Mitochondrial bioenergetics and locomotor activity are altered in zebrafish (Danio rerio) after exposure to the bipyridylium herbicide diquat

Melatonin increased antioxidant capacity to ameliorate growth retardation and intestinal epithelial barrier dysfunction in diquat-challenged piglets

BACKGROUND

Diquat is a common environmental pollutant, which can cause oxidative stress in humans and animals. Diquat exposure causes growth retardation and intestinal damage. Therefore, this study was performed to investigate the effects of melatonin on diquat-challenged piglets.

RESULTS

Dietary supplementation with 2 mg kg-1 melatonin significantly increased the average daily gain and feed conversion rate in piglets. Melatonin increased antioxidant capacity, and improved intestinal epithelial barrier function of duodenum and jejunum in piglets. Moreover, melatonin was found to regulated the expression of immune and antioxidant-related genes. Melatonin also alleviated diquat-induced growth retardation and anorexia in diquat-challenged piglets. It also increased antioxidant capacity, and ameliorated diquat-induced intestinal epithelial barrier injury. Melatonin also regulated the expression of MnSOD and immuner-elated genes in intestinal.

CONCLUSION

Dietary supplementation with 2 mg kg-1 melatonin increased antioxidant capacity to ameliorate diquat-induced oxidative stress, alleviate intestinal epithelial barrier injury, and increase growth performance in weaned piglets. © 2023 Society of Chemical Industry.

Do zebrafish become blind or is it too much red dye in water? Distinguishing the embryo-larval development and physiology effects of DR 60, 73, and 78

Some dyes currently used by the textile, pharmaceutical, food, cosmetic, and photographic industries have been shown to be toxic and/or mutagenic to aquatic life. Most of these dyes resist degradation processes available for treating wastewater, and these processes might generate even more toxic by-products. Despite the large number of available dyes and the large quantity of dyes released into the environment, studies on their toxicity are still scarce. We evaluated and compared the effects in the animal model Danio rerio (zebrafish) of environmentally relevant concentrations of Disperse Red 60 (DR 60), 73 (DR 73), and 78 (DR 78) using the fish embryo acute toxicity (FET) test, morphometric analysis, immunofluorescence imaging, and behavioral parameters. DR 60 caused ocular modifications, while the DR 73 caused non-inflation of the swim bladder (NISB), pericardial edema (PE), scoliosis (S) and abnormal yolk sac (AYS) from at 0.125 mg/L. In behavioral tests, all the dyes induced changes in velocity and time spent swimming of exposed larvae. However, these alterations in behavior seem to be caused by different factors dependent on the dye and its concentration. Nevertheless, behavior seems to add valuable information concerning the hazards analysis of dyes, since it reveals to be the most sensitive group of parameters tested in the current study. In conclusion, of the behavioral and developmental alterations caused by these dyes should be interpreted as an alert for greater attention when registering new dyes and releasing them into the environment. In the particular case of DR 60 the possibility that directly affects the eye of larvae is of great environmental concern, but also from the human health perspective.

Surface-enhanced Raman scattering of a gold core-silver shell-sponge substrate for detection of thiram and diquat

Aiming at the difficulty of traditional pesticide sampling, a low-cost and convenient flexible surface enhanced Raman scattering (SERS) gold core-silver shell-sponge (Au-Ag-sponge) substrate was synthesized by chemical reduction. The SERS substrate consisted of Au-AgNPs and a melamine sponge. The sponge had a rich open pore structure, which could well "capture" Au-AgNPs, generating a large number of "hot spots". The SERS enhancement activity of the flexible substrate was characterized with rhodamine 6G (R6G) Raman probe molecules. The substrate showed good activity to 10-12 M rhodamine 6G with an enhancement factor (EF) of 7.72 × 106. Applying this substrate to the qualitative and quantitative detection of pesticide residues, the results showed that the Raman intensity was well related to the concentration of pesticide solution with the range of 0.1-10 mg L-1 of thiram and 1-10 mg L-1 of diquat. Furthermore, the substrate was analyzed by finite difference time domain (FDTD) simulation and the results were in good agreement with the experimental results. The reason for the difference in Raman signals of pesticide molecules on the same substrate was the different binding modes of Au-AgNPs on the sponge. Finally, we pointed out the advantages of flexible substrates in the field of pesticide residues, as well as future opportunities and challenges.

Biochemical and histopathological alterations in freshwater fish, Labeo rohita (Hamilton, 1822) upon chronic exposure to a commonly used hopper insecticide, triflumezopyrim

The present study has been carried out to see the long-term effects of triflumezopyrim in an Indian major carp, Labeo rohita. Fishes were exposed to sub-lethal concentrations triflumezopyrim insecticide, 1.41 ppm (Treatment 1), 3.27 ppm (Treatment 2) and 4.97 ppm (Treatment 3), respectively for 21 days. The liver, kidney, gills, muscle, and brain tissues of the fish were examined for physiological parameters and biochemical parameters such as catalase (CAT), superoxide dismutase (SOD), lactate dehydrogenase (LDH), malate dehydrogenase (MDH), alanine aminotransaminase (ALT), aspartate aminotransaminase (AST), acetylcholinessterase (AChE), and hexokinase. After 21 days of exposure, the activity CAT, SOD, LDH, MDH and ALT got increased and a drop in the activity of total protein was found in all treatment groups in comparison to the control group. Long-term triflumezopyrim exposure increased ROS production, ultimately leading to oxidative cell damage and inhibiting the antioxidant capabilities of the fish tissues. Histopathological analysis showed alteration in different tissues structures of pesticide treated fishes. Fishes exposed to highest sublethal concentration of the pesticide showed higher damage rate. The present study demonstrated that chronic exposure of fish to different sublethal concentration of triflumezopyrim exerts detrimental effect on the organism.

Experimental models of chemically induced Parkinson's disease in zebrafish at the embryonic larval stage: a systematic review

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra that results in a decrease in dopamine levels, resulting in motor-type disturbances. Different vertebrate models, such as rodents and fish, have been used to study PD. In recent decades, Danio rerio (zebrafish) has emerged as a potential model for the investigation of neurodegenerative diseases due to its homology to the nervous system of humans. In this context, this systematic review aimed to identify publications that reported the utilization of neurotoxins as an experimental model of parkinsonism in zebrafish embryos and larvae. Ultimately, 56 articles were identified by searching three databases (PubMed, Web of Science, and Google Scholar). Seventeen studies using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 4 1-methyl-4-phenylpyridinium (MPP+), 24 6-hydroxydopamine (6-OHDA), 6 paraquat/diquat, 2 rotenone, and 6 articles using other types of unusual neurotoxins to induce PD were selected. Neurobehavioral function, such as motor activity, dopaminergic neuron markers, oxidative stress biomarkers, and other relevant parameters in the zebrafish embryo-larval model were examined. In summary, this review provides information to help researchers determine which chemical model is suitable to study experimental parkinsonism, according to the effects induced by neurotoxins in zebrafish embryos and larvae.

Metabolomics profile of plasma in acute diquat-poisoned patients using gas chromatography-mass spectrometry

Diquat (DQ) has been confirmed to be toxic to humans and responsible for severe health impairment. While to date, very little is known about the toxicological mechanisms of DQ. Thus, investigations to discover the toxic targets and potential biomarkers of DQ poisoning are urgently needed. In this study, a metabolic profiling analysis was conducted to reveal the changes of metabolites of plasma and find out the potential biomarkers of DQ intoxication by GC-MS. First, multivariate statistical analysis demonstrated that acute DQ poisoning can lead to metabolomic changes in human plasma. Then, metabolomics studies showed that 31 of the identified metabolites were significantly altered by DQ. Pathway analysis indicated that three primarily metabolic pathways including phenylalanine, tyrosine and tryptophan biosynthesis, taurine and hypotaurine metabolism, and phenylalanine metabolism were affected by DQ, resulting in the perturbations of phenylalanine, tyrosine, taurine, and cysteine. Finally, the results of receiver operating characteristic analysis showed the above four metabolites could be used as reliable tools for the diagnosis and severity assessments of DQ intoxication. These data provided the theoretical basis for basic research to understand the potential mechanisms of DQ poisoning, and also identified the desirable biomarkers with great potential for clinical applications.

