The impact of biological sex on the response to noise and otoprotective therapies against acoustic injury in mice

Abstract

Background: Noise-induced hearing loss (NIHL) is the most prevalent form of acquired hearing loss and affects about 40 million US adults. Among the suggested therapeutics tested in rodents, suberoylanilide hydroxamic acid (SAHA) has been shown to be otoprotective from NIHL; however, these results were limited to male mice.

Methods: Here we tested the effect of SAHA on the hearing of 10-week-old B6CBAF1/J mice of both sexes, which were exposed to 2 h of octave-band noise (101 dB SPL centered at 11.3 kHz). Hearing was assessed by measuring auditory brainstem responses (ABR) at 8, 16, 24, and 32 kHz, 1 week before, as well as at 24 h and 15-21 days following exposure (baseline, compound threshold shift (CTS) and permanent threshold shift (PTS), respectively), followed by histologic analyses.

Results: We found significant differences in the CTS and PTS of the control (vehicle injected) mice to noise, where females had a significantly smaller CTS at 16 and 24 kHz (p < 0.0001) and PTS at 16, 24, and 32 kHz (16 and 24 kHz p < 0.001, 32 kHz p < 0.01). This sexual dimorphic effect could not be explained by a differential loss of sensory cells or synapses but was reflected in the amplitude and amplitude progression of wave I of the ABR, which correlates with outer hair cell (OHC) function. Finally, the frequency of the protective effect of SAHA differed significantly between males (PTS, 24 kHz, p = 0.002) and females (PTS, 16 kHz, p = 0.003), and the magnitude of the protection was smaller in females than in males. Importantly, the magnitude of the protection by SAHA was smaller than the effect of sex as a biological factor in the vehicle-injected mice.

Conclusions: These results indicate that female mice are significantly protected from NIHL in comparison to males and that therapeutics for NIHL may have a different effect in males and females. The data highlight the importance of analyzing NIHL experiments from males and females, separately. Finally, these data also raise the possibility of effectors in the estrogen signaling pathway as novel therapeutics for NIHL.

Keywords: ABR; B6CBAF1/J mice; Inner ear; Noise-induced hearing loss; SAHA; Sex differences.

Fig. 1

Octave band noise exposure at 101 dB SPL causes PTS in 10-week-old B6CBAF1/J mice. Hearing thresholds were compared between baseline, 24 h, 8 days, and 15 days post-noise exposure. At 24 h post-noise, significant compound threshold shifts are seen across all frequencies tested. Significant permanent threshold shifts are also detected at all frequencies tested at 8 and 15 days post-noise exposure. (**p < 0.01; **** p < 0.0001; ns non-significant)

Fig. 2

Octave band noise exposure at 101 dB SPL causes PTS in male and female mice. a Comparison of baseline hearing threshold between males and females. Female mice present a lower threshold at 32 kHz. b Hearing thresholds were compared between baseline, 24 h, and 15 days post-noise exposure in males (top) and females (bottom). At 24 h and 15 days post-noise exposure, significant compound threshold shifts are seen across all frequencies tested in both males and females. (*p < 0.05; ***p < 0.001; **** < 0.0001; ns non-significant)

Fig. 3

Male and female B6CBAF1/J mice respond differently to 101 dB SPL octave band noise. Threshold shifts were compared between vehicle-treated male and female mice. The dots indicate individual ear threshold shifts, the upper and lower whiskers indicate the maximum and minimum shifts, respectively. a CTS were significantly lower in females when compared to males at 16 and 24 kHz (****p < 0.0001). b PTS values were significantly reduced in females at 16, 24 (****p < 0.001), and 32 kHz (**p < 0.01)

Fig. 4

Differences in the slopes of ABR wave I amplitudes between male and female mice at 16 kHz. Growth of ABR wave I amplitude as a function of increasing stimuli levels at 16 kHz was compared by analysis of the slopes from linear regression of the data (dotted lines). The slopes are shown at baseline, 24 h, and 15 days post-noise exposure in males and females. a Slopes are reduced following noise exposure when compared to baseline for males (left) and females (right). Males partially recover at 15 days when compared to 24 h while females do not. b At baseline (left), the slope value from female wave I amplitude is higher than the value from males. The difference is maintained following noise exposure (right) at 24 h and 15 days. *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001. Error bars indicate S.E.M. ns, non-significant

Fig. 5

Protective effect of SAHA on NIHL—separated by sex. Threshold shifts were compared between SAHA- and vehicle-treated males (left) and females (right) at 24 h (a) and 15 days (b) post-noise exposure. The dots indicate individual ear threshold shifts, the upper and lower whiskers indicate the maximum and minimum shifts, respectively. a CTS values for males (left) and females (right). Male mice are protected only at 24 kHz (***p < 0.001) and female mice only at 16 kHz (*p < 0.05). b PTS values for males (left) and females (right). Females remain protected at 16 kHz (** p < 0.01) and males remain protected at 24 kHz (**p < 0.01)

Fig. 6

Protective effect of SAHA on NIHL—both sexes combined. Threshold shifts were compared between SAHA- and vehicle-treated animals at 24 h (a) and 15 days (b) post-noise exposure. The dots indicate individual ear threshold shifts; the upper and lower whiskers indicate the maximum and minimum shifts, respectively. a CTS values suggested a protective effect of SAHA at 16 kHz (*p = 0.007). b PTS values suggested a protective effect of SAHA at 16 kHz (*p = 0.0095) and 24 kHz (**p = 0.0024)

Fig. 7

Schematic showing the threshold shifts in relation to missing outer hair cells and number of active synapses. Loss of OHC (green) and decrease in the number of active synapses (black) does not correlate with the highest threshold shift (orange) following noise exposure detected at 16 kHz. The difference in threshold shifts between male and female mice is not explained by a difference in OHC loss or active synapse numbers