Comparison of neurobehavioral changes in three inbred strains of mice prenatally exposed to methylmercury

Epigenetic alternations in the SYP and DLG4 genes due to low-level methylmercury exposure during neuronal differentiation in vitro

Methylmercury (MeHg) is an environmental toxin known to damage the central nervous system. When pregnant women ingest seafood, which may contain accumulated MeHg, fetal development may be affected. The embryonic period, a time of major epigenetic change, is susceptible to epigenetic disruptions due to chemical exposure. Therefore, understanding the molecular mechanism underlying MeHg's effects on neuronal development requires consideration of epigenetic factors. In this study, we investigated epigenetic modifications in the synaptophysin (SYP) and discs large MAGUK scaffold protein 4 (DLG4) genes. LUHMES cells were exposed to 1 nM MeHg for 6 days during days 2-8 of neural differentiation. MeHg exposure significantly reduced the number of spikes observed on day 16 of differentiation. Both mRNA and protein expression levels of SYP and DLG4 were significantly decreased by MeHg exposure. Additionally, MeHg treatment reduced acetyl histone H3 levels associated with transcriptional activity in the SYP gene while increasing histone H3 lysine 27 tri-methylation (H3K27me3) levels related to transcriptional repression. Conversely, regarding the DLG4 gene, MeHg exposure increased H3K27me3 levels. Differential changes in DNA methylation (high and low methylation states) were observed in the SYP and DLG4 genes due to MeHg exposure depending on CpG site position. In conclusion, this study suggests that epigenetic changes, particularly histone modifications, contribute to decreased MeHg exposure-induced SYP and DLG4 expression during neuronal differentiation.

Dietary Selenomethionine Reduce Mercury Tissue Levels and Modulate Methylmercury Induced Proteomic and Transcriptomic Alterations in Hippocampi of Adolescent BALB/c Mice

Methylmercury (MeHg) is a well-known environmental contaminant, particularly harmful to the developing brain. The main human dietary exposure to MeHg occurs through seafood consumption. However, seafood also contains several nutrients, including selenium, which has been shown to interact with MeHg and potentially ameliorate its toxicity. The aim of this study was to investigate the combined effects of selenium (as selenomethionine; SeMet) and MeHg on mercury accumulation in tissues and the effects concomitant dietary exposure of these compounds exert on the hippocampal proteome and transcriptome in mice. Adolescent male BALB/c mice were exposed to SeMet and two different doses of MeHg through their diet for 11 weeks. Organs, including the brain, were sampled for mercury analyses. Hippocampi were collected and analyzed using proteomics and transcriptomics followed by multi-omics bioinformatics data analysis. The dietary presence of SeMet reduced the amount of mercury in several organs, including the brain. Proteomic and RNA-seq analyses showed that both protein and RNA expression patterns were inversely regulated in mice receiving SeMet together with MeHg compared to MeHg alone. Several pathways, proteins and RNA transcripts involved in conditions such as immune responses and inflammation, oxidative stress, cell plasticity and Alzheimer’s disease were affected inversely by SeMet and MeHg, indicating that SeMet can ameliorate several toxic effects of MeHg in mice.

DNA methyltransferase- and histone deacetylase-mediated epigenetic alterations induced by low-level methylmercury exposure disrupt neuronal development

Methylmercury (MeHg) is a chemical substance that causes adverse effects on fetal development. However, the molecular mechanisms by which environmental MeHg affects fetal development have not been clarified. Recently, it has been suggested that the toxic effects of chemicals on fetal development are related alterations in epigenetics, such as DNA methylation and histone modification. In order to analyze the epigenetic effects of low-level MeHg exposure on neuronal development, we evaluated neuronal development both in vivo and in vitro. Pregnant mice (C57BL/6J) were orally administrated 3 mg/kg of MeHg once daily from embryonic day 12-14. Fetuses were removed on embryonic day 19 and brain tissues were collected. LUHMES cells were treated with 1 nM of MeHg for 6 days and collected on the last day of treatment. In both in vivo and in vitro samples, MeHg significantly suppressed neurite outgrowth. Decreased acetylated histone H3 (AcH3) levels and increased histone deacetylase (HDAC) 3 and HDAC6 levels were observed in response to MeHg treatment in both in vivo and in vitro experiments. In addition, increased DNA methylation and DNA methyltransferase 1 (DNMT1) levels were observed in both in vivo and in vitro experiments. The inhibition of neurite outgrowth resulting from MeHg exposure was restored by co-treatment with DNMT inhibitor or HDAC inhibitors. Our results suggest that neurological effects such as reduced neurite outgrowth due to low-level MeHg exposure result from epigenetic changes, including a decrease in AcH3 via increased HDAC levels and an increase in DNA methylation via increased DNMT1 levels.

