Milk transfer and tissue uptake of mercury in suckling offspring after exposure of lactating maternal guinea pigs to inorganic or methylmercury

Non-destructive mercury exposure assessment in the Brandt's hedgehog (Paraechinus hypomelas): spines as indicators of endogenous concentrations

Due to its persistence, bioaccumulation characteristics, and toxicity, environmental contamination with mercury (Hg) is of high concern for human health, living organisms, and ecosystems, and its biological monitoring is highly relevant. In this study, the levels of total Hg were measured in organs, tissues, and spines of 50 individuals of Brandt's hedgehog collected in Iran in 2019. The Hg median levels in kidneys, liver, muscle, and spines were 156, 47, 47, and 20 ng/g dry weight, respectively. The results showed a significant positive correlation between the levels of Hg in kidneys and liver (r = 0.519; p < 0.01) and in spines and muscle (r = 0.337, p < 0.01) and kidneys (r = 0.309, p < 0.05). Significant differences (p < 0.05) in Hg levels in organs and tissues were also observed depending on the sex, weight, length, and age of the individuals. In addition, the median levels of total Hg in kidneys of Brandt's hedgehogs from an agricultural ecotype (median 190 ± 65) were significantly higher (p < 0.05) than those collected from a forest ecotype (median 126 ± 50), suggesting that the habitat could have a significant impact on animal contamination.

Neurobehavioral effects of postnatal exposure to low-level mercury vapor and/or methylmercury in mice

This study examined the effects on neurobehavioral function of exposure to low-level mercury vapor (Hg⁰), methylmercury (MeHg) in female mice and the combination of Hg⁰ and MeHg during postnatal development. Postnatal mice were exposed to Hg⁰ at a mean concentration of 0.188 mg/m³ Hg⁰ and supplied with food containing 3.85 μg/g of MeHg from day 2 to day 28 after delivery. The combined exposure group was exposed to both Hg⁰ and MeHg, using the same procedure. When their offspring reached the age of 11 weeks, behavioral analyses were performed. The behavioral effects in mice were evaluated based on locomotive activity and rate of center entries in the open field (OPF), learning activity in the passive avoidance response (PA) and spatial learning ability in the radial maze (RM). Total locomotive activity in the OPF significantly decreased in the Hg⁰, MeHg and combined exposure groups compared with the control group. The proportion of entries to central area in the OPF was significantly higher in the combined exposure group than in the control group, while those in the Hg⁰ or MeHg exposure group did not differ from the control group. Other behavioral tests did not reveal significant differences among the groups. Behavioral anomalies were more distinctive after combined exposure compared to Hg⁰ or MeHg exposure alone. The brain Hg concentration of offspring, immediately after exposure, was highest in the combined exposure group, exceeding 2 μg/g, followed by the MeHg and Hg⁰ exposure groups. Thus, the enhancement of neurobehavioral effects in the combined exposure group was associated with higher brain mercury concentration.

Evaluation of some factors influencing on variability in bioaccumulation of heavy metals in rodents species: Rombomys opimus and Rattus norvegicus from central Iran

In the present study, the effects of sex, age, nutritional status, and habitat use on hair and internal tissue concentrations of some trace metals (Zn, Cu, Ni, Pb and Cd) in brown rat (Rattus norvegicus) and great gerbil (Rhombomys opimus) from Aran-O-Bidgol City in Central Iran were investigated. Five sampling stations in different land-uses were selected for the study. Concentrations of Cd and Pb in liver and whole body concentration as well as concentration of Zn in hair of male rats were significantly higher than females. A significantly higher concentration was only observed for Pb in kidney of male gerbils than females. In numerous cases, age had significantly negative correlations with concentration of metals in different tissues of both species, and was only significantly (and positively) correlated with the concentration of Cd in kidney and Zn in liver of gerbils. This finding may indicate that the early neonatal age is a critical period for metals accumulation. The relationships between nutritional status and metal concentrations for both species were negative, which can be ascribed to the weight-specific metabolic rate that cause to a relatively low uptake of metals per unit of body weight. No significant differences were observed for soil metals concentrations in different sites. So, this suggests that the influence of life history, microhabitat use and foraging behavior did significantly influence on inter and intra specific variation of metals concentration in small mammals. Our observations suggest that hair cannot be used alone for monitoring exposure of metals.

