Developmental exposure to methylmercury and 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB153) affects cerebral dopamine D1-like and D2-like receptors of weanling and pubertal rats

Stochastic marine ecosystem modeling for statistical interpretation of the uncertainty factors used for the determination of tolerable daily intake of polychlorinated biphenyls

This study developed a numerical simulation model for a marine ecosystem to determine accumulation levels of polychlorinated biphenyls (PCBs) in top predators (seabasses). The simulated results were used for calculating probabilities of exceeding tolerable daily intake (TDI) of seafoods enforced by the US and Japanese governments. The model was applied to Tokyo Bay, Japan. Some uncertain parameters used in the model equations were specified stochastically to enable one to produce the probability distributions of the concentrations of accumulated PCB. The calculated probability density functions (PDFs) moderately well reproduced the actual frequency distribution. The daily intakes of PCB via seafood were lower than the TDI in most of the tested years, while they were greater in some of the tested years. The whole ranges of PCB intake were divided into some partial ranges bounded by the PCB intakes determined by dividing the lowest-observed-adverse-effect level (LOAEL) by the uncertainty factors. The cumulative probabilities of the PDFs were computed for these partial ranges to quantify the contributions of each of the uncertainty factors to the safety enhancement. In 2000s, the effect from the uncertainty factor for the conversion from LOAEL to the no-observed-adverse-effect level (NOAEL) was the greatest, and in 2010s and later, uncertainty factors for the interspecies and intraspecies exhibited the most remarkable effect.

Biomass-based stochastic model for the transfer of polychlorinated biphenyls in a marine food web

Clarifying the state of marine pollution is important for evaluating the health risk to predators in the marine food web such as fishes and mammals. Although previous studies have generally modeled marine pollution targeting a single organism, this study focused on the transfer of marine pollution in terms of ecological network. In this study, a biomass-based stochastic model was constructed for the transfer of 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) in a coupled pelagic-benthic system targeting populations of species. Incorporating the populations of species allows for discussions on predation pressure and anthropogenic activities such as fishing. The biomass parameters representing populations of species were set by using published data on the ecosystem in Tokyo Bay, Japan and were represented by probability density distributions to reflect population uncertainty. The PCB153 concentrations computed by the model were then compared with observations for validation. The results showed that the probability density distributions of PCB153 concentrations in marine species had a long tail to the right, which indicated the occurrence probabilities of individual organisms having considerably high contamination levels. A theoretical analysis was performed to quantify the response of species (e.g., fish, plankton, and benthos) to PCB153 accumulation accounting for population uncertainty, which indicated that a reduction in the population of predator species may contribute to the occurrence of high PCB153 concentrations in individual organisms in other species. The obtained probability density distributions were then used to compare the PCB153 human intake through ingesting seafood with the human's acceptable daily intake. The model can quantitatively demonstrate the probability distributions of PCB153 concentrations in marine species and PCB153 intake by humans through seafood, to which more attention should be paid.

Molecular Mechanisms of Environmental Metal Neurotoxicity: A Focus on the Interactions of Metals with Synapse Structure and Function

Environmental exposure to neurotoxic metals and metalloids such as arsenic, cadmium, lead, mercury, or manganese is a global health concern affecting millions of people worldwide. Depending on the period of exposure over a lifetime, environmental metals can alter neurodevelopment, neurobehavior, and cognition and cause neurodegeneration. There is increasing evidence linking environmental exposure to metal contaminants to the etiology of neurological diseases in early life (e.g., autism spectrum disorder) or late life (e.g., Alzheimer’s disease). The known main molecular mechanisms of metal-induced toxicity in cells are the generation of reactive oxygen species, the interaction with sulfhydryl chemical groups in proteins (e.g., cysteine), and the competition of toxic metals with binding sites of essential metals (e.g., Fe, Cu, Zn). In neurons, these molecular interactions can alter the functions of neurotransmitter receptors, the cytoskeleton and scaffolding synaptic proteins, thereby disrupting synaptic structure and function. Loss of synaptic connectivity may precede more drastic alterations such as neurodegeneration. In this article, we will review the molecular mechanisms of metal-induced synaptic neurotoxicity.

