Effects of maternal dietary supplementation with selenite on the postnatal development of rat offspring exposed to methyl mercury in utero

Assessment of the Effects of Organic vs. Inorganic Arsenic and Mercury in Caenorhabditis elegans

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Similar developmental delays and oxidative stress with 20x DMA relative to NaAsO2.

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Dissimilar gene expression and locomotion with organic vs. inorganic mercury.

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Dissimilar unfolded protein responses for organic vs. inorganic arsenic and mercury.

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Across phyla, methylation has opposite effects on arsenic vs. mercury toxicity.

Exposures to mercury and arsenic are known to pose significant threats to human health. Effects specific to organic vs. inorganic forms of these toxic elements are less understood however, especially for organic dimethylarsinic acid (DMA), which has recently been detected in pups of rodent dams orally exposed to inorganic sodium (meta)arsenite (NaAsO2). Caenorhabditis elegans is a small animal alternative toxicity model. To fill data gaps on the effects of DMA relative to NaAsO2, C. elegans were exposed to these two compounds alongside more thoroughly researched inorganic mercury chloride (HgCl2) and organic methylmercury chloride (meHgCl). For timing of developmental milestone acquisition in C. elegans, meHgCl was 2 to 4-fold more toxic than HgCl2, and NaAsO2 was 20-fold more toxic than DMA, ranking the four compounds meHgCl > HgCl2 > NaAsO2 ≫ DMA for developmental toxicity. Methylmercury induced significant decreases in population locomotor activity levels in developing C. elegans. DMA was also associated with developmental hypoactivity, but at >100-fold higher concentrations than meHgCl. Transcriptional alterations in native genes were observed in wild type C. elegans adults exposed to concentrations equitoxic for developmental delay in juveniles. Both forms of arsenic induced genes involved in immune defense and oxidative stress response, while the two mercury species induced proportionally more genes involved in transcriptional regulation. A transgenic bioreporter for activation of conserved proteosome specific unfolded protein response was strongly activated by NaAsO2, but not DMA at tested concentrations. HgCl2 and meHgCl had opposite effects on a bioreporter for unfolded protein response in the endoplasmic reticulum. Presented experiments indicating low toxicity for DMA in C. elegans are consistent with human epidemiologic data correlating higher arsenic methylation capacity with resistance to arsenic toxicity. This work contributes to the understanding of the accuracy and fit-for-use categories for C. elegans toxicity screening and its usefulness to prioritize compounds of concern for further testing.

Advances on the Influence of Methylmercury Exposure during Neurodevelopment

Mercury (Hg) is a toxic heavy-metal element, which can be enriched in fauna and flora and transformed into methylmercury (MeHg). MeHg is a widely distributed environmental pollutant that may be harmful to fish-eating populations through enrichment of aquatic food chains. The central nervous system is a primary target of MeHg. Embryos and infants are more sensitive to MeHg, and exposure to MeHg during gestational feeding can significantly impair the homeostasis of offspring, leading to long-term neurodevelopmental defects. At present, MeHg-induced neurodevelopmental toxicity has become a hotspot in the field of neurotoxicology, but its mechanisms are not fully understood. Some evidence point to oxidative damage, excitotoxicity, calcium ion imbalance, mitochondrial dysfunction, epigenetic changes, and other molecular mechanisms that play important roles in MeHg-induced neurodevelopmental toxicity. In this review, advances in the study of neurodevelopmental toxicity of MeHg exposure during pregnancy and the molecular mechanisms of related pathways are summarized, in order to provide more scientific basis for the study of neurodevelopmental toxicity of MeHg.

Transgenerational Effects of Periconception Heavy Metal Administration on Adipose Weight and Glucose Homeostasis in Mice at Maturity

We previously demonstrated that periconception maternal administration (2 mg/kg body weight each) of cadmium chloride (CdCl2) plus methylmercury (II) chloride (CH3HgCl) impaired glucose homeostasis and increased body weights and abdominal adipose tissue weight of male offspring in the F1 generation. However, transgenerational effects of this exposure have not been studied. Therefore, the effects of periconception Cd+Hg administration on indices of chronic diseases at adulthood in F2-F4 generations were examined. Male and female progeny of Cd+Hg periconceptionally treated females, and offspring of vehicle control females were bred with naïve CD1 mice to obtain F2 offspring, with additional crosses as above to the F4 generation (F1-F4 animals were not administered Cd+Hg). Birth weights and litter size were similar in all generations. Indices of impaired glucose homeostasis were observed in matrilineally descended F2 male offspring, including reduced glucose tolerance, along with increased basal phosphorylation of insulin receptor substrate 1 (IRS1) at serine 307 suggesting altered insulin signaling. Reduced glucose tolerance was also seen in F4 males. Increased body weight and/or abdominal adiposity were observed through the F4 generation in males descended matrilineally from the treated female progenitors. Patrilineally derived F2 females displayed reduced glucose tolerance. Females (F2) patrilineally and matrilineally derived displayed significant kidney enlargement. Periconception administration of cadmium and mercury caused persistent transgenerational effects in offspring through the F4 generation in the absence of continued toxicant exposure, with persistent transgenerational effects inherited specifically through the matrilineal germline.

Oxidative Stress in Methylmercury-Induced Cell Toxicity

Methylmercury (MeHg) is a hazardous environmental pollutant, which elicits significant toxicity in humans. The accumulation of MeHg through the daily consumption of large predatory fish poses potential health risks, and the central nervous system (CNS) is the primary target of toxicity. Despite well-described neurobehavioral effects (i.e., motor impairment), the mechanisms of MeHg-induced toxicity are not completely understood. However, several lines of evidence point out the oxidative stress as an important molecular mechanism in MeHg-induced intoxication. Indeed, MeHg is a soft electrophile that preferentially interacts with nucleophilic groups (mainly thiols and selenols) from proteins and low-molecular-weight molecules. Such interaction contributes to the occurrence of oxidative stress, which can produce damage by several interacting mechanisms, impairing the function of various molecules (i.e., proteins, lipids, and nucleic acids), potentially resulting in modulation of different cellular signal transduction pathways. This review summarizes the general aspects regarding the interaction between MeHg with regulators of the antioxidant response system that are rich in thiol and selenol groups such as glutathione (GSH), and the selenoenzymes thioredoxin reductase (TrxR) and glutathione peroxidase (Gpx). A particular attention is directed towards the role of the PI3K/Akt signaling pathway and the nuclear transcription factor NF-E2-related factor 2 (Nrf2) in MeHg-induced redox imbalance.

Dietary compounds as modulators of metals and metalloids toxicity

A large part of the population is exposed to metals and metalloids through the diet. Most of the in vivo studies on its toxicokinetics and toxicity are conducted by means of exposure through drinking water or by intragastric or intraperitoneal administration of aqueous standards, and therefore they do not consider the effect of the food matrix on the exposure. Numerous studies show that some components of the diet can modulate the toxicity of these food contaminants, reducing their effect on a systemic level. Part of this protective role may be due to a reduction of intestinal absorption and subsequent tissue accumulation of the toxic element, although it may also be a consequence of their ability to counteract the toxicity directly by their antioxidant and/or anti-inflammatory activity, among other factors. The present review provides a compilation of existing information about the effect that certain components of the diet have on the toxicokinetics and toxicity of the metals and metalloids of greatest toxicological importance that are present in food (arsenic, cadmium, lead, and mercury), and of their most toxic chemical species.

Effects of Periconception Cadmium and Mercury Co-Administration to Mice on Indices of Chronic Diseases in Male Offspring at Maturity

BACKGROUND

Long-term exposure to the heavy metals cadmium (Cd) and mercury (Hg) is known to increase the risk of chronic diseases. However, to our knowledge, exposure to Cd and Hg beginning at the periconception period has not been studied to date.

OBJECTIVE

We examined the effect of Cd and Hg that were co-administered during early development on indices of chronic diseases in adult male mice.

