Noise-induced Outer Hair Cells' Dysfunction and Cochlear Damage in Rabbits

Abstract

Background: Outer hair cells' (OHCs') dysfunctions as the extent of temporary and permanent threshold shifts (TTS and PTS) and cochlear damage were assessed in rabbits exposed to continuous noise

Methods: Twelve New Zealand white rabbits were studied in noise (N) (n=6; exposed to continuous noise; 95 dB SPL, 500-8000 Hz for 8 h per day during 5 consecutive days) and control (C) (n=6; not exposed to noise). OHCs' functions were assessed by distortion product otoacoustic emission (DPOAE) level (Ldp) measurements in different periods and comparing TTS and PTS. Animals were anaesthetized by CO2; cochleae were extracted, fixed in 10% formaldehyde for 48 hours, decalcified by 10% nitric acid for 24 hours, and dehydrated, embedded, sectioned 5 µm thickness and stained by Hematoxylin and Eosin for light microscopy.

Results: The most and least Ldp or TTS or PTS were related to 5888.50 Hz and 588.00 Hz respectively in noise subjected rabbits (P<0.05). TTS and PTS were decreased up to 17.79 dB and to 16.01 dB respectively. TTS were more than PTS over all test frequencies, especially at 5888.50 Hz (P<0.05). Ldp or TTS or PTS were found to be equal across ears (P>0.05). Severely vacuolated OHCs, pyknotic IHCs, swollen SC, and slightly thickened BM were found.

Conclusion: Continuous noise extensively led to OHCs' dysfunctions as decreased Ldp (both TTS and PTS) and highly damage to cochlea.

Keywords: Cochlear damage; Distortion product Otoacoustic emissions; Noise-induced Hearing Loss; Outer hair cells' function.

Figure 1. A cross-sectional view of exposure chamber (50×50×50 cm polycarbonate Plexiglas) has been shown. All rabbits were located into this chamber for exposure to noise pollutant (N group) and control group for 8 hr per day during 5 consecutive days. Exposure to noise has been carried out through a noise loudspeaker mounted on the roof of the chamber. Control group is just posed onto the chamber resembled to other groups with identical situations, but without any exposure

Figure 2. Control group. A photograph of the organ of corti of rabbits not exposed to any physical agents, showing healthy and normal cochlear hair cells (OHCs and IHCs), supporting cells (SC), basilar membrane (BM), and tectorial membrane (TM)

Figure 3. Noise group. A photograph of the organ of corti of rabbits exposed to noise, showing severely vacuolated outer hair cells (OHCs) with intensely cell injury as hydropic degeneration type, mild to moderately pyknotic inner hair cells (IHCs), swollen supportive cells, slightly thickened basilar membrane (BM), and not injured tectorial membrane (TM)