Behavior responses of zebrafish (Danio rerio) to aquatic environmental stresses in the characteristic of circadian rhythms

Time is of the essence: The importance of considering biological rhythms in an increasingly polluted world

Biological rhythms have a crucial role in shaping the biology and ecology of organisms. Light pollution is known to disrupt these rhythms, and evidence is emerging that chemical pollutants can cause similar disruption. Conversely, biological rhythms can influence the effects and toxicity of chemicals. Thus, by drawing insights from the extensive study of biological rhythms in biomedical and light pollution research, we can greatly improve our understanding of chemical pollution. This Essay advocates for the integration of biological rhythmicity into chemical pollution research to gain a more comprehensive understanding of how chemical pollutants affect wildlife and ecosystems. Despite historical barriers, recent experimental and technological advancements now facilitate the integration of biological rhythms into ecotoxicology, offering unprecedented, high-resolution data across spatiotemporal scales. Recognizing the importance of biological rhythms will be essential for understanding, predicting, and mitigating the complex ecological repercussions of chemical pollution.

Systematic analysis of circadian disrupting substances with a high-throughput zebrafish circadian behavior screening approach

Circadian rhythm aligns numerous biological functions in majority of animals. Aside from well-known external factors such as the light-dark cycle and temperature, circadian rhythm can also be regulated by rarely explored factors such as synthetic substances. Here, we established a circadian behavior screening approach utilizing zebrafish larvae model, which integrated high-throughput capabilities with automated batch processing. With this approach, we systematically analyzed the circadian disruptive effects of >60 synthetic substances commonly detected in aquatic environment by assessing both the circadian period length and amplitude of circadian behavior, with an exposure concentration set at 100 μg/L. Among tested substances, a series of circadian disrupting compounds (circadian disruptors) were identified. Several categories of the hit compounds can be recognized, such as phthalate (diisopentyl phthalate (DIPP), with 10.1 % and 49.6 % increases for circadian period length and amplitude, respectively), neuroactive substance (mirtazapine, with 10.6 % and 63.1 % increases, respectively), and biocides (thiamethoxam, with 100.3 % increase for amplitude). Among these compounds, DIPP increased circadian period length and amplitude with a high degree. Aside from DIPP, we further examined eleven other phthalates and demonstrated that benzyl butyl phthalate, diisobutyl phthalate and diisohexyl phthalate could also significantly increase the zebrafish circadian period length by 7.9 %, 3.7 % and 8.5 %, respectively. Collectively, the present findings substantiated the feasibility of this high throughput screening strategy for circadian disruptor's discovery and provided novel insights into understanding of the potential risks of synthetic substances.

Application of biological early warning systems in wastewater treatment plants: Introducing a promising approach to monitor changing wastewater composition

Wastewater treatment plants (WWTPs) are a major source of micropollutants to surface waters. Currently, their chemical or biological monitoring is realized by using grab or composite samples, which provides only snapshots of the current wastewater composition. Especially in WWTPs with industrial input, the wastewater composition can be highly variable and a continuous assessment would be advantageous, but very labor and cost intensive. A promising concept are automated real-time biological early warning systems (BEWS), where living organisms are constantly exposed to the water and an alarm is triggered if the organism's responses exceed a harmful threshold of acute toxicity. Currently, BEWS are established for drinking water and surface water but are seldom applied to monitor wastewater. This study demonstrates that a battery of BEWS using algae (Chlorella vulgaris in the Algae Toximeter, bbe Moldaenke), water flea (Daphnia magna in the DaphTox II, bbe Moldaenke) and gammarids (Gammarus pulex in the Sensaguard, REMONDIS Aqua) can be adapted for wastewater surveillance. For continuous low-maintenance operation, a back-washable membrane filtration system is indispensable for adequate preparation of treated wastewater. Only minor deviations in the reaction of the organisms towards treated and filtered wastewater compared to surface waters were detected. After spiking treated wastewater with two concentrations of the model compounds diuron, chlorpyrifos methyl, and sertraline, the organisms in the different BEWS showed clear responses depending on the respective compound, concentration and mode of action. Immediate effects on photosynthetic activity of algae were detected for diuron exposure, and strong behavioral changes in water flea and gammarids after exposure to chlorpyrifos methyl or sertraline were observed, which triggered automated alarms. Different types of data analysis were applied to extract more information out of the specific behavioral traits, than only provided by the vendors algorithms. To investigate, whether behavioral movement changes can be linked to impact other endpoints, the effects on feeding activity of G. pulex were evaluated and results indicated significant differences between the exposures. Overall, these findings provide an important basis indicating that BEWS have the potential to act as alarm systems for pollution events in the wastewater sector.

