Vulnerability of dorsal root neurons and fibers toward methylmercury toxicity: a morphological evaluation

Bee venom Apis mellifera lamarckii rescues blood brain barrier damage and neurobehavioral changes induced by methyl mercury via regulating tight junction proteins expression in rat cerebellum

The objective of the current study is to investigate the protective effect of Egyptian bee venom (BV) against methyl mercury chloride (MMC) induced blood-brain barrier (BBB) damage and neurobehavioral changes. Eighty male Sprague-Dawley rats were randomly grouped into 1st control (C), 2nd BV (0.5 mg/kg S/C for14 days), 3rd MMC (6.7 mg/kg orally/14 days), and 4th MMC + BV group. MMC exposure significantly altered rat cognitive behavior, auditory startle habituation, and swimming performance, increased the exploratory, grooming, and stereotypic behavior. MMC significantly impaired BBB integrity via induction of inflammation, oxidative stress, and down-regulation of tight junction proteins genes (TJPs) mRNA expression levels: Occludin (OCC), Claudins-5 (CLDN5), Zonula occludens-1 (ZO-1), while up-regulated the transforming growth factor-beta (TGF-β) mRNA expression levels. MMC revealed a significantly higher percentage of IgG positive area ratio, a higher index ratio of Iba1, Sox10, and ss-DNA, while index ratio of CD31, neurofilament, and pan neuron showed a significant reduction. Administration of BV significantly regulates the MMC altered behavioral responses, TJPs relative mRNA expression, and the immune-expression markers for specific neural cell types. It could be concluded for the first time that BV retains a promising in vivo protection against MMC-induced BBB dysfunction and neurobehavioral toxicity.

Hypoalgesia and recovery in methylmercury-exposed rats

Methylmercury (MeHg), the causal substrate in Minamata disease, can lead to severe and chronic neurological disorders. The main symptom of Minamata disease is sensory impairment in the four extremities; however, the sensitivity of individual sensory modalities to MeHg has not been investigated extensively. In the present study, we performed stimulus-response behavioral experiments in MeHg-exposed rats to compare the sensitivities to pain, heat, cold, and mechanical sensations. MeHg (6.7 mg/kg/day) was orally administered to 9-week-old Wistar rats for 5 days and discontinued for 2 days, then administered daily for another 5 days. The four behavioral experiments were performed daily on each rat from the beginning of MeHg treatment for 68 days. The pain sensation decreased significantly from day 11 onwards, but recovered to control levels on day 48. Other sensory modalities were not affected by MeHg exposure. These findings suggest that the pain sensation is the sensory modality most susceptive to MeHg toxicity and that this sensitivity is reversible following discontinuation of the exposure.

Methylmercury-induced neural degeneration in rat dorsal root ganglion is associated with the accumulation of microglia/macrophages and the proliferation of Schwann cells

Exposure to organic mercury, especially methylmercury (MeHg), causes Minamata disease, a severe chronic neurological disorder. Minamata disease predominantly affects the central nervous system, and therefore, studies on the mechanisms of MeHg neurotoxicity have focused primarily on the brain. Although the peripheral nervous system is also affected by the organometallic compound and shows signs of neural degeneration, the mechanisms of peripheral MeHg neurotoxicity remain unclear. In the present study, we performed quantitative immunohistochemical analyses of the dorsal root ganglion (DRG) and associated sensory and motor fibers to clarify the mechanisms of MeHg-induced peripheral neurotoxicity in Wistar rats. Methylmercury chloride (6.7 mg/kg/day) was orally administrated for 5 days, followed by 2 days without administration, and this cycle was repeated once again. Seven and 14 days after the beginning of MeHg exposure, rats were anesthetized, and their DRGs and sensory and motor nerve fibers were removed and processed for immunohistochemical analyses. The frozen sections were immunostained for neuronal, Schwann cell, microglial and macrophage markers. DRG sensory neuron somata and axons showed significant degeneration on day 14. At the same time, an accumulation of microglia and the infiltration of macrophages were observed in the DRGs and sensory nerve fibers. In addition, MeHg caused significant Schwann cell proliferation in the sensory nerve fibers. In comparison, there was no noticeable change in the motor fibers. Our findings suggest that in the peripheral nervous system, MeHg toxicity is associated with neurodegenerative changes to DRG sensory neurons and the induction of a neuroprotective and/or enhancement of neurodegenerative host response.

