Late calcium EDTA rescues hippocampal CA1 neurons from global ischemia-induced death

Transient global ischemia induces a delayed rise in intracellular Zn ²⁺, which may be mediated via glutamate receptor 2 (GluR2)-lacking AMPA receptors (AMPARs), and selective, delayed death of hippocampal CA1 neurons. The molecular mechanisms underlying Zn²⁺ toxicity in vivo are not well delineated. Here we show the striking finding that intraventricular injection of the high-affinity Zn²⁺ chelator calcium EDTA (CaEDTA) at 30 min before ischemia (early CaEDTA) or at 48-60 hr (late CaEDTA), but not 3-6 hr, after ischemia, afforded robust protection of CA1 neurons in ∼50% (late CaEDTA) to 75% (early CaEDTA) of animals. We also show that Zn ²⁺ acts via temporally distinct mechanisms to promote neuronal death. Early CaEDTA attenuated ischemia-induced GluR2 mRNA and protein downregulation (and, by inference, formation of Zn²⁺-permeable AMPARs), the delayed rise in Zn²⁺, and neuronal death. These findings suggest that Zn ²⁺ acts at step(s) upstream from GluR2 gene downregulation and implicate Zn²⁺ in transcriptional regulation and/or GluR2 mRNA stability. Early CaEDTA also blocked mitochondrial release of cytochrome c and Smac/DIABLO (second mitochondria-derived activator of caspases/direct inhibitor of apoptosis protein-binding protein with low pI), caspase-3 activity (but not procaspase-3 cleavage), p75^NTR induction, and DNA fragmentation. These findings indicate that CaEDTA preserves the functional integrity of the mitochondrial outer membrane and arrests the caspase death cascade. Late injection of CaEDTA at a time when GluR2 is downregulated and caspase is activated inhibited the delayed rise in Zn²⁺, p75^NTR induction, DNA fragmentation, and cell death. The finding of neuroprotection by late CaEDTA administration has striking implications for intervention in the delayed neuronal death associated with global ischemia.