Immunohistochemical demonstration of serotonin nerve fibers in the corpus striatum of the rat, cat and monkey.

The distribution of serotonin-containing nerve fibers in the corpus striatum of the rat, cat and monkey was studied with modified peroxidase-antiperoxidase method using serotonin antiserum without any pretreatment. In the neostriatum (caudate nucleus and putamen) of all mammalian species investigated, the immunoreactive fibers were distinct varicose fibers in a fine network. The concentration of these fibers was high in the ventral, medial and caudal neostriatum. Especially in the area bounded by the globus pallidus, serotonin fibers were abundant and compactly arranged along the nucleus. In this area of the monkey, a few thick fibers (tract fibers) were intermingled; they ran along the lateral medullary lamina. Such tract fibers were also observed outside the medial medullary lamina and in the central portion of the medial pallidal segment. The paleostriatum (globus pallidus and entopeduncular nucleus) of the rat and cat as well as the medial pallidal segment of the monkey was diffusely innervated with serotonin fibers composed of numerous varicosities and fine intervaricose segments, while in the lateral pallidal segment of the monkey, the distribution of fibers was scanty and partial. Our results hint at the morphological basis of the serotonergic regulation of the extrapyramidal system in mammals.