Brain nitric oxide inactivation is governed by the vasculature

The mechanisms underlying nitric oxide (^•NO) synthesis and inactivation in the brain are essential determinants of ^•NO neuroactivity. Although ^•NO production is well characterized, the pathways of inactivation in vivo remain largely unknown. Here, we characterize the kinetics and the major mechanism of ^•NO inactivation in the rat brain cortex and hippocampus in vivo by measuring locally applied ^•NO with carbon-fiber microelectrodes (CFMs) and ceramic-based microelectrode arrays (MEAs). An apparent first-order clearance was observed in both brain regions, with decay rate constants (k) of ^•NO signals of 0.67 to 0.84 per second, significantly higher than the k obtained in agarose gel (0.099 per second), used as a ^•NO diffusion-control medium. ^•NO half-life in vivo, estimated by mathematical modeling, was 0.42 to 0.75 s. Experiments using MEAs support that the ^•NO diffusion radius is heterogeneous and related to local metabolic activity and vascular density. After global ischemia, k decreased to control values of diffusion in gel, but during anoxia, k decreased only 21%. Additionally, k in brain slices was threefold to fivefold lower than that in vivo, and hemorrhagic shock induced a 53% decrease in k. Overall, the results support that ^•NO scavenging by circulating erythrocytes constitutes the major ^•NO-inactivation pathway in the brain.