Showing papers in "Neurotoxicology in 1998"

Imbalances of trace elements related to oxidative damage in Alzheimer's disease brain.

Abstract: Four elements that have been implicated in free-radical-induced oxidative stress in Alzheimer's disease (AD) were measured by instrumental neutron activation analysis (INAA) in seven brain regions from 58 AD patients and 21 control subjects A statistically significant elevation of iron and zinc was observed in multiple regions of AD brain, compared with controls Mercury was elevated in AD in most regions studied, but the high variability of mercury levels in both AD and control subjects prevented the AD-control difference from reaching significance Selenium, a protective agent against mercury toxicity, was significantly elevated only in AD amygdala The elevation of iron and zinc in AD brain has the potential ofaugmenting neuron degeneration through free radical processes

Toxicokinetics and biochemistry of cadmium with special emphasis on the role of metallothionein.

Abstract: Cadmium is a toxic metal with extremely long biological half-time of 15-20 years in humans. It has for decades been known that cadmium exposure can cause a variety of adverse health effects, among which kidney dysfunction, lung diseases, disturbed calcium metabolism and bone effects are most prominent. Following long term exposure the kidney is the critical organ. Cadmium and its compounds give rise to lung cancer after inhalation and have been classified as human carcinogens. Metallothionein (MT) is a low-molecular -weight protein, 6500Da with high cysteine content and high metal affinity, which plays a major role in the kinetics and metabolism of cadmium. The balance between CdMT and non-bound Cd in renal tissue has been shown to be of crucial importance for expression of toxicity. The most well studied metallothioneins are metallothioneins I and II with their isoforms which are expressed in almost all mammalian tissues. Metallothionein III is expressed in brain and is rich in zinc. Since the blood-brain barrier keeps Cd outside the CNS, reported neurotoxic effects of Cd during development are likely to be secondary to an interference of Cd with Zn-metabolism and not a direct effect of Cd on brain cells. It is therefore of importance to investigate whether neurotoxicity induced by cadmium is related to mechanisms involving MT III in brain.

Ion channels as targets for insecticides

Abstract: Most insecticides are neurotoxicants causing various forms of hyperexcitation and paralysis in animals. A variety of neuroreceptors and ion channels have been identified as the major target sites of these neurotoxic insecticides. This paper gives the highlights of some of the recent development in this area. Pyrethroids keep the sodium channel open for unusually long times causing a prolonged flow of sodium current. The prolonged sodium current elevates and prolongs the depolarizing after-potential which reaches the threshold membrane potential to initiate repetitive after-discharges. We have developed the method with which the percentage of sodium channel population that needs to be modified to cause repetitive after-discharges can be measured accurately. In rat cerebellar Purkinje neurons, only 0.6% of sodium channels needs to be modified for hyperexcitation resulting in a large toxicity amplification. This concept is applicable to other neuroactive drugs that act through the threshold phenomenon. 'The mechanisms of selective toxicity of pyrethroids in mammals and insects have been quantitatively determined to be due mainly to the different sensitivity of the sodium channels to pyrethroids and the negative temperature dependence of pyrethroid action on the sodium channels. The degradation of pyrethroids play only a minor role. The negative temperature dependence of pyrethroid action is due to the increased sodium current flow at low temperature. The major site of action of dieldrin and hexachlorocyclohexane is the GABA A receptor chloride channel complex. Dieldrin exerts a dual action, initial stimulation and subsequent suppression, and the latter is responsible for hyperexcitation of animals. Dieldrin stimulation requires the γ2s subunit in the GABA receptor, whereas dieldrin suppression occurs in the presence or absence of the γ2s subunit.

An integrated hypothesis for the serotonergic axonal loss induced by 3,4-methylenedioxymethamphetamine.

Abstract: Administration of the street drug 3,4-methylenedioxymethamphetamine (MDMA) to various experimental animals has been shown in several laboratories to induce selective damage to serotonergic axons and axon terminals This review examines the current available evidence supporting the development of serotonin (5-HT) neurotoxicity in animals and humans There are a plethora of hypotheses that attempt to explain the mechanisms involved in the development of this serotonergic neurotoxicity An integrated hypothesis incorporating most of the speculated neurotransmitters theorized to be involved in the process is proposed This hypothesis states that MDMA induces the following sequence of events resulting in the serotonergic neurotoxicity: 1 MDMA induces an acute release of 5-HT and dopamine (DA) 2 This acute release is followed by depletion of intraneuronal 5-HT stores 3 The initially released 5-HT activates post-synaptic 5-HT2A/2C receptors located on GABA interneurons resulting in a decrease in GABAegic transmission and increased DA release and synthesis 4 The excessive DA released then may be transported into the depleted 5-HT terminal 5 The DA is then deaminated by monoamine oxidase B (MAO-B) located within the 5-HT terminal This results in free-radical formation and the selective degeneration of the serotonergic axons and axon terminals While there is no clear evidence that human users of the drug are suffering a similar neurotoxicity, data are presented suggesting that there remains cause for concern

Astrocytes as mediators of immune and inflammatory responses in the CNS.

Abstract: The long-standing view that the brain is isolated from the effects of the immune system has recently been challenged, with experimental evidence suggesting that in response to invasion by microorganisms, the CNS can mount its own defense by resident cells, such as the microglia and astrocytes. Both cell types produce and secrete a number of cytokines and therefore can potentially modulate and integrate the communication between hematogenous cells and resident cells of the CNS. This manuscript will commence with a brief overview of astrocytic functions in the CNS, and proceed to discuss astrocytic responses that may regulate CNS inflammation. Specifically, it will address (1) the function of astrocytes as the antigen presenting cells (APCs) of the CNS, and (2) the role afforded by astrocyte-derived cytokines, and astrocytic responses to cytokines secreted elsewhere, in mediating and sustaining immune responses. Finally, some recent experimental evidence on the possibility that astroglial impairment by pathogens may contribute to the etiology of neurologic diseases will be highlighted.

