Zebrafish embryotoxicity test for developmental (neuro)toxicity: Demo case of an integrated screening approach system using anti-epileptic drugs

Retinoic acid signaling pathway perturbation impacts mesodermal-tissue development in the zebrafish embryo: Biomarker candidate identification using transcriptomics

The zebrafish embryo (ZE) model provides a developmental model well conserved throughout vertebrate embryogenesis, with relevance for early human embryo development. It was employed to search for gene expression biomarkers of compound-induced disruption of mesodermal development. We were particularly interested in the expression of genes related to the retinoic acid signaling pathway (RA-SP), as a major morphogenetic regulating mechanism. We exposed ZE to teratogenic concentrations of valproic acid (VPA) and all-trans retinoic acid (ATRA), using folic acid (FA) as a non-teratogenic control compound shortly after fertilization for 4 h, and performed gene expression analysis by RNA sequencing. We identified 248 genes specifically regulated by both teratogens but not by FA. Further analysis of this gene set revealed 54 GO-terms related to the development of mesodermal tissues, distributed along the paraxial, intermediate, and lateral plate sections of the mesoderm. Gene expression regulation was specific to tissues and was observed for somites, striated muscle, bone, kidney, circulatory system, and blood. Stitch analysis revealed 47 regulated genes related to the RA-SP, which were differentially expressed in the various mesodermal tissues. These genes provide potential molecular biomarkers of mesodermal tissue and organ (mal)formation in the early vertebrate embryo.

Nervous system development related gene expression regulation in the zebrafish embryo after exposure to valproic acid and retinoic acid: A genome wide approach

The evaluation of chemical and pharmaceutical safety for humans is moving from animal studies to New Approach Methodologies (NAM), reducing animal use and focusing on mechanism of action, whilst enhancing human relevance. In developmental toxicology, the mechanistic approach is facilitated by the assessment of predictive biomarkers, which allow mechanistic pathways perturbation monitoring at the basis of human hazard assessment. In our search for biomarkers of maldevelopment, we focused on chemically-induced perturbation of the retinoic acid signaling pathway (RA-SP), a major pathway implicated in a plethora of developmental processes. A genome-wide expression screening was performed on zebrafish embryos treated with two teratogens, all-trans retinoic acid (ATRA) and valproic acid (VPA), and a non-teratogen reference compound, folic acid (FA). Each compound was found to have a specific mRNA expression profile with 248 genes commonly dysregulated by both teratogenic compounds but not by FA. These genes were implicated in several developmental processes (e.g., the circulatory and nervous system). Given the prominent response of neurodevelopmental gene sets, and the crucial need to better understand developmental neurotoxicity, our study then focused on nervous system development. We found 62 genes that are potential early neurodevelopmental toxicity biomarker candidates. These results advance NAM-based safety assessment evaluation by highlighting the usefulness of the RA-SP in providing early toxicity biomarker candidates.

Carbamazepine, venlafaxine, tramadol, and their main metabolites: Toxicological effects on zebrafish embryos and larvae

Pharmaceutical compounds and their metabolites are found in natural and wastewater. However, investigation of their toxic effects on aquatic animals has been neglected, especially for metabolites. This work investigated the effects of the main metabolites of carbamazepine, venlafaxine and tramadol. Zebrafish embryos were exposed (0.1-100 µg/L) for 168hpf exposures to each metabolite (carbamazepine-10,11-epoxide, 10,11-dihydrocarbamazepine, O-desmethylvenlafaxine, N-desmethylvenlafaxine, O-desmethyltramadol, N-desmethyltramadol) or the parental compound. A concentration-response relationship was found for the effects of some embryonic malformations. Carbamazepine-10,11-epoxide, O-desmethylvenlafaxine and tramadol elicited the highest malformation rates. All compounds significantly decreased larvae responses on a sensorimotor assay compared to controls. Altered expression was found for most of the 32 tested genes. In particular, abcc1, abcc2, abcg2a, nrf2, pparg and raraa were found to be affected by all three drug groups. For each group, the modelled expression patterns showed differences in expression between parental compounds and metabolites. Potential biomarkers of exposure were identified for the venlafaxine and carbamazepine groups. These results are worrying, indicating that such contamination in aquatic systems may put natural populations at significant risk. Furthermore, metabolites represent a real risk that needs more scrutinising by the scientific community.

