Electron microscopy of the subthalamic nucleus in the baboon. II. Experimental demonstration of pallido-subthalamic...

The present study demonstrates the existence in the baboon of a powerful ipsilateral pallido-subthalamic projection, composed of myelinated fibers. These axons give rise to type I (F) terminals, including all subtypes identified by Hassler et al. (1982), and terminate on the perikarya of the subthalamic relay neurons, on proximal dendrites, and on somatic and dendritic spines. Endings on subthalamic interneurons could not be found. Following experimental lesions in the pallidum externum, these pallido-subthalamic neurons undergo distinctive changes characterized as "pale", "intermediate" and "dark" degeneration, the form of the degeneration depending on the survival times of 3, 4 and 5 days. The ratios between the individual types of degeneration may vary. The ultrastructural features of the pallido-subthalamic terminals, which are believed to be GABA-ergic (Fonnum et al., 1978), are different from those of the GABA-ergic striato-nigral connections (Kim et al., 1971). After pallidum externum lesions, the pale form of degeneration is found in a few instances in the contralateral subthalamic nucleus in the type IX mini-synapses and the type I (F) beak-shaped synapses. There is a considerable convergence of different afferents with specialized synapses on the somata, stem dendrites, and dendritic spines of the neurons of the subthalamic nucleus.