Early chronic low-level methylmercury poisoning in monkeys impairs spatial vision

Visual Characteristics of Adults with Long-Standing History of Dietary Exposure to Mercury in Grassy Narrows First Nation, Canada

Since the 1960s, Grassy Narrows First Nation (Ontario, Canada) has been exposed to methyl mercury (Hg) through fish consumption, resulting from industrial pollution of their territorial waters. This cross-sectional study describes the visual characteristics of adults with documented Hg exposure between 1970 and 1997. Oculo-visual examinations of 80 community members included visual acuity, automated visual fields, optical coherence tomography [OCT], color vision and contrast sensitivity. Median age was 57 years (IQR 51-63) and 55% of participants were women. Median visual acuity was 0.1 logMAR (Snellen 6/6.4; IQR 0-0.2). A total of 26% of participants presented a Visual Field Index inferior to 62%, and qualitative losses assessment showed concentric constriction (18%), end-stage concentric loss (18%), and complex defects (24%). On OCT, retinal nerve fiber layer scans showed 74% of participants within normal/green range. For color testing with the Hardy, Rand, and Rittler test, 40% presented at least one type of color defect, and with the Lanthony D-15 test, median color confusion index was 1.59 (IQR 1.33-1.96). Contrast sensitivity showed moderate loss for 83% of participants. These findings demonstrate important loss of visual field, color vision, and contrast sensitivity in older adults in a context of long-term exposure to Hg in Grassy Narrows First Nation.

Adolescence as a sensitive period for neurotoxicity: Lifespan developmental effects of methylmercury

Neurotoxicity resulting from the environmental contaminant, methylmercury (MeHg), is a source of concern for many human populations that rely heavily on the consumption of fish and rice as stable ingredients in the diet. The developmental period of exposure is important both to the qualitative effects of MeHg and to the dose required to produce those effects. MeHg exposure during the sensitive prenatal period causes deleterious and long-lasting changes in neurodevelopment at particularly low doses. The effects include a wide host of cognitive and behavioral outcomes expressed in adulthood and sometimes not until aging. However, neurotoxic outcomes of methylmercury when exposure occurs during adolescence are only recently revealing impacts on human populations and animal models. This review examines the current body of work and showcases the sensitivity of adolescence, a period that straddles early development and adulthood, to methylmercury neurotoxicity and the implications such toxicity has in our understanding of methylmercury's effects in human populations and animal models.

Current and historical nephric and hepatic mercury concentrations in terrestrial mammals in Poland and other European countries

The long-term anthropogenic release of mercury (Hg) into the environment has led to contamination of the biosphere, with all forms of Hg showing toxic effects and the ability to accumulate in organisms. Since the 1970s, efforts have been made in Western Europe to reduce Hg emissions and for the economic use of Hg, leading to a reduction in Hg exposure to humans and entire ecosystems. The purpose of this research was to present the total mercury (THg) burden in three mustelids (the piscivorous Eurasian otter and American mink, and the invertebrativorous European badger) inhabiting north-western Poland (mostly floodplains) and other European countries (literature data). Moreover, we wanted to investigate whether reductions in the environmental Hg burden in Europe have resulted in reductions in liver and kidney levels in wild terrestrial mammals (Eurasian otter, wild boar, red deer, roe deer, cervids, leporids, rodents, and ecotrophic groups: piscivorous mustelids, non-mustelids whose diets include aquatic prey, canids and other carnivores, omnivores, herbivores), between samples collected before and after 2000. We revealed significantly higher nephric THg levels in roadkilled than in trapped American minks. As roadkilled piscivorous mustelids from the same floodplain had similar hepatic and nephric THg concentrations, we suggest that the European research on Hg ecotoxicology should more often use alien American mink instead of the protected Eurasian otter. Badgers inhabiting Polish and other European floodplains bioaccumulated higher amounts of THg than those from other areas, and as such, may be recommended as bioindicator of mercury soil contamination. Our analysis of abundant data on mammalian hepatic and nephric THg concentrations (excluding non-piscivores mustelids) showed that in 12 of 21 cases, Hg concentrations had dropped significantly since 2000. This data signals a reduction in Hg contamination in terrestrial mammals, such as the Eurasian otter, and may be reason for cautious optimism.

Nonhuman primates as animal models for toxicology research

There is a long history for the use of nonhuman primates in toxicological research. This unit reviews applications in reproductive toxicology and teratology, neural toxicology and neurobehavioral toxicology, immunotoxicology, respiratory (lung) toxicology, and chemical carcinogenesis.

Methylmercury: a potential environmental risk factor contributing to epileptogenesis

Epilepsy or seizure disorder is one of the most common neurological diseases in humans. Although genetic mutations in ion channels and receptors and some other risk factors such as brain injury are linked to epileptogenesis, the underlying cause for the majority of epilepsy cases remains unknown. Gene-environment interactions are thought to play a critical role in the etiology of epilepsy. Exposure to environmental chemicals is an important risk factor. Methylmercury (MeHg) is a prominent environmental neurotoxicant, which targets primarily the central nervous system (CNS). Patients or animals with acute or chronic MeHg poisoning often display epileptic seizures or show increased susceptibility to seizures, suggesting that MeHg exposure may be associated with epileptogenesis. This mini-review highlights the effects of MeHg exposure, especially developmental exposure, on the susceptibility of humans and animals to seizures, and discusses the potential role of low level MeHg exposure in epileptogenesis. This review also proposes that a preferential effect of MeHg on the inhibitory GABAergic system, leading to disinhibition of excitatory glutamatergic function, may be one of the potential mechanisms underlying MeHg-induced changes in seizure susceptibility.

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

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

Options for incorporating children's inhaled dose into human health risk assessment

Increasing attention has been placed on inhalation dosimetry in children because of children's greater air intake rate and unique windows of vulnerability for various toxicants and health outcomes. However, risk assessments have not incorporated this information because dosimetric adjustments have focused upon extrapolation across species rather than across age groups within the human population. The objectives of this study were to synthesize information regarding child/adult intake and dosimetry differences for particles and gases for potential application to risk assessment. Data and models gathered at a 2006 workshop and more recent studies were reviewed to better understand lung development and inhaled dose in children. The results show that child/adult differences exist both on a chemical intake basis and on a deposited or systemic dose basis. These differences can persist for several years and are not captured by standard intraspecies uncertainty factors or by USEPA's reference concentration (RfC) methodology. Options for incorporating children's inhalation exposures into human risk assessments include (1) 3-fold default air intake adjustment for the first 3 years of life with a reduced factor for older children; (2) superseding this default via simplified dosimetry models akin to USEPA's RfC methodology modified for children; (3) utilizing more sophisticated models with better anatomical and air flow descriptions; (4) running these models with input distributions to reflect interchild variability; (5) developing more advanced approaches involving imaging techniques and computational fluid dynamic (CFD) models. These options will enable children's inhaled dose to have a quantitative role in risk assessment that has been lacking and will establish a basis for ongoing research.

