Contribution of beta1- and beta2-adrenergic receptors to cochlear function

Sympathetic innervation is heavily present in the cochlea. However, its role in the development and maintenance of normal hearing remains debatable. Beta1-adrenergic receptors 1 (beta1-ARs) and beta2-adrenergic receptors (beta2-ARs) are two types of receptors that are stimulated by the sympathetic nervous system and are expressed by many cochlear cell types. In this study, we have analyzed the functional consequences of the lack of beta-ARs in the cochlea. We have first evaluated hearing thresholds using auditory brainstem response and distortion product otoacoustic emissions in young and aged knockout mice lacking beta1-ARs, beta2-ARs, or both beta1-ARs and beta2-ARs. Secondly, we tested the sensitivity of these mice to acoustic overexposure.

Hearing tests revealed similar normal thresholds in all beta- adrenergic receptors mice when compared to age matched wild type controls. When exposed to noise mice lacking beta1- but not beta2- AR exhibited a subtle protection of hearing thresholds at the low frequencies only. These results suggest that despite being expressed by many cochlear cell types these beta-ARs are not playing a crucial role in hearing development and maintenance.