A pathological study of Minamata disease in Japan

MR findings of Minamata disease--organic mercury poisoning

We describe MR findings in patients with Minamata disease who have been followed for a long time. All patients examined were affected after daily eating of a large quantity of methylmercury-contaminated seafood, from 1955 to 1958, and showed typical neurological findings. On MR images, the visual cortex, the cerebellar vermis and hemispheres, and the postcentral cortex are significantly atrophic in Minamata disease. The visual cortex is slightly hypointense on T1-weighted images and hyperintense on T2-weighted images, probably representing the pathologic changes of status spongiosus. MRI can demonstrate the lesions located in the calcarine area, cerebellum, and postcentral gyri, which are probably related to three of the characteristic manifestations of this disease: the constriction of the visual fields, ataxia, and sensory disturbance, respectively.

Pathology of Minamata disease

Minamata disease, or methylmercury poisoning, was first discovered in 1956 around Minamata Bay, Kumamoto Prefecture, Japan. A similar epidemic occurred in 1965 along the Agano River, Niigata Prefecture, Japan. The neuropathology of Minamata disease has been well studied; this review focuses on human cases of Minamata disease in Kumamoto Prefecture. Nervous system lesions associated with Minamata disease have a characteristic distribution. In the cerebral cortex, the calcarine cortex was found to be involved in all cases of Minamata disease, particularly along the calcarine fissure. The destruction of nerve tissue was prominent in the anterior portions of the calcarine cortex. Occasionally, the centrifugal route from the visual and visual association areas (internal sagittal stratum) showed secondary degeneration in prolonged cases after acute onset. Postcentral, precentral, and temporal transverse cortices showed similar changes, though they were less severe. Intense lesions in the precentral cortex caused the development of secondary bilateral degeneration of the pyramidal tracts. In the cerebellum, the lesions occurred deeper in the hemisphere. The granule cell population was most affected. In the peripheral nerves, sensory nerves were more affected than motor nerves. Secondary degeneration of Goll's tracts was occasionally seen in prolonged or chronic cases.

Cognitive deficit in 7-year-old children with prenatal exposure to methylmercury

A cohort of 1022 consecutive singleton births was generated during 1986-1987 in the Faroe Islands. Increased methylmercury exposure from maternal consumption of pilot whale meat was indicated by mercury concentrations in cord blood and maternal hair. At approximately 7 years of age, 917 of the children underwent detailed neurobehavioral examination. Neuropsychological tests included Finger Tapping; Hand-Eye Coordination; reaction time on a Continuous Performance Test; Wechsler Intelligence Scale for Children-Revised Digit Spans, Similarities, and Block Designs; Bender Visual Motor Gestalt Test; Boston Naming Test; and California Verbal Learning Test (Children). Clinical examination and neurophysiological testing did not reveal any clear-cut mercury-related abnormalities. However, mercury-related neuropsychological dysfunctions were most pronounced in the domains of language, attention, and memory, and to a lesser extent in visuospatial and motor functions. These associations remained after adjustment for covariates and after exclusion of children with maternal hair mercury concentrations above 10 microgram(s) (50 nmol/g). The effects on brain function associated with prenatal methylmercury exposure therefore appear widespread, and early dysfunction is detectable at exposure levels currently considered safe.

An electrophysiological study of the effects of acute methylmercury chloride exposure on the function of the guinea pig cochlea

The inner ear function of methylmercury chloride (MMC)-exposed guinea-pigs was examined in this study. Previous studies which investigated the function of the eighth cranial nerve and Corti-organ using cochlear microphonics (CM), compound action potential (CAP) and measurement of endocochlear potential (EP) reported ototoxicity following experimental exposure to MMC. In this report, the effect of MMC on the cochlea and the eighth cranial nerve were investigated systematically by measuring CM, action potential (AP), EP and K+ ion concentration of the endolymph. Guinea-pigs were injected with 5 mg/kg MMC (using 0.2% solution) twice a week for 1-3 weeks. The maximum output voltage of AP was decreased by injection of MMC (5 mg/kg x 6). A decrease in the CM maximum output voltage and the elevation of CM pseudothreshold was seen after MMC injection. Changes in EP during 3 min anoxia were observed, especially a decrease in the absolute value of the negative potential. The endolymph K+ ion concentration remained unchanged. These findings indicate that the diffusion potentials decreased and at the same time was reduced the maximum output voltage in CM induced by MMC injection (5 mg/kg x 6) in this experiment.

