The oculomotor internuclear pathway: a double retrograde labeling study

To determine whether oculomotor internuclear neurons (INOs) establish unilateral or bilateral connections with the abducens nucleus (nVI), horseradish peroxidase (HRP) and either DAPI or Nuclear Yellow (NY) were injected into nVI and the oculomotor complex (nIII) examined for retrogradely labeled cells. Such injections resulted in retrograde labeling of a large number of INOs within and just dorsal to nIII in the ventral periaqueductal gray; approximately 10% of HRP-positive cells were also labeled with a fluorescent marker. Within the conditions of the experiment, this frequency of double-labeling was largely independent of the amount of HRP or fluorescent label injected or the degree of spread of label at the injections sites. It was also independent of which fluorochrome was used as a second label. Individual double-labeled cells were scattered within and immediately dorsal to nIII within the distribution of single-labeled INOs. These findings provide strong evidence for a subset of nIII INOs that have bilateral projections to nVI; the low frequency of double-labeling, however, suggests that the population of INOs projecting to both nVI may be small.