Effects of common antiepileptic drugs on teleost fishes

Antiepileptic drugs (AEDs) are globally prescribed to treat epilepsy and many other psychiatric disorders in humans. Their high consumption, low metabolic rate in the human body and low efficiency of wastewater treatment plants (WWTPs) in eliminating these chemicals results in the frequent occurrence of these pharmaceutical drugs in aquatic systems. Therefore, aquatic organisms, including ecologically and economically important teleost fishes, may be inadvertently exposed to these chemicals. Due to their physiological similarity with humans, fishes may be particularly vulnerable to AEDs. Almost all AED drugs are detectable in natural aquatic ecosystems, but diazepam (DZP) and carbamazepine (CBZ) are among the most widely detected AEDs to date. Recent studies suggest that these drugs have a substantial capacity to induce neurotoxicity and behavioral abnormality in fishes. Here we review the current state of knowledge regarding the potential mode of action of DZP and CBZ as well as that of some other AEDs on teleosts and put observable behavioral effects into a mechanistic context. We find that following their intended mode of action in humans, AEDs also disrupt the GABAergic, glutamatergic and serotonergic systems as well as parasympathetic neurotransmitters in fishes. Moreover, AEDs have non-specific modes of action in teleosts ranging from estrogenic activity to oxidative stress. These physiological changes are often accompanied by dose-dependent disruptions of anxiety, locomotor activity, social behaviors, food uptake, and learning and memory, but DZP and CBZ consistently induced anxiolytic effects. Thereby, AED exposure severely compromises individual fitness across teleost fish species, which may lead to population and ecosystem impairment. We also showcase promising avenues for future research by highlighting where we lack data when it comes to effects of certain AEDs, AED concentrations and behavioral endpoints.

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

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

Toxicometabolomics and Biotransformation Product Elucidation in Single Zebrafish Embryos Exposed to Carbamazepine from Environmentally-Relevant to Morphologically Altering Doses

Toxicometabolomics and biotransformation product (bioTP) elucidation were carried out in single zebrafish (ZF) embryos exposed to carbamazepine (CBZ). Exposures were conducted in 96-well plates containing six CBZ concentrations ranging from 0.5 μg/L to 50 mg/L (n = 12 embryos per dose). In the 50 mg/L dose group, 33% of embryos developed edema during the exposure (120 hpf), while hatching was significantly delayed in three of the lower-dose groups (0.46, 3.85, and 445 μg/L) compared to the control at 48 hpf. Toxicometabolomic analysis together with random forest modeling revealed a total of 80 significantly affected metabolites (22 identified via targeted lipidomics and 58 via nontarget analysis). The wide range of doses enabled the observation of both monotonic and nonmonotonic dose responses in the metabolome, which ultimately produced a unique and comprehensive biochemical picture that aligns with existing knowledge on the mode of action of CBZ. The combination of high dose exposures and apical endpoint assessment in single embryos also enabled hypothesis generation regarding the target organ for the morphologically altering insult. In addition, two CBZ bioTPs were identified without additional exposure experiments. Overall, this work showcases the potential of toxicometabolomics and bioTP determination in single ZF embryos for rapid and comprehensive chemical hazard assessment.

The fish early-life stage sublethal toxicity syndrome - A high-dose baseline toxicity response

A large number of toxicity studies report abnormalities in early life-stage (ELS) fish that are described here as a sublethal toxicity syndrome (TxSn_FELS) and generally include a reduced heart rate, edemas (yolk sac and cardiac), and a variety of morphological abnormalities. The TxSn_FELS is very common and not diagnostic for any chemical or class of chemicals. This sublethal toxicity syndrome is mostly observed at high exposure concentrations and appears to be a baseline, non-specific toxicity response; however, it can also occur at low doses by specific action. Toxicity metrics for this syndrome generally occur at concentrations just below those causing mortality and have been reported for a large number of diverse chemicals. Predictions based on tissue concentrations or quantitative-structure activity relationship (QSAR) models support the designation of baseline toxicity for many of the tested chemicals, which is confirmed by observed values. Given the sheer number of disparate chemicals causing the TxSn_FELS and correlation with QSAR derived partitioning; the only logical conclusion for these high-dose responses is baseline toxicity by nonspecific action and not a lock and key type receptor response. It is important to recognize that many chemicals can act both as baseline toxicants and specific acting toxicants likely via receptor interaction and it is not possible to predict those threshold doses from baseline toxicity. We should search out these specific low-dose responses for ecological risk assessment and not rely on high-concentration toxicity responses to guide environmental protection. The goal for toxicity assessment should not be to characterize toxic responses at baseline toxicity concentrations, but to evaluate chemicals for their most toxic potential. Additional aspects of this review evaluated the fish ELS teratogenic responses in relation to mammalian oral LD50s and explored potential key events responsible for baseline toxicity.