In vivo methylmercury exposure induced long-lasting epileptiform activity in layer II/III neurons in cortical slices from the rat

Prenatal and postnatal methylmercury (MeHg) exposure has been shown to increase neuronal excitability and seizure susceptibility. To determine if early postnatal MeHg exposure causes a similar effect, we examined changes in field potentials in layer II/III neurons in cortical slices of rat following in vivo MeHg treatment. Rats received 0 (0.9% NaCl), 0.75 mg/kg/day or 1.5mg/kg/day MeHg subcutaneously for 15 or 30 days beginning on postnatal day 5, after which cortical slices were prepared for field potential recordings. In slices from rats treated with vehicle, single pulse stimulation of layer IV of cortical slices induced a typical field excitatory postsynaptic potential (fEPSP) with a single spike. This type of fEPSPs was also seen in slices from rats with 15 day treatment with 0.75 mg/kg/day or 1.5mg/kg/day MeHg. However, 30-day treatment with either MeHg dose resulted in fEPSPs with multiple spikes (epileptiform activity) in 40% of animals examined. This epileptiform activity remained observable in 50-60% animals in which MeHg exposure had been terminated for 30 days. However, slices from control animals still showed fEPSPs with single spike. Thus, these data suggest that postnatal MeHg exposure in vivo altered neuronal excitability and induced a long-lasting hyperexcitability in cortical neurons.

Delayed acquisition of neonatal reflexes in newborn primates receiving a thimerosal-containing hepatitis B vaccine: influence of gestational age and birth weight

This study examined whether acquisition of neonatal reflexes in newborn rhesus macaques was influenced by receipt of a single neonatal dose of hepatitis B vaccine containing the preservative thimerosal (Th). Hepatitis B vaccine containing a weight-adjusted Th dose was administered to male macaques within 24 h of birth (n = 13). Unexposed animals received saline placebo (n = 4) or no injection (n = 3). Infants were tested daily for acquisition of nine survival, motor, and sensorimotor reflexes. In exposed animals there was a significant delay in the acquisition of root, snout, and suck reflexes, compared with unexposed animals. No neonatal responses were significantly delayed in unexposed animals. Gestational age (GA) and birth weight (BW) were not significantly correlated. Cox regression models were used to evaluate main effects and interactions of exposure with BW and GA as independent predictors and time-invariant covariates. Significant main effects remained for exposure on root and suck when controlling for GA and BW, such that exposed animals were relatively delayed in time-to-criterion. Interaction models indicated there were various interactions between exposure, GA, and BW and that inclusion of the relevant interaction terms significantly improved model fit. This, in turn, indicated that lower BW and/or lower GA exacerbated the adverse effects following vaccine exposure. This primate model provides a possible means of assessing adverse neurodevelopmental outcomes from neonatal Th-containing hepatitis B vaccine exposure, particularly in infants of lower GA or BW. The mechanisms underlying these effects and the requirements for Th requires further study.

Low level postnatal methylmercury exposure in vivo alters developmental forms of short-term synaptic plasticity in the visual cortex of rat

Methylmercury (MeHg) has been previously shown to affect neurotransmitter release. Short-term synaptic plasticity (STP) is primarily related to changes in the probability of neurotransmitter release. To determine if MeHg affects STP development, we examined STP forms in the visual cortex of rat following in vivo MeHg exposure. Neonatal rats received 0 (0.9% NaCl), 0.75 or 1.5 mg/kg/day MeHg subcutaneously for 15 or 30 days beginning on postnatal day 5, after which visual cortical slices were prepared for field potential recordings. In slices prepared from rats treated with vehicle, field excitatory postsynaptic potentials (fEPSPs) evoked by paired-pulse stimulation at 20-200 ms inter-stimulus intervals showed a depression (PPD) of the second fEPSP (fEPSP2). PPD was also seen in slices prepared from rats after 15 day treatment with 0.75 or 1.5 mg/kg/day MeHg. However, longer duration treatment (30 days) with either dose of MeHg resulted in paired-pulse facilitation (PPF) of fEPSP2 in the majority of slices examined. PPF remained observable in slices prepared from animals in which MeHg exposure had been terminated for 30 days after completion of the initial 30 day MeHg treatment, whereas slices from control animals still showed PPD. MeHg did not cause any frequency- or region-preferential effect on STP. Manipulations of [Ca2+](e) or application of the GABA(A) receptor antagonist bicuculline could alter the strength and polarity of MeHg-induced changes in STP. Thus, these data suggest that low level postnatal MeHg exposure interferes with the developmental transformation of STP in the visual cortex, which is a long-lasting effect.

Visual System Manifestations due to Systemic Exposure to Mercury

This article is a summary of the available literature on ocular symptoms due to systemic exposure to mercury. Mercury compounds are first described in terms of their different forms, industrial applications, mechanisms and routes of exposure, toxicity levels, and treatment methods. Eye symptoms are then characterized for organic and inorganic forms of mercury by using the form of various documented case studies of chronic and acute exposure to various mercury compounds.

