Endothelial nitric oxide synthetase (eNOS) in astrocytes: Another source of nitric oxide in neocortex

The distribution of the endothelial form of nitric oxide synthetase (eNOS) was examined in the visual cortex of three species of primate and in the ra t using immunocytochemistry. Labeled cells were found in both the gray and white matter. These cells were stellate in appearance and labeled cell proc esses were seen contacting blood vessels or the pia, suggesting that, by mo rphological criteria, the cells were astrocytes. All eNOS positive cells we re double labeled with an antibody against S100 beta. Although all cells we re double labeled in the white matter, in the gray matter, some S100 beta p ositive cells did not contain detectable levels of eNOS. eNOS positive astr ocytic processes appeared to form prominent and distinctive structures next to neurons, especially in cortical layer IIIC. We postulate that these eNO S-positive structures form astrocytic perisynaptic sheaths on neuronal soma s in the cortex. If this is true, then nitric oxide can influence neuronal transmission directly at axosomatic synapses in the cortex. In addition, th e presence of eNOS in astrocytes and in their processes that contact blood vessels suggests that the link between local cortical activity and changes in cerebral blood flow could be mediated by astrocytic release of nitric ox ide. GLIA 26:280-290, 1999. (C) 1999 Wiley-Liss, Inc.