| Objective To investigate the changes of the Auditory Brainstem Response thresholds in two sides ears of three groups animals and the expression levels of GABA_BR1 in each layer of the primary auditory cortex one week after a loud noise exposure,we attempt to explore the role which GABA_BR1 playing in the pathogenesis of noise induced hearing loss or auditory functional disorder.Methods A total of 39 SD male rats were 9 weeks old and were evenly placed into one of three groups:(1)unexposed control,(2)unilaterally exposed group(left ear)and(3)bilaterally exposed group(n=13/group).In unilaterally exposed group,acoustic stimuli were emitted from a loudspeaker placed about 2cm from the rat’s external auditory meatus of left ear,and an ear plug was inserted into the right ear canal.Rats were unilaterally exposed to a narrow band noise(12 k Hz,127 d B SPL)for 2 hours.Bilaterally exposed animals were exposed not only in the left side,but another experience in the symmetrically right side.Then we measured the changes of auditory brainstem response(ABR)thresholds one week after acoustic trauma,GABABR expression at protein levels in the primary auditory cortex were compared in three groups using quantitative immunohistochemistry and western blotting methods,respectively.Results 1)ABR measurements were performed for all 39 animals for the left ear and right ear one week after noise exposure.A significant increase of threshold shifts at six frequencies(8,12,16,20,24 and 32 k Hz)of three experimental groups was observed in exposed ear only.In unilaterally exposed group,ABR threshold shifts of left ear at each frequency was 64.6±2.4 d B,67.5±3.3 d B,69.3±1.8 d B,65.4±3.7 d B,57.9±4.3 d B,54.6±3.7 d B respectively;in bilaterally exposed group,ABR threshold shifts of left ear at each frequency was 64.3±1.8 d B,67.1±3.2 d B,69.3±1.8 d B,63.9±4.9 d B,60.0±2.8 d B,55.4±3.7 d B,which one found in right ear was 63.4±3.1 d B,66.4±2.3 d B,68.6±2.3 d B,64.6±3.7 d B,59.6±5.0 d B,53.9±4.0 d B,and these alterations were statistically significant(p < 0.05).However,in control group,the ABR threshold were tested before and after noise exposure had no significant difference(p > 0.05).2)The GABA_BR1 protein levels were investigated one week after acoustic trauma.Noise has obvious impact on GABA_BR1 expression in the ipsilateral AC of unilaterally exposed animals(P < 0.05)and bilateral exposed animals(P < 0.05),although a mild decrease was also found in contralateral cortex of unilaterally exposed groups(P > 0.05).In unilaterally exposed animals,significant differences between the left AC and right AC for GABA_BR1 protein level(P < 0.05)was also observed;besides that,we also found that clear distinctions of GABA_BR1 in the left and right cortex compared unilaterally exposed animals(P < 0.05)to bilateral exposed animals(P < 0.05).For another,as compared to unexposed control,significant decreases of staining intensity of GABA_BR1 were observed in the ipsilateral AI layer VI(p < 0.05)of unilaterally exposed animals and the ipsilateral AI layers V-VI(p < 0.05)as well as contralateral AI layer IV(p < 0.05),layer V(p < 0.05)and layer VI(p < 0.01)of bilaterally exposed rats.Conclusion In this study,the ABR thresholds were obviously increase after a loud noise exposure.The expression levels of GABA_BR1 protein in the ipsilateral AC were significant down-regulation,at the same time,the one which in the contralateral cortex was also a mild decrease.It is worthy note that the largest noise-related reductions were found in layer VI.This noise-related change of subunit may contribute to the development of noise induced hearing loss(NIHL). |
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