The morphology of globus pallidus projection neurons in the rat: an intracellular staining study.

The morphology of 23 intracellularly stained projection neurons of rat globus pallidus (GP) was studied in light microscopic preparations. The somatic size of these projection neurons was highly variable. The somatic area ranged from 78 to 353 microns 2. The 23 neurons were divided into aspiny and spiny types, based on the existence of dendritic spines. Sixteen neurons were aspiny and 7 were of the spiny type. The aspiny neurons tended to have a larger soma than the spiny neurons. Fourteen of the 23 projection neurons possessed a discoidal dendritic field with the flat plane parallel to the border between the GP and the neostriatum. All of the 14 neurons having a discoidal dendritic field were of the aspiny type and were located throughout the GP. The other 9 neurons, which include all of the 7 spiny types, had radiating dendritic fields with a variety of shapes and were located only in the medial region of the GP. The axons of a majority (i.e. 21 of 23) of the projection neurons emitted multiple collaterals with large boutons en-passant and boutons terminaux within the GP. The main axons were traced to varying distances from their somata. Four of them were traced into the substantia nigra. Two of these 4 emitted multiple collaterals at various rostro-caudal levels in the entopeduncular nucleus, and all 4 axons had one or two collaterals in the subthalamic nucleus. This study revealed that the rat GP contains two types of projection neurons having different dendritic morphologies. The axon reconstructions indicate that the activity of both types of neurons can influence multiple basal ganglia targets, including the GP itself.