Brain monoaminergic neurotransmission parameters in weanling rats after perinatal exposure to methylmercury and 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB153)

Neurotransmitter disturbances caused by methylmercury exposure: Microbiota-gut-brain interaction

The fetal and early postnatal stages are periods of rapid brain development, during which, methylmercury (MeHg) exposure can cause lasting cognitive impairments. MeHg exposure disrupts neurotransmitter metabolites, which increased susceptibility to neurological responses. However, the neurotoxic mechanism underlying the MeHg-induced disruption of neurotransmitter metabolism requires further exploration. To this end, female Sprague-Dawley (SD) rats were administered NaCl (control group) or MeHg (0.6 mg/kg, 1.2 mg/kg and 2.4 mg/ kg body weight (bw), where the body weight refers to the dams) during the perinatal period, and then changes in neurotransmitter profiles and the gut microbiota of offspring were detected. The results showed that tryptophan (Trp) and tyrosine (Tyr) pathway neurotransmitter metabolites, including serotonin (5-HT), 5-hydroxy indole acetic acid (5-HIAA), N-acetyl-5-hydroxytryptamin (NAS), Tyr, dopamine (DA) and epinephrine (E), were significantly changed, and the Kynurenine/Tryptophan (Kyn/Trp) ratio was increased in the MeHg-treated groups. Meanwhile, acetylcholine (ACh) and neurotransmitters involved in the amino acid pathway were significantly reduced. Notably, MeHg treatment induced a significant reduction in tight junctions in the colon and hippocampal tissue. Furthermore, fecal microbiota analysis indicated that the diversity and composition characteristics were significantly altered by MeHg exposure. Mediation analysis showed that the gut microbiota mediated the effect of MeHg treatment on the neurotransmitter expression profiles. The present findings shed light on the regulatory role of the gut microbiota in MeHg-disrupted neurotransmitter metabolic pathways and the potential impact of perinatal MeHg treatment on the "cross-talk" between the gut and brain.

Impacts of Neonatal Methylmercury on Behavioral Flexibility and Learning in Spatial Discrimination Reversal and Visual Signal Detection Tasks

Early postnatal development in rodents is sensitive to neurotoxic effects of the environmental contaminant, methylmercury. While juvenile and adolescent exposure also produce long-term impairments in behavior, the outcome of neonatal exposure is less understood. Neural development during the neonatal period in rodents is akin to that seen in humans during the third trimester of pregnancy but methylmercury exposure occurring during the neonatal period has not been modeled, partly because breast milk is a poor source of bioavailable methylmercury. To examine this developmental period, male Long-Evans rats were exposed to 0, 80, or 350µg/kg/day methylmercuric chloride from postnatal days 1 to 10, the rodent neonatal period. As adults, behavioral flexibility, attention, memory, and expression of the dopamine transporter in these rats was assessed. Rats exhibited changes in behavioral flexibility assessed in a spatial discrimination reversal procedure. Those rats exposed to the highest dose of methylmercury displayed subtly altered patterns of perseveration compared to control animals. During acquisition of the attention/memory procedure, rats exposed to this dose also had slower acquisition, and achieved lower overall accuracy during training, compared to controls despite neither attention nor memory being affected once the task was acquired. Finally, dopamine transporter expression in the striatum, prefrontal cortex, and hippocampus was unchanged in these adult rats. The results of this study replicate the trend of findings seen with exposure during gestation or during adolescence.

Adolescence as a sensitive period for neurotoxicity: Lifespan developmental effects of methylmercury

Neurotoxicity resulting from the environmental contaminant, methylmercury (MeHg), is a source of concern for many human populations that rely heavily on the consumption of fish and rice as stable ingredients in the diet. The developmental period of exposure is important both to the qualitative effects of MeHg and to the dose required to produce those effects. MeHg exposure during the sensitive prenatal period causes deleterious and long-lasting changes in neurodevelopment at particularly low doses. The effects include a wide host of cognitive and behavioral outcomes expressed in adulthood and sometimes not until aging. However, neurotoxic outcomes of methylmercury when exposure occurs during adolescence are only recently revealing impacts on human populations and animal models. This review examines the current body of work and showcases the sensitivity of adolescence, a period that straddles early development and adulthood, to methylmercury neurotoxicity and the implications such toxicity has in our understanding of methylmercury's effects in human populations and animal models.

Developmental exposure to methylmercury and ADHD, a literature review of epigenetic studies

Attention-deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder that affects the competence of academic performance and social wellness in children and adults. The causes of ADHD are unclear. Both genetic and environmental factors contribute to the development of ADHD. The behavioral impairments in ADHD are associated with epigenetic changes in genes that are important for neurodevelopment. Among environmental causes of ADHD, the neurotoxin methylmercury (MeHg) is associated with an increased risk for ADHD. Developing children are susceptible to neurotoxic effects of prenatal MeHg exposure. Human epidemiology studies have shown that prenatal MeHg exposure could invoke epigenetic changes in genes that are involved in ADHD. In addition, the pathogenesis of ADHD involves dopaminergic system, which is a target of developmental MeHg exposure. MeHg-induced alterations in the dopaminergic system have a profound impact on behavioral functions in adults. As a trace level of MeHg (around nM) can induce long-lasting behavioral alterations, potential mechanisms of MeHg-induced functional changes in the dopaminergic system may involve epigenetic mechanisms. Here, we review the relevant evidence on developmental MeHg exposures and the risk for ADHD. We also point out research gaps in understanding environmental causes of ADHD.

