A reevaluation of midbrain and diencephalic projections to the inferior olive in rat with particular reference to the rubro-olivary pathway.

Projections from the midbrain and caudal diencephalon to the inferior olivary nucleus (ION) in the rat were investigated by using anterograde and retrograde tracing techniques. Particular attention was directed to studying the projection from the red nucleus (RN) to the ION. Tritiated leucine was stereotaxically injected into the RN in six animals. The injection sites and the portion of the medulla containing the ION were processed for autoradiography. In three cases, the injections were largely confined to the RN. In these instances, no terminal labeling was noted in the ION. In the other three animals, the injections spread beyond the RN to include, in one or another of the rats, such areas as the mesencephalic reticular formation (MRF) between the RN and the periaqueductal gray (PAG), the region around the fasciculus retroflexus (FR), the prerubral area, the MRF dorsolateral to the RN, or the region dorsal to the medial geniculate body. In these animals, terminal labeling was observed ipsilaterally in the ION, chiefly in the caudal one-half of the medial accessory olive and in the dorsal lamella and lateral bend of the principal olive. Retrograde transport of lectin-conjugated horseradish peroxidase (WGA-HRP) or the fluorescent dye fast blue (FB), injected into the ION, was used to localize regions in the midbrain and caudal diencephalon containing neurons which project to the inferior olive. Following WGA-HRP injections into the ION, retrogradely labeled neurons were found in the ipsilateral ventral PAG, the MRF between the PAG and the RN, the MRF dorsolateral to the RN, and the area dorsal to the medial geniculate body. Large numbers of heavily labeled neurons were found in the area surrounding the FR. No labeled cells were noted in the RN or the nucleus of Darkschewitsch (ND). Subsequent to FB injections into the ION, fluorescent neurons were observed in the same regions which contained HRP-labeled neurons in the experiments using the enzyme as a tracer. Additionally, very small numbers of FB-labeled neurons were found in the RN and the ND. These results indicate that, in the rat, descending input from the midbrain and caudal diencephalon to the ION arises chiefly from a medially located column of neurons extending from the level of the rostral RN to the region of the FR. Little, if any, input to the ION is derived from the ND or RN in this species, unlike the cat, in which a substantial projection to the ION arises from the ND and the RN.