Prolonged darkness attenuates imidacloprid toxicity through the brain-gut-microbiome axis in zebrafish, Danio rerio

The present study investigated the toxic effects of IMI on brain and gut of zebrafish (Danio rerio) by a combination of transcriptome and microbiome analysis. In addition, the involvement of light/dark period was also evaluated. An acute toxic test was conducted on adult zebrafish weighing 0.45 ± 0.02 g with 4 experimental groups (n = 15): 1) IMI group (Light: Dark = 12: 12 h), 2) prolonged light group (Light: Dark = 20: 4 h), 3) prolonged darkness group (Light: Dark = 4: 20 h) which received 20 mg/L of IMI, and 4) control group, which was not treated with IMI (Light: Dark = 12: 12 h). The results showed that prolonged darkness improved the survival rate of zebrafish upon IMI exposure for 96 h. In the sub-chronic test, zebrafish were divided into the same 4 groups and exposed to IMI at 1 mg/L for 14 d (n = 30). The results showed that IMI induced oxidative stress in both IMI and prolonged light groups by inhibition of antioxidant activities and accumulation of oxidative products. Transcriptome analysis revealed a compromise of antioxidation and tryptophan metabolism pathways under IMI exposure. Several genes encoding rate-limiting enzymes in serotonin and melatonin synthesis were all inhibited in both IMI and LL groups. Meanwhile, significant decrease (P < 0.5) of serotonin and melatonin levels was observed. However, there's remarkable improvement of biochemical and transcriptional status in prolonged darkness group. In addition, microbiome analysis showed great alteration of gut bacterial community structure and inhibition of tryptophan metabolism pathway. Similarly, the gut microbiota dysbiosis induced by IMI was alleviated in prolonged darkness. In summary, sub-chronic IMI exposure induced neurotoxicity and gut toxicity in zebrafish by oxidative stress and impaired the brain-gut-axis through tryptophan metabolism perturbation. Prolonged darkness could effectively attenuate the IMI toxicity probably through maintaining a normal tryptophan metabolism.

Daily rhythms in the behavioural stress response of the zebrafish Danio rerio

In nature, animals are exposed to stressors that occur with different likelihood throughout the day, such as risk of predation and human disturbance. Hence, the stress response is expected to vary plastically to adaptively match these challenges. Several studies have supported this hypothesis in a wide range of vertebrate species, including some teleost fish, mostly through evidence of circadian variation in physiology. However, in teleost fish, circadian variation in behavioural stress responses is less understood. Here, we investigated the daily rhythm of stress response at the behavioural level in the zebrafish Danio rerio. We exposed individuals and shoals to an open field test every 4h over a 24h cycle, recording three behavioural indicators of stress and anxiety levels in novel environments (thigmotaxis, activity and freezing). Thigmotaxis and activity significantly varied throughout the day with a similar pattern, in line with a stronger stress response in the night phase. The same was suggested by analysis of freezing in shoals, but not in individual fish, in which variation appeared mostly driven by a single peak in the light phase. In a control experiment, we observed a set of subjects after familiarisation with the open-field apparatus. This experiment indicated that activity and freezing might present a daily rhythmicity that is unrelated to environmental novelty, and thus to stress responses. However, the thigmotaxis was constant through the day in the control condition, suggesting that the daily variation of this indicator is mostly attributable to the stress response. Overall, this research indicates that behavioural stress response of zebrafish does follow a daily rhythm, although this may be masked using behavioural indicators other than thigmotaxis. This rhythmicity can be relevant to improve welfare in aquaculture and reliability of behavioural research in fish models.

Insights into the circadian rhythm alterations of the novel PFOS substitutes F-53B and OBS on adult zebrafish

As alternatives to perfluorooctane sulfonate (PFOS), 6:2 Cl-PFESA (F-53B) and sodium p-perfluorous nonenoxybenzene sulfonate (OBS) are frequently detected in aquatic environments, but little is known about their neurotoxicity, especially in terms of circadian rhythms. In this study, adult zebrafish were chronically exposed to 1 μM PFOS, F-53B and OBS for 21 days taking circadian rhythm-dopamine (DA) regulatory network as an entry point to comparatively investigate their neurotoxicity and underlying mechanisms. The results showed that PFOS may affect the response to heat rather than circadian rhythms by reducing DA secretion due to disruption of calcium signaling pathway transduction caused by midbrain swelling. In contrast, F-53B and OBS altered the circadian rhythms of adult zebrafish, but their mechanisms of action were different. Specifically, F-53B might alter circadian rhythms by interfering with amino acid neurotransmitter metabolism and disrupting blood-brain barrier (BBB) formation, whereas OBS mainly inhibited canonical Wnt signaling transduction by reducing cilia formation in ependymal cells and induced midbrain ventriculomegaly, finally triggering imbalance in DA secretion and circadian rhythm changes. Our study highlights the need to focus on the environmental exposure risks of PFOS alternatives and the sequential and interactive mechanisms of their multiple toxicities.