Methylmercury induces apoptosis in cultured rat dorsal root ganglion neurons

Methylmercury is known to have devastating effects on the mammalian nervous system. In order to characterize the dose dependence of methylmercury-induced neurotoxicity, we first studied neurite outgrowth from rat dorsal root ganglia explants. In this model, methylmercury inhibited neurite outgrowth with a TD(50) of approximately 0.5 microM. We then used this relationship to optimize dosing for subsequent transcriptional profiling analyses in two independent neuronal model systems: dissociated sensory neurons and PC12 cells. As seen in previous studies, the expression of a number of genes associated with oxidative stress was altered following a 6h challenge with 1 microM methylmercury. When PC12 cells were subjected to a longer exposure (24h), a relative increase was noted in the representation of genes associated with cell cycling and apoptosis. To confirm the presence of apoptosis in cultured neurons, we then applied TUNEL staining and bis-benzimide staining techniques to primary cultures of dissociated sensory neurons. After 24h, 1 microM methylmercury increased both DNA end-labeling (P<0.01) and nuclear fragmentation (P<0.02). The latter effect appeared to be dose-dependent.

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.

Autometallographic mercury correlates with degenerative changes in dorsal root ganglia of rats intoxicated with organic mercury

Organic mercury intoxication in rats produces degenerative changes in the dorsal root ganglia and dorsal nerve roots. In a previous study of rats treated with organic mercury (2 mg/kg ) for 19 days, significant losses of ganglion cells (especially A-cells) and myelinated axons were observed in dorsal nerve roots and there was qualitative evidence of glial cell proliferation and the formation of Nageotte bodies (1). In the present study, the autometallographic silver-enhancement technique, for tracing inorganic mercury bound to sulphide or selenide (AMG-Hg), was applied to tissue sections of dorsal root ganglia and dorsal nerve roots of the same rats used in the earlier study. Satellite cells and macrophages that surrounded ganglion cells and formed Nageotte bodies were heavily labelled by coarse deposits of AMG-Hg, while the labelling of ganglion cells was less pronounced. A-cells were consistently labelled, while B-cells were only occasionally labelled. In the dorsal nerve roots, only a few AMG-Hg deposits could be seen in macrophages. At the ultrastructural level, AMG-Hg was observed within lysosomes of target cells. It is concluded that AMG-Hg is primarily located in glial cells and that the pattern of deposition of AMG-Hg is the same as that for the morphological changes observed in rats intoxicated with organic mercury.

The effect of selenium on the localization of autometallographic mercury in dorsal root ganglia of rats

The autometallographic technique was used to demonstrate the localization of mercury in dorsal root ganglia of adult Wistar rats. The animals were either exposed to mercury vapour, 100 micrograms Hg m-3, 6 h day-1, 5 days per week, or treated with organic mercury in the drinking water, 20 mg CH3HgCl per litre, for 4 weeks. The effect of orally administered sodium selenite on the pattern of intracellular distribution of mercury in these two situations was investigated. In rats exposed to mercury vapour alone, faint staining was present in ganglion cells. The selenite induced a conspicuous increase in the number of stained cells and in the intracellular staining intensity. In rats treated with organic mercury, mercury deposits were detected within ganglion cells and macrophages. The number of mercury-containing cells was increased by co-administration of selenite. In addition, satellite cells, the capsule and vessel walls were faintly stained. Twenty weeks after cessation of the organic mercury treatment, mercury staining was reduced. Again, selenite treatment enhanced staining intensity. When studied using the electron microscope, mercury was restricted to lysosomes, irrespective of treatments. The present study shows that the deposition of autometallographic mercury in the dorsal root ganglia depends on the chemical type of mercury, the co-administration of selenite and the length of the survival period.

