Assessment of the developmental neurotoxicity of compounds by measuring locomotor activity in zebrafish embryos and larvae

Reviewing the use of zebrafish for the detection of neurotoxicity induced by chemical mixtures through the analysis of behaviour

The knowledge and assessment of mixtures of chemical pollutants in the aquatic environment is a complex issue that is often challenging to address. In this review, we focused on the use of zebrafish (Danio rerio), a vertebrate widely used in biomedical research, as a model for detecting the effects of chemical mixtures with a focus on behaviour. Our aim was to summarize the current status of the ecotoxicological research in this sector. Specifically, we limited our research to the period between January 2012 and September 2023, including only those works aimed at detecting neurotoxicity through behavioural endpoints, utilizing zebrafish at one or more developmental stages, from egg to adult. Additionally, we gathered the findings for every group of chemicals involved and summarised data from all the works we included. At the end of the screening process 101 papers were considered eligible for inclusion. Results show a growing interest in zebrafish at all life stages for this kind of research in the last decade. Also, a wide variety of different assays, involving different senses, was used in the works we surveyed, with exposures ranging from acute to chronic. In conclusion, the results of this study show the versatility of zebrafish as a model for the detection of mixture toxicity although, for what concerns behavioural analysis, the lack of standardisation of methods and endpoints might still be limiting.

Dolutegravir and Folic Acid Interaction during Neural System Development in Zebrafish Embryos

Dolutegravir (DTG) is one of the most prescribed antiretroviral drugs for treating people with HIV infection, including women of child-bearing potential or pregnant. Nonetheless, neuropsychiatric symptoms are frequently reported. Early reports suggested that, probably in relation to folic acid (FA) shortage, DTG may induce neural tube defects in infants born to women taking the drug during pregnancy. Subsequent reports did not definitively confirm these findings. Recent studies in animal models have highlighted the association between DTG exposure in utero and congenital anomalies, and an increased risk of neurologic abnormalities in children exposed during in utero life has been reported. Underlying mechanisms for DTG-related neurologic symptoms and congenital anomalies are not fully understood. We aimed to deepen our knowledge on the neurodevelopmental effects of DTG exposure and further explore the protective role of FA by the use of zebrafish embryos. We treated embryos at 4 and up to 144 h post fertilization (hpf) with a subtherapeutic DTG concentration (1 μM) and observed the disruption of the anterior-posterior axis and several morphological malformations in the developing brain that were both prevented by pre-exposure (2 hpf) and rescued by post-exposure (10 hpf) with FA. By whole-mount in situ hybridization with riboprobes for genes that are crucial during the early phases of neurodevelopment (ntl, pax2a, ngn1, neurod1) and by in vivo visualization of the transgenic Tg(ngn1:EGFP) zebrafish line, we found that DTG induced severe neurodevelopmental defects over time in most regions of the nervous system (notochord, midbrain-hindbrain boundary, eye, forebrain, midbrain, hindbrain, spinal cord) that were mostly but not completely rescued by FA supplementation. Of note, we observed the disruption of ngn1 expression in the dopaminergic regions of the developing forebrain, spinal cord neurons and spinal motor neuron projections, with the depletion of the tyrosine hydroxylase (TH)+ dopaminergic neurons of the dorsal diencephalon and the strong reduction in larvae locomotion. Our study further supports previous evidence that DTG can interfere with FA pathways in the developing brain but also provides new insights regarding the mechanisms involved in the increased risk of DTG-associated fetal neurodevelopmental defects and adverse neurologic outcomes in in utero exposed children, suggesting the impairment of dopaminergic pathways.

New developments in pre-clinical models of ALS to guide translation

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder in which selective death of motor neurons leads to muscle weakness and paralysis. Most research has focused on understanding and treating monogenic familial forms, most frequently caused by mutations in SOD1, FUS, TARDBP and C9orf72, although ALS is mostly sporadic and without a clear genetic cause. Rodent models have been developed to study monogenic ALS, but despite numerous pre-clinical studies and clinical trials, few disease-modifying therapies are available. ALS is a heterogeneous disease with complex underlying mechanisms where several genes and molecular pathways appear to play a role. One reason for the high failure rate of clinical translation from the current models could be oversimplification in pre-clinical studies. Here, we review advances in pre-clinical models to better capture the heterogeneous nature of ALS and discuss the value of novel model systems to guide translation and aid in the development of precision medicine.

Recent Advances in Bioimage Analysis Methods for Detecting Skeletal Deformities in Biomedical and Aquaculture Fish Species

Detecting skeletal or bone-related deformities in model and aquaculture fish is vital for numerous biomedical studies. In biomedical research, model fish with bone-related disorders are potential indicators of various chemically induced toxins in their environment or poor dietary conditions. In aquaculture, skeletal deformities are affecting fish health, and economic losses are incurred by fish farmers. This survey paper focuses on showcasing the cutting-edge image analysis tools and techniques based on artificial intelligence that are currently applied in the analysis of bone-related deformities in aquaculture and model fish. These methods and tools play a significant role in improving research by automating various aspects of the analysis. This paper also sheds light on some of the hurdles faced when dealing with high-content bioimages and explores potential solutions to overcome these challenges.

Developmental and neurobehavioral toxicity of 2,2'-methylenebis(6-tert-butyl-4-methylphenol) (antioxidant AO2246) during the early life stage of zebrafish

BACKGROUND

2,2'-Methylenebis (4-methyl-6-tert-butylphenol) (AO2246) is a synthetic phenolic antioxidant extensively used in food packaging bags and cosmetics. Recently, AO2246 was detected with unexpectedly high concentrations in plasma and breast milk samples from pregnant and lactating women. Hence, it is essential to conduct a thorough investigation to evaluate the detrimental effects of AO2246 on biota.

OBJECTIVE

To investigate the developmental and behavioral toxicity of AO2246 in zebrafish, as well as the molecular mechanisms underlying these effects.

METHODS

Zebrafish embryos were exposed to AO2246 at concentrations ranging from 0.05 to 10 μM for up to 6 days postfertilization (dpf). Hatching rate, survival rate, heart rate, and body length were measured. Locomotor behavioral and electrophysiologal analyses were performed. Two fluorescence-labeled transgenic zebrafish lines (endothelium-Tg and macrophage/microglia-Tg) were employed. RNA sequencing was carried out.

RESULTS

AO2246 has a 96-hour LC50 value of 3 μM. The exposure of AO2246 resulted in a significant reduction in both hatching rate and heart rate. Analysis of locomotor behavior demonstrated that larvae exposed to AO2246 doses exceeding 2 μM exhibited a significant decrease in both total distance and mean velocity. Electrophysiological recordings demonstrated a noteworthy reduction in spike activity at a concentration of 3 μM, relative to control conditions. The administration of AO2246 at 3 μM elicited morphological reactivity and immune alteration of the midbrain microglia in the macrophage/microglia-transgenic zebrafish line, indicating a potential contribution of neurological disorders to behavioral defects. RNA sequencing analysis revealed altered gene expression profiles at high AO2246 concentrations, particularly the dysregulation of pathways associated with neuronal function.

CONCLUSIONS

The present study demonstrates that AO2246 exposure elicits developmental and neurobehavioral toxicity in zebrafish larvae. Specifically, exposure to AO2246 was found to cause disturbances in neuronal electrophysiological activity and neurological disorders, which ultimately led to the impairment of locomotor behavior in zebrafish larvae.

