Induction of single-sided deafness in the newborn rat and its consequence for cochlear nucleus volume development

Aim of this study was to induce a single-sided deafness (SSD) in rats before hearing onset. Rats were operated at postnatal day 10 by approaching the tympanic cavity along a retroauricular path without manipulating ossicles or tympanic membrane. The ototoxic aminoglycoside neomycin was injected intracochlearly through the round window membrane on one side. When the animals have reached young adult stages, their hearing threshold was determined by their auditory brainstem response (ABR). Monaural deafening was considered successful when the hearing threshold was at least 95 dB above the threshold of the normal hearing ear. Growing up with one non-functional ear, rats developed a striking anatomical asymmetry of their cochlear nuclei (CN). The CN from age-matched normal hearing brains and from both sides of single-sided deaf brains were cut into series of frontal sections and their volumes calculated. No difference was detected between the volume of the normal hearing CN and the contralateral CN in SSD rats. By contrast, growth retardation was found for the ventral CN on the deaf side to result in a volume of only 57% compared to the normal hearing side. Marginal growth retardation was also observed for the dorsal CN on the deaf side. Thus, loss of sensory activation leads mainly, but not exclusively, to a reduction of tissue volume in the ventral CN of the deaf side, leaving the contralateral side apparently unaffected.