Differential effects of methylmercury on the synthesis of protein species in dorsal root ganglia of the rat

Dorsal root ganglia from control and methylmercury (MeHg)-treated rats were incubated in vitro with 35S-methionine ant the proteins synthesized were analyzed by two-dimensional electrophoresis. The double labelling method, in which proteins of control dorsal root ganglia labelled in vitro with 3H-leucine were added to each of the two samples as an internal standard, was used to minimize unavoidable errors arising from the resolving procedure itself. The results obtained showed that the effect of MeHg on the synthesis of proteins in dorsal root ganglia was not uniform for individual protein species in the latent period of MeHg intoxication. Among 200 protein species investigated, 157 showed inhibition of synthesis close to that of the total proteins in the tissue (68% of the control). Among the remaining protein species, 20 showed real stimulation of synthesis, whereas 7 were moderately inhibited and 16 were inhibited more strongly than the total proteins in the tissue. These results suggest that the effect of MeHg on the synthetic rates for protein species in dorsal root ganglia differs with the species, and that unusual elevation or reduction of the synthesis of some protein species caused by MeHg may lead to impairment of normal nerve functions.

In vivo and in vitro effects of methylmercury on the activities of aminoacyl-tRNA synthetases in rat brain

The activities of six aminoacyl-tRNA synthetase species were determined using enzyme preparations partially purified from the brains of control and methylmercury (MeHg)-treated rats. The activities of Asp-, Leu- and Tyr-tRNA synthetases were significantly reduced in the brains of MeHg-intoxicated rats, whereas those of Lys- and Met-tRNA synthetases remained unchanged. In contrast, the activity of His-tRNA synthetase was significantly increased in the symptomatic phase of MeHg intoxication. The activities of these six aminoacyl-tRNA synthetases in the control brains were affected to different extents on the direct addition of MeHg to the assay system in vitro. No positive correlation was observed between the in vivo and in vitro effects of MeHg on the enzyme activities. These results indicate that the aminoacylation of tRNA is one of the actions of MeHg, which leads to inhibition of protein synthesis, and it is suggested that the syntheses of cellular proteins may be modified in different ways by MeHg, depending on their amino acid compositions.

Protective effects of selenium against methylmercury neurotoxicity: a morphological and biochemical study

The protective effect of selenium (as sodium selenite) against methylmercury neurotoxicity was investigated morphologically and biochemically. Young adult Charles River rats were divided into 4 groups and were treated (i. p. injection) with methylmercury chloride (2 mg MeHg/kg b.w.), sodium selenite (2 mg Se/kg b.w.), both MeHg and Se, or with saline solution for 8 weeks. Four animals from each group were randomly selected for pathological investigation. These animals were sacrificed by intracardiac perfusion. The dorsal root ganglia (DRG) were dissected out for both light and electron microscopic examination. Six animals from each group were injected with 14 C-leucine 3 hours prior to sacrifice. The brains (cerebrum and cerebellum) of these animals were removed, weighed, homogenized and subjected to liquid scintillation analysis. Morphological investigation revealed extensive degenerations of the dorsal root fibers and destructions of neuronal Nissl substances in animals treated with MeHg alone. In MeHg/Se-treated animals, no neuronal change was observed and the nerve fiber damage was also significantly reduced. Proliferative thickenings of the myelin membrane, however, were observed in some dorsal root fibers. These proliferative membrane thickenings tend to protrude into the axonal compartment of the nerve fibers causing compression and shrinkage of the axons. It is believed that this proliferative, membraneous whirling phenomenon may represent a simultaneous destruction/repair condition of the myelin sheaths. Biochemical investigation also demonstrated a significant suppression of amino acid uptake (protein synthesis) in the CNS under the toxic influence of MeHg. However, such suppression effect was largely blocked or alleviated by selenium. Therefore, we have provided both morphological and biochemical evidences on the protective effect of selenium on methylmercury neurotoxicity.