Glyphosate-based herbicide (GBH) causes damage in embryo-larval stages of zebrafish (Danio rerio)

Glyphosate-Based Herbicides (GBH) show risks to the environment and also to aquatic organisms, such as fish. The present work aimed to evaluate the effects of GBH and Pure Glyphosate (PG) exposure on Danio rerio embryos at drinking water concentrations. Zebrafish embryos were exposed to 250, 500, and 1000 μg L^-1 of Roundup Original DI® and pure glyphosate for 96 h. Glyphosate concentration in water, parameters physicochemical water, survival, hatching rate, heart rate, malformations, behavior, and biomarkers were evaluated. We verified that at 6 h post-fertilization (hpf), animals exposed to GBH 500 showed decreased survival as compared to the control. The hatching rate increased in all groups exposed to GBH at 48 hpf as compared to the control group. The embryos exposed did not present changes in the spontaneous movement and touch response. Exposed groups to GBH demonstrated a higher number of malformations in fish embryos as compared to the control. Most malformations were: pericardial edema, yolk sac edema, body malformations, and curvature of the spine. In heart rate, bradycardia occurred in groups exposed, as predicted due to cardiac abnormalities. As biochemical endpoints, we observed a decrease in Glutathione S-transferase (GBH 250, GBH 500 and PG 250) and Acetylcholinesterase (GBH 250 and PG 250) activity. No differences were found between the groups in the concentration of protein, Total Antioxidant Capacity Against Peroxyl Radicals, Lipid peroxidation, Reactive Oxygen Species, Non-protein thiols, and Catalase. In conclusion, the damage in all evaluated stages of development was aggravated by survival and malformations. Therefore, the large-scale use of GBHs, coupled with the permissiveness of its presence could be the cause damage to the aquatic environment affecting the embryonic development of non-target organisms.

Perfluorotetradecanoic Acid (PFTeDA) Induces Mitochondrial Damage and Oxidative Stress in Zebrafish (Danio rerio) Embryos/Larvae

Industrial and consumer products, such as pesticides, lubricants, and cosmetics, can contain perfluorinated compounds (PFCs). Although many short-chain PFCs have been linked to physiological and behavioral changes in fish, there are limited data on longer-chain PFCs. The objective of this study was to determine the potential impact of perfluorotetradecanoic acid (PFTeDA) exposure on zebrafish (Danio rerio) during early developmental stages. We measured several endpoints including gene expression, mitochondrial bioenergetics, and locomotor activity in zebrafish. Survival, timing of hatching, and deformity frequency were unaffected by PFTeDA at the concentrations tested (0.01, 0.1, 1, and 10 µM) over a 7-day exposure period. The expression levels of mitochondrial-related genes (cox1 and mt-nd3) and oxidative stress-related genes (cat, hsp70, and hsp90a) were increased in larval fish with exposure to 10 µM PFTeDA; however, there was no change in oxidative respiration of embryos (i.e., basal respiration and oligomycin-induced ATP-linked respiration). Reactive oxygen species were reduced in larvae treated with 10 µM PFTeDA, coinciding with the increased transcription of antioxidant defense genes. Both the visual motor response test and light-dark preference test were conducted on 7 dpf larvae and yielded no significant findings. This study improves current knowledge regarding toxicity mechanisms for longer-chain PFCs such as PFTeDA.

RNA-sequencing analysis of the Diquat-degrading yeast strain Meyerozyma guilliermondii Wyslmt and the discovery of Diquat degrading genes

Diquat is used in agricultural contexts to control the growth of broadleaf and grassy weeds in both terrestrial and aquatic areas. Diquat can be readily absorbed by the soil and can remain therein for extended periods of time, altering the local microenvironment. In this study, the Meyerozyma guilliermondii Wyslmt yeast strain, which has the capacity to degrade Diquat, was isolated from soil exposed to long-term Diquat treatment. Over a 7-day incubation period, this strain was able to remove 42.51% of available Diquat (100 mg/L). RNA-Seq was performed to assess changes in gene expression in this yeast strain over the course of Diquat degradation, revealing 63 and 151 upregulated and downregulated genes, respectively. KEGG pathway enrichment analysis revealed these genes to be most highly enriched in the carbohydrate metabolism pathway. Through functional annotation and gene expression analyses, we identified seven genes were predicted to be involved in Diquat biodegradation. Results of qRT-PCR assays indicated that the relative mRNA expression levels of these seven genes were significantly higher relative to the control group. Together these analyses led to the identification of DN676 as a candidate Diquat-degrading gene. When a pET-DN676 vector was expressed in E. coli BL21, this strain was able to remove 12.49% of provided Diquat (100 mg/L) over the course of a 7-day incubation. These results thus confirmed that the DN676 gene can promote Diquat degradation, with these studies having yielded an engineered BL21-pET-DN676 bacterial strain capable of degrading Diquat.

Embryonic exposure to benzo[a]pyrene causes age-dependent behavioral alterations and long-term metabolic dysfunction in zebrafish

Polycyclic aromatic hydrocarbons (PAH) are products of incomplete combustion which are ubiquitous pollutants and constituents of harmful mixtures such as tobacco smoke, petroleum and creosote. Animal studies have shown that these compounds exert developmental toxicity in multiple organ systems, including the nervous system. The relative persistence of or recovery from these effects across the lifespan remain poorly characterized. These studies tested for persistence of neurobehavioral effects in AB* zebrafish exposed 5-120 h post-fertilization to a typical PAH, benzo[a]pyrene (BAP). Study 1 evaluated the neurobehavioral effects of a wide concentration range of BAP (0.02-10 μM) exposures from 5 to 120 hpf during larval (6 days) and adult (6 months) stages of development, while study 2 evaluated neurobehavioral effects of BAP (0.3-3 μM) from 5 to 120 hpf across four stages of development: larval (6 days), adolescence (2.5 months), adulthood (8 months) and late adulthood (14 months). Embryonic BAP exposure caused minimal effects on larval motility, but did cause neurobehavioral changes at later points in life. Embryonic BAP exposure led to nonmonotonic effects on adolescent activity (0.3 μM hyperactive, Study 2), which attenuated with age, as well as startle responses (0.2 μM enhanced, Study 1) at 6 months of age. Similar startle changes were also detected in Study 2 (1.0 μM), though it was observed that the phenotype shifted from reduced pretap activity to enhanced posttap activity from 8 to 14 months of age. Changes in the avoidance (0.02-10 μM, Study 1) and approach (reduced, 0.3 μM, Study 2) of aversive/social cues were also detected, with the latter attenuating from 8 to 14 months of age. Fish from study 2 were maintained into aging (18 months) and evaluated for overall and tissue-specific oxygen consumption to determine whether metabolic processes in the brain and other target organs show altered function in late life based on embryonic PAH toxicity. BAP reduced whole animal oxygen consumption, and overall reductions in total basal, mitochondrial basal, and mitochondrial maximum respiration in target organs, including the brain, liver and heart. The present data show that embryonic BAP exposure can lead to neurobehavioral impairment across the life-span, but that these long-term risks differentially emerge or attenuate as development progresses.

