Nutrient-enhanced diet reduces noise-induced damage to the inner ear and hearing loss

Abstract

Oxidative stress has been implicated broadly as a cause of cell death and neural degeneration in multiple disease conditions; however, the evidence for successful intervention with dietary antioxidant manipulations has been mixed. In this study, we investigated the potential for protection of cells in the inner ear using a dietary supplement with multiple antioxidant components, which were selected for their potential interactive effectiveness. Protection against permanent threshold shift (PTS) was observed in CBA/J mice maintained on a diet supplemented with a combination of β-carotene, vitamins C and E, and magnesium when compared with PTS in control mice maintained on a nutritionally complete control diet. Although hair cell survival was not enhanced, noise-induced loss of type II fibrocytes in the lateral wall was significantly reduced (P < 0.05), and there was a trend toward less noise-induced loss in strial cell density in animals maintained on the supplemented diet. Taken together, our data suggest that prenoise oral treatment with the high-nutrient diet can protect cells in the inner ear and reduce PTS in mice. The demonstration of functional and morphologic preservation of cells in the inner ear with oral administration of this antioxidant supplemented diet supports the possibility of translation to human patients and suggests an opportunity to evaluate antioxidant protection in mouse models of oxidative stress-related disease and pathology.

Figure 1

The Diet B combination of nutrients significantly reduced permanent threshold shift (PTS) in treated animals compared to animals maintained on Control Diet; pair-wise comparisons revealed statistically significant differences between animals treated with Diet B and animals maintained on Control diet at 10 and 20 kHz (** p’s<0.01). At 28.3 kHz, there was a trend towards less hearing less in animals maintained on Diet B (0.05

Figure 2

Missing hair cells were detected in the basal regions of the cochlea, with 50% or greater OHC loss in regions corresponding to approximately 30 kHz and above (based on the tonotopic map of the mouse cochlea proposed by 92) in all groups (A: inner hair cells; B: outer hair cells). There were no reliable differences in hair cell count data as a function of treatment. Data are mean ± S.E.M.

Figure 3

A. Example radial view of the cochlear upper basal turn (approx. 10 kHz region) from an animal treated with Diet B. B.-D. show expanded views of insets. The most obvious correlates of NIHL at this location were loss of fibrocytes from the spiral limbus (SpLim in 3A) and from extreme inferior ligament. These losses were not significantly reduced by treatment. Other effects of noise, and their differences by group, were subtle. Stria vascularis (StV in 3B) appeared generally intact, but retained more basal cells in treated mice (see Fig. 4B). Treated mice also showed significantly greater retention of Type II fibrocytes (T_II in 3C; see Fig. 4A). Since hair cell and neuronal losses occurred more basally than the region shown (Fig. 2), the organ of Corti (3D) appeared normal. RM: Reissner’s Membrane; TM: Tectorial Membrane; SpLig: Spiral Ligament; DC: Deiters’ Cells; IP/OP: Inner and Outer Pillar.

Figure 4

Figure 4A. The number of Type II fibrocytes was significantly greater in noise-exposed CBA/J subjects fed Diet B compared to mice maintained on Control Diet, and was equivalent to that reported previously in normal CBA/J mice that had not been exposed to noise (“No Noise”). Figure 4B. Differences in strial cell density by treatment were suggested, but were not statistically reliable. Basal cell density post-noise was generally equivalent to that reported previously in normal CBA/J mice not exposed to noise (“No Noise,” adapted from 86). However, animals fed the Control Diet in this study also showed less noise-induced loss of basal cells than in the previous study. (*=p<0.05). Data are mean ± S.D.

Figure 5

Figure 5A. Average chow consumption (mean ± S.D.) did not systematically vary as a function of diet. Figure 5B. Mice that were maintained on Diet B, the diet with highest nutrient content, had a lower average body weight than mice that were maintained on the Control Diet (*=p<0.05; mean ± S.D.). Figure 5C. When daily weights for individual control animals were evaluated, we found common ending weights to be approximately 30 grams (dashed horizontal line, in panels 5C and 5D). Control animal weights were not routinely measured at earlier times. The vertical line in panels 5C and 5D indicates the day of noise exposure, on day 28 of dietary manipulation. Mice that were maintained on Diet A were generally similar to mice maintained on Control diet, with average ending weights of 32.1 grams (+/− 4.0 grams, S.D.). Figure 5D. When daily weights for individual animals maintained on Diet B were evaluated, we found common ending weights to be approximately 30 grams, although there were two mice that lost weight throughout the study (with no other significant adverse outcomes to distinguish them from other subjects). With the exception of those two subjects, most mice maintained on Diet B lost weight during the initial days during which they were maintained on this diet with subject weights returning to normal a normal range (~30 grams) with increasing time on the modified diet. The mice that lost weight while being maintained on Diet B did not systematically differ from the other mice maintained on Diet B with respect to noise-induced permanent threshold shift. Hearing loss was intermediate to other Diet B subjects.