Open Access

ARTICLE

Cochlear Synaptopathy Following Noise Exposure in Guinea Pigs: Its Electrophysiological and Histological Assessments

Parvane Mahdi¹, Akram Pourbakht^1,*, Vahid Pirhajati Mahabadi², Alireza Karimi Yazdi³, Mahtab Rabbani Anari³, Mohammad Kamali⁴

1 Rehabilitation Research Center, Department of Audiology, School of Rehabilitation Sciences, Iran University of Medical Sciences, Tehran, 15459-13487, Iran
2 Department of Neurosciences, School of Medicine, Iran University of Medical Sciences, Tehran, 14496-14535, Iran
3 Department of Otorhinolaryngology, Tehran University of Medical Sciences, Tehran, 14197-33141, Iran
4 Department of Rehabilitation Management, Iran University of Medical Sciences, Tehran, 15459-13487, Iran

* Corresponding Author: Akram Pourbakht. Email:

Sound & Vibration 2020, 54(3), 163-177. https://doi.org/10.32604/sv.2020.09880

Received 24 January 2020; Accepted 17 May 2020; Issue published 31 August 2020

Abstract

Exposure to high level of noise, may cause the permanent cochlear synaptic degeneration. In present study, a model of noise induced cochlear synaptopathy was established and the electrophysiological and histological metrics for its assessment was designed. 6 guinea pigs were subjected to a synaptopathic noise (octave band of 4 kHz at 104 dB SPL, for 2-h). The amplitude growth curve of Auditory Brainstem Response (ABR) wave-I and wave-III latency shift in presence of noise were calculated. These indexes were considered in pre-exposure, 1 day post exposure (1DPE), 1 week post exposure (1WPE) and 1 month post exposure (1MPE) to noise. Finally, the samples were histologically analyzed. ABR wave-I amplitude was different between pre and 1DPE (p-value ≤ 0.05). However, at 1WPE, it was recovered at low intensities but at 70 dB SPL and above, the differences persisted even till 1MPE. In masked ABR, the latency shift of wave-III was different between pre and 3 post exposure assessments (p-value ≤ 0.05). Electro-microscopic analysis confirmed the synaptic degeneration, as the ribbons were larger than normal, hollow inside, and spherical and irregular in shape, also, the post synaptic density was abnormally thick and missed its flat orientation. These data revealed that noise at level below that can produce permanent hearing loss, can incur synaptic injury. So, noise is considered to be more damaging than previously thought. Accordingly, designing tools for clinical assessment of synaptopathy is beneficial in comprehensive auditory evaluation of those with history of noise exposure and also in hearing protection planning.

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Keywords

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