Methylmercury chloride exposure aggravates proinflammatory mediators and Notch-1 signaling in CD14+ and CD40+ cells and is associated with imbalance of neuroimmune function in BTBR T+ Itpr3tf/J mice

Autism spectrum disorder (ASD) is a severe neurodevelopmental disorder characterized by deficits in social interaction, communication, and repetitive behaviors. A key role for immune dysfunction has been suggested in ASD. Recent studies have indicated that inflammatory mediators and Notch-1 signaling may contribute to the development of ASD. Methylmercury chloride (MeHgCl) is an environmental pollutant that primarily affects the central nervous system, causing neurological alterations. Its effects on immunological responses have not been fully investigated in ASD. In this study, we examined the influence of MeHgCl exposure on inflammatory mediators and Notch-1 signaling in BTBR T⁺ Itpr3tf/J (BTBR) mice, a model of ASD. We examined the effects of MeHgCl on the IL-6-, GM-CSF-, NF-κB p65-, Notch-1-, and IL-27-producing CD14⁺ and CD40⁺ cells in the spleen. We assessed the effect of MeHgCl on IL-6, GM-CSF, NF-κB p65, Notch-1, and IL-27 mRNA levels in brain tissue. We also measured IL-6, GM-CSF, and NF-κB p65 protein expression levels in brain tissue. MeHgCl exposure of BTBR mice significantly increased IL-6-, GM-CSF-, NF-κB p65-, and Notch-1-, and decreased IL-27-producing CD14⁺, and CD40⁺ cells in the spleen. MeHgCl exposure of BTBR mice upregulated IL-6, GM-CSF, NF-κB p65, and Notch-1, and decreased IL-27 mRNA expression levels in brain tissue. Moreover, MeHgCl resulted in elevated expression of the IL-6, GM-CSF, and NF-κB p65 proteins in brain tissue. Taken together, these results indicate that MeHgCl exposure aggravates proinflammatory mediators and Notch-1 signaling which are associated with imbalance of neuroimmune function in BTBR mice.

Coordination Deficits Induced in Young Adult Mice Treated with Methylmercury

Male and female C57BL/6J mice starting at postnatal (P) day 34 were exposed orally to five divided doses totaling 1.0 or 5.0 mg/kg of methylmercury (MeHg; given as methylmercuric chloride) or sterile deionized water in moistened rodent chow. After a 5-day waiting period, control and MeHg-treated mice were subjected to a standard battery of behavior tests for balance and motor coordination. Latency to falling on the accelerating rota-rod was significantly decreased in 5.0 mg/kg MeHg-exposed mice when compared to control mice. In the open field, horizontal exploration with respect to total distance traveled during the first 5 min on the first test day was significantly reduced in 1.0 mg/kg MeHg-exposed mice when compared to control mice. Rearing activity was not affected by MeHg treatment. In the footprint analysis, angle of foot placement measured in 1.0 mg/kg MeHg-treated mice was significantly greater compared to control mice. Base stance and stride length were unaffected by MeHg treatment. On the vertical pole test, 10 mice from each treatment group fell off the pole during the time the pole was shifted from a horizontal position to a vertical position, whereas none of the control mice fell. These results indicate that short-term, low to moderate doses of MeHg in young adult mice can be detrimental to motor coordination and balance.

Adolescent methylmercury exposure affects choice and delay discounting in mice

The developing fetus is vulnerable to low-level exposure to methylmercury (MeHg), an environmental neurotoxicant, but the consequences of exposure during the adolescent period remain virtually unknown. The current experiments were designed to assess the effects of low-level MeHg exposure during adolescence on delay discounting, preference for small, immediate reinforcers over large, delayed ones, using a mouse model. Thirty-six male C57BL/6n mice were exposed to 0, 0.3, or 3.0ppm mercury (as MeHg) via drinking water from postnatal day 21 through 59, encompassing the murine adolescent period. As adults, mice lever pressed for a 0.01-cc droplet of milk solution delivered immediately or four 0.01-cc droplets delivered after a delay. Delays ranged from 1.26 to 70.79s, and all were presented within a session. A model based on the Generalized Matching Law indicated that sensitivity to reinforcer magnitude was lower for MeHg-exposed mice relative to controls, indicating that responding in MeHg-exposed mice was relatively indifferent to the larger reinforcer. Sensitivity to reinforcer delay was reduced (delay discounting was decreased) in the 0.3-ppm group, but not in the 3.0-ppm group, compared to controls. Adolescence is a developmental period during which the brain and behavior may be vulnerable to MeHg exposure. As with gestational MeHg exposure, the effects are reflected in the impact of reinforcing stimuli.

Microphthalmia-associated transcription factor ensures the elongation of axons and dendrites in the mouse frontal cortex

Long interspersed element-1 (LINE-1) is a mammalian transposable element, and its genomic insertion could cause neurological disorders in humans. Incidentally, LINE-1 is present in intron 3 of the microphthalmia-associated transcription factor (Mitf) gene of the black-eyed white mouse (Mitf^mi-bw allele). Mice homozygous for the Mitf^mi-bw allele show the white coat color with black eye and deafness. Here, we explored the functional consequences of the LINE-1 insertion in the Mitf gene using homozygous Mitf^mi-bw mice on the C3H background (C3H-bw mice) or on the C57BL/6 background (bw mice). The open-field test showed that C3H-bw mice moved more irregularly in an unfamiliar environment during the 20-min period, compared to wild-type mice, suggesting the altered emotionality. Moreover, C3H-bw mice showed the lower serum creatinine levels, which may reflect the creatine deficiency. In fact, morphologically abnormal neurons and astrocytes were detected in the frontal cortex of bw mice. The immunohistochemical analysis of bw mouse tissues showed the lower intensity for expression of guanidinoacetate methyltransferase, a key enzyme in creatine synthesis, in neurons of the frontal cortex and in glomeruli and renal tubules. Thus, Mitf may ensure the elongation of axons and dendrites by maintaining creatine synthesis in the frontal cortex.

