Age matters: Developmental stage of Danio rerio larvae influences photomotor response thresholds to diazinion or diphenhydramine

Analyzing the Effects of Age, Time of Day, and Experiment on the Basal Locomotor Activity and Light-Off Visual Motor Response Assays in Zebrafish Larvae

The recent availability of commercial platforms for behavioral analyses in zebrafish larvae based on video-tracking technologies has exponentially increased the number of studies analyzing different behaviors in this model organism to assess neurotoxicity. Among the most commonly used assays in zebrafish larvae are basal locomotor activity (BLA) and visual motor responses (VMRs). However, the effect of different intrinsic and extrinsic factors that can significantly alter the outcome of these assays is still not well understood. In this work, we have analyzed the influence of age (5-8 days post-fertilization), time of day (8:00, 10:00, 12:00, 14:00; 16:00, 18:00, and 20:00 h), and experiment (three experiments performed at different days) on BLA and VMR results (4004 analyses for each behavior) in 143 larvae. The results from both behaviors were adjusted to a random-effects linear regression model using generalized least squares (GLSs), including in the model the effect of the three variables, the second-way interactions between them, and the three-way interaction. The results presented in this manuscript show a specific effect of all three intrinsic factors and their interactions on both behaviors, supporting the view that the most stable time period for performing these behavioral assays is from 10:00 am to 04:00 pm, with some differences depending on the age of the larva and the behavioral test.

Interactive effects between water temperature, microparticle compositions, and fiber types on the marine keystone species Americamysis bahia

Recently, there has been an increasing emphasis on examining the ecotoxicological effects of anthropogenic microparticles (MPs), especially microplastic particles, and related issues. Nevertheless, a notable deficiency exists in our understanding of the consequences on marine organisms, specifically in relation to microfibers and the combined influence of MPs and temperature. In this investigation, mysid shrimp (Americamysis bahia), an important species and prey item in estuarine and marine food webs, were subjected to four separate experimental trials involving fibers (cotton, nylon, polyester, hemp; 3 particles/ml; approximately 200 μm in length) or fragments (low-density Polyethylene: LDPE, polylactic acid: PLA, and their leachates; 5, 50, 200, 500 particles/ml; 1-20 μm). To consider the effects in the context of climate change, three different temperatures (22, 25, and 28 °C) were examined. Organismal growth and swimming behavior were measured following exposure to fragments and microfibers, and reactive oxygen species and particle uptake were investigated after microfiber exposure. To simulate the physical characteristics of MP exposure, such as microfibers obstructing the gills, we also assessed the post-fiber-exposure swimming behavior in an oxygen-depleted environment. Data revealed negligible fragment, but fiber exposure effects on growth. PLA leachate triggered higher activity at 25 °C and 28 °C; LDPE exposures led to decreased activity at 28 °C. Cotton exposures led to fewer behavioral differences compared to controls than other fiber types. The exposure to hemp fibers resulted in significant ROS increases at 28 °C. Microfibers were predominantly located within the gastric and upper gastrointestinal tract, suggesting extended periods of residence and the potential for obstructive phenomena over the longer term. The combination of increasing water temperatures, microplastic influx, and oxidative stress has the potential to pose risks to all components of marine and aquatic food webs.

Assessments of carbon nanotubes toxicities in zebrafish larvae using multiple physiological and molecular endpoints

In recent years, carbon nanotubes (CNTs) have become one of the most promising materials for the technology industry. However, due to the extensive usage of these materials, they may be released into the environment, and cause toxicities to the organism. Here, their acute toxicities in zebrafish embryos and larvae were evaluated by using various assessments that may provide us with a novel perspective on their effects on aquatic animals. Before conducting the toxicity assessments, the CNTs were characterized as multiwall carbon nanotubes (MWCNTs) functionalized with hydroxyl and carboxyl groups, which improved their solubility and dispersibility. Based on the results, abnormalities in zebrafish behaviors were observed in the exposed groups, indicated by a reduction in tail coiling frequency and alterations in the locomotion as the response toward photo and vibration stimuli that might be due to the disruption in the neuromodulatory system and the formation of reactive oxygen species (ROS) by MWCNTs. Next, based on the respiratory rate assay, exposed larvae consumed more oxygen, which may be due to the injuries in the larval gill by the MWCNTs. Finally, even though no irregularity was observed in the exposed larval cardiac rhythm, abnormalities were shown in their cardiac physiology and blood flow with significant downregulation in several cardiac development-related gene expressions. To sum up, although the following studies are necessary to understand the exact mechanism of their toxicity, the current study demonstrated the environmental implications of MWCNTs in particularly low concentrations and short-term exposure, especially to aquatic organisms.

Elucidating the effects of pure glyphosate and a commercial formulation on early life stages of zebrafish using a complete biomarker approach: All-or-nothing!

The search for new methods in the toxicology field has increased the use of early life stages of zebrafish (Danio rerio) as a versatile organism model. Here, we use early stages of zebrafish to evaluate glyphosate as pure active ingredient and within a commercial formulation in terms of oxidative stress. Biomarkers involved in the oxidative status were evaluated along with other markers of neurotoxicity, genotoxicity, cytotoxicity, energy balance and motor performance, and the selected tools were evaluated by its sensitivity in determining early-warning events. Zebrafish embryos exposed to glyphosate active ingredient and glyphosate-based formulation were under oxidative stress, but only the commercial formulation delayed the embryogenesis, affected the cholinergic neurotransmission and induced DNA damage. Both altered the motor performance of larvae at very low concentrations, becoming larvae hypoactive. The energy balance was also impaired, as embryos under oxidative stress had lower lipids reserves. Although data suggest that glyphosate-based formulation has higher toxicity than the active ingredient itself, the most sensitive biomarkers detected early-warning effects at very low concentrations of the active ingredient. Biochemical biomarkers of defense system and oxidative damage were the most sensitive tools, detecting pro-oxidant responses at very low concentrations, along with markers of motor performance that showed high sensitivity and high throughput, suitable for detecting early effects linked to neurotoxicity. Alterations on morphology during embryogenesis showed the lowest sensitivity, thus morphological alterations appeared after several alterations at biochemical levels. Tools evaluating DNA damage and cell proliferation showed mid-sensitivity, but low throughput, thus they could be used as complementary markers.

Reporting and reproducibility: Proteomics of fish models in environmental toxicology and ecotoxicology

Environmental toxicology and ecotoxicology research efforts are employing proteomics with fish models as New Approach Methodologies, along with in silico, in vitro and other omics techniques to elucidate hazards of toxicants and toxins. We performed a critical review of toxicology studies with fish models using proteomics and reported fundamental parameters across experimental design, sample preparation, mass spectrometry, and bioinformatics of fish, which represent alternative vertebrate models in environmental toxicology, and routinely studied animals in ecotoxicology. We observed inconsistencies in reporting and methodologies among experimental designs, sample preparations, data acquisitions and bioinformatics, which can affect reproducibility of experimental results. We identified a distinct need to develop reporting guidelines for proteomics use in environmental toxicology and ecotoxicology, increased QA/QC throughout studies, and method optimization with an emphasis on reducing inconsistencies among studies. Several recommendations are offered as logical steps to advance development and application of this emerging research area to understand chemical hazards to public health and the environment.

Evaluation of Tacrolimus' Adverse Effects on Zebrafish in Larval and Adult Stages by Using Multiple Physiological and Behavioral Endpoints

Tacrolimus (FK506) is a common immunosuppressant that is used in organ transplantation. However, despite its importance in medical applications, it is prone to adverse side effects. While some studies have demonstrated its toxicities to humans and various animal models, very few studies have addressed this issue in aquatic organisms, especially zebrafish. Here, we assessed the adverse effects of acute and chronic exposure to tacrolimus in relatively low doses in zebrafish in both larval and adult stages, respectively. Based on the results, although tacrolimus did not cause any cardiotoxicity and respiratory toxicity toward zebrafish larvae, it affected their locomotor activity performance in light-dark locomotion tests. Meanwhile, tacrolimus was also found to slightly affect the behavior performance, shoaling formation, circadian rhythm locomotor activity, and color preference of adult zebrafish in a dose-dependent manner. In addition, alterations in the cognitive performance of the fish were also displayed by the treated fish, indicated by a loss of short-term memory. To help elucidate the toxicity mechanism of tacrolimus, molecular docking was conducted to calculate the strength of the binding interaction between tacrolimus to human FKBP12. The results showed a relatively normal binding affinity, indicating that this interaction might only partly contribute to the observed alterations. Nevertheless, the current research could help clinicians and researchers to further understand the toxicology of tacrolimus, especially to zebrafish, thus highlighting the importance of considering the toxicity of tacrolimus prior to its usage.