METHODS

Adult female CD1 mice were subcutaneously administered a combination of cadmium chloride (CdCl₂) and methylmercury (II) chloride (CH₃HgCl) (0, 0.125, 0.5, or 2.0 mg/kg body weight each) 4 days before and 4 days after conception (8 days total). Indices of anxiety-like behavior, glucose homeostasis, endocrine and molecular markers of insulin resistance, and organ weights were examined in adult male offspring.

RESULTS

Increased anxiety-like behavior, impaired glucose homeostasis, and higher body weight and abdominal adipose tissue weight were observed in male offspring of treated females compared with controls. Significantly increased serum leptin and insulin concentrations and impaired insulin tolerance in the male offspring of dams treated with 2.0 mg/kg body weight of Cd and Hg suggested insulin resistance. Altered mRNA abundance for genes associated with glucose and lipid homeostasis (GLUT4, IRS1, FASN, ACACA, FATP2, CD36, and G6PC) in liver and abdominal adipose tissues as well as increased IRS1 phosphorylation in liver (Ser 307) provided further evidence of insulin resistance.

CONCLUSIONS

Results suggest that the co-administration of Cd and Hg to female mice during the early development of their offspring (the periconception period) was associated with anxiety-like behavior, altered glucose metabolism, and insulin resistance in male offspring at adulthood.

Sex- and structure-specific differences in antioxidant responses to methylmercury during early development

Methylmercury (MeHg) is a ubiquitous environmental contaminant and neurotoxin, particularly hazardous to developing and young individuals. MeHg neurotoxicity during early development has been shown to be sex-dependent via disturbances in redox homeostasis, a key event mediating MeHg neurotoxicity. Therefore, we investigated if MeHg-induced changes in key systems of antioxidant defense are sex-dependent. C57BL/6J mice were exposed to MeHg during the gestational and lactational periods, modeling human prenatal and neonatal exposure routes. Dams were exposed to 5ppm MeHg via drinking water from early gestational period until postnatal day 21 (PND21). On PND21 a pair of siblings (a female and a male) from multiple (5-6) litters were euthanized and tissue samples were taken for analysis. Cytoplasmic and nuclear extracts were isolated from fresh cerebrum and cerebellum and used to determine thioredoxin (Trx) and glutathione (GSH) levels, as well as thioredoxin reductase (TrxR) and glutathione peroxidase (GPx) activities. The remaining tissue was used for mRNA analysis. MeHg-induced antioxidant response was not uniform for all the analyzed antioxidant molecules, and sexual dimorphism in response to MeHg treatment was evident for TrxR, Trx and GPx. The pattern of response, namely a decrease in males and an increase in females, may impart differential and sex-specific susceptibility to MeHg. GSH levels were unchanged in MeHg treated animals and irrespective of sex. Trx was reduced only in nuclear extracts from male cerebella, exemplifying a structure-specific response. Results from the gene expression analysis suggest posttranscriptional mechanism of sex-specific regulation of the antioxidant response upon MeHg treatment. The study demonstrates for the first time sex-and structure-specific changes in the response of the thioredoxin system to MeHg neurotoxicity and suggests that these differences in antioxidant responses might impart differential susceptibility to developmental MeHg exposure.

Effects of diphenyl diselenide on methylmercury toxicity in rats

This study investigates the efficacy of diphenyl diselenide [(PhSe)2] in attenuating methylmercury- (MeHg-)induced toxicity in rats. Adult rats were treated with MeHg [5 mg/kg/day, intragastrically (i.g.)] and/ or (PhSe)2 [1 mg/kg/day, intraperitoneally (i.p.)] for 21 days. Body weight gain and motor deficits were evaluated prior to treatment, on treatment days 11 and 21. In addition, hepatic and cerebral mitochondrial function (reactive oxygen species (ROS) formation, total and nonprotein thiol levels, membrane potential (ΔΨm), metabolic function, and swelling), hepatic, cerebral, and muscular mercury levels, and hepatic, cerebral, and renal thioredoxin reductase (TrxR) activity were evaluated. MeHg caused hepatic and cerebral mitochondrial dysfunction and inhibited TrxR activity in liver (38,9%), brain (64,3%), and kidney (73,8%). Cotreatment with (PhSe)2 protected hepatic and cerebral mitochondrial thiols from depletion by MeHg but failed to completely reverse MeHg's effect on hepatic and cerebral mitochondrial dysfunction or hepatic, cerebral, and renal inhibition of TrxR activity. Additionally, the cotreatment with (PhSe)2 increased Hg accumulation in the liver (50,5%) and brain (49,4%) and increased the MeHg-induced motor deficits and body-weight loss. In conclusion, these results indicate that (PhSe)2 can increase Hg body burden as well as the neurotoxic effects induced by MeHg exposure in rats.

Relationships between trace element concentrations in chorionic tissue of placenta and umbilical cord tissue: potential use as indicators for prenatal exposure

The role of the placenta was assessed by comparing the profiles of methylmercury (MeHg), inorganic mercury (I-Hg), lead (Pb), cadmium (Cd), selenium (Se), zinc (Zn), and copper (Cu) in freeze-dried chorionic tissue of the placenta and umbilical cord tissue. The significance of the placenta and cord tissue as predictors of prenatal exposure to these trace elements in pregnant women and newborns was also examined by comparing the element profiles among placenta and cord tissue, and maternal and cord blood red blood cells (RBCs). The samples were collected from 48 mother-child pairs at birth in the general population of Japanese. The concentrations of all elements, except for MeHg, were significantly higher in placenta than in cord tissue. In particular, the Cd showed the highest placenta vs. cord tissue ratio (59:1), followed by I-Hg (2.4:1), indicating that the placental barrier works most strongly against Cd among the examined toxic elements. Contrary to the other elements, the MeHg concentration in cord tissue was significantly higher (1.6 times) than that in placenta, indicating its exceptionally high placental transfer. The MeHg in placenta showed significant correlations with total mercury (T-Hg) in maternal and cord RBCs (rs=0.80 and 0.91, respectively). The MeHg in cord tissue also showed significant correlations with T-Hg in maternal and cord RBCs (rs=0.75 and 0.85, respectively). Therefore, both placenta and cord tissue are useful for predicting maternal and fetal exposure to MeHg. The Se concentration in placenta showed significant but moderate correlations with that in maternal and cord RBCs (rs=0.38 and 0.57, respectively). The Pb, Zn, and Cu concentrations in placenta and cord tissue showed no significant correlations with those in maternal and cord RBCs. As an exception, the Cd concentration in placenta showed a moderate but significant correlation (rs=0.41) with that in maternal RBCs, suggesting that the placenta is useful for predicting maternal exposure to Cd during gestation.

Chronic exposure of adult rats to low doses of methylmercury induced a state of metabolic deficit in the somatosensory cortex

Because of the ever-increasing bioaccumulation of methylmercury (MeHg) in the marine food chain, human consumers are exposed to low doses of MeHg continually through seafood consumption. Epidemiological studies strongly suggest that chronic prenatal exposure to nanomolar of MeHg has immense negative impacts on neurological development in neonates. However, effects of chronic exposure to low doses (CELDs) of MeHg in adult brains on a molecular level are unknown. The current study aims to investigate the molecular effects of CELD of MeHg on adult somatosensory cortex in a rat model using proteomic techniques. Young adult rats were fed with a low dose of MeHg (40 μg/kg body weight/day) for a maximum of 12 weeks. Whole proteome expression of the somatosensory cortex (S1 area) of normal rats and those with CELD to MeHg were compared. Levels of MeHg, total calcium, adenosine triphosphate (ATP), and pyruvate were also measured. Comparative proteomic studies of the somatosensory cortexes revealed that 94 proteins involved in the various metabolic processes (including carbohydrate metabolism, generation of precursors for essential metabolites, energy, proteins, cellular components for morphogenesis, and neurotransmission) were down-regulated. Consequently, levels of important end products of active metabolism including ATP, pyruvate, and total calcium were also found to be significantly reduced concomitantly. Our results showed that CELD of MeHg induced a state of metabolic deficit in the somatosensory cortex of adult rats.