Polychlorinated environmental toxicants affect sphingolipid metabolism during neurogenesis in vitro

Sphingolipids (SLs) are important signaling molecules and functional components of cellular membranes. Although SLs are known as crucial regulators of neural cell physiology and differentiation, modulations of SLs by environmental neurotoxicants in neural cells and their neuronal progeny have not yet been explored. In this study, we used in vitro models of differentiated neuron-like cells, which were repeatedly exposed during differentiation to model environmental toxicants, and we analyzed changes in sphingolipidome, cellular morphology and gene expression related to SL metabolism or neuronal differentiation. We compared these data with the results obtained in undifferentiated neural cells with progenitor-like features. As model polychlorinated organic pollutants, we used 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3'-dichlorobiphenyl (PCB11) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153). PCB153 revealed itself as the most prominent deregulator of SL metabolism and as potent toxicant during early phases of in vitro neurogenesis. TCDD exerted only minor changes in the levels of analysed lipid species, however, it significantly changed the rate of pro-neuronal differentiation and deregulated expression of neuronal markers during neurogenesis. PCB11 acted as a potent disruptor of in vitro neurogenesis, which induced significant alterations in SL metabolism and cellular morphology in both differentiated neuron-like models (differentiated NE4C and NG108-15 cells). We identified ceramide-1-phosphate, lactosylceramides and several glycosphingolipids to be the most sensitive SL species to exposure to polychlorinated pollutants. Additionally, we identified deregulation of several genes related to SL metabolism, which may be explored in future as potential markers of developmental neurotoxicity.

Selective dopaminergic effects on attention and memory in male mice exposed to Methylmercury during adolescence

Gestational exposure to methylmercury disrupts dopamine-mediated behavior and produces heightened sensitivity to monoamine agonists later in life. This has been reported and replicated following both pre- and post-natal exposure. Impacts of methylmercury when exposure occurs during the sensitive period of adolescence, a key period of dopaminergic development, remain underexplored. There have been variable results thus far in studies investigating links between adolescent exposure to methylmercury and alterations in executive function and altered sensitivity to monoamine agonists. The current study was designed to investigate adolescent exposure by exposing male mice to 0, 0.3, or 3 ppm methylmercury during adolescence and training them in a hybrid task to assess two executive functions, attention and memory, in adulthood. Behavior in these animals was probed with a range of doses of the dopamine agonist, d-amphetamine, and the norepinephrine agonist, desipramine. Attention and memory in these mice were sensitive to disruption by d-amphetamine and interacted with methylmercury exposure. Choice latencies were also longer in the MeHg-exposed mice. Desipramine did not affect behavior in these animals nor did it interact with methylmercury. It is concluded that methylmercury-related inhibition of behavior observed in this study were differentially sensitive to acute disruption in dopamine, but not norepinephrine, neurotransmission.

Developmental Exposure to PCB153 (2,2',4,4',5,5'-Hexachlorobiphenyl) Alters Circadian Rhythms and the Expression of Clock and Metabolic Genes

Polychlorinated biphenyls (PCBs) are highly persistent and ubiquitously distributed environmental pollutants. Based on their chemical structure, PCBs are classified into non-ortho-substituted and ortho-substituted congeners. Non-ortho-substituted PCBs are structurally similar to dioxin and their toxic effects and mode of action are well-established. In contrast, very little is known about the effects of ortho-substituted PCBs, particularly, during early development. The objective of this study is to investigate the effects of exposure to an environmentally prominent ortho-substituted PCB (2,2',4,4',5,5'-hexachlorobiphenyl; PCB153) on zebrafish embryos. We exposed zebrafish embryos to 3 different concentrations of PCB153 starting from 4 to 120 hours post-fertilization (hpf). We quantified gross morphological changes, behavioral phenotypes, gene expression changes, and circadian behavior in the larvae. There were no developmental defects during the exposure period, but starting at 7 dpf, we observed spinal deformity in the 10 μM PCB153 treated group. A total of 633, 2227, and 3378 differentially expressed genes were observed in 0.1 μM (0.036 μg/ml), 1 μM (0.36 μg/ml), and 10 μM (3.6 μg/ml) PCB153-treated embryos, respectively. Of these, 301 genes were common to all treatment groups. KEGG pathway analysis revealed enrichment of genes related to circadian rhythm, FoxO signaling, and insulin resistance pathways. Behavioral analysis revealed that PCB153 exposure significantly alters circadian behavior. Disruption of circadian rhythms has been associated with the development of metabolic and neurological diseases. Thus, understanding the mechanisms of action of environmental chemicals in disrupting metabolism and other physiological processes is essential.

Prenatal exposure to PCBs and neurological and sexual/pubertal development from birth to adolescence

Several chemical compounds are resistant to degradation and end up in the food chain. One group of these chemicals is polychlorinated biphenyls (PCBs) which are used as flame retardants and plasticizers. Although PCBs were banned several decades ago, PCBs are still found in environmental media, including in the body of humans. PCBs are transferred from mother to fetus via the placenta during pregnancy. Considering that the prenatal period is a sensitive period during which essential developmental processes take place, exposure to environmental chemicals might have considerable and permanent consequences for outcomes in later life. The aim of this review is to provide an update on the latest insights on the effects of prenatal exposure to PCBs on neurological, sexual and pubertal development in children. We give an overview of recent literature, and discuss it in the light of the findings in a unique Dutch birth cohort, with data on both neurological and pubertal development into adolescence. The findings in the studies included in this review, together with the findings in the Dutch cohort, demonstrate that prenatal exposure to PCBs can interfere with normal child development, not only during the perinatal period, but up to and including adolescence. Higher prenatal exposure to PCBs was found to be both negatively and positively associated with neurodevelopmental outcomes. Regarding pubertal development, higher prenatal PCB exposure was found to be associated with more advanced pubertal development, also in the Dutch cohort, whereas other studies also found delayed pubertal development. These findings raise concern regarding the effects of man-made chemical compounds on child development. They further contribute to the awareness of how environmental chemical compounds can interfere with child development and negatively influence healthy ageing.

Endocrine active metals, prenatal stress and enhanced neurobehavioral disruption

Metals, including lead (Pb), methylmercury (MeHg) and arsenic (As), are long-known developmental neurotoxicants. More recently, environmental context has been recognized to modulate metals toxicity, including nutritional state and stress exposure. Modulation of metal toxicity by stress exposure can occur through shared targeting of endocrine systems, such as the hypothalamic-pituitary-adrenal axis (HPA). Our previous rodent research has identified that prenatal stress (PS) modulates neurotoxicity of two endocrine active metals (EAMs), Pb and MeHg, by altering HPA and CNS systems disrupting behavior. Here, we review this research and further test the hypothesis that prenatal stress modulates metals neurotoxicity by expanding to test the effect of developmental As ± PS exposure. Serum corticosterone and behavior was assessed in offspring of dams exposed to As ± PS. PS increased female offspring serum corticosterone at birth, while developmental As exposure decreased adult serum corticosterone in both sexes. As + PS induced reductions in locomotor activity in females and reduced response rates on a Fixed Interval schedule of reinforcement in males, with the latter suggesting unique learning deficits only in the combined exposure. As-exposed males showed increased time in the open arms of an elevated plus maze and decreased novel object recognition whereas females did not. These data further confirm the hypothesis that combined exposure to chemical (EAMs) and non-chemical (PS) stressors results in enhanced neurobehavioral toxicity. Given that humans are exposed to multiple environmental risk factors that alter endocrine function in development, such models are critical for risk assessment and public health protection, particularly for children.

