Synaptophysin in the cochlear nucleus following acoustic trauma

Chinchillas are notable for a low-frequency hearing range similar to that of humans and a marked sensitivity to loud noise. A single noise exposure that produces cochlear damage may lead to progressive loss of synaptic endings in the cochlear nucleus, followed by new axonal growth. As an index of synaptic regulation during such changes, we have examined the expression of a synaptic vesicle protein, synaptophysin, in the cochlear nucleus following a damaging acoustic stimulus in adult chinchillas. With one ear protected by a plug, following a 3-h exposure to an octave-band noise of 108 dB sound pressure level, centered at 4 kHz, the unprotected cochlea and the cochlear nuclei exhibited degeneration of hair cells and axons over periods of 7, 14, 30, 90, and 150 days. Axonal degeneration, as revealed by a silver degeneration method, was heavy ipsilateral to the cochlear damage, but sparse degeneration also appeared on the contralateral, unexposed side. Synaptophysin immunostaining underwent a major, bilateral decline in the anteroventral and posteroventral cochlear nuclei, interrupted at intervening periods by transient increases in the numbers of stained structures. A distinction in staining between large perisomatic structures and smaller puncta in the neuropil and between the dorsal and the ventral zones of the ventral cochlear nuclei revealed some variations in the response and degree of recovery of synaptophysin staining. These findings could best be explained by degeneration of synaptic endings followed by new growth of terminals and by regulatory changes in the levels of synaptophysin expression and synaptic vesicle accumulation over time.