Effects of Age and Exposure Duration on the Sensitivity of Early Life Stage Fathead Minnow (Pimephales promelas) to Waterborne Propranolol Exposure

Propranolol is a heavily prescribed, nonspecific beta-adrenoceptor (bAR) antagonist frequently found in wastewater effluents, prompting concern over its potential to adversely affect exposed organisms. In the present study, the transcriptional responses of 4, 5, and 6 days postfertilization (dpf) ±1 h fathead minnow, exposed for 6, 24, or 48 h to 0.66 or 3.3 mg/L (nominal) propranolol were characterized using RNA sequencing. The number of differentially expressed genes (DEGs) was used as an estimate of sensitivity. A trend toward increased sensitivity with age was observed; fish >7 dpf at the end of exposure were particularly sensitive to propranolol. The DEGs largely overlapped among treatment groups, suggesting a highly consistent response that was independent of age. Cluster analysis was performed using normalized count data for unexposed and propranolol-exposed fish. Control fish clustered tightly by age, with fish ≥7 dpf clustering away from younger fish, reflecting developmental differences. When clustering was conducted using exposed fish, in cases where propranolol induced a minimal or no transcriptional response, the results mirrored those of the control fish and did not appreciably cluster by treatment. In treatment groups that displayed a more robust transcriptional response, the effects of propranolol were evident; however, fish <7 dpf clustered away from older fish, despite having similar numbers of DEGs. Increased sensitivity at 7 dpf coincided with developmental milestones with the potential to alter propranolol pharmacokinetics or pharmacodynamics, such as the onset of exogenous feeding and gill functionality as well as increased systemic expression of bAR. These results may have broader implications because toxicity testing often utilizes fish <4 dpf, prior to the onset of these potentially important developmental milestones, which may result in an underestimation of risk for some chemicals. Environ Toxicol Chem 2024;00:1-14. Published 2023. This article is a U.S. Government work and is in the public domain in the USA.

Integrating morphological, biochemical, behavioural, and molecular approaches to investigate developmental toxicity triggered by tebuthiuron in zebrafish (Danio rerio)

Tebuthiuron (TBU), a phenylurea herbicide, is widely applied in agricultural and non-agricultural soils. Because TBU resists degradation, it can contaminate water and reach the biota once it is released into the environment. However, the potential toxic effects of TBU on aquatic developing organisms have been poorly studied. By taking advantage of the early-life stages of zebrafish (Danio rerio), we have combined morphological, biochemical, behavioural, and molecular approaches to investigate the developmental toxicity triggered by environmentally relevant concentrations (from 0.1 to 1000 μg/L) of TBU. Exposure to TBU did not elicit morphological abnormalities but it significantly delayed hatching. In addition, TBU altered the frequency of tail coils in one-day post-fertilization (dpf) old embryos. Moreover, TBU exposure during four days significantly inhibited the whole body AChE activity of larvae. At the molecular level, TBU did not significantly affect the mRNA levels of four genes (elavl3, gfap, gap43, and shha) that play key roles during the neurodevelopment of zebrafish. By assessing the motor responses to repeated light-dark stimuli, 6 dpf larvae exposed to TBU displayed hyperactivity, showing greater travelling distance during the dark periods. Our categorization of swimming speed revealed an interesting finding - after the light was turned off, the exposed larvae abandoned the freezing mode (<2 mm/s) and travelled mainly at cruising speed (2-20 mm/s), showing that the larval hyperactivity did not translate into higher swimming velocity. Overall, our results offer new insights into the TBU toxicity to developing organisms, namely effects in AChE activity and hyperactivity, providing support data for future studies considering environmental risk assessment of this herbicide.

Comparative Assessment of the Toxicity of Brominated and Halogen-Free Flame Retardants to Zebrafish in Terms of Tail Coiling Activity, Biomarkers, and Locomotor Activity

BDE-47, a flame retardant that is frequently detected in environmental compartments and human tissues, has been associated with various toxic effects. In turn, information about the effects of aluminum diethyl-phosphinate (ALPI), a halogen-free flame retardant from a newer generation, is limited. This study aims to assess and compare the toxicity of BDE-47 and ALPI to zebrafish by analyzing the tail coiling, locomotor, acetylcholinesterase activities, and oxidative stress biomarkers. At 3000 µg/L BDE-47, the coiling frequency increased at 26-27 h post-fertilization (hpf), but the burst activity (%) and mean burst duration (s) did not change significantly. Here, we considered that the increased coiling frequency is a slight neurotoxic effect because locomotor activity was impaired at 144 hpf and 300 µg/L BDE-47. Moreover, we hypothesized that oxidative stress could be involved in the BDE-47 toxicity mechanisms. In contrast, only at 30,000 µg/L did ALPI increase the catalase activity, while the motor behavior during different developmental stages remained unaffected. On the basis of these findings, BDE-47 is more toxic than ALPI.

Optimizing assays of zebrafish larvae swimming performance for drug discovery

INTRODUCTION

Zebrafish larvae are one of the few vertebrates amenable to large-scale drug discovery screens. Larval swimming behavior is often used as an outcome variable and many fields of study have developed assays for evaluating swimming performance. An unintended consequence of this wide interest is that details related to assay methodology and interpretation become scattered across the literature. The aim of this review is to consolidate this information, particularly as it relates to high-throughput approaches.

AREAS COVERED

The authors describe larval swimming behaviors as this forms the basis for understanding their experimentally evoked swimming or spontaneous activity. Next, they detail how swimming activity can serve as an outcome variable, particularly in the multi-well formats used in large-scale screening studies. They also highlight biological and technical factors that can impact the sensitivity and variability of these measurements.

EXPERT OPINION

Careful attention to animal husbandry, experimental design, data acquisition, and interpretation of results can improve screen outcomes by maximizing swimming activity while minimizing intra- and inter-larval variability. The development of more sensitive, quantitative methods of assessing swimming performance that can be incorporated into high-throughput workflows will be important in order to take full advantage of the zebrafish model.

Experimental arena size alters larval zebrafish photolocomotor behaviors and influences bioactivity responses to a model neurostimulant

Zebrafish behavior is increasingly common in biomedical and environmental studies of chemical bioactivity. Multiple experimental arena sizes have been used to measure photolocomotion in zebrafish depending on age, endpoints observed, and instrumentation, among other factors. However, the extent to which methodological parameters may influence naïve behavioral performance and detection of behavioral changes is poorly understood. Here we measured photolocomotion and behavioral profiles of naïve larval zebrafish across arena sizes. We then performed concentration response studies with the model neurostimulant caffeine, again across various arena dimensions. We found total swimming distance of unexposed fish to increase logarithmically with arena size, which as related to circumference, area, and volume. Photomotor response during light/dark transitions also increased with arena size. Following caffeine exposure, total distance travelled was significantly (p < 0.001) affected by well size, caffeine treatment (p < 0.001), and the interaction of these two experimental factors (p < 0.001). In addition, behavioral response profiles showed differences between 96 well plates and larger well sizes. Biphasic response, with stimulation at lower concentrations and refraction at the highest concentration, was observed in dark conditions for the 96 well size only, though almost no effects were identified in the light. However, swimming behavior was significantly (p < 0.1) altered in the highest studied caffeine treatment level in larger well sizes during both light and dark periods. Our results indicate zebrafish swim more in larger arenas and arena size influences behavioral response profiles to caffeine, though differences were mostly observed between very small and large arenas. Further, careful consideration should be given when choosing arena size, because small wells may lead to restriction, while larger wells may differentially reflect biologically relevant effects. These findings can improve comparability among experimental designs and demonstrates the importance of understanding confounding methodological variables.

