Nitric Oxide Synthases I and II Are Localized in Neurons

Recent studies [3] revealed that both nitric oxide (NO) synthase (NOS) isoforms NOS I and NOS II in the rostral ventrolateral medulla (RVLM), where sympathetic premotor neurons are located [18], are tonically active under physiologic conditions. The prevalence of NOS I over NOS II activity at the RVLM and the associated dominance of sympathoexcitation over sympathoinhibition may underlie the maintenance of basal sympathetic vasomotor outflow and stable blood pressure by endogenous NO. Chang et al. [4] validated this operational scheme by demonstrating that whereas all three known NOS isoforms are expressed in the ventrolateral medulla at mRNA or protein levels under basal conditions, only NOS I or II is present in RVLM neurons, and NOS III is associated primarily with blood vessels. These observations provide an important basis for further studies of the role of NO at the RVLM in central cardiovascular regulation.