Glutamate induces oxidative stress not mediated by glutamate receptors or cystine transporters: protective effect of melatonin and other antioxidants

Glutamate is responsible for most of the excitatory synaptic activity and o xidative stress induction in the mammalian brain. This amino acid is increa sed in the substantia nigra in parkinsonism due to the lack of dopamine res traint to the subthalamic nucleus. Parkinson's disease also shows an increa se of iron levels in the substantia nigra and a decrease of glutathione, th e antioxidant responsible for the ascorbate radical recycling. Considered t ogether, these facts could make the antioxidant ascorbate behave as a pro-o xidant in parkinsonism. Since both glutamate and ascorbate are present in t he synaptosomes and neurons of substantia nigra, we tested 1) if glutamate is able to induce oxidative stress independently of its excitatory activity , and 2) if ascorbate may have synergistic effects with glutamate when thes e two molecules co-exist. Brains were homogenized in order to disrupt membr anes and render membrane receptors and intracellular signaling pathways non -functional. In these homogenates glutamate induced lipid peroxidation, ind icating that this amino acid also may cause oxidative stress not mediated b y its binding to glutamate receptors or cystine transporters. Ascorbate als o induced lipid peroxidation thus behaving as a pro-oxidant. Both substance s together produced an additive effect but they did not synergize. Given th at melatonin is a potent physiological antioxidant with protective effects in models of neurotoxicity, we tested the role of this secretory product on the pro-oxidant effect of both compounds given separately or in combinatio n. We also checked the protective ability of several other, antioxidants. P harmacological doses of melatonin (millimolar), es estrogens, pinoline and trolox (micromolar) prevented the oxidant. effect of glutamate, ascorbate, and the combination of both substances. Potential therapeutic application o f these results is discussed.