Nigrocaudate and nigroputaminal projections in the monkey

Following injections of tracers into the caudate nucleus, the putamen or into both structures of the same cerebral hemisphere, retrograde cell labeling in the nigral complex of the squirrel monkey has been analyzed. Our goal was to investigate a possible relation of the retrograde neuronal labeling in the substantia nigra pars compacta (SNc) with the histochemical compartmentalization of this structure detected with the aid of the acetylcholinesterase (AChE) technique. Our results confirmed that in the squirrel monkey there was a very precise topography in the nigrostriatal projection system as a whole, and also in the nigrocaudate and nigroputaminal projection systems considered separately. In the cases with injections of two different tracers, we were not able to find any double-labeled nigral cell. Nigrocaudate and nigroputaminal projecting cells generally formed independent groups which at some nigral levels clearly interdigitated, as shown by the group of Parent. However, the presence of nigral cells labeled either by one or another tracer within the same histochemical compartment of the SNc confirmed that the segregation of nigroputaminal and nigrocaudate projecting cells did not explain the AChE compartmentalization of the SNc. Based on our experiments and on the literature we suggest that the histochemical compartmentalization of the SNc could be related, at least in part, to the subdivision of the striatum into AChE-poor striosomes and an AChE-rich extrastriosomal matrix.