Windows of sensitivity to toxic chemicals in the motor effects development

Many chemicals currently used are known to elicit nervous system effects. In addition, approximately 2000 new chemicals introduced annually have not yet undergone neurotoxicity testing. This review concentrated on motor development effects associated with exposure to environmental neurotoxicants to help identify critical windows of exposure and begin to assess data needs based on a subset of chemicals thoroughly reviewed by the Agency for Toxic Substances and Disease Registry (ATSDR) in Toxicological Profiles and Addenda. Multiple windows of sensitivity were identified that differed based on the maturity level of the neurological system at the time of exposure, as well as dose and exposure duration. Similar but distinct windows were found for both motor activity (GD 8-17 [rats], GD 12-14 and PND 3-10 [mice]) and motor function performance (insufficient data for rats, GD 12-17 [mice]). Identifying specific windows of sensitivity in animal studies was hampered by study designs oriented towards detection of neurotoxicity that occurred at any time throughout the developmental process. In conclusion, while this investigation identified some critical exposure windows for motor development effects, it demonstrates a need for more acute duration exposure studies based on neurodevelopmental windows, particularly during the exposure periods identified in this review.

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

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

Insertion of long interspersed element-1 in the Mitf gene is associated with altered neurobehavior of the black-eyed white Mitf(mi-bw) mouse

Microphthalmia-associated transcription factor (Mitf) is required for the differentiation of melanoblasts of the neural crest origin. The mouse homozygous for the black-eyed white (Mitf(mi-bw) ) allele is characterized by white-coat color and deafness with black eye, due to the loss of melanoblasts during embryonic development. The Mitf(mi-bw) allele carries an insertion of long interspersed element-1 (L1) in intron 3 of the Mitf gene, which may cause the deficiency of melanocyte-specific Mitf-M. Here, we show that the L1 insertion results in the generation of alternatively spliced Mitf-M mRNA species, such as Mitf-M mRNA lacking exon 3, exon 4 or both exons 3 and 4, each of which encodes Mitf-M protein with an internal deletion. Transient expression assays showed the loss of or reduction in function of each aberrant Mitf-M protein and the dominant negative effect of Mitf-M lacking exon 4 that encodes an activation domain. Thus, the L1 insertion may decrease the expression level of functional Mitf-M. Importantly, Mitf-M mRNA is expressed in the wild-type mouse brain, with the highest expression level in the hypothalamus. Likewise, aberrant Mitf-M mRNAs are expressed in the bw mouse brain. The bw mice show the altered neurobehavior under a stressful environment, suggesting the role of Mitf-M in sensory perception.

Sub-acute administration of (S)-dimethyl 2-(3-(phenyltellanyl) propanamido) succinate induces toxicity and oxidative stress in mice: unexpected effects of N-acetylcysteine

The organic tellurium compound (S)-dimethyl 2-(3-(phenyltellanyl) propanamide) succinate (TeAsp) exhibits thiol-peroxidase activity that could potentially offer protection against oxidative stress. However, data from the literature show that tellurium is a toxic agent to rodents. In order to mitigate such toxicity, N-acetylcysteine (NAC) was administered in parallel with TeAsp during 10 days. Mice were separated into four groups receiving daily injections of (A) vehicle (PBS 2.5 ml/kg, i.p. and DMSO 1 ml/kg, s.c.), (B) NAC (100 mg/kg, i.p. and DMSO s.c.), (C) PBS i.p. and TeAsp (92.5 μmol/kg, s.c), or (D) NAC plus TeAsp. TeAsp treatment started on the fourth day. Vehicle or NAC-treated animals showed an increase in body weight whereas TeAsp caused a significant reduction. Contrary to expected, NAC co-administration potentiated the toxic effect of TeAsp, causing a decrease in body weight. Vehicle, NAC or TeAsp did not affect the exploratory and motor activity in the open-field test at the end of the treatment, while the combination of NAC and TeAsp produced a significant decrease in these parameters. No DNA damage or alterations in cell viability were observed in leukocytes of treated animals. Treatments produced no or minor effects on the activities of antioxidant enzymes catalase, glutathione peroxidase and glutathione reductase, whereas the activity of the thioredoxin reductase was decreased in the brain and increased the liver of the animals in the groups receiving TeAsp or TeAsp plus NAC. In conclusion, the toxicity of TeAsp was potentiated by NAC and oxidative stress appears to play a central role in this process.