The extent of mercury (Hg) exposure among Saudi mothers and their respective infants

A total of 1016 healthy Saudi mothers and their respective infants (aged 3-12 months) were recruited from 57 Primary Health Care Centers (PHCCs) in Riyadh, Saudi Arabia, to evaluate the extent of mercury (Hg) exposure and predict its sources in the healthy Saudi population. Total Hg levels were measured in maternal urine, breast milk, blood, and hair and in the infants' urine and hair. Only 1.9% of the mothers had urinary Hg (UHg)>10 μg/l, the limit for asymptomatic adults recommended by the World Health Organization, but the median (0.99 μg/l) was higher than in other countries. Also, 49.3% of the mothers had UHg>1 μg/l, the German reference value for adults. Median infant UHg was 0.729 μg/l, and 77 and 93 % of the infants had levels higher than 0.4 and 0.1 μg/l, the reference values of the Centers for Disease Control and Prevention and for Germany, respectively. The median Hg level in breast milk was 0.884 μg/l. Even though 43.2% of the milk samples were above the background level for Hg in human milk (1 μg/l), our results were lower than those reported from other countries. Median maternal total Hg in blood was 0.637 μg/l, and only 0.4 and 6.9% of samples were higher than the Hg reference levels of 5.8 μg/l of the Environmental Protection Agency (EPA) and of 2 μg/l for Germany, respectively. Total Hg levels in hair (HHg) varied widely among mothers and infants, but only 3.9% of the mothers and 2.8% of the infants had HHg>1 μg/g (the EPA reference level). Median HHg values were 0.117 μg/g dry weight in mothers and 0.1 μg/g dry weight in infants; both were lower than in other countries. The Hg levels in mothers and their respective infants were relatively low, but our results were consistent with other studies indicating that dental amalgam fillings and fish consumption were the main predictors of maternal Hg exposure. Among the several biomarkers of Hg exposure, Hg levels in maternal hair and urine were the strongest predictors of infant exposure. The lack of an association between Hg in breast milk and Hg in infant urine and hair suggested that the infants were exposed to Hg predominately during pregnancy rather than during breastfeeding. We expect that our data can serve as a baseline for further biomonitoring and follow-up studies, particularly of the long-term impact of Hg on childhood neurodevelopment.

Distinct response of lactating and nonlactating rats exposed to inorganic mercury on hepatic δ-aminolevulinic acid dehydratase activity

This study investigated if lactating and nonlactating rats presented differences in relation to hepatic sensitivity to HgCl2 and the potential preventive role of ZnCl2. Lactating (days 3-12 of lactation) and nonlactating rats received 27 mg/kg ZnCl2 for five consecutive days and 5 mg/kg HgCl2 for five subsequent days. Lactating and nonlactating rats exposed to HgCl2 presented a decrease in food intake, a decrease in plasma alanine aminotransferase (ALT), and an increase in hepatic Hg levels when compared to the control group. Only lactating rats exposed to HgCl2 presented an increase in hepatic δ-aminolevulinic acid dehydratase activity. On the other hand, only nonlactating rats exposed to HgCl2 presented an increase in plasma aspartate aminotransferase (AST). ZnCl2 pre-exposure partially protected the increase in plasma AST activity presented by nonlactating rats and potentiated the liver Hg accumulation in lactating rats. Pups from the Sal-Hg and Zn-Hg groups showed a decrease in absolute liver weight and an increase in liver Hg levels. Summarizing, this study demonstrated that lactating rats presented distinct biochemical responses compared to nonlactating rats exposed to HgCl2 when hepatic parameters were evaluated.

Mercury speciation in hair of children in three communities of the Amazon, Brazil

Children from riverside communities located downstream of gold mining areas may be chronically exposed to relatively high levels of MeHg through the consumption of fish of this region. The objective of this study was to evaluate and compare levels of THg and MeHg in hair of children less than 12 years in communities near mines in the municipality of Itaituba and in communities far from prospecting areas in the city of Abaetetuba. The communities of Itaituba (Barreiras and São Luiz do Tapajós) had THg mean levels of 5.64±5.55  μg·g(-1) (0.43-27.82) and 11.41±7.16  μg.g(-1) (1.08-28.17), respectively, and an average count of MeHg relative to THg of 92.20% and 90.27%, respectively. In the Maranhão community, the THg average concentrations results were 2.27±2.11  μg·g(-1) (0.13-9.54) and the average values were 93.17% for MeHg. Children of Itaituba had average levels of mercury above the limit established by the World Health Organization (10  μg·g(-1)) and the strong correlation coefficient between the communities (R=0.968 and P=0.0001) suggests the hair as an excellent biomarker of human exposure to organic mercury in riverside populations of the Tapajós, which has the intake of fish daily as main source of protein dietary.

[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.

Bioaccumulation of lead, mercury, and cadmium in the greater white-toothed shrew, Crocidura russula, from the Ebro Delta (NE Spain): sex- and age-dependent variation

We quantified bioaccumulation of lead, mercury, and cadmium in bones from 105 greater white-toothed shrews (Crocidura russula) collected at the Ebro Delta, a polluted area, and the Medas Islands, a control site. Lead and mercury levels varied with site, age, and sex, although statistical significances depended on each factor. Globally, shrews from the polluted area exhibited significantly higher concentrations of Pb and Hg. Increment of Pb with age was particularly remarkable in wetland animals and was interpreted in relation to human activities, namely hunting. Unlike males, females from the Ebro Delta maintained low Hg levels, which were associated with gestation and lactation. Cadmium levels did not differ between sites, sexes, or ages. This study provides the first data on heavy metals in mammals from this wetland and suggests that C. russula is a good bioindicator of metal pollution. We concluded that sex and age may represent an important source of variation in the bioaccumulation of these metals in wild populations.

Subchronic mercury treatment of rats in different phases of ontogenesis: functional effects on the central and peripheral nervous system

Electrophysiological changes caused by inorganic mercury administration during the pre- and/or postnatal development were studied. Pregnant female Wistar rats were treated, by gavage, with 0.4, 0.8 or 1.6 mg/kg mercury (HgCl2 diluted in distilled water): 1/ from day 5 to 15 during pregnancy (P protocol); 2/ from day 5 to 15 of pregnancy+for 4 weeks of lactation (P+L protocol); 3/ from day 5 to 15 of pregnancy+for 4 weeks of lactation, and the offspring were further treated for 8 weeks post-weaning (P+L+P protocol). Electrophysiological parameters (electrocorticogram, cortical evoked potentials, conduction velocity and refractory periods of peripheral nerve) of the male offspring from dams in the groups treated according to the above protocols were investigated at the age of 12 weeks. The rats' spontaneous and evoked electrophysiological activity underwent dose- and treatment-dependent changes following the treatment (increased frequency of spontaneous activity, lengthened latencies and duration of evoked potentials, lower conduction velocity of the peripheral nerve, etc.). In the same rats, however, the treatment failed to cause major signs of general intoxication. The results emphasize the functional neurotoxic risk arising from the continuous presence of inorganic mercury in the human environment, and point to possible use of early functional changes for monitoring the effects of mercury.

Evaluation of changes in methylmercury accumulation in the developing rat brain and its effects: a study with consecutive and moderate dose exposure throughout gestation and lactation periods

Methylmercury (MeHg) can be transferred to the fetus through the placenta and to newborn offspring through breast milk. The higher mercury (Hg) accumulation and susceptibility to toxicity in the fetus than in the mother during the gestation period is well known. However, the contribution of MeHg exposure through breast milk to the brain Hg concentration in offspring is not clear. The purposes of this study were to evaluate the changes in Hg concentration in the brain of offspring and its effects on the developing rat brain, based on consecutive and moderate doses of MeHg throughout gestation and lactation. Adult female rats were given a diet containing 5 ppm Hg (as MeHg) for 8 weeks. The administration level was thought not to cause adverse effects in adult rats. The rats were then mated and subsequently given the same diet throughout gestation and after parturition. The newborn offspring were placed with the mothers until postnatal day 30. The offspring were exposed to MeHg throughout their intrauterine life through the placenta, and during the postnatal developing phase via contaminated milk. Furthermore, they were given the same diet containing MeHg for 2 months following weaning. On the day of parturition, the concentration of Hg in the brains of newborns was 1.4 times higher than that in the mothers. During the suckling period the concentration in the brain of the offspring rapidly declined to 1/5 of that at birth, suggesting that MeHg transport by milk was limited while the brain and body volumes increased rapidly. The concentration increased gradually again after the offspring started the contaminated diet. In behavioral tests performed at 5 and 6 weeks of age, MeHg-exposed rats showed a significant deficit in motor coordination in the rotarod test and a learning disability in the passive avoidance response test, compared with controls. Histopathologically, focal cerebellar dysplasia, including the heterotopic location of Purkinje cells and granule cells, was observed. These abnormalities may be induced by the effect of highly accumulated MeHg in the brain during the gestation period. Thus, although offspring are subjected to consecutive and moderate dose MeHg exposure throughout both the gestation and suckling periods, the risk is especially high during gestation but may decrease during lactation.

