High quality RNA extraction of the mammalian cochlea for qRT-PCR and transcriptome analyses

Comparison of five commercial kits for isolation of total RNA in samples of WSSV-infected shrimp

Viral diseases are the most serious threat to the expansion and development of shrimp aquaculture. Rapid diagnosis of the white spot syndrome virus (WSSV), a lethal shrimp pathogen, is essential to restrict its spread and reduce the mortality of infected shrimp. This virus has globally affected the shrimp farming industry, with a devastating economic impact. Several studies have focused on the expression of WSSV transcripts to understand the molecular mechanisms governing the pathological development of the disease. Since gene expression studies and molecular diagnostics at the early stages of infection depend on the efficient isolation of high-quality RNA, the extraction methods should be carefully selected. However, previous comparisons of the performance of RNA isolation kits have yet to be systematically investigated. In this study, 5 commercial RNA extraction methods were compared in WSSV-infected shrimp. The highest total RNA yield (ng mg-1 tissue) was obtained using TRIzol. Even though the 260/280 nm absorption ratios showed significant differences, the methods showed good purity values (>2.0). RNA integrity was evaluated in a denaturing agarose gel electrophoresis, and degradation was observed after the total RNA samples were treated with DNase I. Finally, the method that allowed the earlier detection of WSSV transcripts by qRT-PCR was the Zymo Direct-zol RNA MiniPrep kit. This study shows that the amount of observed (or estimated) WSSV transcripts might be affected because of the RNA isolation method. In addition, these results may contribute to improve the accuracy of the results obtained in gene expression studies, for more sensitive and robust detection of WSSV.

Systemic gene therapy rescues retinal dysfunction and hearing loss in a model of Norrie disease

Deafness affects 5% of the world's population, yet there is a lack of treatments to prevent hearing loss due to genetic causes. Norrie disease is a recessive X-linked disorder, caused by NDP gene mutation. It manifests as blindness at birth and progressive sensorineural hearing loss, leading to debilitating dual sensory deprivation. To develop a gene therapy, we used a Norrie disease mouse model (Ndptm1Wbrg ), which recapitulates abnormal retinal vascularisation and progressive hearing loss. We delivered human NDP cDNA by intravenous injection of adeno-associated viral vector (AAV)9 at neonatal, juvenile and young adult pathological stages and investigated its therapeutic effects on the retina and cochlea. Neonatal treatment prevented the death of the sensory cochlear hair cells and rescued cochlear disease biomarkers as demonstrated by RNAseq and physiological measurements of auditory function. Retinal vascularisation and electroretinograms were restored to normal by neonatal treatment. Delivery of NDP gene therapy after the onset of the degenerative inner ear disease also ameliorated the cochlear pathology, supporting the feasibility of a clinical treatment for progressive hearing loss in people with Norrie disease.

Long-term NAD+ supplementation prevents the progression of age-related hearing loss in mice

Age-related hearing loss (ARHL) is the most common sensory disability associated with human aging. Yet, there are no approved measures for preventing or treating this debilitating condition. With its slow progression, continuous and safe approaches are critical for ARHL treatment. Nicotinamide Riboside (NR), a NAD+ precursor, is well tolerated even for long-term use and is already shown effective in various disease models including Alzheimer's and Parkinson's disease. It has also been beneficial against noise-induced hearing loss and in hearing loss associated with premature aging. However, its beneficial impact on ARHL is not known. Using two different wild-type mouse strains, we show that long-term NR administration prevents the progression of ARHL. Through transcriptomic and biochemical analysis, we find that NR administration restores age-associated reduction in cochlear NAD+ levels, upregulates biological pathways associated with synaptic transmission and PPAR signaling, and reduces the number of orphan ribbon synapses between afferent auditory neurons and inner hair cells. We also find that NR targets a novel pathway of lipid droplets in the cochlea by inducing the expression of CIDEC and PLIN1 proteins that are downstream of PPAR signaling and are key for lipid droplet growth. Taken together, our results demonstrate the therapeutic potential of NR treatment for ARHL and provide novel insights into its mechanism of action.

