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 th e nuclear protein Fos, the translation product of the c-fos proto-onco gene. Patterns were compared in ICs of immature and mature rats and in mature rats which experienced auditory deprivation beginning on day 1 4, an age near the developmental onset of hearing. Neonatal hearing lo sses, caused here by exposure to potentially deafening noise, are know n to result in audiogenic seizure susceptibility in neonatal rats. The se seizures can be triggered only by high-frequency stimuli and are be lieved to be initiated in IC. Thus, it seemed possible that susceptibi lity might depend on derangements of topographic frequency representat ion due to neonatal auditory deprivation. The band-like frequency-resp onse domains; characteristic of adult IC, were found to be poorly diff erentiated in ICs of immature rats. On day 12, only lower-frequency st imuli induced discrete bands of Fos immunoreactivity while responses t o higher frequencies remained exceptionally diffuse within ventral por tions of IC. Only after day 24 did responses to the highest frequencie s also appear mature. Furthermore, most significantly, adult rats whic h were transiently deafened on day 14, retained the more voluminous re sponse patterns which were characteristic of immature IC. Because freq uency selectivity in cochlea also develops by a low-to-high frequency sequence, results are consistent with a hypothesis that topographic or ganization arises in IC by an activity-dependent process. Whereas neon atal noise exposure also conferred audiogenic seizure susceptibility, it appears the arrest of tonotopic organization of IC is the probable basis of this reflex epilepsy.