Jørgen Drasbæk Schiønning

Selective degeneration of dorsal root ganglia and dorsal nerve roots in methyl mercury-intoxicated rats: a stereological study

Abstract The components of the nervous system of rats that are most critically affected by methyl mercury are still a matter of debate. A recent stereological study of rats with typical symptoms resulting from methyl mercury intoxication demonstrated that the morphology of cerebellar granule cells and Purkinje cells were unchanged at the light microscopic level, even though there was pronounced degeneration of myelinated axons in dorsal nerve root nerves. In the present study, unbiased stereological methods were used to quantify morphological changes in the dorsal root ganglion, and dorsal and ventral nerve roots of the rats used in the previous study. The rats were treated with methyl mercury (2 mg daily/kg, per os) for a 19-day period that was followed by a 32-day period without treatment. The means of the total numbers of A-cell and B-cell perikarya in the dorsal root ganglion of the intoxicated rats were reduced by 60% and 24%, respectively. The mean volume of A-cell perikarya in rats of the experimental group was reduced by 22%, whereas the mean volume of B-cell perikarya was the same in the two groups. In the experimental group, the total number of myelinated axons in the dorsal nerve roots was reduced by 60%, whereas no difference was found in the ventral nerve roots. The areas of axon and myelin sheath, dorsal and ventral nerve roots were not affected. This study demonstrates that extensive loss of dorsal root ganglion cells and myelinated axons in dorsal nerve roots precedes light microscopical changes in the ventral nerve roots and the cerebellum of rats intoxicated with methyl mercury.

Neuron loss in cerebellar cortex of rats exposed to mercury vapor: a stereological study

Abstract Mercury vapor produces tremor in humans and experimental animals. We have previously reported that mercury vapor intoxication over an 8-week period induces only subtle changes in dorsal root ganglia and nerve roots in rats. In the present study we have carried out stereological analyses of the cerebellum of the same rats, and demonstrated significant losses of Purkinje cells (12.7%, 2P = 0.005) and granule cells (15.6%, 2P = 0.016). All sizes of Purkinje cells were lost with an equal probability, i.e. there were no indication of any preferential loss of any subpopulation of the neurons. The volume of the granular cell layer was significantly reduced (18.9%, 2P = 0.015), whereas the volumes of the molecular layer and the white matter were unchanged. Previous stereological studies have demonstrated that methyl mercury intoxication primarily induces degeneration in the peripheral nervous system, while sparing the cerebellum. We therefore suggest that metallic mercury vapor and methyl mercury have different toxicological profiles in rats, where metallic mercury vapor mainly affects the central nervous system and methyl mercury mainly affects the peripheral nervous system.

Autometallographic mapping of mercury deposits in the spinal cord of rats treated with inorganic mercury.

SummaryThe autometallographic method has been used in conjunction light and electron microscopy to determine the exact localization of mercury in the rat spinal cord. Adult male Wistar rats were treated intraperitoneally with accumulative doses of mercuric chloride (100–200 μg HgCl2 daily). Transverse sections of the first cervical segment (C1), fifth cervical segment (C5), sixth thoracic segment (T6), and first lumbar segment (L1) of the spinal cord were examined. The distribution pattern of mercury was dose dependent. In ventral horn motoneurons and neurons of nucleus dorso-medialis (C1) pronounced staining was found after a total dosage of 1200 μg HgCl2. In nucleus intermedio-lateralis (T6, L1) and nucleus cervicalis centralis (C1) stained neurons were first seen after 2600 μg HgCl2. Ultrastructurally, mercury deposits were exclusively located in lysosomes of neurons, astrocytes, endothelial cells, and ependymal cells.

A stereological study of dorsal root ganglion cells and nerve root fibers from rats exposed to mercury vapor

Abstract Although mercury vapor is known to produce tremor and peripheral neuropathy, neuropathological studies of the effects of the vapor are few in number. The aim of the present study has been to evaluate the effect of mercury vapor on the morphology of the dorsal root ganglion and the spinal nerve roots. Adult male rats were exposed to mercury vapor for 33 days. The exposed rats developed somatic signs of intoxication and became increasingly irritable. The total numbers and volumes of A- and B-cell perikarya in the dorsal root ganglia, the total number of myelinated axons in the roots, and the cross-sectional areas of axon and myelin in the nerve roots were estimated using unbiased stereological principles. The mean cross-sectional area of myelin associated with nerve fibers in dorsal nerve roots of the exposed group was significantly reduced by 20% (2P = 0.014). A tendency towards a reduction was seen in axon area of myelinated nerve fibers in the dorsal nerve roots (2P = 0.087) and in the total numbers and mean volume of A-cell perikarya (2P = 0.059 and 2P = 0.087, respectively). No differences between the two test groups were found for any of the parameters measured in B-cells and ventral nerve roots. It is concluded that mercury vapor, in a dose sufficient to produce intoxication, induces only minor changes in dorsal root ganglion and nerve roots in rats.

