Neuroprotection by group I mGlu receptors in a rat hippocampal slice model of cerebral ischemia is associated with the PI3K-Akt signaling pathway: a novel postconditioning strategy?

Ischemic postconditioning is defined as a repetitive series of brief interruptions of reperfusion applied immediately after ischemia. In this study, postconditioning was investigated by first exposing rat organotypic hippocampal slices to 30min oxygen-glucose deprivation (OGD), which promotes selective CA1 pyramidal cell death, and 5min later to either a brief period (3min) of OGD or to a low dose (10microM) of 3,5-dihydroxyphenylglycine (DHPG) for 30min. Both protocols attenuated CA1 neuronal injury, as revealed 24h later by measuring the intensity of propidium iodide fluorescence in this region. The beneficial effects were observed when DHPG postconditioning was applied up to 15min after OGD, but not at later time points, and was not additive with the neuroprotective effects of a preconditioning DHPG treatment. The attenuation of the OGD-induced CA1 injury evoked by postconditioning was prevented when mGlu1 and mGlu5 receptor antagonists and inhibitors of phosphatidylinositol 3-kinase and Akt activity were present in the incubation medium during the 5min recovery period after OGD and the 30min exposure to DHPG. The PI3K inhibitor was also able to prevent the reduction of NMDA toxicity induced by the DHPG treatment. Finally, DHPG increased the phosphorylation of Akt in a transient and mGlu1/mGlu5-dependent manner. Our results show that activation of the mGlu1/mGlu5-PI3K-Akt signaling pathway plays a crucial role in the mechanisms of postconditioning evoked by DHPG and point to this strategy as a possible novel therapeutic tool for stroke and cerebral ischemia.