Transcriptome signatures of wastewater effluent exposure in larval zebrafish vary with seasonal mixture composition in an effluent-dominated stream

Wastewater treatment plant (WWTP) effluent-dominated streams provide critical habitat for aquatic and terrestrial organisms but also continually expose them to complex mixtures of pharmaceuticals that can potentially impair growth, behavior, and reproduction. Currently, few biomarkers are available that relate to pharmaceutical-specific mechanisms of action. In the experiment reported in this paper, zebrafish (Danio rerio) embryos at two developmental stages were exposed to water samples from three sampling sites (0.1 km upstream of the outfall, at the effluent outfall, and 0.1 km below the outfall) during base-flow conditions from two months (January and May) of a temperate-region effluent-dominated stream containing a complex mixture of pharmaceuticals and other contaminants of emerging concern. RNA-sequencing identified potential biological impacts and biomarkers of WWTP effluent exposure that extend past traditional markers of endocrine disruption. Transcriptomics revealed changes to a wide range of biological functions and pathways including cardiac, neurological, visual, metabolic, and signaling pathways. These transcriptomic changes varied by developmental stage and displayed sensitivity to variable chemical composition and concentration of effluent, thus indicating a need for stage-specific biomarkers. Some transcripts are known to be associated with genes related to pharmaceuticals that were present in the collected samples. Although traditional biomarkers of endocrine disruption were not enriched in either month, a high estrogenicity signal was detected upstream in May and implicates the presence of unidentified chemical inputs not captured by the targeted chemical analysis. This work reveals associations between bioeffects of exposure, stage of development, and the composition of chemical mixtures in effluent-dominated surface water. The work underscores the importance of measuring effects beyond the endocrine system when assessing the impact of bioactive chemicals in WWTP effluent and identifies a need for non-targeted chemical analysis when bioeffects are not explained by the targeted analysis.

Assessment of eco-toxic effects of commonly used water disinfectant on zebrafish (Danio rerio) swimming behaviour and recovery responses: an early-warning biomarker approach

Eco-toxicity profiles for commonly used disinfectants were lacking. Available traditional toxicity techniques have some limitations (assessments and ethical issues). Behaviour toxicology is a promising research area towards early warning and non-invasive approaches. We studied the potential eco-toxic effects of sodium hypochlorite (NaOCl) on the swimming behaviour of zebrafish. Zebrafish were exposed to different concentrations (Treatment I, Treatment II, Treatment III, and Treatment IV) of NaOCl for 360 h. Recovery study (144 h) was conducted for NaOCl treatment groups. The swimming behaviour of zebrafish was quantified efficiently using an online monitoring system (OMS). OMS dataset was processed for determination of behavioural differences by MATLAB and SPSS. Compared to the control group, the swimming strength of zebrafish under NaOCl treatments declined significantly (p < 0.001). Avoidance behaviour has occurred on zebrafish under NaOCl exposure periods. Furthermore, NaOCl toxicity also adjusted circadian rhythms on zebrafish. Zebrafish swimming strength was significantly (p < 0.001) improved under-recovery periods. Moreover, normal diurnal patterns have occurred. NaOCl could cause behavioural abnormalities in non-target organisms. Continuous exposure to common disinfectants could cause external and internal stress on non-target organisms, resulting in behavioural changes and circadian rhythm adjustments. Continuous changes in behavioural and circadian rhythms might reduce organisms' fitness and adaptation capacity. This study highlights (1) the importance of computer-based toxicity assessments, and (2) swimming behaviour is an early warning biomarker for eco-toxicity studies.

Environmental Co-Exposure to Potassium Perchlorate and Cd Caused Toxicity and Thyroid Endocrine Disruption in Zebrafish Embryos and Larvae (Danio rerio)

The increasing pollution of aquatic habitats with anthropogenic compounds has led to various test strategies to detect hazardous chemicals. However, information on the effects of pollutants on the thyroid system in fish, which is essential for growth, development, and parts of reproduction, is still scarce. Modified early life-stage tests were carried out with zebrafish exposed to the known thyroid inhibitor potassium perchlorate (0.1, 1, 1.5, 2, 2.5, and 5 mM) to identify adverse effects on embryo development. The endogenous antioxidant defense mechanism is one of the key functions of the thyroid gland; in this regard, we examined the co-exposure to potassium perchlorate (KClO₄), which could disrupt thyroid function, with cadmium (Cd), a known pro-oxidant compound. Zebrafish embryos were exposed to control KClO₄ 1 mM and Cd 0.5 μM for 96 h after fertilization (hpf) individually and in combination. The morphological alteration, body length, and messenger RNA (mRNA) expression related to thyroid function and oxidative stress, thyroid hormone levels, and malondialdehyde were measured. Significant down-regulation of mRNAs related to thyroid function (thyroid hormone receptor-alpha (THRα), thyroid hormone receptor-beta (THRβ), haematopoietically expressed homeobox (hhex)) and decreased thyroxin (T4) levels were observed after co-exposure to KClO₄ and Cd, but this was not observed in the individually treated groups. These results suggest that co-exposure to KClO₄ and Cd could affect antioxidant defense mechanisms and potentially normally increase Cd toxicity on mRNA expression, altering the thyroid functions important in zebrafish embryonic developmental stages.

Translational relevance of forward genetic screens in animal models for the study of psychiatric disease

Psychiatric disorders represent a significant burden in our societies. Despite the convincing evidence pointing at gene and gene-environment interaction contributions, the role of genetics in the etiology of psychiatric disease is still poorly understood. Forward genetic screens in animal models have helped elucidate causal links. Here we discuss the application of mutagenesis-based forward genetic approaches in common animal model species: two invertebrates, nematodes (Caenorhabditis elegans) and fruit flies (Drosophila sp.); and two vertebrates, zebrafish (Danio rerio) and mice (Mus musculus), in relation to psychiatric disease. We also discuss the use of large scale genomic studies in human populations. Despite the advances using data from human populations, animal models coupled with next-generation sequencing strategies are still needed. Although with its own limitations, zebrafish possess characteristics that make them especially well-suited to forward genetic studies exploring the etiology of psychiatric disorders.

