Serotonergic innervation of the primate claustrum

The claustrum is reciprocally and topographically connected with all functional areas of the cerebral cortex. Different cortical areas differ in the source, density, and laminar distribution of serotonergic innervation, with visual cortex receiving an especially rich serotonergic innervation. We asked if there were likewise differences in serotonergic innervation in different regions of the claustrum. We analyzed 50-μm coronal sections through the claustrum of the macaque monkey processed using standard immunohistochemical techniques and an antibody to serotonin. We found labeled fibers throughout the dorsal-ventral and anterior-posterior extent of the claustrum. A few fibers were relatively straight and lacked varicosities. Most fibers had varicosities; the size, shape, and spacing of varicosities varied among fibers and even along a single fiber. Some stained fibers partially encircled cells, and varicosities were seen in close apposition to the cell bodies. There was a major difference between dorsal and ventral claustrum in the pattern of stained fibers. In the ventral, visual, claustrum, stained segments of axons were short and randomly arranged relative to each other, and there were many stained puncta. In the more dorsal, nonvisual claustrum, many fibers ran in a dorsal-ventral direction, along the long axis of the claustrum, and could be followed for long distances.