Metabolomics analysis of the potential toxicological mechanisms of diquat dibromide herbicide in adult zebrafish (Danio rerio) liver

Although diquat is a widely used water-soluble herbicide in the world, its sublethal adverse effects to fish have not been well characterised. In this study, histopathological examination and biochemical assays were applied to assess hepatotoxicity and combined with gas chromatography-mass spectrometry (GC-MS)-based metabolomics analysis to reveal overall metabolic mechanisms in the liver of zebrafish (Danio rerio) after diquat exposure at concentrations of 0.34 and 1.69 mg·L^-1 for 21 days. Results indicated that 1.69 mg·L^-1 diquat exposure caused cellular vacuolisation and degeneration with nuclear abnormality and led to the disturbance of antioxidative system and dysfunction in the liver. No evident pathological injury was detected, and changes in liver biochemistry were not obvious in the fish exposed to 0.34 mg·L^-1 diquat. Multivariate statistical analysis revealed differences between profiles obtained by GC-MS spectrometry from control and two treatment groups. A total of 17 and 22 metabolites belonging to different classes were identified following exposure to 0.34 and 1.69 mg·L^-1 diquat, respectively. The metabolic changes in the liver of zebrafish are mainly manifested as inhibition of energy metabolism, disorders of amino acid metabolism and reduction of antioxidant capacity caused by 1.69 mg·L^-1 diquat exposure. The energy metabolism of zebrafish exposed to 0.34 mg·L^-1 diquat was more inclined to rely on anaerobic glycolysis than that of normal zebrafish, and interference effects on lipid metabolism were observed. The metabolomics approach provided an innovative perspective to explore possible hepatic damages on fish induced by diquat as a basis for further research.

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.

Temporal exposure to malathion: Biochemical changes in the Amazonian fish tambaqui, Colossoma macropomum

The main toxicity mechanism of organophosphate insecticides such as malathion is the acetylcholinesterase enzyme inhibition. However, fish responses to organophosphates may vary depending on the activation of different defense mechanisms as well as the length of exposure. As such, the evaluation of acetylcholinesterase activity, in combination with the evaluation of biotransformation and antioxidants enzymes levels, is useful for indicating damage in fish exposed to this insecticide. Moreover, evaluating mitochondrial activity might evidence how the hierarchic responses occur in relation to the length of time that the fish is exposed. Therefore, the aim of our study is to evaluate whether the length of exposure to malathion differentially affects the biochemical responses of tambaqui. Our hypothesis is that the physiological alterations due to exposure are time dependent. Fish were exposed to sublethal concentrations of the insecticide during 6, 12, 24, 36, and 48 h. Contrary to expectations, there was no acetylcholinesterase activity inhibition during the experiment, which indicates an absence of neurotoxicity. Phase II biotransformation mechanism was activated early, especially in the liver. Oxidative damage was evident in the first hours of exposure and was concurrent with the activation of antioxidant enzymes. Mitochondrial bioenergetics were differentially affected by the length of exposure. The data suggest that the tambaqui regulates mitochondrial respiration differently over time, seeking to maintain homeostasis and ATP demand, and ensures the activation of response mechanisms, thus minimizing oxidative damage and avoiding the neurotoxicity of malathion.

Exposure to acetochlor impairs swim bladder formation, induces heat shock protein expression, and promotes locomotor activity in zebrafish (Danio rerio) larvae

Acetochlor is one of the most widely used herbicides in the world, however, there are few data on the sub-lethal effects of acetochlor on early developmental stages of fish. To address this, we measured survival, deformity, swim bladder formation, embryo oxygen consumption rates, reactive oxygen species (ROS) levels, transcripts (related to swim bladder formation, oxidative damage response, and apoptosis) and behavior responses following exposure to acetochlor (0.001 µM up to 125 µM). Exposure to acetochlor at concentrations 50 µM and above exerted 100% mortality after 3 dpf, and significantly reduced the size of the swim bladder (25 µM). In embryos, basal respiration, oligomycin-induced ATP production, and maximal respiration were decreased 30-60% following a 24 h exposure to 125 μM acetochlor. Acetochlor did not affect ROS levels up to 25 µM in larvae with acute exposure. Acetochlor at 25 µM increased mRNA levels of bax1, hsp70, and hsp90a by ~4-fold in larval zebrafish. In both the visual motor response and light-dark preference test, 25 µM acetochlor increased locomotor activity of larval fish. At lower exposure concentrations, 100 and 1000 nM acetochlor increased the mean time spent in the dark zone, suggesting promotion of anxiolytic behavior. This study presents a comprehensive evaluation of sublethal effects of acetochlor, spanning molecular responses to behavior, which can be used to refine risk assessment decisions for aquatic environments.

Behavioral and developmental toxicity assessment of the strobilurin fungicide fenamidone in zebrafish embryos/larvae (Danio rerio)

Strobilurin fungicides are among the most widely used in the world and have characteristics that include high water solubility and toxicity to aquatic organisms. While several studies report on mechanisms of toxicity of strobilurins in fish, there are no data on the sub-lethal toxicity of fish to the fungicide fenamidone. To address this gap, survival and hatch rate, deformities, mitochondrial bioenergetics, expression of oxidative stress and apoptotic genes, and behavior (locomotor activity and anxiolytic-related behaviors) were assessed in zebrafish embryos and larvae following exposure to fenamidone. Fenamidone negatively affected development of zebrafish embryos, causing a delay of hatching time at concentrations of 2.5 and 5 μM. Fenamidone caused morphological deformities in zebrafish, including pericardial edema, yolk sac edema, tail deformities, and spinal curvature. Exposure to 1.5 μM fenamidone reduced surface area of swim bladder in larvae at 6 dpf. Fenamidone significantly reduced oxygen consumption rates of embryos; 5 μM fenamidone decreased basal respiration (~85%), oligomycin induced ATP-linked respiration (~70%), FCCP-induced maximal respiration (~75%) and non-mitochondrial respiration (~90%) compared to controls. Sod2 mRNA levels were decreased by fenamidone in larval fish. Locomotor activity was significantly decreased in zebrafish larvae following exposure to 2 μM fenamidone but there was no evidence for anxiolytic nor anxiety-related behaviors (exposures of 100 nM up to 1.5 µM). This study addresses a data gap for potential risks associated with fenamidone exposure in developing fish.

In vivo toxicological assessment of diquat dibromide: cytotoxic, genotoxic, and biochemical approach

Diquat dibromide is a comprehensive herbicide commonly used in the cultivation of cotton, soybeans, and other crops to combat unwanted weeds. In this study, the half-maximal effective concentration (EC₅₀) value of diquat dibromide was determined 60 mg/L in the Allium root growth inhibition test. ½ × EC₅₀ (30 mg/L), EC₅₀ (60 mg/L), and 2 × EC₅₀ (120 mg/L) concentrations of diquat dibromide were applied to Allium cepa L. bulbs for 72 h to investigate the dose-dependent toxic effects. To determine the toxic effects cytogenetic, biochemical and physiological parameters were used. Physiological effects were investigated by determination of the percentage of rooting, relative injury rate, root length, and weight gain. Genetic effects were evaluated by the frequency of chromosomal abnormalities (CAs), micronucleus (MN) formation, mitotic index (MI) rate, and comet assay. Biochemical parameters were evaluated with antioxidant enzyme activities and lipid peroxidation by determining malondialdehyde (MDA) level, superoxide dismutase (SOD) activity, catalase (CAT) activity, and glutathione (GSH) level. Also, chlorophyll pigment contents (a, b, and total) in green leaves were calculated to elucidate the effect of diquat dibromide on plants and the biosphere. The findings show that increasing doses of diquat dibromide caused a decrease in all physiological parameters and MI ratio, promoting MN and CAs and tail DNA formation in genetic parameters. It was determined by the increases in MDA level, SOD, and CAT activities and decreases in GSH levels that diquat dibromide administration caused oxidative stress depending on the dose. Also, chlorophyll pigment levels (a, b, and total) measured in leaf tissues decreased with the application dose. Considering that the toxic effects caused by diquat dibromide and that organisms other than unwanted plants will be exposed during the application, its use should be abandoned and biocontrol methods should be used instead. In cases where use is compulsory, doses that will not harm the environment and organisms should be determined and used.