Depression of neuromuscular transmission in methylmercury-poisoned rats: a glass microelectrode and single fiber electromyography study

Changes in neuromuscular transmission were examined in methylmercury (MeHg)-poisoned rats, given a total oral dose of 60 mg CH3HgCl at 5 mg/kg/day. A microelectrode study was done on the 21st day. The mean quantal content and mean values of the immediately available pool of ACh in the MeHg-poisoned rats were reduced as compared to those in the control rats, but the mean values for the release probability of ACh did not differ significantly. Stimulation single fiber electromyography (SFEMG) was done on the biceps femoris muscle at 1, 5, 10 and 20 Hz on the 28th day. Both a significant and consistent increase in jitter were found at increasing stimulation rates in the MeHg-poisoned rats. The SFEMG findings suggest presynaptic involvement due to accelerated depletion of ACh. We confirmed that neuromuscular transmission is depressed in MeHg-poisoned rats in vivo and in vitro.

Lymphocyte proliferative response and tissue distribution of methylmercury sulfide and chloride in exposed rats

The immunotoxic effects and tissue distribution of different forms of methylmercury compounds were studied in rats. Methylmercury sulfide or methylmercury chloride was fed to rats at concentrations of 5 or 500 microg/L in drinking water for 8 wk. T-cell lymphocyte proliferative response to phytohemagglutinin (PHA) and determination of tissue distribution of mercury by gas chromatography using electron capture were assayed. Four different forms of mercury compounds were employed: MeHgS-, (MeHg)2S, (MeHg)3S+, and MeHgCl. Results indicated that exposure to methylmercury significantly enhanced lymphocyte responsiveness in most of the exposed groups at the low concentration of 5 microg/L, with the highest proliferative response (fourfold increase) in the MeHgCl group. At 500 microg/L, a significant decrease in the lymphocyte proliferative response was observed in the (MeHg)3S+ and MeHgCl groups; conversely, the MeHgS(-)- and (MeHg)2S-exposed animals had a modest increase of the lymphocyte proliferative response. The largest concentrations of all four mercury forms were detected in the kidney and spleen. The levels of mercury found in kidney, spleen, liver, brain, and testis were lower in the MeHgCl group than in those exposed to (MeHg)2S and (MeHg)3S+. These data indicate that the organ distribution of mercury and immune alteration may vary according to the chemical structure of the compound. This observation may have important implications in humans potentially exposed to low levels of methylmercury present in the environment, since the immune system plays an important regulatory role in the host-defense mechanisms.

Metallothionein induction protects swollen rat primary astrocyte cultures from methylmercury-induced inhibition of regulatory volume decrease

Metallothionein (MT) proteins have been postulated to play a role in the detoxification of heavy metals. Since methylmercury (MeHg) preferentially accumulates in astrocytes, and MT-1 and MT-2 are astrocyte-specific MT isoforms, we investigated the ability of MTs to attenuate MeHg-induced cytotoxicity. The toxic effects of MeHg on astrocytes were investigated in a model of regulatory volume decrease (RVD) in which the cells are swollen by exposure to a hypotonic buffer. Preexposure to CdCl2 (1 microM) for 72, 96 or 120 h, prior to acute exposure to hypotonic buffer and MeHg (10 microM) led to a time-dependent increase in the intracellular levels of astrocyte MT proteins. The acute MeHg-induced inhibition of RVD was significantly, and almost fully reversed by preexposure to CdCl2. This reversal was time-dependent, 120-h preexposure to CdCl2 producing the greatest reversibility. Furthermore, the ability of astrocytes to efficiently volume regulate in the presence of MeHg-containing hypotonic buffer was highly correlated (r = 0.99) with the intracellular levels of MT proteins. The release of [3H]taurine, an osmolyte involved in the RVD process was also measured. The inhibitory effect of MeHg on [3H]taurine in swollen cells was significantly, and fully reversed by CdCl2 preexposure. The study suggests that astrocytes induced to express high levels of MT proteins are resistant to the acute inhibitory effect of MeHg on RVD.

Variability of brain lesions in rats administered methylmercury at various postnatal development phases

The effect of methylmercury chloride (MMC) on the developing rat nervous system was studied by light microscopy. Rats on postnatal day 2 (P2), P15 and P60 were administered 10 mg/kg/day MMC orally for 10 days. In newborn (after P2) rats, there was no abnormal activity or body weight loss. Young (after P15) rats showed weight loss on the 9th day after starting MMC, and subsequently unsteadiness, gait disturbance and paroxysmal convulsions appeared. In adult rats, weight loss began on the 6th day after starting MMC, and the hind-limb crossing phenomenon was induced on the 13th day. Histopathologically, minimal damage was found in the hippocampus and brainstem in newborn rats. In young rats, widespread neuronal degeneration was observed in the cerebral neocortex, CA3 and CA4 regions of the hippocampus, neostriatum, red nucleus, and various brainstem nuclei. In adult rats, neuronal damage was most extensive in the cerebellum and spinal dorsal nerve roots. These findings indicate that neuronal vulnerability to MMC exposure differs depending on the postnatal developmental stage and the brain region in the rat.

