Bisphenol A affects axonal growth, musculature and motor behavior in developing zebrafish

Advancing statistical treatment of photolocomotor behavioral response study data

Fish photolocomotor behavioral response (PBR) studies have become increasingly prevalent in pharmacological and toxicological research to assess the environmental impact of various chemicals. There is a need for a standard, reliable statistical method to analyze PBR data. The most common method currently used, univariate analysis of variance (ANOVA), does not account for temporal dependence in observations and leads to incomplete or unreliable conclusions. Repeated measures ANOVA, another commonly used method, has drawbacks in its interpretability for PBR study data. Because each observation is collected continuously over time, we instead consider each observation to be a function and apply functional ANOVA (FANOVA) to PBR data. Using the functional approach not only accounts for temporal dependency but also retains the full structure of the data and allows for straightforward interpretation in any subregion of the domain. Unlike the traditional univariate and repeated measures ANOVA, the FANOVA that we propose is nonparametric, requiring minimal assumptions. We demonstrate the disadvantages of univariate and repeated measures ANOVA using simulated data and show how they are overcome by applying FANOVA. We then apply one-way FANOVA to zebrafish data from a PBR study and discuss how those results can be reproduced for future PBR studies.

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.

Effects and mechanisms of bisphenols exposure on neurodegenerative diseases risk: A systemic review

Environmental bisphenols (BPs) pose a global threat to human health because of their extensive use as additives in plastic products. BP residues are increasing in various environmental media (i.e., water, soil, and indoor dust) and biological and human samples (i.e., serum and brain). Both epidemiological and animal studies have determined an association between exposure to BPs and an increased risk of neurodegenerative diseases (e.g., Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis), including cognitive abnormalities and behavioral disturbances. Hence, understanding the biological responses to different BPs is essential for prevention, and treatment. This study provides an overview of the underlying pathogenic molecular mechanisms as a valuable basis for understanding neurodegenerative disease responses to BPs, including accumulation of misfolded proteins, reduction of tyrosine hydroxylase and dopamine, abnormal hormone signaling, neuronal death, oxidative stress, calcium homeostasis, and inflammation. These findings provide new insights into the neurotoxic potential of BPs and ultimately contribute to a comprehensive health risk evaluation.

Vitamin A ameliorates valproic acid-induced autism-like symptoms in developing zebrafish larvae by attenuating oxidative stress and apoptosis

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by social deficits and repetitive/stereotyped behaviors. Prenatal exposure to valproic acid (VPA) has been reported to induce ASD-like symptoms in human and rodents. However, the etiology and pathogenesis of ASD have not been well elucidated. This study aimed to explore the mechanisms underlying VPA-induced ASD-like behaviors using zebrafish model and investigated whether vitamin A could prevent VPA-induced neurotoxicity. Here, zebrafish embryos were exposed to 0, 25 and 50 μM VPA from 4 to 96 h post fertilization (hpf) and the neurotoxicity was assessed. Our results showed that VPA affected the normal development of zebrafish larvae and induced ASD-like behaviors, including reduced locomotor activity, decreased distance near conspecifics, impaired social interaction and repetitive swimming behaviors. Exposure to VPA decreased the GFP signal in transgenic HuC:egfp zebrafish according to the negative effect of VPA on the expression of neurodevelopmental genes. In addition, VPA enhanced oxidative stress by promoting the production of reactive oxygen species (ROS) and hydrogen peroxide (H2O2) and inhibiting the activity of superoxide dismutase, then triggered apoptosis by upregulation of apoptotic genes. These adverse outcomes were mitigated by vitamin A, suggesting that vitamin A rescued VPA-induced ASD-like symptoms by inhibiting oxidative stress and apoptosis. Overall, this study identified vitamin A as a promising strategy for future therapeutic regulator of VPA-induced ASD-like behaviors.

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.

Swimming against ALS: How to model disease in zebrafish for pathophysiological and behavioral studies

Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease that leads to progressive disability and motor impairment. Existing therapies provide modest improvements in patient survival, raising a need for new treatments for ALS. Zebrafish is a promising model animal for translational and fundamental research in ALS - it is an experimentally tractable vertebrate, with high homology to humans and an ample experimental toolbox. These advantages allow high-throughput study of behavioral and pathophysiological phenotypes. The last decade saw an increased interest in modelling ALS in zebrafish, leading to the current abundance and variety of available methods and models. Additionally, the rise of gene editing techniques and toxin combination studies has created novel opportunities for ALS studies in zebrafish. In this review, we address the relevance of zebrafish as a model animal for ALS studies, the strategies for model induction and key phenotypical evaluation. Furthermore, we discuss established and emerging zebrafish models of ALS, analyzing their validity, including their potential for drug testing, and highlighting research opportunities in this area.

Toxicity and Estrogenicity of Bisphenol TMC in Oryzias melastigma via In Vivo and In Silico Studies

Bisphenol 4-[1-(4-hydroxyphenyl)-3,3,5-trimethylcyclohexyl] phenol (BPTMC), as a substitute for bisphenol A, has been detected in environments. However, the ecotoxicological data of BPTMC are extremely scarce. Here, the lethality, developmental toxicity, locomotor behavior, and estrogenic activity of BPTMC at different concentrations (0.25-2000 μg/L) in marine medaka (Oryzias melastigma) embryos were examined. In addition, the in silico binding potentials of O. melastigma estrogen receptors (omEsrs) with BPTMC were assessed by docking study. Low-concentration BPTMC exposure (including an environmentally relevant concentration, 0.25 μg/L) resulted in stimulating effects, including hatching rate, heart rate, malformation rate, and swimming velocity. However, elevated concentrations of BPTMC led to an inflammatory response, changed heart rate and swimming velocity in the embryos and larvae. In the meantime, BPTMC (including 0.25 μg/L) altered the concentrations of estrogen receptor, vitellogenin, and endogenous 17 β-estradiol as well as the transcriptional levels of estrogen-responsive genes in the embryos or/and larvae. Furthermore, elaborate tertiary structures of omEsrs were built by ab initio modeling, and BPTMC exerted potent binding potential with three omEsrs with -47.23, -49.23, and -50.30 kJ/mol for Esr1, Esr2a, and Esr2b, respectively. This work suggests that BPTMC has potent toxicity and estrogenic effects in O. melastigma.

