Serotonergic innervation of the primate claustrum

The claustrum is reciprocally and topographically connected with all functi onal areas of the cerebral cortex. Different cortical areas differ in the s ource, density, and laminar distribution of serotonergic innervation, with visual cortex receiving an especially rich serotonergic innervation. We ask ed if there were likewise differences in serotonergic innervation in differ ent regions of the claustrum. We analyzed 50-mum coronal sections through t he claustrum of the macaque monkey processed using standard immunohistochem ical techniques and an antibody to serotonin. We found labeled fibers throu ghout the dorsal-ventral and anterior-posterior extent of the claustrum. A few fibers were relatively straight and lacked varicosities. Most fibers ha d varicosities; the size, shape, and spacing of varicosities varied among f ibers and even along a single fiber. Some stained fibers partially encircle d cells, and varicosities were seen in close apposition to the cell bodies. There was a major difference between dorsal and ventral claustrum in the p attern of stained fibers. In the ventral, visual, claustrum, stained segmen ts of axons were short and randomly arranged relative to each other, and th ere were many stained puncta. In the more dorsal, nonvisual claustrum, many fibers ran in a dorsal-ventral direction, along the long axis of the claus trum, and could be followed for long distances. (C) 2001 Elsevier Science I nc.