Mice lacking serum paraoxonase are susceptible to organophosphate toxicity and atherosclerosis

Serum paraoxonase (PON1) is an esterase that is associated with high-density lipoproteins (HDLs) in the plasma; it is involved in the detoxification of organophosphate insecticides such as parathion and chlorpyrifos. PON1 may also confer protection against coronary artery disease by destroying pro-inflammatory oxidized lipids present in oxidized low-density lipoproteins (LDLs). To study the role of PON1 in vivo, we created PON1-knockout mice by gene targeting. Compared with their wild-type littermates, PON1-deficient mice were extremely sensitive to the toxic effects of chlorpyrifos oxon, the activated form of chlorpyrifos, and were more sensitive to chlorpyrifos itself. HDLs isolated from PON1-deficient mice were unable to prevent LDL oxidation in a co-cultured cell model of the artery wall, and both HDLs and LDLs isolated from PON1-knockout mice were more susceptible to oxidation by co-cultured cells than the lipoproteins from wild-type littermates. When fed on a high-fat, high-cholesterol diet, PON1-null mice were more susceptible to atherosclerosis than their wild-type littermates.

Fibroblast heterogeneity of signal transduction mechanisms to complement-C1q. Analyses of calcium mobilization, inositol phosphate accumulation, and protein kinases-C redistribution

Fibroblasts of healthy and granulation gingiva are phenotypically heterogeneous with regard to binding C1q collagen-like (cC1qR) or C1q globular-heads (gC1qR) regions, respectively. Here, isolated fibroblast subsets, expressing either the cC1qR or the gC1qR phenotype, were stimulated with C1q, and assessed for changes in cytosolic free calcium [Ca2+]i, accumulation of inositol trisphosphate (IP3), and redistribution of Ca2+-dependent protein kinases-C (cPKCs) from cytosol to membranes. Changes in [Ca2+]i were determined using Indo-1 fluorescence in combination with adhering cell analysis and sorting (ACAS) cytometry. Accumulation of IP3 was quantified using a competitive radioreceptor binding assay. Redistribution of cPKCs was evaluated by immunoblotting with antibodies to PKCalpha/betaI-betaII/gamma. Subsets manifested different fluctuations in [Ca2+]i levels 20 seconds after C1q-stimulation in the presence of millimolar concentrations of external calcium. Whereas cC1qR fibroblasts responded with a 38% over baseline [Ca2+]i increase which was sustained for 20 to 30 minutes, gC1qR fibroblasts responded with a higher (264% over baseline) and more rapid (2 to 3 minutes) transient. Likewise, subsets exhibited different kinetics of IP3 accumulation. Whereas cC1qR fibroblasts responded with an IP3 increase of 32 +/- 3 pmol/10(4) cells over baseline after 5 seconds stimulation, gC1qR fibroblasts responded after 15 to 20 seconds with a lower increase (13 +/- 0.8 IP3 pmol/10(4) cells over baseline). Subsets differed in cPKCs redistribution which peaked in gC1qR-membranes 30 seconds after stimulation and remained sustained between 10 and 30 minutes. No cPKC redistribution was detectable in stimulated cC1qR-cells. We conclude that fibroblasts are heterogeneous in phosphoinositide-Ca2+ signaling and cPKC redistribution to C1q, and suggest that these differences may affect activities of normal and granulation gingiva.

Signal transduction in environmental neurotoxicity

Signal transduction is the process by which specific information is transferred from the cell surface to the cytosol and ultimately to the nucleus, leading to changes in gene expression. Since these chains of biochemical and molecular steps control the normal function of each cell, disruption of these processes would have a significant impact on cell physiology. Some of the major signal transduction pathways are briefly reviewed. The interactions of four chemicals (lead, ethanol, polychlorinated biphenyls, and trimethyltin) with different cell signaling systems, particularly the phospholipid hydrolysis/protein kinase C pathway, are discussed. The possible causal relationship of such cellular and molecular interactions with known signs and symptoms of neurotoxicity are highlighted.

