Evidence for a pathway that facilitates nitric oxide diffusion in the brain.

Nitric oxide (()NO) is a diffusible messenger that conveys information based on its concentration dynamics, which is dictated by the interplay between its synthesis, inactivation and diffusion. Here, we characterized ()NO diffusion in the rat brain in vivo. By direct sub-second measurement of ()NO, we determined the diffusion coefficient of ()NO in the rat brain cortex. The value of 2.2×10(-5)cm(2)/s obtained in vivo was only 14% lower than that obtained in agarose gel (used to evaluate ()NO free diffusion). These results reinforce the view of ()NO as a fast diffusing messenger but, noticeably, the data indicates that neither ()NO diffusion through the brain extracellular space nor homogeneous diffusion in the tissue through brain cells can account for the similarity between ()NO free diffusion coefficient and that obtained in the brain. Overall, the results support that ()NO diffusion in brain tissue is heterogeneous, pointing to the existence of a pathway that facilitates ()NO diffusion, such as cell membranes and other hydrophobic structures.