Glycine immunoreactivity of multipolar neurons in the ventral cochlear nucleus which project to the dorsal cochlear nucleus

Certain distinct populations of neurons in the dorsal cochlear nucleus are inhibited by a neural source that is responsive to a wide range of acoustic frequencies. In this study, we examined the glycine immunoreactivity of tw o types of ventral cochlear nucleus neurons (planar and radiate) in the rat which project to the dorsal cochlear nucleus (DCN) and thus, might be resp onsible for this inhibition. Previously, we proposed that planar neurons pr ovided a tonotopic and narrowly tuned input to the DCN, whereas radiate neu rons provided a broadly tuned input and thus, were strong candidates as the som ce of broadband inhibition (Doucet and Ryugo [1997] J. Comp. Neurol. 3 85:245-264). We tested this idea by combining retrograde labeling and glyci ne immunohistochemical protocols. Planar and radiate neurons were first ret rogradely labeled by injecting biotinylated dextran amine into a restricted region of the dorsal cochlear nucleus. The labeled cells were visualized u sing streptavidin conjugated to indocarbocyanine (Cy3), a fluorescent marke r. Sections that contained planar or radiate neurons were then processed fo r glycine immunocytochemistry using diaminobenzidine as the chromogen. Immu nostaining of planar neurons was light, comparable to that of excitatory ne urons (pyramidal neurons in the DCN), whereas immunostaining of radiate neu rons was dark, comparable to that of glycinergic neurons (cartwheel cells i n the dorsal cochlear nucleus and principal cells in the medial nucleus of the trapezoid body). These results are consistent with the hypothesis that radiate neurons in the ventral cochlear nucleus subserve the wideband inhib ition observed in the dorsal cochlear nucleus. (C) 1999 Wiley-Liss, Inc.