Methylmercury and nutrition: adult effects of fetal exposure in experimental models

Human exposure to the life-span developmental neurotoxicant, methylmercury (MeHg), is primarily via the consumption of fish or marine mammals. Fish are also excellent sources of important nutrients, including selenium and n-3 polyunsaturated fatty acids (PUFAs), such as docosahexaenoic acid (DHA). Laboratory models of developmental MeHg exposure can be employed to assess the roles of nutrients and MeHg and to identify potential mechanisms of action if the appropriate exposure measures are used. When maternal exposure is protracted, relationships between daily intake and brain mercury are consistent and orderly across species, even when large differences in blood:brain ratios exist. It is well established that low-level developmental MeHg produces sensory deficits. Recent studies also show that perseveration in reversal-learning tasks occurs after gestational exposures that produce low micromolar concentrations in the brain. A no-effect level has not been identified for this effect. These exposures do not affect the acquisition or performance of discrimination learning, set shifting (extradimensional shift), or memory. Reversal-learning deficits may be related to enhanced impact of reinforcers as measured using progressive ratio reinforcement schedules, an effect that could result in perseveration. Also reported is enhanced sensitivity to dopamine reuptake inhibitors and diminished sensitivity to pentobarbital, a GABA(A) agonist. Diets rich in PUFAs or selenium do not protect against MeHg's effects on reversal learning but, by themselves, may diminish variability in performance, enhance attention or psychomotor function and may confer some protection against age-related deficits in these areas. It is hypothesized that altered reward processing, dopamine and GABAergic neurotransmitter systems, and cortical regions associated with choice and perseveration are especially sensitive to developmental MeHg at low exposure levels. Human testing for MeHg's neurotoxicity should emphasize these behavioral domains.

Electrophysiological evidence for impairment of contrast sensitivity in mercury vapor occupational intoxication

Contrast sensitivity (CS) was evaluated in 41 former workers from a lamp manufacturing plant who were on disability retirement due to exposure to mercury and 14 age-matched controls. The CS was measured monocularly using the sweep visual evoked potential (sVEP) paradigm at 6 spatial frequencies (0.2, 0.8, 2.0, 4.0, 15.0, and 30 cpd). Statistical difference (p<0.05) was found between the controls and the patient right and left eyes for 2.0 and 4.0 cpd. According the results in those spatial frequencies the eyes were classified in best and worst. Statistical differences were found between the controls and the best eyes for 2.0 and 4.0 cpd and for 0.8, 2.0, and 4.0 cpd for their worst eyes. No correlation was found between CS results and the time of exposure (mean=8.9 yr+/-4.1), time away from the mercury source (mean=6.0 yr+/-3.9), urinary mercury level at the time of work (mean=40.6 microg/g+/-36.3) or with the mercury level at the CS measurement time (mean=1.6 microg/g+/-1.1). We show the first evidence of a permanent impairment in CS measured objectively with the sVEP. Our data complement the previous psychophysical works reporting a diffuse impairment in the CS function showing a CS reduction in the low to middle spatial frequencies. In conclusion, non-reversible CS impairment was found in occupational exposure to mercury vapor. We suggest that CS measurement should be included in studies of the mercury effects of occupational exposure.

Neurobehavioral assessment of rats exposed to low doses of PCB126 and methyl mercury during development

Epidemiological and laboratory studies have suggested that polychlorinated biphenyls (PCBs) and methyl mercury (MeHg) may have additive or synergistic effects on CNS function. Aim of this study was to characterize the effects of exposure to low levels of MeHg (0.5mg/kgday in drinking water) and PCB126 (100ng/kgday in food), alone and in combination, on neurobehavioral development in Wistar rats. Dams were treated from gestational day 7 to post-natal day (PND) 21. Animals were tested for developmental landmarks and reflexes (PND1-21), attention deficits (PND40), locomotor activity (PND30, 110), spatial learning (PND75), coordination and balance (PND90), object discrimination (PND80), anxiety (PND100), and conditioned learning (PND110). Parameters related to pregnancy, sex ratio at birth, and physical development (at weaning) did not differ among groups, though PCB126 decreased number of pups at birth. A slight delay in negative geotaxis was found in female rats in all treatment groups. No significant effects were seen in attention, coordination and balance, object discrimination, and spatial and conditioned learning. Increased motor activity was present in PCB126-treated male and in MeHg+PCB-treated female rats in the elevated plus maze test, and in PCB126-treated male rats in the open field test (PND110). The results do not support the hypothesis that co-exposure to MeHg and PCB126 results in additive or synergistic effects. This finding is in agreement with more recent in vitro and in vivo studies.

Spatial and visual discrimination reversals in adult and geriatric rats exposed during gestation to methylmercury and n-3 polyunsaturated fatty acids

Fish contain essential long chain polyunsaturated fatty acids (PUFAs), particularly docosahexaenoic acid (DHA), an omega-3 (or n-3) PUFA, but are also the main source of exposure to methylmercury (MeHg), a potent developmental neurotoxicant. Since n-3 PUFAs support neural development and function, benefits deriving from a diet rich in n-3s have been hypothesized to protect against deleterious effects of gestational MeHg exposure. To determine whether protection occurs at the behavioral level, female Long-Evans rats were exposed, in utero, to 0, 0.5, or 5ppm of Hg as MeHg via drinking water, approximating exposures of 0, 40, and 400 microgHg/kg/day and producing 0, 0.29, and 5.50ppm of total Hg in the brains of siblings at birth. They also received pre- and postnatal exposure to one of two diets, both based on the AIN-93 semipurified formulation. A "fish-oil" diet was high in, and a "coconut-oil" diet was devoid of, DHA. Diets were approximately equal in alpha-linolenic acid and n-6 PUFAs. As adults, the rats were first assessed with a spatial discrimination reversal (SDR) procedure and later with a visual (nonspatial) discrimination reversal (VDR) procedure. MeHg increased the number of errors to criterion for both SDR and VDR during the first reversal, but effects were smaller or non-existent on the original discrimination and on later reversals. No such MeHg-related deficits were seen when the rats were retested on SDR after 2 years of age. These results are consistent with previous reports and hypotheses that gestational MeHg exposure produces perseverative responding. No interactions between diet and MeHg were found, suggesting that n-3 PUFAs do not guard against these behavioral effects. Brain Hg concentrations did not differ between the diets, either. In geriatric rats, failures to respond were less common and response latencies were shorter for rats fed the fish-oil diet, suggesting that exposure to a diet rich in n-3s may lessen the impact of age-related declines in response initiation.

Prenatal methylmercury exposure affects spatial vision in adult monkeys

Decades of research have demonstrated that exposure to methylmercury (MeHg), a ubiquitous environmental pollutant, can have both early and long-term neurobehavioral consequences in exposed offspring. The present study assessed visual functioning in adult macaque monkeys (Macaca fascicularis) exposed in utero to 0, 50, 70, or 90 microg/kg/day of MeHg hydroxide. Twenty-one full-term, normal birth weight offspring (9 controls, 12 exposed) were tested at approximately 11-14.5 years of age on a visual contrast sensitivity task. A forced-choice tracking procedure was utilized with spatial frequencies of 1, 4, 10, and 20 cycles per degree of visual angle. On each test session, a single spatial frequency was presented across five levels of contrast, each differing by 3 dB. Methylmercury-exposed monkeys exhibited reduced contrast sensitivity thresholds, particularly at the higher spatial frequencies. The degree of visual impairment was not related to MeHg body burden or clearance and almost half of the exposed animals were unimpaired. The results from this study demonstrate that chronic in utero MeHg exposure, at subclinical levels, is associated with permanent adverse effects on spatial vision in adult monkeys.