The effect of methylmercury (CH3HgCl) on the production of endothelium-derived relaxing factor (EDRF) by cultured human umbilical vascular endothelial cells based on its anti-aggregatory effect on human platelets

The effect of methylmercury (CH3HgCl) on the production of endothelium-derived relaxing factor (EDRF) by cultured human umbilical vascular endothelial cells (HUVECs) based on its anti-aggregatory effect on human platelets was examined. HUVECs were harvested from umbilical veins by collagenase treatment. The platelet aggregation test was performed with cuvettes lined with HUVECs. Platelet aggregation induced by 0.05 units thrombin/ml was inhibited in the presence of HUVECs. This HUVEC-dependent anti-platelet aggregatory effect was enhanced by the addition of bradykinin (10 nmol/L), which stimulates the production of EDRF. Indomethacin (IND, 1 mumol/L) reduced the HUVEC-dependent anti-platelet aggregatory effect. The effect of NG-monomethyl-L-arginine L-NMMA, 100 mumol/L), an inhibitor of nitric oxide synthase (NOS) in endothelial cells, on HUVECs pretreated with IND showed almost complete platelet aggregation similar to results without HUVECs. The anti-platelet aggregatory effect of HUVECs pretreated with IND seemed to depend mainly on EDRF. Methylmercury (MeHg) (20-50 mumol/L) induced dose-dependent platelet aggregation in cuvettes, without HUVECs. Methylmercury (30 mumol/L) induced less platelet aggregation in the presence of HUVECs than in their absence. The degree of inhibitory effect by HUVECs on MeHg-induced platelet aggregation was reduced dose-dependently (30-50 mumol/L MeHg). Methylmercury-induced platelet aggregation at 50 mumol/L MeHg with or without HUVECs was similar. These findings suggest that this simple new experimental system is useful for assessing the production of EDRF by HUVECs, and show that MeHg inhibits the production of EDRF by HUVECs, which may be involved in the etiology of cardiovascular diseases such as hypertension and arteriosclerosis.

Inhibitory effect of methylmercury on migration and tube formation by cultured human vascular endothelial cells

The effect of methylmercury chloride (MeHg) on migration and tube formation by cultured human umbilical vein endothelial cells (HUVECs) was quantitatively analyzed. The distance of endothelial cell outgrowth from the scraped edge of a monolayer was measured. HUVEC outgrowth was inhibited by MeHg (1.0-5.0 microM) treatment in a dose-dependent manner. Tube formation was studied by culturing the cells on gelled basement membrane matrix (Matrigel). Treatment of HUVECs with 0.1-5.0 microM MeHg for 24 h inhibited tube formation dose-dependently. These results suggest that migration and tube formation by HUVECs are susceptible to MeHg cytotoxicity, and that MeHg could be injurious to endothelial cell function, which may be involved in the pathogenesis of arteriosclerosis.

Minamata disease: methylmercury poisoning in Japan caused by environmental pollution

Minamata disease (M. d.) is methylmercury (MeHg) poisoning that occurred in humans who ingested fish and shellfish contaminated by MeHg discharged in waste water from a chemical plant (Chisso Co. Ltd.). It was in May 1956, that M. d. was first officially "discovered" in Minamata City, south-west region of Japan's Kyushu Island. The marine products in Minamata Bay displayed high levels of Hg contamination (5.61 to 35.7 ppm). The Hg content in hair of patients, their family and inhabitants of the Shiranui Sea coastline were also detected at high levels of Hg (max. 705 ppm). Typical symptoms of M. d. are as follows: sensory disturbances (glove and stocking type), ataxia, dysarthria, constriction of the visual field, auditory disturbances and tremor were also seen. Further, the fetus was poisoned by MeHg when their mothers ingested contaminated marine life (named congenital M. d.). The symptom of patients were serious, and extensive lesions of the brain were observed. While the number of grave cases with acute M. d. in the initial stage was decreasing, the numbers of chronic M. d. patients who manifested symptoms gradually over an extended period of time was on the increase. For the past 36 years, of the 2252 patients who have been officially recognized as having M. d., 1043 have died. This paper also discusses the recent remaining problems.

Mercuric(II) chloride modulates single-channel properties of carbachol-activated Cl- channels in cultured neurons of Aplysia californica

1. The effect of mercuric(II) chloride on kinetic parameters of carbachol-activated single chloride channels were studied in cultured neurons of the marine mollusk, Aplysia californica. 2. Single neurons of Aplysia were cultured in L-15 medium containing 1 mM beta-D-xyloside, which improved the success rate for gigaseal formation by 46%. Carbachol-activated single chloride channels were recorded in the cell-attached patch clamp configuration. Recordings with control solution (1 microM carbachol) and with test solution (1 microM carbachol + 1 microM HgCl2) were performed successively on the same neuron. 3. In both the control and the test solution the open and closed time distributions were fitted with a double-exponential function. However, kinetic analysis revealed that Hg2+ caused a significant reduction of the mean closed time (10.37 +/- 1.08 vs. 3.32 +/- 0.02 msec) and of the second time constant tau 2 of the closed time distribution (2.09 +/- 0.05 vs. 0.66 +/- 0.5 msec). The reduction of tau 2, i.e., fewer events in the longer closed state under the action of Hg2+, may be the physical cause for the reduction of the mean closed time and thus underlies the increased open probability p0 (0.13 +/- 0.01 vs. 0.29 +/- 0.01 msec) of carbachol-activated chloride channels. 4. Inorganic Hg2+ affects the acetylcholine receptor at lower concentrations than previously reported.

