Some effects of methyl mercury salts on the rabbit nervous system

The toxicology of mercury and its compounds

A concentrated review on the toxicology of inorganic mercury together with an extensive review on the neurotoxicology of methylmercury is presented. The challenges of using inorganic mercury in dental amalgam are reviewed both regarding the occupational exposure and the possible health problems for the dental patients. The two remaining "mysteries" of methylmercury neurotoxicology are also being reviewed; the cellular selectivity and the delayed onset of symptoms. The relevant literature on these aspects has been discussed and some suggestions towards explaining these observations have been presented.

Widespread calcium deposits, as detected using the alizarin red S technique, in the nervous system of rats treated with dimethyl mercury

It has been reported that the alizarin red S technique may be used to visualize both intracellular and extracellular calcium deposits. Using this method histologic observations of the nervous system were made in rats that were given dimethyl mercury at 5 mg/kg per day for 12 consecutive days, and killed on days 1, 4, 7, 10, 12, 24, 32, 49, 100 and 140 (day 0 was the day that the final dose was administered). Neuronal degeneration with calcium deposition was found in the nervous system from day 4 onward. In the cerebellum alizarin red S-positive granules became gradually larger with time after dimethyl mercury administration, and large calcospherites were observed from day 32 onward. In contrast, the visualization of calcium deposits in the cerebral cortex was restricted to days 10-12. Calcium deposits were found in the ascending axons of the dorsal root ganglion neurons (dorsal fascicles of the spinal cord), but not in their perikarya. These findings suggest that widespread calcium deposition could occur in the nervous system following dimethyl mercury exposure, and that in the rat the mechanism of calcium deposition differs depending upon the brain region.

Selective involvement of large motor neurons in the spinal cord of rats treated with methylmercury

Mercury is thought to be a possible epidemiological factor for the pathogenesis of motor neuron disease, since it has been reported that metallic, inorganic and organic mercury causes a syndrome clinically resembling amyotrophic lateral sclerosis. We administered 10 mg/kg/day methylmercury chloride to adult rats for 10 consecutive days. The hind-limbs became flaccid and atrophic, and 14 out of the 34 rats had died by the 18th day after methylmercury treatment began. Light microscopical examination of the large motor neurons in the spinal anterior horn revealed cytoplasmic vacuolation and loss of Nissl substance on the 14th day, and neuronophagia appeared on the 16th day. On the 18th day, the loss of large motor neurons was almost complete, whereas small to medium-sized neurons were preserved. Silver acetate autometallography to detect mercury revealed the selective accumulation of mercury in the large motor neurons. These findings suggest that although a high dose is required, organic mercury can cause the definite loss of large spinal motor neurons in rats.

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.

Metabolism of methylmercury in rabbits and hamsters

The metabolism of 203Hg-labeled methylmercury chloride (MeHg) has been studied in rabbits and hamsters. Rabbits were administered 1.6 mumols MeHgCl/kg bw intravenously, and hamsters 40 mumols/kg bw orally. Urine and feces were collected daily and groups of four animals killed after 1 h, 1 d, or 7 d. The concentration of 203Hg in blood, liver, kidney, spleen, lung, heart, and brain was determined by gamma counting. In both animal species, the clearance of 203Hg in the brain was slower than in other tissues. In the rabbits the brain 203Hg concentration increased during the whole experimental period. Rabbits excreted 203Hg primarily in feces (about 20% of the dose within 1 wk), and much less in urine (less than 2%). In contrast, hamsters very efficiently excreted 203Hg in urine (50% in 1 wk). The fecal excretion was similar to that of the rabbits. Separation of inorganic Hg and MeHg in urine from hamsters by ion exchange chromatography showed that about 90% of the urinary 203Hg was excreted as MeHg. These findings show that rabbits and hamsters are interesting experimental animal systems for studying the metabolism of MeHg.

Changes in the distribution of histochemically localized mercury in the CNS and in tissue levels of organic and inorganic mercury during the development of intoxication in methylmercury treated rats

The development of neurotoxicity in rats after exposure to methylmercuric chloride was monitored using behavioural indices. At selected time points the cellular localization of mercury and the relative amounts of organic and inorganic mercury were determined in several regions of the CNS, and in some non-neural tissues. The CNS showed an affinity for organic mercury, the levels of inorganic mercury remaining low throughout symptomatic intoxication. Histopathological changes were not closely related to the regional tissue content of the organic or inorganic forms, nor to mercury localized histochemically at the cellular level. The stained deposits, which had focal cytoplasmic distribution, appeared in glial cells initially then in larger neurones as the intoxication progressed. These observations may represent changes in the mercury content of different cell types or reflect differences in the way that they handle a similar burden of mercury. A transitory accumulation of mercury in glial cells may be a factor contributing to the occurrence of a latent period and sequestration of mercury in cytoplasmic organelles may serve to protect some cell types from injury.

Effects of methylmercury chloride on rat incisor odontoblasts and dentinogenesis

Methylmercury chloride, a well-known nerve cell toxicant, was administered to 40 rats in 2 groups, in doses of 10 mg/kg and 20 mg/kg. Histomorphologic investigation of maxillary incisors after 2 and 4 days revealed granular and hydropic changes in parts of the odontoblastema. Deposition of interglobular dentin was seen after 7 and 11 days. The most vulnerable cells seemed to be the mantle dentin-producing odontoblasts, and the alterations found are probably due to interference with the rough endoplasmic reticulum.

Early indices of methyl mercury toxicity and their use in treatment evaluation

Mercury distribution, food consumption, body weight, and in vivo protein synthesis were compared as criteria for evaluating the efficacy of D-penicillamine (DPA) in treating experimental methyl mercury (MM) intoxication. Female rats were orally administered MM hydroxide at 40 mg/kg and, after a 7- to 8-d latency period, displayed characteristic neurological signs of MM intoxication. Within 24 h of MM exposure food consumption decreased 75% causing a 12-g drop in body weight, and synthesis of whole blood and kidney protein increased. Protein synthesis in liver was increased 39% by MM after 3 d, and that in cerebellum was decreased 15% after 7 d. Treatment with DPA (1.2 g/kg.d sc on d 2, 3, and 4) prevented the appearance of neurological signs. DPA lowered the Hg content of all tissues; restored food consumption of control levels; increased the onset and amount of body weight gain; returned synthesis of blood, liver, and kidney proteins to control levels; and prevented the decrease in protein synthesis in cerebellum. By itself, DPA produced a transitory decrease in both food consumption and body weight, which could be prevented with vitamin B6. Vitamin B6 antagonized DPA's reversal of MM's action on protein synthesis. Furthermore, DPA and/or vitamin B6 had a variety of effects on protein synthesis in control rats. Thus it was not possible to use protein synthesis to predict the efficacy of the combination of DPA and vitamin B6 as found for the parameters of food consumption, body weight, and Hg distribution. Since changes in body weight and food consumption were the earliest and most pronounced and consistent responses to MM and effective DPA treatment, they were considered the best criteria for evaluating treatment efficacy in experimental MM poisoning in rats.

A Golgi and electron-microscopic study of cerebellum in methylmercury-poisoned neonatal mice

Neonatal C57BL/6J mice were injected with 5 mg/kg body weight of 203Hg-labeled methylmercuric chloride on postnatal days 3, 4, and 5, totaling 15 mg/kg body weight per animal. The experimental and control animals were sacrificed on postnatal by 15. Whole body radioactivity of 203Hg progressively increased during the 3-day injection period and reached the peak level and remained at peak levels until the time of sacrifice. This indicates a lack of clearance of 203Hg by neonatal mice during the period examined in this study. Golgi preparations of cerebella of MeHg-treated animals revealed significant reduction in dendritic arborization of Purkinje cells. Ultrastructurally, the vascular endothelium showed attenuation with increased electron density and frequent vacuolization of cytoplasm. Marked swelling of perivascular glia was noted in most of the capillaries throughout the cerebella of MeHg-treated animals.

Accumulation and removal of Hg203 in different regions of the rat brain

We have studied the brain regional distribution of methyl mercury following intravenous administration of CH3 203HgCl in rat. Early peak levels were obtained in cerebellum, medulla oblongata and midbrain. The efficacy of removal of 203Hg by different chelators is also region dependent. The most efficient chelator for brain mercury proved to be mesodimercaptosuccinic acid.