Ears Employ 'Defenses' to Prevent Acoustic Traumas

Progressive hearing loss is a common disease amongst Djs, who live most of their lives in the loud noises of control rooms in clubs or discos around the world. After years and years of prolonged exposure to strong noises, their ear muscles deteriorate, and, in the end, total deafness sets in. Researchers now believe they've discovered a protective mechanism that the human ear employs, in order to save itself from the damage made by loud noises. The automated response allows less sound into the ear, thus saving the eardrum and adjacent bones in the inner part of the organ.

"There's some uncertainty in the field about what this sound-limiting system is used for. Now we've definitively shown that this system functions in part to prevent acoustic trauma," the leader of a new study that appears in this week's issue of the journal PLoS Biology, Paul Fuchs, PhD, who is also the otolaryngology-head and neck surgery professor in the Center for Sensory Biology from the Institute for Basic Biomedical Sciences, at the Johns Hopkins Medical Institutions, says.

During their study, the researchers have focused on a specific protein, nAChR, believed to regulate the amount of sound so-called sensory "hair cells" of the ear. The release of the protein is triggered by the brain, when it interprets a noise as being too loud. The hair cells decrease their sensitivity, and the sounds are heard a bit diminished. To test its hypothesis, the team has modified this protein in mice, and then subjected them to various hearing tests.

"This point mutation was designed to produce a so-called gain of function in which the inhibitory effect of ACh should be greater than normal," Fuchs explains. He notes that, in the case of mice with the modified protein, the level of auditory inhibition was a lot larger than in control batches, and that the animals showed an incapacity of hearing soft noises, as opposed to their peers.

When subjecting the "altered" mice to more than 100 decibels of noise, the team observed a significantly lower damage to the ear in those subjects with the modified protein, compared with the others. "One hundred decibels, for me, is painfully loud, and conversation is impossible. But sound levels in night clubs or rock concerts can be that high, and extended exposure to sound at that volume can cause hearing loss," Fuchs reveals.

"We think this pathway could be a therapeutic target for protecting from sound damage. So far, there is little or no specific pharmacology of hearing. We're still learning how the inner ear works. The encouraging news is that molecular mechanisms like the hair cell's nAChR frequently involve unique gene products, so there is a real chance of finding ear-specific drugs in the future," the researcher concludes.