Parkinson's disease is characterized by selective loss of dopaminergic neurons in the substantia nigia a by the appearance of Lewy bodies Fibrillar alpha-synuclein is the main component of Lewy bodies Previous studies have Suggested that dopamine promotes alpha-synuclein oligomerization and that partially aggregated or oligomeric alpha-synuclein could be cytotoxic To confirm this hypothesis using cell Cultures. we performed size exclusion chromatography as a pretreatment method prior to Western blotting to more clearly detect a small amount of alpha-synuclein oligomers in wild-type alpha-synuclein-overexpressing SH-SY5Y cells. Using this method, we confirmed that stable overexpression of alpha-synuclein in SH-SY5Y cells indeed increased the amounts of alpha-synuclein oligomers in these cells and exposure of the cells to dopamine for 6 h facilitated alpha-synuclein oligomerization. These dopamine-induced alpha-synuclein oligomers continued to exist for the following 24 h. However, the dopamine-treated cells did not undergo cell death or apoptosis in spite of the presence of increased oligomeric alpha-synuclein. Our data may contribute to the understanding of the mechanisms underlying alpha-synuclein oligomer formation and its suspected cytotoxicity toward dopaminergic neurons (C) 2009 Elsevier Inc All rights reserved