GABA receptor-channel complex as a target site of mercury, copper, zinc, and lanthanides

1. The GABAA receptor-chloride channel complex has been shown to be modulated by a variety of chemicals. Scores of chemicals with diverse and unrelated structures augment the GABA-induced chloride current, while some other chemicals suppress the current. Certain heavy metals and a variety of polyvalent cations increase or decrease the current in a potent and efficacious manner. 2. We have studied the mechanisms whereby mercury, copper, zinc, and lanthanides modulated the GABA system by whole-cell and single-channel patch clamp techniques as applied to the rat dorsal root ganglion neurons in primary culture. 3. Mercuric chloride augmented the GABA-induced current to 115% of control at 0.1 microM and to 270% of control at 100 microM. It also generated a slowly developing inward current carried by a variety of ions. In contrast, methylmercury suppressed the GABA-induced current. The potent stimulation of the GABA system by mercuric chloride is deemed important in mercury intoxication. 4. Copper and zinc suppressed the GABA-induced current with an EC50 of 16 and 19 microM, respectively. They bound to a common site on the external surface of the GABA receptor-channel complex. 5. Lanthanum augmented the GABA-induced current with an EC50 of 230 microM by increasing the affinity of the receptor for GABA. It bound to a site on or near the external surface of the GABA receptor-channel complex which is different from the sites for GABA, barbiturates, benzodiazepines, picrotoxin, and copper/zinc. 6. Six other lanthanides with larger atomic numbers also exerted the same stimulatory effect with their efficacies increasing with the atomic number. 7. Single-channel analyses have revealed that the augmentation of whole-cell current by terbium, a lanthanide, is due to three actions: an increase in the overall mean open time, a decrease in the overall mean closed time, and an increase in the overall mean burst time.

Methylmercury poisoning: long-term clinical, radiological, toxicological, and pathological studies of an affected family

For 3 months in 1969 a family in the United States that included a pregnant mother consumed pork containing methylmercury. Children, aged 20, 13, and 8 years and a neonate, developed severe neurological signs. Twenty-two years later, the 2 oldest had cortical blindness or constricted visual fields, diminished hand proprioception, choreoathetosis, and attentional deficits. Magnetic resonance images showed tissue loss in the calcarine and parietal cortices and cerebellar folia. The youngest had quadriplegia, blindness, and severe mental retardation until their deaths. The brain of the 8-year-old who died at age 30 showed cortical atrophy, neuronal loss, and gliosis, most pronounced in the paracentral and parietooccipital regions. The total mercury level in formalin-fixed, left occipital cortex was 1,974 ng/gm as measured by atomic absorption. Regional brain mercury levels correlated with extent of brain damage. A control patient had 38.5 ng of mercury/gm in the occipital cortex. Systemic organs in the patient and a control subject had comparable mercury levels. In mercury-intoxicated rats, we found that only 5 to 10% of total brain mercury was lost by formalin fixation. Brain inorganic mercury in the patient ranged from 82 to 100%. Since inorganic mercury crosses the blood-brain barrier poorly, biotransformation of methyl to inorganic mercury may have occurred after methylmercury crossed the blood-brain barrier, accounting for its persistence in brain and causing part of the brain damage.

Identification and partial characterization of a glycoprotein species with high affinity for methylmercury in peripheral nervous tissues of man and experimental animals

A small amount of a glycoprotein species (21-kDa glycoprotein) with high affinity for methylmercury (MeHg) was detected in the post-nuclear or post-mitochondrial supernatant fraction of the homogenate of rat sciatic nerve on electrophoresis and autoradiography after binding of Me203Hg to the fraction. The 21-kDa glycoprotein was also found in the subcellular fractions of mouse, hamster, guinea pig, rabbit and human peripheral nervous tissues. Experiments with the cellular fractions of the tissues revealed that the 21-kDa glycoprotein is localized mainly in the myelin fraction, whereas it was not found in the cellular fractions of brain, spinal cord and nonneural tissues, such as kidney and liver. The specific binding activity of the 21-kDa glycoprotein with MeHg was 12-15 fold that of the major myelin protein, Po. It was shown that the interaction of the 21-kDa glycoprotein with MeHg was mediated through sulfhydryl groups in experiments with iodoacetamide and dithiothreitol. The amino acid compositions of the rat and human 21-kDa glycoproteins were similar but very different from that of a typical metallothionein. The N-terminal amino acid sequences of the two components of the rat 21-kDa glycoprotein were identical to those of P0 and PMP-22, respectively. The in vitro binding of MeHg was also observed in the myelin fraction obtained from the sciatic nerves of MeHg-dosed rats.

