The use of tocofersolan as a rescue agent in larval zebrafish exposed to benzo[a]pyrene in early development

PAHs as environmental pollutants and their neurotoxic effects

Polycyclic aromatic hydrocarbons (PAHs), which are widely present in incompletely combusted air particulate matter <2.5 μm (PM2.5), tobacco and other organic materials, can enter the human body through various routes and are a class of environmental pollutants with neurotoxic effects. PAHs exposure can lead to abnormal development of the nervous system and neurobehavioral abnormalities in animals, including adverse effects on the nervous system of children and adults, such as a reduced learning ability, intellectual decline, and neural tube defects. After PAHs enter cells of the nervous system, they eventually lead to nervous system damage through mechanisms such as oxidative stress, DNA methylation and demethylation, and mitochondrial autophagy, potentially leading to a series of nervous system diseases, such as Alzheimer's disease. Therefore, preventing and treating neurological diseases caused by PAHs exposure are particularly important. From the perspective of the in vitro and in vivo effects of PAHs exposure, as well as its effects on human neurodevelopment, this paper reviews the toxic mechanisms of action of PAHs and the corresponding prevention and treatment methods to provide a relevant theoretical basis for preventing the neurotoxicity caused by PAHs, thereby reducing the incidence of diseases related to the nervous system and protecting human health.

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.

Vitamin E alleviates pyraclostrobin-induced toxicity in zebrafish (Danio rerio) and its potential mechanisms

Strobilurin fungicides (SFs) are commonly used in agriculture worldwide and frequently detected in aquatic environments. High toxicity of SFs to aquatic organisms has caused great concerns. To explore whether vitamin E (VE) can relieve the toxicity caused by pyraclostrobin (PY), zebrafish were exposed to PY with or without VE supplementation. When co-exposure with VE (20 μM), the 96 h-LC50 values of PY to zebrafish embryos, adult, and the 24 h-LC50 value of PY to larvae increased from 43.94, 58.36 and 38.16 μg/L to 64.72, 108.62 and 72.78 μg/L, respectively, indicating that VE significantly decreased the toxicity of PY to zebrafish at different life stages. In addition, VE alleviated the deformity symptoms (pericardial edema and brain damage), reduced speed and movement distance, and decreased heart rate caused by 40 μg/L PY in zebrafish larvae. Co-exposure of PY with VE significantly reduced PY-caused larval oxidative stress and immunotoxicity via increasing the activities of superoxide dismutase, catalase and level of glutathione, as well as reducing the malondialdehyde production and the expression levels of Nrf2, Ucp2, IL-8, IFN and CXCL-C1C. Meanwhile, the expression levels of gria4a and cacng4b genes, which were inhibited by PY, were significantly up-regulated after co-exposure of PY with VE. Moreover, co-exposure with VE significantly reversed the increased mitochondrial DNA copies and reduced ATP content caused by PY in larvae, but had no effect on the expression of cox4i1l and activity of complex III that reduced by PY, suggesting VE can partially improve PY-induced mitochondrial dysfunction. In conclusion, the potential mechanisms of VE alleviating PY-induced toxicity may be ascribed to decreasing the oxidative stress level, restoring the functions of heart and nervous system, and improving the immunity and mitochondrial function in zebrafish.

Embryonic exposures to cadmium and PAHs cause long-term and interacting neurobehavioral effects in zebrafish

