Chronic exposure of juvenile rats to environmental noise impairs hippocampal cell proliferation in adulthood

Visual EMDR stimulation mitigates acute varied stress effects on morphology of hippocampal neurons in male Wistar rats

Introduction

Stress is a pervasive health concern known to induce physiological changes, particularly impacting the vulnerable hippocampus and the morphological integrity of its main residing cells, the hippocampal neurons. Eye Movement Desensitization and Reprocessing (EMDR), initially developed to alleviate emotional distress, has emerged as a potential therapeutic/preventive intervention for other stress-related disorders. This study aimed to investigate the impact of Acute Variable Stress (AVS) on hippocampal neurons and the potential protective effects of EMDR.

Methods

Rats were exposed to diverse stressors for 7 days, followed by dendritic morphology assessment of hippocampal neurons using Golgi-Cox staining.

Results

AVS resulted in significant dendritic atrophy, evidenced by reduced dendritic branches and length. In contrast, rats receiving EMDR treatment alongside stress exposure exhibited preserved dendritic morphology comparable to controls, suggesting EMDR's protective role against stressinduced dendritic remodeling.

Conclusions

These findings highlight the potential of EMDR as a neuroprotective intervention in mitigating stress-related hippocampal alterations.

Developmental programming by prenatal sounds: insights into possible mechanisms

In recent years, the impact of prenatal sound on development, notably for programming individual phenotypes for postnatal conditions, has increasingly been revealed. However, the mechanisms through which sound affects physiology and development remain mostly unexplored. Here, I gather evidence from neurobiology, developmental biology, cellular biology and bioacoustics to identify the most plausible modes of action of sound on developing embryos. First, revealing often-unsuspected plasticity, I discuss how prenatal sound may shape auditory system development and determine individuals' later capacity to receive acoustic information. I also consider the impact of hormones, including thyroid hormones, glucocorticoids and androgen, on auditory plasticity. Second, I review what is known about sound transduction to other - non-auditory - brain regions, and its potential to input on classical developmental programming pathways. Namely, the auditory pathway has direct anatomical and functional connectivity to the hippocampus, amygdala and/or hypothalamus, in mammals, birds and anurans. Sound can thus trigger both immediate and delayed responses in these limbic regions, which are specific to the acoustic stimulus and its biological relevance. Third, beyond the brain, I briefly consider the possibility for sound to directly affect cellular functioning, based on evidence in earless organisms (e.g. plants) and cell cultures. Together, the multi-disciplinary evidence gathered here shows that the brain is wired to allow multiple physiological and developmental effects of sound. Overall, there are many unexplored, but possible, pathways for sound to impact even primitive or immature organisms. Throughout, I identify the most promising research avenues for unravelling the processes of acoustic developmental programming.

Early-life noise exposure causes cognitive impairment in a sex-dependent manner by disrupting homeostasis of the microbiota-gut-brain axis

Epidemiological investigations show that noise exposure in early life is associated with health and cognitive impairment. The gut microbiome established in early life plays a crucial role in modulating developmental processes that subsequently affect brain function and behavior. Here, we examined the impact of early-life exposure to noise on cognitive function in adolescent rats by analyzing the gut microbiome and metabolome to elucidate the underlying mechanisms. Chronic noise exposure during early life led to cognitive deficits, hippocampal injury, and neuroinflammation. Early-life noise exposure showed significant difference on the composition and function of the gut microbiome throughout adolescence, subsequently causing axis-series changes in fecal short-chain fatty acid (SCFA) metabolism and serum metabolome profiles, as well as dysregulation of endothelial tight junction proteins, in both intestine and brain. We also observed sex-dependent effects of microbiota depletion on SCFA-related beneficial bacteria in adolescence. Experiments on microbiota transplantation and SCFA supplementation further confirmed the role of intestinal bacteria and related SCFAs in early-life noise-exposure-induced impairments in cognition, epithelial integrity, and neuroinflammation. Overall, these results highlight the homeostatic imbalance of microbiota-gut-brain axis as an important physiological response toward environmental noise during early life and reveals subtle differences in molecular signaling processes between male and female rats.