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.

Does methylmercury intoxication induce arteriosclerosis in humans? A pathological investigation of 22 autopsy cases in Niigata, Japan

In order to clarify whether or not arterio- and/or arteriolosclerosis is induced or exacerbated in patients with methylmercury (Me-Hg) intoxication, the pathological features of arteries and arterioles in specific areas in 22 patients and 36 control subjects were examined qualitatively and quantitatively. Vessels investigated were: (1) small arteries and arterioles in the subarachnoid space and cortex of the postcentral gyrus, transverse temporal gyrus, first visual area and cerebellar vermis, as well as the myocardium and renal cortex; (2) the lateral striate artery; (3) the internal carotid, anterior, middle and posterior cerebral, basilar and vertebral arteries, as well as the coronary and renal arteries; and (4) the aorta. The arteriosclerotic changes observed in the patients with Me-Hg intoxication were indistinguishable both qualitatively and quantitatively from those of controls. The results indicate that Me-Hg intoxication does not induce or exacerbate sclerotic changes in arteries and arterioles. Thus, the peculiar neurological symptoms and neuropathological features of Me-Hg intoxication are thought to be induced not by ischemia but by selective primary degeneration of the neurons in specific regions.

Mercury neurotoxicity: mechanisms of blood-brain barrier transport

Mercury exists in a wide variety of physical and chemical states, each of which has unique characteristics of target organ toxicity. The classic symptoms associated with exposure to elemental mercury vapor (Hg0) and methylmercury (CH3Hg+; MeHg) involve the central nervous system (CNS), while the kidney is the target organ for the mono- and divalent salts of mercury (Hg+ and Hg++, respectively). Physical properties and redox potentials determine the qualitative and quantitative differences in toxicity among inorganic mercury compounds, while the ability of MeHg to cross the blood-brain barrier accounts for its accumulation in the CNS and a clinical picture that is dominated by neurological disturbances. This review gives an up-to-date account of mercury's physical and chemical properties and its interaction with biologically active sites pertinent to transport across the blood-brain barrier, a major regulator of the CNS millieu.

The auditory system in methyl mercurial intoxication: a neuropathological investigation on 14 autopsy cases in Niigata, Japan

The auditory system in 14 autopsy cases of methyl mercurial intoxication in Niigata was examined neuropathologically. In addition to degeneration of the transverse temporal gyrus, there was also either significant loss of small myelinated fibers or decrease of large neurons in the cochlear nerve, ventral cochlear nucleus and inferior colliculus. The degree of decrease was higher in acute patients than in the chronic, and was not correlative to the severity of cerebrovascular sclerosis. The hearing impairment in methyl mercurial intoxication may be induced by the combined degeneration of the neurons or nerve fibers of these structures in both acute and chronic patients.

Neurotological follow-up studies upon Minamata disease

During 1986-87, 35 patients with organic mercury intoxication, who had been recognized as having Minamata disease by the Arbitration Committee in Niigata during 1968-78, were followed up neurotologically in Kido Hospital, Niigata, Japan. Characteristic follow-up features were served, as follows. 1. In 6 cases, spontaneous nystagmus had disappeared, but had newly appeared in 9 cases. However, positional nystagmus was markedly improved in 15 cases. 2. In OKN tests, especially in the vertical OKN test, relatively significant deterioration was observed at the follow-up examinations. 3. There was little significant change in the caloric nystagmus test and body-equilibrium tests. 4. In pure-tone audiometry, 16 out of 58 ears showed deterioration of hearing (28%), while 4 ears showed improvement (7%). In the neurotological findings there was much variation among the patients themselves. Therefore, these variable findings should be considered as being caused not by duration of the intoxication alone, but also by ageing factors in the patients.

Uptake of methylmercury in the rat brain: effects of amino acids

Aspects of the regulation of methylmercury (MeHg) transport across the blood-brain barrier (BBB) were investigated in the in vivo Long-Evans female rat. Fifteen s after intracarotid injection, brain 203Hg concentration was significantly increased in animals injected with 0.05 mM [203Hg]MeHgCl plus 0.1 mM L-cysteine compared with controls (P less than 0.05). This L-cysteine-enhanced 203Hg brain uptake was abolished by coinjections of [203Hg]MeHgCl with 0.1 mM L-cysteine-L-methionine, or 0.1 mM L-cysteine plus AT-125 (alpha S, 5S-alpha-amino-3-chloro-4,5-dihydro-5-isoxazolacetic acid), an irreversible inhibitor of gamma-glutamyl transpeptidase. Coinjections of rats with [203Hg]MeHgCl and 0.1 mM D-cysteine did not result in an increase in the rate of 203Hg uptake compared with controls. Furthermore, [203Hg]MeHg uptake at 15 s after intracarotid injections in the rat was stereospecific to the neutral amino acid carrier, as 203Hg uptake across the BBB was not inhibited by coinjections of [203Hg]MeHgCl with aspartic acid, an acidic amino acid. These results indicate the presence in brain capillaries of a transport system capable of selectively mediating MeHg uptake across the brain capillary endothelial cell membrane.

