Comparison of mercury accumulation among the brain, liver, kidney, and the brain regions of rats administered methylmercury in various phases of postnatal development

Brain methylmercury uptake in fetal, neonate, weanling, and adult rats

Fetuses and neonates are known to be highly susceptible to methylmercury (MeHg) toxicity, but little is known about the relative uptake of MeHg from blood to the developing brain. We measured time-course changes in mercury (Hg) concentrations in the brain of fetal, neonate, weanling, and adult rats after an injection of 0.08 μg (0.4 nmol) Hg/g MeHg. In the prenatal experiment, MeHg was subcutaneously injected to pregnant dams on embryonic days 17, 18, 18.5, 19, 19.5, or 20, and Hg concentrations in tissues were measured in both mothers and fetuses on embryonic day 21 (1 day before parturition). Brain Hg levels in fetuses peaked 2 days after injection and were approximately 1.5 times higher than in mothers. In the postnatal experiment, the same MeHg dose was injected subcutaneously to male rats on postnatal days 1 (neonates), 35 (weanlings), or 56 (adults). Mercury concentrations in tissues were measured 1, 2, 3, 4, 5, or 6 days after the injection. Brain Hg levels peaked most rapidly in neonates, and were approximately 1.5 times higher than levels in weanlings or adults. Throughout the examined period, peak Hg levels in the brain and the Hg brain/blood ratio 24 h after injection were highest in fetuses, followed by the levels in neonates, and decreased with life stage. These findings suggest that relatively higher brain MeHg uptake is an important factor in the vulnerability of fetuses and neonates to MeHg exposure.

Mercury speciation in brain tissue of polar bears (Ursus maritimus) from the Canadian Arctic

Methylmercury (MeHg) is a neurotoxicant that has been found at elevated concentrations in the Arctic ecosystem. Little is known about its internal dose in wildlife such as polar bears. We measured concentrations of mercury (Hg) in three different brain regions (cerebellum, frontal lobe and brain stem) of 24 polar bears collected from the Nunavik, Canada between 2000 and 2003. Speciation of Hg was measured by High Performance Liquid Chromatography coupled to Inductively Coupled Plasma Mass Spectroscopy (HPLC-ICP-MS). Concentrations of mean total Hg in brain tissue were up to 625 times lower (0.28 ± 0.07 mg kg(-1) dry weight (dw) in frontal lobe, 0.23 ± 0.07 mg kg(-1) dw in cerebellum and 0.12 ± 0.0 3mg kg(-1) dw in brain stem) than the mean total Hg concentration previously reported in polar bear liver collected from Eastern Baffin Island. Methylmercury (MeHg) accounted for 100% of the Hg found in all three brain regions analyzed. These results suggest that polar bear might reduce the toxic effects of Hg by limiting the uptake into the brain and/or decrease the rate of demethylation so that Hg can be excreted from the brain more easily. The toxicokinetics and the blood-brain-barrier mechanisms of polar bears are still unknown and further research is required.

Effects of postnatal exposure to methylmercury on spatial learning and memory and brain NMDA receptor mRNA expression in rats

The extreme vulnerability of developing nervous system to methylmercury (MeHg) is well documented. Still unclear is the consequence of different postnatal period exposure to MeHg. We investigated the critical postnatal phase when MeHg induced neurotoxicity in rats and the underlying mechanism. Rats were given 5mg/(kg day) methylmercury chloride (MMC) orally on postnatal day (PND) 7, PND14, PND28, and PND60 for consecutive 7 days. A control group was treated with 0.9% sodium chloride solution 5 ml/(kg day) instead. On PND69, spatial learning and memory was evaluated by Morris water maze test. Behavior deficits were found in MMC-treated rats of PND7 and PND14 groups (p<0.01). N-methyl-D-aspartate (NMDA) receptor 2 subunits mRNA expressions were evaluated 3 days after the last administration. In hippocampus, the mRNA expression of NR2A and NR2B decreased, but the NR2C expression increased in PND14 group following MMC-treatment (p<0.01). In cerebral cortex, mRNA expression of NR2A decreased, with NR2C expression elevating in PND14 group following MMC-treatment (p<0.05). These observations suggest that the postnatal exposure to MeHg during PND7-20 could cause neurobehavioral deficits which extend to adulthood. Furthermore, the abnormal expression of NMDAR 2 subunits might associate with the impairment.

Possible involvement of cathepsin B released by microglia in methylmercury-induced cerebellar pathological changes in the adult rat

There is increasing evidence that cathepsin B (CB), a lysosomal cysteine protease, is one of the toxic molecules that are secreted by activated microglia. We herein provide evidence that CB released by activated microglia may play a role in the methylmercury (MeHg)-induced pathological changes observed in the cerebellum of the adult rat. Pathological changes tended to progress slowly after treatment with MeHg (5 mg/kg) for 12 consecutive days. At 5 days after the final treatment of MeHg, there was a mild pyknotic change of the granule cells, whereas a marked accumulation of activated microglia was observed in the granule cell layer of the lingual and central lobe. At 8 days after the final treatment, intense pyknotic changes of the granule cells and the accumulation of activated microglia were observed throughout the cerebellar vermis. CB first significantly increased at 3 days after the final treatment of MeHg as the mature form. CB mainly increased in activated microglia which accumulated in the granule cell layer. The coadministration of CA074, an irreversible CB inhibitor, with MeHg significantly reduced the severity of pyknotic changes of the granule cells. Furthermore, primary cultured microglia secreted the mature CB in the culture medium following cellular activation. These observations strongly suggest that CB secreted by activated microglia is thus closely associated with the MeHg-induced severe pyknotic changes of the cerebellar granule cells. The treatment of CA074 could be a potentially effective therapeutic intervention to prevent the pathological changes in the cerebellum caused by ingestion of MeHg-contaminated food.

