In Situ 3D-Imaging of the Inner Ear Synapses with a Cochlear Implant.

Kathrin Malfeld,Holger A Volk,Nina Armbrecht,Thomas Lenarz,Verena Scheper

doi:10.3390/life11040301

Abstract

In recent years sensorineural hearing loss was found to affect not exclusively, nor at first, the sensory cells of the inner ear. The sensory cells’ synapses and subsequent neurites are initially damaged. Auditory synaptopathies also play an important role in cochlear implant (CI) care, as they can lead to a loss of physiological hearing in patients with residual hearing. These auditory synaptopathies and in general the cascades of hearing pathologies have been in the focus of research in recent years with the aim to develop more targeted and individually tailored therapeutics. In the current study, a method to examine implanted inner ears of guinea pigs was developed to examine the synapse level. For this purpose, the cochlea is made transparent and scanned with the implant in situ using confocal laser scanning microscopy. Three different preparation methods were compared to enable both an overview image of the cochlea for assessing the CI position and images of the synapses on the same specimen. The best results were achieved by dissection of the bony capsule of the cochlea.

Highlights

Sensorineural hearing loss (SNHL) is primarily caused by the initial loss of hair cells in the Organ of Corti of the inner ear
With the established method for confocal laser scanning microscopy (CLSM) of the intact guinea pig cochlea based on the green autofluorescence [29] it was possible to generate a good overview image (Figure 2A)
In this study the specimen was stained against the hair cell ribbon synapses using antibodies against C-terminal binding protein 2 (CtBP2) and PSD95

Summary

Introduction

Sensorineural hearing loss (SNHL) is primarily caused by the initial loss of hair cells in the Organ of Corti of the inner ear. In some cases of SNHL the ribbon synapses between the inner hair cells and the peripheral neurites of the SGN are identified as being initially traumatized, resulting in a synaptopathy [4,5,6,7]. Noise damage may lead to hearing loss—without hair cell loss but with a severe synaptopathy. Sometimes there is no threshold shift but a reduction of the wave I amplitude in the auditory brainstem response [8]. This phenomenon is called “hidden hearing loss” and the affected patients struggle with hearing in noisy environments

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Conclusion