Investigating the neurotoxicity of environmental pollutants using zebrafish as a model organism: A review and recommendations for future work

Myclobutanil induces neurotoxicity by activating autophagy and apoptosis in zebrafish larvae (Danio rerio)

Myclobutanil (MYC), a typical broad-spectrum triazole fungicide, is often detected in surface water. This study aimed to explore the neurotoxicity of MYC and the underlying mechanisms in zebrafish and in PC12 cells. In this study, zebrafish embryos were exposed to 0, 0.5 and 1 mg/L of MYC from 4 to 96 h post fertilization (hpf) and neurobehavior was evaluated. Our data showed that MYC decreased the survival rate, hatching rate and heart rate, but increased the malformation rate and spontaneous movement. MYC caused abnormal neurobehaviors characterized by decreased swimming distance and movement time. MYC impaired cerebral histopathological morphology and inhibited neurogenesis in HuC:egfp transgenic zebrafish. MYC also reduced the activities of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), and downregulated neurodevelopment related genes (gfap, syn2a, gap43 and mbp) in zebrafish and PC12 cells. Besides, MYC activated autophagy through enhanced expression of the LC3-II protein and suppressed expression of the p62 protein and autophagosome formation, subsequently triggering apoptosis by upregulating apoptotic genes (p53, bax, bcl-2 and caspase 3) and the cleaved caspase-3 protein in zebrafish and PC12 cells. These processes were restored by the autophagy inhibitor 3-methyladenine (3-MA) both in vivo and in vitro, indicating that MYC induces neurotoxicity by activating autophagy and apoptosis. Overall, this study revealed the potential autophagy and apoptosis mechanisms of MYC-induced neurotoxicity and provided novel strategies to counteract its toxicity.

Mechanistic insights into the parental co-exposure of T-2 toxin and epoxiconazole on the F1 generation of zebrafish (Danio rerio)

Mycotoxins and pesticides frequently coexist in agricultural commodities on a global scale. The potential transgenerational consequences induced by these substances pose a significant threat to human health. However, there is a lack of data concerning the effects of co-contamination by these chemicals in the F1 generation following parental exposure. This investigation delved into the mixture effects of T-2 toxin (T-2) and epoxiconazole (EPO) on the offspring of zebrafish (Danio rerio). The findings revealed that exposure across generations to a combination of T-2 and EPO resulted in toxicity in the larvae of the F1 generation. This was demonstrated by a significant increase in the levels or activities of malondialdehyde (MDA), thyroxine (T4), Caspase3, and cas9, along with a decrease in the levels of cyp19a, ERα, and ERβ. These outcomes suggested that cross-generational exposure to T-2 and EPO in D. rerio disrupted oxidative balance, induced cell apoptosis, and affected the endocrine system. Moreover, these effects were magnified when the F1 generation was continuously exposed to these compounds. Notably, these adverse effects could persist in subsequent generations without additional exposure. This study underscored the potential dangers associated with the simultaneous presence of T-2 and EPO on the development of fish offspring and the resulting environmental hazards to aquatic ecosystems. These findings emphasized the significant health risks posed by cross-generational exposure and highlighted the need for additional legislative measures to address these concerns.

Reviewing the use of zebrafish for the detection of neurotoxicity induced by chemical mixtures through the analysis of behaviour

The knowledge and assessment of mixtures of chemical pollutants in the aquatic environment is a complex issue that is often challenging to address. In this review, we focused on the use of zebrafish (Danio rerio), a vertebrate widely used in biomedical research, as a model for detecting the effects of chemical mixtures with a focus on behaviour. Our aim was to summarize the current status of the ecotoxicological research in this sector. Specifically, we limited our research to the period between January 2012 and September 2023, including only those works aimed at detecting neurotoxicity through behavioural endpoints, utilizing zebrafish at one or more developmental stages, from egg to adult. Additionally, we gathered the findings for every group of chemicals involved and summarised data from all the works we included. At the end of the screening process 101 papers were considered eligible for inclusion. Results show a growing interest in zebrafish at all life stages for this kind of research in the last decade. Also, a wide variety of different assays, involving different senses, was used in the works we surveyed, with exposures ranging from acute to chronic. In conclusion, the results of this study show the versatility of zebrafish as a model for the detection of mixture toxicity although, for what concerns behavioural analysis, the lack of standardisation of methods and endpoints might still be limiting.