Immunotoxicity and transcriptome analysis of zebrafish embryos exposure to Nitazoxanide

Nitazoxanide (NTZ) is a broad-spectrum immunomodulatory drug, and little information is about the immunotoxicity of aquatic organisms induced by NTZ. In the present study, reduced body length and decreased yolk sac absorption in the NTZ-treated group were observed. Meanwhile, the number of innate immune cells and adaptive immune cells was substantially reduced upon NTZ exposure, and the migration and retention of macrophages and neutrophils in the injured area were inhibited. Following NTZ stimulation, oxidative stress levels in the zebrafish increased obviously. Mechanistically, RNA-seq, a high-throughput method, was performed to analyze the global expression of differentially expressed genes (DEGs) in zebrafish embryos treated with NTZ. 531 DEGs were identified by comparative transcriptome analysis, including 121 up-regulated and 420 down-regulated genes in zebrafish embryos after NTZ exposure. The transcriptome sequences were further subjected to the Kyoto Encyclopedia of Genes and Genomes (KEGG) and gene ontology (GO) and analysis, showing phototransduction and metabolic pathway, respectively, and were most enriched. In addition, some immune-related genes were inhibited after NTZ exposure. RNA-seq results confirmed by qRT-PCR were used to verify the expression of the 6 selected genes. The other immune-related genes such as two pro-inflammatory cytokines (IL-1β, tnfα) and two chemokines (CXCL8b.3, CXCL-c1c) were further confirmed and were differentially regulated after NTZ exposure. In summary, NTZ exposure could lead to immunotoxicity and increased ROS in zebrafish embryos, this study provides valuable information for future elucidating the molecular mechanism of exogenous stimuli-induced immunotoxicity in aquatic ecosystems.

Embryonic exposures to chemicals acting on brain aromatase lead to different locomotor effects in zebrafish larvae

Pathways underlying neurodevelopmental effects of endocrine disruptors (EDs) remain poorly known. Expression of brain aromatase (aroB), responsible for estrogen production in the brain of teleosts, is regulated by estrogenic EDs and could play a role in their behavioral effects. We exposed zebrafish eleutheroembryos (0-120 h post-fertilization) to various concentrations of 16 estrogenic chemicals (incl. bisphenols and contraceptives), and of 2 aroB inhibitors. Behavior was monitored using a photomotor response test procedure. Both aroB inhibitors (clotrimazole and prochloraz) and a total of 6 estrogenic EDs induced significant behavioral alterations, including DM-BPA, BPC and BPS-MPE, three bisphenol substitutes which behavioral effects were, to our knowledge, previously unknown. However, no consensus was reported on the effects among tested substances. It appears that behavioral changes could not be linked to groups of substances defined by their specificity or potency to modulate aroB expression, or by their structure. Altogether, behavioral effects of estrogenic EDs in 120 h post-fertilization larvae appear unrelated to aroB but are nonetheless not to be neglected in the context of environmental safety.

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.

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.

The sensitivity of the zebrafish embryo coiling assay for the detection of neurotoxicity by compounds with diverse modes of action

In the aim to determine neurotoxicity, new methods are being validated, including tests and test batteries comprising in vitro and in vivo approaches. Alternative test models such as the zebrafish (Danio rerio) embryo have received increasing attention, with minor modifications of the fish embryo toxicity test (FET; OECD TG 236) as a tool to assess behavioral endpoints related to neurotoxicity during early developmental stages. The spontaneous tail movement assay, also known as coiling assay, assesses the development of random movement into complex behavioral patterns and has proven sensitive to acetylcholine esterase inhibitors at sublethal concentrations. The present study explored the sensitivity of the assay to neurotoxicants with other modes of action (MoAs). Here, five compounds with diverse MoAs were tested at sublethal concentrations: acrylamide, carbaryl, hexachlorophene, ibuprofen, and rotenone. While carbaryl, hexachlorophene, and rotenone consistently induced severe behavioral alterations by ~ 30 h post fertilization (hpf), acrylamide and ibuprofen expressed time- and/or concentration-dependent effects. At 37-38 hpf, additional observations revealed behavioral changes during dark phases with a strict concentration-dependency. The study documented the applicability of the coiling assay to MoA-dependent behavioral alterations at sublethal concentrations, underlining its potential as a component of a neurotoxicity test battery.

Developmental safety of nirmatrelvir in zebrafish (Danio rerio) embryos

BACKGROUND

Nirmatrelvir, in combination with ritonavir, is one of the first orally available antiviral treatment for coronavirus disease 2019 (COVID-19). Symptomatic pregnant women are at increased risk for severe illness and complications that can affect the developing baby. No malformations or lower embryo-fetal survival have been observed when nirmatrelvir were administered to pregnant rats and rabbits. Safety evaluation of drugs used for treating COVID-19 also in pregnancy is urgent for public health, then in this study we further investigated nirmatrelvir developmental toxicity using zebrafish as in vivo model.

MATERIAL AND METHODS

Using the standardized Fish Embryo Toxicity (FET) test, we first determined the lethal concentration 50 (LC50), exposing embryos from gastrula stage up to 120 hr post fertilization (hpf) and daily recording lethality. Then, we exposed embryos to five doses comprising the human therapeutic one and up to the LC50 (25 μM). Morphology was evaluated at 72 and 120 hpf.

RESULTS

Nirmatrelvir did not affect survival rate and did not induce morphological defects up to the human therapeutic dose. Exposure at higher doses (2.4× and 3× the human C_max ) however resulted in decreased hatching rate, reduced growth, slower heartbeat with pericardial edema, reduction of eye dimension, absence of the swim bladder and disruption of the anterior-posterior axis, with lack of tail detachment, spinal curvature and straight and smaller head.

CONCLUSIONS

Our findings in zebrafish embryos add further information about developmental nirmatrelvir safety. Further studies are needed for pharmacological safety assessment of nirmatrelvir exposure during pregnancy.

Global occurrence and aquatic hazards of antipsychotics in sewage influents, effluent discharges and surface waters

Despite increasing reports of pharmaceuticals in surface waters, aquatic hazard information remains limited for many contaminants, particularly for sublethal, chronic responses plausibly linked to molecular initiation events that are largely conserved across vertebrates. Here, we critically examined available refereed information on the occurrence of 67 antipsychotics in wastewater effluent and surface waters. Because the majority of sewage remains untreated around the world, we also examined occurrence in sewage influents. When sufficient information was available, we developed probabilistic environmental exposure distributions (EEDs) for each compound in each matrix by geographic region. We then performed probabilistic environmental hazard assessments (PEHAs) using therapeutic hazard values (THVs) of each compound, due to limited sublethal aquatic toxicology information for this class of pharmaceuticals. From these PEHAs, we determined predicted exceedances of the respective THVs for each chemical among matrices and regions, noting that THV values of antipsychotic contaminants are typically lower than other classes of human pharmaceuticals. Diverse exceedances were observed, and these aquatic hazards varied by compound, matrix and geographic region. In wastewater effluent discharges and surface waters, sulpiride was the most detected antipsychotic; however, percent exceedances of the THV were minimal (0.6%) for this medication. In contrast, we observed elevated aquatic hazards for chlorpromazine (30.5%), aripiprazole (37.5%), and perphenazine (68.7%) in effluent discharges, and for chlorprothixene (35.4%) and flupentixol (98.8%) in surface waters. Elevated aquatic hazards for relatively understudied antipsychotics were identified, which highlight important data gaps for future environmental chemistry and toxicology research.

Susceptibility of male reproductive system to bisphenol A, an endocrine disruptor: Updates from epidemiological and experimental evidence

Bisphenol A (BPA) is an omnipresent environmental pollutant. Despite being restrictions in-force for its utilization, it is widely being used in the production of polycarbonate plastics and epoxy resins. Direct, low-dose, and long-term exposure to BPA is expected when they are used in the packaging of food products and are used as containers for food consumption. Occupationally, workers are typically exposed to BPA at higher levels and for longer periods during the manufacturing process. BPA is a known endocrine disruptor chemical (EDC), that causes male infertility, which has a negative impact on human life from emotional, physical, and societal standpoints. To minimize the use of BPA in numerous consumer products, efforts and regulations are being made. Despite legislative limits in numerous nations, BPA is still found in consumer products. This paper examines BPA's overall male reproductive toxicity, including its impact on the hypothalamic-pituitary-testicular (HPT) axis, hormonal homeostasis, testicular steroidogenesis, sperm parameters, reproductive organs, and antioxidant defense system. Furthermore, this paper highlighted the role of non-monotonic dose-response (NMDR) in BPA exposure, which will help to improve the overall understanding of the harmful effects of BPA on the male reproductive system.