A clean process for phosphorus recovery and gallium enrichment from phosphorus flue dust by sodium carbonate roasting

Phosphorus flue dust (PFD) is a solid waste product from phosphorus (P) production that contains P and is enriched with gallium (Ga). The recovery of these valuable components not only protects the environment, but also reduces resource waste. This study aimed to develop a green and efficient method to recover P and enriched Ga from PFD. The effects of different parameters on the P leaching rate and Ga loss rate during Na₂CO₃ roasting and water leaching were investigated and optimized. The reaction mechanisms during the experiment were characterized, revealing that the P-containing compounds in PFD mainly transformed into water-soluble Na₃PO₄. Furthermore, the leaching rate of P reached 85.38%, while Ga was mainly concentrated in the residue and its loss rate was only about 1%. Ga content in the residue reached about 0.1%. An attempt was made to recover Na⁺ and PO₄^3- from the aqueous solution by evaporative crystallization and XRD analysis showed that the main phase of the crystallization product was Na₂HPO₄. The proposed process is technically simple, only Na₂CO₃ is added and no hazardous substances are generated, and represents a new method for recovering P and enriching Ga from PFD.

Tetrabromobisphenol S alters the circadian rhythm network in the early life stages of zebrafish

Tetrabromobisphenol S (TBBPS), an emerging brominated flame retardant (BFR) has been widely detected in the environment, and may potentially pose environmental risks. However, data on the occurrence and toxic effects of TBBPS are limited. Circadian rhythms govern multiple behavioral and physiological processes, and their disruption is closely associated with various pathological conditions. Little is known about the potential for TBBPS to perturb circadian rhythm networks or circadian-driven locomotor behavior. In the present study, behavior assays and gene expression analysis based on circadian rhythm pathways were designed to investigate the potential circadian rhythm impairments and subsequent adverse effects caused in 120 h post-fertilization (hpf) zebrafish larvae by TBBPS. The development of embryos was inhibited by TBBPS exposure even at concentrations below the maximal non-lethal concentration (MNLC, 3.47 mg/L). Our results indicated remarkable alterations in the expression of several key circadian rhythm genes due to TBBPS exposure. Compared to control, the expression of per1a, per1b, per3, cry2, and csnk1da was increased, while the expression of clocka, clockb, cry4, cry1ba, arntl1a, and cank1db was decreased. Significant alterations of the circadian rhythm network could be observed in the zebrafish embryos. TBBPS exposure also significantly affected the behavioral responses of larvae. Our findings suggest the circadian rhythm network could be a potential target of TBBPS. Further study is needed to explore whether the transcriptional alterations in circadian rhythm translate into physiological effects.

Phenomics Approach to Investigate Behavioral Toxicity of Environmental or Occupational Toxicants in Adult Zebrafish (Danio rerio)

Over the last few years, environmental pollution, especially water pollution, has become a serious issue worldwide. Thus, methods that can help us understand the impact and effects of these pollutants, especially on aquatic animals, are needed. Behavioral assessment has emerged as a crucial tool in toxicology and pharmacology because many studies have shown, in multiple animal models, that various pharmacological compounds can alter behavior, with many of the findings being translatable to humans. Moreover, behavior study can also be used as a suitable indicator in the ecotoxicological risk assessment of pollutants. Several model organisms, especially rodent models, have been extensively employed for behavior studies. However, assessments using this model are generally time consuming, expensive, and require extensive facilities for housing experimental animals. Moreover, behavioral studies typically use different measurements and assessment tools, making comparisons difficult. In addition, even though behavioral phenomics has the potential to comprehensively illustrate the toxicities of chemicals, there is only a limited number of studies focusing on animal behavior using such a global approach. Here, we describe a phenomics approach that can be used to investigate the impact of pollutants using zebrafish. The approach consists of several behavioral tests, including response to a novel environment, mirror-reflection image, predator fish, and conspecifics, after exposure to a test chemical. Phenotype fingerprinting, a method for summarizing individual phenotypes based on the results of the behavioral tests, is then conducted to reduce data complexity and display the pattern of each compound on behavioral phenotypes in zebrafish. This approach may be useful to researchers studying the potential adverse effects of different pollutants. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Novel tank test Basic Protocol 2: Shoaling test Basic Protocol 3: Aggression test (mirror biting test) Basic Protocol 4: Social interaction test Basic Protocol 5: Fear response test Basic Protocol 6: PCA and heatmap clustering.

