Evidence supporting a role for programmed cell death in focal cerebral ischemia in rats

Cells die by one of two mechanisms, necrosis or programmed cell death. Necrosis has been implicated in stroke and occurs when the cytoplasmic membrane is compromised. Programmed cell death requires protein synthesis and often involves endonucleolytic cleavage of the cellular DNA. We assessed the potential contribution of programmed cell death to ischemia-induced neuronal death.

Cycloheximide (protein synthesis inhibitor; 1 mg/kg per 24 hours) or vehicle (1 mL/kg per 24 hours) was continuously infused into the right cerebral ventricle of spontaneously hypertensive rats. Neocortical focal ischemia was produced by tandem occlusion of the right common carotid artery and the ipsilateral middle cerebral artery. After 24 hours the brain was stained with 2% 2,3,5-triphenyltetrazolium and the ischemic zone quantitated. Protein synthesis was determined by [3H]methionine incorporation into acid-precipitated protein. DNA integrity was determined in isolated DNA by gel electrophoresis and in whole cells by flow cytometry.

Continuous cycloheximide infusion caused approximately 70% reduction in cortical protein synthesis. Cycloheximide also reduced the size of the infarction produced by focal cerebral ischemia when compared with controls (ischemic brain volume, 147.5 +/- 25.9 and 188.7 +/- 16.8 mm3 for cycloheximide and saline, respectively; P < .01), suggesting that protein synthesis may contribute to cell death. Purified DNA from the ischemic zone showed evidence of endonucleolytic degradation when fractionated by gel electrophoresis. Flow cytometric analysis demonstrated increased propidium iodide fluorescence in intact cells isolated from ischemic cortex, indicating an increased accessibility of degraded DNA to the intercalating dye.

New protein synthesis appears to contribute to ischemic cell death in which endonucleolytic DNA degradation is apparent. These observations implicate programmed cell death in ischemic injury and may open unique therapeutic approaches for the preservation of neurons in stroke.