Cocktail effects of emerging contaminants on zebrafish: Nanoplastics and the pharmaceutical diphenhydramine

Nanoplastics (NPLs) became ubiquitous in the environment, from the air we breathe to the food we eat. One of the main concerns about the NPLs risks is their role as carrier of other environmental contaminants, potentially increasing their uptake, bioaccumulation and toxicity to the organisms. Therefore, the main aim of this study was to understand how the presence of polystyrene NPLs (∅ 44 nm) will influence the toxicity (synergism, additivity or antagonism) of the antihistamine diphenhydramine (DPH), towards zebrafish (Danio rerio) embryos, when in dual mixtures. After 96 hours (h) exposure, at the organismal level, NPLs (0.015 or 1.5 mg/L) + DPH (10 mg/L) induced embryo mortality (90%) and malformations (100%) and decreased hatching (80%) and heartbeat rates (60%). After 120 h exposure, NPLs (0.015 or 1.5 mg/L) + DPH (0.01 mg/L) decreased larvae swimming distance (30-40%). At the biochemical level, increased glutathione S-transferases (55-122%) and cholinesterase (182-343%) activities were found after 96 h exposure to NPLs (0.015 or 1.5 mg/L) + DPH (0.01 mg/L). However, catalase (CAT) activity remained similar to the control group in the mixtures, inhibiting the effects detected after the exposure to 1.5 mg/L NPLs alone (increased 230% of CAT activity). In general, the effects of dual combination - NPLs + DPH (even at concentrations as low as 10 μg/L of DPH) - were more harmful than the correspondent individual exposures, showing the synergistic interactions of the dual mixture and answering to the main question of this work. The obtained results, namely the altered toxicity patterns of NPLs + DPH compared with the individual exposures, show the importance of an environmental risk assessment considering NPLs as a co-contaminant due to the potential NPLs role as vector for other contaminants.

Swimming Behavior of Daphnia magna Is Altered by Pesticides of Concern, as Components of Agricultural Surface Water and in Acute Exposures

Pesticides with novel modes of action including neonicotinoids and anthranilic diamides are increasingly detected in global surface waters. Little is known about how these pesticides of concern interact in mixtures at environmentally relevant concentrations, a common exposure scenario in waterways impacted by pesticide pollution. We examined effects of chlorantraniliprole (CHL) and imidacloprid (IMI) on the sensitive invertebrate, Daphnia magna. Exposures were first performed using surface waters known to be contaminated by agricultural runoff. To evaluate the seasonal variation in chemical concentration and composition of surface waters, we tested surface water samples taken at two time points: during an extended dry period and after a first flush storm event. In surface waters, the concentrations of CHL, IMI, and other pesticides of concern increased after first flush, resulting in hypoactivity and dose-dependent photomotor responses. We then examined mortality and behavior following single and binary chemical mixtures of CHL and IMI. We detected inverse photomotor responses and some evidence of synergistic effects in binary mixture exposures. Taken together, this research demonstrates that CHL, IMI, and contaminated surface waters all cause abnormal swimming behavior in D. magna. Invertebrate swimming behavior is a sensitive endpoint for measuring the biological effects of environmental pesticides of concern.

Feeding behavior of early life stages of the zebrafish Danio rerio is altered by exposure to glyphosate

Glyphosate levels and the transfer of glyphosate across trophic levels have rarely been studied in zooplankton. The food preferences of zebrafish during the first-feeding stage (which is critical for the survival of organisms), were analyzed because of the requirement for live food. Larval survival begins to be affected when glyphosate intake exceeds 0.3666 µg/larvae/day, in the case that only the food is contaminated; if the medium is also contaminated, the effects on survival start from 0.2456 µg/larvae/day. It was shown that glyphosate was more likely to be incorporated through the medium than through the food (zooplankton), which supports the results of previous studies that have ruled out the potential for biomagnification. The bioconcentration factor (BCF) of glyphosate was determined using an ELISA tests specific to measure glyphosate in the fish D. rerio, the rotifers Brachionus calyciflorus and Lecane papuana, and the cladoceran Ceriodaphnia dubia. The experimental design consisted in exposing seven zebrafish adults per replica (four replicates) in three treatments 1, 5, and 10 mg/L of glyphosate for 96 h to obtain bioconcentration factors in the gills, liver, and muscle. These concentrations were selected as potential glyphosate concentrations right after application as double highest reported concentration. Glyphosate levels in zooplankton can represent up to 6.26% of the total weight of rotifers (BFC = 60.35) and in zebrafish adult organs were less than 8 µg/mg of tissue (BCF values < 6). Although glyphosate does not biomagnify, our results suggest that glyphosate affected the dynamics between zooplankton and zebrafish larvae, diminishing survival and feeding rates, given that zooplankton species bioconcentrate glyphosate in large quantities. The BCF values found in this contribution are higher than expected. Glyphosate exposure affected energy metabolism and feeding behavior of zebrafish larvae, which presented high mortality rates at environmentally relevant concentrations.

Efficiency of Several Cytochrome P450 Biomarkers in Highlighting the Exposure of Daphnia magna to an Organophosphate Pesticide

The cytochromes P450 (CYP450) represent a major enzyme family operating mostly in the first step of xenobiotic detoxification in aquatic organisms. The ability to measure these CYP450 enzymes' activities provides a crucial tool to understand organisms' response to chemical stressors. However, research on CYP450 activity measurement is still limited and has had variable success. In the present study, we optimize, compile, and compare existing scientific information and techniques for a series of CYP450 biomarkers (EROD, MROD, ECOD, APND, and ERND) used on Daphnia magna. Additionally, we explored these CYP450 biomarkers' activities through the first 5 days of life of daphnids, providing a link between their age and sensitivity to chemicals. In the experiment, daphnids were exposed to an organophosphate pesticide (diazinon) from birth to measure the molecular response of the detoxification process. Our results suggest EROD as the most applicable biomarker for organisms such as D. magna, with a higher organophosphate detoxification rate in daphnids that are 2 and 5 days old. Additionally, a larger body size allowed a more accurate EROD measurement; hence, we emphasize the use of 5-day-old daphnids when analyzing their detoxification response.

Toxicity Assessment of an Anti-Cancer Drug of p-Toluene Sulfonamide in Zebrafish Larvae Based on Cardiovascular and Locomotion Activities

p-Toluene sulfonamide (p-TSA), a small molecular drug with antineoplastic activity is widely gaining interest from researchers because of its pharmacological activities. In this study, we explored the potential cardio and neural toxicity of p-TSA in sublethal concentrations by using zebrafish as an in vivo animal model. Based on the acute toxicity assay, the 96hr LC50 was estimated as 204.3 ppm, suggesting the overall toxicity of p-TSA is relatively low in zebrafish larvae. For the cardiotoxicity test, we found that p-TSA caused only a minor alteration in treated larvae after no overall significant alterations were observed in cardiac rhythm and cardiac physiology parameters, as supported by the results from expression level measurements of several cardiac development marker genes. On the other hand, we found that acute p-TSA exposure significantly increased the larval locomotion activity during the photomotor test while prolonged exposure (4 days) reduced the locomotor startle reflex activities in zebrafish. In addition, a higher respiratory rate and blood flow velocity was also observed in the acutely treated fish groups compared to the untreated group. Finally, by molecular docking, we found that p-TSA has a moderate binding affinity to skeletal muscle myosin II subfragment 1 (S1), ATPase activity, actin- and Ca2+-stimulated myosin S1 ATPase, and v-type proton ATPase. These binding interactions between p-TSA and proteins offer insights into the potential molecular mechanism of action of p-TSA on observed altered responses toward photo and vibration stimuli and minor altered vascular performance in the zebrafish larvae.

Identifying Behavioral Response Profiles of Two Common Larval Fish Models to a Salinity Gradient

Salinization of aquatic systems is an emerging global issue projected to increase in magnitude, frequency, and duration with climate change and landscape modifications. To consider influences of salinity on locomotor activity of common fish models, we examined behavioral response profiles of two species, zebrafish (Danio rerio) and fathead minnow (Pimephales promelas), across a gradient of sodium chloride. Following each experiment, behavior was recorded with automated tracking software and then behavioral response variables, including locomotor (e.g., distance traveled, number of movements, duration of movements) and photolocomotor changes, were examined at several speed thresholds (bursting, cruising, freezing) to identify potential salinity responses. Zebrafish responses were significantly (p < 0.05) reduced at the highest treatment level (5.78 g/L) for multiple behavioral endpoints during both dark and light phases; however, fathead minnow responses were more variable and not consistently concentration dependent. Future efforts are needed to understand behavioral response profiles in combination with anthropogenic contaminants and natural toxins across the freshwater to marine continuum, considering salinization of inland waters, sea level rise, and transport of anthropogenic contaminants and algal toxins from inland waters to coastal systems.

