Auditory hair cells release the excitatory neurotransmitter glutamate at their basal pole where they form synapses with afferent fibers. The underlying mechanisms that control and regulate the release of glutamate from hair cells, and how this release elicits action potential spikes in the afferent fibers, are still poorly understood. Part of the reason for this is the small size, fragility, and inaccessibility of adult mammalian inner hair cells and their afferent fibers. We propose here to study fundamental aspects of synaptic transmission from auditory hair cells in the adult bullfrog amphibian papilla. This unique in vitro preparation allows us to routinely access single hair cells and their afferent fibers for high-time-resolution patch-clamp electrophysiology and structure/function studies. We propose to use paired recordings of the hair cell and its connected afferent fiber to study multiquantal glutamate release and simultaneously to measure membrane capacitance changes from the hair cell to assay the exocytosis of synaptic vesicles. We will pursue three Specific Aims: First, we hypothesize that hair cells contain three distinct readily releasable pools of synaptic vesicles that have morphological correlates. Electrophysiological and serial electron microscopy reconstruction studies will be undertaken to determine the size, efficiency of release, Ca-dependence, recruitment and recovery rates, and short-term plasticity of these vesicle pools. Second, we hypothesize that the depletion of a small but fast releasing pool of vesicles accounts for the rapid phase of spike firing adaptation, whereas the second and slower phase of spike adaptation depends on vesicle recruitment from the synaptic ribbon and cytoplasm. Paired recordings will be used to determine vesicle release rates, which will then be compared to afferent fiber spike rates evoked by the same hair-cell stimulus protocol. We will also determine the identity and properties of the ionic currents present at the afferent fiber to better understand how afferent fibers trigger spikes. Finally, we hypothesize that evoked multiquantal EPSC amplitudes become effectively Ca-independent when hair cells are stimulated by sinusoidal-like stimuli that mimic pure tone sounds. We propose that this regime of Ca-independent multiquantal release allows spike synchronization to occur at a given characteristic sound frequency even as stimulus intensity changes. This grant will thus lead to fundamental insights on how the hair cell synapse encodes information about the timing and intensity of sound via the rate, latency, and timing of action potential spikes in the afferent fibers.

Public Health Relevance

More than thirty million Americans suffer from significant hearing deficits and most of these impairments are due to damaged hair cells in the inner ear. Our ability to treat this hearing loss, however, has been greatly impaired by a poor understanding of hair cell synapses, and of the mechanisms that generate action potential spikes in the auditory nerve fibers. This proposal uses a novel adult auditory hair cell synapse preparation to study fundamental aspects of hair cell synaptic physiology, so it will further our basic understanding of how to excite different auditory nerve fibers artificially with cochlea implants (devices that can partially restore hearing by bypassing the damaged hair cells to directly stimulate the auditory nerve) by using more physiologically relevant patterns of stimulation that match the original information rates of healthy adult hair cells.

National Institute of Health (NIH)
National Institute on Deafness and Other Communication Disorders (NIDCD)
Research Project (R01)
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Auditory System Study Section (AUD)
Program Officer
Cyr, Janet
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Oregon Health and Science University
Schools of Medicine
United States
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Li, Geng-Lin; Cho, Soyoun; von Gersdorff, Henrique (2014) Phase-locking precision is enhanced by multiquantal release at an auditory hair cell ribbon synapse. Neuron 83:1404-17
Graydon, Cole W; Cho, Soyoun; Diamond, Jeffrey S et al. (2014) Specialized postsynaptic morphology enhances neurotransmitter dilution and high-frequency signaling at an auditory synapse. J Neurosci 34:8358-72
Kim, Mean-Hwan; Li, Geng-Lin; von Gersdorff, Henrique (2013) Single Ca2+ channels and exocytosis at sensory synapses. J Physiol 591:3167-78
Graydon, Cole W; Cho, Soyoun; Li, Geng-Lin et al. (2011) Sharp Ca²? nanodomains beneath the ribbon promote highly synchronous multivesicular release at hair cell synapses. J Neurosci 31:16637-50
Cho, Soyoun; Li, Geng-Lin; von Gersdorff, Henrique (2011) Recovery from short-term depression and facilitation is ultrafast and Ca2+ dependent at auditory hair cell synapses. J Neurosci 31:5682-92
Li, Geng-Lin; Keen, Erica; Andor-Ardo, Daniel et al. (2009) The unitary event underlying multiquantal EPSCs at a hair cell's ribbon synapse. J Neurosci 29:7558-68
Leao, R M; von Gersdorff, H (2009) Synaptic vesicle pool size, release probability and synaptic depression are sensitive to Ca2+ buffering capacity in the developing rat calyx of Held. Braz J Med Biol Res 42:94-104
Spirou, G A; Chirila, F V; von Gersdorff, H et al. (2008) Heterogeneous Ca2+ influx along the adult calyx of held: a structural and computational study. Neuroscience 154:171-85
Kim, Jun Hee; Sizov, Igor; Dobretsov, Maxim et al. (2007) Presynaptic Ca2+ buffers control the strength of a fast post-tetanic hyperpolarization mediated by the alpha3 Na(+)/K(+)-ATPase. Nat Neurosci 10:196-205
Hermann, Joachim; Pecka, Michael; von Gersdorff, Henrique et al. (2007) Synaptic transmission at the calyx of Held under in vivo like activity levels. J Neurophysiol 98:807-20

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