An HRP-TMB ultrastructural study of rubral afferents in the rat.

The projections from the deep cerebellar nuclei and the sensorimotor cortex to the red nucleus were studied in the rat using anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (HRP-WGA). The anterogradely transported HRP-WGA was visualized ultrastructurally by using a modification of the tetramethylbenzidine (TMB) histochemical technique of Carson and Mesulam ('82). Following injection of HRP-WGA into the sensorimotor cortex, ultrastructural examination of anterograde labeling in the ipsilateral red nucleus revealed labeled synaptic terminals located on small-diameter dendrites of the parvocellular region. These terminals made asymmetrical contacts and contained round vesicles. HRP-WGA placement in the nucleus lateralis resulted in anterograde labeling of synaptic terminals which made asymmetrical contacts with small- to medium-sized dendrites of the parvocellular red nucleus. Similar placements in the nucleus interpositus gave rise to anterograde labeling of synaptic terminals which made asymmetrical contacts with somata and proximal dendrites of magnocellular neurons. In addition, retrograde labeling of magnocellular neurons was also observed following HRP-WGA placements in the nucleus interpositus. Anterogradely labeled interpositorubral synaptic terminals were located on retrogradely labeled rubrocerebellar neurons. The rat red nucleus thus receives topographically organized afferents which are characterized by their specificity in location at the cellular level.