Influence of the perivitelline space on the quantification of internal concentrations of chemicals in eggs of zebrafish embryos (Danio rerio)

Physiology-informed toxicokinetic model for the zebrafish embryo test developed for bisphenols

Zebrafish embryos (ZFE) is a widely used model organism, employed in various research fields including toxicology to assess e.g., developmental toxicity and endocrine disruption. Variation in effects between chemicals are difficult to compare using nominal dose as toxicokinetic properties may vary. Toxicokinetic (TK) modeling is a means to estimate internal exposure concentration or dose at target and to enable extrapolation between experimental conditions and species, thereby improving hazard assessment of potential pollutants. In this study we advance currently existing TK models for ZFE with physiological ZFE parameters and novel experimental bisphenol data, a class of chemicals with suspected endocrine activity. We developed a five-compartment model consisting of water, plastic, chorion, yolk sack and embryo in which surface area and volume changes as well as the processes of biotransformation and blood circulation influence mass fluxes. For model training and validation, we measured internal concentrations in ZFE exposed individually to BPA, bisphenol AF (BPAF) and Z (BPZ). Bayesian inference was applied for parameter calibration based on the training data set of BPZ. The calibrated TK model predicted internal ZFE concentrations of the majority of external test data within a 5-fold error and half of the data within a 2-fold error for bisphenols A, AF, F, and tetrabromo bisphenol A (TBBPA). We used the developed model to rank the hazard of seven bisphenols based on predicted internal concentrations and measured in vitro estrogenicity. This ranking indicated a higher hazard for BPAF, BPZ, bisphenol B and C (BPB, BPC) than for BPA.

Uptake and Biotransformation of the Tire Rubber-derived Contaminants 6-PPD and 6-PPD Quinone in the Zebrafish Embryo (Danio rerio)

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD) is a widely used antioxidant in tire rubber known to enter the aquatic environment via road runoff. The associated transformation product (TP) 6-PPD quinone (6-PPDQ) causes extreme acute toxicity in some fish species (e.g., coho salmon). To interpret the species-specific toxicity, information about biotransformation products of 6-PPDQ would be relevant. This study investigated toxicokinetics of 6-PPD and 6-PPDQ in the zebrafish embryo (ZFE) model. Over 96 h of exposure, 6-PPD and 6-PPDQ accumulated in the ZFE with concentration factors ranging from 140 to 2500 for 6-PPD and 70 to 220 for 6-PPDQ. A total of 22 TPs of 6-PPD and 12 TPs of 6-PPDQ were tentatively identified using liquid chromatography coupled to high-resolution mass spectrometry. After 96 h of exposure to 6-PPD, the TPs of 6-PPD comprised 47% of the total peak area (TPA), with 4-hydroxydiphenylamine being the most prominent in the ZFE. Upon 6-PPDQ exposure, >95% of 6-PPDQ taken up in the ZFE was biotransformed, with 6-PPDQ + O + glucuronide dominating (>80% of the TPA). Among other TPs of 6-PPD, a reactive N-phenyl-p-benzoquinone imine was found. The knowledge of TPs of 6-PPD and 6-PPDQ from this study may support biotransformation studies in other organisms.

Biotransformation, metabolic response, and toxicity of UV-234 and UV-326 in larval zebrafish (Danio rerio)

Benzotriazole ultraviolet stabilizers (BUVSs) are emerging pollutants that are widely detected in aquatic ecosystems. While structure-dependent effects of BUVSs are reported, the relationship between biotransformation and toxicity outcomes remains unclear. In this study, zebrafish embryos were exposed to two common BUVSs (UV-234 and UV-326) at 1, 10, and 100 µg/L for up to 7 days. Comparison of their uptake and biotransformation revealed that the bioaccumulation capacity of UV-234 was higher than that of UV-326, while UV-326 was more extensively biotransformed with additional conjugation reactions. However, UV-326 showed low metabolism due to inhibited phase II enzymes, which may result in the comparable internal concentrations of both BUVSs in larval zebrafish. Both BUVSs induced oxidative stress while decreased MDA, suggesting the disturbance of lipid metabolism. The subsequent metabolomic profiling revealed that UV-234 and UV-326 exerted different effects on arachidonic acid, lipid, and energy metabolism. However, both BUVSs negatively impacted the cyclic guanosine monophosphate / protein kinase G pathway. This converged metabolic change resulted in comparable toxicity of UV-234 and UV-326, which was confirmed by the induction of downstream apoptosis, neuroinflammation, and abnormal locomotion behavior. These data have important implications for understanding the metabolism, disposition, and toxicology of BUVSs in aquatic organisms.

