New research from the Karolinska Institute in Sweden indicates that the cochlea houses a biological circadian clock that influences the degree of hearing damage that occurs after exposure to noise, and that controls genes known to regulate circadian rhythms.
In the study, researchers at the were able to show that the degree of damage to hearing from noise exposure depends on the time of day or night of the exposure. When measuring the activity of the auditory nerve in mice, a major area damaged by noise-inducing hearing loss, the researchers found that mice exposed to moderate noise levels at night had permanent hearing damage, whereas mice exposed to similar noise levels during the day did not.
“The most important finding of the study is that hearing sensitivity to noise trauma differs over 24 hours,” said Barbara Canlon, PhD, professor of auditory physiology in the department of physiology and pharmacology at the Karolinska Institute and senior author of the study.
Given that mice, who are nocturnal animals, experience greater hearing damage from night noise exposure, Dr. Canlon said the implications for humans is that noise trauma would be more damaging during the day when compared with nighttime exposure.
The researchers then looked at the levels of brain-derived neurotropic factor (BDNF), a growth hormone known to protect auditory nerve cells, in the mice and found higher levels of this hormone in response to day noise versus night noise. To test whether the BDNF-mediated signaling regulates noise sensitivity throughout the day, the researchers administered a selective BDNF receptor (tropomyosin-related kinase type B [TrkB]) to the mice during the night. They found that the TrkB agonist protected the mice and allowed for full recovery of hearing thresholds after night noise over exposure.
“These findings highlight the coupling of circadian rhythms and the TrKB receptor for the successful prevention and treatment of NIHL,” said the researchers.