Implication of glutamate in the expression of inducible nitric oxide synthase after oxygen and glucose deprivation in rat forebrain slices

Nitric oxide synthesis by inducible nitric oxide synthase (iNOS) has been p ostulated to contribute to ischemia-reperfusion neurotoxicity. The expressi on of this enzyme has been demonstrated in cells present in the postischemi c brain. The mechanisms of iNOS expression after cerebral ischemia are a su bject of current research. We therefore decided to investigate whether glut amate, which is released in ischemia and is implicated in neurotoxicity, mi ght be involved in the mechanisms by which oxygen and glucose deprivation ( OGD) leads to the expression of iNOS in rat forebrain slices. In this model , we have shown previously that 20 min of OGD causes the expression of iNOS . We have now found that the NMDA receptor antagonist MK-801 blocks the exp ression of iNOS, suggesting that the activation of the NMDA subtype of glut amate receptor is implicated in the mechanisms that lead 20 the expression of this isoform, Moreover, we have found that glutamate alone could trigger the induction process, as shown by the appearance of a Ca2+-independent NO S activity and by the detection of iNOS mRNA and protein in slices exposed to glutamate. Glutamate-dependent iNOS expression was concentration-depende nt and was blocked by EGTA and by the inhibitors of nuclear factor kappa B (NF-kappa B) activation pyrrolidine dithiocarbamate and MG132. In addition, glutamate induced NF-kappa B translocation to the nucleus, an effect that was inhibited by MG132, Taken together, our data suggest that activation of NMDA receptors by glutamate released in ischemia is involved in the expres sion of iNOS in rat forebrain slices via a Ca2+-dependent activation of the transcription factor NF-kappa B. To our knowledge, this is the first repor t showing an implication of excitatory amino acids in the expression of iNO S caused by ischemia.