Long-term survival of LGR5 expressing supporting cells after severe ototoxic trauma in the adult mouse cochlea

Introduction

The leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5) is a tissue resident stem cell marker, which it is expressed in supporting cells (SCs) in the organ of Corti in the mammalian inner ear. These LGR5+ SCs can be used as an endogenous source of progenitor cells for regeneration of hair cells (HCs) to treat hearing loss and deafness. We have recently reported that LGR5+ SCs survive 1 week after ototoxic trauma. Here, we evaluated Lgr5 expression in the adult cochlea and long-term survival of LGR5+ SCs following severe hearing loss.

Methods

Lgr5GFP transgenic mice and wild type mice aged postnatal day 30 (P30) and P200 were used. P30 animals were deafened with a single dose of furosemide and kanamycin. Seven and 28 days after deafening, auditory brainstem responses (ABRs) were recorded. Cochleas were harvested to characterize mature HCs and LGR5+ SCs by immunofluorescence microscopy and quantitative reverse transcription PCR (q-RT-PCR).

Results

There were no significant age-related changes in Lgr5 expression when comparing normal-hearing (NH) mice aged P200 with P30. Seven and 28 days after ototoxic trauma, there was severe outer HC loss and LGR5 was expressed in the third row of Deiters' cells and in inner pillar cells. Seven days after induction of ototoxic trauma there was an up-regulation of the mRNA expression of Lgr5 compared to the NH condition; 28 days after ototoxic trauma Lgr5 expression was similar to NH levels.

Discussion

The presence of LGR5+ SCs in the adult mouse cochlea, which persists after severe HC loss, suggests potential regenerative capacity of endogenous cochlear progenitor cells in adulthood. To our knowledge, this is the first study showing not only long-term survival of LGR5+ SCs in the normal and ototoxically damaged cochlea, but also increased Lgr5 expression in the adult mouse cochlea after deafening, suggesting long-term availability of potential target cells for future regenerative therapies.

RNA Extraction from Cartilage: Issues, Methods, Tips

The increase in degenerative diseases involving articular cartilage has pushed research to focus on their pathogenesis and treatment, exploiting increasingly complex techniques. Gene expression analyses from tissue are representative of the in vivo situation, but the protocols to be applied to obtain a reliable analysis are not completely cleared through customs. Thus, RNA extraction from fresh samples and specifically from musculoskeletal tissue such as cartilage is still a challenging issue. The aim of the review is to provide an overview of the techniques described in the literature for RNA extraction, highlighting limits and possibilities. The research retrieved 65 papers suitable for the purposes. The results highlighted the great difficulty in comparing the different studies, both for the sources of tissue used and for the techniques employed, as well as the details about protocols. Few papers compared different RNA extraction methods or homogenization techniques; the case study reported by authors about RNA extraction from sheep cartilage has not found an analog in the literature, confirming the existence of a relevant blank on studies about RNA extraction from cartilage tissue. However, the state of the art depicted can be used as a starting point to improve and expand studies on this topic.

Development and in vivo validation of small interfering RNAs targeting NOX3 to prevent sensorineural hearing loss

The reactive oxygen species (ROS)-generating enzyme NOX3 has recently been implicated in the pathophysiology of several acquired forms of sensorineural hearing loss, including cisplatin-, noise- and age-related hearing loss. NOX3 is highly and specifically expressed in the inner ear and therefore represents an attractive target for specific intervention aiming at otoprotection. Despite the strong rationale to inhibit NOX3, there is currently no specific pharmacological inhibitor available. Molecular therapy may represent a powerful alternative. In this study, we developed and tested a collection of small interfering (si) RNA constructs to establish a proof of concept of NOX3 inhibition through local delivery in the mouse inner ear. The inhibitory potential of 10 different siRNA constructs was first assessed in three different cells lines expressing the NOX3 complex. Efficacy of the most promising siRNA construct to knock-down NOX3 was then further assessed in vivo, comparing middle ear delivery and direct intracochlear delivery through the posterior semi-circular canal. While hearing was completely preserved through the intervention, a significant downregulation of NOX3 expression in the mouse inner ear and particularly in the spiral ganglion area at clinically relevant levels (>60%) was observed 48 h after treatment. In contrast to successful intracochlear delivery, middle ear administration of siRNA failed to significantly inhibit Nox3 mRNA expression. In conclusion, intracochlear delivery of NOX3-siRNAs induces a robust temporal NOX3 downregulation, which could be of relevance to prevent predictable acute insults such as cisplatin chemotherapy-mediated ototoxicity and other forms of acquired hearing loss, including post-prevention of noise-induced hearing loss immediately after trauma. Successful translation of our concept into an eventual clinical use in humans will depend on the development of atraumatic and efficient delivery routes into the cochlea without a risk to induce hearing loss through the intervention.