Mechanisms of Action Point Towards Combined PBDE/NDL-PCB Risk Assessment

At present, human risk assessment of the structurally similar non-dioxin-like (NDL) PCBs and polybrominated diphenylethers (PBDEs) is done independently for both groups of compounds. There are however obvious similarities between NDL-PCBs and PBDEs with regard to modulation of the intracellular calcium homeostasis (basal calcium levels, voltage-gated calcium channels, calcium uptake, ryanodine receptor) and thyroid hormone (TH) homeostasis (TH levels and transport). which are mechanisms of action related to neurobehavioral effects (spontaneous activity, habituation and learning ability). There also similarities in agonistic interactions with the hepatic nuclear receptors PXR and CAR. Several effects on developmental (reproductive) processes have also been observed, but results were more dispersed and insufficient to compare both groups of compounds. The available mechanistic information is sufficient to warrant a dose addition model for NDL-PCBs and PBDEs, including their hydroxylated metabolites.Although many of the observed effects are similar from a qualitative point of view for both groups, congener or tissue specific differences have also been found. As this is a source of uncertainty in the combined hazard and risk assessment of these compounds, molecular entities involved in the observed mechanisms and adverse outcomes associated with these compounds need to be identified. The systematical generation of (quantitative) structure-activity information for NDL-PCBs and PBDEs on these targets (including potential non-additive effects) will allow a more realistic risk estimation associated with combined exposure to both groups of compounds during early life. Additional validation studies are needed to quantify these uncertainties for risk assessment of NDL-PCBs and PBDEs.

PCB95 and PCB153 change dopamine levels and turn-over in PC12 cells

Polychlorinated biphenyls (PCB) exposure at low chronic levels is a significant public health concern. Animal and epidemiological studies indicate that low PCB body burden may cause neurotoxicity and be a risk factor for neurodegenerative diseases. In the current study, we measured the ability of two non-dioxin like PCBs, 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) and 2,2'3,5',6-pentachlorobiphenyl (PCB95), to alter dopamine (DA) levels and metabolism using the dopaminergic PC12 cell line. Our hypothesis is that treatment of PC12 cells with non-toxic concentrations of PCB153 or PCB95 for 12 and 24 h will have different effects due to different congener structures. Levels of DA and of 3,4-dihydroxyphenylacetaldehyde (DOPAL), 3, 4-dihyroxylphenylethanol (DOPET), and 3,4-dihyroxylphenylacetic acid (DOPAC) metabolite, gene expression of the dopamine synthesis enzyme tyrosine hydroxylase (TH) and the vesicular monoamine transporter (VMAT2), and gene expression of the anti-oxidant enzymes Cu/Zn and Mn superoxide oxidase (Cu/ZnSOD, MnSOD), glutathione peroxidase (GPx) and catalase were determined. PCB153 decreased intracellular and extracellular levels of DA after 12 h exposure and this was consistent with an increase in DA metabolites. After 24 h, the level of DA in medium increased compared to the control. In contrast, PCB95 exposure increased the intracellular DA level and decreased DA in medium consistent with a down-regulation of VMAT2 expression at 12 h. After 24 h exposure, PCB95 increased DA levels in media. Expression of TH mRNA increased slightly following 12 h but not at 24 h exposure. MnSOD mRNA increased up to 6-7 fold and Cu/ZnSOD increased less than two-fold after treatment with both congeners. Catalase expression was up-regulated following 24 h exposure to PCB153 and PCB95, but GPx expression was down-regulated after 12 h exposure to PCB95 only. These results suggest that PCB153 and PCB95 are neurotoxic and affect DA turnover with structure-dependent differences between these two congeners.

Methylmercury impairs canonical dopamine metabolism in rat undifferentiated pheochromocytoma (PC12) cells by indirect inhibition of aldehyde dehydrogenase

The environmental neurotoxicant methylmercury (MeHg) disrupts dopamine (DA) neurochemical homeostasis by stimulating DA synthesis and release. Evidence also suggests that DA metabolism is independently impaired. The present investigation was designed to characterize the DA metabolomic profile induced by MeHg, and examine potential mechanisms by which MeHg inhibits the DA metabolic enzyme aldehyde dehydrogenase (ALDH) in rat undifferentiated PC12 cells. MeHg decreases the intracellular concentration of 3,4-dihydroxyphenylacetic acid (DOPAC). This is associated with a concomitant increase in intracellular concentrations of the intermediate metabolite 3,4-dihydroxyphenylaldehyde (DOPAL) and the reduced metabolic product 3,4-dihydroxyethanol. This metabolomic profile is consistent with inhibition of ALDH, which catalyzes oxidation of DOPAL to DOPAC. MeHg does not directly impair ALDH enzymatic activity, however MeHg depletes cytosolic levels of the ALDH cofactor NAD(+), which could contribute to impaired ALDH activity following exposure to MeHg. The observation that MeHg shunts DA metabolism along an alternative metabolic pathway and leads to the accumulation of DOPAL, a reactive species associated with protein and DNA damage, as well as cell death, is of significant consequence. As a specific metabolite of DA, the observed accumulation of DOPAL provides evidence for a specific mechanism by which DA neurons may be selectively vulnerable to MeHg.