The neurotoxicity of polychlorinated biphenyls

Abstract: Like dioxin, some polychlorinated biphenyl (PCB) congeners produce toxicity by binding to an aryl hydrocarbon (Ah) receptor. Other PCB congeners that have little or no activity at the Ah receptor have been shown to accumulate in the brain following in vivo exposure and decrease dopamine content. Subsequent research has found that non-dioxin-like PCBs also interfere with calcium homeostatic mechanisms and intracellular second messenger systems in vitro in neuronal cultures and brain subcellular fractions. The biological significance of these effects of PCBs in nervous system preparations is not known, although a number of calcium-dependent processes are important for nervous system function and development. Structure-activity relationship (SAR) studies based on measures of PCB-induced alterations in protein kinase C (PKC) translocation and Ca2+-buffering indicate that congeners with chlorine substitutions at the ortho-position are active in vitro, while non-ortho congeners are relatively inactive. Subsequent research has found that chloride substitution patterns that favor non-co-planarity are associated with activity in nervous system preparations. Recent in vivo studies in adults have shown that repeated exposure to a PCB mixture Aroclor 1254 increases translocation of PKC and decreases Ca2+-buffering in the brain. Increased levels of ortho-substituted non-coplanar PCB congeners were observed in the brains of Aroclor 1254-treated animals relative to vehicle controls. Current research is focusing on the possibility that PCB-induced alterations in calcium homeostasis and intracellular second messengers may be related to the developmental neurotoxicity of PCBs.

A case-control study of occupational and environmental risk factors for Parkinson's disease in the Emilia-Romagna region of Italy.

Abstract: A questionnaire-based case-control study was carried out on 86 patients with neurologist-confirmed idiopathic Parkinson's disease (PD) and 86 controls similar in sex and age The control group was recruited in outpatient specialist centers of the same University Hospital (glaucoma, psoriasis vulgaris, essential arterial hypertension and renal diseases) Exposure was defined as occupational or residential contact with a given factor for at least 10 consecutive years prior to the onset of PD Smoking habits were defined by exclusion of those subjects who never smoked The following risk factors were identified: cranial trauma (OR: 288; 95% CI: 098-849), well water use (OR: 278; 95% CI: 146-528) and occupational exposure to industrial chemicals (OR: 213; 95% CI: 116-391) Among industrial chemicals, only organic solvents were identified as significant risk factors for PD (OR : 278, 95% CI : 123-626) Whereas no exposure to neurotoxic metals occurred among controls, making the assessment of the OR impossible, exposure pesticides and herbicides was similar in the two groups (OR : 115; 95% C : 056-2-36) Smoking habits was negatively associated with PD (OR: 041; 95% CI: 022-075), confirming the "protective" role of tobacco smoking suggested by many studies As a whole, these results support the role of environmental factors in the etiology of PD

Neural lesions in the rat and their relationship to EEG delta activity following seizures induced by the nerve agent soman.

Abstract: This study describes the neural structures damaged following exposure to the nerve agent soman, shows there are time-dependent differences in the extent of damage in certain structures, and relates seizure-induced increases in delta band (0-3.5 Hz) electroencephalographic (EEG) activity with severity of subsequent neuropathology. Rats, instrumented to record cortical EEG activity, were pretreated with the oxime HI-6 (125 mg/kg, ip) and then challenged with soman (180 ug/kg, sc). All animals developed continuous epileptiform seizures that lasted in excess of 4 hr. Groups of animals were perfused 1, 3, 10 or 30 days following exposure. Paraffin-embedded brains were stained with hematoxylin and eosin; thirty-four neural structures were examined and scored for neural damage. All cortical areas sustained damage, with piriform and perirhinal cortices exhibiting the most severe. Subcortical limbic areas (amygdala, amygdala-piriform transition zone, hippocampus, claustrum) and various thalamic nuclei were most consistently and severely damaged in all animals regardless ofsurvival time. Brainstem structures, cerebellum, spinal cord, and other motor output nuclei were never damaged. It was found that some structures were rated as more severely damaged when evaluated at shorter survival times. Severity of neural damage was related to high levels of EEG delta power recorded 24 hr after exposure; power during the acute seizure or 24 hr body weight loss did not predict lesion severity. Sections between A-P-0.8 to -4.8 contain cortical and subcortical structures that can be readily and reproducibly evaluated for brain damage.

In vitro effect of the cysteine metabolites homocysteic acid, homocysteine and cysteic acid upon human neuronal cell lines.

Abstract: Cysteine (CYS) is a non-essential amino acid which elicits excitotoxic properties via the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor. CYS levels are known to be elevated in association with neurological disease such as Alzheimers Disease (AD) and Parkinsons Disease (PD). We have previously reported studies investigating the toxicity of CYS and its major metabolite cysteinesulfinic acid (CSA) to human neuronal cell lines in vitro and in continuation of this we now report the toxicity of other compounds (Homocysteic Acid, HCA; Homocysteine, HCYS; and Cysteic Acid, CA) in the CYS metabolic pathway. Three cell lines, all of human origin and derived from separate discrete areas of the brain were used in the neurotoxicity assays. Lactate dehydrogenase (LDH) release was assayed as a measure of cell death. The cell lines investigated showed varying degrees of toxic responses which were the reverse of those seen when they were exposed to CYS or CSA. The SK.N.SH (Neuroblastoma) cell line, which exhibits a high toxic response to CYS and CSA, gave a low toxic response to HCA and CA while the TE 671 (Medulloblastoma) cell line, which exhibits a low toxic response to CYS and CSA, showed a high toxic response to HCYS, HCA and CA. However, the U-87 MG (Glioblastoma) cell line, which has a median toxic response to CYS and CSA, also has median response to HCYS, HCA and CA. These results show that toxic responses are cell-type specific for CYS and its metabolites and this may be reflected in the patterns of neurodegeneration observed in such diseases as AD and PD. HCYS is selectively toxic to medulloblastoma cells; this may explain why high HCYS levels result in neural tube defects in prenatal humans, where the same cell-type is involved.