A Systematic Review to Compare Chemical Hazard Predictions of the Zebrafish Embryotoxicity Test With Mammalian Prenatal Developmental Toxicity

Originally developed to inform the acute toxicity of chemicals on fish, the zebrafish embryotoxicity test (ZET) has also been proposed for assessing the prenatal developmental toxicity of chemicals, potentially replacing mammalian studies. Although extensively evaluated in primary studies, a comprehensive review summarizing the available evidence for the ZET's capacity is lacking. Therefore, we conducted a systematic review of how well the presence or absence of exposure-related findings in the ZET predicts prenatal development toxicity in studies with rats and rabbits. A two-tiered systematic review of the developmental toxicity literature was performed, a review of the ZET literature was followed by one of the mammalian literature. Data were extracted using DistillerSR, and study validity was assessed with an amended SYRCLE's risk-of-bias tool. Extracted data were analyzed for each species and substance, which provided the basis for comparing the 2 test methods. Although limited by the number of 24 included chemicals, our results suggest that the ZET has potential to identify chemicals that are mammalian prenatal developmental toxicants, with a tendency for overprediction. Furthermore, our analysis confirmed the need for further standardization of the ZET. In addition, we identified contextual and methodological challenges in the application of systematic review approaches to toxicological questions. One key to overcoming these challenges is a transition to more comprehensive and transparent planning, conduct and reporting of toxicological studies. The first step toward bringing about this change is to create broad awareness in the toxicological community of the need for and benefits of more evidence-based approaches.

Effects of new antiseizure drugs on seizure activity and anxiety-like behavior in adult zebrafish

Several studies with larvae and adult zebrafish have shown that old and new antiseizure drugs (ASDs) produce discrepant results in seizure tests, locomotor activity or anxiety models. In this study, the pentylenetetrazole seizure test (PTZ) was performed to assess the effectiveness of four new ASDs: lamotrigine (LTG), topiramate (TPM), felbamate (FBM), and levetiracetam (LEV) in the subsequent stages of seizures in adult fish. All ASDs were administered intraperitoneally (i.p.). The time of maximal anticonvulsant effect and the dose-response relationship of the drugs were assessed. The effects of studied ASDs on the locomotor activity and the anxiety-like behavior in the color preference test were also investigated. Furthermore, drug concentrations in zebrafish homogenates were determined. LTG, TPM, and LEV significantly increased the seizure latency at three subsequent stages of seizures (SI-SIII), while FBM was effective only at SI. Locomotor activity decreased after TPM treatment. TPM and FBM exhibited a strong anxiolytic-like effect in the color preference test. LEV at the highest dose tested had a weak anxiolytic-like effect. The HPLC analysis showed average concentrations of the studied ASDs in the fish body during their maximum anticonvulsant activity. The present study shows that FBM cannot inhibit all subsequent PTZ seizure stages in the adult fish. Except for LTG, the studied drugs affected the anxiety-like behavior of treated animals. Furthermore, only TPM significantly changed locomotion parameters. Our findings support the need to accurately characterize the efficacy of new ASDs at different stages of the PTZ-induced seizures in adult zebrafish.

Developmental Neurotoxicity of Environmentally Relevant Pharmaceuticals and Mixtures Thereof in a Zebrafish Embryo Behavioural Test

Humans are exposed daily to complex mixtures of chemical substances via food intake, inhalation, and dermal contact. Developmental neurotoxicity is an understudied area and entails one of the most complex areas in toxicology. Animal studies for developmental neurotoxicity (DNT) are hardly performed in the context of regular hazard studies, as they are costly and time consuming and provide only limited information as to human relevance. There is a need for a combination of in vitro and in silico tests for the assessment of chemically induced DNT in humans. The zebrafish (Danio rerio) embryo (ZFE) provides a powerful model to study DNT because it shows fast neurodevelopment with a large resemblance to the higher vertebrate, including the human system. One of the suitable readouts for DNT testing in the zebrafish is neurobehaviour (stimulus-provoked locomotion) since this provides integrated information on the functionality and status of the entire nervous system of the embryo. In the current study, environmentally relevant pharmaceuticals and their mixtures were investigated using the zebrafish light-dark transition test. Zebrafish embryos were exposed to three neuroactive compounds of concern, carbamazepine (CBZ), fluoxetine (FLX), and venlafaxine (VNX), as well as their main metabolites, carbamazepine 10,11-epoxide (CBZ 10,11E), norfluoxetine (norFLX), and desvenlafaxine (desVNX). All the studied compounds, except CBZ 10,11E, dose-dependently inhibited zebrafish locomotor activity, providing a distinct behavioural phenotype. Mixture experiments with these pharmaceuticals identified that dose addition was confirmed for all the studied binary mixtures (CBZ-FLX, CBZ-VNX, and VNX-FLX), thereby supporting the zebrafish embryo as a model for studying the cumulative effect of chemical mixtures in DNT. This study shows that pharmaceuticals and a mixture thereof affect locomotor activity in zebrafish. The test is directly applicable in environmental risk assessment; however, further studies are required to assess the relevance of these findings for developmental neurotoxicity in humans.