Sex-dependent and non-monotonic enhancement and unmasking of methylmercury neurotoxicity by prenatal stress

Methylmercury (MeHg) and prenatal stress (PS) are risk factors for neurotoxicity that may co-occur in human populations. Because they also share biological substrates and can produce common behavioral deficits, this study examined their joint effects on behavioral and neurochemical effects in male and female rats. Dams had access to 0, 0.5 or 2.5ppm MeHg chloride drinking water from two to three weeks prior to breeding through weaning. Half of the dams in each of these treatment groups also underwent PS on gestational days 16-17. This yielded 6 groups/gender: 0-NS, 0-PS, 0.5-NS, 0.5-PS, 2.5-NS, and 2.5-PS. Behavioral testing began in young adulthood and included fixed interval (FI) schedule-controlled behavior, novel object recognition (NOR) and locomotor activity, behaviors previously demonstrated to be sensitive to MeHg and/or mediated by brain mesocorticolimbic dopamine glutamate systems targeted by both MeHg and PS. Behavioral deficits were more pronounced in females and included impaired NOR recognition memory only under conditions of combined MeHg and PS, while non-monotonic reductions in FI response rates occurred, with greatest effects at the 0.5ppm concentration; the less reduced 2.5ppm FI response rates were further reduced under conditions of PS (2.5-PS). Correspondingly, many neurochemical changes produced by MeHg were only seen under conditions of PS, particularly in striatum in males and in hippocampus and nucleus accumbens in females, regions of significance to the mediation of FI and NOR performance. Collectively these findings demonstrate sex-dependent and non-monotonic effects of developmental MeHg exposure that can be unmasked or enhanced by PS, particularly for behavioral outcomes in females, but for both sexes in neurochemical changes, that were observed at MeHg exposure concentrations that did not influence either reproductive outcomes or maternal behavior. Thus, assessment of risks associated with MeHg may be underestimated in the absence of other extant risk factors with which it may share common substrates and effects.

Behavioral changes following PCB 153 exposure in the spontaneously hypertensive rat - an animal model of Attention-Deficit/Hyperactivity Disorder

Background

Attention-Deficit/Hyperactivity Disorder (ADHD) is a behavioral disorder affecting 3-5% of children. Although ADHD is highly heritable, environmental factors like exposure during early development to various toxic substances like polychlorinated biphenyls (PCBs) may contribute to the prevalence. PCBs are a group of chemical industrial compounds with adverse effects on neurobiological and cognitive functioning, and may produce behavioral impairments that share significant similarities with ADHD. The present study examined the relation between exposure to PCB 153 and changes in ADHD-like behavior in an animal model of ADHD, the spontaneously hypertensive rats (SHR/NCrl), and in Wistar Kyoto (WKY/NHsd) controls.

Methods

SHR/NCrl and WKY/NHsd, males and females, were orally given PCB 153 dissolved in corn oil at around postnatal day (PND) 8, 14, and 20 at a dosage of 1, 3 or 6 mg/kg bodyweight at each exposure. The control groups were orally administered corn oil only. The animals were behaviorally tested for exposure effects from PND 37 to 64 using an operant procedure.

Results

Exposure to PCB 153 was associated with pronounced and long-lasting behavioral changes in SHR/NCrl. Exposure effects in the SHR/NCrl depended on dose, where 1 mg/kg tended to reduce ADHD-like behaviors and produce opposite behavioral effects compared to 3 mg/kg and 6 mg/kg, especially in the females. In the WKY/NHsd controls and for the three doses tested, PCB 153 exposure produced a few specific behavioral changes only in males. The data suggest that PCB 153 exposure interacts with strain and sex, and also indicate a non-linear dose–response relation for the behaviors observed.

Conclusions

Exposure to PCB 153 seems to interact with several variables including strain, sex, dose, and time of testing. To the extent that the present findings can be generalized to humans, exposure effects of PCB 153 on ADHD behavior depends on amount of exposure, where high doses may aggravate ADHD symptoms in genetically vulnerable individuals. In normal controls, exposure may not constitute an environmental risk factor for developing the full range of ADHD symptoms, but can produce specific behavioral changes.

Early-life exposure to methylmercury in wildtype and pdr-1/parkin knockout C. elegans

We examined the impact of early-life exposure to methylmercury (MeHg) on Caenorhabditis elegans (C. elegans) pdr-1 mutants, addressing gene-environment interactions. We tested the hypothesis that early-life exposure to MeHg and knockout (KO) of pdr-1 (mammalian: parkin/PARK2) exacerbates MeHg toxicity and damage to the dopaminergic (DAergic) system. pdr-1KO worms showed increased lethality and decreased lifespan following MeHg exposure. Mercury (Hg) content, measured with inductively coupled plasma-mass spectrometry was increased in pdr-1KO worms compared to wildtype (N2) controls. 2'7' dichlorodihydrofluorescein diacetate assay revealed a significant increase in reactive oxygen species in both strains following MeHg exposure; however, while N2 worms showed an increase in skn-1 transcript levels following MeHg exposure, there was no difference in skn-1 induction in pdr-1KO worms. Dopamine-dependent behavioral analysis revealed an effect of MeHg on N2 wildtype worms, but no effect on pdr-1KO worms. Taken together, these results suggest that pdr-1KO worms are more sensitive to MeHg than wildtype worms, but MeHg does not exacerbate behavioral changes related to the absence of pdr-1.

Platelet oxygen consumption as a peripheral blood marker of brain energetics in a mouse model of severe neurotoxicity

Interactions of chemicals with cerebral cellular systems are often accompanied by similar changes involving components in non-neural tissues. On this basis, indirect strategies have been developed to investigate neural cell function parameters by methods using accessible cells, including platelets and/or peripheral blood lymphocytes. Therefore, here it was investigated whether peripheral blood markers may be useful for assessing the central toxic effects of methylmercury (MeHg). For this purpose, we investigated platelet mitochondrial physiology in a well-established mouse model of MeHg-induced neurotoxicity, and correlated this peripheral activity with behavioural and central biochemical parameters. In order to characterize the cortical toxicity induced by MeHg (20 and 40 mg/L in drinking water, 21 days), the behavioral parameter namely, short-term object recognition, and the central mitochondrial impairment assessed by measuring respiratory complexes I-IV enzyme activities were determined in MeHg-poisoned animals. Neurotoxicity induced by MeHg exposure provoked compromised cortical activity (memory impairment) and reduced NADH dehydrogenase, complex II and II-III activities in the cerebral cortex. These alterations correlated with impaired systemic platelet oxygen consumption of intoxicated mice, which was characterized by reduced electron transfer activity and uncoupled mitochondria. The data brought here demonstrated that impaired systemic platelet oxygen consumption is a sensitive and non-invasive marker of the brain energy deficits induced by MeHg poisoning. Finally, brain and platelets biochemical alterations significantly correlated with cognitive behavior in poisoned mice. Therefore, it could be proposed the use of platelet oxygen consumption as a peripheral blood marker of brain function in a mouse model MeHg-induced neurotoxicity.

