Effects Of Noise-Induced Hearing Loss On The Expression Of Nucleic Acid And Protein In Immediate Early Genes Related To Spatial Learning And Memory In Adult Mice

Background and goal:Cognitive impairment(CI)is a serious health problem in modern society,especially affecting the quality of life of aging population.Hearing loss(HL)is one of the most common neurological disorders.Several epidemiological studies has confirmed that HL is an independent risk factor for CI because it can promote or accelerate the decline of cognitive function with aging.In elderly population,HL often appears as sensorineural hearing loss(SNHL).One of the major cause for SNHL is excessive noise exposure.Since the noiseinduced hearing loss(NIHL)can be established in laboratory setting in a controllable manner,our lab uses this method to investigate the relationship between HL and cognitive function.The connection between auditory system and hippocampus,a key brain area of cognitive function,provides an important theoretical basis for studying the mechanism of HL affecting cognitive function.The effect of noise on learning and memory,and the neurogenesist in hippocampus have been reported in many studies.However,most of them focused on effect of noise-induced oxidative stress to cognitive functions,but ignored the role of HL itself.In the previous study in my lab,we confirmed the effect of NIHL,independent of its oxidative stress,in promoting the decline of cognitive function.To prove this,NIHL was established by exposing to a single short-term intense noise in adult mice.The mice showed only a transient increase in stress response after the noise exposure.The number of neural stem cells in the dentate gyrus(DG)region of hippocampus was not impacted during and shortly after this period.However,the spatial learning and memory abilities were found to be decreased 3 months after the noise exposure,which indicates the independent effect of HL.Further,the decline in spatial cognitive abilities of mice is associated with a decrease in neurogenesis in the DG region.We found that in the hippocampal of NIHL animals,the reduced cell proliferation and the reduced complexity of dendrites,as well as the reduction of the promoting effect of learning activity on neurogenesis diminished in many aspects,including(1)the survival of the newly proliferated cells,(2)the neuronal differentiation and(3)the integration of neurons into neuronal network.To further study the mechanism of NIHL affecting cognitive function,this study mainly examined the potential effects of NIHL on the expression of several IEGs and its upstream transcription factor CREB induced by MWM training.Methods:In this study,75 healthy male CBA/CaJ mice at the age of 6 and 8 weeks old were used as experimental subjects.They were randomly divided into control group and noise exposed group.The mice in the noise group were exposed to an intense broadband noise at 123 dB SPL for 2 hours.Auditory brainstem response(ABR)was used to detect the auditory thresholds of all experimental animals at 1-month post noise(1MPN)and 3 months post noise(3MPN)to evaluate the degree of HL and to determine whether the NIHL-mouse model was successfully established.After 3MPN ABR,the mice in each group were divided into two sub-groups depending on whether or not MWM training was given.The mice in the trained subgroups were further divided into 5,30 and 90 min subgroups after the MWM training.The hippocampus tissues were harvested to obtain RANs and proteins from each groups and the expression of several IEGs and their transcriptional key factor CREB were observed at both mRNA and protein levels by real-time quantitative PCR(RT-qPCR)and Western blot(WB)respectively.Results:The ABR results showed that the a moderate-to-severe HL was established in the mice exposed to the noise and the HL was stable between 1MPN and 3MPN.In consistent with the previous result,the NIHL resulted in a poorer performance in spatial learning at 3MPN in the noise-exposed mice.However,no significant difference in spatial memory was seen between the groups.Among the 6 targeted genes observed,Npas4 is known as the one that is specifically responsive to the learning activties,while the expression of other IEGs and their transcriptional key factor CREB can be initiated by non-learning related activities.The MWM training induced a transient increase of mRNA only in Npas4 out of the 6 targeted genes observed,and no significant effect of MWM training was seen at protein le vel for any IEGs and CREB observed.Importantly,the transient increase in Npas4 mRNA was only seen in the control group but not in the mice with NIHL.Conclusions:The lack of training induced transient increase of Npas4 in the NIHL group suggests that the impact of NIHL on cognitive function is likely initiated from the depression of learning-related activation of IEGs.The null effect of training on other IEGs and CREB indicates that the expression of training induced IEGs may vary with the locations and cell types in hippocampus,besides,the samples used in this experiment for PCR and Western blot were from the intact hippocampus,the induction effect may have been diluted or covered.