Connections of the paraflocculus of the cerebellum with the superior colliculus in the rat brain

Abstract

The interconnections between the paraflocculus of the cerebellum and the superior colliculus (SC) of the adult male Wistar albino rat were traced utilising the retrograde and anterograde transport of horseradish peroxidase conjugated with wheatgerm agglutinin (WGA-HRP). The study comprises three series of experiments. In the first series designed to trace the connections between the paraflocculus and the pre- and deep cerebellar nuclei, microiontophoretic injections of WGA-HRP filled either the entire paraflocculus or one of its subdivisions (ventral or dorsal paraflocculus). The results showed that, the ventral and dorsal paraflocculus receive afferents from distinct and separate regions of pontine nuclei, nucleus reticularis tegmenti pontis (Rtp) and the inferior olive. The efferents from the ventral and dorsal parafloccular subdivisions project to circumscribed regions of the lateral and posterior interpositus cerebellar nuclei. In the second series of experiments, unilateral iontophoretic WGA-HRP injections were placed at varying depths, through the rostrocaudal extent of SC and the tectal connections with the brainstem, and deep cerebellar nuclei were traced. The SC projects to well circumscribed regions of the pontine nuclei, Rtp and the inferior olive and receives afferents from the lateral and posterior interpositus cerebellar nuclei. In the third series of experiments involving combined injections of WGA-HRP into SC and the ventral paraflocculus, the collicular and ventral parafloccular connections were traced. The ventral paraflocculus receives afferents from regions of pons and Rtp which in turn receive inputs from the SC and projects to the lateral and posterior interpositus cerebellar nuclei which in turn give rise to efferents to the SC. These findings demonstrate that the ventral paraflocculus is linked to the contralateral superior colliculus via (i) a crossed afferent tecto-ponto/Rtp-parafloccular pathway, and (ii) a crossed efferent paraflocculo-nucleo-tectal pathway.