Light and electron microscopic observations of a direct projection from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat.

A direct projection from rat mesencephalic trigeminal nucleus (Vme) neurons to the hypoglossal nucleus (XII) motoneurons was studied using a double labeling method of anterogradely biotinylated dextran amine (BDA) tracing combined with retrogradely horseradish peroxidase (HRP) transport at both light and electron microscopic levels. BDA was iontophoresed unilaterally into the caudal Vme, and 7 days later HRP was injected into the ipsilateral tongue to label hypoglossal motoneurons. The BDA-labeled fibers were seen descended along Probst' tract and were traced to the caudal medulla. In this course, the fibers gave off axon collaterals bearing varicosities in the trigeminal motor nucleus (Vmo), the parvicellular reticular formation (PCRt), the dorsomedial portions of the subnuclei of oralis (Vodm) and interpolaris (Vidm) and in the XII ipsilaterally. The labeling of terminals was most dense in the PCRt at the levels of caudal pons and rostral medulla, which displayed a "dumbbell-shaped" form in the transverse planes. In the XII, labeled terminals were distributed mainly in the dorsal compartment of the nucleus. One hundred sixty-eight appositions made by BDA-labeled terminals on HRP-labeled motoneurons were seen in the dorsal compartment (71%) and in the lateral subcompartment (24%) of the ventral XII. Under electron microscopy BDA-labeled boutons containing clear, spherical synaptic vesicles were found to form synaptic contacts with the somata and dendrites of hypoglossal motoneurons with asymmetric specializations. The present study provides new evidence that the trigeminal proprioceptive afferent neurons terminate in the XII and make synaptic contacts with their motoneurons.