DEGENERATION OF AXONS IN THE BRAIN-STEM OF THE CHINCHILLA AFTER AUDITORY OVERSTIMULATION

The patterns of axonal degeneration following acoustic overstimulation of the cochlea were traced in the brainstem of adult chinchillas. The Nauta-Rasmussen method for axonal degeneration was used following sur vivals of 1-32 days after a 105 min exposure to an octave-band noise w ith a center frequency of 4 kHz and a sound pressure level of 108 dB. Hair-cell and myelinated nerve-fiber loss were assessed in the cochlea . The cochleotopic pattern of terminal degeneration in the ventral coc hlear nucleus correlated with the sites of myelinated fiber and inner- hair-cell loss: this correlation was less rigorous with outer-hair-cel l loss, especially in the dorsal cochlear nucleus. These results are c onsistent with a dystrophic process with a slow time course depending on hair-cell loss and/or direct cochlear nerve-fiber damage. However, in a number of cases with no damage in the apical cochlea, fine fiber degeneration occurred with a faster course in low-frequency regions in the dorsal cochlear nucleus and, transynaptically, in a non-cochleoto pic pattern in the superior olive and inferior colliculus. These findi ngs suggest that neuronal hyperactivity plays a role in the central de generation following acoustic overstimulation, possibly by an excitoto xic process.