The C57BL/6J (C57) mouse is a model for human presbycusis, demonstrating progressive sensorineural hearing pathology with onset after maturation of its auditory system. Previous work found that tuning curves and tonotopic organization in the central auditory system (CAS) of aging C57 mice undergo significant changes, but these depend upon the location of neurons within a CAS nucleus, as related to the normal tonotopic organization and the CAS structure considered--sensitivity of neurons in the anterior and posterior ventral nuclei of the cochlear nucleus (AVCN, PVCN) are more deleteriously affected than those in the dorsal cochlear nucleus (DCN) and central nucleus of the inferior colliculus (ICC). The general aims of the proposed project are to extend the earlier (multiple-unit) tuning curve data using higher resolution single-unit recordings; expand our understanding of presbycusic changes to suprathreshold responses of CAS neurons; and develop a clearer picture of the relationship of peripheral and CAS responses in aging C57 mice. Extracellular single-unit recordings will be obtained from neurons of C57 mice across an age range of optimum sensitivity to severe hearing loss and from like-aged CBA/J mice (which show little age-related loss). Responses will be obtained from the eighth nerve, AVCN, PVCN, DCN, and IC. A tuning curve, suprathreshold post-stimulus time histograms, click responses, and spontaneous activity rate will be obtained for each unit. For each animal, an eighth nerve audiogram, quantitative evaluation of spiral ganglion cells, and qualitative evaluation of other cochlear structures and the middle ear will be obtained. Quantitative measures will be derived from response properties and neurons will also be categorized (temporal discharge patterns present; response area type). These measures will be compared statistically as a function of subject groups, degree of hearing loss, and anatomical location of neurons. This will provide the first systematic data on what CAS response changes can occur in an animal model of presbycusis and should, therefore, be considered in a comprehensive understanding of perceptual dysfunctions of people with age-related hearing loss.
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