The Impact of Persistent Noise Exposure under Inflammatory Conditions

The aim of this study was to investigate the impact of noise exposure in an intensive care unit (ICU) environment on the development of postoperative delirium in a mouse model that mimics the ICU environment. Additionally, we aimed to identify the underlying mechanisms contributing to delirium and provide evidence for reducing the risk of delirium. In this study, to mimic an ICU environment, lipopolysaccharide (LPS)-injected sepsis mouse models were exposed to a 75 dB noise condition. Furthermore, we assessed neurobehavioral function and observed the level of neuroinflammatory response and blood-brain barrier (BBB) integrity in the hippocampal region. The LPS-injected sepsis mouse model exposed to noise exhibited increased anxiety-like behavior and cognitive impairment. Moreover, severe neuroinflammation and BBB disruption were detected in the hippocampal region. This study provides insights suggesting that persistent noise exposure under systemic inflammatory conditions may cause cognitive dysfunction and anxiety- like behavior via the mediation of BBB disruption and neuroinflammation. As a result, we suggest that the detailed regulation of noise exposure may be required to prevent the development of postoperative delirium.

Acoustic Stress Induces Opposite Proliferative/Transformative Effects in Hippocampal Glia

The hippocampus is a brain region crucially involved in regulating stress responses and highly sensitive to environmental changes, with elevated proliferative and adaptive activity of neurons and glial cells. Despite the prevalence of environmental noise as a stressor, its effects on hippocampal cytoarchitecture remain largely unknown. In this study, we aimed to investigate the impact of acoustic stress on hippocampal proliferation and glial cytoarchitecture in adult male rats, using environmental noise as a stress model. After 21 days of noise exposure, our results showed abnormal cellular proliferation in the hippocampus, with an inverse effect on the proliferation ratios of astrocytes and microglia. Both cell lineages also displayed atrophic morphologies with fewer processes and lower densities in the noise-stressed animals. Our findings suggest that, stress not only affects neurogenesis and neuronal death in the hippocampus, but also the proliferation ratio, cell density, and morphology of glial cells, potentially triggering an inflammatory-like response that compromises their homeostatic and repair functions.

The hearing hippocampus

The hippocampus has a well-established role in spatial and episodic memory but a broader function has been proposed including aspects of perception and relational processing. Neural bases of sound analysis have been described in the pathway to auditory cortex, but wider networks supporting auditory cognition are still being established. We review what is known about the role of the hippocampus in processing auditory information, and how the hippocampus itself is shaped by sound. In examining imaging, recording, and lesion studies in species from rodents to humans, we uncover a hierarchy of hippocampal responses to sound including during passive exposure, active listening, and the learning of associations between sounds and other stimuli. We describe how the hippocampus' connectivity and computational architecture allow it to track and manipulate auditory information - whether in the form of speech, music, or environmental, emotional, or phantom sounds. Functional and structural correlates of auditory experience are also identified. The extent of auditory-hippocampal interactions is consistent with the view that the hippocampus makes broad contributions to perception and cognition, beyond spatial and episodic memory. More deeply understanding these interactions may unlock applications including entraining hippocampal rhythms to support cognition, and intervening in links between hearing loss and dementia.

Sex Differences in the Expression of c-fos in a Rat Brain after Exposure to Environmental Noise

Noise is an inarticulate stimulus that threatens health and well-being. It compromises audition and induces a strong stress response that activates the brain at several levels. In the present study, we expose male and female rats to environmental noise in order to investigate if acute or chronic stimulation produces differential brain activation patterns. The animals were exposed to a rat’s audiogram-fitted adaptation of a noisy environment and later sacrificed to quantify the expression of the brain activity marker c-fos. Additionally, the serum corticosterone (CORT) levels were measured to elucidate possible the stress-related effects of noise. It was found that environmental noise differentially increased the serum CORT levels in male and female rats. We identified 17 brain regions outside the classical auditory circuits with a high expression of c-fos, including the hypothalamus, prefrontal cortex, habenular complex, septum, cingulate cortex, nucleus accumbens, insular cortex, amygdala, and hippocampus. Overall, we evidenced that females exhibit less intense c-fos expression in most of the examined areas. We concluded that females might be less affected by the changes produced by environmental noise.