Distribution of mercury 203 in pregnant rats and their fetuses following systemic infusions with thiol-containing amino acids and glutathione during late gestation

To investigate the effect of amino acids and the tripeptide glutathione (GSH) on tissue uptake of methylmercury (MeHg) in the developing rat fetus in utero, pregnant rats were continuously infused into the external jugular vein with 0.1 mM L-cysteine, 0.1 mM L-leucine, 0.1 mM GSH or saline commencing on day 17 of gestation. This was followed at 24, 48, and 72 hours by external jugular infusion of 50 microM [203Hg]-MeHgCl administered in 1 ml over 1 hour. Pups were surgically removed from the uterus on gestational day 21. Whole body, brain, kidney, liver, and placental 203Hg radioactivity was measured by means of gamma-spectrometry. Brain 203Hg concentration in pups exposed in utero to L-cysteine was significantly higher compared with pups exposed to saline (P less than 0.05). Brain 203Hg concentration in pups exposed in utero to L-leucine and GSH was significantly depressed compared with pups exposed to saline (P less than 0.05). Kidney 203Hg concentration was not significantly changed in all treatment groups compared with controls. Liver 203Hg concentration was significantly depressed in L-leucine- and GSH-treated pups compared with controls (P less than 0.05). Placental 203Hg concentration was not affected by any treatment compared with controls. These effects occurred despite no difference in total 203Hg body burden among pups, irrespective of the treatment. In addition, infusion with L-cysteine resulted in a significant increase in 203Hg brain concentration in dams compared with controls, and 203Hg brain concentration in L-leucine- and GSH-treated dams was significantly depressed compared with controls. Thus 203Hg distribution in both adult and developing animals is altered by chronic amino acid or GSH infusions and suggests that MeHg uptake may be mediated through the formation of a cysteine-MeHg complex which is transported across the blood-brain barrier by the neutral amino acid carrier transport system.

Hydrocephalus following prenatal methylmercury poisoning

Prenatal methylmercury poisoning of C57BL/6J mice was followed by the development of communicating hydrocephalus in 15% to 25% of surviving offspring. Although examination of the serially sectioned cerebral aqueduct in hydrocephalic animals revealed the presence of stenosis, complete occlusion of the lumen was not observed. The ependymal epithelium of the cerebral aqueduct was preserved, and there was no evidence of periaqueductal inflammation or reactive gliosis. Edema and vacuolar change were, however, observed subependymally. The cerebral white matter, which bore the brunt of the degenerative changes seen in hydrocephalic brains, showed edema, spongy degeneration, gross cystic change and loss of parenchyma. In addition, ependymal cells and choroid plexus epithelial cells in Hg-treated animals contained large amounts of mercury within their cytoplasm, and it is possible that this may have contributed to the development of hydrocephalus by causing disturbances of CSF homeostasis. We believe that the appearance of aqueductal stenosis in Hg-intoxicated animals represents the result rather than the cause of the hydrocephalus.

Abnormal neuronal distribution within the cerebral cortex after prenatal methylmercury intoxication

C57BL/6J mice were poisoned with methylmercury during pregnancy, and the location of heavily labeled neurons generated at embryonic day 16 was determined by tritiated thymidine autoradiography of the cerebral cortex of offspring at postnatal day 10. Camera lucida plotting of the distribution of radioactively labeled cortical neurons revealed statistically significant differences between control and methylmercury-treated groups. While control animals showed regular and tight packing of labeled neurons within the upper part of the cortical layer II, in methylmercury-treated animals such neurons were irregularly distributed throughout cortical layers II and III. Short-term intermittent and long-term low-dosage regimens of intoxication produced similar results. These findings support the hypothesis that prenatal methylmercury poisoning results in abnormal neuronal migration and anomalous cortical cytoarchitectonic patterning within the developing brain and provide a possible morphological basis for some of the neurobehavioral abnormalities that may be observed in association with sublethal prenatal intoxication in humans.