Multi-biomarker approach and IBR index to evaluate the effects of bisphenol A on embryonic stages of zebrafish (Danio rerio)

This study assessed the effects of Bisphenol A in embryonic stages of zebrafish, applying an IBR multi-biomarker approach that included alterations in growth and oxidative status and relates it with the expression of Nrf1, Nrf2, Wnt3a, Wnt8a, COX-2, Qdpra, and DKK1 genes. For this purpose, we exposed zebrafish embryos to eight environmentally relevant concentrations of BPA (220, 380, 540, 700, 860, 1180, 1340, and 1500 ng L^-1) until 96 h post-fertilization. Our results show that BPA induces several malformations in embryos (developmental delay, hypopigmentation, tail malformations, and on), leading to their death. The LC₅₀, EC₅₀ of malformations, and teratogenic index (TI) were 1234.60 ng L^-1, 987.77 ng L^-1, and 1.25, respectively; thus, this emerging contaminant is teratogenic. Regarding oxidative stress and gene expression, we demonstrated BPA altered oxidative status and the gene expression in embryos of Danio rerio.

High glucose-induced ROS-accumulation in embryo-larval stages of zebrafish leads to mitochondria-mediated apoptosis

In recent decades, diabetes mellitus has become a major chronic disease threatening human health worldwide, and the age of patients tends to be younger; however, the pathogenesis remains unclear, resulting in many difficulties in its treatment. As an ideal model animal, zebrafish can simulate the processes of human diabetes well. In this study, we successfully established a model of diabetic zebrafish larvae in a previous work. Furthermore, transcriptome analysis was completed, and the results suggested that 10.59% of differentially expressed genes (DEGs) related to the apoptosis pathway need to be considered. Then, glucose-induced developmental toxicity, reactive oxygen species (ROS) accumulation, antioxidant system function, apoptosis and mitochondrial dysfunction were measured in zebrafish larvae. We hope that this study will provide valuable reference information for type 2 juvenile diabetes treatment.

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.

Bisphenol A Exposure Induces Sensory Processing Deficits in Larval Zebrafish during Neurodevelopment

Because of their ex utero development, relatively simple nervous system, translucency, and availability of tools to investigate neural function, larval zebrafish are an exceptional model for understanding neurodevelopmental disorders and the consequences of environmental toxins. Furthermore, early in development, zebrafish larvae easily absorb chemicals from water, a significant advantage over methods required to expose developing organisms to chemical agents in utero Bisphenol A (BPA) and BPA analogs are ubiquitous environmental toxins with known molecular consequences. All humans have measurable quantities of BPA in their bodies. Most concerning, the level of BPA exposure is correlated with neurodevelopmental difficulties in people. Given the importance of understanding the health-related effects of this common toxin, we have exploited the experimental advantages of the larval zebrafish model system to investigate the behavioral and anatomic effects of BPA exposure. We discovered that BPA exposure early in development leads to deficits in the processing of sensory information, as indicated by BPA's effects on prepulse inhibition (PPI) and short-term habituation (STH) of the C-start reflex. We observed no changes in locomotion, thigmotaxis, and repetitive behaviors (circling). Despite changes in sensory processing, we detected no regional or whole-brain volume changes. Our results show that early BPA exposure can induce sensory processing deficits, as revealed by alterations in simple behaviors that are mediated by a well-defined neural circuit.

Does Bisphenol A Confer Risk of Neurodevelopmental Disorders? What We Have Learned from Developmental Neurotoxicity Studies in Animal Models

Substantial evidence indicates that bisphenol A (BPA), a ubiquitous environmental chemical used in the synthesis of polycarbonate plastics and epoxy resins, can impair brain development. Clinical and epidemiological studies exploring potential connections between BPA and neurodevelopmental disorders in humans have repeatedly identified correlations between early BPA exposure and developmental disorders, such as attention deficit/hyperactivity disorder and autism spectrum disorder. Investigations using invertebrate and vertebrate animal models have revealed that developmental exposure to BPA can impair multiple aspects of neuronal development, including neural stem cell proliferation and differentiation, synapse formation, and synaptic plasticity-neuronal phenotypes that are thought to underpin the fundamental changes in behavior-associated neurodevelopmental disorders. Consistent with neuronal phenotypes caused by BPA, behavioral analyses of BPA-treated animals have shown significant impacts on behavioral endophenotypes related to neurodevelopmental disorders, including altered locomotor activity, learning and memory deficits, and anxiety-like behavior. To contextualize the correlations between BPA and neurodevelopmental disorders in humans, this review summarizes the current literature on the developmental neurotoxicity of BPA in laboratory animals with an emphasis on neuronal phenotypes, molecular mechanisms, and behavioral outcomes. The collective works described here predominantly support the notion that gestational exposure to BPA should be regarded as a risk factor for neurodevelopmental disorders.

