Effects of chronic noise stress on spatial memory of rats in relation to neuronal dendritic alteration and free radical-imbalance in hippocampus and medial prefrontal cortex

Ambient Air Pollution, Noise, and Late-Life Cognitive Decline and Dementia Risk

Exposure to ambient air pollution and noise is ubiquitous globally. A strong body of evidence links air pollution, and recently noise, to cardiovascular conditions that eventually may also affect cognition in the elderly. Data that support a broader influence of these exposures on cognitive function during aging is just starting to emerge. This review summarizes current findings and discusses methodological challenges and opportunities for research. Although current evidence is still limited, especially for chronic noise exposure, high exposure has been associated with faster cognitive decline either mediated through cerebrovascular events or resulting in Alzheimer's disease. Ambient environmental exposures are chronic and affect large populations. While they may yield relatively modest-sized risks, they nevertheless result in large numbers of cases. Reducing environmental pollution is clearly feasible, though lowering levels requires collective action and long-term policies such as standard setting, often at the national level as well as at the local level.

The Links Between the Gut Microbiome, Aging, Modern Lifestyle and Alzheimer's Disease

Gut microbiome is a community of microorganisms in the gastrointestinal tract. These bacteria have a tremendous impact on the human physiology in healthy individuals and during an illness. Intestinal microbiome can influence one's health either directly by secreting biologically active substances such as vitamins, essential amino acids, lipids et cetera or indirectly by modulating metabolic processes and the immune system. In recent years considerable information has been accumulated on the relationship between gut microbiome and brain functions. Moreover, significant quantitative and qualitative changes of gut microbiome have been reported in patients with Alzheimer's disease. On the other hand, gut microbiome is highly sensitive to negative external lifestyle aspects, such as diet, sleep deprivation, circadian rhythm disturbance, chronic noise, and sedentary behavior, which are also considered as important risk factors for the development of sporadic Alzheimer's disease. In this regard, this review is focused on analyzing the links between gut microbiome, modern lifestyle, aging, and Alzheimer's disease.

Low cognitive competence as a vulnerability factor for behavioral despair in rats

Elucidating the multi-faceted relationship between cognitive competence and affective states is a major pursuit in behavioral sciences. Mood disorders constitute a good research model for this question, as cognitive impairment may accompany clinical depression and persist after full remission. This suggests cognitive dysfunction as an etiological factor of depression, rather than an epiphenomenon. Complementing clinical studies, animal models utilizing well-controlled, systematic paradigms are essential to elucidate the complex relationship between cognitive competence and affective states. In current set of experiments, we investigated the extent to which cognitive competence determines the stress response in Wistar rats by utilizing two well-established spatial memory paradigms with different degrees of complexity together with the forced swim test. We revealed that rats with low cognitive competence as assessed by learning performance in the Y-Maze, but not in the radial arm maze, were significantly more vulnerable to behavioral despair. In contrast, rats with high cognitive competence were resilient to the negative effects of the forced swim test, irrespective of the spatial memory task used. These results point to a nonlinear relationship between spatial memory performance and behavioral despair, suggesting that different types of cognitive functioning may have differential effects on affective processes.

Life-Course Contribution of Prenatal Stress in Regulating the Neural Modulation Network Underlying the Prepulse Inhibition of the Acoustic Startle Reflex in Male Alzheimer's Disease Mice

The prepulse inhibition (PPI) of the acoustic startle reflex (ASR), as an index of sensorimotor gating, is one of the most extensively used paradigms in the field of neuropsychiatric disorders. Few studies have examined how prenatal stress (PS) regulates the sensorimotor gating during the lifespan and how PS modifies the development of amyloid-beta (Aβ) pathology in brain areas underlying the PPI formation. We followed alternations in corticosterone levels, learning and memory, and the PPI of the ASR measures in APPNL-G-F/NL-G-F offspring of dams exposed to gestational noise stress. In-depth quantifications of the Aβ plaque accumulation were also performed at 6 months. The results indicated an age-dependent deterioration of sensorimotor gating, long-lasting PS-induced abnormalities in PPI magnitudes, as well as deficits in spatial memory. The PS also resulted in a higher Aβ aggregation predominantly in brain areas associated with the PPI modulation network. The findings suggest the contribution of a PS-induced hypothalamic-pituitary-adrenal (HPA) axis hyperactivity in regulating the PPI modulation substrates leading to the abnormal development of the neural protection system in response to disruptive stimuli. The long-lasting HPA axis dysregulation appears to be the major underlying mechanism in precipitating the Aβ deposition, especially in brain areas contributed to the PPI modulation network.

Transcriptome analysis of the hippocampus in environmental noise-exposed SAMP8 mice reveals regulatory pathways associated with Alzheimer's disease neuropathology

Background

Chronic noise exposure is one environmental hazard that is associated with genetic susceptibility factors that increase Alzheimer’s disease (AD) pathogenesis. However, the comprehensive understanding of the link between chronic noise stress and AD is limited. Herein, we investigated the effects of chronic noise exposure on AD-like changes in senescence-accelerated mouse prone 8 (SAMP8).

Methods

A total of 30 male SAMP8 mice were randomly divided into the noise-exposed group, the control group, and aging group (positive controls), and mice in the exposure group were exposed to 98 dB SPL white noise for 30 consecutive days. Transcriptome analysis and AD-like neuropathology of hippocampus were examined by RNA sequencing and immunoblotting. Enzyme-linked immunosorbent assay and real-time PCR were used to further determine the differential gene expression and explore the underlying mechanisms of chronic noise exposure in relation to AD at the genome level.

Results

Chronic noise exposure led to amyloid beta accumulation and increased the hyperphosphorylation of tau at the Ser202 and Ser404 sites in young SAMP8 mice; similar observations were noted in aging SAMP8 mice. We identified 21 protein-coding transcripts that were differentially expressed: 6 were downregulated and 15 were upregulated after chronic noise exposure; 8 genes were related to AD. qPCR results indicated that the expression of Arc, Egr1, Egr2, Fos, Nauk1, and Per2 were significantly high in the noise exposure group. These outcomes mirrored the results of the RNA sequencing data.

Conclusions

These findings further revealed that chronic noise exposure exacerbated aging-like impairment in the hippocampus of the SAMP8 mice and that the protein-coding transcripts discovered in the study may be key candidate regulators involved in environment-gene interactions.

Crucial role for Nox2 and sleep deprivation in aircraft noise-induced vascular and cerebral oxidative stress, inflammation, and gene regulation

Aims

Aircraft noise causes endothelial dysfunction, oxidative stress, and inflammation. Transportation noise increases the incidence of coronary artery disease, hypertension, and stroke. The underlying mechanisms are not well understood. Herein, we investigated effects of phagocyte-type NADPH oxidase (Nox2) knockout and different noise protocols (around-the-clock, sleep/awake phase noise) on vascular and cerebral complications in mice.

