Requirement for nitric oxide activation of p21 ras yextracellular regulated kinase in

receptor- and NO-dependent, but cGMP-independent, manner. We demonstrate that Ras activity is necessary and sufficient for OGD tolerance in neurons. Pharmacological inhibition of Ras, as well as a dominant negative mutant Ras, block OGD preconditioning whereas a constitutively active form of Ras promotes neuropro- tection against lethal OGD insults. In contrast, the activity of phosphatidyl inositol 3-kinase is not required for OGD precondi- tioning because inhibition of phosphatidyl inositol 3-kinase with a chemical inhibitor or with a dominant negative mutant does not have any effect on the development of OGD tolerance. Further- more, using recombinant adenoviruses and pharmacological inhib- itors, we show that downstream of Ras the extracellular regulated kinase cascade is required for OGD preconditioning. Our observa- tions indicate that activation of the Rasyextracellular regulated kinase cascade by NO is a critical mechanism for the development of OGD tolerance in cortical neurons, which may also play an important role in ischemic preconditioning in vivo.