Astrocytic functions and physiological reactions to injury: the potential to induce and/or exacerbate neuronal dysfunction--a forum position paper.

Abstract: This forum highlights the wide diversity of astrocytic functions which maintain CNS homeostasis, well beyond the originally proposed passive cytoskeletal support role for these cells Astrocytic potential in modulating damage and repair is also reflected in this forum While astrocytes may potentially play a primary role in epilepsy, and in neurodegenerative disorders such as Huntington's diseases, HIV, and demyelination, one needs to keep in mind that to a large extent evidence supporting involvement of astrocytes in these diseases is derived from in vitro studies Observations on regional heterogeneity and functional specialization of astrocytes also suggest that astrocytes have adapted to perform functions specific to their respective residence site Therefore, it is necessary to identify potentially damaging consequences of astrocytic functions in vivo, although these analyses will be undoubtedly extremely complex Expanded investigations on astrocytic involvement in neurotoxicity and neurodegeneration is clearly warranted, and as new experimental tools are developed it is likely that further strides will be made in our understanding of astrocyte functions, both in health and disease

Neurotoxicity of ammonia and glutamate: molecular mechanisms and prevention.

Abstract: Ammonia is a main factor in the pathogenesis of hepatic encephalopathy We found that acute ammonia toxicity is mediated by activation of NMDA receptors Chronic moderate hyperammonemia prevents acute ammonia toxicity in rats Chronic exposure of cultured neurons to 1 mM ammonia leads to impaired response of the NMDA receptor to activation by its agonists (due to decreased protein kinase C-mediated phosphorylation) and prevents glutamate (Glu) neurotoxicity Compounds that prevent ammonia toxicity in mice (eg carnitine) also prevent Glu toxicity in cultured neurons These compounds did not prevent activation of NMDA receptor or the rise of Ca2+ They interfered with subsequent steps in the toxic process The protective effect of carnitine is mediated by activation of metabotropic Glu receptors Agonists of mGluRs, especially of mGluR5, prevent Glu toxicity Agonists of muscarinic receptors also prevent Glu toxicity and there seems to be an interplay between muscarinic and metabotropic Glu receptors in the protective effect We have tried to identify intracellular events involved in the process of neuronal death It is known that the rise of Ca2+ is an essential step Glu leads to depletion of ATP; some compounds (eg carnitine) prevent Glu-induced neuronal death without preventing ATP depletion: additional events are required for neuronal death Glu induces activation of Na+/K+-ATPase, which could be involved in the toxic process Inhibitors of protein kinase C, calcineurin or nitric oxide synthase prevent Glu toxicity Our results indicate that Glu toxicity can be prevented at different steps or by activating receptors coupled to the transduction pathways interfering with the toxic process Agents acting on these steps could prevent excitotoxicity in vivo in animals

Blood mercury and auditory neuro-sensory responses in children and adults in the Nambija gold mining area of Ecuador.

Abstract: This study investigated blood mercury (B-Hg) levels and the auditory neuro-sensory status of children and adults in the remote Andean settlement of Nambija, Ecuador where Hg is used in the extensive gold mining operations. The mean B-Hg level in 75 Nambija (Study Area) inhabitants (36 children and 39 adults) was 17.5 micrograms/L (SD = 11.0) vs 3.0 micrograms/L (SD = 1.6) in a second group of 34 subjects (15 children and 19 adults) in a non-gold mining area (Reference Area), the difference being statistically significant (p < 0.0001). Neuro-otological examinations revealed 34 subjects (45%) with complaints of headaches and/or memory loss, 3 cases of severe neurological impairment and 4 cases of middle ear pathology. Audiological tests on 40 persons in the Study Area (21 children and 19 adults) showed hearing thresholds ranging from normal to mildly abnormal at 2, 3, 4, 6 and 8 kHz for children, and normal to severely abnormal for adults. Correlation coefficients showed a significant relationship between B-Hg level and hearing level in children at 3 kHz in the right ear, and at no frequency for adults. Auditory brainstem evoked responses (ABR) on subjects in the Study Area showed a significant correlation between B-Hg and the I-III interpeak latency on the right side. The results indicated that the study population of the Nambija gold mining area had abnormally elevated B-Hg levels, and may be at neurological risk from exposure to methylmercury (MeHg) from the consumption of contaminated food and possibly from elemental Hg vapors inhaled during amalgam burning in the gold extraction process.

Heat stress, even extreme, does not induce penetration of pyridostigmine into the brain of guinea pigs.

