The organization of prefrontocaudate projections and their laminar origin in the macaque monkey: a retrograde study using HRP-gel.

The organization of the prefrontocaudate connection in the macaque monkey was studied with a polyacrylamide gel of horseradish peroxidase that was injected, after callosotomy, into the head of the caudate nucleus. Tissues were processed for peroxidase activity with tetramethylbenzidine. Retrogradely labeled cells appeared bilaterally in the prefrontal cortex, and ipsilaterally in the premotor area, frontal operculum, insula, and rostral region of the temporal cortex. In the prefrontal cortex, labeled cells were located in cortical layers 2-6. In layer 2, labeled cells were occasionally seen. In layer 3, a considerable number of labeled cells were seen and were mainly distributed in its lower part. They were pyramidal cells. Layer 4 contained a considerable number of labeled cells that were pyramidal or round. Somal sizes of these labeled cells were from 5 to 15 micron, but none of the cells less than 5 micron were labeled. Layer 5 was densely packed with labeled cells, though at a higher density in its upper part (5a). They were pyramidal cells. In layer 6, pyramidal or fusiform cells were labeled at a lower density than in layer 5. The laminar pattern and density of labeled cells were different among various regions of the prefrontal cortex, but, in general, the supragranular labeling tended to frequently occur in the regions containing the numerous labeled cells. There was a topographical relationship between the prefrontal regions containing the majority of labeled cells and the injection sites in the head of the caudate nucleus: the medial orbitofrontal cortex and the medial surface extending downward from the rostral sulcus project ipsilaterally to the medialmost portion of the ventromedial part of the head of the nucleus, the lateral orbitofrontal cortex projects to the ventromedial part of the head of the nucleus, and the dorsolateral prefrontal cortex ventral to the principal sulcus projects to the central part of the head of the nucleus. Our findings in the macaque monkey indicate that the corticocaudate connection is similar in laminar organization of efferent neurons to the corticocortical connections.