Transplantation of fetal mesencephalic and medullary raphe tissues into the cerebellum of denervated adult rats--an immunohistochemical study.

Pieces of mesencephalic and medullary raphe tissues were transplanted into the cerebella of 5,6-dihydroxytryptamine-treated adult rats. The extent of axonal outgrowth of serotonergic and dopaminergic neurons in the grafts was immunohistochemically studied. At 3 months after transplantation, numerous dopaminergic neurons with many processes extending within the graft were detected in the mesencephalic raphe graft, but not in the medullary raphe graft. In contrast, both the mesencephalic and medullary raphe grafts contained numerous serotonergic neurons and a dense plexus of their fibers. The outgrowing serotonergic fibers from the mesencephalic raphe graft showed a hyperinnervation pattern in the cerebellar cortex adjacent to the graft. Furthermore, a glomerulus-like accumulation of serotonergic fibers was observed in the granular layer. In the cases of medullary raphe grafts, the relatively abundant outgrowing serotonergic fibers showed a laminar organization in the cerebellar cortex near the graft, which was similar to the normal distributional pattern. These results indicate that serotonergic and dopaminergic neurons located within the mesencephalic raphe graft clearly differed from each other in their ability to extend their processes into the host cerebellum, which provides further evidence for the existence of specific interactions between outgrowing serotonergic fibers and their terminal fields (targets).