Assessment of diphenhydramine toxicity - Is its mode of action conserved between human and zebrafish?

The main aim of the study is to evaluate the effects of the pharmaceutical diphenhydramine (DPH) on embryo-larvae Danio rerio across distinct levels of organization - individual and subcellular - and correlate those effects with the DPH mode of action (MoA) assessed by in silico analysis. An embryos heartbeat rate reduction was observed at 10 mg/L DPH, but 0.001 to 10 mg/L did not significantly affect the zebrafish survival, hatching and morphology. Larvae swimming distance decreased (hypoactivity) at 1 and 10 mg/L DPH. Moreover, the straightforward movements decrease and the increase in the zigzag movements or movements with direction changes, shown an erratic swimming behavior. Energy budgets decreased for lipid (0.01 mg/L DPH) and carbohydrate (10 mg/L DPH) contents. Cholinesterase (neural function) and glutathione S-transferase (Phase II biotransformation/antioxidant processes) increased their activities at 10 mg/L DPH, where a decrease in the total glutathione content (antioxidant system) was observed. DNA damage was found at 0.01 and 10 mg/L DPH. However, a DNA repair occurred after subsequent 72 h in clean media. The in silico study revealed a relevant conservation between human and zebrafish DPH target molecules. These data provide a valuable ecotoxicological information about the DPH effects and MoA to non-target organisms.

Getting more out of the zebrafish light dark transition test

In (eco-)toxicological studies the light/dark transition (LDT) test is one of the most frequently used behaviour assays with zebrafish eleutheroembryos. However, study results vary regarding data presentation and analysis and mostly focus on a limited amount of the recorded data. In this study, we investigated whether monitoring two behavioural outcomes (time and distance moved) together with analysing multiple parameters can improve test sensitivity and data interpretation. As a proof of principle 5-day old zebrafish (Danio rerio) eleutheroembryos exposed to either endocrine disruptors (EDs) or acetylcholine esterase (AChE) inhibitors were investigated. We analysed conventional parameters such as mean and sum and implemented additional endpoints such as minimum or maximum distance moved and new parameters assessing the bursting response of eleutheroembryos. Furthermore, changes in eleutheroembryonic behaviour during the moment of the light to dark transition were added. To improve data presentation control-normalised results were displayed in radar charts, enabling the simultaneous presentation of different parameters in relation to each other. This enabled us to identify parameters most relevant to a certain behavioural response. A cut off threshold using control data was applied to identify parameters that were altered in a biological relevant manner. Our approach was able to detect effects on different parameters that remained undetected when analysis was done using conventional bar graphs on - in most cases analysed - averaged, mean distance moved values. By combining the radar charts with additional parameters and by using control-based thresholds, we were able to increase the test sensitivity and promote a deeper understanding of the behaviour response of zebrafish eleutheroembryos in the LDT test and thereby increased its usability for behavioural toxicity studies.

Assessment of the Preventive Effect of L-carnitine on Post-statin Muscle Damage in a Zebrafish Model

Statins, such as lovastatin, are lipid-lowering drugs (LLDs) that have been used to treat hypercholesterolaemia, defined as abnormally elevated cholesterol levels in the patient’s blood. Although statins are considered relatively safe and well tolerated, recipients may suffer from adverse effects, including post-statin myopathies. Many studies have shown that supplementation with various compounds may be beneficial for the prevention or treatment of side effects in patients undergoing statin therapy. In our study, we investigated whether L-carnitine administered to zebrafish larvae treated with lovastatin alleviates post-statin muscle damage. We found that exposure of zebrafish larvae to lovastatin caused skeletal muscle disruption observed as a reduction of birefringence, changes in muscle ultrastructure, and an increase in atrogin-1. Lovastatin also affected heart performance and swimming behaviour of larvae. Our data indicated that the muscle-protective effect of L-carnitine is partial. Some observed myotoxic effects, such as disruption of skeletal muscle and increase in atrogin-1 expression, heart contraction could be rescued by the addition of L-carnitine. Others, such as slowed heart rate and reduced locomotion, could not be mitigated by L-carnitine supplementation.

Characterization of locomotor phenotypes in zebrafish larvae requires testing under both light and dark conditions

Despite growing knowledge, much remains unknown regarding how signaling within neural networks translate into specific behaviors. To pursue this quest, we need better understanding of the behavioral output under different experimental conditions. Zebrafish is a key model to study the relationship between network and behavior and illumination is a factor known to influence behavioral output. By only assessing behavior under dark or light conditions, one might miss behavioral phenotypes exclusive to the neglected illumination setting. Here, we identified locomotor behavior, using different rearing regimes and experimental illumination settings, to showcase the need to assess behavior under both light and dark conditions. Characterization of free-swimming zebrafish larvae, housed under continuous darkness or a day/night cycle, did not reveal behavioral differences; larvae were most active during light conditions. However, larvae housed under a day/night cycle moved a shorter distance, had lower maximum velocity and maximum acceleration during the startle response under light conditions. Next, we explored if we could assess behavior under both dark and light conditions by presenting these conditions in sequence, using the same batch of larvae. Our experiments yielded similar results as observed for naïve larvae: higher activity during light conditions, regardless of order of illumination (i.e. dark-light or light-dark). Finally, we conducted these sequenced illumination conditions in an experimental setting by characterizing behavioral phenotypes in larvae following neuromast ablation. Depending on the illumination during testing, the behavioral phenotype following ablation was characterized differently. In addition, the results indicate that the order in which the light and dark conditions are presented has to be considered, as habituation may occur. Our study adds to existing literature on illumination-related differences in zebrafish behavior and emphasize the need to explore behavioral phenotypes under both light and dark condition to maximize our understanding of how experimental permutations affect behavior.

Anticonvulsant activity of melatonin and its success in ameliorating epileptic comorbidity-like symptoms in zebrafish

Epilepsy is one of common neurological disorders, greatly distresses the well-being of the sufferers. Melatonin has been used in clinical anti-epileptic studies, but its effect on epileptic comorbidities is unknown, and the underlying mechanism needs further investigation. Herein, by generating PTZ-induced zebrafish seizure model, we carried out interdisciplinary research using neurobehavioral assays, bioelectrical detection, molecular biology, and network pharmacology to investigate the activity of melatonin as well as its pharmacological mechanisms. We found melatonin suppressed seizure-like behavior by using zebrafish regular locomotor assays. Zebrafish freezing and bursting activity assays revealed the ameliorative effect of melatonin on comorbidity-like symptoms. The preliminary screening results of neurobehavioral assays were further verified by the expression of key genes involved in neuronal activity, neurodevelopment, depression and anxiety, as well as electrical signal recording from the midbrain of zebrafish. Subsequently, network pharmacology was introduced to identify potential targets of melatonin and its pathways. Real-time qPCR and protein-protein interaction (PPI) were conducted to confirm the underlying mechanisms associated with glutathione metabolism. We also found that melatonin receptors were involved in this process, which were regulated in response to melatonin exposure before PTZ treatment. The antagonists of melatonin receptors affected anticonvulsant activity of melatonin. Overall, current study revealed the considerable ameliorative effects of melatonin on seizure and epileptic comorbidity-like symptoms and unveiled the underlying mechanism. This study provides an animal model for the clinical application of melatonin in the treatment of epilepsy and its comorbidities.

Evaluation of Effects of Ractopamine on Cardiovascular, Respiratory, and Locomotory Physiology in Animal Model Zebrafish Larvae

Ractopamine (RAC) is a beta-adrenoceptor agonist that is used to promote lean and increased food conversion efficiency in livestock. This compound has been considered to be causing behavioral and physiological alterations in livestock like pig. Few studies have addressed the potential non-target effect of RAC in aquatic animals. In this study, we aimed to explore the potential physiological response after acute RAC exposure in zebrafish by evaluating multiple endpoints like locomotor activity, oxygen consumption, and cardiovascular performance. Zebrafish larvae were subjected to waterborne RAC exposure at 0.1, 1, 2, 4, or 8 ppm for 24 h, and the corresponding cardiovascular, respiratory, and locomotion activities were monitored and quantified. In addition, we also performed in silico molecular docking for RAC with 10 zebrafish endogenous β-adrenergic receptors to elucidate the potential acting mechanism of RAC. Results show RAC administration can significantly boost locomotor activity, cardiac performance, oxygen consumption, and blood flow rate, but without affecting the cardiac rhythm regularity in zebrafish embryos. Based on structure-based flexible molecular docking, RAC display similar binding affinity to all ten subtypes of endogenous β-adrenergic receptors, from adra1aa to adra2db, which are equivalent to the human one. This result suggests RAC might act as high potency and broad spectrum β-adrenergic receptors agonist on boosting the locomotor activity, cardiac performance, and oxygen consumption in zebrafish. To validate our results, we co-incubated a well-known β-blocker of propranolol (PROP) with RAC. PROP exposure tends to minimize the locomotor hyperactivity, high oxygen consumption, and cardiac rate in zebrafish larvae. In silico structure-based molecular simulation and binding affinity tests show PROP has an overall lower binding affinity than RAC. Taken together, our studies provide solid in vivo evidence to support that RAC plays crucial roles on modulating cardiovascular, respiratory, and locomotory physiology in zebrafish for the first time. In addition, the versatile functions of RAC as β-agonist possibly mediated via receptor competition with PROP as β-antagonist.