A genetic polymorphism of MAO-B modifies the association of cigarette smoking and Parkinson's disease

In a population-based case-control study, we found a reversal of the association of cigarette smoking with Parkinson's disease (PD) in relation to the monoamine oxidase B intron 13 genetic polymorphism. A reduced PD risk related to pack-years of smoking was detected for persons with the G allele, whereas an opposite effect was found among persons with the A allele. These results indicate an unexplained interaction between cigarette smoking and this genetic polymorphism.

Neurotoxicity testing: a discussion of in vitro alternatives

A large number of chemicals may exert adverse effects on the central and/or peripheral nervous system. A commonly recommended strategy for neurotoxicity testing is that of a tiered approach aimed at identifying and characterizing the neurotoxicity of a compound. Guidelines exist in the United States and other countries that define the tests to be utilized in tier 1 testing. To address problems related to the increasing cost and time required for toxicity testing, the increasing number of chemicals being developed, and the concern of animal welfare activists, attention is currently being devoted to in vitro alternatives. This paper addresses the use of in vitro systems in neurotoxicology, and their potential role in a general strategy for neurotoxicity testing. The advantages and disadvantages of in vitro approaches for mechanistic studies and for screening of neurotoxicants are discussed. Suggestions for further validation studies are proposed.

Interaction of blood lead and delta-aminolevulinic acid dehydratase genotype on markers of heme synthesis and sperm production in lead smelter workers

The gene that encodes gamma-aminolevulinic acid dehydratase (ALAD) has a polymorphism that may modify lead toxicokinetics and ultimately influence individual susceptibility to lead poisoning. To evaluate the effect of the ALAD polymorphism on lead-mediated outcomes, a cross-sectional study of male employees from a lead-zinc smelter compared associations between blood lead concentration and markers of heme synthesis and semen quality with respect to ALAD genotype. Male employees were recruited via postal questionnaire to donate blood and urine for analysis of blood lead, zinc protoporphyrin (ZPP), urinary coproporphyrin (CPU), and ALAD genotype, and semen samples for semen analysis. Of the 134 workers who had ALAD genotypes completed, 114 (85%) were ALAD1-1 (ALAD1) and 20 (15%) were ALAD1-2 (ALAD2). The mean blood lead concentrations for ALAD1 and ALAD2 were 23.1 and 28.4 microg/dl (p = 0.08), respectively. ZPP/heme ratios were higher in ALAD1 workers (68.6 vs. 57.8 micromol/ml; p = 0.14), and the slope of the blood lead ZPP linear relationship was greater for ALAD1 (2.83 vs. 1.50, p = 0.06). No linear relationship between CPU and blood lead concentration was observed for either ALAD1 or ALAD2. The associations of blood lead concentration with ZPP, CPU, sperm count, and sperm concentration were more evident in workers with the ALAD1 genotype and blood lead concentrations >/= 40 microg/dl. The ALAD genetic polymorphism appears to modify the association between blood lead concentration and ZPP. However, consistent modification of effects were not found for CPU, sperm count, or sperm concentration.

The effects of ethanol on glial cell proliferation: relevance to the fetal alcohol syndrome

Exposure to ethanol during pregnancy is detrimental to brain development. Individuals affected by the Fetal Alcohol Syndrome present a number of central nervous system dysfunctions including microencephaly and mental retardation. Studies on the mechanisms of ethanol's developmental neurotoxicity have focused on its interaction with neurons; however, emerging evidence is suggesting that ethanol can significantly affect glial cells as well. A number of in vitro studies have shown that ethanol can inhibit the proliferation of various glial cells (mostly primary astrocytes or astrocytoma cells) at relatively high concentrations (100-200 mM). On the other hand, proliferation induced by some, but not all mitogens, is inhibited by low concentrations (10-50 mM) of ethanol. These inhibitory effects of ethanol may contribute to its developmental neurotoxicity observed following in vivo exposure. Animal models have indeed shown that ethanol causes microencephaly when given during the brain growth spurt, a period of brain development characterized by astroglial proliferation and maturation.