The Role of Estrogen and Thyroid Hormones in Zebrafish Visual System Function

Visual system development is a highly complex process involving coordination of environmental cues, cell pathways, and integration of functional circuits. Consequently, a change to any step, due to a mutation or chemical exposure, can lead to deleterious consequences. One class of chemicals known to have both overt and subtle effects on the visual system is endocrine disrupting compounds (EDCs). EDCs are environmental contaminants which alter hormonal signaling by either preventing compound synthesis or binding to postsynaptic receptors. Interestingly, recent work has identified neuronal and sensory systems, particularly vision, as targets for EDCs. In particular, estrogenic and thyroidogenic signaling have been identified as critical modulators of proper visual system development and function. Here, we summarize and review this work, from our lab and others, focusing on behavioral, physiological, and molecular data collected in zebrafish. We also discuss different exposure regimes used, including long-lasting effects of developmental exposure. Overall, zebrafish are a model of choice to examine the impact of EDCs and other compounds targeting estrogen and thyroid signaling and the consequences of exposure in visual system development and function.

Effects of common environmental endocrine-disrupting chemicals on zebrafish behavior

Environmental endocrine-disrupting chemicals (EDCs), a type of exogenous organic pollutants, are ubiquitous in natural aquatic environments. Therefor, this review focused on the use of the zebrafish as a model to explore the effect of different EDCs on behavior, as well as the molecular mechanisms that drive these effects. Furthermore, our study summarizes the current knowledge on the neuromodulatory effects of different EDCs in zebrafish. This study also reviews the current state of zebrafish behavior research, in addition to the potential mechanisms of single and mixed pollutant-driven behavioral dysregulation at the molecular level, as well as the applications of zebrafish behavior experiments for neuroscience research. This review broadens our understanding of the influence of EDCs on zebrafish behavior and provides guidance for future research.

Bisphenol A induced toxicity in blood cells of freshwater fish Channa punctatus after acute exposure

The widespread use of bisphenol A (BPA) has led to its ubiquity in the natural environment. It is extensively incorporated into different industrial products and is associated with deleterious health effects on both public and wildlife. The current trial was conducted to determine the toxic potential of bisphenol A using various parameters viz haematological, biochemical, and cytological in freshwater fish Channa punctatus. For this purpose, fish were exposed to 1.81 mg/l (1/4 of LC₅₀) and 3.81 mg/l (1/2 of LC₅₀) of BPA along with positive (acetone) and negative controls (water) for 96 h. The blood samples were collected at 24, 48, 72, and 96 h post-exposure. Compared to the control group, fish after acute exposure to BPA showed a significant decrease in HB content, number of red blood cells, PCV values whereas a significant increase in WBCs count was recorded with an increase in the exposure period. Besides, oxidative stress (determined as malondialdehyde content) increased as BPA concentration increased. Further, the activity of different antioxidant enzymes like catalase, and superoxide dismutase decreased significantly after treatment. Results also showed significantly increased frequency of morphological alterations, nuclear changes, and increased DNA damage potential of BPA in red blood cells. Further structural analysis of erythrocytes in maximally damaged group using Scanning Electron Microscopy was performed. The study concludes that BPA exhibits genotoxic activity and oxidative stress could be one of the mechanisms leading to genetic toxicity.

Zebrafish Optomotor Response and Morphology Are Altered by Transient, Developmental Exposure to Bisphenol-A

Estrogen-specific endocrine disrupting compounds (EDCs) are potent modulators of neural and visual development and common environmental contaminants. Using zebrafish, we examined the long-term impact of abnormal estrogenic signaling by testing the effects of acute, early exposure to bisphenol-A (BPA), a weak estrogen agonist, on later visually guided behaviors. Zebrafish aged 24 h postfertilization (hpf), 72 hpf, and 7 days postfertilization (dpf) were exposed to 0.001 μM or 0.1 μM BPA for 24 h, and then allowed to recover for 1 or 2 weeks. Morphology and optomotor responses (OMRs) were assessed after 1 and 2 weeks of recovery for 24 hpf and 72 hpf exposure groups; 7 dpf exposure groups were additionally assessed immediately after exposure. Increased notochord length was seen in 0.001 μM exposed larvae and decreased in 0.1 μM exposed larvae across all age groups. Positive OMR was significantly increased at 1 and 2 weeks post-exposure in larvae exposed to 0.1 μM BPA when they were 72 hpf or 7 dpf, while positive OMR was increased after 2 weeks of recovery in larvae exposed to 0.001 μM BPA at 72 hpf. A time-delayed increase in eye diameter occurred in both BPA treatment groups at 72 hpf exposure; while a transient increase occurred in 7 dpf larvae exposed to 0.1 μM BPA. Overall, short-term developmental exposure to environmentally relevant BPA levels caused concentration- and age-dependent effects on zebrafish visual anatomy and function.

Zebrafish (Danio rerio) using as model for genotoxicity and DNA repair assessments: Historical review, current status and trends

Genotoxic pollutants lead to both DNA damage and changes in cell repair mechanisms. Selecting suitable biomonitors is a fundamental step in genotoxicity studies. Thus, zebrafish have become a popular model used to assess the genotoxicity of different pollutants in recent years. They have orthologous genes with humans and hold almost all genes involved in different repair pathways. Therefore, the aim of the current study is to summarize the existing literature on zebrafish using as model system to assess the genotoxicity of different pollutants. Revised data have shown that comet assay is the main technique adopted in these studies. However, it is necessary standardizing the technique applied to zebrafish in order to enable better result interpretation and comparisons. Overall, pollutants lead to single-strand breaks (SSB), double-strand breaks (DSB), adduct formation, as well as to changes in the expression of genes involved in repair mechanisms. Although analyzing repair mechanisms is essential to better understand the genotoxic effects caused by pollutants, few studies have analyzed repair capacity. The current review reinforces the need of conducting further studies on the role played by repair pathways in zebrafish subjected to DNA damage. Revised data have shown that zebrafish are a suitable model to assess pollutant-induced genotoxicity.

