Prenatal methyl mercury exposure in relation to neurodevelopment and behavior at 19 years of age in the Seychelles Child Development Study.

BACKGROUND: Fish are important sources of protein and contain a variety of nutrients, such as n-3 long-chain polyunsaturated fatty acids (PUFA), essential for normal brain development. Nevertheless, all fish also contain methyl mercury (MeHg), a known neurotoxicant in adequate dosage. Our studies of the Seychelles Child Development Study (SCDS) Main Cohort enrolled in 1989-1990 (n=779) have found no consistent pattern of adverse MeHg effects at exposures achieved by daily fish consumption. Rather, we have observed evidence of improved performance on some cognitive endpoints as prenatal MeHg exposure increases in the range studied. These observations cannot be related to MeHg and may reflect the role of unmeasured covariates such as essential nutrients present in fish. To determine if these associations persist into young adulthood, we examined the relationship between prenatal MeHg exposure, recent PUFA exposure and subjects' neurodevelopment and behavior at 19 years of age. METHODS: We examined 533 participants using the following test battery: the Profile of Mood States-Bipolar (POMS-Bi); Finger Tapping; Kaufman Brief Intelligence Test (K-BIT); measures of Fine Motor Control and Complex Perceptual Motor Control; and Visual Spatial Contrast Sensitivity. We collected the following covariates: maternal IQ, family life course stressors, socioeconomic status, and subjects' recent postnatal MeHg, sex, and computer use. Primary analyses (based on N=392-475) examined covariate-adjusted associations in multiple linear regression models with prenatal MeHg as the primary exposure measure. Secondary analyses additionally adjusted for total n-6 and fish-related n-3 PUFA measured in the subjects' serum at the 19-year examination. RESULTS: Study participants had a mean prenatal MeHg exposure of 6.9 ppm, and a mean recent postnatal exposure of 10.3 ppm. There were no adverse associations between prenatal MeHg and any of the measured endpoints. For recent postnatal MeHg exposure, however, adverse associations were observed for Finger Tapping (non-dominant hand) among women and for the K-BIT Matrices for both sexes, with or without adjustment for PUFA. CONCLUSION: Our findings continue to provide no evidence for an adverse effect of prenatal MeHg exposure on development in a cohort that consumes fish daily. Observations for postnatal MeHg exposure will need to be confirmed using more comprehensive exposure measures.

The biological monitoring of prenatal exposure to methylmercury.

Several biological media have been used as indicators of the fetal body burden of methylmercury and the levels in the primary target tissue, the developing brain. These media include maternal hair and blood. The relative merits of these media will be considered both with regard to current knowledge of the physiology of mercury disposition in the body and also the practicality of field application with respect to sample, collection, transport, storage and processing.

Altered operant responding for motor reinforcement and the determination of benchmark doses following perinatal exposure to low-level 2,3,7,8-tetrachlorodibenzo-p-dioxin.

Pregnant Holtzman rats were exposed to a single oral dose of 0, 20, 60, or 180 ng/kg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the 18th day of gestation. Their adult female offspring were trained to respond on a lever for brief opportunities to run in specially designed running wheels. Once they had begun responding on a fixed-ratio 1 (FR1) schedule of reinforcement, the fixed-ratio requirement for lever pressing was increased at five-session intervals to values of FR2, FR5, FR10, FR20, and FR30. We examined vaginal cytology after each behavior session to track estrous cyclicity. Under each of the FR values, perinatal TCDD exposure produced a significant dose-related reduction in the number of earned opportunities to run, the lever response rate, and the total number of revolutions in the wheel. Estrous cyclicity was not affected. Because of the consistent dose-response relationship at all FR values, we used the behavioral data to calculate benchmark doses based on displacements from modeled zero-dose performance of 1% (ED(01)) and 10% (ED(10)), as determined by a quadratic fit to the dose-response function. The mean ED(10) benchmark dose for earned run opportunities was 10.13 ng/kg with a 95% lower bound of 5.77 ng/kg. The corresponding ED(01) was 0.98 ng/kg with a 95% lower bound of 0.83 ng/kg. The mean ED(10) for total wheel revolutions was calculated as 7.32 ng/kg with a 95% lower bound of 5.41 ng/kg. The corresponding ED(01) was 0.71 ng/kg with a 95% lower bound of 0.60. These values should be viewed from the perspective of current human body burdens, whose average value, based on TCDD toxic equivalents, has been calculated as 13 ng/kg.

