Absence of Early Neuronal Death in the Olivocochlear System Following Acoustic Overstimulation

Abstract

This study was conducted to examine possible effects of noise trauma on olivocochlear (OC) neurons. Anesthetized rats were exposed to a continuous 10 kHz pure tone at 120 dB sound pressure level for 2 hrs. The effects of treatment were verified by recordings of auditory brainstem response and distortion product otoacoustic emission. Three or 8 days after acoustic trauma, rats received unilateral injections of an aqueous solution of the retrograde neuronal tracer Fluorogold (FG) into the scala tympani to identify OC neurons (OCN). Five days after FG injection, brains were perfusion-fixed, and brainstem sections were cut and analyzed with respect to FG-labeled neurons. We found that, in both groups, numbers of OCN were similar to that of controls. The incubation of a second set of sections with antibodies against choline-acetyltransferase (the enzyme responsible for acetylcholine synthesis) showed the cholinergic neurons of the brainstem, however, without suggesting differences between groups. Our study, the first to investigate noise trauma effects on identified OCN, revealed that no visible alterations occurred in 2 weeks following trauma, neither in identified OCN nor in cholineacetyltransferase-immunofluorescence. At this time, auditory brainstem response and distortion product otoacoustic emission measurements showed severe signs of hearing loss. The mechanisms leading to hearing loss upon noise trauma apparently do not involve degeneration of OCN. Anat Rec, 299:103-110, 2016. V C 2015 Wiley Periodicals, Inc.Abbreviations used: ABR 5 auditory brainstem response; ChAT 5 choline-acetyltransferase; DPOAE 5 distortion product otoacoustic emission; FG 5 Fluorogold; IHC 5 inner hair cells; LOC/MOC 5 lateral/medial olivocochlear neurons; LSO 5 lateral superior olive; OC 5 olivocochlear; OCN 5 olivocochlear neurons; OHC 5 outer hair cells; PBS 5 phosphate-buffered 0.9% saline; RPO 5 rostral paraolivary region; RT 5 room temperature; SOC 5 superior olivary complex; SPL 5 sound pressure level; VNTB 5 ventral nucleus of the trapezoid body Grant sponsor: R€ ottger-Stiftung; Grant sponsor: HoffmannKlose-Stiftung.

Morphofunctional alterations in the olivocochlear efferent system of the genetic audiogenic seizure-prone hamster GASH:Sal

“…This morphometric analysis was carried out in retrogradely labeled neurons after injections of FG into the cochlea. Consistently with previous track-tracing studies in rodents [21,44,45], our injections generated retrograde labeling of the three types of olivocochlear neurons: LOC, MOC, and shell neurons. We also obtained a distribution pattern of olivocochlear neurons at the ipsicontralateral and rostrocaudal levels, in the GASH:Sal and control hamsters, which was similar to that reported in other rodents [21,[46][47][48][49].…”

“…This might be explained because deficits in ABRs and DPOAEs do not necessarily involve degeneration of olivocochlear neurons. Supporting this argument, a recent study using ABR and DPOAE tests combined with FG injections in the cochlea indicated that, in acoustic trauma, LOC and MOC neurons remain intact despite hair cell dysfunction [44]. Our statistical comparative analysis of the soma area indicated that the MOC, LOC, and shell neurons of the ipsilateral side were significantly smaller in the GASH:Sal than in controls, as well as MOC neurons of the contralateral side.…”

“…One of the main goals of the present study was to determine whether the functional alterations observed in the peripheral auditory activity of the GASH:Sal have their corresponding anatomical correlations. A considerable number of studies used ABR and DPOAE tests to assess damage in the cochlea and in the brainstem auditory pathway, as well as an indicator of OHC function and the status of the olivocochlear efferent system [42][43][44]. To further explore our ABR and DPOAE results in the GASH:Sal, we performed a morphological study of the efferent olivocochlear system using a multi-technical approach that included a scanning electron microscopic analysis of the organ of Corti, a morphometric analysis of the olivocochlear neurons, and a 3D reconstruction of the LSO.…”