Xestospongins: potent membrane permeable blockers of the inositol 1,4,5-trisphosphate receptor

Xestospongins (Xe's) A, C, D, araguspongine B, and demethylxestospongin B, a group of macrocyclic bis-1-oxaquinolizidines isolated from the Australian sponge, Xestospongia species, are shown to be potent blockers of IP3-mediated Ca2+ release from endoplasmic reticulum vesicles of rabbit cerebellum. XeC blocks IP3-induced Ca2+ release (IC50 = 358 nM) without interacting with the IP3-binding site, suggesting a mechanism that is independent of the IP3 effector site. Analysis of Pheochromocytoma cells and primary astrocytes loaded with Ca2+-sensitive dye reveals that XeC selectively blocks bradykinin- and carbamylcholine-induced Ca2+ efflux from endoplasmic reticulum stores. Xe's represent a new class of potent, membrane permeable IP3 receptor blockers exhibiting a high selectivity over ryanodine receptors. Xe's are a valuable tool for investigating the structure and function of IP3 receptors and Ca2+ signaling in neuronal and nonneuronal cells.

Association of a polymorphism in intron 13 of the monoamine oxidase B gene with Parkinson disease

Monoamine oxidase B (MAO-B) is an enzyme that has relevance for Parkinson disease (PD) because of its roles in catabolizing dopamine and potentially activating exogenous neurotoxicants. A polymorphism of the gene encoding MAO-B has been identified as a single base change (A or G) in intron 13 of the X chromosome. The A allele was previously associated with an approximately twofold risk of PD. The present study compared A and G allele frequencies between newly diagnosed idiopathic PD cases and a control group free of neurodegenerative diseases. All study subjects were Caucasian. Cases were 37 men and 25 women, age 37-80 years; controls were 50 men and 29 women, age 45-82 years. MAO-B genotype was determined by the allele-specific polymerase chain reaction on DNA extracted from peripheral lymphocytes. In complete contrast to previous studies, elevated risks were detected with the G allele. The age-adjusted odds ratio for the G allele in males was 1.87 ((95% confidence interval) 0.78-4.47). Among females the age-adjusted odds ratios were 5.00 ((95% confidence interval) 1.13-22.1) for the GA genotype and 5.60 ((95% confidence interval) 1.01-30.9) for the GG genotype. These findings, although of limited statistical precision, suggest that the G allele of this MAO-B polymorphism may relate to PD risk.

Basic toxicology of pesticides

This chapter reviews basic aspects of the health effects and mechanisms of action of the major classes of pesticides: insecticides, herbicides, rodenticides, and fungicides. Specific pesticides in each class are described, with historical detail and depictions of chemical structure.

Paraoxonase (PON1) gene in mice: sequencing, chromosomal localization and developmental expression

Serum paraoxonase hydrolyses the toxic metabolites of several organophosphorus insecticides, as well as the nerve agents soman and sarin. We have previously shown that elevated serum paraoxonase levels protect mice against organophosphate toxicity. In the present study, we determined the cDNA sequence and chromosomal location of the mouse paraoxonase gene, as well as its developmental expression in mice and rats. The mouse cDNA encodes a protein of 355 amino acids and shows 81% identity with the human sequence. In situ hybridization demonstrated that the mouse paraoxonase gene maps to chromosome 6, a region conserved with the paraoxonase region of chromosome 7q21-22 in humans. Serum paraoxonase activities toward three substrates, paraoxon, chlorpyrifos-oxon and diazoxon, were very low at birth and increased with age reaching adult levels at 20 days in mice and 25 days in rats. The increase of serum paraoxonase activity in developing animals correlates well with the increased resistance to organophosphate poisoning that has been reported in previous studies.

