Treatment with DAPT, an inhibitor of the Notch-activating enzyme, gamma-secretase is known to reduce damage to ischemic brain. However, the molecular mechanisms supporting this therapeutic effect are not fully understood. Here we demonstrated that Notch/RBP-J signaling is activated in NG2(+) glial progenitors and reactive astrocytes such as GFAP(+) cells, Nestin(+) cells and RC2(+) cells, using Notch/RBP-J signaling reporter mice. 3-day DAPT treatment reduced the number of reactive astrocytes but not NG2(+) glial progenitors. BrdU labeling experiments have shown that this reduction was due to decreased proliferation of reactive astrocytes. DAPT inhibited nuclear-translocation of Olig2, which is indispensable for proliferation and differentiation of reactive astrocytes. These findings suggest that Notch signaling might promote proliferation and differentiation of reactive astrocytes through the regulation of nucleo-cytoplasmic translocation of Olig2. (C) 2013 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved.