RESUMEN
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 2âhours to 4 to 8âkHz noise at either 95 or 105âdB SPL. Immediately afterward a 4âµl bolus of neurotrophins (brain-derived neurotrophic factor 1âµg/µl, and neurotrophin-3 1âµg/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 105âdB 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 32âkHz 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 95âdB, but amplitude growth recovered in neurotrophin-treated ears, this reaching statistical significance in response to 16âkHz 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 95âdB noise. These findings suggest that hidden hearing loss may be reduced by providing trophic support to the cochlea after injury.