Development of frequency-selective domains in inferior colliculus of normal and neonatally noise-exposed rats

Topographic patterns of pure-tone responses in inferior colliculus (IC) of Wistar rats were mapped using immunohistochemical staining for the nuclear protein Fos, the translation product of the c-fos proto-oncogene. Patterns were compared in ICs of immature and mature rats and in mature rats which experienced auditory deprivation beginning on day 14, an age near the developmental onset of hearing. Neonatal hearing losses, caused here by exposure to potentially deafening noise, are known to result in audiogenic seizure susceptibility in neonatal rats. These seizures can be triggered only by high-frequency stimuli and are believed to be initiated in IC. Thus, it seemed possible that susceptibility might depend on derangements of topographic frequency representation due to neonatal auditory deprivation. The band-like frequency-response domains, characteristic of adult IC, were found to be poorly differentiated in ICs of immature rats. On day 12, only lower-frequency stimuli induced discrete bands of Fos immunoreactivity while responses to higher frequencies remained exceptionally diffuse within ventral portions of IC. Only after day 24 did responses to the highest frequencies also appear mature. Furthermore, most significantly, adult rats which were transiently deafened on day 14, retained the more voluminous response patterns which were characteristic of immature IC. Because frequency selectivity in cochlea also develops by a low-to-high frequency sequence, results are consistent with a hypothesis that topographic organization arises in IC by an activity-dependent process. Whereas neonatal noise exposure also conferred audiogenic seizure susceptibility, it appears the arrest of tonotopic organization of IC is the probable basis of this reflex epilepsy.