Developmental mercury exposure elicits acute hippocampal cell death, reductions in neurogenesis, and severe learning deficits during puberty

Effect of Lycium bararum polysaccharides on methylmercury-induced abnormal differentiation of hippocampal stem cells

The aim of the present study was to observe the effects of a general extract of Lycium bararum polysaccharides (LBPs) on methylmercury (MeHg)-induced damage in hippocampus neural stem cells (hNSCs). The hippocampal tissues of embryonic day 16 Sprague-Dawley rats were extracted for the isolation, purification and cloning of hNSCs. Following passage and proliferation for 10 days, the cells were allocated at random into the following groups: Control, LBPs, MeHg and MeHg + LBPs. MTT and microtubule-associated protein 2 (MAP-2)/glial fibrillary acidic protein/Hoechst immunofluorescence tests were performed to detect the differentiation and growth of hNSCs in the various groups. The differentiation rate of MeHg-treated hNSCs and the perimeter of MAP-2-positive neurons were 3.632±0.63% and 62.36±5.58 µm, respectively, significantly lower compared with the control group values of 6.500±0.81% and 166±8.16 µm (P<0.05). Furthermore, the differentiation rate and the perimeter of MAP-2-positive neurons in LBPs groups cells was 7.75±0.59% and 253.3±11.21 µm, respectively, significantly higher compared with the control group (P<0.05). The same parameters in the MeHg + LBPs group were 5.92±0.98% and 111.9±6.07 µm, respectively, significantly higher than the MeHg group (P<0.05). The astrocyte differentiation rates in the MeHg and MeHg + LBPs group were 41.19±2.14 and 34.58±1.70, respectively (P<0.05). These results suggest that LBPs may promote the generation and development of new neurons and inhibit the MeHg-induced abnormal differentiation of astrocytes. Thus, LBPs may be considered to be a potential new treatment for MeHg-induced neurotoxicity in hNSCs.

Early Developmental Low-Dose Methylmercury Exposure Alters Learning and Memory in Periadolescent but Not Young Adult Rats

Few studies have assessed the effects of developmental methylmercury (MeHg) exposure on learning and memory at different ages. The possibility of the amelioration or worsening of the effects has not been sufficiently investigated. This study aimed to assess whether low-dose MeHg exposure in utero and during suckling induces differential disturbances in learning and memory of periadolescent and young adult rats. Four experimental groups of pregnant Sprague-Dawley rats were orally exposed to MeHg or vehicle from gestational day 5 to weaning: (1) control (vehicle), (2) 250 μg/kg/day MeHg, (3) 500 μg/kg/day MeHg, and (4) vehicle, and treated on the test day with MK-801 (0.15 mg/kg i.p.), an antagonist of the N-methyl D-aspartate receptor. The effects were evaluated in male offspring through the open field test, object recognition test, Morris water maze, and conditioned taste aversion. For each test and stage assessed, different groups of animals were used. MeHg exposure, in a dose-dependent manner, disrupted exploratory behaviour, recognition memory, spatial learning, and acquisition of aversive memories in periadolescent rats, but alterations were not observed in littermates tested in young adulthood. These results suggest that developmental low-dose exposure to MeHg induces age-dependent detrimental effects. The relevance of decreasing exposure to MeHg in humans remains to be determined.

MeHg Developing Exposure Causes DNA Double-Strand Breaks and Elicits Cell Cycle Arrest in Spinal Cord Cells

The neurotoxicity caused by methylmercury (MeHg) is well documented; however, the developmental neurotoxicity in spinal cord is still not fully understood. Here we investigated whether MeHg affects the spinal cord layers development. Chicken embryos at E3 were treated in ovo with 0.1 μg MeHg/50 μL saline solution and analyzed at E10. Thus, we performed immunostaining using anti-γ-H2A.X to recognize DNA double-strand breaks and antiphosphohistone H3, anti-p21, and anti-cyclin E to identify cells in proliferation and cell cycle proteins. Also, to identify neuronal cells, we used anti-NeuN and anti-βIII-tubulin antibodies. After the MeHg treatment, we observed the increase on γ-H2A.X in response to DNA damage. MeHg caused a decrease in the proliferating cells and in the thickness of spinal cord layers. Moreover, we verified that MeHg induced an increase in the number of p21-positive cells but did not change the cyclin E-positive cells. A significantly high number of TUNEL-positive cells indicating DNA fragmentation were observed in MeHg-treated embryos. Regarding the neuronal differentiation, MeHg induced a decrease in NeuN expression and did not change the expression of βIII-tubulin. These results showed that in ovo MeHg exposure alters spinal cord development by disturbing the cell proliferation and death, also interfering in early neuronal differentiation.

Thimerosal induces apoptotic and fibrotic changes to kidney epithelial cells in vitro

Thimerosal is an ethyl mercury-containing compound used mainly in vaccines as a bactericide. Although the kidney is a key target for mercury toxicity, thimerosal nephrotoxicity has not received the same attention as other mercury species. The aim of this study was to determine the potential cytotoxic mechanisms of thimerosal on human kidney cells. Human kidney proximal tubular epithelial (HK2) cells were exposed for 24 h to thimerosal (0-2 µM), and assessed for cell viability, apoptosis, and cell proliferation; expression of proteins Bax, nuclear factor-κB subunits, and transforming growth factor-β1 (TGFβ1); mitochondrial health (JC-1, MitoTracker Red CMXRos); and fibronectin levels (enzyme-linked immunosorbent assay). Thimerosal diminished HK2 cell viability and mitosis, promoted apoptosis, impaired the mitochondrial permeability transition, enhanced Bax and TGFβ1 expression, and augmented fibronectin secretion. This is the first report about kidney cell death and pro-fibrotic mechanisms promoted by thimerosal. Collectively, these in vitro results demonstrate that (1) thimerosal induces kidney epithelial cell apoptosis via upregulating Bax and the mitochondrial apoptotic pathway, and (2) thimerosal is a potential pro-fibrotic agent in human kidney cells. We suggest that new evidence on toxicity as well as continuous surveillance in terms of fibrogenesis is required concerning thimerosal use.

