A potential cure for permanent deafness has been found by scientists using a drug that stimulates the inner ear.
The drug, codenamed LY411575, triggers the regeneration of sensory hair cells.
Until now it has not been possible to restore the cells once they have been lost due to factors such as loud noise exposure, infection and toxic drugs.
This type of deafness, often suffered by rock musicians and DJs, is generally assumed to be irreversible.
Scientists succeeded in partially restoring hearing to mice that had been deafened by loud noise.
Although the research is at an early stage, they believe it could lead to effective treatments for acute noise-induced deafness in humans.
The tiny sensory hairs in the cochlea are vital to hearing. Sound vibrations transferred from the eardrum shake the hairs, causing nerve messages to be fired to the brain.
Without the hairs, the hearing pathway is blocked and no signals are received by the brain's auditory centre.
While birds and fish are capable of regenerating sound-sensing hair cells, mammals are not.
The new approach involves reprogramming inner ear cells by inhibiting a protein called Notch.
Previous laboratory research had shown that Notch signals help prevent stem cells in the cochlea transforming themselves into new sensory hair cells.
The drug LY411575 suppresses Notch. Mice with noise-induced hearing loss generated functioning sensory hair cells after the drug was injected into their damaged cochleas.
Lead researcher Dr Albert Edge, from Harvard Medical School in the US, said: "We show that hair cells can be regenerated from the surrounding cells in the cochlea. These cells, called supporting cells, transdifferentiate into hair cells after inhibition of the Notch signalling pathway, and the new hair cell generation results in a recovery of hearing in the region of the cochlea where the new hair cells appear.
"The significance of this study is that hearing loss is a huge problem affecting 250 million worldwide."
Details of the study are reported in the journal Neuron.
A green fluorescent protein was used to label the newly generated hair cells.
Electronic measurements of auditory brainstem responses confirmed that three months after treatment, lost hair cells had been replaced and were working.
Improvement in hearing was seen over a wide range of frequencies.
Dr Edge added: "The missing hair cells had been replaced by new hair cells after the drug treatment, and analysis of their location allowed us to correlate the improvement in hearing to the areas where the hair cells were replaced.
"We're excited about these results because they are a step forward in the biology of regeneration and prove that mammalian hair cells have the capacity to regenerate. With more research, we think that regeneration of hair cells opens the door to potential therapeutic applications in deafness."
Vivienne Michael, chief executive of the charity Deafness Research UK, said: "As always, we have to be cautious about new research findings but this US research is extremely encouraging.
"At the moment there is no way of reversing eight in 10 cases of hearing loss, including noise-induced deafness and the progressive deafness so many of us experience as we age - hearing aids are the only answer.
"These results show just how important it is to increase the investment in research into medical treatments that could prevent or reverse hearing loss and improve the quality of life for the millions of people affected."