ULTRASTRUCTURE OF AFFERENT INNERVATION AT INNER HAIR CELLS IN MOUSE

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The structure and underlying physiology of afferent innervation in the mammalian cochlea is only partially understood in cat and is essentially unknown in mouse. The increasing importance of mouse as an animal model in hearing research necessitates a more detailed analysis of the critical synaptic interface between the inner hair cell (IHC), the transducer of sound, and auditory nerve fibers. Dr. Francis¿ preliminary studies of afferent terminal and synaptic ultrastructure at this interface in the C57BL/6J mouse have revealed two morphologically distinct groups of afferent endings. One of these endings is a conventional bouton, whereas the other has novel morphological features including the presence of a deep sulcus apparently created by the invagination of the outer membrane. This ending has been called the folded ending and accounts for about a half of the terminals that form afferent synapses with IHCs at 8, 16 and 32 kHz sites. This morphology has been observed in 3 out of 4 C57BL/6J mice studied so far. Dr. Francis proposes a study in which electron tomography is used to elucidate the 3-dimensional structure of the sulcus in folded terminals and particularly its relationship to the outer membrane of the nerve terminal. The sulcus partially compartmentalizes the terminal and appears to sequester mitochondria and other organelles. Electron tomography would be helpful in showing these complex relationships, which may provide insight into the functional role of this unusual structure. Dr. Francis also proposes to examine the spatial relationships between components of the afferent synapse including the pre-synaptic body, and vesicles bound to both the pre-synaptic membrane and ribbon. This study will provide valuable information about the synaptic machinery (especially the releasable pool of vesicles) and would find immediate use in on-going studies of afferent synaptic transmission in this animal.