Pharmaceutical Assessment Suggests Locomotion Hyperactivity in Zebrafish Triggered by Arecoline Might Be Associated with Multiple Muscarinic Acetylcholine Receptors Activation

Arecoline is one of the nicotinic acid-based alkaloids, which is found in the betel nut. In addition to its function as a muscarinic agonist, arecoline exhibits several adverse effects, such as inducing growth retardation and causing developmental defects in animal embryos, including zebrafish, chicken, and mice. In this study, we aimed to study the potential adverse effects of waterborne arecoline exposure on zebrafish larvae locomotor activity and investigate the possible mechanism of the arecoline effects in zebrafish behavior. The zebrafish behavior analysis, together with molecular docking and the antagonist co-exposure experiment using muscarinic acetylcholine receptor antagonists were conducted. Zebrafish larvae aged 96 h post-fertilization (hpf) were exposed to different concentrations (0.001, 0.01, 0.1, and 1 ppm) of arecoline for 30 min and 24 h, respectively, to find out the effect of arecoline in different time exposures. Locomotor activities were measured and quantified at 120 hpf. The results showed that arecoline caused zebrafish larvae locomotor hyperactivities, even at a very low concentration. For the mechanistic study, we conducted a structure-based molecular docking simulation and antagonist co-exposure experiment to explore the potential interactions between arecoline and eight subtypes, namely, M1a, M2a, M2b, M3a, M3b, M4a, M5a, and M5b, of zebrafish endogenous muscarinic acetylcholine receptors (mAChRs). Arecoline was predicted to show a strong binding affinity to most of the subtypes. We also discovered that the locomotion hyperactivity phenotypes triggered by arecoline could be rescued by co-incubating it with M1 to M4 mAChR antagonists. Taken together, by a pharmacological approach, we demonstrated that arecoline functions as a highly potent hyperactivity-stimulating compound in zebrafish that is mediated by multiple muscarinic acetylcholine receptors.

Antidepressant Screening Demonstrated Non-Monotonic Responses to Amitriptyline, Amoxapine and Sertraline in Locomotor Activity Assay in Larval Zebrafish

Antidepressants are well-known drugs to treat depression and major depressive disorder for humans. However, the misuse and abuse of antidepressants keep increasing with several side effects reported. The aim of this study was to assess the potential adverse effects of 18 antidepressants by monitoring zebrafish larval locomotor activity performance based on the total distance traveled, burst movement count, and total rotation count at four dark-light intercalated phases. In general, zebrafish larvae displayed sedative effects after antidepressant exposure by showing a significant reduction in all of the locomotor activity-related endpoints. However, three antidepressants i.e., amitriptyline, amoxapine, and sertraline were able to trigger a significantly high locomotor activity in zebrafish larvae during the light cycle. These differences might be due to the pharmacologic differences among the antidepressants. In addition, since each antidepressant possesses a different dosage range from the other, overdoses of these antidepressants might also be the causes of these differences. Furthermore, based on these results, a further study was conducted to observe the effect of these three antidepressants in lower concentrations. From the results, biphasic effects in terms of zebrafish larval locomotor activity were demonstrated by these drugs. Even though further studies are still required to validate the mechanism, these findings indicate that these antidepressants might share a common mechanism responsible for their effects on zebrafish larval locomotor activity although there were some differences in potency of these effects.

Tdp1 protects from topoisomerase 1-mediated chromosomal breaks in adult zebrafish but is dispensable during larval development

Deficiency in the DNA end-processing enzyme, tyrosyl-DNA phosphodiesterase 1 (TDP1), causes progressive neurodegeneration in humans. Here, we generated a tdp1 knockout zebrafish and confirmed the lack of TDP1 activity. In adulthood, homozygotes exhibit hypersensitivity to topoisomerase 1 (Top1) poisons and a very mild locomotion defect. Unexpectedly, embryonic tdp1 ^-/- zebrafish were not hypersensitive to Top1 poisons and did not exhibit increased Top1-DNA breaks. This is in contrast to the hypersensitivity of Tdp1-deficient vertebrate models reported to date. Tdp1 is dispensable in the zebrafish embryo with transcript levels down-regulated in response to Top1-DNA damage. In contrast, apex2 and ercc4 (xpf) transcripts were up-regulated. These findings identify the tdp1^-/- zebrafish embryo as the first vertebrate model that does not require Tdp1 to protect from Top1-DNA damage and identify apex2 and ercc4 (xpf) as putative players fulfilling this role. It highlights the requirement of distinct DNA repair factors across the life span of vertebrates.

Differential influences of (±) anatoxin-a on photolocomotor behavior and gene transcription in larval zebrafish and fathead minnows

BACKGROUND

Though anatoxin-a (antx-a) is a globally important cyanobacterial neurotoxin in inland waters, information on sublethal toxicological responses of aquatic organisms is limited. We examined influences of (±) antx-a (11-3490 μg/L) on photolocomotor behavioral responses and gene transcription associated with neurotoxicity, oxidative stress and hepatotoxicity, in two of the most common alternative vertebrate and fish models, Danio rerio (zebrafish) and Pimephales promelas (fathead minnow). We selected environmentally relevant treatment levels from probabilistic exposure distributions, employed standardized experimental designs, and analytically verified treatment levels using isotope-dilution liquid chromatography tandem mass spectrometry. Caffeine was examined as a positive control.

RESULTS

Caffeine influences on fish behavior responses were similar to previous studies. Following exposure to (±) antx-a, no significant photolocomotor effects were observed during light and dark transitions for either species. Though zebrafish behavioral responses profiles were not significantly affected by (±) antx-a at the environmentally relevant treatment levels examined, fathead minnow stimulatory behavior was significantly reduced in the 145-1960 μg/L treatment levels. In addition, no significant changes in transcription of target genes were observed in zebrafish; however, elavl3 and sod1 were upregulated and gst and cyp3a126 were significantly downregulated in fathead minnows.

CONCLUSION

We observed differential influences of (±) antx-a on swimming behavior and gene transcription in two of the most common larval fish models employed for prospective and retrospective assessment of environmental contaminants and water quality conditions. Sublethal responses of fathead minnows were consistently more sensitive than zebrafish to this neurotoxin at the environmentally relevant concentrations examined. Future studies are needed to understand such interspecies differences, the enantioselective toxicity of this compound, molecular initiation events within adverse outcome pathways, and subsequent individual and population risks for this emerging water quality threat.

Multiple Screening of Pesticides Toxicity in Zebrafish and Daphnia Based on Locomotor Activity Alterations

Pesticides are widely used to eradicate insects, weed species, and fungi in agriculture. The half-lives of some pesticides are relatively long and may have the dire potential to induce adverse effects when released into the soil, terrestrial and aquatic systems. To assess the potential adverse effects of pesticide pollution in the aquatic environment, zebrafish (Danio rerio) and Daphnia magna are two excellent animal models because of their transparent bodies, relatively short development processes, and well-established genetic information. Moreover, they are also suitable for performing high-throughput toxicity assays. In this study, we used both zebrafish larvae and water flea daphnia neonates as a model system to explore and compare the potential toxicity by monitoring locomotor activity. Tested animals were exposed to 12 various types of pesticides (three fungicides and 9 insecticides) for 24 h and their corresponding locomotor activities, in terms of distance traveled, burst movement, and rotation were quantified. By adapting principal component analysis (PCA) and hierarchical clustering analysis, we were able to minimize data complexity and compare pesticide toxicity based on locomotor activity for zebrafish and daphnia. Results showed distinct locomotor activity alteration patterns between zebrafish and daphnia towards pesticide exposure. The majority of pesticides tested in this study induced locomotor hypo-activity in daphnia neonates but triggered locomotor hyper-activity in zebrafish larvae. According to our PCA and clustering results, the toxicity for 12 pesticides was grouped into two major groups based on all locomotor activity endpoints collected from both zebrafish and daphnia. In conclusion, all pesticides resulted in swimming alterations in both animal models by either producing hypo-activity, hyperactivity, or other changes in swimming patterns. In addition, zebrafish and daphnia displayed distinct sensitivity and response against different pesticides, and the combinational analysis approach by using a phenomic approach to combine data collected from zebrafish and daphnia provided better resolution for toxicological assessment.