Prenatal exposure to polychlorinated biphenyls and child neuropsychological development in 4-year-olds: an analysis per congener and specific cognitive domain

Polychlorinated biphenyls (PCB) are synthetic organochlorine compounds with potential neurotoxic effects. Although negative effects on neuropsychological development have been observed in previous studies on PCB exposure, there are inconsistencies in these effects at current exposure levels of these compounds which are much lower than for previous generations. This study aimed to disentangle the effects of prenatal and postnatal PCB exposure on neuropsychological development at 4 years of age. This study is based on a population-based birth cohort design established in Menorca (Spain) as part of the INMA [Environment and Childhood] Project. We assessed general neuropsychological development using the McCarthy Scales of Children's Abilities (MCSA). A total of 422 4-year old children were assessed with the MCSA. Levels of PCBs were measured in cord blood (n=405) and in blood samples taken at 4 years (n=285). We found no statistically significant effects of the sum of prenatal PCBs on MCSA scores. Nevertheless, individual congener analyses yielded significant detrimental effects of prenatal PCB153 on the majority of MCSA scores, while no effects were reported for other congeners. The levels of PCBs at 4 years of age were not associated with neuropsychological development. Thus, prenatal exposure to low-level concentrations of PCBs, particularly PCB153, was associated with an overall deleterious effect on neuropsychological development at 4 years of age, including negative effects on executive function, verbal functions and visuospatial abilities, but not on motor development.

Alteration of serotonin system by polychlorinated biphenyls exposure

Although commercial production of polychlorinated biphenyls (PCBs) was banned in 1979, PCBs continue to be an environmental and health concern due to their high bioaccumulation and slow degradation rates. In fact, PCBs are still present in our food supply (fish, meat, and dairy products). In laboratory animals, exposure to single PCB congener or to mixtures of different congeners induces a variety of physiological alterations. PCBs cross the placenta and even exposure at low level is harmful for the foetus by leading to neurodevelopment alterations. Serotonin system which regulates many physiological functions from platelet activation to high cerebral processes and neurodevelopment is one of the targets of PCBs toxicity. The effects of PCBs exposure on serotonin system have been investigated although to a lesser extent compared to its effect in other neurotransmitter systems. This review provides a summary of the results concerning the impact of PCBs exposure (in vitro and in vivo) on serotonin system. Further research is needed to correlate specific deficits with PCB-induced changes in the serotonin system.

Application of Neurochemical Markers for Assessing Health Effects after Developmental Methylmercury and PCB Coexposure

Cholinergic muscarinic receptors (MRs) and monoamine oxidase activity (MAO-B), expressed both in brain and blood cells, were investigated in animals and exposed subjects to assess (i) MeHg (0.5–1 mg/kg/day GD7-PD7) and/or PCB153 (20 mg/kg/day GD10–GD16) effects on cerebellar MAO-B and MRs, and lymphocyte MRs, in dams and offspring 21 days postpartum; (ii) MAO-B in platelets and MRs in lymphocytes of a Faroese 7-year-old children cohort, prenatally exposed to MeHg/PCBs. Animal Data. MAO-B was altered in male cerebellum by MeHg, PCB153, and their combination (35%, 45%, and 25% decrease, resp.). Cerebellar MRs were enhanced by MeHg alone in dams (87%) and male pups (27%). PCB153 alone and in mixture did not modify cerebellar MRs. Similarly to brain, lymphocyte MRs were enhanced in both dams and offspring by MeHg alone. All changes were caused by 1 MeHg mg/kg/day, the lower dose was ineffective. Human Data. Both biomarkers showed homogeneous distributions within the cohort (MRs, range 0.1–36.78 fmol/million cells; MAO-B, 0.95–14.95 nmol/mg protein/h). No correlation was found between the two biomarkers and neurotoxicant concentrations in blood (pre- and postnatally).

Brain proteome alterations of Atlantic cod (Gadus morhua) exposed to PCB 153

Polychlorinated biphenyls (PCBs) are still widespread environmental pollutants that bioaccumulate and biomagnify in the aquatic food chains despite the ban on their production. They constitute a class of 209 possible congeners with different chlorination pattern of the biphenyl ring structure resulting in many different toxicities and mechanisms of toxicity. The neurotoxicity of PCBs is relatively poorly understood, and biomarkers for their neurotoxic effects are lacking. We have carried out a proteomic analysis of brain tissue from Atlantic cod (Gadus morhua) exposed to 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153, ortho-substituted and non-coplanar), a previously demonstrated neurotoxic congener and the most prevalent congener in biological samples. The fish received 0, 0.5, 2 and 8 mg/kg PCB 153 by intraperitoneal injection, half of the dose on the first day and the second half after one week, and were exposed for two weeks in total. Using a 2-DE approach we found 56 protein spots to be 20% or more (≤ 0.8-fold or ≥ 1.2-fold) significantly different between at least one of the three PCB 153-exposed groups and the control group, and 27 of these were identified by MALDI-TOF MS and MS/MS. Approximately 80% of the differentially regulated proteins may be associated with a non stressor-specific response and/or have previously been classified as notoriously differentially regulated in 2-DE/MS based proteomics studies, such as alterations/responses in energy metabolism, cytoskeleton, protein synthesis, protein degradation (ubiquitin-proteasome system), cellular growth, cycle and death (14-3-3 protein), and (surprisingly) axon guidance (dihydropyrimidinase-like 2 (=collapsin response mediator protein 2, CRMP-2)). The six remaining affected proteins include the strongest up-regulated protein, pyridoxal kinase (essential for synthesis of neurotransmitters such as dopamine, serotonin and GABA), nicotinamide phosphoribosyl-transferase (involved in protection against axonal degeneration) and protein phosphatase 1 (controls brain recovery by synaptic plasticity). The last three of these six proteins (deltex, Rab14 and sorting nexin 6) may preliminarily identify involvement of the Notch signaling pathway and endosomal function in PCB 153-induced neurotoxicity. Our findings constitute novel clues for further research on PCB 153 mode of action in brain, and a proper selection of proteins may, following validation, be applicable in a panel of biomarkers for aquatic environmental monitoring.