Effects of Acute Diquat Poisoning on Liver Mitochondrial Apoptosis and Autophagy in Ducks

Diquat (DQ) is an effective herbicide and is widely used in agriculture. Due to persistent and frequent applications, it can enter into aquatic ecosystem and induce toxic effects to exposed aquatic animals. The residues of DQ via food chain accumulate in different tissues of exposed animals including humans and cause adverse toxic effects. Therefore, it is crucial and important to understand the mechanisms of toxic effects of DQ in exposed animals. We used ducks as test specimens to know the effects of acute DQ poisoning on mechanisms of apoptosis and autophagy in liver tissues. Results on comparison of various indexes of visceral organs including histopathological changes, apoptosis, autophagy-related genes, and protein expression indicated the adverse effects of DQ on the liver. The results of our experimental trial showed that DQ induces non-significant toxic effects on pro-apoptotic factors like BAX, BAK1, TNF-α, caspase series, and p53. The results revealed that anti-apoptotic gene Parkin was significantly upregulated, while an upward trend was also observed for Bcl2, suggesting that involvement of the anti-apoptotic factors in ducklings plays an important role in DQ poisoning. Results showed that DQ significantly increased the protein expression level of the autophagy factor Beclin 1 in the liver. Results on key autophagy factors like LC3A, LC3B, and p62 showed an upward trend at gene level, while the protein expression level of both LC3B and p62 reduced that might be associated with process of translation affected by the pro-apoptotic components such as apoptotic protease that inhibits the occurrence of autophagy while initiating cell apoptosis. The above results indicate that DQ can induce cell autophagy and apoptosis and the exposed organism may resist the toxic effects of DQ by increasing anti-apoptotic factors.

Behavioral changes occur earlier than redox alterations in developing zebrafish exposed to Mancozeb

As agriculture expands to provide food and wellbeing to the world's growing population, there is a simultaneous increasing concern about the use of agrochemicals, which can harm non-target organisms, mainly in the aquatic environment. The fungicide Mancozeb (MZ) has been used on a large-scale and is a potent inducer of oxidative stress. Therefore, there is an urgent need for the development of more sensitive biomarkers designed to earlier biomonitoring of this compound. Here we tested the hypothesis that behavioral changes induced by sublethal MZ concentrations would occur first as compared to biochemical oxidative stress markers. Embryos at 4 h post-fertilization (hpf) were exposed to Mancozeb at 5, 10 and 20 μg/L. Controls were kept in embryo water only. Behavioral and biochemical parameters were evaluated at 24, 28, 72, and 168 hpf after MZ exposure. The results showed that MZ significantly altered spontaneous movement, escape responses, swimming capacity, and exploratory behavior at all exposure times. However, changes in ROS steady-stead levels and the activity of antioxidant enzymes were observable only at 72 and 168 hpf. In conclusion, behavioral changes occurred earlier than biochemical alterations in zebrafish embryos exposed to MZ, highlighting the potential of behavioral biomarkers as sensitive tools for biomonitoring programs.

Picolinafen exerts developmental toxicity via the suppression of oxidative stress and angiogenesis in zebrafish embryos

Picolinafen, a phytoene desaturase-inhibiting herbicide, has been used since 2001 to control the growth of broadleaf weeds. Picolinafen has lower solubility and volatility, and shows lower toxicity to non-target insect species than other types of herbicide. Although picolinafen has been detected in lakes near urban environments and induces chronic toxicity in the mammals, birds, and some aquatic organisms, no study has investigated the toxicity or mode of action of picolinafen in zebrafish. In this study, we demonstrated the lethality and acute LC₅₀ value of picolinafen towards zebrafish embryos. Picolinafen hampered the development of embryos by the induction of morphological abnormalities via apoptosis. Additionally, picolinafen suppressed the generation of reactive oxygen species and angiogenesis. Also, the angiogenesis related genes, flt1 and flt4 mRNA expression was decreased in zebrafish embryos. This study provides a mechanistic understanding of the developmental toxicity of picolinafen in vertebrates.

Residual molecular and behavioral effects of the phenylpyrazole pesticide fipronil in larval zebrafish (Danio rerio) following a pulse embryonic exposure

Pesticides are typically applied to crops as acute applications, and residual effects of such intermittent exposures are not often characterized in developing fish. Fipronil is an agricultural pesticide that inhibits γ-amino-butyric acid (GABA) gated chloride channels. In this study, zebrafish (Danio rerio) embryos were exposed for 48 h (starting at ~3 h post fertilization, hpf) to various concentrations of fipronil (0.02 μg/L up to 4000 μg/L). Following this acute exposure, a subset of fish was transferred to clean water for a 7-day depuration phase. We hypothesized that a pulse exposure to fipronil during critical periods of central nervous system development would adversely affect fish later in life. After a 48 hour pulse exposure, survival was reduced in embryos exposed to 2 μg fipronil/L or greater. However, there was no further mortality during the depuration phase, nor were there changes in body length nor notochord length in larvae 9 dpf (days post-fertilization) compared to controls. Additional experiments were carried out at higher concentrations over 96 h (up to 4 dpf) to also elucidate developmental effects and teratogenicity of fipronil (43.7 μg/L up to 4370 μg/L). Fipronil at these higher concentrations significantly impacted the development of zebrafish, and the following morphometric and teratogenic effects were observed in 4 dpf fish; reduced body length, yolk sac and pericardial edema, reduced midbrain length, reduced optic and otic diameter, and truncation of the lower jaw. In depurated fish, we hypothesized that there would exist residual effects of exposure at the molecular level. Transcriptome profiling was therefore conducted on 9 dpf depurated larvae exposed initially for 48 h to one dose of either 0.2 μg/L, 200 μg/L or 2000 μg/L fipronil. The expression of gene networks associated with glycogen and omega-3-fatty acid metabolism were decreased in larvae exposed to each of the three concentrations of fipronil, suggesting metabolic disruption. Moreover, transcriptomics revealed that fipronil suppressed gene networks related to light-dark adaptation, photoperiod sensing, and circadian rhythm. Based on these data, we tested fish for altered behavioral responses in a Light-Dark preference test. Larvae exposed to >200 μg fipronil/L as embryos showed fewer number of visits (20-30% less) to the dark zone compared to controls. Larvae also spent a lower amount of time in the dark zone compared to controls, suggesting that fipronil strengthened dark avoidance behavior which is indicative of anxiety. This study demonstrates that a short pulse exposure to fipronil can affect transcriptome networks for metabolism, circadian rhythm, and response to light in fish after depuration, and these molecular responses are hypothesized to be related to aberrant behavioral effects observed in the light-dark preference test.

Assessing the toxicity of the benzamide fungicide zoxamide in zebrafish (Danio rerio): Towards an adverse outcome pathway for beta-tubulin inhibitors

Commercial benzamide fungicides are applied to crops to control damage caused by oomycete fungi and are used as veterinary pharmaceuticals in aquaculture. The mechanism of action of these fungicides is to induce mitotic arrest via binding to beta-tubulin, thus inhibiting tubulin polymerization. However, there are little toxicity data available for benzimidazole fungicides in fish. To address this knowledge gap, we conducted zebrafish embryo toxicity tests to assess deformities, survival, and sub-lethal responses following exposure to zoxamide (0, 0.5, 1.0, 2.5, 5.0 and 10 μM zoxamide). We hypothesized that skeletal deformities would be prevalent in zebrafish due to its mechanism of inhibiting beta-tubulin polymerization. Zoxamide was relatively toxic to zebrafish embryos and larvae, and survival was reduced ∼50 % at 2 days post fertilization (dpf) with 10 μM exposure and over time at 6 dpf, 2.5 μM exposure reduced survival by ∼20 %. Frequency of hatch was also reduced/delayed in zebrafish at 3 dpf with >2.5 μM zoxamide. Pericardial edema, body length shortening, and spine curvature were observed in larvae exposed to >5 μM. Mitochondrial bioenergetics were assessed in ∼30 hpf embryos (24-hour exposure) using an XFe24 Flux Analyzer and regression analysis revealed a negative relationship between basal respiration and zoxamide concentration. Superoxide dismutase 1 and caspase 3 mRNA levels were both decreased in 6 dpf larvae exposed to 2.5 μM zoxamide, but were not changed in expression at 0.5 nor 1 μM zoxamide. Continuous 6-day exposure to zoxamide reduced larval activity at 2.5 μM; conversely a 24-hour exposure (at 5-6 dpf) induced hyperactivity at 5 μM suggesting dose and time dependent effects on fish behavior. Based on sub-lethal endpoints, we conceptualize an adverse outcome pathway for chemicals that inhibit tubulin polymerization.