A Novel Function of the Lysophosphatidic Acid Receptor 3 (LPAR3) Gene in Zebrafish on Modulating Anxiety, Circadian Rhythm Locomotor Activity, and Short-Term Memory

Lysophosphatidic acid (LPA) is a small lysophospholipid molecule that activates multiple cellular functions through pathways with G-protein-coupled receptors. So far, six LPA receptors (LPAR1 to LPAR6) have been discovered and each one of them can connect to the downstream cell message-transmitting network. A previous study demonstrated that LPA receptors found in blood-producing stem cells can enhance erythropoietic processes through the activation of LPAR3. In the current study, newly discovered functions of LPAR3 were identified through extensive behavioral tests in lpar3 knockout (KO) zebrafish. It was found that the adult lpar3 KO zebrafish display an abnormal movement orientation and altered exploratory behavior compared to that of the control group in the three-dimensional locomotor and novel tank tests, respectively. Furthermore, consistent with those results, in the circadian rhythm locomotor activity test, the lpar3 KO zebrafish showed a lower level of angular velocity and average speed during the light cycles, indicating an hyperactivity-like behavior. In addition, the mutant fish also exhibited considerably higher locomotor activity during the dark cycle. Supporting those findings, this phenomenon was also displayed in the lpar3 KO zebrafish larvae. Furthermore, several important behavior alterations were also observed in the adult lpar3 KO fish, including a lower degree of aggression, less interest in conspecific social interaction, and looser shoal formation. However, there was no significant difference regarding the predator avoidance behavior between the mutant and the control fish. In addition, lpar3 KO zebrafish displayed memory deficiency in the passive avoidance test. These in vivo results support for the first time that the lpar3 gene plays a novel role in modulating behaviors of anxiety, aggression, social interaction, circadian rhythm locomotor activity, and memory retention in zebrafish.

Nutrients and salinity influence Prymnesium parvum (UTEX LB 2797) elicited sublethal toxicity in Pimephales promelas and Danio rerio

The magnitude, frequency, and duration of harmful algal blooms (HABs) are increasing worldwide, primarily due to climate change and anthropogenic activities. Prymnesium parvum is a euryhaline and eurythermal HAB forming species that has expanded throughout North America, resulting in massive fish kills. Previous aquatic ecology and toxicology efforts supported an understanding of conditions resulting in P. parvum HABs and fish kills; however, the primary endpoint selected for these studies was acute mortality. Whether adverse sublethal responses to P. parvum occur in fish are largely unknown. To begin to address this question, molecular and biochemical oxidative stress (OS) biomarker responses and photomotor behavioral alterations were investigated in two common fish models, the fathead minnow (Pimephales promelas) and zebrafish (Danio rerio). Varying nutrient and salinity conditions influenced P. parvum related OS biomarkers and fish behavioral responses in zebrafish and fathead minnows, which were heightened by nonoptimal conditions for P. parvum growth. Such sublethal observations present important considerations for future aquatic assessments and management of P. parvum HABs.

A critical evaluation of comparative regulatory strategies for monitoring pharmaceuticals in recycled wastewater

Pharmaceuticals are a subset of micropollutants, present in the environment in trace concentrations. Because of their persistent nature, these chemicals are of particular concern. Little is known about how mixtures of pharmaceutical residues, found in WWTP effluents, affect the environment or public health. Yet, numerous studies show negative outcomes for both aquatic and terrestrial organisms, suggesting that they are given both to bioaccumulation and uptake in plants. Israel leads the world in effluent reuse (86%), almost exclusively utilized for purposes of agricultural irrigation. Pharmaceuticals, however, are not included in Israel's water regulatory oversight or management, essentially creating an epidemiological experiment among its citizens and environment. Globally, these compounds also are not commonly subject to monitoring or regulation. This study reviews and analyzes water policies and regulation worldwide that address the presence of pharmaceuticals in water resources, with a particular focus on Australia, Singapore, Switzerland, and the USA. Furthermore, the study investigates the reasons why these chemicals are not yet regulated in Israel. Based on a comprehensive evaluation of the data and analysis of the regulatory rationale in other countries, a list of recommended pharmaceutical standards that should be measured and monitored in Israel's wastewater treatment system is proposed. The suggested prioritization criteria should be at the heart of a new regulatory agenda for controlling pharmaceutical contamination in wastewater.

Toward Less Hazardous Industrial Compounds: Coupling Quantum Mechanical Computations, Biomarker Responses, and Behavioral Profiles To Identify Bioactivity of SN2 Electrophiles in Alternative Vertebrate Models

Sustainable molecular design of less hazardous chemicals promises to reduce risks to public health and the environment. Computational chemistry modeling coupled with alternative toxicology models (e.g., larval fish) present unique high-throughput opportunities to understand structural characteristics eliciting adverse outcomes. Numerous environmental contaminants with reactive properties can elicit oxidative stress, an important toxicological response associated with diverse adverse outcomes (i.e., cancer, diabetes, neurodegenerative disorders, etc.). We examined a common chemical mechanism (bimolecular nucleophilic substitution (S_N2)) associated with oxidative stress using property-based computational modeling coupled with acute (mortality) and sublethal (glutathione, photomotor behavior) responses in the zebrafish (Danio rerio) and the fathead minnow (Pimephales promelas) models to identify whether relationships exist among biological responses and molecular attributes of industrial chemicals. Following standardized methods, embryonic zebrafish and larval fathead minnows were exposed separately to eight different S_N2 compounds for 96 h. Acute and sublethal responses were compared to computationally derived in silico chemical descriptors. Specifically, frontier molecular orbital energies were significantly related to acute LC₅₀ values and photomotor response (PMR) no observed effect concentrations (NOECs) in both fathead minnow and zebrafish. This reactivity index, LC₅₀ values, and PMR NOECs were also significantly related to whole body glutathione (GSH) levels, suggesting that acute and chronic toxicity results from protein adduct formation for S_N2 electrophiles. Shared refractory locomotor response patterns among study compounds and two alternative vertebrate models appear informative of electrophilic properties associated with oxidative stress for S_N2 chemicals. Electrophilic parameters derived from frontier molecular orbitals were predictive of experimental in vivo acute and sublethal toxicity. These observations provide important implications for identifying and designing less hazardous industrial chemicals with reduced potential to elicit oxidative stress through bimolecular nucleophilic substitution.

Metabolic profiling of Daphnia magna exposure to a mixture of hydrophobic organic contaminants in the presence of dissolved organic matter

The hydrophobic organic contaminants triclosan, triphenyl phosphate (TPhP) and diazinon sorb to dissolved organic matter (DOM) and this may alter their bioavailability and toxicity. ¹H nuclear magnetic resonance (NMR)-based metabolomics was used to investigate how DOM at 1 and 5 mg organic carbon/L may alter the metabolome of Daphnia magna from exposure to equitoxic mixtures of triclosan, TPhP and diazinon. These contaminants have different modes of action toward D. magna. The contaminant concentrations in each mixture were an equal percentage of their lethal concentration to 50% of the population (LC₅₀) values, which equates to 1250 μg/L TPhP, 330 μg/L triclosan and 0.9 μg/L diazinon. The ternary mixture exposure at 1% LC₅₀ values did not alter the D. magna metabolome. Contaminant mixture exposures at 5%, 10%, and 15% LC₅₀ values decreased glucose, serine and glycine concentrations and increased asparagine and threonine concentrations, suggesting disruptions in energy metabolism. The contaminant mixture had a unique mode of action in D. magna and DOM at 1 and 5 mg organic carbon/L did not change this mode of action. The estimated sorption of triclosan, TPhP or diazinon to DOM at 1 or 5 mg organic carbon/L in this experimental design was calculated to be <50% for each contaminant. This suggests that the mode of action of the contaminant mixture was not altered by DOM because the two environmentally relevant concentrations of DOM may have not substantially altered contaminant bioavailability. Our results indicate that DOM may not inevitably mitigate or alter the sub-lethal toxicity of a mixture of hydrophobic organic contaminants. This indicates the complexity of predicting the molecular-level toxicity of environmental mixtures. For adequate risk assessment of freshwater ecosystems, it is vital to account for the combined sub-lethal toxicity of an environmental mixture of contaminants.

