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

Biomarkers of methylmercury neurotoxicity and neurodevelopmental features: A systematic review

The issue of MeHg contamination is a significant concern due to its detrimental impact on the environment. This study aimed to thoroughly investigate the effects of MeHg on neurodevelopmental biomarkers, as there is a lack of systematic reviews in this area. We conducted a comprehensive search of three databases (PubMed, Scopus, and Web of Science) and found 522 records, which were then meticulously reviewed by two independent reviewers. A total of 66 studies were included, with biomarkers related to oxidative stress, neurotransmission, inflammation, epigenetics, and apoptosis being the most prominent. The results of both in vitro and in vivo models indicate that antioxidant enzymes and other oxidative stress-related markers are indeed, altered following MeHg exposure. Moreover, MeHg exposure causes significant disruptions to neurotransmitter levels, activities of neurotransmitter synthesis enzymes, receptor densities, and proteins involved in synaptic function. Proinflammatory biomarkers are consistently overexpressed in both MeHg-treated cells and the brains of exposed rats. Furthermore, studies on DNA methylation and biomarker activity suggest that MeHg exposure may lead to neurotoxicity and neurodevelopmental issues via perturbations to epigenetic markers and the apoptosis pathway.

Chronic exposure to methylmercury induces puncta formation in cephalic dopaminergic neurons in Caenorhabditis elegans

The neurotransmitter dopamine is a neuromodulator in the positive and negative regulation of brain circuits. Dopamine insufficiency or overload has been implicated in aberrant activities of neural circuits that play key roles in the pathogenesis of neurological and psychiatric diseases. Dopaminergic neurons are vulnerable to environmental insults. The neurotoxin methylmercury (MeHg) produces dopaminergic neuron damage in rodent as well as in Caenorhabditis elegans (C. elegans) models. Previous studies have demonstrated the utility of C. elegans as an alternative and complementary experimental model in dissecting out mechanism of MeHg-induced dopaminergic neurodegeneration. However, a sensitive pathological change that marks early events in neurodegeneration induced by environmental level of MeHg, is still lacking. By establishing a chronic exposure C. elegans model, for the first time, we have shown the propensity of MeHg (5 μM, 10 days) to induce bright puncta of dat-1::mCherry aggreagtes in the dendrites of cephalic (2 CEPs) dopaminergic neurons in a dose- and time-dependent manner, while these changes were not found in other dopaminergic neurons: anterior deirids (2 ADEs) and posterior deirids (2 PDEs), cholinergic neurons (2 AIYs) or glutamatergic neurons (2 PVDs). The bright puncta appear as an aggregation of mCherry proteins accumulating in dendrites. Further staining shows that the puncta were not inclusions in lysosome, or amyloid protein aggregates. In addition, features of the puncta including enlarged sphere shape (0.5-2 μm diameters), bright and accompanying with the shrinkage of the dendrite suggest that the puncta are likely composed of homologous mCherry molecules packaged at the dendritic site for exportation. Moreover, in the glutathione S-transferase 4 (gst-4) transcriptional reporter strain and RT-PCR assay, the expression levels of gst-4 and tubulins (tba-1 and tba-2) genes were not significantly modified under this chronic exposure paradigm, but gst-4 did show significant changes in an one day exposure paradigm. Collectively, these results suggest that CEP dopaminergic neurons are a sensitive target of MeHg, and the current exposure paradigm could be used as a model to investigate mechanism of dopaminergic neurotoxicity.

Neurotransmitter amines and antioxidant agents in neuronal protection against methylmercury-induced cytotoxicity in primary cultures of mice cortical neurons