Valproic acid stimulates proliferation of glial precursors during cortical gliogenesis in developing rat

Valproic acid (VPA) is a neurotherapeutic drug prescribed for seizures, bipolar disorder, and migraine, including women of reproductive age. VPA is a well-known teratogen that produces congenital malformations in many organs including the nervous system, as well as later neurodevelopmental disorders, including mental retardation and autism. In developing brain, few studies have examined VPA effects on glial cells, particularly astrocytes. To investigate effects on primary glial precursors, we developed new cell culture and in vivo models using frontal cerebral cortex of postnatal day (P2) rat. In vitro, VPA exposure elicited dose-dependent, biphasic effects on DNA synthesis and proliferation. In vivo VPA (300 mg/kg) exposure from P2 to P4 increased both DNA synthesis and cell proliferation, affecting primarily astrocyte precursors, as >75% of mitotic cells expressed brain lipid-binding protein. Significantly, the consequence of early VPA exposure was increased astrocytes, as both S100-β+ cells and glial fibrillary acidic protein were increased in adolescent brain. Molecularly, VPA served as an HDAC inhibitor in vitro and in vivo as enhanced proliferation was accompanied by increased histone acetylation, whereas it elicited changes in culture in cell-cycle regulators, including cyclin D1 and E, and cyclin-dependent kinase (CDK) inhibitors, p21 and p27. Collectively, these data suggest clinically relevant VPA exposures stimulate glial precursor proliferation, though at higher doses can elicit inhibition through differential regulation of CDK inhibitors. Because changes in glial cell functions are proposed as mechanisms contributing to neuropsychiatric disorders, these observations suggest that VPA teratogenic actions may be mediated through changes in astrocyte generation during development. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 780-798, 2016.

Engrailed-2 (En2) deletion produces multiple neurodevelopmental defects in monoamine systems, forebrain structures and neurogenesis and behavior

Many genes involved in brain development have been associated with human neurodevelopmental disorders, but underlying pathophysiological mechanisms remain undefined. Human genetic and mouse behavioral analyses suggest that ENGRAILED-2 (EN2) contributes to neurodevelopmental disorders, especially autism spectrum disorder. In mouse, En2 exhibits dynamic spatiotemporal expression in embryonic mid-hindbrain regions where monoamine neurons emerge. Considering their importance in neuropsychiatric disorders, we characterized monoamine systems in relation to forebrain neurogenesis in En2-knockout (En2-KO) mice. Transmitter levels of serotonin, dopamine and norepinephrine (NE) were dysregulated from Postnatal day 7 (P7) to P21 in En2-KO, though NE exhibited the greatest abnormalities. While NE levels were reduced ∼35% in forebrain, they were increased 40 -: 75% in hindbrain and cerebellum, and these patterns paralleled changes in locus coeruleus (LC) fiber innervation, respectively. Although En2 promoter was active in Embryonic day 14.5 -: 15.5 LC neurons, expression diminished thereafter and gene deletion did not alter brainstem NE neuron numbers. Significantly, in parallel with reduced NE levels, En2-KO forebrain regions exhibited reduced growth, particularly hippocampus, where P21 dentate gyrus granule neurons were decreased 16%, suggesting abnormal neurogenesis. Indeed, hippocampal neurogenic regions showed increased cell death (+77%) and unexpectedly, increased proliferation. Excess proliferation was restricted to early Sox2/Tbr2 progenitors whereas increased apoptosis occurred in differentiating (Dcx) neuroblasts, accompanied by reduced newborn neuron survival. Abnormal neurogenesis may reflect NE deficits because intra-hippocampal injections of β-adrenergic agonists reversed cell death. These studies suggest that disruption of hindbrain patterning genes can alter monoamine system development and thereby produce forebrain defects that are relevant to human neurodevelopmental disorders.

Antioxidant effects of Dendropanax morbifera Léveille extract in the hippocampus of mercury-exposed rats

Background

Dendropanax morbifera Léveille has been employed for the treatment of infectious diseases using folk medicine. In this study, we evaluated the antioxidant effects of a leaf extract of Dendropanax morbifera Léveille in the hippocampus of mercury-exposed rats.

Methods

Seven-week-old Sprague–Dawley rats received a daily intraperitoneal injection of 5 μg/kg dimethylmercury and/or oral Dendropanax morbifera Léveille leaf extract (100 mg/kg) for 4 weeks. Animals were sacrificed 2 h after the last dimethylmercury and/or leaf extract treatment. Mercury levels were measured in homogenates of hippocampal tissue, a brain region that is vulnerable to mercury toxicity. In addition, we measured reactive oxygen species production, lipid peroxidation levels, and antioxidant levels in these hippocampal homogenates.

Results

Treatment with Dendropanax morbifera Léveille leaf extract significantly reduced mercury levels in hippocampal homogenates and attenuated the dimethylmercury-induced increase in the production of reactive oxygen species and formation of malondialdehyde. In addition, this leaf extract treatment significantly reversed the dimethylmercury-induced reduction in the hippocampal activities of Cu, Zn-superoxide dismutase, catalase, glutathione peroxidase, and glutathione-S-transferase.

Conclusion

These results suggest that a leaf extract of Dendropanax morbifera Léveille had strong antioxidant effects in the hippocampus of mercury-exposed rats.

Neuroprotection elicited by nerve growth factor and brain-derived neurotrophic factor released from astrocytes in response to methylmercury

The protective roles of astrocytes in neurotoxicity induced by environmental chemicals, such as methylmercury (MeHg), are largely unknown. We found that conditioned medium of MeHg-treated astrocytes (MCM) attenuated neuronal cell death induced by MeHg, suggesting that astrocytes-released factors can protect neuronal cells. The increased expression of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) was observed in MeHg-treated astrocytes. NGF and BDNF were detected in culture media as homodimers, which are able to bind specific tyrosine kinase receptors, tropomyosin related kinase (Trk) A and TrkB, respectively. The TrkA antagonist and TrkB antagonist abolished the protective effects of MCM in neuronal cell death induced by MeHg. Taken together, astrocytes synthesize and release NGF and BDNF in response to MeHg to protect neurons from MeHg toxicity. This study is considered to show a novel defense mechanism against MeHg-induced neurotoxicity.

Hippocampal developmental vulnerability to methylmercury extends into prepubescence

The developing brain is sensitive to environmental toxicants such as methylmercury (MeHg), to which humans are exposed via contaminated seafood. Prenatal exposure in children is associated with learning, memory and IQ deficits, which can result from hippocampal dysfunction. To explore underlying mechanisms, we have used the postnatal day (P7) rat to model the third trimester of human gestation. We previously showed that a single low exposure (0.6 μg/gbw) that approaches human exposure reduced hippocampal neurogenesis in the dentate gyrus (DG) 24 h later, producing later proliferation and memory deficits in adolescence. Yet, the vulnerable stem cell population and period of developmental vulnerability remain undefined. In this study, we find that P7 exposure of stem cells has long-term consequences for adolescent neurogenesis. It reduced the number of mitotic S-phase cells (BrdU), especially those in the highly proliferative Tbr2+ population, and immature neurons (Doublecortin) in adolescence, suggesting partial depletion of the later stem cell pool. To define developmental vulnerability to MeHg in prepubescent (P14) and adolescent (P21) rats, we examined acute 24 h effects of MeHg exposure on mitosis and apoptosis. We found that low exposure did not adversely impact neurogenesis at either age, but that a higher exposure (5 μg/gbw) at P14 reduced the total number of neural stem cells (Sox2+) by 23% and BrdU+ cells by 26% in the DG hilus, suggesting that vulnerability diminishes with age. To determine whether these effects reflect changes in MeHg transfer across the blood brain barrier (BBB), we assessed Hg content in the hippocampus after peripheral injection and found that similar levels (~800 ng/gm) were obtained at 24 h at both P14 and P21, declining in parallel, suggesting that changes in vulnerability depend more on local tissue and cellular mechanisms. Together, we show that MeHg vulnerability declines with age, and that early exposure impairs later neurogenesis in older juveniles.

