Changes in the synapses of spiral ganglion cells in the rostral anteroventral cochlear nucleus of the waltzing guinea pig following hair cell loss.

Between 10 and 60 days of age in the waltzing guinea pig, there is a genetically induced loss of all hair cells in the organ of Corti. About 43% of the spiral ganglion cells degenerate between 30 and 60 days of age. After 90 days of age, there is no further loss of spiral ganglion cells. Both Type I and II ganglion cells remain and are without afferent input. The terminals of these ganglion cells in the rostral AVCN, the end bulbs of Held, are normal until 30 days of age. During the period of ganglion cell loss degenerating end bulbs are seen. After 60 days of age, when most ganglion cell degeneration is complete, the remaining end bulbs have fewer synaptic vesicles and their synaptic junctions are flattened. The channels of enlarged extracellular space, which normally surround each synaptic junction or small groups of junctions, are only infrequently present. In freeze-fracture replicas of the rostral AVCN of waltzing guinea pigs after hair cell loss, the number of large, non-aggregate particles on the external leaflet of the principal cell opposite the end bulb is increased, and the number of perisynaptic aggregates is decreased compared to waltzing guinea pigs 10 days of age. The junctional aggregates are unaltered. These changes in the presynaptic terminal and postsynaptic membrane may be related to the loss of afferent input to the spiral ganglion cells, suggesting that activity is important for maintaining the synapse.