Methylmercury (MeHg) is an environmental toxicant with detrimental effects on the developing brain and adult nervous system. The main mechanisms identified include oxidative stress, changes in intracellular calcium, mitochondrial changes, inhibition of glutamate uptake, of protein synthesis and disruption of microtubules. However, little is known about mechanisms of protection against MeHg neurotoxicity. We found that resveratrol (10 μM) and ascorbic acid (200 μM) protected MeHg-induced cell death in primary cultures of cortical neurons. In this work, we aimed at finding additional targets that may be related to MeHg mode of action in cell toxicity with special emphasis in cell protection. We wonder whether neurotransmitters may affect the MeHg effects on neuronal death. Our findings show that neurons exposed to low MeHg concentrations exhibit less mortality if co-exposed to 10 μM dopamine (DA). However, DA metabolites, HVA (homovanillic acid) and DOPAC (3,4-dihydroxyphenylacetic acid) are not responsible for such protection. Furthermore, both DA D1 and D2 receptors agonists showed a protective effect against MeHg toxicity. It is striking though that DA receptor antagonists SKF83566 (10 μM) and haloperidol (10 μM) did not inhibit DA protection against MeHg. In addition, the protective effect of 10 μM DA against MeHg-induced toxicity was not affected by additional organochlorine pollutants exposure. Our results also demonstrate that cells exposed to MeHg in presence of 100 μM acetylcholine (ACh), show an increase in cell mortality at the "threshold value" of 100 nM MeHg. Finally, norepinephrine (10 μM) and serotonin (20 μM) also had an effect on cell protection. Altogether, we propose to further investigate the additional mechanisms that may be playing an important role in MeHg-induced cytotoxicity.

Methylmercury induces an initial increase in GABA-evoked currents in Xenopus oocytes expressing α1 and α6 subunit-containing GABAA receptors

Early onset effects of methylmercury (MeHg) on recombinant α₁β₂γ_2S or α₆β₂γ_2S subunit-containing GABA_A receptors were examined. These are two of the most prevalent receptor types found in cerebellum-a consistent target of MeHg-induced neurotoxicity. Heterologously expressed receptors were used in order to: (1) isolate receptor-mediated events from extraneous effects of MeHg due to stimulation of the receptor secondary to increased release of GABA seen with MeHg in neurons in situ and (2) limit the phenotypes of GABA_A receptors present at one time. Initial changes in I_GABA in Xenopus laevis oocytes expressing either α₁β₂γ_2S or α₆β₂γ_2S receptors were compared during continuous bath application of MeHg. A time-dependent increase in I_GABA mediated by both receptor subtypes occurred following the first 25-30min of MeHg (5μM) exposure. In α₆β₂γ_2S containing receptors, the MeHg-induced increase in I_GABA was less pronounced compared to that mediated by α₁β₂γ_2S containing receptors, although the pattern of effects was generally similar. Washing with MeHg-free solution reversed the increase in current amplitude. Application of bicuculline at the time of peak potentiation of I_GABA rapidly and completely reversed the MeHg-induced currents. Therefore these MeHg-increased inward currents are mediated specifically by the two subtypes of GABA_A receptors and appear to entail direct actions of MeHg on the receptor. However bicuculline did not affect stimulation by MeHg of oocyte endogenous Cl^- -mediated current, which presumably results from increased [Ca²⁺]_i. Thus, MeHg initially potentiates I_GABA in oocytes expressing either α₁β₂γ_2S or α₆β₂γ_2S receptors prior to its more defined later effects, suggesting that MeHg may initially interact directly with GABA_A receptors in a reversible manner to cause this potentiation.

Exposure to low dose of cinnabar (a naturally occurring mercuric sulfide (HgS)) caused neurotoxicological effects in offspring mice

Cinnabar, a naturally occurring mercuric sulfide (HgS), has long been used in Chinese mineral medicine for more than 2000 years. Although mercury is well-known for its toxicity, whether cinnabar induces neurotoxicity, especially in infants and children, is unknown. The purpose of this study was to explore the neurotoxic effects of low-dose of cinnabar (10 mg/kg/day) on developing mice. The results revealed neurobehavioral defects in F1-C-Cin group, which were associated with Hg accumulation, increased NO(x) levels in whole blood, and Na(+)/K(+)-ATPase activities in brain tissues. F1- and F2-Cin-V groups were found to increase brain Hg contents and prominent neurobehavioral defects compared with F1-C-V group, suggesting that the fetal brain was more susceptible to irreversible effects for cinnabar-induced damage. Moreover, F1- and F2-Cin-Cin groups had severely neurobehavioral dysfunctions, closely correlated with the further alteration of NO(x) levels and Na(+)/K(+)-ATPase activities than F1- and F2-C-Cin groups. Effects in F2-Cin-Cin group were more significant than those in F1-Cin-Cin group. In conclusion, this study demonstrates that exposure to low-dose of cinnabar during the perinatal and developmental stages results in irreversible and severe injuries of the neurotoxicity in offspring, and NO(x) and Na(+)/K(+)-ATPase activities may exist potential and useful biomarkers for neurotoxicity-induced by low-doses of mercuric compounds.