Biological monitoring of iodine, a water disinfectant for long-term space missions.

In order to establish guidelines for exposure of astronauts to iodine, used as a water disinfectant in space, we studied the usefulness of hair, saliva, and urine for biological monitoring in humans and in the human hair/nude mouse model. The monitoring of iodine in patients that received 150 mCi of Na131I (carrier-free) showed similar patterns of elimination for blood, saliva, and urine. The mean correlation coefficient (r) between iodine elimination for blood/saliva was 0.99, for blood/urine, 0.95, and for saliva/urine, 0.97. The absolute value of iodine concentrations in urine revealed marked variability, which was corrected by adjusting for creatinine levels. The autoradiographic studies of human hair demonstrated that iodine is rapidly incorporated into external layers of the hair root and can be removed easily during washing. These data were confirmed after iodine exposure using the human hair/nude mouse model. Hair does not provide satisfactory information about exposure due to unstable incorporation of iodine. The most useful medium for biological monitoring of astronauts exposed to high doses of iodine in drinking water is urine, when adjusted for creatinine, and saliva, if quantitative evaluation of flow rate is provided.

Combined effect of tin and lead on heme biosynthesis in rats.

The aim of the present study was to investigate the combined effect of tin (SnCl2) and lead Pb(CH3COO)2 on activity of heme biosynthesis enzymes [delta-aminolevulinic acid synthetase (ALA-S) and heme oxygenase] in liver and kidneys, as well as iron (Fe) and copper (Cu) concentration in serum of rats. The experiment was performed on female rats which received 2 mg Sn/kg and 3.5 mg Pb/kg separately and jointly intraperitoneally (ip) for 5 days and per os (po) at single dose (100 mg Sn/kg and 17.5 mg Pb/kg). Lead induced ALA-S in liver and kidney both after ip and po administration; tin, however, induced ALA-S only after ip administration in liver of rats. The activity of heme oxygenase was induced after Sn po and ip administration in liver and kidneys and Pb administration (ip) in kidneys. Sn and Pb administered jointly caused a significant increase of Cu (ip), whereas Sn (po) decreased this metal level in serum of rats. Kidneys proved to be the organ in which the highest degree of examined enzyme induction took place. Pb is responsible for ALA-S, whereas Sn is responsible for induction of heme oxygenase activity in this organ, especially after per os administration. No additive effect on ALA-S and heme oxygenase activities of Pb and Sn combined was noticed.

Comparison of tin and lead toxic action on erythropoietic system in blood and bone marrow of rabbits.

The aim of this work was to assess and compare morphological changes in blood and bone marrow of rabbits after per os (po) or intraperitoneal (ip) administration of equimolar doses of tin or lead. The experiment was performed on female rabbits that were divided into four groups of six animals each, and received stannous chloride SnCl2 x 2 H2O (Merck) or lead acetate Pb(CH3COO)2 (POCh Gliwice) in equimolar doses (ip--17/microM/kg) or per os (po--85/microM/kg). Group I was administered SnCl2 ip at the dose of 2 mg Sn/kg every day for 3 mo, group II Pb(CH3COO)2 ip at a dose of 3.5 mg Pb/kg every day for 3 wk, group III po SnCl2 (10 mg Sn/kg), and group IV po Pb(CH3COO)2 (17.5 mg Pb/kg), both for 4 mo. The morphological factors hemoglobin (Hb), hematocrit (Ht), erythrocyte (Ercs), and reticulocyte counts, MCV, MCH, MCHC, and erythropoietic system in bone marrow aspirates with sideroblast count, iron concentration, TIBC, and SI were estimated. Tin caused hemolytic anemia depending on abnormal iron utilization. After ip administration of tin, anemia was observed during the whole time of the study, whereas after po exposure, transient anemia was noticed. It has been proven that the mechanism of toxic action of tin on hematopoietic system is similar to the toxic effect of lead.

Protective effect of zinc on heme biosynthesis disturbances in rabbits after administration per os of tin.