Zebrafish as model for studies in dentistry

INTRODUCTION

Over the last years, zebrafish has gained prominence in the biomedical community. It is currently considered one of the best vertebrate animal models for various types of studies, such as toxicology and developmental biology.

OBJECTIVE

The aim of this study was to conduct a literature review on the use of zebrafish in dentistry and whether this animal model could be a viable alternative for performing different types of studies in this area.

METHODS

A literature search was performed using the PubMed, Lilacs, Embase, and Dentistry and Oral Sciences Source. The keywords used as search terms were zebrafish and dentistry. The selection criteria were articles published in English that used zebrafish as an animal model in dentistry, oral health, and craniofacial growth/development.

RESULTS

The electronic search of literature yielded 421 articles. After the analysis of the abstracts, 29 articles were selected for an in-depth analysis and reading of the full text.

CONCLUSIONS

All studies included in this review confirm zebrafish's excellence as an animal model for various types of dentistry studies, as well as assisting and complementing other studies involving mammals.

Potency matters: Impacts of embryonic exposure to nAChR agonists thiamethoxam and nicotine on hatching success, growth, and neurobehavior in larval zebrafish

Thiamethoxam (TM) is a neonicotinoid insecticide that acts as a nicotinic acetylcholine receptor (nAChR) agonist. While designed to specifically target invertebrate nAChRs, recent studies have reported adverse effects of neonicotinoid exposure in early life-stage fish. This study examined the health and neurobehavioral impacts of chronic exposure to various concentrations of TM or nicotine (NIC) in early life zebrafish (Danio rerio) in conjunction with in-silico molecular docking to compare their ligand-receptor interactions with vertebrate nAChR. Chronic exposure to both reduced survival by approximately 20% (163 µg TM/l) and 25-100% (≥0.49 µg NIC/l). Hatching and growth were impaired following exposure to ≥0.21 µg TM/l or 4.9 µg NIC/l. Both TM and NIC produced morphological and behavioral indicators of neurotoxicity, with more potent effects following NIC exposure. NIC impaired embryonic motor activity by 40% (49 µg NIC/l), while both TM and NIC significantly altered predator escape response in larvae, specifically the latency and the initial burst movement of the response were impacted. Molecular docking predicted variations in the type and strength of interactions that occur between NIC or TM and vertebrate nAChR. These findings demonstrate that chronic exposure to TM might impact general health and neurobehavior of early-stage zebrafish. Our data support hypotheses that TM presents low affinity for vertebrate nAChR but may still pose an adverse risk to larval fish growth and neurobehavior.

A review and assessment of cyanobacterial toxins as cardiovascular health hazards

Eutrophicated waters frequently support bloom-forming cyanobacteria, many of which produce potent cyanobacterial toxins (cyanotoxins). Cyanotoxins can cause adverse health effects in a wide range of organisms where the toxins may target the liver, other internal organs, mucous surfaces and the skin and nervous system. This review surveyed more than 100 studies concerning the cardiovascular toxicity of cyanotoxins and related topics. Over 60 studies have described various negative effects on the cardiovascular system by seven major types of cyanotoxins, i.e. the microcystin (MC), nodularin (NOD), cylindrospermopsin (CYN), anatoxin (ATX), guanitoxin (GNTX), saxitoxin (STX) and lyngbyatoxin (LTX) groups. Much of the research was done on rodents and fish using high, acutely toxin concentrations and unnatural exposure routes (such as intraperitoneal injection), and it is thus concluded that the emphasis in future studies should be on oral, chronic exposure of mammalian species at environmentally relevant concentrations. It is also suggested that future in vivo studies are conducted in parallel with studies on cells and tissues. In the light of the presented evidence, it is likely that cyanotoxins do not constitute a major risk to cardiovascular health under ordinary conditions met in everyday life. The risk of illnesses in other organs, in particular the liver, is higher under the same exposure conditions. However, adverse cardiovascular effects can be expected due to indirect effects arising from damage in other organs. In addition to risks related to extraordinary concentrations of the cyanotoxins and atypical exposure routes, chronic exposure together with co-existing diseases could make some of the cyanotoxins more dangerous to cardiovascular health.

Melatonin attenuates polystyrene microplastics induced motor neurodevelopmental defect in zebrafish (Danio rerio) by activating nrf2 - isl2a Axis

Microplastics, a new type of ecological pollutant, have now become a major environmental concern worldwide. Polystyrene microplastics (PS), one of the most abundant form of microplastics, cause deleterious effects across species. Melatonin (MT), which is secreted by pineal gland, exhibits protective role against pollutant-induced damage. However, whether MT could ameliorate PS-induced neurodevelopmental toxicity remain unclear. In our study, zebrafish embryos were treated with PS (0.5, 25 mg/L) in the presence or absence of MT (1 μM) from 4 h post-fertilization (hpf) to 144 hpf. Locomotion behavior, oxidative stress, apoptosis, proliferation and development of caudal primary (Cap) motoneuron axon were analyzed. Gene expression was determined by qRT-PCR or whole-mount in situ hybridization. Results showed that PS exposure significantly reduced swimming speed of zebrafish larvae and induced excessive reactive oxygen species (ROS), apoptosis and aberrant proliferation. In addition, PS treatment markedly shortened the length of Cap motoneuron axons and decreased expression of neurodevelopment related genes. While, MT administration considerably rescued the neurodevelopmental toxicity of PS. Mechanistically, MT activated nrf2 (nuclear factor-E2-related factor 2) - isl2a (ISL LIM homeobox 2a) axis to antagonize the side effects of PS. In all, our findings suggest that PS exposure during early life lead to aberrant neurodevelopment of zebrafish, and MT might be a therapeutic option for protecting such disorder.

Zebrafish Is a Powerful Tool for Precision Medicine Approaches to Neurological Disorders

Personalized medicine is currently one of the most promising tools which give hope to patients with no suitable or no available treatment. Patient-specific approaches are particularly needed for common diseases with a broad phenotypic spectrum as well as for rare and yet-undiagnosed disorders. In both cases, there is a need to understand the underlying mechanisms and how to counteract them. Even though, during recent years, we have been observing the blossom of novel therapeutic techniques, there is still a gap to fill between bench and bedside in a patient-specific fashion. In particular, the complexity of genotype-to-phenotype correlations in the context of neurological disorders has dampened the development of successful disease-modifying therapeutics. Animal modeling of human diseases is instrumental in the development of therapies. Currently, zebrafish has emerged as a powerful and convenient model organism for modeling and investigating various neurological disorders. This model has been broadly described as a valuable tool for understanding developmental processes and disease mechanisms, behavioral studies, toxicity, and drug screening. The translatability of findings obtained from zebrafish studies and the broad prospect of human disease modeling paves the way for developing tailored therapeutic strategies. In this review, we will discuss the predictive power of zebrafish in the discovery of novel, precise therapeutic approaches in neurosciences. We will shed light on the advantages and abilities of this in vivo model to develop tailored medicinal strategies. We will also investigate the newest accomplishments and current challenges in the field and future perspectives.