Methods and results

C57BL/6j and Nox2^−/− (gp91phox^−/−) mice were exposed to aircraft noise (maximum sound level of 85 dB(A), average sound pressure level of 72 dB(A)) around-the-clock or during sleep/awake phases for 1, 2, and 4 days. Adverse effects of around-the-clock noise on the vasculature and brain were mostly prevented by Nox2 deficiency. Around-the-clock aircraft noise of the mice caused the most pronounced vascular effects and dysregulation of Foxo3/circadian clock as revealed by next generation sequencing (NGS), suggesting impaired sleep quality in exposed mice. Accordingly, sleep but not awake phase noise caused increased blood pressure, endothelial dysfunction, increased markers of vascular/systemic oxidative stress, and inflammation. Noise also caused cerebral oxidative stress and inflammation, endothelial and neuronal nitric oxide synthase (e/nNOS) uncoupling, nNOS mRNA and protein down-regulation, and Nox2 activation. NGS revealed similarities in adverse gene regulation between around-the-clock and sleep phase noise. In patients with established coronary artery disease, night-time aircraft noise increased oxidative stress, and inflammation biomarkers in serum.

Conclusion

Aircraft noise increases vascular and cerebral oxidative stress via Nox2. Sleep deprivation and/or fragmentation caused by noise triggers vascular dysfunction. Thus, preventive measures that reduce night-time aircraft noise are warranted.

Exposure of Developing Male Rats to One or Multiple Noise Sessions and Different Housing Conditions: Hippocampal Thioredoxin Changes and Behavioral Alterations

Exposure of developing rats to noise has shown to induce hippocampal-related behavioral alterations that were prevented after a week of housing in an enriched environment. However, neither the effect of repeated exposures nor its impact on key endogenous antioxidants had been studied yet. Thus, the aim of the present work was to reveal novel data about hippocampal oxidative state through the measurement of possible age-related differences in the levels of hippocampal thioredoxins in rats exposed to noise at different developmental ages and subjected to different schemes and housing conditions. In addition, the possibility that oxidative changes could underlie hippocampal-related behavioral changes was also analyzed. Developing male Wistar rats were exposed to noise for 2 h, either once or for 5 days. Upon weaning, some animals were transferred to an enriched cage for 1 week, whereas others were kept in standard cages. One week later, auditory and behavioral assessments, as well as measurement of hippocampal thioredoxin, were performed. Whereas no changes in the auditory function were observed, significant behavioral differences were found, that varied according to the age, scheme of exposure and housing condition. In addition, a significant increase in Trx-1 levels was found in all noise-exposed groups housed in standard cages. Housing animals in an enriched environment for 1 week was effective in preventing most of these changes. These findings suggest that animals become less susceptible to undergo behavioral alterations after repeated exposure to an environmental challenge, probably due to the ability of adaptation to an unfavorable condition. Moreover, it could be hypothesized that damage to younger individuals could be more easily prevented by a housing manipulation.

Environmental noise exposure modifies astrocyte morphology in hippocampus of young male rats

Background:

Chronic exposure to noise induces changes on the central nervous system of exposed animals. Those changes affect not only the auditory system but also other structures indirectly related to audition. The hippocampus of young animals represents a potential target for these effects because of its essential role in individuals’ adaptation to environmental challenges.

Objective:

The aim of the present study was to evaluate hippocampus vulnerability, assessing astrocytic morphology in an experimental model of environmental noise (EN) applied to rats in pre-pubescent stage.

Materials and Methods:

Weaned Wistar male rats were subjected to EN adapted to the rats’ audiogram for 15 days, 24 h daily. Once completed, plasmatic corticosterone (CORT) concentration was quantified, and immunohistochemistry for glial fibrillary acidic protein was taken in hippocampal DG, CA3, and CA1 subareas. Immunopositive cells and astrocyte arborizations were counted and compared between groups.

Results:

The rats subjected to noise exhibited enlarged length of astrocytes arborizations in all hippocampal subareas. Those changes were accompanied by a marked rise in serum CORT levels.

Conclusions:

These findings confirm hippocampal vulnerability to EN and suggest that glial cells may play an important role in the adaptation of developing the participants to noise exposure.

Effects of combined traffic noise on the synaptic ultrastructure and expressions of p-CaMKII and NMDAR1 in the hippocampus of young SD rats

In order to explore the effects of combined traffic noise (CTN) on learning and memory function, young Sprague-Dawley (SD) rats were exposed to CTN from highway and high-speed railway for 52 days, whose day-night equivalent continuous A-weighted sound pressure level (L_dn) was 70 dB(A) (corresponding sound pressure level was 80 dB). The synaptic ultrastructure and the expressions of phosphorylated calcium/calmodulin-dependent protein kinase II (p-CaMKII) and N-methyl-D-aspartate receptor 1 (NMDAR1 or NR1) in the hippocampus were tested by transmission electron microscopy (TEM) and Western blot, respectively. Results showed that there was no significant difference in the synaptic ultrastructure and the expressions of p-CaMKII and NR1 in the hippocampus of young rats between the experimental group and control group. Compared with single high-speed railway noise (HSRN) with L_dn of 70 dB(A), CTN had less influences on learning and memory function, which was closely related to smaller intermittency of CTN and less anxiety caused by CTN. In comparison with white noise with a sound pressure level of 80 dB, CTN had less impacts on learning and memory function, which was mainly associated with CTN's smaller R-weighted sound pressure level based on rats' auditory sensitivity.

Animal Models of Early-Life Adversity

From the prenatal period throughout the first years of life, the brain undergoes its most rapid development, a period during which it is highly sensitive to external experiences. The timing of brain development differs from one region to another, as it also differs between substrates, neurotransmitter systems, and central endocrine circuitries. These discontinuities are part of the "critical periods of brain development." Early-life adversity (ELA), such as exposure to infection, maternal deprivation, and substance use, disrupts the programmed brain development, yielding a myriad of deviations in brain circuitry, stress responsivity, cognitive function, and general health. This is applicable to both humans and animal models.In our laboratory, several experimental animal designs have been developed that allow investigating the long-lasting consequences of ELA on brain function, cognitive and emotional development, and the risk to develop stress-related psychopathology later in adulthood. This book chapter will provide a review of such animal models, in particular, designs related to infections (LPS-induced), the quality of mother-infant relationship (maternal deprivation and separation), and substance use (ethanol intoxication). The behavior tests, biochemical, and immunohistochemistry assays applied after ELA will be explained. The behavioral tests encompass the open-field, elevated plus maze, forced swim, sucrose preference, Y-maze, object recognition, and Morris water maze tests. These experiments allow the assessment of several outcomes of interest, pertaining to locomotor activity, anxiety-like symptoms, depressive-like symptoms, working memory, recognition memory, spatial memory, and learning performance. The biochemical assays are employed to measure the level of oxidative stress and inflammation in brain areas after application of adversity. Immunohistochemistry puts into perspective the degree of immunoreactivity in the brain subjected to adversity. The findings from our laboratory indicate that the nature and timing of exposure play a critical role in sensitivity to develop neurodevelopmental disorders.

Chronic noise exposure exacerbates AD-like neuropathology in SAMP8 mice in relation to Wnt signaling in the PFC and hippocampus

Non-genetic environmental hazards are thought to be associated with genetic susceptibility factors that increase Alzheimer’s disease (AD) pathogenesis. Aging and chronic noise exposure have been considered important factors in the AD. Here, we investigated the impact of chronic noise exposure on the AD-like neuropathology in the senescence-accelerated prone mouse (SAMP8) and the underlying mechanisms of such effects. We examined the consequences of AD-like neuropathology in 3-month-old SAMP8 mice using low- and high-intensity noise exposure and 8-month-old SAMP8 mice as aging positive controls. Immunoblotting and immunohistochemistry were conducted to examine AD-like pathological changes and potential mechanisms. Chronic noise exposure led to progressive overproduction of Aβ and increased the hyperphosphorylation of tau at Ser396, Thr205, and Thr231 sites in the hippocampus and the prefrontal cortex (PFC) in young SAMP8 mice, similar to that observed in aging SAMP8 mice. Both noise exposure and aging could cause a significant downregulation in Wnt signaling expression. These findings demonstrate that chronic noise stress exacerbated AD-like neuropathology, possibly by disrupting Wnt signaling and triggering aberrant tau hyperphosphorylation and Aβ in the PFC and hippocampus.

