Nigropedunculopontine projection in the rat: An Anterograde tracing study with phaseolus vulgaris‐leucoagglutinin (PHA‐L)

The termination of the substantia nigra pars reticulata efferents in the nucleus tegmenti pedunculopontinus was studied in the rat by using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Both large and small injections of PHA-L in various portions of the substantia nigra pars reticulata labeled varicose fibers in the ipsilateral and contralateral nucleus tegmenti pedunculopontinus, subnucleus dissipatus as well as in the ipsilateral nucleus tegmenti pedunculopontinus, subnucleus compactus. However, the bulk of the nigral fibers appeared to terminate in the medial two-thirds of the ipsilateral subnucleus dissipatus of the pedunculopontine nucleus and exhibited a discrete dorsoventral topographical pattern. The terminal plexus displayed patches of uneven density, which was partly due to the numerous fiber bundles passing through the pedunculopontine nucleus, but also to an obvious preference of nigral fibers for some cells. Electron microscopic examination confirmed that nearly all of the varicosities observed in the light microscope contained synaptic vesicles and represented either terminal boutons or boutons en passant. The labeled boutons were elongated (average length: 1.5 μm) and consistently contained a prominent group of mitochondria. The results suggest that the nigral input to the nucleus tegmenti pedunculopontinus may be directed toward specific subpopulation(s) of pedunculopontine neurons and may influence not only cells in the subnucleus dissipatus, but also in the subnucleus compactus.