Clin Exp Otorhinolaryngol.  2021 Feb;14(1):61-68. 10.21053/ceo.2019.01662.

Estrogen Replacement Reduces Hearing Threshold Shifts and Cochlear Hair Cell Loss After Acoustic Overexposure in Ovariectomized Rats

• Kim MT
• Lee JH
• Carpena NT
• Lee MY
• Chung PS
• Jung JY

• KMID: 2512882
• DOI: http://doi.org/10.21053/ceo.2019.01662

Abstract

Objectives
. The relationship of estrogen (the primary female sex hormone) with hearing function has been studied in both humans and animals. However, whether estrogen levels affect hearing remains uncertain. Therefore, in this study, we investigated changes in the vulnerability of hearing to acoustic overexposure in ovariectomized female rats.
Methods
. Eighteen 8-week-old female Sprague-Dawley rats were separated into four groups as follows: sham ovariectomy (OP), OP only, and OP treated with low (10 µg/kg) or high doses (100 µg/kg) of estrogen. Rats in the estrogen replacement groups were given two intraperitoneal injections. Hearing thresholds were measured before noise exposure, and at 1 day and 2 weeks after exposure.
Results
. The hearing thresholds of the sham OP and OP-only groups were not significantly different. However, both estrogen groups showed a lower threshold shift than the OP-only group. Histological immunostaining analyses showed that hair cell loss in the 32 kHz region was more severe in the sham OP group than in the OP-only group. Furthermore, there was little or no hair cell loss in either estrogen replacement group and significantly more hair cell loss in the OP-only group.
Conclusion
. These results suggest that estrogen replacement may reduce the vulnerability of hearing to noise exposure in menopausal women.

Keyword

Hearing Loss; Estrogen Replacement Therapy; Ovariectomy

Figure

• 

  Fig. 1. Experimental schedule. Ovariectomy or sham surgery was performed 2 weeks before noise exposure (brown). Estrogen was injected 1 day before and 3 days after noise exposure (purple). Auditory brainstem response (ABR) was measured before and at 1 day and 2 weeks after noise exposure (green). The animals were sacrificed after the final ABR measurements (red).

• 

  Fig. 2. Hearing thresholds increased after noise exposure in all groups. Hearing thresholds were measured before and 1 day after noise exposure. (A) Baseline hearing thresholds were normal in all experimental groups. (B) Hearing thresholds increased significantly 1 day after noise exposure in all groups and reached >80 dB SPL. Error bars indicate standard deviation. SPL, sound pressure level; OP, ovariectomy; ET1, low-dose estrogen replacement therapy; ET2, high-dose estrogen replacement therapy.

• 

  Fig. 3. Hearing thresholds were measured 2 weeks after noise exposure (NE), and threshold shifts from baseline were calculated. (A) Hearing thresholds among the different groups varied 2 weeks after NE. Significant differences were found between the OP-only and the estrogen replacement therapy groups at all test frequencies. (B) The threshold shifts between baseline and 2 weeks after NE varied among all groups. In addition, the threshold shift data between the OP-only and the estrogen replacement therapy groups differed significantly at all test frequencies. At 16 kHz, there was also a difference between the sham OP and ET2 groups. Error bars indicate standard deviation. SPL, sound pressure level; OP, ovariectomy; ET1, low-dose estrogen replacement therapy; ET2, high-dose estrogen replacement therapy. *P<0.05, **P<0.01.

• 

  Fig. 4. Cochlear hair cells and the auditory nerve. Three parts of the basilar membrane (shown as the relative distance from the apex, %), corresponding to different auditory brainstem response frequencies (8, 16, and 32 kHz), were immunostained for hair cells (myosin VIIa, red) and nerve fibers (neurofilament-heavy, green). The sham OP group showed severe loss of both inner (IHCs) and outer hair cells (OHCs) at 32 kHz, slight loss of OHCs at 16 kHz, but intact hair cells in the 8 kHz region of the basilar membrane. The OP-only group showed almost complete hair cell loss in the 32 kHz region, severe OHC and slight IHC loss in the 16 kHz region, and moderate OHC and IHC loss in the 16 kHz region of the basilar membrane. For the estrogen replacement groups, both OHCs and IHCs remained nearly intact in every region except for a slight loss of OHCs in the ET1 group in the 32 kHz region. Scale bar=100 µm. OP, ovariectomy; ET1, low-dose estrogen replacement therapy; ET2, high-dose estrogen replacement therapy.

• 

  Fig. 5. Quantification of inner hair cells (IHCs; A) and outer hair cells (OHCs; B) in the cochlea. The number of IHCs and OHCs within a square area 200 µm in length were counted at three regions within the cochlea. There were no significant differences in the numbers of IHCs or OHCs among the groups. Error bars indicate standard deviation. OP, ovariectomy; ET1, low-dose estrogen replacement therapy; ET2, high-dose estrogen replacement therapy. *P<0.05.

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