THE TOPOGRAPHICAL ORGANIZATION OF DESCENDING PROJECTIONS FROM THE CENTRAL NUCLEUS OF THE INFERIOR COLLICULUS IN GUINEA-PIG

We describe the descending projections from the central nucleus of the inferior colliculus (CMC) in guinea pig. Focal injections of the trac er biocytin, made in physiologically defined frequency regions of the CNIC, labelled laminated axonal terminal fields in the ipsilateral dor sal nucleus of the lateral lemniscus, and bilaterally in the ventral n ucleus of the trapezoid body and the dorsal cochlear nucleus. Labellin g was also present in the rostral periolivary nucleus, but we could no t distinguish a clear border between the terminal fields in this nucle us and those in the ventral nucleus of the trapezoid body. Labelling o bserved in the ventral nucleus of the lateral lemniscus, and to a less er extent in the dorsal nucleus of the lateral lemniscus, was accompan ied by retrogradely labelled somata and therefore we cannot conclude u nequivocally that the CNIC projects to these lemniscal nuclei. Where t he labelling was ordered topographically, its position varied as a fun ction of the best frequency at the injection site. High-frequency regi ons in the CNIC project to the medial parts of the ventral nucleus of the trapezoid body and dorsal cochlear nucleus, while low-frequency re gions in the CNIC project to the lateral parts of the ventral nucleus of the trapezoid body and dorsal cochlear nucleus. Additional axonal l abelling with terminal boutons, but with no apparent topographical arr angement, was present in the ipsilateral horizontal cell group, sagulu m, and also bilaterally in the superficial granule cell layer of the v entral cochlear nucleus and layer 2 of the dorsal cochlear nucleus. Ou r findings are consistent with the existence of tonotopically organise d feedback projections from the CNIC to the brainstem nuclei that proj ect to it.