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, IK,f, that transforms the immature IHC from a regenerative, spiking pacemaker to a high-frequency signal transmitter. Expression of IK,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 IK,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.

Recovery of hearing and vocal behavior after hair-cell regeneration

Postmitotic hair-cell regeneration in the inner ear of birds provides an opportunity to study the effect of renewed auditory input on auditory perception, vocal production, and vocal learning in a vertebrate. We used behavioral conditioning to test both perception and vocal production in a small Australian parrot, the budgerigar. Results show that both auditory perception and vocal production are disrupted when hair cells are damaged or lost but that these behaviors return to near normal over time. Precision in vocal production completely recovers well before recovery of full auditory function. These results may have particular relevance for understanding the relation between hearing loss and human speech production especially where there is consideration of an auditory prosthetic device. The present results show, at least for a bird, that even limited recovery of auditory input soon after deafening can support full recovery of vocal precision.