Distinguishing excitotoxic from apoptotic neurodegeneration in the developing rat brain

Much confusion has arisen recently over the question of whether excitotoxic neuronal degeneration can be considered an apoptotic phenomenon. Here, we addressed this question by using ultrastructural methods and DNA fragmentation analysis to compare a prototypic apoptotic in vivo central nervous system cell death process (physiologic cell death in the developing rat brain) with several central nervous system cell death processes in the in vivo infant rat brain that are generally considered excitotoxic (degeneration of hypothalamic neurons after subcutaneous administration of glutamate and acute neurodegeneration induced by hypoxia/ischemia or by concussive head trauma). We found by ultrastructural analysis that glutamate induces neurodegenerative changes in the hypothalamus that are identical to acute changes induced in the infant rat brain by either hypoxia/ischemia or head trauma, and that these changes are fundamentally different both in type and sequence from those associated with physiologic cell death (apoptosis). In addition, we show by ultrastructural analysis that concussive head trauma induces both excitotoxic and apoptotic neurodegeneration, the excitotoxic degeneration being very acute and localized to the impact site, and the apoptotic degeneration being delayed and occurring in regions distant from the impact site. Thus, in the head trauma model, excitotoxic and apoptotic degeneration can be distinguished not only by ultrastructural criteria but by their temporal and spatial patterns of expression. Whereas ultrastructural analysis provided an unambiguous means of distinguishing between excitotoxic and apoptotic neurodegeneration in each example analysed in this study, DNA fragmentation analysis (TUNEL staining or gel electrophoresis) was of no value because these tests were positive for both processes.