Effects of chronic noise exposure on the microbiome-gut-brain axis in senescence-accelerated prone mice: implications for Alzheimer's disease

Background

Chronic noise exposure is associated with neuroinflammation and gut microbiota dysregulation and increases the risk of Alzheimer’s disease (AD). Environmental hazards are also thought to be associated with genetic susceptibility factors that increase AD pathogenesis. However, there is limited experimental evidence regarding the link between chronic noise stress and microbiome-gut-brain axis alterations, which may be closely related to AD development.

Methods

The aim of the present study was to systematically investigate the effects of chronic noise exposure on the microbiome-gut-brain axis in the senescence-accelerated mouse prone 8 (SAMP8) strain. We established SAMP8 mouse models to examine the consequences of noise exposure on the microbiome-gut-brain axis. Hippocampal amyloid-β (Aβ) assessment and the Morris water maze were used to evaluate AD-like changes, 16S ribosomal RNA sequencing analyses were used for intestinal flora measurements, and assessment of endothelial tight junctions and serum neurotransmitter and inflammatory mediator levels, as well as fecal microbiota transplant, was conducted to explore the underlying pathological mechanisms.

Results

Chronic noise exposure led to cognitive impairment and Aβ accumulation in young SAMP8 mice, similar to that observed in aging SAMP8 mice. Noise exposure was also associated with decreased gut microbiota diversity and compositional alterations. Axis-series studies showed that endothelial tight junction proteins were decreased in both the intestine and brain, whereas serum neurotransmitter and inflammatory mediator levels were elevated in young SAMP8 mice exposed to chronic noise, similar to the observations made in the aging group. The importance of intestinal bacteria in noise exposure-induced epithelial integrity impairment and Aβ accumulation was further confirmed through microbiota transplantation experiments. Moreover, the effects of chronic noise were generally intensity-dependent.

Conclusion

Chronic noise exposure altered the gut microbiota, accelerated age-related neurochemical and inflammatory dysregulation, and facilitated AD-like changes in the brain of SAMP8 mice.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1223-4) contains supplementary material, which is available to authorized users.

Physical enrichment enhances memory function by regulating stress hormone and brain acetylcholinesterase activity in rats exposed to restraint stress

To study the effects of stress on mental health activity is of great importance in neuropsychological studies as it may affect the lifelong performance related to brain and overall health and wellbeing of an individual. It is observed very often that exposure to stress during early life can alter the brain function which may reflect as cognitive disability. Impairment of memory is associated with increased oxidative stress which is due to enhanced production of free radicals that may lead to lipid peroxidation and disintegration of cell structure and functions. Exposure to enriched environment has shown to enhance spatial learning and memory, although the underlying mechanism covering the regulation of antioxidant capacity is limited. Here we investigated short and long term memory using Morris water maze before and after giving restraint stress procedure in rats exposed to social and physically enriched environment. Levels of malondialdehyde (MDA), activity of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and acetylcholinesterase (AChE) in brain tissue were estimated. Plasma corticosterone was also determined after decapitation. Results demonstrated that rats pre-exposed to physical along with social enrichment showed improved short and long term memory as compared to control group. However, restraint stress exerted differential effects in socially and physically enriched groups. Reduced lipid peroxidation and decreased activity of SOD, GPx and AChE were observed in physically enriched rats subjected to stress as compared to stressed rats kept in social environment. Levels of corticosterone were also found to be significantly reduced in rats kept in physically enriched environment. This study shows the beneficial effects of environmental enrichment on learning and spatial memory by reducing oxidative stress via reducing lipid peroxidation and regulation of antioxidant enzymes in rats.

Chronic noise-exposure exacerbates insulin resistance and promotes the manifestations of the type 2 diabetes in a high-fat diet mouse model

Epidemiological studies have revealed that noise exposure was associated with an increased risk of type 2 diabetes mellitus (T2DM). However, the exact nature of that association remains to be elucidated. The present study is designed to examine the effects of chronic noise exposure on the development of T2DM in combination with a high-fat-diet (HFD) in mice. Here we show that chronic noise exposure at 85 dB SPL (4 h /day, below the safety limit for occupational noise exposure) exaggerated multiple metabolic abnormalities induced by HFD in C57BL/6J male mice, including worsened glucose intolerance, insulin resistance, fasting hyperglycemia and dyslipidemia. Furthermore, noise exposure exhibited a paradoxical impact on fat accumulation and circulating levels of free fatty acid, indicating a potential stimulating effect of noise on lipolysis. These results provide first in vivo supporting evidence for the causative role of noise exposure in diabetogenesis and pinpoint a noise-associated increase in blood free fatty acid levels as a possible mediator accelerating the effect of noise on the development of insulin resistance and T2DM.

The effect of noise exposure on insulin sensitivity in mice may be mediated by the JNK/IRS1 pathway

Background

Epidemiological studies have suggested that noise exposure may increase the risk of type 2 diabetes mellitus (T2DM), and experimental studies have demonstrated that noise exposure can induce insulin resistance in rodents. The aim of the present study was to explore noise-induced processes underlying impaired insulin sensitivity in mice.

Methods

Male ICR mice were randomly divided into four groups: a control group without noise exposure and three noise groups exposed to white noise at a 95-dB sound pressure level for 4 h/day for 1, 10, or 20 days (N1D, N10D, and N20D, respectively). Systemic insulin sensitivity was evaluated at 1 day, 1 week, and 1 month post-noise exposure (1DPN, 1WPN, and 1MPN) via insulin tolerance tests (ITTs). Several insulin-related processes, including the phosphorylation of Akt, IRS1, and JNK in the animals’ skeletal muscles, were examined using standard immunoblots. Biomarkers of inflammation (circulating levels of TNF-α and IL-6) and oxidative stress (SOD and CAT activities and MDA levels in skeletal muscles) were measured via chemical analyses.

Results

The data obtained in this study showed the following: (1) The impairment of systemic insulin sensitivity was transient in the N1D group but prolonged in the N10D and N20D groups. (2) Noise exposure led to enhanced JNK phosphorylation and IRS1 serine phosphorylation as well as reduced Akt phosphorylation in skeletal muscles in response to exogenous insulin stimulation. (3) Plasma levels of TNF-α and IL-6, CAT activity, and MDA concentrations in skeletal muscles were elevated after 20 days of noise exposure.

Conclusions

Impaired insulin sensitivity in noise-exposed mice might be mediated by an enhancement of the JNK/IRS1 pathway. Inflammation and oxidative stress might contribute to insulin resistance after chronic noise exposure.

