Immunohistochemical demonstration of serotonin-containing nerve fibers in the inferior olivary complex of the rat, cat and monkey.

By the use of a modified peroxidase-antiperoxidase immunohistochemical method, the distribution of serotonin-containing nerve fibers was studied in the inferior olivary complex of the rat, cat and monkey. The entire inferior olivary complex of all three species contained serotonin fibers, and species-related differences in the distribution of serotonin fibers were observed. In the rat, the overall density of serotonin fibers was sparser than that in the other two species; the highest concentration of varicose serotonergic fibers was noted in the lateral portion of the dorsal accessory olive. In the cat, the densest distribution of serotonin fibers occurred in the caudal portion of the medial accessory olive, the dorsomedial cell column and the lateral portion of the dorsal accessory olive, where intervaricose segments of serotonin-immunoreactive fibers were not so distinct. In the monkey, the caudal medial accessory olive, the lateral portion of the dorsal accessory olive and the dorsal as well as the lateral lamella of the principal olive showed a maximum density of serotonin fibers. Apart from the influence of afferent serotonergic projections to the cerebellum, serotonergic neurons of the brainstem are considered to affect Purkinje cells via neurons of the inferior olivary complex projecting to the cerebellum particularly in the cat and monkey.