Sustained activation of ERK signaling in astrocytes is critical for neuronal injury‐induced monocyte chemoattractant protein‐1 production in rat corticostriatal slice cultures

We previously demonstrated that N‐methyl‐d ‐aspartate (NMDA) treatment (50 μm , 3 h) induced astrocytic production of monocyte chemoattractant protein‐1 (MCP‐1, CCL2), a CC chemokine implicated in ischemic and excitotoxic brain injury, in rat corticostriatal slice cultures. In this study, we investigated the signaling mechanisms for NMDA‐induced MCP‐1 production in slice cultures. The results showed a close correlation between NMDA‐induced neuronal injury and MCP‐1 production, and an abrogation of NMDA‐induced MCP‐1 production in NMDA‐pretreated slices where neuronal cells had been eliminated. These results collectively indicate that NMDA‐induced neuronal injury led to astrocytic MCP‐1 production. NMDA‐induced MCP‐1 production was significantly inhibited by U0126, an inhibitor of extracellular signal‐regulated kinase (ERK). Immunostaining for phosphorylated ERK revealed that transient neuronal ERK activation was initially induced and subsided within 30 min, followed by sustained ERK activation in astrocytes. Treatment with U0126 during only the early phase (U0126 was washed out at 15 or 30 min after NMDA administration) suppressed early activation of ERK in neuronal cells, but not later activation of ERK in astrocytes. In this case, MCP‐1 production was not suppressed, suggesting that activation of neuronal ERK is not necessary for MCP‐1 production. In contrast, delayed application of U0126 at 3 h after the beginning of NMDA treatment inhibited MCP‐1 production to the same degree as that observed when U0126 was applied from 3 h before NMDA administration. These findings suggest that sustained activation of the ERK signaling pathway in astrocytes plays a key role in neuronal injury‐induced MCP‐1 production.