PBTK modeled perfluoroalkyl acid kinetics in zebrafish eleutheroembryos suggests impacts on bioconcentrations by chorion porosity dynamics

The zebrafish eleutheroembryo (zfe) is widely used as a model to characterize the toxicity of chemicals. However, analytical methods are still missing to measure organ concentrations. Therefore, physiologically-based toxicokinetic (PBTK) modeling may overcome current limitations to help understand the relationship between toxic effects and internal exposure in various organs. A previous PBTK model has been updated to include the chorionic transport barrier and its permeabilization, hatching dynamics within a zfe population over development, and active mediated transport mechanisms. The zfe PBTK model has been calibrated using measured time-dependent internal concentrations of PFBA, PFHxS, PFOA, and PFOS in a zfe population and evaluated using external datasets from the literature. Calibration was successful with 96% of the predictions falling within a 2-fold range of the observed concentrations. The external dataset was correctly estimated with about 50% of the predictions falling within a factor of 3 of the observed data and 10% of the predictions are out of the 10-fold error. The calibrated model suggested that active mediated transport differs between PFAS with a sulfonic and carboxylic acid functional end groups. This PBTK model predicts well the fate of PFAS with various physicochemical properties in zfe. Therefore, this model may improve the use of zfe as an alternative model in toxicokinetic-toxicodynamic studies and help to refine and reduce zfe-based experiments, while giving insights into the internal kinetics of chemicals.

Toxicokinetic Modeling of Per- and Polyfluoroalkyl Substance Concentrations within Developing Zebrafish (Danio rerio) Populations

Per- and polyfluoroalkyl substances (PFAS) are pervasive environmental contaminants, and their relative stability and high bioaccumulation potential create a challenging risk assessment problem. Zebrafish (Danio rerio) data, in principle, can be synthesized within a quantitative adverse outcome pathway (qAOP) framework to link molecular activity with individual or population level hazards. However, even as qAOP models are still in their infancy, there is a need to link internal dose and toxicity endpoints in a more rigorous way to further not only qAOP models but adverse outcome pathway frameworks in general. We address this problem by suggesting refinements to the current state of toxicokinetic modeling for the early development zebrafish exposed to PFAS up to 120 h post-fertilization. Our approach describes two key physiological transformation phenomena of the developing zebrafish: dynamic volume of an individual and dynamic hatching of a population. We then explore two different modeling strategies to describe the mass transfer, with one strategy relying on classical kinetic rates and the other incorporating mechanisms of membrane transport and adsorption/binding potential. Moving forward, we discuss the challenges of extending this model in both timeframe and chemical class, in conjunction with providing a conceptual framework for its integration with ongoing qAOP modeling efforts.

Zebrafish Embryos and Larvae as Alternative Animal Models for Toxicity Testing

Prerequisite to any biological laboratory assay employing living animals is consideration about its necessity, feasibility, ethics and the potential harm caused during an experiment. The imperative of these thoughts has led to the formulation of the 3R-principle, which today is a pivotal scientific standard of animal experimentation worldwide. The rising amount of laboratory investigations utilizing living animals throughout the last decades, either for regulatory concerns or for basic science, demands the development of alternative methods in accordance with 3R to help reduce experiments in mammals. This demand has resulted in investigation of additional vertebrate species displaying favourable biological properties. One prominent species among these is the zebrafish (Danio rerio), as these small laboratory ray-finned fish are well established in science today and feature outstanding biological characteristics. In this review, we highlight the advantages and general prerequisites of zebrafish embryos and larvae before free-feeding stages for toxicological testing, with a particular focus on cardio-, neuro, hepato- and nephrotoxicity. Furthermore, we discuss toxicokinetics, current advances in utilizing zebrafish for organ toxicity testing and highlight how advanced laboratory methods (such as automation, advanced imaging and genetic techniques) can refine future toxicological studies in this species.