Sub-lethal toxicity assessment of the phenylurea herbicide linuron in developing zebrafish (Danio rerio) embryo/larvae

Due to run-off and rain events, agrochemicals can enter water catchments, exerting endocrine disruption effects and toxicity to aquatic organisms. Linuron is a phenylurea herbicide used to control a wide variety of vegetative weeds in agriculture in addition to residential applications. However, there are few studies that quantify its toxicity to early developmental stages of fish. The objectives of this study were to assess the acute toxicity of linuron to zebrafish embryos/larvae by measuring mortality, morphological deformities, oxidative respiration, gene expression, and locomotor activity via the Visual Motor Response test. Zebrafish embryos at ~6-h post-fertilization (hpf) were exposed to either embryo rearing medium (ERM), or one dose of 0.625, 1.25, 2.5, 5, and 10 μM linuron for up to 7 days post-fertilization (dpf) depending on the assay. Zebrafish larvae exposed to linuron displayed pericardial edema, yolk sac edema, and spinal curvature. Oxidative respiration assessments in embryos using the Agilent XF^e24 Flux Analyzer revealed that linuron decreased mean basal respiration and oligomycin-induced ATP-linked respiration in 30 hpf embryos at 20 μM after a 24-hour exposure. In 7 dpf larvae, transcript abundance was determined for 6 transcripts that have a role in oxidative respiration (atp06, cox1, cox4-1, cox5a1, cytb, and nd1); the relative abundance of these transcripts was not altered with linuron treatment. A Visual Motor Response test was conducted on 7 dpf larvae to determine whether linuron (0.625 to 5 μM) impaired locomotor activity. Larval activity in the dark period decreased in a dose dependent manner and there were indications of hypoactivity as low as 1.25 μM. Transcript abundance was thus determined for tyrosine hydroxylase (th1) and glutamic acid decarboxylase 67 (gad1b), two rate limiting enzymes that control the production of dopamine and gamma-aminobutyric acid respectively. The mRNA levels of gad1b (p = 0.019) were reduced with increasing concentrations of linuron while th1 (p = 0.056) showed a similar decreasing trend, suggesting that neurotransmitter biosynthesis may be altered with exposure to linuron. This study improves knowledge related to the toxicity mechanisms for linuron and is the first to demonstrate that this anti-androgenic chemical impairs oxidative respiration and exerts neurotoxic effects associated with neurotransmitter biosynthesis during early development. These data are significant for environmental risk assessment of agrochemicals.

Morphometric and proteomic responses of early-life stage rainbow trout (Oncorhynchus mykiss) to the aquatic herbicide diquat dibromide

The objective of this study was to examine the acute toxicity and sub-lethal effects of the commercial formulation of diquat dibromide, Reward® Landscape and Aquatic Herbicide, on multiple life stages of rainbow trout. The continuous exposure 96 h LC₅₀ derived for juvenile feeding fry aged 85 d post-hatch was 9.8 mg/L. Rainbow trout eyed embryos and juvenile feeding fry were also exposed to concentrations of Reward® ranging from 0.12 to 10 mg/L during two 24 h pulse exposures separated by 14 d of rearing in fresh water to mimic the manufacturers instructions for direct applications to water bodies. Decreased survival and body morphometrics were evident at 9.3 mg/L during the embryo/alevin exposures, but not in feeding juveniles, indicating a higher sensitivity of the early life stage fish. Quantitative proteomics and subnetwork enrichment analyses were conducted in the livers for both life stages to evaluate protein profiles after exposure to 0.37 mg/L diquat via Reward® exposure. Unique protein profiles were revealed for pre-feeding swim-up fry and for feeding juvenile fish, reflecting differences between the two life stages in sub-cellular responses after diquat dibromide exposure. Hepatic proteome effects were more dramatic in the pre-feeding swim-up fry with 315 proteins differentially expressed between the control and exposed fish while in the later life stage feeding fry, only 84 proteins were different after Reward® exposure. Exposure to Reward® significantly increased RNA/mRNA processes, induced activation of Atk/mTOR and caspase activity, and altered energy homeostasis. Proteomic alterations are associated with reduced growth observed in embryo/alevin at higher exposure concentrations, offering insight into key events underlying growth impairment within the adverse outcome pathway framework. This study is the first to report the sub-cellular and whole organism level effects of diquat dibromide in a commercial formulation and demonstrates that concentrations based on aquatic application rates alter the hepatic proteome.

Toxicity assessment of the herbicide acetochlor in the human liver carcinoma (HepG2) cell line

Acetochlor is a high-volume herbicide used on a global scale and toxicity assessments are needed to define its potential for adverse effects in wildlife and humans. This study was conducted to determine the effects of acetochlor on human liver carcinoma cells (HepG2), a cell model widely used to assess the potential for chemical hepatotoxicity. Experiments were conducted at concentrations ranging 0-800 μM acetochlor over a 12 to 48h period to quantify underlying mechanisms of toxicity. Our data indicate that acetochlor suppressed HepG2 cell proliferation in both a concentration- and time-dependent manner. Acetochlor induced reactive oxygen species (ROS) generation more than 700% with exposure to 400 μM acetochlor, and acetochlor decreased the activities and levels of anti-oxidant responses (superoxide dismutase, glutathione) following exposure to 100 μM, 200 μM and 400 μM acetochlor. Acetochlor also (1) induced HepG2 cell damage through apoptotic-signaling pathways; (2) enhanced intracellular free Ca²⁺ concentration (>400%); (3) decreased mitochondrial transmembrane potential (∼77%), and reduced ATP levels (∼65%) following exposure to 400 μM acetochlor compared to untreated cells. Notably, cell cycle progression was blocked at G0/G1 phase in HepG2 cells when treated for 24 h with 400 μM acetochlor. Taken together, acetochlor induced significant cytotoxicity toward HepG2 cells, and the underlying toxicity mechanisms appear to be related to ROS generation, mitochondrial dysfunction and disruption in the cell cycle regulation. These data contribute to toxicity assessments for acetochlor, a high-use herbicide, to quantify risk to wildlife and human health.

Ecological risk of pesticide contamination in a Brazilian river located near a rural area: A study of biomarkers using zebrafish embryos

Aquatic environments are affected by the use of pesticides in agricultural areas near rivers. To assess the impact of pesticide residues on affected environments Danio rerio (zebrafish) embryos have become an alternative model for biomonitoring studies. In the present study, zebrafish embryos were used as bioindicator of water quality in the Vacacaí river, located in the city of Santa Maria, southern Brazil. We hypothesized that it would be possible to observe changes in the biomarkers tested in the embryos. Exposures were performed over a total of eight months during the year 2018 using water collected in a river located near agricultural areas. Twenty-four pesticides were found in river water samples. The most frequently found were atrazine, quinclorac and clomazone. During exposure (96 h) spontaneous movement, the heart rate and hatching rate were evaluated. After the exposure time the embryos were euthanized for biochemical assays. We analyzed biomarkers such as thiobarbituric acid reactive substance (TBARS), acetylcholinesterase (AChE), glutathione S-transferase (GST) and catalase (CAT). We observed increases in GST and TBARS, especially during periods of major water contamination such as January, February, October, and November. Pesticides can affect the development of native species that reproduce during periods of high agricultural production. These results demonstrate the potential use of biochemical parameters combined with developmental and behavioral analyses in zebrafish embryos for biomonitoring studies.

