The cochlea as an independent neuroendocrine organ: expression and possible roles of a local hypothalamic-pituitary-adrenal axis-equivalent signaling system

Hearing Impairment and Allostatic Load in Older Adults

Importance

Allostatic load, the cumulative strain that results from the chronic stress response, is associated with poor health outcomes. Increased cognitive load and impaired communication associated with hearing loss could potentially be associated with higher allostatic load, but few studies to date have quantified this association.

Objective

To investigate if audiometric hearing loss is associated with allostatic load and evaluate if the association varies by demographic factors.

Design, Setting, Participants

This cross-sectional survey used nationally representative data from the National Health and Nutrition Examination Survey. Audiometric testing was conducted from 2003 to 2004 (ages 20-69 years) and 2009 to 2010 (70 years or older). The study was restricted to participants aged 50 years or older, and the analysis was stratified based on cycle. The data were analyzed between October 2021 and October 2022.

Exposure

A 4-frequency (0.5-4.0 kHz) pure tone average was calculated in the better-hearing ear and modeled continuously and categorically (<25 dB hearing level [dB HL], no hearing loss; 26-40 dB HL, mild hearing loss; ≥41 dB HL, moderate or greater hearing loss).

Main Outcome and Measures

Allostatic load score (ALS) was defined using laboratory measurements of 8 biomarkers (systolic/diastolic blood pressure, body mass index [calculated as weight in kilograms divided by height in meters squared], and total serum and high-density lipoprotein cholesterol, glycohemoglobin, albumin, and C-reactive protein levels). Each biomarker was assigned a point if it was in the highest risk quartile based on statistical distribution and then summed to yield the ALS (range, 0-8). Linear regression models adjusted for demographic and clinical covariates. Sensitivity analysis included using clinical cut points for ALS and subgroup stratification.

Results

In 1412 participants (mean [SD] age, 59.7 [5.9] years; 293 women [51.9%]; 130 [23.0%] Hispanic, 89 [15.8%] non-Hispanic Black, and 318 [55.3%] non-Hispanic White individuals), a modest association was suggested between hearing loss and ALS (ages 50-69 years: β = 0.19 [95% CI, 0.02-0.36] per 10 dB HL; 70 years or older: β = 0.10 [95% CI, 0.02-0.18] per 10 dB HL) among non-hearing aid users. Results were not clearly reflected in the sensitivity analysis with clinical cut points for ALS or modeling hearing loss categorically. Sex-based stratifications identified a stronger association among male individuals (men 70 years or older: β = 0.22 [95% CI, 0.12-0.32] per 10 dB HL; women: β = 0.08 [95% CI, -0.04 to 0.20] per 10 dB HL).

Conclusion and Relevance

The study findings did not clearly support an association between hearing loss and ALS. While hearing loss has been shown to be associated with increased risk for numerous health comorbidities, its association with the chronic stress response and allostasis may be less than that of other health conditions.

Pharmacological Modulation of Energy and Metabolic Pathways Protects Hearing in the Fus1/Tusc2 Knockout Model of Mitochondrial Dysfunction and Oxidative Stress

Tightly regulated and robust mitochondrial activities are critical for normal hearing. Previously, we demonstrated that Fus1/Tusc2 KO mice with mitochondrial dysfunction exhibit premature hearing loss. Molecular analysis of the cochlea revealed hyperactivation of the mTOR pathway, oxidative stress, and altered mitochondrial morphology and quantity, suggesting compromised energy sensing and production. Here, we investigated whether the pharmacological modulation of metabolic pathways using rapamycin (RAPA) or 2-deoxy-D-glucose (2-DG) supplementation can protect against hearing loss in female Fus1 KO mice. Additionally, we aimed to identify mitochondria- and Fus1/Tusc2-dependent molecular pathways and processes critical for hearing. We found that inhibiting mTOR or activating alternative mitochondrial energetic pathways to glycolysis protected hearing in the mice. Comparative gene expression analysis revealed the dysregulation of critical biological processes in the KO cochlea, including mitochondrial metabolism, neural and immune responses, and the cochlear hypothalamic-pituitary-adrenal axis signaling system. RAPA and 2-DG mostly normalized these processes, although some genes showed a drug-specific response or no response at all. Interestingly, both drugs resulted in a pronounced upregulation of critical hearing-related genes not altered in the non-treated KO cochlea, including cytoskeletal and motor proteins and calcium-linked transporters and voltage-gated channels. These findings suggest that the pharmacological modulation of mitochondrial metabolism and bioenergetics may restore and activate processes critical for hearing, thereby protecting against hearing loss.

