Ultrastructural demonstration of constitutive nitric oxide synthase (cNOS) in neocortical glial cells and glial perisynaptic sheaths

Abstract

We investigated the localization of the constitutive isoform of nitric oxide synthase (cNOS) in the rat visual cortex by electron microscopy, using a pre-embedding immunohistochemical method. NOS-immunoreactivity was demonstrated in very few somata, dendrites and axon terminals of nerve cells, and also in a few glial cells. The morphological characteristics of labelled glial cells enabled us to identify them as astrocytes. In comparison to the enzyme activity in neurons, the glial cells show a very weak reactivity which was not detectable at the light microscopical level. Immunoreactivity was located in the perikaryon and in the processes of astrocytes. Qualitative analyses with the electron microscope indicate that NOS-positive astroglial cells constitute a very small proportion in the rat visual cortex, whereas the great majority of astrocytes is NOS-negative. An unexpected observation was the localization of NOS immunoreactivity in astroglial sheath-like structures around some synaptic junctions. These results suggest that NO may act very locally at some synapses, and that specialized glial cells are involved in the NO-mediated mechanisms.