Developmental exposure to either polycyclic aromatic hydrocarbons (PAHs) or heavy metals has been shown to cause persisting and overlapping neurobehavioral effects in animal models. However, interactions between these compounds have not been well characterized, despite their co-occurrence in a variety of environmental media. In two companion studies, we examined the effects of developmental exposure to cadmium (Cd) with or without co-exposure to prototypic PAHs benzo[a]pyrene (BaP, Exp. 1) or fluoranthene (FA, Exp. 2) using a developing zebrafish model. Zebrafish embryos were exposed to Cd (0-0.3 μM), BaP (0-3 μM), FA (0-1.0 μM), or binary Cd-PAH mixtures from 5 to 122 h post fertilization (hpf). In Exp. 1, Cd and BaP produced independent effects on an array of outcomes and interacting effects on specific outcomes. Notably, Cd-induced deficits in dark-induced locomotor stimulation were attenuated by BaP co-exposure in the larval motility test and BaP-induced hyperactivity was attenuated by Cd co-exposure in the adolescent novel tank test. Likewise, in Exp. 2, Cd and FA produced both independent and interacting effects. FA-induced increases on adult post-tap activity in the tap startle test were attenuated by co-exposure with Cd. On the predator avoidance test, FA- and 0.3 μM Cd-induced hyperactivity effects were attenuated by their co-exposure. Taken together, these data indicate that while the effects of Cd and these representative PAHs on zebrafish behavior were largely independent of one another, binary mixtures can produce sub-additive effects for some neurobehavioral outcomes and at certain ages. This research emphasizes the need for detailed risk assessments of mixtures containing contaminants of differing classes, and for clarity on the mechanisms which allow cross-class toxicant interactions to occur.

Developmental exposure to polycyclic aromatic hydrocarbons (PAHs): Focus on benzo[a]pyrene neurotoxicity

Polycyclic Aromatic Hydrocarbons (PAHs) are a class of ubiquitous organic compounds produced during the incomplete combustion or pyrolysis of organic material. Dietary source is the main route for PAH human exposure by environmental contamination, food industrial processing or domestic cooking methods. The most studied PAH is benzo[a]pyrene (B[a]P), due to its harmful and multiple effects on human health: in addition to its well-known carcinogenic effects, emerging evidence indicates that B[a]P also induces neurotoxicity earlier and at lower doses than B[a]P-induced carcinogenicity making B[a]P neurotoxicity relevant to human health risk assessment. Developmental neurotoxicity of B[a]P has indeed received increasing attention: both human and experimental studies provide evidence of detrimental effects of prenatal or early postnatal B[a]P exposure, even at low doses. Indeed, in some of the multi-dose animal studies, maximal adverse effects were observed at lower B[a]P doses, according to a non-monotonic dose-response curve typical of endocrine-disrupting compounds. In substantial agreement with epidemiological studies, both rodents and zebrafish developmentally exposed to B[a]P exhibit long-term changes in multiple behavioural domains, in the absence of overt toxicological effects at birth (e.g. body weight and morphologic abnormalities). Notably, most targeted behavioural responses converge on locomotor activity and emotional profile, often, but not always, leading to a disinhibitory/hyperactive profile.

Proper modulation of AHR signaling is necessary for establishing neural connectivity and oligodendrocyte precursor cell development in the embryonic zebrafish brain

2,3,7,8-tetrachlorodibenzo-[p]-dioxin (TCDD) is a persistent global pollutant that exhibits a high affinity for the aryl hydrocarbon receptor (AHR), a ligand activated transcription factor. Epidemiological studies have associated AHR agonist exposure with multiple human neuropathologies. Consistent with the human data, research studies using laboratory models have linked pollutant-induced AHR activation to disruptions in learning and memory as well as motor impairments. Our understanding of endogenous AHR functions in brain development is limited and, correspondingly, scientists are still determining which cell types and brain regions are sensitive to AHR modulation. To identify novel phenotypes resulting from pollutant-induced AHR activation and ahr2 loss of function, we utilized the optically transparent zebrafish model. Early embryonic TCDD exposure impaired embryonic brain morphogenesis, resulted in ventriculomegaly, and disrupted neural connectivity in the optic tectum, habenula, cerebellum, and olfactory bulb. Altered neural network formation was accompanied by reduced expression of synaptic vesicle 2. Loss of ahr2 function also impaired nascent network development, but did not affect gross brain or ventricular morphology. To determine whether neural AHR activation was sufficient to disrupt connectivity, we used the Gal4/UAS system to express a constitutively active AHR specifically in differentiated neurons and observed disruptions only in the cerebellum; thus, suggesting that the phenotypes resulting from global AHR activation likely involve multiple cell types. Consistent with this hypothesis, we found that TCDD exposure reduced the number of oligodendrocyte precursor cells and their derivatives. Together, our findings indicate that proper modulation of AHR signaling is necessary for the growth and maturation of the embryonic zebrafish brain.

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.