Acceleration of age-related hearing loss by early noise exposure: evidence of a misspent youth

Abstract

Age-related and noise-induced hearing losses in humans are multifactorial, with contributions from, and potential interactions among, numerous variables that can shape final outcome. A recent retrospective clinical study suggests an age-noise interaction that exacerbates age-related hearing loss in previously noise-damaged ears (Gates et al., 2000). Here, we address the issue in an animal model by comparing noise-induced and age-related hearing loss (NIHL; AHL) in groups of CBA/CaJ mice exposed identically (8-16 kHz noise band at 100 dB sound pressure level for 2 h) but at different ages (4-124 weeks) and held with unexposed cohorts for different postexposure times (2-96 weeks). When evaluated 2 weeks after exposure, maximum threshold shifts in young-exposed animals (4-8 weeks) were 40-50 dB; older-exposed animals (> or =16 weeks) showed essentially no shift at the same postexposure time. However, when held for long postexposure times, animals with previous exposure demonstrated AHL and histopathology fundamentally unlike unexposed, aging animals or old-exposed animals held for 2 weeks only. Specifically, they showed substantial, ongoing deterioration of cochlear neural responses, without additional change in preneural responses, and corresponding histologic evidence of primary neural degeneration throughout the cochlea. This was true particularly for young-exposed animals; however, delayed neuropathy was observed in all noise-exposed animals held 96 weeks after exposure, even those that showed no NIHL 2 weeks after exposure. Data suggest that pathologic but sublethal changes initiated by early noise exposure render the inner ears significantly more vulnerable to aging.

Figure 1.

Young (4–8 weeks) mice are more vulnerable to noise damage than old (96 weeks) mice. Each age group was exposed to high-level noise, and threshold shifts were measured by ABR and DPOAE 2 weeks later. Threshold shifts (calculated relative to age-matched, unexposed cohorts) are greater in young-exposed ears by both measures. Data are expressed as means ± SE. For the numbers of animals in each group, see Table 1. The gray bar denotes the pass band of the noise-exposure stimulus.

Figure 2.

Vulnerability to noise decreases dramatically between 8 and 16 weeks of age. A, B, Maximum threshold shifts (i.e., shifts at 16 kHz) seen at 2 weeks after exposure by ABR (A) and DPOAE (B) for all ages at exposure. C, D, In unexposed control ears, thresholds at 16 kHz do not show large change between 8 and 16 weeks. Data are means ± SE and are plotted as a function of age on a logarithmic scale. For the numbers of animals in each group, see Table 1.

Figure 3.

Early NIHL exacerbates AHL when measured by ABR (A, C) but not by DPOAE (B, D). A, B, NIHL in animals exposed at 6 weeks (white circles; replotted from Fig. 1) is defined as thresholds at 2 weeks after exposure relative to unexposed 6 week controls; 96 weeks later, aggregate NIHL/AHL in animals exposed at 6 weeks is also calculated relative to unexposed 6 week controls. C, D, AHL in unexposed animals (white squares) is simply the difference between thresholds at 102 versus 6 weeks; AHL for the noise-exposed group (gray diamonds) removes the initial NIHL component, i.e., it is the difference between thresholds at 96 versus 2 weeks after exposure (the difference between the curves in A and B). Data are means ± SE. For the numbers of animals in each group, see Table 1. The arrows above the points indicate that at least 50% of animals from this group at this frequency lacked responses at the highest SPLs presented; thus, the threshold shift may be underestimated. For additional explanation, see Materials and Methods.

Figure 4.

Progressive threshold shifts as a function of age in animals initially exposed to noise at different ages: 6 weeks (white circles), 16 weeks (gray squares), 32 weeks (gray diamonds), 64 weeks (gray triangles), or 96 weeks (black triangles). Shifts are shown at 16 kHz (right), the frequency of maximum initial shift, and 8 kHz (left), a frequency showing minimal initial shift. Age-corrected shifts are defined as the difference between the measured threshold and the thresholds of unexposed animals of similar age. Data are plotted as a function of age at test; thus, an animal exposed at 16 weeks and held for 32 weeks will be plotted at 48 weeks of age. The numbers of animals in each group are given in Table 1.

Figure 5.

When examined 2 weeks after exposure, the only histopathology is loss of type IV fibrocytes: compare circled regions of B and C. A and B show the upper basal turn of an ear exposed at 6 weeks and tissues processed at 8 weeks. The region of the high-power view in B is indicated by the box in A. C shows the normal appearance of the type IV fibrocytes at the same cochlear region.

Figure 6.

Primary neuronal degeneration was seen in mice that were exposed and allowed to survive for many months. The degeneration, seen as decreased density of spiral ganglion cells (heavy black circles), although inner and outer hair cells (light black circles) are still present, is visible in cases exposed at 6 weeks and aged to 96 weeks (D) but not in cases exposed at 96 weeks and evaluated at 98 weeks (B) or in unexposed animals tested at 96 weeks (C) or in cases exposed at 6 weeks and tested at 8 weeks (A). All images are from the upper basal turn. Scale bar in B applies to A–D.

Figure 7.

The sensory epithelium appears normal in animals from all exposure groups, even with high-power DIC optics. Images are from the same four cases shown in Figure 6. Each image is focused on an inner hair cell nucleus (e.g., white arrow in B). Stereocilia on inner hair cells are also in focus (e.g., black arrow in B), and, in some cases, the basolateral membrane of the inner hair cell is visible (e.g., white arrowhead in B). Three rows of outer hair cells are seen in all images (e.g., white arrows in D); however, not all rows are in focus. Outer hair cell stereocilia in mouse are generally too small to be visible in this material. The scale bar in A applies to A–D.

Figure 8.

Semiquantitative analysis of cochlear histopathology in groups of exposed and unexposed animals performed by an observer blind to the exposure history and threshold measures. Analysis included estimates of inner and outer hair cell loss, spiral ganglion cell loss, and loss of type IV fibrocytes. Each histogram shows means and SEs or the estimates of fractional cell survival. Estimates were made in three cochlear regions, as indicated, corresponding to the three regions seen in a midcochlear section. The numbers of animals in each group are given under each column letter. “Unexposed Test Young” animals were tested at 7.5 weeks; “Expose Young Test Young” were exposed at 5.5 weeks and tested at 7.5 weeks; “Expose Young Test Old” animals were exposed at 5 weeks and tested at 100 weeks; “Expose Old Test Old” animals were exposed at 124 weeks and tested at 126 weeks; and “Unexposed Test Old” animals were tested at 105 weeks.