Introduction
The molecular mechanisms by which preconditioning stimuli induce tolerance to ischemia remain poorly understood. Preconditioning with cortical spreading depression (CSD) increases the expression of proinflammatory cytokines, which themselves have been shown to induce tolerance to ischemia. Expression of cytokines is normally linked to upregulation of feedback inhibitors of inflammation, a negative feedback loop that limits the severity, duration, and spatial dissemination of inflammation 1 . The objective of the present study was to determine whether CSD increases the expression of feedback inhibitors of inflammation that may protect the brain against ischemia-induced inflammatory demage.
Methods
Multiple episodes of CSD were evoked in Sprague-Dawley rats (n=9) by applying 2 M KCl to the frontal cortex of one hemisphere for 2 hr. At various times after CSD, mRNAs encoding feedback inhibitors of inflammation were measured in several regions of the cerebral cortex using Northern blot analysis.
Results
CSD caused rapid and robust elevation of mRNA encoding tristetraprolin (TTP), a protein that promotes the degradation of transcripts encoding inflammatory cytokines. Cortical levels of TTP mRNA increased 2.9-fold (p<0.01) at the end of the KCl-application period and 1.8-fold (p<0.01) after recovery for 2 hr. CSD also caused a marked elevation in mRNA encoding SOCS3 (suppressor of cytokine signaling-3), a protein that suppresses inflammatory signal transduction by binding and blocking Janus kinases. Levels of SOCS3 mRNA increased 3.2-fold (p<0.01) at the end of the KCl-application period, 1.8-fold (p<0.01) after recovery for 2 hr, and 2.2-fold (p<0.01) after recovery for 24 hr. The elevation of both TTP and SOCS3 mRNAs was greatest in the frontal cortex (site of KCl application).
Conclusions
These results demonstrate that CSD elevates mRNAs encoding TTP and SOCS3, proteins that suppress inflammation. Upregulation of these endogenous feedback inhibitors would be expected to suppress the inflammatory response to a subsequent episode of ischemia and, thus, diminish the overall extent of ischemic brain damage. Induced suppression of inflammation by blocking inflammatory cytokine production and their signal transduction is a novel mechanism that may contribute significantly to the induction of tolerance to ischemia after CSD and other preconditioning stimuli.