Degradation of the nonylphenol aqueous solution by strong ionization discharge: evaluation of degradation mechanism and the water toxicity of zebrafish

Nonylphenol (NP) is a typical environmental endogenous disrupter with low concentration and high toxicity. This paper describes the mechanism of NP degradation in solution by strong ionization dielectric barrier discharge (SIDBD). Furthermore, the degradation performance of NP by SIDBD was tested by changing the equipment voltage, the initial concentration of NP in aqueous solution, pH, and inorganic ions. Degradation pathways of NP were detected using a high-performance liquid chromatography-mass spectrometer. The biological effects of NP degradation were assessed by detecting indicators of embryonic development in zebrafish (survival rate, fetal movement, heartbeat, the body length, behavior, deformity) and adult fish (sex differentiation, weight, ovarian testes pathological section analysis). The results showed when the input O₂ was 5 L/min and the voltage was 3.2 kV, the degradation efficiency of NP can reach 99.0% after 60 min of experiment. Equipment voltage, initial concentration of NP in solution, pH, inorganic ions and other factors can influence the degradation efficiency of NP by DBD. At the higher concentration of NP, the greater influence on embryonic development in zebrafish was noticed. Although the effects of NP on zebrafish sex differentiation were not obvious, it showed significant male weight inhibition and decrease in sperm number.

Enantioselective toxic effects of mefentrifluconazole in the early life stage of zebrafish (Danio rerio)

The research on the enantioselective toxic effects of chiral pesticides on non-target aquatic organisms has attracted more and more attention. This study investigated the enantioselective toxic effects of mefentrifluconazole (MFZ) on acute toxicity, developmental toxicity, locomotor behaviors, and the mRNA relative expression levels of genes related to neurodevelopment and cardiac development in zebrafish embryos or larvae. The 96-h lethal concentration 50 (LC₅₀ ) values (exposed to racemate and enantiomers of MFZ, that is, rac-MFZ/(-)-MFZ/(+)-MFZ) were 1.010, 1.552, and 0.753 mg/L for embryo, and 0.753, 1.187, and 0.553 mg/L for larvae. The rac-MFZ/(-)-MFZ/(+)-MFZ can affect the heart development of zebrafish embryos, accompanied by heart rate inhibition, yolk sac deformities, pericardial deformities, and down-regulation of genes related to cardiotoxicity in larvae in an enantioselective manner. Moreover, the rac-MFZ/(-)-MFZ/(+)-MFZ also can affect the neural development of zebrafish embryos, accompanied by autonomic movement inhibition, swimming speed and swimming distance abnormalities, and down-regulation of genes related to neurotoxicity in larvae in an enantioselective manner. For all toxicity endpoints, the effect of the (+)-MFZ to early-staged zebrafish were significantly greater than that of (-)-MFZ. These results will help distinguishing the difference of MFZ enantiomers to zebrafish, and provide scientific reference for improving the risk assessment of chiral pesticides MFZ.

Developmental Neurotoxicity and Behavioral Screening in Larval Zebrafish with a Comparison to Other Published Results

With the abundance of chemicals in the environment that could potentially cause neurodevelopmental deficits, there is a need for rapid testing and chemical screening assays. This study evaluated the developmental toxicity and behavioral effects of 61 chemicals in zebrafish (Danio rerio) larvae using a behavioral Light/Dark assay. Larvae (n = 16-24 per concentration) were exposed to each chemical (0.0001-120 μM) during development and locomotor activity was assessed. Approximately half of the chemicals (n = 30) did not show any gross developmental toxicity (i.e., mortality, dysmorphology or non-hatching) at the highest concentration tested. Twelve of the 31 chemicals that did elicit developmental toxicity were toxic at the highest concentration only, and thirteen chemicals were developmentally toxic at concentrations of 10 µM or lower. Eleven chemicals caused behavioral effects; four chemicals (6-aminonicotinamide, cyclophosphamide, paraquat, phenobarbital) altered behavior in the absence of developmental toxicity. In addition to screening a library of chemicals for developmental neurotoxicity, we also compared our findings with previously published results for those chemicals. Our comparison revealed a general lack of standardized reporting of experimental details, and it also helped identify some chemicals that appear to be consistent positives and negatives across multiple laboratories.

Embryonic Exposure to Thiamethoxam Reduces Survival and Alters Neurobehavior of Fathead Minnows

Thiamethoxam is a commonly used neonicotinoid insecticide that acts as a nicotinic acetylcholine receptor (nAChR) agonist. Although vertebrates are less sensitive to neonicotinoid insecticides than invertebrates, some neonicotinoids have been shown to cause neurobehavioral changes in larval fishes. In the present study, we examine the neurobehavioral toxicity of acute and chronic exposure to environmentally relevant concentrations of thiamethoxam in fathead minnows at two different life stages. Whereas acute exposure of embryos to thiamethoxam does not appear to stimulate spontaneous contractions within 1 min, chronic exposure of embryos to 1.57 µg or more thiamethoxam/L caused increased mortality as well as a subtle increase in spontaneous contraction frequency (SCF), which was negatively correlated with early hatching success. Chronic exposure of embryos to 155 µg thiamethoxam/L impaired predator escape response, and chronic exposure to 0.02-14.61 µg thiamethoxam/L impaired foraging efficiency of some fish. Fathead minnows exposed to thiamethoxam beginning post hatch did not experience changes to measured health or neurobehavioral indicators. Taken together, our findings indicate that embryonic life stages are more sensitive to thiamethoxam exposure than later larval life stages. Because early exposure to thiamethoxam can cause deficits in predatory escape behaviors and may impair foraging success, further study of the potential direct and nondirect impacts of thiamethoxam on wild fish populations is warranted. Environ Toxicol Chem 2022;41:1276-1285. © 2022 SETAC.

Specificity of time- and dose-dependent morphological endpoints in the fish embryo acute toxicity (FET) test for substances with diverse modes of action: the search for a "fingerprint"

The fish embryo acute toxicity (FET) test with the zebrafish (Danio rerio) embryo according to OECD TG 236 was originally developed as an alternative test method for acute fish toxicity testing according to, e.g., OECD TG 203. Given the versatility of the protocol, however, the FET test has found application beyond acute toxicity testing as a common tool in environmental hazard and risk assessment. Whereas the standard OECD guideline is restricted to four core endpoints (coagulation as well as lack of somite formation, heartbeat, and tail detachment) for simple, rapid assessment of acute toxicity, further endpoints can easily be integrated into the FET test protocol. This has led to the hypothesis that an extended FET test might allow for the identification of different classes of toxicants via a "fingerprint" of morphological observations. To test this hypothesis, the present study investigated a set of 18 compounds with highly diverse modes of action with respect to acute and sublethal endpoints. Especially at higher concentrations, most observations proved toxicant-unspecific. With decreasing concentrations, however, observations declined in number, but gained in specificity. Specific observations may at best be made at test concentrations ≤ EC10. The existence of a "fingerprint" based on morphological observations in the FET is, therefore, highly unlikely in the range of acute toxicity, but cannot be excluded for experiments at sublethal concentrations.

Road Runoff Characterization: Ecotoxicological Assessment Combined with (Non-)Target Screenings of Micropollutants for the Identification of Relevant Toxicants in the Dissolved Phase

Road runoff (RR) is an important vector of micropollutants towards groundwater and soils, threatening the environment and ecosystems. Through combined chemical and biological approaches, the purpose of this study was to get insights on specific toxicants present in RR from two sites differing by their traffic intensity and their toxicological risk assessment. Non-target screening was performed by HRMS on RR dissolved phase. Ecotoxicological risk was evaluated in a zebrafish embryos model and on rat liver mitochondrial respiratory chain. Specific HRMS fingerprints were obtained for each site, reflecting their respective traffic intensities. Several micropollutants, including 1,3-diphenylguanidine (DPG) and benzotriazole (BZT) were identified in greater concentrations at the high-traffic site. The origin of DPG was confirmed by analyzing HRMS fingerprints from shredded tires. RR samples from each site, DPG and BZT were of relatively low toxicity (no mortality) to zebrafish embryos, but all generated distinct and marked stress responses in the light–dark transition test, while DPG/BZT mixes abolished this effect. The moderate-traffic RR and DPG inhibited mitochondrial complex I. Our study highlights (i) the unpredictability of pollutants cocktail effect and (ii) the importance of a multi-approaches strategy to characterize environmental matrices, essential for their management at the source and optimization of depollution devices.