Abstract: Stress due to forced swimming was recently shown to allow penetration of pyridostigmine (PYR) into the brain of mice. Accordingly, it was suggested that in troops exposed to emotional stress under conditions of war, as during the Gulf War, the BBB may have unexpectedly become permeable to PYR thus leading to an increased frequency of CNS symptoms. In this study, the entry of PYR into the brain was investigated in guinea pigs subjected to different heat stress levels. In a first group, guinea pigs were maintained at room temperature for 2 hours, their core temperature remaining stable at about 39.8 degrees C. In a second group, animals were placed in a climatic chamber in order to keep their core temperature at 41.5 degrees C for 2 hours. In a third group, animals were subjected to a high ambient temperature (42.6 degrees C) during about 2 hours and developed heatstroke symptoms, their core temperature progressively increasing and reaching around 44.3 degrees C. In each group, the stress of the animals was assessed by measuring the increase of plasma cortisol level. PYR (0.2 mg/kg, s.c.) was injected 90 minutes after beginning the experiment. Penetration of the drug into the brain was examined by measurement of acetylcholinesterase (AChE) activity in the cortex, the striatum and the hippocampus of the animals 30 minutes after PYR administration. A passage of this drug into the brain was also evaluated autoradiographically after i.v. injection of tritiated PYR 90 minutes after the beginning of the experiment (100 microCi/animal). Whatever the group examined, no entry of PYR into the CNS could be detected. Exposure to an ambient temperature at 42.6 degrees C for 2 hours resulted by itself in a partial inhibition of cerebral AChE activity. Our results, which agree with previous data obtained in humans exposed to heat stress, are opposite to the recent research showing a central passage of PYR in mice following a forced swim stress test. This demonstrated that the penetration of PYR into the brain of rodents under stress depends on the experimental conditions used (animal species, nature of the stressor, etc.). Extrapolations to humans of results primarily obtained in rodents about central passage of a drug under stress must thus be done very carefully.

Action of the pyrethroid insecticide cypermethrin on rat brain IIa sodium channels expressed in xenopus oocytes.

Abstract: Pyrethroid insecticides bind to a unique site on voltage-dependent sodium channels and prolong sodium currents, leading to repetitive bursts of action potentials or use-dependent nerve block. To further characterize the site and mode of action of pyrethroids on sodium channels, we injected synthetic mRNA encoding the rat brain IIa sodium channel alpha subunit, either alone or in combination with synthetic mRNA encoding the rat sodium channel beta1 subunit, into oocytes of the frog Xenopus laevis and assessed the actions of the pyrethroid insecticide [1R,cis,alphaS]-cypermethrin on expressed sodium currents by two-electrode voltage clamp. In oocytes expressing only the rat brain IIa alpha subunit, cypermethrin produced a slowly-decaying sodium tail current following a depolarizing pulse. In parallel experiments using oocytes expressing the rat brain IIa alpha subunit in combination with the rat beta1 subunit, cypermethrin produced qualitatively similar tail currents following a depolarizing pulse and also induced a sustained component of the sodium current measured during a step depolarization of the oocyte membrane. The voltage dependence of activation and steady-state inactivation of the cypermethrin-dependent sustained current were identical to those of the peak transient sodium current measured in the absence of cypermethrin. Concentration-response curves obtained using normalized tail current amplitude as an index of the extent of sodium channel modification by cypermethrin revealed that coexpression of the rat brain IIa alpha subunit with the rat beta1 subunit increased the apparent affinity of the sodium channel binding site for cypermethrin by more than 20-fold. These results confirm that the pyrethroid binding site is intrinsic to the sodium channel alpha subunit and demonstrate that coexpression of the rat brain IIa alpha subunit with the rat beta1 subunit alters the apparent affinity of this site for pyrethroids.

Prosaptide prevents paclitaxel neurotoxicity

Abstract: Paclitaxel (Taxol), a chemotherapeutic agent used to treat breast and ovarian tumors, has been reported to induce a predominantly sensory neuropathy. Co-treatment with neurotrophic factors and paclitaxel has been proposed for preventing or reversing paclitaxel-induced peripheral neuropathy. Prosaposin, the precursor of saposins A, B, C and D was recently identified as a neurotrophic factor and was reported to facilitate nerve regeneration in vivo. Peptides (prosaptides) encompassing the neurotrophic sequence located in the saposin C domain, have neurotrophic activity similar to the holoprotein (O'Brien et al. 1995). In the present study, we investigated the effect of a 14-mer prosaptide, TX14(A), or a 22-mer prosaptide, 769P, on paclitaxel-induced neutrotoxicity in vitro and in vivo. Paclitaxel treatment (1 microM) decreased cell viability of both PC12 and Schwann cells. TX14(A) (10 nM) prevented paclitaxel-induced loss of cell viability in PC12 cells but not in Schwann cells. Systemic injections (i.p.) of paclitaxel (1.2 mg/kg/day) given five times per week for three weeks (cumulative dose 18 mg/kg) or given every third day (25, 12.5 and 12.5 mg/kg) for 10 ten days (cumulative dose 50 mg/kg) in adult rats induced thermal hypoalgesia that was not accompanied by morphological changes in the sciatic nerve or changes of nerve conduction velocity. Co-administration of paclitaxel with prosaptides (cumulative dose 3 or 10 mg/kg) prevented paclitaxel-induced thermal hypoalgesia. In the short-term high dose study, paclitaxel treated rats lost 10% of their body weight, had reduced erythrocyte counts, hematocrit and hemoglobin concentrations which were not prevented by treatment with prosaptide. TX14(A) did not diminish paclitaxel cytotoxicity of breast cancer cells in vitro. These findings suggest that prosaptide prevents the neurotoxic effects of paclitaxel while not interfering with its anti-tumor activity.

Mechanisms underlying the aluminum-induced potentiation of the pro-oxidant properties of transition metals.

