Down-regulation of neuronal nitric oxide synthase by nitric oxide after oxygen-glucose deprivation in rat forebrain slices

The precise role that nitric oxide (NO) plays in the mechanisms of ischemic brain damage remains to be established. The expression of the inducible is oform (iNOS) of NO synthase (NOS) has been demonstrated not only in blood a nd glial cells using in vivo models of brain ischemia-reperfusion but also in neurons in rat forebrain slices exposed to oxygen-glucose deprivation (O GD). We have used this experimental model to study the effect of OGD on the neuronal isoform of NOS (nNOS) and iNOS. In OGD-exposed rat forebrain slic es, a decrease in the calcium-dependent NOS activity was found 180 min afte r the OGD period, which was parallel to the increase during this period in calcium-independent NOS activity, Both dexamethasone and cycloheximide, whi ch completely inhibited the induction of the calcium-independent NOS activi ty, caused a 40-70% recovery in calcium-dependent NOS activity when compare d with slices collected immediately after OGD, The NO scavenger oxyhemoglob in produced complete recovery of calcium-dependent NOS activity, suggesting that NO formed after OGD is responsible for this down-regulation. Consiste ntly, exposure to the NO donor (Z)-1-[(2-aminoethyl)-N-(2-ammonioethyl)amin o]diazen-1-ium-1,2-diolate (DETA-NONOate) for 180 min caused a decrease in the calcium-dependent NOS activity present in control rat forebrain slices. Furthermore, OGD and DETA-NONOate caused a decrease in level of both nNOS mRNA and protein. In summary, our results indicate that iNOS expression dow n-regulates nNOS activity in rat brain slices exposed to OGD. These studies suggest important and complex interactions between NOS isoforms, the eluci dation of which may provide further insights into the physiological and pat hophysiological events that occur during and after cerebral ischemia.