Mercury 203 distribution in pregnant and nonpregnant rats following systemic infusions with thiol-containing amino acids

Near-term pregnant (gestational day 17) and nonpregnant Long-Evans female rats were continuously infused into the external jugular vein with 0.1 mmole/hour L-cysteine, 0.1 mmole/hour L-leucine, or saline. At 24, 48, and 72 hours, 50 mumole/hour [203Hg]-MeHgCl was administered over 1 hour. Total 203Hg body burden, brain, kidney, liver, and blood 203Hg concentrations were determined at 96 hours by gamma scintillation spectrometry. Despite significantly greater 203Hg whole body retention in the pregnant animals 203Hg concentrations in blood, brain, kidney, and liver were higher in nonpregnant rats. In addition, brain 203Hg concentrations in both pregnant and virgin rats were significantly higher in L-cysteine-treated rats compared with controls. These results suggest that the fetus may act as a "sink" for MeHg, thus decreasing 203Hg concentrations in maternal blood, brain, kidney, and liver. Furthermore, the data indicate that brain uptake of methylmercury in both pregnant and nonpregnant rats is enhanced by chronic L-cysteine infusion, lending support to the hypothesis that methylmercury in the rat may be translocated across the blood-brain barrier by the neutral amino acid carrier transport system.

Mechanism of methylmercury cytotoxicity

Although a large number of epidemiological, clinical, and pathological studies on methylmercury intoxication have been published, these investigations have not been able to elucidate the detailed mechanisms by which the metal alkyl causes a wide variety of biological dysfunctions. Thus, the cultured cells which are free from the influence of whole body complexities, such as absorption, distribution, metabolism, etc., which complicate the interpretation of in vivo experimental results, attract the attention of many scientists who are interested in clarifying the mode of toxic action of methylmercury. The aim of this article is to review the recent studies on the toxicity of methylmercury at the cellular level and to outline the mechanisms which have been proposed to be responsible for cell injuries.

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.

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.

Effect of mercuric chloride on some biochemical and physiological parameters of the freshwater murrel, Channa punctatus

The freshwater murrel, Channa punctatus, was exposed to a sublethal concentration of mercuric chloride (3 micrograms/liter) for 120 days and the following effects were examined: changes in the levels of glucose and lactic acid in blood and of glycogen and lactic acid in liver and muscles; rate of absorption of glucose from the intestine; and changes in the activities of glucose-6-phosphatase (G-6-Pase), hexokinase, lactate dehydrogenase (LDH), pyruvate dehydrogenase (PDH), succinate dehydrogenase (SDH), malate dehydrogenase (MDH), glutamate dehydrogenase (GDH), L-amino acid oxidase (AO), and xanthine oxidase (XO) in brain, gills, intestine, kidney, liver, and muscles. Mercury-treated fish were hypoglycemic and hypolactemic. The glycogen content of liver and muscles remained unaltered but the muscle lactic acid level decreased significantly. The rate of intestinal absorption of glucose was reduced significantly by exposure to mercury. G-6-Pase activity was decreased in all the tissues. Hexokinase activity also decreased in mercury-exposed fish but it was significant only in intestine, kidney, and liver. The activities of LDH, PDH, SDH, and MDH also were decreased significantly except LDH in brain and MDH in kidney where an insignificant decrease and an insignificant increase, respectively, were recorded. GDH and AO activities were elevated in most of the tissues except GDH in gills, and AO in gills and muscles where a decrease was observed. XO activity in brain, gills, and kidneys was significantly elevated, but no marked alteration was noted in other tissues.

Cerebellar degeneration caused by high-dose cytosine arabinoside: a clinicopathological study

Twenty-four patients with leukemia or lymphoma refractory to conventional chemotherapy were given a course of systemic, high-dose cytosine arabinoside (3 gm/m2 every 12 hours for twelve doses). Four patients developed cerebellar degeneration during treatment. Ataxia of gait and limb movements, dysarthria, and nystagmus appeared five to seven days after the first dose, worsened over the next two to three days, and then remained stable for two to six days. Incomplete improvement occurred over the following one to two weeks. Postmortem examination disclosed loss of Purkinje cells in the depths of cortical sulci with relative preservation of those at the crests of folia and those in the most posterior inferior portions of the cerebellum. Other patients developed a mild, reversible cerebellar syndrome over the same time course as that of the irreversible disorder. Manifestations ranged from nystagmus alone to dysarthria and unsteadiness of gait without limb ataxia. We conclude that cytosine arabinoside at this dosage causes a cerebellar degeneration with characteristic clinical and pathological features. Among the present patients with refractory hematological malignancies, such degeneration occurred with an incidence of 16.7%, more than twice that reported in previous series.

Inter-relation between environmental monitoring data, human exposure and health effects

To determine whether a population has been affected by a chemical, evidence of exposure must be established. The mere presence of a chemical in the surroundings of a population may not, in all instances, result in actual exposure. Not all such exposures will cause health effects; nor is it always possible to establish that illness has or will result from exposure to chemicals. The inability to establish health effects in humans cannot a priori be translated to mean that a specific chemical is harmless. On the other hand, it must be determined whether health studies would be fruitful. If exposure was so minimal that no health effects are expected, then no health studies should be conducted.