Investigation into the sub-lethal effects of the triazole fungicide triticonazole in zebrafish (Danio rerio) embryos/larvae

Global use of azole fungicides is expected to increase over the next several years. Triticonazole is a triazole fungicide that is used for turf protection, residential, and other commercial applications. As such, it can enter local rural and urban water systems via run-off and rain events. Early life stages of aquatic organisms can be susceptible to pesticides that enter the water, but in the case of triticonazole, data on the potential for subacute toxicity are lacking. Here, we determined the effects of triticonazole on development, oxygen consumption rates, and locomotor activity in zebrafish to address this knowledge gap. Wild-type zebrafish (ABTu strain) embryos and larvae were exposed to triticonazole (1-100 μM) in early development for different lengths of time depending on the assay conducted. Triticonazole did not affect survival nor induce significant deformity (pericardial edema, skeletal defects) in zebrafish at doses up to 100 μM. Oxygen consumption rate was measured in embryos after 24 and 48 hour exposure to triticonazole beginning at ∼6 hpf using the XF_e flux analyzer. Triticonazole did not affect basal respiration, oligomycin-induced ATP linked respiration, FCCP-induced maximum respiration, proton leak, spare capacity, nor non-mitochondrial respiration at doses up to 100 μM for 24 hours, even for exposure up to 250 μM for 48 hours. To determine whether the fungicide affected larval swimming activity, the visual motor response test was conducted following triticonazole exposure for 6 days. Larval zebrafish exposed to triticonazole showed hypoactivity in the dark following a 100 μM treatment, suggesting that the fungicide can affect the locomotor activity of zebrafish, albeit at relatively high levels. Given the fact that sublethal biological responses were absent at lower environmentally relevant concentrations, we conclude that triticonazole, relative to other triazole fungicides and types of pesticides, exhibits a relatively low risk of toxicity to the early life stages of fish.

Developmental toxicity in embryo-larval zebrafish (Danio rerio) exposed to strobilurin fungicides (azoxystrobin and pyraclostrobin)

Azoxystrobin and pyraclostrobin are broad spectrum strobilurin fungicides that have been measured in the aquatic environment. Strobilurins inhibit mitochondrial respiration by binding to the mitochondrial respiratory complex III. The goal of this study was to investigate mitochondrial dysfunction and oxidative stress in the developing zebrafish from exposure to azoxystrobin and pyraclostrobin. Exposure studies were performed where zebrafish embryos were exposed to azoxystrobin and pyraclostrobin at 0.1, 10, 100 μg/L from 4 hpf to 48 hpf to measure mitochondrial dysfunction and oxidative stress mRNA transcripts, and 5 dpf to measure movement, growth, oxygen consumption, enzymatic activities, and mRNA transcripts. Results from this study indicated that there was a significant reduction in both basal and maximal respiration at 48 hpf in zebrafish exposed to 100 μg/L of pyraclostrobin. There was no difference in oxidative stress or apoptotic mRNA transcripts at 48 hpf, indicating that the two strobilurins were acting first on mitochondrial function and not directly through oxidative stress. At 5 dpf, standard body length was significantly reduced with exposure to pyraclostrobin and azoxystrobin exposure as compared to the control. These reductions in apical endpoints corresponded with increases in oxidative stress and apoptotic mRNA transcripts in treatment groups at 5 dpf indicating that strobilurins' exposure followed the adverse outcome pathway for mito-toxicants. Our results indicate that strobilurins can decrease mitochondrial function, which in turn lead to diminished growth and movement.

The organochlorine pesticide toxaphene reduces non-mitochondrial respiration and induces heat shock protein 70 expression in early-staged zebrafish (Danio rerio)

Toxaphene is a restricted-use pesticide produced by reacting chlorine gas with camphene. It was heavily used as a pesticide for agricultural purposes in the 1960-1970s, but despite being banned >30 years ago, it can remain elevated in the soil due to its resistance to metabolic degradation; this has led to longstanding concerns about elevated levels of toxaphene and other organochlorine pesticides (OCPs) in the environment. The objective of this study were to determine the effects of waterborne exposure to toxaphene on early life stages of zebrafish. Based on the LC₅₀, zebrafish embryos were exposed to control (embryo rearing media or DMSO) or to one dose of toxaphene ranging between 0.011 and 111.1 μg/mL from 6 h post fertilization (hpf) up to 120 hpf. Significant mortality and hatch time delays were observed in embryos exposed to toxaphene (at or above 0.11 and 1.11 μg/mL, depending on the assay). Higher prevalence of deformities was noted at higher doses (≥0.011 μg/mL), and these included pericardial edema and skeletal deformities. As energy production is important for normal development, mitochondrial bioenergetics were assessed in embryos following toxaphene exposure. Embryos exposed to 11.1 or 111 μg/mL toxaphene for 24 h showed lower non-mitochondrial respiration (~30%) compared to both solvent and no treatment controls. Expression of transcripts related to oxidative damage responses and apoptosis were measured and heat shock protein 70 was significantly increased with 111 μg/mL toxaphene (14.5 fold), while the expression levels of caspase 3, caspase 9, and superoxide dismutase 1 were not changed. These data demonstrate that developmental deformities induced by toxaphene include pericardial edema and skeletal deformity, and that toxaphene can affect oxidative phosphorylation in early staged zebrafish.

The psychoactive cathinone derivative pyrovalerone alters locomotor activity and decreases dopamine receptor expression in zebrafish (Danio rerio)

INTRODUCTION

Pyrovalerone (4-methyl-β-keto-prolintane) is a synthetic cathinone (beta-keto-amphetamine) derivative. Cathinones are a concern as drugs of abuse, as related street drugs such as methylenedioxypyrovalerone have garnered significant attention. The primary mechanism of action of cathinones is to inhibit reuptake transporters (dopamine and norepinephrine) in reward centers of the central nervous system.

METHODS

We measured bioenergetic, behavioral, and molecular responses to pyrovalerone (nM-µM) in zebrafish to evaluate its potential for neurotoxicity and neurological impairment.

RESULTS

Pyrovalerone did not induce any mortality in zebrafish larvae over a 3- and 24-hr period; however, seizures were prevalent at the highest dose tested (100 µM). Oxidative phosphorylation was not affected in the embryos, and there was no change in superoxide dismutase 1 expression. Following a 3-hr treatment to pyrovalerone (1-100 µM), larval zebrafish (6d) showed a dose-dependent decrease (70%-90%) in total distance moved in a visual motor response (VMR) test. We interrogated potential mechanisms related to the hypoactivity, focusing on the expression of dopamine-related transcripts as cathinones can modulate the dopamine system. Pyrovalerone decreased the expression levels of dopamine receptor D1 (~60%) in larval zebrafish but did not affect the expression of tyrosine hydroxylase, dopamine active transporter, or any other dopamine receptor subunit examined, suggesting that pyrovalerone may regulate the expression of dopamine receptors in a specific manner.

DISCUSSION

Further studies using zebrafish are expected to reveal new insight into molecular mechanisms and behavioral responses to cathinone derivates, and zebrafish may be a useful model for understanding the relationship between the dopamine system and bath salts.

