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M Aschner

Publications and source records attributed to M Aschner.

At least 73 records · Page 4Linked to original sources
PubMed1994

Astrocytes as potential modulators of mercuric chloride neurotoxicity.

1. MC has been shown to inhibit the uptake of L-glutamate and increase D-aspartate release from preloaded astrocytes in a dose-dependent fashion. 2. Two sulfhydryl (SH-)-protecting agents; reduced glutathione (GSH), a cell membrane-nonpenetrating compound, and the membrane permeable dithiothreitol (DTT), have been shown consistently to reverse the above effects. MC-induced D-aspartate release is completely inhibited by the addition of 1 mM DTT or GSH during the actual 5-min perfusion period with MC (5 microM); when added after MC treatment, DTT fully inhibits the MC-induced D-aspartate release, while GSH does not. 3. Neither DTT nor GSH, in the absence of MC, have any effect on the rate of astrocytic D-aspartate release. Other studies demonstrate that although MC treatment (5 microM) does not induce astrocytic swelling, its addition to astrocytes swollen by exposure to hypotonic medium leads to their failure to volume regulate. 4. Omission of calcium from the medium greatly potentiates the effect of MC on astrocytic D-aspartate release, an effect which can be reversed by cotreatment of astrocytes with the dihydropyridine Ca(2+)-channel antagonist nimodipine (10 microM), indicating that one possible route of MC entry into the cells is through voltage-gated L-type channels.

Animals↗
PubMed1994

Manganese (Mn) transport across the rat blood-brain barrier: saturable and transferrin-dependent transport mechanisms.

Using a single capillary pass technique, the transport of manganese (Mn) across the rat blood-brain barrier (BBB) was characterized. Initial rate measurements (15 s) of Mn2+ [0-1000 microM] accumulation in rat brains clearly indicated saturation kinetics by both 1/v vs. 1/s plots, and plots of v vs. [s]. Common carotid injection of freshly mixed Mn2+ with transferrin at a 1:10 molar ratio did not result in a significant change in the initial rate of Mn brain levels compared with injection of Mn2+ alone. However, when Mn2+ was incubated at 25 degrees C in the presence of transferrin at a 1:10 ratio for up to 5 days prior to common carotid injection, the initial rate of Mn uptake by brain was incubation-time-dependent, increasing linearly with prolonged incubations. These findings suggest that the saturable component of divalent Mn transport into brain represents but one of the transport mechanisms for Mn across the BBB. A second transport system for Mn may occur by a transferrin-conjugated Mn transport system.

Animals↗
PubMed1994

Potassium and taurine release are highly correlated with regulatory volume decrease in neonatal primary rat astrocyte cultures.

Neonatal rat primary astrocyte cultures were swollen by exposure to hypotonic buffer. Using an electrical impedance method for determination of cell volume coupled with on-line measurements of efflux of radioactive ions or amino acids, we have investigated the role of K+ (using 86Rb), taurine, and D-aspartate (an analogue of glutamate) in regulatory volume decrease (RVD). Addition of 1 mM quinine, 10 microM nimodipine, 100 microM BAPTA-AM, 10 microM trifluoperazine, or a calcium-free buffer significantly (p < 0.0001) inhibited RVD. This was accompanied by inhibition of 86Rb release but an increase in D-[3H]-aspartate release, which was proportional to the degree to which RVD was inhibited. These results support a regulatory role for calcium in RVD and show that inhibition of calcium entry from the extracellular fluid, intracellular calcium sequestration, inhibition of calcium-activated K+ channels, and inhibition of calmodulin all inhibit RVD. Because D-[3H]aspartate efflux profiles increase as RVD is inhibited, it is unlikely that D-aspartate release is a main determinant of RVD. In contrast, [3H]taurine release was increased by 1 mM quinine and inhibited by 10 microM trifluoperazine. The net release of K+ and taurine is highly correlated with the degree of RVD, implicating a regulatory role for both K+ and taurine release in RVD.

Animals↗
PubMed1994

Astrocytes as mediators of methylmercury neurotoxicity: effects on D-aspartate and serotonin uptake.

