Connectivity of striatal grafts implanted into the ibotenic acid-lesioned striatum—III. Efferent projecting graft neurons and their relation to host afferents within the grafts

Abstract

Efferent projections of intrastriatally implanted striatal neurons have been studied using a combination of anterograde and retrograde axonal tracers. Adult rats subjected to a unilateral ibotenic acid lesion of the head of the caudate putamen received cell suspension grafts obtained from 14–15-day-old striatal primordia. Three and a half to 20 months after transplantation the rats received either intra-transplant injections of the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin or injections of fluorescent retrograde tracers, Fluoro-Gold and rhodamine-labelled latex beads, into the host globus pallidus and substantia nigra. Injections of Phaseolus vulgaris leucoagglutinin located entirely within the grafts labelled axons that ramified extensively within the tissue itself, as well as axons that extended caudally, across the graft-host border, along the myelinated fascicles of the internal capsule to arborize in the medial parts of the host globus pallidus. A few axons also reached the entopeduncular nucleus. Injections of Fluoro-Gold into the host globus pallidus labelled large numbers of graft neurons, which had a prominent patchy distribution and were most abundant in the caudal portions of the grafts. Clear retro-grade labelling was also seen after injection of Fluoro-Gold or rhodamine beads into the host substantia nigra, although the number of labelled graft neurons was 30–50 times lower than that seen after pallidal injections. Combined injections of Fluoro-Gold into the pallidus and rhodamine beads into the nigra showed that the vast majority of cells labelled from the nigra were also labelled by Fluoro-Gold from the pallidus. In some of the grafted and Fluoro-Gold-injected animals, the fetal donor tissue had been labelled with [ 3H]thymidine prior to transplantation. Many examples of neurons labelled with both [ 3H]thymidine and Fluoro-Gold were found after tracer injections into the host globus pallidus, and double-labelled neurons were identified also after Fluoro-Gold injections into the host substantia nigra. In several animals retrograde tracing was combined with labelling of host dopaminergic afferents (by tyrosine hydroxylase immunohistochemistry) and cortical afferents (by injections of Phaseolus vulgaris leucoagglutinin into the host frontal cortex). Comparison of adjacent sections revealed a striking overlap between the patches of Fluoro-Gold-labelled graft neurons (labelled from the host pallidum) and the dense patches of tyrosine hydroxylase-positive terminals. In addition, many of the Fluoro-Gold-labelled cell patches received a high density of cortical afferents labelled by Phaseolus vulgaris leucoagglutinin. The results thus demonstrate that the intrastriatal striatal grafts are capable of establishing efferent projections to the host globus pallidus and to a lesser extent also to the entopeduncular nucleus and the substantia nigra. The close association between the efferent projecting graft nerons and the dopaminergic and cortical afferent inputs from the host suggests that the grafted striatal neurons become anatomically and functionally integrated into the neuron-depleted host striatal circuitry.