Developmental Toxicity and Biotransformation of Two Anti-Epileptics in Zebrafish Embryos and Early Larvae

The zebrafish (Danio rerio) embryo is gaining interest as a bridging tool between in-vitro and in-vivo developmental toxicity studies. However, cytochrome P450 (CYP)-mediated drug metabolism in this model is still under debate. Therefore, we investigated the potential of zebrafish embryos and larvae to bioactivate two known anti-epileptics, carbamazepine (CBZ) and phenytoin (PHE), to carbamazepine-10,11-epoxide (E-CBZ) and 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH), respectively. First, zebrafish were exposed to CBZ, PHE, E-CBZ and HPPH from 5¼- to 120-h post fertilization (hpf) and morphologically evaluated. Second, the formations of E-CBZ and HPPH were assessed in culture medium and in whole-embryo extracts at different time points by targeted LC-MS. Finally, E-CBZ and HPPH formation was also assessed in adult zebrafish liver microsomes and compared with those of human, rat, and rabbit. The present study showed teratogenic effects for CBZ and PHE, but not for E-CBZ and HPPH. No HPPH was detected during organogenesis and E-CBZ was only formed at the end of organogenesis. E-CBZ and HPPH formation was also very low-to-negligible in adult zebrafish compared with the mammalian species. As such, other metabolic pathways than those of mammals are involved in the bioactivation of CBZ and PHE, or, these anti-epileptics are teratogens and do not require bioactivation in the zebrafish.

Minimum reporting standards based on a comprehensive review of the zebrafish embryo teratogenicity assay

Reproductive toxicity chemical safety assessment involves extensive use of vertebrate animals for regulatory testing purposes. Although alternative methods such as the zebrafish embryo teratogenicity assay (identified in the present manuscript by the acronym ZETA) are promising for replacing tests with mammals, challenges to regulatory application involve lack of standardization and incomplete validation. To identify key protocol aspects and ultimately support improving this situation, a comprehensive review of the literature on the level of harmonization/standardization and validation status of the ZETA has been conducted. The gaps and needed advances of the available ZETA protocols were evaluated and discussed with respect to its applicability as an alternative approach for teratogenicity assessment. Based on the review outcomes, a set of minimum reporting standards for the experimental protocol is proposed. Together with other initiatives towards implementation of alternative approaches at the screening and regulatory levels, the application of minimum reporting requirements is anticipated to support future method standardization and validation, as well as identifying potential improvement aspects. Present findings are expected to ultimately support advancing the ongoing validation initiatives towards the regulatory acceptance of the ZETA.

Pharmacokinetics in Zebrafish Embryos (ZFE) Following Immersion and Intrayolk Administration: A Fluorescence-Based Analysis

Zebrafish embryos (ZFE) have increasingly gained in popularity as a model to perform safety screenings of compounds. Although immersion of ZFE is the main route of exposure used, evidence shows that not all small molecules are equally absorbed, possibly resulting in false-negative readouts and incorrect conclusions. In this study, we compared the pharmacokinetics of seven fluorescent compounds with known physicochemical properties that were administered to two-cell stage embryos by immersion or by IY microinjection. Absorption and distribution of the dyes were followed at various timepoints up to 120 hpf by spatiotemporal fluorescence imaging. The concentration (10 µM) and dose (2 mg/kg) used were selected as quantities typically applied in preclinical experiments and zebrafish studies. The data show that in the case of a lipophilic compound (log D: 1.73) the immersion procedure resulted in an intrabody exposure which is similar or higher than that seen after the IY microinjection. In contrast, zero to low intrabody exposure was reached after immersion of the embryos with less lipophilic compounds. In the latter case IY microinjection, a technical procedure that can be easily automated, is highly recommended.

Yolk Sac of Zebrafish Embryos as Backpack for Chemicals?

The zebrafish embryo (Danio rerio) has developed into one of the most important nonsentient animal models for the hazard assessments of chemicals, but the processes governing its toxicokinetics (TK) are poorly understood. This study compares the uptake of seven test compounds into the embryonic body and the yolk sac of the zebrafish embryo using TK experiments, a dialysis approach, thermodynamic calculations, and kinetic modeling. Experimental data show that between 95% (4-iodophenol) and 67% (carbamazepine) of the total internal amount in 26 h post fertilization (hpf) embryos and between 80 and 49% in 74 hpf embryos were found in the yolk. Thus, internal concentrations determined for the whole embryo overestimate the internal concentration in the embryonic body: for the compounds of this study, up to a factor of 5. Partition coefficients for the embryonic body and a one-compartment model with diffusive exchange were calculated for the neutral test compounds and agreed reasonably with the experimental data. For prevalently ionic test compounds at exposure pH (bromoxynil, paroxetine), however, the extent and the speed of uptake were low and could not be modeled adequately. A better understanding of the TK of ionizable test compounds is essential to allow assessment of the validity of this organismic test system for ionic test compounds.