Abstract: Aluminum is known to enhance the ability of iron to promote the generation of reactive oxygen species (ROS) but the mechanism subserving this is unknown. In an attempt to understand the means by which this potentiation occurs, several types of experiment have been conducted. It was found that iron must be in the ferrous form for aluminum-based stimulation of ROS to take place in a cerebral cortical synaptosomal-mitochondrial fraction. The ability of other transition metals of varying valences, copper and chromium, to catalyze formation of ROS was also increased in the presence of aluminum. Catalase but not superoxide dismutase blocked such stimulation suggesting hydrogen peroxide as an intermediate. The formation of aluminosilicates in the presence of brain tissue did not enhance iron-stimulated ROS formation. Furthermore, configurational changes of proteins which have been proposed to account for this phenomenon, do not appear to be a key element since iron-aluminum potentiation could be observed using protein-free liposomal micelles bearing an external negative charge.

Metallothionein (MT) isoforms in the central nervous system (CNS): regional and cell-specific distribution and potential functions as an antioxidant.

Abstract: Similar to the "housekeeping" functions ascribed to MTs in other tissues, central nervous system (CNS) metallothioneins (MTs) are implicated in metal metabolism, cellular repair processes, growth and differentiation, where they are likely to serve as the source of zinc for newly synthesized apoenzymes, as well as regulator molecules in gene expression. Additional likely functions of MTs include control of intracellular redox potential, and metal detoxification. This manuscript will focus on (1) the distribution of MT isoforms within the CNS, with particular emphasis on the cell-specific localization of MTs and its significance, (2) emerging accounts on the function of MT as an antioxidant, and (3) the relationship between MT and ethanol (EtOH)-induced neurotoxicity in a model system of neonatal rat primary astrocyte cultures.

Reversal by desferrioxamine of tau protein aggregates following two days of treatment in aluminum-induced neurofibrillary degeneration in rabbit: implications for clinical trials in Alzheimer's disease.

Abstract: A clinical trial in patients with Alzheimer's disease has indicated that frequent intramuscular (i.m.) treatment with desferrioxamine (DFO) slows progression of the disease. Confirmatory trials have not been carried out, partly because of the rigors of twice daily intramuscular injections over a period of 2 years, even though the initial report gave promising results. The aim of the present study was to determine an optimal DFO treatment protocol in an animal model exhibiting Alzheimer's-like intraneuronal protein aggregates, previously shown to be partially reversed by such treatment. New Zealand white rabbits were injected intracisternally with either aluminum (Al) maltolate or with saline on day 0. Intramuscular injections of DFO were given to selected rabbits for 2 days prior to sacrifice on days 4, 6 or 8. Bielschowsky's silver impregnation demonstrated widespread neurofibrillary degeneration (NFD) in neuronal cell bodies and neurites of brain and spinal cord from Al-treated rabbits. Monoclonal antibodies Tau-2, AT8, PHF-1 and Alz-50, all of which characteristically stain neurofibrillary tangles associated with Alzheimer's disease, strongly labeled the Al-induced NFD. The number of positive neurons and staining intensities were much less in rabbits treated with Al and subsequently with DFO, than in animals only given Al. Control rabbit receiving intracisternal saline were negative for NFD. The results of quantitative immunohistochemistry using image analysis confirmed that immunostaining densities with all tau mAbs were higher in Al-treated than in Al-DFO-treated or in saline-treated controls. Furthermore, it appears that hyperphosphorylation of tau does not make this protein resistant to degradation once Al has been removed by DFO treatment. The effectiveness of only two days of DFO treatment in reversing Al-induced neurofibrillary degeneration suggests that further clinical trials of DFO for treatment of Alzheimer's disease should be attempted using much less frequent administration of DFO than in the initial study (Crapper McLachlan et al., 1991).

Organophosphate-induced acetylcholinesterase inhibition and embryonic retinal cell necrosis in vivo in the teleost (Oryzias latipes).

Abstract: Recent monitoring of the Sacramento- San Joaquin River system (CA) indicates that levels of the organophosphate pesticide, diazinon, exceed National Academy of Science guidelines and these levels result in toxicity in USEPA acute toxicity tests with Ceriodaphnia dubia. Since organophosphates (OPs) inhibit acetylcholinesterase (AChE), the present study examined the effects of diazinon on the embryonic nervous system of model teleost, medaka, Oryzias latipes. Preliminary histological screens revealed limited retinal cell necrosis in control embryos with apparent increased necrosis in diazinon - exposed embryos. Subsequently, embryos were exposed to 1.8 x 10 -5 , 4.4 x 10 -5 , or to 8.8 x 10 -5 M diazinon and replicates were frozen for biochemical analysis or were fixed for histopathological analysis at days 3, 5, and 7 of development. Diazinon exposure significantly inhibited AChE activity within whole embryos and in homogenates of retinas from treated animals. Histological examination ofembryos indicated that as the retina underwent differentiation into distinct cell layers, between days 5 and 7, small foci of necrotic cells became apparent within the inner nuclear layer and isolated individual pyknotic cells were observed in the ganglion layer. Quantification of foci of necrotic cells revealed that 8.8 x 10 -5 M diazinon increased number and area of these lesions. Enzyme histochemistry localized AChE activity to regions equivalent to sites of necrosis. Separate exposures of embryos to the OP, diisopropylphosphorofluoridate, produced large foci of necrotic cells at sites equivalent to those seen following diazinon exposure.

A review of methylmercury and child development.

