Mechanisms of ribbon synapse function and plasticity

Research Questions and Significance Primary auditory neurons (spiral ganglion neurons) innervate the receptor cells (hair cells) in the cochlea (the hearing part of inner ear) with a special type of synapses called ribbon synapses that are characterized by the pre-synaptic ribbon structure. Three questions will be addressed. (1) The ribbon is known to facilitate fast neurotransmitter release and therefore critical for encoding fast change of signals—referred as temporal processing ability. However, exactly how the ribbon synapses fulfill this job is not clear. Specifically, there are 15-30 ribbon synapses around each inner hair cell (IHC), and morphology differences are seen for synapses at different locations around an IHC. However, it is not clear if neurons that synapse at the different locations are different in their temporal processing ability. (2) The synaptic ribbons in retina photo receptor cells are dynamic: they disassembled in light and reassembled in dark. It is not clear if this dynamic process exists in IHCs to serve intensity coding of the cochlea. (3) Recently, we demonstrated an interesting synaptic plasticity during the repair process of ribbon synapses after noise-induced damage. This involves breaking down, regeneration and the interaction between ribbons and post-synaptic terminals for the reallocation of the synapses. Functionally, the repaired synapses show temporal processing deficits. However the natures and the mechanisms for the repair and how it impacts the temporal processing remain to be explored. Research on these questions will provide insight how ribbon morphology is connected with the function; and how the ribbon contributes to intensity and temporal coding in the cochlea. The study on the mechanisms and the consequence of the synaptic plasticity will lead the manipulation or control of the plasticity to beneficial direction. Objectives and Research Plan The first goal is to understand the mechanisms of ribbon synapse for temporal processing of IHCs. Previously, the morphology differences were identified in immunohistology. We will use transmission electronic microscope (TEM) to further identify morphology differences in ribbons and post-synaptic terminals around IHCs. I will explore how the morphology differences are related with the temporal processing ability that will be verified in the single unit activities of neurons innervating different positions of IHCs. The second goal is to explore if the ribbons in IHCs are dynamically broken down by acoustic stimulation at the level for normal communication (70-90 dB SPL) and reassembled in quiet. I will compare the morphology of ribbons across different time points re: sound stimulation using TEM. Molecular biological methods will also be used to identify the changes in structure configuration of ribbons (the changes in ribbon protein organization). Functional observation will also used in association with morphology and molecular biology. The third goal is to explore the mechanisms of synaptic plasticity during the repair. I will first identify if the regeneration of ribbons require the synthesis of new proteins, or a simply reassembly of broken ribbons. I will investigate the molecular nature of the shifting from physiological breaking down and the damage, as well as the potential gene control of the repair. I will identify at TEM level if repaired ribbons attract post-synaptic terminals during the re-establishment of the synapses and the relocation of them during the repair as indicated by my recent study. Previous observation in gross evoked responses has shown the temporal processing deficit of repaired ribbon synapses. I will further identify this at single unit level and explore the molecular bases for the deficit.