Environmentally relevant concentrations of boscalid exposure affects the neurobehavioral response of zebrafish by disrupting visual and nervous systems

Boscalid is a persistent fungicide that is frequently detected in surface waters and may be neurotoxic to aquatic organisms. Herein, we evaluated the effects of environmentally relevant boscalid concentrations to zebrafish to explore its potentially neurotoxic mechanisms of effect. Behavioral responses (swimming, phototaxis, and predation), histopathology, transcriptomics, biochemical parameter analysis and gene expression of larval and adult zebrafish following boscalid treatment were assessed. We found that boscalid significantly inhibited the locomotor ability and phototactic response of larvae after an 8-d exposure, and altered the locomotor activity, predation trajectories and ability in adults after a 21-d exposure. It was noted that predation rates of zebrafish were significantly decreased by 30% and 100% after exposure to 0.1 and 1.0 mg/L boscalid, respectively. Adverse alterations in the cell differentiation of eyes and brain injury were also observed in both larvae and adults following boscalid exposure. The expression of genes related to neurodevelopment, neurotransmission, eye development, and visual function, in conjunction with RNA-Seq results, indicated that boscalid may impair visual phototransduction and nervous system processes in larval zebrafish. Conclusively, boscalid exposure may affect the neurobehavioral response of zebrafish by impairing proper visual and nervous system function.

A comprehensive system for detection of behavioral change of D. magna exposed to various chemicals

The purpose of the present study was to develop a sensitive and comprehensive method, based on D. magna swimming behavior, for toxicity assessment of environmental chemicals. Firstly, D. magna swimming in several chambers with different diameters were compared to determine the most suitable container, and then baseline behaviors during light/dark periods as well as reactions to light/dark switching and vibration stimulation were determined. Secondly, after exposure to sub-lethal concentrations of the selected 42 typical chemicals, which were classified into heavy metals, pesticides, fungicides and flame retardants, the alterations in the swimming parameters were evaluated. Our results indicated the 48-well plate was the most suitable chamber for behavioral monitoring of D. magna, and specific responsive patterns of D. magna neonates to light/dark switching and vibration stimulation were observed. The results of the behavioral assays of chemicals suggested that D. magna was the most sensitive to methylmercury-chloride and then to abamectin and chlorpyrifos. The three chemicals at several to dozens of ng/L significantly changed swimming behaviors of D. magna. Furthermore, the alteration in the behavioral parameters (average swimming speed, etc.) induced by the selected chemicals could be ascribed to various modes of actions, confirming the reliability and practicability of the monitoring method.

Fetal Bisphenol-A Induced Changes in Murine Behavior and Brain Gene Expression Persisted in Adult-aged Offspring

In utero Bisphenol A (BPA) exposure has been linked to many deficits during brain development, including sexual differentiation, behavior, and motor coordination. Yet, how BPA induces these disorders and whether its effects are long lasting are largely unknown. In this study, using a mouse model, we demonstrated that in utero exposure to an environmentally relevant dose of BPA induced locomotor deficits, anxiety-like behavior, and declarative memory impairments that persisted into old age (18 months). Compared to the control animals, the BPA-exposed mice had a significant decrease in locomotor activity, exploratory tendencies, and long-term memory, and an increase in anxiety. The global brain gene expression profile was altered permanently by BPA treatment and showed regional and sexual differences. The BPA-treated male mice had more changes in the hippocampus, while female mice experienced more changes in the cortex. Overall, we demonstrate that in utero exposure to BPA induces permanent changes in brain gene expression in a region-specific and sex-specific manner, including a significant decrease in locomotor activity, learning ability, long-term memory, and an increase in anxiety. Fetal/early life exposures permanently affect neurobehavioral functions that deteriorate with age; BPA exposure may compound the effects of aging.

Impacts of bisphenol A analogues on zebrafish post-embryonic brain

Bisphenol A (BPA) is a widely studied and well-recognised endocrine-disrupting chemical, and one of the current issues is its safe replacement by various analogues. Using larva zebrafish as a model, the present study reveals that moderate and chronic exposure to BPA analogues such as bisphenol S, bisphenol F and bisphenol AF may also affect vertebrate neurodevelopment and locomotor activity. Several parameters of embryo-larval development were investigated, such as mortality, hatching, number of mitotically active cell, as defined by 5-bromo-2'-deoxyuridine incorporation and proliferative cell nuclear antigen labelling, aromatase B protein expression in radial glial cell and locomotor activity. Our results show that exposure to several bisphenol analogues induced an acceleration of embryo hatching rate. At the level of the developing brain, a strong up-regulation of the oestrogen-sensitive Aromatase B was also detected in the hypothalamic region. This up-regulation was not associated with effects on the numbers of mitotically active progenitors nor differentiated neurones in the preoptic area and in the nuclear recessus posterior of the hypothalamus zebrafish larvae. Furthermore, using a high-throughput video tracking system to monitor locomotor activity in zebrafish larvae, we show that some bisphenol analogues, such as bisphenol AF, significantly reduced locomotor activity following 6 days of exposure. Taken together, our study provides evidence that BPA analogues can also affect the neurobehavioural development of zebrafish.