Inhibition of muscarinic receptor-stimulated glial cell proliferation by ethanol

Acetylcholine and other muscarinic agonists stimulate the proliferation of rat cortical astrocytes and 132 1N1 human astrocytoma cells by activating muscarinic m3 cholinergic receptors. Ethanol was a potent inhibitor of carbachol-stimulated proliferation, measured by [3H]thymidine incorporation, with an IC50 of 10 mM. On the other hand, basal and serum-stimulated proliferation of astrocytes and astrocytoma cells was inhibited by ethanol with lower potency (IC50 = 200-250 mM). Concentration-response experiments with carbachol, in the presence of 10 mM ethanol, suggested that inhibition of proliferation by the alcohol was of the noncompetitive type. Experiments with acetaldehyde and with the alcohol dehydrogenase inhibitor 4-methylpyrazole suggested that the inhibitory effect of alcohol was due to ethanol itself and not to its metabolite acetaldehyde. Proliferation of astrocytoma cells induced by carbachol and the inhibitory effects of ethanol were also confirmed by flow cytometry using the 5-bromodeoxyuridine-Hoechst 33258 method. Ethanol (10 mM) had no effect on proliferation induced by 50 micrograms/ml insulin and 100 ng/ml platelet-derived growth factor BB; on the other hand, the mitogenic effect of 1 mM histamine, 100 U/ml interleukin-1, and 100 ng/ml 12-O-tetradecanoylphorbol 13-acetate were inhibited by approximately 50%. These results indicate that proliferation of glial cells induced by muscarinic agonists is especially sensitive to the inhibitory effect of ethanol. This action of ethanol may be relevant to its developmental neurotoxicity, particularly microencephaly, which is one of the common features of the fetal alcohol syndrome.

Interaction of mercury compounds with muscarinic receptor subtypes in the rat brain

The in vitro effects of mercuric chloride (HgCl2) and methylmercury (CH3HgOH) on the M1 and M2 muscarinic receptor subtypes were investigated in rat brain cortical membranes. HgCl2 and CH3HgOH were almost equipotent in inhibiting the binding of [3H]telenzepine to M1 receptors (IC50s = 2.2 and 3.4 microM, respectively). Conversely, HgCl2 was a thirty-fold more potent inhibitor of [3H]AF-DX 384 binding to M2 sites than CH3HgOH (lC50s = 5 and 149 microM, respectively). In all cases HgCl2 showed steep and monophasic inhibition curves, whereas those of CH3HgOH were biphasic (M1) or shallow (M2). CH3HgOH-induced inhibition of both [3H]telenzepine and [3H]AF-DX 384 binding was of the competitive type, while HgCl2 caused a pronounced reduction of the Bmax value associated with a small change in affinity. CH3HgOH also decreased the affinity of the agonist carbachol for M1 and M2 receptors, while inorganic mercury had minimal effects on the carbachol dose-response curves. These results indicate that inorganic and organic mercury differ in their interaction with muscarinic receptor subtypes and that M1 receptors may represent a preferential target for their effects.

Urinary mercapturic acid diastereoisomers in rats subchronically exposed to styrene and ethanol

Styrene is stereoselectively oxidized by cytochrome P450 to its reactive metabolite, styrene oxide. The (R)- and (S)-enantiomers of styrene oxide can be conjugated with glutathione (GSH) to both (R)- and (S)-diastereoisomers of the specific mercapturic acids, N-acetyl-S-(1-phenyl-2-hydroxyethyl)-L-cysteine (M1) and N-acetyl-S-(2-phenyl-2-hydroxyethyl)-L-cysteine (M2). Several investigations have indicated different toxic potential of the (R)- and (S)-configurations of styrene oxide and its GSH- and N-acetyl-conjugates. In this study the mercapturic acid diastereoisomers were measured in the urine of rats exposed to styrene in combination with ethanol, a good inducer of styrene metabolism. Male Sprague-Dawley rats were given an isocaloric liquid diet containing ethanol (5% w/v) for 3 weeks. Starting from the 2nd week, the animals were also exposed to styrene vapours (300 ppm, 6 h/day, 5 days/week) in a dynamic exposure chamber. Both the (R)- and (S)-diastereoisomers of the M1 and M2 as well as the conventional biomarkers, mandelic acid (MA) and phenylglyoxylic acid (PGA) were measured in urinary samples. Approximately 30 and 25% reduction of the levels of brain non-protein sulfhydryls (NPS) was observed in the animals given styrene and ethanol, respectively, while the combined ethanol and styrene treatment resulted in a 60% decrease. Ethanol consumption also resulted in higher urinary levels of the M1-R, M1-S and M2 metabolites associated with increased M1-R/S ratio and higher urinary MA excretion compared to animals treated with styrene. These results suggest that the urinary mercapturic acid diastereoisomers may be used as a noninvasive tool to examine stereoselective patterns of styrene metabolism in vivo, as well as their alterations caused by ethanol. These compound-specific mercapturic acids may also be valuable indicators of styrene-induced disorders of GSH homeostasis in nonaccessible organs.

