Understanding the roles of the efferent system in the inner ear

BOERO, L; PLAZAS, PV; GOUTMAN, JD; ELGOYHEN, AB; GÓMEZ CASATI, ME

Huerta Grande, Cordoba

Congreso; Sociedad Argentina de Neurociencias; 2013

Hearing loss affects about 10% of the overall population and 42% of the population above 75 years of age. Such impairment is usually permanent and results from a variety of causes, including loud sounds. Here, we intend to address the issue of noise-induced hearing loss and the role of the efferent olivocochlear system in this process. To this end, we are using a murine model of enhanced noise protection, the Chrna9L9´T knock-in, a mouse in which the alpha9 nicotinic receptor subunit bears a mutation and leads to enhanced medial efferent activity. We tested how efferent innervation is rewired after noise trauma in wild type, Chrna9L9´T knock-in and alpha9 knockout mice. We exposed mice to loud sounds (1-16 kHz, 100 dB SPL, 1hr) and measured auditory brainstem responses (ABR), which reflect synchronized discharges from neurons along the auditory pathway. To test outer hair cell function, we recorded the distortion product otoacoustic emissions (DPOAEs). After trauma, knockouts and wild types showed large ABRs and DPOAEs threshold shifts. Suprathreshold ABR amplitudes were reduced, even in animals with almost normal threshold sensitivity. However, Chrna9L9´T knock-in mice were resistant to the same noise exposure. We used immunohistochemistry to visualize efferent neurons and found disorganized terminals after trauma compared to controls. These findings will contribute to the understanding of how normal hair cell function is affected by loud noise and the role of the efferent system.