Changes in body burden of mercury, lead, arsenic, cadmium and selenium in infants during early lactation in comparison with placental transfer

The developing brains of both fetuses and infants are susceptible to environmental contaminants. However, the contribution of breast-feeding to the element body burden in infants remains unclear. The main objective of this study was to investigate the changes in body burden of elements such as methylmercury, lead, arsenic, cadmium, and selenium in infants during a 3-month breast-feeding period compared with placental transfer of the elements. Element concentrations were measured in maternal and umbilical cord (fetus) red blood cells (RBCs) at parturition and in infant RBCs at 3 months. Most of the mercury in RBCs is in the methyl form, and the total mercury concentration in RBCs reflects methylmercury exposure. The mercury level in cord RBCs was approximately 1.5 times higher than that in mothers, while in infants, it declined by approximately 60% after 3-months' breast-feeding. The cord selenium level was similar to the maternal level, but declined approximately 75% after 3-months' breast-feeding in infants. Lead and arsenic concentrations in cord RBCs were about 60% of the maternal levels, and remained constant until the 3-month study period. The cadmium level in cord RBCs was about 20% of that in maternal RBCs, and remained almost constant until the end of the 3-month study period. In conclusion, although pregnant women should pay attention to avoid high methylmercury exposure, element exposure through breast-feeding does not pose any great concern in this population.

The prenatal toxic effect of methylmercury on the development of the appendicular skeleton of rat fetuses and the protective role of vitamin E

Methylmercury (MeHg) is an environmental contaminant that is found in many ecosystems. Many studies reported that MeHg toxicity is accompanied by increased lipid peroxidation that may lead to oxidative damage to DNA, RNA, and proteins. Vitamin E is considered as the most effective antioxidant preventing lipid peroxidation. The aim of this study was to evaluate the effects of MeHg exposure during pregnancy on the development of the appendicular skeleton in rat fetuses and whether vitamin E administration could reduce this toxicity. Positively mated adult female Sprague-Dawley rats were used and divided into the following experimental groups: control group, received only deionized water, and four MeHg treated groups received 1 mg of MeHg/kg/d, 2 mg of MeHg/kg/d, 1 mg of MeHg/kg/d plus 150 mg of vitamin E/kg/d, and 2 mg of MeHg/kg/d, plus 150 mg of vitamin E/kg/d starting from Day 0 of gestation. On Day 20 of gestation, the fetuses from the pregnant rats were extracted and the fetal growth parameters were evaluated. Skeletal evaluation of ossification of both fore- and hind-limbs, and coxal bones were undertaken. Results showed that treatment with MeHg caused adverse effects on fetal growth parameters and ossification of the bones. The coadministration of vitamin E with MeHg revealed an improvement in these parameters. These results suggest that vitamin E may ameliorate some aspects of MeHg developmental toxicity. The underlying and human health implications warrant further investigations.

Assessment of mercury and selenium concentrations in captive bottlenose dolphin's (Tursiops truncatus) diet fish, blood, and tissue

Concentrations of total mercury (Hg) and selenium (Se) were determined in diet fish and whole blood and tissue samples from seven bottlenose dolphins (Tursiops truncatus) housed at the National Aquarium Baltimore (NAB). In addition, concentrations of monomethylmercury (CH(3)Hg(+)) were determined in diet fish and dolphins' tissue samples. The data were compared with the values found in wild populations to better understand how the dietary Hg and Se uptake rates affect the Hg and Se levels in dolphins. The diet fish total Hg concentrations ranged between 14 and 47 ng g(-1) and were markedly lower than for similar fish found in Florida, South Carolina, and other aquaria. CH(3)Hg(+) accounted for 85 to 91% of the total Hg found in diet fish. The diet fish Se concentrations ranged between 270 and 800 ng g(-1), indicating excess molar concentrations of Se over Hg. The Hg concentration range in the blood of NAB dolphins was 27-117 ng g(-1) and the concentrations were about one order of magnitude and several factors lower than the concentrations found in the blood of wild bottlenose dolphins in Florida and in South Carolina, respectively. The total Hg and CH(3)Hg(+) in tissue samples were also significantly lower than the reported values obtained from wild populations of bottlenose dolphins. The differences in the Hg concentrations in the dolphins' blood may be due to the different levels of Hg atmospheric deposition in the area where the dolphins' diet fish were found. The Se concentration range in the blood of NAB dolphins was 221-297 ng g(-1) which was two factors lower than the values found in wild populations. The lower Hg levels, as well as higher Se:Hg molar ratios in the blood of NAB dolphins, suggest that NAB dolphins may be less susceptible to the potential neurotoxicity from the CH(3)Hg(+) in their blood.

Recent evidence from epidemiological studies on methylmercury toxicity

More than fifty years have passed since the outbreak of Minamata disease, and large-scale methylmercury poisoning due to industrial effluents or methylmercury-containing fungicide intoxication has scarcely happened in developed countries. On the other hand, widespread environmental mercury contamination has occurred in gold and mercury mining areas of developing countries. In this article, we provided an overview of recent studies addressing human health effects of methylmercury, which we searched using the PubMed of the US National Library of Medicine. The following suggestions were obtained for low-level methylmercury exposure: (1) In recent years, the proportion of human studies addressing methylmercury has tended to decrease. (2) Prenatal exposure to methylmercury through fish intake, even at low levels, adversely affects child development after adjusting for polychlorinated biphenyls and maternal fish intake during pregnancy, whereas maternal seafood intake has some benefits. (3) Long-term methylmercury exposure through consumption of fish such as bigeye tuna and swordfish may pose a potential risk of cardiac events involving sympathovagal imbalance. (4) In measuring methylmercury levels in preserved umbilical cord collected from inhabitants born in Minamata areas between 1945 and 1989, the elevated concentrations (≥1 mg/g) were observed mainly in inhabitants born between 1947 and 1968, and the peak coincided with the peak of acetaldehyde production in Minamata. (5) Since some developing countries appear to be in similar situations to Japan in the past, attention should be directed toward early recognition of a risky agent and precautions should be taken against it.

Mercury levels in maternal and cord blood and attained weight through the 24 months of life

Birth weight is a strong determinant of attained weight at early ages. Until now, many studies have reported that low birth weight corresponds with high mercury levels. However, the relationship between mercury exposure and attained weight of infant has not been well studied. Therefore, the aim of the present study was to assess the degree of prenatal exposure to mercury by measuring the total mercury levels in maternal and cord blood, and examine the relationship between the mercury level during pregnancy and the attained weight of infant during the first 24 months of life. The prospective cohort study of Mothers and Children's Environmental Health (MOCEH) was built up in 2006, and 921 mother-infant pairs were recruited. Information on the socio-demographic characteristics, health behavior and environmental exposure were collected from an interview with trained nurses. After delivery, infants and mothers were followed up at 6, 12 and 24 months and the weights of the infants were measured. The mercury concentrations in the late maternal blood (β=-0.19. p=0.05) and cord blood (β=-0.36. p=0.01) were negatively associated with the infants' attained weight over the first 24 months of age. The infants' attained weight in the small for their gestational age (SGA) group was lower than the normal birth weight group at the highest quartile of the mercury level. Therefore, efforts should be made to reduce the mercury level in the maternal blood at late pregnancy and cord blood. Further research on the possible harmful effects of prenatal mercury exposure on postnatal growth is recommended.

Biochemical factors modulating cellular neurotoxicity of methylmercury

Methylmercury (MeHg), an environmental toxicant primarily found in fish and seafood, poses a dilemma to both consumers and regulatory authorities, given the nutritional benefits of fish consumption versus the possible adverse neurological damage. Several studies have shown that MeHg toxicity is influenced by a number of biochemical factors, such as glutathione (GSH), fatty acids, vitamins, and essential elements, but the cellular mechanisms underlying these complex interactions have not yet been fully elucidated. The objective of this paper is to outline the cellular response to dietary nutrients, as well as to describe the neurotoxic exposures to MeHg. In order to determine the cellular mechanism(s) of toxicity, the effect of pretreatment with biochemical factors (e.g., N-acetyl cysteine, (NAC); diethyl maleate, (DEM); docosahexaenoic acid, (DHA); selenomethionine, SeM; Trolox) and MeHg treatment on intercellular antioxidant status, MeHg content, and other endpoints was evaluated. This paper emphasizes that the protection against oxidative stress offered by these biochemical factors is among one of the major mechanisms responsible for conferring neuroprotection. It is therefore critical to ascertain the cellular mechanisms associated with various dietary nutrients as well as to determine the potential effects of neurotoxic exposures for accurately assessing the risks and benefits associated with fish consumption.

