Effects of neonatal methylmercury exposure on development of nucleic acids and proteins in rat brain: regional specificity

Effect of Bacopa monniera extract on methylmercury-induced behavioral and histopathological changes in rats

Methylmercury (MeHg) is a well-recognized environmental contaminant with established health risk to human beings by fish and marine mammal consumption. Bacopa monniera (BM) is a perennial herb and is used as a nerve tonic in Ayurveda, a traditional medicine system in India. This study was aimed to evaluate the effect of B. monniera extract (BME) on MeHg-induced toxicity in rat cerebellum. Male Wistar rats were administered with MeHg orally at a dose of 5 mg/kg b.w. for 21 days. Experimental rats were given MeHg and also administered with BME (40 mg/kg, orally) 1 h prior to the administration of MeHg for 21 days. After treatment period, MeHg exposure significantly decreases the body weight and also caused the following behavioral changes. Decrease tail flick response, longer immobility time, significant decrease in motor activity, and spatial short-term memory. BME pretreatment reverted the behavioral changes to normal. MeHg exposure decreases the DNA and RNA content in cerebellum and also caused some pathological changes in cerebellum. Pretreatment with BME restored all the changes to near normal. These findings suggest that BME has a potent efficacy to alleviate MeHg-induced toxicity in rat cerebellum.

Sensitivity of dopaminergic neuron differentiation from stem cells to chronic low-dose methylmercury exposure

Perinatal exposure to low doses of methylmercury (MeHg) can cause adult neurological symptoms. Rather than leading to a net cell loss, the toxicant is assumed to alter the differentiation and neuronal functions such as catecholaminergic transmission. We used neuronally differentiating murine embryonic stem cells (mESC) to explore such subtle toxicity. The mixed neuronal cultures that formed within 20 days contained a small subpopulation of tyrosine hydroxylase (TH)-positive neurons with specific dopaminergic functions such as dopamine transport (DAT) activity. The last 6 days of differentiation were associated with the functional maturation of already preformed neuronal precursors. Exposure to MeHg during this period downregulated several neuronal transcripts, without affecting housekeeping genes or causing measurable cell loss. Profiling of mRNAs relevant for neurotransmitter systems showed that dopamine receptors were coordinately downregulated, whereas known counterregulatory systems such as galanin receptor 2 were upregulated. The chronic (6 days) exposure to MeHg, but not shorter incubation periods, attenuated the expression levels of endogenous neurotrophic factors required for the maturation of TH cells. Accordingly, the size of this cell population was diminished, and DAT activity as its signature function was lost. When mixed lineage kinase activity was blocked during MeHg exposure, DAT activity was restored, and the reduction of TH levels was prevented. Thus, transcriptional profiling in differentiating mESC identified a subpopulation of neurons affected by MeHg, and a pharmacological intervention was identified that specifically protected these cells.

Methylmercury Alters Eph and Ephrin Expression During Neuronal Differentiation of P19 Embryonal Carcinoma Cells

Developmental exposure to methylmercury (MeHg) induces a spectrum of neurological impairment characterized by cognitive disturbance, sensory/motor deficit, and diffuse structural abnormalities of the brain. These alterations may arise from neural path-finding errors during brain development, resulting from disturbances in the function of morphoregulatory guidance molecules. The Eph family of tyrosine kinase receptors and their ligands, the ephrins, guide neuronal migration and neurite pathfinding mainly via repulsive intercellular interactions. The present study examined the effects of MeHg on mRNA and protein expression profiles of Ephs and ephrins in the P19 embryonal carcinoma (EC) cell line and its neuronal derivatives. Undifferentiated control P19 cells displayed low- to undetectable levels of mRNA for ephrins or Ephs, with the sole exception of EphA2 which was highly expressed. Upon differentiation into neurons, the ephrin expression increased progressively through day 10. Similarly, expression of the Ephs, including EphsA3, -A4, -A8, -B2, -B3, -B4, and -B6, increased significantly. In contrast, EphA2 expression decreased in day 2, 6 and 10 control neurons. Treatment with MeHg did not affect the expression of mRNA for ephrins or Ephs in undifferentiated P19 cells. However, treatment of differentiating neurons with MeHg for 24 h caused consistent increases in ligand mRNA expression, particularly ephrin-A5, -A6, -B1, and -B2. Similarly, MeHg induced variable increases in mRNA expression of receptors EphA2, -A3, -B3, and -B6. A trend toward a concentration-response relationship was observed for the alterations in Eph receptor mRNA expression although increases at the low and mid concentrations did not reach statistical significance. Immunoblots for ligand and receptor proteins mirrored the increases in the mRNA levels at the 0.5 and 1.5 microM MeHg concentrations but showed decreased protein levels compared to controls at the 3.0 microM concentration. Alterations in the Eph/ephrin family of repulsion molecules may represent an important mechanism in developmental MeHg neurotoxicity.

