Tracking tonotopic changes in the auditory system following noise-induced hearing loss

Unilateral noiseinduced hearing loss results in significant changes throughout the auditory system, but the manifestation of these changes depends on the nuclei under study. For instance, significant changes in the distribution of characteristic frequencies are observed in the inferior colliculus and primary auditory cortex following noiseinduced hearing loss, but only in the auditory cortex are these changes thought to be due to plastic reorganisation. Determining how these changes develop at different levels of the auditory pathway may shed light on the mechanisms responsible for the cortical plastic effects. Moreover, a growing body of evidence suggests that hearing loss and its accompanying neuronal changes are also involved in tinnitus. Thus, understanding the development of neuronal changes following noiseinduced hearing loss may aid us in our understanding of the neural basis of tinnitus. Accordingly, we examined neuronal changes at three different levels of the auditory pathway and at different time points up to 6 months following exposure to a damaging narrow band noise. Male Long Evans rats (n = 16) were unilaterally exposed to a 115 dB SPL 16 kHz 1/10th octave bandpass noise for 1hour. Principally, we were interested in the frequency representation and spontaneous activity of neurons in cochlear nucleus, inferior colliculus and auditory cortex. We recorded simultaneously from each of these structures using multichannel electrodes. Six unexposed rats served as controls. Hearing was assessed before and after the noise trauma procedure using auditory brainstem response (ABR) audiograms. Tone pips (144 kHz, 50 ms duration, 080 dB SPL, 1 Hz presentation rate) were used to obtain frequency tuning curves. At 30 days following the noise treatment the majority of multiunit clusters recorded in auditory cortex had two peaks in their frequency tuning curves (912 kHz and 3035 kHz), which bordered the spectral range of the noisetrauma stimulus. Similar changes were also evident when we recorded from animals 6 months after noise trauma. Less pronounced tonotopic changes were observed in the IC. The only reliable effect shown in the cochlear nucleus was an absence of neuronal activity in response to 16 kHz stimulation. In summary, the pattern of tonotopic changes is evident in the first month following noise exposure and remains unchanged for up to 6 months.