Cell survival and cell death in the auditory nerve

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Spiral ganglion neurons (SGNs) are the primary afferent neurons that carry auditory information from the inner hair cells (IHCs) of the cochlea to the central nervous system. Although degeneration of SGNs is known to occur in response to injury and with age, little is known about the sequence of cellular or molecular events underlying this pathology in vivo. Nuclear factor - KB (NF KB) is a transcription factor that is known to regulate apoptosis in response to insults in many cell types, including neurons. NF KB is also associated with intracellular Ca2+ regulation, the dysfunction of which is an important factor in neuronal excitotoxicity and apoptosis. The goal of this project is to determine the role of NF KB in the survival of SGNs following acute insults. Wild type and NF KB knockout mice will be used as animal models. These knockout mice show a progressive hearing loss with age that is closely correlated with accelerated degeneration of SGNs. Moreover, the pathology of the SGN radial dendrites in this knockout suggests that excessive excitotoxicity is present in the afferent dendrites at IHC synapses. Based on these observations, we hypothesize that NFKB plays an anti-apoptotic role in the protection of SGNs from degeneration after acute injury, and that the underlying mechanism of this protection is that NF KB activity helps maintain Ca2+ homeostasis in SGNs to reduce excitotoxic effects. These hypotheses will be tested with two specific aims. The first aim will determine whether activation of NF KB in vivo protects SGNs from degeneration in response to acute noise and ouabain exposures. The noise and ouabain exposures allow differing assessments of the processes of excitotoxicity and apoptosis underlying SGN degeneration. The second aim will determine whether NFKB activity is required to maintain Ca2+ homeostasis in SGNs after these acute insults. These experiments employ electrophysiological, histopathological and immunoflourescence techniques, along with those using electrophoretic mobility shift assay (EMSA) and real-time reverse transcription polymerase chain reaction (RT-PCR). The results of these studies will lead to a better understanding of the cellular and molecular mechanisms of SGN degeneration and will contribute to the development of novel approaches to the prevention and treatment of sensorineural hearing loss in humans.