The alterations in neurotransmitters and their metabolites in discrete brain regions in the rats after inhalation of disinfectant, glutaraldehyde or ortho-phthalaldehyde for 4 weeks

Glutaraldehyde (GA) and ortho-phtalaldehyde (OPA) have been widely used as major components of disinfectants in hospitals. We evaluated the alterations in GA or OPA in rats after subacute inhalation exposure by determining levels of neurotransmitters (norepinephrine [NE], dopamine [DA], DA metabolites, dihydroxyphenylacetic acid [DOPAC] and homovanillic acid [HVA], indoleamine serotonin [5-HT] and 5-HT metabolite, 5-hydroxyindoleacetic acid [5-HIAA]) in discrete brain regions using high performance liquid chromatography (HPLC) equipped with an electrochemical detector. Female Wistar rats were exposed to 0, 50, 100, or 200 ppb gaseous GA or OPA by inhalation for 1 h per day, 5 d per week for 4 wk. Following the exposure, the brain of each rat was removed and dissected into cerebrum, cerebellum, medulla oblongata, midbrain, corpus striatum and hypothalamus. The neurotransmitters and their metabolites were extracted from each brain region, and determined by HPLC. Regarding GA, the daily water intake of the 50 or the 200 ppb exposed groups was significantly lower than that of the control. DA and 5-HIAA levels in the medulla oblongata among the GA exposed groups were significantly lower than those of the control. For OPA, the mean final body weight and daily food intake of the 100 or 200 ppb exposed groups were significantly lower than those of the control. The mean DA concentrations in the cerebrum in the groups exposed to OPA were significantly lower than those of the control. OPA may modulate DA metabolism in the cerebrum of female rats. The levels GA or OPA that induced alienations in neurotransmitters were comparable to those levels usually found in hospitals, further studies are warranted to evaluate the of safety of disinfectants containing GA or OPA.

Methylmercury and nutrition: adult effects of fetal exposure in experimental models

Human exposure to the life-span developmental neurotoxicant, methylmercury (MeHg), is primarily via the consumption of fish or marine mammals. Fish are also excellent sources of important nutrients, including selenium and n-3 polyunsaturated fatty acids (PUFAs), such as docosahexaenoic acid (DHA). Laboratory models of developmental MeHg exposure can be employed to assess the roles of nutrients and MeHg and to identify potential mechanisms of action if the appropriate exposure measures are used. When maternal exposure is protracted, relationships between daily intake and brain mercury are consistent and orderly across species, even when large differences in blood:brain ratios exist. It is well established that low-level developmental MeHg produces sensory deficits. Recent studies also show that perseveration in reversal-learning tasks occurs after gestational exposures that produce low micromolar concentrations in the brain. A no-effect level has not been identified for this effect. These exposures do not affect the acquisition or performance of discrimination learning, set shifting (extradimensional shift), or memory. Reversal-learning deficits may be related to enhanced impact of reinforcers as measured using progressive ratio reinforcement schedules, an effect that could result in perseveration. Also reported is enhanced sensitivity to dopamine reuptake inhibitors and diminished sensitivity to pentobarbital, a GABA(A) agonist. Diets rich in PUFAs or selenium do not protect against MeHg's effects on reversal learning but, by themselves, may diminish variability in performance, enhance attention or psychomotor function and may confer some protection against age-related deficits in these areas. It is hypothesized that altered reward processing, dopamine and GABAergic neurotransmitter systems, and cortical regions associated with choice and perseveration are especially sensitive to developmental MeHg at low exposure levels. Human testing for MeHg's neurotoxicity should emphasize these behavioral domains.

Motor function following developmental exposure to PCBS and/or MEHG

Recent studies raise concern for combined exposure to polychlorinated biphenyls (PCBs) and methylmercury (MeHg), two environmental contaminants that are found in fish and seafood. Past accidental poisonings in humans show that exposure to high levels of either contaminant is associated with motor impairments, including alterations in cerebellar functions such as balance and coordination. Epidemiological studies of lower level exposures suggest some neuromotor impairment in exposed children, but the majority of these studies have focused on cognitive endpoints rather than examining a full-range of motor function. In particular, the cerebellum could be a sensitive target for combined PCB and MeHg toxicity. MeHg exposure during development damages the cerebellum along with cortical areas, and PCBs may also cause cerebellar damage via thyroid hormone disruption during development. In addition, in vitro studies report interactive effects of PCBs and MeHg on ryanodine-sensitive calcium signaling. Ryanodine receptors are found especially within the cerebellum, and alterations in calcium signaling within the cerebellum could impair long-term depression and subsequent motor learning. This article reviews the motor impairments reported in humans and laboratory animals following exposure to PCBs and/or MeHg during development. There is need for a better understanding of the interactive effects of PCBs and MeHg, especially in regard to motor function.