Assessment of the Effects of Bisphenol A on Dopamine Synthesis and Blood Vessels in the Goldfish Brain

Bisphenol A (BPA) is an abundant contaminant found in aquatic environments. While a large number of toxicological studies have investigated the effects of BPA, the potential effects of BPA exposure on fish brain have rarely been studied. To understand how BPA impacts goldfish brains, we performed a transcriptome analysis of goldfish brains that had been exposed to 50 μg L⁻¹ and 0 μg L⁻¹ BPA for 30 days. In the analysis of unigene expression profiles, 327 unigenes were found to be upregulated and 153 unigenes were found to be downregulated in the BPA exposure group compared to the control group. Dopaminergic signaling pathway-related genes were significantly downregulated in the BPA exposure group. Furthermore, we found that serum dopamine concentrations decreased and TUNEL (terminal deoxynucleotidyl transferase 2-deoxyuridine, 5-triphosphate nick end labeling) staining was present in dopamine neurons enriched regions in the brain after BPA exposure, suggesting that BPA may disrupt dopaminergic processes. A KEGG analysis revealed that genes involved in the fluid shear stress and atherosclerosis pathway were highly significantly enriched. In addition, the qRT-PCR results for fluid shear stress and atherosclerosis pathway-related genes and the vascular histology of the brain showed that BPA exposure could damage blood vessels and induce brain atherosclerosis. The results of this work provide insights into the biological effects of BPA on dopamine synthesis and blood vessels in goldfish brain and could lay a foundation for future BPA neurotoxicity studies.

Neurodevelopmental toxicity assessments of alkyl phenanthrene and Dechlorane Plus co-exposure in zebrafish

Alkyl phenanthrene (A-Phen) and Dechlorane Plus (DP) are ubiquitous environmental pollutants that widely co-exist in the environment. It has been established that both A-Phen and DP elicit neurotoxicity, but the potential interactive toxicity of these contaminants is not well-known. To determine whether a mixture of A-Phen and DP would exhibit interactive effects on neurodevelopment, we co-exposed 3-methylphenanthrene (3-MP), a representative of A-Phen, with DP. Our results illustrated that exposure to 5 or 20 μg/L 3-MP alone or in combination with 60 μg/L DP caused neurobehavioral anomalies in zebrafish. In accordance with the behavioral deficits, 3-MP alone or co-exposed with DP significantly decreased axonal growth of secondary motoneurons, altered intracellular Ca²⁺ homeostasis and induced cell apoptosis in the muscle of zebrafish. Additionally, 3-MP alone or co-exposed with DP significantly increased reactive oxygen species (ROS) and the mRNA levels of apoptosis-related genes. These findings indicate that 3-MP alone or co-exposed with DP induces neurobehavioral deficits through the combined effects on neuronal connectivity and muscle function. Chemical analysis revealed significant increases in 3-MP and DP bioaccumulation in zebrafish co-exposed with 3-MP and DP. Elevated bioaccumulation resulting from mixture exposure may represent a significant contribution of the synergistic effects observed in combined chemical exposure.

Associations Between Behavioral Effects of Bisphenol A and DNA Methylation in Zebrafish Embryos

Endocrine-disrupting contaminants have been associated with aberrant changes in epigenetic pathways in animals. In this study, zebrafish embryos were exposed bisphenol A (BPA) to search for associations between behavior and epigenetic mechanisms in fish. For concentration-dependent responses, embryos were exposed to a range of BPA concentrations (0.1 nM to 30 μM). Embryos were analyzed for locomotor activity at 3-, 4-, and 5-days post fertilization (dpf) in response to changing light conditions. Based on concentration-dependent effects on behavior and gene expression, 10 μM BPA [from 24 to 96 hours post fertilization (hpf)] was used for a whole-genome bisulfite sequencing (WGBS) study searching for genome-wide impacts on DNA methylation. Over the examined concentration ranges, hyperactivity was demonstrated for exposures to 0.001 μM BPA in comparison to embryos exposed to lower or higher BPA concentrations. Transcriptional analysis showed significant effects at >0.01 μM BPA for two genes related to DNA methylation (dnmt1, cbs). BPA exposure did not significantly affect global DNA methylation, but 20,474 differentially methylated (DM) sites in 4,873 genes were identified by WGBS analysis. Most DM sites were identified within gene bodies. The genes with the most DM sites were all protocadherin 2 gamma subfamily genes, related to axon targeting, synaptic development and neuronal survival. KEGG pathways most significantly affected by BPA exposure were phosphatidylinositol signaling system, followed by VEGF and MAPK signaling pathways. This study shows that BPA can affect zebrafish embryo swimming activity at very low concentrations as well as affecting numerous methylated sites in genes which are overrepresented in functionally relevant metabolic pathways. In conclusion, altered methylation patterns of genes associated with nervous system development might lead to abnormal swimming activity.

Associations Between Behavioral Effects of Bisphenol A and DNA Methylation in Zebrafish Embryos

Endocrine-disrupting contaminants have been associated with aberrant changes in epigenetic pathways in animals. In this study, zebrafish embryos were exposed bisphenol A (BPA) to search for associations between behavior and epigenetic mechanisms in fish. For concentration-dependent responses, embryos were exposed to a range of BPA concentrations (0.1 nM to 30 μM). Embryos were analyzed for locomotor activity at 3-, 4-, and 5-days post fertilization (dpf) in response to changing light conditions. Based on concentration-dependent effects on behavior and gene expression, 10 μM BPA [from 24 to 96 hours post fertilization (hpf)] was used for a whole-genome bisulfite sequencing (WGBS) study searching for genome-wide impacts on DNA methylation. Over the examined concentration ranges, hyperactivity was demonstrated for exposures to 0.001 μM BPA in comparison to embryos exposed to lower or higher BPA concentrations. Transcriptional analysis showed significant effects at >0.01 μM BPA for two genes related to DNA methylation (dnmt1, cbs). BPA exposure did not significantly affect global DNA methylation, but 20,474 differentially methylated (DM) sites in 4,873 genes were identified by WGBS analysis. Most DM sites were identified within gene bodies. The genes with the most DM sites were all protocadherin 2 gamma subfamily genes, related to axon targeting, synaptic development and neuronal survival. KEGG pathways most significantly affected by BPA exposure were phosphatidylinositol signaling system, followed by VEGF and MAPK signaling pathways. This study shows that BPA can affect zebrafish embryo swimming activity at very low concentrations as well as affecting numerous methylated sites in genes which are overrepresented in functionally relevant metabolic pathways. In conclusion, altered methylation patterns of genes associated with nervous system development might lead to abnormal swimming activity.