Abstract: Methylmercury (MeHg) is a neurotoxin present in both fresh and saltwater fish throughout the world Increased levels of MeHg can be found in individuals who regularly consume fish The developing brain is very sensitive to the deleterious effects of MeHg, and prenatal exposure can occur when the mother has a diet high in fish If the level of MeHg exposure achieved by eating fish adversely affects the fetus or child's neurological development it could have far reaching public health implications Studies of human prenatal MeHg poisoning in Iraq suggest that MeHg levels achieved by eating fish may affect neurological development even when the fish MeHg levels are not elevated by obvious pollution Studies in fish eating populations have identified adverse neurological and developmental outcomes, but these findings have not been consistent Additional studies are presently underway to determine whether consistent adverse outcomes can be identified using more sensitive testing methods and examining children older than in previous studies This review examines studies of human prenatal and postnatal MeHg exposure Studies of poisoning episodes where children are symptomatic and studies of fish eating populations where no symptoms are apparent will be addressed Individuals around the world depend on fish as a protein source and increasing evidence suggests that regular fish consumption has cardiovascular benefits It is not presently clear whether MeHg exposures from a high fish diet adversely affect children's neurological development, but it is an important question to answer

Preliminary evidence of neurotoxicity associated with eating fish from the Upper St. Lawrence River Lakes.

Abstract: Pollution of hydrographic basins has affected the flora and fauna that thrive in these aquatic ecosystems, and fish, which constitute an important food resource, often contain a plethora of potentially toxic chemicals. In a major research project on early neurotoxic effects ofenvironmental exposure to manganese among residents in Southwest Quebec, fish consumption from 2 lakes of the Upper St. Lawrence River System, was surveyed as a potential confounding factor. Participants were selected using a random, stratified sampling strategy from lists of the Quebec Health Plan. Following exclusions, 273 men and women between 20-69 years were retained for the present analysis. A total of 103 (37.7%) reported eating fish from the Upper St. Lawrence. Although fisheaters and non-fisheaters were similar for most socio-demographic variables, significantly more fisheaters (65.2%) reported consuming alcoholic beverages as compared to non-fisheaters (42.4%) (Chi Sq. <0.01). To eliminate this possible bias, fisheaters were matched to non-fisheaters for the variables sex, alcohol consumption (never or occasionally vs. regularly), age (±5y) and education (±2y). A total of 63 matched pairs were thus created. Paired analyses (t-test or Signed Rank) showed that fisheaters had higher levels of blood organic mercury and lead. Analysis of nervous system functions revealed that both groups performed similarly on tests of sensory function, visual memory and recognition, fine motor performance and some motor tests, but fisheaters performed significantly more poorly (p<0.05) on tests requiring cognitive flexibility, word naming, auditory recall, and more complex motor tasks. The profile of deficits is consistent with diminished capacity for information processing. These observations were made within a study that was not specifically designed to examine the effects of fish eating from these two lakes, and the characterization of fish dietary habits has many limitations. Nevertheless, the findings are sufficiently compelling to warrant further studies, since fish from the Upper St. Lawrence Lakes are known to contain multiple neurotoxic substances.

Entry of low doses of mercury vapor into the nervous system.

Abstract: Inorganic mercury remains within neurons indefinitely and has been implicated in some human neurodegenerative diseases. We were interested in finding the lowest dose of mercury vapor that resulted in mercury deposition in neurons. Female BALB/c mice were exposed to 25 micrograms mercury/m3 for 2-20 hr or 500 micrograms mercury/m3 for 5-240 min. To see if female mouse neurons were more susceptible to mercury vapor than male neurons, male and female BALB/c mice were exposed to 50 micrograms mercury/m3 for 4-24 hr. Mice were perfused with formalin 1-30 weeks after exposure and paraffin sections of brain, spinal cord and kidney were stained for mercury with silver nitrate autometallography. On light microscopy, spinal motor neurons contained mercury granules after 12 hr exposure to 25 micrograms mercury/m3 or after 30 min exposure to 500 micrograms mercury/m3. Mercury remained in motor neurons 30 weeks after exposure. In female mice, mercury was seen in motor neurons at half the exposure times of male mice. In conclusion, low doses of mercury vapor, well within WHO guidelines for safe human occupational exposure, enter and remain within motor neurons of mice.

Genetic polymorphisms in Parkinson's disease.

Abstract: The search for genetic polymorphisms relevant to Parkinson's disease etiology and pathogenesis has been motivated by recent thinking emphasizing the potential significance of gene-environment interactions. Especially influential to this research have been the MPTP model of PD induction, hypotheses concerning oxidative stressor reactions, and epidemiological observations of an inverse relation between cigarette smoking and PD risk. This brief review summarizes trends in genetic polymorphism research, with examples provided by investigations of cytochrome P450 enzymes, monoamine oxidase, superoxide dismutase, and mitochondrial genes.

Developmental neurotoxicity and immunotoxicity of 2,4,6-Tribromophenol in wistar rats

Abstract: Pregnant Wistar rats were exposed to 2,4,6-Tribromophenol (TBP) by whole body inhalation (0, 0.03, 0.1, 0.3, 1.0 mg/m3, 24 hr/day, 7 days/weeks, from day 1 to 21 of gestation). Significant decreases in orientation reactions were noted at concentrations of 1.0 mg/m3 (p 0.05) toward decreased horizontal movement and emotionality in the open field and increased electrical impulse skin pain threshold (SPT) were observed. No significant exposure-related differences in the nonspecific immunological status (phagocytosis and blood anti-microbe activity) of pregnant rats were seen after the exposure. Preimplantation and postimplantation embryo losses were significantly increased in a dose-dependent manner and were seen in all treated groups except the lowest concentration (0.03 mg/m3) group. Signs of retarded fetal skeletal development and increased frequencies of visceral abnormalities were found at concentrations of 0.1 and 1.0 mg/m3. Significant effects were found for lower incisor eruption and ear unfolding at a concentration of 0.3 mg/m3. The grooming behavior of 30-day old male progeny was significantly less than control in all experimental groups. Grooming behavior in female subjects exposed to a concentration of 0.3 mg/m3 and emotionality in subjects exposed to a concentration of 1 mg/m3 were decreased significantly. At 60 days of age emotional reactions were significantly decreased in female subjects from the 0.03, 0.3 and 1.0 mg/m3 groups. SPT was significantly increased in the 1 mg/m3 group for both male and female pups. Thus, evidence of CNS depression influence of TBP both in maternal and offspring groups was found. The NOEL (No Observed Effect Level) for developmental neurotoxicity is thus < 0.03 mg/m3, and the NOEL for maternal neurotoxicity is 0.3 mg/m3. These results suggest that exposure to TBP for 24 hr/day throughout gestation may cause developmental neurotoxicity, embryotoxicity and fetotoxicity, but not immunotoxicity.

