Acoustic injury and TRPV1 expression in the cochlear spiral ganglion

Acoustic trauma not only produces temporary and permanent hearing loss but is a common cause of chronic tinnitus. Recent work indicated a possible role for the transient receptor potential channel vanilloid subfamily type 1 (TRPV1) in modulating the effects of cochlear injury. In our research, we investigated the effects of acoustic damage on TRPV1 expression in spiral ganglion neurons of adult rats. After exposing them unilaterally to noise, we extracted cochleas and processed the spiral ganglion for TRPV1 expression at four posttrauma intervals (2 hours, 24 hours, 12 days, and 16.9 months). We measured TRPV1 immunodensity in the apical, middle, and basal turns of the cochlea. We found a significant interaction (p = .039) between posttrauma interval and regional cochlear receptor expression: For survival intervals between 24 hours and 2 weeks, TRPV1 density increased in all cochlear regions; at the longest survival interval (16.9 months), TRPV1 density was dramatically reduced in the basal region. We also psychophysically tested the long-survival subjects, which showed evidence of 20-kHz tonal tinnitus. These results suggest that TRPV1 may participate after cochlear injury in a signal cascade that is responsible for the neuroplastic events leading to tinnitus and hyperacusis.