Large chromosomal deletions and impaired homologous recombination repairing in HEK293T cells exposed to polychlorinated biphenyl 153

Background

Polychlorinated biphenyls (PCBs) are persistent pollutants with carcinogenesis and mutagenesis effects which have been closely associated with PCBs-induced DNA damage. However, the detailed DNA damage events and corresponding pathway alterations under PCBs poisoning is still not well understood.

Methods

Whole-genome sequencing (WGS) and RNA sequencing (RNA-seq) were used to explore genome wide variations and related pathway changes in HEK293T cells that challenged by 15 µM PCB153 for 96 h in vitro. Double strand breaks (DSBs) were measured by 53BP1 foci detection, altered pathways were confirmed by quantitative real-time PCR (qPCR).

Results

The results indicated that abundant copy number variations (CNVs), including four duplications and 30 deletions, occurred in PCB153-exposed HEK293T cells. Multiple large fragment deletions (>1 Mb) involving up to 245 Mb regions on many chromosomes. Missense mutations were found in six tumor susceptibility genes, two of which are key members participating in homologous recombination (HR) repair response, BRCA1 and BRCA2. RNA-seq data showed that PCB153 poisoning apparently suppressedHR repairing genes. Besides, 15 µM PCB153 exposure significantly increased 53BP1 foci formation and effectively reduced BRCA1, RAD51B and RAD51C expression, indicating an elevated DSBs and impaired HR repairing.

Conclusion

This study firstly reported multiple large chromosomal deletions and impaired HR repairing in PCB153-exposed HEK293T cells, which provided a new insight into the understanding of early response and the mechanism underlying PCB153 genotoxicity. The chromosomal instabilities might be related to the impaired HR repairing that induced by PCB153; however, further investigations, especially on actual toxic effects of human body, are needed to confirm such speculation.

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.

Chronic exposure to methylmercury induces puncta formation in cephalic dopaminergic neurons in Caenorhabditis elegans

The neurotransmitter dopamine is a neuromodulator in the positive and negative regulation of brain circuits. Dopamine insufficiency or overload has been implicated in aberrant activities of neural circuits that play key roles in the pathogenesis of neurological and psychiatric diseases. Dopaminergic neurons are vulnerable to environmental insults. The neurotoxin methylmercury (MeHg) produces dopaminergic neuron damage in rodent as well as in Caenorhabditis elegans (C. elegans) models. Previous studies have demonstrated the utility of C. elegans as an alternative and complementary experimental model in dissecting out mechanism of MeHg-induced dopaminergic neurodegeneration. However, a sensitive pathological change that marks early events in neurodegeneration induced by environmental level of MeHg, is still lacking. By establishing a chronic exposure C. elegans model, for the first time, we have shown the propensity of MeHg (5 μM, 10 days) to induce bright puncta of dat-1::mCherry aggreagtes in the dendrites of cephalic (2 CEPs) dopaminergic neurons in a dose- and time-dependent manner, while these changes were not found in other dopaminergic neurons: anterior deirids (2 ADEs) and posterior deirids (2 PDEs), cholinergic neurons (2 AIYs) or glutamatergic neurons (2 PVDs). The bright puncta appear as an aggregation of mCherry proteins accumulating in dendrites. Further staining shows that the puncta were not inclusions in lysosome, or amyloid protein aggregates. In addition, features of the puncta including enlarged sphere shape (0.5-2 μm diameters), bright and accompanying with the shrinkage of the dendrite suggest that the puncta are likely composed of homologous mCherry molecules packaged at the dendritic site for exportation. Moreover, in the glutathione S-transferase 4 (gst-4) transcriptional reporter strain and RT-PCR assay, the expression levels of gst-4 and tubulins (tba-1 and tba-2) genes were not significantly modified under this chronic exposure paradigm, but gst-4 did show significant changes in an one day exposure paradigm. Collectively, these results suggest that CEP dopaminergic neurons are a sensitive target of MeHg, and the current exposure paradigm could be used as a model to investigate mechanism of dopaminergic neurotoxicity.