Glutamate-stimulated ROS production in neuronal cultures: interactions with lead and the cholinergic system.

Abstract: Oxidative stress may be an important factor in several pathological brain conditions. A contributing factor in many such conditions is excessive glutamate release, and subsequent glutamatergic neuronal stimulation, that causes increased production of reactive oxygen species (ROS), oxidative stress, excitotoxicity and neuronal damage. Glutamate release is also associated with illnesses such as Alzheimer's disease, stroke, and brain injury. Glutamate may interact with an environmental toxin, lead, and this interaction may result in neuronal damage. Glutamate-induced ROS production is greatly amplified by lead in cultured neuronal cells. Alterations in protein kinase C (PKC) activity seem to be important both for glutamate-induced ROS production, and for the amplification of glutamate-induced ROS production by lead. It is possible that the neurotoxic effects of lead are amplified through glutamate-induced neuronal excitation. Cholinergic stimulation can also trigger ROS production in neuronal cells. PKC seems to play a key-role also in cholinergic-induced ROS production superoxide anion being the primary reactive oxygen species. There seems to be a close relationship between the responses of cholinergic muscarinic and glutamatergic receptors because glutamate receptor antagonists inhibit cholinergic-induced activation of human neuroblastoma cells. Glutamatergic neuronal stimulation may be a common final pathway in several brain conditions in which oxidative stress and ensuing excitotoxicity plays a role.

Genetically determined susceptibility to organophosphorus insecticides and nerve agents: developing a mouse model for the human PON1 polymorphism.

Abstract: Several organophosphorus insecticides and nerve agents are detoxified through the cytochrome P450/paraoxonase (PON1) pathway. PON1 is an HDL-associated enzyme encoded as a 355 amino acid protein in humans. The PON1 Arg192 isoform hydrolyzes paraoxon rapidly while the Gln192 isoform hydrolyzes this compound slowly. Both isoforms hydrolyze phenylacetate and chlorpyrifos oxon at approximately the same rate. We recently found that the effect of this polymorphism is dramatically reversed for sarin hydrolysis. The PON1 Arg192 isoform has virtually no sarinase activity while the Gln192 isoform has substantial activity. The Gln192 isoform also hydrolyzes diazoxon and soman faster than the Arg192 isoform. In addition to the large differences in rates of hydrolysis observed for some OP substrates by the two PON1 isoforms, there is also a large variability in serum PON1 concentrations that is stable over time between individuals. Thus, two factors govern the PON1 status of a given individual, the PON1 genotype as well as the amount of protein expressed from each allele. A two-dimensional enzyme analysis provides an excellent assessment of an individual's PON1 status, ie. the position 192 genotype as well as phenotype, or level of serum PON1 (Nature Genet 14:334-336). Do these interindividual differences in rates of substrate hydrolysis by PON1 reflect an individual's sensitivity or resistance to OP compounds processed through the P450/PON1 pathway? Injection of purified PON1 into mice clearly demonstrates the protective effect of having high serum levels of PON1 against toxicity by chlorpyrifos oxon or chlorpyrifos. Preliminary experiments with PON1 knockout mice, on the other hand, clearly demonstrate that low PON1 levels result in dramatically increased sensitivity to chlorpyrifos oxon. Attempts to express human PON1 in mice from constructs containing either of the human PON1 cDNA sequences were unsuccessful, despite the generation of the respective transgenic mice.

Field screening of blood lead levels in remote Andean villages.

Abstract: Blood lead (PbB) levels were investigated in chronically lead (Pb) exposed Andean children and adults living in a highly Pb contaminated area of Ecuador where Pb glazing of ceramics is prevalent. A comparative study was made of the PbB levels of Pb-glazing and non-Pb-glazing families living in close proximity, using three PbB analysis techniques. Fifty-one, 50-microl blood samples from children and adults were analyzed in the field by a finger-stick capillary screening technique using the portable ESA LeadCare Blood Lead Testing System (LCS). Venous blood samples of 2-4 ml were collected from the same 51 participants and analyzed in the laboratory by inductively coupled plasma mass spectrometry (ICP-MS) and by graphite furnace atomic absorption spectrometry (AAS). The median PbB levels for the Pb-glazing group as determined by the ICP-MS, AAS and LCS techniques were 37.2 microg/dl (range 11.6-101.0), 32.0 microg/dl (range 8.0-70.0 microg/dl) and 44.0 microg/dl (range 19.0-105.0), respectively. The median PbB levels for the non-Pb-glazing group were 9.2 microg/dl (range 5.0-21.7) with ICP-MS, 9.0 microg/dl (range 4.3-32.0) with AAS, and 11.3 microg/dl (range 7.3-21.1) with LCS. The differences in PbB levels between the Pb glazing and non-Pb glazing groups were statistically significant (p = < .0001) for each PbB analysis method. Correlations between paired samples were: LCS and ICP-MS: r = 0.913, LCS and AAS: r = 0.829, and ICP-MS and AAS: r = 0.905. The results suggest that neighboring Pb glazing and non-Pb glazing families have significantly different PbB levels, and that the portable LCS field technique may be useful for screening and periodic monitoring of relatively low and high PbB levels of persons in remote high altitude Andean areas.