Maternal exposure to mercury chloride during pregnancy and lactation affects the immunity and social behavior of offspring

Developmental HgCl2 exposures of F1 offspring (H-2(q/s)) from unsociable SJL/J (H-2(s)) dams with high susceptibility to Hg-induced autoimmunity (SFvF1) and from highly sociable FVB/NJ (FVB; H-2(q)) dams with lower susceptibility to Hg-induced autoimmunity (FvSF1) were investigated. Hg exposure increased the serum IgG levels of all offspring at postnatal day 21 (pnd21) and of SJL/J dams but not of FVB dams. Serum IgG anti-brain antibody (Ab) levels of pnd21 SFvF1 offspring and SJL dams were higher than those of the FvSF1 offspring and FVB dams, but Hg only increased the titers of the FVB dams and their offspring. Hg significantly elevated the presence of IgG in all brain regions of the pnd21 SFvF1 offspring, and the SFvF1 offspring had greater amounts of IgG in the brain than the FvSF1 offspring, which had Hg-induced increases in only two brain regions. Cytokine levels were elevated in the brain regions of Hg-treated pnd21 SFvF1 but not of FvSF1 offspring, and SFvF1 females had more brain regions expressing cytokines than the males. At pnd70, the serum IgG, serum antibrain Abs, amounts of brain IgG, and brain cytokine levels of all of the Hg-treated offspring were equivalent to those of their appropriate controls, suggesting that developmental Hg exposure did not induce chronic immunological effects. However, the social behaviors of Hg-exposed SFvF1 offspring at pnd70 were significantly impaired, and SFvF1 females displayed greater decline in social behaviors than males, suggesting that the higher neuroinflammation of SFvF1 females earlier in life is associated with the altered behavior. Thus, developmental Hg exposure induces long-lasting effects on social behavior of offspring, which is dependent on sex and genetics and the induction of neuroinflammation.

Adult brain nitrergic activity after concomitant prenatal exposure to ethanol and methyl mercury

Pregnant rats were exposed to ethanol (EtOH) and/or methyl mercury (MeHg) during fetal brain development. Nitrergic activity was quantified by densitometric measurement of formazan deposits in the hippocampus, cerebellum and striatum of two-month-old offspring following histochemical assay for NADPH-diaphorase (NADPH-d) activity. Compared to control subjects, an increase in nitrergic activity was found in the molecular layer of dentate gyrus and in the lacunosum molecular and stratum radiatum of CA1 (cornus amoni 1) in the EtOH+MeHg group, whereas a single administration of EtOH increased the activity in all striatal segments. The cerebellum seems to be less sensitive at this time-point to intoxication, and presented an increase only at the molecular layer of EtOH-exposed animals when compared to the MeHg and EtOH+MeHg groups (ANOVA, one-way followed by Tukey's test, p<0.05 or p<0.01). Taken together, results suggest that developmental exposure to EtOH and MeHg, singularly or in combination, alters nitrergic activity in adult rat in different ways depending on the region and layer of the central nervous system (CNS), and that these alterations might be related to different local metabolic properties.

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.

Behavioral, morphological, and biochemical changes after in ovo exposure to methylmercury in chicks

Methylmercury (MeHg) is an environmental pollutant known to induce neurotoxicity in several animal species, including humans. However, studies focusing the effects of MeHg poisoning in chicks were based on phenomenological approaches and did not delve into the molecular mechanisms. The purpose of this study was to evaluate the postnatal consequences of the in ovo exposure to MeHg on behavioral, morphological and biochemical parameters in chicks. At the fifth embryonic day (E5), Gallus domesticus eggs were submitted to a single injection of 0.1 microg MeHg/0.05 ml saline. After treatment, the eggs returned to the incubator until hatching (E21). From first to fifth postnatal days (PN 1-PN 5), the MeHg-treated chicks showed lower frequency of exploratory movements and a significantly higher frequency of wing and anomalous movements. Cerebellar glutathione (GSH) levels and the activities of the GSH-related enzymes GSH reductase and GSH peroxidase were significantly higher (70, 72, and 80%, respectively) in MeHg exposed chicks in comparison to controls. Mercury impregnation was densest in the granular layer, followed by the Purkinje and molecular layers of treated chicks. A significant reduction of the number of Purkinje cells, as well as a greater distance between these cells were observed in chicks of MeHg group. Our results disclose that the prehatching exposure to MeHg induced motor impairments, which were correlated to histological damage and alterations on the cerebellar GSH system's development from PN 1 to PN 5.

Effects of methylmercury on postnatal neurobehavioral development in mice

Eighty ICR mice were randomly assigned to one of four groups given daily intraperitoneal injections of 0, 0.1, 1 or 3 mg/kg MeHg chloride respectively from postnatal days (PD) 15-17, and then tested with the Morris water maze on PD45. After that the mice were sacrificed by cervical dislocation, and the protein levels of NMDA receptor subtypes in the hippocampus were measured by Western blot analysis. A significant increase in the latency (F=2.88, P<0.05) before finding the platform was observed in the 1 and 3 mg/kg MeHg exposure groups. Further, the 3 mg/kg MeHg exposure group also had a longer swim distance (F=2.97, P<0.05) for finding the platform. In the probe test, the MeHg exposure groups displayed a smaller number of platform crossings when the hidden platform was moved, but this did not reach statistical significance. Western blot analysis results showed significant increases in the levels of NR1, NR2A and NR2B proteins of the hippocampus in the 1 and 3 mg/kg MeHg exposure groups. Overall, the current study found that MeHg exposure at 1 and 3 mg/kg doses during the postnatal brain growth spurt induces subtle and persistent learning deficits, and the neurobehavioral abnormalities of MeHg-exposed mice might be ascribed to alteration of the gene expression of specific NMDA receptor subunits in the hippocampus.

