Preservation of synapses on principal cells of the central nucleus of the inferior colliculus with aging in the CBA mouse.

The light and electron microscopic features of principal neurons of the central nucleus of the inferior colliculus were quantitated in the CBA mouse. Three age groups of mice were examined, including young (3 months), middle-aged (8 months) and old (25 months). No changes were noted in the size of the principal neurons over the age range examined. At the ultrastructural level, synapses on the somata of the principal neurons showed no change in the number or type of synapses, the length of synaptic apposition nor the size of synaptic terminal area. These results are in contrast with the moderately severe synapse loss which we previously reported in the C57BL/6 mouse strain, a strain which has a genetic deficit producing progressive sensorineural hearing loss starting in young adulthood (Kazee et al., 1995). In contrast, hearing is quite well-preserved across the lifespan in the CBA mouse strain, making this a useful animal to study the intrinsic effects of aging in the auditory system versus the effects of sensorineural hearing loss. The preservation of synapses on principal neurons in this strain suggests that synaptic loss is not an inevitable event in aging, but may be related to the preservation of peripheral auditory function and input to the neurons.