Elsevier

Hearing Research

Volume 139, Issues 1–2, January 2000, Pages 13-30
Hearing Research

Histopathological differences between temporary and permanent threshold shift1

https://doi.org/10.1016/S0378-5955(99)00163-XGet rights and content

Abstract

The structural changes associated with noise-induced temporary threshold shift (TTS) were compared to the damage associated with permanent threshold shift (PTS). A within-animal paradigm involving survival-fixation was used to minimize problems with data interpretation from interanimal variability in response to noise. Auditory brainstem response thresholds for clicks and tone pips were determined pre- and 1–2 h post-exposure in 11 chinchillas. The animals were exposed for 24 h to an octave band of noise with a center frequency of 4 kHz and a sound pressure level of 86 dB. Three animals (0/0-day) had both cochleas terminal-fixed 2–3 h post-exposure. Two animals (27/27-day) had threshold shifts determined every other day for 1 week, every week thereafter, and underwent terminal-fixation of both cochleas 27 days after exposure. Six animals (0/n-day) had threshold shifts determined in both ears upon removal from the noise; their left cochlea was then survival-fixed 2–3 h post-exposure. Threshold shifts were determined in their right ear every 2–3 days until their hearing either returned to pre-exposure values or stabilized at a reduced level at which time their right cochlea was terminal-fixed (4–13 days post-exposure). All cochleas were prepared as plastic-embedded flat preparations. Missing hair cells were counted and supporting cells and nerve fibers were evaluated throughout the organ of Corti using phase-contrast microscopy. Post-exposure, all animals had moderate TTSs in their left and right ears which averaged 43 dB for 4–12 kHz. In the 0/0-day animals, the only abnormality which correlated with TTS was a buckling of the pillar bodies. In the 0/n-day animals, their left cochlea (survival-fixed 2–3 h post-exposure) had outer hair cell (OHC) stereocilia which were not embedded in the tectorial membrane in the region of the TTS whereas OHC stereocilia were embedded in the tectorial membrane throughout the cochleas of non-noise-exposed, survival-fixed controls. Three of six right cochleas (terminal-fixed 4–13 days post-exposure) from the 0/n-day animals developed a PTS and two of these cochleas had focal losses of inner and outer hair cells and afferent nerve fibers at the corresponding frequency location. The other cochlea with PTS had buckled pillars in the corresponding frequency region. These results suggest that with moderate levels of noise exposure, buckling of the supporting cells results in an uncoupling of the OHC stereocilia from the tectorial membrane which results in a TTS. The mechanisms resulting in TTS appear to be distinct from those that produce permanent hair cell damage and a PTS.

Introduction

For many years, animals have been used to study noise-induced temporary threshold shift (TTS) and its relation to permanent threshold shift (PTS). Despite the large number of studies, there are a number of conflicting findings which have led to different hypotheses concerning mechanisms of noise damage. Postulated mechanisms include mechanical damage (e.g., Lurie, 1942, Hawkins, 1971), ischemia (e.g., Hawkins, 1971, Yamane et al., 1995), excitotoxic damage (e.g., Pujol, 1992), metabolic exhaustion (e.g., Lim and Dunn, 1979), and ionic imbalance in the inner-ear fluids (e.g., Bohne, 1976, Bohne and Rabbitt, 1983).

Previous noise studies generally focused on pathological alterations in only one or two cell types, including: (1) sensory cells of the organ of Corti (OC) (e.g., Lim and Melnick, 1971); (2) sensory-cell stereocilia (e.g., Hunter-Duvar, 1977); (3) nerve fibers in the cochlea, particularly in the area below the inner hair cells (IHC), including the inner spiral bundle and radial afferents (e.g., Spoendlin, 1971); and (4) the stria vascularis and cochlear vasculature (e.g., Hawkins, 1971). The results from some earlier studies, summarized by species, are listed in Table 1. In many of these studies, it is difficult to distinguish reversible from irreversible alterations and acute from chronic effects because function was not always determined in the cochleas that were examined morphologically, and the long-term effects of the different exposures were not always reported. Additional noise studies using alligator lizards and birds have been published. However, these studies are not listed in Table 1 because of the fundamental differences in the organization of the basilar papilla compared to the organ of Corti, especially with respect to supporting cells and fluid spaces.