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.

Global scanning of selective serotonin reuptake inhibitors: occurrence, wastewater treatment and hazards in aquatic systems

As the global population becomes more concentrated in urban areas, resource consumption, including access to pharmaceuticals, is increasing and chemical use is also increasingly concentrated. Unfortunately, implementation of waste management systems and wastewater treatment infrastructure is not yet meeting these global megatrends. Herein, pharmaceuticals are indicators of an urbanizing water cycle; antidepressants are among the most commonly studied classes of these contaminants of emerging concern. In the present study, we performed a unique global hazard assessment of selective serotonin reuptake inhibitors (SSRIs) in water matrices across geographic regions and for common wastewater treatment technologies. SSRIs in the environment have primarily been reported from Europe (50%) followed by North America (38%) and Asia-Pacific (10%). Minimal to no monitoring data exists for many developing regions of the world, including Africa and South America. From probabilistic environmental exposure distributions, 5th and 95th percentiles for all SSRIs across all geographic regions were 2.31 and 3022.1 ng/L for influent, 5.3 and 841.6 ng/L for effluent, 0.8 and 127.7 ng/L for freshwater, and 0.5 and 22.3 ng/L for coastal and marine systems, respectively. To estimate the potential hazards of SSRIs in the aquatic environment, percent exceedances of therapeutic hazard values of specific SSRIs, without recommended safety factors, were identified within and among geographic regions. For influent sewage and wastewater effluents, sertraline exceedances were observed 49% and 29% of the time, respectively, demonstrating the need to better understand emerging water quality hazards of SSRIs in urban freshwater and coastal ecosystems. This unique global review and analysis identified regions where more monitoring is necessary, and compounds requiring toxicological attention, particularly with increasing aquatic reports of behavioral perturbations elicited by SSRIs.

Temporal trends in organophosphorus pesticides use and concentrations in river water in Japan, and risk assessment

We reviewed organophosphorus pesticide use in Japan between 1982 and 2016 using data from the National Institute of Environmental Studies. Organophosphorus pesticide concentrations in river water throughout Japan were taken from the literature, and risk assessments were performed for some organophosphorus pesticides based on risk quotients and hazard quotients. Assessments were performed for 20 common pesticides, including insecticides, fungicides, and herbicides. The amounts used decreased in the order: insecticides > herbicides > fungicides. Organophosphorus insecticide and fungicide use have decreased over the last four decades, but organophosphorus herbicide use has increased. During this period, annual organophosphorus pesticide use was the highest for chlorpyrifos (105,263 tons/year) and the lowest for glyphosate-sodium (8 tons/year). The ecotoxicological risk assessment indicated that diazinon and fenitrothion posed strong risks to the Japanese aquatic environment, and chlorpyrifos and malathion have moderate risks. None of the pesticides that were assessed posed significant risks to humans. Continued use of organophosphorus pesticides in Japan may cause strong risks to aquatic environments. These risks should be reassessed periodically.

pH-Dependent Uptake and Sublethal Effects of Antihistamines in Zebrafish (Danio rerio) Embryos

Reported off-target effects of antihistamines in humans draw interest in ecotoxicity testing of first- and second-generation antihistamines, the latter of which have fewer reported side effects in humans. Because antihistamines are ionizable compounds, the pH influences uptake and toxicity and thus is highly relevant when conducting toxicity experiments. Zebrafish embryo toxicity tests were performed with the 3 first-generation antihistamines ketotifen, doxylamine, and dimethindene and the 2 second-generation antihistamines cetirizine and levocabastine at pH 5.5, 7.0, and 8.0. We detected effects on survival, phenotype, swimming activity, and heart rate for 4 antihistamines with the exception of levocabastine, which did not show any lethal or sublethal effects. When compared to lethal concentrations, effect concentrations neither of phenotype malformation nor of swimming activity or heart rate deviated by more than a factor of 10 from lethal concentrations, indicating that all sublethal effects were fairly nonspecific. First-generation antihistamines are weak bases and showed decreasing external effect concentrations with increasing neutral fraction, accompanied by increased uptake in the fish embryo. As a result, internal effect concentrations were independent from external pH. The pH-dependent toxicity originates from speciation-dependent uptake, with neutral species taken up in higher amounts than the corresponding ionic species. Cetirizine, which shifts from a zwitterionic to an anionic state in the measured pH range, did not show any pH-dependent uptake or toxicity. Environ Toxicol Chem 2019;00:1-11. © 2019 SETAC.

Subtle morphometric, behavioral and gene expression effects in larval zebrafish exposed to PFHxA, PFHxS and 6:2 FTOH

Recent studies of perfluoroalkylated substances (PFASs) have focused on the toxicity of long chain PFASs, such as PFOS or PFOA, which have been demonstrated to cause an array of developmental and behavioral effects. However, less is known about low molecular weight PFASs and alternatives. This study examined the morphometric and behavioral effects in zebrafish following developmental exposures of C6 PFASs: perfluorohexanoic acid, PFHxA, perfluorohexane sulfonate, PFHxS, and 6:2 fluorotelomer alcohol, 6:2 FTOH. Embryos were exposed to 0.02-20 μM concentrations of these compounds from the high stage (˜3 h post fertilization, hpf) until 120 hpf. Morphometric and gene expression endpoints were examined at 120 hpf. Genes selected for analysis were previously shown to be altered in zebrafish developmentally exposed to PFOS and PFOA. Additionally, exposed larvae were transferred to clean water and reared until 14 days post fertilization, dpf, when behavioral assays were completed and morphometric endpoints examined. While PFHxA was found to be the most acutely toxic at 120 hpf, few morphometric effects were observed. Gene expression was the most sensitive endpoint with significant increased tgfb1a, bdnf, and ap1s1 expression observed with PFHxA exposure. PFHxS exposure produced morphometric effects in the larvae, specifically increased length and yolk sac area at 2 and 20 μM. This phenotype persisted to the 14 dpf time point, where these larvae additionally displayed decreased distance traveled and crosses through the center of the arena of the behavioral assay. Exposure to 6:2 FTOH caused no morphometric effects at 120 hpf, and this compound was the least acutely toxic. However, expression of both tgfb1a and bdnf were increased by greater than 2 fold change at this time point. Effects also persisted to 14 dpf where a significant increase in distance traveled and velocity were observed in the behavioral assay. This study demonstrates effects on behavioral, morphometric and gene expression endpoints with developmental PFHxA, PFHxS, and 6:2 FTOH exposures in zebrafish.

Influence of Diltiazem on Fathead Minnows Across Dissolved Oxygen Gradients

Water resources in many arid to semi-arid regions are stressed by population growth and drought. Growing populations and climatic changes are influencing contaminant and water chemistry dynamics in urban inland waters, where flows can be dominated by, or even dependent on, wastewater effluent discharge. In these watersheds, interacting stressors such as dissolved oxygen and environmental contaminants (e.g., pharmaceuticals) have the potential to affect fish physiology and populations. Recent field observations from our group identified the calcium channel blocker (CCB) diltiazem in fish plasma exceeding human therapeutic doses (e.g., C_min ) in aquatic systems impaired because of nonattainment of dissolved oxygen water quality standards. Therefore our study objectives examined: 1) standard acute and chronic effects of dissolved oxygen and diltiazem to fish, 2) influences of dissolved oxygen at criteria levels deemed protective of aquatic life on diltiazem toxicity to fish, and 3) whether sublethal effects occur at diltiazem water concentrations predicted to cause a human therapeutic level (therapeutic hazard value [THV]) in fish plasma. Dissolved oxygen × diltiazem co-exposures significantly decreased survival at typical stream, lake, and reservoir water quality standards of 5.0 and 3.0 mg dissolved oxygen/L. Dissolved oxygen and diltiazem growth effects were observed at 2 times and 10 times lower than median lethal concentration (LC50) values (1.7 and 28.2 mg/L, respectively). Larval fathead minnow (Pimephales promelas) swimming behavior following low dissolved oxygen and diltiazem exposure generally decreased and was significantly reduced in light-to-dark bursting distance traveled, number of movements, and duration at concentrations as low as the THV. Individual and population level consequences of such responses are not yet understood, particularly in older organisms or other species; however, these findings suggest that assessments with pharmaceuticals and other cardioactive contaminants may underestimate adverse outcomes in fish across dissolved oxygen levels considered protective of aquatic life. Environ Toxicol Chem 2018;37:2835-2850. © 2018 SETAC.

