Projections from the Cerebral Cortex to the Red Nucleus of the Guinea-pig. A Retrograde Tracing Study

The origin and the topographic distribution of corticorubral (CR) projections in the guinea-pig were studied by using the retrograde axonal transport of a tracer, colloidal gold-labelled, enzymatically inactive horseradish peroxidase conjugated to wheat-germ agglutinin (WGAapoHRP - Au), which was injected in the red nucleus (RN). It was found that the bulk of the CR projections arise from layer V neurons of the agranular frontal cortex in both its medial (Agm) and lateral (Agl) subdivisions; in the Agm labelled neurons are preferentially located in the upper part of layer V, whereas in the Agl they are more concentrated in the central band of the layer. Fewer projections originate from areas of the granular parietal and the agranular cingulate and retrobulbar cortices. CR projections have a bilateral origin, with a large ipsilateral predominance. The pattern of retrograde cortical labelling observed after injection of WGAapoHRP - Au in different portions of the RN indicates that CR projections are distributed throughout the entire rostrocaudal extent of the nucleus, but are slightly more concentrated in the rostral parvocellular area. The morphological arrangement of CR projections in the guinea-pig, as demonstrated in the present study, shows several analogies with other mammals. The functional characteristics of the cortical areas in which CR neurons are located indicate that CR projections may play a significant role in the central organization of movement.