Epigenetic biomarkers as tools for chemical hazard assessment: Gene expression profiling using the model Danio rerio

Recent reports raise the concern that exposure to several environmental chemicals may induce persistent changes that go beyond the exposed organisms, being transferred to subsequent generations even in the absence of the original chemical insult. These changes in subsequent non-exposed generations have been related to epigenetic changes. Although highly relevant for hazard and risk assessment, biomarkers of epigenetic modifications that can be associated with adversity, are still not integrated into hazard assessment frameworks. Here, in order to validate new biomarkers of epigenetic modifications in a popular animal model, zebrafish embryos were exposed to different concentrations of Bisphenol A (0.01, 0.1, 1 and 10 mg/L) and Valproic Acid (0.8, 4, 20 and 100 mg/L), two chemicals reported to alter the modulation of the epigenome. Morphological abnormalities and epigenetic changes were assessed at 80 hours-post fertilization, including DNA global methylation and gene expression of both DNA and histone epigenetic modifications. Gene expression changes were detected at concentrations below those inducing morphological abnormalities. These results further support the importance of combining epigenetic biomarkers with apical endpoints to improve guidelines for chemical testing and hazard assessment, and favour the integration of new biomarkers of epigenetic modifications into the standardized OECD test guideline 236 with zebrafish embryos.

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

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

Neurotoxic effects in zebrafish embryos by valproic acid and nine of its analogues: the fish-mouse connection?

Since teratogenicity testing in mammals is a particular challenge from an animal welfare perspective, there is a great need for the development of alternative test systems. In this context, the zebrafish (Danio rerio) embryo has received increasing attention as a non-protected embryonic vertebrate in vivo model. The predictive power of zebrafish embryos for general vertebrate teratogenicity strongly depends on the correlation between fish and mammals with respect to both overall general toxicity and more specific endpoints indicative of certain modes-of-action. The present study was designed to analyze the correlation between (1) effects of valproic acid and nine of its analogues in zebrafish embryos and (2) their known neurodevelopmental effects in mice. To this end, zebrafish embryos exposed for 120 h in an extended version of the acute fish embryo toxicity test (FET; OECD TG 236) were analyzed with respect to an extended list of sublethal endpoints. Particular care was given to endpoints putatively related to neurodevelopmental toxicity, namely jitter/tremor, deformation of sensory organs (eyes) and craniofacial deformation, which might correlate to neural tube defects caused by valproic acid in mammals. A standard evaluation of lethal (LC according to OECD TG 236) and sublethal toxicity (EC) merely indicated that four out of ten compounds tested in zebrafish correlate with positive results in mouse in vivo studies. A detailed assessment of more specific effects, however, namely, jitter/tremor, small eyes and craniofacial deformation, resulted in a correspondence of 75% with in vivo mouse data. A refinement of endpoint analysis from an integration of all observations into one LC_x or EC_x data (as foreseen by current ecotoxicology-driven OECD guidelines) to a differential evaluation of endpoints specific of selected modes-of-action thus increases significantly the predictive power of the zebrafish embryo model for mammalian teratogenicity. However, for some of the endpoints observed, e.g., scoliosis, lordosis, pectoral fin deformation and lack of movement, further experiments are required for the identification of underlying modes-of-action and an unambiguous interpretation of their predictive power for mammalian toxicity.

The α1-adrenoceptor inhibitor ρ-TIA facilitates net hunting in piscivorous Conus tulipa

Cone snails use separately evolved venoms for prey capture and defence. While most use a harpoon for prey capture, the Gastridium clade that includes the well-studied Conus geographus and Conus tulipa, have developed a net hunting strategy to catch fish. This unique feeding behaviour requires secretion of "nirvana cabal" peptides to dampen the escape response of targeted fish allowing for their capture directly by mouth. However, the active components of the nirvana cabal remain poorly defined. In this study, we evaluated the behavioural effects of likely nirvana cabal peptides on the teleost model, Danio rerio (zebrafish). Surprisingly, the conantokins (NMDA receptor antagonists) and/or conopressins (vasopressin receptor agonists and antagonists) found in C. geographus and C. tulipa venom failed to produce a nirvana cabal-like effect in zebrafish. In contrast, low concentrations of the non-competitive adrenoceptor antagonist ρ-TIA found in C. tulipa venom (EC₅₀ = 190 nM) dramatically reduced the escape response of zebrafish larvae when added directly to aquarium water. ρ-TIA inhibited the zebrafish α₁-adrenoceptor, confirming ρ-TIA has the potential to reverse the known stimulating effects of norepinephrine on fish behaviour. ρ-TIA may act alone and not as part of a cabal, since it did not synergise with conopressins and/or conantokins. This study highlights the importance of using ecologically relevant animal behaviour models to decipher the complex neurobiology underlying the prey capture and defensive strategies of cone snails.

