Changes in the tonotopic map of the dorsal cochlear nucleus in hamsters with hair cell loss and radial nerve bundle degeneration

Abstract

Hamsters were exposed to an intense tone (10 kHz) at levels and durations sufficient to cause hair cell loss and radial nerve bundle degeneration. A previous study reported changes in the tonotopic map of the dorsal cochlear nucleus (DCN) in hamsters with tone-induced stereocilia loss. Such changes appear similar to those observed by others in the auditory nerve following acoustic trauma, and suggest that the map alterations have a peripheral origin. However, the potential for tonotopic map reorganization after more severe lesions involving cellular degeneration in the cochlea has not yet been determined. The purpose of the present study was to determine how the tonotopic map of the DCN appears in animals with severe cochlear injury involving hair cell loss and radial nerve bundle degeneration. Neural population thresholds and tonotopic organization were mapped over the surface of the DCN in normal unexposed animals and those showing tone-induced lesions. The results indicate that cochlear lesions characterized mainly by radial bundle degeneration in a restricted portion of the organ of Corti cause changes in a corresponding region of the tonotopic map which reflect primarily changes in the shape and thresholds of neural tuning curves. In many cases the center of the lesion was represented in the DCN as a distinct characteristic frequency (CF) gap in the tonotopic map in which responses were either extremely weak or absent. In almost all cases the map area representing the center of the lesion was bordered by an expanded region of near-constant CF, a feature superficially suggestive of map reorganization (i.e., plasticity). However, these expanded map areas had abnormal tip thresholds and showed other features suggesting that their CFs had been shifted downward by distortion and deterioration of their original tips. Such changes in neural tuning following tone-induced loss of anatomical input to the central auditory pathway are similar to those observed in our previous study and by others in the auditory nerve following less severe acoustic trauma, and thus would seem to have a peripheral origin. Thus, changes in the DCN tonotopic map can be explained by peripheral modifications and do not seem to involve plastic changes (i.e., reorganization).