The expression of the protooncogenes, c-fos, jun B, c-jun, and jun D was investigated in a rat focal cerebral ischemia model by Northern analysis and in situ hybridization. Severe ischemia (reduction of regional blood flow by 88-92%) in this model is confined to cerebral cortex irrigated by the right middle cerebral artery. Ischemia for 30 minutes, which caused only slight cortical damage (infarct size, < 10 mm3), induced both jun B and c-fos mRNAs exclusively in the right cerebral cortex. Ischemia for 90 minutes, which led to large cortical infarction (infarct size, > 140 mm3), also induced the expression of these two genes in the right cerebral cortex as well as the ipsilateral hippocampus. The latter sustained very mild ischemia (reduction of regional blood flow by 10-20%). The coinduction of jun B and c-fos expression occurred immediately after reperfusion and peaked at 60 minutes after reperfusion. The expression of c-jun was enhanced in a similar pattern, but at a much lower magnitude. In contrast, no change in jun D expression was observed. Nuclear run-on assays indicated that the increase in c-fos, jun B, and c-jun mRNA levels was due to the increase of transcription rate in these genes. Mobility shift assays showed a basal DNA binding activity of transcription factor AP-1 in the right cerebral cortex. Ischemia for 30 or 90 minutes followed by reperfusion for 4 hours resulted in a four- to sixfold increase of AP-1 binding activity. The enhanced DNA binding activity persisted for as long as 24 hours.(ABSTRACT TRUNCATED AT 250 WORDS)