Projection from the deep cerebellar nuclei to the pedunculopontine nucleus in the squirrel monkey.

Large injections of the anterograde tracer biocytin in the deep nuclei of the cerebellum of squirrel monkeys (Saimiri sciureus) led to a massive labeling of the superior cerebellar peduncle fibers which could be followed up to their major termination site in the thalamus. Along their course through the brainstem, biocytin-labeled fibers emitted fine collaterals that arborized profusely within the entire rostrocaudal extent of the pedunculopontine nucleus (PPN). These fibers were long, slightly varicose, and broke off into numerous shorter and thinner fibers whose terminal portions consisted of a few large varicosities that were often closely apposed to dendrites and cell bodies of PPN neurons. Some PPN cells that were contacted displayed immunoreactivity for choline acetyltransferase. Ultrastructural analysis revealed that synapses formed by cerebellar fibers in PPN were of the asymmetric type and occurred predominantly on dendrites of PPN neurons. Thus, beside the well established cerebellothalamic projection, our findings reveal the existence of a cerebellotegmental projection, whereby the cerebellum may influence the basal ganglia and/or the thalamus via a relay in PPN.