A proof of concept study demonstrating that environmental levels of carbamazepine impair early stages of chick embryonic development

Carbamazepine (CBZ) is an anticonvulsant drug used for epilepsy and other disorders. Prescription of CBZ during pregnancy increases the risk for congenital malformations. CBZ is ubiquitous in effluents and persistent during wastewater treatment. Thus, it is re-introduced into agricultural ecosystems upon irrigation with reclaimed wastewater. People consuming produce irrigated with reclaimed wastewater were found to be exposed to CBZ. However, environmental concentrations of CBZ (μgL^-1) are magnitudes lower than its therapeutic levels (μgml^-1), raising the question of whether and how environmental levels of CBZ affect embryonic development. The chick embryo is a powerful and highly sensitive amniotic model system that enables to assess environmental contaminants in the living organism. Since the chick embryonic development is highly similar to mammalians, yet, it develops in an egg, toxic effects can be directly analyzed in a well-controlled system without maternal influences. This research utilized the chick embryo to test whether CBZ is embryo-toxic by using morphological, cellular, molecular and imaging strategies. Three key embryonic stages were monitored: after blastulation (st.1HH), gastrulation/neurulation (st.8HH) and organogenesis (st.15HH). Here we demonstrate that environmental relevant concentrations of CBZ impair morphogenesis in a dose- and stage- dependent manner. Effects on gastrulation, neural tube closure, differentiation and proliferation were exhibited in early stages by exposing embryos to CBZ dose as low as 0.1μgL^-1. Quantification of developmental progression revealed a significant difference in the total score obtained by CBZ-treated embryos compared to controls (up to 5-fold difference, p<0.05). Yet, defects were unnoticed as embryos passed gastrulation/neurulation. This study provides the first evidence for teratogenic effect of environmental-relevant concentrations of CBZ in amniotic embryos that impair early but not late stages of development. These findings call for in-depth risk analysis to ensure that the environmental presence of CBZ and other drugs is not causing irreversible ecological and public-health damages.

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.

Comparative behavioral toxicology with two common larval fish models: Exploring relationships among modes of action and locomotor responses

Behavioral responses inform toxicology studies by rapidly and sensitively detecting molecular initiation events that propagate to physiological changes in individuals. These behavioral responses can be unique to chemical specific mechanisms and modes of action (MOA) and thus present diagnostic utility. In an initial effort to explore the use of larval fish behavioral response patterns in screening environmental contaminants for toxicity and to identify behavioral responses associated with common chemical specific MOAs, we employed the two most common fish models, the zebrafish and the fathead minnow, to define toxicant induced swimming activity alterations during interchanging photoperiods. Though the fathead minnow (Pimephales promelas) is a common model for aquatic toxicology research and regulatory toxicology practice, this model has received little attention in behavioral studies compared to the zebrafish, a common biomedical model. We specifically compared behavioral responses among 7 different chemicals (1-heptanol, phenol, R-(-)-carvone, citalopram, diazinon, pentylenetetrazole (PTZ), and xylazine) that were selected and classified based on anticipated MOA (nonpolar narcosis, polar narcosis, electrophile, specific mechanism) according to traditional approaches to examine whether these comparative responses differ among chemicals with various structure-based predicted toxicity. Following standardized experimental guidelines, zebrafish embryos and fathead minnow larvae were exposed for 96 h to each compound then were observed using digital behavioral analysis. Behavioral observations included photomotor responses, distance traveled, and stimulatory, refractory and cruising locomotor activity. Though fathead minnow larvae displayed greater behavioral sensitivity to 1-heptanol, phenol and citalopram, zebrafish were more sensitive to diazinon and R-(-)-carvone. Both fish models were equally sensitive to xylazine and PTZ. Further, the pharmaceuticals citalopram and xylazine significantly affected behavior at therapeutic hazard values, and each of the seven chemicals elicited unique behavioral response profiles. Larval fish behaviors appear useful as early tier diagnostics to identify mechanisms and pathways associated with diverse biological activities for chemicals lacking mechanistic data.

Plausibility of the zebrafish embryos/larvae as an alternative animal model for autism: A comparison study of transcriptome changes

Autism spectrum disorder (ASD) is a serious neurodevelopmental disorder characterized by impaired or abnormal social interaction and communication and by restricted and repetitive behaviour. ASD is highly prevalent in Asia, Europe, and the United States, and the frequency of ASD is growing each year. Recent epidemiological studies have indicated that ASD may be caused or triggered by exposure to chemicals in the environment, such as those in the air or water. Thus, toxicological studies are needed to examine chemicals that might be implicated. However, the experimental efficiency of existing experimental models is limited, and many models represent challenges in terms of animal welfare. Thus, alternative ASD animal models are necessary. To address this, we examined the efficacy of the zebrafish embryo/larva as an alternative model of ASD. Specifically, we exposed zebrafish to valproic acid (0, 12.5, 25, 50, or 100 μM), which is a chemical known to induce autism-like effects. We then analysed subsequent developmental, behavioural, and transcriptomic changes. We found that 100 μM and 50 μM valproic acid decreased the hatching rate and locomotor activity of zebrafish embryos/larvae. Transcriptomic analysis revealed significant alterations in a number of genes associated with autism, such as adsl, mbd5, shank3, and tsc1b. Additionally, we found changes in gene ontology that were also reported in previous studies. Our findings indicate that zebrafish embryos/larvae and humans with ASD might have common physiological pathways, indicating that this animal model may represent an alternative tool for examining the causes of and potential treatments for this illness.