Hippocampal developmental vulnerability to methylmercury extends into prepubescence

The developing brain is sensitive to environmental toxicants such as methylmercury (MeHg), to which humans are exposed via contaminated seafood. Prenatal exposure in children is associated with learning, memory and IQ deficits, which can result from hippocampal dysfunction. To explore underlying mechanisms, we have used the postnatal day (P7) rat to model the third trimester of human gestation. We previously showed that a single low exposure (0.6 μg/gbw) that approaches human exposure reduced hippocampal neurogenesis in the dentate gyrus (DG) 24 h later, producing later proliferation and memory deficits in adolescence. Yet, the vulnerable stem cell population and period of developmental vulnerability remain undefined. In this study, we find that P7 exposure of stem cells has long-term consequences for adolescent neurogenesis. It reduced the number of mitotic S-phase cells (BrdU), especially those in the highly proliferative Tbr2+ population, and immature neurons (Doublecortin) in adolescence, suggesting partial depletion of the later stem cell pool. To define developmental vulnerability to MeHg in prepubescent (P14) and adolescent (P21) rats, we examined acute 24 h effects of MeHg exposure on mitosis and apoptosis. We found that low exposure did not adversely impact neurogenesis at either age, but that a higher exposure (5 μg/gbw) at P14 reduced the total number of neural stem cells (Sox2+) by 23% and BrdU+ cells by 26% in the DG hilus, suggesting that vulnerability diminishes with age. To determine whether these effects reflect changes in MeHg transfer across the blood brain barrier (BBB), we assessed Hg content in the hippocampus after peripheral injection and found that similar levels (~800 ng/gm) were obtained at 24 h at both P14 and P21, declining in parallel, suggesting that changes in vulnerability depend more on local tissue and cellular mechanisms. Together, we show that MeHg vulnerability declines with age, and that early exposure impairs later neurogenesis in older juveniles.

Methylmercury exposure during early Xenopus laevis development affects cell proliferation and death but not neural progenitor specification

Methylmercury (MeHg) is a widespread environmental toxin that preferentially and adversely affects developing organisms. To investigate the impact of MeHg toxicity on the formation of the vertebrate nervous system at physiologically relevant concentrations, we designed a graded phenotype scale for evaluating Xenopus laevis embryos exposed to MeHg in solution. Embryos displayed a range of abnormalities in response to MeHg, particularly in brain development, which is influenced by both MeHg concentration and the number of embryos per ml of exposure solution. A TC50 of ~50μg/l and LC50 of ~100μg/l were found when maintaining embryos at a density of one per ml, and both increased with increasing embryo density. In situ hybridization and microarray analysis showed no significant change in expression of early neural patterning genes including sox2, en2, or delta; however a noticeable decrease was observed in the terminal neural differentiation genes GAD and xGAT, but not xVGlut. PCNA, a marker for proliferating cells, was negatively correlated with MeHg dose, with a significant reduction in cell number in the forebrain and spinal cord of exposed embryos by tadpole stages. Conversely, the number of apoptotic cells in neural regions detected by a TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay was significantly increased. These results provide evidence that disruption of embryonic neural development by MeHg may not be directly due to a loss of neural progenitor specification and gene transcription, but to a more general decrease in cell proliferation and increase in cell death throughout the developing nervous system.

Hippocampal ER stress and learning deficits following repeated pyrethroid exposure

Endoplasmic reticulum (ER) stress is implicated as a significant contributor to neurodegeneration and cognitive dysfunction. Previously, we reported that the widely used pyrethroid pesticide deltamethrin causes ER stress-mediated apoptosis in SK-N-AS neuroblastoma cells. Whether or not this occurs in vivo remains unknown. Here, we demonstrate that repeated deltamethrin exposure (3 mg/kg every 3 days for 60 days) causes hippocampal ER stress and learning deficits in adult mice. Repeated exposure to deltamethrin caused ER stress in the hippocampus as indicated by increased levels of C/EBP-homologous protein (131%) and glucose-regulated protein 78 (96%). This was accompanied by increased levels of caspase-12 (110%) and activated caspase-3 (50%). To determine whether these effects resulted in learning deficits, hippocampal-dependent learning was evaluated using the Morris water maze. Deltamethrin-treated animals exhibited profound deficits in the acquisition of learning. We also found that deltamethrin exposure resulted in decreased BrdU-positive cells (37%) in the dentate gyrus of the hippocampus, suggesting potential impairment of hippocampal neurogenesis. Collectively, these results demonstrate that repeated deltamethrin exposure leads to ER stress, apoptotic cell death in the hippocampus, and deficits in hippocampal precursor proliferation, which is associated with learning deficits.

Preliminary morphological and immunohistochemical changes in rat hippocampus following postnatal exposure to sodium arsenite

The effects of arsenic exposure during rapid brain growth period (RBGP) (postnatal period 4-11) on pyramidal neurons of cornu ammonis (specifically CA1 and CA3 regions) and granule cells of dentate gyrus (DG) of rat hippocampus were studied. Wistar rat pups, subdivided into the control (group I) and the experimental groups (group II, III, and IV), received distilled water and sodium arsenite (aqueous solution of 1.0, 1.5, and 2.0 mg/kg body weight, respectively) by intraperitoneal (i.p.) route. On postnatal day (PND) 12, the animals were sacrificed and brain tissue obtained. Paraffin sections (8 μm thick) stained with Cresyl Violet (CV) were observed for morphological and morphometric parameters. Arsenic induced programmed cell death (apoptosis) was studied using Terminal deoxyribonucleotidyl transferase mediated dUTP biotin Nick End Labeling (TUNEL) technique on the paraffin sections. Microscopy revealed decreased number and isolation of pyramidal neurons in superficial layers, misalignments of pyramidal cells in stratum pyramidale (SP) of CA1 and CA3 in experimental group III and IV, and presence of polymorphic cells in subgranular zone of ectal limb of dentate gyrus (suggestive of arsenic induced proliferation and migration of granule cells in the dentate gyrus). Morphometric assessments quantified and confirmed the microscopic findings. The mean nuclear area of pyramidal cells was increased and cell density was decreased in the CA1, CA3, and DG of experimental groups in comparison to the control group. Increase in the TUNEL positive cells in DG was observed in the experimental group IV, suggestive of increased apoptosis. These observations confirm vulnerability of pyramidal (CA1, CA3) and granule cells (DG) of hippocampus during RBGP.