Joint toxicity of lead and cadmium on the behavior of zebrafish larvae: An antagonism

Although the individual toxicity of lead (Pb) and cadmium (Cd) was intensively studied, little is known about their joint toxicity on the development of circadian behavioral rhythm. Therefore, we co-exposed zebrafish to Pb and Cd to investigate the alterations of behavioral rhythm and the potential mechanism. Inductively coupled plasma mass spectrometry analysis was used to detect the internal exposure level of heavy metals. The behavioral rhythm was monitored by a video-track tracking system. The changes of gene expression regarding melatonin-related molecules and clock genes were analyzed by quantitative polymerase chain reaction and JTK-Cycle analysis. The results showed that the level of Pb2+ and Cd2+ accumulated in the co-exposure group were significantly lower than that in the Pb or Cd group. Exposed to Pb reduced the locomotor activity; the behavioral rhythms were disrupted by Cd, while the pattern in the co-exposure group showed an antagonistic effect on locomotor activity and behavioral rhythm. The expression rhythm of aanat1 was disturbed and the expression levels of mtnr1aa and mtnr1bb were decreased by co-exposure treatment, but mtnr1c was increased in Pb and Cd group, respectively. Exposure to Cd caused the disruption of expression rhythm in clock genes, like clock1b, clock2, and cry1b, while only the rhythm of clock2 was disrupted in the co-exposure group. The results suggest that the behavioral rhythm disruption caused by Cd exposure is associated with the disturbance of certain circadian genes, whereas Pb exposure only abates the locomotor activity; an antagonistic effect on the behavioral pattern when co-exposed zebrafish larvae to Pb and Cd.

Environmental chemicals affect circadian rhythms: An underexplored effect influencing health and fitness in animals and humans

Circadian rhythms control the life of virtually all organisms. They regulate numerous aspects ranging from cellular processes to reproduction and behavior. Besides the light-dark cycle, there are additional environmental factors that regulate the circadian rhythms in animals as well as humans. Here, we outline the circadian rhythm system and considers zebrafish (Danio rerio) as a representative vertebrate organism. We characterize multiple physiological processes, which are affected by circadian rhythm disrupting compounds (circadian disrupters). We focus on and summarize 40 natural and anthropogenic environmental circadian disrupters in fish. They can be divided into six major categories: steroid hormones, metals, pesticides and biocides, polychlorinated biphenyls, neuroactive drugs and other compounds such as cyanobacterial toxins and bisphenol A. Steroid hormones as well as metals are most studied. Especially for progestins and glucocorticoids, circadian dysregulation was demonstrated in zebrafish on the molecular and physiological level, which comprise mainly behavioral alterations. Our review summarizes the current state of knowledge on circadian disrupters, highlights their risks to fish and identifies knowledge gaps in animals and humans. While most studies focus on transcriptional and behavioral alterations, additional effects and consequences are underexplored. Forthcoming studies should explore, which additional environmental circadian disrupters exist. They should clarify the underlying molecular mechanisms and aim to better understand the consequences for physiological processes.

The specification of zebrafish (Danio rerio) heart electrocardiogram index characteristic responses to different types of pollutants

Water quality was highly affected by common pollutants. Metals, pesticides and small molecules are ubiquitous pollutants. Advancement in engineering technology (computer-based monitoring systems) increased the efficiency of quantifying toxicity of different chemicals in an organism. The cardiovascular system reflects internal and external stress of an organism, and electrocardiogram (ECG) data reliably measure external stress. As ECG data can accurately reflect the physiological conditions of organisms, and zebrafish (Danio rerio) are considered to be good models for cardiovascular research, it is hypothesized that ECG parameters of zebrafish could indicate the toxicity of water-borne chemicals. To achieve this, we treated zebrafish with different concentrations of target chemicals (CuSO₄, C₁₀H₁₉O₆PS₂ and NH₄Cl) for 48 h and ECG data were measured. P-wave, R-wave, T-wave, PR-interval, QRS-complex and QT-interval data were the focus of this study. The results of self-organizing maps and Pearson correlation analysis indicate that the QRS-complex can be used as an indicator for CuSO₄ stress. The QT-interval could be used to assess the C₁₀H₁₉O₆PS₂ stress. The QT-interval and P-wave can be used to evaluate the NH₄Cl stress. Responses of zebrafish ECG parameters were identical with other vertebrate model, and were specific to toxicant types. It is proved that zebrafish heart ECG index could be used as a potential indicator in early detection of environmental stress.

Circadian rhythms in zebrafish (Danio rerio) behaviour and the sources of their variability

Over recent decades, changes in zebrafish (Danio rerio) behaviour have become popular quantitative indicators in biomedical studies. The circadian rhythms of behavioural processes in zebrafish are known to enable effective utilization of energy and resources, therefore attracting interest in zebrafish as a research model. This review covers a variety of circadian behaviours in this species, including diurnal rhythms of spawning, feeding, locomotor activity, shoaling, light/dark preference, and vertical position preference. Changes in circadian activity during zebrafish ontogeny are reviewed, including ageing-related alterations and chemically induced variations in rhythmicity patterns. Both exogenous and endogenous sources of inter-individual variability in zebrafish circadian behaviour are detailed. Additionally, we focus on different environmental factors with the potential to entrain circadian processes in zebrafish. This review describes two principal ways whereby diurnal behavioural rhythms can be entrained: (i) modulation of organismal physiological state, which can have masking or enhancing effects on behavioural endpoints related to endogenous circadian rhythms, and (ii) modulation of period and amplitude of the endogenous circadian rhythm due to competitive relationships between the primary and secondary zeitgebers. In addition, different peripheral oscillators in zebrafish can be entrained by diverse zeitgebers. This complicated orchestra of divergent influences may cause variability in zebrafish circadian behaviours, which should be given attention when planning behavioural studies.