Early-Life Stress Impairs Perception and Neural Encoding of Rapid Signals in the Auditory Pathway

During developmental critical periods (CPs), early-life stress (ELS) induces cognitive deficits and alters neural circuitry in regions underlying learning, memory, and attention. Mechanisms underlying critical period plasticity are shared by sensory cortices and these higher neural regions, suggesting that sensory processing may also be vulnerable to ELS. In particular, the perception and auditory cortical (ACx) encoding of temporally-varying sounds both mature gradually, even into adolescence, providing an extended postnatal window of susceptibility. To examine the effects of ELS on temporal processing, we developed a model of ELS in the Mongolian gerbil, a well-established model for auditory processing. In both male and female animals, ELS induction impaired the behavioral detection of short gaps in sound, which are critical for speech perception. This was accompanied by reduced neural responses to gaps in auditory cortex, the auditory periphery, and auditory brainstem. ELS thus degrades the fidelity of sensory representations available to higher regions, and could contribute to well-known ELS-induced problems with cognition.SIGNIFICANCE STATEMENT In children and animal models, early-life stress (ELS) leads to deficits in cognition, including problems with learning, memory, and attention. Such problems could arise in part from a low-fidelity representation of sensory information available to higher-level neural regions. Here, we demonstrate that ELS degrades sensory responses to rapid variations in sound at multiple levels of the auditory pathway, and concurrently impairs perception of these rapidly-varying sounds. As these sound variations are intrinsic to speech, ELS may thus pose a challenge to communication and cognition through impaired sensory encoding.

Conditional Ablation of Glucocorticoid and Mineralocorticoid Receptors from Cochlear Supporting Cells Reveals Their Differential Roles for Hearing Sensitivity and Dynamics of Recovery from Noise-Induced Hearing Loss

Endogenous glucocorticoids (GC) are known to modulate basic elements of cochlear physiology. These include both noise-induced injury and circadian rhythms. While GC signaling in the cochlea can directly influence auditory transduction via actions on hair cells and spiral ganglion neurons, evidence also indicates that GC signaling exerts effects via tissue homeostatic processes that can include effects on cochlear immunomodulation. GCs act at both the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). Most cell types in the cochlea express both receptors sensitive to GCs. The GR is associated with acquired sensorineural hearing loss (SNHL) through its effects on both gene expression and immunomodulatory programs. The MR has been associated with age-related hearing loss through dysfunction of ionic homeostatic balance. Cochlear supporting cells maintain local homeostatic requirements, are sensitive to perturbation, and participate in inflammatory signaling. Here, we have used conditional gene manipulation techniques to target Nr3c1 (GR) or Nr3c2 (MR) for tamoxifen-induced gene ablation in Sox9-expressing cochlear supporting cells of adult mice to investigate whether either of the receptors sensitive to GCs plays a role in protecting against (or exacerbating) noise-induced cochlear damage. We have selected mild intensity noise exposure to examine the role of these receptors related to more commonly experienced noise levels. Our results reveal distinct roles of these GC receptors for both basal auditory thresholds prior to noise exposure and during recovery from mild noise exposure. Prior to noise exposure, auditory brainstem responses (ABRs) were measured in mice carrying the floxed allele of interest and the Cre recombinase transgene, but not receiving tamoxifen injections (defined as control (no tamoxifen treatment), versus conditional knockout (cKO) mice, defined as mice having received tamoxifen injections. Results revealed hypersensitive thresholds to mid- to low-frequencies after tamoxifen-induced GR ablation from Sox9-expressing cochlear supporting cells compared to control (no tamoxifen) mice. GR ablation from Sox9-expressing cochlear supporting cells resulted in a permanent threshold shift in mid-basal cochlear frequency regions after mild noise exposure that produced only a temporary threshold shift in both control (no tamoxifen) f/fGR:Sox9iCre+ and heterozygous f/+GR:Sox9iCre+ tamoxifen-treated mice. A similar comparison of basal ABRs measured in control (no tamoxifen) and tamoxifen-treated, floxed MR mice prior to noise exposure indicated no difference in baseline thresholds. After mild noise exposure, MR ablation was initially associated with a complete threshold recovery at 22.6 kHz by 3 days post-noise. Threshold continued to shift to higher sensitivity over time such that by 30 days post-noise exposure the 22.6 kHz ABR threshold was 10 dB more sensitive than baseline. Further, MR ablation produced a temporary reduction in peak 1 neural amplitude one day post-noise. While supporting cell GR ablation trended towards reducing numbers of ribbon synapses, MR ablation reduced ribbon synapse counts but did not exacerbate noise-induced damage including synapse loss at the experimental endpoint. GR ablation from the targeted supporting cells increased the basal resting number of Iba1-positive (innate) immune cells (no noise exposure) and decreased the number of Iba1-positive cells seven days following noise exposure. MR ablation did not alter innate immune cell numbers at seven days post-noise exposure. Taken together, these findings support differential roles of cochlear supporting cell MR and GR expression at basal, resting conditions and especially during recovery from noise exposure.

