Limited studies in humans and in animal models have investigated the neurotoxic risks related to a gestational exposure to diesel exhaust particles (DEP) on the embryonic brain, especially those regarding monoaminergic systems linked to neurocognitive disorders. We previously showed that exposure to DEP alters monoaminergic neurotransmission in fetal olfactory bulbs and modifies tissue morphology along with behavioral consequences at birth in a rabbit model. Given the anatomical and functional connections between olfactory and central brain structures, we further characterized their impacts in brain regions associated with monoaminergic neurotransmission. At gestational day 28 (GD28), fetal rabbit brains were collected from dams exposed by nose-only to either a clean air or filtered DEP for 2 h/day, 5 days/week, from GD3 to GD27. HPLC dosage and histochemical analyses of the main monoaminergic systems, i.e., dopamine (DA), noradrenaline (NA), and serotonin (5-HT) and their metabolites were conducted in microdissected fetal brain regions. DEP exposure increased the level of DA and decreased the dopaminergic metabolites ratios in the prefrontal cortex (PFC), together with sex-specific alterations in the hippocampus (Hp). In addition, HVA level was increased in the temporal cortex (TCx). Serotonin and 5-HIAA levels were decreased in the fetal Hp. However, DEP exposure did not significantly modify NA levels, tyrosine hydroxylase, tryptophan hydroxylase or AChE enzymatic activity in fetal brain. Exposure to DEP during fetal life results in dopaminergic and serotonergic changes in critical brain regions that might lead to detrimental potential short-term neural disturbances as precursors of long-term neurocognitive consequences.

DNA methylation is one of the most important epigenetic modifications and is closely related with several biological processes such as regulation of gene transcription and the development of non-malignant diseases. The prevailing dogma states that DNA methylation in eukaryotes occurs essentially through 5-methylcytosine but recently adenine methylation was also found to be present in eukaryotes. In mouse embryonic stem cells, 6-methyladenine was associated with the repression and silencing of genes, particularly in the X-chromosome, known to play an important role in cell fate determination. Here, we have demonstrated that 6mA is a ubiquitous eukaryotic epigenetic modification that is put in place during epigenetically sensitive periods such as embryogenesis and foetal development. In somatic cells there are clear tissue specificity in 6mA levels, with the highest 6mA levels being observed in the brain. In zebrafish, during the first 120h of embryo development, from a single pluripotent cell to an almost fully formed individual, 6mA levels steadily increase. An identical pattern was observed over embryonic days 7-21 in the mouse. Furthermore, exposure to a neurotoxic environmental pollutant during the same early life period may led to a decrease in the levels of this modification in female rats. The identification of the periods during which 6mA epigenetic marks are put in place increases our understanding of this mammalian epigenetic modification, and raises the possibility that it may be associated with developmental processes.

Background: Airborne pollution, especially from diesel exhaust (DE), is known to have a negative effect on the central nervous system in exposed human populations. However, the consequences of gestational exposure to DE on the fetal brain remain poorly explored, with various effects depending on the conditions of exposure, as well as little information on early developmental stages. We investigated the short-term effects of indirect DE exposure throughout gestation on the developing brain using a rabbit model. We analyzed fetal olfactory tissues at the end of gestation and tested behaviors relevant to pups’ survival at birth. Pregnant dams were exposed by nose-only inhalation to either clean air or DE with a content of particles (DEP) adjusted to 1 mg/m3 by diluting engine exhaust, for 2 h/day, 5 days/week, from gestational day 3 (GD3) to day 27 (GD27). At GD28, fetal olfactory mucosa, olfactory bulbs and whole brains were collected for anatomical and neurochemical measurements. At postnatal day 2 (PND2), pups born from another group of exposed or control
female were examined for their odor-guided behavior in response to the presentation of the rabbit mammary pheromone 2-methyl-3-butyn-2-ol (2MB2).
Results: At GD28, nano-sized particles were observed in cilia and cytoplasm of the olfactory sensory neurons in the olfactory mucosa and in the cytoplasm of periglomerular cells in the olfactory bulbs of exposed fetuses. Moreover, cellular and axonal hypertrophies were observed throughout olfactory tissues. Concomitantly, fetal serotoninergic and dopaminergic systems were affected in the olfactory bulbs. Moreover, the neuromodulatory homeostasis was disturbed in a sex-dependent manner in olfactory tissues. At birth, the olfactory sensitivity to 2MB2 was reduced in exposed PND2 pups.
Conclusion: Gestational exposure to DE alters olfactory tissues and affects monoaminergic neurotransmission in fetuses’ olfactory bulbs, resulting in an alteration of olfactory-based behaviors at birth. Considering the anatomical and functional continuum between the olfactory system and other brain structures, and due to the importance of monoamine neurotransmission in the plasticity of neural circuits, such alterations could participate to disturbances in higher integrative structures, with possible long-term neurobehavioral consequences.

Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants originating from incompletecombustion processes. Humans are mainly exposed through contaminated food ingestion. PAHs are neu-rotoxic compounds both for human and rodents, and may be found in placenta, umbilical cord blood andbreast milk, suggesting that early exposure may impact developing central nervous system.In a previous study we showed that PAH exposure during both gestation and lactation periods in ratsincreased anxiety-related behaviours and decreased cerebral metabolism in several key structures linkedto the limbic system on male pups at the adult stage. The aim of the present study was to assess the effectsof an exclusive gestational PAH exposure on the same aspects of brain functionality. Female rats wereexposed through diet to a 16 PAH mixture at doses of 2 g/kg/day or 200 g/kg/day during gestation.Late neurotoxic effects were evaluated by carrying out behavioural and cognitive tests and histochemicalanalyses using cytochrome oxidase activity as a cerebral metabolism marker in different brain areas.The results of this study revealed that behaviour and cerebral metabolism on prenatally PAH exposedadult rats was not significantly affected by the exposure to these pollutants. Finally this work highlights that the exposure period to pollutants such as PAHs at very early stages ofdevelopment play a key role on the neurological impairment induced.

Evidence is now growing that early-life environmental pollutant exposure during the critical period of brain development may be an important risk factor, contributing to the emergence of neurobehavioral disorders later in life (Grova et al., 2019). In this context, our team previously highlighted that a daily exposure of rat pups to the α isomer of HBCDD, a brominated flame retardant largely added to polystyrene building materials, during gestation and lactation (66 ng/kg/day) induced disturbances in locomotor maturation, exploratory activity and level of anxiety over the first 6 weeks of postnatal life (Maurice et al., 2015). The present study therefore aims at evaluating the developmental neurotoxicity of an early exposure to this chemical that is considered as a compound of high concern for human health, in comparison with valproic acid (VPA), a common anti-epileptic drug known to induce developmental disorders and contribute to the emergence of autism spectrum disorders. HBCDD-exposed dams were administered daily p.o. from GD0 to PND21 with 100 ng/kg/day of α-HBCDD isomer in oil, whereas the two other groups received the vehicle only. At GD12, VPA-treated rats received a single i.p. injection of VPA at a dose of 600 mg/kg whereas the two other groups were injected with the vehicle only. Pups were tested for their early behavioral development from PND3 to PND21 using a standardized test battery. At PND21, brains were collected and cortex removed for further analysis. The results pointed out the ability of both compounds to induce subtle significant behavioral changes during the neurodevelopment with the reduction in the time spent to grasp a rotating grid in VPA-exposed pups at PND9-11 and the increase in the time to move back in the neogative geotaxis task in the HBCDD-treated rats at PND8-10. No significant modification in the olfactory discriminative test (PN9-PND11) has been observed among groups. Cortical protein expression was analyzed for the neuroinflammation and synaptic plasticity, demonstrating a significant increase in the level of glial fibrillary acidic protein (GFAP) associated with a diminution of synaptophysin in the VPA-treated pups whereas HBCDD-exposed rats showed only an increasing level of expression of GFAP. In conclusion, both results suggest the ability of both compounds to impair slightly the brain and behavior development of rat pups in a different way according to the chemicals. The measurements of the cytochrome oxydase activity in various brain regions, currently under progress, should provide us additional information on the impact of an early exposure to HBCDD or VPA on behavioral impairments later in life.