Toxicokinetics of Perfluorinated Alkyl Acids Influences Their Toxic Potency in the Zebrafish Embryo ( Danio rerio)

Perfluorinated alkyl acids (PFAA) are highly persistent and bioaccumulative and have been associated with several adverse health effects. The chemical structure mainly differs in two ways: the length of the hydrophobic alkyl chain and the type of hydrophilic end group. Little is known how the chemical structure affects the toxicokinetics (TK) in different organisms. We studied the TK of four PFAA (PFOS, PFHxS, PFOA, and PFBA) with different chain lengths (4-8 carbons) and functional groups (sulfonic and carboxylic acid) in zebrafish ( Danio rerio) embryo. The time courses of the external (ambient water) and internal concentrations were determined at three exposure concentrations from 2 up to 120 h postfertilization (hpf). Three of the four PFAA showed a biphasic uptake pattern with slow uptake before hatching (around 48 hpf) and faster uptake thereafter. A two-compartment TK model adequately described the biphasic uptake pattern, suggesting that the chorion functions as an uptake barrier until 48 hpf. The bioconcentration factors (BCF) determined at 120 hpf varied widely between PFAA with averages of approximately 4000 (PFOS), 200 (PFHxS), 50 (PFOA), and 0.8 (PFBA) L kg dry weight^-1, suggesting that both the alkyl chain length and the functional group influence the TK. The differences in toxic potency were reduced by 3 orders of magnitude when comparing internal effect concentrations instead of effective external concentrations.

Influence of pH on the uptake and toxicity of β-blockers in embryos of zebrafish, Danio rerio

ß-Blockers are weak bases with acidity constants related to their secondary amine group. At environmental pH they are protonated with the tendency to shift to their neutral species at more alkaline pH. Here we studied the influence of pH from 5.5 to 8.6 on the toxicity of the four ß-blockers atenolol, metoprolol, labetalol and propranolol in zebrafish embryos, relating toxicity not only in a conventional way to external aqueous concentrations but also to measured internal concentrations. Besides lethality, we evaluated changes in swimming activity and heartbeat, using the Locomotor Response (LMR) method and the Vertebrate Automated Screening Technology (VAST) for high throughput imaging. Effects of metoprolol, labetalol and propranolol were detected on phenotype, heart rate and swimming activity. External effect concentrations decreased with increasing neutral fraction for all three pharmaceuticals, attributed by an enhanced uptake of the neutral species in comparison to the corresponding charged form. The LC₅₀ of metoprolol decreased by a factor of 35 from 1.91 mM with almost complete cationic state at pH 7.0 to 0.054 mM with 8% neutral fraction at pH 8.6. For propranolol the LC₅₀ of 2.42 mM at pH 5.5 was even 100 fold higher than the LC₅₀ at pH 8 with 0.023 mM where 3% were neutral fraction. No effects were detected in the zebrafish embryo exposed to atenolol. The internal concentrations for metoprolol and propranolol were quantified at non-toxic concentrations and at the LC₁₀. Apparent bioconcentration factors (BCF) ranged from 1.96 at pH 7.0 to 32.0 at pH 8.6 for metoprolol and from 1.86 at pH 5.5 to 169 at pH 8.0 for propranolol. The BCFs served to predict the internal effect concentrations from the measured external effect concentrations. Internal effect concentrations of metoprolol and propranolol were in a similar range for all pH-values and for all endpoints. Interestingly, the internal effect concentrations were in the internal concentration range of baseline toxicity, which suggests that the effects of the ß-blockers are rather unspecific, even for sublethal effects on heart rate. In summary, our data confirm that the pH-dependent toxicity related to external concentrations can be explained by toxicokinetic effects and that the internal effect concentrations are pH-independent.

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.

Meta-analysis of fish early life stage tests-Association of toxic ratios and acute-to-chronic ratios with modes of action

