Evidence of collateral axonal projections to the superior olivary complex

In this study we investigated the collateral axonal projections to the superior olivary complex using the combined anterograde and retrograde transport of wheat-germ agglutinin conjugated with horseradish peroxidase. Small injections of this tracer were placed in the lateral or medial superior olivary nuclei in cats, and the location of anterograde label in the alternate nuclei of the superior olivary complex was determined. Injections of [3H]leucine were also placed in these nuclei for control purposes. After wheat-germ agglutinin-horseradish peroxidase injections in the lateral superior olivary nucleus anterograde label was observed bilaterally in the medial superior olivary nuclei. Likewise, after injections in the medial superior olivary nucleus anterograde label was observed in the contralateral medial and lateral superior olivary nuclei. The topography of the anterograde label was always precise and varied predictably as a function of the injection site. Most retrogradely labeled cells were located in the ipsilateral anteroventral cochlear nucleus and the medial nucleus of the trapezoid body. Various interpretations of the data are considered. Our primary conclusion is that cells in the anteroventral cochlear nucleus are a major source of collaterals to both the ipsilateral and contralateral nuclei of the superior olivary complex.