Distribution of efferent neurons projecting to the tectum and cerebellum in the rat prepositus hypoglossi nucleus.

Distributions of efferent neurons projecting to the superior colliculus (SC), the pretectum, and the cerebellar cortex were studied in the rat prepositus hypoglossi nucleus (PRH) by retrograde labeling with wheat germ agglutinin-horseradish peroxidase (WGA-HRP). After injection of the tracer into the unilateral SC, small- and medium-sized neurons were labeled throughout the entire rostrocaudal extent of the PRH, with the highest proportion in the middle part of the nucleus. After application of WGA-HRP to the unilateral pretectal region, including the nucleus of the optic tract, numerous labeled neurons appeared in the bilateral PRH. The highest distribution was found in the most rostral part of this nucleus. Furthermore, small neurons of the supragenual nucleus of the facial nerve also were labeled. Injection of WGA-HRP into lobules VI-VII of the cerebellar vermis resulted in the labeling of mainly medium-sized neurons in the bilateral PRH, primarily in the caudal third. WGA-HRP-labeled neurons were less frequently observed in the PRH after unilateral injection into the flocculus and paraflocculus. Retrograde double labeling by fluorescent dyes showed that a few neurons in the middle part of the PRH sent divergent axons to vermal lobules VI-VII and the SC.