In this study we address the effects of methylmercuric chloride (MeHgCl), a metal that is preferentially sequestered in astrocytes, on 5-HT and glutamate/aspartate uptake by rat primary astrocyte cultures. Quantitative autoradiography (ARG) combined with glial acidic fibrillary protein (GFAP) immunocytochemistry, as well as intact-cell (bulk) measurements of radiolabel uptake of these neurotransmitters were performed in 7- and 21-day-old primary astrocyte cultures. MeHg (10 microM for 30 min) treatment of astrocytes (21 days in culture) significantly inhibited the Na(+)-dependent and fluoxetine-sensitive [3H]5-HT uptake. D-aspartate uptake in 7- and 21-day-old cultures was even more sensitive to MeHg, leading to > 99% inhibition of D-aspartate uptake by astrocytes (30 min; 10 microM MeHg). These results imply that the Na(+)-dependent and fluoxetine-sensitive 5-HT uptake, as well as the Na(+)-dependent L-glutamate/D-aspartate uptake systems in primary astrocyte cultures are sensitive to low concentrations of MeHg. Since astrocytic removal of glutamate (and aspartate) and 5-HT from the extracellular space in situ is crucial to the maintenance of chemical homeostasis, MeHg-induced uptake inhibition of 5-HT and aspartate could have cytotoxic effects on neighboring neurons.

Animals↗
PubMed1993

Stimulation of D-aspartate efflux by mercuric chloride from rat primary astrocyte cultures.

Mercuric chloride (HgCl2; MC) was shown to increase D-aspartate release from preloaded astrocytes in a dose-dependent fashion. Two sulfhydryl (-SH) protecting agents, a cell membrane non-penetrating compound, reduced glutathione (GSH), and the membrane-permeable dithiothreitol (DTT), were found to inhibit the stimulatory action of MC on the efflux of radiolabeled D-aspartate. MC-induced D-aspartate release was completely inhibited by the addition of 1 mM DTT or GSH during the actual 5 min perfusion period with MC (5 microM). However, when added after MC treatment, this inhibition could not be sustained by GSH, while DTT fully inhibited the MC-induced release of D-aspartate. Neither DTT nor GSH alone had any effect on the rate of astrocytic D-aspartate release. Accordingly, it is postulated that the stimulatory effect exerted by MC on astrocytic D-aspartate release is associated with vulnerable -SH groups located within, but not on the surface of the cell membrane. Omission of Na+ from the perfusion solution did not accelerate MC-induced D-aspartate release, suggesting that reversal of the D-aspartate carrier can not be invoked to explain MC-induced D-aspartate release. Furthermore, MC did not appear to be associated with astrocytic swelling.

Animals↗
PubMed1993

Lead increases inositol 1,4,5-trisphosphate levels but does not interfere with calcium transients in primary rat astrocytes.

Alteration of receptor-mediated signal transduction pathways by inorganic lead (Pb) has been postulated to contribute to the neurotoxicity of this environmental toxicant, some of these effects involving astrocytes. As Pb is known to mimic Ca2+ in various biological systems or alter Ca(2+)-mediated cellular processes, we analyzed the effect of Pb exposure on alpha 1 receptor activated astrocytic phosphoinositide metabolism and Ca2+ responses in primary astrocyte cultures prepared from cerebral cortex of 1-day-old rats. Exposure to norepinephrine (NE; 10-100 microM) resulted in a significant increase in astrocytic inositol 1,4,5-trisphosphate levels, concomitant with an increase in intracellular Ca2+ levels. Fifteen minute exposure to Pb (10 microM lead acetate) significantly increased inositol 1,4,5-trisphosphate generation compared with controls, both in the presence and absence of NE. However, the inositol 1,4,5-trisphosphate-mediated Ca2+ transients following NE stimulation was unaltered in the presence of Pb (1-100 microM). NE-evoked intracellular Ca2+ responses, both in the presence and absence of extracellular Ca2+ did not differ between control and Pb-treated astrocytes. Additional studies failed to demonstrate the occurrence of Pb influx into astrocytes within the first 12 min of exposure such that Ca2+ responses would be directly affected. It therefore appears unlikely that astrotoxic effects of Pb are mediated via direct changes in intracellular Ca2+ transients.

Animals↗
PubMed1993

The role of sulfhydryl groups and calcium in the mercuric chloride-induced inhibition of glutamate uptake in rat primary astrocyte cultures.

Inhibition by mercuric chloride (MC) of the astrocytic uptake of the excitotoxic neurotransmitter L-glutamate (L-GLU) has been postulated to contribute to MC neurotoxicity. In the present study, we analyzed the ability of two sulfhydryl (SH)-protecting agents: a cell membrane non-penetrating compound-reduced glutathione (GSH), and the membrane permeable dithiothreitol (DTT), to reverse the inhibitory action of MC on the initial rate of uptake of radiolabelled GLU (100 microM) in primary cultures of rat astrocytes. MC at 5 microM concentration reduced the uptake to 46% of control when present in the incubation medium during the 5 min of actual uptake, and to 27% of control when astrocytes were preincubated for 30 min in HEPES buffer containing MC prior to GLU uptake measurements. GLU uptake inhibition caused by 30 min preincubation with MC was partly relieved by the addition of 1 mM DTT during the actual 5 min uptake period. However, this inhibition could not be reversed by 1 mM GSH. Accordingly, it is postulated that the inhibitory effect exerted by MC on GLU uptake is associated with vulnerable SH groups located within, but not on the surface of the cell membrane. Neither 5 microM N-ethylmaleimide (NEM) nor 5 microM or 25 microM iodoacetate (IA) affected GLU uptake, indicating steric hindrance of the access of these two sulfhydryl reagents to the SH groups critical for the uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Methylmercury-induced alterations in excitatory amino acid transport in rat primary astrocyte cultures.

