Neuronal nitric oxide synthase regulates regional brain perfusion in healthy humans
Abstract Aims Neuronal nitric oxide synthase (nNOS) is highly expressed within the cardiovascular and nervous systems. Studies in genetically modified mice suggest roles in brain blood flow regulation while dysfunctional nNOS signaling is implicated in cerebrovascular ischemia and migraine. Previous human studies have investigated the effects of non-selective NOS inhibition but there has been no direct investigation of the role of nNOS in human cerebrovascular regulation. We hypothesised that inhibition of the tonic effects of nNOS would result in global or localized changes in cerebral blood flow, as well as changes in functional brain connectivity. Methods and Results We investigated the acute effects of a selective nNOS inhibitor, S-methyl-L-thiocitrulline (SMTC), on cerebral blood flow and brain functional connectivity in healthy human volunteers (n=19). We performed a randomised, placebo-controlled, crossover study with either intravenous SMTC or placebo, using magnetic resonance imaging protocols with arterial spin labelling and functional resting state neuroimaging. Conclusions These data suggest a fundamental physiological role of nNOS in regulating regional cerebral blood flow and functional connectivity in the human hippocampus. Our findings have relevance to the role of nNOS in the regulation of cerebral perfusion in health and disease. Translational Perspective Animal models have implicated dysfunctional nNOS-mediated signaling in neurovascular and neurodegenerative conditions. This first study of the effects of a selective nNOS inhibitor, S-methyl-thiocitrulliune (SMTC), on the physiological regulation of human cerebral blood flow and brain functional connectivity opens the way to investigation of the effects of nNOS in cerebrovascular disease states. We also demonstrate that an acute infusion of SMTC is safe yet biologically active within the healthy human brain. The therapeutic potential of modulating the nNOS pathway in the brain could now be investigated; e.g. whether inhibition of nNOS activity is valuable in settings of post-ischemia excitotoxicity.