Towards Therapeutic Alternatives for Mercury Neurotoxicity in the Amazon: Unraveling the Pre-Clinical Effects of the Superfruit Açaí (Euterpe oleracea, Mart.) as Juice for Human Consumption

Methylmercury (MeHg) exposure is a serious problem of public health, especially in the Amazon. Exposure in riverine populations is responsible for neurobehavioral abnormalities. It was hypothesized that consumption of Amazonian fruits could protect by reducing mercury accumulation. This work analyzed the effects of commercial samples of Euterpe oleracea (EO) for human consumption (10 μL/g) against MeHg i.p. exposure (2.5 mg/Kg), using neurobehavioral (open field, rotarod and pole tests), biochemical (lipid peroxidation and nitrite levels), aging-related (telomerase reverse transcriptase (TERT) mRNA expression) and toxicokinetic (MeHg content) parameters in mice. Both the pole and rotarod tests were the most sensitive tests accompanied by increased lipid peroxidation and nitrite levels in brains. MeHg reduced TERT mRNA about 50% demonstrating a strong pro-aging effect. The EO intake, similar to that of human populations, prevented all alterations, without changing the mercury content, but avoiding neurotoxicity and premature aging of the Central Nervous System (CNS). Contrary to the hypothesis found in the literature on the possible chelating properties of Amazonian fruits consumption, the effect of EO would be essentially pharmacodynamics, and possible mechanisms are discussed. Our data already support the regular consumption of EO as an excellent option for exposed Amazonian populations to have additional protection against MeHg intoxication.

Alterations to the Intestinal Microbiome and Metabolome of Pimephales promelas and Mus musculus Following Exposure to Dietary Methylmercury

Mercury is a global contaminant, which may be microbially transformed into methylmercury (MeHg), which bioaccumulates. This results in potentially toxic body burdens in high trophic level organisms in aquatic ecosystems and maternal transfer to offspring. We previously demonstrated effects on developing fish including hyperactivity, altered time-to-hatch, reduced survival, and dysregulation of the dopaminergic system. A link between gut microbiota and central nervous system function in teleosts has been established with implications for behavior. We sequenced gut microbiomes of fathead minnows exposed to dietary MeHg to determine microbiome effects. Dietary exposures were repeated with adult CD-1 mice. Metabolomics was used to screen for metabolome changes in mouse brain and larval fish, and results indicate effects on lipid metabolism and neurotransmission, supported by microbiome data. Findings suggest environmentally relevant exposure scenarios may cause xenobiotic-mediated dysbiosis of the gut microbiome, contributing to neurotoxicity. Furthermore, small-bodied teleosts may be a useful model species for studying certain types of neurodegenerative diseases, in lieu of higher vertebrates.

Neurotransmitter amines and antioxidant agents in neuronal protection against methylmercury-induced cytotoxicity in primary cultures of mice cortical neurons

Methylmercury (MeHg) is an environmental toxicant with detrimental effects on the developing brain and adult nervous system. The main mechanisms identified include oxidative stress, changes in intracellular calcium, mitochondrial changes, inhibition of glutamate uptake, of protein synthesis and disruption of microtubules. However, little is known about mechanisms of protection against MeHg neurotoxicity. We found that resveratrol (10 μM) and ascorbic acid (200 μM) protected MeHg-induced cell death in primary cultures of cortical neurons. In this work, we aimed at finding additional targets that may be related to MeHg mode of action in cell toxicity with special emphasis in cell protection. We wonder whether neurotransmitters may affect the MeHg effects on neuronal death. Our findings show that neurons exposed to low MeHg concentrations exhibit less mortality if co-exposed to 10 μM dopamine (DA). However, DA metabolites, HVA (homovanillic acid) and DOPAC (3,4-dihydroxyphenylacetic acid) are not responsible for such protection. Furthermore, both DA D1 and D2 receptors agonists showed a protective effect against MeHg toxicity. It is striking though that DA receptor antagonists SKF83566 (10 μM) and haloperidol (10 μM) did not inhibit DA protection against MeHg. In addition, the protective effect of 10 μM DA against MeHg-induced toxicity was not affected by additional organochlorine pollutants exposure. Our results also demonstrate that cells exposed to MeHg in presence of 100 μM acetylcholine (ACh), show an increase in cell mortality at the "threshold value" of 100 nM MeHg. Finally, norepinephrine (10 μM) and serotonin (20 μM) also had an effect on cell protection. Altogether, we propose to further investigate the additional mechanisms that may be playing an important role in MeHg-induced cytotoxicity.