Experimental Protocol for Examining Behavioral Response Profiles in Larval Fish: Application to the Neuro-stimulant Caffeine

Fish models and behaviors are increasingly used in the biomedical sciences; however, fish have long been the subject of ecological, physiological and toxicological studies. Using automated digital tracking platforms, recent efforts in neuropharmacology are leveraging larval fish locomotor behaviors to identify potential therapeutic targets for novel small molecules. Similar to these efforts, research in the environmental sciences and comparative pharmacology and toxicology is examining various behaviors of fish models as diagnostic tools in tiered evaluation of contaminants and real-time monitoring of surface waters for contaminant threats. Whereas the zebrafish is a popular larval fish model in the biomedical sciences, the fathead minnow is a common larval fish model in ecotoxicology. Unfortunately, fathead minnow larvae have received considerably less attention in behavioral studies. Here, we develop and demonstrate a behavioral profile protocol using caffeine as a model neurostimulant. Though photomotor responses of fathead minnows were occasionally affected by caffeine, zebrafish were markedly more sensitive for photomotor and locomotor endpoints, which responded at environmentally relevant levels. Future studies are needed to understand comparative behavioral sensitivity differences among fish with age and time of day, and to determine whether similar behavioral effects would occur in nature and be indicative of adverse outcomes at the individual or population levels of biological organization.

Morphometric analysis and neuroanatomical mapping of the zebrafish brain

Large-scale genomic studies have recently identified genetic variants causative for major neurodevelopmental disorders, such as intellectual disability and autism. However, determining how underlying developmental processes are affected by these mutations remains a significant challenge in the field. Zebrafish is an established model system in developmental neurogenetics that may be useful in uncovering the mechanisms of these mutations. Here we describe the use of voxel-intensity, deformation field, and volume-based morphometric techniques for the systematic and unbiased analysis of gene knock-down and environmental exposure-induced phenotypes in zebrafish. We first present a computational method for brain segmentation based on transgene expression patterns to create a comprehensive neuroanatomical map. This map allowed us to disclose statistically significant changes in brain microstructure and composition in neurodevelopmental models. We demonstrate the effectiveness of morphometric techniques in measuring changes in the relative size of neuroanatomical subdivisions in atoh7 morphant larvae and in identifying phenotypes in larvae treated with valproic acid, a chemical demonstrated to increase the risk of autism in humans. These tools enable rigorous evaluation of the effects of gene mutations and environmental exposures on neural development, providing an entry point for cellular and molecular analysis of basic developmental processes as well as neurodevelopmental and neurodegenerative disorders.

Development of a sensitive and quantitative capillary LC-UV method to study the uptake of pharmaceuticals in zebrafish brain

The present study explores the potential of 10-day-old zebrafish (Danio rerio) as a predictive blood-brain-barrier model using a set of 7 pharmaceutical agents. For this purpose, zebrafish were incubated with each of these 7 drugs separately via the route of immersion and the concentration reaching the brain was determined by applying a brain extraction procedure allowing isolation of the intact brain from the head of the zebrafish larvae. Sample analysis was performed utilizing capillary ultra-high performance liquid chromatography (cap-UHPLC) on a Pepmap RSLC C18 capillary column (150 mm × 300 μm, d_p = 2 μm) coupled to a variable wavelength UV detector. Gradient separation was performed in 28 min at a flow rate of 5 μL/min and the optimal injection volume was determined to be 1 μL. The brain extraction procedure was established for the zebrafish strain TG898 exhibiting red fluorescence of the brain, allowing control of the integrity of the extracted parts. Quantitative experiments carried out on pooled samples of six zebrafish (n = 6) demonstrated the selective semipermeable nature of the blood-brain barrier after incubating the zebrafish at the maximum tolerated concentration for the investigated pharmaceuticals. The obtained brain-to-trunk ratios ranged between 0.3 for the most excluded compound and 1.2 for the pharmaceutical agent being most accumulated in the brain of the fish. Graphical abstract Workflow of brain extraction to study the uptake of pharmaceuticals in the brain of zebrafish larvae.