Cerebral gene expression in response to single or combined gestational exposure to methylmercury and selenium through the maternal diet

Controversy remains regarding the safety of consuming certain types of seafood, particularly during pregnancy. While seafood is rich in vital nutrients, it may also be an important source of environmental contaminants such as methylmercury (MeHg). Selenium (Se) is one essential element present in seafood, hypothesised to ameliorate MeHg toxicity. The aim of the present study was to ascertain the impact of Se on MeHg-induced cerebral gene expression in a mammalian model. Microarray analysis was performed on brain tissue from 15-day-old mice that had been exposed to MeHg throughout development via the maternal diet. The results from the microarray analysis were validated using qPCR. The exposure groups included: MeHg alone (2.6 mg kg(-1)), Se alone (1.3 mg kg(-1)), and MeHg + Se. MeHg was presented in a cysteinate form, and Se as Se-methionine, one of the elemental species occurring naturally in seafood. Eight genes responded to Se exposure alone, five were specific to MeHg, and 63 were regulated under the concurrent exposure of MeHg and Se. Significantly enriched functional classes relating to the immune system and cell adhesion were identified, highlighting potential ameliorating mechanisms of Se on MeHg toxicity. Key developmental genes, such as Wnt3 and Sparcl1, were also identified as putative ameliorative targets. This study, utilising environmentally realistic forms of toxicants, delivered through the natural route of exposure, in association with the power of transcriptomics, highlights significant novel information regarding putative pathways of selenium and MeHg interaction in the mammalian brain.

Interaction between GSTM1/GSTT1 polymorphism and blood mercury on birth weight

BACKGROUND

Mercury (Hg) is toxic to both the reproductive and nervous systems. In addition, glutathione S-transferases (GSTs), which conjugate glutathione to a variety of electrophilic compounds, are involved in the detoxification of Hg.

OBJECTIVE

In this study we examined the association between prenatal exposure to Hg and birth weight as well as the influence of GST polymorphisms.

METHODS

The total Hg concentration in maternal and cord blood was measured from 417 Korean women and newborns in the Mothers and Children's Environmental Health study from 2006 to 2008. Information on birth weight was collected from the patients' medical records. The genotyping of glutathione S-transferase M1 (GSTM1) and glutathione S-transferase T1 (GSTT1) polymorphisms was carried out using polymerase chain reaction. Regression analysis was performed to determine the association between the blood Hg concentration and birth weight in mothers with GSTM1 and GSTT1 polymorphisms.

RESULTS

The geometric mean levels of Hg in the maternal blood during late pregnancy and in cord blood were 3.30 microg/L and 5.53 microg/L, respectively. For mothers with the GSTT1 null genotype, elevated Hg levels in maternal blood during late pregnancy were associated with an increased risk of lower birth weight. For mothers with both GSTM1 and GSTT1 null genotype, both maternal and cord blood Hg levels were associated with lower birth weight.

CONCLUSIONS

This study suggests that the interactions of Hg with GSTM1 and GSTT1 polymorphisms play a role in reducing birth weight.

Mercury and heavy metal profiles of maternal and umbilical cord RBCs in Japanese population

Mercury (Hg) and other heavy metal profiles, namely, lead (Pb), arsenic (As), cadmium (Cd), and selenium (Se) were investigated in maternal and umbilical cord (cord) red blood cells (RBCs) at parturition in Japanese population. Correlation coefficients of Hg, Pb, As, Cd, and Se between maternal and cord RBCs were 0.91, 0.79, 0.89, 0.31, and 0.76, respectively, and the respective means of cord/maternal RBCs ratios were 1.63, 0.52, 0.62, 0.12, and 1.18. These results indicate that fetal exposure to these metals (excluding Cd) strongly reflected each maternal exposure level. Among these metals, the placental transfer of methylmercury (MeHg) seemed to be extremely high but that of Cd to be limited. Hg showed positive correlations with Se in maternal RBCs but not in cord RBCs, and the Se/Hg molar ratio was lower in the latter, suggesting that the protective effects of Se against MeHg are less expected in fetuses than in mothers.

Developmental selenomethionine and methylmercury exposures affect zebrafish learning

Methylmercury (MeHg) is a ubiquitous environmental pollutant and has been shown to affect learning in vertebrates following relatively low exposures. Zebrafish were used to model long-term learning deficits after developmental MeHg exposure. Selenomethionine (SeMet) co-exposure was used to evaluate its role in neuroprotection. Embryos were exposed from 2 to 24h post fertilization to (1) MeHg without SeMet, (2) SeMet without MeHg and (3) in combination of MeHg and SeMet. In case (1), the levels of MeHg were 0.00, 0.01, 0.03, 0.06, 0.10, and 0.30 microM. In case (2), the levels of SeMet were 0.00. 0.03, 0.06, 0.10, and 0.30 microM. In case (3), co-exposure levels of (MeHg, SeMet) were (0.03, 0.03), (0.03, 0.06), (0.03, 0.10), (0.03, 0.30), (0.10, 0.03), (0.10, 0.06), (0.10, 0.10), and (0.10, 0.30) microM. Learning functions were tested in individual adults, 4 months after developmental exposure using a spatial alternation paradigm with food delivery on alternating sides of the aquarium. Low levels of MeHg (<0.1 microM) exposure delayed learning in treated fish; fish exposed to higher MeHg levels were unable to learn the task; SeMet co-exposure did not prevent this deficit. These data are consistent with findings in laboratory rodents. The dorsal and lateral telencephalon are the primary brain regions in fish involved in spatial learning and memory. Adult telencephalon cell body density decreased significantly at all MeHg exposures >0.01 microM MeHg. SeMet co-exposure ameliorated but did not prevent changes in telencephalon cell body density. In summary, MeHg affected both learning and brain structure, but SeMet only partially reversed the latter.

Mercury and Selenium - A Review on Aspects Related to the Health of Human Populations in the Amazon

Mercury (Hg) toxicity is governed by cellular thiol compounds and its capacity to generate reactive oxygen radicals and oxidative stress. Selenium (Se) plays a key role in the prevention of the toxic effects of Hg by modulating the activity of several Se-dependent enzymes, including glutathione peroxidase (GSH-Px). In addition, dietary Se can reduce Hg toxicity by directly interacting with either Hg(II) or methylmercury (MeHg) to form inert products, such as HgSe complexes.. Although experimental and environmental data have indicated a protective role for selenium against Hg toxicity, human data are more limited and somewhat conroversial In the Amazon Region of Brazil, Hg pollution is rampant as a result of gold (Au) mining and other anthropogenic factors, leading to pervasive release of large quantities of metallic Hg⁰ into the environment. Exposure to Hg in this region is associated with direct occupational exposure in the gold mining industry, as well as consumption by in inhabitants of riverside communities of a diet rich in MeHg-contaminated fish. Human exposure to MeHg in the Amazon through the diet has been monitored by measuring Hg and MeHg in hair samples. In this paper, we review the environmental contamination of Hg in the Amazon and detail human exposures in populations of this region. We conclude with a brief synopsis on Se levels in the Amazon population and provide a brief review of data available on the interaction between Hg and Se in this region. Overall, the literature supports the notion that low environmental Se is linked to susceptibility to Hg toxicity and that Se levels could be used as a bioindicator to monitor the health of Hg exposed subjects. However, in light of the limited human data on this subject, further epidemiological studies are needed to clarify how changes in Se levels modify the toxicity of environmental Hg.

