(II) (0.1-8 μM) in single or binary exposures in very soft, moderately hard or very hard water. The whole body burdens of Cd(II) and Cu(II) reflect the net effect of biouptake and elimination, mortality was the indicator of toxicity, and whole body major ion content was measured to assess ion regulatory functions. Cu(II) was found to be more toxic than Cd(II) for zebrafish, and Cu(II) and Cd(II) exhibited a significant synergistic effect. The toxicity of metal ions increased upon decreasing the ionic strength of the exposure medium, probably due to elevated competition between metal ions with other cations in hard water and increased activity of Ca²⁺ pathways in soft water treatments. Whole body metal accumulation and the accumulation rate of both Cu and Cd increased as the metal ion concentration in the exposure medium increased. Nevertheless, neither parameter explained the observed synergistic effect on mortality. Finally, we observed a significant loss of whole body Na⁺ in fish which died during the metal exposure compared to surviving fish, irrespective of exposure conditions. Such an effect was not observed for other major cations (K⁺, Ca²⁺ and Mg²⁺). This observation suggests that, under the applied exposure conditions, survival was correlated to the capacity of the organism to maintain Na⁺ homeostasis.

Zebrafish as an integrative vertebrate model to identify miRNA mechanisms regulating toxicity

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Zebrafish are an established vertebrate model for toxicity studies.

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Zebrafish have a fully sequenced genome and the capability to create genetic models.

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Zebrafish have over 80 % homology for genes related to human disease.

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Functions of miRNAs in the zebrafish genome are being characterized.

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Zebrafish are ideal for mechanistic studies on how miRNAs regulate toxicity.

Zebrafish (Danio rerio) are an integrative vertebrate model ideal for toxicity studies. The zebrafish genome is sequenced with detailed characterization of all life stages. With their genetic similarity to humans, zebrafish models are established to study biological processes including development and disease mechanisms for translation to human health. The zebrafish genome, similar to other eukaryotic organisms, contains microRNAs (miRNAs) which function along with other epigenetic mechanisms to regulate gene expression. Studies have now established that exposure to toxins and xenobiotics can change miRNA expression profiles resulting in various physiological and behavioral alterations. In this review, we cover the intersection of miRNA alterations from toxin or xenobiotic exposure with a focus on studies using the zebrafish model system to identify miRNA mechanisms regulating toxicity. Studies to date have addressed exposures to toxins, particulate matter and nanoparticles, various environmental contaminants including pesticides, ethanol, and pharmaceuticals. Current limitations of the completed studies and future directions for this research area are discussed.

The effect of copper on behaviour, memory, and associative learning ability of zebrafish (Danio rerio)

Copper is an essential element in many biological processes, but may exert toxic effects at levels surplus to metabolic requirements. Herein we assess the effect of copper on zebrafish behaviour using two assays, namely the novel tank diving test and a T-maze test with food reward. Novel tank diving tests were conducted on days 0, 4, and 10 of a 10 day Cu exposure (at concentrations of 0.77 μM (25% of the 240 h LC50) and 1.52 μM (50% of the 240 h LC50) to assess the alterations of behavioural responses in repeating novel tank diving assays and the effect of Cu on these patterns. Results demonstrate habituation to novelty, which is an indicator of spatial memory. Copper exposure had no effect on the latency of entry into the upper zones of the tank, nor on the total time spent therein, but did cause a greater number of freezing bouts in comparison to the control group. Additionally, Cu exposure had no effect on the habituation responses of zebrafish. Using the T-maze assay, we tested the effect of prior exposure to Cu for 10 days on subsequent behavioural trainings. The T-maze protocol was based on associative learning, where a visual stimulus (colour) was linked with a natural stimulus (food). Results of the control group showed that zebrafish are able to perform associative learning tasks. Moreover, Cu was found to negatively affect the associative learning capabilities. Specifically, while zebrafish in the control group achieved a significant number of correct choices (leading to food reward) throughout the T-maze training, such a trend was not observed for Cu exposed fish. Thus at the exposure concentrations and exposure times considered herein, Cu has no determinative impact on instinctual behavioural responses of zebrafish in repeated novel tank diving assays but does limit the associative learning capabilities.

