Dopaminergic innervation of striatal grafts placed into different sites of normal striatum: differences in the tyrosine hydroxylase immunoreactive growth pattern.

When patients with Parkinson's disease initially show symptoms, approximately 80-85% of their dopaminergic nerve fibers in the striatum have degenerated. It is thus of importance to develop strategies to try to rescue the remaining dopaminergic neurons and to stimulate them to induce sprouting. In this study the goal was to examine whether the different subgroups of dopaminergic neurons in the ventral mesencephalon projecting to the basal ganglia have different sprouting capacities when stimulated by the trophic effect of a fetal striatal graft. Lateral ganglionic eminence was implanted into the lateral ventricle, the midportion of dorsal striatum, globus pallidus, or ventral striatum. Solid tissue pieces from 13- to 15-mm fetuses were stereotactically implanted into adult female Sprague-Dawley rats. At postgrafting week 4 the animals were perfused and processed for tyrosine hydroxylase (TH) immunohistochemistry. Transplants placed in the lateral ventricle were TH-negative, except for two cases with TH-positive fibers where the ependymal layer was disrupted, thereby allowing direct contact between the graft and the adjacent host striatum. The transplants placed into dorsal striatum were innervated by small patches of dopaminergic nerve fibers. Areas between the TH-positive patchy structures remained TH-negative. In grafts placed into globus pallidus, both patchy structures and a less dense TH-positive nerve fiber network was noted. The TH-positive growth pattern in transplants placed in ventral striatum was also divided into patchy and widespread growth. Grafts placed in globus pallidus and ventral striatum revealed significantly larger areas of TH-positive innervation compared with that measured in grafts placed in dorsal striatum and the lateral ventricle. In conclusion, it is possible to induce sprouting of TH-immunoreactive nerve fibers from all areas examined. The most potent areas to initiate dopaminergic growth were the globus pallidus and ventral striatum, where both a patchy dense and a widespread, less dense growth was induced. Thus, if using a trophic stimulus to induce sprouting from remaining dopaminergic nerve fibers in Parkinson's disease, the preferential target to induce sprouting would be ventromedial striatum and growth would be guided toward dorsal striatum owing to the enhanced dopaminergic growth properties in the ventromedial areas.