Lead potentiates cytokine- and glutamate-mediated increases in permeability of the blood-brain barrier.

Abstract: We have measured the transendothelial electrical resistance across the blood-brain barrier (BBB) with a microelectrode technique and determined the effects of subcutaneous injections (five injections over ten days) of lipopolysaccharide (LPS, 100 ng/g), recombinant mouse interleukin-6 (IL-6, 5 ng/g), and/or inorganic lead (lead, 2.5 5 micrograms/g) on the ion permeability of arterioles in the temporoparietal cortex of anaesthetized mice between 10 and 40 days of age. In controls the electrical resistance increased with age. It was decreased in animals treated with IL-6, but unaffected by lead at the different ages studied. In IL-6 treated mice, repeated neonatal exposure to lead (five injections between 2 and 10 days after birth) caused a delay in the increase in arteriole resistance with age. LPS injections caused a 36% increase in ion permeability of the BBB in twenty-day-old mice, and lead potentiated this effect of LPS. Intra-arterial injections of glutamate did not alter vascular resistance, but topical applications of glutamate on the cerebrum caused a reversible decrease in the resistance in mice not treated with lead, and an irreversible decrease in mice treated with lead. Injections of glutamate in the lumen of arterial vessels in the parietal and temporoparietal brain areas of mice pretreated with lead and LPS, plus a topical application of glutamate, caused depolarization of neurons in the temporoparietal cortex. These results suggest that disruption of the BBB can allow serum glutamate to penetrate the brain, causing further disruption of the BBB, and that lead irreversibly potentiates this cascade of harmful events.

Insect (Locusta migratoria migratorioides) test monitoring the toxicity of cyanobacteria

Abstract: An insect test was developed to investigate the toxicity of cyanobacteria. The African locust, Locusta migratoria migratorioides R.F. was used as a test animal instead of mouse. The cyanobacteria tested were Aphanizomenon flos-aque, Anabaena aphanizomenoides, Cylindrospermopsis raciborskii, Microcystis aeruginosa. The toxicity of authentic microcystin-LR was also tested. Cyanobacteria producing toxins killed the animals when the homogenized cell suspension was injected into the animals. The locust test proved to be more sensitive than the mouse test. The LD50 values of the different cyanobacteria for locusts and for mice, respectively were the following: 90 microg/animal (60 mg/kg) and 8000 microg/animal (320 mg/kg), for Aphanizomenon flos-aquae; 255 microg/animal (170.2 mg/kg) and 3750 microg/animal (150 mg/kg), for Anabaena aphanizomenoides; 195 microg/animal (131.4 mg/kg) and 5750 microg/animal (230 mg/kg), for Cylindrospermopsis raciborskii; 22.5 microg/animal (15 mg/kg) and 6000 microg/ animal (240 mg/kg), for Microcystis aeruginosa. In locusts the LD50 value for authentic microcystin-LR was 0.2 microg/animal (130 mg/kg). Since the weight of the mice is 15 to 20 times larger than that of the locusts, hence less toxic cells are needed to kill the locusts. The locust test is cheaper than the mouse test, large number of animals can be used in the experiments and the LD50 values can be estimated more precisely. The toxicity of C. raciborskii was significantly lower when the lyophilized cells were extracted in methanol (LD50 = 767 mg/kg), instead of NaCl solution (LD50 = 131.4 mg/kg).

Evaluation of subchronic neurotoxicity of n-butyl acetate vapor.

Abstract: n-Butyl acetate, a common industrial solvent, was selected by the US EPA as a chemical of concern for neurotoxicity as part of the Multisubstance Rule for the Testing of Neurotoxicity. The neurotoxic potential of n-butyl acetate was investigated in Sprague-Dawley rats using a functional observational battery, motor activity, neurohistopathology, and schedule-controlled operant behavior (SCOB) as indicators of neurotoxicity. Animals were exposed to concentrations of 0, 500, 1500, or 3000 ppm of n-butyl acetate for 6 hours per day for 65 exposures over 14 weeks. Functional observational battery and motor activity values for ad libitum-fed male and female rats were measured during Weeks -1, 4, 8, and 13. SCOB testing of food-restricted animals, using a multiple fixed ratio/fixed interval schedule, was conducted daily prior to each exposure to maintain the operant behavior; the data from Weeks -1, 4, 8, and 13 were evaluated for evidence of neurotoxicity. Transient signs of sedation and hypoactivity were observed only during exposure to the 1500 and 3000 ppm concentrations. The only signs of systemic toxicity were reduced body weights for the 3000 ppm ad libitum-fed groups and occasionally for the female 1500 ppm ad libitum-fed group. No evidence of neurotoxicity was seen during the functional observational battery examinations. Motor activity for the 3000 ppm male group was significantly (p < or = 0.05) higher than for the control group only during Week 4. No significant differences were observed among groups for Weeks 8 and 13. No significant differences in motor activity values were observed for female rats. No significant differences were seen in operant behavior at any test vapor concentration. Microscopic evaluations of sections from the brain, spinal cord (cervical and lumbar regions), dorsal and ventral spinal roots, dorsal root ganglia, sciatic nerve, and tibial nerve of animals in the control and 3000 ppm groups did not indicate any treatment-related effects. In conclusion, there was no evidence of cumulative neurotoxicity based on the functional observational battery, motor activity, neurohistopathology, and schedule-controlled operant behavior endpoints. The data presented here are relevant to the neurotoxicity risk assessment of n-butanol due to the rapid hydrolysis of n-butyl acetate in vivo.