# Cochlear Ribbon Synapses in Aged Gerbils

## TL;DR

This study examines how aging affects the structure of synapses in the inner ear of gerbils, revealing age-related losses in specific synapse types.

## Contribution

The study identifies age-related synapse loss in the cochlear base, specifically affecting low-spontaneous-rate auditory nerve fibers.

## Key findings

- Quiet-aged gerbils showed about 20% loss of afferent ribbon synapses.
- Synapse loss at the basal, high-frequency location predominantly affected modiolar-located synapses.
- Age did not affect the subtle differences in synaptic volumes between modiolar and pillar sides.

## Abstract

In mammalian hearing, type-I afferent auditory nerve fibers comprise the basis of the afferent auditory pathway. They are connected to inner hair cells of the cochlea via specialized ribbon synapses. Auditory nerve fibers of different physiological types differ subtly in their synaptic location and morphology. Low-spontaneous-rate auditory nerve fibers typically connect on the modiolar side of the inner hair cell, while high-spontaneous-rate fibers are typically found on the pillar side. In aging and noise-damaged ears, this fine-tuned balance between auditory nerve fiber populations can be disrupted and the functional consequences are currently unclear. Here, using immunofluorescent labeling of presynaptic ribbons and postsynaptic glutamate receptor patches, we investigated changes in synaptic morphology at three different tonotopic locations along the cochlea of aging gerbils compared to those of young adults. Quiet-aged gerbils showed about 20% loss of afferent ribbon synapses. While the loss was random at apical, low-frequency cochlear locations, at the basal, high-frequency location it almost exclusively affected the modiolar-located synapses. The subtle differences in volumes of pre- and postsynaptic elements located on the inner hair cell’s modiolar versus pillar side were unaffected by age. This is consistent with known physiology and suggests a predominant, age-related loss in the low-spontaneous-rate auditory nerve population in the cochlear base, but not the apex.