European demonstration program on the effect-based and chemical identification and monitoring of organic pollutants in European surface waters

Growing concern about the adverse environmental and human health effects of a wide range of micropollutants requires the development of novel tools and approaches to enable holistic monitoring of their occurrence, fate and effects in the aquatic environment. A European-wide demonstration program (EDP) for effect-based monitoring of micropollutants in surface waters was carried out within the Marie Curie Initial Training Network EDA-EMERGE. The main objectives of the EDP were to apply a simplified protocol for effect-directed analysis, to link biological effects to target compounds and to estimate their risk to aquatic biota. Onsite large volume solid phase extraction of 50 L of surface water was performed at 18 sampling sites in four European river basins. Extracts were subjected to effect-based analysis (toxicity to algae, fish embryo toxicity, neurotoxicity, (anti-)estrogenicity, (anti-)androgenicity, glucocorticoid activity and thyroid activity), to target analysis (151 organic micropollutants) and to nontarget screening. The most pronounced effects were estrogenicity, toxicity to algae and fish embryo toxicity. In most bioassays, major portions of the observed effects could not be explained by target compounds, especially in case of androgenicity, glucocorticoid activity and fish embryo toxicity. Estrone and nonylphenoxyacetic acid were identified as the strongest contributors to estrogenicity, while herbicides, with a minor contribution from other micropollutants, were linked to the observed toxicity to algae. Fipronil and nonylphenol were partially responsible for the fish embryo toxicity. Within the EDP, 21 target compounds were prioritized on the basis of their frequency and extent of exceedance of predicted no effect concentrations. The EDP priority list included 6 compounds, which are already addressed by European legislation, and 15 micropollutants that may be important for future monitoring of surface waters. The study presents a novel simplified protocol for effect-based monitoring and draws a comprehensive picture of the surface water status across Europe.

Development of a sensitive and quantitative UHPLC-MS/MS method to study the whole-body uptake of pharmaceuticals in zebrafish

An analytical procedure to measure the whole-body uptake of pharmaceuticals in zebrafish has been developed using state-of-the-art methodologies. A sample preparation procedure for 9 pharmaceuticals displaying a variety in physicochemical properties was developed using 10-day old zebrafish (TG898). For an efficient homogenization of the samples and subsequent recovery of the compounds of interest, different amounts of organic solvents in combination with acidic modifiers were added to zebrafish samples. Samples were subsequently processed using a powerful bath sonicator and centrifuged. Supernatant was then removed and evaporated in a vacuum oven before being reconstituted in a mobile phase-like solvent. Samples were analyzed using ultra-high performance liquid chromatography (UHPLC) on an Acquity BEH C₁₈ column (100 × 2.1mm, d_p=1.7µm) coupled to a Waters Xevo TQ-S mass spectrometer. For this purpose, a generic gradient was run, wherein the percentage of acetonitrile was varied from 3% to 82% in 10.5min at a flow rate of 0.41mL/min. Linearity of the method was demonstrated for all compounds (R² > 0.997) in a practically relevant concentration range. Matrix effects were between 81% and 106%, except for amitriptyline (51%). Using this method, it was demonstrated that a sample pretreatment using 1:2 (v/v) water:methanol in combination with 0.1% formic acid resulted in acceptable recoveries between 74% and 100% for all compounds. Together with the obtained lower limits of quantification of the analytical method (between 0.005 and 1.5ng/mL), this allowed the use of a single zebrafish to study the whole-body uptake of a particular drug, after incubating zebrafish at the maximum tolerated concentration for this drug.

Environmental concentration of carbamazepine accelerates fish embryonic development and disturbs larvae behavior

Environmental pollution caused by pharmaceuticals has been recognized as a major threat to the aquatic ecosystems. Carbamazepine, as the widely prescribed antiepileptic drug, has been frequently detected in the aquatic environment and has created concerns about its potential impacts in the aquatic organisms. The effects of carbamazepine on zebrafish embryos were studied by examining their phenotype, behavior and molecular responses. The results showed that carbamazepine disturbed the normal growth and development of exposed zebrafish embryos and larvae. Upon exposure to carbamazepine at 1 μg/L, the hatching rate, body length, swim bladder appearance and yolk sac absorption rate were significantly increased. Embryos in treatment groups were more sensitive to touch and light stimulation. At molecular level, exposure to an environmentally relevant concentration (1 μg/L) of carbamazepine disturbed the expression pattern of neural-related genes of zebrafish embryos and larvae. This study suggests that the exposure of fish embryo to antiepileptic drugs, at environmentally relevant concentrations, affects their early development and impairs their behavior. Such impacts may have future repercussions by affecting fish population structure.