In the present study the lowest p.o. doses of tin affecting heme biosynthesis in rabbits were determined and the protective effect of zinc on these disorders was evaluated. The experiment was performed on female rabbits who received per os single doses of SnCl2 x 2 H2O (10, 100, and 200 mg Sn/kg) and ZnSO4 (50 mg Zn/kg s.c.). The activities of delta-aminolevulinic acid dehydratase (ALA-D) in the whole blood, free erythrocyte propoporhyrins, urine delta-aminolevulinic acid, and coproporphyrins (CP-U) were determined. In animals administered tin at a dose of 100 mg Sn/kg, ALA-D activity decreased by about 80% and two- to threefold increases in the ALA and CP concentrations in urine were observed. A protective effect of zinc with respect to ALA-D activity was noticed in both groups (100 and 200 mg Sn/kg) after combined administration of both metals. Results of an interaction between zinc and tin were also observed to reduce ALA levels in urine, whereas zinc did not protect against an effect of tin on CP excretion.

Disturbances in heme biosynthesis in rabbits after administration per os of low doses of tin or lead.

The experiment was performed on female rabbits that received per os equimolar doses (17 microM Me/kg) of SnCl2 x 2 H2O or Pb (CH3 COO)2 every day for 5 d. The activity of delta-aminolevulinic acid dehydratase (ALA-D) in the whole blood, liver, kidneys, brain, spleen, and bone marrow, concentration of free erythrocyte protoporphyrins (FEP), activity of delta-aminolevulinic acid synthetase (ALA-S) in the liver and bone marrow, urine delta-aminolevulinic acid (ALA-U), and coproporphyrins (CP-U) were determined. Lead and tin concentrations in the blood were estimated. Lead caused a significant inhibition of ALA-D in the blood, increased FEP concentration, and ALA and CP excretion in urine of rabbits. Lead also decreased ALA-D activity in the bone marrow and in the liver, and did not change ALA-S activity in the liver and bone marrow. Tin did not change any of the examined indices. Tin doses applied in the present study, maintained within the limits of permissible standards of metal levels in human diet, did not affect the process of heme biosynthesis in rabbits.

Effects of tin and lead on organ levels of essential minerals in rabbits.

The effect of tin and lead on levels of essential metals (Zn, Cu, Ca, Fe) in rabbit tissues was compared in relation to the route of administration. Animals received intraperitoneally, or per os, SnCl2 (2 mg Sn/kg) or Pb(CH3COO)2 (3.5 mg Pb/kg) every day for 5 d or for 1 mo. Copper, zinc, iron, and calcium were determined by AAS in the liver, kidneys, spleen, brain, bone marrow, and blood; lead and tin concentration were measured in the blood of animals. Tin and lead administered per os caused either no changes or the decreased concentration of endogenous metals in several tissues. The other route of administration (ip) of both metals generally contributed to the increased storage of essential elements. Blood tin levels of tin treated animals were only about less than or equal to 1/10 of blood lead concentrations of rabbits exposed to lead.

Effects of interaction between 65Zn, cadmium, and copper in rats.

Distribution and retention of zinc in the presence of cadmium and copper was studied in rats exposed repeatedly to these metals. The experiment was performed on white rats of the Wistar strain. The animals were divided into four groups/five rats each: 1) 65ZnCl2; 2) 65ZnCl2 + CdCl2; 3) 65ZnCl2 + CuCl2; and 4) control group. Rats were administered sc every other day for two weeks: 65ZnCl2-5 mg Zn/kg; CdCl2-0,3 Cd/kg; and CuCl2-2 mg Cu/kg. The zinc content was measured in rat tissues by gamma-counting. Effect of Cd and Cu on subcellular distribution of zinc in the kidney and liver and on the level of metallothionein were also examined. Whole body retention of zinc under the influence of cadmium was lower than that observed in animals treated with zinc alone. However, copper increased twofold the whole body retention of zinc. Cadmium elevated the accumulation of zinc only in the kidneys nuclear fraction and liver soluble fraction. In the kidneys and liver, copper elevated the accumulation of zinc, in the nuclear, mitochondrial, and soluble fractions. The level of metallothionein-like proteins (MT) in the kidneys after a combined supply of zinc and copper was significantly increased with respect to the group of animals treated with zinc alone. These results indicated complex interactions between cadmium, copper, and zinc that can affect the metabolism of each of the metals.

Zinc-selenium interaction in the rat.

