MPP+ toxicity in rat striatal slices: Relationship between non-selective effects and free radical production

Incubations of rat striatal slices have been used to assay MPP⁺ neurotoxicity. MPP⁺, at concentrations of 1 mM or higher, caused a marked increase in hydroxyl radicals, measured as malondialdehyde (MDA) accumulation, but not in nitric oxide production. At these doses, MPP⁺ showed an effect on dopamine terminals, causing a massive dopamine decrease, and on non-neuronal glial cells, where a marked reduction in glutamine synthetase activity was detected. At lower concentrations (25 μM), the toxic effect on dopaminergic endings was maintained without increasing malondialdehyde concentrations or inhibiting glutamine synthetase activity. The effect on glutamine synthetase was prevented by the addition to the medium of 0.5% dimethyl sulfoxide, a hydroxyl-radical scavenger, but this did not protect the effect of dopamine depletion. We propose that non-selective effects of MPP⁺, at doses of 1 mM or higher, are mediated by extracellular overproduction of hydroxyl radicals. The main factor responsible for this overproduction would not be the released dopamine but rather the MPP⁺ itself, through non selective inhibition of the mitochondrial respiratory chain or through a redox cycling that can trigger oxygen radical production.