NEUROTRANSMITTER-RELATED GENE-EXPRESSION IN INTRASTRIATAL STRIATAL TRANSPLANTS .2. CHARACTERIZATION OF EFFERENT PROJECTING GRAFT NEURONS

The phenotypic characteristics of identified graft neurons in intrastriatal striatal transplants which give rise to efferent projections innervating the host brain were examined using a combination of in situ hybridization histochemistry and fluorescent retrograde tracing. Cell suspension grafts of embryonic day 14-15 rat striatal primordia (including both the medial and lateral ganglionic eminences) were implanted into the previously excitotoxically lesioned striatum of adult rats, and after longer than one year the retrograde tracer Fluoro-Gold was injected bilaterally into either the globus pallidus or the substantial nigra. Injections into the globus pallidus resulted in significant retrograde labelling of graft neurons within most of the experimental animals, whereas very few graft cells were labelled after the nigral injections. The vast majority of the neurons retrogradely labelled from the globus pallidus occurred in clusters or patches in the caudal half of the transplants, which corresponded well with DARPP-32 messenger RNA expressing (i.e. striatal) regions of the grafts. Indeed, within these Fluoro-Gold labelled graft patches, the proportion of retrogradely labelled found to contain DARPP-32 messenger RNA identical to that observed in the intact striatum after similar pallidal injections (93%). In addition, some Fluoro-Gold-labelled cells were found scattered outside the DARPP-32-positive cell clusters; these cells were overall larger and rarely (c. 9%) DARPP-32 messenger RNA-positive. Messenger RNA encoding for glutamate decarboxylase (which was found in 95% of Fluoro-Gold labelled neurons in the intact striatum) was detected in almost all retrogradely labelled graft neurons located in both the DARPP-32-positive patches of retrograde labelling (93%) and in the DARPP-32-negative regions (82%). In the intact striatum, neurons labelled after pallidal injections of Fluoro-Gold were observed to express preproenkephalin messenger RNA to a greater extent than preprotachykinin messenger RNA (81% vs 21%). Conversely, within the grafts, retrogradely labelled neurons in the patches of Fluoro-Gold-labelled cells were more often found to contain preprotachykinin messenger RNA (50%) than preproenkephalin messenger RNA (21%). The Fluoro-Gold-labelled cells scattered outside the patches of retrograde labelling rarely expressed either preproenkephalin or preprotachykinin messenger RNA. Fluoro-Gold injections into the host substantia nigra resulted in very few retrogradely labelled graft neurons; however, many (85%) of these cells were observed to express glutamate decarboxylate messenger RNA, while only rarely were they observed to contain either DARPP-32, preproenkephalin or preprotachykinin messenger RNAs (c. 10%). In the intact striatum, virtually all cells retrogradely labelled after nigral Fluoro-Gold injections were both DARPP-32 and glutamate decarboxylase messenger RNA-positive (96% and 94%, respectively) and they preferentially expressed preprotachykinin messenger RNA (85%) over preproenkephalin messenger RNA (12%). These data demonstrate that the vast majority of neurons projecting out of striatal grafts express phenotypic characteristics of GABAergic striatal projection neurons. Thus, the ability to extend axons along the internal capsule toward the host globus pallidus appears to be a specific property of the medium-sized GABAergic neurons present within the striatal regions of the grafts. On a more detailed level of analysis, however, the pattern of connectivity of the enkephalin-and substance P-containing subtypes may, at least in part, be aberrant.