Parental exposure to antidepressants has lasting effects on offspring? A case study with zebrafish

Fish have common neurotransmitter pathways with humans, exhibiting a significant degree of conservation and homology. Thus, exposure to fluoxetine makes fish potentially susceptible to biochemical and physiological changes, similarly to what is observed in humans. Over the years, several studies demonstrated the potential effects of fluoxetine on different fish species and at different levels of biological organization. However, the effects of parental exposure to unexposed offspring remain largely unknown. The consequences of 15-day parental exposure to relevant concentrations of fluoxetine (100 and 1000 ng/L) were assessed on offspring using zebrafish as a model organism. Parental exposure resulted in offspring early hatching, non-inflation of the swimming bladder, increased malformation frequency, decreased heart rate and blood flow, and reduced growth. Additionally, a significant behavioral impairment was also found (reduced startle response, basal locomotor activity, and altered non-associative learning during early stages and a negative geotaxis and scototaxis, reduced thigmotaxis, and anti-social behavior at later life stages). These behavior alterations are consistent with decreased anxiety, a significant increase in the expression of the monoaminergic genes slc6a4a (sert), slc6a3 (dat), slc18a2 (vmat2), mao, tph1a, and th2, and altered levels of monoaminergic neurotransmitters. Alterations in behavior, expression of monoaminergic genes, and neurotransmitter levels persisted until offspring adulthood. Given the high conservation of neuronal pathways between fish and humans, data show the possibility of potential transgenerational and multigenerational effects of pharmaceuticals' exposure. These results reinforce the need for transgenerational and multigenerational studies in fish, under realistic scenarios, to provide realistic insights into the impact of these pharmaceuticals.

Utility of zebrafish-based models in understanding molecular mechanisms of neurotoxicity mediated by the gut-brain axis

The gut microbes perform several beneficial functions which impact the periphery and central nervous systems of the host. Gut microbiota dysbiosis is acknowledged as a major contributor to the development of several neuropsychiatric and neurological disorders including bipolar disorder, depression, anxiety, Parkinson's disease, Alzheimer's disease, attention deficit hyperactivity disorder, and autism spectrum disorder. Thus, elucidation of how the gut microbiota-brain axis plays a role in health and disease conditions is a potential novel approach to prevent and treat brain disorders. The zebrafish (Danio rerio) is an invaluable vertebrate model that possesses conserved brain and intestinal features with those of humans, thus making zebrafish a valued model to investigate the interplay between the gut microbiota and host health. This chapter describes current findings on the utility of zebrafish in understanding molecular mechanisms of neurotoxicity mediated via the gut microbiota-brain axis. Specifically, it highlights the utility of zebrafish as a model organism for understanding how anthropogenic chemicals, pharmaceuticals and bacteria exposure affect animals and human health via the gut-brain axis.

The impact of ammonia and microcystin-LR on neurobehavior and glutamate/gamma-aminobutyric acid balance in female zebrafish (Danio rerio): ROS and inflammation as key pathways

Ammonia and microcystin-LR (MC-LR) are both toxins that can be in eutrophic waters during cyanobacterial blooms. While previous studies have focused on the effects of ammonia exposure on fish neurobehavioral toxicity, little attention has been given to the effects of MC-LR and combined exposures to both. This study exposed adult female zebrafish to ammonia (30 mg/L) and MC-LR (10 μg/L) alone and in combination for 30 days to investigate their neurotoxic effects and underlying mechanisms. Behavioral results showed that exposure to ammonia and MC-LR, both alone and in combination, led to decreased locomotor activity and increased anxiety in fish. Histomorphological analysis revealed the formation of thrombi and vacuolization in the brain across all exposure groups. Exposure to ammonia and MC-LR resulted in significant increases in MDA contents, decreases in Mn-SOD activities, and alterations in GSH contents compared to the control. Single and combined exposure to ammonia and MC-LR also induced the release of inflammatory factors (IL-1β and TNF-α) by activating the NOD/NF-κB signaling pathway. Furthermore, both ammonia and MC-LR significantly changed the expression of genes related to the glutamatergic and GABAergic systems, elevated Glu and GABA contents, as well as increased the Glu/GABA ratio, indicating that a shift towards increased Glu levels. Overall, these findings suggested that exposure to MC-LR and ammonia, individually and in combination, could decrease locomotor activity and increase anxiety of female zebrafish. This was likely due to brain damage from over-activated ROS and the release of pro-inflammatory cytokines, which led to a disruption in the balance of glutamatergic and GABAergic systems. However, there was no significant interaction between MC-LR and ammonia in fish neurobehavioral toxicity.

