A primate model revealed specific age-related changes in spiral ganglion neurons in the cochlea.

Type I spiral ganglion neurons, which play a major role in hearing by conveying electrical sound signals to the brain, are divided into electrophysiological and molecular subtypes. The differences in vulnerability among the subtypes reported in rodents are of pathophysiological importance for age-related hearing loss. However, the changes in spiral ganglion neurons in primates and humans have not been established. In this study, we investigated the age-related changes in the spiral ganglion neurons of the common marmoset, a primate model animal. Here, we show that the overall number of spiral ganglion neurons decreased in middle-aged individuals and was most pronounced during the basal turn. However, no subtype-specific reduction, as reported in rodents, was identified. Our observations indicate that aging-related changes observed in the cochlea of rodents are not always applicable to primates. The usefulness and importance of common marmosets in the study of age-related hearing loss are also highlighted.