Linking Mitochondrial Dysfunction to Organismal and Population Health in the Context of Environmental Pollutants: Progress and Considerations for Mitochondrial Adverse Outcome Pathways

Mitochondria are key targets of many environmental contaminants, because specific chemicals can interact directly with mitochondrial proteins, lipids, and ribonucleic acids. These direct interactions serve as molecular initiating events that impede adenosine triphosphate production and other critical functions that mitochondria serve within the cell (e.g., calcium and metal homeostasis, apoptosis, immune signaling, redox balance). A limited but growing number of adverse outcome pathways (AOPs) have been proposed to associate mitochondrial dysfunction with effects at organismal and population levels. These pathways involve key events such as altered membrane potential, mitochondrial fission/fusion, and mitochondrial DNA damage, among others. The present critical review and analysis reveals current progress on AOPs involving mitochondrial dysfunction, and, using a network-based computational approach, identifies the localization of mitochondrial molecular initiating events and key events within multiple existing AOPs. We also present 2 case studies, the first examining the interaction between mitochondria and immunotoxicity, and the second examining the role of early mitochondrial dysfunction in the context of behavior (i.e., locomotor activity). We discuss limitations in our current understanding of mitochondrial AOPs and highlight opportunities for clarifying their details. Advancing our knowledge of key event relationships within the AOP framework will require high-throughput datasets that permit the development and testing of chemical-agnostic AOPs, as well as high-resolution research that will enhance the mechanistic testing and validation of these key event relationships. Given the wide range of chemicals that affect mitochondria, and the centrality of energy production and signaling to ecologically important outcomes such as pathogen defense, homeostasis, growth, and reproduction, a better understanding of mitochondrial AOPs is expected to play a significant, if not central, role in environmental toxicology. Environ Toxicol Chem 2019;38:1625-1634. © 2019 SETAC.

Auxinic herbicides induce oxidative stress on Cnesterodon decemmaculatus (Pisces: Poeciliidae)

Pesticides might increase the production of reactive oxygen species (ROS). Dicamba (DIC) and 2,4-dichlorophenoxyacetic acid (2,4-D) are auxinic herbicides commonly applied in agroecosystems to control unwanted weeds. We analysed the oxidative damage exerted on the fish Cnesterodon decemmaculatus by an acute exposure to DIC- and 2,4-D-based herbicides formulations Banvel® and DMA®, respectively. The Endo III- and Fpg-modified alkaline comet assay was employed for detecting DNA damage caused by oxidative stress, whereas enzymatic and non-enzymatic biomarkers such as the activities of catalase (CAT), glutathione-S-transferase (GST), acetylcholinesterase (AChE), and glutathione content (GSH) were used to assess antioxidant response to these two herbicides. At the DNA level, results demonstrate that both auxinic herbicides induce oxidative damage at purines level. An increase on CAT and GST activities were detected in 48 h- and 96 h-treated specimens with both auxinics. GSH content decreased in fish exposed to DIC during 48 h and to 2,4-D after 96 h of exposure. Additionally, a diminished AChE activity in specimens treated with DIC and 2,4-D was observed only after 96 h. Total protein content decreased in fish exposed to both auxinics during 96 h. These results represent the first evaluation of oxidative damage related to DIC and 2,4-D exposure on a fish species as the Neotropical freshwater teleost C. decemmaculatus.

Sub-lethal effects of the triazole fungicide propiconazole on zebrafish (Danio rerio) development, oxidative respiration, and larval locomotor activity

Propiconazole is a triazole fungicide used in agriculture. Via run-off, it can enter the aquatic environment, and can adversely affect organisms. However, data are scarce on how propiconazole may affect early developmental life stages of fish. The objectives of this study were to evaluate the potential sub-lethal effects of propiconazole during zebrafish development. Wildtype zebrafish (ABTu strain) embryos and larvae were exposed to propiconazole (0.1-100 μM) for up to 150 hours post fertilization (hpf) depending upon the endpoint measured. Propiconazole decreased survival and induced hypopigmentation in fish at 100 μM compared to the water and solvent controls. Pericardial edema was also noted in embryos and larvae (beginning at 2-3 dpf) exposed to 100 μM propiconazole. To visualize the effects of propiconazole on the circulatory system in more detail, we exposed transgenic zebrafish (globin-LCR:eGFP) to the fungicide. Hematopoietic changes were observed within 48 h of exposure to 100 μM, and localization of blood cells in the cardic region became diffuse, indicating pooling of blood in the pericardial region. We measured oxidative respiration in embryos as sufficient ATP is needed for development. Exposure to 100 μM propiconazole (~6-30 hpf) reduced basal respiration (~50%), oligomycin-induced ATP linked respiration (~70%), proton leak (~30%), and non-mitochondrial respiration (~50%), indicating compromised mitochondrial bioenergetics. A Visual Motor Response (VMR) test was used to measure dark photokinesis behavior in larval fish exposed to propiconazole for a 6-day period. Larval fish exposed to the highest concentration in the assay (10 μM) showed evidence of hypoactivity. This study demonstrates that propiconazole can induce hypopigmentation in zebrafish, disrupt mitochondrial bioenergetics, and can alter locomotor activity. However, these sub-lethal responses were observed at concentrations above what is typically detected in the environment.

Developmental neurotoxicity of maneb: Notochord defects, mitochondrial dysfunction and hypoactivity in zebrafish (Danio rerio) embryos and larvae

Broad applications and exposure to the fungicide maneb can lead to toxicity in non-target organisms. Maneb is also associated with neurogenerative diseases such as Parkinson's disease (PD). The objectives of this study were to determine the acute toxicity of maneb to zebrafish by measuring mitochondrial bioenergetics, locomotor activity, and the expression of genes related to the oxidative damage response, as well as those related to dopamine signaling due to its association with PD. Zebrafish embryos at 6 h post-fertilization (hpf) were exposed to either solvent control (0.1% DMSO, v/v), or one dose of 0.1, 0.5, 1.0 and 10.0 µM maneb for 96 h. Maneb was moderately toxic to zebrafish embryos, and had a 96-h LC₅₀ value of 4.29 μM (~ 1.14 mg/L). Maneb induced a dose-dependent increase in mortality, decreased hatching rate, and increased notochord deformity rate at both 1.0 and 10.0 µM after 72 and 96 h. Total body length was also significantly reduced with 1.0 µM maneb. A 50-60% decrease in mean basal oxygen consumption rate was also observed in embryos following a 24 hpf exposure to 10.0 µM maneb but oligomycin-induced ATP production and FCCP-induced maximum respiration remained unaffected. No change was detected in the expression levels of genes associated with oxidative stress (sod1 and sod2), nor those related to dopamine synthesis (th1), dopamine transporter (dat), dopamine receptors (drd1, drd2a, drd3, and drd4b). Thus, modifying the expression of these transcripts may not be a mechanism for maneb-induced developmental toxicity in zebrafish. To assess the potential for neurotoxicity, a dark photokinesis assay was conducted in larvae following 7 d exposure to 0.1, 0.5 and 1.0 μM maneb. Larvae exposed to 0.5 and 1.0 μM maneb showed signs related to hypoactivity, and this reduced activity is hypothesized to be associated with notochord defects as this deformity was prevalent at higher concentrations of maneb. Overall, these data demonstrate that maneb negatively affects embryonic development (i.e. notochord development), affects basal oxygen consumption rates of embryos, and induces hypoactivity in larval fish. This study improves understanding regarding the developmental neurotoxicity of the fungicide maneb to zebrafish.

Developmental toxicity of the fungicide ziram in zebrafish (Danio rerio)

Ziram is a broad spectrum pesticide that belongs to the class of dimethyl-dithiocarbamate (DTC) fungicides. The objectives of this study were to assess the effects of ziram in developing zebrafish. Ziram was highly toxic to zebrafish embryos, with a 96-h LC₅₀ value of 1082.54 nM (∼0.33 mg/L). Zebrafish embryos at 6 h post-fertilization (hpf) were exposed to solvent control (0.1% DMSO), or one dose of 1, 10, 100, and 1000 nM ziram for 96 h. Ziram induced lethality in a dose-dependent manner, decreased hatching rate and heartbeat, and caused wavy deformities at 72 and 96 hpf at 100 and 1000 nM. Basal oxygen consumption rates of zebrafish at 24 hpf were decreased with 1000 nM, suggesting that ziram affects oxidative phosphorylation. We also measured the expression of transcripts associated with the oxidative stress response (sod1 and sod2) and dopamine receptor signaling at ∼96 h of exposure. There was no difference in the expression of genes related to oxidative stress, nor those related to the dopamine system. Locomotor activity was also assessed in larval zebrafish (7 dpf), and ziram increased total activity, the velocity in light zone, and total distance moved at 10 nM, while it decreased the mean time spent in the dark zone at 1 and 10 nM. Behavioral responses were dependent upon the time point and clutch examined. These data demonstrate that ziram negatively impacts embryonic development (i.e. mortality, hatching, heartbeat and notochord development) of zebrafish, decreases basal respiration of embryos, and alters behavioral responses in larvae.