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.

Direct and indirect effects of chemical contaminants on the behaviour, ecology and evolution of wildlife

Chemical contaminants (e.g. metals, pesticides, pharmaceuticals) are changing ecosystems via effects on wildlife. Indeed, recent work explicitly performed under environmentally realistic conditions reveals that chemical contaminants can have both direct and indirect effects at multiple levels of organization by influencing animal behaviour. Altered behaviour reflects multiple physiological changes and links individual- to population-level processes, thereby representing a sensitive tool for holistically assessing impacts of environmentally relevant contaminant concentrations. Here, we show that even if direct effects of contaminants on behavioural responses are reasonably well documented, there are significant knowledge gaps in understanding both the plasticity (i.e. individual variation) and evolution of contaminant-induced behavioural changes. We explore implications of multi-level processes by developing a conceptual framework that integrates direct and indirect effects on behaviour under environmentally realistic contexts. Our framework illustrates how sublethal behavioural effects of contaminants can be both negative and positive, varying dynamically within the same individuals and populations. This is because linkages within communities will act indirectly to alter and even magnify contaminant-induced effects. Given the increasing pressure on wildlife and ecosystems from chemical pollution, we argue there is a need to incorporate existing knowledge in ecology and evolution to improve ecological hazard and risk assessments.

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.

Differential uptake of and sensitivity to diphenhydramine in embryonic and larval zebrafish

The zebrafish fish embryo toxicity (FET) test is increasingly employed for alternative toxicity studies, yet our previous research identified increased sensitivity of zebrafish slightly older than embryos employed in FET methods (0-4 d postfertilization [dpf]). We identified rapid steady-state accumulation of diphenhydramine across zebrafish embryo and larval stages. However, significantly (p < 0.05) lower accumulation was observed at 48 h compared to 96 h in chorionated and dechorionated embryos (0-4 dpf), but not in zebrafish at 7 to 11 and 14 to 18 dpf. Increased uptake and toxicity of diphenhydramine was further observed in zebrafish at 7 to 11 and 14 to 18 dpf compared with 0-4 dpf embryos with chorion or dechorionated, which indicates that differential zebrafish sensitivity with age is associated with accumulation resulting from gill and other toxicokinetic and toxicodynamic changes during development. Environ Toxicol Chem 2018;37:1175-1181. © 2017 SETAC.

Urbanization, environment and pharmaceuticals: advancing comparative physiology, pharmacology and toxicology

Pharmaceuticals are routinely reported in the environment, which indicates an increasingly urban water cycle and highlights a global megatrend. Physicochemical properties and intrinsic biological activity of medicines routinely differ from conventional organic contaminants; thus, diverging applicability domains often challenge environmental chemistry and toxicology computational tools and biological assays originally developed to address historical chemical stressors. Because pharmacology and toxicology information is more readily available for these contaminants of emerging concern than other chemicals in the environment, and many drug targets are conserved across species, leveraging mammalian drug discovery, safety testing and clinical pharmacology information appears useful to define environmental risks and to design less hazardous industrial chemicals. Research is needed to advance biological read across, which promises to reduce uncertainties during chemical assessment aimed at protecting public health and the environment. Whereas such comparative information has been critical to advance an understanding of pharmaceutical hazards and risks in urban ecosystems, studies of medicines with fish and other ecotoxicological models are reciprocally benefiting basic and translational efforts, advancing comparative mechanistic toxicology, and providing robust comparative bridges for integrating conservation and toxicology.

Acute exposure to an environmentally relevant concentration of diclofenac elicits oxidative stress in the culturally important galaxiid fish Galaxias maculatus

Diclofenac is a nonsteroidal anti-inflammatory drug (NSAID) of growing concern in aquatic environments worldwide; nevertheless, knowledge of its effects on aquatic biota is restricted to a few model species with limited information regarding its mechanisms of impact. In the present study, diclofenac accumulation, its effects on metabolic rate, ionoregulation, and oxidative stress were examined at environmentally relevant (0.17 µg L^-1 ) and elevated (763 µg L^-1 ) concentrations in a culturally and economically important galaxiid fish, inanga (Galaxias maculatus), from the Southern Hemisphere. This species is among the most widespread freshwater fish in the world but its sensitivity to emerging contaminants is unknown. Following an acute 96-h exposure, bioconcentration of diclofenac was measured in the inanga whole-body, resulting in an estimated bioconcentration factor of 87 for the 0.17-µg L^-1 exposure concentration, approaching values where transfer through the food chain should be considered. Lipid peroxidation in the liver was significantly elevated at both 0.17- and 763-µg L^-1 exposure concentrations but lipid peroxidation in the kidney and gill decreased after diclofenac exposure. Catalase activity was also elevated in the liver of inanga but activity decreased in the gill. There were no effects of diclofenac on metabolic rate or ion (sodium and calcium) influx rates. These data indicate that toxicologically relevant adverse outcomes and bioconcentration of diclofenac at environmentally relevant levels warrant additional study in this important fish. Environ Toxicol Chem 2018;37:224-235. © 2017 SETAC.

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

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

Reduced anxiety is associated with the accumulation of six serotonin reuptake inhibitors in wastewater treatment effluent exposed goldfish Carassius auratus

Pharmaceuticals and personal care products (PPCPs) have been found in wastewater treatment plant (WWTP) effluents and their recipient watersheds. To assess the potential of WWTP effluents to alter fish behaviour, we caged male goldfish (Carassius auratus) for 21-days at three sites along a contamination gradient downstream from a WWTP which discharges into Cootes Paradise Marsh, on the western tip of Lake Ontario. We also included a fourth caging site as an external reference site within Lake Ontario at the Jordan Harbour Conservation Area. We then measured concentrations of PPCPs and monoamine neurotransmitters in caged goldfish plasma, and conducted behavioural assays measuring activity, startle response, and feeding. We detected fifteen different PPCPs in goldfish plasma including six serotonin reuptake inhibitors (amitriptyline, citalopram, fluoxetine/norfluoxetine, sertraline, venlafaxine, and diphenhydramine). Plasma concentrations of serotonin were significantly greater in plasma of fish caged closer to the WWTP effluent outfall site. The fish caged near and downstream of the WWTP effluent were bolder, more exploratory, and more active overall than fish caged at the reference site. Taken together, our results suggest that fish downstream of WWTPs are accumulating PPCPs at levels sufficient to alter neurotransmitter concentrations and to also impair ecologically-relevant behaviours.

Fish embryo toxicity test, threshold approach, and moribund as approaches to implement 3R principles to the acute fish toxicity test

The acute fish toxicity test (AFT) is requested by EU legal frameworks for hazard classification and risk assessment. AFT is one of the few regulatory required tests using death as an endpoint. This paper reviews efforts made to reduce, refine and replace (3Rs) AFT. We make an inventory of information requirements for AFT, summarize studies on 3Rs of AFT and give recommendations. The fish embryo toxicity test (FET) is proposed as a replacement of AFT and analyses have focused on two aspects: assessing the capacity of FET in predicting AFT and defining the applicability domain of FET. Six comparison studies have consistently shown a strong correlation of FET and AFT. In contrast, the applicability domain of FET has not yet been fully defined. FET has not yet been accepted as a replacement of AFT by any EU legal frameworks to fulfill information requirements because FET is insensitive to some chemicals. It is recommended that the outlier chemicals that do not correlate between FET and AFT should be further investigated. When necessary, additional FET data should be generated. Another effort to reduce and refine AFT is incorporation of FET into the threshold approach. Furthermore, moribund as an endpoint of fish death has been introduced in revising AFT guideline to reduce the duration of suffering for refinement. This endpoint, however, needs further work on the link of moribund and death. Global regulatory acceptance of the moribund endpoint would be critical for this development.