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.

Neurobehavioural and molecular changes induced by methylmercury exposure during development

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

Does low mercury containing skin-lightening cream (fair & lovely) affect the kidney, liver, and brain of female mice?

Fair & Lovely is an over-the-counter skin-lightening cream sold widely in Saudi markets. Its mercury content is 0.304+/-0.316 microg/g, in the range of 0.102 to 0.775 microg/g. This study was designed to evaluate its toxic effects on mice. The cream was applied on mice for a period of 1 month at different intervals. Mercury levels were measured in the liver, kidney, and brain tissue samples of a total of 75 adult female CD1 mice by Atomic Absorption Spectrophotometer coupled to a Vapor Generator Accessory. The mean mercury concentrations in the tissues of the treated mice were 0.193+/-0.319 microg/g; whereas for the control group, it was 0.041 microg/g+/-0.041microg/g. While the kidney was found to have the highest mercury content, the brain was found to have the lowest content. Treated mice showed a significant reduction in body weight. Marked histological changes were clearly noted in the kidney and, to a lesser extent, in the brain and liver. These results indicate that although Fair & Lovely mercury content is less than the U.S. Food and Drug Administration (FDA) permissible limits histopathological changes in the brain, kidney, and liver tissues are evidence of its possible toxicity.

[My experience in mercury toxicology: behavioral teratology study of the effects of prenatal exposure to environmental pollutants]

I would like to express sincere thanks to all the people involved in awarding the 2005 Congress Award. It is a great honor to be given the prize. I have been interested in mercury toxicology since I was a graduate student and mercury toxicology has been my main subject of focus for 30 years. I am proud to continue the research in this field. In this presentation I would like to summarize some of my research works and to talk anecdotes related to the research works. Soon after I became involved in this research field, I came across "behavioral teratology", which is a novel field of study whose focus is understanding the postnatal consequences of exposure to harmful agents in utero. Spyker and her colleagues clearly showed postnatal behavioral deviations in mouse offspring exposed to a low dose of methylmercury prenatally. In various animal experiments, I elucidated the subtle consequences that appear postnatally. I found that these consequences are modified by many factors such as selenium status, PCBs, and heat, because the degree of exposure to produce these consequences can be extremely small. These experiments simulate actual human life and the results will be useful to assess the risk of methylmercury exposure through fish consumption in the human population, since fish eating populations are exposed to these factors at a low dose. Recently, cohort studies elucidating the possible effects of prenatal exposure to environmental pollutants in the postnatal period have been established and ongoing. When a conclusion is drawn, it is expected to serve as basic information that is useful for the risk assessment of methylmercury and other environmental pollutants. This will give great gratification to scientists in environmental health and preventive medicine.

Biomarkers of exposure and effect as indicators of the interference of selenomethionine on methylmercury toxicity

The present study was conducted to clarify the interference of selenomethionine (SeMet) on methylmercury (MeHg) toxicity through the evaluation of changes in biomarkers of exposure and effect in rats exposed to MeHg and co-exposed to MeHg and SeMet. Male Wistar rats received two intraperitoneally (i.p.) administrations, either MeHg (1.5mg/kg body weight), SeMet alone (1mg/kg body weight) or combined MeHg and SeMet, followed by 3 weeks of rat urine collection and neurobehavioural assays. The effects of different administrations were investigated by the quantification of total mercury in kidney and brain, analysis of urinary porphyrins, determination of hepatic GSH and evaluation of motor activity functions (rearing and ambulation). MeHg exposure resulted in a significant increase of urinary porphyrins during the 3 weeks of rat urine collection, where as it caused a significant decrease in motor activity only at the first day after cessation of rat exposure. Additionally, SeMet co-exposure was able to normalize the porphyrins excretion, and a tendency to restore rat motor activity was observed, on the first day after cessation of exposure. Brain and kidney mercury levels increased significantly in rats exposed to MeHg; however, in co-exposed rats to SeMet no significant changes in Hg levels were found as compared to rats exposed to MeHg alone. Hence, the present study shows that urinary porphyrins are sensitive and persistent indicators of MeHg toxicity and demonstrates for the first time that SeMet reduces its formation. Finally, these results confirm that the mechanism of interaction between SeMet and MeHg cannot be explained by the reduction of Hg levels in target organs and suggestions are made to clarify the interference of SeMet on MeHg toxicity.