Fluazinam impairs oxidative phosphorylation and induces hyper/hypo-activity in a dose specific manner in zebrafish larvae

Fluazinam is a pyridinamine fungicide that induces oxidative stress and mitochondrial damage in cells, and it has been reported to be neurotoxic. To characterize the biological effects of fluazinam, we assessed mitochondrial bioenergetics, dopamine system expression, and behavior of early life staged zebrafish (0.01 μM-0.5 μM). Fluazinam at environmentally-relevant levels did not induce sub-lethal effects in larvae, but at the LC₅₀ (0.5 μM), fluazinam decreased basal and ATP-linked respiration significantly in embryos. As mitochondria are directly related to redox homeostasis and apoptosis, the expression of genes related to oxidative stress and apoptosis were measured. Superoxide dismutase 2 (sod2), heat stock protein 70 (hsp70), bcl2-associated X protein (bax), and caspase 9 (casp9) mRNA levels were up-regulated by 0.5 μM fluazinam. Taken together, there was evidence for mitochondrial dysfunction and oxidative damage at the highest concentration of fluazinam (0.5 μM) tested. As there are reports for fluazinam-induced neurotoxicity in dopamine synthesizing cells, transcriptional targets in the dopamine system were assessed in the zebrafish. Tyrosine hydroxylase 1 (th1) and dopamine receptor 2a (drd2a) mRNA levels were decreased by 0.5 μM fluazinam, suggesting that this fungicide may affect the dopaminergic system. To further assess the potential for fluazinam-mediated neuromodulation, the dark photokinesis response was assessed in larvae following exposure. Larvae exposed to 0.1 μM fluazinam showed hyperactivity, while larvae exposed to 0.2 and 0.3 μM showed hypo-activity. This study demonstrates that fluazinam disrupts mitochondrial bioenergetics in zebrafish, inducing an oxidative stress response, and aberrant behaviors in larvae that are dose dependent.

Comparative toxicity of three phenolic compounds on the embryo of fathead minnow, Pimephales promelas

Phenols are classified as polar narcotics, which are thought to cause toxicity by non-specific mechanisms, possibly by disrupting membrane structure and function. Here we test three phenolic chemicals, phenol, 2,4-dichlorphenol and pentachlorophenol on embryo development, heartbeat rate and mitochondrial respiration in fathead minnow (Pimephales promelas). While these chemicals have been used on isolated mitochondria, they have not yet been used to verify respiration in intact embryos. Mitochondrial respiration in intact embryos was measured after optimizing the Seahorse XFe24 Extracellular Flux Analyzer. Heartbeat rate and mitochondrial respiration patterns of fathead minnow embryos at different developmental stages were also characterized. Exposures of embryos at developmental stage 20 occurred for 24 h with five concentrations of each phenolic compound ranging from 0.85 to 255 μM for phenol, 0.49 to 147 μM for 2,4-dichlorophenol and 0.3 to 90 μM for pentachlorophenol. Exposure to phenol at the concentrations tested had no effects on development, heartbeat or mitochondrial respiration. However, both 2,4-dichlorophenol and pentachlorophenol showed dose-dependent effects on development, heartbeat rate, and mitochondrial respiration, with the effects occurring at lower concentrations of pentachlorophenol, compared to 2,4-dichlorophenol, highlighting the higher toxicity of the more chlorinated phenols. Both 2,4-dichlorophenol and pentachlorophenol decreased basal mitochondrial respiration of embryos and ATP production. These results indicate that higher chlorinated phenolic chemicals cause developmental toxicity in fathead minnow embryos by decreasing mitochondrial respiration and heartbeat rate.

High-throughput assessment of oxidative respiration in fish embryos: Advancing adverse outcome pathways for mitochondrial dysfunction

Mitochondrial dysfunction is a prevalent molecular event that can result in multiple adverse outcomes. Recently, a novel high throughput method to assess metabolic capacity in fish embryos following exposure to chemicals has been adapted for environmental toxicology. Assessments of oxygen consumption rates using the Seahorse XF(e) 24/96 Extracellular Flux Analyzer (Agilent Technologies) can be used to garner insight into toxicant effects at early stages of development. Here we synthesize the current state of the science using high throughput metabolic profiling in zebrafish embryos, and present considerations for those wishing to adopt high throughput methods for mitochondrial bioenergetics into their research. Chemicals that have been investigated in zebrafish using this metabolic platform include herbicides (e.g. paraquat, diquat), industrial compounds (e.g. benzo-[a]-pyrene, tributyltin), natural products (e.g. quercetin), and anti-bacterial chemicals (i.e. triclosan). Some of these chemicals inhibit mitochondrial endpoints in the μM-mM range, and reduce basal respiration, maximum respiration, and spare capacity. We present a theoretical framework for how one can use mitochondrial performance data in zebrafish to categorize chemicals of concern and prioritize mitochondrial toxicants. Noteworthy is that our studies demonstrate that there can be considerable variation in basal respiration of untreated zebrafish embryos due to clutch-specific effects as well as individual variability, and basal oxygen consumption rates (OCR) can vary on average between 100 and 300 pmol/min/embryo. We also compare OCR between chorionated and dechorionated embryos, as both models are employed to test chemicals. After 24 h, dechorionated embryos remain responsive to mitochondrial toxicants, although they show a blunted response to the uncoupling agent carbonylcyanide-4-trifluoromethoxyphenylhydrazone (FCCP); dechorionated embryos are therefore a viable option for investigations into mitochondrial bioenergetics. We present an adverse outcome pathway framework that incorporates endpoints related to mitochondrial bioenergetics. High throughput bioenergetics assays conducted using whole embryos are expected to support adverse outcome pathways for mitochondrial dysfunction.

An ecotoxicological view on neurotoxicity assessment

The numbers of potential neurotoxicants in the environment are raising and pose a great risk for humans and the environment. Currently neurotoxicity assessment is mostly performed to predict and prevent harm to human populations. Despite all the efforts invested in the last years in developing novel in vitro or in silico test systems, in vivo tests with rodents are still the only accepted test for neurotoxicity risk assessment in Europe. Despite an increasing number of reports of species showing altered behaviour, neurotoxicity assessment for species in the environment is not required and therefore mostly not performed. Considering the increasing numbers of environmental contaminants with potential neurotoxic potential, eco-neurotoxicity should be also considered in risk assessment. In order to do so novel test systems are needed that can cope with species differences within ecosystems. In the field, online-biomonitoring systems using behavioural information could be used to detect neurotoxic effects and effect-directed analyses could be applied to identify the neurotoxicants causing the effect. Additionally, toxic pressure calculations in combination with mixture modelling could use environmental chemical monitoring data to predict adverse effects and prioritize pollutants for laboratory testing. Cheminformatics based on computational toxicological data from in vitro and in vivo studies could help to identify potential neurotoxicants. An array of in vitro assays covering different modes of action could be applied to screen compounds for neurotoxicity. The selection of in vitro assays could be guided by AOPs relevant for eco-neurotoxicity. In order to be able to perform risk assessment for eco-neurotoxicity, methods need to focus on the most sensitive species in an ecosystem. A test battery using species from different trophic levels might be the best approach. To implement eco-neurotoxicity assessment into European risk assessment, cheminformatics and in vitro screening tests could be used as first approach to identify eco-neurotoxic pollutants. In a second step, a small species test battery could be applied to assess the risks of ecosystems.