The interactive effect of copper(II) and conspecific alarm substances on behavioural responses of zebrafish (Danio rerio)

Environmental contaminants such as metal ions can have detrimental effects on aquatic organisms at the molecular, organismal and population levels. In the present work, we examined the interactive effect of Cu(II) and conspecific alarm substance on zebrafish behavioural responses utilizing the novel tank diving assay. To this end, 3 novel tank diving tests (on day 0, 3 and 10 of the experimental phase) were conducted on zebrafish in 4 experimental groups: (1) control: no Cu(II) and no alarm substance, (2) Cu(II) only: exposed to 0.78 μM Cu(II) (25 % of the 240 h LC50) in the home tank for 10 days, (3) alarm substance only: exposed to alarm substance for 6 min concomitant with behavioural testing, and (4) Cu(II) + alarm substance: exposed to 0.78 μM Cu(II) in the home tank for 10 days and treated with alarm substance for 6 min during the behavioural testing. Results showed robust habituation response of zebrafish. Exposure to Cu(II) did not affect the behavioural phenotypes of zebrafish in the novel tank diving test or habituation responses. Alarm substance treatment evoked strong anxiety-like behaviour. Finally, zebrafish in the Cu(II) + alarm substance group lost their sensitivity to alarm substance in repeated novel tank assays throughout the concomitant Cu(II) exposure; this observation is tentatively ascribed to Cu(II)-induced olfactory impairment.

Behavioral toxicity of tannery effluent in zebrafish (Danio rerio) used as model system

The ecotoxicity of untreated tannery effluent (UTE) in several animal models has been reported; however, its effects on fish behavior, and neurotoxicity, remain unknown. Thus, the hypothesis that the chronic exposure to UTE can induce behavioral changes in adult zebrafish (Danio rerio) representatives, even when it is highly diluted in water, was tested. Animals exposed to 0.1% and 0.3% UTE for 30 days showed behavioral changes in visual social preference tests through their co-specific and antipredator defensive responses, which had indicated neurotoxic actions. Zebrafish exposed to UTE appeared to have not co-specific preference when it is paired with Poecilia sphrenops. In addition, only animals in the control group showed aversive behavior in the presence of the herein used predatory stimulus (Oreochromis niloticus). However, Cr, Na and Mg bioaccumulation was higher in zebrafish exposed to 0.1% and 0.3% UTE, although anxiogenic and anxiolytic effects were not observed in the models exposed to UTE in the novel tank diving or aggressiveness-increase-in-the-mirror tests. This outcome allowed associating the exposure to the pollutant and bioaccumulation with the observed behavioral changes. The present study is pioneer in scientifically evidencing the sublethal impact caused by chronic exposure to UTE in experimental environment simulating realistic aquatic pollution conditions. Accordingly, results in the current research should motivate further investigations to broaden the knowledge about the real magnitude of UTE biological impacts on the aquatic biota.

Ecotoxicity Assessment of Fe3O4 Magnetic Nanoparticle Exposure in Adult Zebrafish at an Environmental Pertinent Concentration by Behavioral and Biochemical Testing

Magnetic Nanoparticles (MNPs) are widely being investigated as novel promising multifunctional agents, specifically in the fields of development for theranostics, electronics, waste water treatment, cosmetics, and energy storage devices. Unique, superior, and indispensable properties of magnetization, heat transfer, and melting temperature make MNPs emerge in the field of therapeutics in future healthcare industries. However, MNPs ecotoxicity as well as behavioral toxicity is still unexplored. Ecotoxicity analysis may assist investigate MNPs uptake mechanism and its influence on bioavailability under a given set of environmental factors, which can be followed to investigate the biomagnification of MNPs in the environment and health risk possessed by them in an ecological food chain. In this study, we attempted to determine the behavioral changes in zebrafishes at low (1 ppm) or high (10 ppm) concentration levels of Fe₃O₄ MNPs. The synthesized Fe₃O₄ MNPs sized at 15 nm were characterized by the transmission electron microscope (TEM), the superconducting quantum interference device (SQUID) magnetometer, and the multiple behavior tests for novel tank, mirror biting, conspecific social interaction, shoaling, circadian rhythm, and short-term memory of zebrafish under MNPs chronic exposure were demonstrated. Low concentration MNP exposure did not trigger alteration for majority behavioral and biochemical tests in adult zebrafish. However, tight shoal groups were observed at a high concentration of MNPs exposure along with a modest reduction in fish exploratory behavior and a significant reduction in conspecific social interaction behavior. By using enzyme-linked immunosorbent assays (ELISA), we found a high dose of MNPs exposure significantly elevated cortisol, acetylcholine, and catalase levels while reducing serotonin, acetylcholine esterase, and dopamine levels in the brain. Our data demonstrates chronic MNPs exposure at an environmentally-relevant dose is relatively safe by supporting evidence from an array of behavioral and biochemical tests. This combinational approach using behavioral and biochemical tests would be helpful for understanding the MNPs association with anticipated colloids and particles effecting bioavailability and uptake into cells and organisms.