Fish early life stage (ELS) tests (Organisation for Economic Co-operation and Development test guideline 210) are widely conducted to estimate chronic fish toxicity. In these tests, fish are exposed from the embryonic to the juvenile life stages. To analyze whether certain modes of action are related to high toxic ratios (i.e., ratios between baseline toxicity and experimental effect) and/or acute-to-chronic ratios (ACRs) in the fish ELS test, effect concentrations (ECs) for 183 compounds were extracted from the US Environmental Protection Agency's ecotoxicity database. Analysis of ECs of narcotic compounds indicated that baseline toxicity could be observed in the fish ELS test at similar concentrations as in the acute fish toxicity test. All nonnarcotic modes of action were associated with higher toxic ratios, with median values ranging from 4 to 9.3 × 10⁴ (uncoupling < reactivity < neuromuscular toxicity < methemoglobin formation < endocrine disruption < extracellular matrix formation inhibition). Four modes of action were also found to be associated with high ACRs: 1) lysyl oxidase inhibition leading to notochord distortion, 2) putative methemoglobin formation or hemolytic anemia, 3) endocrine disruption, and 4) compounds with neuromuscular toxicity. For the prediction of ECs in the fish ELS test with alternative test systems, endpoints targeted to the modes of action of compounds with enhanced toxic ratios or ACRs could be used to trigger fish ELS tests or even replace these tests. Environ Toxicol Chem 2018;37:955-969. © 2018 SETAC.

Kinetics of glucocorticoid exposure in developing zebrafish: A tracer study

In the current study the dynamics of glucocorticoid uptake by zebrafish chorionated embryos from the surrounding medium were studied, using 2.5 μM cortisol or dexamethasone solutions complemented with their tritiated variant. We measured the uptake of radioactive cortisol by embryos during a 1 h submersion. Interestingly, the signal in chorionated embryos was 85% (exposure: 1-2 hpf) or 78% (exposure: 48-49 hpf) of the signal present in an equal volume medium. By comparing embryos measured without chorion, we found that 18-20% of the radioactivity present in chorionated embryos is actually bound to the chorion or located in the perivitelline space. Consequently, embryonic tissue contains radioactivity levels of 60% of a similar volume of medium after 1 h incubation. During early developmental stages (1-48 hpf) exposure of more than 24 h in cortisol was needed to achieve radioactivity levels similar to an equal volume of medium within the embryonic tissue and more than 48 h for dexamethasone. In glucocorticoid-free medium, radioactivity dropped rapidly below 10% for both glucocorticoids, suggesting that the major portion of the embryonic radioactivity was a result of simple diffusion. During later developmental stages (48-96 hpf) initial uptake dynamics were similar, but showed a decrease of tissue radioactivity to 20% of an equal volume of medium after hatching, probably due to development and activation of the hypothalamic pituitary interrenal axis. Uptake is dependent on the developmental stage of the embryo. Furthermore, the presence of the chorion during exposure should be taken into account even when small lipophilic molecules are being tested.

Toxicokinetics of Polar Chemicals in Zebrafish Embryo (Danio rerio): Influence of Physicochemical Properties and of Biological Processes

The time-resolved uptake of 17 nonionic and ionic polar compounds (logD ≤ 2) with a diversity of functional groups into zebrafish embryos (ZFE) was studied over 96 h of exposure. Among them were pharmaceuticals, pesticides and plant active ingredients. Uptake rates for the diffusion controlled passive uptake through the ZFE membrane ranged from 0.02 to 24 h(-1) for the nonionic compounds and were slower for ionic compounds (<0.008-0.08 h(-1)). The study compounds did not enrich much in the ZFE (median bioconcentration factor of 1, max. 7). Biotransformation significantly influenced the internal concentration of some of the test compounds over time (benzocaine, phenacetin, metribuzin, phenytoin, thiacloprid, valproic acid). For benzocaine, valproic acid and phenacetin several transformation products (TPs) were observed by LC-MS already at early life-stages (before 28 hpf); for benzocaine the TPs comprised >90% of the initial amount taken up into the ZFE. For six compounds internal concentrations remained very low (rel. int. conc. < 0.2). Besides biotransformation (sulfamethoxazole), poor membrane permeability (cimetidine, colchicine) and also affinity to efflux transporters (atropine and chloramphenicol) are the likely reasons for these low internal concentrations. This study outlines that the uptake of polar compounds into ZFE is influenced by their physicochemical properties. However, biological processes, biotransformation and, likely, efflux can strongly affect the internal concentrations already in early developmental stages of the ZFE. This should be considered in future toxicokinetic modeling. The evaluation of the toxicity of chemicals by ZFE requires toxicokinetic studies of the test compounds and their TPs to increase comparability to effects in fish.

