Organisation of connections between the zona incerta and the interposed nucleus

We have examined the organisation of connections between the zona incerta ( ZI), a small diencephalic nucleus deriving from the ventral thalamus, and t he interposed nucleus (Int) of the cerebellum. Injections of the tracer cho lera toxin subunit B were made into either the ZI or Int of Sprague Dawley rats by using stereotaxic coordinates. We have two major findings. First, t here is a heavy projection from Int to ZI; there is also a small projection back to Int from ZI. After injections into Int, labelled terminals and cel ls tend to concentrate within the medial region of each of the cytoarchitec tonically defined sectors of ZI. Second, there is an unusual laterality of connectivity between the ZI and the Int. The projection from the Int to the ZI is mainly contralateral, whilst the ZI projection back to the Int is ma inly ipsilateral. In conclusion, our results indicate that the Int of the c erebellum provides a rich source of afferents to the ZI, rendering the latt er in a key position to integrate information from the Int together with ma ny other types of subcortical information it receives, particularly from th e brainstem.