Connectivity of striatal grafts implanted into the ibotenic acid-lesioned striatum--I. Subcortical afferents.

Subcortical afferents to transplants of fetal striatal tissue, implanted into the excitotoxically lesioned striatum of adult recipient rats, were studied with retrograde and anterograde axonal tracers and immunohistochemistry. One week after a striatal ibotenic acid lesion, involving most of the head of the caudate-putamen, a suspension of fetal striatal tissue (embryonic day 14-15) was injected into the lesioned area. In one group of rats, the ibotenic acid lesion was preceded (10 days) by large intrastriatal injections of True Blue, with injection sites matching the area to be lesioned. This was done to retrogradely pre-label the host brain afferents to the area of the striatum later to be lesioned and grafted. At 3 or 6 months post-transplantation, small injections (50 nl) of rhodamine-labelled latex beads were made into the striatal grafts. In animals where the injections were confined to the graft, retrogradely labelled host brain neurons were found in the thalamus, the substantia nigra, amygdala and dorsal raphe nucleus. Double-labelling analysis revealed that the vast majority of the rhodamine bead-labelled neurons also contained True Blue, which indicates that the host afferents to the graft, to a large extent, were derived from the neurons which normally project to the area of the caudate-putamen which was lesioned by the ibotenic acid injection. To further substantiate these observations a second group of lesioned and grafted animals received unilateral wheatgerm agglutinin-horseradish peroxidase injections into the ipsilateral host thalamus at 4 months post-transplantation in order to anterogradely label the host thalamostriatal axons. In a third group of animals serotonin immunocytochemistry was performed in order to detect possible afferents from the raphe nuclei. In contrast to the serotonin-containing fibers, which were fairly evenly distributed throughout the graft tissue, the peroxidase-labelled thalamic afferents were most prominent in the peripheral zones of the grafts and they were densely aggregated at the graft-host interface. The combined results provide evidence that the intrastriatal grafts receive afferents from the host substantia nigra, thalamus, amygdala and dorsal raphe nucleus, but with different distributions. The afferents from the substantia nigra, amygdala and raphe nuclei seem to distribute throughout the grafted tissue, although they are most dense in the peripheral parts, whereas the thalamic afferents are largely confined to the peripheral areas of the transplants and to the graft-host interface.