Specific outgrowth from neurons of ventral mesencephalic grafts to the catecholamine-depleted striatum of adult hosts.

Grafts of fetal ventral mesencephalon including substantia nigra have been used to correct some motor deficits produced by unilateral destruction of the dopaminergic nigrostriatal pathway in rats. Histochemical studies have shown that dopaminergic neurons within the graft send processes from the graft to the host neuropil, wherein they form synapses. The results of numerous immunocytochemical studies indicate, however, that a large proportion of neurons in grafts are not catecholaminergic. Whether or not the nondopaminergic neurons in grafts project to the host brain is unknown. The purpose of the present study was to combine immunocytochemistry and retrograde tracing with fluorogold to identify the cell types which project from grafts to the host striatum. Tissue from the ventral mesencephalon of E15 fetuses was placed into the 6-hydroxydopamine denervated striatum of graft recipients. Six weeks to 6 months following transplantation, fluorogold was pressure injected under stereotaxic control immediately adjacent to the ventral mesencephalic grafts; after 4 days CNS tissue was prepared for light microscopic immunocytochemistry. Ventral mesencephalic grafts contained cell bodies immunoreactive for enkephalin, GAD, substance P, and serotonin in addition to those immunoreactive for tyrosine hydroxylase. Some cells of each immunochemically defined type were retrogradely labeled by the fluorogold injection into the host brain. Nevertheless, more catecholaminergic and serotonergic cells projected from grafts to the fluorogold injection site than did other cell types. Since many of the nonmonoaminergic neurons in grafts are probably projection neurons, our results suggest that the extent of neurite outgrowth from grafted cells is influenced by the surrounding target tissue.