Differential activation of c‐fos and caspase‐3 in hippocampal neuron subpopulations following neonatal hypoxia‐ischemia

Neonatal hypoxia‐ischemia (HI) induces immediate early gene (IEG) c‐fos expression as well as neuron death. The precise role of IEGs in neonatal HI is unclear. We investigated the temporal and spatial patterns of c‐Fos expression in postnatal day 7 mice after unilateral carotid ligation and exposure to 8% oxygen. mRNA levels of c‐fos quantitated by real‐time polymerase chain reaction (PCR) increased nearly 40‐fold (log 1.2 ± 0.4) in the ipsilateral hippocampus 3 hr following neonatal HI, then returned to basal levels within 12 hr, although no change was observed in c‐jun mRNA. Frozen coronal brain sections were stained with cresyl violet or used for immunohistochemical detection of c‐Fos, cleaved caspase‐3, glial fibrillary acidic protein (GFAP), and the mature neuron marker NeuN. c‐Fos immunoreactivity increased throughout the injured hippocampus 3 hr after HI but became restricted to the CA2–3 subregion and the dentate gyrus (DG) at 6–12 hr and declined by 24 hr. In contrast, cleaved (activated) caspase‐3 immunoreactivity was most abundant in the ipsilateral CA1 region at 3–6 hr after neonatal HI, then became more prominent in CA2–3 and DG. Double‐labeling experiments showed c‐Fos and cleaved caspase‐3 immunoreactivity localized in spatially distinct neuron subpopulations. Prominent c‐Fos immunoreactivity was observed in surviving CA2–3 and external granular DG neurons, and robust cleaved caspase‐3 immunoreactivity was observed in pyknotic CA1, CA2–3, and subgranular DG neurons. The differential expression of c‐Fos in HI‐resistant hippocampal subpopulations vs. cleaved caspase‐3 in dying neurons suggests a neuroprotective role for c‐Fos expression in neonatal HI. © 2007 Wiley‐Liss, Inc.