Postnatal exposure to PCB 153 and PCB 180, but not to PCB 52, produces changes in activity level and stimulus control in outbred male Wistar Kyoto rats

Background

Polychlorinated biphenyls (PCBs) are a class of organic compounds that bioaccumulate due to their chemical stability and lipophilic properties. Humans are prenatally exposed via trans-placental transfer, through breast milk as infants, and through fish, seafood and fatty foods as adolescents and adults. Exposure has several reported effects ranging from developmental abnormalities to cognitive and motor deficiencies. In the present study, three experimental groups of rats were orally exposed to PCBs typically found in human breast milk and then behaviorally tested for changes in measures of stimulus control (percentage lever-presses on the reinforcer-producing lever), activity level (responses with IRTs > 0.67 s), and responses with short IRTs (< 0.67 s).

Methods

Male offspring from Wistar Kyoto (WKY/NTac) dams purchased pregnant from Taconic Farms (Germantown, NY) were orally given PCB at around postnatal day 8, 14, and 20 at a dose of 10 mg/kg body weight at each exposure. Three experimental groups were exposed either to PCB 52, PCB 153, or PCB 180. A fourth group fed corn oil only served as controls. From postnatal day 25, for 33 days, the animals were tested for behavioral changes using an operant procedure.

Results

PCB exposure did not produce behavioral changes during training when responding was frequently reinforced using a variable interval 3 s schedule. When correct responses were reinforced on a variable interval 180 s schedule, animals exposed to PCB 153 or PCB 180 were less active than controls and animals exposed to PCB 52. Stimulus control was better in animals exposed to PCB 180 than in controls and in the PCB 52 group. Also, the PCB 153 and PCB 180 groups had fewer responses with short IRTs than the PCB 52 group. No effects of exposure to PCB 52 were found when compared to controls.

Conclusions

Exposure to PCBs 153 and 180 produced hypoactivity that continued at least five weeks after the last exposure. No effects of exposure to PCB 52 were observed.

Recent Advance in Biosensors for microRNAs Detection in Cancer

MicroRNAs (miRNAs) are short non-protein-coding RNA molecules that regulate the expression of a wide variety of genes. They act by sequence-specific base pairing in the 3′ untranslated region (3′UTR) of the target mRNA leading to mRNA degradation or translation inhibition. Recent studies have implicated miRNAs in a wide range of biological processes and diseases including development, metabolism and cancer, and revealed that expression levels of individual miRNAs may serve as reliable molecular biomarkers for cancer diagnosis and prognosis. Therefore, a major challenge is to develop innovative tools able to couple high sensitivity and specificity for rapid detection of miRNAs in a given cell or tissue. In this review, we focus on the latest innovative approaches proposed for miRNA profiling in cancer and discuss their advantages and disadvantages.

Discriminative stimulus effects of cocaine and amphetamine in rats following developmental exposure to polychlorinated biphenyls (PCBs)

Polychlorinated biphenyls (PCBs) are environmental neurotoxicants known to affect the brain dopaminergic (DA) system. This project investigated whether developmental exposure to PCBs would alter the discriminative stimulus effects of psychostimulant drugs known to act on the DA system. Female Long-Evans rats were orally exposed to 0, 3, or 6 mg/kg/day of an environmentally relevant PCB mixture from four weeks prior to breeding through weaning of their litters on PND 21. When they reached adulthood one male and female/litter were trained to discriminate cocaine (10.0 mg/kg, IP) from saline by repeatedly pairing cocaine injections with reinforcement on one operant response lever, and saline injections with reinforcement on the other lever. After response training, generalization tests to four lower doses of cocaine (7.5, 5.0, 2.5, and 1.25 mg/kg, IP) and to amphetamine (1.0, 0.5, 0.25, and 0.125 mg/kg, IP) were given two days/week, with additional training dose days in-between. Percent responding of the PCB-exposed rats on the cocaine-paired lever was significantly higher than that of controls for the highest generalization dose of cocaine, and lower than that of controls for the highest dose of amphetamine. Response rate and percent responding on the cocaine lever did not differ among the exposure groups on the days when the training dose of cocaine was given, suggesting that the generalization test results were not due to pre-existing differences in discrimination ability or rate of responding. These findings suggest that developmental PCB exposure can alter the interoceptive cues of psychostimulants.

Toxicology of Alkylmercury Compounds

Methylmercury is a global pollutant and potent neurotoxin whose abundance in the food chain mandates additional studies on the consequences and mechanisms of its toxicity to the central nervous system. Formulation of our new hypotheses was predicated on our appreciation for (a) the remarkable affinity of mercurials for the anionic form of sulfhydryl (-SH) groups, and (b) the essential role of thiols in protein biochemistry. The present chapter addresses pathways to human exposure of various mercury compounds, highlighting their neurotoxicity and potential involvement in neurotoxic injury and neurodegenerative changes, both in the developing and senescent brain. Mechanisms that trigger these effects are discussed in detail.

Early developmental effects of separate or combined perinatal exposure to methylmercury (MeHg) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) in the rat

OBJECTIVES

Methylmercury (MeHg) and polychlorinated biphenyls (PCBs) are ubiquitous and persistent environmental pollutants and food contaminants. Both are neurotoxic, especially for the developing nervous system.

MATERIAL AND METHODS

Female rats were exposed from day 7 of pregnancy up to day 21 after the delivery to MeHg in drinking water, PCB 153 per os or MeHg+PCB 153. Assessment of the exposure effects in mothers included food and water intake, body weight and reproduction success. Assessment of the progeny comprised determination of body weight, time of pinna detachment, eye opening, incisor eruption, and the negative geotaxis, grip strength and righting reflex.