Assessing sub-lethal effects of the dinitroaniline herbicide pendimethalin in zebrafish embryos/larvae (Danio rerio)

Pendimethalin is a dinitroaniline herbicide used to control broadleaf weeds by inhibiting the formation of microtubules during cell division. Its use on a variety of crops leads to its potential entry into aquatic environments, but little is known about its sub-lethal toxicity to early developmental stages of aquatic vertebrates. To address this knowledge gap, we assessed the toxicity of pendimethalin to zebrafish embryos and larvae by measuring mortality, developmental abnormalities, oxidative respiration, reactive oxygen species, gene expression, and locomotor activity following exposure to the herbicide throughout early development. Embryos at ~6 h post-fertilization (hpf) were exposed to either a solvent control (0.1% DMSO, v/v), embryo rearing medium (ERM), or one dose of either 1, 2.5, 5, or 25 μM pendimethalin for up to 7-days post fertilization depending on the bioassay. Exposure to 25 μM pendimethalin resulted in high prevalence of spinal curvature, tail malformations, pericardial edema, and yolk sac edema at 4 dpf, while exposure to 5 μM pendimethalin induced pericardial edema and lordosis in the fish exposed over 7 dpf. Exposure to pendimethalin up to 5 μM did not negatively impact oxidative respiration (e.g., basal respiration, oligomycin-induced ATP production) in embryos following a 24-h exposure. Pendimethalin did not induce reactive oxygen species at concentrations of 1-5 μM. Levels of transcripts associated with oxidative respiration and damage response were altered in 7d-larvae; cox1 mRNA was increased in larvae fish exposed to 1 μM while cox5a1 and sod2 mRNA were decreased with 2.5 μM exposure. The Visual Motor Response (VMR) test for light-dark response revealed that larval activity in the dark period was reduced for zebrafish exposed to >1 μM pendimethalin compared to ERM and DMSO solvent control groups. These data inform on the sub-lethal toxicity of pendimethalin to early stages of fish embryos and larvae.

Subchronic Exposure to Environmental Concentrations of Chlorpyrifos Affects Swimming Activity of Rainbow Trout Larvae

Chlorpyrifos (CPF), an organophosphorous pesticide, can be found in aquatic ecosystems at concentrations of up to several hundred nanograms per liter because of water runoff from treated crops. While some studies have shown that low concentrations of CPF may have adverse effects on aquatic species, comparatively little is known about its effect on fish embryos and larvae. To investigate the developmental effects of CPF, rainbow trout (Oncorhynchus mykiss) eyed-stage embryos were exposed in semistatic conditions to 0.3 and 3 µg/L of CPF up to the end of the sac-fry stage, 3 weeks, at 12 °C. Several endpoints were analyzed including survival, hatching delay, hatching success, biometry, swimming activity, DNA damage, lipid peroxidation, protein carbonyl content, acetylcholinesterase (AChE) activity, and gene expression. At the end of the 3-week exposure, larvae exposed to the highest concentration of CPF were less mobile compared to the control and the lowest CPF conditions. No significant differences in AChE activity were observed in either set of CPF conditions compared to control, but it was significantly reduced for larvae exposed to 3 µg/L compared to those exposed to 0.3 µg/L of CPF. Expression of genes that encoded estrogen receptor beta was downregulated for larvae exposed to both CPF concentrations. Expression of cytochrome P450 family 19 subfamily A member 1 was also significantly repressed but only on larvae exposed to the highest concentration of CPF. Our results indicated that subchronic exposure to environmental concentrations of CPF could lead to sublethal effects on early-life stages of rainbow trout, especially effects on swimming activity that could affect foraging activity and escaping from predators. Environ Toxicol Chem 2021;40:3092-3102. © 2021 SETAC.

Developmental Exposure to Low Concentrations of Methylmercury Causes Increase in Anxiety-Related Behaviour and Locomotor Impairments in Zebrafish

Methylmercury (MeHg) is a ubiquitous pollutant shown to cause developmental neurotoxicity, even at low levels. However, there is still a large gap in our understanding of the mechanisms linking early-life exposure to life-long behavioural impairments. Our aim was to characterise the short- and long-term effects of developmental exposure to low doses of MeHg on anxiety-related behaviours in zebrafish, and to test the involvement of neurological pathways related to stress-response. Zebrafish embryos were exposed to sub-acute doses of MeHg (0, 5, 10, 15, 30 nM) throughout embryo-development, and tested for anxiety-related behaviours and locomotor activity at larval (light/dark locomotor activity) and adult (novel tank and tap assays) life-stages. Exposure to all doses of MeHg caused increased anxiety-related responses; heightened response to the transition from light to dark in larvae, and a stronger dive response in adults. In addition, impairment in locomotor activity was observed in the higher doses in both larvae and adults. Finally, the expressions of several neural stress-response genes from the HPI-axis and dopaminergic system were found to be disrupted in both life-stages. Our results provide important insights into dose-dependent differences in exposure outcomes, the development of delayed effects over the life-time of exposed individuals and the potential mechanisms underlying these effects.

Defining drinking water metal contaminant mixture risk by coupling zebrafish behavioral analysis with citizen science

Contaminated drinking water is an important public health consideration in New England where well water is often found to contain arsenic and other metals such as cadmium, lead, and uranium. Chronic or high level exposure to these metals have been associated with multiple acute and chronic diseases, including cancers and impaired neurological development. While individual metal levels are often regulated, adverse health effects of metal mixtures, especially at concentrations considered safe for human consumption remain unclear. Here, we utilized a multivariate analysis that examined behavioral outcomes in the zebrafish model as a function of multiple metal chemical constituents of 92 drinking well water samples, collected in Maine and New Hampshire. To collect these samples, a citizen science approach was used, that engaged local teachers, students, and scientific partners. Our analysis of 4016 metal-mixture combinations shows that changes in zebrafish behavior are highly mixture dependent, and indicate that certain combinations of metals, especially those containing arsenic, cadmium, lead, and uranium, even at levels considered safe in drinking water, are significant drivers of behavioral toxicity. Our data emphasize the need to consider low-level chemical mixture effects and provide a framework for a more in-depth analysis of drinking water samples. We also provide evidence for the efficacy of utilizing citizen science in research, as the broader impact of this work is to empower local communities to advocate for improving their own water quality.

TCMacro: A Simple and Robust ImageJ-Based Method for Automated Measurement of Tail Coiling Activity in Zebrafish

Tail coiling is a reflection response in fish embryos that can be used as a model for neurotoxic analysis. The previous method to analyze fish tail coiling is largely based on third-party software. In this study, we aim to develop a simple and cost-effective method called TCMacro by using ImageJ macro to reduce the operational complexity. The basic principle of the current method is based on the dynamic change of pixel intensity in the region of interest (ROI). When the fish tail is moving, the average intensity is increasing. In time when the fish freeze, the peak of mean intensity is maintaining at a relatively low level. By using the optimized macro settings and excel VBA scripts, all the tail coiling measurement processes can be archived with few operation steps with high precision. Three major endpoints of tail coiling counts, tail coiling duration and tail coiling intervals can be obtained in batch. To validate this established method, we tested the potential neurotoxic activity of Tricaine (methanesulfonate, MS-222) and psychoactive compound of caffeine. Zebrafish embryos after Tricaine exposure displayed significantly less tail coiling activity in a dose-dependent manner, and were comparable to manual counting through the Wilcoxon test and Pearson correlation double validation. Zebrafish embryos after caffeine exposure displayed significantly high tail coiling activity. In conclusion, the TCMacro method presented in this study provides a simple and robust method that is able to measure the relative tail coiling activities in zebrafish embryos in a high-throughput manner.