Modeling Environmentally-Induced Motor Neuron Degeneration in Zebrafish

Zebrafish have been used to investigate motor neuron degeneration, including as a model system to examine the pathogenesis of amyotrophic lateral sclerosis (ALS). The use of zebrafish for this purpose has some advantages over other in vivo model systems. In the current paper, we show that bisphenol A (BPA) exposure in zebrafish embryos results in motor neuron degeneration with affected motor function, reduced motor axon length and branching, reduced neuromuscular junction integrity, motor neuron cell death and the presence of activated microglia. In zebrafish, motor axon length is the conventional method for estimating motor neuron degeneration, yet this measurement has not been confirmed as a valid surrogate marker. We also show that reduced motor axon length as measured from the sagittal plane is correlated with increased motor neuron cell death. Our preliminary timeline studies suggest that axonopathy precedes motor cell death. This outcome may have implications for early phase treatments of motor neuron degeneration.

Developmental and behavioral alterations in zebrafish embryonically exposed to valproic acid (VPA): An aquatic model for autism

Autism spectrum disorder (ASD) has complex neurodevelopmental impairments and origins that are linked to both genetic and environmental factors. Hence, there is an urgency to establish animal models with ASD-like characteristics to understand the underlying mechanisms of ASD. Prenatal exposure to valproic acid (VPA) produced ASD-like symptoms in humans, rats, and recently zebrafish. The present study investigated the use of VPA exposure to generate an ASD model in zebrafish. Early life stage exposures produced ASD-like phenotypes in the developing brain development and behavioral changes in embryonic and larval zebrafish. Our findings revealed that treating zebrafish embryos with VPA starting at 8h post fertilization (hpf) resulted in significant: increase in the ASD macrocephalic phenotype; hyperactivity of embryo/larvae movement behaviors; and increases of ASD-like larval social behaviors. Further analysis showed increases in cell proliferation, the proportion of mature newborn neurons, and neural stem cell proliferation in the brain region, which may contribute to the brain overgrowth and macrocephaly observed following VPA exposure. Our study demonstrated that VPA exposure generates ASD-like phenotypes and behaviors, indicating that zebrafish is an alternative model to investigate underlying ASD mechanisms.

Bisphenol A Induces Oxidative Stress in Bone Marrow Cells, Lymphocytes, and Reproductive Organs of Holtzman Rats

Bisphenol A (BPA) is an estrogenic chemical used in the production of polycarbonate plastics and epoxy resins. Our earlier studies have demonstrated that BPA is a potent reproductive and genotoxic agent and affects the normal physiological functions. The objective of this study was to evaluate whether exposure to BPA induces oxidative stress. The male Holtzman rats were orally gavaged with BPA (0.01 mg and 5.0 mg/kg/bw) over the period of 6 days. Animals were euthanized by cervical dislocation at the end of the treatments; bone marrow cells and blood lymphocytes were aspirated; testis and epididymis were collected, immediately frozen in liquid nitrogen, and stored at −80°C. These samples were utilized for the determination of lipid peroxidation and various antioxidant enzymes such as superoxide dismutase, catalase, and nonenzymatic reduced glutathione. The results demonstrated that BPA caused an increase in lipid peroxidation and a decrease in activity of various enzymatic and nonenzymatic antioxidants in bone marrow cells, blood lymphocytes, and testicular and epididymal tissues. The findings of the current study suggest that BPA exposure induced oxidative stress, which could be one of the possible mechanisms causing reproductive and genetic toxicity.

Bioconcentration pattern and induced apoptosis of bisphenol A in zebrafish embryos at environmentally relevant concentrations

Bisphenol A (BPA) is a well-known endocrine-disrupting chemical that is ubiquitously present in the environment. In the present study, 4-h post-fertilization (hpf) zebrafish embryos were exposed to various environmentally relevant concentrations of BPA (0.1, 1, 10, 100, and 1000 μg/L) until 72 and 168 hpf, and the accumulation pattern of BPA and its potential to induce toxicity through apoptosis were determined. Compared to BPA concentrations in larvae at 168 hpf, BPA concentrations in embryos exposed until 72 hpf were at relatively higher levels (p < 0.05) with higher bioconcentration factor (BCF) values. The nonlinear fitting analysis indicated that the BCF values of BPA in fish embryos/larvae were significantly correlated to the log10-transformed BPA exposure concentrations in water in an inverse concentration-dependent manner. Fish accumulated more BPA as the exposure concentrations increased; however, their accumulation capacity of BPA declined and tended to be saturated in the high exposure groups of BPA. Moreover, caspase-3 activity was significantly induced upon BPA exposure at 0.1, 1, 10, and 100 μg/L BPA at 72 hpf, and also at 10 and 100 μg/L BPA at 168 hpf. Correspondingly, exposure to 10 and 100 μg/L of BPA significantly increased the DNA fragmentation in the extracted DNA at 168 hpf as determined by DNA ladder analysis. In addition, the expression patterns of four genes related to apoptosis including caspase-3, bax, p53, and c-jun were significantly up-regulated (p < 0.05) in fish embryos/larvae upon BPA exposure at 72 and 168 hpf. Our results revealed that low and environmentally relevant concentrations of BPA could be significantly accumulated in zebrafish and induced apoptosis with involvement of the regulation of caspase-3 and other apoptosis-related genes.

