Substantia nigra transplants into denervated striatum of the rat: ultrastructure of graft and host interconnections.

A number of recent experiments suggest that grafted dopaminergic neurons provide functional input to a host caudoputamen which previously had been deprived of its dopaminergic input. The purpose of the present study was to determine whether tyrosine hydroxylase immunoreactive processes which originate in the graft participate in morphologically identifiable synapses in the host neuropil. Prior to transplantation, adult Sprague-Dawley rats received unilateral injections of 6-hydroxydopamine into the medial forebrain bundle. Animals were screened for the success of striatal denervation by a test of apomorphine-induced rotation. Transplants of fetal substantia nigra then were placed into cavities in the caudoputamen. After a 6-8 month survival period, animals were perfused and prepared for tyrosine hydroxylase immunocytochemistry. No evidence of sprouting of the host catecholoaminergic system was observed, even after long survival times. Both pre- and postsynaptic immunoreactive elements were clearly present in the host caudoputamen. Immunoreactive axons made synaptic contact with unlabeled dendrites; immunoreactive dendrites were postsynaptic to unlabeled axon terminals. The present results suggest that both host-to-graft and graft-to-host synapses are present in the host caudoputamen.