Dangerous connections: biochemical and behavioral traits in Daphnia magna and Daphnia longispina exposed to ecologically relevant amounts of paracetamol

Exposure of nontarget organisms to therapeutic agents can cause distinct toxic effects, even at low concentrations. Paracetamol is a painkiller drug, widely used in human and veterinary therapies, being frequently found in the aquatic compartment in considerable amounts. Its toxicity has already been established for some species, but its full ecotoxicological potential is still not sufficiently described. To characterize the ecotoxicity of paracetamol, the present study evaluated several parameters, such as acute immobilization (EC₅₀ calculation), biochemical alterations, and behavioral effects, in two species of freshwater microcrustaceans of the genus Daphnia (D. magna and D. longispina). To increase the relevance of the data obtained, animals were exposed to levels of paracetamol similar to those already reported to occur in the wild. Data showed antioxidant responses in both species, namely an increase of catalase and GSTs activities in D. magna. On the contrary, effects of paracetamol on D. longispina included only an impairment of GSTs activity. Despite the absence of anticholinesterasic effects, behavioral modifications were also observed. This set of data indicates that realistic levels of paracetamol may trigger the activation of the antioxidant defense system of freshwater crustaceans, causing changes in behavioral traits (increase in swimming time, but with a reduction in swimming distance) of unknown etiology that are likely to affect normal life traits of wild populations.

Investigating the Predictive Power of Three Potential Sublethal Endpoints for the Fathead Minnow Fish Embryo Toxicity Test: Snout-Vent Length, Eye Size, and Pericardial Edema

The fish embryo acute toxicity (FET) test is known to be less sensitive than the fish acute test for some chemicals, including neurotoxicants. Thus, there is an interest in identifying additional endpoints that can improve FET test performance. The goal of this project was to advance alternative toxicity testing methods by determining whether select developmental abnormalities-snout-vent length, eye size, and pericardial area-are linked to adverse alterations in ecologically-relevant behaviors and delayed mortality. Fathead minnow (Pimephales promelas) FET tests were conducted with 3,4-dicholoroaniline, cadmium, and perfluorooctanesulfonic acid (PFOS) and developmental abnormalities were quantified. Surviving eleutheroembryos were reared in clean water to 14 days post fertilization (dpf), during which time behaviors and mortality were evaluated. None of the abnormalities evaluated were predictive of behavioral alterations; however, embryos with ≥14% reductions in length or ≥3.54-fold increases in pericardial area had an 80% chance of mortality by 14 dpf. When these abnormalities were used as markers of mortality, the LC₅₀s for cadmium and PFOS were less than those calculated using only standardized FET test endpoints and similar to those obtained via larval fish tests, indicating that the snout-vent length and pericardial area warrant consideration as standard FET test endpoints.

Zebrafish as a Translational Model: An Experimental Alternative to Study the Mechanisms Involved in Anosmia and Possible Neurodegenerative Aspects of COVID-19?

The Coronavirus disease-2019 (COVID-19) presents a variability of clinical symptoms, ranging from asymptomatic to severe respiratory and systemic conditions. In a cohort of patients, the Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2), beyond the classical respiratory manifestations, induces anosmia. Evidence has suggested SARS-CoV-2-induced anosmia can be the result of neurodegeneration of the olfactory pathway. Neurologic symptoms associated with COVID-19 have been reported; however, the precise mechanism and possible long-lasting effects remain poorly investigated. Preclinical models are valuable tools for describing and testing new possible treatments for neurologic disorders. In this way, the zebrafish (Danio rerio) organism model represents an attractive tool in the field of neuroscience, showing economic and logistic advantages besides genetic and physiologic similarities with mammalian, including the brain structure and functions. Besides, its external embryonic development, high availability of eggs, and fast development allows easy genetic manipulation and fast replications. In the present review, we suggest that the zebrafish model can be advantageous to investigate the neurologic features of COVID-19.

Does the food processing contaminant acrylamide cause developmental neurotoxicity? A review and identification of knowledge gaps

There is a worldwide concern on adverse health effects of dietary exposure to acrylamide (AA) due to its presence in commonly consumed foods. AA is formed when carbohydrate rich foods containing asparagine and reducing sugars are prepared at high temperatures and low moisture conditions. Upon oral intake, AA is rapidly absorbed and distributed to all organs. AA is a known human neurotoxicant that can reach the developing foetus via placental transfer and breast milk. Although adverse neurodevelopmental effects have been observed after prenatal AA exposure in rodents, adverse effects of AA on the developing brain has so far not been studied in humans. However, epidemiological studies indicate that gestational exposure to AA impair foetal growth and AA exposure has been associated with reduced head circumference of the neonate. Thus, there is an urgent need for further research to elucidate whether pre- and perinatal AA exposure in humans might impair neurodevelopment and adversely affect neuronal function postnatally. Here, we review the literature with emphasis on the identification of critical knowledge gaps in relation to neurodevelopmental toxicity of AA and its mode of action and we suggest research strategies to close these gaps to better protect the unborn child.

Impact of environmental neurotoxic: current methods and usefulness of human stem cells

The development of central nervous system is a highly coordinated and complex process. Any alteration of this process can lead to disturbances in the structure and function of the brain, which can cause deficits in neurological development, resulting in neurodevelopmental disorders, including, for example, autism or attention-deficit hyperactivity disorder. Exposure to certain chemicals during the fetal period and childhood is known to cause developmental neurotoxicity and has serious consequences that persist into adult life. For regulatory purposes, determination of the potential for developmental neurotoxicity is performed according the OECD Guideline 426, in which the test substance is administered to animals during gestation and lactation. However, these animal models are expensive, long-time consuming and may not reflect the physiology in humans; that makes it an unsustainable model to test the large amount of existing chemical products, hence alternative models to the use of animals are needed. One of the most promising methods is based on the use of stem cell technology. Stem cells are undifferentiated cells with the ability to self-renew and differentiate into more specialized cell types. Because of these properties, these cells have gained increased attention as possible therapeutic agents or as disease models. Here, we provide an overview of the current models both animal and cellular, available to study developmental neurotoxicity and review in more detail the usefulness of human stem cells, their properties and how they are becoming an alternative to evaluate and study the mechanisms of action of different environmental toxicants.

Zebrafish (Danio rerio) as an excellent vertebrate model for the development, reproductive, cardiovascular, and neural and ocular development toxicity study of hazardous chemicals

In the past decades, the type of chemicals has gradually increased all over the world, and many of these chemicals may have a potentially toxic effect on human health. The zebrafish, as an excellent vertebrate model, is increasingly used for assessing chemical toxicity and safety. This review summarizes the efficacy of zebrafish as a model for the study of developmental toxicity, reproductive toxicity, cardiovascular toxicity, neurodevelopmental toxicity, and ocular developmental toxicity of hazardous chemicals, and the transgenic zebrafish as biosensors are used to detect the environmental pollutants.

Low-dose methylmercury exposure impairs the locomotor activity of zebrafish: Role of intestinal inositol metabolism

Methylmercury (MeHg) is a ubiquitous environmental toxicant with neurotoxic effects. Although its neurotoxicity had been more studied, the role of gut microbiota remains unclear. In this study, adult zebrafish and larvae were exposed to MeHgCl at the dose of 0, 1 and 10 ng/mL. MeHgCl exposure impaired the locomotor activity via upregulation of apoptosis and autophagy related genes in the brain. Intestinal and cerebral metabolome indicated that phosphatidylinositol signaling system and inositol phosphate metabolism pathways were significantly impacted in adult zebrafish upon MeHgCl exposure. The levels of myo-inositol (MI) in the intestine and brain were decreased and positively correlated. 16 S rRNA sequencing data from adult zebrafish showed that MeHgCl exposure also shifted the structure of gut microbiota and reduced the relative abundance of Bacteroidetes and Proteobacteria, which were further identified at genus level as Aeromonas and Cetobacterium. Further functional analysis indicated that MeHgCl disrupted inositol phosphate metabolism of gut microbiota. Notably, MI supplementation restored the impairment of locomotor activity and inhibited the upregulation of apoptosis and autophagy related genes, such as bcl-2 and atg5. Thus, this study not only revealed the key role of gut microbiota in MeHgCl-mediated neurotoxicity but also gave new insights into antagonizing its toxicity.

