NEW GROWTH OF AXONS IN THE COCHLEAR NUCLEUS OF ADULT CHINCHILLAS AFTER ACOUSTIC TRAUMA

This study determined the effect of acoustic overstimulation of the ad ult cochlea on axons in the cochlear nucleus, Chinchillas were exposed to an octave-band noise centered at 4 kHz set 108 dB sound pressure l evel for 1.75 h. One chinchilla was never exposed to the noise, and se veral others had one ear protected by an ear plug or prior removal of the malleus and incus. Exposure of unprotected ears caused loss of inn er and outer hair cells and myelinated nerve fibers, mostly in the bas al half of the cochlea, Cochlear nerve fiber degeneration, ipsilateral to the exposed ears, was traced to regions of the cochlear nucleus re presenting the damaged parts of the cochlea. In silver impregnations o f a deafferented zone ir the posteroventral cochlear nucleus, the conc entration of axons decreased by 43% after 1 month and by 54% after 2 m onths. However, by 8 months, the concentration of thinner axons, with diameters of less than 0.46 mu m, increased by 46-90% over that; at 2 months. The concentration of axons with larger diameters did not chang e, Between 2 and 8 months small axonal endings appeared next to neuron al cell bodies. This later increase of thinner axons and endings is co nsistent with a reactive growth of new axons of relatively small diame ter, The emergence of small perisomatic boutons suggests that the new axons formed synaptic endings, which might contribute to an abnormal r eorganization of the central auditory system and to the pathological c hanges that accompany acoustic overstimulation. (C) 1997 Academic Pres s.