Extended role of necrotic cell death after hypoxia-ischemia-induced neurodegeneration in the neonatal rat

Abstract

The relative contribution of apoptosis and necrosis after neonatal hypoxia-ischemia (HI) is still a matter of debate. Here we determined the time course of necrotic cell death after neonatal HI and its relationship to caspase-3 activation and apoptotic cell death. Necrosis was evaluated by intracerebroventricular injection of propidium iodide (PI) before sacrificing the animal and processing brain sections for caspase-3 immunohistochemistry and TUNEL assay. PI-positive cells were found starting from 30 min after HI and increased rapidly in different brain areas. PI co-localized with the neuronal-specific nuclear marker NeuN but not with GFAP indicating that the dye label neurons with damaged plasma membrane but not reactive astrocytes. In the cerebral cortex 24 h after HI, the superficial layers showed cells with strong caspase-3 and TUNEL staining and with nuclei having apoptotic morphology whereas the deep layers of the cortex and the hippocampus showed cells with necrotic features. At later times, cells of the superficial layers were positive to PI, caspase-3, TUNEL and cathepsin-B. These data indicate that necrosis has an extended role in the progression of brain injury after neonatal HI and that a different spectrum of suicidal programs can be activated in the same cell. The extended period of caspase-3 activation in PI-positive necrotic cells supports the possibility that the apoptotic-to-necrotic continuum may ensue as the result of an incomplete execution of the apoptotic program.