A stereological study of dorsal root ganglion cells and nerve root fibers from rats treated with inorganic mercury

Abstract Unbiased stereological methods have been used to quantify the effects of inorganic mercury on the morphology of the fifth lumbar dorsal root ganglion cells and nerve root fibers. Adult male Wistar rats were treated with intraperitoneal injections of mercuric chloride (0.15 mg daily) for 30 days. The total numbers and mean volumes of A- and B-cell perikarya were estimated using the optical fractionator and the vertical rotator techniques. The total numbers of myelinated axons in the ventral and the dorsal roots were estimated with the two-dimensional fractionator technique and the areas of axon and myelin were estimated using the two-dimensional nucleator technique. No differences were found for any parameters in experimental and control animals, indicating that inorganic mercury intoxication alters neither the numbers or sizes of dorsal root ganglion cells and nerve root fibers nor the amount of myelin associated with the nerve fibers.

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 μg 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.

Differentiation of silver-enhanced mercury and gold in tissue sections of rat dorsal root ganglia.

SummaryAutometallography was used in conjunction with light and electron microscopy to detect traces of gold and mercury in the dorsal root ganglia of rats treated with sodium aurothiomalate and mercuric chloride. In order to differentiate between gold and mercury in tissue sections, the gold accumulations were removed by potassium cyanide, leaving mercury sulphides/selenides as the only possible catalysts for autometallographic development. With this technique, it is now possible to differentiate between all tissue metals capable of initiating the autometallographic process, i.e. gold, vesicular zinc, and sulphides and selenides of mercury and silver.

Experimental neurotoxicity of mercury Autometallographic and stereologic studies on rat dorsal root ganglion and spinal cord

This review distinguishes between the three major forms of mercury present today in our environment, namely metallic, inorganic, and organic mercury. Metallic mercury refers to vapor of metallic mercury (Hg?, inorganic mercury refers to mercuric mercury, e.g., mercuric chloride (HgCl2), and organic mercury is used to refer to shortchain alkyl mercury compounds, e.g., methylmercuric chloride (CH3HgC1). The chemical form of mercury determines both its neurotoxicologic and general toxicologic characters (see reviews on the pharmacology and toxicology of mercury by Clarkson (1972,1997), Magos (1975), Chang (1977,1980), Petering andTepper (1976), Kark (1979), Berlin (1986), Miura and Imura (1987), Eccles and Annau (1987), Komulainen (1988), WHO (1990, 1991). Organic mercury is recognized as a potent neurotoxin and has been implicated in several mass health disasters during this century (WHO 1991). In humans, the clinical manifestations of organic mercury intoxication are numerous. The most characteristic early features of neurologic dysfunction include ataxia and disturbances in sensory and visual systems (Takeuchi et a1 1962; Chang 1977; Marsh 1979; Annau and Eccles 1987). Experimental rats intoxicated with organic mercury develop both an ataxic gait and hind leg crossing phenomenon after a latency period (Cavanagh and Chen 1971; Klein et a1 1972; Herman et a1 1973; Hargreaves et a1 1985). Today, the general population is exposed to small amounts of methylmercury through the consumption of food, particularly seafood (WHO 1990), though this is not considered to be a significant health risk. Whether or not the continuous leakage of metallic and inorganic mercury from dental amalgam fillings shown by Lorscheider and Vimy (1990), is neurotoxic is still a controversial issue. One environmental problem of concern today is occupational exposure to metallic mercury vapor which has been shown to result in tremor, peripheral polyneuropathy, and both motor and sensory disturbances (Albers et a1 1982; 1988; Levine et a1 1982; Shapiro et a1 1982; Singer et a1 1987; Chapman et a1 1990; Ehrenberg et a1 1991; Ellingsen et a1 1993). Prolonged exposure to high concentrations of metallic mercury has also been shown to induce tremor in rats, rabbits, and pigeons (Armstrong et a1 1963; Fukuda 1971; Kishi et a1 1978). Research on inorganic mercury has focused primarily on its nephrotoxic effects, including both glomerular and tubular damages (Berlin 1986). Its neurotoxic potential is in general considered to be small. Recently, however, inorganic mercury has been suspected of playing a role in the

Autometallographic detection of mercury in rat spinal cord after treatment with organic mercury.

SummaryAutometallography was used to localize mercury in rat spinal cord after intraperitoneal administration of methylmercuric chloride (200 μg CH3HgCl daily). The technique permits small amounts of mercury sulfides and mercury selenides to be visualized by silverenhancement. Mercury deposits were observed by light microscopy only in neurons. In all of the spinal cord segments selected (first cervical segment, C1; fifth cervical segment, C5; sixth thoracic segment, T6; and first lumbar segment, L1) the mercury was observed with cumulative dosages of 6000 μg CH3HgCl and greater. Laminae VII, VIII, and IX contained the majority of stained neurons, whereas laminae IV, V, VI, and X had a relatively lower density of mercury-containing neurons. Stained neurons were confined to specific cell groups, such as Clarke’s column, nucleus intermediolateralis, nucleus cervicalis centralis, and nucleus dorsomedialis. At the ultrastructural level, mercury deposits were restricted to lysosomes of neurons and occasional accumulations in the lysosomes of ependymal cells.