Hypoxia-Induced Apoptotic Cell Death is Prevented by Oestradiol Via Oestrogen Receptors in the Developing Central Nervous System

The neuroprotective effects of oestrogens have been demonstrated against a variety of insults, including excitotoxicity, oxidative stress and cerebral ischemia under certain conditions. However, the molecular mechanisms underlying oestrogen neuroprotection are still unclear. We aimed to determine whether 17β‐oestradiol (E2) administration post‐hypoxia (p‐hx) was neuroprotective and whether these actions were mediated through oestrogen receptors (ER). For this purpose, 12‐embyonic day‐old chickens were subjected to acute hypoxia [8% (O2), 60 min], followed by different reoxygenation periods. To test the neuroprotective effect of E2 and its mechanism, embryos were injected 30 min after the end of hypoxia with E2 alone or with ICI 182 780, a competitive antagonist of ER. Cytochrome c (cyt c) release, an indicator of mitochondrial apoptotic pathway, was measured by western blot in optic lobe cytosolic extracts. DNA fragmentation by TUNEL fluorescence and caspase‐3 fragmentation by immunofluorescence were detected on optic lobe sections. Acute hypoxia produces a significant increase in cyt c release from mitochondria at 4 h p‐hx, followed by an increase in TUNEL positive cells 2 h later (6 h p‐hx). Administration of E2 (0.5 mg/egg) produced a significant decrease in cytosolic cyt c levels at 4 h p‐hx, in casapse‐3 activation and in TUNEL positive cells at 6 h p‐hx compared to vehicle treated embryos. In the E2‐ICI 182 780 treated embryos, cyt c release, caspase‐3 fragmentation and TUNEL positive cells were similar to the hypoxic embryos, thus suggesting the requirement of an E2–ER interaction for E2 mediated neuroprotective effects. In conclusion, E2 prevents hypoxia‐induced cyt c release and posterior cell death and these effects are mediated by oestrogen receptors.