RESULTS

The following effects of the exposures were observed: A) MeHg: 0.5 mg/kg/day - no effect on maternal health status and reproduction. In the progeny: faster incisor eruption and hastened negative geotaxis development. MeHg 2.0 mg/kg/day: In mothers: signs of MeHg toxicity (reduced food intake and body weight, ataxia) during lactation. In the progeny: reduced rate of body weight increase, accelerated incisor eruption but delayed development of the righting reflex. B) PCB 153 exposure: 1.0 mg/kg/day: no effect on maternal health status, reproduction success or morphological and physical development of the progeny; 5.0 mg/kg/day: no effect on maternal health status and reproduction. In the progeny: accelerated growth in females, faster pinna detachment and incisor eruption but delayed development of the grip strength. C) MeHg+PCB153 exposure: none overt effect was noted in mothers or in their progeny.

CONCLUSION

The results confirm the ability of a low level perinatal exposure to MeHg or PCB 153 to affect the early development in the rat. They have not provided, however, an evidence of a synergistic interaction of these contaminants. To the contrary, the results suggest that, at least under the conditions prevailing in the present study, MeHg and PCB 153 interact antagonistically.

Gene expression profiles in the brain of the neonate mouse perinatally exposed to methylmercury and/or polychlorinated biphenyls

Methylmercury (MeHg) and polychlorinated biphenyls (PCBs) are environmentally persistent neurodevelopmental toxicants. The primary source of human exposure is the consumption of contaminated fish, seafood and marine mammals. However, little is known about the molecular mechanisms of MeHg and PCB toxicities and interactions between these contaminants. We investigated the functional profiles of differently expressed genes in the brains of offspring mice perinatally exposed to MeHg and/or PCBs to elucidate how these contaminants interact with each other. Pregnant mice (C57BL/6) were divided into four groups by exposure: (1) vehicle control, (2) MeHg alone, (3) PCBs alone, (4) MeHg + PCBs. Gene expression analysis of the brains of offspring mice was carried out with 4 x 44 K whole mouse genome's microarrays (Agilent) on postnatal day 1. The gene expression pattern of the MeHg exposure-group differed from that of the PCB-exposure group. The MeHg + PCB group expressed a larger number of genes, most of which were not expressed in the MeHg group or PCB group. It was revealed that gene expression was greatly increased, and the most altered genes were found with co-exposure. The genes were related to the functional categories of development, inflammation, calcium ion homeostasis, signal transduction, the ubiquitin-proteasome pathway and detoxication. The ubiquitin-proteasome system and detoxication categories might function for protection against the toxicity induced by co-exposure to MeHg and PCBs. These results suggest that co-exposure does not simply exacerbate the toxicity of MeHg alone or PCB alone, but stimulates a protection system.

Methylmercury neurotoxicity is associated with inhibition of the antioxidant enzyme glutathione peroxidase

In this study, we investigated the involvement of glutathione peroxidase-GPx in methylmercury (MeHg)-induced toxicity using three models: (a) in mouse brain after treatment with MeHg (40 mg/L in drinking water), (b) in mouse brain mitochondrial-enriched fractions isolated from MeHg-treated animals, and (c) in cultured human neuroblastoma SH-SY5Y cells. First, adult male Swiss mice exposed to MeHg for 21 days showed a significant decrease in GPx activity in the brain and an increase in poly(ADP-ribose) polymerase cleavage, an index of apoptosis. Second, in mitochondrial-enriched fractions isolated from MeHg-treated mice, there was a significant reduction in GPx activity and a concomitant decrease in mitochondrial activity and increases in ROS formation and lipid peroxidation. Incubation of mitochondrial-enriched fractions with mercaptosuccinic acid, a GPx inhibitor, significantly augmented the toxic effects of MeHg administered in vivo. Incubation of mitochondrial-enriched fractions with exogenous GPx completely blocked MeHg-induced mitochondrial lipid peroxidation. Third, SH-SY5Y cells treated for 24 h with MeHg showed a significant reduction in GPx activity. There was a concomitant significant decrease in cell viability and increase in apoptosis. Inhibition of GPx substantially enhanced MeHg toxicity in the SH-SY5Y cells. These results suggest that GPx is an important target for MeHg-induced neurotoxicity, presumably because this enzyme is essential for counteracting the pro-oxidative effects of MeHg both in vitro and in vivo.

No changes in lymphocyte muscarinic receptors and platelet monoamine oxidase-B examined as surrogate central nervous system biomarkers in a Faroese children cohort prenatally exposed to methylmercury and polychlorinated biphenyls

Experimental evidence suggests that monoamine oxidase B (MAO-B) and muscarinic cholinergic receptors (mAChRs) are involved in the pathogenesis of neurotoxicity caused by methylmercury and polychlorinated biphenyls (PCBs). Blood samples from 7-year-old exposed children were analyzed for platelet MAO-B and lymphocyte mAChRs as potential markers of exposure to these neurotoxicants. The blood neurotoxicity biomarkers were compared with prenatal and current exposures and with neuropsychological test results. Both biomarkers showed homogeneous distributions within this cohort (mAChR, range 0.04-36.78 fmol/million cells; MAO-B, 0.95-14.95 nmol mg(-1) protein h(-1)). No correlation was found between the two biomarkers and either blood neurotoxicant concentrations or clinical findings. MAO-B and mAChR sensitivity may not be sufficiently high to assess early, subclinical responses to low/moderate methylmercury and/or PCB exposure, whereas these markers are significantly altered in sustained exposure scenarios, as shown by clinical studies in drug addicts or patients treated with psychopharmacological agents.