Biochemical markers of neurotoxicity. A review of mechanistic studies and applications

Neurotoxicology presents major challenges to the development of biological markers in accordance to conventional research strategies. Because of the inaccessibility of the nervous system, one of the proposed alternatives is the study of biochemical signals in peripheral tissues which can easily and ethically be obtained in humans, and which could represent surrogate indicators of equivalent parameters in the nervous tissue. Considerable scientific support to this approach is provided by the results of recent investigations in major areas of pharmacology and psychobiology. Studies examining parameters of neurotransmission and second messenger systems in peripheral blood cells, and variations in the peripheral body fluid content of endogenous substances reflecting nervous tissue dysfunction or damage are presented in this paper as examples of efforts toward rational development and validation of novel indicators of nervous system toxicity. Cholinergic muscarinic receptors and calcium signalling in peripheral blood lymphocytes, myelin basic protein in cerebrospinal fluid, and blood polyamines are discussed as potential surrogate indicators based on the results of in vitro or in vivo animal studies of neurotoxic metals (mercury, triethyltin), pesticides (disulfoton), drugs of abuse (d-fenfluramine) and model epileptogenic compounds (kainic acid). Data from investigations examining serum prolactin, type B monoamine oxidase (MAO-B) and dopamine beta-hydroxylase (DBH) in workers occupationally exposed to manganese, lead or styrene are also presented. Although research in this field is still at its very early stage, current evidence suggests that (i) certain neurochemical markers may be valuably used in animal studies as a complement to conventional laboratory tests to augment their sensitivity or predictivity; (ii) a mechanistic research approach is required to establish which markers offer the greatest promise for application in human biomonitoring.

Biomarker research in neurotoxicology: the role of mechanistic studies to bridge the gap between the laboratory and epidemiological investigations

There is an increasing interest in the development and validation of biomarkers for use in biochemical/molecular epidemiological studies. Though the area of neurotoxicology has received much attention in the past several years, it still lags behind with regard to the development of biomarkers, particularly those of health effects and susceptibility. This review discusses several aspects of biomarker research as it relates to neurotoxic compounds and focuses on selected agents (organophosphorus insecticides, styrene, n-hexane, carbon disulfide, acrylamide), which have been the subject of a number of investigations in animals and humans. While traditional biomonitoring approaches and novel techniques (e.g., hemoglobin adducts) provide several measurements for monitoring exposure to neurotoxic chemicals, potential markers of genetic susceptibility have been seldom investigated in a neurotoxicology context. Furthermore, the complexity of the nervous system, together with the multiplicity of end points and the limited knowledge of the exact mechanism(s) of action of neurotoxicants, has led to only limited advancements in the development of biomarkers for neurotoxic effects. Significant progress in this area will depend upon an increased understanding of the cellular, biochemical, and molecular targets directly involved in neurotoxicity.

