Investigating Voltage-Gated Sodium Channel Expression in Rat Spiral Ganglion Neurons Following Noise Induced Hearing Loss

Noise exposure has been shown to elevate hearing thresholds by damaging the cochlea through hair cell damage and the excitotoxic, excessive release of glutamate at the hair cell afferent spiral ganglion neuron (SGN) synapse. This excitotoxicity results in the loss of synaptic innervation of the neuron from the hair cell. In somatosensory neurons a similar axotomy results in altered voltage-gated sodium channel (Nav) expression. This study investigates whether similar changes in Nav expression occur in rat SGN following noise exposure. This study has shown the expression of Nav1.1, 1.6 and 1.7 in normal rat SGN using RT-PCR and immunohistochemistry. The noise exposure protocol, 2 sessions presenting a single tone (14.8kHz) at a modest 110dB SPL for 2 hours with 48 hours between each session, resulted with elevated hearing thresholds of the sound-exposed rats of 18±4dB and 24±3dB at 24kHz and 30kHz respectively, with insignificant elevation at frequencies below 16kHz. RT-PCR showed no additional Nav isoforms present in the sound-exposed cochlea. Quantitative PCR showed a significant decrease in Nav1.6 mRNA expression level, reduced by 56% compared to normal hearing animals. Nav1.1 mRNA was significantly reduced by 29% and Nav1.7 mRNA was elevated by 20% when compared to control cochleae. Immunohistochemistry of sound-exposed cochleae showed increased Nav1.1 staining along the peripheral processes of the SGN. Also, staining for Nav1.7 in the SGN was altered in sound-exposed rats compared to control animals, with the majority of sound-exposed animals showing an increase in darker stained neurons. Nav1.6 SGN cell body staining was not altered between sound-exposed and control neurons using immunohistochemistry. SGN cell body counting showed no decrease in neuronal number following sound exposure compared to control cochleae. These results show that Nav expression in rat SGN alters following sound exposure, which may modify neuronal function in deafened animals.