To determine whether methylmercury (MeHg) has the potential to interfere with homeostatic functions in neonatal rat cortical primary astrocyte cultures, the effects of MeHg on the uptake and efflux of both L-glutamate and D-aspartate were examined. Uptake of both of these excitatory amino acids (EAAs) was significantly (P < 0.05) reduced in the presence of MeHg concentrations as low as 10(-5) M. Efflux of both glutamate and aspartate from preloaded astrocytes was also increased by MeHg in a dose- and time-dependent fashion. Since in our earlier studies we had found that MeHg causes dose-dependent astrocytic swelling, which could have been the mechanism of the increased efflux, we examined whether blockage of conductive ion fluxes, which have been implicated in astrocytic swelling, could reverse the MeHg-induced increase in L-glutamate and D-aspartate efflux. Three compounds which inhibit the hypotonic-media-induced efflux of EAA, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), L-644,711 (a derivative of ethacrynic acid) and furosemide were tested at their maximal concentrations for their ability to reverse MeHg-induced EAA efflux. Only furosemide (5 mM) could sustain the reversal for the entire 120 min duration of the efflux measurement. Since hypotonic-media swelling-induced release of EAAs is inhibited by these anion inhibitors (in the following rank order: L-644,711 > SITS > furosemide), we conclude that different mechanisms account for EAA release from primary astrocyte cultures during MeHg exposure as compared to hypotonic media-induced efflux.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗
PubMed1993

Astrocytes: targets and mediators of chemical-induced CNS injury.

It is now well established that a reciprocal relationship exists between neurons and astrocytes, and that this association is vital for mutual differentiation, development, and functioning of both cell types. It had also become apparent that perturbations in astrocytic function may lead to deleterious consequences in juxtaposed neurons. It is therefore possible that neuronal damage induced by chemicals or neuropathic disease involves dissociation of astrocytic-neuronal interactions. The purpose of this review is to explore astrocytic-neuronal interactions, focusing on potential sites of neurotoxicant actions. In developing this thesis, we briefly examine the functional interactions between astrocytes and neurons, followed by specific examples of astrocyte-mediated neurotoxicity.

Animals↗
PubMed1992

Interactions of trimethyl tin (TMT) with rat primary astrocyte cultures: altered uptake and efflux of rubidium, L-glutamate and D-aspartate.

Studies were undertaken to assess the effects of trimethyl tin (TMT) on metabolic functions in primary neonatal rat cultured astrocytes. Concentrations as low as 10(-5) M TMT significantly inhibited the initial rate (1 min) of uptake of 86RbCl, used as a tracer for K+. TMT also markedly inhibited the initial rate (1 min) of Na(+)-dependent uptake of L-[3H]glutamate and D-[3H]aspartate, and stimulated the release of intracellular 86Rb+, -[3H]glutamate and D-[3H]aspartate in a dose-dependent fashion. These observations support the hypothesis that the astrocyte plasma membrane is potentially an important target for TMT's toxic effect and specifically that small concentrations of this organometal can inhibit the ability of astrocytes to maintain a transmembrane K+ gradient. This would be expected to compromise the ability of astrocytes to control extracellular K+ either by spatial buffering or active uptake, and exacerbate on-going swelling. Increased levels of glutamate and aspartate in the extracellular fluid upon release from astrocytes may play an important role in TMT neurotoxicity.

Animals↗
PubMed1992

Cellular and molecular effects of trimethyltin and triethyltin: relevance to organotin neurotoxicity.

Many of the neurotoxic aspects of organotin exposure have been described. Organotin exposure culminates in its accumulation in the CNS and PNS. The clinical picture is dominated by neurological disturbances; yet, the primary basis for their neurotoxicity is unknown. Trimethyltin (TMT) is primarily a CNS neurotoxin affecting neurons within the hippocampal pyramidal band and the fascia dentata. Triethyltin (TET) is a neurotoxin that produces a pathological picture dominated by brain and spinal cord edema. The first part of this review summarizes the current understanding of the interaction of TMT and TET with biologically active sites in the induction of neurotoxicity. In the second part, several hypotheses for the differential neurotoxic effects of these organotins and their shortcomings are discussed.