Using zebrafish in systems toxicology for developmental toxicity testing

With the high cost and the long-term assessment of developmental toxicity testing in mammals, the vertebrate zebrafish has become a useful alternative model organism for high-throughput developmental toxicity testing. Zebrafish is also very favorable for the 3R perspective in toxicology; however, the methodologies used by research groups vary greatly, posing considerable challenges to integrative analysis. In this review, we discuss zebrafish developmental toxicity testing, focusing on the methods of chemical exposure, the assessment of morphological abnormalities, housing conditions and their effects on the production of healthy embryos, and future directions. Zebrafish as a systems toxicology model has the potential to elucidate developmental toxicity pathways, and to provide a sound basis for human health risk assessments.

Embryological exposure to valproic acid induces social interaction deficits in zebrafish (Danio rerio): A developmental behavior analysis

Changes in social behavior are associated with brain disorders, including mood disorders, stress, schizophrenia, Alzheimer's disease, and autism spectrum disorders (ASD). Autism is a complex neurodevelopmental disorder characterized by deficits in social interaction, impaired communication, anxiety, hyperactivity, and the presence of restricted interests. Zebrafish is one of the most social vertebrates used as a model in biomedical research, contributing to an understanding of the mechanisms that underlie social behavior. Valproic acid (VPA) is used as an anti-epileptic drug and mood stabilizer; however, prenatal VPA exposure in humans has been associated with an increased incidence of autism and it can also affect fetal brain development. Therefore, we conducted a behavioral screening at different periods of zebrafish development at 6, 30, 70, and 120dpf (days postfertilization) after VPA exposure in the early development stage to investigate social behavior, locomotion, aggression, and anxiety. VPA (48μM) exposure during the first 48hpf (hours postfertilization) did not promote changes on survival, morphology, and hatching rate at 24hpf, 48hpf, and 72hpf. The behavioral patterns suggest that VPA exposure induces changes in locomotor activity and anxiety at different developmental periods in zebrafish. Furthermore, a social interaction deficit is present at 70dpf and 120dpf. VPA exposure did not affect aggression in the adult stage at 70dpf and 120dpf. This is the first study that demonstrated zebrafish exposed to VPA during the first 48h of development exhibit deficits in social interaction, anxiety, and hyperactivity at different developmental periods.

Prenatal factors associated with autism spectrum disorder (ASD)

Autism spectrum disorder (ASD) affecting about 1% of all children is associated, in addition to complex genetic factors, with a variety of prenatal, perinatal and postnatal etiologies. We discuss the known associated prenatal factors affecting the fetus throughout pregnancy; whenever relevant, also summarize some animal data. Among the maternal diseases in pregnancy associated with ASD are pregestational and/or gestational diabetes mellitus (PGDM, GDM), maternal infections (i.e. rubella, cytomegalovirus (CMV)), prolonged fever and maternal inflammation, which cause changes in a variety of inflammatory cytokines. Among the drugs are valproic acid, thalidomide, and possibly misoprostol and serotonin reuptake inhibitors (SSRIs). Associations were described with ethanol, and possibly cocaine, heavy metals heavy smoking and Folic acid deficiency. Heavy exposure to pesticides and air pollution during pregnancy was recently associated with ASD. We need more epidemiologic data to establish many of these associations; if proven, they might be promising avenues for prevention.

The proteomic and genomic teratogenicity elicited by valproic acid is preventable with resveratrol and α-tocopherol

BACKGROUND

Previously, we reported that valproic acid (VPA), a common antiepileptic drug and a potent teratogenic, dowregulates RBP4 in chicken embryo model (CEM) when induced by VPA. Whether such teratogenicity is associated with more advanced proteomic and genomic alterations, we further performed this present study.

METHODOLOGY/PRINCIPAL FINDINGS

VPA (60 µM) was applied to 36 chicken embryos at HH stage 10 (day-1.5). Resveratrol (RV) and vitamin E (vit E) (each at 0.2 and 2.0 µM) were applied simultaneously to explore the alleviation effect. The proteins in the cervical muscles of the day-1 chicks were analyzed using 2D-electrophoresis and LC/MS/MS. While the genomics associated with each specific protein alteration was examined with RT-PCR and qPCR. At earlier embryonic stage, VPA downregulated PEBP1 and BHMT genes and at the same time upregulated MYL1, ALB and FLNC genes significantly (p<0.05) without affecting PKM2 gene. Alternatively, VPA directly inhibited the folate-independent (or the betaine-dependent) remethylation pathway. These features were effectively alleviated by RV and vit E.

CONCLUSIONS

VPA alters the expression of PEBP1, BHMT, MYL1, ALB and FLNC that are closely related with metabolic myopathies, myogenesis, albumin gene expression, and haemolytic anemia. On the other hand, VPA directly inhibits the betaine-dependent remethylation pathway. Taken together, VPA elicits hemorrhagic myoliposis via these action mechanisms, and RV and vit E are effective for alleviation of such adverse effects.