Neural stem cell apoptosis after low-methylmercury exposures in postnatal hippocampus produce persistent cell loss and adolescent memory deficits

The developing brain is particularly sensitive to exposures to environmental contaminants. In contrast to the adult, the developing brain contains large numbers of dividing neuronal precursors, suggesting that they may be vulnerable targets. The postnatal day 7 (P7) rat hippocampus has populations of both mature neurons in the CA1-3 region as well as neural stem cells (NSC) in the dentate gyrus (DG) hilus, which actively produce new neurons that migrate to the granule cell layer (GCL). Using this well-characterized NSC population, we examined the impact of low levels of methylmercury (MeHg) on proliferation, neurogenesis, and subsequent adolescent learning and memory behavior. Assessing a range of exposures, we found that a single subcutaneous injection of 0.6 µg/g MeHg in P7 rats induced caspase activation in proliferating NSC of the hilus and GCL. This acute NSC death had lasting impact on the DG at P21, reducing cell numbers in the hilus by 22% and the GCL by 27%, as well as reductions in neural precursor proliferation by 25%. In contrast, non-proliferative CA1-3 pyramidal neuron cell number was unchanged. Furthermore, animals exposed to P7 MeHg exhibited an adolescent spatial memory deficit as assessed by Morris water maze. These results suggest that environmentally relevant levels of MeHg exposure may decrease NSC populations and, despite ongoing neurogenesis, the brain may not restore the hippocampal cell deficits, which may contribute to hippocampal-dependent memory deficits during adolescence.

Neuropathology and animal models of autism: genetic and environmental factors

Autism is a heterogeneous behaviorally defined neurodevelopmental disorder. It is defined by the presence of marked social deficits, specific language abnormalities, and stereotyped repetitive patterns of behavior. Because of the variability in the behavioral phenotype of the disorder among patients, the term autism spectrum disorder has been established. In the first part of this review, we provide an overview of neuropathological findings from studies of autism postmortem brains and identify the cerebellum as one of the key brain regions that can play a role in the autism phenotype. We review research findings that indicate possible links between the environment and autism including the role of mercury and immune-related factors. Because both genes and environment can alter the structure of the developing brain in different ways, it is not surprising that there is heterogeneity in the behavioral and neuropathological phenotypes of autism spectrum disorders. Finally, we describe animal models of autism that occur following insertion of different autism-related genes and exposure to environmental factors, highlighting those models which exhibit both autism-like behavior and neuropathology.

Adult hippocampal neurogenesis and mRNA expression are altered by perinatal arsenic exposure in mice and restored by brief exposure to enrichment

Arsenic is a common and pervasive environmental contaminant found in drinking water in varying concentrations depending on region. Exposure to arsenic induces behavioral and cognitive deficits in both human populations and in rodent models. The Environmental Protection Agency (EPA) standard for the allotment of arsenic in drinking water is in the parts-per-billion range, yet our lab has shown that 50 ppb arsenic exposure during development can have far-reaching consequences into adulthood, including deficits in learning and memory, which have been linked to altered adult neurogenesis. Given that the morphological impact of developmental arsenic exposure on the hippocampus is unknown, we sought to evaluate proliferation and differentiation of adult neural progenitor cells in the dentate gyrus after 50 ppb arsenic exposure throughout the perinatal period of development in mice (equivalent to all three trimesters in humans) using a BrdU pulse-chase assay. Proliferation of the neural progenitor population was decreased by 13% in arsenic-exposed mice, but was not significant. However, the number of differentiated cells was significantly decreased by 41% in arsenic-exposed mice compared to controls. Brief, daily exposure to environmental enrichment significantly increased proliferation and differentiation in both control and arsenic-exposed animals. Expression levels of 31% of neurogenesis-related genes including those involved in Alzheimer's disease, apoptosis, axonogenesis, growth, Notch signaling, and transcription factors were altered after arsenic exposure and restored after enrichment. Using a concentration previously considered safe by the EPA, perinatal arsenic exposure altered hippocampal morphology and gene expression, but did not inhibit the cellular neurogenic response to enrichment. It is possible that behavioral deficits observed during adulthood in animals exposed to arsenic during development derive from the lack of differentiated neural progenitor cells necessary for hippocampal-dependent learning. This study is the first to determine the impact of arsenic exposure during development on adult hippocampal neurogenesis and related gene expression.

Protective effect of salicylic acid on Hg(0) intoxication in mice

Elemental mercury (Hg(0)) is a hazardous metal with significant human exposure through diverse sources. In this study, the role of salicylic acid (SA) was assessed against Hg(0)-induced injury in mice, with the aim of screening alternative clinical drugs to prevent or treat Hg(0) poisoning. An exposure to Hg(0) (1.0 mg/m(3) in a glass box) for 2 h per day for successive 15 d significantly increased Hg accumulation in mouse brain and lung, inhibited the animal growth and altered the neurobehavior such as impairing the spatial learning and memory in the Barnes maze test. However, although oral SA (5.5 mg/kg body weight) during the Hg(0) exposure did not reduce the Hg levels in these organs, it effectively counteracted the Hg(0)-induced growth inhibition, and improved the behavioral performance, accompanied by a series of ameliorations in the antioxidative defense and anti-inflammatory response. For instance, when compared with control, Hg(0)-inhaled animals had significant decreases in the activities of superoxide dismutase and peroxidase, and in the levels of reduced form of glutathione and the ratio to its oxidized form, concomitantly with a high accumulation of hydrogen peroxide and malondialdehyde in the brain and lung. However, these values in Hg(0) + SA-exposed animals were comparable with the basal levels in control. Likewise, interleukin-6 in the brain and lung of Hg(0)-exposed animals were dramatically elevated, whereas it was maintained to the basal level in Hg(0) + SA-exposed animals. These data suggested that application of SA could protect mice against Hg(0)-induced injury.

Altered behavior of neonatal northern watersnakes (Nerodia sipedon) exposed to maternally transferred mercury

Little is known about effects of maternally transferred contaminants in snakes. The purpose of this study was to evaluate sublethal effects of maternally transferred mercury (Hg) on neonatal northern watersnakes (Nerodia sipedon). We captured 31 gravid females along a historically Hg-contaminated river. Following birth, we measured litter Hg concentrations and assessed locomotor performance, foraging ability (i.e., number of prey eaten, latency to first strike, strike efficiency, and handling time), and learning (i.e., change in foraging measures over time) in their offspring (n = 609). Mercury concentrations in offspring negatively correlated with motivation to feed and strike efficiency. Over time, strike efficiency and latency to strike decreased for all snakes in the study. However, offspring from contaminated areas maintained consistently lower efficiencies than reference individuals. This study is the first to examine sublethal behavioral effects of maternally transferred contaminants in snakes and suggests that maternally transferred Hg negatively affects offspring behavior.

