Researchers hope that as well as improving sound quality for people with implants, the approach will allow safe and localised gene therapy for neurological disorders.
Cochlear implants have allowed more than 300,000 profoundly deaf people worldwide to experience sound, but listening to music and picking out voices from background noise can still cause problems.
'People with cochlear implants do well with understanding speech, but their perception of pitch can be poor, so they often miss out on the joy of music', says Professor Gary Housley from the University of New South Wales, who led the research.
'Ultimately, we hope that after further research, people who depend on cochlear implant devices will be able to enjoy a broader dynamic and tonal range of sound'.
Nerve endings in the ear are known to regenerate in the presence of proteins called neurotrophins, but these cannot be delivered into the cochlea using standard viral gene therapy vectors or with drugs.
Instead, the researchers injected DNA encoding a growth factor called BDNF into deaf guinea pigs' cochleas. In order to make cells receptive to the DNA entering them, researchers sent electric pulses to the electrodes of the cochlea implant.
The cells produced neurotrophin and this allowed the auditory nerve to regrow and branch out.
Guinea pigs are often used as a hearing model in scientific research because their cochleas are similar in size to human ones. Future research will need to investigate whether the new branches stabilise after they are developed, or if they will spread.
The researchers believe that this novel approach for delivering gene therapy could be applied to neurological and psychiatric disorders such as Parkinson's disease and depression.
'Our work has implications far beyond hearing disorders', said co-author Dr Matthias Klugmann. 'Gene therapy has been suggested as a treatment concept even for devastating neurological conditions and our technology provides a novel platform for safe and efficient gene transfer into tissues as delicate as the brain'.