Some of the conflicts about mechanisms of damage appear to be the result of interanimal variations in susceptibility to noise. All individuals exposed to the same damaging noise do not sustain identical amounts of hair cell loss or develop the same PTS (e.g., Miller et al., 1963, Taylor et al., 1965). In order to minimize problems with data interpretation due to variations in susceptibility, a ‘survival-fixation’ technique was developed in which the two cochleas of an experimental animal are preserved at different post-exposure times (Bohne et al., 1999). Each animal exposed to noise binaurally sustains nearly equivalent primary damage in its left and right cochleas (Bohne et al., 1986). Thus, by fixing its cochleas at different times, a particular noise-exposed animal can provide two ‘snapshots’ of the natural progression (or resolution) of noise damage in which the differences found between its cochleas can be directly related to recovery time.

The study reported here employed the survival-fixation technique to determine if TTS and PTS represent different stages along a continuum of damage or if the histopathological correlates of TTS are distinct from those of PTS.

Section snippets

Animals

Eleven 1–3-year-old chinchillas were used in this study. The animals were purchased from a commercial supplier (Moulton Chinchilla Ranch, Rochester, MN) and housed in a quiet animal facility prior to and after their experimental manipulations. Six additional survival-fixed animals that were not exposed to noise served as controls. Their left cochlea was survival-fixed at day 0. Their right cochlea was terminal-fixed at day 14. The animals were then killed after which both cochleas were removed

Non-noise-exposed controls

The survival-fixed control cochleas were found to have some artifacts when compared to terminal-fixed controls including the loss of cuticular plate substance from hair cell apices and headplate substance from the pillar heads (Bohne et al., 1999). An interesting finding from apex to base in the survival-fixed controls was that the OHC stereocilia were firmly embedded in the tectorial membrane (TM) at the level of its fibrils, but were no longer attached to the OHC apices (not shown in Bohne et

Correlates of TTS

A number of studies have described disarrayed, splayed, fused, collapsed or floppy stereocilia on the IHCs or OHCs as correlates of TTS (e.g., Table 1), whereas in the present study, this alteration was not found. Many of the previous studies used scanning electron microscopy (SEM) to examine the cochleas. Techniques used to prepare tissue for SEM examination are harsh, cause considerable shrinkage (e.g., Hayat, 1978) and may result in subcellular artifacts, especially in cells that are already

Conclusions

(1) The survival-fixation technique is useful for examining the relationship between the OHC stereocilia and the tectorial membrane.

(2) There was less variability in the magnitude of TTS than in the magnitude of PTS after the 24-h noise exposure.

(3) In the 0/0-day cochleas which were terminal-fixed 2–3 h post-exposure, the only consistent pathological change in the region of the TTS was buckling of the pillar bodies. In the 0-day cochleas which were survival-fixed 2–3 h post-exposure, the OHC

Acknowledgments

This work was supported by the Deafness Research Foundation, the American Heart Association, and the Department of Otolaryngology at Washington University School of Medicine. We would like to especially thank Thomas J. Watkins for his excellent technical assistance, Chris S. Martinez for performing the organ of Corti height measurements, and Dr. Charles J. Tseng for his assistance with surgery, post-op care of the chinchillas, and constructive criticisms concerning the manuscript.

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    A portion of this study formed the basis of a pre-doctoral M.A. thesis in Biological Sciences, Washington University School of Medicine (A.S.N.). This study was presented in part at the 22nd Midwinter Research Meeting of the Association for Research in Otolaryngology, February 1999.

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