Downregulation of REST in the cochlea contributes to age-related hearing loss via the p53 apoptosis pathway

Age-related hearing loss (AHL) is the most common sensory disorder amongst the elderly population. Although the degeneration of spiral ganglion neurons (SGNs) and hair cells (HCs) is considered to play a critical role in AHL, the mechanism has not been fully outlined. The repressor element 1-silencing transcription factor (REST) has recently been associated with mediating cell death in neurodegenerative diseases. However, whether REST induces degeneration of cochlear HCs and SGNs to contribute to AHL remains unknown. Here, we report that REST expression was decreased in HCs and SGNs in AHL mice. Conditional deletion of Rest in HCs and SGNs of 2-month-old mice resulted in hearing loss accompanied by the upregulation of p53, TNFR1(tumor necrosis factor receptor-1), and cleaved caspase-3. The p53 inhibitor pifithrin-α significantly attenuated SGN and HC damage and rescued hearing impairment in Rest cKO mice. Furthermore, downregulation of REST by H₂O₂ treatment induced apoptosis in the House Ear Institute Organ of Corti 1 cell, through the upregulation of p53. In contrast, overexpression of REST reversed the changes in p53 expression. In addition, REST was further shown to bind directly to the p53 promoter site, thereby inhibiting the effect of p53. Finally, in aged mice, the p53 inhibitor significantly reduced loss of HCs and SGNs, and subsequently improved hearing. In summary, our findings indicate that REST has a protective role in AHL, and that its deficiency upregulates p53 and induces cochlear cell apoptosis, which that leads to deafness.

NADPH Oxidase 3 Deficiency Protects From Noise-Induced Sensorineural Hearing Loss

The reactive oxygen species (ROS)-generating NADPH oxidase NOX3 isoform is highly and specifically expressed in the inner ear. NOX3 is needed for normal vestibular development but NOX-derived ROS have also been implicated in the pathophysiology of sensorineural hearing loss. The role of NOX-derived ROS in noise-induced hearing loss, however, remains unclear and was addressed with the present study. Two different mouse strains, deficient in NOX3 or its critical subunit p22phox, were subjected to a single noise exposure of 2 h using an 8-16 kHz band noise at an intensity of 116-120 decibel sound pressure level. In the hours following noise exposure, there was a significant increase in cochlear mRNA expression of NOX3 in wild type animals. By using RNAscope in situ hybridization, NOX3 expression was primarily found in the Rosenthal canal area, colocalizing with auditory neurons. One day after the noise trauma, we observed a high frequency hearing loss in both knock-out mice, as well as their wild type littermates. At day seven after noise trauma however, NOX3 and p22phox knockout mice showed a significantly improved hearing recovery and a marked preservation of neurosensory cochlear structures compared to their wild type littermates. Based on these findings, an active role of NOX3 in the pathophysiology of noise-induced hearing loss can be demonstrated, in line with recent evidence obtained in other forms of acquired hearing loss. The present data demonstrates that the absence of functional NOX3 enhances the hearing recovery phase following noise trauma. This opens an interesting clinical window for pharmacological or molecular intervention aiming at post prevention of noise-induced hearing loss.

