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 the interposed nucleus (Int) of the cerebellum. Injections of the tracer cholera 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, there 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 cells tend to concentrate within the medial region of each of the cytoarchitectonically 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 mainly ipsilateral. In conclusion, our results indicate that the Int of the cerebellum provides a rich source of afferents to the ZI, rendering the latter in a key position to integrate information from the Int together with many other types of subcortical information it receives, particularly from the brainstem.