CHANGES IN COCHLEAR FUNCTION AND SYNAPTIC PROTEIN EXPRESSION IN NOISE INDUCED COCHLEAR SYNAPTOPATHY
As people age, the ability to understand fast speech and discern conversation in background noise becomes more difficult, despite normal audiological testing results. Perceptual difficulties in subjects with normal audiometric thresholds are referred to as hidden hearing loss (HHL), and are based on animal models that show noise can damage the delicate ribbon synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) without causing a permanent threshold shift (PTS). The issue of whether or not damage to the ribbon synapses can be reversed has been controversial. Therefore, the focus of this project was to verify molecular changes that may serve as further evidence of synaptic repair. Using Western Blotting to analyze proteins and Reverse Transcription quantitative Polymerase Chain Reaction (RT-qPCR) for messenger ribonucleic acid (mRNA) analysis, several proteins of the ribbon synapse were studied at several time points after noise exposure in a guinea pig model. In addition, other members in our lab evaluated how the cochlear responses (using compound action potential, CAP) from signals with large dynamic level changes (by utilizing amplitude modulation, AM) at a relatively high sound level (80 dB SPL) was impaired with noise induced hidden hearing loss (NIHHL). The number of synapses were also counted by other members in the lab to confirm the consistency of the model used in this project as compared with that found in previous studies. There is a trend of up-regulation in the expression of ribbon protein at the RNA level one week post-noise exposure. This could suggest a synaptic repair, following the timeline previously found in guinea pigs after NIHHL (Song et al., 2016). However, the RNA level of GAP43, a critical protein for synaptic formation and plasticity, was found down-regulated in this thesis study, which could oppose prior findings of up-regulation of the protein itself (Dodson & Mohuiddin, 2000). Additionally, concern about the use of reference genes was raised and discussed in this thesis for the RNA analysis. Morphologically, the synapse counts per IHC (synapse density) were similar to what was reported in guinea pigs as evaluated one month after the noise exposure: there was a significant reduction in synapses at the high-frequency region (between 8-32 kHz). Responses of the compound action potential (CAP) at high modulation frequencies (~1 kHz) to signals with dynamic amplitude changes remained highly depressed one month after the noise exposure. The reduction of the AM CAP amplitude appeared to be larger than the reduction of the synapse count. This is consistent with the idea that the noise exposure selectively damages the synapses innervating a special group of auditory nerve fibers that are important for the coding of signals with a larger dynamic range at high sound level. The results of this study further suggest that the repaired synapses are functionally abnormal.