Retrograde analysis of cerebellar inputs to ventrolateral, intralaminar and midline thalamic nuclei in macaque monkeys

We injected retrograde tracers (cholera toxin b and fluorescent dextrans) in distinct thalamic nuclei to examine the origin of their cerebellar inputs. In a previous study (Evrard Craig, SfN 2006), we showed that the posterior ventral part of the ventral lateral nucleus (VLpv) receives topographically organized cerebellar inputs from the anterior two thirds of the dentate nucleus (DN), the entire anterior interposed nucleus (NIA) and the dorsal portion of the posterior interposed nucleus (NIP). In the present study, injections were made in the posterior dorsal part of the ventral lateral nucleus (VLpd), area X (X), the central lateral nucleus (CL), the centre median/parafascicular complex (CM/Pf) and the medial dorsal nucleus (MD). Injections in VLpd, X and CL produced abundant labeling in DN, NIA and NIP. The cerebellar labeling from these thalamic nuclei clearly overlapped with the labeling obtained after injection in VLpv but also included the caudal third of DN and the ventral portion of NIP. Injections in Pf labeled relatively few cells that were widely scattered in DN, especially its caudal half. By contrast, injections in CM produced abundant labeling in all parts of DN, NIA and NIP. Injections in MD labeled neurons specifically confined to the ventral posterior portion of DN and to the ventral portion of NIP. These two regions always remained unlabeled after large injections in VLpv and are thus clearly distinct portions of the deep cerebellar nuclei. The ventral posterior portion of DN might correspond to the “non-motor” portion of DN labeled after injections of transneuronal retrograde tracer in prefrontal cortex (Middleton Strick, J Neurosci 2001 15: 700-12). Overall, these results show that cerebellar inputs to different thalamic nuclei originate from overlapping regions of the deep cerebellar nuclei. Whether these overlapping regions contain neurons that send collateral projections to several thalamic nuclei or intermingled neurons that project to only one thalamic nucleus remains to be tested