Efferent projections of the subthalamic nucleus in the rat: Light and electron microscopic analysis with the PHA‐L method

Efferent projections of rat subthalamic nucleus were studied by use of the axonal transport of phaseolus vulgaris-leucoagglutinin (PHA-L), and the results were analyzed with light and electron microscopes. PHA-L injections in the subthalamic nucleus (STH) resulted in heavy labeling of fiber plexus with en passant boutons and terminals in the pallidal complex, i.e., the entopeduncular nucleus (EP), the globus pallidus (GP) and the ventral pallidum (VP), and the substantia nigra pars reticulata (SNR). Labeling in GP was characterized by two distinct bands of labeled terminals oriented dorsoventrally, whereas labeling in SNR was patchy. STH efferents to the pallidum and SNR displayed a mediolateral topographic organization. With regard to dorsoventral organization, projections to GP were inverted, but those to SNR were not. There were moderate projections to the neostriatum and sparse projections to the frontal cortex, substantia innominata, substantia nigra pars compacta (SNC), pedunculopontine tegmental nucleus, ventral part of the central gray matter including the dorsal raphe nucleus, and the mesencephalic and pontine reticular formation. PHA-L injections in the zona incerta and the lateral hypothalamic area resulted in fiber and terminal labelings in many structures, including the basal forebrain, EP, SNC, and other brainstem areas that overlap with some of the terminal sites of STH projections. Ultrastructural observations of PHA-L labeled processes in GP and SNR revealed that STH terminals in both structures contained small pleomorphic vesicles and formed asymmetrical contacts. These contacts were mainly on dendritic shafts, but some were on somata. It also was observed that the myelinated axons of STH neurons lost their myelin after reaching their target areas and the synaptic boutons arose from relatively thin unmyelinated axons.