Exclusive prenatal exposure to a 16 PAH mixture does not impact anxiety-related behaviours and regional brain metabolism in adult male rats: A role for the period of exposure in the modulation of PAH neurotoxicity

Neurotoxicity of Polycyclic Aromatic Hydrocarbons: A Systematic Mapping and Review of Neuropathological Mechanisms

Several studies present the neurotoxic effects of polycyclic aromatic hydrocarbons (PAHs), a class of environmental pollutants capable of causing neurological deficits. However, a collective review approach to this research topic is scarce. This study presents the effect of PAHs on the central nervous system using a bibliometric approach. The neuropathological mechanisms of PAHs are also highlighted. Published articles were searched for in the Scopus and Web of Science databases from January 1979 to December 2020 using the keywords ‘polycyclic aromatic hydrocarbons’ and ‘neurotoxicity’. The total number of documents retrieved from both databases was 338. Duplicated documents (80) were excluded and 258 articles were used for the final analysis. Our findings revealed that there has been a significant increase in research outputs on this topic in the last ten years. The countries with the highest scientific productivity in this area are USA, China, France and Italy. The result also showed that, in the past few years, global scientific output in research relating to PAH neurotoxicity focused on neurodegeneration, cholinergic function, neurodevelopmental toxicity, behavioural studies, oxidative stress, neuroprotection and therapeutic intervention using different experimental models, including zebrafish, neuronal cell lines, Caenorhabditis elegans and rats. Recent studies also revealed the neuroprotective roles of some natural products against PAH-induced neurotoxicity. However, more investigation involving clinical trials is required to emphasize the observed neurotoxic effects.

Dose makes poison: Insights into the neurotoxicity of perinatal and juvenile exposure to environmental doses of 16 priority-controlled PAHs

Whether chronic exposure to environmental doses of polycyclic aromatic hydrocarbons (PAHs) can lead to neurotoxic effects is still unclear. Hence, the neurotoxic effects of perinatal and juvenile exposure to 16 priority-controlled PAHs were investigated. The mice were treated with 0, 0.5, 18.75, 50, 1875 μg/kg/day of PAHs corresponding to various population exposure concentrations from gestation to postnatal day 60. Urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG) and hippocampal and cortical neurotransmitter levels were determined using liquid chromatography-tandem mass spectrometry. Typical indicators or outcome of neurotoxicity, including, spatial learning and memory ability, hippocampal long-term potentiation (LTP) and dendritic spine density were evaluated via Morris water maze tests, electrophysiological experiments and Golgi-Cox assays, respectively. The results showed that exposure to different levels of PAH could not increase oxidative DNA damage level. Mice exposed to 0.5, 50 and 1875 μg/kg/day PAHs had significantly longer escape latency than the control group only on the 1st day (p < 0.05). The number of platform crossings and the time spent in target quadrant were similar between the control and the PAHs-exposed mice. Compared with the control mice, only those exposed to 50 μg/kg/day PAHs had significantly lower LTP in hippocampal CA1 region and dendritic spine density in hippocampal DG region (p < 0.05). Except for serotonin, no significant difference in hippocampal and cortical neurotransmitter concentrations was observed between the control and PAHs-exposed groups. Taken together, perinatal and juvenile exposure to environmental doses of PAHs had no profound effect on spatial learning and memory abilities, hippocampal LTP, dendritic spines density, and neurotransmitter levels. These unexpected findings were quite different from previous in vivo studies which commonly used 2-3 orders of magnitude higher PAHs doses to treat animals. Thus, the environmental dose is a crucial reference for future toxicological research to reveal the actual toxic mechanisms and human health effects of PAHs exposure.

Phenanthrene induces autism-like behavior by promoting oxidative stress and mTOR pathway activation

Autism is thought to be associated with both environmental and genetic factors. Phenanthrene (Phe) makes up a relatively high proportion of the low-ring polycyclic aromatic hydrocarbons. However, the association between exposure to Phe and Autism remain unclear. In this study, the effect and mechanisms of phenanthrene exposure on autistic behavior were investigated. Three-week-old male Kunming mice were exposed to doses of 5, 50, or 500 μg/kg/d Phe for 22 days. Exposure to phenanthrene induced a marked decrease in the activity of the mice in the central area in the open field test, and caused a significant decrease in communication with unfamiliar mice in the three-chambered social test. The hippocampus of the mice exposed to high concentrations of Phe showed pathological changes. Exposure to phenanthrene induced an increase in the levels of ROS and a decrease in levels of glutathione, and caused a significant decrease in the expression of Shank3 and Beclin1. This also led to an increase in the phosphorylation levels of Akt and mTOR. However, administering Rapamycin or vitamin E, inhibited the oxidative stress and activation of the mTOR pathway induced by Phe exposure, effectively alleviating the above-mentioned autistic-like anxious social behaviors. These results indicate that exposure to phenanthrene will lead to autism-like behavior. The underlying mechanism involves oxidative stress and the mTOR pathway.

