Hidden Hearing Loss Induced By Different Modes Of Noise Exposure

Objective: Different noise methods were used to establish animal models of noise-induced hearing loss,to explore whether exposed to repeated noise at low-intensity noise(100dB SPL)can do a accumulate damage to auditory synapses in mice;Does the ribbon synapses in the guinea pig cochlea show resistance to low-intensity(106dB SPL)noise after pre-exposed to toughening noise(85dB SPL).Through the establishment of animal models with different noise exposure,the damage of auditory synaptic morphology and function by different noise methods was studied.To explore whether the damage of auditory synapses can accumulate after repeated exposure to trauma noise(100dB SPL);and whether the auditory synapses in cochlea exhibits resistance to higher intensity(106dB SPL)noise after pre-exposure to low intensity(85dB SPL)noise.Methods: In repeated noise exposure experiment,C57 mice with normal hearing were used to perform two 100 dB SPL(4k-22 kHz)noise exposures,and the two noise exposure intervals were 4 weeks.The experimental end points were 1 day,3 weeks and4 weeks after the first noise exposure and 1 day and 4 weeks after the second noise exposure.Two control group was set corresponding with 4 weeks after the first exposure and 4 weeks after the second exposure respectively.To explore the effect of pre-low-intensity noise exposure on synapses,albino guinea pigs with normal hearing were used for 8 hours of 85 dB SPL noise exposure per day for 3 consecutive days,and106 dB SPL(4k-22 kHz)intensity exposure for 2 hours after a week.Observe the effect of long-term low-intensity noise pre-exposure on the number and function of synapses1 day and 3 weeks after traumatic noise exposure.Identify changes in the number of synapses by immunofluorescence staining Ct BP2;assess changes in animal synapse function by auditory electrophysiology tests,including auditory brainstem evoked potential(Auditory brainstem response),compound action potential(including compound action potential),including Click / TB 20 kHz / TB 16 kHz / Amplitude modulation(AM)signal evoked potential.Results: In the repeated low-intensity noise exposure experiment,after a single noise exposure(100dB SPL exposure for 2 hours),the ABR threshold of mice did not change significantly,and the TB 20 kHz/AM CAP showed there was no significant difference,but the Click CAP showed an overall decline in animal hearing function;in terms of morphology,there was a temporary loss of 49% in the number of synapses after a single noise exposure.Then the synapses were repaired and rebuilt,but by the fourth week there were still 32% permanent loss of synapses.After the second noise exposure,the ABR result showed a significant thresholdshift;in terms of morphology,the number of synapses showed a temporary loss of 21.6%,and a permanent loss of13.1%.In experiments pre-exposed to toughen noise,low-intensity noise exposure after toughened noise exposure failed to cause changes in the guinea pig ABR threshold;while click /TB 16 kHz and AM CAP responses are significantly decreased.Morphologically,low intensity noise exposure after toughening noise exposure resulted in a 39.2% reduction in temporary synapse loss,and a 9.3% reduction in permanent synapse loss.Conclusion: After second low-intensity noise exposure,the ABR threshold of the mice increased significantly,thus the CAP could not be extracted.Synapse was damaged,showing cumulative damage effects,but there is no statistical difference;pre-exposure to toughen noise can significantly increase the damage to hearing function of guinea pigs caused by low-intensity noise exposure,while temporary and permanent synapse loss is significantly reduced.