The beneficial and toxic effects of selenium on zebrafish. A systematic review of the literature

Selenium is an important and essential trace element in organisms, but its effects on organisms are also a "double-edged sword". Selenium deficiency or excess can endanger the health of humans and animals. In order to thoroughly understand the nutritional value and toxicity hazards of selenium, researchers have conducted many studies on the model animal zebrafish. However, there is a lack of induction and summary of relevant research on which selenium acts on zebrafish. This paper provides a review of the reported studies. Firstly, this article summarizes the benefits of selenium on zebrafish from three aspects: Promoting growth, Enhancing immune function and anti-tumor ability, Antagonizing some pollutants, such as mercury. Then, three aspects of selenium toxicity to zebrafish are introduced: nervous system and behavior, reproductive system and growth, and damage to some organs. This article also describes how different forms of selenium compounds have different effects on zebrafish health. Finally, prospects for future research directions are presented.

Vitamin A ameliorates valproic acid-induced autism-like symptoms in developing zebrafish larvae by attenuating oxidative stress and apoptosis

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by social deficits and repetitive/stereotyped behaviors. Prenatal exposure to valproic acid (VPA) has been reported to induce ASD-like symptoms in human and rodents. However, the etiology and pathogenesis of ASD have not been well elucidated. This study aimed to explore the mechanisms underlying VPA-induced ASD-like behaviors using zebrafish model and investigated whether vitamin A could prevent VPA-induced neurotoxicity. Here, zebrafish embryos were exposed to 0, 25 and 50 μM VPA from 4 to 96 h post fertilization (hpf) and the neurotoxicity was assessed. Our results showed that VPA affected the normal development of zebrafish larvae and induced ASD-like behaviors, including reduced locomotor activity, decreased distance near conspecifics, impaired social interaction and repetitive swimming behaviors. Exposure to VPA decreased the GFP signal in transgenic HuC:egfp zebrafish according to the negative effect of VPA on the expression of neurodevelopmental genes. In addition, VPA enhanced oxidative stress by promoting the production of reactive oxygen species (ROS) and hydrogen peroxide (H2O2) and inhibiting the activity of superoxide dismutase, then triggered apoptosis by upregulation of apoptotic genes. These adverse outcomes were mitigated by vitamin A, suggesting that vitamin A rescued VPA-induced ASD-like symptoms by inhibiting oxidative stress and apoptosis. Overall, this study identified vitamin A as a promising strategy for future therapeutic regulator of VPA-induced ASD-like behaviors.

Zebrafish neuromast sensory system: Is it an emerging target to assess environmental pollution impacts?

Environmental pollution poses risks to ecosystems. Among these risks, one finds neurotoxicity and damage to the lateral line structures of fish, such as the neuromast and its hair cells. Zebrafish (Danio rerio) is recommended as model species to be used in ecotoxicological studies and environmental biomonitoring programs aimed at assessing several biomarkers, such as ototoxicity. However, little is known about the history of and knowledge gaps on zebrafish ototoxicity. Thus, the aim of the current study is to review data available in the scientific literature about using zebrafish as animal model to assess neuromast toxicity. It must be done by analyzing the history and publication category, world production, experimental design, developmental stages, chemical classes, neuromasts and hair cell visualization methods, and zebrafish strains. Based on the results, number, survival and fluorescence intensity of neuromasts, and their hair cells, were the parameters oftentimes used to assess ototoxicity in zebrafish. The wild AB strain was the most used one, and it was followed by Tübingen and transgenic strains with GFP markers. DASPEI was the fluorescent dye most often applied as method to visualize neuromasts, and it was followed by Yo-Pro-1 and GFP transgenic lines. Antibiotics, antitumorals, metals, nanoparticles and plant extracts were the most frequent classes of chemicals used in the analyzed studies. Overall, pollutants can harm zebrafish's mechanosensory system, as well as affect their behavior and survival. Results have shown that zebrafish is a suitable model system to assess ototoxicity induced by environmental pollution.

