GluR2 protein synthesis and metabolism in rat hippocampus following transient ischemia and ischemic tolerance induction.

In this study we have determined the metabolic half-life, protein synthesis and expression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR2 in the hippocampus of the living rat. Synthesized proteins were pulse labeled in vivo using intracarotid infusion or intrahippocampal injection of L-[(35)S] labeled amino acids, and the GluR2 protein immunoprecipitated in order to measure the tracer incorporation at different survival time-points. A limited time course study suggested a metabolic half-life of 144 and 108 h in the CA1 region in control animals following carotid artery infusion and intrahippocampal injection, respectively. Twenty-four hours following a moderate ischemic insult, GluR2 protein synthesis was decreased significantly in both the CA1 and DG/CA3 region, whereas the total protein synthesis was decreased significantly only in the CA1 region. Twenty-four hours following ischemic tolerance induction, a significant increase in GluR2 expression was found in the CA1 region using quantitative Western blotting, while no change was found in the dentate gyrus (DG)/CA3 or in expression of GluR1 protein. Data from labeling experiments did not reveal the reason for the increased amount of GluR2 in the CA1 region of the tolerant animals. This study shows that following global ischemia the GluR2 synthesis is decreased both in the CA1 and DG/CA3, which, together with the found GluR2 metabolic half-life, contradict a selective loss of GluR2 protein as a triggering mechanism for the delayed CA1 pyramidal cell death. Twenty-four hours following tolerance induction, we found an increased GluR2 expression in the CA1 region, suggesting that GluR2 plays a role in the acquisition of ischemic tolerance. Our study suggests the ability of neurons to regulate the AMPA receptor subunit expression through changes in protein synthesis and stability.