The interrelations between cell groups in the caudal diencephalon of the rat projecting to the striatum and to the medulla oblongata.

In order to investigate the topographical relationships between the caudal diencephalic cells of origin of ascending and descending projections in the rat, one fluorescent retrograde tracer was injected into the striatum and another into the medulla oblongata. The medullary injections were mainly centered in the inferior olive. Cells labeled from the striatal injections densely filled the thalamic parafascicular nucleus. Cells labeled from the medullary injections were seen ventrally to the fasciculus retroflexus in the subparafascicular nucleus. The two populations were mixed in a small area at the ventromedial border of the fasciculus retroflexus. No double labeled cells were observed. The present results indicate that caudal diencephalic cells which ascend to the striatum are different from those descending to the medulla oblongata and that they partially overlap.