Acetylcholine as a mitogen: muscarinic receptor-mediated proliferation of rat astrocytes and human astrocytoma cells

The mitogenic effect of muscarinic receptor agonists in glial cells has been characterized in rat cortical astrocytes and human 132 1N1 astrocytoma cells. The muscarinic receptor agonist carbachol caused a dose- and time-dependent increase in proliferation, as measured by [3H]thymidine incorporation. The mitogenic effect was mimicked by several muscarinic, but not nicotinic receptor agonists, and was blocked by muscarinic receptor antagonists. Reverse transcription-polymerase chain reaction (RT-PCR) experiments indicated the presence of m2, m3 and to a lesser degree, m5 muscarinic receptor mRNA in both astrocytes and astrocytoma cells. Proliferation experiments with subtype-specific muscarinic receptor antagonists suggest that carbachol-induced proliferation is due to activation of muscarinic M3 receptors. The phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) also stimulated glial cell proliferation. Down-regulation of protein kinase C, or the protein kinase C antagonist 1,5-(isoquinolynsulfanyl)-2-methylpiperazine dihydrochloride (H7) blocked proliferation induced by either TPA or carbachol. Of other neurotransmitters tested, histamine caused glial cell proliferation, norepinephrine and gamma-aminobutyric acid were ineffective, while serotonin and glutamate inhibited basal or serum-stimulated proliferation.

Effects of ethanol administration on cerebral non-protein sulfhydryl content in rats exposed to styrene vapour

Glutathione (GSH) and other non-protein sulfhydryls (NPS) are known to protect cells from oxidative stress and from potentially toxic electrophiles formed by biotransformation of xenobiotics. This study examined the effect of a simultaneous administration of styrene and ethanol on NPS content and lipid peroxidation in rat liver and brain. Hepatic cytochrome P450 and cytochrome b5 content, aniline hydroxylase and aminopyrine N-demethylase activities as well as the two major urinary metabolites of styrene, mandelic and phenylglyoxylic acids were also measured. Groups of rats given ethanol for 3 weeks in a liquid diet were exposed, starting from the second week, to 326 ppm of styrene (6 h daily, 5 days a week, for 2 weeks). In control pair-fed animals, styrene produced about 30% depletion of brain NPS and 50% depletion of hepatic NPS. Subchronic ethanol treatment did not affect hepatic NPS levels, but caused 23% depletion of brain NPS. Concomitant administration of ethanol and styrene caused a NPS depletion in brain tissue in the order of 60%. These results suggest that in the rat, simultaneous exposure to ethanol and styrene may lead to considerable depletion of brain NPS. This effect is seen when both compounds are given on a subchronic basis, a situation which better resembles possible human exposure.

Interaction of ethanol and anoxia with muscarinic receptor--stimulated phosphoinositide metabolism during brain development

The mechanism(s) by which ethanol induces alterations in brain development may involve direct actions (e.g. changes in specific biochemical pathways), or indirect effects, such as cerebral hypoxia resulting from ethanol - induced circulatory changes. Since both ethanol and hypoxia are known to affect the metabolism of phosphoinositides, which has been suggested as a possible target for ethanol's developmental neurotoxicity, in the present study we have investigated the in vitro effects of both severe hypoxia (anoxia) and ethanol (alone or in combination) on muscarinic receptor-stimulated phosphoinositide metabolism in cerebral cortex slices from neonatal rats. Anoxia markedly inhibited carbachol - stimulated phosphoinositide metabolism in adult rats (67%), but only slightly (10%) in neonatal animals. Reoxygenation reversed the effect of anoxia at both ages. On the other hand, ethanol's inhibitory effect was pronounced in neonatal rats only, and was additive to that of anoxia. The presence of ethanol did not affect the recovery of carbachol - stimulated phosphoinositide metabolism following anoxia and reoxygenation. These results indicate that ethanol and anoxia differently and independently affect muscarinic receptor - stimulated phosphoinositide metabolism and may mutually contribute to the CNS effects observed following developmental ethanol exposure.

