Noradrenergic innervation of peptidergic interneurons in the rat visual cortex

Previous studies have shown that cortical interneurons, presumably GABAergi c, are among the targets of the noradrenaline (NA)-containing cortical affe rents and that NA interacts with neuropeptides at various cellular levels. The present study attempts to characterize further the cortical targets of the NA afferents by examining, at the light and the electron microscopic le vel, the anatomical relationships of the NA fibers with three subpopulation s of cortical interneurons, those containing somatostatin (SRIF), neuropept ide Y (NPY) or vasoactive intestinal polypeptide (VIP). For this purpose, a double preembedding immunoprocedure with antibodies against NA and SRIF, N PY or VIP was combined with the gold-substituted silver peroxidase method. Light microscopic examination showed that NA fibers contact perikarya and p roximal dendrites of the SRIF, NPY and VIP neurons. However, NA fibers, whi le found to form pericellular arrays around NPY neurons and, to a lesser ex tent, around SRIF neurons, were seen to target VIP cortical cells with sing le terminal varicosities. Electron microscopy revealed that all peptidergic populations examined represent synaptic targets for the NA fibers. The NAe rgic synapses, localized onto the cell body and proximal dendrites of the p eptidergic neurons, were always of the symmetrical variety. Results of the present study provide the morphological basis for the explanation of the fu nctional interaction between the NA cortical afferent system and the intrin sic cortical elements.