Low-salt diet increases mRNA expression of aldosterone-regulated transporters in the intermediate portion of the endolymphatic sac

The endolymphatic sac is a small sac-shaped organ at the end of the membranous labyrinth of the inner ear. The endolymphatic sac absorbs the endolymph, in which the ion balance is crucial for inner ear homeostasis. Of the three sections of the endolymphatic sac, the intermediate portion is the center of endolymph absorption, particularly sodium transport, and is thought to be regulated by aldosterone. Disorders of the endolymphatic sac may cause an excess of endolymph (endolymphatic hydrops), a histological observation in Meniere's disease. A low-salt diet is an effective treatment for Meniere's disease, and is based on the assumption that the absorption of endolymph in the endolymphatic sac abates endolymphatic hydrops through a physiological increase in aldosterone level. However, the molecular basis of endolymph absorption in each portion of the endolymphatic sac is largely unknown because of difficulties in gene expression analysis, resulting from its small size and intricate structure. The present study combined reverse transcription-quantitative polymerase chain reaction and laser capture microdissection techniques to analyze the difference of gene expression of the aldosterone-controlled epithelial Na⁺ channel, thiazide-sensitive Na⁺-Cl^- cotransporter, and Na⁺, K⁺-ATPase genes in the three individual portions of the endolymphatic sac in a rat model. A low-salt diet increased the expression of aldosterone-controlled ion transporters, particularly in the intermediate portion of the endolymphatic sac. Our findings will contribute to the understanding of the physiological function of the endolymphatic sac and the pathophysiology of Meniere's disease.

Disruption of Hars2 in Cochlear Hair Cells Causes Progressive Mitochondrial Dysfunction and Hearing Loss in Mice

Mutations in a number of genes encoding mitochondrial aminoacyl-tRNA synthetases lead to non-syndromic and/or syndromic sensorineural hearing loss in humans, while their cellular and physiological pathology in cochlea has rarely been investigated in vivo. In this study, we showed that histidyl-tRNA synthetase HARS2, whose deficiency is associated with Perrault syndrome 2 (PRLTS2), is robustly expressed in postnatal mouse cochlea including the outer and inner hair cells. Targeted knockout of Hars2 in mouse hair cells resulted in delayed onset (P30), rapidly progressive hearing loss similar to the PRLTS2 hearing phenotype. Significant hair cell loss was observed starting from P45 following elevated reactive oxygen species (ROS) level and activated mitochondrial apoptotic pathway. Despite of normal ribbon synapse formation, whole-cell patch clamp of the inner hair cells revealed reduced calcium influx and compromised sustained synaptic exocytosis prior to the hair cell loss at P30, consistent with the decreased supra-threshold wave I amplitudes of the auditory brainstem response. Starting from P14, increasing proportion of morphologically abnormal mitochondria was observed by transmission electron microscope, exhibiting swelling, deformation, loss of cristae and emergence of large intrinsic vacuoles that are associated with mitochondrial dysfunction. Though the mitochondrial abnormalities are more prominent in inner hair cells, it is the outer hair cells suffering more severe cell loss. Taken together, our results suggest that conditional knockout of Hars2 in mouse cochlear hair cells leads to accumulating mitochondrial dysfunction and ROS stress, triggers progressive hearing loss highlighted by hair cell synaptopathy and apoptosis, and is differentially perceived by inner and outer hair cells.

Redox activation of excitatory pathways in auditory neurons as mechanism of age-related hearing loss

Age-related hearing (ARHL) loss affects a large part of the human population with a major impact on our aging societies. Yet, underlying mechanisms are not understood, and no validated therapy or prevention exists. NADPH oxidases (NOX), are important sources of reactive oxygen species (ROS) in the cochlea and might therefore be involved in the pathogenesis of ARHL. Here we investigate ARHL in a mouse model. Wild type mice showed early loss of hearing and cochlear integrity, while animals deficient in the NOX subunit p22^phox remained unaffected up to six months. Genes of the excitatory pathway were down-regulated in p22^phox-deficient auditory neurons. Our results demonstrate that NOX activity leads to upregulation of genes of the excitatory pathway, to excitotoxic cochlear damage, and ultimately to ARHL. In the absence of functional NOXs, aging mice conserve hearing and cochlear morphology. Our study offers new insights into pathomechanisms and future therapeutic targets of ARHL.