Acute exposure to methylmercury at two developmental windows: Focus on neurobehavioral and neurochemical effects in rat offspring

The neurobehavioral and neurochemical effects produced by prenatal methylmercury exposure (8 mg/kg, gestational-days 8 or 15), were investigated in rats. On postnatal day 40, animals exposed to methylmercury and tested in the open field arena, showed a reduction in the number of rearings, whereas the number of crossings and resting time was not altered with respect to the age-matched control rats. The methylmercury-exposed groups showed a lower level of exploratory behavior as well as an impairment in habituation and working memory when subjected to the novel object exploration task. The neophobia displayed by methylmercury-exposed rats is unlikely to be attributed to a higher degree of anxiety. Prenatal methylmercury exposure did not affect motor coordination or motor learning in 40-day-old rats subjected to the balance task on a rotating rod, and it did not impair the onset of reflexive behavior in pups screened for righting reflex, cliff aversion and negative geotaxis. In cortical cell cultures from pups exposed to methylmercury during gestation, basal extracellular glutamate levels were higher, whereas the KCl-evoked extracellular glutamate levels were lower than that measured in cultures from rats born to control mothers. In addition, a higher responsiveness of glutamate release to N-methyl-D-aspartic acid receptor activation was evident in cortical cell cultures from pups born from methylmercury-treated dams than in cultures obtained from control rats. The present results suggest that acute maternal methylmercury exposure induces, in rat offspring, subtle changes in short-term memory as well as in exploratory behavior. These impairments seem to be associated to alterations of cortical glutamatergic signaling.

Activation of mu-calpain in developing cortical neurons following methylmercury treatment

In order to examine the possible involvement of mu-calpain in methylmercury (MeHg)-induced neurotoxicity in developing cortical neurons, we performed biochemical and immunohistochemical studies utilizing two antibodies which specifically recognize the 150-kDa mu-calpain-specific alpha-spectrin breakdown product (SBDP) and the active form of mu-calpain in rats on postnatal day 16. Soluble fractions of the cerebral cortex from control rats exhibited slight immunoreactivity for SBDP. Although the amount of SBDP in the cerebral cortex was only slightly increased the day after the final treatment of MeHg (10 mg/kg) for 3 or 7 consecutive days, there was a prominent accumulation of SBDP 3 days after the final treatment of MeHg for 7 consecutive days. On the other hand, the 76-kDa isoform of mu-calpain gradually increased after chronic treatment of MeHg, but markedly decreased 3 days after the final treatment of MeHg for 7 consecutive days. At this stage, many cortical neurons were densely stained with anti-SBDP antibody. The delayed increase in SBDP corresponded well with the delayed nature of the MeHg-induced neurotoxicity. When MK-801 (0.1 mg/kg), a non-competitive antagonist of N-methyl-D-aspartate (NMDA), was administered intraperitoneally with MeHg for 7 consecutive days, both neuronal damage and accumulation of SBDP were markedly depressed in the cerebral cortex 3 days after the final treatment. Our results indicate that mu-calpain activation and mu-calpain-mediated proteolysis of alpha-spectrin preceded neuronal damage in the developing cerebral cortex induced by chronic treatment of MeHg.

Involvement of enhanced sensitivity of N-methyl-d-aspartate receptors in vulnerability of developing cortical neurons to methylmercury neurotoxicity

The developing cortical neurons have been well documented to be extremely vulnerable to the toxic effect of methylmercury (MeHg). In the present study, a possible involvement of N-methyl-D-aspartate (NMDA) receptors in MeHg neurotoxicity was examined because the sensitivity of cortical neurons to NMDA neurotoxicity has a similar developmental profile. Rats on postnatal day 2 (P2), P16, and P60 were orally administered MeHg (10 mg/kg) for 7 consecutive days. The most severe neuronal damage was observed in the occipital cortex of P16 rats. When MK-801 (0.1 mg/kg), a non-competitive antagonist of NMDA, was administered intraperitoneally with MeHg, MeHg-induced neurodegeneration was markedly ameliorated. Furthermore, there was a marked accumulation of nitrotyrosine, a reaction product of peroxynitrite and L-tyrosine, after chronic treatment of MeHg in the occipital cortex of P16 rats. The accumulation of nitrotyrosine was also significantly suppressed by MK-801. In the present electrophysiological study, the amplitude of synaptic responses mediated by NMDA receptors recorded in cortical neurons of P16 rats was significantly larger than those from P2 and P60 rats. These observations strongly suggest that a generation of peroxynitrite through activation of NMDA receptors is a major causal factor for MeHg neurotoxicity in the developing cortical neurons. Furthermore, enhanced sensitivity of NMDA receptors may make the cortical neurons of P16 rats most susceptible to MeHg neurotoxicity.

Widespread neuronal degeneration in rats following oral administration of methylmercury during the postnatal developing phase: a model of fetal-type minamata disease

The neurotoxicity of methylmercury (MeHg) treatment during the postnatal developing phase in rats was studied. Rats on postnatal day 1 were orally administered 5 mg/kg/day methylmercury chloride (MMC) for more than 30 consecutive days. Body weight loss began 26 days after MMC was administered, and severe paralysis of the hind-limbs and unsteadiness appeared subsequently. Histopathologically, the widespread neuronal degeneration was observed in the cerebral neocortex, neostriatum, red nucleus, brainstem, cerebellum and spinal dorsal root ganglia on day 32. The widespread distribution of the lesions was quite similar to that in fetal cases of MeHg intoxication in Minamata, Japan. These findings suggest that MMC treatment during the postnatal development phase in rats produce a good model of fetal-type Minamata disease.