Zinc provides new clue for why loud noise causes hearing loss

Exposure to loud noises may affect our hearing by disrupting levels of zinc in our inner ears, a study in mice suggests. Therapies that mitigate this could be used to treat or even prevent such damage, for example if taken before a rock concert.

Loud noises can cause cells in the inner ear to die. This has long been known to affect hearing, but the mechanism behind it is less clear.

Thanos Tzounopoulos at the University of Pittsburgh, Pennsylvania, suspected it might have something to do with free-moving zinc, which plays an important role in the neurological communication of our senses.

Most of the body’s zinc is attached to proteins, but the rest works as a communication signal between organs, especially the brain, says Tzounopoulos. The highest concentration of free zinc in the body is in the cochlea, the snail-shaped structure in the inner ear that converts vibrations into electrical signals, which are then interpreted as sound.

To learn more, Tzounopoulos and his colleagues tested free zinc levels in young mice that had been genetically modified to produce biological markers that flag the transportation of free zinc throughout the body.

After hearing noises at 100 decibels – as loud as a bulldozer or motorcycle – for 2 hours straight, the mice had significant hearing loss within the next 24 hours, says Tzounopoulos.

The researchers found that these mice had greater amounts of free zinc in between and around the cells in their cochlea after the sound blast compared with before, as well as in comparison to a group of control mice that hadn’t heard the loud sounds.

“There is a very robust upregulation of zinc, in terms of quantity, but also in terms of spatial covering of the area,” he says. “It goes everywhere.”

The zinc appears to have been released from certain cells in the cochlea after detaching from the proteins that normally bind it, says Tzounopoulos. The free zinc ultimately leads to cell damage and disrupts normal communication between cells, he says.

To see if reducing free zinc levels could protect hearing, Tzounopoulos and his team treated another group of mice with a zinc-trapping compound, either by injecting it in their abdomens or by placing a slow-release implant in their inner ears. The mice then heard the same loud noise for 2 hours. Both groups experienced much less hearing loss.

With further research, zinc-trapping pills, drops or slow-release implants might one day help prevent or treat inner ear damage from noise trauma, says Tzounopoulos.

“You could go to a concert or to combat and you could take a pill,” he says. “Or you might have an accident, and they could have these compounds in the ER [emergency room] to give you to help mitigate the damage.”

Future studies should also determine how long after noise exposure people could benefit from such a zinc-trapping therapy, says team member Amantha Thathiah, also at the University of Pittsburgh.