Cerebellar connections with the motor cortex and the arcuate premotor area: an analysis employing retrograde transneuronal transport of WGA-HRP.

We have employed transneuronal transport to examine the anatomical relationships between the deep cerebellar nuclei and 2 cortical motor areas: the primary motor cortex and the arcuate premotor area (APA). In the same animals, we have also examined the patterns of labeling in the thalamus and the red nucleus to provide evidence for the potential routes of transneuronal transport to the cerebellum. When the appropriate technical procedures were employed, cortical injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) resulted in transneuronal labeling within portions of the contralateral deep cerebellar nuclei. Injections into the primary motor cortex labeled neurons in the dentate and in the 2 subdivisions of the interpositus. Injections into the APA labeled neurons in the dentate and in only the posterior subdivision of the interpositus. In most instances, dentate neurons were more intensely labeled following the cortical injections than interpositus neurons. The transneuronal labeling observed in the dentate nucleus was topographically organized. The dentate region that was labeled following injections into the "arm area" of the APA was caudal and ventral to the dentate region that was labeled following injections into the "arm area" of the primary motor cortex. This observation provides evidence for two "arm areas" in the dentate: one anatomically related to the APA, and the other related to the primary motor cortex. More than one route of transport may be responsible for the labeling of cerebellar neurons. We propose that the labeling observed in the dentate nucleus reflects the pattern of connections in the cerebellothalamocortical pathways that link the dentate with the cerebral cortex. Thus, our observations support the concept proposed by Schell and Strick (J. Neurosci. 4:539-560, '84)--that the cortical targets of the dentate nucleus include both the primary motor cortex and the APA.