Insulin reduces cerebral ischemia/reperfusion injury in the hippocampus of diabetic rats: a role for glycogen synthase kinase-3β

Abstract

Objective. There is evidence that insulin reduces brain injury evoked by ischemia/reperfusion (I/R). However, the molecular mechanisms underlying the protective effects of insulin remain unknown. Insulin is a well-known inhibitor of glycogen synthase kinase-3β (GSK-3β). Here we investigate the role of GSK-3β inhibition on I/R-induced cerebral injury in a rat model of insulinopenic diabetes.

Research Design and Methods. Streptozotocin-induced diabetic rats were subjected to 30 min occlusion of common carotid arteries followed by 1 h or 24 h reperfusion. Insulin (2-12 IU/kg, i.v.) or the selective GSK-3β inhibitor, TDZD-8 (0.1-3 mg/kg, i.v.), were administered during reperfusion.

Results. Insulin or TDZD-8 dramatically reduced infarct volume and levels of S100B protein, a marker of cerebral injury. Both drugs induced phosphorylation of the Ser9-residue, thereby, inactivating GSK-3β in the rat hippocampus. Insulin, but not TDZD-8, lowered blood glucose. The hippocampi of the drug-treated animals displayed reduced oxidative stress at 1 h reperfusion as shown by the decreased generation of reactive oxygen species and lipid peroxidation. I/R-induced activation of nuclear factor-κB was attenuated by both drug treatments. At 24 h reperfusion, TDZD-8 and insulin significantly reduced plasma levels of tumor-necrosis-factor-α, neutrophil infiltration, measured as myeloperoxidase activity and intercellular-adhesion-molecule-1 expression, and cyclooxygenase-2 and inducible-NO-synthase expression.

Conclusions. Acute administration of insulin or TDZD-8 reduced cerebral I/R injury in diabetic rats. We propose that the inhibitory effect on the activity of GSK-3β contributes to the protective effect of insulin, independently of any effects on blood glucose.