Modification of glutamate- effects on neuroblastoma cells in culture by heavy metals

Uptake of glutamate and cysteine in C-6 glioma cells and in cultured astrocytes

The uptake of glutamate in rat glioma C-6 cells and cultured astrocytes derived from rat cerebral hemispheres was found to be mediated by a Na(+)-dependent and a Na(+)-independent system. The Na(+)-dependent system was inhibited by aspartate and was consistent with the commonly occurring system designated system X-AG. The Na(+)-independent system was inhibited by cystine and was consistent with system x-c described in various types of cells in the periphery. It was also found that quisqualate selectively and competitively interfered with the Na(+)-independent glutamate uptake. In C-6 cells, the glutamate uptake via systems X-AG and x-c accounted for approximately 35% and 55% of the total uptake, respectively, at 0.05 mM glutamate. In cultured astrocytes, the glutamate uptake via system X-AG was very potent, whereas the uptake via system xc- was relatively weak and its contribution to the total uptake of glutamate seemed almost negligible. However, in both C-6 cells and astrocytes, system xc- was necessary for the uptake of cystine, another substrate of system xc-. Cystine in the culture medium was an essential precursor of glutathione, and the inhibition of the cystine uptake by excess glutamate as a competitor led to a severe deficiency in glutathione, followed by cell degeneration.

Glutamate-induced inward current in a clonal neuroblastoma cell line

The presence of glutamate-induced electrophysiological responses was examined in eight clonal neuroblastoma cell lines with the whole-cell voltage clamp technique. Only N2A cells responded to glutamate superfusion with a concentration-dependent, reversible inward current. Superfusion with the analogues kainic acid, quisqualic acid and N-methyl-D-aspartic acid also evoked inward currents but they had a smaller amplitude. The results indicate that N2A neuroblastoma cells could serve as an in vitro model to study the functional properties of glutamate receptors and associated ion channels.

Effects of manganese oxide on monkeys as revealed by a combined neurochemical, histological and neurophysiological evaluation

Four monkeys were exposed to a total of 8 g each of manganese as oxide by repetitive subcutaneous injections during 5 months, after which they were left for 1 week to 6 months before they were sacrificed. All animals developed hyperactive behaviour after about 2 months. About 5 months after the start of the exposure the animals became hypoactive with an unsteady gait, and subsequently an action tremor appeared in some of the animals. The animals lost power in both upper and lower limbs and the movements of the hands and feet were very clumsy. The serum content of manganese rose 10-40 times during the exposure time and the content in brain was generally increased more than 10 times, with the highest content found in globus pallidus and putamen. The observed neurochemical effects were also largest in globus pallidus and putamen. In these regions there was a considerable depletion of dopamine and 3,4-dihydroxyphenylacetic acid, while the homovanillic acid content remained almost unchanged. A severe neuronal cell loss was observed in globus pallidus but not in other regions. This is in accordance with results from the most recent neuropathological study of a human suffering from chronic manganese poisoning [Yamada et al. (1986) Acta Neuropathol 70: 273-278] where globus pallidus was devoid of neuronal cells while the content of pigmented cells in substantia nigra was normal. Our data suggest a reduction in number of dopaminergic nerve terminals, as the activity of the dopamine synthesizing enzyme DOPA-decarboxylase was also lowered.(ABSTRACT TRUNCATED AT 250 WORDS)

The mechanism of glutamate-induced degeneration of cultured Huntington's disease and control fibroblasts

Human cultured skin fibroblasts undergo rapid cellular degeneration and cell death when exposed to moderate levels of glutamate (10-30 mM). Huntington's disease (HD) skin fibroblast cultures are more sensitive to the toxic effects of glutamate (Gray et al. 1980) and elucidation of the toxic mechanism may bear on degenerative processes occurring in the HD brain. Glutamate toxicity was found to be inversely related to cystine content in the culture medium. Supplementing culture media with cystine effectively suppressed toxicity of L-Glu and that of a closely related but more potent analogue, L-homocysteic acid (L-HCA). Glutamate and L-HCA treatment provoked a rapid and marked decrease in cellular glutathione levels prior to cell death. Depletion of cellular glutathione with buthionine sulfoximine potentiated the toxicity of glutamate and L-HCA. Glutamate- or L-HCA-induced cell death also could be reduced or inhibited by co-treatment with antioxidants, thereby suggesting that free radical-induced peroxidative damage may ultimately be responsible for the loss of cell viability. While no site for the differential sensitivity of HD and normal fibroblasts to glutamate was clearly discerned, our results suggest that HD fibroblasts are more susceptible to peroxidative damage. The increased deposition of lipofuscin (Tellez-Nagel et al. 1974) and abnormal neuronal membrane ultrastructure seen in HD (Roizin et al. 1979) suggest similar degenerative processes may occur in HD brain.

