Alpha-synuclein has been implicated in the pathology of certain neurodegenerative diseases, including Parkinson disease (PD) and dementia with Lewy bodies (LBs). Overexpression of human alpha-synuclein in neuronal cells reduces cell viability, but the precise cellular and molecular mechanisms remain poorly understood. Gap junctional intercellular communication (GJIC) is thought to be essential for maintaining cellular homeostasis and growth control. In the present study, the effect of alpha-synuclein overexpression on GJIC in human dopaminergic neuroblastoma SH-SY5Y cells was investigated. Cells overexpressing wild-type alpha-synuclein were more vulnerable to hydrogen peroxide and 6-hydroxydopamine. GJIC was decreased in cells overexpressing alpha-synuclein. In addition, alpha-synuclein binds directly to connexin-32 (Cx32). As such, the post-translational modification of Cx32 was enhanced in cells overexpressing alpha-synuclein. These findings suggest that alpha-synuclein can modulate GJIC in a dopaminergic neuronal cell line through specific binding to Cx32.