Increased extracellular glutamate evoked by 1-Methyl-4-phenylpyridinium (MPP⁺) in the rat striatum is not essential for dopaminergic neurotoxicity and is not derived from released glutathione

A number of studies have implicated the interactions of the excitatory amino acid L-glutamate (Glu) with its ionotropic and metabotropic receptors as important components of the mechanism underlying the dopaminergic neurotoxicity of 1-methyl-4-phenylpyridinium (MPP⁺). Furthermore, microdi-alysis experiments have demonstrated that perfusion of relatively high concentrations of MPP⁺ into the rat striatum evoke a delayed, massive release of Glu. Interestingly, perfusion of MPP⁺ also mediates a similar release of glutathione (GSH). Together, these observations raise the possibility that the rise of extracellular Glu mediated by MPP⁺ may be the result of hydrolysis of released GSH by γ-glutamyl transpeptidase (γ-GT). In the present investigation it is demonstrated that perfusions of solutions of 0.7 and 1.3 mM MPP⁺ dissolved in artificial cerebro-spinal fluid into the rat striatum evoke neurotoxic damage to dopaminergic terminals, assessed by both a two-day test/challenge procedure and tyro-sine hydroxylase immunoreactivity, but without the release of Glu. Perfusions of ≥ 2.5 mM MPP⁺ cause more extensive dopaminergic neurotoxicity and a dose-dependent release of Glu. However, neither this release of Glu nor MPP⁺-induced dopaminergic neurotoxicity are blocked by the irreversible γ-GT inhibitor acivicin. Together, these observations indicate that a rise of extracellular levels of Glu is not essential for the dopaminergic neurotoxicity of MPP⁺. Furthermore, the rise of extracellular Glu caused by perfusion of ≥2.5 mM MPP⁺ is not the result of the γ-GT-mediated hydrolysis of released GSH. It is possible that the rise of extracellular levels of Glu, L-aspartate, L-glycine and L-taurine evoked by perfusions of ≥2.5 mM MPP⁺ into the rat striatum may reflect, at least in part, the release of these amino acids from astrocytes.