Projections of auditory nerve in the cat as seen by anterograde transport methods

Abstract

Ascending projections of the auditory nerve of the cat were studied by autoradiographic methods after injection of labeled precursors into the cochlea. In animals in which [ 3H]leucine was injected, transport to the cochlear nucleus was observed after survival periods of 2–7 days, but there was no evidence of transport to more central structures in the auditory pathways. In these cases the pattern of labeling around cell bodies depended on the type of cell and on the location of the cell within subdivisions of the cochlear nucleus. For example, there was dense perisomatic labeling around spherical cells in the anteroventral cochlear nucleus and around octopus cells in the posteroventral cochlear nucleus. Some other types of cells in subdivisions of both anteroventral and posteroventral cochlear nucleus had little perisomatic labeling. In the dorsal cochlear nucleus, the most densely labeled area consisted of a band just central to the layer of fusiform cells. Labeled fibers were not found peripheral to this band in the granular layer, molecular layer and peripheral part of the layer of fusiform cells. In the dorsal nucleus there was no evidence of dense perisomatic labeling such as was found around some cells in the ventral divisions of the cochlear nucleus. Following injections of a mixture of [ 3H]proline and [ 3H]fucose into the cochlea, labeling was seen in the superior olivary complex. Transneuronal transport was suggested as the explanation for this finding because of (1) the choice of the labeled precursors, (2) the length of the survival periods (14 or 20 days), and (3) the evidence that isotope was within neurons in the cochlear nucleus. We concluded that under appropriate conditions, unique patterns of silver grains are found around different types of cells in the various subdivisions of the cochlear nucleus, apparently because fibers and terminals of the auditory nerve become saturated with the radioisotope. As a result, a highly detailed survey of projections of the auditory nerve to each subdivision of the cochlear nucleus is revealed at the light-microscopic level. Under different experimental conditions labeled fibers are found in the superior olivary complex, but these conditions are those most conducive to transneuronal transport and so such experiments cannot be used as evidence of projection of auditory nerve fibers beyond the cochlear nucleus.