Mercury-induced externalization of phosphatidylserine and caspase 3 activation in human liver carcinoma (HepG2) cells

Apoptosis arises from the active initiation and propagation of a series of highly orchestrated specific biochemical events leading to the demise of the cell. It is a normal physiological process, which occurs during embryonic development as well as in the maintenance of tissue homeostasis. Diverse groups of molecules are involved in the apoptosis pathway and it functions as a mechanism to eliminate unwanted or irreparably damaged cells. However, inappropriate induction of apoptosis by environmental agents has broad ranging pathologic implications and has been associated with several diseases including cancer. The toxicity of several heavy metals such as mercury has been attributed to their high affinity to sulfhydryl groups of proteins and enzymes, and their ability to disrupt cell cycle progression and/or apoptosis in various tissues. The aim of this study was to assess the potential for mercury to induce early and late-stage apoptosis in human liver carcinoma (HepG2) cells. The Annexin-V and Caspase 3 assays were performed by flow cytometric analysis to determine the extent of phosphatidylserine externalization and Caspase 3 activation in mercury-treated HepG2 cells. Cells were exposed to mercury for 10 and 48 hours respectively at doses of 0, 1, 2, and 3 microg/mL based on previous cytotoxicity results in our laboratory indicating an LD50 of 3.5 +/- 0.6 microg/mL for mercury in HepG2 cells. The study data indicated a dose response relationship between mercury exposure and the degree of early and late-stage apoptosis in HepG2 cells. The percentages of cells undergoing early apoptosis were 0.03 +/- 0.03%, 5.19 +/- 0.04%, 6.36 +/- 0.04%, and 8.84 +/- 0.02% for 0, 1, 2, and 3 microg/mL of mercury respectively, indicating a gradual increase in apoptotic cells with increasing doses of mercury. The percentages of Caspase 3 positive cells undergoing late apoptosis were 3.58 +/- 0.03%, 17.06 +/- 0.05%, 23.32 +/- 0.03%, and 34.51 +/- 0.01% for 0, 1, 2, and 3 microg/mL of mercury respectively, also indicating a gradual increase in Caspase positive cells with increasing doses of mercury.

Developmental exposure to methylmercury alters learning and induces depression-like behavior in male mice

To investigate the long-term effects of developmental exposure to methylmercury (MeHg), pregnant mice were exposed to at 0.5 mg MeHg/kg/day via drinking water from gestational day 7 until day 7 after delivery. The behavior of offspring was monitored at 5-15 and 26-36 weeks of age using an automated system (IntelliCage) designed for continuous long-term recording of the home cage behavior in social groups and complex analysis of basic activities and learning. In addition, spontaneous locomotion, motor coordination on the accelerating rotarod, spatial learning in Morris water maze, and depression-like behavior in forced swimming test were also studied. The analysis of behavior performed in the IntelliCage without social deprivation occurred to be more sensitive in detecting alterations in activity and learning paradigms. We found normal motor function but decreased exploratory activity in MeHg-exposed male mice, especially at young age. Learning disturbances observed in MeHg-exposed male animals suggest reference memory impairment. Interestingly, the forced swimming test revealed a predisposition to depressive-like behavior in the MeHg-exposed male offspring. This study provides novel evidence that the developmental exposure to MeHg can affect not only cognitive functions but also motivation-driven behaviors.

Lactational exposure to inorganic mercury: evidence of neurotoxic effects

This study examined the effects of inorganic mercury (mercuric chloride - HgCl2) exposure exclusively through maternal milk on biochemical parameters related to oxidative stress (glutathione and thiobarbituric acid reactive substances levels, glutathione peroxidase and glutathione reductase activities) in the cerebellum of weanling mice. These parameters were also evaluated in the cerebellum of mothers, which were subjected to intraperitoneal injections of HgCl2 (0, 0.5 and 1.5 mg/kg, once a day) during the lactational period. Considering the relationship between cerebellar function and motor activity, the presence of motor impairment was also evaluated in the offspring exposed to HgCl2 during lactation. After treatments (at weaning), pups lactationally exposed to inorganic mercury showed high levels of mercury in the cerebellar tissue, as well as significant impairment in motor performance in the rotarod task and decreased locomotor activity in the open field. Offspring and dams did not show changes in cerebellar glutathione levels or glutathione peroxidase activity. In pups, lactational exposure to inorganic mercury significantly increased cerebellar lipoperoxidation, as well as the activity of cerebellar glutathione reductase. However, these phenomena were not observed in dams. These results indicate that inorganic mercury exposure through maternal milk is capable of inducing biochemical changes in the cerebellum of weanling mice, as well as motor deficit and these phenomena appear to be related to the pro-oxidative properties of inorganic mercury.

Cerebellar thiol status and motor deficit after lactational exposure to methylmercury

This study examined the exclusive contribution of methylmercury (MeHg) exposure through maternal milk on biochemical parameters related to the thiol status (glutathione (GSH) levels, glutathione peroxidase (GPx) and glutathione reductase (GR) activities) in the cerebellums of suckling mice. The same biochemical parameters were also evaluated in the cerebellums of mothers, which were submitted to a direct oral exposure to MeHg (10 mg/L in drinking water). With regard to the relationship between cerebellar function and motor activity, the presence of signs of motor impairment was also evaluated in the offspring exposed to MeHg during lactation. After the treatment (at weaning period), the pups lactationally exposed to MeHg showed increased levels of mercury in the cerebellum compared to pups in the control group and a significant impairment in the motor performance in the rotarod apparatus. In addition, these pups showed decreased levels of GSH in the cerebellum compared to pups in the control group. In dams, MeHg significantly increased the levels of cerebellar GSH and the activities of cerebellar GR. However, this was not observed in pups. This study indicates that (1) the exposure of lactating mice to MeHg causes significant impairments in motor performance in the offspring which may be related to a decrease in the cerebellar thiol status and (2) the increased GSH levels and GR activity, observed only in the cerebellums of MeHg-exposed dams, could represent compensatory pathophysiologic responses to the oxidative effects of MeHg toward endogenous GSH.