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.

Fish pollutants MeHg and Aroclor cause permanent structural damage in male gonads and kidneys after prepubertal exposure

This study investigated whether or not prepubertal exposure to the fish contaminants methylmercury (MeHg) and the polychlorinated bisphenol Aroclor in low doses interferes with the histomorphometry of the testes, epididymis, liver and kidneys in rats. Wistar male rats, 21 days old, were allocated into the following: control (n = 17, received corn oil), MeHg (n = 17, received MeHg at 0.5 mg/kg/day), Aroclor (n = 17, received Aroclor at 1.0 mg/kg/day), low mix (n = 18, received MeHg at 0.05 mg/kg/day and Aroclor at 0.1 mg/kg/day), high mix (n = 18, received MeHg at 0.5 mg/kg/day and Aroclor at 1.0 mg/kg/day). Dosing continued from post natal day (PND) 23 to 53, by gavage. Euthanasia was performed on PND 53; or, after an interval of 62 days without exposure to chemicals, on PND 115. The degree of maturation of the seminiferous epithelium was delayed in chemical-exposed groups and testicular interstitial oedema was observed at adulthood. The pattern of male gonad organization was changed in the Aroclor group on PND 53 and in all treated groups at adulthood. The animals from Aroclor, low mix and high mix groups showed a reduction in the number of Sertoli cells. Histological evidence of renal injury was observed in all chemical-exposed groups in both ages. A probable target for MeHg and Aroclor in the reproductive system was Sertoli cells, in which possible dysfunctions could be linked to the other testicular alterations. Curiously, the main deleterious effects were late outcomes, along with the absence of synergistic interaction of MeHg and Aroclor in the parameters investigated. In conclusion, fish pollutants MeHg and Aroclor caused permanent structural damage in male gonads and kidneys after prepubertal exposure, without showing clear chemical interactions.

Delayed effects of methylmercury on the mitochondria of dopaminergic neurons and developmental toxicity in zebrafish larvae (Danio rerio)

Methylmercury (MeHg) is a known neurotoxicant affecting the central nervous system but effects on dopaminergic (DA) neurons are not well understood. Wild-type zebrafish (Danio rerio) and two transgenic lines: Tg(dat:eGFP) expressing enhanced green fluorescent protein (eGFP) in DA neuron clusters and Tg(dat:tom20 MLS-mCherry) expressing red fluorescence (mCherry) targeted to mitochondria of DA neurons were used to evaluate the effects of micromolar MeHg exposure on DA neuron and whole animal motor function during early development. Three-day-old larvae were exposed to micromolar concentrations of MeHg (0.03, 0.06, and 0.3μM) in system water. Exposure to 0.3μM MeHg caused mortality and significant morphological abnormalities including edema, curvature of the spine, and hemorrhages in zebrafish larvae after a 48h exposure period. At 0.06μM MeHg, the appearance of morphological abnormalities was delayed for 72h and far less severe, whereas 0.03μM MeHg did not cause any morphological defects or mortalities. A delayed but significant reduction in locomotor ability and mCherry fluorescence in specific brain regions in the 0.06μM MeHg exposed larvae suggests that DA neuron function rather than neuron numbers was compromised. Double immunolabeling with tyrosine hydroxylase and pan neural staining showed no effect of MeHg exposure. We have established Tg(dat:tom20 MLS-mCherry) zebrafish larvae as a model which can be used to assess MeHg neurotoxicity and that exposure to low dose MeHg (0.06μM) during development may predispose DA neurons to impairment caused by changes in mitochondrial dynamics.