Relationship Between Chronic Noise Exposure, Cognitive Impairment, and Degenerative Dementia: Update on the Experimental and Epidemiological Evidence and Prospects for Further Research

Degenerative dementia, of which Alzheimer's disease is the most common form, is characterized by the gradual deterioration of cognitive function. The events that trigger and promote degenerative dementia are not clear, and treatment options are limited. Experimental and epidemiological studies have revealed chronic noise exposure (CNE) as a potential risk factor for cognitive impairment and degenerative dementia. Experimental studies have indicated that long-term exposure to noise might accelerate cognitive dysfunction, amyloid-β deposition, and tau hyperphosphorylation in different brain regions such as the hippocampus and cortex. Epidemiological studies are increasingly examining the possible association between external noise exposure and dementia. In this review, we sought to construct a comprehensive summary of the relationship between CNE, cognitive dysfunction, and degenerative dementia. We also present the limitations of existing evidence as a guide regarding important prospects for future research.

Effects of early noise exposure on hippocampal-dependent behaviors during adolescence in male rats: influence of different housing conditions

Central nervous system (CNS) development is a very complex process that can be altered by environmental stimuli such as noise, which can generate long-term auditory and/or extra-auditory impairments. We have previously reported that early noise exposure can induce hippocampus-related behavioral alterations in postnatal day (PND) 28 adolescent rats. Furthermore, we recently found biochemical modifications in the hippocampus (HC) of these animals that seemed to endure even in more mature animals (i.e. PND35) and that have not been studied along with behavioral correlates. Thus, the aim of this work was to reveal novel data about the effects of early noise exposure on hippocampal-dependent behaviors in more mature animals. Additionally, extended enriched environment (EE) housing was evaluated to determine its capacity to induce behavioral modifications, either by its neuroprotective ability or the greater stimulation that it generates. Male Wistar rats were exposed to different noise schemes at PND7 or PND15. Upon weaning, some animals were transferred to EE whereas others were kept in standard cages. At PND35, different hippocampal-dependent behavioral assessments were performed. Results showed noise-induced behavioral changes that differed according to the scheme and age of exposure used. In addition, housing in an EE was effective either in preventing some of these changes or in inducing the appearance of new behavioral modifications. These findings suggest that CNS development would be sensitive to the effects of different type of environmental stimuli such as noise or enriched housing, leading to maladaptive behavioral changes that last even until adolescence.

Noise-induced hippocampal oxidative imbalance and aminoacidergic neurotransmitters alterations in developing male rats: Influence of enriched environment during adolescence

Living in big cities might involuntarily expose people to high levels of noise causing auditory and/or extra-auditory impairments, including adverse effects on central nervous system (CNS) areas such as the hippocampus. In particular, CNS development is a very complex process that can be altered by environmental stimuli. We have previously shown that noise exposure of developing rats can induce hippocampal-related behavioral alterations. However, noise-induced biochemical alterations had not been studied yet. Thus, the aim of this work was to assess whether early noise exposure can affect rat hippocampal oxidative state and aminoacidergic neurotransmission tone. Additionally, the effectiveness of an enriched environment (EE) as a neuroprotective strategy was evaluated. Male Wistar rats were exposed to different noise schemes at 7 or 15 days after birth. Upon weaning, some animals were transferred to an EE whereas others were kept in standard cages. Short- and long-term measurements were performed to evaluate reactive oxygen species, thioredoxins levels and catalase activity as indicators of hippocampal oxidative status as well as glutamic acid decarboxylase and a subtype of glutamate transporter to evaluate aminoacidergic neurotransmission tone. Results showed noise-induced changes in hippocampal oxidative state and aminoacidergic neurotransmission markers that lasted until adolescence and differed according to the scheme and the age of exposure. Finally, EE housing was effective in preventing some of these changes. These findings suggest that CNS development seems to be sensitive to the effects of stressors such as noise, as well as those of an environmental stimulation, favoring prompt and lasting molecular changes.

Neuroanatomical changes of the medial prefrontal cortex of male pups of Wistar rat after prenatal and postnatal noise stress

