MAPK1 and hyperhomocysteinemia: This is the first study to evaluate the effect of levodopa-induced hyperhomocysteinemia on neurogenesis in vitro and in vivo The major findings were (1) elevated homocysteine by levodopa treatment exerted an anti-proliferative effect on NPCs in the SVZ, (2) levodopa-induced apoptosis in the NPCs is mediated by ERK signaling pathways possibly via the NMDA receptor, and (3) the dopamine agonist produced a greater increase in neurogenesis compare to levodopa.