Metabolic Abnormalities of Erythrocytes as a Risk Factor for Alzheimer's Disease

Alzheimer's disease (AD) is a slowly progressive, neurodegenerative disorder of uncertain etiology. According to the amyloid cascade hypothesis, accumulation of non-soluble amyloid β peptides (Aβ) in the Central Nervous System (CNS) is the primary cause initiating a pathogenic cascade leading to the complex multilayered pathology and clinical manifestation of the disease. It is, therefore, not surprising that the search for mechanisms underlying cognitive changes observed in AD has focused exclusively on the brain and Aβ-inducing synaptic and dendritic loss, oxidative stress, and neuronal death. However, since Aβ depositions were found in normal non-demented elderly people and in many other pathological conditions, the amyloid cascade hypothesis was modified to claim that intraneuronal accumulation of soluble Aβ oligomers, rather than monomer or insoluble amyloid fibrils, is the first step of a fatal cascade in AD. Since a characteristic reduction of cerebral perfusion and energy metabolism occurs in patients with AD it is suggested that capillary distortions commonly found in AD brain elicit hemodynamic changes that alter the delivery and transport of essential nutrients, particularly glucose and oxygen to neuronal and glial cells. Another important factor in tissue oxygenation is the ability of erythrocytes (red blood cells, RBC) to transport and deliver oxygen to tissues, which are first of all dependent on the RBC antioxidant and energy metabolism, which finally regulates the oxygen affinity of hemoglobin. In the present review, we consider the possibility that metabolic and antioxidant defense alterations in the circulating erythrocyte population can influence oxygen delivery to the brain, and that these changes might be a primary mechanism triggering the glucose metabolism disturbance resulting in neurobiological changes observed in the AD brain, possibly related to impaired cognitive function. We also discuss the possibility of using erythrocyte biochemical aberrations as potential tools that will help identify a risk factor for AD.

Neuroimmune Impacts of Early-Life Stress on Development and Psychopathology

Maltreatment and trauma in childhood, termed early-life stress (ELS), has long-term effects on the immune system. ELS impacts immune signaling at the time of exposure but also disrupts the developmental trajectory of certain immunological processes, both in the periphery and in the brain. One consequence of these early alterations is a heightened immune response to stressors later in life. However, chronic and sustained inflammatory response can also lead to excitotoxicity and prevent typical brain development. In this chapter, we discuss current progress toward understanding the contribution of neuroimmune signaling to ELS-attributable dysfunction or maladaptation with a focus on postnatal experiences. To do so we first present an operational definition of ELS. Then, we offer a brief overview of the immune system and neuroimmune development, followed by a section discussing the interaction between immunity, childhood trauma, and mental disorders in humans. We present evidence from animal models about immune alterations after ELS and discuss the ways in which ELS-induced immune changes ultimately affect brain and behavior, as well as the importance of individual differences and future directions in this field. Taken together, we submit that when encountered with ELS, some core brain circuits could develop differently via various mechanisms involving dysfunctional immune reprograming. However, given the remarkable plasticity of both the brain and the immune system, many of the deleterious effects of ELS may be mitigated with interventions that account for sex and target neuroimmune interactions over the lifespan.

A single episode of high intensity sound inhibits long-term potentiation in the hippocampus of rats

Exposure to loud sounds has become increasingly common. The most common consequences of loud sound exposure are deafness and tinnitus, but emotional and cognitive problems are also associated with loud sound exposure. Loud sounds can activate the hipothalamic-pituitary-adrenal axis resulting in the secretion of corticosterone, which affects hippocampal synaptic plasticity. Previously we have shown that long-term exposure to short episodes of high intensity sound inhibited hippocampal long-term potentiation (LTP) without affecting spatial learning and memory. Here we aimed to study the impact of short term loud sound exposure on hippocampal synaptic plasticity and function. We found that a single minute of 110 dB sound inhibits hippocampal Schaffer-CA1 LTP for 24 hours. This effect did not occur with an 80-dB sound exposure, was not correlated with corticosterone secretion and was also observed in the perforant-dentate gyrus synapse. We found that despite the deficit in the LTP these animals presented normal spatial learning and memory and fear conditioning. We conclude that a single episode of high-intensity sound impairs hippocampal LTP, without impairing memory and learning. Our results show that the hippocampus is very responsive to loud sounds which can have a potential, but not yet identified, impact on its function.

Behavioral Animal Model of the Emotional Response to Tinnitus and Hearing Loss

Increased prevalence of emotional distress is associated with tinnitus and hearing loss. The underlying mechanisms of the negative emotional response to tinnitus and hearing loss remain poorly understood, and it is challenging to disentangle the emotional consequences of hearing loss from those specific to tinnitus in listeners experiencing both. We addressed these questions in laboratory rats using three common rodent anxiety screening assays: elevated plus maze, open field test, and social interaction test. Open arm activity in the elevated plus maze decreased substantially after one trial in controls, indicating its limited utility for comparing pre- and post-treatment behavior. Open field exploration and social interaction behavior were consistent across multiple sessions in control animals. Individual sound-exposed and salicylate-treated rats showed a range of phenotypes in the open field, including reduced entries into the center in some subjects and reduced locomotion overall. In rats screened for tinnitus, less locomotion was associated with higher tinnitus scores. In salicylate-treated animals, locomotion was correlated with age. Sound-exposed and salicylate-treated rats also showed reduced social interaction. These results suggest that open field exploratory activity is a selective measure for identifying tinnitus distress in individual animals, whereas social interaction reflects the general effects of hearing loss. This animal model will facilitate future studies of the structural and functional changes in the brain pathways underlying emotional distress associated with hearing dysfunction, as well as development of novel interventions to ameliorate or prevent negative emotional responses.

Prenatal noise stress impairs HPA axis and cognitive performance in mice

Noise stress is a common environmental pollutant whose adverse effect on offspring performance has been less studied. This study was novel in terms of using “noise” as a prenatal stress compared with physical stress to explore the effect of stress during gestation on HPA axis activation, cognitive performance, and motor coordination, as well as in investigating the effect of behavioral assessments on the corticosterone (CORT) levels. Three groups of C57BL/6 mice with a gestational history of either noise stress (NS), physical stress (PS), or no stress were examined in several behavioral tests. Plasma CORT level was significantly higher before starting the behavioral tests in NS group than the two other groups. It was significantly increased after the behavioral tests in both prenatal stressed groups relative to the controls. Stress caused anxiety-like behavior and reduced learning and memory performance in both stressed groups compared to the controls, as well as decreased motor coordination in the NS group relative to the other groups. The findings suggested that: prenatal NS severely changes the HPA axis; both prenatal stressors, and particularly NS, negatively impair the offspring’s cognitive and motor performance; and, they also cause a strong susceptibility to interpret environmental experiences as stressful conditions.

Loud Noise Exposure Produces DNA, Neurotransmitter and Morphological Damage within Specific Brain Areas

Exposure to loud noise is a major environmental threat to public health. Loud noise exposure, apart from affecting the inner ear, is deleterious for cardiovascular, endocrine and nervous systems and it is associated with neuropsychiatric disorders. In this study we investigated DNA, neurotransmitters and immune-histochemical alterations induced by exposure to loud noise in three major brain areas (cerebellum, hippocampus, striatum) of Wistar rats. Rats were exposed to loud noise (100 dBA) for 12 h. The effects of noise on DNA integrity in all three brain areas were evaluated by using Comet assay. In parallel studies, brain monoamine levels and morphology of nigrostriatal pathways, hippocampus and cerebellum were analyzed at different time intervals (24 h and 7 days) after noise exposure. Loud noise produced a sudden increase in DNA damage in all the brain areas under investigation. Monoamine levels detected at 7 days following exposure were differently affected depending on the specific brain area. Namely, striatal but not hippocampal dopamine (DA) significantly decreased, whereas hippocampal and cerebellar noradrenaline (NA) was significantly reduced. This is in line with pathological findings within striatum and hippocampus consisting of a decrease in striatal tyrosine hydroxylase (TH) combined with increased Bax and glial fibrillary acidic protein (GFAP). Loud noise exposure lasting 12 h causes immediate DNA, and long-lasting neurotransmitter and immune-histochemical alterations within specific brain areas of the rat. These alterations may suggest an anatomical and functional link to explain the neurobiology of diseases which prevail in human subjects exposed to environmental noise.