Gestational exposure to methylmercury alters the developmental pattern of trk-like immunoreactivity in the rat brain and results in cortical dysmorphology

Nerve growth factor signal transduction mediated through the trk receptor has been implicated in neuronal growth, differentiation, and survival. In this study, we examined the effects of gestational exposure to the developmental neurotoxicant methylmercury (CH3Hg) on the ontogeny of trk-immunoreactivity (IR). Long-Evans dams were dosed on gestational days 6-15 (p.o.) with 0, 1, or 2 mg/kg CH3Hg dissolved in saline. Pups were sacrificed and perfused with buffered paraformaldehyde on postnatal days (PND) 1, 4, 10, 21 and 85. The brains were sectioned sagitally, Nissl-stained or stained immunohistochemically for trk receptors or glial fibrillary acidic protein (GFAP), and examined throughout the medial to lateral extent of the brain. The greatest density of IR in neural cell bodies was seen in the olfactory bulb, hippocampus, cerebral, and cerebellar cortex, striatum, septum, nucleus basalis, inferior colliculus, pons, and brain stem nuclei. trk IR was not limited to nerve cell bodies, with prominent axonal and dendritic staining in the brainstem, neocortex, hippocampus, cerebellum, and olfactory tract. The regional pattern of trk IR varied in an age-dependent manner. In controls, trk-like IR appeared to peak in most regions between PND4-10 and decreased dramatically after PND21. This age-related difference in trk IR was supported by western blot analysis of PND10 and adult neocortex. This reduced and more adult-like pattern of trk IR was apparent on PND21 with some persistent trk-like IR in the olfactory bulb, hippocampus, neocortex, cerebellum and basal forebrain. In contrast to the normal regional patterns of trk IR, CH3Hg produced a dose-related decrease in trk-like IR in the absence of overt maternal toxicity or neonatal toxicity. CH3Hg-induced decreases in trk-like IR were especially apparent during the early postnatal period when trk IR was the greatest. The effects of CH3Hg exposure were restricted regionally, with the largest decrease in trk-like IR apparent in cortical regions, basal forebrain nuclei, and brain stem nuclei. Subsequent to the effects of CH3Hg on cortical trk-like IR were alterations in the development of cortical laminae on PND10 and 21 of neocortex. These alterations were characterized by quantifiable decreases in cell density, cell size and the widths of the layers of posterior neocortex. Not all of the CH3Hg-induced effects were characterized by decreased trk-like IR. Robust increases in trk IR in glial cells in the corpus callosum and brain stem were observed coincident with increased GFAP IR in cells of similar morphology. The present results localize the cellular and regional ontogeny of trk and suggest that developmental exposure to CH3Hg alters the normal ontogeny of this trophic factor receptor which may be associated with the developmental neurotoxicity of this chemical.