Motor function following developmental exposure to PCBS and/or MEHG

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

Comparison of blood and brain mercury levels in infant monkeys exposed to methylmercury or vaccines containing thimerosal

Thimerosal is a preservative that has been used in manufacturing vaccines since the 1930s. Reports have indicated that infants can receive ethylmercury (in the form of thimerosal) at or above the U.S. Environmental Protection Agency guidelines for methylmercury exposure, depending on the exact vaccinations, schedule, and size of the infant. In this study we compared the systemic disposition and brain distribution of total and inorganic mercury in infant monkeys after thimerosal exposure with those exposed to MeHg. Monkeys were exposed to MeHg (via oral gavage) or vaccines containing thimerosal (via intramuscular injection) at birth and 1, 2, and 3 weeks of age. Total blood Hg levels were determined 2, 4, and 7 days after each exposure. Total and inorganic brain Hg levels were assessed 2, 4, 7, or 28 days after the last exposure. The initial and terminal half-life of Hg in blood after thimerosal exposure was 2.1 and 8.6 days, respectively, which are significantly shorter than the elimination half-life of Hg after MeHg exposure at 21.5 days. Brain concentrations of total Hg were significantly lower by approximately 3-fold for the thimerosal-exposed monkeys when compared with the MeHg infants, whereas the average brain-to-blood concentration ratio was slightly higher for the thimerosal-exposed monkeys (3.5 +/- 0.5 vs. 2.5 +/- 0.3). A higher percentage of the total Hg in the brain was in the form of inorganic Hg for the thimerosal-exposed monkeys (34% vs. 7%). The results indicate that MeHg is not a suitable reference for risk assessment from exposure to thimerosal-derived Hg. Knowledge of the toxicokinetics and developmental toxicity of thimerosal is needed to afford a meaningful assessment of the developmental effects of thimerosal-containing vaccines.

Colour vision and contrast sensitivity losses of mercury intoxicated industry workers in Brazil

We evaluated vision loss in workers from fluorescent lamp industries (n=39) who had retired due to intoxication with mercury vapour and had been away from the work situation for several years (mean=6.32 years). An age-matched control group was submitted to the same tests for comparison. The luminance contrast sensitivity (CSF) was measured psychophysically and with the sweep visual evoked potential (sVEP) method. Chromatic red-green and blue-yellow CSFs were measured psychophysically. Colour discrimination was assessed with the Farnsworth-Munsell 100-hue test, Lanthony D-15d test and Cambridge Colour Vision Test. Patient data showed significantly lower scores in all colour tests compared to controls (p<.001). The behavioural luminance CSF of the patients was lower than that of controls (p<.001 at all frequencies tested). This result was confirmed by the electrophysiologically measured sweep VEP luminance CSF except at the highest frequencies-a difference that might be related to stimulus differences in the two situations. Chromatic CSFs were also statistically significantly lower for the patients than for the controls, for both chromatic equiluminant stimuli: red-green (p<.005) and blue-yellow (p<.04 for all frequencies, except 2 cycles per degree (cpd), the highest spatial frequency tested) spatial gratings. We conclude that exposure to elemental mercury vapour is associated with profound and lasting losses in achromatic and chromatic visual functions, affecting the magno-, parvo- and koniocellular visual pathways.

Multifocal and full-field electroretinogram changes associated with color-vision loss in mercury vapor exposure

We evaluated the color vision of mercury-contaminated patients and investigated possible retinal origins of losses using electroretinography. Participants were retired workers from a fluorescent lamp industry diagnosed with mercury contamination (n= 43) and age-matched controls (n= 21). Color discrimination was assessed with the Cambridge Colour Test (CCT). Retinal function was evaluated by using the ISCEV protocol for full-field electroretinography (full-field ERG), as well as by means of multifocal electroretinography (mfERG). Color-vision losses assessed by the CCT consisted of higher color-discrimination thresholds along the protan, deutan, and tritan axes and significantly larger discrimination ellipses in mercury-exposed patients compared to controls. Full-field ERG amplitudes from patients were smaller than those of the controls for the scotopic responseb-wave, maximum response, sum of oscillatory potentials (OPs), 30-Hz flicker response, and light-adapted cone response. OP amplitudes measured in patients were smaller than those of controls for O2 and O3. Multifocal ERGs recorded from ten randomly selected patients showed smaller N1–P1 amplitudes and longer latencies throughout the 25-deg central field. Full-field ERGs showed that scotopic, photopic, peripheral, and midperipheral retinal functions were affected, and the mfERGs indicated that central retinal function was also significantly depressed. To our knowledge, this is the first demonstration of retinal involvement in visual losses caused by mercury toxicity.

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.

Neurobehavioral toxicity of methylmercury and PCBs Effects-profiles and sensitive populations

A large and growing body of literature is available on the neurotoxicity of methylmercury and PCBs as expressed in the behavior of both humans and laboratory animals. Methylmercury and PCBs will be compared with PCBs with attention directed at overlaps and distinctions in their profiles of neurotoxicity. It is possible with methylmercury and, to a lesser extent, with PCBs to characterize the sensory, motor, and cognitive consequences of exposure. Methylmercury is emerging as a life-span developmental neurotoxicant: adverse effects of exposure have been identified in development and during aging in human populations as well as in laboratory animals. Less is known about the PCBs on this count. While the mechanisms of neurotoxicity are not understood for either class of compounds, emerging clues are pointing to the possibility of overlap in some mechanisms of neurotoxicity.