A comparison of presynaptic and postsynaptic dopaminergic agonists on inhibitory control performance in rats perinatally exposed to PCBs

Polychlorinated Biphenyls (PCBs) are very stable environmental contaminants whose exposure induces a number of health and cognitive concerns. Currently, it is well known that PCB exposure leads to poor performance on inhibitory control tasks. It is also well known that dopamine (DA) depletion within medial prefrontal cortex (mPFC) leads to poor performance on inhibitory control tasks. However, what is not well established is whether or not the inhibitory control problems found following PCB exposure are mediated by DA depletion in mPFC. This study was an investigation into the link between perinatal exposure to PCBs, the effect of this exposure on DA neurotransmission in the mPFC, and inhibitory-control problems during adulthood using a rodent model. The current study served to determine if microinjections of different DA agonists (the presynaptic DA transporter inhibitor and vesicular monoamine transporter agonist bupropion, the postsynaptic DA receptor 2 (DAD2) agonist quinpirole, and the postsynaptic DA receptor 1 (DAD1) agonist SKF81297) directly into the mPFC would differentially improve performance on an inhibitory control task in rats perinatally exposed to an environmentally relevant PCB mixture. Findings suggest several significant sex-based differences on differential reinforcement of low rates (DRL) 15 performance as well as some evidence of differential effectiveness of the DA agonists based on PCB exposure group.

Endocrine-disrupting actions of PCBs on brain development and social and reproductive behaviors

Polychlorinated biphenyls are among the most well-studied endocrine-disrupting chemicals (EDCs) for their neurobehavioral effects, especially neurodevelopment and cognitive performance. In addition, past research has demonstrated effects of PCBs on circulating hormones and associated changes in reproductive behaviors. This article will focus on recent advances that have been made in characterizing developmental PCB effects on reproductive function, broader social and affective behaviors, and the neuroendocrine mechanisms behind such changes. In general, PCBs seem to inhibit reproductive function by suppressing multiple aspects of the associated hypothalamic circuitry. Additionally, PCBs may also reduce motivation for social behaviors and induce depressive-like symptoms via overall reductions in dopaminergic and glutamatergic functions in the limbic system. However, more work with human-relevant exposure paradigms is needed to fully support these conclusions.

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.

Genetic polymorphisms of catechol-O-methyltransferase modify the neurobehavioral effects of mercury in children

Mercury (Hg) is neurotoxic and children may be particularly susceptible to this effect. A current major challenge is identification of children who may be uniquely susceptible to Hg toxicity because of genetic disposition. This study examined the hypothesis that genetic variants of catechol-O-methyltransferase (COMT) that are reported to alter neurobehavioral functions that are also affected by Hg in adults might modify the adverse neurobehavioral effects of Hg exposure in children. Five hundred and seven children, 8-12 yr of age at baseline, participated in a clinical trial to evaluate the neurobehavioral effects of Hg from dental amalgam tooth fillings. Subjects were evaluated at baseline and at seven subsequent annual intervals for neurobehavioral performance and urinary Hg levels. Following the clinical trial, genotyping assays were performed for single-nucleotide polymorphisms (SNPs) of COMT rs4680, rs4633, rs4818, and rs6269 on biological samples provided by 330 of the trial participants. Regression-modeling strategies were employed to evaluate associations between allelic status, Hg exposure, and neurobehavioral test outcomes. Similar analysis was performed using haplotypes of COMT SNPs. Among girls, few interactions for Hg exposure and COMT variants were found. In contrast, among boys, numerous gene-Hg interactions were observed between individual COMT SNPs, as well as with a common COMT haplotype affecting multiple domains of neurobehavioral function. These findings suggest increased susceptibility to the adverse neurobehavioral effects of Hg among children with common genetic variants of COMT, and may have important implications for strategies aimed at protecting children from the potential health risks associated with Hg exposure.