Studying Toxicants as Single Chemicals: Does this Strategy Adequately Identify Neurotoxic Risk?

Despite the fact that virtually all chemicals exposure of humans are to mixtures, and that these mixed exposures occur in the context of numerous other risk modifiers, our current understanding of human health risks is based almost entirely on the evaluation of chemicals studied in isolation. This paper describes findings from our collaborative studies that prompt questions about these approaches in the context of neurotoxicology. The first section describes studies investigating the interactions of maternal Pb exposure with maternal stress. Examined across a range of outcome measures, it shows that maternal Pb can modulate the effects of maternal stress, and, conversely, stress modifies the effects of Pb. Further, effects of Pb+stress could be detected in the absence of an effect of either risk factor alone, and, moreover, the profile of effects of Pb alone differs notably from that of Pb+stress. Collectively, interactions were not systematic, but differed by brain region, gender and outcome measure. A second section describes outcomes of studies examining combined exposures to the pesticides paraquat (PQ) and maneb (MB) during development which likewise reveal potentiated effects of combined exposures. They also demonstrate examples of both progressive and cumulative neurotoxicity, including a marked vulnerability following gestational exposure to MB, to the effects of PQ, a pesticide with no structural relationship to MB. The ability of current hazard identification and risk assessment approaches to adequately identify and encompass such effects remains an important unanswered question. One consideration proposed for further evaluating potential interactions that may be of significance for the nervous system is based on a multi-hit hypothesis. It hypothesizes that the brain may readily compensate for the effects of an individual chemical itself acting on a particular target system, but when multiple target or functional sites within that one system are attacked by different mechanisms (i.e., multiple chemical exposures or chemical exposures combined with other risk factors), homeostatic capabilities may be restricted, thereby leading to sustained or cumulative damage.

Autism: a novel form of mercury poisoning

Autism is a syndrome characterized by impairments in social relatedness and communication, repetitive behaviors, abnormal movements, and sensory dysfunction. Recent epidemiological studies suggest that autism may affect 1 in 150 US children. Exposure to mercury can cause immune, sensory, neurological, motor, and behavioral dysfunctions similar to traits defining or associated with autism, and the similarities extend to neuroanatomy, neurotransmitters, and biochemistry. Thimerosal, a preservative added to many vaccines, has become a major source of mercury in children who, within their first two years, may have received a quantity of mercury that exceeds safety guidelines. A review of medical literature and US government data suggests that: (i) many cases of idiopathic autism are induced by early mercury exposure from thimerosal; (ii) this type of autism represents an unrecognized mercurial syndrome; and (iii) genetic and non-genetic factors establish a predisposition whereby thimerosal's adverse effects occur only in some children.

Serotonin production in lymphocytes and mercury intolerance

Patients with suspected illness due to mercury in dental amalgam were classified as tolerant or intolerant depending on their psychosomatic responses following in vivo epicutaneous provocation with low doses (patch test doses) of metallic mercury and phenylmercuric acetate. Ten intolerant patients and nine tolerant patients plus seven healthy amalgam-free and metal non-allergic controls were recruited to the study. Peripheral blood lymphocytes were exposed in vitro to three concentration of mercuric chloride (0.92, 1.83 and 3.68 microM) with and without 10 microg phytohaemagglutinine (PHA)/ml and the release of serotonin into the supernatant was measured. Lymphocytes exposed only to HgCl(2) showed no significant dose-dependent increase of serotonin, but the response of the tolerant patients was significantly higher compared with the controls. No other differences were found. Co-culture with mercuric chloride and PHA showed a statistically significant dose-dependant release of serotonin, but no differences between the three clinical groups could be detected. Thus, our results could not validate the concept of mercury tolerance and intolerance.

