NMDA receptor regulation of nNOS phosphorylation and induction of neuron death

Stimulation of NMDA receptors activates neuronal nitric oxide synthase (nNOS) and the production of nitric oxide (NO). Dephosphorylation of nNOS increases nNOS enzymatic activity. We have examined the regulation of nNOS phosphorylation in rat cortical neurons following NMDA receptor activation. We show that nNOS is constitutively phosphorylated and that NMDA receptor activation decreases the level of nNOS phosphorylation by a mechanism that is blocked specifically by NMDA receptor antagonists and inhibitors of the Ca2+-regulated phosphatases calcineurin and PP1/PP2A. Using quantitative digital microscopy, we show that NMDA receptor activation induces the accumulation of nitrotyrosine, a measure of nNOS activity, and TdT-mediated fluorescein-dUTP nick end labeling (TUNEL) positivity, a measure of cell death. A calcineurin inhibitor blocked the increase in both TUNEL and nitrotyrosine positivity. Notably, TUNEL was increased in those neurons that were most strongly positive for nitrotyrosine. We conclude that NMDA receptor activation induces death of neurons by a cell autonomous pathway involving nNOS dephosphorylation by a calcineurin-dependent mechanism.