Both caspase-dependent and caspase-independent pathways may be involved inhippocampal CA1 neuronal death because of loss of cytochrome c from mitochondria in a rat forebrain ischemia model

In a rat forebrain ischemia model, the authors examined whether loss of cyt ochrome c from mitochondria correlates with ischemic hippocampal CAI neuron al death and how cytochrome c release may shape neuronal death. Forebrain i schemia was induced by bilateral common carotid artery occlusion with simul taneous hypotension for 10 minutes. After reperfusion, an early rapid deple tion of mitochondrial cytochrome c and a late phase of diffuse redistributi on of cytochrome c occurred in the hippocampal CAI region, but not in the d entate gyrus and CA3 regions. Intracerebroventricular administration of Z-D EVD-FMK, a relatively selective caspase-3 inhibitor, provided limited but s ignificant protection against ischemic neuronal damage on day 7 after reper fusion. Treatment with 3 minutes of ischemia (ischemic preconditioning) 48 hours before the 10-minute ischemia attenuated both the early and late phas es of cytochrome c redistribution. In another subset of animals treated wit h cycloheximide, a general protein synthesis inhibitor, the late phase of c ytochrome c redistribution was inhibited, whereas most hippocampal CAI neur ons never regained mitochondrial cytochrome c. Examination of neuronal surv ival revealed that ischemic preconditioning prevents, whereas cycloheximide only delays, ischemic hippocampal CA1 neuronal death. DNA fragmentation de tected by terminal deoxytransferase-mediated dUTP-nick end labeling (TUNEL) in situ was largely attenuated by ischemic preconditioning and moderately reduced by cycloheximide. These results indicate that the loss of cytochrom e c from mitochondria correlates with hippocampal CA1 neuronal death after transient cerebral ischemia in relation to both caspase-dependent and -inde pendent pathways. The amount of mitochondrial cytochrome c regained mag det ermine whether ischemic hippocampal CAI neurons survive or succumb to late- phase death.