Early Loss of Spiral Ganglion Neurons in the Auditory System after Noise Trauma

Abstract

Introduction: Noise-induced hearing loss is one of the most frequent recognized occupational diseases. The time course of the involved pathologies is still under investigation. Several studies have demonstrated an acute damage of the sensory tissue, but only few experiments investigated the degeneration of (type I) spiral ganglion neurons (SGNs), representing the primary neurons in the auditory system. The aim of the present study was to investigate the time course of SGN degeneration within a 7-day period after traumatic noise exposure starting immediately after trauma. Methods: Young adult normal hearing mice were noise exposed for 3 h with a broadband noise (5–20 kHz) at 115 dB SPL. Auditory threshold shift was measured by auditory brainstem recordings, and SGN densities were analyzed at different time points during the first week after acoustic trauma. Results: Significant reduction of SGN densities was detected and is accompanied by a significant hearing loss. Degeneration starts within hours after the applied trauma, further progressing within days post-exposure. Discussion: Early neurodegeneration in the auditory periphery seems to be induced by direct overstimulation of the auditory nerve fibers. SGN loss is supposed to be a result of inflammatory responses and neural deprivation, leading to permanent hearing loss and auditory processing deficits.