Developmental exposure to PCBs and/or MeHg: effects on a differential reinforcement of low rates (DRL) operant task before and after amphetamine drug challenge

The current study assessed the effects of developmental PCB and/or MeHg exposure on an operant task of timing and inhibitory control and determined if amphetamine (AMPH) drug challenges differentially affected performance. Long-Evans rats were exposed to corn oil (control), PCBs alone (1 or 3 mg/kg), MeHg alone (1.5 or 4.5 ppm), the low combination (1 mg/kg PCBs+1.5 ppm MeHg), or the high combination (3 mg/kg PCBs+4.5 ppm MeHg) throughout gestation and lactation. An environmentally relevant, formulated PCB mixture was used. Male and female offspring were trained to asymptotic performance on a differential reinforcement of low rates (DRL) operant task as adults. PCB-exposed groups had a lower ratio of reinforced to non-reinforced responses than controls. Groups exposed to MeHg alone were not impaired and the deficits observed in PCB-exposed groups were not seen when PCBs were co-administered with MeHg. AMPH was less disruptive to responding in males receiving PCBs alone, MeHg alone, and 1.0 mg/kg PCB+1.5 ppm MeHg. Paradoxically, the disruption in responding by AMPH in males given 3.0 mg/kg PCB+4.5 ppm MeHg did not differ from controls. Exposed females from all treatment groups did not differ from controls in their AMPH response. Overall, the findings suggest that developmental exposure to PCBs can decrease DRL performance. Co-exposure to MeHg seemed to mitigate the detrimental effects of PCBs on performance. The finding that the disruptive effects of AMPH on DRL performance were lessened in some groups of exposed males suggests that alterations in dopaminergic functioning may have a role in behavioral changes seen after perinatal PCB and MeHg exposure.

Developmental exposure to polychlorinated biphenyls or methylmercury, but not to its combination, impairs the glutamate-nitric oxide-cyclic GMP pathway and learning in 3-month-old rats

Prenatal exposure to polychlorinated biphenyls (PCBs) or methylmercury (MeHg) contaminated food may affect brain development, leading to long-term alterations in cognitive function. Both types of contaminants, PCBs and MeHg, are often found together contaminating food, especially fish in some polluted areas. Exposure to combinations of neurotoxicants may exert different effects on the developing nervous system than exposure to individual contaminants. Developmental exposure (during pregnancy and lactation) to PCB126 or PCB153 impairs learning ability when the rats are 3 months old. Impairment of learning seems to be a consequence of impairment of the function of the glutamate-nitric oxide (NO)-cGMP pathway in brain in vivo. The aims of the present work were 1) to assess whether perinatal exposure to MeHg also affects the function of the glutamate-NO-cGMP pathway in brain in vivo analyzed by in vivo brain microdialysis and/or the ability to learn the Y maze task when the rats are 3 months old, and 2) to assess whether perinatal exposure to combinations of MeHg with PCB153 or PCB126 potentiates, decreases or does not modify the effects of the individual neurotoxicants. Perinatal exposure to PCB126, PCB153 or MeHg impaired the function of the glutamate-NO-cGMP pathway in cerebellum and learning ability. However, co-exposure to PCB126+MeHg or PCB153+MeHg inhibits the impairment of the pathway or learning ability. These results support that the function of this pathway modulates learning of the Y maze task. Moreover, they show that co-exposure to these PCBs and MeHg does not exacerbate, but reduces the effects on the ability to learn this task.

Neurodevelopmental toxicity of methylmercury: Laboratory animal data and their contribution to human risk assessment

Methylmercury (MeHg) is one of the most significant public health hazards. The clinical findings in the victims of the Japanese and Iraqi outbreaks have disclosed the pronounced susceptibility of the developing brain to MeHg poisoning. This notion has triggered worldwide scientific attention toward the long-term consequences of prenatal exposure on child development in communities with chronic low level dietary exposure. MeHg neurodevelopmental effects have been extensively investigated in laboratory animals under well-controlled exposure conditions. This article provides an updated overview of the main neuromorphological and neurobehavioral changes reported in non-human primates and rodents following developmental exposure to MeHg. Different aspects of MeHg's effects on the immature organism are reported, with particular reference to the delayed onset of symptoms and the persistency of central nervous system (CNS) injury/dysfunction. Particular attention is paid to the comparative toxicity assessment across species, and to the degree of concordance/discordance between human and animal data. The contribution of animal studies to define the role of potential effect modifiers and variables on MeHg dose-response relationships is also addressed. The ultimate goal is to discuss the relevance of laboratory animal results, as a complementary tool to human data, with regard to the human risk assessment process.

Human developmental neurotoxicity of methylmercury: impact of variables and risk modifiers

Methylmercury (MeHg) is a widespread environmental and food toxicant which has long been known to affect neurodevelopment in both humans and experimental animals. Risk assessment for MeHg is mainly based on human data coming from the massive episodes of poisoning in Japan and Iraq, as well as from large scale epidemiological studies concerning childhood development and neurotoxicity in relation to in utero exposure in various fish eating communities around the world. Despite the extensive literature and research, the threshold dose for MeHg neurotoxic effects is still unclear, in particular when it comes to subtle effects on neurobehaviour. In this article clinical and epidemiological findings concerning the neurodevelopmental toxicity of MeHg are reviewed. Much attention is focussed on the potential impact of factors, such as diet and nutrition, gender, pattern of exposure and co-exposure to other neurotoxic pollutants, which may modulate MeHg toxic effects. These factors, together with the notion that some symptoms may ensue or exacerbate with aging, contribute to the difficulties in the definition of safe levels for developmental exposure.

Neurobehavioural and molecular changes induced by methylmercury exposure during development

There is an increasing body of evidence on the possible environmental influence on neurodevelopmental and neurodegenerative disorders. Both experimental and epidemiological studies have demonstrated the distinctive susceptibility of the developing brain to environmental factors such as lead, mercury and polychlorinated biphenyls at levels of exposure that have no detectable effects in adults. Methylmercury (MeHg) has long been known to affect neurodevelopment in both humans and experimental animals. Neurobehavioural effects reported include altered motoric function and memory and learning disabilities. In addition, there is evidence from recent experimental neurodevelopmental studies that MeHg can induce depression-like behaviour. Several mechanisms have been suggested from in vivo- and in vitro-studies, such as effects on neurotransmitter systems, induction of oxidative stress and disruption of microtubules and intracellular calcium homeostasis. Recent in vitro data show that very low levels of MeHg can inhibit neuronal differentiation of neural stem cells. This review summarises what is currently known about the neurodevelopmental effects of MeHg and consider the strength of different experimental approaches to study the effects of environmentally relevant exposure in vivo and in vitro.