P7, P14 or P21 rats were treated with a single subcutaneous injections of vehicle or MK801 (1 mg/kg) and at 8 hours later animals were sacrificed. Expression of activated caspase-3 was determined by immunohistochemistry in the S1 (A, B, C, and D) and in layer II of M1/M2 (E, F, G and H): (A and E) P7 vehicle; (B and F) P7 8 hr-MK801; (C and G) P14 8 hr-MK801; and (D and H) P21 8 hr-MK801. Vehicle not shown for P14 and P21. Arrows - activated caspase-3-positive cells; scale bar is 50 μm.