Retention, dynamics of 75Se and 65Zn distribution, and elimination were studied in rats after separate or joint single doses of these metals. White female Wistar rats were divided into four groups (fifteen rats each). Group I received Na2(75)SeO3 (0.1 mg Se/kg i.g.), group II received Na2(75)SeO3 + ZnCl2 (5 mg Zn/kg s.c.), group III received 65ZnCl2, and group IV received 65ZnCl2 + Na2SeO3. The zinc and selenium contents in the tissues were estimated during 120 h after administration; excretion in urine and feces of animals was determined throughout the experiment. Combined administration of zinc and selenium resulted in an enhanced selenium retention in the brain, spleen, kidneys, blood, lungs, and heart. A selenium-induced increase in the concentration of zinc was noted in the bowels, blood, liver, kidneys, spleen, brain, and lungs. The effects of the zinc/selenium interaction were visible especially in the lowered level of excretion of these elements. Zinc induced a decrease in the excretion of selenium in urine, with no concomitant changes in the excretion in feces. However, a visible decrease in the excretion of zinc in the feces was observed in the presence of selenium. The present results indicate an occurrence of clear-cut interaction effects between zinc and selenium administered simultaneously in the rat.

Interaction of tin and zinc in some processes of heme biosynthesis in rabbits.

Female rabbits (2.5-3.0 kg) were divided into five groups. Group I (control) received ip 0.9% NaCl, every day for 5 days; group II received ip SnCl2 (dose of 2 mg Sn/kg) every day for 5 days; group III was given single sc injection of ZnSO4 (dose of 50 mg Zn/kg); group IV, a single sc injection of ZnSO4 followed by a single ip injection of SnCl2 on the next day; and group V received a single ip injection of SnCl2 followed next day by a single sc injection of ZnSO4. Eighteen hours after the first dose of SnCl2 an inhibition of delta-aminolevulinic acid dehydratase (ALA-D) by 60% was found in the blood of group II, while increases of urinary excretion of coproporphyrin (CP-U) and delta-aminolevulinic acid (ALA-U) with respect to the control group took place not earlier than after the fourth dose of SnCl2 (correlation coefficients, respectively: ALA-D/ALA-U r = 0.91; CP-U/ALA-D r = 0.70; CP-U/ALA-U r = 0.81). The single dose of ZnSO4 resulted in an increase of ALA-D by about 60% while administration of SnCl2 to the animals in group IV decreased this value down to physiologic levels. Then an increased tendency was noted again for the activity of this enzyme. Administration of SnCl2 and ZnSO4 (group V) resulted in a decrease of ALA-D activity and in an increase in CP-U.

Aminolevulinic acid dehydratase activity in the blood of rats exposed to tin and zinc.

Ninety rats of the Wistar strain were divided into 18 groups (5 animals in each). Stannous chloride was administered to animals by different routes (subcutaneously, intraperitoneally, intragastrically). Zinc was given intraperitoneally and lead was given subcutaneously, both every other day. Control animals received 0.9% NaCl. Aminolevulinic acid dehydratase (ALAD) activity was clearly decreased due to the double tin dose (total dose 4 mg Sn/kg), whereas 7 doses (altogether 14 mg Sn/kg) resulted in almost complete enzyme inhibition. ALAD inhibition in rat blood was similar, whatever the SnCl2 administration route. Greater inhibition was noted when this enzyme measured at pH 6.8. Zinc administered intraperitoneally (without tin) was also found to inhibit ALAD activity. The zinc doses applied did not protect ALAD activity against inhibition by tin, administered either intragastrically or intraperitoneally.

Effect of interaction between 65Zn, mercury and selenium in rats (retention, metallothionein, endogenous copper).

Interaction of zinc with mercuric chloride and sodium selenite was studied in the rat at the organ and subcellular levels (liver and kidneys). Zinc chloride was administered subcutaneously at dose of 5 mg Zn/kg, mercury chloride into the tail vein at a dose of 0.5 mg Hg/kg (both metals every other day during 2 weeks) and sodium selenite intragastrically, at doses of 0.1 mg Se/kg, every day. Zinc retention in the rat did not exceed 20% and was unchanged in the presence of mercury. An interaction effect was reflected by an increased whole-body retention of zinc by selenium, mercury, and selenium. In the presence of selenium no peak of metallothionein-like proteins stimulated by zinc or mercury was found in the soluble fraction of the kidneys. The metallothionein level did not differ from that typical for control group animals, too. A significant increase in the level of endogenous copper was found only in the kidneys of rats exposed to zinc in the presence of mercury and selenium.