Review of neurobehavioral deficits and river fish consumption from the Tapajós (Brazil) and St. Lawrence (Canada)

Our research group is carrying out studies on neurobehavioral changes associated with eating fish from the Upper St. Lawrence River (Québec, Canada) and the Lower Tapajós River (Brazilian Amazon). Here, these studies are reviewed with respect to exposure, effects and intervention. Although mercury (Hg) levels in piscivorous fish are similar in both regions, in the Amazon, fish constitutes the dietary mainstay, while in Quebec, fish consumption is primarily occasional. Mercury exposure of Amazonian fish-eaters was considerably higher than Québec (median blood total Hg: 28 and 1 μg/l, respectively), but fish from the St. Lawrence contain multiple contaminants. For the Tapajós River, increasing hair Hg was associated with reduced motor and visual functions. Comparison of neurobehavioral performance of Québec fish-eaters and non fish-eaters showed a consistent pattern of information processing slowing among the former; these deficits were not related to blood methyl Hg levels. Early changes associated with exposure can be used to trigger intervention. Since fish provide important essential nutrients, mitigation must balance the beneficial and harmful effects. In Canada, advisories from environmental and health agencies consider both these aspects. In the Amazon, we are currently involved in a participatory research whose goal is to reduce Hg absorption, while maintaining fish consumption.

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.

Determination of a site-specific reference dose for methylmercury for fish-eating populations

Environmental risk-management decisions in the U.S. involving potential exposures to methylmercury currently use a reference dose (RfD) developed by the U.S. Environmental Protection Agency (USEPA). This RfD is based on retrospective studies of an acute poisoning incident in Iraq in which grain contaminated with a methylmercury fungicide was inadvertently used in the baking of bread. The exposures, which were relatively high but lasted only a few months, were associated with neurological effects in both adults (primarily paresthesia) and infants (late walking, late talking, etc.). It is generally believed that the developing fetus represents a particularly sensitive subpopulation for the neurological effects of methylmercury. The USEPA derived an RfD of 0.1 microg/kg/day based on benchmark dose (BMD) modeling of the combined neurological endpoints reported for children exposed in utero. This RfD included an uncertainty factor of 10 to consider human pharmacokinetic variability and database limitations (lack of data on multigeneration effects or possible long-term sequelae of perinatal exposure). Alcoa signed an Administrative Order of Consent for the conduct of a remedial investigation/feasibility study (RI/FS) at their Point Comfort Operations and the adjacent Lavaca Bay in Texas to address the effects of historical discharges of mercury-containing wastewater. In cooperation with the Texas Natural Resource Conservation Commission and USEPA Region VI, Alcoa conducted a baseline risk assessment to assess potential risk to human health and the environment. As a part of this assessment. Alcoa pursued the development of a site-specific RfD for methylmercury to specifically address the potential human health effects associated with the ingestion of contaminated finfish and shellfish from Lavaca Bay. Application of the published USEPA RfD to this site is problematic; while the study underlying the RfD represented acute exposure to relatively high concentrations of methylmercury, the exposures of concern for the Point Comfort site are from the chronic consumption of relatively low concentrations of methylmercury in fish. Since the publication of the USEPA RfD, several analyses of chronic exposure to methylmercury in fish-eating populations have been reported. The purpose of the analysis reported here was to evaluate the possibility of deriving an RfD for methylmercury, specifically for the case of fish ingestion, on the basis of these new studies. In order to better support the risk-management decisions associated with developing a remediation approach for the site in question, the analysis was designed to provide information on the distribution of acceptable ingestion rates across a population, which could reasonably be expected to be consistent with the results of the epidemiological studies of other fish-eating populations. Based on a review of the available literature on the effects of methylmercury, a study conducted with a population in the Seychelles Islands was selected as the critical study for this analysis. The exposures to methylmercury in this population result from chronic, multigenerational ingestion of contaminated fish. This prospective study was carefully conducted and analyzed, included a large cohort of mother-infant pairs, and was relatively free of confounding factors. The results of this study are essentially negative, and a no-observed-adverse-effect level (NOAEL) derived from the estimated exposures has recently been used by the Agency for Toxic Substances and Disease Registry (ATSDR) as the basis for a chronic oral minimal risk level (MRL) for methylmercury. In spite of the fact that no statistically significant effects were observed in this study, the data as reported are suitable for dose-response analysis using the BMD method. Evaluation of the BMD method used in this analysis, as well as in the current USEPA RfD, has demonstrated that the resulting 95% lower bound on the 10% benchmark dose (BMDL) represents a conservative estimate of the traditional NOAEL, and that it is superior to the use of "average" or "grouped" exposure estimates when dose-response information is available, as is the case for the Seychelles study. A more recent study in the Faroe Islands, which did report statistically significant associations between methylmercury exposure and neurological effects, could not be used for dose-response modeling due to inadequate reporting of the data and confounding from co-exposure to polychlorinated biphenyls (PCBs). BMD modeling over the wide range of neurological endpoints reported in the Seychelles study yielded a lowest BMDL for methylmercury in maternal hair of 21 ppm. This BMDL was then converted to an expected distribution of daily ingestion rates across a population using Monte Carlo analysis with a physiologically based pharmacokinetic (PBPK) model to evaluate the impact of interindividual variability. The resulting distribution of ingestion rates at the BMDL had a geometric mean of 1.60 microg/kg/day with a geometric standard deviation of 1.33; the 1st, 5th, and 10th percentiles of the distribution were 0.86, 1.04, and 1.15 microg/kg/day. In place of the use of an uncertainty factor of 3 for pharmacokinetic variability, as is done in the current RfD, one of these lower percentiles of the daily ingestion rate distribution provides a scientifically based, conservative basis for taking into consideration the impact of pharmacokinetic variability across the population. On the other hand, it was felt that an uncertainty factor of 3 for database limitations should be used in the current analysis. Although there can be high confidence in the benchmark-estimated NOAEL of 21 ppm in the Seychelles study, some results in the New Zealand and Faroe Islands studies could be construed to suggest the possibility of effects at maternal hair concentrations below 10 ppm. In addition, while concerns regarding the possibility of chronic sequelae are not supported by the available data, neither can they be absolutely ruled out. The use of an uncertainty factor of 3 is equivalent to using a NOAEL of 7 ppm in maternal hair, which provides additional protection against the possibility that effects could occur at lower concentrations in some populations. Based on the analysis described above, the distribution of acceptable daily ingestion rates (RfDs) recommended to serve as the basis for site-specific risk-management decisions at Alcoa's Point Comfort Operations ranges from approximately 0.3 to 1.1 microg/kg/day, with a population median (50th percentile) of 0.5 microg/kg/day. By analogy with USEPA guidelines for the use of percentiles in applications of distributions in exposure assessments, the 10th percentile provides a reasonably conservative measure. On this basis, a site-specific RfD of 0.4 microg/kg/day is recommended.

