Calpain inhibition prevents Bax activation and translocation after excitotoxicity. A, B, Treatment of oligodendrocytes with 10 μm AMPA (A) or 3 μm kainate (KA) (B), in the presence of CTZ or GYKI53655, respectively, activates calpain. This effect was prevented by the calpain inhibitors PD150606, calpeptin, or MDL 28170. At different times after treatment, calpain activity was measured in live cells using the Calpain-Glo Protease Assay, and the results are shown as a percentage with respect to the control (untreated cells; 100%). C, Bax activation and translocation were inhibited by PD150606, a calpain inhibitor. Oligodendrocytes were treated for 15 min with 10 μm AMPA plus 100 μm CTZ and loaded for 15 min with Mitotracker Orange to identify mitochondria (red). Cells were processed for Bax immunofluorescence using the Bax-NT antibody, which only recognized activated Bax. Exposure to AMPA triggered Bax activation (green) and its translocation into mitochondria (yellow). Those effects were prevented by the calpain inhibitor. Scale bar, 20 μm. *p < 0.05, **p < 0.01, ***p < 0.001.