Importance of cysteine residues in the thyroid hormone transporter MCT8

The thyroid hormone (TH) transporter monocarboxylate transporter 8 (MCT8) is crucial for brain development as demonstrated by the severe psychomotor retardation in patients with MCT8 mutations. MCT8 contains 10 residues of the reactive amino acid cysteine (Cys) whose functional roles were studied using the Cys-specific reagent p-chloromercurybenzenesulfonate (pCMBS) and by site-directed mutagenesis. Pretreatment of JEG3 cells with pCMBS resulted in a dose- and time-dependent decrease of subsequent T3 uptake. Pretreatment with dithiothreitol did not affect TH transport or its inhibition by pCMBS. However, pCMBS inhibition of MCT8 was reversed by dithiothreitol. Inhibition of MCT8 by pCMBS was prevented in the presence of T3. The single and double mutation of C481A and C497A did not affect T3 transport, but the single mutants were less sensitive and the double mutant was completely insensitive to pCMBS. Similar effects on MCT8 were obtained using HgCl2 instead of pCMBS. In conclusion, we have identified Cys481 and Cys497 in MCT8 as the residues modified by pCMBS or HgCl2. These residues are probably located at or near the substrate-recognition site in MCT8. It remains to be investigated whether MCT8 function is regulated by modification of these Cys residues under pathophysiological conditions.

De novo synthesized estradiol protects against methylmercury-induced neurotoxicity in cultured rat hippocampal slices

Background

Estrogen, a class of female sex steroids, is neuroprotective. Estrogen is synthesized in specific areas of the brain. There is a possibility that the de novo synthesized estrogen exerts protective effect in brain, although direct evidence for the neuroprotective function of brain-synthesized estrogen has not been clearly demonstrated. Methylmercury (MeHg) is a neurotoxin that induces neuronal degeneration in the central nervous system. The neurotoxicity of MeHg is region-specific, and the molecular mechanisms for the selective neurotoxicity are not well defined. In this study, the protective effect of de novo synthesized 17β-estradiol on MeHg-induced neurotoxicity in rat hippocampus was examined.

Methodology/Principal Findings

Neurotoxic effect of MeHg on hippocampal organotypic slice culture was quantified by propidium iodide fluorescence imaging. Twenty-four-hour treatment of the slices with MeHg caused cell death in a dose-dependent manner. The toxicity of MeHg was attenuated by pre-treatment with exogenously added estradiol. The slices de novo synthesized estradiol. The estradiol synthesis was not affected by treatment with 1 µM MeHg. The toxicity of MeHg was enhanced by inhibition of de novo estradiol synthesis, and the enhancement of toxicity was recovered by the addition of exogenous estradiol. The neuroprotective effect of estradiol was inhibited by an estrogen receptor (ER) antagonist, and mimicked by pre-treatment of the slices with agonists for ERα and ERβ, indicating the neuroprotective effect was mediated by ERs.

Conclusions/Significance

Hippocampus de novo synthesized estradiol protected hippocampal cells from MeHg-induced neurotoxicity via ERα- and ERβ-mediated pathways. The self-protective function of de novo synthesized estradiol might be one of the possible mechanisms for the selective sensitivity of the brain to MeHg toxicity.

N-acetyl cysteine treatment reduces mercury-induced neurotoxicity in the developing rat hippocampus

Mercury is an environmental toxicant that can disrupt brain development. However, although progress has been made in defining its neurotoxic effects, we know far less about available therapies that can effectively protect the brain in exposed individuals. We previously developed an animal model in which we defined the sequence of events underlying neurotoxicity: Methylmercury (MeHg) injection in postnatal rat acutely induced inhibition of mitosis and stimulated apoptosis in the hippocampus, which later resulted in intermediate-term deficits in structure size and cell number. N-acetyl cysteine (NAC) is the N-acetyl derivative of L-cysteine used clinically for treatment of drug intoxication. Here, based on its known efficacy in promoting MeHg urinary excretion, we evaluated NAC for protective effects in the developing brain. In immature neurons and precursors, MeHg (3 μM) induced a >50% decrease in DNA synthesis at 24 hr, an effect that was completely blocked by NAC coincubation. In vivo, injection of MeHg (5 μg/g bw) into 7-day-old rats induced a 22% decrease in DNA synthesis in whole hippocampus and a fourfold increase in activated caspase-3-immunoreactive cells at 24 hr and reduced total cell numbers by 13% at 3 weeks. Treatment of MeHg-exposed rats with repeated injections of NAC abolished MeHg toxicity. NAC prevented the reduction in DNA synthesis and the marked increase in caspase-3 immunoreactivity. Moreover, the intermediate-term decrease in hippocampal cell number provoked by MeHg was fully blocked by NAC. Altogether these results suggest that MeHg toxicity in the perinatal brain can be ameliorated by using NAC, opening potential avenues for therapeutic intervention.

Oxoguanine glycosylase 1 (OGG1) protects cells from DNA double-strand break damage following methylmercury (MeHg) exposure

Methylmercury (MeHg) is a potent neurotoxin, teratogen, and probable carcinogen, but the underlying mechanisms of its actions remain unclear. Although MeHg causes several types of DNA damage, the toxicological consequences of this macromolecular damage are unknown. MeHg enhances oxidative stress, which can cause various oxidative DNA lesions that are primarily repaired by oxoguanine glycosylase 1 (OGG1). Herein, we compared the response of wild-type and OGG1 null (Ogg1(-/-)) murine embryonic fibroblasts to environmentally relevant, low micromolar concentrations of MeHg by measuring clonogenic efficiency, cell cycle arrest, DNA double-strand breaks (DSBs), and activation of the DNA damage response pathway.Ogg1(-/-) cells exhibited greater sensitivity to MeHg than wild-type controls, as measured by the clonogenic assay, and showed a greater propensity for MeHg-initiated apoptosis. Both wild-type and Ogg1(-/-) cells underwent cell cycle arrest when exposed to micromolar concentrations of MeHg; however, the extent of DSBs was exacerbated in Ogg1(-/-) cells compared with that in wild-type controls. Pretreatment with the antioxidative enzyme catalase reduced levels of DSBs in both wild-type and Ogg1(-/-) cells but failed to block MeHg-initiated apoptosis at micromolar concentrations. Our findings implicate reactive oxygen species mediated DNA damage in the mechanism of MeHg toxicity; and demonstrate for the first time that impaired DNA repair capacity enhances cellular sensitivity to MeHg. Accordingly, the genotoxic properties of MeHg may contribute to its neurotoxic and teratogenic effects, and an individual's response to oxidative stress and DNA damage may constitute an important determinant of risk.