Recent evidences showed that, noise stress causes abnormal changes in structure and function of central nervous system (CNS). The Current study was conducted to evaluate some stereological parameters of the medial prefrontal cortex (mPFC) of male pups of Wistar rat after prenatal and early postnatal noise stress. 18 pregnant Wistar rats were randomly divided into prenatal noise-exposed (NE) group, postnatal NE group, and controls. Male pups of NE groups were exposed to noise 100 dB at the frequency ranges of 500-8000 Hz, 4 h per day from gestational day one (GD1) to GD21 for the prenatal NE group, and from postnatal day one (PND1) to PND21 in the postnatal NE group. The Control group animals were maintained under standard condition without noise stimulation. Corticosterone level in plasma was measured using ELISA technique. Changes of the neurons and non-neurons cells number and volume of the mPFC were evaluated by stereological analysis. Tunnel assay was also used for detection of apoptotic cells. Increase in plasma corticosterone level, decrease in the number of neurons, and increase in the apoptotic cells number were observed in both NE groups. Decrease in volume of mPFC and also in non-neurons cells number was observed in the prenatal NE group. An increase in the non-neurons number was seen in the postnatal NE group. Data of the current comparative study showed that, noise stress during prenatal and early postnatal periods can induce the abnormal alteration in some stereological parameters of mPFC in male pups of Wistar rat. These negative alterations were more remarkable after prenatal noise stress.

Does hearing loss lead to dementia? A review of the literature

Recent studies have revealed a correlation between aging-related hearing loss and the likelihood of developing Alzheimer Disease. However, it is not yet known if the correlation simply reflects the fact that these two disorders share common risk factors or whether there is a causal link between them. The answer to this question carries therapeutic implications. Unfortunately, it is not possible to study the question of causality between aging-related hearing loss and dementia in human subjects. Here, we evaluate the research surrounding induced-hearing loss in animal models on non-auditory cognition to help infer if there is any causal evidence linking hearing loss and a more general dementia. We find ample evidence that induction of hearing loss in animals produces cognitive decline, particularly hippocampal dysfunction. The data suggest that noise-exposure produces a toxic milieu in the hippocampus consisting of a spike in glucocorticoid levels, elevations of mediators of oxidative stress and excitotoxicity, which as a consequence induce cessation of neurogenesis, synaptic loss and tau hyperphosphorylation. These data suggest that hearing loss can lead to pathological hallmarks similar to those seen in Alzheimer's Disease and other dementias. However, the rodent data do not establish that hearing loss on its own can induce a progressive degenerative dementing illness. Therefore, we conclude that an additional "hit", such as aging, APOE genotype, microvascular disease or others, may be necessary to trigger an ongoing degenerative process such as Alzheimer Disease.

Male/female Differences in Radial Arm Water Maze Execution After Chronic Exposure to Noise

Introduction:

Noise is one of the main sources of discomfort in modern societies. It affects physiology, behavior, and cognition of exposed subjects. Although the effects of noise on cognition are well known, gender role in noise-cognition relationship remains controversial.

Aim:

We analyzed the effects of noise on the ability of male and female rats to execute the Radial Arm Water Maze (RAWM) paradigm.

Materials and Methods:

Male and female Wistar rats were exposed to noise for 3 weeks, and the cognitive effects were assessed at the end of the exposure. RAWM execution included a three-day training phase and a reversal-learning phase conducted on the fourth day. Escape latency, reference memory errors, and working memory errors were quantified and compared between exposed and non-exposed subjects.

Results:

We found that male rats were in general more affected by noise. Execution during the three-day learning phase evidenced that male exposed rats employed significantly more time to acquire the task than the non-exposed. On the other hand, the exposed females solved the paradigm in latencies similar to control rats. Both, males and females diminished their capacity to execute on the fourth day when re-learning abilities were tested.

Conclusion:

We conclude that male rats might be less tolerable to noise compared to female ones and that spatial learning may be a cognitive function comparably more vulnerable to noise.

Male rats exhibit higher pro-BDNF, c-Fos and dendritic tree changes after chronic acoustic stress

Prolonged or intense exposure to environmental noise (EN) has been associated with a number of changes in auditory organs as well as other brain structures. Notably, males and females have shown different susceptibilities to acoustic damage as well as different responses to environmental stressors. Rodent models have evidence of sex-specific changes in brain structures involved in noise and sound processing. As a common effect, experimental models have demonstrated that dendrite arborizations reconfigure in response to aversive conditions in several brain regions. Here, we examined the effect of chronic noise on dendritic reorganization and c-Fos expression patterns of both sexes. During 21 days male and female rats were exposed to a rats' audiogram-fitted adaptation of a noisy environment. Golgi-Cox and c-Fos staining were performed at auditory cortices (AC) and hippocampal regions. Sholl analysis and c-Fos counts were conducted for evidence of intersex differences. In addition, pro-BDNF serum levels were also measured. We found different patterns of c-Fos expression in hippocampus and AC. While in AC expression levels showed rapid and intense increases starting at 2 h, hippocampal areas showed slower rises that reached the highest levels at 21 days. Sholl analysis also evidenced regional differences in response to noise. Dendritic trees were reduced after 21 days in hippocampus but not in AC. Meanwhile, pro-BDNF levels augmented after EN exposure. In all analyzed variables, exposed males were the most affected. These findings suggest that noise may exert differential effects on male and female brains and that males could be more vulnerable to the chronic effects of noise.