Zebrafish breeding program: genetic parameters estimates for growth traits

The objective of this study was to evaluate the genetic parameters of two generations of zebrafish breeding program. The base population was formed by crossing individuals of six commercial stocks of zebrafish, resulting in a nucleus with 60 families. Two generations were evaluated, with a total of 780 and 781 individuals for the first and second generation, respectively. The selection was made based on the mean genetic value of each family, followed by mass selection of the breeders. Mathematical models that considered the fixed (age, density in the larval stage, sex, and generation) and random (animal additive genetics, common to full-sibs, and residual) effects were evaluated using BLUPF90 program family. Weight and total length were used as response variables. Total length was the best selection criterion because it had a higher heritability (0.30) than weight (0.22). There was a high common to full-sib effect, especially in the first generation of animals. For second-generation data, the heritability was 0.26 for total length, as well as a lower common to full-sib effect for length. The best model obtained for this evaluation was considering all effects, being age and density as first and second polynomial, respectively. The genetic and phenotypic correlations for weight and length were 0.87 and 0.75, respectively. These results indicate that genetic breeding using total length as the selection criterion may produce a larger and heavier zebrafish strain.

In vivo and in vitro effects on cholinesterase of blood of Oreochromis mossambicus by copper

The present study is aimed to evaluate the effects of sub-acute toxicity testing of copper sulphate (CuSO₄), on behavioural, histological and biochemical changes of the Oreochromis mossambicus (black tilapia) blood tissues. The effects were assessed according to the previous results on sub-acute toxicity test after exposing fish to several concentrations (0.0, 2.5, 5.0, and 10.0 mg/L). The observations of scanning electron microscope, and transmission electron microscope studies revealed severe histopathological changes on the surface and the cellular changes in blood tissues, respectively. The morphological alterations in blood involved irregular structure of red blood cell and blood clot formation. CuSO₄ affected the biochemical alteration of the blood cholinesterase also known as serum cholinesterase (ChE). Blood ChE inhibited up to 80% of activity when exposed to 10.0 mg/L CuSO₄. The findings from this study can further improve the quality standards of aquaculture industry and the fundamental basis in selecting suitable strains among freshwater fish species to be used as bioindicator.

Neurobehavioral Alterations in Zebrafish Due to Long-Term Exposure to Low Doses of Inorganic Arsenic

Inorganic arsenic (As) is one of the most ubiquitous and toxic substances with widespread health effects on human populations and biodiversity. Although arsenic is a frequent surface water pollutant, there is scant evidence about neurotoxicity in aquatic species in different stages of development. In the present study, we investigated the neurobehavioral effects of chronic exposure to environmentally relevant doses of arsenic. We exposed zebrafish to 50 and 500 ppb during the larval, juvenile, and adult stage (from 4 h to 150 days postfertilization). We then used broad behavioral screening to evaluate motor function, social behavior, learning and memory, and anxiety-like behaviors. Our results show that arsenic exposure to 500 ppb alters motor function from the embryo to the adult stage. Furthermore, during the adult phase, associative learning and the sensorimotor response are affected with both high and low doses of As, respectively. Notably, exposure to 500 ppb of As induces behaviors associated with anxiety, during the juvenile and adult phase but not the larval stage, without changes in whole-body cortisol levels. These results indicate that chronic exposure to arsenic during their lifespan is capable of producing alterations in different behavioral markers in aquatic vertebrates.

Nickel exposure alters behavioral parameters in larval and adult zebrafish

Nickel is a heavy metal that, at high concentrations, leads to environmental contamination and causes health problems. We evaluated the effects of NiCl₂ exposure on cognition and behavior in larval and adult zebrafish. Larval and adult zebrafish were exposed to NiCl₂ concentrations (0.025, 2.0, 5.0, and 15.0mg/L) or water (control) in two treatment regimens: acute and subchronic. Larvae were exposed to NiCl₂ for 2h (acute treatment: 5-day-old larvae treated for 2h, tested after treatment) or 11days (subchronic treatment: 11-day-old larvae treated since fertilization, tested at 5, 8 and 11days post-fertilization, dpf). Adults were exposed for 12h (acute treatment) or 96h (subchronic treatment) and were tested after the treatment period. In both regimens, exposed zebrafish showed concentration-dependent increases in body nickel levels compared with controls. For larvae, delayed hatching, decreased heart rate and morphological alterations were observed in subchronically treated zebrafish. Larvae from subchronic treatment tested at 5dpf decrease distance and mean speed at a low concentration (0.025mg/L) and increased at higher concentrations (5.0 and 15.0mg/L). Subchronic treated larvae decrease locomotion at 15.0mg/L at 8 and 11dpf, whereas decreased escape responses to an aversive stimulus was observed at 2.0, 5.0 and 15.0mg/L in all developmental stages. For adults, the exploratory behavior test showed that subchronic nickel exposure induced anxiogenic-like behavior and decrease aggression, whereas impaired memory was observed in both treatments. These results indicate that exposure to nickel in early life stages of zebrafish leads to morphological alterations, avoidance response impairment and locomotor deficits whereas acute and subchronic exposure in adults resulst in anxiogenic effects, impaired memory and decreased aggressive behavior. These effects may be associated to neurotoxic actions of nickel and suggest this metal may influence animals' physiology in doses that do not necessarily impact their survival.