Acoustic stapedius muscle reflex in mercury-exposed Andean children and adults

CONCLUSION

The results suggested mercury (Hg)-induced anomalies in the brainstem-mediated acoustic stapedius muscle reflex in children.

OBJECTIVES

Hg exposure has been associated with hearing impairment and brainstem anomalies. Acoustic stapedius reflex (ASR) thresholds, growth functions, decay/adaptation times, and behavioral auditory thresholds were used to screen Andean children and adults for Hg-induced auditory brainstem and facial nerve impairment.

METHODS

Fifty-one participants, which included 22 children (aged 6-17 years) and 29 adults (aged 19-83 years) living in gold mining areas of Ecuador where Hg is widely used in amalgamation, were screened using ASR immittance procedures.

RESULTS

Mean blood mercury (HgB) level in the children was 15.6 μg/L (SD, 21.3; median, 7 μg/L; range, 2.0-89 μg/L), and in the adults 8.5 μg/L (SD, 7.1; median, 6 μg/L; range, 2.0-32 μg/L). Mean contralateral ASR thresholds (ASRT) for the screening frequency of 2000 Hz in the children (39 ears) was 92.9 dB HL (SD, 6.1; range, 80-105 dB HL), and in the adults (53 ears) 90.0 dB HL (SD, 6.4; range, 65-105 dB HL). The ASRT in the children increased significantly with HgB level (rho = 0.433; p = 0.008).

Proliferation potential of human amniotic fluid stem cells differently responds to mercury and lead exposure

There are considerable gaps in our knowledge on cell biological effects induced by the heavy metals mercury (Hg) and lead (Pb). In the present study we aimed to explore the effects of these toxicants on proliferation and cell size of primary human amniotic fluid stem (AFS) cells. Monoclonal human AFS cells were incubated with three dosages of Hg and Pb (single and combined treatment; ranging from physiological to cytotoxic concentrations) and the intracellular Hg and Pb concentrations were analyzed, respectively. At different days of incubation the effects of Hg and Pb on proliferation, cell size, apoptosis, and expression of cyclins and the cyclin-dependent kinase inhibitor p27 were investigated. Whereas we found Hg to trigger pronounced effects on proliferation of human AFS cells already at low concentrations, anti-proliferative effects of Pb could only be detected at high concentrations. Exposure to high dose of Hg induced pronounced downregulation of cyclin A confirming the anti-proliferative effects observed for Hg. Co-exposure to Hg and Pb did not cause additive effects on proliferation and size of AFS cells, and on cyclin A expression. Our here presented data provide evidence that the different toxicological effects of Pb and Hg on primary human stem cells are due to different intracellular accumulation levels of these two toxicants. These findings allow new insights into the functional consequences of Pb and Hg for mammalian stem cells and into the cell biological behavior of AFS cells in response to toxicants.

The chemokine CCL2 protects against methylmercury neurotoxicity

Industrial pollution due to heavy metals such as mercury is a major concern for the environment and public health. Mercury, in particular methylmercury (MeHg), primarily affects brain development and neuronal activity, resulting in neurotoxic effects. Because chemokines can modulate brain functions and are involved in neuroinflammatory and neurodegenerative diseases, we tested the possibility that the neurotoxic effect of MeHg may interfere with the chemokine CCL2. We have used an original protocol in young mice using a MeHg-contaminated fish-based diet for 3 months relevant to human MeHg contamination. We observed that MeHg induced in the mice cortex a decrease in CCL2 concentrations, neuronal cell death, and microglial activation. Knock-out (KO) CCL2 mice fed with a vegetal control food already presented a decrease in cortical neuronal cell density in comparison with wild-type animals under similar diet conditions, suggesting that the presence of CCL2 is required for normal neuronal survival. Moreover, KO CCL2 mice showed a pronounced neuronal cell death in response to MeHg. Using in vitro experiments on pure rat cortical neurons in culture, we observed by blockade of the CCL2/CCR2 neurotransmission an increased neuronal cell death in response to MeHg neurotoxicity. Furthermore, we showed that sod genes are upregulated in brain of wild-type mice fed with MeHg in contrast to KO CCL2 mice and that CCL2 can blunt in vitro the decrease in glutathione levels induced by MeHg. These original findings demonstrate that CCL2 may act as a neuroprotective alarm system in brain deficits due to MeHg intoxication.

Methylmercury (MeHg) elicits mitochondrial-dependent apoptosis in developing hippocampus and acts at low exposures

The developing brain is particularly sensitive to environmental teratogens, such as methylmercury (MeHg), which may induce cell death. Although several mechanisms of MeHg-induced apoptosis have been defined in culture models, pathways mediating caspase-3 activation in vivo remain unclear, especially in the developing hippocampus. To explore apoptotic mechanisms, Sprague-Dawley rats were exposed to 5 μg/g MeHg or PBS vehicle on postnatal day 7 (P7) and the hippocampus was assessed at various times for levels of apoptotic proteins. MeHg induced a 38% increase in Bax protein and an increase in cytosolic cytochrome c at 4h, followed by later increases in caspase-9 (40% at 12h; 33% at 24h) and caspase-8 (33% at 24h), compared to controls. MeHg also induced an increase in executioner caspase-3, a protease activated by both mitochondrial-dependent caspase-9 and mitochondrial-independent caspase-8. To further define pathways, we used a forebrain culture model and found that the MeHg-induced increases in caspase-3 and caspase-8 were completely blocked by a caspase-9-specific inhibitor, while caspase-9 induction was unperturbed by the caspase-8 inhibitor. These observations suggest that MeHg acts primarily through the mitochondrial-dependent cascade to activate caspase-3 in forebrain precursors, a pathway that may contribute to previously documented neurotoxicity in developing hippocampus. In turn, using the endpoint protein, caspase-3, as a sensitive marker for neural injury, we were able to detect hippocampal cell death in vivo at ten-fold lower levels of MeHg exposure (0.6 μg/g) than previously reported. Thus mitochondrial-dependent cell death in the hippocampus may serve as a sensitive index for teratogenic insults to the developing brain.