Distribution of methylmercury and inorganic mercury in neonate hamsters dosed with methylmercury during fetal life

The study of the effects of methylmercury (MeHg) contamination has intensified since the MeHg outbreaks in Japan and Iraq. However, most of these studies used high doses of MeHg to obtain its most severe effects. This work identified the MeHg targets in neonate hamsters after administration of two different single oral doses of radiolabeled methylmercury chloride (Me203HgCl) to two groups of pregnant hamsters. The low-dose group received 0.05 microg of 203Hg as Me203HgCl per gram of body weight and the radioactivity was 6.6x10(3) Bq. The high-dose group received 0.55 microg of 203Hg as Me203HgCl per gram of body weight with 2x104 Bq. In the first day of life, the brain was the organ with the highest Me203Hg concentration, followed by the liver, kidneys, and intestines. However, these values of Me203Hg decreased at different rates in all organs until 12 days of life, whereas total 203Hg in the body did not change during the same period. This suggests an age-dependent Me203Hg body distribution instead of an elimination of Me203Hg from the body.

Lactational exposure and neonatal kinetics of methylmercury and inorganic mercury in mice

The concentration of mercury in milk and the distribution pattern in the sucking pup was followed over time after administration of a single iv injection of 0.5 mg/kg body wt of 203Hg-labeled methylmercuric chloride or mercuric chloride to lactating mice on Day 10 of lactation. Mercury concentrations in milk of the dams and in whole body, blood, plasma, GI-tract, liver, kidneys, and brain of the offspring were followed up to 11 days after dosing (until lactational Day 21). Following the inorganic mercury dose to the dams, most of the mercury in milk was delivered to the pups during the first 24 h, but the maximum mercury concentration in plasma and tissues of pups was not reached until 7 days after dosing, indicating a prolonged absorption of inorganic mercury in the sucking pup. Pups of dams given methylmercury were exposed to a much lower and constant mercury concentration in milk. The estimated accumulated mercury dose via milk per pup of dams given methylmercury was less than half of that estimated after the inorganic mercury dose. When the accumulated dose via milk from methylmercury-exposed dams was compared to the amount of mercury in pup's carcass (whole body minus GI-tract including content), it was revealed that almost all mercury delivered via milk was absorbed, and that the suckling pups had a very low elimination of mercury until lactational Day 17. Lactational exposure following a maternal methylmercury or inorganic mercury dose resulted in almost similar mercury concentrations in liver, kidneys, and plasma of the suckling, but higher concentrations in brain (as most 14 times) and also twice as high mercury body burden in the methylmercury group. Thus, differences in kinetics indicate that lactational exposure of methylmercury is a greater hazard for the breast-fed infant than inorganic mercury.

Kinetics of methylmercury and inorganic mercury in lactating and nonlactating mice

