We performed coimmunoprecipitation with rat brains as described previously [22 (link)]. After rats were anesthetized and decapitated, brains were removed. The cerebellum was homogenized in cold isotonic sucrose homogenization buffer containing 0.32 M sucrose, 10 mM HEPES, pH 7.4, 2 mM ethylenediaminetetraacetic acid (EDTA), and a protease/phosphatase inhibitor cocktail (Thermo Scientific, Rochester, NY). Homogenates were centrifuged at 800×g for 10 min at 4 °C. P2 pellets (synaptosomal fraction) were obtained by centrifuging the supernatant for 15 min (10,000×g) at 4 °C. Washed P2 was resuspended in the sucrose homogenization buffer containing Triton X-100 (0.5 %, v/v). The suspension was lysed and incubated with gentle rotation for 20 min at 4 °C. Samples were centrifuged for 20 min at 32,000×g to yield the pellet enriched with synaptic membranes. These pellets were solubilized in sucrose-Triton buffer containing 1 % sodium deoxycholate and a protease/phosphatase inhibitor cocktail for 1 h at 4 °C. Solubilized proteins were incubated with a rabbit antibody against ERK1/2 or mGluR1a. The complex was precipitated with 50 % protein A or G agarose/sepharose bead slurry (GE Healthcare). Proteins were separated on Novex 4–12 % gels and probed with a mouse antibody against ERK1/2 or mGluR1a.