Serotonergic axons are widely distributed in the primate forebrain and represent the most abundant ascending projection from the reticular formation. Immunocytochemical methods have been utilized to examine the density, laminar distribution and morphology of serotonergic axons in both primary projection (motor, somatosensory) and association areas (dorsolateral prefrontal, area 5) as well as in the hippocampus and in cingulate cortex of rhesus and cynomolgus macaques. Serotonergic axons are present in all areas of cortex examined, and all cortical layers receive serotonergic afferents. However, the intracortical distribution of serotonergic axon terminals is not uniform; rather, different regions of cortex exhibit dissimilarities in both the density and laminar distribution of serotonergic axons. Thus, there are local patterns of serotonin innervation that are characteristic of each cortical area. Highly diverse patterns of serotonin innervation are found in heterotypical areas of cortex; more subtle variations are present among homotypical areas. Two morphologic types of serotonergic axon terminals, fine and beaded, are present in all cortical areas, and they typically exhibit different laminar distributions: in most areas of neocortex, beaded axons predominate in layer I while fine axons predominate in layers II-VI. However, exceptions to this pattern were observed in primary visual cortex and in the hippocampal formation. The distinctive local patterns of serotonin innervation observed in this study indicate that raphe-cortical projections are likely to have differential influences on particular cytoarchitectonic areas of cerebral cortex in the primate. Moreover, the discrete laminar distribution of serotonin axons suggests that serotonergic projections selectively innervate particular neuronal elements in cerebral cortex. The present findings suggest that the two classes of serotonergic axons, fine and beaded, which have different patterns of termination, affect different sets of cortical neurons. In addition, these two serotonergic projections may be associated with different sets of serotonergic receptors and thus produce selective effects on cortical function.
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