Changes in the cat cochlear nucleus following neonatal deafening and chronic intracochlear electrical stimulation

Abstract

The effects of chronic intracochlear electrical stimulation on the cochlear nucleus (CN) were studied in eight cats that were neonatally deafened by daily intramuscular injections of neomycin. Profound hearing loss was confirmed in each animal by auditory brainstem response (ABR) and frequency following response (500 Hz) testing. Five of the kittens were implanted unilaterally with a scala tympani electrode array at ages 8–16 weeks. These kittens were stimulated daily for four hours at 2 dB above the evoked ABR threshold, over a period of three months, and subsequently euthanized for histological analysis at 26–32 weeks of age. The three remaining deaf kittens were maintained without stimulation over prolonged periods in order to study the long-term consequences of neonatal deafening, and were euthanized at 66–133 weeks of age. This study compares the CN of these deafened experimental animals and the CN of normal adult cats. Three experimental parameters were examined: CN volume, cross-sectional area of spherical cells in the rostral anteroventral cochlear nucleus (AVCN), and spherical cell density in this same region. The CN in animals that received electrical stimulation showed significant bilateral degenerative changes in all three measured parameters. Total nuclear volume was reduced by 35–36%, spherical cell size was reduced by 20–26%, and spherical cell density decreased by 36–42%, as compared to the normal cat CN. Comparisons were also made in the stimulated animals between CN ipsilateral to the stimulated cochlea and the contralateral, unstimulated CN. Although CN volume and cell density were not significantly different between the two sides, the spherical cells ipsilateral to the stimulated cochlea were on average 6% larger than cells in the contralateral, unstimulated CN. This difference was statistically significant (paired Student's t-test, P = 0.035). After neonatal long-term deafening, there was highly significant shrinkage of about 42% in total CN volume, a 38% reduction in mean spherical cell size, and a 57% decrease in spherical cell density in the AVCN, compared to the CN of the normal adult cat. From these data it is concluded that neonatal deafening induces severe reduction in CN volume and a decrease in AVCN spherical cell area and density that are progressive over many months. The spherical cell shrinkage that was induced by deafness was mitigated slightly by chronic intracochlear electrical stimulation; however the other effects of deafening were not prevented or reversed by chronic stimulation.