Developmental and neurobehavioral effects of perinatal exposure to polychlorinated biphenyls in mice

Because behavioral deficits associated with gestational exposure to polychlorinated biphenyls (PCBs) have been a concern, we studied the developmental and neurobehavioral effects of perinatal exposure to Aroclor 1254 (A1254), a commercial mixture of PCBs, in mice. The PCB mixture (A1254; 0, 6, 18, and 54 mg/kg body weight) was administered to pregnant mice (C57BL/6Cr) every 3 days by gavage from gestational day (GD) 6 to postnatal day (PND) 20. Compared with the control, treatment with A1254 did not alter the maternal body weight during the gestation and lactation periods. The body weight of the offspring did not differ among treatments. To assess the effects on offspring following such exposure, physical and neurobehavioral development (i.e., pinna detachment, hair growth, eye opening, incisor eruption, grasp reflex, righting reflex, walking, negative geotaxis, and cliff avoidance) was observed before weaning. At PND 7, poor adult-like responses in negative geotaxis were observed in all exposed groups. When the offspring were at 8-week old, the PCB-treated (18 mg/kg body weight) mice showed a decreased walking speed in the open-field test, and a prolonged time to reach the platform in the water maze test. Spontaneous locomotion activity was not affected by PCB exposure at 9 weeks . These results showed that perinatal exposure to PCBs produces several behavioral alterations in mice. Although dose-dependent changes were not observed, the neurobehavioral effects such as a decreased walking speed in the open-field test and a prolonged time to reach the platform in the water maze test remained in adulthood after the seeming recovery from the transient delay in development before weaning.

Methylmercury increases S100B content in rat cerebrospinal fluid

S100B, a calcium binding protein physiologically produced and released by astrocytes, has been used as a peripheral marker of brain damage. Here, we investigated the effects of subcutaneous injections of methylmercury chloride (MeHg-5mg/kg), an environmental neurotoxicant, on S100B protein content in cerebrospinal fluid (CSF) of adult rats. In addition, the performance of animals in an open field (number of squares crossing and rearings) was also analyzed in order to obtain a possible link between alteration in S100B protein content in CSF and parameters related to neurological injury. MeHg treatment increased serum mercury and S100B protein levels in the CSF. A decrease in the numbers of crossings and rearings was observed in MeHg-treated animals when compared to control group, which suggests a possible neurological injury. The present data show, for the first time, increased S100B levels in CSF after exposure to a neurotoxic metal. Authors discuss the possibility of astrocytic involvement in MeHg-induced neurotoxicity.

Motor impairment induced by oral exposure to methylmercury in adult mice

The effects of oral exposure to methylmercury chloride (MeHg) on locomotor control and activity in adult mice were investigated in the present study. MeHg was diluted in drinking water (0, 20 and 40mg/L - as methylmercury chloride) and locomotion (spontaneous locomotor activity) and motor impairment tests (beam walking, footprint and clasping) were performed at 7, 14 and 21 days after the beginning of the treatment. MeHg exposure caused a significant decrease in spontaneous locomotor activity and this effect was dose- and time-dependent. Significant dose- and duration-dependent increases in beam walking latency were observed following chronic MeHg exposure. Furthermore, dose- and duration-dependent locomotor deficits on footprint coordination were also observed. Taken together, these results show that MeHg-induced impairment on locomotor activity is not limited to exposures that take place during neural development. We discuss the possible relationship between our findings and the similar clinical signs observed in adult humans exposed to MeHg.

Neurobehavioral changes in mice chronically exposed to methylmercury during fetal and early postnatal development

Pregnant C57BL/6 mice were chronically treated with 0, 4, 6, or 8 ppm of methylmercury chloride (MeHg) in drinking water during fetal and early postnatal development. Four behavioral functions were analyzed in female and male offspring between the age of 6 and 12 weeks: motor coordination learning on the rotarod; training to spatial alternation in the standard T maze followed by a working memory test with delays; spontaneous locomotion and rearings in the open field; reference and working memory assessment in the modified T maze [Behav. Neurosci. 102 (1988) 635]. Chronic perinatal treatment with MeHg resulted in moderate brain levels of mercury near birth which rapidly decreased during nursing. MeHg exerted an effect on the performance of females, but not of males, on two of the four measurements. All treated females exhibited less locomotion than control mice when the open field was new, but not in the following four sessions when the environment was becoming increasingly familiar. Working memory was impaired in females treated with 6 and 8 ppm of MeHg in the modified T maze, but not on the test with delays in the standard T maze. Taken together, these results show that chronic exposure to MeHg during fetal and postnatal development had sex-dependent effects on horizontal exploration and on working memory in the modified T maze, and no effects on motor coordination learning and reference memory.