Effects of L-glutamate on viabilities of cultured diploid skin fibroblasts and lymphocytes. Increased toxicity not observed in Huntington's disease

The toxicity of L-glutamic acid toward cultured fibroblasts and lymphocytes from age-matched, sex-matched controls, patients with Huntington's disease (HD) and patients with other diseases has been studied. In an initial non-blinded study, L-glutamic acid (15 mM) exerted a significant toxic effect on HD fibroblasts, decreasing viability by approximately 60% after 48 h exposure. The magnitude of the toxic effect on HD fibroblasts as a group was significantly different from the mean effect on the normal control group (P less than 0.003) and non-HD control group (P less than 0.004). However, there was variability in the sensitivity of a given fibroblast culture to L-glutamate. This toxic effect was also seen in several normal control and non-HD control fibroblasts. In a second blinded study using cultured fibroblasts from HD patients and age-matched, sex-matched controls, we were unable to distinguish between HD and control cultures. No difference in L-glutamate toxicity was observed between control and HD lymphocytes in short-term cultures. We conclude that the toxicity of L-glutamate is not specific for HD cells and that this experimental approach will be of little value in identifying cells from patients with HD.

Sensitive and insensitive states of cultured glioma cells to glutamate damage

Cytotoxic effects of L-glutamate and related compounds were investigated on rat glioma C6 cells in vitro. Within 12-24 h, addition of glutamate to the culture medium, resulted in degeneration of the C6 cells. The ED50 for glutamate-induced damage was about 4 mM. Seventeen structural analogues of glutamate, including agonists and antagonists for glutamate receptors as well as glutamate-uptake inhibitor, were examined concerning their toxicity on C6 cells. Among them, L-aminoadipic acid, DL-aminopimelic acid, DL-homocysteic acid, L-cysteic acid, quisqualic acid, L-glutamic acid diethyl ester and 2-amino-4-phosphonobutyric acid elicited similar degeneration at comparable concentrations. The D-isomer of glutamate was not cytotoxic. Following differentiation of C6 cells with 1 mM dibutyryl cyclic AMP or 3 mM sodium butyrate, they were no longer susceptible to L-glutamate and L-aminoadipate. C6 cells treated with 10 microM hydrocortisone, which is known to induce glutamine synthetase activity, were also resistant to L-glutamate, but not to L-aminoadipate. The decomposition of cellular DNA in glutamate-treated cultures was confirmed by flow cytometer analysis. The results demonstrate that the sensitivity of C6 cells to glutamate-induced cytotoxicity was modified by cellular metabolic conditions. This indicates that cultured glioma C6 cells are a useful model system to investigate the molecular mechanism of glutamate gliotoxicity in vitro.

Neurochemical, pharmacological, and developmental studies on cerebellar receptors for dicarboxylic amino acids

Specific binding of L-[3H]glutamate ([3H]Glu) and L[3H]Asp) to cerebellar membranes represented a time-, temperature-, pH- and protein-dependent interaction which was both saturable and reversible. Binding sites for both radioligands appeared maximally enriched in synaptosomal fractions isolated by gradient centrifugation. Kinetically derived dissociation constant (K off/K on = Kd) for [3H]Glu binding to this fraction indicated high-affinity (433 nM). Competition experiments employing analogs of excitatory amino acids, including new antagonists, helped identify binding sites for [3H]Glu and [3H]Asp as receptors with differential pharmacological specificities. Membrane freezing reduced numbers of both receptor types, but binding activity could be recovered partially by incubation at 37 degrees C. Glu receptors exhibited a pronounced deleterious sensitivity to thiol modifying reagents and L-Glu (50-1000 microM) provided protection against these compounds during co-incubation with cerebellar membranes. It is suggested that cold storage may induce partially reversible receptor inactivation by promoting sulfhydryl group/bond modification. Rat cerebellar glutamatergic function (endogenous Glu content, Glu uptake and receptor sites) exhibited an apparent ontogenetic peak between days 8-12 postpartum with a plateauing profile from day 30 to adulthood. The accelerated development (days 8-12) coincides with the first demonstrable Glu release and kainic acid neurotoxicity, as described previously.