Protectiveness of Cu water quality criteria against impairment of behavior and chemo/mechanosensory responses: An update

A meta-analysis was conducted of studies that reported behavior and chemo/mechanosensory responses by fish, amphibians, and aquatic invertebrates in Cu-containing waters and also reported sufficient water chemistry for calculation of hardness-based and biotic ligand model (BLM)-based water quality criteria (WQC) for Cu. The calculated WQC concentrations were then compared with the corresponding 20% impairment concentrations (IC20) of Cu for those behavior and chemo/mechanosensory responses. The hardness-based acute and chronic WQC for Cu would not have been protective (i.e., the IC20 would have been lower than the WQC) in 33.6 and 26.2%, respectively, of the 107 combined behavior- and chemo/mechanosensory-response cases that also had adequate water chemistry data for BLM-based WQC calculations (32.7% inconclusive). In comparison, the BLM-based acute and chronic WQC for Cu would not have been protective in only 10.3 and 4.7%, respectively, of the same 107 cases (29.9% inconclusive). To improve evaluations of regulatory effectiveness, researchers conducting aquatic Cu toxicity tests should measure and report complete BLM-input water chemistry and bracket the hardness-based and BLM-based WQC concentrations for Cu that would be applicable in their exposure waters. This meta-analysis demonstrates that, overall, the BLM-based WQC for Cu were considerably more protective than the hardness-based WQC for Cu against impairment of behavior and chemo/mechanosensory responses. Environ Toxicol Chem 2018;37:1260-1279. © 2018 SETAC.

In vivo toxic effects of 4-methoxy-5-hydroxy-canthin-6-one in zebrafish embryos via copper dyshomeostasis and oxidative stress

Dysfunction of copper homeostasis can lead to a host of disorders, which might be toxic sometimes. 4-Methoxy-5-hydroxy-canthin-6-one (CAN) is one of the major constituents from Picrasma quassioides and responsible for its therapeutic effects. In this work, we evaluated the toxic effect of CAN (7.5μM) on zebrafish embryos. CAN treatment decreased survival, delayed hatching time and induced malformations (loss of pigmentation, pericardial edema, as well as hematologic and neurologic abnormalities). Besides, exogenous copper supplementation rescued the pigmentation and cardiovascular defects in CAN-treated embryos. Further spectroscopic studies revealed a copper-chelating activity of CAN. Then its regulation on the expressions of copper homeostasis related genes also be analyzed. In addition, CAN lowered the total activity of SOD, elevated the ROS production and altered the oxidative related genes transcriptions, which led to oxidative stress. In conclusion, we demonstrated that CAN (7.5μM) might exert its toxic effects in zebrafish embryos by causing copper dyshomeostasis and oxidative stress. It will give insight into the risk assessment and prevention of CAN-mediated toxicity.

Alterations of larval photo-dependent swimming responses (PDR): New endpoints for rapid and diagnostic screening of aquatic contamination

Detection and toxicity assessment of waterborne contaminants are crucial for protecting human health and the environment. Development of easy-to-implement, rapid and cost-effective tools to measure anthropogenic effects on watersheds are critical for responsible management, particularly in times of increasing development and urbanization. Traditionally, environmental toxicology has focused on limited endpoints, such as lethality and fertility, which are directly affecting population levels. However, more sensitive readings are needed to assess sub-lethal effects. Monitoring of contaminant-induced behavior alterations was proposed before, but is difficult to implement in the wild and performing it in aquatic laboratory models seem more suited. For this purpose, we adapted a photo-dependent swimming response (PDR) that was previously described in zebrafish larva. We first asked if PDR was present in other aquatic animals. We measured PDR in larvae from two freshwater prawn species (Macrobrachium rosenbergii, MR, and Macrobrachium carcinus, MC) and from another fish the fathead minnow (FHM, Pimephales promelas). In all, we found a strong and reproducible species-specific PDR, which is arguing that this behavior is important, therefore an environmental relevant endpoint. Next, we measured PDR in fish larvae after acute exposure to copper, a common waterborne contaminant. FHM larvae were hyperactive at all tested concentrations in contrast to ZF larvae, which exhibited a concentration-dependent hyperactivity. In addition to this well-accepted anxiety-like behavior, we examined two more: photo-stimulated startle response (PSSR) and center avoidance (CA). Both were significantly increased. Therefore, PDR measures after acute exposure to this waterborne contaminant provided as sensitive readout for its detection and toxicity assessment. This approach represents an opportunity to diagnostically examine any substance, even when present in complex mixtures like ambient surface waters. Mechanistic studies of toxicity using the extensive molecular tool kit of ZF could be a direct extension of such approaches.