Apoptotic mechanisms after repeated noise trauma in the mouse medial geniculate body and primary auditory cortex

Systemic health effects of noise exposure

Noise, any unwanted sound, is pervasive and impacts large populations worldwide. Investigators suggested that noise exposure not only induces auditory damage but also produces various organ system dysfunctions. Although previous reviews primarily focused on noise-induced cardiovascular and cerebral dysfunctions, this narrow focus has unintentionally led the research community to disregard the importance of other vital organs. Indeed, limited studies revealed that noise exposure impacts other organs including the liver, kidneys, pancreas, lung, and gastrointestinal tract. Therefore, the aim of this review was to examine the effects of noise on both the extensively studied organs, the brain and heart, but also determine noise impact on other vital organs. The goal was to illustrate a comprehensive understanding of the systemic effects of noise. These systemic effects may guide future clinical research and epidemiological endpoints, emphasizing the importance of considering noise exposure history in diagnosing various systemic diseases.

Neurodegeneration after repeated noise trauma in the mouse lower auditory pathway

High intensity noise exposure leads to a permanent shift in auditory thresholds (PTS), affecting both peripheral (cochlear) tissue and the central auditory system. Studies have shown that a noise-induced hearing loss results in significant cell loss in several auditory structures. Degeneration can be demonstrated within hours after noise exposure, particularly in the lower auditory pathway, and continues to progress over days and weeks following the trauma. However, there is limited knowledge about the effects of recurring acoustic trauma. Repeated noise exposure has been demonstrated to increase neuroplasticity and neural activity. Thus, the present study aimed to investigate the influence of a second noise exposure on the cytoarchitecture of key structures of the auditory pathway, including spiral ganglion neurons (SGN), the ventral and dorsal cochlear nucleus (VCN and DCN, respectively), and the inferior colliculus (IC). In the experiments, young adult normal hearing mice were exposed to noise once or twice (with the second trauma applied one week after the initial exposure) for 3 h, using broadband white noise (5 - 20 kHz) at 115 dB SPL. The cell densities in the investigated auditory structures significantly decreased in response to the initial noise exposure compared to unexposed control animals. These findings are consistent with earlier research, which demonstrated degeneration in the auditory pathway within the first week after acoustic trauma. Additionally, cell densities were significantly decreased after the second trauma, but this effect was only observed in the VCN, with no similar effects seen in the SGN, DCN, or IC. These results illustrate how repeated noise exposure influences the cytoarchitecture of the auditory system. It appears that an initial noise exposure primarily damages the lower auditory pathway, but surviving cellular structures may develop resistance to additional noise-induced injury.

Blast Exposure Causes Long-Term Degeneration of Neuronal Cytoskeletal Elements in the Cochlear Nucleus: A Potential Mechanism for Chronic Auditory Dysfunctions

Blast-induced auditory dysfunctions including tinnitus are the most prevalent disabilities in service members returning from recent combat operations. Most of the previous studies were focused on the effect of blast exposure on the peripheral auditory system and not much on the central auditory signal-processing regions in the brain. In the current study, we have exposed rats to single and tightly coupled repeated blasts and examined the degeneration of neuronal cytoskeletal elements using silver staining in the central auditory signal-processing regions in the brain at 24 h, 14 days, 1 month, 6 months, and 1 year. The brain regions evaluated include cochlear nucleus, lateral lemniscus, inferior colliculus, medial geniculate nucleus, and auditory cortex. The results obtained indicated that a significant increase in degeneration of neuronal cytoskeletal elements was observed only in the left and right cochlear nucleus. A significant increase in degeneration of neuronal cytoskeletal elements was observed in the cochlear nucleus at 24 h and persisted through 1 year, suggesting acute and chronic neuronal degeneration after blast exposure. No statistically significant differences were observed between single and repeated blasts. The localized degeneration of neuronal cytoskeletal elements in the cochlear nucleus suggests that the damage could be caused by transmission of blast shockwaves/noise through the ear canal and that use of suitable ear protection devices can protect against acute and chronic central auditory signal processing defects including tinnitus after blast exposure.

Age-related and noise-induced hearing loss alters grasshopper mouse (Onychomys) vocalizations

Age-related and noise-induced hearing loss disorders are among the most common pathologies affecting Americans across their lifespans. Loss of auditory feedback due to hearing disorders is correlated with changes in voice and speech-motor control in humans. Although rodents are increasingly used to model human age- and noise-induced hearing loss, few studies have assessed vocal changes after acoustic trauma. Northern grasshopper mice (Onychomys leucogaster) represent a candidate model because their hearing sensitivity is matched to the frequencies of long-distance vocalizations that are produced using vocal fold vibrations similar to human speech. In this study, we quantified changes in auditory brainstem responses (ABRs) and vocalizations related to aging and noise-induced acoustic trauma. Mice showed a progressive decrease in hearing sensitivity across 4-32 kHz, with males losing hearing more rapidly than females. In addition, noise-exposed mice had a 61.55 dB SPL decrease in ABR sensitivity following a noise exposure, with some individuals exhibiting a 21.25 dB recovery 300-330 days after noise exposure. We also found that older grasshopper mice produced calls with lower fundamental frequency. Sex differences were measured in duration of calls with females producing longer calls with age. Our findings indicate that grasshopper mice experience age- and noise- induced hearing loss and concomitant changes in vocal output, making them a promising model for hearing and communication disorders.

