Spiral ganglion cell endings in the cochlear nucleus of young and old rats

The spiral ganglion cells (SGCs) forming the auditory nerve have been shown to degenerate with age in both human and animal models, presumably resulting in post-synaptic sites in the cochlear nucleus (CN) that have lost their inputs. The present study examined the morphological changes in the surviving SGC endings in the CN of aged animals. The auditory nerves of 2-3 MO and 25-26 MO male, Fisher 344 rats were anterogradely labeled with horseradish peroxidase and the CN prepared histochemically. This resulted in Golgi-like labeling of afferent fibers and their terminals. All endings within each section were drawn and the area and number of components per ending were measured. Young and old animals both had about the same proportion of "simple", "string" and "complex" endings within the ventral CN, with complex endings being predominant in both age groups. The area of many complex endings was greater in the old animals with some endings being twice as large as any seen in young animals. There was no evidence of smaller endings in the old animals, suggesting that endings are not shrinking with age. A comparison of the number of components per complex ending revealed significantly more complexity in the endings of aged animals. Following the degeneration of SGCs it seems likely that the remaining cells, by increasing the area and altering the shape of their central terminals, may cover some of the post-synaptic sites made available by degenerated endings in aged animals.