Non-plastic reorganization of frequency coding in the inferior colliculus of the rat following noise-induced hearing loss

It is well established that restricted mechanical lesions of the cochlea result in reorganization of the tonotopic map in the auditory thalamus and cortex, but it is unclear whether acoustic trauma produces similar effects at earlier stages of the auditory pathways. To test whether the tonotopic map is reorganized after acoustic trauma at the midbrain level, i.e. the inferior colliculus (IC), we exposed rats to an acoustic trauma and let them survive for at least 5 weeks to ensure that we produced a permanent threshold shift. Experiments were carried out in urethane-anesthetized animals 35-296 days after the traumatic exposure. The acoustic lesions were assessed by measuring the compound action potential. We mapped the frequency organization of the IC using multiunit recordings. In addition, we recorded frequency response areas (FRAs) when a single unit was isolated (N=142). The results show that acoustic trauma produces a persistent reorganization of the tonotopic map and that the normal stepwise representation of sound frequency in the IC is profoundly disrupted. Although the reorganization in the IC is similar to that previously described in the cortex and thalamus in that the affected area appears to be invaded by the adjacent normal frequencies, changes in thresholds and FRAs in these regions are different from those in the forebrain. We conclude that most of the changes can be explained by the residual-response hypothesis [Irvine DR, Rajan R, Smith S (2003) Effects of restricted cochlear lesions in adult cats on the frequency organization of the inferior colliculus. J Comp Neurol 467:354-374]. Plastic reorganization of frequency response areas and tonotopic organization does not seem to occur at the midbrain level following acoustic trauma in adult animals in a manner similar to that previously shown in the auditory cortex. Maintaining the stability of the neuronal circuitry for frequency coding in the IC may be important for the treatment of noise-induced hearing loss.