Short- and long-term behavioral impairments related to anxiety, sexual and social behavior have recently been demonstrated in rats daily exposed during gestation and lactation (GD0 to PND21) to α-hexabromocyclododecane (α-HBCDD, 66 ng/kg/day of body weight), a brominated flame retardant of very high concern. The present study is aimed at examining the effects of such exposure on the potent mechanism leading to the phenotypes observed in the cerebellum of male pups at PND14. This brain region is known for its high sensitivity to environmental disturbances occurring essentially during the early phase of brain development. In the cerebellum, we initially assumed that perinatal exposure to α-HBCDD may i) lead to epigenetic changes in the 6-methyl Adenine (6-mA), which has been identified as genuine epigenetic mark through different techniques (LC-MS/MS, DotBlot and immunochemistry) and ii) induce neuroinflammation which could also exert key influence in neuronal dysfunction. The results revealed that α-HBCDD is able to alter the expression levels of proteins associated with neuroinflammation such as GFAP (+10% compared with controls for males and -18% compared to controls for females) and S100β (+ 19% compared with controls in males, p<0.05) which could later interfere in brain development and functioning. Concomitantly, a decrease in males (-30%) in 6mA has been noticed in this part of the brain, suggesting the ability of this contaminant to induce early reduction in DNA methylation. Corresponding IlluminaR sequencing proved that many differential methylated regions (DMR) can be identified on the different chromosomes at PND14 with a particular attention to be paid to changes on the Y and mitochondrial chromosome. This early decrease in 6mA signal intensity was also observed in the cerebellum at the adult stage (PND270) for both female and male exposed animals compared with controls, with a significant interaction between HBCDD and sex (p<0.01). Detailed gene analysis, currently under evaluation, should enable us to understand how the impact of early life α-HBCDD exposure on gene expression induces chronic neuroinflammation and changes in neurotransmission pathways with the occurrence of behavioural impairments later in life as a consequence.

Short- and long-term behavioral impairments related to anxiety, sexual and social behavior have been recently demonstrated in rats daily exposed during gestation and lactation (GD0 to PND21) to α-hexabromocyclododecane (α-HBCDD, 66 ng/kg/day of body weight), a brominated flame retardant of very high concern. The present study aimed at examining the effects of such exposure on potent mechanism leading to the observed phenotypes in the cerebellum of male pups at PND14. This part of the brain is known as a structure of high sensitivity to environmental disturbances occurring during the early phase of brain development and of later maturation compared to other brain regions. In cerebellum, we assume that a perinatal exposure to α-HBCDD may i) lead to epigenetic changes of the 6-methyl Adenine (6-mA) that has been identified as genuine epigenetic mark through different techniques (LC-MS/MS, DotBlot and immunochemistry) and ii) induce neuroinflammation which could also play a key role in neuronal dysfunction. The results showed that α-HBCDD is able to change the expression levels of proteins associated to neuroinflammation such as GFAP (+10% compared to controls in male and -18% compared to control in females) and S100β (+ 19% compared to controls in male, p<0.05) which later on could interfere with brain development and functioning. Concomitantly, a decrease in male (-30%) in 6mA has been noticed in this part of the brain, suggesting the ability of this contaminant to induce an early reduction of DNA methylation. Corresponding IlluminaR sequencing proved that many differential methylated regions (DMR) can be identified on the different chromosomes at PND14 with a particular attention to be put on changes on the Y and mitochondrial chromosome. Results under progress showed that this early decrease in 6mA signal intensity was also stated in the cerebellum at the adult stage (PND270) for both female and male-exposed animals compared to controls with a significant interaction between HBCDD and sex (p<0.01). Detailed gene analysis, currently under evaluation, should enable us to understand how the impact of early life α-HBCDD exposure on gene expression may induce chronic neuroinflammation and changes in neurotransmission pathways with the occurrence of behavioural impairments later in life as a consequence.