Immunohistochemical study on the localization of serotonin fibers and terminals in the spinal cord of the monkey (Macaca fuscata).

A modified procedure of PAP-immunohistochemistry with the use of a rabbit antiserum against serotonin was applied to investigate the pattern of serotonin-containing nerve fibers in the spinal cord of the monkey, Macaca fuscata. The majority of descending serotonin fibers in the white matter is located immediately below the pia mater in the ventrolateral funiculi. Lamina I and the outer zone of lamina II are supplied with numerous serotonin fibers. In the intermediate gray, two prominent bundles composed of longitudinal fibers, i.e., lateral and medial longitudinal serotonin bundles, were recognized at the lateral column and in the vicinity of the central canal, respectively. The motoneurons of the anterior horn are encompassed by fine networks of serotonin fibers and terminals. The results obtained from studies with the monkey spinal cord closely resemble those characteristic of the dog spinal cord as presented in a previous paper, except for portions of the lumbar level. In segments L3-L4, intercalated cell groups between the medial and lateral motor nuclei receive particularly rich inputs of serotonin fibers in the same manner as the neurons of the nucleus intermediolateralis. This peculiar finding may suggest the presence of a specialized nucleus in the anterior column of the simian and also human spinal cord.