Methods for studying nonhuman primates in neurobehavioral toxicology and teratology

The behavioral repertoire of nonhuman primates is highly evolved and includes advanced problem-solving capabilities, complex social relationships, and sensory acuity equal or superior to humans. These factors make nonhuman primates valuable animal models for studies of the functional effects of neurotoxicants. This review provides descriptions of tests designed to study learning, memory, schedule-controlled behavior, information processing, social behavior, sensory functioning, and visual-motor coordination and/or visuospatial orientation in macaque monkeys. Whenever possible, the results of studies in primate behavioral toxicology are provided for individual test measures. The primate model is especially useful for studies of developmental exposures because monkeys, like humans, have relatively prolonged periods of gestation, infancy, and adolescence. In recognition of this, a special section is provided for tasks that are specifically designed to study behavioral processes in infant monkeys.

Neurotoxic effects of low-level methylmercury contamination in the Amazonian Basin

Many studies have demonstrated mercury contamination in the Amazonian ecosystem, particularly in fish, a dietary mainstay of populations in this region. The present study focused on potential health effects of this low-level methylmercury exposure. The study was carried out in a village on the Tapajós River, a tributary of the Amazon, on 91 adults inhabitants (15-81 years), whose hair mercury levels were inferior to 50 mu/g. Performance on a neurofunctional test battery and clinical manifestations of nervous system dysfunction were examined in relation to hair mercury concentrations. Near visual contrast sensitivity and manual dexterity, adjusted for age, decreased significantly with hair mercury levels (P < 0.05), while there was a tendency for muscular fatigue to increase and muscular strength to decrease in women. For the most part, clinical examinations were normal, however, hair mercury levels were significantly higher (P < 0.05) for persons who presented disorganized movements on an alternating movement task and for persons with restricted visual fields. These results suggest dose-dependent nervous system alterations at hair mercury levels below 50 micrograms/g, previously considered a threshold for clinical effects. The profile of dysfunction in this adult population is consistent with the current knowledge on methyl-mercury poisoning. The long-term implications of these findings are unknown and need to be addressed.

Lack of effect of methylmercury exposure from birth to adulthood on information processing speed in the monkey

Although it is established that developmental methylmercury exposure produces severe motor and sensory impairment, the effect on cognitive function is less clear. To explore this issue, monkeys with robust methylmercury-induced deficits in visual, auditory, and somatosensory function were tested on a series of tasks assessing central processing speed, which is highly correlated with intelligence in humans. Five monkeys (Macaca fascicularis) were dosed from birth to 7 years of age with 50 micrograms/kg/day of mercury as methylmercuric chloride. Blood mercury levels were stable at 0.8-1.1 micrograms/g until cessation of dosing. When they were 20 years old, these monkeys and four age- and rearing-matched controls were tested on a series of simple and complex reaction time tasks. The monkey sat in a primate chair with a stainless steel bar centered at waist height. Four push buttons equidistant from the steel bar were mounted on a vertical Plexiglas panel in front of the monkey. The monkey was required to make contact with the bar, then release the bar and push the appropriate button in response to a change in stimulus conditions. For the first task (simple reaction time), the monkey was required to respond on a button when it changed from unlit to red. The monkey then performed a sequence of complex reaction time tasks: two-button, four-button, and several tasks of increasing complexity using four buttons and multiple colors. For each task, the latency to release the bar after the stimulus change (central processing speed) and to move the hand from the bar to the button (motor speed) were determined. Lastly, the monkey was required to make the quickest possible motor response on the simple reaction time task. There were no differences between groups on any aspect of the experiment. These data provide further evidence for absence of cognitive impairment in monkeys exposed developmentally to methylmercury.

Visual functions in 6-year-old children in relation to lead and mercury levels

Within a larger comparative environmental health screening program in East and West Germany we investigated functions of the developing visual system in field experiments in a total of 384 children living in three different areas. Visual functions were assessed neurophysiologically by visual-evoked potentials (VEPs) and psychophysically by measuring the contrast sensitivity (CS). Blood lead concentrations and urinary mercury levels were used as markers of environmental and/or amalgam-derived exposure, respectively. The relationships among lead and mercury concentrations and the neurophysiological and psychophysical outcomes were investigated by means of linear regression analysis. After adjusting for confounding effects, statistically significant lead-related changes were found only for some of the VEP interpeak latencies, while some of the CS values were significantly reduced with increasing mercury concentrations. All other outcome variables were not significantly related to lead or mercury levels. It is concluded that even at blood lead levels in the range of 14 to 174 micrograms/l and at very low urinary mercury levels subtle changes in visual system functions can be measured.

Fixed interval/fixed ratio performance in adult monkeys exposed in utero to methylmercury

Previous studies in monkeys and rodents have shown the fixed interval/fixed ratio (FI/FR) schedule to be a sensitive indicator of neurotoxicity. In the present study, monkeys (Macaca fascicularis) were exposed in utero to methylmercury (MeHg). Maternal doses of MeHg of 50, 70, or 90 micrograms/kg b.wt./day resulted in infant blood mercury levels at birth ranging from 1.04 to 2.45 ppm. Monkeys were tested on a multiple FI/FR schedule of reinforcement at 8-10 years of age. Four FI/FR cycles were run per session. Pause time and run rate were calculated for FI and FR components, as well as FI quarter-life and local FI response rates. MeHg treatment and sex effects were investigated by fitting a linear orthogonal polynomial regression to each monkey's profile across sessions and performing two-way ANOVAs on the resulting linear and intercept terms. There were no treatment-related effects on either the FI or FR component for pause time or run rate. Analysis of the quarter-life revealed a significant treatment by sex effect as well as a main effect for sex. Post hoc t-tests revealed a significant difference in quarter-life of treated male and female monkeys and a marginal difference between treated and control males. The FI run rate of the male monkeys was significantly greater than that of the female monkeys whereas the FR run rate of the males was marginally greater. These results indicate that there may be a differential effect of MeHg on male and female monkeys, which could be interpreted as an effect on temporal discrimination. Overall, adult monkeys exposed to in utero MeHg exhibited a very limited sex-related effects on the FI/FR intermittent schedule of reinforcement.

