-The diverse signaling events following mitochondrial depolarization in neurons are not clear. We examined for the first time the effects of mitochondrial depolarization on mitochondrial function, intracellular calcium, neuronal nitric oxide synthase (nNOS) activation, and nitric oxide (NO) production in cultured neurons and perivascular nerves. Cultured rat primary cortical neurons were studied on 7-10 days in vitro, and endothelium-denuded cerebral arteries of adult Sprague-Dawley rats were studied ex vivo. Diazoxide and BMS-191095 (BMS), activators of mitochondrial K ATP channels, depolarized mitochondria in cultured neurons and increased cytosolic calcium levels. However, the mitochondrial oxygen consumption rate was unaffected by mitochondrial depolarization. In addition, diazoxide and BMS not only increased the nNOS phosphorylation at positive regulatory serine 1417 but also decreased nNOS phosphorylation at negative regulatory serine 847. Furthermore, diazoxide and BMS increased NO production in cultured neurons measured with both fluorescence microscopy and electron spin resonance spectroscopy, which was sensitive to inhibition by the selective nNOS inhibitor 7-nitroindazole (7-NI). Diazoxide also protected cultured neurons against oxygen-glucose deprivation, which was blocked by NOS inhibition and rescued by NO donors. Finally, BMS induced vasodilation of endothelium denuded, freshly isolated cerebral arteries that was diminished by 7-NI and tetrodotoxin. Thus pharmacological depolarization of mitochondria promotes activation of nNOS leading to generation of NO in cultured neurons and endothelium-denuded arteries. Mitochondrial-induced NO production leads to increased cellular resistance to lethal stress by cultured neurons and to vasodilation of denuded cerebral arteries. MITOCHONDRIAL MEMBRANE POTENTIAL is a critical regulator of cellular activity and survival and is controlled by a variety of factors including the ATP-sensitive potassium (mitoK ATP ) channels located on the inner mitochondrial membrane (2). Pharmacological activators of mitoK ATP channels, such as BMS-191095 (BMS) and diazoxide, decrease the mitochondrial membrane potential by facilitating the influx of potassium from cytosol into the matrix (2). Signaling events following mitochondrial depolarization appear to be diverse in the various cell types comprising the neurovascular unit (cerebral vascular endothelium and smooth muscle, perivascular neurons, parenchymal neurons, and glia). Both BMS and diazoxide applications result in dilation of isolated cerebral arteries; however, individual effects on vascular smooth muscle and endothelium, which contribute to the overall vascular effect, are completely different (20,22,35,42). Thus BMS and diazoxide relax cerebral vascular smooth muscle cells via generation of localized calcium sparks by sarcoplasmic reticulum linked to mitochondrial depolarization, resulting in decreased global intracellular Ca 2ϩ ([Ca 2ϩ ] i ) (20, 42). Conversely, mitochondrial depolarization in cerebral endothe...