Behavioral changes and mercury concentrations in tissues of rats exposed to mercury vapor

Elemental mercury neurotoxicity and clinical recovery of function: A review of findings, and implications for occupational health

This paper assessed approximately 30 studies, mostly involving occupationally exposed subjects, concerning the extent to which those who developed elemental mercury (Hg)-induced central and/or peripheral neurotoxicities from chronic or acute exposures recover functionality and/or performance. While some recovery occurred in the vast majority of cases, the extent of such recoveries varied considerably by individual and endpoint. Factors accounting for the extensive inter-individual variation in toxicity and recovery were not specifically assessed such as age, gender, diet, environmental enrichment, chelation strategies and dose-rate. While the data indicate that psychomotor endpoints often show substantial and relatively rapid (i.e., 2-6 months) recovery and that neuropsychological endpoints display slower and less complete recovery, generalizations are difficult due to highly variable study designs, use of different endpoints measured between studies, different Hg exposures based on blood/urine concentrations and Hg dose-rates, the poor capacity for replicating findings due to the unpredictable/episodic nature of harmful exposures to elemental Hg, and the inconsistency of the initiation of studies after induced toxicities and the differing periods of follow up during recovery periods. Finally, there is strikingly limited animal model literature on the topic of recovery/reversibility of elemental Hg toxicity, a factor which significantly contributes to the overall marked uncertainties for predicting the rate and magnitude of recovery and the factors that affect it.

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

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

Immunotoxicity of mercury: Pathological and toxicological effects

Mercury (Hg) is toxic and hazardous metal that causes natural disasters in the earth's crust. Exposure to Hg occurs via various routes; like oral (fish), inhalation, dental amalgams, and skin from cosmetics. In this review, we have discussed the sources of Hg and its potential for causing toxicity in humans. In addition, we also review its bio-chemical cycling in the environment; its systemic, immunotoxic, genotoxic/carcinogenic, and teratogenic health effects; and the dietary influences; as well as the important considerations in risk assessment and management of Hg poisoning have been discussed in detail. Many harmful outcomes have been reported, which will provide more awareness.

Health effects of ingestion of mercury-polluted urban soil: an animal experiment

Rio Grande, the southernmost Brazilian port and industrial center, is marked by mercury-polluted ground cover. This pollution varies spatially, with levels exceeding 1,000 μg kg(-1) in 30% of the urban territory. The risk of Hg impact as a result of deliberate and involuntary geophagy is increased by restrained urban conditions in combination with the large proportion of the population living at low-income levels. Laboratory tests have demonstrated that ingestion of Hg-polluted soil by rats results in significant alterations in animal health such as stagnation in body weight increase, and significant mercury accumulation in the liver and kidney. The consumption of Hg-contaminated urban soil also provoked changes in hematological profiles of experimental animals by increasing the number of platelets. The present study indicates the potential for the local population of Rio Grande living in mercury-polluted districts, specifically young children, to experience health disturbances.

Behavioral changes in metallothionein-null mice after the cessation of long-term, low-level exposure to mercury vapor

The neurobehavioral changes in wild-type and metallothionein (MT)-null mice after the cessation of long-term, low-level exposure to Hg0 were investigated. MT-null and wild-type females were continuously (24 h/day) exposed to mercury vapor (Hg0) at 0.055 mg/m3 (range: 0.043-0.073 mg/m3), which was similar to the current threshold limit value (TLV), for 29 weeks. The effects on behavior, such as locomotor activity in the open field (OPF), learning ability in the passive avoidance response (PA) and spatial learning ability in the Morris water maze (MM) were examined immediately and 12 weeks after the cessation of exposure. Immediately after the exposure had ceased, total locomotor activity in OPF was decreased in the both strain of mice, although the MT-null mice appeared to show more distinct effect. In the PA test, the exposed animals of both strains showed learning impairment as compared to un-exposed mice. Twelve weeks after the cessation of exposure, the locomotor activity in OPF was elevated in the exposed mice of both strains, while the learning ability in the PA test appeared normal in both strains. Spatial learning ability was not affected at all. Immediately after the exposure had ceased, the brain mercury concentration of the exposed wild-type mice was 1.75 microg/g, twofold of that in the MT-null mice. In 12 weeks, brain mercury levels decreased to approximately 1/20 of those in immediately after the exposure in both of the strains. These results for the first time indicated that long-term, low-level exposure to Hg0 could exert neurobehavioral effects, which were not reversible even after a long exposure-free period. Whereas the effects on learning ability were presumably transient, the effects on spontaneous behavior as evaluated in OPF were persistent. Finally, the MT-null mice seemed more susceptible to Hg0-induced neurotoxicity than the wild-type mice, confirming our previous results.

Inorganic mercury changes the fate of murine CNS stem cells

Stem cells isolated from the central nervous system of both embryonic and adult mice can generate neurons and glia through the activation of different patterns of differentiation in dependence of exposure to appropriate epigenetic signals. On the other hand, environmental conditions might affect the proliferation, migration, and differentiation of these cells. We report here, for the first time, that inorganic mercury affects the proliferative and differentiative capacity of adult neuronal stem cells (ANSCs). Actually, inorganic mercury increases apoptosis in ASNC. Furthermore, in stem cell-derived astrocytes, high levels of the 70 kDa heat shock protein (HSP-70) occur, while the levels of GTP-beta-tubulin activity dramatically decrease. Interestingly, when induced to differentiate, inorganic mercury modifies morphological proprieties of astrocytes, while the neuron population is reduced. These results demonstrate that inorganic mercury produces toxicity in the ANSC-derived neuronal population and affects the biological properties of the glial-derived population.

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.