Does selenium modify neurobehavioural impacts of developmental methylmercury exposure in mice?

There is controversy as to whether low-level chronic exposure to methylmercury (MeHg) through maternal fish consumption may cause subtle effects in the developing child, owing in part to the potential ameliorating effects of beneficial seafood nutrients. The aim of the present investigation was to assess the ameliorating potential of selenium (Se; as the naturally occurring methionine complex) on the neurobehavioural toxicity of foodborne MeHg (as the naturally occurring cysteinate) in prenatally exposed mice. Pups from dams exposed to a diet containing 3mg/kg of MeHg fed throughout gestation showed delayed fur development and impaired performance in a motor function assessment. These effects were not apparent in pups born to dams concurrently exposed to Se (at 1.3mg/kg). These results, using natural dietary forms of the elements administered through the relevant exposure pathway, suggest only minor impacts of MeHgCys on neurobehaviour, and possible amelioration of these effects by Se.

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

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

Trace element levels in foetus-mother pairs of short-beaked common dolphins (Delphinus delphis) stranded along the French coasts

Tissues of foetus-mother pairs of common dolphins (Delphinus delphis) stranded along the French coasts (Bay of Biscay and English Channel) were analysed for their Cd, Cu, Hg, Se and Zn contents. In the kidneys, foetal Cd levels were extremely low, and strong relationships between Cu and Zn suggested the involvement of metallothioneins since early foetal life. The results also indicated a limited maternal transfer of Hg during pregnancy since levels in the tissues of foetuses were below 1 microg g(-1) w.wt. However, hepatic Hg levels in foetuses increased with body length, and were also proportionate to maternal hepatic, renal and muscular Hg levels. Lastly, affinities between Hg and Se in tissues would participate in Hg neutralisation in both mothers--through tiemannite granules--and fetuses--through reduced glutathione--counteracting the toxic effects linked to the particularly high quantities of methyl-Hg to which marine mammals are naturally exposed.

Selenomethionine reduces visual deficits due to developmental methylmercury exposures

Developmental exposures to methylmercury (MeHg) have life-long behavioral effects. Many micronutrients, including selenium, are involved in cellular defenses against oxidative stress and may reduce the severity of MeHg-induced deficits. Zebrafish embryos (<4 h post fertilization, hpf) were exposed to combinations of 0.0-0.30 microM MeHg and/or selenomethionine (SeMet) until 24 hpf then placed in clean medium. Fish were tested as adults under low light conditions ( approximately 60 microW/m(2)) for visual responses to a rotating black bar. Dose-dependent responses to MeHg exposure were evident (ANOVA, P<0.001) as evidenced by reduced responsiveness, whereas SeMet did not induce deficits except at 0.3 microM. Ratios of SeMet:MeHg of 1:1 or 1:3 resulted in responses that were indistinguishable from controls (ANOVA, P<0.001). No gross histopathologies were observed (H&E stain) in the retina or optic tectum at any MeHg concentration. Whole-cell, voltage-gated, depolarization-elicited outward K(+) currents of bipolar cells in intact retina of slices adult zebrafish were recorded and outward K(+) current amplitude was larger in bipolar cells of MeHg-treated fish. This was due to the intense response of cells expressing the delayed rectifying I(K) current; cells expressing the transient I(A) current displayed a slight trend for smaller amplitude among MeHg-treated fish. Developmental co-exposure to SeMet reduced but did not eliminate the increase in the MeHg-induced I(K) response, however, I(A) responses increased significantly over MeHg-treated fish to match control levels. Electrophysiological deficits parallel behavioral patterns in MeHg-treated fish, i.e., initial reactions to the rotating bar were followed by periods of inactivity and then a resumption of responses.

Dependence of blood indices of selenium and mercury on estimated fish intake in a national survey of British adults

OBJECTIVE

Contributions of fish and other foods to variance of selenium and mercury status were studied in British adults.

SETTING AND DESIGN

Plasma and red-cell selenium and whole-blood mercury concentrations were measured during the National Diet and Nutrition Survey of Adults aged 19-64 years in mainland Britain, 2000-2001 (n = 1216). Food intake was weighed for seven consecutive days, and foods were combined in groups for data analysis. Four subsidiary groups characterised fish intakes: fried white fish, 'other' white fish, shellfish and oily fish.

RESULTS

Geometric means and 5-95% ranges were: for whole-blood mercury, 5.61 (1.30-22.2) nmol l(-1); for plasma selenium, 1.09 (0.83-1.43) micromol l(-1); for red-cell selenium, 1.64 (1.14-2.40) micromol l(-1). Twenty-eight per cent had no fish intake recorded during 7 days; the remaining 72% had a median intake of 237 g over the 7-day period, 5-95% range 45-780 g. Total fish intake was strongly and directly correlated with blood mercury, and moderately with red-cell and plasma selenium. Thus, sqrt(total fish intake) was correlated with: loge(blood Hg), t = +19.7; loge(plasma Se), t = +9.8; and loge(red-cell Se), t = +9.6, all P < 0.0001. All three biochemical (mercury and selenium) indices were strongly correlated with oily fish intake, and moderately correlated with shellfish and 'other' ( = non-fried) white fish, but none was significantly correlated with fried white fish. Blood mercury was strongly and directly correlated with red-cell and plasma selenium, and both increased with age.

CONCLUSIONS

Dietary fish, especially oily fish, is a strong predictor of blood mercury and selenium in British adults.

Gender differences in the disposition and toxicity of metals

There is increasing evidence that health effects of toxic metals differ in prevalence or are manifested differently in men and women. However, the database is small. The present work aims at evaluating gender differences in the health effects of cadmium, nickel, lead, mercury and arsenic. There is a markedly higher prevalence of nickel-induced allergy and hand eczema in women compared to men, mainly due to differences in exposure. Cadmium retention is generally higher in women than in men, and the severe cadmium-induced Itai-itai disease was mainly a woman's disease. Gender differences in susceptibility at lower exposure are uncertain, but recent data indicate that cadmium has estrogenic effects and affect female offspring. Men generally have higher blood lead levels than women. Lead accumulates in bone and increased endogenous lead exposure has been demonstrated during periods of increased bone turnover, particularly in women in pregnancy and menopause. Lead and mercury, in the form of mercury vapor and methylmercury, are easily transferred from the pregnant women to the fetus. Recent data indicate that boys are more susceptible to neurotoxic effects of lead and methylmercury following exposure early in life, while experimental data suggest that females are more susceptible to immunotoxic effects of lead. Certain gender differences in the biotransformation of arsenic by methylation have been reported, and men seem to be more affected by arsenic-related skin effect than women. Experimental studies indicate major gender differences in arsenic-induced cancer. Obviously, research on gender-related differences in health effects caused by metals needs considerable more focus in the future.

Prenatal methylmercury exposure increases responding under clocked and unclocked fixed interval schedules of reinforcement

Recent experiments have suggested that developmental methylmercury exposure produces perseverative behavior in adulthood. In the present experiment, interactions between developmental low-level methylmercury (MeHg) and nutritionally relevant dietary selenium (Se) on operant behavior and its persistence were examined in aged animals. Female rats were exposed, in utero, to 0, 0.5, or 5 ppm mercury as MeHg via drinking water, approximating mercury exposures of 0, 40, and 400 microg/kg/day. They also received both pre- and chronic post-natal exposure to a diet that was marginal (0.06 ppm) or rich (0.6 ppm) in Se, a nutrient believed to protect against MeHg's toxicity. This created a 2 (chronic Se)x3 (gestational MeHg) full factorial design, with 6-8 female rats per cell. At eleven months of age, a multiple schedule consisting of alternating fixed interval (FI) and clocked FI (CFI) components was arranged. The CFI component was divided into 5, 24-second bins, each associated with a different auditory stimulus, providing a "clock." Low and high response rates were evaluated using the initial 40% (bins 1 and 2) and last 20% (bin 5) of the FI and CFI components, respectively. Rats exposed to 5 ppm Hg made more responses than the other two groups during the last 20% of the intervals, regardless of selenium exposure or presence of the clock stimuli. They did not differ from the other groups during the initial 40% of the FI and CFI components. Following reinforcement omission for half of the intervals at 21 months of age, the 5 ppm Hg group continued to respond at higher rates than the other groups in both components.