Associations of long-term exposure to air pollution and road traffic noise with cognitive function-An analysis of effect measure modification

BACKGROUND

Adverse effects of traffic-related air pollution (AP) and noise on cognitive functions have been proposed, but little is known about their interactions and the combined effect of co-exposure.

METHODS

Cognitive assessment was completed by 4086 participants of the population-based Heinz Nixdorf Recall cohort study using five neuropsychological subtests and an additively calculated global cognitive score (GCS). We assessed long-term residential concentrations for size-fractioned particulate matter (PM) and nitrogen oxides with land use regression. Road traffic noise (weighted 24-h (L_DEN) and night-time (L_NIGHT) means) was assessed according to the EU directive 2002/49/EC. Linear regression models adjusted for individual-level characteristics were calculated to estimate effect modification of associations between AP and noise with cognitive function. We used multiplicative interaction terms and categories of single or double high exposure, dichotomizing the potential effect modifier at the median (AP) or at an a priori defined threshold (road traffic noise).

RESULTS

In fully adjusted models, high noise exposure increased the association of AP with cognitive function. For example, for an interquartile range increase of PM_2.5 (IQR 1.43), association s with GCS were: estimate (β)=-0.16 [95% confidence interval: -0.33; 0.01] and β=-0.48 [-0.72; -0.23] for low and high L_DEN, respectively. The association of noise with GCS was restricted to highly AP-exposed participants. We observed stronger negative associations in those participants with double exposure compared to the addition of effect estimates of each single exposure.

CONCLUSIONS

Our study suggests that AP and road traffic noise might act synergistically on cognitive function in adults.

Effects of environmental noise exposure on DNA methylation in the brain and metabolic health

Environmental noise exposure is associated with adverse effects on human health including hearing loss, heart disease, and changes in stress-related hormone levels. Alteration in DNA methylation in response to environmental exposures is a well-known phenomenon and it is implicated in many human diseases. Understanding how environmental noise exposures affect DNA methylation patterns may help to elucidate the link between noise and adverse effects on health. In this pilot study we examined the effects of environmental noise exposure on DNA methylation of genes related to brain function and investigated whether these changes are related with metabolic health. We exposed four groups of male Wistar rats to moderate intensity noise (70-75dB with 20-4000Hz) at night for three days as short-term exposure, and for three weeks as long-term exposure. Noise exposure was limited to 45dB during the daytime. Control groups were exposed to only 45dB, day and night. We measured DNA methylation in the Bdnf, Comt, Crhr1, Mc2r, and Snca genes in tissue from four brain regions of the rats (hippocampus, frontal lobe, medulla oblongata, and inferior colliculus). Further, we measured blood pressure and body weight after long-term noise exposure. We found that environmental noise exposure is associated with gene-specific DNA methylation changes in specific regions of the brain. Changes in DNA methylation are significantly associated with changes in body weight (between Bdnf DNA methylation and Δ body weight: r=0.59, p=0.018; and between LINE-1 ORF DNA methylation and Δ body weight: =-0.80, p=0.0004). We also observed that noise exposure decreased blood pressure (p=0.038 for SBP, p=0.017 for DBP and p 0. 017 for MAP) and decreased body weight (β=-26g, p=0.008). In conclusion, environmental noise exposures can induce changes in DNA methylation in the brain, which may be associated with adverse effects upon metabolic health through modulation of response to stress-related hormones.

Antioxidant mediated response of Scoparia dulcis in noise-induced redox imbalance and immunohistochemical changes in rat brain

Noise has been regarded as an environmental/occupational stressor that causes damages to both auditory and non-auditory organs. Prolonged exposure to these mediators of stress has often resulted in detrimental effect, where oxidative/nitrosative stress plays a major role. Hence, it would be appropriate to examine the possible role of free radicals in brain discrete regions and the "antioxidants" mediated response of S. dulcis. Animals were subjected to noise stress for 15 days (100 dB/4 hours/day) and estimation of endogenous free radical and antioxidant activity were carried out on brain discrete regions (the cerebral cortex, cerebellum, brainstem, striatum, hippocampus and hypothalamus). The result showed that exposure to noise could alleviate endogenous free radical generation and altered antioxidant status in brain discrete regions when compared to that of the control groups. This alleviated free radical generation (H2O2 and NO) is well supported by an upregulated protein expression on immunohistochemistry of both iNOS and nNOS in the cerebral cortex on exposure to noise stress. These findings suggest that increased free radical generation and altered anti-oxidative status can cause redox imbalance in the brain discrete regions. However, free radical scavenging activity of the plant was evident as the noise exposed group treated with S. dulcis[200 mg/(kg·b·w)] displayed a therapeutic effect by decreasing the free radical level and regulate the anti-oxidative status to that of control animals. Hence, it can be concluded that the efficacy of S. dulcis could be attributed to its free radical scavenging activity and anti-oxidative property.

Role of Scoparia dulcis linn on noise-induced nitric oxide synthase (NOS) expression and neurotransmitter assessment on motor function in Wistar albino rats

Noise pollution is one of the most widespread and fast growing environmental and occupational menaces in the modern era. Exposure to noise above 100dB is not adaptable through the brain homeostatic mechanism. Yet, the detrimental effects of noise have often been ignored. Developing reliable animal models to understand the neurobiology of noise stress and advance our research in the field of medicine to impede this growing stressor is needed. In this study experimental animals were divided into four groups, (i) Control and (ii) S. dulcis extract (200mg/kgbw) treated control group. (iii) To mimic the influence of noise, animals in this group were exposed to noise stress (100dB/4h/day) for 15days and finally, (iv) Noise exposed treated with S. dulcis extract (200mg/kgbw) group. Rota-rod and narrow beam performance results showed impaired motor co-ordination in noise exposed group on both 1st and 15th day when compared to controls. This impaired motor function on exposure to noise could be attributed to the altered norepinephrine, dopamine and serotonin levels in both the striatum and cerebellum. Moreover, the motor impaired associated changes could also be attributed to upregulated nNOS and iNOS protein expression in the cerebellum resulting in increased nitric oxide radical production. This increased reactive free radicals species can initiate lipid peroxidation mediated changes in the cerebellar Purkinje cells, which is responsible for initiating inhibitory motor response and ultimately leading to impaired motor co-ordination. Treatment with S. dulcis extract (200mg/kgbw) could control motor impairment and regulate neurotransmitter level as that of control groups when compared to noise exposed group. One key aspect of therapeutic efficacy of the plant could have resulted due to attenuated lipid peroxidation mediated damages on the cerebellar Purkinje cells thereby regulating motor impairment. Thus, targeting the antioxidant and free radicals scavenging properties of the plant could serve as a potential therapeutic to combat this environmental stressor.