Fumonisin B1-induced increases in neurotransmitter metabolite levels in different brain regions of BALB/c mice

Fumonisin B1, a toxin produced by Fusarium moniliforme, causes a variety of diseases in animals, including those involving the central nervous system, such as equine leukoencephalomalacia (ELEM). The changes of biogenic amines may reflect fumonisin B1 neurotoxicity. It was previously reported that consumption of feed contaminated with Fusarium moniliforme cultures produced an elevation of 5-hydroxyindoleacetic acid (5-HIAA), the major metabolite of 5-hydroxytryptamine (5-HT), in whole rat brains. In a subsequent study from the same laboratory, rats given fumonisin B1 orally for 4 weeks showed no changes in neurotransmitter levels of the whole brain. In the current study, groups of five male BALB/c mice were injected with fumonisin B1 subcutaneously at doses of 0, 0.25, 0.75, 2.25, 6.75 mg kg-1 body weight daily for 5 days. One day after the last treatment, their brains were dissected into cerebrum, cerebellum, medulla oblongata, midbrain, corpus striatum and hypothalamus. Levels of norepinephrine (NE), dopamine (DA), DA metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and 5-HT and 5-HIAA were determined. A significant elevation of HVA was observed in mice treated with high doses of fumonisin B1 in most brain regions. In striatum, a decrease of 5-HT was observed by the fumonisin B1 treatment. Ratios of neurotransmitters to metabolites such as HVA/DA and 5-HIAA/5-HT were elevated in several brain regions of the treated groups. An accumulation of neurotransmitter metabolites is suggestive of increased neuronal activity or interference with their efflux from cells.

In vitro and in vivo effects of HgCl2 on synaptosomal ATP diphosphohydrolase (EC 3.6.1.5) from cerebral cortex of developing rats

The aim of the present investigation was to evaluate the in vitro (10-500 microM) and in vivo (1-21 subcutaneous injections of 2.5 mg/kg each) effects of HgCl2 on the ATP diphosphohydrolase activity (EC 3.6.1.5; apyrase) of synaptosomes from cerebral cortex of rats at different ages (5, 11, 18 and 25 days of life). The in vitro results showed that HgCl2 (from 10 to 500 microM) inhibited the hydrolysis of both substrates by the synaptosomal enzyme at all ages studied. In contrast, HgCl2 injected in vivo did no affected the normal ontogeny of ATP and ADP hydrolysis. The hydrolysis of both nucleotides increased at the same rate as a function of age in control and HgCl2-treated rats (the specific activity of enzyme increased about 5-fold from the first week of postnatal life of weaning). The results of the present study demonstrated that in vitro HgCl2 inhibited the enzyme, but was ineffective when tested in vivo. Probably the absence of an in vivo effect is due to the low permeability of blood-brain barrier to inorganic forms of mercury.

Correlation of striatal fluorodopa uptake in the MPTP monkey with dopaminergic indices

Striatal 18F-6-fluorodopa (FD) uptake constants were measured by positron emission tomography in (1) normal cynomolgus monkeys and (2) a series of cynomolgus and rhesus monkeys that had received intracarotid infusions of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). After the animals were killed, the number and average size of dopaminergic neurons in the substantia nigra pars compacta were measured. Striatal levels of dopamine and its metabolites, and the striatal activities of the dopaminergic synthetic enzymes, were also determined. The striatal FD uptake constants showed highly significant positive correlations with both number and size of dopaminergic neurons, indicating atrophy of surviving neurons in MPTP-treated animals. The uptake constants also showed significant positive correlations with striatal levels of dopamine, total catecholamines, and the activities of the synthetic enzymes. Both histochemical and biochemical data on tyrosine hydroxylase suggested some contralateral enzyme loss in these MPTP-treated monkeys, as well as decreased enzyme activity in surviving neurons on the lesioned side. However, residual enzyme activities were apparently not rate limiting to striatal FD uptake. It is concluded that PET-FD measurements by positron emission tomography provide a good index of the integrity of the nigrostriatal pathway.

Human positron emission tomographic [18F]fluorodopa studies correlate with dopamine cell counts and levels

Postmortem counts of dopaminergic cell densities in the substantia nigra (5 subjects) and striatal levels of dopamine (DA) and its metabolites (6 subjects) were determined on 1 parkinsonian (PD), 3 progressive supranuclear palsy (PSP), 1 amyotrophic lateral sclerosis, and 1 Alzheimer's case who had been positron emission tomography scanned with 6-[18F]fluorodopa during life. [18F]Fluorodopa uptake rate constants, which presumably depend on the number of functioning striatal DA terminals, were strictly proportional to cell densities (significant correlation with zero intercept) and also correlated significantly with striatal DA levels but with an intercept indicating greater losses of DA than of terminals in PSP and PD. Postmortem data on 6 PD, 1 PSP, and 9 neuronally normal controls substantiated the significant correlation between cell counts and DA levels, with the latter being the more depressed in pathological cases.

