Reduction in sharpness of frequency tuning but not endocochlear potential in aging and noise-exposed BALB/cJ mice.

Schuknecht proposed categories for human age-related hearing loss (ARHL) based upon whether the primary degeneration involves the organ of Corti (sensory ARHL), spiral ganglion cells (neural), stria vascularis (strial), or a combination of these (mixed). Genetically standardized mouse ARHL models can help validate Schuknecht's framework and clarify the underlying cellular processes. Much recent work has focused on the mouse Ahl locus, which promotes both ARHL and noise-induced hearing loss. On the C57BL/6 inbred background, Ahl has been associated with degeneration of organ of Corti, afferent neurons, and stria vascularis/spiral ligament, suggesting that it promotes mixed (sensory/neural/strial) ARHL. Some cochlear degeneration in C57BL/6 mice could be caused by genes other than Ahl, however. The question of what constitutes Ahl-related pathology can be addressed by comparing C57BL/6 mice with other strains that carry the same allele, including BALB/c substrains. We examined the effects of aging and broadband noise exposure in inbred BALB/cJ mice (1.5-13.0 mos) using measures of frequency tuning (compound action potential tuning curves) (CAPTCs), strial function (endocochlear potential recording, EP), and light microscopy. Aging and noise led to generally similar physiological and anatomical changes. Reductions in sensitivity and sharpness of frequency tuning were not consistently linked to hair cell loss, reduction in the EP, or changes in the lateral wall. Instead they appeared best explained by alterations in supporting cells in the basal half of the cochlear and in the spiral limbus in the apex. These results emphasize the importance of cell types other than hair cells in cochlear pathology. They also indicate that Ahl does not necessarily promote a strial form of ARHL.