The serotonin innervation of the cerebral cortex in the rat—an immunohistochemical analysis

The serotonergic innervation of the cerebral cortex in the rat has been studied by immunohistochemistry employing an antibody directed against the neurotransmitter, serotonin. The dorsal raphe, median raphe and B9 cell groups contain intensely labelled neuronal perikarya. Bundles of large diameter axons suggestive of fibers of passage are observed in successive sections as they ascend through the midbrain tegmentum, medial forebrain bundle, diagonal band and supracallosal stria en route to the cortex. In addition, a lateral pathway to the cerebral cortex traversing the ansa peduncularis is visualized. All regions of the cerebral cortex appear to be innervated by serotonergic axons which have a distinctive morphology: they are fine (0.1–0.5 μm), varicose, and extremely convoluted. Serotonergic axons of passage are thicker and comparatively straight. Throughout the lateral neocortex, as well as in the anterior cingulate cortex, serotonergic axons form a densely arborizing plexus through all cortical layers. Contrary to earlier reports, based on histofluorescence, describing a sparse innervation of the cortex with most of the fibers found in the molecular layer, the present study reveals that the innervation is relatively uniform across all cortical layers. In most of the cortex the density of serotonin-containing axons exceeds that of noradrenergic fibers. A distinctive and different pattern of serotonin innervation is found in the posterior cingulate cortex (cytoarchitectonic field RSg): the serotonergic axons are restricted largely to lamina I and III. A restricted laminar pattern also characterizes the innervation of the hippocampus; dense axonal plexuses occur in the outer rim of the dentate hilus and in the stratum lacunosum-moleculare. The serotonergic afferents to the cortex appear to have at least two different modes of distribution, a relatively uniform pattern in the anterior cingulate and the lateral neocortex and a restricted, laminar pattern in the posterior cingulate and the hippocampus.The density and extent of the serotonin innervation is such that the raphe neurons may contact every cell in the cortex. The widespread arborization of serotonin axons contrasts with the spatially restricted termination of thalamic afferents. The distribution of serotonin-containing fibers also differs substantially from the terminal patterns of noradrenergic and dopaminergic fibers. The differences in axonal morphology and distribution amongst the monoamine afferents reflect differences in their contributions to cortical circuitry. The present findings indicate that the serotonin-containing neurons may exert a profound and global, but not necessarily uniform, influence upon cortical function.