Neuroprotective effects of sildenafil against oxidative stress and memory dysfunction in mice exposed to noise stress

Noise exposure has been well characterized as an environmental stressor, and is known to have auditory and non-auditory effects. Phosphodiesterase 5 (PDE5) inhibitors affect memory and hippocampus plasticity through various signaling cascades which are regulated by cGMP. In this study, we investigated the effects of sildenafil on memory deficiency, neuroprotection and oxidative stress in mice caused by chronic noise exposure. Mice were exposed to noise for 4h every day up to 14days at 110dB SPL of noise level. Sildenafil (15mg/kg) was orally administered 30min before noise exposure for 14days. Behavioral assessments were performed using novel object recognition (NOR) test and radial arm maze (RAM) test. Higher levels of memory dysfunction and oxidative stress were observed in noise alone-induced mice compared to control group. Interestingly, sildenafil administration increased memory performance, decreased oxidative stress, and increased neuroprotection in the hippocampus region of noise alone-induced mice likely through affecting memory related pathways such as cGMP/PKG/CREB and p25/CDK5, and induction of free radical scavengers such as SOD1, SOD2, SOD3, Prdx5, and catalase in the brain of stressed mice.

Effects of Noise Exposure on Systemic and Tissue-Level Markers of Glucose Homeostasis and Insulin Resistance in Male Mice

BACKGROUND

Epidemiological studies have indicated that noise exposure is associated with an increased risk of type 2 diabetes mellitus (T2DM). However, the nature of the connection between noise exposure and T2DM remains to be explored.

OBJECTIVES

We explored whether and how noise exposure affects glucose homeostasis in mice as the initial step toward T2DM development.

METHODS

Male ICR mice were randomly assigned to one of four groups: the control group and three noise groups (N20D, N10D, and N1D), in which the animals were exposed to white noise at 95 decibel sound pressure level (dB SPL) for 4 hr per day for 20 successive days, 10 successive days, or 1 day, respectively. Glucose tolerance and insulin sensitivity were evaluated 1 day, 1 week, and 1 month after the final noise exposure (1DPN, 1WPN, and 1MPN). Standard immunoblots, immunohistochemical methods, and enzyme-linked immunosorbent assays (ELISA) were performed to assess insulin signaling in skeletal muscle, the morphology of β cells, and plasma corticosterone levels.

RESULTS

Noise exposure for 1 day caused transient glucose intolerance and insulin resistance, whereas noise exposure for 10 and 20 days had no effect on glucose tolerance but did cause prolonged insulin resistance and an increased insulin response to glucose challenge. Akt phosphorylation and GLUT4 translocation in response to exogenous insulin were decreased in the skeletal muscle of noise-exposed animals.

CONCLUSIONS

Noise exposure at 95 dB SPL caused insulin resistance in male ICR mice, which was prolonged with longer noise exposure and was likely related to the observed blunted insulin signaling in skeletal muscle.

CITATION

Liu L, Wang F, Lu H, Cao S, Du Z, Wang Y, Feng X, Gao Y, Zha M, Guo M, Sun Z, Wang J. 2016. Effects of noise exposure on systemic and tissue-level markers of glucose homeostasis and insulin resistance in male mice. Environ Health Perspect 124:1390-1398; http://dx.doi.org/10.1289/EHP162.

Effects of chronic noise on mRNA and protein expression of CRF family molecules and its relationship with p-tau in the rat prefrontal cortex

Chronic noise exposure has been associated with Alzheimer's disease (AD)-like pathological changes, such as tau hyperphosphorylation and β-amyloid peptide accumulation in the prefrontal cortex (PFC). Corticotropin-releasing factor (CRF) is the central driving force in the stress response and a regulator of tau phosphorylation via binding to CRF receptors (CRFR). Little is known about the CRF system in relation to noise-induced AD-like changes in the PFC. The aim of this study was to explore the effects of chronic noise exposure on the CRF system in the PFC of rats and its relationship to tau phosphorylation. Male Wistar rats were randomly divided into control and noise exposure groups. The CRF system was evaluated following chronic noise exposure (95dB sound pressure level white noise, 4h/day×30days). Chronic noise significantly accelerated the progressive overproduction of corticosterone and upregulated CRF and CRFR1 mRNA and protein, both of which persisted 7-14days after noise exposure. In contrast, CRFR2 was elevated 3-7days following the last stimulus. Double-labeling immunofluorescence co-localized p-tau with CRF in PFC neurons. The results suggest that chronic noise exposure elevates the expression of the CRF system, which may contribute to AD-like changes.

Tualang Honey Attenuates Noise Stress-Induced Memory Deficits in Aged Rats

Ageing and stress exposure may lead to memory impairment while oxidative stress is thought to be one of the underlying mechanisms involved. This study aimed to investigate the potential protective effects of Tualang honey supplementation on memory performance in aged rats exposed to noise stress. Tualang honey supplementation was given orally, 200 mg/kg body weight for 28 days. Rats in the stress group were subjected to loud noise, 100 dB(A), 4 hours daily for 14 days. All rats were subjected to novel object recognition test for evaluation of memory performance. It was observed that the rats subjected to noise stress exhibited significantly lower memory performance and higher oxidative stress as evident by elevated malondialdehyde and protein carbonyl levels and reduction of antioxidant enzymes activities compared to the nonstressed rats. Tualang honey supplementation was able to improve memory performance, decrease oxidative stress levels, increase brain-derived neurotrophic factor (BDNF) concentration, decrease acetylcholinesterase activity, and enhance neuronal proliferation in the medial prefrontal cortex (mPFC) and hippocampus. In conclusion, Tualang honey protects against memory decline due to stress exposure and/or ageing via enhancement of mPFC and hippocampal morphology possibly secondary to reduction in brain oxidative stress and/or upregulation of BDNF concentration and cholinergic system.

Noise induced hearing loss impairs spatial learning/memory and hippocampal neurogenesis in mice

Hearing loss has been associated with cognitive decline in the elderly and is considered to be an independent risk factor for dementia. One of the most common causes for acquired sensorineural hearing loss is exposure to excessive noise, which has been found to impair learning ability and cognitive performance in human subjects and animal models. Noise exposure has also been found to depress neurogenesis in the hippocampus. However, the effect is mainly attributed to the oxidant stress of noise on the cognitive brain. In the present study, young adult CBA/CAJ mice (between 1.5 and 2 months of age) were briefly exposed a high sound level to produce moderate-to-severe hearing loss. In both the blood and hippocampus, only transient oxidative stress was observed after noise exposure. However, a deficit in spatial learning/memory was revealed 3 months after noise exposure. Moreover, the deficit was correlated with the degree of hearing loss and was associated with a decrease in neurogenesis in the hippocampus. We believe that the observed effects were likely due to hearing loss rather than the initial oxidant stress, which only lasted for a short period of time.