Stimulation-evoked dopamine release in the nucleus accumbens following cocaine administration in rats perinatally exposed to polychlorinated biphenyls

Exposure to polychlorinated biphenyls (PCBs) alters brain dopamine (DA) concentrations and DA receptor/transporter function, suggesting the reinforcing properties of drugs of abuse acting on the DA system may be affected by PCB exposure. Female Long-Evans rats were orally exposed to 0, 3, or 6 mg/kg/day PCBs from 4 weeks prior to breeding until litters were weaned on postnatal day 21. In vivo fixed potential amperometry (FPA) was used in adult anesthetized offspring to determine whether perinatal PCB exposure altered (1) presynaptic DA autoreceptor (DAR) sensitivity, (2) electrically evoked nucleus accumbens (NAc) DA efflux following administration of cocaine, and (3) the rate of depletion of presynaptic DA stores. One adult male and female littermate were tested using FPA following a single injection of cocaine (20 mg/kg ip), whereas a second adult male and female littermate were tested following the last of seven daily cocaine injections of the same dose. The carbon fiber recording microelectrode was positioned in the NAc core, and DA oxidation currents (i.e., DA release) evoked by brief stimulation of the medial forebrain bundle (MFB) were quantified before and after administration of cocaine. PCB-exposed rats exhibited enhanced stimulation-evoked DA release (relative to baseline) following a single injection of cocaine. Although nonexposed controls exhibited typical DA sensitization following repeated cocaine administration, this effect was attenuated in PCB-exposed rats. In addition, DAR sensitivity was higher (males only), and the rate of depletion of presynaptic DA stores was greater in PCB-exposed animals relative to nonexposed controls. These results indicate that perinatal PCB exposure can modify DA synaptic transmission in the NAc in a manner previously shown to alter the reinforcing properties of cocaine.

Developmental exposure to polychlorinated biphenyls reduces amphetamine behavioral sensitization in Long-Evans rats

PCBs have long been known to affect dopamine (DA) function in the brain. The current study used an amphetamine behavioral sensitization paradigm in rats developmentally exposed to PCBs. Long-Evans rats were given perinatal exposure to 0, 3, or 6mg/kg/day PCBs and behavioral sensitization to d-amphetamine (AMPH) was assessed in one adult male and female/litter. Non-exposed (control) males showed increasing locomotor activity to repeated injections of 0.5mg/kg AMPH, typical of behavioral sensitization. PCB-exposed males showed greater activation to the initial acute AMPH injection, but sensitization occurred later and was blunted relative to controls. Sensitization in control females took longer to develop than in the males, but no exposure-related differences were observed. Analysis of whole brain and serum AMPH content following a final IP injection of 0.5mg/kg revealed no differences among the exposure groups. Overall, these results indicated developmental PCB exposure can alter the motor-stimulating effects of repeated AMPH injections. Males developmentally exposed to PCBs appeared to be pre-sensitized to AMPH, but quickly showed behavioral tolerance to the same drug dose. Results also revealed the behavioral effect was not due to exposure-induced alterations in AMPH metabolism following PCB exposure.

S-Mercuration of rat sorbitol dehydrogenase by methylmercury causes its aggregation and the release of the zinc ion from the active site

We previously developed a screening method to identify proteins that undergo aggregation through S-mercuration by methylmercury (MeHg) and found that rat arginase I is a target protein for MeHg (Kanda et al. in Arch Toxicol 82:803-808, 2008). In the present study, we characterized another S-mercurated protein from a rat hepatic preparation that has a subunit mass of 42 kDa, thereby facilitating its aggregation. Two-dimensional SDS-polyacrylamide gel electrophoresis and subsequent peptide mass fingerprinting using matrix-assisted laser desorption and ionization time-of-flight mass spectrometry revealed that the 42 kDa protein was NAD-dependent sorbitol dehydrogenase (SDH). With recombinant rat SDH, we found that MeHg is covalently bound to SDH through Cys44, Cys119, Cys129 and Cys164, resulting in the inhibition of its catalytic activity, release of zinc ions and facilitates protein aggregation. Mutation analysis indicated that Cys44, which ligates the active site zinc atom, and Cys129 play a crucial role in the MeHg-mediated aggregation of SDH. Pretreatment with the cofactor NAD, but not NADP or FAD, markedly prevented aggregation of SDH. Such a protective effect of NAD on the aggregation of SDH caused by MeHg is discussed.

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).