Effect of methylmercury and inorganic mercury on the nerve growth factor-induced neurite outgrowth in chick embryonic sensory ganglia

Methylmercury inhibited the fiber outgrowth in chick embryonic sensory ganglia in the presence of nerve growth factor (NGF) by 50% at 2 x 10(-6) M and completely depressed it at 5 x 10(-6) M. Whereas inorganic mercury completely inhibited the fiber outgrowth at 10(-4) M. The possible role of microtubules is discussed.

The cerebral lesions in a patient with generalized glutathione deficiency and pyroglutamic aciduria (5-oxoprolinuria)

The clinical and pathologic features of a male patient with generalized glutathione deficiency and pyroglutamic aciduria are presented. The patient died at the age of 28 years. He was mentally retarded from infancy and developed progressive tremor, retardation of movement, and ataxia as from the age of 16. Neuropathologic examination of the brain disclosed a selective atrophy of the granule cell layer of the cerebellum and focal lesions in the visual cortex and the thalamus. The type and distribution of the lesions resembled those seen after mercury intoxication. However, in our patient the damage was probably caused by the lack of protection of glutathione against oxidative damage in the brain. Possible treatment of this rare metabolic disorder might include external supply of an antioxidant, e.g., a thiol capable of penetrating the blood brain barrier.

Loss of visual suppression of caloric nystagmus in cats

Loss of visual suppression (VS) of caloric nystagmus was produced after creating flocculus lesions. The flocculus receives visual signals through a climbing fiber pathway via the inferior olive (IO) and through a mossy fiber pathway (MF) presumably via the superior colliculus (SC). In order to elucidate the prefloccular nuclei responsible for VS of caloric nystagmus, VS of caloric nystagmus was investigated after making lesions in such nuclei as the SC and the IO in 42 cats. After the IO lesion, VS of caloric nystagmus was revealed in all IO-lesioned cats throughout the whole experimental course. After the SC lesion, loss of VS was constantly observed and persisted in 7 out of 9 cats. Hence, the MF pathway via the SC is believed to be the most likely candidate for the immediate modification of the vestibuloocular reflex by visual stimuli.

Embryotoxic effects of methylmercuric chloride administered to mice and rats during orangogenesis

Pregnant C57BL mice were given a daily dose of 7.5, 6.0, 5.0 or 2.5 mg/kg methylmercuric chloride (MMC) orally from the sixth through the thirteenth day of pregnancy. Pregnant Wistar rats were given a daily dose of 7.5, 5.0 or 2.5 mg/kg MMC orally from the seventh through the fourteenth day of pregnancy. Fetal examinations were performed on days 18 and 20 in mice and rats, respectively. In mice, the dose of 7.5 mg/kg was embryocidal. The dose of 6.0 mg/kg also caused a high incidence of fetal death, decreased fetal weight and a marked increase in malformations. A dose of 5.0 mg/kg caused a decrease in fetal weight and a marked increase in the incidence of malformations. The most common malformations were cleft palate and fused thoracic vertebrae. In rats, the dose of 7.5 mg/kg caused a high incidence of fetal death and malformations. The most frequent malformations were cleft palate, generalized edema, brain lesions and wavy ribs. A dose of 5.0 mg/kg caused a significant decrease in fetal weight and a significant increase in the incidence of malformations.

A pathological study of prolonged cases of Minamata disease. With particular reference to 83 autopsy cses

This study consists of 83 autopsy cases including 64 prolonged cases of Minamata disease. Lesions were severe in the prolonged cases with an acute onset, while they tended to be mild in those with chronic onset. Cerebral cortex showed loss of nerve cells in many of the former, with the cortex often being in a loosening or spongy state, while the thinning-out or decrease of neurons not exceeding 30% of all cells was frequent in the latter. Glial reaction was also intense in the former, but relatively weak in the latter; gradual loss of degenerative neurons without glial reaction was not rare. A given selectivity was found in the localization of lesions of the cerebral cortex, but it was not so conspicuous in those with a chronic onset as in those with an acute onset. Secondary changes corresponding to the lesions of the cortex appeared in the medulla. Cerebellar lesions in the chronic onset cases, as compared with those in the acute onset ones, were milder. The selectively localized area was diminished; the nodulus, uvula and lingula of the vermis and the medial surface of the semilunar lobules were the most likely to be damaged. Of the spinal peripheral nerves, damages to the spinal sensory nerve were found, which were more predominant than those to the motor nerve system, and in which was noted prominent regeneration and repair. The deposited mercury found in organs tended to remain in the nervous system and the kidneys during the prolonged course of this disease, and the prolonged accumulation of mercury in the nervous system appeared to have an influence on the pathogenesis of chronic Minamata disease.