Whole blood mercury and selenium concentrations in a selected Austrian population: does gender matter?

BACKGROUND

Data on mercury exposure of the Austrian population were inadequate. This study was performed to determine the causal factors underlying mercury exposure and selenium concentrations, and to estimate the gender-related health impacts.

METHODOLOGY

Venous blood samples of 78 women and 81 men were drawn at the Austrian Red Cross, Vienna. Mercury contents in acid-digested whole blood samples were measured after amalgam enrichment by CV-AAS, and selenium by AAS (heated quartz-cell) after hydrid formation.

RESULTS

The average total mercury blood content of Austrians was low (2.38+/-1.55 microgL(-1); N=152). Mercury and selenium concentrations were not different between the genders (P>0.05) but we observed discrepancies regarding the causal factors. Mercury levels in men were influenced not only by fish consumption but also by age, education level, and amalgam fillings, whereas in women, only the diet (fish/seafood, red wine consumption) determined blood mercury (P<0.05). Moreover, only the males indicated a depressive effect of dental amalgam on hematocrit (P<0.05). Regarding selenium, age and alcohol consumption led to lower concentrations in men, whereas a high-level education had the opposite effect; no determinant was found for women. For the whole study group, a significant effect of chronic disease on selenium levels could be detected (P<0.05). 18% of women and 13% of men showed marginal selenium deficiency (blood selenium<65 microgL(-1)). Selenium and mercury concentrations were not correlated.

CONCLUSIONS

Our results indicate the need to evaluate and integrate gender-related findings in metal toxicology and trace element research, because different causal factors require different preventive measures to reduce mercury exposure and the risk of low selenium concentrations. Future research is needed on the gender- and age-related differences in fish/seafood consumption habits, the modifications of mercury toxicokinetics through sex hormones, the selenium supply in Austria, and the clinical relevance of a low selenium status.

Gestational exposure to methylmercury and selenium: effects on a spatial discrimination reversal in adulthood

Selenium, a nutrient, and methylmercury, a developmental neurotoxicant, are both found in fish. There are reports that selenium sometimes ameliorates methylmercury's neurotoxicity, but little is known about the durability of this protection after low-level gestational exposure. Developmental methylmercury exposure disrupts behavioral plasticity, and these effects extend well into adulthood and aging. The present experiment was designed to examine interactions between developmental low-level methylmercury and nutritionally relevant dietary selenium on discrimination reversals in adulthood. Female rats were exposed, in utero, to 0, 0.5, or 5 ppm mercury as methylmercury via drinking water, approximating mercury exposures of 0, 40, and 400 microg/kg/day. They also received both prenatal and postnatal exposure to a diet containing selenium from casein only (0.06 ppm) or 0.6 ppm selenium, creating a 2 (chronic Se)x3 (gestational MeHg) full factorial design, with six to eight rats per cell. Behavior was evaluated with a spatial discrimination procedure using two levers and sucrose reinforcers. All groups acquired the original discrimination similarly. Rats exposed to low selenium (0.06 ppm), regardless of MeHg exposure, required more sessions to complete the first reversal and made more omissions during this reversal than high selenium (0.6 ppm) animals, but the two diet groups did not differ on subsequent reversals. Rats exposed to MeHg, regardless of selenium exposure, made more errors than controls on the first and third reversals, which was away from the original discrimination. MeHg-exposed animals also had shorter choice latencies than controls during the first session of a reversal. Low selenium increased the number of omissions during a reversal, whereas high MeHg exposure produced perseverative responding (errors) on the lever that was reinforced during the original discrimination. However, there was no interaction between selenium and MeHg exposure.

Motor function following developmental exposure to PCBS and/or MEHG

Recent studies raise concern for combined exposure to polychlorinated biphenyls (PCBs) and methylmercury (MeHg), two environmental contaminants that are found in fish and seafood. Past accidental poisonings in humans show that exposure to high levels of either contaminant is associated with motor impairments, including alterations in cerebellar functions such as balance and coordination. Epidemiological studies of lower level exposures suggest some neuromotor impairment in exposed children, but the majority of these studies have focused on cognitive endpoints rather than examining a full-range of motor function. In particular, the cerebellum could be a sensitive target for combined PCB and MeHg toxicity. MeHg exposure during development damages the cerebellum along with cortical areas, and PCBs may also cause cerebellar damage via thyroid hormone disruption during development. In addition, in vitro studies report interactive effects of PCBs and MeHg on ryanodine-sensitive calcium signaling. Ryanodine receptors are found especially within the cerebellum, and alterations in calcium signaling within the cerebellum could impair long-term depression and subsequent motor learning. This article reviews the motor impairments reported in humans and laboratory animals following exposure to PCBs and/or MeHg during development. There is need for a better understanding of the interactive effects of PCBs and MeHg, especially in regard to motor function.

Selenoprotein W as molecular target of methylmercury in human neuronal cells is down-regulated by GSH depletion

Methylmercury (MeHg) is well known as a neurotoxic chemical. However, little is mentioned about its neurotoxic mechanism or molecular target in human neuronal cells in particular. We show in this study that exposure of human neuronal cell line, SH-SY5Y, to MeHg dose- and time-dependently impairs viability and mRNA expression of selenoprotein W (SeW) with a significant difference, unlike other selenoenzymes such as, SeP, GPX4, 5DI, and 5'DI. Using real-time RT PCR, the influence of selenium (Se) and glutathione (GSH) on SeW expression was also investigated. While Se depletion caused a weakly reduced SeW mRNA levels, additional Se caused an increase of SeW mRNA levels. Although 2 mM GSH had induced a weak shift on SeW level, the expression of SeW mRNA was down-regulated in SH-SY5Y cells treated with 25 microM BSO, an inhibitor of GSH synthesis. To understand the relationship between a decrease of SeW expression and intracellular GSH and ROS, we measured the concentration of intracellular GSH and ROS in cells treated to 1.4 microM MeHg using fluorescence based assays. A positive correlation was found between SeW mRNA level and intracellular GSH but no significant correlation was observed between intracellular ROS and SeW mRNA level or intracellular GSH contents. Therefore, we suggest that SeW is the novel molecular target of MeHg in human neuronal cells and down-regulation of this selenoenzyme by MeHg is dependent not on generation of ROS but on depletion of GSH.

Prenatal exposure to methylmercury and child development: influence of social factors

BACKGROUND

In the Seychelles Child Development Main Study (SCDS), at 19 months of age, enhanced MDI scores were associated with increasing methylmercury (MeHg) exposure in higher caregiver IQ groups at several levels of family income. We performed a similar analysis of the 66-month evaluations to determine if the modifying influences of social and environmental factors were consistent with those previously observed.

METHODS

Prenatal MeHg exposure was determined by analysis of maternal hair growing during pregnancy. Children in the cohort (N=711) were evaluated for cognitive ability (McCarthy Scales of Children's Abilities), language development (Preschool Language Scale), drawing and copying (Bender Gestalt Test), scholastic achievement (the Woodcock-Johnson Test of Achievement), and behavior (the Child Behavior Checklist). Interactions between prenatal MeHg exposure and caregiver intelligence, socioeconomic status (SES), home environment, and gender were examined by multiple regression analysis.

RESULTS

The median prenatal MeHg exposure was 5.9 ppm (range=0.5-26.7 ppm). Gender significantly influenced the association between prenatal exposure and drawing and copying; however, the effects were not consistent. Prenatal exposure interacted with one or more social or environmental covariates for general cognitive ability, overall language ability, and prearithmetic achievement. Again, the effects were not consistent across either endpoints or covariate categories.

CONCLUSIONS

Evidence of a small influence by social and environmental variables at 66 months is neither internally consistent nor consistent with earlier results. Overall, a consistent pattern of effect modification (EM) has not been observed, suggesting that the results may be due to chance.