The uses of zebrafish (Danio rerio) as an in vivo model for toxicological studies: A review based on bibliometrics

An in vivo model is necessary for toxicology. This review analyzed the uses of zebrafish (Danio rerio) in toxicology based on bibliometrics. Totally 56,816 publications about zebrafish from 2002 to 2023 were found in Web of Science Core Collection, with Toxicology as the top 6 among all disciplines. Accordingly, the bibliometric map reveals that "toxicity" has become a hot keyword. It further reveals that the most common exposure types include acute, chronic, and combined exposure. The toxicological effects include behavioral, intestinal, cardiovascular, hepatic, endocrine toxicity, neurotoxicity, immunotoxicity, genotoxicity, and reproductive and transgenerational toxicity. The mechanisms include oxidative stress, inflammation, autophagy, and dysbiosis of gut microbiota. The toxicants commonly evaluated by using zebrafish model include nanomaterials, arsenic, metals, bisphenol, and dioxin. Overall, zebrafish provide a unique and well-accepted model to investigate the toxicological effects and mechanisms. We also discussed the possible ways to address some of the limitations of zebrafish model, such as the combination of human organoids to avoid species differences.

Long-term effects of embryonic exposure to benzophenone-3 on neurotoxicity and behavior of adult zebrafish

Benzophenone-3 (BP-3) is the most widely used ultraviolet filter (UV filter) in industries to avoid UV radiation damage. BP-3 is added to most sunscreens to protect the skin, hair, and lips from sun rays. It results in continuous discharge into aquatic environments, leading to aquatic biota and human's continuous exposure. Consequences of BP-3 exposure on the physiology and behavior of aquatic animals, mainly zebrafish, have been investigated, including their neurotoxic effects. However, little is known about its consequences in long-term developmental endpoints. This study aimed to investigate the long-term effects of embryonic BP-3 exposure on biomarkers of neurotoxicity in zebrafish. For this, we exposed embryos to 5, 10, and 20 μg∙L-1 BP-3 concentration and let fish grow to adulthood (5mpf). We evaluated anxiety-like behavior, social preference, aggressiveness, and enzymatic activity of the antioxidant defenses system and neurotoxic biomarkers (Glutathione S-transferase -GST, catalase -CAT, and acetylcholinesterase -AChE) in adult zebrafish. Enzymatic activities were also investigated in larvae immediately after BP-3 exposure. Animals early exposed to BP-3 presented anxiety-like behaviors and decreased social preference, but aggressiveness was not altered. In general, exposure to BP-3 leads to altered enzymatic activity, which persists into adulthood. GST activity increased in embryos and adults, while CAT activity decreased in both life stages. AChE activity enhanced only at the larval stage (96 hpf). The long-term behavioral and biochemical effects of BP-3 highlight the need for abolishing or restricting the compound from personal care products, which are continually disposed into the environment and threaten the biota and human health.

The Neurotoxic Effect of Environmental Temperature Variation in Adult Zebrafish (Danio rerio)

Neurotoxicity consists of the altered functionality of the nervous system caused by exposure to chemical agents or altered chemical-physical parameters. The neurotoxic effect can be evaluated from the molecular to the behavioural level. The zebrafish Danio rerio is a model organism used in many research fields, including ecotoxicology and neurotoxicology. Recent studies by our research group have demonstrated that the exposure of adult zebrafish to low (18 °C) or high (34 °C) temperatures alters their brain proteome and fish behaviour compared to control (26 °C). These results showed that thermal variation alters the functionality of the nervous system, suggesting a temperature-induced neurotoxic effect. To demonstrate that temperature variation can be counted among the factors that generate neurotoxicity, eight different protein datasets, previously published by our research group, were subjected to new analyses using an integrated proteomic approach by means of the Ingenuity Pathway Analysis (IPA) software (Release December 2022). The datasets consist of brain proteome analyses of wild type adult zebrafish kept at three different temperatures (18 °C, 26 °C, and 34 °C) for 4 days (acute) or 21 days (chronic treatment), and of BDNF+/- and BDNF-/- zebrafish kept at 26 °C or 34 °C for 21 days. The results (a) demonstrate that thermal alterations generate an effect that can be defined as neurotoxic (p value ≤ 0.05, activation Z score ≤ -2 or ≥2), (b) identify 16 proteins that can be used as hallmarks of the neurotoxic processes common to all the treatments applied and (c) provide three protein panels (p value ≤ 0.05) related to 18 °C, 34 °C, and BDNF depletion that can be linked to anxiety-like or boldness behaviour upon these treatments.