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.

Hippocampal Mechanisms Underlying Impairment in Spatial Learning Long After Establishment of Noise-Induced Hearing Loss in CBA Mice

Sensorineural hearing loss (SNHL) has been demonstrated in many clinical reports as a risk factor that promotes the development of cognitive impairment. However, the underlying neurological mechanisms are not clear. Noise exposure is one of the most common causes of SNHL. Although noise exposure causes relatively less damage to general health as compared with other methods for creating hearing loss (such as ototoxicity), it does impair cognitive function. Many studies have shown that the noise-induced cognitive impairment occur via the oxidative stress induced by the noise. In those studies, the effects of the noise-induced hearing loss induced (NIHL) were not addressed. Previously, we have demonstrated in the CBA/CaJ mouse model that oxidative stress was transient after a brief noise exposure, but the NIHL was permanent. In addition, NIHL was followed by a declined cognitive function and decreased hippocampal neurogenesis that were developed long after the oxidative stress disappeared. Therefore, NIHL can cause cognitive impairment independent of its stress effect and can serve as a model to investigate the relationship between hearing loss and the development of cognitive impairment. In the present study, we further demonstrated that the oxidative stress produced by the brief noise exposure did not damage the stem cell bank of hippocampus that was evaluated shortly after the noise exposure. In addition to the reduction in the rate of cell proliferation in hippocampus that was found previously, we found that the NIHL significantly reduced the promoting effect of learning activity on various stages of hippocampal neurogenesis, accompanied by the reduction in learning-induced expression of immediate early genes (IEGs) in hippocampus. Since the MWM-tested spatial function does not directly require auditory input, the results provide evidence for the maintenance role of auditory input on the cognitive function; the reduction of IEG expression that is required in memory-formation may be the initial step in blocking the effect of learning activity on neurogenesis in subjects with NIHL.

Substance-use initiation moderates the effect of stress on white-matter microstructure in adolescents

BACKGROUND

While childhood stress may contribute risk to substance-use initiation and differences in brain white-matter development, understanding of the potential impact of substance-use initiation on the relationship between experienced stress and white-matter microstructure remains limited.

OBJECTIVES

This study examined whether substance-use initiation moderated the effect of perceived stress on white-matter differences using measures of primary white-matter fiber anisotropy.

METHODS

Forty adolescents (age 14.75 ± .87 years) were assessed on the Perceived Stress Scale, and 50% were determined to have presence of substance-use initiation. White-matter microstructure was examined using primary-fiber orientations anisotropy, which may reflect white-matter integrity, modeled separately from other fiber orientations in the same voxels. Analyses were conducted on regions of interest previously associated with childhood stress and substance use.

RESULTS

Lower perceived stress and presence of substance-use initiation were related to greater right cingulum primary-fiber measures. Substance-use-initiation status moderated the association between perceived stress and right cingulum primary-fiber measures, such that higher perceived stress was associated with lower right cingulum primary-fiber anisotropy in adolescents without substance-use initiation, but not in those with substance-use initiation.

CONCLUSIONS AND SCIENTIFIC SIGNIFICANCE

Findings in primary-fiber anisotropy suggest differences in right cingulum white-matter integrity is associated with substance-use initiation in higher-stress adolescents. This reflects a possible pre-existing risk factor, an impact of early substance use, or a combination thereof. Examination of potential markers associated with substance-use initiation in white-matter microstructure among stress-exposed youth warrant additional investigation as such biomarkers may inform efforts relating to tailored interventions. (Am J Addict 2018;27:217-224).