Metabolism of clofibric acid in zebrafish embryos (Danio rerio) as determined by liquid chromatography-high resolution-mass spectrometry

The zebrafish embryo (ZFE) is increasingly used in ecotoxicology research but detailed knowledge of its metabolic potential is still limited. This study focuses on the xenobiotic metabolism of ZFE at different life-stages using the pharmaceutical compound clofibric acid as study compound. Liquid chromatography with quadrupole-time-of-flight mass spectrometry (LC-QToF-MS) is used to detect and to identify the transformation products (TPs). In screening experiments, a total of 18 TPs was detected and structure proposals were elaborated for 17 TPs, formed by phase I and phase II metabolism. Biotransformation of clofibric acid by the ZFE involves conjugation with sulfate or glucuronic acid, and, reported here for the first time, with carnitine, taurine, and aminomethanesulfonic acid. Further yet unknown cyclization products were identified using non-target screening that may represent a new detoxification pathway. Sulfate containing TPs occurred already after 3h of exposure (7hpf), and from 48h of exposure (52hpf) onwards, all TPs were detected. The detection of these TPs indicates the activity of phase I and phase II enzymes already at early life-stages. Additionally, the excretion of one TP into the exposure medium was observed. The results of this study outline the high metabolic potential of the ZFE with respect to the transformation of xenobiotics. Similarities but also differences to other test systems were observed. Biotransformation of test chemicals in toxicity testing with ZFE may therefore need further consideration.

Arsenic (III, V), indium (III), and gallium (III) toxicity to zebrafish embryos using a high-throughput multi-endpoint in vivo developmental and behavioral assay

Gallium arsenide (GaAs), indium gallium arsenide (InGaAs) and other III/V materials are finding increasing application in microelectronic components. The rising demand for III/V-based products is leading to increasing generation of effluents containing ionic species of gallium, indium, and arsenic. The ecotoxicological hazard potential of these streams is unknown. While the toxicology of arsenic is comprehensive, much less is known about the effects of In(III) and Ga(III). The embryonic zebrafish was evaluated for mortality, developmental abnormalities, and photomotor response (PMR) behavior changes associated with exposure to As(III), As(V), Ga(III), and In(III). The As(III) lowest observable effect level (LOEL) for mortality was 500 μM at 24 and 120 h post fertilization (hpf). As(V) exposure was associated with significant mortality at 63 μM. The Ga(III)-citrate LOEL was 113 μM at 24 and 120 hpf. There was no association of significant mortality over the tested range of In(III)-citrate (56-900 μM) or sodium citrate (213-3400 μM) exposures. Only As(V) resulted in significant developmental abnormalities with LOEL of 500 μM. Removal of the chorion prior to As(III) and As(V) exposure was associated with increased incidence of mortality and developmental abnormality suggesting that the chorion may normally attenuate mass uptake of these metals by the embryo. Finally, As(III), As(V), and In(III) caused PMR hypoactivity (49-69% of control PMR) at 900-1000 μM. Overall, our results represent the first characterization of multidimensional toxicity effects of III/V ions in zebrafish embryos helping to fill a significant knowledge gap, particularly in Ga(III) and In(III) toxicology.

Visualizing Compound Distribution during Zebrafish Embryo Development: The Effects of Lipophilicity and DMSO

The predictability of the zebrafish embryo model is highly influenced by internal exposure of the embryo/larva. As compound uptake is likely to be influenced by factors such as lipophilicity, solvent use, and chorion presence, this article focuses on investigating their effects on compound distribution within the zebrafish embryo. To visualize compound uptake and distribution, zebrafish embryos were exposed for 96 hr, starting at 4 hr postfertilization, to water-soluble dyes: Schiff's reagent (logP -4.63), Giemsa stain (logP -0.77), Van Gierson stain (logP 1.64), Cresyl fast violet (logP 3.5), Eosine Y (logP 4.8), Sudan III (logP 7.5), and Oil red O (logP 9.81), with and without 1% dimethyl-sulfoxide (DMSO). Three additional compounds were used to analytically determine the uptake and distribution: Acyclovir (logP -1.56), Zidovudine (logP 0.05), and Metoprolol Tartrate Salt (logP 1.8). Examinations were performed every 24 hr. Both methods (visualization and specific analysis) showed that exposure to higher logP values results in higher compound uptake. Specific analysis showed that for lipophilic compounds >90% of compound is taken up by the embryo. For hydrophilic compounds, >90% of compound within the complete egg could not be associated to embryo or chorion and is probably distributed into the perivitelline space. Overall, internal exposure analyses on at least two occasions (i.e., before and after hatching) is crucial for interpretation of zebrafish embryotoxicity data, especially for compounds with extreme logP values. DMSO did not affect exposure when examined with the visualization method, however, this method might be not sensitive enough to draw hard conclusions.