Distribution of efferent neurons projecting to the superior colliculus and the cerebellar vermal lobule Ⅸ in the spinal and principal sensory trigeminal nuclei of the rat

Distributions of efferent neurons projecting to the superior colliculus (SC) and the posterior vermis of the cerebellum were studied in the rat trigeminal sensory nuclei by retrograde labeling with wheat germ agglutinin-horseradish peroxidase (WGA-HRP). After unilateral injection of the tracer into the SC, small to medium-sized neurons in the principal sensory (5SNPR) and spinal trigeminal nuclei (5SP) were contralaterally labeled. A large number of the labeled neurons were found in the border between the interpolar (5SPip) and the oralis subnucleus (5SPo) of 5SP and in the rostral part of 5SPo. Very few neurons were labeled in the rostral tip of the caudal subnucleus of 5SP (5SPc). Labeled neurons in the 5SNPR and 5SP were detected after WGA-HRP injection into the vermal lobule Ⅸ. The labeled neurons were densely distributed in the dorsomedial region of 5SPip and the caudal 5SPo, but less frequently in 5SNPR. No neurons were labeled in 5SPc. Retrograde double labeling by fluorescent dyes showed that a small number of neurons in the ventral part of 5SNPR sent divergent axons to both lobule Ⅸ and SC