New growth of axons in the cochlear nucleus of adult chinchillas after acoustic trauma

Abstract

This study determined the effect of acoustic overstimulation of the adult cochlea on axons in the cochlear nucleus. Chinchillas were exposed to an octave-band noise centered at 4 kHz at 108 dB sound pressure level for 1.75 h. One chinchilla was never exposed to the noise, and several others had one ear protected by an ear plug or prior removal of the malleus and incus. Exposure of unprotected ears caused loss of inner and outer hair cells and myelinated nerve fibers, mostly in the basal half of the cochlea. Cochlear nerve fiber degeneration, ipsilateral to the exposed ears, was traced to regions of the cochlear nucleus representing the damaged parts of the cochlea. In silver impregnations of a deafferented zone in the posteroventral cochlear nucleus, the concentration of axons decreased by 43% after 1 month and by 54% after 2 months. However, by 8 months, the concentration of thinner axons, with diameters of less than 0.46 microm, increased by 46-90% over that at 2 months. The concentration of axons with larger diameters did not change. Between 2 and 8 months small axonal endings appeared next to neuronal cell bodies. This later increase of thinner axons and endings is consistent with a reactive growth of new axons of relatively small diameter. The emergence of small perisomatic boutons suggests that the new axons formed synaptic endings, which might contribute to an abnormal reorganization of the central auditory system and to the pathological changes that accompany acoustic overstimulation.