Single-axon tracing study of neurons of the external segment of the globuspallidus in primate

Axonal projections arising from the external segment of the globus pallidus (GPe) in cynomolgus monkeys (Macaca fascicularis) were mapped after labeli ng small pools (5-15 cells) of neurons with biotinylated dextran amine. Sev enty-six single axons were reconstructed from serial sagittal sections with a camera lucida. The majority of labeled GPe cells displayed long, aspiny, and poorly branched dendrites that arborized mostly along the sagittal pla ne, whereas others showed dendrites radiating in all directions. Numerous G Pe axone emitted short, intranuclear collaterals that arborized close to th eir parent cell body. Based on their axonal targets, four distinct types of GPe projection neurons have been identified: 1) neurons that project to th e internal segment of the globus pallidus (GPi), the subthalamic nucleus (S TN), and the substantia nigra, pars reticulata (SNr; 13.2%); 2) neurons; th at target the GPi and the STN (18.4%); 3) neurons that project to the STN a nd the SNr (52.6%); and 4) neurons that target the striatum (15.8%). Labele d GPe axons displayed large varicosities that often were closely apposed to the somata and proximal dendrites of STN, GPi, and SNr neurons. At striata l levels, however, GPe axons displayed small axonal varicosities that did n ot form perineuronal nets. These results suggest that the GPe is an importa nt integrative locus in primate basal ganglia. This nucleus harbors several subtypes of projection neurons that are endowed with a highly patterned se t of collaterals. This organization allows single GPe neurons to exert a mu ltifarious effect not only on the STN, which is the claimed GPe target, but also on the two major output structures of the basal ganglia, the SNr and the GPi. J. Comp. Neurol. 417:17-31, 2000. (C) 2000 Wiley-Liss, Inc.