Exposure to Music Alters Cell Viability and Cell Motility of Human Nonauditory Cells in Culture

Although music is part of virtually all cultures in the world, little is known about how it affects us. Since the beginning of this century several studies suggested that the response to music, and to sound in general, is complex and might not be exclusively due to emotion, given that cell types other than auditory hair cells can also directly react to audible sound. The present study was designed to better understand the direct effects of acoustic vibrations, in the form of music, in human cells in culture. Our results suggest that the mechanisms of cell growth arrest and/or cell death induced by acoustic vibrations are similar for auditory and nonauditory cells.

Dental noise exposed mice display depressive-like phenotypes

Background

Studies have indicated that depressive disorders are observed frequently in dentists. It’s suggested that dentists encounter numerous sources of stress in their professional career. We noticed that the noises in dental environments are very unpleasant. The animal modeling studies suggested that stressful noise could produce depressive-like phenotypes in rodent animals. We hypothesize that the dental noise may be one of the primary stressors causing depressive disorders in dentists.

Results

We treated C57BL/6 mice with programmatically played wide-spectrum dental noise for 8 h/day at 75 ± 10 dB SPL level for 30 days, and then tested the behaviors. After exposure to dental noise, animals displayed the depressive-like phenotypes, accompanied by inhibition of neurogenesis in hippocampus. These deficits were ameliorated by orally administered with antidepressant fluoxetine.

Conclusions

Our results suggested that dental noise could be one of the primary stressors for the pathogenesis of depressive disorders and the dental noise mouse model could be used in further depression studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0229-z) contains supplementary material, which is available to authorized users.

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.

Hippocampal cytogenesis and spatial learning in senile rats exposed to chronic variable stress: effects of previous early life exposure to mild stress

In this study, we exposed adult rats to chronic variable stress (CVS) and tested the hypothesis that previous early-life exposure to stress changes the manner in which older subjects respond to aversive conditions. To this end, we analyzed the cytogenic changes in the hippocampus and hippocampal-dependent spatial learning performance. The experiments were performed on 18-month-old male rats divided into four groups as follows: Control (old rats under standard laboratory conditions), Early-life stress (ELS; old rats who were exposed to environmental noise from postnatal days, PNDs 21–35), CVS + ELS (old rats exposed to a chronic stress protocol who were previously exposed to the early-life noise stress) and CVS (old rats who were exposed only to the chronic stress protocol). The Morris Water Maze (MWM) was employed to evaluate the spatial learning abilities of the rats at the end of the experiment. Immunohistochemistry against 5′Bromodeoxyuridine (BrdU) and glial fibrillar acidic protein (GFAP) was also conducted in the DG, CA1, CA2 and CA3 regions of the hippocampus. We confocally analyzed the cytogenic (BrdU-labeled cells) and astrogenic (BrdU + GFAP-labeled cells) changes produced by these conditions. Using this procedure, we found that stress diminished the total number of BrdU+ cells over the main proliferative area of the hippocampus (i.e., the dentate gyrus, DG) but increased the astrocyte phenotypes (GFAP + BrdU). The depleted BrdU+ cells were restored when the senile rats also experienced stress at the early stages of life. The MWM assessment demonstrated that stress also impairs the ability of the rats to learn the task. This impairment was not present when the stressful experience was preceded by the early-life exposure. Thus, our results support the idea that previous exposure to mild stressing agents may have beneficial effects on aged subjects.

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.

Can cell proliferation of umbilical cord blood cells reflect environmental exposures?

Environmental hazards were shown to have an impact on cell proliferation (CP). We investigated CP of lymphocytes in umbilical cord blood in relation to prenatal environmental exposures in a sample of 346 Arab-Bedouin women giving birth in a local hospital. Information on subjects' addresses at pregnancy, potential household exposures and demographical status was collected in an interview during hospitalization. This population is usually featured by high rates of neonatal morbidity and multiple environmental exposures, originating from the local industrial park (IP), household hazards and frequent male smoking. A geometric mean CP ratio 2.17 (2.06; 2.29), and was high in women residing in a direction of prevailing winds from the local IP (p value = 0.094) and who gave birth during fall-winter season (p value = 0.024). Women complaining on disturbing exposure to noise had lower CP (p value = 0.015), compared to other women. CP was not indicative of neonatal morbidity. However, our findings suggest that CP of umbilical cord might be modified by environmental exposures. A long-term follow-up of the children is required to assess their developmental outcomes.

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.

