SINGLE STRIATOFUGAL AXONS ARBORIZING IN BOTH PALLIDAL SEGMENTS AND INTHE SUBSTANTIA-NIGRA IN PRIMATES

The striatofugal fiber system in primates is believed to be composed o f separate subsystems terminating in either the external (GPe) or inte rnal (GPi) segment of the globus pallidus, or in the substantia nigra (SN). At variance with this concept is the present demonstration of si ngle biocytin-labeled striatofugal axons that arborize in the three ma jor target structures of the striatum in cynomolgus monkeys. Out of ni ne single-labeled axons that were analyzed in detail, one terminated e xclusively in GPe, another in both GPe and GPi, whereas the rest arbor ized in GPe, GPi and SN. The axons that branched in the three sites ha d one preferential recipient structure where they arborized profusely and formed typical woolly fibers. These findings suggest that, in cont rast to previous beliefs based on results of retrograde double-labelin g studies, most striatofugal axons arborize within more than one stria tal target structures in primates.