M. Charles Liberman1
1Eaton-Peabody Laboratories, Mass. Eye and Ear, Boston, USA

Most hearing impairment in adults arises from damage to the sensory cells and/or nerve fibers of the inner ear.  This talk will summarize recent research on animal models and human autopsy material showing that, in both noise-induced and age-related hearing loss, the synaptic terminals of cochlear nerve fibers degenerate first, leaving their peripheral targets, the inner hair cells, partially disconnected from the brain. This primary neural degeneration has little effect on hearing thresholds (the audiogram) but affects discrimination of complex sounds like speech.  Because the cell bodies and central projections of the cochlear neurons survive long after loss of their synaptic connections, there is a therapeutic window for repair, as has been shown in animal models using both local delivery, and virally mediated overexpression, of neurotrophins.  The endogenous capacity for repair is low in mice but high in guinea pigs, and the differences may provide further clues for therapeutic approaches.

Acknowledgements: Supported by grants from the NIH (R01 DC00188 and P50 DC015857)