Long-term recovery from hippocampal-related behavioral and biochemical abnormalities induced by noise exposure during brain development. Evaluation of auditory pathway integrity

Sound is an important part of man's contact with the environment and has served as critical means for survival throughout his evolution. As a result of exposure to noise, physiological functions such as those involving structures of the auditory and non-auditory systems might be damaged. We have previously reported that noise-exposed developing rats elicited hippocampal-related histological, biochemical and behavioral changes. However, no data about the time lapse of these changes were reported. Moreover, measurements of auditory pathway function were not performed in exposed animals. Therefore, with the present work, we aim to test the onset and the persistence of the different extra-auditory abnormalities observed in noise-exposed rats and to evaluate auditory pathway integrity. Male Wistar rats of 15 days were exposed to moderate noise levels (95-97 dB SPL, 2 h a day) during one day (acute noise exposure, ANE) or during 15 days (sub-acute noise exposure, SANE). Hippocampal biochemical determinations as well as short (ST) and long term (LT) behavioral assessments were performed. In addition, histological and functional evaluations of the auditory pathway were carried out in exposed animals. Our results show that hippocampal-related behavioral and biochemical changes (impairments in habituation, recognition and associative memories as well as distortion of anxiety-related behavior, decreases in reactive oxygen species (ROS) levels and increases in antioxidant enzymes activities) induced by noise exposure were almost completely restored by PND 90. In addition, auditory evaluation shows that increased cochlear thresholds observed in exposed rats were re-established at PND 90, although with a remarkable supra-threshold amplitude reduction. These data suggest that noise-induced hippocampal and auditory-related alterations are mostly transient and that the effects of noise on the hippocampus might be, at least in part, mediated by the damage on the auditory pathway. However, we cannot exclude that a different mechanism might be responsible for the observed hippocampal-related changes.

Role of sound stimulation in reprogramming brain connectivity

Sensory stimulation has a critical role to play in the development of an individual. Environmental factors tend to modify the inputs received by the sensory pathway. The developing brain is most vulnerable to these alterations and interacts with the environment to modify its neural circuitry. In addition to other sensory stimuli, auditory stimulation can also act as external stimuli to provide enrichment during the perinatal period. There is evidence that suggests that enriched environment in the form of auditory stimulation can play a substantial role in modulating plasticity during the prenatal period. This review focuses on the emerging role of prenatal auditory stimulation in the development of higher brain functions such as learning and memory in birds and mammals. The molecular mechanisms of various changes in the hippocampus following sound stimulation to effect neurogenesis, learning and memory are described. Sound stimulation can also modify neural connectivity in the early postnatal life to enhance higher cognitive function or even repair the secondary damages in various neurological and psychiatric disorders. Thus, it becomes imperative to examine in detail the possible ameliorating effects of prenatal sound stimulation in existing animal models of various psychiatric disorders, such as autism.

Effects of 85 dB noise exposure on macro and trace element levels in plasma and brain areas of rats

In this study, our hypothesis was that workplace noise can alter brain element levels like immobilization and light-dark cycle shift, and we aimed to investigate the effects of workplace noise on element levels of brain areas as well as changes in blood of chronically noise-exposed rats. Twenty-four rats were randomly divided into three groups. The first group was the control. The second group (noise exposed) was subjected to daily 8 h 85 dB workplace noise for 15 days, and the third group (noise exposed plus normal condition) was also subjected to the same noise exposure and then returned to normal condition for 15 days. In noise-exposed group, Na, K, Ca, Mg , Fe, Cu, and Zn levels in plasma significantly increased when compared with controls (p = 0.03 for Na; p = 0.005 for K and Mg; p = 0.002 for Ca and Fe; p = 0.01 for Cu and Zn). In this group, Fe level of temporal lobe significantly increased (p = 0.021) while Mg level significantly increased in frontal (p = 0.021) and temporal (p = 0.001) lobes when compared with controls. In the noise-exposed plus normal condition group, plasma Na, K, Fe, Cu, and Zn levels were greatly similar to controls. But, plasma Ca and Mg levels significantly decreased when compared with controls (p = 0.002 and p = 0.007) and noise-exposed group (p = 0.002 and p = 0.001). Fe level of temporal lobe significantly increased when compared with controls (p = 0.001) and noise exposed group (p = 0.001). These alterations may be attributed to impaired intake of water and micronutrients or their excretions, emerging by acoustic stress.