There has been an expeditious increase in anthropogenic noise ever since the industrial revolution. As a result, noise-induced hearing loss has rapidly increased worldwide. The World Health Organization has even suggested that prolonged noise exposure is likely responsible for upwards of one-third of hearing losses. In this recent study, Zebrafish were used as a research model in an effort to better understand the effect of noise on the human auditory system. Zebrafish (Danio rerio) are an effective model organism since genes known to be associated with human deafness are highly conserved in this species. Additionally, noise-induced hearing loss can be attributed to hair cell damage after exposure to loud noise in both humans and zebrafish. Specifically in this study, zebrafish were used in order to examine certain auditory changes after continuous noise exposure.
There were three types of investigation that took place in this study:
- The analysis of the frequency-specific noise-induced hearing loss in both larvae and adult zebrafish
- The investigation on whether hearing loss in zebrafish is affected by sex
- The measurement of cortisol levels in noise-exposed male and female adult zebrafish
In part one, five-day post-fertilized larvae were used for the experiment in tandem with three-month-old, adult zebrafish. Three different frequencies (200 Hz, 1 kHz, and white noise) were used to cause noise-induced hair cell damage for both adult zebrafish and larvae. Their goal was to understand whether a particular frequency of noise damages hair cells more than others. 
Zebrafish are able to sense sound through their inner ear and lateral line hair cells; therefore, these were the parts of the ear examined with a confocal microscope in both larvae and the adult zebrafish after the noise exposure experiment. The results showed that the larvae had noise-induced hair cell changes in the lateral line but not in the inner ear after exposure to all frequencies. In contrast, the adult zebrafish experienced a decrease in the number of hair cells in a specific region of the inner ear (the saccule) when exposed to both 1 kHz and white noise. However, a decrease in hair cells in another region of the inner ear (the caudal) was found in the adult zebrafish exposed to 200 Hz noise. These findings suggest that the age/developmental stages of zebrafish can have an effect on the part of the auditory system that gets damaged as a result of stimuli like continuous loud noise.
In part two of this study, the differences in noise-induced hearing loss based on sex were assessed. Both the male and the female adult zebrafish were raised in the same tank and exposed to the same noise conditions. The findings showed that only male zebrafish experienced damage to the saccule after exposure to a continuous 1 kHz tone. These results indicated that male zebrafish are more vulnerable to noise than female zebrafish (at least at the frequency measured).
In part three of this study, the researcher’s aim was to investigate if stress could be a cause of sex-based differences in noise-induced hearing loss. More specifically, the researchers tested whether noise exposure resulted in different levels of cortisol based on sex. The zebrafish were split into the following four groups: control males, control females, noise-exposed males, and noise-exposed females. Their cortisol levels were measured using cortisol ELISA kits after the exposure treatment. The results showed that the cortisol levels were significantly higher in the noise-exposed male group compared to the noise-exposed female group or the control male group. Whereas the noise-exposed female group showed no difference in cortisol levels when compared to the control female group. These findings echo the findings from part two of the study; the sex of the zebrafish may influence the auditory system’s response to abnormal environmental stimuli.
Overall the study demonstrated that, not only is noise-induced hearing loss frequency-dependent but that the degree of hearing loss is affected by sex in zebrafish. These findings mirror characteristics of hearing loss in humans since males are reported to be more severely impacted by noise-induced hearing loss. Historically, sex was not an independent variable considered when performing studies related to hearing loss. Interestingly enough, it was not until 2016 that the National Institute of Health announced a mandate to include sex as a biological variable in research. In the last six years, there have been a plethora of studies, including this one, that has emphasized the need to consider sex in hearing loss studies. We live in a world that grows louder by the day; additional studies involving animals from other taxonomic groups should be considered moving forward.
- Eunjung Han, Dae Hyeok Lee, Saemi Park, Yoon Chan Rah, Hae-Chul Park, Jee Woong Choi, and June Choi. Noise-induced hearing loss in zebrafish model: Characterization of tonotopy and sex-based differences. Hearing Research, Volume 418, 2022. https://doi.org/10.1016/j.heares.2022.108485.
- Image credit: Jill Neimark (2020). Researching Regeneration Through the Zebrafish. Brainfacts.org
- Image credit: Liyir Tasar (2020). Noise pollution: a silent killer. Shortpedia Voices