Noise exposure of immature rats can induce different age-dependent extra-auditory alterations that can be partially restored by rearing animals in an enriched environment

It has been previously shown that different extra-auditory alterations can be induced in animals exposed to noise at 15 days. However, data regarding exposure of younger animals, that do not have a functional auditory system, have not been obtained yet. Besides, the possibility to find a helpful strategy to restore these changes has not been explored so far. Therefore, the aims of the present work were to test age-related differences in diverse hippocampal-dependent behavioral measurements that might be affected in noise-exposed rats, as well as to evaluate the effectiveness of a potential neuroprotective strategy, the enriched environment (EE), on noise-induced behavioral alterations. Male Wistar rats of 7 and 15 days were exposed to moderate levels of noise for two hours. At weaning, animals were separated and reared either in standard or in EE cages for one week. At 28 days of age, different hippocampal-dependent behavioral assessments were performed. Results show that rats exposed to noise at 7 and 15 days were differentially affected. Moreover, EE was effective in restoring all altered variables when animals were exposed at 7 days, while a few were restored in rats exposed at 15 days. The present findings suggest that noise exposure was capable to trigger significant hippocampal-related behavioral alterations that were differentially affected, depending on the age of exposure. In addition, it could be proposed that hearing structures did not seem to be necessarily involved in the generation of noise-induced hippocampal-related behaviors, as they were observed even in animals with an immature auditory pathway. Finally, it could be hypothesized that the differential restoration achieved by EE rearing might also depend on the degree of maturation at the time of exposure and the variable evaluated, being younger animals more susceptible to environmental manipulations.

Noise exposure and oxidative balance in auditory and extra-auditory structures in adult and developing animals. Pharmacological approaches aimed to minimize its effects

Noise coming from urban traffic, household appliances or discotheques might be as hazardous to the health of exposed people as occupational noise, because may likewise cause hearing loss, changes in hormonal, cardiovascular and immune systems and behavioral alterations. Besides, noise can affect sleep, work performance and productivity as well as communication skills. Moreover, exposure to noise can trigger an oxidative imbalance between reactive oxygen species (ROS) and the activity of antioxidant enzymes in different structures, which can contribute to tissue damage. In this review we systematized the information from reports concerning noise effects on cell oxidative balance in different tissues, focusing on auditory and non-auditory structures. We paid specific attention to in vivo studies, including results obtained in adult and developing subjects. Finally, we discussed the pharmacological strategies tested by different authors aimed to minimize the damaging effects of noise on living beings.

Inhibition of long-term potentiation in the schaffer-CA1 pathway by repetitive high-intensity sound stimulation

High-intensity sound can induce seizures in susceptible animals. After repeated acoustic stimuli changes in behavioural seizure repertoire and epileptic EEG activity might be seen in recruited limbic and forebrain structures, a phenomenon known as audiogenic kindling. It is postulated that audiogenic kindling can produce synaptic plasticity events leading to the spread of epileptogenic activity to the limbic system. In order to test this hypothesis, we investigated if long-term potentiation (LTP) of hippocampal Schaffer-CA1 synapses and spatial navigation memory are altered by a repeated high-intensity sound stimulation (HISS) protocol, consisting of one-minute 120 dB broadband noise applied twice a day for 10 days, in normal Wistar rats and in audiogenic seizure-prone rats (Wistar Audiogenic Rats - WARs). After HISS all WARs exhibited midbrain seizures and 50% of these animals developed limbic recruitment, while only 26% of Wistar rats presented midbrain seizures and none of them had limbic recruitment. In naïve animals, LTP in hippocampal CA1 neurons was induced by 50- or 100-Hz high-frequency stimulation of Schaffer fibres in slices from both Wistar and WAR animals similarly. Surprisingly, HISS suppressed LTP in CA1 neurons in slices from Wistar rats that did not present any seizure, and inhibited LTP in slices from Wistar rats with only midbrain seizures. However HISS had no effect on LTP in CA1 neurons from slices of WARs. Interestingly HISS did not alter spatial navigation and memory in both strains. These findings show that repeated high-intensity sound stimulation prevent LTP of Schaffer-CA1 synapses from Wistar rats, without affecting spatial memory. This effect was not seen in hippocampi from audiogenic seizure-prone WARs. In WARs the link between auditory stimulation and hippocampal LTP seems to be disrupted which could be relevant for the susceptibility to seizures in this strain.

Protective effect of the orientin on noise-induced cognitive impairments in mice

There is increasing evidence that chronic noise stress impairs cognition and induces oxidative stress in the brain. Recently, orientin, a phenolic compound abundant in some fruits, millet, and herbs, has been shown to have antioxidant properties. This study investigated the potential effects of orientin against chronic noise-induced cognitive decline and its underlying mechanisms. A moderate-intensity noise exposure model was used to investigate the effects of orientin on behavior and biochemical alterations in mice. After 3 weeks of the noise exposure, the mice were treated with orientin (20mg/kg and 40 mg/kg, oral gavage) for 3 weeks. The chronic noise exposure impaired the learning and memory in mice in the Morris water maze and step-through tests. The noise exposure also decreased exploration and interest in a novel environment in the open field test. The administration of orientin significantly reversed noise-induced alterations in these behavior tests. Moreover, the orientin treatment significantly improved the noise-induced alteration of serum corticosterone and catecholamine levels and oxidative stress in the hippocampus and prefrontal cortex. Furthermore, the orientin treatment ameliorated the noise-induced decrease in brain-derived neurotrophic factor and synapse-associated proteins (synaptophysin and postsynaptic density protein 95) in the hippocampus and prefrontal cortex. Thus, orientin exerts protective effects on noise-induced cognitive decline in mice, specifically by improving central oxidative stress, neurotransmission, and increases synapse-associated proteins. Therefore, supplementation with orientin-enriched food or fruit could be beneficial as a preventive strategy for chronic noise-induced cognitive decline.

Hippocampal signaling pathways are involved in stress-induced impairment of memory formation in rats

Stress is a potent modulator of hippocampal-dependent memory formation. The aim of the present study was to assess the role of hippocampal signaling pathways in stress-induced memory impairment in male Wistar rats. The animals were exposed to acute elevated platform (EP) stress and memory formation was measured by a step-through type passive avoidance task. The results indicated that post-training or pre-test exposure to EP stress impaired memory consolidation or retrieval respectively. Using western blot analysis, it was found that memory retrieval was associated with the increase in the levels of phosphorylated cAMP-responsive element binding protein (P-CREB), peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) and its downstream targets in the hippocampus. In contrast, the stress exposure decreased the hippocampal levels of these proteins. In addition, stress-induced impairment of memory consolidation or retrieval was associated with the decrease in the P-CREB/CREB ratio and the PGC-1α level in the hippocampus. On the other hand, the hippocampal level of nuclear factor E2-related factor 2 (Nrf2) and gamma-glutamylcysteine synthetase (γ-GCS) which are the master regulators of defense system were decreased by the stress exposure. The increased hippocampal levels of Nrf2 and it׳s downstream was observed during memory retrieval, while stress-induced impairment of memory consolidation or retrieval inhibited this hippocampal signaling pathway. Overall, these findings suggest that down-regulation of CREB/PGC-1α signaling cascade and Nrf2 antioxidant pathways in the hippocampus may be associated with memory impairment induced by stress.

Spatial learning and memory deficits in young adult mice exposed to a brief intense noise at postnatal age

Noise pollution is a major hazardous factor to human health and is likely harmful for vulnerable groups such as pre-term infants under life-support system in an intensive care unit. Previous studies have suggested that noise exposure impairs children's learning ability and cognitive performance and cognitive functions in animal models in which the effect is mainly attributed to the oxidant stress of noise on the cognitive brain. The potential role of noise induced hearing loss (NIHL), rather than the oxidant stress, has also been indicated by a depression of neurogenesis in the hippocampus long after a brief noise exposure, which produces only a tentative oxidant stress. It is not clear if noise exposure and NIHL during early development exerts a long term impact on cognitive function and neurogenesis towards adulthood. In the present study, a brief noise exposure at high sound level was performed in neonatal C57BL/6J mice (15 days after birth) to produce a significant amount of permanent hearing loss as proved 2 months after the noise. At this age, the noise-exposed animals showed deteriorated spatial learning and memory abilities and a reduction of hippocampal neurogenesis as compared with the control. The averaged hearing threshold was found to be strongly correlated with the scores for spatial learning and memory. We consider the effects observed are largely due to the loss of hearing sensitivity, rather than the oxidant stress, due to the long interval between noise exposure and the observations.