French infant total diet study: Dietary exposure to heat-induced compounds (acrylamide, furan and polycyclic aromatic hydrocarbons) and associated health risks

A total diet study (TDS) was conducted between 2010 and 2016 to assess the risk associated with chemicals in food of non-breast-fed children from 1 to 36 months living in France. Food samples were collected, prepared "as consumed", and analyzed for chemicals of public health interest. Acrylamide, furan and polycyclic aromatic hydrocarbons (PAHs) were analyzed as heat-induced compounds produced mainly during thermal processing of foods. Dietary exposure was assessed for 705 representative children using food consumptions recorded through a 3-consecutive-days record. As all calculated margins of exposure (MOE) for PAHs exceeded 10 000, dietary exposure of the infant and toddler population was deemed tolerable with regard to the carcinogenic risk. Conversely, the exposure levels to acrylamide and furan were considered as of concern, requiring management measures to reduce the exposure essentially by reducing the formation of heat-induced compounds during food production or preparation processes. Efforts should mainly focus on major contributors to the exposure, i.e. sweet and savoury biscuits and bars, and potatoes and potato products for acrylamide, baby jars of vegetables, with or without meat or fish for acrylamide and furan.

Early-Life Benzo[a]Pyrene Exposure Causes Neurodegenerative Syndromes in Adult Zebrafish (Danio rerio) and the Mechanism Involved

There is increasing recognition of the importance of early-life environmental exposures in health disorders at later-life stages. The aim of this study was to evaluate whether early-life exposure to benzo[a]pyrene (BaP) could induce neurodegenerative syndromes at later-life stages in zebrafish. Embryos were exposed to BaP at doses of 0, 0.05, 0.5, 5, and 50 nM from early embryogenesis to 96 h post-fertilization (hpf), then transferred to clean water and maintained for 365 days. We found that BaP decreased locomotor and cognitive ability, neurotransmitter levels of dopamine, 3,4-dihydroxyphenylacetic acid and norepinephrine; and induced loss of dopaminergic neurons and resulted in neurodegeneration. Additionally, BaP increased amyloid β protein and cell apoptosis in the adult zebrafish brain. Further, DNA methyltransferase 1 (DNMT1) and DNMT3a were up-regulated in 96 hpf larvae and the adult brain. MeDIP-sequencing data of the 96 hpf larvae identified 235 differentially methylated genes in promoter, with the fold change > 1.5. Guanylate cyclase 2F (gucy2f) and dopamine receptor D4 related sequence (drd4-rs) were hypermethylation promoters, whereas zinc finger C4H2 domain (zc4h2) was a hypomethylation promoter in 96 hpf larvae and the adult brain. The mRNA levels of gucy2f and drd4-rs were down-regulated, and zc4h2 was up-regulated. Our findings suggested that the lasting modifications of DNA methylation were associated with neurodegenerative syndromes in adult zebrafish as a result of early-life BaP exposure.

Comparative developmental toxicity of a comprehensive suite of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental contaminants that occur in complex mixtures. Several PAHs are known or suspected mutagens and/or carcinogens, but developmental toxicity data is lacking for PAHs, particularly their oxygenated and nitrated derivatives. Such data are necessary to understand and predict the toxicity of environmental mixtures. 123 PAHs were assessed for morphological and neurobehavioral effects for a range of concentrations between 0.1 and 50 µM, using a high throughput early-life stage zebrafish assay, including 33 parent, 22 nitrated, 17 oxygenated, 19 hydroxylated, 14 methylated, 16 heterocyclic, and 2 aminated PAHs. Additionally, each PAH was evaluated for AHR activation, by assessing CYP1A protein expression using whole animal immunohistochemistry (IHC). Responses to PAHs varied in a structurally dependent manner. High-molecular weight PAHs were significantly more developmentally toxic than the low-molecular weight PAHs, and CYP1A expression was detected in five distinct tissues, including vasculature, liver, skin, neuromasts and yolk.

Changes in Brain Monoamines Underlie Behavioural Disruptions after Zebrafish Diet Exposure to Polycyclic Aromatic Hydrocarbons Environmental Mixtures

Zebrafish were exposed through diet to two environmentally relevant polycyclic aromatic hydrocarbons (PAHs) mixtures of contrasted compositions, one of pyrolytic (PY) origin and one from light crude oil (LO). Monoamine concentrations were quantified in the brains of the fish after six month of exposure. A significant decrease in noradrenaline (NA) was observed in fish exposed to both mixtures, while a decrease in serotonin (5HT) and dopamine (DA) was observed only in LO-exposed fish. A decrease in metabolites of 5HT and DA was observed in fish exposed to both mixtures. Several behavioural disruptions were observed that depended on mixtures, and parallels were made with changes in monoamine concentrations. Indeed, we observed an increase in anxiety in fish exposed to both mixtures, which could be related to the decrease in 5HT and/or NA, while disruptions of daily activity rhythms were observed in LO fish, which could be related to the decrease in DA. Taken together, these results showed that (i) chronic exposures to PAHs mixtures disrupted brain monoamine contents, which could underlie behavioural disruptions, and that (ii) the biological responses depended on mixture compositions.