Animals↗
PubMed1992

Manganese uptake and efflux in cultured rat astrocytes.

Astrocytes play a central role in manganese (Mn) regulation in the CNS. Using primary astrocyte cultures from neonatal rat brains, these studies demonstrate a specific high-affinity transport system for Mn2+. Saturation kinetics are clearly indicated by both 1/v versus 1/s plots (Km = 0.30 +/- 0.03 microM; Vmax = 0.30 +/- 0.02 nmol/mg of protein/min) and plots of v versus [s]. Several divalent cations (Co2+, Zn2+, and Pb2+) failed to inhibit the initial rate of 54Mn2+ uptake. In contrast, extracellular Ca2+ at 10 microM decreased 54Mn2+ uptake. Exchange with extracellular Mn2+ was not obligatory for the efflux of 54Mn2+ into extracellular medium because efflux occurred into Mn(2+)-free extracellular medium, but efflux of 54Mn2+ was enhanced when astrocytes were equilibrated in the presence of unlabeled Mn2+. Efflux of 54Mn2+ was biphasic with both a rapid and a slow component. Efflux was most rapid during the first 10 min of incubation, with 27.5 +/- 2.2% of 54Mn2+ transported extracellularly, and 37.2 +/- 1.2% of preloaded 54Mn2+ was retained by the astrocytes at 120 min. These studies show, for the first time, that mammalian astrocytes can transport Mn via a specific transport system.

Animals↗
PubMed1991

Interactions of methylmercury with rat primary astrocyte cultures: methylmercury efflux.

Methylmercury (MeHg) efflux from rat astrocyte cultures was studied to complement our previous studies on uptake of MeHg in these cells. Exchange with extracellular MeHg was not obligatory for the efflux of [203Hg]MeHg into the extracellular media, because efflux occurred into MeHg-free extracellular media, but stimulation of [203Hg]MeHg net efflux was shown when astrocytes were equilibrated in the presence of 'cold' MeHg and graded concentrations of L-cysteine. Net efflux of MeHg was most rapid for the first 5 min, and approximately 20% of preloaded [203Hg]MeHg was lost from the astrocytes by 60 min. Uptake of [203Hg]MeHgCl was maximal by 30 min and did not increase when the loading period was extended up to 4 h. However, the total amount of intracellular 203Hg that was available for net efflux gradually decreased as the duration of the preloading period increased. MeHg net efflux from astrocytes was unchanged when [203Hg]MeHgCl preloaded astrocytes were equilibrated in hypotonic buffer, suggesting that unlike ions and amino acids swollen astrocytes remain impervious to MeHg efflux. Thus, the main MeHg efflux transport system is apparently specific for the MeHg-L-cysteine conjugate and represents transport by the same neutral amino acid System L that facilitates its uptake.

Animals↗
PubMed1991

Effects of mercury and lead on rubidium uptake and efflux in cultured rat astrocytes.

Astrocytes readily sequester lead and mercury (8, 10, 19, 22). Accordingly, studies were undertaken to assess the effects of lead and mercury on homeostatic functions in neonatal rat brain primary astrocyte cultures. Both inorganic and organic mercury, but not lead, significantly inhibited the initial rate (5 min) of uptake of 86RbCl, used as a tracer for K+, at concentrations of 10-100 microM. Mercury and to a lesser extent lead also stimulated the efflux of intracellular 86Rb+ at 10-500 microM. These observations suggest that the astrocyte plasma membrane may be an important target for lead and mercury, and that relatively low concentrations of these heavy metals should inhibit the ability of astrocytes to maintain a transmembrane K+ gradient.

Animals↗
PubMed1991

Manganese neurotoxicity: cellular effects and blood-brain barrier transport.

The observations by Couper in 1837 are acknowledged as the earliest description of the toxic syndrome associated with chronic manganese (Mn) exposure. Since that time, many of the neurotoxic aspects of manganism have been described, yet, the primary basis for its neurotoxicity remains unknown. Recent evidence corroborates the original hypothesis by Maynard and Cotzias (82) which invokes the mitochondrion as the target organelle for Mn cytotoxicity which is primarily expressed as a perturbation in Ca2+ homeostasis. Despite recognition that excessive Mn exposure culminates in Mn accumulation in the CNS and a clinical picture dominated by neurological disturbances, the role of the blood-brain barrier in the CNS uptake of Mn has received little attention. Accordingly, the first part of this review summarizes the current understanding of the interaction of Mn with biologically active sites in the induction of Mn cytotoxicity. The second part of this review summarizes what is known about Mn transport across the blood-brain barrier, a major regulator of the CNS milieu, with the contention that the rate and extent of Mn transport across the blood-brain barrier modulates its neurotoxicity.

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