Thyroid hormone receptor β-dependent expression of a potassium conductance in inner hair cells at the onset of hearing

To elucidate the role of thyroid hormone receptors (TRs) α1 and β in the development of hearing, cochlear functions have been investigated in mice lacking TRα1 or TRβ. TRs are ligand-dependent transcription factors expressed in the developing organ of Corti, and loss of TRβ is known to impair hearing in mice and in humans. Here, TRα1-deficient (TRα1(-/-)) mice are shown to display a normal auditory-evoked brainstem response, indicating that only TRβ, and not TRα1, is essential for hearing. Because cochlear morphology was normal in TRβ(-/-) mice, we postulated that TRβ regulates functional rather than morphological development of the cochlea. At the onset of hearing, inner hair cells (IHCs) in wild-type mice express a fast- activating potassium conductance, I(K,f), that transforms the immature IHC from a regenerative, spiking pacemaker to a high-frequency signal transmitter. Expression of I(K,f) was significantly retarded in TRβ(-/-) mice, whereas the development of the endocochlear potential and other cochlear functions, including mechanoelectrical transduction in hair cells, progressed normally. TRα1(-/)- mice expressed I(K,f) normally, in accord with their normal auditory-evoked brainstem response. These results establish that the physiological differentiation of IHCs depends on a TRβ-mediated pathway. When defective, this may contribute to deafness in congenital thyroid diseases.