Mechanisms underlying melatonin-mediated prevention of fenvalerate-induced behavioral and oxidative toxicity in zebrafish

The neurotoxic effects attributed to the pesticide fenvalerate (FEN) are well-established. The aim of this study was to determine whether melatonin (MLT) was able to protect against FEN-induced behavior, oxidative stress, apoptosis, and neurogenesis using zebrafish (Danio rerio) model. Zebrafish exposed to 100 μg/L FEN for 120 h exhibited decreased swimming activity accompanied by downregulated expression of neurogenesis-related genes (Dlx2, Shha, Ngn1, Elavl3, and Gfap), suggesting that neurogenesis were impaired. In addition, FEN exposure significantly elevated oxidative stress as evidenced by increased malondialdehyde levels, as well as activities of Cu/Zn superoxide dismutase (Cu/Zn SOD), catalase, and glutathione peroxidase. Acridine orange staining demonstrated that embryos treated with FEN for 120 h significantly enhanced apoptosis mainly in the brain. FEN also produced upregulation of the expression of the pro-apoptotic genes (Bax, Fas, caspase 8, caspase 9, and caspase 3) and decreased expression of the anti-apoptotic gene Bcl-2. MLT significantly attenuated the FEN-mediated oxidative stress, modulated apoptotic-regulating genes, and diminished apoptotic responses. Further, MLT blocked the FEN-induced effects on swimming behavior as well as on neurogenesis-related genes. In conclusion, MLT protected against FEN-induced developmental neurotoxicity and apoptosis by inhibiting pesticide-mediated oxidative stress in zebrafish.

Toward the Design of Less Hazardous Chemicals: Exploring Comparative Oxidative Stress in Two Common Animal Models

Sustainable molecular design of less hazardous chemicals presents a potentially transformative approach to protect public health and the environment. Relationships between molecular descriptors and toxicity thresholds previously identified the octanol-water distribution coefficient, log D, and the HOMO-LUMO energy gap, ΔE, as two useful properties in the identification of reduced aquatic toxicity. To determine whether these two property-based guidelines are applicable to sublethal oxidative stress (OS) responses, two common aquatic in vivo models, the fathead minnow (Pimephales promelas) and zebrafish (Danio rerio), were employed to examine traditional biochemical biomarkers (lipid peroxidation, DNA damage, and total glutathione) and antioxidant gene activation following exposure to eight structurally diverse industrial chemicals (bisphenol A, cumene hydroperoxide, dinoseb, hydroquinone, indene, perfluorooctanoic acid, R-(-)-carvone, and tert-butyl hydroperoxide). Bisphenol A, cumene hydroperoxide, dinoseb, and hydroquinone were consistent inducers of OS. Glutathione was the most consistently affected biomarker, suggesting its utility as a sensitivity response to support the design of less hazardous chemicals. Antioxidant gene expression (changes in nrf2, gclc, gst, and sod) was most significantly (p < 0.05) altered by R-(-)-carvone, cumene hydroperoxide, and bisphenol A. Results from the present study indicate that metabolism of parent chemicals and the role of their metabolites in molecular initiating events should be considered during the design of less hazardous chemicals. Current empirical and computational findings identify the need for future derivation of sustainable molecular design guidelines for electrophilic reactive chemicals (e.g., SN2 nucleophilic substitution and Michael addition reactivity) to reduce OS related adverse outcomes in vivo.

Estrogenic Effects of Several BPA Analogs in the Developing Zebrafish Brain

Important set of studies have demonstrated the endocrine disrupting activity of Bisphenol A (BPA). The present work aimed at defining estrogenic-like activity of several BPA structural analogs, including BPS, BPF, BPAF, and BPAP, on 4- or 7-day post-fertilization (dpf) zebrafish larva as an in vivo model. We measured the induction level of the estrogen-sensitive marker cyp19a1b gene (Aromatase B), expressed in the brain, using three different in situ/in vivo strategies: (1) Quantification of cyp19a1b transcripts using RT-qPCR in wild type 7-dpf larva brains exposed to bisphenols; (2) Detection and distribution of cyp19a1b transcripts using in situ hybridization on 7-dpf brain sections (hypothalamus); and (3) Quantification of the cyp19a1b promoter activity in live cyp19a1b-GFP transgenic zebrafish (EASZY assay) at 4-dpf larval stage. These three different experimental approaches demonstrated that BPS, BPF, or BPAF exposure, similarly to BPA, significantly activates the expression of the estrogenic marker in the brain of developing zebrafish. In vitro experiments using both reporter gene assay in a glial cell context and competitive ligand binding assays strongly suggested that up-regulation of cyp19a1b is largely mediated by the zebrafish estrogen nuclear receptor alpha (zfERα). Importantly, and in contrast to other tested bisphenol A analogs, the bisphenol AP (BPAP) did not show estrogenic activity in our model.

Low dose exposure to Bisphenol A alters development of gonadotropin-releasing hormone 3 neurons and larval locomotor behavior in Japanese Medaka

Accumulating evidence indicates that chronic low dose exposure to Bisphenol A (BPA), an endocrine disruptor, may disrupt normal brain development and behavior mediated by the gonadotropin-releasing hormone (GnRH) pathways. While it is known that GnRH neurons in the hypothalamus regulate reproductive physiology and behavior, functional roles of extra-hypothalamic GnRH neurons remain unclear. Furthermore, little is known whether BPA interacts with extra-hypothalamic GnRH3 neural systems in vulnerable developing brains. Here we examined the impact of low dose BPA exposure on the developing GnRH3 neural system, eye and brain growth, and locomotor activity in transgenic medaka embryos and larvae with GnRH3 neurons tagged with GFP. Fertilized eggs were collected daily and embryos/larvae were chronically exposed to 200ng/ml of BPA, starting at 1 day post fertilization (dpf). BPA significantly increased fluorescence intensity of the GnRH3-GFP neural population in the terminal nerve (TN) of the forebrain at 3dpf, but decreased the intensity at 5dpf, compared with controls. BPA advanced eye pigmentation without affecting eye and brain size development, and accelerated times to hatch. Following chronic BPA exposure, 20dpf larvae showed suppression of locomotion, both in distance covered and speed of movement (47% and 43% reduction, respectively). BPA-induced hypoactivity was accompanied by decreased cell body sizes of individual TN-GnRH3 neurons (14% smaller than those of controls), but not of non-GnRH3 neurons. These novel data demonstrate complex neurobehavioral effects of BPA on the development of extra-hypothalamic GnRH3 neurons in teleost fish.

