COMPARISON OF THE NEUROTOXICITY OF DIHYDROXYPHENYLALANINE STEREOISOMERS IN CULTURED DOPAMINE NEURONS

Oxidant stress resulting from excess dopamine (DA) may contribute to t he development and progression of Parkinson's disease (PD). Free radic als resulting from the enzymatic metabolism of DA are most often discu ssed in this regard. However, levodopa (L-DOPA) and DA can also underg o autooxidation, producing free radicals as well as cytotoxic metaboli tes, We evaluated the neurotoxic effects of the two stereoisomers of L -DOPA to differentiate between enzyme-mediated and autooxidation mecha nisms, Various concentrations of D- or L-DOPA (1 mM through 10 nM) wer e added to freshly harvested rostral mesencephalic tegmentum cultures. After 72 h, the cultures were fixed and stained for tyrosine hydroxyl ase (TH), The number of TH-immunoreactive (THir) neurons was then asse ssed and used as an index of DA neuron survival. Both D- and L-DOPA in duced a dose-dependent loss of THir neurons (F-10,F-21 = 135.75, p < 0 .0001 and F-10,F-21 = 142.53, p < 0.0001, respectively) with ED(50) va lues of 10(-5.3) and 10(-5.2) M, respectively, The dose-response curve s for each drug were not significantly different from one another (F-1 ,F-43 = 0.09, p > 0.05), Moreover, both drugs killed THir as well as n on-THir cells at high concentrations, suggesting a nonspecific toxic e ffect, These data are most consistent with an enzyme-independent, auto oxidation-mediated mechanism for DA neuron loss.