Lack of correlation between the effects of transient exposure to glutamate and those of hypoxia/reoxygenation in immature neurons in vitro

To assess the influence of brain immaturity on the effects of oxygen deprivation and the participation of excitotoxicity, the consequences of a 6-h exposure to either hypoxia (95% N2/5% CO2) or 100 microM glutamate were studied in cultured fetal rat forebrain neurons taken at two maturational stages, i.e., 6 and 13 days in vitro. Cells were examined for their morphology, viability, energy metabolism reflected by 2-D-[3H]deoxyglucose uptake, and protein synthesis assessed by [3H]leucine incorporation. Apoptosis and necrosis were scored using the fluorescent dye 4,6-diamidino-2-phenylindole. Whereas 6-day-old neurons responded to a 6-h hypoxia by transient hypermetabolism, biphasic increase in protein synthesis, and cycloheximide-sensitive apoptotic death within 72 h postexposure, glutamate did not affect cell characteristics by the same time. In 13-day-old neurons, hypoxia induced both apoptosis (8.2%) and necrosis (22.3%). At this age, glutamate definitely reduced energy metabolism (26%) and protein synthesis (17%) by the end of exposure. The percentage of necrotic neurons reached 40.7%, but the rate of apoptosis was unchanged compared with controls. Therefore, excitotoxicity cannot account for hypoxia-induced injury in immature neurons, but its participation is suggested in older cells by the suppression of the necrotic component of hypoxia by glutamate receptor antagonists at 13 days.