Organophosphorus-based chemical additives induced behavioral changes in zebrafish (Danio rerio): Swimming activity is a sensitive stress indicator

Organophosphorus flame retardants (OPFRs) have been extensively used as chemical additives in polymer based consumer products. Among them, Isopropylphenyl phosphate (IPPP) and tripropyl phosphate (TPP) are predominant, which have potential to cause neuro-toxic effects on non-target organisms. As behavior (swimming activity) response is the first adjustment due to neurotoxic stress on the fitness of fish. In this study, the quantified swimming activity of zebrafish (Danio rerio) under IPPP and TPP exposure in an online monitoring system was investigated to assess the neurotoxin effects under long-term exposure periods, no swimming anomalies were observed in the control group. Whereas, in the OPFR exposures ((treatment I: 5 μg/L and treatment II: 25 μg/L), a series of anomalies were identified. Hyperactivity was shown in IPPP treatment I group (5 μg/L), whereas zebrafish swimming activity was declined throughout the study period in IPPP treatment II (25 μg/L), and TPP groups (5 μg/L and 25 μg/L) when compared to the control group. Circadian rhythm was not affected in the present study. The results of the present study indicated that the fitness of test individuals was a valid biomarker for eco-toxicity assessment under unescapable conditions. Hypoactivity of zebrafish signified the neurotoxic effects of IPPP and TPP. A concentration based improvement in swimming activity was observed under recovery conditions, which suggested that recovery capacity along with toxicity responses could be a comprehensive non-invasive technique to assess the eco-toxicity of waterborne chemicals.

Simultaneous eco-toxicity assessment technique using an online monitoring system: effects of different environmental factors on swimming behavior of zebrafish (Danio rerio)

Environmental factors, such as photoperiod and temperature were the main limiting factors for the survival of organisms in the nature environment. Changes in environmental factors are well predicted but determining their effects on organisms are challenging hot topic in the field of eco-toxicology. Thus, technology based eco-toxicity assessment was focused worldwide. In this research, the effects of different temperatures (15 °C, 22 °C, 30 °C, 32 °C, and 35 °C) and photoperiods (dark and light periods) on the continuous behavior responses of Zebrafish (Danio rerio) were investigated using an online monitoring system (OMS). We designed a new fish chamber with sensors to measure the behavior responses of zebrafish under different conditions. Data obtained from the OMS could be assessed for factors such as difference in swimming behavior, circadian rhythm, and avoidance behavior using latest software (MATLAB). The observed behavior anomalies on zebrafish under different temperatures and continuous photoperiods were statically significant (p < 0.05). We conclude that the new designed fish chamber (behavior sensors) is good in sensing behavioral responses of zebrafish under different conditions. The fish behavior strength could be a potential biomarker to assess the effects of environmental factors. The present study would be a basic platform for assessing the effects of different stressors simultaneously on swimming behavior of zebrafish.

Parental exposure to cadmium chloride causes developmental toxicity and thyroid endocrine disruption in zebrafish offspring

Cadmium is a common heavy metal pollutant. Previous studies have found that long-term cadmium exposure can cause damage to multiple organs/systems in humans and experimental animals; however, there are few studies that elucidate its effects on offspring development, discuss whether it can be transmitted to offspring from the parent, and debate whether it affects the functional development of the thyroid hormone system in offsprings. In this study, sexually mature zebrafish were exposed to different concentrations of cadmium chloride (0.01 μmol/L, 0.1 μmol/L, and 1 μmol/L) to study reproductive toxicity. It was found that parental zebrafish exposed to 1 μmol/L of cadmium chloride produced offsprings with different degrees of malformation. At 5 days post-fertilization (dpf), the levels of 3,5,3'-triiododenosine (T3) and thyroxine (T4) in the zebrafish were decreased. At 10 dpf, the T4 and T3 levels in the zebrafish of the offspring were significantly reduced. At the same time, the expression of thyroid receptor (trα and trβ) genes in five dpf larvae was significantly up-regulated in the 1 μmol/L treatment group relative to the control group. The mRNAs of thyroid hormone synthesis and metabolism-related genes (tshβ, dio1, dio2, ugt1ab, and ttr) were significantly up-regulated in the 0.1 μmol/L and 1 μmol/L treatment groups. This study demonstrates that parental cadmium chloride exposure produces reproductive toxicity in zebrafish and that the effects can be transferred from the parent to the offspring, resulting in developmental toxicity in the thyroid endocrine system.

Progesterone affects the transcription of genes in the circadian rhythm signaling and hypothalamic-pituitary-gonadal axes and changes the sex ratio in crucian carp (Carassius auratus)

Progesterone (P4) is an extensively applied progestin in human and veterinary medicine that has been widely detected in ambient aquatic environments, which can be detrimental to the health of aquatic organisms. Here we investigate the long-term effects of P4 on the transcription of genes related to the circadian rhythm signaling pathway and hypothalamic-pituitary-gonadal (HPG) axes in the crucian carp, which may have a potentially negative on endocrine-disrupting and sex differentiation impacts. Our results suggest that the expression of genes associated with the circadian rhythm signaling pathway are altered following exposure for 10, 20, 30, 40, 50 and 60 d, leading to disorders in the endocrine system disorders and the regulation of HPG axes-related gene expression. These maladies may affect gonadal development and the reproductive systems of crucian carp and provide a plausible mechanism for the observed change in sex ratio toward females after 180 d.

Comprehensive fitness evaluation of common carp (Cyprinus carpio L.) after twelve weeks of atrazine exposure

Atrazine is frequently detected in surface waters and negatively affect physiological function in aquatic organisms. Even though numerous authors have intensively studied its toxicity, only limited information concerning the long-term fish exposure is available. The aim of this study was evaluation of chronic exposure in common carp. Fish were exposed to a range of atrazine concentrations (0.3 - environmentally relevant concentration; 300; 1000; and 3000 µg/l) for twelve weeks. The potential impact of atrazine exposure was studied using various markers (behaviour; biometrical characteristics; haematological, biochemical and oxidative stress indices and histopathological changes). Most alterations were recorded at the highest concentration (3000 µg/l) which is probably due to a combined effect of both the herbicide exposure and intensive parasite infection development during second week. This group was cancelled after three weeks due to adverse health status, which partly confirmed atrazine immunotoxicity. Chronic exposure resulted in long-term reduction in feed intake followed by a significant decrease in body weight and morphological changes in gill at 1000 µg/l. At the same concentration, significant alterations in haematological (e.g. increase in erythrocyte, leukocyte, lymphocytes and neutrophil counts as well as decrease in monocyte counts) and biochemical (e.g. changes in enzyme activities; increase in glucose; decrease in ammonia) indices were documented, especially during first three weeks. Similar trend, but not so intensive, was observed at 300 µg/l. Moreover, significant changes were observed in various indicators of oxidative stress. These alterations were highly variable with both increasing and decreasing trends depending on dose and analysed tissue. Significant changes, especially in white blood profile, enzyme activities and oxidative stress indices, were proven even at 0.3 µg/l. Obtained results indicate that chronic atrazine exposure of common carp can negatively influence many indicators of health status such as behaviour, immune response, haematological and biochemical profile, oxidative stress indices and organ histopathology.

Toxicity testing of pesticides in zebrafish-a systematic review on chemicals and associated toxicological endpoints

The use of zebrafish (Danio rerio) has arisen as a promising biological platform for toxicity testing of pesticides such as herbicides, insecticides, and fungicides. Therefore, it is relevant to assess the use of zebrafish in models of exposure to investigate the diversity of pesticide-associated toxicity endpoints which have been reported. Thus, this review aimed to assess the recent literature on the use of zebrafish in pesticide toxicity studies to capture data on the types of pesticide used, classes of pesticides, and zebrafish life stages associated with toxicity endpoints and phenotypic observations. A total of 352 articles published between September 2012 and May 2019 were curated. The results show an increased trend in the use of zebrafish for testing the toxicity of pesticides, with a great diversity of pesticides (203) and chemical classes (58) with different applications (41) being used. Furthermore, experimental outcomes could be clustered in 13 toxicity endpoints, mainly developmental toxicity, oxidative stress, and neurotoxicity. Organophosphorus, pyrethroid, azole, and triazine were the most studied classes of pesticides and associated with various toxicity endpoints. Studies frequently opted for early life stages (embryos and larvae). Although there is an evident lack of standardization of nomenclatures and phenotypic alterations, the information gathered here highlights associations between (classes of) pesticides and endpoints, which can be used to relate mechanisms of action specific to certain classes of chemicals.

Combined treatment of melatonin and sodium tanshinone IIA sulfonate reduced the neurological and cardiovascular toxicity induced by deltamethrin in zebrafish

The pyrethroid insecticide deltamethrin has been reported to have an effect on vertebrate development and cardiovascular disease. Sodium tanshinone IIA sulfonate (STS) is considered to have cardioprotective effects and melatonin is known to regulate sleep-waking cycles. In this experiment, we used transgenic zebrafish Tg (kdrl:mCherry) and Tg (myl7:GFP) to investigate whether STS and melatonin could reverse the cardiovascular toxicity and neurotoxicity induced by deltamethrin. Zebrafish embryos were exposed to 25 μg/L deltamethrin at 10 hpf and treated with 100 mmol/L STS and 1 μmol/L melatonin showed that deltamethrin treatment affected normal cardiovascular development. In situ hybridization and qRT-PCR results showed that deltamethrin could interfere with the normal expression of cardiovascular development-related genes vegfr2, shh, gata4, nkx2.5, causing functional defects in the cardiovascular system. In addition, deltamethrin could affect the sleep-waking behavior of larvae, increasing the activity of larvae, decreasing the rest behavior and the expression of hcrt, hcrtr, aanat2 were down-regulated. The addition of melatonin and STS can significantly alleviate cardiovascular toxicity and sleep-waking induced by deltamethrin, while restoring the expression of related genes to normal levels. Our study demonstrates the role of STS and melatonin in protecting cardiovascular and sleep-waking behavior caused by deltamethrin.