Mercury distribution and renal metallothionein induction after subchronic oral exposure in rats

The effects of long-term daily intake of low and high levels of mercury on its organ distribution and binding to renal metallothionein (MT) in male rats were studied. The animals were exposed to mercuric chloride labelled with 203Hg via drinking water for 8 weeks (5, 50 and 500 microM Hg). The greatest concentration of mercury was found in the kidneys. Similar levels of radioactivity in the buccal cavity and oesophagus were also observed by whole-body autoradiography. In the kidneys, the mercury was accumulated in the outer stripe of the outer zone of the medulla and, to a minor degree, in the renal cortex. Almost 50% the total renal mercury was associated to MT. The binding capacity of the renal MT for mercury tends to saturate with increasing doses, thus this means that the capacity of the kidneys to accumulate mercury is limited.

Mercury toxicokinetics in Wistar rats exposed to elemental mercury vapour: modeling and computer simulation

The kinetics of total mercury (Hg) absorption, distribution and elimination in Wistar rats exposed for long periods to elemental mercury vapour (Hg zero) in the Idrija mercury mine were studied. From the experimental data base a compartmental model was built as a framework for experimental data interpretation and prediction of organ mercury levels under different conditions. Using the model the exposures of rats under conditions comparable to those of professionally exposed workers (mercury miners, workers in the chloralkali industry) and individuals with amalgam fillings were simulated.

Residual neurobehavioural effects associated with chronic exposure to mercury vapour

To find the residual effects of long term exposure to mercury vapour, neurobehavioural tests were given to ex-mercury miners about 18 years after the end of mercury exposure. Seventy six male ex-mercury miners who had been exposed to high concentrations of mercury vapour (over 1.0 mg/m3) and with a history of mercury intoxication were compared with controls matched for age (within 3 years), sex, and education. Although the extent of the workers' symptoms caused by mercury poisoning, termed erethismus merculialis, decreased considerably after the end of exposure, matched paired comparison showed that performances of motor coordination, simple reaction time, and short term memory had deteriorated significantly in the exposed group. Multiple linear regression analysis of exposure variables with neurological examination measures showed positive correlations between poorer neurological performance and variables related to mercury exposure. Thus the duration of exposure correlated with poorer performance of hand-eye coordination, tapping, and a colour card reading test. Job categories classified by exposure to mercury also had a significant negative correlation with these performances. The period of years after the end of exposure had a significant correlation with better performance of reaction time and digit span. On the other hand, the history of intoxication itself had no significant correlation with any of the current neurobehavioural performances. These results suggest that there are slight but persistent effects on neurobehavioural function, especially on motor coordination, among mercury miners even more than 10 years after the end of exposure.

Mercuric chloride-induced alterations of levels of noradrenaline, dopamine, serotonin and acetylcholine esterase activity in different regions of rat brain during postnatal development

Wistar rats were fed mercuric chloride, 4 mg/kg body weight per day chronically from postnatal day 2 to 60 by gastric intubation. Mercury consumption was then discontinued until 170 days to allow time for recovery. Since mercury caused reduction in body weight, an underweight group was also included besides the normal saline group. Levels of noradrenaline (NA), dopamine (DA), 5-hydroxytryptamine (5-HT) and the activity of acetylcholine esterase (AChE) were assayed in various brain regions in different age groups. By 60 days of age, the mercury group showed elevations of NA levels in olfactory bulb (OB), visual cortex (VC) and brain stem (BS) but not in striatum-accumbens (SA) and hippocampus (HI). DA levels were also increased in OB, HI, VC and BS but not in SA. AChE activity was decreased in the mercury group only in HI and VC at 20 days of age. The Mercury group showed no behavioural abnormality outwardly; however, operant conditioning revealed a deficiency in performance. Nevertheless, all these changes disappeared after discontinuation of mercury intake. Thus the changes occurring in the brain at this level of oral mercuric chloride intake seem to reflect adaptive neural mechanisms rather than pathological damage.

Correlation of dental amalgam with mercury in brain tissue

Data from this project demonstrate a positive correlation between the number of occlusal surfaces of dental amalgam and mercury levels in the brain (p less than .0025 in white matter). This is indirect evidence suggesting that mercury from dental amalgam fillings may contribute to the body burden of mercury in the brain. The toxic levels of mercury in human tissues have not been sufficiently investigated and the amount of mercury in human brain tissue from dental amalgam may or may not be clinically significant. Nevertheless, dental amalgam exposure should be considered in monitoring sources of mercury accumulation in human brain tissue.

An examination of the oxidation of mercury vapor by rat brain homogenate

The oxidation of mercury vapor (Hg degrees) to divalent inorganic mercury (Hg2+) was studied in rat brain homogenates. By using a "degassing" method, it was possible to speciate the mercury present in the homogenate and, for the first time, to measure the rate of oxidation as a function of the substrate (Hg degrees) concentration. Mercury oxidation was first-order with respect to substrate concentration at all concentrations tested, and the first-order rate constant for the oxidation process was proportional to homogenate concentration. The role of catalase compound I in mercury vapor oxidation by brain homogenate was examined by observing the effects of two inhibitors of catalase (catalase compound I) on homogenate mercury-oxidizing activity and catalase activity. Sodium azide (50 mM) completely inhibited both mercury-oxidizing activity and catalase activity. Aminotriazole (3-amino-1H-1,2,4-triazole) (50 mM) completely inhibited only mercury-oxidizing activity; some residual catalase activity was found in the aminotriazole-treated homogenate. It was concluded that catalase compound I plays a major role in the oxidation of Hg degrees, but the possibility that catalase-independent pathways make a minor contribution cannot be excluded.