Age-related changes in cochleas of mongolian gerbils.

The effects of aging on the gerbil cochlea were studied in 16 animals raised in a quiet environment. Animals were tested at ages ranging from 33 to 36 months, the approximate average lifespan of gerbils in our colony. Hearing sensitivity was assessed by measures of whole-nerve compound action potential (CAP) thresholds and surface preparations of the organ of Corti were subsequently examined by light microscopy for losses of sensory hair cells. These quiet-aged animals showed a wide range of hair-cell losses and threshold shifts. Outer hair cells often showed significant losses while inner hair cells were rarely absent. All animals had some threshold shift, especially at frequencies above 4 kHz. These shifts ranged from 1 to 68 dB. At high frequencies, threshold shifts often occurred without hair-cell losses at corresponding cochlear locations. At low frequencies, threshold shifts seldom reflected the losses of hair cells commonly found in the cochlear apex. Thus, the correlation of specific hair-cell losses and CAP threshold shifts at corresponding frequencies was poor. On the other hand, the total number of missing hair cells, irrespective of location, was a good, general indicator of the hearing capacity in a given ear. It appears that the factor or factors that makes cochleas susceptible to hair-cell loss with increasing age also affects other cochlear mechanisms that are necessary for normal functioning of the ear.