The continuous physiological changes of zebrafish (Danio rerio) based on metabolism under controlled thallium stress

In this research, the continuous physiological changes of zebrafish (Danio rerio) in 0.1 μg/L thallium (Tl) in 15 days were investigated. The results showed that Tl(I) stress had a significant positive linear correlation with zebrafish ammonia nitrogen excretion (ANE) (p < 0.001), and the mean value of ANE in Tl(I) treatment (435 ± 227 mg/kg/h) was approximately 2 times higher than in the control group (239 ± 168 mg/kg/h), which suggested that ANE was suitable for Tl(I) stress assessment. A substantial difference based on oxygen consumption rate (OCR) between the control group (587 ± 112 mg/kg/h) and Tl(I) treatment (260 ± 88 mg/kg/h) with a high significance p < 0.001 could be observed, and the results indicated that Tl(I) played a negative role in OCR of zebrafish. The characteristics of both ANE and OCR changes under slight Tl(I) stress could be reflected by the ammonia quotient (AQ). It was noteworthy that AQ increased rapidly in first 6 h from 0.66 to 4.50, which was 3 times higher than 1.2, indicating rapid increase in both anaerobic energy utilization and protein metabolism in 0.1 μg/L Tl(I) exposure. It is concluded that the physiological changes of zebrafish based on metabolism can be regarded as a sensitive biological indicator of Tl(I) pollution, which could work as a substitute of potassium that disrupts the normal biological metabolism in the process of transport.

Potential effects of internal physio-ecological changes on the online biomonitoring of water quality: The behavior responses with circadian rhythms of zebrafish (Danio rerio) to different chemicals

The online biomonitoring of aquatic accidental pollution is very important to realize the assessment of complex toxicity. However, the monitoring results would be affected greatly by the internal physio-ecological changes of test organisms, and circadian rhythms might contribute greatly to this kind of effects. In the present study, the behavior responses of zebrafish (Danio rerio) to different concentrations of Deltamethrin, Atrazine, and Thallium (Tl) in 15 days were investigated using an online behavior monitoring system. The results showed that the average behavior strength (BS) value of dark period (0.71 ± 0.16) was lower than that of light period (0.88 ± 0.09) in the control group. Similar pattern was observed in all other treatments with negative relationship between exposure concentrations and mean BS values. It is concluded that the 24 h circadian rhythms in the behavior responses of zebrafish (Danio rerio) could be observed clearly in the online biomonitoring system, and the online monitoring results would be affected obviously in the characteristics of behavior periodicity abnormal and time delay. Therefore, it is suggested that internal physio-ecological characteristics of organisms must be considered once they have the chance to play roles in bio-induced technologies. More investigations are warranted to clear the effects of internal physio-ecological changes on the exported results.

Deltamethrin exposure induces oxidative stress and affects meiotic maturation in mouse oocyte

Pyrethroid insecticides are commonly used as insecticides and considered to be less toxic to mammals, but may still impair the reproduction of animals and humans. The aim of this research was to evaluate the tendency of deltamethrin induced oxidative stress and its effects on meiosis, apoptosis and autophagy of mouse oocytes in vitro maturation after deltamethrin exposure. Especially, the maturation rate of oocytes decreased significantly after 14 h exposure of deltamethrin in concentration-dependent manners, which was manifested as abnormal spindle morphology and DNA double strand breaks. Oxidative stress was found in mouse oocytes exposed to deltamethrin, as shown by changes in the expression of CAT and SOD2. Our results also show that deltamethrin affects the quality of oocytes by causing abnormal mitochondrial distribution and by decreasing mitochondrial membrane potential. The apoptosis of oocyte regulated by the expression of Bax and Bcl-2 protein was obviously affected by deltamethrin. Compared with the control group, the expression of key regulatory factors in the autophagy pathway, LC3, Atg12, Atg14, and Beclin, increased in the experimental group. In summary, these results revealed that deltamethrin might inhibit the maturation of mouse oocytes and adversely affect the survival of oocytes.

The online monitoring and assessment of thallium stress using oxygen consumption rate and carbon dioxide excretion rate of zebrafish (Danio rerio)

An online monitoring and assessment system of metabolism to measure oxygen consumption rate (OC) and carbon dioxide (CO₂) excretion rate (CR) of zebrafish (Danio rerio) was used to illustrate changes in stressful states in 15 days' (360 h) 0.1 μg/L Tl exposure. Tl had a significant inhibition on zebrafish OC and CR (p < 0.01). OC was more suitable for Tl stress assessment than CR, considering that the OC response was more stable and discernible from the control comparing with CR. However, CR is a suitable alternative to characterize toxic effects on different metabolic substrates. Both OC and CR were integrated to present the respiratory quotient (RQ) analysis. RQ was efficient in differentiating between CO₂ produced by respiration in the control group (RQ less than 0.7) and CO₂ used for urination or stored in tissues after Tl exposure (some RQs larger than 1.0). Circadian rhythm was observed in RC and CR in the controls and persisted in 0.1 μg/L Tl treatments. The rhythm was relatively more disordered in CR. OC and CR would be suitable for indicating physiological stress in the online system as sensitive physiological indices.