Developmental exposure to mepanipyrim induces locomotor hyperactivity in zebrafish (Danio rerio) larvae

Mepanipyrim is a widely used fungicide, and residues of mepanipyrim are frequently detected in commodities. However, the neurotoxicity and underlying mechanisms of mepanipyrim are still insufficiently understood. In this study, zebrafish embryos at 0.5-1.0 post-fertilization hours (hpf) were exposed to 0.1, 1, 10 and 100 μg/L mepanipyrim for 7 days. Our results showed that mepanipyrim could cause the locomotor hyperactivity and increase the concentration of γ-amino butyric acid (GABA) and the Na⁺/K⁺- and Ca²⁺-ATPase activities in zebrafish larvae. We have conducted the RNA-sequence and RT-qPCR to analyze the gene expressions. The mRNA expression levels of calcium/sodium ion conduction associated genes were observably up-regulated, demonstrating that mepanipyrim could enhance the cell energy metabolism, the synaptic transmission and skeletal muscle contraction, which were consistent with the locomotor hyperactivity. Meanwhile, exposure to mepanipyrim could significantly change the gene expression levels of gad1, bdnf, nlgn1, and type A and B GABA receptors in zebrafish larvae. This is the first study focusing on the underlying mechanisms of the neurotoxic effects that are induced by mepanipyrim.

Differential responses of larval zebrafish to the fungicide propamocarb: Endpoints at development, locomotor behavior and oxidative stress

The fungicide propamocarb (PM) is widely used to protect cucumbers, tomatoes and other plants from pathogens. According to previous studies, PM could be detected in the aquatic system in some area. However, the toxic effects of PM on zebrafish received very limited attention. In this study, we examined the toxic effects of various concentration of PM on the endpoints of development, locomotor behavior and oxidative stress in larval zebrafish. It was observed that PM exposure delayed embryonic development, inhibited hatchability at 60 and 72 h postfertilization and increased heart rate. After PM exposure, the larval zebrafish showed abnormal free swimming behavior and the swimming behavior in response to light-dark transition, indicating that PM had the potential to induce neurotoxicity. Moreover, PM exposure also affected the enzymatic activity of acetylcholinesterase and dopamine and the transcriptional level of genes related to neurotoxicity. In addition, PM exposure not only affects catalase (CAT), glutathione peroxidase (GPX), and glutathione S-transferase (GST) activity but also affects the transcription level of various genes. We believed that PM induced oxidative stress was also a possible reason to cause neurotoxicity in larval zebrafish. In summary, our results suggested that PM could disturb the endpoints at development, locomotor behavior and oxidative stress in larval zebrafish.

Acid mine drainage (AMD) treatment by neutralization: Evaluation of physical-chemical performance and ecotoxicological effects on zebrafish (Danio rerio) development

Acid mine drainage (AMD) represents a major problem in the mining industry worldwide due to the risk of water and soil pollution. Its active treatment involves the addition of alkaline reagents such as NaOH or Ca(OH)₂ to increase the pH and precipitate the dissolved metals, although substantial amounts of dissolved ions might persists. Under a remediation approach, the aim of this work was to assess the chemical and physical characteristics of treated effluent and to evaluate its ecotoxicological effects on zebrafish (Danio rerio) embryonic and larval stages, through developmental, functional, morphological, and behavioral end-points. The studied AMD sample, highly associated with pyrite, presented high sulfate and dissolved metal ions content and was submitted to the following treatment conditions: NaOH - pH 7.0 and 8.7, and Ca(OH)₂ - pH 7.0 and 8.7. All neutralizing treatments resulted in a satisfactory reduction of the metals concentration, with best results achieved using Ca(OH)₂ at pH 8.7; although Mn and As still remained above or very near the discharge maximum limits according to Brazilian legislation. Therefore, an additional step was employed to Mn and As adsorption by algal biomass. Regarding in-vivo toxicological assays, no significant lethality was recorded in all treated AMD groups, although adverse effects were observed in all endpoints analyzed. Ca(OH)₂ groups performed closer to control than NaOH-treated groups. The additional polishing stage treatment with the algae Scenesmus sp. allowed tenuous improvements in terms of removal of residual amounts of As and Mn but not in the toxicological characteristics of treated AMD.

Acute exposure of zebrafish (Danio rerio) larvae to environmental concentrations of selected antidepressants: Bioaccumulation, physiological and histological changes

Antidepressants have been detected in surface waters worldwide at ng-μg/L concentration. These compounds can exert adverse effects on fish even at low levels. But, all previous analyses have concentrated on adult fish. The aim of the study was to assess the effect of environmental concentrations of sertraline, paroxetine, fluoxetine and mianserin, and their mixtures on such unusual endpoints as physiological and histological changes of zebrafish (Danio rerio) larvae. We also determined the bioconcentration of the pharmaceuticals. Fish Embryo Toxicity test was used to analyze the influence on developmental progression. Histological sections were stained with hematoxylin and eosin. Proliferating cells in liver were determined immunohistochemically by detection of Proliferating Cell Nuclear Antigens. The bioconcentration factor was measured by liquid chromatography coupled to mass spectrometry. Pharmaceuticals were used at low, medium and high concentrations in mixtures and at medium concentration as single compound. Exposure to the analyzed pharmaceuticals increased the rate of abnormal embryo and larvae development, accelerated the hatching time and affected the total hatching rate. Three-times lower proliferation of hepatocytes was observed in larvae exposed to paroxetine, mianserin, sertraline and the mixture of the pharmaceuticals at the highest concentrations. The highest bioaccumulation factor (BCF) was obtained for sertraline. The BCF of the analyzed compounds was higher if the organisms were exposed to the mixtures than to single pharmaceuticals. To conclude, the exposure of zebrafish larvae to selected antidepressants and their mixtures may cause disturbances in the organogenesis of fish even at environmental concentrations.

Analysis of tail coiling activity of zebrafish (Danio rerio) embryos allows for the differentiation of neurotoxicants with different modes of action

In (eco)toxicology, there is a critical need for efficient methods to evaluate the neurotoxic potential of environmental chemicals. Recent studies proposed analysis of early coiling activity in zebrafish embryos as a powerful tool for the identification of neurotoxic compounds. In order to demonstrate that the analysis of early tail movements of zebrafish embryos allows for the discrimination of neurotoxicants acting via different mechanisms, the present study investigated the effects of four different neurotoxicants on the embryogenesis (fish embryo toxicity test) and early tail coiling movements of zebrafish embryos. Cadmium predominantly increased the frequency of tail coiling at the late pharyngula stage. Dichlorvos delayed embryonic development and caused convulsive tail movements resulting in prolonged duration of tail coils. Embryos exposed to teratogenic concentrations of fluoxetine and citalopram displayed absence of spontaneous tail movements at 24 h post-fertilization. In contrast, a non-teratogenic test concentration of citalopram decreased coiling frequency at multiple time points. Results demonstrated that the analysis of tail coiling movements of zebrafish embryos has the potential to discriminate neurotoxic compounds with different primary modes of action. In addition, chemical-induced effects on coiling activity were shown to potentially overlap with effects on embryogenesis. Further studies are needed to clarify the interplay of unspecific developmental toxicity of neurotoxic chemicals and effects resulting from specific neurotoxic mechanisms.

