SYNAPTOPHYSIN IMMUNOREACTIVITY IN THE COCHLEAR NUCLEUS AFTER UNILATERAL COCHLEAR OR OSSICULAR REMOVAL

This study determined if unilateral cochlear removal in adult guinea p igs led to synaptic loss followed by synaptogenesis in the cochlear nu cleus (CN) and if unilateral middle ear ossicle removal led to synapti c loss in the CN. Synaptic endings were identified immunohistochemical ly, using a monoclonal antibody to synaptophysin. Immunolabeling was q uantified densitometrically in the CN 4-161 days after cochlear remova l and 161 days after ossicle removal. Fiber degeneration was visualize d with the Nauta-Rasmussen silver method. Tissue shrinkage was measure d from drawings of CN sections. Compared to the contralateral side, im munolabeling density ipsilaterally was reduced by 4 days in the anteri or division of the anteroventral CN (a-AVCN) and by 7 days in the ante rior part of the posteroventral CN (a-PVCN). At 7 days, preterminal fi ber degeneration was abundant in both areas. These findings were consi stent with the loss of cochlear nerve endings and fibers. At later tim es, immunolabeling density recovered. In the a-AVCN, tissue shrinkage explained approximately half the recovery of staining density; the res t was attributed to synaptogenesis. In the a-PVCN, the entire recovery was attributed to tissue shrinkage. In the polymorphic layer of the d orsal CN, immunostaining density increased transiently at 4 days, whil e at 7 days preterminal fiber degeneration was abundant. A net loss of synaptic endings was not detected immunohistochemically. The increase d immunostaining density may reflect a transient growth of immature pr ocesses or presynaptic endings. Ossicle removal produced a deficit in immunolabeling density only in the ipsilateral a-PVCN, without fiber d egeneration, suggesting a loss of presynaptic endings or of synaptophy sin expression. (C) 1997 Wiley-Liss, Inc.