LONG-TERM DEGENERATION IN THE COCHLEAR NERVE AND COCHLEAR NUCLEUS OF THE ADULT CHINCHILLA FOLLOWING ACOUSTIC OVERSTIMULATION

Adult chinchillas were exposed once to an octave-band noise, centered at 4 kHz, and allowed to survive for 16 days or for 1, 2, 4, and 8 mon ths. Axonal degeneration was mapped in the cochlear nucleus, using the Nauta-Rasmussen silver method, and related to hair cell damage and to loss of myelinated nerve fibers in the osseous spiral lamina of the c ochlea. Axonal degeneration in the dorsal cochlear nucleus had already reached a peak by 16 days and disappeared after 1 month. Meanwhile, m yelinated nerve fiber degeneration in the cochlea extended basally, fo llowed 2 weeks to 2 months later by spread of axonal degeneration into the corresponding high-frequency region of the ventral cochlear nucle us. Axonal degeneration occurred early in the low-frequency region of the ventral cochlear nucleus, followed 2-4 weeks later by spread of my elinated fiber degeneration into more apical regions of the cochlea. N ew degeneration of axons in the cochlear nerve and in the ventral coch lear nucleus continued to occur for up to 8 months after stimulation. These findings imply that plastic changes in the central auditory path ways could play a role in the long-term effects of cochlear damage and acoustic overstimulation, possibly leading to a chronic neurodegenera tive condition in the ear and in the brain. (C) 1998 Wiley-Liss, Inc.