Progenitor cell therapy for acquired pediatric nervous system injury: Traumatic brain injury and acquired sensorineural hearing loss

While cell therapies hold remarkable promise for replacing injured cells and repairing damaged tissues, cell replacement is not the only means by which these therapies can achieve therapeutic effect. For example, recent publications show that treatment with varieties of adult, multipotent stem cells can improve outcomes in patients with neurological conditions such as traumatic brain injury and hearing loss without directly replacing damaged or lost cells. As the immune system plays a central role in injury response and tissue repair, we here suggest that multipotent stem cell therapies achieve therapeutic effect by altering the immune response to injury, thereby limiting damage due to inflammation and possibly promoting repair. These findings argue for a broader understanding of the mechanisms by which cell therapies can benefit patients.

Apoptosis in the cochlear nucleus and inferior colliculus upon repeated noise exposure

The time course of apoptosis and the corresponding neuronal loss was previously shown in central auditory pathway of mice after a single noise exposure. However, repeated acoustic exposure is a major risk factor for noise-induced hearing loss. The present study investigated apoptosis by terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end labeling (TUNEL) assay after a second noise trauma in the ventral and dorsal cochlear nucleus and central nucleus of the inferior colliculus. Mice [Naval Medical Research Institute (NMRI) strain] were noise exposed [115 dB sound pressure level, 5-20 kHz, 3 h) at day 0. A double group received the identical noise exposure a second time at day 7 post-exposure and apoptosis was either analyzed immediately (7-day group-double) or 1 week later (14-day group-double). Corresponding single exposure groups were chosen as controls. No differences in TUNEL were seen between 7-day or 14-day single and double-trauma groups. Interestingly, independent of the second noise exposure, apoptosis increased significantly in the 14-day groups compared to the 7-day groups in all investigated areas. It seems that the first noise trauma has a long-lasting effect on apoptotic mechanisms in the central auditory pathway that were not largely influenced by a second trauma. Homeostatic mechanisms induced by the first trauma might protect the central auditory pathway from further damage during a specific time slot. These results might help to understand the underlying mechanisms of different psychoacoustic phenomena in noise-induced hearing loss.

Repeated Moderate Sound Exposure Causes Accumulated Trauma to Cochlear Ribbon Synapses in Mice

Repeated induction of a temporary threshold shift (TTS) may result in a permanent threshold shift (PTS) and is thought to be associated with early onset of age-related hearing loss (ARHL). The possibility that a PTS might be induced by administration of repeated TTS-inducing noise exposures (NEs) over a short period during early adulthood has not been formally investigated. We aimed to investigate possible cumulative acoustic overstimulation effects that permanently shift the auditory threshold. Young adult C57BL/6J mice were exposed twice to moderate white noise in an experimental design that minimized the effects of aging. The first exposure resulted in a reversible noise-induced hearing loss (NIHL) measured as recoverable alterations in auditory brainstem response (ABR) thresholds, waveform amplitudes, and numbers of ribbon synapses. The second NE with the same parameters caused persistent threshold shifts, wave I amplitude reductions, wave IV/I ratio enhancements, and synaptic losses, even though recovery time sufficient for a TTS had been provided. The pattern of PTS resembled NIHL since the observed impairments tonotopically followed the power spectrum of the noise insult, rather than ARHL, which distributes at higher frequencies. No significant changes were observed in the control group as the mice aged. To conclude, our results demonstrate a cumulative effect of repetitive TTS-inducing NE on hearing function and synaptic plasticity that does not cause premature ARHL, thereby providing insight into the pathophysiological mechanisms underlying NIHL and ARHL.

Hyperbaric Oxygen Treatment Ameliorates Hearing Loss and Auditory Cortex Injury in Noise Exposed Mice by Repressing Local Ceramide Accumulation

Noise-induced hearing loss (NIHL) relates closely to auditory cortex (AC) injury, so countermeasures aiming at the AC recovery would be of benefit. In this work, the effect of hyperbaric oxygen treatment on NIHL was elucidated, which was imposed on mice before (HBOP), during (HBOD) or after (HBOA) noise exposure. Morphology of neurons was assayed by hematoxylin-eosin or Nissl staining. Ceramide (Cer) level was measured through immunohistochemistry analysis. Apoptotic neurons were counted using transferase-mediated dUTP nick end labeling (TUNEL) staining. We demonstrated that the intense, broad band noise raised the threshold of auditory brainstem response, evoked neuronal degeneration or apoptosis and triggered the Cer accumulation in AC, all of which were restored significantly by HBOP, but not HBOD or HBOA. Cer over-generation reversed the advantages of HBOP significantly, while its curtailment recapitulated the effect. Next, noise exposure raised the superoxide or malondialdehyde (MDA) production which was blocked by HBOP or Cer repression. Oxidative control not only attenuated the hearing loss or neurodegeneration but, in turn, reduced the Cer formation significantly. In summary, mutual regulation between Cer and oxidative stress underlies the HBOP’s curative effect on hearing loss and neuronal damage in noise-exposed mice.

[Noise-induced neurodegeneration in the central auditory pathway : An overview of experimental studies in a mouse model]

BACKGROUND

A noise trauma induces central nervous system pathologies, which generate deficits in hearing and perception of sound.

OBJECTIVE

Are degenerative mechanisms in the central auditory system a direct impact of overstimulation or an effect of acoustic deprivation?

MATERIALS AND METHODS

Detection of cell death in a mouse model of noise-induced hearing loss at different times after single or repeated noise exposure.

RESULTS

A single noise exposure (3 h, 115 dB SPL, 5-20 kHz) induces acute (≤1 day) and long-term (observation period 14 days) degeneration, particularly in subcortical structures. Repeated noise trauma is followed by pathologies in the auditory thalamus and cortex.

CONCLUSION

Noise has a direct impact on basal structures of the central auditory system; a protection of cortical areas is possibly due to inhibitory neuronal projections. Degenerative mechanisms in higher structures of the pre-damaged system point to an increased impairment of complex processing of acoustic information.