Mechanisms of methylmercury-induced neurotoxicity: evidence from experimental studies

Neurological disorders are common, costly, and can cause enduring disability. Although mostly unknown, a few environmental toxicants are recognized causes of neurological disorders and subclinical brain dysfunction. One of the best known neurotoxins is methylmercury (MeHg), a ubiquitous environmental toxicant that leads to long-lasting neurological and developmental deficits in animals and humans. In the aquatic environment, MeHg is accumulated in fish, which represent a major source of human exposure. Although several episodes of MeHg poisoning have contributed to the understanding of the clinical symptoms and histological changes elicited by this neurotoxicant in humans, experimental studies have been pivotal in elucidating the molecular mechanisms that mediate MeHg-induced neurotoxicity. The objective of this mini-review is to summarize data from experimental studies on molecular mechanisms of MeHg-induced neurotoxicity. While the full picture has yet to be unmasked, in vitro approaches based on cultured cells, isolated mitochondria and tissue slices, as well as in vivo studies based mainly on the use of rodents, point to impairment in intracellular calcium homeostasis, alteration of glutamate homeostasis and oxidative stress as important events in MeHg-induced neurotoxicity. The potential relationship among these events is discussed, with particular emphasis on the neurotoxic cycle triggered by MeHg-induced excitotoxicity and oxidative stress. The particular sensitivity of the developing brain to MeHg toxicity, the critical role of selenoproteins and the potential protective role of selenocompounds are also discussed. These concepts provide the biochemical bases to the understanding of MeHg neurotoxicity, contributing to the discovery of endogenous and exogenous molecules that counteract such toxicity and provide efficacious means for ablating this vicious cycle.

Induction of autoimmunity to brain antigens by developmental mercury exposure

A.SW mice, which are known to be prone to mercury (Hg)-induced immune nephritis, were assessed for their ability to develop autoimmunity to brain antigens after developmental exposure to Hg. Maternal drinking water containing subclinical doses of 1.25μM methyl Hg (MeHg) or 50μM Hg chloride (HgCl(2)) were used to evaluate developmental (exposure from gestational day 8 to postnatal day 21) induction of immune responses to brain antigens. Only HgCl(2) induced autoantibody production; the HgCl(2)-exposed offspring showed an increased number of CD4(+) splenic T cells expressing CD25 and V(β) 8.3 chains, and the brain-reactive immunoglobulin G (IgG) antibodies were predominantly against nuclear proteins (30 and 34 kD). The antibodies were deposited in all brain regions. Although male and female A.SW mice exposed to HgCl(2) showed deposition of IgG in multiple brain regions, inflammation responses were observed only in the cerebellum (CB) of female A.SW mice; these responses were associated with increased levels of exploratory behavior. The developmental exposure to MeHg also induced inflammation in the CB and increased exploratory behavior of the female A.SW mice, but the change did not correlate with increased IgG in the brain. Interestingly, the non-Hg-exposed female A.SW mice habituated (adapted to the information and/or stimuli of a new environment) more than the male A.SW mice during exploratory behavior assessment, and the Hg exposure eliminated the habituation (i.e., no changes in behavior with subsequent trials), making the female behaviors more like those of the male A.SW mice. Additionally, gender differences in A.SW brain cytokine expressions prior to Hg exposure were eliminated by the Hg exposure.

Effects of low level of methylmercury on proliferation of cortical progenitor cells

Methylmercury (MeHg) is a potent environmental neurotoxin that shows toxicity to developing central nervous system (CNS), causing brain damage in children even at low exposure levels. However, the mechanisms for its effect on CNS are not well understood. In current study, primary cultures of progenitor cells from embryonic cerebral cortex were used as a model system to study the potential effect and the underlying mechanism of MeHg on neural progenitor cells. Results showed that, in cultured cortical progenitor cells, 48-h exposure to low-level of MeHg (at 2.5 nM, 5 nM and 50 nM, respectively) caused G1/S cell cycle arrest in a dose-dependent manner without inducing cell death. Interestingly, the expression of cyclin E, which promotes G1/S transition, but not cyclin D1 and CDK2, was selectively downregulated by exposure of MeHg. In addition, low-level of MeHg inhibited the maintenance of ERK1/2 phosphorylation, possibly by abolishing the late phase ERK1/2 activation induced by bFGF. Thus, MeHg may induce proliferation inhibition and cell cycle arrest of neural progenitor cells via regulating cyclin E expression and perturbing a pathway that involves ERK1/2.

Animal models of autism spectrum disorders: information for neurotoxicologists

Recent findings derived from large-scale datasets and biobanks link multiple genes to autism spectrum disorders. Consequently, novel rodent mutants with deletions, truncations and in some cases, overexpression of these candidate genes have been developed and studied both behaviorally and biologically. At the Annual Neurotoxicology Meeting in Rochester, NY in October of 2008, a symposium of clinicians and basic scientists gathered to present the behavioral features of autism, as well as strategies to model those behavioral features in mice and primates. The aim of the symposium was to provide researchers with up-to-date information on both the genetics of autism and how they are used in differing in vivo and in vitro animal models as well as to provide a background on the environmental exposures being tested on several animal models. In addition, researchers utilizing complementary approaches, presented on cell culture, in vitro or more basic models, which target neurobiological mechanisms, including Drosophila. Following the presentation, a panel convened to explore the opportunities and challenges of using model systems to investigate genetic and environment interactions in autism spectrum disorders. The following paper represents a summary of each presentation, as well as the discussion that followed at the end of the symposium.

Mitochondrial dysfunction, impaired oxidative-reduction activity, degeneration, and death in human neuronal and fetal cells induced by low-level exposure to thimerosal and other metal compounds

Thimerosal (ethylmercurithiosalicylic acid), an ethylmercury (EtHg)-releasing compound (49.55% mercury (Hg)), was used in a range of medical products for more than 70 years. Of particular recent concern, routine administering of Thimerosal-containing biologics/childhood vaccines have become significant sources of Hg exposure for some fetuses/infants. This study was undertaken to investigate cellular damage among in vitro human neuronal (SH-SY-5Y neuroblastoma and 1321N1 astrocytoma) and fetal (nontransformed) model systems using cell vitality assays and microscope-based digital image capture techniques to assess potential damage induced by Thimerosal and other metal compounds (aluminum (Al) sulfate, lead (Pb)(II) acetate, methylmercury (MeHg) hydroxide, and mercury (Hg)(II) chloride) where the cation was reported to exert adverse effects on developing cells. Thimerosal-associated cellular damage was also evaluated for similarity to pathophysiological findings observed in patients diagnosed with autistic disorders (ADs). Thimerosal-induced cellular damage as evidenced by concentration- and time-dependent mitochondrial damage, reduced oxidative-reduction activity, cellular degeneration, and cell death in the in vitro human neuronal and fetal model systems studied. Thimerosal at low nanomolar (nM) concentrations induced significant cellular toxicity in human neuronal and fetal cells. Thimerosal-induced cytoxicity is similar to that observed in AD pathophysiologic studies. Thimerosal was found to be significantly more toxic than the other metal compounds examined. Future studies need to be conducted to evaluate additional mechanisms underlying Thimerosal-induced cellular damage and assess potential co-exposures to other compounds that may increase or decrease Thimerosal-mediated toxicity.

