The functional age of hearing loss in a mouse model of presbycusis. II. Neuroanatomical correlates.

This report relates patterns of age-related outer hair cell (OHC) loss to auditory behavioral deficits in C57BL/6J mice. Hair cell counts were made from serial sections of the cochlear partition in three subject groups representing young (2-3 months), middle (8-9 months), and old ages (12-13 months). The cochlear location of OHC counts was determined from three-dimensional computerized reconstructions of the serial sections. Comparisons of the topographic distribution of surviving OHCs across the subject groups confirmed an orderly base-to-apex progression of cochlear degeneration that is well known in this mouse strain. All mice appeared to follow the same progression of OHC loss, although subjects showed considerable variation in the rate at which they advanced through a uniform sequence of structural changes. Behavioral implications of the magnitude and location of OHC loss were investigated by correlating the histological status of individual mice with sound detection thresholds from the same subjects [Hear. Res. 183 (2003) 44-56]. The analysis revealed regionalized patterns of OHC loss that were correlated with frequency-dependent changes in hearing thresholds, and validates the use of 'functional age' as an indicator of age-related cochlear degeneration and dysfunction. In the absence of physiologically defined cochlear frequency maps for C57BL/6J mice, these structure-function correlation techniques offer an alternative approach for linking anatomical results to hearing abilities.