Changes in projections to the inferior colliculus following early hearing loss in rats

The purpose of this study was to investigate the effects of early hearing loss on the anatomy of the central auditory system, specifically, the ascending projections to the inferior colliculus (IC). We compared normal animals with animals deafened during early development by administration of amikacin, an ototoxic antibiotic that is known to destroy the hair cells in the inner ear. The amikacin was injected subcutaneously every day from postnatal days P7 to P16. A retrograde tract tracer, Fluoro-Gold (FG), was then injected unilaterally directly into the IC at either 4 weeks of age or 12 weeks of age. After axonal transport the animals were sacrificed and their brains were prepared for histology. The FG labeled neurons in the cochlear nucleus (CN) and the dorsal nucleus of lateral lemniscus (DNLL) were counted for each of the animals in the two age groups. For deaf animals sacrificed at 4 weeks of age there was a significant reduction in the number of FG labeled neurons that was limited to the ventral CN ipsilateral to the tracer injection. For deaf animals sacrificed at 12 weeks of age, however, there was a significant decrease in the number of labeled cells in both dorsal and ventral CN on both sides of the brain. In DNLL there was no change in the number or pattern of labeled neurons. The results show that neonatal deafness reduces the number of labeled neurons projecting from the CN to the IC with the effect being more evident during later stages of deafness. In contrast, there are no significant changes in the projections from DNLL to IC.