Chlorpyrifos interferes with cell development in rat brain regions

Chlorpyrifos, one of the most widely used pesticides, exhibits greater toxicity during development than in adulthood. We administered chlorpyrifos to neonatal rats in doses spanning the threshold for systemic toxicity and examined developing brain regions (brainstem, forebrain, cerebellum) for signs of interference with cell development using markers for cell packing density and cell number (DNA concentration and content) and cell size (protein/DNA ratio). Neonatal rats given 5 mg/kg of chlorpyrifos on postnatal days 1-4 showed significant mortality and the survivors exhibited severe cell loss in the brainstem; brainstem growth was maintained by enlargement of the remaining cells. This effect was not seen at 1 mg/kg, a dose that did not compromise survival or growth, nor was there any adverse effect at either dose in the forebrain, despite the fact that both brainstem and forebrain possess comparable cholinergic projections. When chlorpyrifos was administered later, on days 11-14, the major target for cell loss shifted from the brainstem to the forebrain and in this case, effects were seen at doses that did not compromise survival or growth. The loss of forebrain cell number occurred between 15 and 20 days of age rather than during the chlorpyrifos treatment. The cerebellum differed from the other regions in that it showed short-term elevations of DNA after chlorpyrifos exposure in either early or late postnatal periods; nevertheless, values then regressed to subnormal in parallel with the loss of cells in other regions. Thus, chlorpyrifos likely causes delayed cell death. Although regions rich in cholinergic projections, such as brainstem and forebrain, may be more affected than noncholinergic regions (cerebellum), the maturational timetable of each region (brainstem earliest, forebrain intermediate, cerebellum last) appears to be more important in setting the window of vulnerability. These results indicate that, even when growth or survival are unaffected, chlorpyrifos produces cellular deficits in the developing brain that could contribute to behavioral abnormalities.

Fetal dexamethasone exposure alters macromolecular characteristics of rat brain development: a critical period for regionally selective alterations?

Fetal glucocorticoid exposure retards postnatal growth and evokes abnormalities of nervous system structure and function. To examine the underlying mechanisms, we administered 0.2 or 0.8 mg/kg of dexamethasone to pregnant rats on gestational days 17, 18, and 19 and assessed brain region cell development with indices of DNA content (total cell numbers), DNA concentration (cell packing density), and protein/DNA ratio (relative cell size). Dexamethasone evoked deficits of pup body and brain region weights, but the brain regions displayed growth-sparing associated initially with preservation of cell numbers (normal or elevated DNA content and concentration), at the expense of relative cell size (decreased protein/DNA). Subsequently, brain cell acquisition lagged behind that of controls, with deficits in DNA and elevations of protein/DNA. In midbrain + brainstem and in cerebellum, cell markers returned to normal by weaning. However, the forebrain showed persistent elevations of DNA and reduced protein/DNA, indicative of replacement of neurons with glia. Because the treatment period coincided with the timing of neuronal cell replication in the forebrain, but not in the other regions, these results suggest that the critical period for lasting deficits of dexamethasone coincides with the peak of neuronal mitosis.

Combined effects of low-level methylmercury and x-radiation on the developing mouse cerebellum

The effects of low-level methylmercury and x-radiation in combination on the developing mouse cerebellum were studied. Pregnant BALB/cJcl mice were divided into two groups. Dams of a group were orally given 9 micrograms Hg/g body weight of methylmercuric chloride on d 17 of pregnancy. The other dams were not treated. They were allowed to give birth, and the male pups were exposed to a single x-radiation at doses of 0.125 or 0.5 Gy on the day following birth. Sham-exposed pups were also prepared. The pups were killed at various postexposure periods, and their cerebella were removed and processed for microscopy and mercury analysis. Mercury was retained in the pup cerebellum at levels of 4-8 micrograms/g for 4 d and then lowered. Appearance and incidence of cell death in the external granular layer (EGL) were similar in groups exposed to 0.125 Gy alone or 0.125 Gy and methylmercury in combination. Fourteen percent of cells in the EGL were killed by exposure to 0.5 Gy with or without methylmercury. Restoration of the EGL from the damage was slightly retarded by methylmercury, but by 10 d of age these cerebella overtook the normal ones in development. These findings indicate that methylmercury does not modify the apoptotic cell death caused by x-radiation in the EGL but retards the tissue restoration from the damage.

Enhanced electroretinogram in cats induced by exposure to mercury acetate

The present study was undertaken in order to verify whether, and how, retinal functions are affected by subacute poisoning with organic mercury. Mercury acetate in various concentrations (0.025-0.25 mg/kg per day) was injected subcutaneously every second day to adult cats (N = 20) throughout a 2.5-4.0-week period. The electroretinogram (ERG) was recorded and the Hg2+ concentrations in the blood were determined. In nearly 90% of the intoxicated cats an enhanced electroretinogram (scotopic b-wave amplitude) was found as compared to its level in the normal control cats (N = 10). The latency of the ERG was found to be appropriately shorter, up to a maximal difference of nearly 20% in comparison to the controls. Hg2+ was present in the blood of the exposed cats during a 2.5-month period following the exposure. It is concluded that exposure to mercury acetate induces a permanent increase in the excitability level of the cat's retina.

