Turning down the sound signal in the inner ear

10 February 2009

Listening to loud music over headphones or working in a noisy environment are just two examples of activities that can lead to tinnitus and noise-induced hearing loss.

Over-exposure to loud sounds causes damage to the sensory hair cells in the inner ear, but now a team of researchers in Argentina and the USA may have found a way to reduce the damage.

There are two types of sensory cells in the cochlea: inner hair cells and outer hair cells, named for the minute hair-like strands at the end of the cell. The inner hair cells turn the sound signals they receive into electrical signals and relay them on to the brain. In contrast, the outer hair cells have an inherent property known as electromotility by which they amplify the incoming sound. The brain, in turn, sends information to the outer hair cells and regulates the way these outer hair cells function.

An important part of this feedback system from the brain is the neurotransmitter acetylcholine (ACh). This is used as a message that tells the outer hair cells to turn their amplification down. The new research, published online in the journal PLoS Biology, shows that a mutation in a gene for the ACh receptor can make the message to turn it down more effective.

Julian Taranda and colleagues found that mice with the mutation had less hearing loss after exposure to noise than mice without the mutation.

Encouragingly, the mutation didn't introduce any new effects to the ear, it just enhanced the natural protective feedback system that dampens down the cochlear amplifier.

“The most important result was that we could show that the feedback system protects the cochlea from acoustic injury. This makes it a great target for pharmacological prevention strategies” said Dr A. Belén Elgoyhen PhD, Principal Investigator at the Institute for Research on Genetic Engineering and Molecular Biology at the University of Buenos Aires, and senior author of the paper.

A useful next step for research would now be to try to develop a drug that can mimic the effect of the mutation. Having a protective treatment like this could mean that being able to go to concerts or working with DIY power tools can be done without the worry of ringing ears and hearing loss