Further observations on Tau-positive glia in the brains with progressive supranuclear palsy

The previously reported unusual, Tau-positive glia with astrocytic morphology seen in brain tissues from cases of progressive supranuclear palsy (PSP) were re-examined immunohistochemically using antibodies to CD44 and vimentin, as well as Alz-50. Four brains of PSP cases, one of whom had atypical clinical features, were examined. All four cases showed the unusual glia which were positive to Alz-50 and anti-CD44 antibodies, but negative to anti-vimentin antibody. Ultrastructurally, they had either paired nucleated or lobulated nuclei and the cytoplasm frequently contained lipofuscin pigment. The CD44 was located on the surface of the cell bodies and their processes. Such glia were most numerous in the striatum in all cases. They also appeared in the cortex and some subcortical nuclei in the three typical cases. They were not seen in the lower brain stem or cerebellum. In their morphological characteristics and regionally specific appearance, these unusual glia seemed similar to the Alzheimer type I glia which are commonly seen in hepatic encephalopathy or Wilson's disease.

Autoradiographic study on dopamine uptake sites and their correlation with dopamine levels and their striata from patients with Parkinson disease, Alzheimer disease, and neurologically normal controls

An autoradiographic study of labeled mazindol binding to presumed dopamine (DA) uptake sites in the striatum was done in 7 Parkinson (PD), 6 Alzheimer (AD), 1 Huntington disease (HD), and 4 neurologically normal cases. Large and significant decreases of specific binding were found in PD in both caudate (to 32% of control) and putamen (to 16%), with no significant effect in AD or HD. Nonspecific binding was a large proportion of total binding in all cases. In the 12 cases where both binding data and DA levels were available, they showed highly significant linear correlations in both caudate and putamen. Autoradiographic studies on D1, D2, and muscarinic binding sites in the PD, AD and control striata revealed no significant group differences.

Prediction of the volume of distribution of 7-hydroxycoumarin in man from in vitro and ex vivo data obtained in rat

The essential parameter to estimate the first dose size of a drug in man is the volume of distribution. For a drug that has never been used in man before, estimates of the volume of distribution can only be obtained from animals and in vitro data. The purpose of this study was to compare various approaches presented in the literature for predicting the volume of distribution at steady state (VSS) and the terminal phase volume of distribution (Vd beta) in man. A lipophilic active metabolite of coumarin, 7-hydroxycoumarin (7OHC), was selected for this investigation. This compound is extensively metabolized in both the central and peripheral compartments. Of the six methods evaluated, only an empirical allometric approach yielded a reasonable estimate of VSS. All methods underestimated VSS and none of the applicable methods were able to predict Vd beta. The reason for this discrepancy may be due to the fact that the calculation of VSS in man was done assuming elimination from the central compartment.

Some immunohistochemical features of argyrophilic grain dementia with normal cortical choline acetyltransferase levels but extensive subcortical pathology and markedly reduced dopamine

Detailed immunohistochemical and biochemical studies are reported on two cases of progressive dementia showing no Alzheimer-type pathology but extensive argyrophilic grains as described previously by Braak and Braak. These cases had no specific clinical features, and the pathology of these brains showed subcortical gliosis (proliferation of astrocytes and microglia) without significant neuronal losses. Interesting novel immunohistochemical findings were the profuse appearance of complement-activated oligodendrocytes and oligodendroglial microtubular masses. Their appearance seems to indicate oligodendroglial reactions to widespread damage of myelinated axons. Cortical levels of choline acetyltransferase were normal, but striatal levels of dopamine and its metabolites were markedly reduced. This disease may be consistent with the criteria for progressive subcortical gliosis.