Organic mercury compounds: human exposure and its relevance to public health

Humans may be exposed to organic forms of mercury by either inhalation, oral, or dermal routes, and the effects of such exposure depend upon both the type of mercury to which exposed and the magnitude of the exposure. In general, the effects of exposure to organic mercury are primarily neurologic, while a host of other organ systems may also be involved, including gastrointestinal, respiratory, hepatic, immune, dermal, and renal. While the primary source of exposure to organic mercury for most populations is the consumption of methylmercury-contaminated fish and shellfish, there are a number of other organomercurials to which humans might be exposed. The antibacterial and antifungal properties of organomercurials have resulted in their long use as topical disinfectants (thimerosal and merbromin) and preservatives in medical preparations (thimerosal) and grain products (both methyl and ethyl mercurials). Phenylmercury has been used in the past in paints, and dialkyl mercurials are still used in some industrial processes and in the calibration of certain analytical laboratory equipment. The effects of exposure to different organic mercurials by different routes of exposure are summarized in this article.

Effects of maternal-melatonin treatment on open-field behaviors and hypertensive phenotype in spontaneously hypertensive rats' (SHR) offspring

Effects of maternal-melatonin treatment in spontaneously hypertensive rats (SHR) were investigated in their offspring. Pregnant SHR were given drinking water with/without melatonin (20 micrograms melatonin/ml tap water) during pregnancy and the lactation period. Maternal-melatonin treatment did not cause changes in body weights during 7 to 27 weeks. Melatonin administration up to weaning period via mother caused a decrease in systolic blood pressure (BP) during 11 to 27 weeks in their offspring compared with those of control group. Open-field behaviors in the offspring were observed at 24 weeks age. Both the control and treatment groups had ratios of central and peripheral locomotion of 30% and 70%, respectively. The treatment group exhibited less total locomotor activity and rearing than the control group did, whereas more latency was exhibited in the treatment group compared with that of the control group. These findings suggest that maternal-melatonin administration may modify open-field behaviors as well as the hypertensive phenotype in their progeny.

Neurobehavioral changes in mice treated with methylmercury at two different stages of fetal development

Pregnant C57BL/6 mice were orally given daily doses of 4 or 6 mg/kg of methylmercury chloride (MeHg) or vehicle during either gestational days 7-9 (GD7-9) or days 12-14 (GD12-14). Their female offspring were tested between 6 and 16 weeks of age on a variety of behavioral tasks. Motor coordination on the rotarod and visual discrimination learning in the Y maze were not affected by administration of MeHg either at GD7-9 or at GD12-14. In the open field, the total number of square crossings was lower in mice treated with 4 and 6 mg/kg of MeHg at GD12-14 than in control mice whether the environment was new or familiar, but prenatal administration of MeHg at GD7-9 had no effect on this measure. Administration of MeHg either at GD7-9 or at GD12-14 had no effect on the percentage of central square crossings or on the frequency of rearings in the open field. On spatial alternation training in the T maze, both treated groups in Condition GD7-9 and the group treated with 6 mg/kg at GD12-14 required more sessions to reach the learning criterion than their respective vehicle groups. When spatial alternation was tested with delays, treated groups did not differ from their respective control groups. In the radial arm maze, the performance of mice treated at GD7-9 was normal, but reference memory and working memory were impaired by administration of MeHg at GD12-14. In mice treated with 4 mg/kg of MeHg, reference memory was impaired only on the first block of trials, whereas in mice treated with 6 mg/kg, the deficit persisted on all blocks of trials. Overall, these results indicate that prenatal administration of MeHg at GD12-14 had more detrimental effects on behavioral performance than administration at GD7-9. It reduced locomotor activity and impaired reference memory for egocentric and allocentric spatial information as well as working memory for places.

Methods to identify and characterize developmental neurotoxicity for human health risk assessment. I: behavioral effects

Alterations in nervous system function after exposure to a developmental neurotoxicant may be identified and characterized using neurobehavioral methods. A number of methods can evaluate alterations in sensory, motor, and cognitive functions in laboratory animals exposed to toxicants during nervous system development. Fundamental issues underlying proper use and interpretation of these methods include a) consideration of the scientific goal in experimental design, b) selection of an appropriate animal model, c) expertise of the investigator, d) adequate statistical analysis, and e) proper data interpretation. Strengths and weaknesses of the assessment methods include sensitivity, selectivity, practicality, and variability. Research could improve current behavioral methods by providing a better understanding of the relationship between alterations in motor function and changes in the underlying structure of these systems. Research is also needed to develop simple and sensitive assays for use in screening assessments of sensory and cognitive function. Assessment methods are being developed to examine other nervous system functions, including social behavior, autonomic processes, and biologic rhythms. Social behaviors are modified by many classes of developmental neurotoxicants and hormonally active compounds that may act either through neuroendocrine mechanisms or by directly influencing brain morphology or neurochemistry. Autonomic and thermoregulatory functions have been the province of physiologists and neurobiologists rather than toxicologists, but this may change as developmental neurotoxicology progresses and toxicologists apply techniques developed by other disciplines to examine changes in function after toxicant exposure.