Hypoxia-tolerant neonatal CA1 neurons: relationship of survival to evoked glutamate release and glutamate receptor-mediated calcium changes in hippocampal slices

Abstract

Neurons in the neonatal mammalian brain survive greater degrees of hypoxic stress than those in the mature brain. To investigate how developmental changes in glutamate receptor-mediated neurotoxicity contribute to this difference, we measured hypoxia-evoked glutamate release, glutamate receptor contribution to hypoxia-evoked intracellular calcium changes, and survival of hypoxia-/ischemia-sensitive CA1 neurons in rat hippocampus. Glutamate release was measured by a fluorescence assay, calcium changes in CA1 neurons with fura-2, and cell viability using Nissl and fluorescence staining with calcein-AM/ethidium homodimer, all in 300-micron thick hippocampal slices from 3-30 post-natal day (PND) rats. Glutamate released from PND 3-7 slices during hypoxia (PO2 = 5 mmHg) was only one third that of PND 18-22 slices. In PND 3-7 slices, survival of CA1 neurons after 5 min of hypoxia and 6 h of recovery was significantly greater than in PND 18-22 slices (viability indices 0.60 and 0.28, respectively, (p < 0.05). Five min of anoxia significantly altered Nissl staining pattern and morphology of CA1 neurons in PND 18-22 but not PND 3-7 slices. Hypoxia (PO2 = 5 mm Hg) caused three to five times greater increases in [Ca2+]i in PND 18-22 slices than in PND 3-7 slices (p < 0.001). During re-oxygenation, [Ca2+]i returned to baseline in PND 3-7 slices, but remained elevated in PND 18-22 slices. Glutamate receptor-mediated calcium changes in CA1 during hypoxia were 33% and 62% of the total calcium change in PND 3-7 and PND 18-22 CA1, respectively. We conclude that survival of CA1 neurons in PND 3-7 slices following hypoxic stress is associated with smaller increases and enhanced recovery of [Ca2+]i, less accumulation of glutamate, and less glutamate receptor-mediated calcium influx than in PND 18-22 slices.