Projections to the cochlear nuclei from principal cells in the medial nucleus of the trapezoid body in guinea pigs.

Spherical and globular cells in the cochlear nucleus provide input to the cell groups in the superior olivary complex devoted to the analysis of binaural cues. Descending projections from the superior olivary complex appear to inhibit the spherical and globular cells. It is not known which of the numerous cell types in the superior olive provide this descending input, but recent studies have shown that some of the cells are located in the medial nucleus of the trapezoid body (MTB). The present experiments were designed to determine whether the MTB projections arise from principal cells, which are known to play a role in sound localization, and to determine whether their projections terminate on spherical or globular cells. Principal cells in the MTB are characterized by their contacts with synaptic specializations called calyces, which arise from the axons of cells in the contralateral cochlear nucleus. In the first experiment, a fluorescent tracer was injected into one cochlear nucleus to label the calyces anterogradely. A different tracer was injected into the opposite cochlear nucleus to label cells retrogradely in the MTB. In every case, some of the labeled cells were enveloped by a labeled calyx, demonstrating that principal cells do project to the cochlear nucleus. In the second experiment, fluorescent tracers were injected into different parts of the cochlear nucleus. Analysis of the distribution of labeled cells suggested that MTB projections selectively target the globular cell region of the cochlear nucleus. In a third experiment, the axonal arborizations arising from this projection were labeled with biocytin or wheat germ agglutinin conjugated to horseradish peroxidase. Labeled boutons appeared to contact globular cells but not spherical cells. Multipolar cells in the ventral cochlear nucleus and cells in the dorsal cochlear nucleus were also contacted. The results suggest that MTB projections to the cochlear nucleus arise largely from principal cells and contact, at least in part, cells in the cochlear nucleus that give rise to ascending pathways involved in sound localization.