Human fetal mesencephalic tissue grafted to dopamine-denervated striatum of athymic rats: light- and electron-microscopical histochemistry and in vivo chronoamperometric studies.

Human fetal mesencephalic tissue obtained from elective first-trimester abortions was grafted to 6-hydroxydopamine-denervated striatum of athymic (nude) rats. After 3-6 months, the transplants were evaluated by light and electron microscopy using antibodies against tryosine hydroxylase (TH), human specific Thy-1 (Thy-1), 5-hydroxytryptamine (5-HT), and laminin. In vivo chronoamperometric studies of K+-induced release of electroactive species were done prior to the histochemical evaluations. At the light microscopical level, Thy-1-immunoreactivity was evenly distributed throughout the entire transplants. Thy-1-immunoreactive nerve fibers were observed radiating from the graft into the host striatum. In sections that were double-stained with antibodies against Thy-1 and TH, such nerve fibers contained both markers. Also 5-HT-immunoreactive cells were found in the grafts with processes both in the grafts and radiating into host neuropil. Laminin immunohistochemistry showed an even distribution of capillaries in the graft with less density than in host brain, suggesting immaturity of graft tissue. At the ultrastructural level, TH-immunoreactive axons made symmetric contacts with unlabeled dendritic shafts and dendritic spines within the host brain. A few asymmetric contacts with TH-immunoreactive axons were seen. 5-HT-immunoreactive terminals made both symmetric and asymmetric contacts with unlabeled dendritic shafts and spines. In vivo chronoamperometry using local application of K+ revealed average signals that were lower on the transplanted side than in control striatum. However, close to the grafts significant amounts of the K+-evoked signal amplitudes were as large as 1.3 microM, and the ratio of the reduction to oxidation currents suggested release of a mixture of dopamine and 5-HT. Taken together, this study shows that human fetal mesencephalic tissue pieces survive grafting into nude rats, develop normal vascularization, and express coexistence of TH- and Thy-1-immunoreactivity. Human TH- and 5-HT-immunoreactive nerve fibers form synapses in host striatum and release monoamine neurotransmitters.