Attenuated growth of breast-fed children exposed to increased concentrations of methylmercury and polychlorinated biphenyls

Breast-feeding has been linked to slowed postnatal growth. Although the basis for this "weanling's dilemma" is unclear, environmental contaminants in human milk may be of relevance. We studied a Faroese birth cohort of 182 singleton children, born at term in 1994-95. Concentrations of mercury in cord blood and of polychlorinated biphenyls in maternal milk were measured, and duration of breast-feeding was recorded. At 18 months, children who had been exclusively breast-fed for at least 6 months weighed 0.59 kg less [95% confidence interval (CI) = 0.03, 1.16 kg] and were 1.50 cm [95% CI = 0.52, 2.47 cm] shorter than those not breast-fed. However, calculated transfer of contaminants from human milk fully explained the attenuated growth. Irrespective of duration of breast-feeding, a doubling of the mercury concentration in cord blood was associated with a decrease in weight at 18 months by 0.19 kg (95% CI = 0.03, 0.35 kg) and in height by 0.26 cm (95% CI = -0.02, 0.55 cm). Weight and height at 42 months showed the same tendencies, but the main effect occurred before 18 months of age. Thus, in communities with increased contaminant exposures, risks associated with lactational transfer of toxicants to the infant must be considered when judging the benefits of prolonged breast-feeding.

Neurotoxicity of organomercurial compounds

Mercury is a ubiquitous contaminant, and a range of chemical species is generated by human activity and natural environmental change. Elemental mercury and its inorganic and organic compounds have different toxic properties, but all them are considered hazardous in human exposure. In an equimolecular exposure basis, organomercurials with a short aliphatic chain are the most harmful compounds and they may cause irreversible damage to the nervous system. Methylmercury (CH(3)Hg(+)) is the most studied following the neurotoxic outbreaks identified as Minamata disease and the Iraq poisoning. The first description of the CNS pathology dates from 1954. Since then, the clinical neurology, the neuropathology and the mechanisms of neurotoxicity of organomercurials have been widely studied. The high thiol reactivity of CH(3)Hg(+), as well as all mercury compounds, has been suggested to be the basis of their harmful biological effects. However, there is clear selectivity of CH(3)Hg(+) for specific cell types and brain structures, which is not yet fully understood. The main mechanisms involved are inhibition of protein synthesis, microtubule disruption, increase of intracellular Ca(2+) with disturbance of neurotransmitter function, oxidative stress and triggering of excitotoxicity mechanisms. The effects are more damaging during CNS development, leading to alterations of the structure and functionality of the nervous system. The major source of CH(3)Hg(+) exposure is the consumption of fish and, therefore, its intake is practically unavoidable. The present concern is on the study of the effects of low level exposure to CH(3)Hg(+) on human neurodevelopment, with a view to establishing a safe daily intake. Recommendations are 0.4 micro g/kg body weight/day by the WHO and US FDA and, recently, 0.1 micro g/kg body weight/day by the US EPA. Unfortunately, these levels are easily attained with few meals of fish per week, depending on the source of the fish and its position in the food chain.

Brain sites of movement disorder: genetic and environmental agents in neurodevelopmental perturbations

In assessing and assimilating the neurodevelopmental basis of the so-called movement disorders it is probably useful to establish certain concepts that will modulate both the variation and selection of affliction, mechanisms-processes and diversity of disease states. Both genetic, developmental and degenerative aberrations are to be encompassed within such an approach, as well as all deviations from the necessary components of behaviour that are generally understood to incorporate "normal" functioning. In the present treatise, both conditions of hyperactivity/hypoactivity, akinesia and bradykinesia together with a constellation of other symptoms and syndromes are considered in conjunction with the neuropharmacological and brain morphological alterations that may or may not accompany them, e.g. following neonatal denervation. As a case in point, the neuroanatomical and neurochemical points of interaction in Attention Deficit and Hyperactivity disorder (ADHD) are examined with reference to both the perinatal metallic and organic environment and genetic backgrounds. The role of apoptosis, as opposed to necrosis, in cell death during brain development necessitates careful considerations of the current explosion of evidence for brain nerve growth factors, neurotrophins and cytokines, and the processes regulating their appearance, release and fate. Some of these processes may possess putative inherited characteristics, like alpha-synuclein, others may to greater or lesser extents be endogenous or semi-endogenous (in food), like the tetrahydroisoquinolines, others exogenous until inhaled or injested through environmental accident, like heavy metals, e.g. mercury. Another central concept of neurodevelopment is cellular plasticity, thereby underlining the essential involvement of glutamate systems and N-methyl-D-aspartate receptor configurations. Finally, an essential assimilation of brain development in disease must delineate the relative merits of inherited as opposed to environmental risks not only for the commonly-regarded movement disorders, like Parkinson's disease, Huntington's disease and epilepsy, but also for afflictions bearing strong elements of psychosocial tragedy, like ADHD, autism and Savantism.

Modification of mercury toxicity by selenium: practical importance?

The interaction between mercury and selenium may involve a variety of toxicologically and biochemically distinct processes. In this paper, the interaction between inorganic mercury and sodium selenite, the interaction most extensively studied, as well as the interaction between methylmercury (MeHg) and selenium, the interaction perhaps most significant for non-occupational human populations, will be discussed. It has been shown that the former interaction can be understood as a modification of the kinetic behavior of inorganic mercury by selenite, but this interaction may occur only under very limited conditions. On the other hand, the mechanism of the latter interaction is largely unknown, and kinetic modification appears to play only a minor role. An interaction between MeHg and seleno-proteins or a possible interaction between the inorganic mercury, resulting from the demethylation of MeHg, and the selenium may be important. Compared to the experimental findings, little evidence of the toxicological modification of MeHg by selenium was obtained in epidemiological studies.

Methylmercury neurotoxicity in cultures of human neurons, astrocytes, neuroblastoma cells

Neurotoxic effects of methylmercury, were investigated in vitro in primary cultures of human neurons and astrocytes isolatedfrom human fetal brain and in the human neuroblastoma cell line SH-SY5Y. The protection provided by agents with antioxidant properties was tested in these cultures to examine the oxidative stress mechanism of methylmercury poisoning. After 24 h of exposure to methylmercury, LC50 values were 6.5, 8.1 and 6.9 microM for human neurons, astrocytes and neuroblastoma cells, respectively, and the degree of cell damage increased at longer exposure times. Depletion of the cellular pool of reduced glutathione (GSH) by treatment with buthionine sulfoximine potentiated methylmercury cytotoxicity in all three cell types; neuroblastoma cells were the most sensitive. Addition of GSH extracellularly blocked methylmercury neurotoxicity in all cell types. The major beneficial effect of GSH could be attributed to its capacity to form conjugates with methylmercury, which reduces the availability of these organometallic molecules to the cells and facilitates their efflux. Cysteine protected astrocytes and neuroblastoma cells from methylmercury neurotoxicity, while selenite, Vitamin E and catalase produced some minor protective effects in three cell types, particularly in neurons. The present study showed that the human neural cells tested had differential responses to methylmercury: astrocytes were resistant to methylmercury neurotoxicity and neurons were more most responsive to protection afforded by antioxidants among the three cell types.

Prenatal exposure to methylmercury changes dopamine-modulated motor activity during early ontogeny: age and gender-dependent effects

We have shown previously that prenatal exposure of rats to 0.5 mg/kg/day of methylmercury (MeHg) produces gender-dependent changes in motor activity in adulthood. In the present study we have investigated whether changes in motor activity could also be found during early ontogeny of the offspring. Pregnant rats were treated with MeHg from day 7 of pregnancy to day 7 of lactation. The habituation to a novel environment (spontaneous activity) and the response to stimulation of the dopaminergic system were studied on postnatal day 14 and 21. Measures of spontaneous activity showed a slight increase in MeHg-prenatal exposed male and female rats at 14 days, but not at 21 days. Following administration of U91356A, a selective dopamine D(2) receptor agonist, a significantly lower dopamine-mediated locomotor activity was observed in the 21 day old MeHg-treated males, but not in females. These results show that prenatal exposure to MeHg alters postjunctional dopaminergic activity during the period of maturation of the dopamine system in the brain. Moreover, the gender-dependent susceptibility previously found in adulthood is already evident at the prepubertal stage.