The effect of the aquatic contaminants bisphenol-A and PCB-95 on the zebrafish lateral line

Environmental toxicants such as bisphenol-A (BPA) and polychlorinated biphenyls (PCBs) are prevalent in our water supply, soil, and many food products and can profoundly affect the central nervous system. Both BPA and PCBs can disrupt endocrine signaling, which is important for auditory development and function, but the effect of these toxicants on the auditory periphery is not understood. In this study we investigated the effect of PCB-95 and BPA on lateral line development, function, and regeneration in larval zebrafish. The lateral line is a system of mechanosensory hair cells on the exterior of the fish that are homologous to the hair cells located in the mammalian inner ear. We found that PCB-95 had no effect on lateral line development or hair cell survival. BPA also did not affect lateral line development, but instead had a significant effect on both hair cell survival and regeneration. BPA-induced hair cell loss is both dose- and time-dependent, with concentrations of 1 μM or higher killing lateral line hair cells during a 24h exposure period. Pharmacologic manipulation experiments suggest that BPA kills hair cells via activation of oxidative stress pathways, similar to prior reports of BPA toxicity in other tissues. We also observed that hair cells killed with neomycin, a known ototoxin, failed to regenerate normally when BPA was present, suggesting that BPA in aquatic environments could impede innate regenerative responses in fishes. Collectively, these data demonstrate that BPA can have detrimental effects on sensory systems, both in aquatic life and perhaps in terrestrial organisms, including humans.

Current concepts in neuroendocrine disruption

In the last few years, it has become clear that a wide variety of environmental contaminants have specific effects on neuroendocrine systems in fish, amphibians, birds and mammals. While it is beyond the scope of this review to provide a comprehensive examination of all of these neuroendocrine disruptors, we will focus on select representative examples. Organochlorine pesticides bioaccumulate in neuroendocrine areas of the brain that directly regulate GnRH neurons, thereby altering the expression of genes downstream of GnRH signaling. Organochlorine pesticides can also agonize or antagonize hormone receptors, adversely affecting crosstalk between neurotransmitter systems. The impacts of polychlorinated biphenyls are varied and in many cases subtle. This is particularly true for neuroedocrine and behavioral effects of exposure. These effects impact sexual differentiation of the hypothalamic-pituitary-gonadal axis, and other neuroendocrine systems regulating the thyroid, metabolic, and stress axes and their physiological responses. Weakly estrogenic and anti-androgenic pollutants such as bisphenol A, phthalates, phytochemicals, and the fungicide vinclozolin can lead to severe and widespread neuroendocrine disruptions in discrete brain regions, including the hippocampus, amygdala, and hypothalamus, resulting in behavioral changes in a wide range of species. Behavioral features that have been shown to be affected by one or more these chemicals include cognitive deficits, heightened anxiety or anxiety-like, sociosexual, locomotor, and appetitive behaviors. Neuroactive pharmaceuticals are now widely detected in aquatic environments and water supplies through the release of wastewater treatment plant effluents. The antidepressant fluoxetine is one such pharmaceutical neuroendocrine disruptor. Fluoxetine is a selective serotonin reuptake inhibitor that can affect multiple neuroendocrine pathways and behavioral circuits, including disruptive effects on reproduction and feeding in fish. There is growing evidence for the association between environmental contaminant exposures and diseases with strong neuroendocrine components, for example decreased fecundity, neurodegeneration, and cardiac disease. It is critical to consider the timing of exposures of neuroendocrine disruptors because embryonic stages of central nervous system development are exquisitely sensitive to adverse effects. There is also evidence for epigenetic and transgenerational neuroendocrine disrupting effects of some pollutants. We must now consider the impacts of neuroendocrine disruptors on reproduction, development, growth and behaviors, and the population consequences for evolutionary change in an increasingly contaminated world. This review examines the evidence to date that various so-called neuroendocrine disruptors can induce such effects often at environmentally-relevant concentrations.

Gestational bisphenol A exposure and testis development

Virtually all humans are exposed to bisphenol A (BPA). Since BPA can act as a ligand for estrogen receptors, potential hazardous effects of BPA should be evaluated in the context of endogenous estrogenic hormones. Because estrogen is metabolized in the placenta, developing fetuses are normally exposed to very low endogenous estrogen levels. BPA, on the other hand, passes through the placenta and might have distinct adverse consequences during the sensitive stages of fetal development. Testicular gametogenesis and steroidogenesis begin early during fetal development. These processes are sensitive to estrogens and play a role in determining the number of germ stem cells, sperm count, and male hormone levels in adulthood. Although studies have shown a correlation between BPA exposure and perturbed reproduction, a clear consensus has yet to be established as to whether current human gestational BPA exposure results in direct adverse effects on male genital development and reproduction. However, studies in animals and in vitro have provided direct evidence for the ability of BPA exposure to influence male reproductive development. This review discusses the current knowledge of potential effects of BPA exposure on male reproductive health and whether gestational exposure adversely affects testis development.