A re-examination of the rubro-olivary tract in the cat,using horseradish peroxidase as a retrograde and an anterograde neuronal tracer

Most authors using free horseradish peroxidase as a retrograde tracer have had difficulties in demonstrating a rubro-olivary tract. Injections of this tracer into the inferior olive in animals of different species resulted in only a few retrogradely labelled rubral cells within the rostral part of the ipsilateral nucleus. The present study, which is based on the highly sensitive technique of Mesulam (1978), confirms this observation. Injections of free horseradish peroxidase (Serva or Sigma VI type) into the dorsal lamella of the principal olive in eleven cats gave a positive retrograde labelling of rostral rubral cells in only three animals, with a very few cells in each case. In contrast to this, olivary injections of horseradish peroxidase labelled lectin (wheat germ agglutinin) result in an abundance of retrogradely labelled cells within the rostral part of the ipsilateral red nucleus, the great majority of these cells being of the small and medium-sized type. Many of the retrogradely labelled rubral cells are only faintly stained. However, free horseradish peroxidase appears to be well suited as an anterograde tracer for a demonstration of the rubro-olivary tract.The findings are discussed in relation to the observation that in the cat only peroxidase conjugated to lectin can be used successfully as a retrograde tracer for a visualization of the rubro-olivary connection.