The precise role that nitric oxide (NO) plays in the mechanisms of ischemic brain damage remains to be established. The expression of the inducible isoform (iNOS) of NO synthase (NOS) has been demonstrated not only in blood and glial cells using in vivo models of brain ischemia-reperfusion but also in neurons in rat forebrain slices exposed to oxygen-glucose deprivation (OGD). 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 slices, a decrease in the calcium-dependent NOS activity was found 180 min after the OGD period, which was parallel to the increase during this period in calcium-independent NOS activity. Both dexamethasone and cycloheximide, which completely inhibited the induction of the calcium-independent NOS activity, caused a 40-70% recovery in calcium-dependent NOS activity when compared with slices collected immediately after OGD. The NO scavenger oxyhemoglobin produced complete recovery of calcium-dependent NOS activity, suggesting that NO formed after OGD is responsible for this down-regulation. Consistently, exposure to the NO donor (Z)-1-[(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-iu m-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 down-regulates nNOS activity in rat brain slices exposed to OGD. These studies suggest important and complex interactions between NOS isoforms, the elucidation of which may provide further insights into the physiological and pathophysiological events that occur during and after cerebral ischemia.