Exposures of zebrafish through diet to three environmentally relevant mixtures of PAHs produce behavioral disruptions in unexposed F1 and F2 descendant

The release of polycyclic aromatic hydrocarbons (PAHs) into the environment has increased very substantially over the last decades. PAHs are hydrophobic molecules which can accumulate in high concentrations in sediments acting then as major secondary sources. Fish contamination can occur through contact or residence nearby sediments or though dietary exposure. In this study, we analyzed certain physiological traits in unexposed fish (F1) issued from parents (F0) exposed through diet to three PAH mixtures at similar and environmentally relevant concentrations but differing in their compositions. For each mixture, no morphological differences were observed between concentrations. An increase in locomotor activity was observed in larvae issued from fish exposed to the highest concentration of a pyrolytic (PY) mixture. On the contrary, a decrease in locomotor activity was observed in larvae issued from heavy oil mixture (HO). In the case of the third mixture, light oil (LO), a reduction of the diurnal activity was observed during the setup of larval activity. Behavioral disruptions persisted in F1-PY juveniles and in their offspring (F2). Endocrine disruption was analyzed using cyp19a1b:GFP transgenic line and revealed disruptions in PY and LO offspring. Since no PAH metabolites were dosed in larvae, these findings suggest possible underlying mechanisms such as altered parental signaling molecule and/or hormone transferred in the gametes, eventually leading to early imprinting. Taken together, these results indicate that physiological disruptions are observed in offspring of fish exposed to PAH mixtures through diet.

Chronic dietary exposure to pyrolytic and petrogenic mixtures of PAHs causes physiological disruption in zebrafish--part II: behavior

In the last 10 years, behavior assessment has been developed as an indicator of neurotoxicity and an integrated indicator of physiological disruption. Polycyclic aromatic hydrocarbon (PAH) release into the environment has increased in recent decades resulting in high concentrations of these compounds in the sediment of contaminated areas. We evaluated the behavioral consequences of long-term chronic exposure to PAHs, by exposing zebrafish to diets spiked with three PAH fractions at environmentally relevant concentrations. Fish were exposed to these chemicals from their first meal (5 days postfertilization) until they became reproducing adults (at 6 months old). The fractions used were representative of PAHs of pyrolytic (PY) origin and of two oils differing in composition (a heavy fuel oil (HO) and a light crude oil (LO)). Several tests were carried out to evaluate circadian spontaneous swimming activity, responses to a challenge (photomotor response), exploratory tendencies, and anxiety levels. We found that dietary PAH exposure was associated with greater mobility, lower levels of exploratory activity, and higher levels of anxiety, particularly in fish exposed to the HO fraction and, to a lesser extent, the LO fraction. Finally, our results indicate that PAH mixtures of different compositions, representative of situations encountered in the wild, can induce behavioral disruptions resulting in poorer fish performance.

Detection of Medium-Sized Polycyclic Aromatic Hydrocarbons via Fluorescence Energy Transfer

Reported herein is the use of proximity-induced non-covalent energy transfer for the detection of medium-sized polycyclic aromatic hydrocarbons (PAHs). This energy transfer occurs within the cavity of γ-cyclodextrin in various aqueous environments, including human plasma and coconut water. Highly efficient energy transfer was observed, and the efficiency of the energy transfer is independent of the concentration of γ-cyclodextrin used, demonstrating the importance of hydrophobic binding in facilitating such energy transfer. Low limits of detection were also observed for many of the PAHs investigated, which is promising for the development of fluorescence-based detection schemes.

Short-term effects of a perinatal exposure to a 16 polycyclic aromatic hydrocarbon mixture in rats: assessment of early motor and sensorial development and cerebral cytochrome oxidase activity in pups

Humans are exposed to polycyclic aromatic hydrocarbons (PAHs), a family of ubiquitous neurotoxic pollutants, mainly through ingestion of contaminated food. Developing organisms can be exposed also to PAHs due to the ability of these compounds to pass through the placental barrier as well as through the breast milk. Previous animal studies have reported that the exposure of rats to a 16 PAH mixture at environmental doses strictly limited to gestation did not induce any long-lasting consequences, whereas gestational and lactational PAH exposure induced long-term behavioral and cerebral metabolic effects. In the present study, short-term effects of exposures to the same PAH mixture during gestation, or during gestation and lactation, were assessed by evaluating motor and sensory development of rat pups, and by measuring cerebral cytochrome oxidase activity (a marker of energetic metabolism) in different brain areas. Brain levels of PAHs and some monohydroxylated metabolites were also evaluated in pups at birth and at 21 days of postnatal life. No significant short-term modifications of behavioral development and of cerebral metabolism were observed following an early PAH exposure whatever the dose and the period of exposure. Surprisingly, the same brain levels of concentration of PAHs and metabolites were observed in control and exposed pups in both studies. These analytical results raise the difficulty in overcoming environmental contamination of control animals and the choice of such controls in experimental studies which focus on neurotoxicity of exposure to low levels of pollutants.