Cytomegalovirus infection of the developing brain alters catecholamine and indoleamine metabolism

Tissue concentrations of noradrenaline (NA), serotonin (5-HT), dopamine (DA) and selected metabolites were measured in the spinal cord, cerebellum, cerebral cortex and caudate-putamen of developing mice following intraventricular inoculation with murine cytomegalovirus (MCMV) on postnatal day 10. MCMV-infected animals exhibited transient signs of neurological impairment, including apparent hypertonicity of hindlimb extensors and abnormal gait, beginning on days 14-16 and continuing for 3-5 days. At the onset of neurological impairment, tissue concentrations of NA were significantly reduced in the spinal cord (20%), cerebellum (32%) and cerebral cortex (40%) of infected animals. Levels of 5-HT were significantly increased in the caudate-putamen (50%), while 5-hydroxyindoleacetic acid (5-HIAA) was increased in both the spinal cord (94%) and caudate-putamen (65%). The ratio of 5-HIAA/5-HT, which is frequently used as an estimate of turnover of 5-HT, was significantly increased in the spinal cord (90%) at the onset of neurological impairment. In the caudate-putamen of MCMV-infected animals, there were significant increases in the tissue levels of DA (37%), homovanillic acid (HVA, 41%) and 3,4-dihydroxyphenylacetic acid (DOPAC, 34%). All neurochemical parameters were normal in the MCMV-infected animals by postnatal day 70, approximately 50 days after the resolution of neurological signs. These results indicate transient alterations in monoamine metabolism in the developing nervous system during the pathogenesis of cytomegalovirus-induced movement and postural disorders.

In vitro effect of organic and inorganic mercury on the serotonergic system

Interactions of organic (methyl mercury) and mineral (acetate, chloride, nitrate: Hg2+) mercury salts with the serotonergic systems were studied in vitro using synaptosomal fractions prepared from rat brain cortex preloaded with [3H]5-HT. In the absence of calcium, mercury salts induced a spontaneous release of the amine corresponding to a total depletion of the synaptosomal content. EC50 for the organic and mineral forms of the ion were 66 +/- 2 microM and 107 +/- 16 microM respectively. In the presence of calcium (2.4 mM) a similar pattern of release was observed except that an additional release was induced by mineral mercury at low concentration (EC50 = 8.4 +/- 1.3 microM) which corresponded to a maximal release of 18.6 +/- 3.5% of the synaptosomal content. Therefore, the latter release appears calcium-dependent. Parallelly, the effects of mercury salts have been examined on the binding of [3H]5-HT to its high affinity 5-HT1 sites; organic and mineral mercury inhibited the binding with IC50 of 27.8 +/- 3.2 microM respectively. These results show that mercury ions interact with the serotonergic system by different mechanisms depending on their concentrations and their ionic forms.

Methylmercury-induced movement and postural disorders in developing rat: high-affinity uptake of choline, glutamate, and gamma-aminobutyric acid in the cerebral cortex and caudate-putamen

Subcutaneous administration of methylmercuric chloride to neonatal rats resulted in movement and postural disorders during the fourth postnatal week. Sodium-dependent high-affinity uptake of radiolabeled choline, glutamate, and gamma-aminobutyric acid (GABA) was measured in homogenates of cerebral cortex and caudate-putamen. There was a significant decrease in the uptake of [3H]choline in the cerebral cortex, but not in the caudate-putamen, at the onset of neurological impairment (73-75%) and at one subclinical stage of toxicity (58-64%). No significant differences in [3H]glutamate uptake were detected in either region. The uptake of [3H]GABA in the presence of 1 mM beta-alanine, which was employed to inhibit the glial uptake process, was reduced significantly in both the cerebral cortex and caudate-putamen at the onset of neurological impairment (50-62%) and at one subclinical stage (40-51%). This decrease in [3H]GABA uptake is consistent with the results of previous studies using this animal model, which demonstrated a preferential degeneration of GABAergic neurons in the cerebral cortex and caudate-putamen of methylmercury-treated animals. Because the high-affinity uptake of choline is the rate-limiting step for acetylcholine synthesis by cholinergic neurons, the decrease in [3H]choline uptake may reflect an abnormal development of cholinergic innervation of the cerebral cortex.

Increased cytochrome oxidase activity of mesencephalic neurons in developing rats displaying methylmercury-induced movement and postural disorders

Subcutaneous administration of the neurotoxin methylmercuric chloride to developing rats produced movement and postural disorders during the 4th postnatal week. Cytochrome oxidase histochemistry revealed an increase in the oxidative metabolic activity of small neurons within the magnocellular red nucleus (RMC) and the interrubral mesencephalon. A concurrent suppression of cytochrome oxidase activity in the large neurons and neuropil of RMC was apparent relative to controls. Decortication on postnatal day 3 did not alter the course of motor impairment or the cytochrome oxidase histopathology, suggesting that the role of neocortex in the pathogenesis of methylmercury-induced movement and postural disorders is minimal.