Retrograde transport of [3H]glycine from the cochlear nucleus to the superior olive in the guinea pig

The origins of descending glycinergic projections to the guinea pig cochlear nucleus were investigated using retrograde labelling techniques. To identify the cell groups that provide descending projections to the cochlear nucleus, horseradish peroxidase, a nonspecific retrograde neuronal marker, was injected into the cochlear nucleus. After 24 or 48 hours, labelled cell bodies were evident bilaterally in all of the periolivary nuclei that surround the lateral and medial superior olive. The largest numbers of labelled neurons were located in the ventral nucleus of the trapezoid body bilaterally and in the lateral nucleus of the trapezoid body and dorsal periolivary nucleus ipsilaterally. Labelled cells were also present in the inferior colliculus bilaterally and in the contralateral cochlear nucleus. [3H]Glycine was employed as a retrograde tracer to identify the cell groups providing descending glycinergic projections to the cochlear nucleus. Three to 48 hours after injection of 19, 190, or 380 microM [3H]glycine into the cochlear nucleus, retrogradely labelled cell bodies were observed ipsilaterally in all of the periolivary nuclei. No labelled neurons were found in the inferior colliculus. After injections of the highest concentration of [3H]glycine, labelled cells were also found contralaterally in the ventral and lateral nuclei of the trapezoid body and also in the contralateral cochlear nucleus. We conclude that descending glycinergic projections to the cochlear nucleus originate mostly in ipsilateral periolivary cell groups. Minor glycinergic projections originate from the contralateral cochlear nucleus and also from the contralateral ventral and lateral nuclei of the trapezoid body.