Cochlear Synaptopathy Following Noise Exposure in Guinea Pigs: Its Electrophysiological and Histological Assessments

Exposure to high level of noise,may cause the permanent cochlear synaptic degeneration.In present study,a model of noise induced cochlear synaptopathy was established and the electrophysiological and histological metrics for its assessment was designed.6 guinea pigs were subjected to a synaptopathic noise(octave band of 4 kHz at 104 dB SPL,for 2-h).The amplitude growth curve of Auditory Brainstem Response(ABR)wave-I and wave-III latency shift in presence of noise were calculated.These indexes were considered in pre-exposure,1 day post exposure(1DPE),1 week post exposure(1WPE)and 1 month post exposure(1MPE)to noise.Finally,the samples were histologically analyzed.ABR wave-I amplitude was different between pre and 1DPE(p-value≤0.05).However,at 1WPE,it was recovered at low intensities but at 70 dB SPL and above,the differences persisted even till 1MPE.In masked ABR,the latency shift of wave-III was different between pre and 3 post exposure assessments(p-value≤0.05).Electro-microscopic analysis confirmed the synaptic degeneration,as the ribbons were larger than normal,hollow inside,and spherical and irregular in shape,also,the post synaptic density was abnormally thick and missed its flat orientation.These data revealed that noise at level below that can produce permanent hearing loss,can incur synaptic injury.So,noise is considered to be more damaging than previously thought.Accordingly,designing tools for clinical assessment of synaptopathy is beneficial in comprehensive auditory evaluation of those with history of noise exposure and also in hearing protection planning.

Presenting a Signal to Noise Ratio Model Based on the Combined Effect ofSound Pressure Level/frequency, Exposure Time and Oral Potassium:Experimental Study in Rats

Exposure to noise can lead to anatomical,nonauditory,and auditory impacts.The auditory influence of noise exposure is manifested in the form of Noise-induced hearing loss(NIHL).The current study aimed at present a signal to noise ratio model of otoacoustic emission of rats’ears in the light of the combined effect of sound pressure level,sound frequency,exposure time,and potassium concentration of the used water.In total,36 adult male rates,whose age varied from 3 to 4 months and had a weight of 200±50 g,were randomly divided into 12 groups,with each group consisting of 3 rats.The rats in both groups(case and control groups)were exposed to SPLs of 85,95,and 105 dB,emitted from sources that generated white noise.A distortion product otoacoustic emission(DPOAE)machine(4000 I/O manufactured by Homoth of Germany)was utilized to gauge the signal to noise ratio(SNR)of otoacoustic emissions of rats’ears at various frequencies in an acoustic room.The inclusion criterion was SNR≥6 dB.The collected data were fed into the Statistical Package for Social Sciences(SPSS)version 18,followed by conducting descriptive and inferential data analysis procedures.The results of SNR analysis indicated that over 82%of all data had SNRs that were equal to or greater than 6 dB.These data were considered as acceptable response.Furthermore,SPL and sound frequency had significant associations with SNR(P<0.0001).Exposure time also significantly correlated with SNR(P=0.008).However,the potassium concentration of the used water had no significant correlation with SNR(P=0.97).High sound pressure levels result in lower DPOAE.Furthermore,higher frequency leads to higher SNR.On the contrary,longer exposure time reduces SNR.Finally,the potassium concentration of the used water has no effect on SNR.