Ultrastructural relationships between noradrenergic nerve fibers and non-neuronal elements in the rat cerebral cortex

Pharmacological and biochemical data suggest that noradrenaline (NA)-containing fibers not only regulate the activity of cortical neurons but also influence the functional state of non-neuronal elements. In the present study, immunocytochemistry with an antiserum against NA, followed by silver-gold intensification of the immunoreaction end-product, was employed to examine the ultrastructural relationships between the NA fiber system and the intraparenchymal blood vessels, oligodendrocytes, and astrocytes in the rat visual cortex. Electron microscopy revealed a large number of fine varicose NA fibers to be in intimate contact with cortical capillaries. Examination of single thin sections showed that NA boutons were usually separated from the capillary wall by a fine astroglial sleeve. However, serial section analysis revealed that the continuity of the astrocytic end feet was interrupted at sites, resulting in direct apposition of the perivascular NA fibers to the capillary basal lamina. Noradrenergic fibers were found to contact both types of macroglial cells. Single or clustered oligodendrocytes in intimate contact with NA fibers were observed throughout the cortical depth. Individual contacts could be followed in more than six successive thin sections, and oligodendrocyte plasma membrane frequently exhibited a light thickening at the sites of the NA fiber apposition. NA fiber-astroglial relationships were largely encountered in supragranular layers. In these layers, astrocytic cell bodies were characteristically outlined by fine varicose NA fibers. However, no plasma membrane differentiations were observed at the sites of intimate NA fiber apposition. The present ultrastructural findings provide the anatomical substrate for the control exerted by the NA fiber system over cortical microvasculature and macroglia.