Synaptic physiology of the cochlear sound encoding

Synaptic physiology of the cochlear sound encoding

Tzu-Lun Wang

Doctoral thesis

Date of Examination: 2013-02-07

Date of issue: 2013-11-27

Advisor: Moser, Tobias Prof. Dr.

Referee: Göpfert, Martin Prof. Dr.

Referee: Wolf, Fred Prof. Dr.

Persistent Address: http://hdl.handle.net/11858/00-1735-0000-0001-BAB4-8

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Abstract

English

Sound intensity is encoded as action potential firing rates by spiral ganglion neurons. These neurons show different rate-level functions with different auditory thresholds, and are thought to collectively encode the large dynamic range of the auditory stimuli. Hair cell ribbon synapses drive the action potential firing in spiral ganglion neurons through release of glutamate that is triggered by voltage-gated Ca2+ channels. Therefore, the investigation of presynaptic Ca2+ signaling and its relationship to synaptic ribbons might help to explain how inner hair cells (IHCs) decompose auditory information at their heterogeneous ribbon synapses thereby driving neurons with different rate-level functions. In this study, patch-clamp and Ca2+ imaging by spinning disk confocal microscopy were performed to investigate the presynaptic Ca2+ signaling at all active zones of a given IHC. Presynaptic Ca2+ microdomains and ribbons varied in their properties within and among cells. Moreover, their fluorescence intensities were positively correlated. Additionally, we assessed the spatial distribution of the properties of synaptic ribbon and Ca2+ microdomain relative to the circumferential position of the inner hair cell. Larger ribbons and low voltage-sensitivity Ca2+ microdomain exhibit the preference to localize to the modiolar (neural) side. Our results imply that larger synaptic ribbons are associated with more Ca2+ channels, which is expected to enhance neurotransmitter release at those synapses. Interestingly, previous studies (Merchan-Perez & Liberman, 1996) on cat cochlea suggested that high spontaneous rate auditory nerve fibers mainly innervate the pillar (abneural) face of inner hair cells. If conserved among species, our finding of high voltage-sensitivity Ca2+ microdomains on the pillar face seems support the high spontaneous-rate neuron innervating this face. Therefore, the hair cell ribbon synapses might utilize different voltage-sensitivities of their Ca2+ channels to determine the sound coding by the postsynaptic spiral ganglion neurons.

Keywords: ribbon synapse; calcium; hair cell; sound level