Chronic Noise Exposure Acts Cumulatively to Exacerbate Alzheimer's Disease-Like Amyloid-β Pathology and Neuroinflammation in the Rat Hippocampus

A putative etiological association exists between noise exposure and Alzheimer’s disease (AD). Amyloid-β (Aβ) pathology is thought to be one of the primary initiating factors in AD. It has been further suggested that subsequent dysregulation of Aβ may play a mechanistic role in the AD-like pathophysiology associated with noise exposure. Here, we used ELISA, immunoblotting, cytokine arrays, and RT-PCR, to examine both hippocampal Aβ pathology and neuroinflammation in rats at different time points after noise exposure. We found that chronic noise exposure significantly accelerated the progressive overproduction of Aβ, which persisted for 7 to 14 days after the cessation of exposure. This effect was accompanied by up-regulated expression of amyloid precursor protein (APP) and its cleavage enzymes, β- and γ-secretases. Cytokine analysis revealed that chronic noise exposure increased levels of tumor necrosis factor-α and the receptor for advanced glycation end products, while decreasing the expression of activin A and platelet-derived growth factor- AA. Furthermore, we found persistent elevations of glial fibrillary acidic protein and ionized calcium-binding adapter molecule 1 expression that closely corresponded to the noise-induced increases in Aβ and neuroinflammation. These studies suggest that lifelong environmental noise exposure may have cumulative effects on the onset and development of AD.

Early-life exposure to noise reduces mPFC astrocyte numbers and T-maze alternation/discrimination task performance in adult male rats

In this experiment, we evaluated the long-term effects of noise by assessing both astrocyte changes in medial prefrontal cortex (mPFC) and mPFC-related alternation/discrimination tasks. Twenty-one-day-old male rats were exposed during a period of 15 days to a standardized rats' audiogram-fitted adaptation of a human noisy environment. We measured serum corticosterone (CORT) levels at the end of the exposure and periodically registered body weight gain. In order to evaluate the long-term effects of this exposure, we assessed the rats' performance on the T-maze apparatus 3 months later. Astrocyte numbers and proliferative changes in mPFC were also evaluated at this stage. We found that environmental noise (EN) exposure significantly increased serum CORT levels and negatively affected the body weight gain curve. Accordingly, enduring effects of noise were demonstrated on mPFC. The ability to solve alternation/discrimination tasks was reduced, as well as the number of astroglial cells. We also found reduced cytogenesis among the mPFC areas evaluated. Our results support the idea that early exposure to environmental stressors may have long-lasting consequences affecting complex cognitive processes. These results also suggest that glial changes may become an important element behind the cognitive and morphological alterations accompanying the PFC changes seen in some stress-related pathologies.

Noise stress changes mRNA expressions of corticotropin-releasing hormone, its receptors in amygdala, and anxiety-related behaviors

Noise is a psychological, environmental stressor that activates limbic sites in the brain. Limbic sites such as the amygdala and the amygdaloid corticotropin-releasing hormone (CRH) system play an important role in integrating stress response. We investigated the association between noise exposures, CRH-related molecules in the amygdala, and behavioral alterations. In total 54 Sprague-Dawley rats were divided into the following three groups: Control (CON), acute noise exposure (ANE), and chronic noise exposure (CNE). The ANE group was exposed to 100 dB white noise only once in 4 h and the CNE group was exposed to the same for 4 h per day for 30 days. Expression profiles of CRH and its receptors CRH-R1 and CRH-R2 were analyzed by quantitative real-time polymerase chain reaction (qPCR). The same stress procedure was applied to the ANE and CNE groups for behavior testing. The anxiety responses of the animals after acute and chronic stress exposure were measured in the defensive withdrawal test. CNE upregulated CRH and CRH-R1 mRNA levels but downregulated CRH-R2 mRNA levels. ANE led to a decrease in both CRH-R1 and CRH-R2 expression. In the defensive withdrawal test, while the ANE increased, CNE reduced anxiety-like behaviors. The present study shows that the exposure of rats to white noise (100 dB) leads to behavioral alterations and molecule-specific changes in the CRH system. Behavioral alterations can be related to these molecular changes in the amygdala.

Tualang honey improves memory performance and decreases depressive-like behavior in rats exposed to loud noise stress

Recent evidence has exhibited dietary influence on the manifestation of different types of behavior induced by stressor tasks. The present study examined the effects of Tualang honey supplement administered with the goal of preventing or attenuating the occurrence of stress-related behaviors in male rats subjected to noise stress. Forty-eight adult male rats were randomly divided into the following four groups: i) nonstressed with vehicle, ii) nonstressed with Tualang honey, iii) stressed with vehicle, and iv) stressed with honey. The supplement was given once daily via oral gavage at 0.2 g/kg body weight. Two types of behavioral tests were performed, namely, the novel object recognition test to evaluate working memory and the forced swimming test to evaluate depressive-like behavior. Data were analyzed by a two-way analysis of variance (ANOVA) using IBM SPSS 18.0. It was observed that the rats subjected to noise stress expressed higher levels of depressive-like behavior and lower memory functions compared to the unexposed control rats. In addition, our results indicated that the supplementation regimen successfully counteracted the effects of noise stress. The forced swimming test indicated that climbing and swimming times were significantly increased and immobility times significantly decreased in honey-supplemented rats, thereby demonstrating an antidepressant-like effect. Furthermore, cognitive function was shown to be intensely affected by noise stress, but the effects were counteracted by the honey supplement. These findings suggest that subchronic exposure to noise stress induces depressive-like behavior and reduces cognitive functions, and that these effects can be attenuated by Tualang honey supplementation. This warrants further studies to examine the role of Tulang honey in mediating such effects.

Is rosuvastatin protective against on noise-induced oxidative stress in rat serum?

Noise, one of the main components of modern society, has become an important environmental problem. Noise is not only an irritating sound, but also a stress factor leading to serious health problems. In this study, we have investigated possible effects of rosuvastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor, thought to have an antioxidant effect, on noise-induced oxidative stress in the serum of rat models. Thirty-two male Wistar albino rats were used. In order to ease their adaptation, 2 weeks before the experiment, the rats were divided into four groups (with eight rats per each group): Noise exposure plus rosuvastatin usage, only noise exposure, only rosuvastatin usage and control. After the data had been collected, oxidant (Malondialdehyde, nitric oxide [NO], protein carbonyl [PC]) and antioxidant (superoxide dismutase [SOD], glutathione peroxidase [GSH-PX], catalase [CAT]) parameters were analyzed in the serum. Results indicated that SOD values were found to be significantly lower, while PC values in serum were remarkably higher in the group that was exposed to only noise. GSH-Px values in serum dramatically increased in the group on which only rosuvastatin was used. During noise exposure, the use of rosuvastatin caused significantly increased CAT values, whereas it resulted in reduced PC and NO values in serum. In conclusion, our data show that noise exposure leads to oxidative stress in rat serum; however, rosuvastatin therapy decreases the oxidative stress caused by noise exposure.