The elimination of mercury was followed for 9 days (Days 10-19 of lactation) in milk and/or 21 days in blood and plasma of lactating and nonlactating mice administered a single iv injection of either 203Hg-labeled methylmercuric chloride or 203Hg-labeled mercuric chloride (0.5 mg Hg/kg body wt). Demethylation of methylmercury to inorganic mercury was taken into consideration by analyzing the data with a combined pharmacokinetic model based on the assumption of constant blood plasma ratios for methylmercury and inorganic mercury. A three-compartment model fitted the blood and plasma concentrations vs time profiles for both compounds. Plasma clearance and volume of distribution at steady state for methylmercury were 95. 3 ml/h/kg and 18,500 ml/kg, respectively, in lactating mice, and significantly higher than in nonlactating mice with values of 47.1 ml/h/kg and 9400 ml/kg, respectively. The terminal half-lives of methylmercury in plasma were similar, 170 h in lactating and 158 h in nonlactating mice. No differences were observed between the pharmacokinetic parameters in lactating and nonlactating mice administered inorganic mercury. The lactational transfer of mercury was more efficient following administration of inorganic mercury than after administration of methylmercury, with a five times higher peak concentration in milk, higher milk:plasma concentration ratios, and 8% of the administered dose excreted in milk compared with 4% for methylmercury. Mercury concentrations in milk following an iv dose of inorganic mercury decreased with a terminal half-life of 107 h, whereas after administration of methylmercury, the concentration of total mercury in milk remained at an almost constant level during the whole period of investigation. There was a nonlinear relationship between mercury in milk and plasma following inorganic mercury administration. It is suggested that inorganic mercury enters the mammary gland by a carrier-mediated transport system, which is saturated at high plasma levels of inorganic mercury. The present study shows that physiological changes during lactation alter the pharmacokinetics for methylmercury in mice but not for inorganic mercury.

Immune system alteration in the rat after indirect exposure to methyl mercury chloride or methyl mercury sulfide

Methyl mercury is a well-recognized health hazard. It is an environmental contaminant that accumulates in the food chain. The primary source of mercury exposure for humans is through the consumption of contaminated fish. We studied the effects of indirect methyl mercury exposure on the immune system of Sprague-Dawley rats. The effects of different forms of methyl mercury on immune system development were studied in Sprague-Dawley rats at 6 and 12 weeks of age. Rats were indirectly exposed to mercury during gestation and during nursing by exposing pregnant rats to either 5 or 500 micrograms/liter of methyl mercury chloride (CH3HgCl) or 5 micrograms/liter of methyl mercury sulfide [(CH3Hg)2S] in their drinking water. Total body, splenic, and thymic weights were measured, and NK cell cytolytic activity and lymphoproliferative response to T and B cell mitogens were evaluated in the offspring. At 6 weeks of age, total body and splenic weights were significantly increased in both high- and low-dose methyl mercury chloride-exposed groups. Rats exposed to methyl mercury sulfide had a significant increase in thymic weight at 6 weeks of age. At 12 weeks, the total body and organ weights were not different from controls. The lymphocyte proliferative response of splenocytes to PWM was enhanced at 6 weeks in both CH3HgCl exposed groups and not affected in the (CH3Hg)2S exposed group. NK cell activity was not affected in either group at 6 weeks of age. At age 12 weeks, NK cell activity was statistically significantly decreased by 56.6% in both CH3HgCl-exposed groups and not affected in the (CH3Hg)2S-exposed rats. The lymphocyte proliferative response of splenocytes to the B cell mitogen pokeweed remained increased in the CH3HgCl groups. Indirect exposure of rats (during gestation and nursing) to different forms of methyl mercury reveals that chloride forms have prolonged predominantly enhancing effects on lymphoproliferative response of splenocytes, followed by significant depression of NK cell activity.

Lactational exposure to methylmercury in the hamster

Syrian Golden hamster dams were administered 203Hg-labelled methyl mercury (MeHg; 1.6 mumol/kg) 1 day after parturition and milk was collected twice during the 1st week. The excretion of 203Hg in milk and the uptake, retention and tissue distribution of 203Hg in the pups was studied using gamma counting. The fraction of inorganic Hg in milk and in the kidneys of the pups was determined following separation of inorganic Hg and MeHg by ion exchange chromatography. The concentration of 203Hg in milk on the 1st day after MeHg administration was 0.12 nmol/g. 203Hg was mainly (80-90%) excreted as MeHg during the first 6 days of lactation. The whole body and tissue concentration of 203Hg in the pups increased for 10-15 days and decreased thereafter. The content of 203Hg in the pelt and the fraction of inorganic Hg in the kidney increased throughout the study period (4 weeks). The excretion of MeHg in milk corresponded to at least 5% of the dose administered to the dam. Our study demonstrates that breast milk may be a significant source of MeHg exposure during the critical neonatal period.