Applying Neurotrophins to the Round Window Rescues Auditory Function and Reduces Inner Hair Cell Synaptopathy After Noise-induced Hearing Loss.

Otology & Neurotology

PubMedID: 27631825

Sly DJ, Campbell L, Uschakov A, Saief ST, Lam M, O'Leary SJ. Applying Neurotrophins to the Round Window Rescues Auditory Function and Reduces Inner Hair Cell Synaptopathy After Noise-induced Hearing Loss. Otol Neurotol. 2016;37(9):1223-30.
HYPOTHESIS
Applying neurotrophins to the round window immediately after a single noise exposure will prevent noise-induced hidden hearing loss.

BACKGROUND
Loud noise can eliminate neural connections between inner hair cells and their afferent neurons (thereby diminishing sound perception) without causing a detectable change on audiogram. This phenomenon is termed hidden hearing loss.

METHODS
Guinea pigs were exposed for 2?hours to 4 to 8?kHz noise at either 95 or 105?dB SPL. Immediately afterward a 4?µl bolus of neurotrophins (brain-derived neurotrophic factor 1?µg/µl, and neurotrophin-3 1?µg/µl) was delivered to the round window of one ear, and saline to the other. Auditory brainstem responses to pure-tone pips were acquired preoperatively, and at 1 and 2 weeks' postexposure. Cochleae were removed and whole mounted for immunohistochemical analysis, with presynaptic ribbons of inner hair cells and associated postsynaptic glutamatergic AMPA receptors identified using CtBP2 and GluA2 antibodies respectively.

RESULTS
After exposure to 105?dB noise, threshold did not change, but the amplitude growth of the auditory brainstem response was significantly reduced in control ears in response to 16 and 32?kHz tones. The amplitude growth was also reduced neurotrophin ears, but to a lesser degree and the reduction was not significant. Similar results were obtained from control ears exposed to 95?dB, but amplitude growth recovered in neurotrophin-treated ears, this reaching statistical significance in response to 16?kHz tones. There were significantly more presynaptic ribbons, postsynaptic glutamate receptors, and colocalized ribbons after neurotrophin treatment.

CONCLUSION
A single dose of neurotrophins delivered to the round window reduced synaptopathy and recovered high-frequency hearing in ears exposed to 95?dB noise. These findings suggest that hidden hearing loss may be reduced by providing trophic support to the cochlea after injury.