Clethodim exposure induced development toxicity and behaviour alteration in early stages of zebrafish life

Clethodim is one of the most widely used herbicides in agriculture, however, its potential toxic effects on organisms and the underlying toxicity mechanism are still poorly understood. In this study, zebrafish embryos at 6 h post-fertilization (hpf) were exposed to 10 mg/L, 20 mg/L, and 30 mg/L clethodim for up to 24 hpf, and zebrafish larvae at 6 days post-fertilization (dpf) were exposed to the same density gradient for 24 h. Our results showed that clethodim could cause head and cardiovascular malformations in embryos: blurred brain ventricles, unapparent brain regions, condensation of nucleus and cytoplasm in brain cells, increased intercellular space, developmental malformations of eyes and ears, reduced neonatal neurons, disorder migration of neural ridge cells; morphological aberrations of the vascular ICM, slowing of heart beat and blood flow, reduction of circulating red blood cells, and delayed development of head and tail blood vessels. These defects could be a result of clethodim-induced oxidative stress and decreased acetylcholinesterase (AChE) activity, which in turn affected the expression of neurodevelopmental genes, decreased ATPase activity, and ultimately led to developmental malformations. The swimming behaviour of zebrafish larvae was observed to decrease with increasing concentration of clethodim exposure, but the angular velocity and mobility increased. These could be due to reduced AChE activity and disturbed gene expression of GABA, dopamine and glutamatergic neurotransmitter systems, which thus altered the locomotor behaviour. In summary, we found that clethodim induces developmental toxicity and neurotoxicity in zebrafish embryos and larvae.

Time-course of coiling activity in zebrafish (Danio rerio) embryos exposed to ethanol as an endpoint for developmental neurotoxicity (DNT) - Hidden potential and underestimated challenges

Detection of developmental neurotoxicity (DNT) has been recognized as a major challenge by regulatory bodies and science. In search of sensitive and specific test methods, spontaneous tail coiling of embryonic zebrafish has been recommended as a promising tool for identification of DNT-inducing chemicals. The present study was designed to develop a protocol for a prolonged test to study neurotoxicity during the entire development of coiling movement in zebrafish embryos. Ambient illumination was found to modulate coiling activity from the very onset of tail movements representing the earliest behavioral response to light possible in zebrafish. In the dark, embryos displayed increased coiling activity in a way known from photokinesis, a stereotypical element of the visual motor response. Elevated coiling activity during dark phases allows for the development of test strategies that integrate later coiling movements under the control of a further developed nervous system. Furthermore, zebrafish embryos were exposed to ethanol, and coiling activity was analyzed according to the new test protocol. Exposure of embryos to non-teratogenic concentrations of ethanol (0.4-1%) resulted in a delay of the onset of coiling activity and heartbeat. Moreover, ethanol produced a dose-dependent increase in coiling frequency at 26 h post-fertilization, indicating the involvement of neurotoxic mechanisms. Analysis of coiling activity during prolonged exposure allowed for (1) attributing effects on coiling activity to different mechanisms and (2) preventing false interpretation of results. Further research is needed to verify the potential of this test protocol to distinguish between different mechanisms of neurotoxicity.

Embryotoxicity of ozonated diclofenac, carbamazepine, and oxazepam in zebrafish (Danio rerio)

Pharmaceutical residues are polluting the surface water environments worldwide. Sewage and wastewater treatment, therefore, needs to be improved in order to remove pharmaceutical residues from the effluent. One such treatment improvement is effluent ozonation. Even though ozonation has proven to be very efficient in reducing pharmaceutical parent compound concentrations in wastewater effluents, much remains unclear regarding potentially toxic ozonation by-product (OBP) formation. In this study, we sought to elucidate the aquatic toxicity of ozonated pharmaceuticals in zebrafish (Danio rerio) embryos in a static 144 h post fertilization (hpf) fish embryotoxicity (ZFET) assay. Three pharmaceuticals commonly detected in wastewater effluents, i.e. carbamazepine, diclofenac, and oxazepam, were selected for testing. Toxicity was assessed before and after 1 min ozonation (0.053 mg L^-1 peak O₃ concentration) and 10 min ozonation (0.147 mg L^-1 peak O₃ concentration). Chemical analysis showed that carbamazepine and diclofenac were largely removed by ozone (90 ± 11% and 97 ± 3.8%), whereas oxazepam was removed to a lesser extent (19 ± 5.7%). The ZFET assay revealed diverging toxicities. Diclofenac embryotoxicity decreased with increasing ozonation. Oxazepam did not cause embryotoxicity in the ZFET assay either pre- or post ozonation, but larvae swimming activity was affected at 144 hpf. Carbamazepine embryotoxicity, on the other hand, increased with increasing ozonation. Chemical analysis showed the formation of two OBPs (carbamazepine-10,11-epoxide and 10,11-dihydrocarbamazepine), possibly explaining the increased embryotoxicity. The results of this study highlight the importance of new chemical and toxicological knowledge regarding the formation of OBPs in post-ozonated effluents.

Identifying sublethal endpoints for evaluating neurotoxic compounds utilizing the fish embryo toxicity test

Fish embryos are increasingly being utilized in aquatic toxicity testing, as evidenced by the Organisation for Economic Co-operation and Development's approval of the fish embryo acute toxicity (FET) test. However, the FET test only allows for the estimation of acute toxicity, whereas other test methods such as the larval growth and survival (LGS) test allow for the estimation of both acute and chronic toxicity. Additionally, it has been demonstrated that the FET test is less sensitive than other test methods for some neurotoxic compounds. To address these limitations, efforts to identify sublethal endpoints that increase FET test sensitivity and allow for the prediction of sublethal adverse effects have begun. As such, the objectives of the current study were 1) to compare estimated LC50 values from the FET and LGS test for three known neurotoxicants: fluoride (F), nickel (Ni), and cadmium (Cd) and 2) to evaluate the responsiveness of potential sublethal endpoints for the FET test related to growth (i.e., wet weight and snout-vent length), neurological development (i.e., spontaneous contraction frequency and eye size), and cardiovascular function (i.e., heart rate and pericardial area). The calculated LC50 values from the F and Cd FET test were significantly higher than those from the LGS test, demonstrating that the FET test is less sensitive than the LGS test for neurotoxic compounds. Only Cd exposure resulted in alterations in any of the sublethal endpoints investigated. Embryos/eleutheroembryos exposed to Cd displayed alterations in length, eye size, and pericardial area at concentrations five-fold less than the estimated LC50 value, suggesting that for Cd the inclusion of these sublethal endpoints would improve the sensitivity of the FET test. Overall, these results provide evidence that for some neurotoxicants, the inclusion of sublehtal endpoints may improve the utility of the FET test; however, further research utilizing a broader range of neurotoxicants with differing mechanisms of action, is needed to fully establish such endpoints in the context of routine FET test.

Comparative effects of mercury chloride and methylmercury exposure on early neurodevelopment in zebrafish larvae

Mercury (Hg) is a ubiquitous environmental toxicant with important public health implications. Hg causes neurotoxicity through astrocytes, Ca2+, neurotransmitters, mitochondrial damage, elevations of reactive oxygen species and post-translational modifications. However, the similarities and differences between the neurotoxic mechanisms caused by different chemical forms of Hg remain unclear. Zebrafish embryos were exposed to methylmercury (MeHgCl) or mercury chloride (HgCl2) (0, 4, 40, 400 nM) up for 96 h. HgCl2 exposure could significantly decrease survival rate, body length and eye size, delay the hatching period, induce tail bending and reduce the locomotor activity, and these effects were aggravated in the MeHgCl group. The compounds could increase the number of apoptotic cells in the brain and downregulate the expression of Shha, Ngn1 and Nrd, which contribute to early nervous development. The underlying mechanisms were investigated by metabolomics data. Galactose metabolism, tyrosine metabolism and starch and sucrose metabolism pathways were disturbed after HgCl2 or MeHgCl exposure. In addition, the levels of three neurotransmitters including tyrosine, dopamine and tryptophan were reduced after HgCl2 or MeHgCl exposure. Oxidative stress is related to metabolite changes, such as changes in the putrescine, niacinamide and uric acid contents in the HgCl2 group, and squalene in the MeHgCl group. These data indicated that downregulation of these genes and abnormal metabolic profile and pathways contribute to the neurotoxicity of HgCl2 and MeHgCl.