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Objective: Mice with genetic deletion of endothelial (eNOS; protective) and neuronal (nNOS; detrimental) nitric oxide synthase isoforms exhibit dramatically opposite consequences of ischemic brain injury. nNOS has been identified recently in endothelial cells, however, its functional significance is unclear. Our objective was to identify nNOS and characterize its functional role in primary brain microvascular endothelial cells (MECs).Methods and Results: MECs from humans (hMECs), rats (rMECs), and mice (mMECs) along with cultured primary rat cortical neurons were used. In addition, rat brain microvessels were freshly isolated. Transendothelial electrical resistance (TEER) measurements of monolayers of hMECs cultured in transwells were used to quantitate in vitro blood-brain barrier (BBB) integrity. Immunocytochemistry identified von Willebrand factor, eNOS, and nNOS in MECs but stained negative for glial (GFAP) and neuronal (Neu1) markers. PCR studies confirmed the expression of eNOS and nNOS mRNA in MECs and microvessels. We utilized electron spin resonance spectrometry to measure reactive oxygen species (ROS) (1-Hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethyl-pyrrolidine; CMH) and NO (colloid Fe(DETC)2). Inhibition of nNOS (N-ω-Propyl-L-arginine and ARL-17477) reduced ROS but increased NO levels in MECs and rat brain microvessels. In contrast, eNOS inhibitor (L-N5-(1-Iminoethyl)ornithine) increased ROS but reduced NO levels. Inhibition of nNOS in neurons, similarly increased ROS and decreased NO levels. siRNA targeting rat nNOS in rMECs was able to knockdown nNOS mRNA as well as ROS levels. BBB studies of hMECs treated with NOS inhibitors followed by oxygen-glucose deprivation (OGD) revealed that nNOS inhibition increased TEER at baseline and promoted TEER recovery following OGD. In contrast, eNOS inhibition had no effect on TEER at baseline but weakly albeit transiently helps in the post-OGD recovery of BBB function.Conclusions: Thus, we identified a constitutively active nNOS in MECs that is functionally distinct from the nNOS isoform expressed in neurons and eNOS. In addition, nNOS inhibition enhances the BBB integrity and affords protection against anoxic-injury induced impairment of BBB function.