Acrylamide induces neurotoxicity in zebrafish (Danio rerio) via NLRP3-mediated pyroptosis

Acrylamide (ACR) is widely used in water treatment, cosmetics, dyes, paper manufacturing, and other industries. Evidence suggests that ACR exposure causes selective neurotoxicity in humans. The primary symptoms include extremity numbness, skeletal muscle weakness, and ataxia, skeletal muscle weakness. An experimental zebrafish (Danio rerio) embryo model was used in this study to assess the impact of ACR toxicity on the development of the zebrafish nervous system. The results showed that neurodevelopmental disorders, inflammatory reactions, and oxidative stress were common in zebrafish exposed to ACR. Furthermore, ACR exposure induces pyroptotic phenotypical nerve cells, pyroptosis-related protein activation, and inflammasome NLR family pyrin domain-containing 3 (NLRP3) expression. Caspy and Caspy2 expression was knocked down via CRISPR/Cas9 to further investigate the pyroptotic mechanism, showing that these two targets alleviated the inflammatory reaction and neurodevelopmental disorder caused by ACR. Moreover, the Caspy-mediated classic pathway may be vital for the pyroptosis caused by ACR. In conclusion, this study is the first to show that ACR can activate NLRP3 inflammation to cause neurotoxicity in zebrafish via the Caspy pathways, which differs from the traditional exogenous infection model.

Curcumin protects against fenvalerate-induced neurotoxicity in zebrafish (Danio rerio) larvae through inhibition of oxidative stress

Fenvalerate (FEN), a typical type II pyrethroid pesticide, is widely used in agriculture. FEN has been detected in the environment and human body. However, the neurotoxicity of FEN has not been well elucidated. This study aimed to explore the mechanisms underlying FEN-induced neurotoxicity using the zebrafish (Danio rerio) model. We also investigated whether curcumin (CUR), a polyphenol antioxidant that exhibits neuroprotective properties, can prevent FEN-induced neurotoxicity. Here, zebrafish embryos were exposed to 0, 3.5, 7 and 14 μg/L of FEN from 4 to 96 h post fertilization (hpf) and neurotoxicity was assessed. Our results showed that FEN decreased the survival rate, heart rate, body length and spontaneous movement, and increased malformation rate. FEN caused neurobehavioral alterations, including decreased swimming distance and velocity, movement time and clockwise rotation times. FEN also suppressed neurogenesis in transgenic HuC:egfp zebrafish, reduced cholinesterase activity and downregulated the expression of neurodevelopment related genes (elavl3, gfap, gap43 and mbp). In addition, FEN enhanced oxidative stress via excessive reactive oxygen species and antioxidant enzyme inhibition, then triggered apoptosis by upregulation of apoptotic genes (p53, bcl-2, bax and caspase 3). These adverse outcomes were alleviated by CUR, indicating that CUR mitigated FEN-induced neurotoxicity by inhibiting oxidative stress. Overall, this study revealed that CUR ameliorated FEN-induced neurotoxicity via its antioxidant, indicating a promising protection of CUR against environmental pollutant-induced developmental anomalies.

Neonicotinoid Imidacloprid Affects the Social Behavior of Adult Zebrafish by Damaging Telencephalon Neurons through Oxidation Stress, Inflammation, and Apoptosis

The neonicotinoid imidacloprid is a widely used insecticide worldwide. We assessed the effects of acute and chronic imidacloprid exposure on the social behavior of adult zebrafish. We assembled simple apparatus to detect 2D locomotion: a single camera capture system and two specially designed water tanks. We then used the tracking and heat maps of the behavior trajectories of zebrafish subjected to sham and imidacloprid exposure and compared their social behavior. Furthermore, histomorphology and immunohistochemistry of their brain tissue sections were performed to clarify possible neurotoxicity due to imidacloprid exposure in our adult zebrafish. Our results showed that imidacloprid exposure significantly reduced the zebrafish's swimming speed, distance traveled, acceleration, and deceleration. The longer the imidacloprid exposure, the more severe the locomotor behavior disability. Furthermore, imidacloprid exposure significantly reduced heterosexual attractive behavior between the different sexes, as well as defensive alert behavior among males. Our histomorphology and immunohistochemistry evidence showed imidacloprid exposure may lead to neuronal oxidative stress, inflammation, apoptosis, and damage in the telencephalon of adult zebrafish. Thus, we suggested that neonicotinoid imidacloprid exposure can damage the telencephalon neurons of adult zebrafish through oxidative stress, inflammation, and apoptosis and then affect the social behavior of adult zebrafish.