Embryonic substantia nigra grafts innervate embryonic striatal co-grafts in preference to mature host striatum.

Embryonic substantia nigra grafts partially reinnervate the dopamine-denervated corpus striatum when implanted adjacent to that structure. This reinnervation is generally limited to a small portion of the denervated striatum and does not completely compensate for the behavioral effects of a 6-hydroxydopamine lesion of the substantia nigra. This limited reinnervation may be due to the fact that adult denervated striatum is not an ideal target for dopaminergic neurites. To test this hypothesis, embryonic striatum and embryonic substantia nigra were implanted together into the lateral ventricle of adult rats, adjacent to the denervated striatum. Five months after transplantation, fluorescence histochemistry showed that the embryonic striatal grafts were exclusively reinnervated with little or no reinnervation of the adult host striatum. When substantia nigra was implanted without embryonic striatal co-grafts, reinnervation of the host striatum was observed. We conclude that embryonic striatum is a better target tissue than adult denervated striatum for developing dopaminergic neurites and hypothesize that this difference may be due to the presence or the absence of specific trophic factors.