Exposure to methyl mercury from birth to adulthood impairs high-frequency hearing in monkeys

Hearing deficits are a frequent consequence of both developmental and adult methyl mercury exposure in humans. However, a detailed characterization of these deficits has not been performed in either humans or animals. Cynomolgus monkeys (Macaca fascicularis) were dosed from birth to 7 years of age with 50 micrograms/kg/day of mercury as methyl mercuric chloride. Steady-state blood mercury levels during dosing were 0.6-0.9 ppm. When monkeys were 14 years old, pure tone detection thresholds were determined by a psychophysical procedure. Control monkeys exhibited thresholds at frequencies between 125 and 31,500 Hz comparable to previously published values for macaques. One methyl mercury-treated monkey exhibited normal detection thresholds at all frequencies. Three treated monkeys were impaired at the second highest frequency tested (25,000 Hz) and therefore were not tested at 31,500 Hz. The fifth treated monkey displayed severely elevated thresholds at middle frequencies (10,000-12,500 Hz), precluding testing at higher frequencies. These results indicate a selective high-frequency deficit in monkeys exposed to methyl mercury from birth to adulthood and not exposed to methyl mercury in the 7 intervening years before auditory testing. These findings extend previous results in this group of monkeys in which deficits in spatial and temporal visual function were observed.

Behavioral effects of lead in monkeys tested during infancy and adulthood

A total of 12 monkeys (Macaca fascicularis) were dosed orally from birth with 0 or 2000 micrograms/kg/day of lead as lead acetate. Blood lead concentrations of treated monkeys peaked at an average of 115 micrograms/dl by 100 days of age and decreased to a steady state level of 33 micrograms/dl after withdrawal of infant formula at 270 days of age. At 5-6 months of age, they were tested on a nonspatial discrimination reversal paradigm. At 2.5-3.0 years of age, they were tested on a series of nonspatial discrimination reversal problems, including irrelevant cues. As adults, performance was assessed on a differential reinforcement of low rate (DRL) schedule of reinforcement, a spatial delayed alternation task, and during training on a visual discrimination task for a visual psychophysics experiment. There were no or marginal deficits on the discrimination reversal task during infancy. Although lead-treated monkeys were impaired on this task as juveniles, they were less impaired than would have been predicted based on their history of blood lead concentrations. Treated monkeys exhibited decreased interresponse times and a greater ratio of responses per reinforcement on the DRL schedule compared to controls. Four of five treated monkeys were unable to learn the visual discrimination task without a remedial training procedure in which the relevant visual stimuli were arranged to appear as if they were on the response buttons. Treated monkeys were unimpaired on the delayed spatial alternation task. The results are interpreted as suggestive of an interaction between the behavioral history of the monkeys as infants with the results of later behavioral testing.

Nonhuman primates in behavioral toxicology: issues of validity, ethics and public health

The small, but vital, niche of nonhuman primates in neurotoxicology is examined. Several models of sensory and cognitive function have been especially useful with primates. Their sensitivity to low doses is clear. The validity of data from these models is indicated by their high correlation with data from intoxicated and normal humans, by the degree to which they approximate job functions and other vital human performances, and by their ability to document specific changes in behavioral function which correlate well with morphological and biochemical effects. The use of primates for this research is justified by the absence of adequate alternatives using nonprimate species, in vitro tests or computer programs. A series of experiments on the effects of methylmercury is used to illustrate ethical and scientific issues concerning research with primates. Recent trends are illustrated by data with trimethyltin.

Methylmercury developmental neurotoxicity: a comparison of effects in humans and animals

A qualitative and quantitative comparison of the neuropathological and neurobehavioral effects of early methylmercury (MeHg) exposure is presented. The focus of the qualitative comparison is the examination of how specific end-points (and categories of behavioral functions) compare across species. The focus of the quantitative comparison is the investigation of the relationship between MeHg exposure, target-organ dose and effects in humans and animals. The results of the comparisons are discussed in the context of the adequacy of the proposed EPA neurotoxicity battery to characterize the risk of MeHg to humans. The comparisons reveal several qualitative and quantitative similarities in the neuropathological effects of MeHg on humans and animals at high levels of exposure. Reports of neuropathological effects at lower levels are available for animals only, precluding any comparison. At high levels of exposure, specific neurobehavioral end-points affected across species are also similar. Effects at lower levels of exposure are similar if categories of neurobehavioral functioning are compared. Changes in the EPA test battery consistent with the results of the comparisons are discussed.

Effects of developmental exposure to methyl mercury on spatial and temporal visual function in monkeys

Detailed characterization of several aspects of visual function was made in two groups of monkeys exposed developmentally to methyl mercury. One group of monkeys (Macaca fascicularis) was dosed from birth onward with 50 micrograms/kg/day of mercury as methyl mercury. Another group was exposed in utero by dosing the mother with 10, 25, or 50 micrograms/kg/day of mercury as methyl mercury, and postnatally until 4.0-4.5 years of age with the same dose the mother had received. Spatial and temporal visual function was tested in both groups. Spatial visual deficits observed in the group dosed beginning postnatally were reported previously (Rice and Gilbert, 1982, Science, 216, 759-761). Monkeys exposed in utero plus postnatally exhibited impaired high- and low-luminance spatial vision. They also exhibited deficits in low-frequency high-luminance temporal vision, while low-luminance temporal vision was superior to that of control monkeys. Monkeys in which exposure began at birth displayed superior low-luminance temporal vision, while high-luminance temporal vision was not impaired. These effects were observed in the absence of constriction of visual fields. These data suggest that the pattern of visual deficits produced by developmental exposure to methyl mercury may be different from that in the adult, and that the developing visual system may be able to remodel as a result of early insult by a neurotoxic agent.

