Deaf mice saw their hearing restored using a new form of therapy that could hold promise for humans. The process was carried out in rodents who lost part of their hearing due to excessive noise.

A protein called Neurotrophin-3 (NT3) was found to play a critical role in the transmission of signals between the ear and brain. The protein works in the connection between the hair cells and nerve cells in the ear, forming a super-fast connection known as the ribbon synapse. When researchers increased the concentration of this chemical in mice, hearing improved for the rodents treated.

"It has become apparent that hearing loss due to damaged ribbon synapses is a very common and challenging problem, whether it's due to noise or normal aging. We began this work 15 years ago to answer very basic questions about the inner ear, and now we have been able to restore hearing after partial deafening with noise, a common problem for people. It's very exciting," Gabriel Corfas from the University of Michigan said.

Approximately 50 million Americans suffer partial deafness in at least one ear, which is often caused by damage to the ribbon synapse. Hearing loss was brought on in about half of these patients - 26 million - by excessive noise, resulting in the loss of recognition of many high frequency noises. Hearing loss becomes more common with age, with the condition being present in 30 percent of subjects aged 65-74, increasing to 47 percent of those over age 75.

Cells deep within ears manufacture the NT3 protein, and investigators stimulated these cells in mice, through the application of tamoxifen, in order to produce more of the chemical. Mice treated with the substance regained hearing within two weeks of the process.

Mice in the study only suffered from partial hearing loss, and researchers are uncertain whether or not this technique could be effective in restoring hearing in rodents - or humans - who experience a complete loss of the sense.

Temporary damage to the ribbon synapse can often occur in people who attend a loud concert or are exposed to other excessive noise. The partial hearing loss that can happen in these conditions usually fades away over a couple days, restoring normal hearing.

Human beings could benefit from the study, and researchers are already investigating some drugs which could raise NT3 levels in people. A drug-based method of raising levels of the protein in humans would provide the easiest means of treatment.

Restoration of hearing in mice deafened by noise was detailed in the journal eLife.