The posterior pretectal nucleus: evidence for a direct projection to the inferior olive of the cat.

In our horseradish peroxidase (HRP) study of the afferents to the medullary raphe nuclei in the cat, HRP uptake by damaged axons en route to the inferior olive (IO) was thought to be responsible for retrograde labelling of specific midbrain nuclei. To control for such indirect labelling, HRP was injected iontophoretically into the inferior olive. The location of retrogradely labelled neurons was related to the specific locus of HRP injection within the IO. Injection of HRP into the caudal dorsal accessory olive resulted in dense neuronal labelling in the ipsilateral caudal pole of the posterior pretectal nucleus (PPN). There was no labelling of the nucleus of Darkschevitch (Dk), interstitial nucleus of Cajal (ICA) or Edinger-Westphal nucleus (EW). In contrast, an injection focussed more rostrally, into the rostral, dorsal accessory, the medial accessory and the principal olive, produced dense labelling of Dk, ICA and EW; there was much less PPN labelling. It is concluded that labelling of Dk and PPN after HRP injections rostral to the IO, is due, at least in part, to uptake of HRP by damaged medial longitudinal fasciculus axons en route to the inferior olive. The direct PPN-inferior olivary projection provides a potential disynaptic retino-cerebellar connection, which may be involved in rapidly timed eye-body coordinate movements.