Methylmercury-induced movement and postural disorders in developing rat: loss of somatostatin-immunoreactive interneurons in the striatum

Tissue concentrations of the neuropeptide somatostatin and the specific activities of glutamic acid decarboxylase (GAD) were measured in several regions of the central nervous system in young rats, following chronic postnatal administration of methylmercuric chloride. By the beginning of the fourth postnatal week, these animals exhibited clinical signs of a mixed spastic/dyskinetic syndrome with visual deficits. At the onset of neurological impairment, a significant decrease in GAD activity was detected in the occipital cortex (48-49%) and striatum (45-50%) when compared to either normal or weight-matched controls. At one subclinical stage of toxicity, decreased GAD activity was detected only in the occipital cortex (29-30%). Tissue levels of somatostatin did not change significantly in the occipital cortex of methylmercury-treated animals at any stage of the experiment. However, somatostatin levels in the striatum were significantly reduced at the onset of neurological impairment (55-57%) and at one subclinical stage of toxicity (49-54%). Immunohistochemistry for somatostatin- and neuropeptide Y-immunoreactive neurons confirmed a marked loss of cells in the dorsolateral region of the striatum with atrophy of the surviving neurons. In the cerebral cortex of methylmercury-treated animals the morphology and distribution of somatostatin-positive neurons appeared normal. In view of the reported co-localization of GAD and somatostatin in some non-pyramidal neurons of the cerebral cortex, these results indicate that methylmercury-induced lesions of the developing cerebral cortex involve a subpopulation of GABAergic neurons which are not co-localized with somatostatin. In the striatum, where GAD and somatostatin are not co-localized within the same neurons, methylmercury-induced lesions involve both GABAergic and somatostatin-positive neurons.

Methodological approaches to primate behavioral toxicological testing

The monkey may often be the best model with which to characterize low-level effects of neurotoxicants, including those produced by developmental exposure. The nervous system of the monkey is very similar to the human and, like humans, monkeys have a long period of gestation, infancy, and sexual immaturity during which the nervous system continues to develop. The sensory systems of monkeys are very similar to humans, and intellectual capabilities may be tested that are not possible using other common laboratory species. The effects of low-level developmental exposure to neurotoxicants have been studied in the cynomolgus monkey using operant conditioning techniques to detect subtle defects. Intermittent schedules, a standard tool in behavioral pharmacology, have proved sensitive indicators of toxicity produced by developmental lead exposure. Such intellectual functions as learning, memory, adaptability, and distractibility have been explored by techniques including discrimination reversal, matching to sample, and delayed alternation. Such techniques have revealed impairment produced by lead similar to that observed in lead-exposed children. Visual deficits produced by methylmercury have been revealed by psychophysical techniques, in the absence of any obvious signs of toxicity.

Methylmercury effects on reproduction and offspring size at birth

This article describes a study of the toxic, reproductive, and developmental effects of chronic methylmercury (MeHg) exposure in Macaca fascicularis monkeys. Adult and infant monkeys were studied using procedures to assess maternal and newborn blood Hg concentrations, menstrual cyclicity, conception rate, reproductive outcome, maternal toxicity, and offspring size at birth. Maternal intakes of 0, 50, 70, or 90 micrograms/kg/d MeHg hydroxide were studied. Maternal blood Hg concentrations reached equilibrium by 10 weeks of exposure. The half-life of blood Hg for adult females ranged from 15 to 40 days (mean = d) and did not vary with dose. Maternal MeHg exposure did not affect the length of the menstrual cycle or the conception rate. Maternal MeHg exposure did significantly reduce the number of viable deliveries at blood Hg concentrations above 1.5 ppm. Maternal blood Hg concentrations at delivery were significantly lower than newborn concentrations. No effect of maternal MeHg exposure on offspring size at birth was observed. Maternal toxicity was related to blood Hg concentrations above 2.0 ppm following approximately one year of exposure. Results indicate that MeHg exposure can affect reproductive outcome at levels that do not cause overt toxicity.

Methylmercury poisoning of the developing nervous system in the rat: decreased activity of glutamic acid decarboxylase in cerebral cortex and neostriatum

The specific activities of glutamic acid decarboxylase (GAD) and choline acetyltransferase (ChAT) were measured in 6 regions of the central nervous system in young rats, following chronic postnatal administration of methylmercuric chloride. These rats exhibited signs of neurological impairment which included visual deficits, ataxia, spasticity and myoclonus. At the onset of neurological impairment, there was a significant reduction in GAD activity in the occipital cortex (43%), frontal cortex (37%) and caudate-putamen (42%). Preceding the onset of neurological impairment, diminished GAD activity was detected only in the occipital cortex. In the cerebellum, thalamus and spinal cord, GAD activities were normal throughout the experiment. No significant differences in ChAT activity were detected in any of the 6 regions. These results are consistent with a preferential involvement of GABAergic neurons in methylmercury-induced lesions of the cerebral cortex and neostriatum.

Synaptic degeneration in rat visual cortex after neonatal administration of methylmercury

Neonatal rats received subcutaneous injections of methylmercuric chloride in physiological saline (5 mg Hg/kg/day) beginning on postnatal day 5 and continuing until a given animal demonstrated a persistent loss of body weight during 48 h. Clinical signs of visual impairment were observed in all methylmercury-treated animals within 2 to 3 days of the initial weight loss (postnatal days 19 to 23). Four animals exhibited myoclonic jerking of the hind limbs, and one of these animals demonstrated a generalized motor seizure. Electron microscopy of visual cortex in the nonconvulsive animals revealed a selective degeneration of axon terminals forming symmetric synapses. Degenerating neurons were concentrated in layer IV with a more diffuse distribution in other cortical laminae. Dendritic spines were not found on degenerating dendritic profiles, although apparently normal axon terminals were observed to form synapses on degenerating dendritic shafts. These morphologic criteria suggest that aspinous or sparsely-spinous stellate neurons, the inhibitory GABAergic interneurons of the neocortex, are selectively impaired in methylmercury-induced lesions of the developing visual cortex.

Review of the health effects of methylmercury

The study critically reviews recent data relating to the health effects of methylmercury in man and the attendant dose-response relationships. New data obtained from animal studies, including pre-and postnatal exposure, are also examined. The consumption of fish and fish produce represents the major source of methylmercury exposure in the general population. Reported mercury concentrations in fish throughout the world are examined, particularly in the Mediterranean Sea. Here there is limited knowledge of methylmercury intake in critically exposed populations such as fishermen, employees of the fish industries and their families. The measurement of mercury in hair is now regarded as the most useful indicator of exposure but more experimental data are still required to increase the value of this index. The threshold levels of methylmercury in blood, hair and for dietary intake, as estimated by the World Health Organization, have been largely endorsed. However, new information from Japan and Canada suggests the existence of a latency period for some effects, so that the frequency or probability of their occurrence is inversely related to the duration of exposure. Incorporation of such findings would therefore lead to the designation of lower threshold values than are presently recognized. Pregnant women and the fetus have been identified as groups that are at special risk. The fetal blood mercury level is up to twice that of the mother and the sensitivity of both mother and fetus may be higher than in non-pregnant adults. This should be taken into account when assigning protective threshold concentrations.