The Effects Of Noise-induced Hearing Loss On Spatial Learning/Memory, Hippocampal Neurogenesis And Immediate Early Genes Expression In Adult Mice

Background and goal:Hearing Loss (HL) and cognitive impairment (CI) are two major categories of neurological disorders with high incidence and serious impact on the quality of human life. The relationships between HL and CI have attracted the attention in medicine for long time. In recent years, serial clinical investigations have shown that HL is an independent risk factor for all-cause dementia, which may be facilitated in its development by HL. However, due to the inevitable confound factors in clinical research (such as the involvement of common causes, the influence of listening ability on cognitive test results, the ethical limitations of research in human subjects), the relationships between HL and CI and the mechanisms for HL to promote CI are not clear at all, and need to be verified in animal experiments. The noise-induced impairment on cognitive function has been reported in many studies, in which the CI was associated with the suppression on hippocampal neurogenesis. However, those previous studies focused on the noise induced stress as the reason of hippocampus damage, and the role of noise induced hearing loss (NIHL) was completely ignored. The preliminary study in our lab showed that only a transient stress response was generated by a noise exposure limited in a short duration; In contrast, decline in cognitive function and hippocampal neurogenesis was found long after the recovery of the stress responses. Those results strongly suggest the role of HL instead of stress. In the present study, a similar NIHL-mouse model was used to observe the impact of HL on spatial learning and memory. The impact of the NIHL on hippocampal neurogenesis was investigated by dynamic observation in many aspects across multiple time points in order to verify whether HL promotes the development of CI via the impairment on hippocampus.Method:The subject and the main tests:healthy adult male CBA mice were used as experimental objects. They were divided into control and noise exposure group in which broadband noise was given at 123 dB SPL for 2h. The hearing threshold was evaluated by auditory brainstem responses (ABR) to evaluate the degree of HL. Spatial learning/memory function was evaluated with the test of Morris water maze (MWM), which also served as a training method for learning and memory. The neurogenesis of hippocampus was examined in many aspects with different methods, including the proliferation, survive, neuronal differentiation by using different markers, the dendrite tree density by Sholl analysis, as well as the expression of several immediate early expression genes (IEGs) related to learning and memory using qPCR.Experimental grouping and design:To rule out the impact of stress, the size of hippocampal stem cell bank and the proliferation rate were compared between groups shortly after the noise exposure. The main examinations were carried out 3 months after the noise exposure (5 months of age). Animals in each group were divided into two sub-groups based upon whether MWM was examined (trained versus not-trained). Brain tissue was harvested at designed time after MWM to exam all the items related to hippocampal neurogenesis.Analysis of the results:The quantities of hippocampal stem cells and newly proliferated cells were compared between the noise and the control groups to rule out the impact of stress. At the 3 months after the noise exposure, the impact of NIHL on spatial learning/memory and hippocampal neurogenesis were verified by comparisons between the NIHL and control groups and further evaluated with correlation analysis. Within each group, the rescue effect of MWM training on the survival of newly generated cells was demonstrated by the comparison between trained and not-trained sub-groups. Furthermore, the NIHL-related reduction in the rescue effect was verified by the comparison between the noise and the control group. Finally, similar analyses were applied to the impact of NIHL on the expression of IEGs.Results:Binaural moderate to severe NIHL was established in CBA mice by a brief and intensive noise exposure. Within a week after the noise exposure, no significant differences were seen in the number of neural stem cells and newborn cells in hippocampus between the two groups, suggesting that the noise exposure used in this study did not impair hippocampal cell proliferation via stress. At the 3 month point after the noise exposure, we found that the animals with NIHL (1) were significantly poorer in spatial learning/memory as tested in MWM; (2) showed reduced effect of MWM training on hippocampal newborn cell survival, neuronal differentiation, integration into neural network, and the expression of IEGs, (3) had less developed dendrite tree.The results summarized above suggest that the NIHL (rather than stress) employed in this study impair the spatial learning/memory functions of the animals; the results are independent of stress. The cognitive function test we used does not require intact auditory function. However, the reduction of auditory input due to hearing loss may have impair the hippocampal function in general, as evidence in the reduction of cell proliferation, and dendrite tree development. Furthermore, the HL impair the effect of the non-auditory behavioral training in promoting the survival of the newborn cells, neuronal differentiation, and network formation and the induced expression of memory related genes. The reduction in the learning/memory related gene expression may be the initial mechanism for the suppression in the later stage of neurogenesis in hippocampus (including the neuronal differentiation, dendrite tree development and network formation).