The initiator of neuroexcitotoxicity and ferroptosis in ischemic stroke: Glutamate accumulation

Glutamate plays an important role in excitotoxicity and ferroptosis. Excitotoxicity occurs through over-stimulation of glutamate receptors, specifically NMDAR, while in the non-receptor-mediated pathway, high glutamate concentrations reduce cystine uptake by inhibiting the System Xc-, leading to intracellular glutathione depletion and resulting in ROS accumulation, which contributes to increased lipid peroxidation, mitochondrial damage, and ultimately ferroptosis. Oxidative stress appears to crosstalk between excitotoxicity and ferroptosis, and it is essential to maintain glutamate homeostasis and inhibit oxidative stress responses in vivo. As researchers work to develop natural compounds to further investigate the complex mechanisms and regulatory functions of ferroptosis and excitotoxicity, new avenues will be available for the effective treatment of ischaemic stroke. Therefore, this paper provides a review of the molecular mechanisms and treatment of glutamate-mediated excitotoxicity and ferroptosis.

Expression Patterns of the Neuropeptide Urocortin 3 and Its Receptor CRFR2 in the Mouse Central Auditory System

Sensory systems have to be malleable to context-dependent modulations occurring over different time scales, in order to serve their evolutionary function of informing about the external world while also eliciting survival-promoting behaviors. Stress is a major context-dependent signal that can have fast and delayed effects on sensory systems, especially on the auditory system. Urocortin 3 (UCN3) is a member of the corticotropin-releasing factor family. As a neuropeptide, UCN3 regulates synaptic activity much faster than the classic steroid hormones of the hypothalamic-pituitary-adrenal axis. Moreover, due to the lack of synaptic re-uptake mechanisms, UCN3 can have more long-lasting and far-reaching effects. To date, a modest number of studies have reported the presence of UCN3 or its receptor CRFR2 in the auditory system, particularly in the cochlea and the superior olivary complex, and have highlighted the importance of this stress neuropeptide for protecting auditory function. However, a comprehensive map of all neurons synthesizing UCN3 or CRFR2 within the auditory pathway is lacking. Here, we utilize two reporter mouse lines to elucidate the expression patterns of UCN3 and CRFR2 in the auditory system. Additional immunolabelling enables further characterization of the neurons that synthesize UCN3 or CRFR2. Surprisingly, our results indicate that within the auditory system, UCN3 is expressed predominantly in principal cells, whereas CRFR2 expression is strongest in non-principal, presumably multisensory, cell types. Based on the presence or absence of overlap between UCN3 and CRFR2 labeling, our data suggest unusual modes of neuromodulation by UCN3, involving volume transmission and autocrine signaling.

Urocortin 3 signalling in the auditory brainstem aids recovery of hearing after reversible noise-induced threshold shift

KEY POINTS

Ongoing, moderate noise exposure does not instantly damage the auditory system but may cause lasting deficits, such as elevated thresholds and accelerated ageing of the auditory system. The neuromodulatory peptide urocortin-3 (UCN3) is involved in the body's recovery from a stress response, and is also expressed in the cochlea and the auditory brainstem. Lack of UCN3 facilitates age-induced hearing loss and causes permanently elevated auditory thresholds following a single 2 h noise exposure at moderate intensities. Outer hair cell function in mice lacking UCN3 is unaffected, so that the observed auditory deficits are most likely due to inner hair cell function or central mechanisms. Highly specific, rather than ubiquitous, expression of UCN3 in the brain renders it a promising candidate for designing drugs to ameliorate stress-related auditory deficits, including recovery from acoustic trauma.