Ethanol inhibits muscarinic receptor-stimulated phosphoinositide metabolism and calcium mobilization in rat primary cortical cultures

In recent years, it has been hypothesized that muscarinic receptor-stimulated phosphoinositide (PI) metabolism may represent a relevant target for the developmental neurotoxicity of ethanol. Age-, brain region-, and receptor-specific inhibitory effects of ethanol on this system have been found, both in vitro and after in vivo administration. As a direct consequence of this action, alterations of calcium homeostasis would be expected, through alterations of inositol trisphosphate formation, which mediates intracellular calcium mobilization. In the present study, the effects of ethanol (50-500 mM) on carbachol-stimulated PI metabolism and free intracellular calcium levels were investigated in rat primary cortical cultures, by measuring release of inositol phosphates and utilizing the two calcium probes fluo-3 and indo-1 on an ACAS (Adherent Cell Analysis and Sorting) Laser Cytometer. Ethanol exerted a concentration-dependent inhibition of carbachol-stimulated PI metabolism. In addition, ethanol's inhibitory effect paralleled the temporal development of the muscarinic receptor signal transduction system, with the strongest inhibition (25-50%) occurring when maximal stimulation by carbachol occurs (days 5-7). Ethanol also exerted a concentration-dependent decrease in free intracellular calcium levels following carbachol stimulation. Both initial calcium spike amplitude, seen in all responsive cells, as well as the total number of cells responding to carbachol, were decreased by ethanol. The inhibitory effects of ethanol seemed dependent upon preincubation time, in that a longer preincubation (30 min) with the lowest dose (50 mM), showed almost the same decrease in responding cell number and reduction in spike amplitude in responding cells, as a shorter incubation (10 min) with the highest ethanol dose (500 mM). The specificity of the response to carbachol was demonstrated by blocking the response with 10 microM atropine. Moreover, experiments with carbachol in calcium-free buffer with 1 mM EGTA indicated that the initial calcium spike was due to intracellular calcium mobilization from intracellular stores. Since calcium is believed to play important roles in cell proliferation and differentiation, these results support the hypothesis that this intracellular signal-transduction pathway may be a target for ethanol, contributing to its developmental neurotoxicity.

Identification of m3, m4 and m5 subtypes of muscarinic receptor mRNA in human blood mononuclear cells

In this study we made use of the Reverse transcription-polymerase chain reaction to analyze the expression of mRNA for the five subtypes of muscarinic acetylcholine receptors in human blood mononuclear cells. mRNA for m3, m4 and m5 subtypes was detected, while mRNA for m1 and m2 muscarinic receptors was not found. Similar results were obtained for three different healthy human subjects studied. Interestingly, the m5 subtype was expressed at higher levels in blood mononuclear cells than in cerebral cortex. To our knowledge this is the first time that m5 muscarinic receptor mRNA has been found outside of the central nervous system.

Postnatal development of muscarinic receptor-stimulated phosphoinositide metabolism in mouse cerebral cortex: sensitivity to ethanol

An enhanced coupling of cholinergic muscarinic receptors to phosphoinositide metabolism had been previously observed in brain from immature rat. This study reports that the postnatal development of muscarinic receptor-stimulated phosphoinositide metabolism is also enhanced in cerebral cortex slices from immature Swiss-Webster and Balb-c mice, as compared to adults. Response to the agonist carbachol was lower on postnatal day 3, peaked between days 5 and 12 and then declined to adult levels. Density of muscarinic binding sites, measured with the M1 ligand [3H]telenzepine on postnatal day 7, was, on the other hand, only half of the adult value. Phosphoinositide hydrolysis stimulated by glutamate decreased with age, while that elicited by norepinephrine increased. These results are also similar to those previously reported in the rat. Ethanol has been found to inhibit muscarinic receptor-stimulated phosphoinositide metabolism in rat brain in an age-dependent manner. This was confirmed in mouse brain, where ethanol inhibited this response in cerebral cortex of immature but not adult animals. These results indicate that the enhanced muscarinic receptor-stimulated phosphoinositide metabolism, which coincides with the brain growth spurt, is similar in rats and mice. Mice may be a useful species in which to genetically manipulate muscarinic receptors to gain a better understanding of their potential role in brain development.

