Hypoxia-induced ischemic tolerance in neonatal rat brain involves enhanced ERK1/2 signaling

Hypoxic preconditioning (HP) 24 h before hypoxic-ischemic (HI) injury confers significant neuroprotection in neonatal rat brain. Recent studies have shown that the mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-kinase (PI3K) intracellular signaling pathways play a role in the induction of tolerance to ischemic injury in heart and brain. To study the role of MAPK (ERK1/2, JNK, p38MAPK) and PI3K/Akt/GSK3beta signaling pathways in hypoxia-induced ischemic tolerance, we examined the brains of newborn rats at different time points after exposure to sublethal hypoxia (8% O-2 for 3 h). Immunoblot analysis showed that HP had no effect on the levels of phosphorylated Akt, GSK3beta, JNK and p38MAPK. In contrast, significantly increased levels of phosphorylated ERK1/2 were observed 0.5 h after HP. Double immunofluorescence staining showed that hypoxia-induced ERK1/2 phosphorylation was found mainly in microvessels throughout the brain and in astrocytes in white matter tracts. Inhibition of hypoxia-induced ERK1/2 pathway with intracerebral administration of U0126 significantly attenuated the neuroprotection afforded by HP against HI injury. These findings suggest that activation of ERK1/2 signaling may contribute to hypoxia-induced tolerance in neonatal rat brain in part by preserving vascular and white matter integrity after HI.