ABSTRACT

Environmental acoustic noise is omnipresent in our modern society, with sound levels that are considered non-damaging still causing long-lasting or permanent changes in the auditory system. The small neuromodulatory peptide urocortin-3 (UCN3) is the endogenous ligand for corticotropin-releasing factor receptor type 2 and together they are known to play an important role in stress recovery. UCN3 expression has been observed in the auditory brainstem, but its role remains unclear. Here we describe the detailed distribution of UCN3 expression in the murine auditory brainstem and provide evidence that UCN3 is expressed in the synaptic region of inner hair cells in the cochlea. We also show that mice with deficient UCN3 signalling experience premature ageing of the auditory system starting at an age of 4.7 months with significantly elevated thresholds of auditory brainstem responses (ABRs) compared to age-matched wild-type mice. Following a single, 2 h exposure to moderate (84 or 94 dB SPL) noise, UCN3-deficient mice exhibited significantly larger shifts in ABR thresholds combined with maladaptive recovery. In wild-type mice, the same noise exposure did not cause lasting changes to auditory thresholds. The presence of UCN3-expressing neurons throughout the auditory brainstem and the predisposition to hearing loss caused by preventing its normal expression suggests UCN3 as an important neuromodulatory peptide in the auditory system's response to loud sounds.

Melatonin Attenuates Noise Stress-induced Gastrointestinal Motility Disorder and Gastric Stress Ulcer: Role of Gastrointestinal Hormones and Oxidative Stress in Rats

BACKGROUND/AIMS

There are increasing evidences for gastrointestinal motility disorder (GIMD) and gastric stress ulcer induced by noise stress. The present study was to investigate the reversed effect of melatonin on GIMD and gastric stress ulcer induced by noise stress and potential mechanism.

METHODS

Noise stress was induced on rats, and melatonin (15 mg/kg) was administered to rats by intraperitoneal injection. Differences were assessed in gastric residual rate (GRR), small intestine propulsion rate (SPR), Guth injury score, cortisol, gastrointestinal hormones (calcitonin-gene-related peptide and motilin) and oxidative stress markers (superoxide dismutase and malondialde hyde) in blood plasma as well as gastric mucosa homogenate with or without melatonin. The pathological examination of gastric mucosa was also performed.

RESULTS

The GRR and SPR were improved by noise stress compared with control (P < 0.05). The pathological examination and Guth injury score revealed gastric stress ulcer. Moreover, the levels of cortisol, motilin and malondialdehyde in blood plasma and ma-londialdehyde in gastric mucosa homogenate were increased by noise stress (P < 0.05). CGRP and superoxide dismutase activ-ity in both of blood plasma and gastric mucosa homogenate were significantly decreased (P< 0.05). Furthermore, melatonin reversed changes in GRR, SPR, pathological examination, Guth injury score, cortisol, motilin, CGRP, superoxide dismutase activity and malondialdehyde (P < 0.05).

CONCLUSIONS

Melatonin is effective in reversing the GIMD and gastric stress ulcer induced by noise stress. The underlying mechanism may be involved in oxidative stress and gastrointestinal hormones.

Noise-induced hearing loss in the 21st century: A research and translational update

Millions of people worldwide are exposed to harmful levels of noise daily in their work and leisure environment. This makes noise-induced hearing loss (NIHL) a major occupational health risk globally. NIHL is the second most common form of acquired hearing loss after age-related hearing loss and is itself a major contributing factor to presbycusis. Temporary threshold shifts, once thought to be relatively harmless and recoverable, are now known to cause permanent cochlear injury leading to permanent loss of hearing sensitivity. This article reviews the current understanding of the cellular and molecular pathophysiology of NIHL with latest findings from animal models. Therapeutic approaches to protect against or to mitigate NIHL are discussed based on their proposed action against these known mechanisms of cochlear injury. Successes in identifying genes that predispose individuals to NIHL by candidate gene association studies are discussed with matched gene knockout animal models. This links to exciting developments in experimental gene therapy to replace and regenerate lost hair cells and post-noise otoprotective therapies currently being investigated in clinical trials. The aim is to provide new insights into current and projected future strategies to manage NIHL; bench to bedside treatment is foreseeable in the next 5 to 10 years.