Ischemic preconditioning promotes neuroprotection via cyclooxygenase-2 induction in vitro

Introduction

Ischemic preconditioning (IPC) is a well established endogenous protective mechanism in which a brief, sublethal ischemic stimulus can reduce cell injury caused by subsequent lethal ischemic injury. Although numerous signaling pathways appear to be implicated for IPC in the brain, the effective protective mediators have not been fully characterized. Some studies demonstrated that upreglulation of cyclooxgynase-2 (COX-2), prostaglandin synthase, is associated with IPC and that COX-2 derived prostaglandins (PGs) plays a role in the cardioprotective effects ¹ . It is also known that prostaglandins are neuroprotective in cerebral ischemia ² . However, the roles of COX-2 and COX-2 derived PGs during ischemic tolerance in brain remains undefined. In this study, we hypothesize that COX-2 play a key role in the protective mechanisms that ensue after IPC.

Methods and Results

To determine the roles of COX-2 during the late phase of IPC, we used two different in vitro models: organotypic hippocampal slice cultures (OHSC) and mixed cortical neuronal cultures (MCN). OHSC and MCN are prepared from neonatal rat (9–11 days old) and rat embryo (18–19 days old), respectively. We examined whether COX-2 expression is induced after IPC in MCN. First, we determined that 1 hr of IPC followed by 48 hr of reperfusion prior to 2 hr of a lethal ischemic insult (oxygen glucose deprivation-OGD) promoted significant neuroprotection in MCN (n=24, p<0.01). Next, we determined whether COX-2 was induced after IPC. MCN were preconditioned by 1 hr of OGD and then, cells were isolated at 8 hr, 15 hr and 24 hr of reperfusion. We found that COX-2 was induced at 15 hr and decreased at 24 hr following IPC. Previously, we have shown that a sublethal ischemic insult of 15 min, followed by 48 hr of reperfusion prior to 40 min of lethal ischemic insult induced neuroprotection in the CA1 region of hippocampus in OHSC ³ . Based on this study, we administered NS 398 (COX-2 selective inhibitor, 10 μM and 20 μM) to OHSC for 48 hr of reperfusion after IPC. We found that inhibition of COX-2 activity (NS 398 20 μM) following IPC abolished the IPC mediated neuroprotection in CA1 region of hippocampus (n=8, p<0.05).

Conclusions

These results suggest that transient induction of COX-2 expression after IPC might activate the signal transduction pathway that leads to neuronal survival following subsequent lethal ischemic injury.