Oxidative and non-oxidative mechanisms of neuronal cell death and apoptosis by L-3,4-dihydroxyphenylalanine (L-DOPA) and dopamine

1 The present study was designed to evaluate the nature of intervening agents in L-DOPA- and dopamine-induced neurotoxicity in Neuro-2A cells. 2 In the absence of cells and in conditions of light protection, at 37degreesC, L-DOPA or dopamine (1 mm) in culture medium degraded spontaneously in a time-dependent manner, this being prevented by ascorbic acid (200 mum) and other antioxidants, namely glutathione (1 mm), N-acetyl-L-cysteine (1 mm), sodium metabisulphite (200 mum), but not N-ter-butyl-alpha-phenylnitrone (1 mm) and deferoxamine (100 mum). 3 The viability of Neuro-2A cells declined following treatment with L-DOPA or dopamine in. a concentration- and time-dependent manner. The decrease in cell viability by L-DOPA (10+/-4% of control) or dopamine (15+/-4% of control) was markedly attenuated by antioxidants (ascorbic acid, glutathione, N-acetyl-L-cysteine and sodium metabisulphite). Autoxidation of L-DOPA or dopamine was accompanied by the formation of H2O2 in a time-dependent manner, this being completely prevented by ascorbic acid at 24 h or markedly reduced at 48 h. 4 Protective effects of 100 U ml(-1) catalase (40+/-1% of control) against L-DOPA-induced cell death were lower than those conferred by 200 mum ascorbic acid (70+/-3 % of control). Catalase-induced protection (59+/-5% of control) against dopamine-induced cell death was similar to that conferred by 200 pm ascorbic acid (57+/-4% of control). L-DOPA-induced neuronal cell death was also accompanied by increases in caspase-3 activity, this being insensitive to ascorbic acid. Dopamine-induced increase in caspase-3 activity occurred only when autoxidation of the amine was prevented by ascorbic acid. 5 It is suggested that in addition to generation of H2O2 and quinone formation, L-DOPA- and dopamine-induced cell death may result from induction of apoptosis, as evidenced by increases in caspase-3 activity. Dopamine per se induces apoptosis by a mechanism independent of oxidative stress, as evidenced by the fact that increases in caspase-3 activity occurred only when autoxidation of the amine was prevented.