Characterization of the effects of methylmercury on Caenorhabditis elegans

The rising prevalence of methylmercury (MeHg) in seafood and in the global environment provides an impetus for delineating the mechanism of the toxicity of MeHg. Deleterious effects of MeHg have been widely observed in humans and in other mammals, the most striking of which occur in the nervous system. Here we test the model organism, Caenorhabditis elegans (C. elegans), for MeHg toxicity. The simple, well-defined anatomy of the C. elegans nervous system and its ready visualization with green fluorescent protein (GFP) markers facilitated our study of the effects of methylmercuric chloride (MeHgCl) on neural development. Although MeHgCl was lethal to C. elegans, induced a developmental delay, and decreased pharyngeal pumping, other traits including lifespan, brood size, swimming rate, and nervous system morphology were not obviously perturbed in animals that survived MeHgCl exposure. Despite the limited effects of MeHgCl on C. elegans development and behavior, intracellular mercury (Hg) concentrations (<or=3 ng Hg/mg protein) in MeHgCl-treated nematodes approached levels that are highly toxic to mammals. If MeHgCl reaches these concentrations throughout the animal, this finding indicates that C. elegans cells, particularly neurons, may be less sensitive to MeHgCl toxicity than mammalian cells. We propose, therefore, that C. elegans should be a useful model for discovering intrinsic mechanisms that confer resistance to MeHgCl exposure.

Prenatal methylmercury exposure: effects on stress response during active learning

The long-term impact of prenatal methylmercury (MeHg) exposure on the stress response during active learning was investigated. Pregnant rats were gavage fed MeHg (8 mg/kg) on gestational day 15. Ninety-day-old rats born to both MeHg- and saline-treated dams were subjected to an active avoidance test. The active avoidance-experienced rats (AAERs) with prenatal exposure to MeHg showed significant impairment in learning ability and exhibited higher levels of corticosterone than the untreated AAERs. The present findings suggest that the abnormal increase in plasma corticosterone levels could contribute to the poor performance of MeHg-treated AAERs in this learning task.

Interference of ethanol and methylmercury in the developing central nervous system

Studies involving alcohol and its interactions with other neurotoxicants represent the focus of several works of research due to the fact that the use of alcohol can sometimes leads to serious health problems. Fetal exposure to alcohol and mercury has a high incidence in some regions of Brazil, where there are pregnant women who are alcoholics and live in mining areas. This work was conducted to examine the effects of combined exposure to ethanol (EtOH) and methylmercury (MeHg) in rats during the development of the central nervous system (CNS). Experimental behavioral animal models/tests were used in order to examine locomotion, anxiety, depression and memory. Pregnant rats received tap water or EtOH 22.5% w/v (6.5 g/kg per day), by gavage) during pregnancy and breast-feeding. On the 15th day of pregnancy, some groups received 8 mg/kg of MeHg (by gavage). The groups were as follows: control, EtOH, MeHg and EtOH+MeHg. The experimental results showed that the EtOH, MeHg and EtOH+MeHg groups reduced the percentage of frequency and time spent in the open arms entries of the elevated plus-maze (EPM) test, when compared to the control group. This result suggests an anxiogenic behavioral response. The MeHg group increased locomotor activity in the arena and the immobility time in the forced swimming test, suggestive of depression-like behavior. The EtOH+MeHg group showed greater reductions in the percentages of frequency and time spent in the open arms entries in the EPM test, suggesting a sedative-behavior since the frequency of enclosed arm entries was affected. In the inhibitory avoidance task, the EtOH+MeHg group reduced the latency of the step-down response onto the grid floor, suggesting a cognitive and behavior dysfunctions. Taken together, the results suggest that EtOH and/or MeHg intoxication during the developing CNS may be a risk for deficits related to locomotor impairment, anxiety, depression and neurocognitive functions. There is a possibility that EtOH may prevent some of the MeHg responses, but the precise mechanism of action involved in this process needs to be considered for future research.

Developmental exposure to methylmercury elicits early cell death in the cerebral cortex and long-term memory deficits in the rat

Experiments were performed to assess the neurotoxic effects induced by prenatal acute treatment with methylmercury on cortical neurons. To this purpose, primary neuronal cultures were obtained from cerebral cortex of neonatal rats born to dams treated with methylmercury (4 and 8 mg/kg by gavage) on gestational day 15, the developmental stage critical for cortical neuron proliferation. Prenatal exposure to methylmercury 8 mg/kg significantly reduced cell viability and caused either apoptotic or necrotic neuronal death. Moreover, this exposure level resulted in abnormal neurite outgrowth and retraction or collapse of some neurites, caused by a dissolution of microtubules. The severe and early cortical neuron damage induced by methylmercury 8 mg/kg treatment correlated with long term memory impairment, since adult rats (90 days of age) born to dams treated with this dose level showed a significant deficit in the retention performance when subjected to a passive avoidance task. Prenatal exposure to methylmercury 4 mg/kg significantly increased the neuronal vulnerability to a neurotoxic insult. This was determined by measuring the increment of chromatin condensation induced by glutamate, at a concentration (30 microM) able to induce an excitotoxic damage. This exposure level eliciting apoptotic death did not result in cognitive dysfunctions. In conclusion, the methylmercury-induced disruption of glutamate pathway during critical windows of brain development may interfere with cell fate and proliferation resulting in a more or less severe cortical lesions associated or not with loss of function later in life, depending on the exposure levels. Therefore, the early biochemical effects and long-term behavioral changes elicited by high methylmercury levels suggest that the developing brain is impaired in its ability to recover following toxic insult, and the initial effects on cortical neurons may lead to permanent cognitive dysfunctions.

The methylmercury-L-cysteine conjugate is a substrate for the L-type large neutral amino acid transporter

Methylmercury (MeHg) is a potent neurotoxin. The mechanism(s) that governs MeHg transport across the blood-brain barrier and other biological membranes remains unclear. This study addressed the role of the L-type large neutral amino acid transporter, LAT1, in MeHg transport. Studies were carried out in CHO-k1 cells. Over-expression of LAT1 in these cells was associated with enhanced uptake of [(14)C]-MeHg when treated with L-cysteine, but not with the D-cysteine conjugate. In the presence of excess L-methionine, a substrate for LAT1, L-cysteine-conjugated [(14)C]-MeHg uptake was significantly attenuated. Treatment of LAT-1 over-expressing CHO-k1 cells with L-cysteine-conjugated MeHg was also associated with increased leakage of lactate dehydrogenase into the media as well as reduced cell viability measured by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction assay. In contrast, knock-down of LAT1 decreased the uptake of l-cysteine-conjugated MeHg and attenuated the effects of MeHg on lactate dehydrogenase leakage and CHO-k1 cell viability. These results indicate that the MeHg-L-cysteine conjugate is a substrate for the neutral amino acid transporter, LAT1, which actively transports MeHg across membranes.