Heterogeneity of cholinergic muscarinic receptors coupled to phosphoinositide metabolism in immature rat brain

The effects of muscarinic agonists and antagonists on phosphoinositide (PtdIns) metabolism were examined in the cerebral cortex and brainstem of 7-day-old rats, in order to evaluate the role of muscarinic receptor subtypes in this process. Additionally, comparative experiments were performed in cortices from adult animals. Accumulation of [3H]inositol phosphates ([3H]InsPs) in [3H]inositol pre-labeled brain slices was taken as an index of PtdIns hydrolysis. In neonatal cortex, maximal stimulation induced by the full agonists acetylcholine, carbachol and methacholine was 8-10 fold over basal [3H]InsPs accumulation. The effect of the partial agonists bethanechol, pilocarpine and oxotremorine varied from 3 to 4 fold over basal. Smaller responses to cholinergic stimulation were found in the brainstem and in the adult cortex. In neonatal cortex, muscarinic antagonists inhibited the stimulatory responses with the following order of potency: 4-DAMP > pirenzepine > AF-DX 116 approximately p-F-HHSiD. Pirenzepine inhibition of full agonist-induced [3H]InsPs accumulation showed biphasic curves, with two thirds of the response being inhibited with high affinity. When partial agonists were used, the resulting pirenzepine curves were better described by interaction at one high affinity site. No differences were found between immature and adult rats in the effect of pirenzepine on [3H]InsPs accumulation induced by carbachol, methacholine, or bethanechol. Inhibition by pirenzepine of PtdIns hydrolysis induced by carbachol or methacholine showed biphasic curves also in the brainstem. In this area, only one third of the response was inhibited with high affinity, and p-F-HHSiD was more potent as an antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical markers of neurotoxicity: research strategies and epidemiological applications

A great deal of attention has been given in the past few years to the development and validation of biomarkers for non-cancer endpoints, to be used in human epidemiological studies. In this research, as they apply to the field of neurotoxicology, will be discussed. As biomarkers are often divided into indicators of exposure, effect and susceptibility, one example for each of these classes is presented. Measurements of hemoglobin adducts were developed as a way to monitor exposure to acrylamide (a peripheral neurotoxicant) in animals as well as humans, and have been successfully applied in a field study in occupationally exposed workers. Activity of monoamine oxidase B (MAO-B) in platelets was found to be inversely correlated with the levels of exposure to styrene, suggesting that this biochemical measurement may be a useful effect-related biomarker, though additional studies are needed to understand the mechanistic implications of these findings. Serum paraoxonase activity has been shown in animals to modulate the toxicity of organophosphorus insecticides. Studies in humans have lead to the identification of a genetic polymorphism for this enzyme and to define its underlying molecular basis. Epidemiological studies are needed to determine the relevance of this polymorphism in determining susceptibility to organophosphate toxicity.

Mechanisms of neurotoxicity: applications to human biomonitoring

Interactions of chemicals with cerebral neurotransmitters, receptors, and second messenger systems are often accompanied by similar changes involving components in non-neural tissues. On this basis, indirect strategies have been developed to investigate neural cell function parameters by methods using accessible cells such as platelets or peripheral blood lymphocytes. The validity of certain surrogate markers of biochemical events occurring in the nervous system has been documented by recent studies in both laboratory animals and humans. Although experience with neurotoxicants is still limited, advantages and limitations of methods using peripheral blood cells as indicators of chemically-induced nervous system changes have been documented by a number of studies in psychopharmacology and biological psychiatry. Applicability of this approach in conventional population studies of environmental chemicals remains to be demonstrated. However, recent data regarding the action of low doses of mercury and organophosphates on receptors and signal transduction pathways in peripheral lymphocytes suggest useful applications of certain surrogate markers in mechanistic studies of neurotoxicity in vivo and, possibly, in assessing early biochemical effects of neurotoxicants in humans.