In utero methylmercury exposure differentially affects the activities of selenoenzymes in the fetal mouse brain

Pregnant ICR mice were subcutaneously injected with 0,5, or 3x3 mg Hg/kg of methylmercury (MeHg) on days 12,13, and 14(G12-14) of gestation and were sacrificed on G17. Activity of selenoenzymes, including glutathione peroxidase (GPx) and 5'- or 5-iodothyronine deiodinases (5'-DI, 5-DI), was determined in fetal brain and placenta. MeHg did not affect the concentration of Se in these tissues, while it significantly inhibited the activity of GPx in the fetal brain and placenta, but not in the maternal brain. Although the levels of thyroid hormones in the maternal and fetal plasma were not affected by MeHg, 5-DI decreased and 5'-DI increased in the fetal brain, as if they had responded to hypothyroidism. Because the level of T4 in the fetal plasma was not affected by MeHg, these changes in enzymatic activities may result in a harmful excess of T3 in the fetal brain. In addition, 5-DI activity was increased in the placenta of MeHg-treated mice. These effects of prenatal MeHg exposure on fetal and placental DIs differed from those of dietary-induced Se deficiency, where the activities of DIs were decreased or not affected. Further evaluation of the effect of MeHg on selenoenzymes, especially 5-DIs, is warranted.

In utero exposure to methylmercury and Se deficiency converge on the neurobehavioral outcome in mice

Pregnant female ICR mice, maintained on torula-based diets containing various amounts of Se (0.02, 0.05, or 0.4 mg/kg diet), were given methyl-mercury (MeHg; 0, 5, or 9 mg Hg/kg in total) on the 12-14th days of gestation. The neurobehavioral function of the offspring born to these dams was evaluated with respect to reflex and motor development, thermal preference, and open-field activity. Se deficiency per se as well as exposure to MeHg exerted additive or synergistic effects on the neurobehavioral functions examined. The group of mice most affected was the group given the lowest amount of Se and the highest dose of MeHg. Thus, the neurobehavioral outcome of in utero MeHg exposure and Se deficiency converged. Although the dietary level of Se did not affect the Hg concentration in the fetal brain, the Se concentration and the activity of glutathione peroxidase, a selenoenzyme, were severely depressed by MeHg in the neural tissue. The possibility that functional Se deficiency by MeHg exposure partly accounts for the neurobehavioral toxicity of MeHg is discussed.

Effects of prenatal alcohol exposure on activity and learning in Sprague-Dawley rats

Nulliparous pregnant Sprague-Dawley rats were exposed to ethanol via a liquid diet technique (FAE, fetal alcohol exposure) or administered a fixed amount of control diet from gestational day 11 to day 21. The offspring, at 2-3 months of age, were studied in tests of mechanically monitored motor activity and learning acquisition in an automatized testing cage requiring an instrumental discriminative response, where the ability to learn and relearn correlations of a light signal to water presentation was monitored. A significantly reduced activity (i.e. ramp mounting behaviour) in a novel situation was obtained in the FAE group compared to controls. The initial disruption of ramp mounting behaviour could reflect alterations in either habituation to a novel test situation, altered neophobia, or some retardation in associating these responses with the outcome of water-availability. Adult FAE rats (six months of age) showed a tendency towards a lowered acquisition performance (p = 0.06) when tested in a circular Morris-type swim maze, but no detectable differences were shown in a motor activity test chamber situation.

Prenatal coexposure to metallic mercury vapour and methylmercury produce interactive behavioural changes in adult rats

Pregnant rats were 1) administered methyl mercury (MeHg) by gavage, 2 mg/kg/day during days 6-9 of gestation, 2) exposed by inhalation to metallic mercury (Hg degrees) vapour (1.8 mg/m3 air for 1.5 h per day) during gestation days 14-19, 3) exposed to both MeHg by gavage and Hg degrees vapour by inhalation (MeHg + Hg degrees), or 4) were given combined vehicle administration for each of the two treatments (control). The inhalation regimen corresponded to an approximate dose of 0.1 mg Hg degrees/kg/day. Clinical observations and developmental markers up to weaning showed no differences between any of the groups. Testing of behavioural function was performed between 4 and 5 months of age and included spontaneous motor activity, spatial learning in a circular bath, and instrumental maze learning for food reward. Offspring of dams exposed to Hg degrees showed hyperactivity in the motor activity test chambers over all three parameters: locomotion, rearing and total activity; this effect was potentiated in the animals of the MeHg + Hg degrees group. In the swim maze test, the MeHg + Hg degrees and Hg degrees groups evidenced longer latencies to reach a submerged platform, which they had learned to mount the day before, compared to either the control or MeHg groups. In the modified, enclosed radial arm maze, both the MeHg + Hg degrees and Hg degrees groups showed more ambulations and rearings in the activity test prior to the learning test. During the learning trial, the same groups (i.e., MeHg + Hg degrees and Hg degrees) showed longer latencies and made more errors in acquiring all eight pellets. Generally, the results indicate that prenatal exposure to Hg degrees causes alterations to both spontaneous and learned behaviours, suggesting some deficit in adaptive functions. Coexposure to MeHg, which by itself did not alter these functions at the dose given in this study, served to significantly aggravate the changes.

The effect of mercury vapour on cholinergic neurons in the fetal brain: studies on the expression of nerve growth factor and its low- and high-affinity receptors

The effects of mercury vapour on the production of nerve growth factor during development have been examined. Pregnant rats were exposed to two different concentrations of mercury vapour during either embryonic days E6-E11 (early) or E13-E18 (late) in pregnancy, increasing the postnatal concentration of mercury in the brain from 1 ng/g tissue to 4 ng/g tissue (low-dose group) or 11 ng/g (high-dose group). The effect of this exposure in offspring was determined by looking at the NGF concentration at postnatal days 21 and 60 and comparing these levels to age-matched controls from sham-treated mothers. Changes in the expression of mRNA encoding NGF, the low- and high-affinity receptors for NGF (p75 and p140 trk, respectively) and choline acetyltransferase (ChAT) were also determined. When rats were exposed to high levels of mercury vapour during early embryonic development there was a significant (62%) increase in hippocampal NGF levels at P21 accompanied by a 50% decrease of NGF in the basal forebrain. The expression of NGF mRNA was found to be unaltered in the dentate gyrus. The expression of p75 mRNA was significantly decreased to 39% of control levels in the diagonal band of Broca (DB) and to approximately 50% in the medial septal nucleus (MS) whereas no alterations in the level of trk mRNA expression were detectable in the basal forebrain. ChAT mRNA was slightly decreased in the DB and MS, significantly in the striatum. These findings suggest that low levels of prenatal mercury vapour exposure can alter the levels of the NGF and its receptors, indicating neuronal damage and disturbed trophic regulations during development.

A comparison of the lactational and transplacental deposition of mercury in offspring from methylmercury-exposed mice. Effect of seleno-L-methionine

Females exposed to methylmercury expose their offspring to mercury across the placenta as well as through milk. The relative importance of these two routes of exposure has hitherto been unresolved. Using a cross-fostering model with female mice, the transplacental and lactational exposures to mercury were evaluated separately. In female mice exposed to low, non-toxic levels of methylmercury in the drinking water the deposition of mercury in offspring before birth was quantitatively more important than later transfer of mercury from milk to offspring. Seleno-L-methionine supplementation of the dams increased the whole-body deposition in offspring. As methylmercury is anticipated to be absorbed completely and the young mice are unable to excrete mercury, these data indicate that seleno-L-methionine affects the kinetics of the inorganic mercury pool, which, due to demethylating processes, is present in both blood and milk of methylmercury-exposed females.