Methylmercury-induced movement and postural disorders in developing rat: regional analysis of brain catecholamines and indoleamines

Subcutaneous administration of methylmercury (MeHg) to rats during early postnatal development resulted in movement and postural disorders by day 22-24. Tissue concentrations of norepinephrine (NE), serotonin (5-HT), dopamine (DA) and selected metabolites were measured in the cerebral cortex, spinal cord and caudate-putamen at the onset of neurological impairment and at two subclinical stages of toxicity. In the cerebral cortex there was a significant increase in tissue concentrations of 5-HT (54-81%) and 5-hydroxyindoleacetic acid (HIAA, 133-178%) at the onset of neurological impairment. Similar increases were detected in the spinal cord for 5-HT (19-43%) and HIAA (98-123%) as well as an increase in the concentration of NE (42-51%). In the caudate-putamen there were significant increases in the concentrations of NE (98-116%), HIAA (108-124%) and DA (28-29%) with a significant decrease in the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC, 20-27%); however, tissue levels of homovanillic acid (HVA) did not change significantly. Many of these changes were detected at subclinical stages of MeHg toxicity. The ratio of HIAA/5-HT, which is frequently used as an estimate of turnover for 5-HT, was significantly increased in all 3 tissues at the onset of neurological impairment (38-94%) and at one subclinical stage (47-114%). The ratio of (DOPAC + HVA)/DA was significantly decreased in caudate-putamen at all 3 stages of toxicity (18-40%). These changes indicate altered metabolism in aromatic amine systems in the developing central nervous system during the pathogenesis of MeHg-induced movement and postural disorder.

Teratogenic interactions between methylmercury and mitomycin-C in mice

Pregnant mice were given p.o. various nonteratogenic doses (0, 2.5, 5 and 10 mg/kg) of methylmercuric chloride on day 9 of pregnancy, and then injected i.p. with a teratogenic dose (4 mg/kg) of mitomycin-C on day 10. Major malformations produced by mitomycin-C alone were cervical rib and vertebral anomaly, polydactyly of the hindlimb and tail anomaly. Combined treatment significantly increased the incidence of these malformations, showing the dose-effect relationship of methylmercury, whereas methylmercury alone is known not to produce such malformations. When mitomycin-C treatment alone was performed on day 9.5 of pregnancy, only vertebral anomalies increased in incidence. Therefore, mitomycin-C teratogenicity in terms of the manifestation of cervical rib, polydactyly and tail anomaly, but not vertebral anomaly, was suggested to be enhanced by methylmercury. A considerable number of foetuses showed cleft palate involvement following combined treatments, but not by either chemical alone. Cleft palate is known to be a major malformation in mice that is caused by methylmercury, and mitomycin-C also induces cleft palate. Therefore, the two chemicals might have affected foetuses additively and thereby induced cleft palate.

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.

Functional consequences of prenatal methylmercury exposure: effects on renal and hepatic responses to trophic stimuli and on renal excretory mechanisms

The effects of prenatal exposure to methylmercury on the functional development of renal and hepatic response systems was examined in the developing rat. Methylmercury produced an elevation of basal activity of renal ornithine decarboxylase (ODC, an enzyme involved in regulation of cellular maturation) and an eventual relative hypertrophy; liver ODC was reduced and hypertrophy was not evident. In contrast, the reactivity of liver ODC to trophic stimulants (vasopressin, isoproterenol) was markedly enhanced by prenatal methylmercury exposure, whereas renal ODC responses were much less affected (vasopressin) or actually reduced (isoproterenol). Targeted actions of methylmercury on renal excretory function were also prominent, with increased fractional excretions of urea and electrolytes and an eventual reduction in glomerular filtration as assessed by creatinine clearance. In addition, the reactivity of the kidney to challenge with desmopressin was altered coincidentally with the appearance of the effects on basal clearance mechanisms. These studies show that doses of methylmercury ordinarily associated with selective actions on development of neurobehavioral patterns also influence the functional ontogeny of other organ systems; furthermore, the fact that the target tissues are different for prenatal vs. postnatal methylmercury exposure, indicates that the functional teratology of methylmercury exhibits critical periods of sensitivity.