Axonal branching pattern of neurons of the subthalamic nucleus in primates

Axonal projections arising from the subthalamic nucleus (STN) in cynomolgus monkeys (Macaca fascicularis) were traced after labeling small pools (5-15 cells) of neurons with biotinylated dextran amine. Seventy-five single axo ns were reconstructed from serial sagittal sections with a camera lucida. M ost of the STN labeled cells displayed five to eight long, sparsely spined dendrites that arborized mostly along the main axis of the nucleus. Based o n their axonal targets, five distinct types of STN projection neurons have been identified: 1) neurons projecting to the substantia nigra pars reticul ata (SNr), the internal (GPi) and external (GPe) segments of the globus pal lidus (21.3%); 2) neurons targeting SNr and GPe (2.7%); 3) neurons projecti ng to GPi and GPe (48%); 4) neurons targeting GPe only (10.7%); and 5) neur ons with axons that coursed toward the sriatum, but whose terminal arboriza tion could not be visualized in detail. (17.3%). Axons of the first two typ es bifurcated into rostral subthalamopallidal and caudal pallidonigral bran ches. However, the majority of STN axone had only a single branch that cour sed rostrally toward the pallidum and striatum. These results reveal that, in contrast to current beliefs, the primate STN is not a monolithic entity. This nucleus harbors several subtypes of projection neurons, each endowed with a highly patterned set of collaterals. This organization allows STN ne urons to exert a multifarious effect not only on the GPe, with which the ST N is reciprocally connected, but also on the two major output structures of the basal ganglia, the SNr and the GPi. J. Comp. Neurol. 424:142-152, 2000 . (C) 2000 Wiley-Liss, Inc.