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Mice devoid of NADPH oxidase (NOX) activity are protected from age-related hearing loss.

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Cochlear NOX expression shows a similar pattern in mouse and human.

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NOX3, the predominant NOX isoform in the cochlea, is mostly expressed in auditory neurons.

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NOX-deficient auditory neurons show decreased transcription of glutamatergic pathway and are protected from excitotoxicity.

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NOX-mediated gene regulation within auditory neurons contributes to age-related hearing loss.

Circadian Regulation of Cochlear Sensitivity to Noise by Circulating Glucocorticoids

The cochlea possesses a robust circadian clock machinery that regulates auditory function. How the cochlear clock is influenced by the circadian system remains unknown. Here, we show that cochlear rhythms are system driven and require local Bmal1 as well as central input from the suprachiasmatic nuclei (SCN). SCN ablations disrupted the circadian expression of the core clock genes in the cochlea. Because the circadian secretion of glucocorticoids (GCs) is controlled by the SCN and GCs are known to modulate auditory function, we assessed their influence on circadian gene expression. Removal of circulating GCs by adrenalectomy (ADX) did not have a major impact on core clock gene expression in the cochlea. Rather it abolished the transcription of clock-controlled genes involved in inflammation. ADX abolished the known differential auditory sensitivity to day and night noise trauma and prevented the induction of GABA-ergic and glutamate receptors mRNA transcripts. However, these improvements were unrelated to changes at the synaptic level, suggesting other cochlear functions may be involved. Due to this circadian regulation of noise sensitivity by GCs, we evaluated the actions of the synthetic glucocorticoid dexamethasone (DEX) at different times of the day. DEX was effective in protecting from acute noise trauma only when administered during daytime, when circulating glucocorticoids are low, indicating that chronopharmacological approaches are important for obtaining optimal treatment strategies for hearing loss. GCs appear as a major regulator of the differential sensitivity to day or night noise trauma, a mechanism likely involving the circadian control of inflammatory responses.

Inner Hair Cell and Neuron Degeneration Contribute to Hearing Loss in a DFNA2-Like Mouse Model

DFNA2 is a progressive deafness caused by mutations in the voltage-activated potassium channel KCNQ4. Hearing loss develops with age from a mild increase in the hearing threshold to profound deafness. Studies using transgenic mice for Kcnq4 expressed in a mixed background demonstrated the implication of outer hair cells at the initial phase. However, it could not explain the last phase mechanisms of the disease. Genetic backgrounds are known to influence disease expressivity. To unmask the cause of profound deafness phenotype, we backcrossed the Kcnq4 knock-out allele to the inbred strain C3H/HeJ and investigated inner and outer hair cell and spiral ganglion neuron degeneration across the lifespan. In addition to the already reported outer hair cell death, the C3H/HeJ strain also exhibited inner hair cell and spiral ganglion neuron death. We tracked the spatiotemporal survival of cochlear cells by plotting cytocochleograms and neuronal counts at different ages. Cell loss progressed from basal to apical turns with age. Interestingly, the time-course of cell degeneration was different for each cell-type. While for outer hair cells it was already present by week 3, inner hair cell and neuronal loss started 30 weeks later. We also established that outer hair cell loss kinetics slowed down from basal to apical regions correlating with KCNQ4 expression pattern determined in wild-type mice. Our findings indicate that KCNQ4 plays differential roles in each cochlear cell-type impacting in their survival ability. Inner hair cell and spiral ganglion neuron death generates severe hearing loss that could be associated with the last phase of DFNA2.

Circadian regulation of auditory function

The circadian system integrates environmental cues to regulate physiological functions in a temporal fashion. The suprachiasmatic nucleus, located in the hypothalamus, is the master clock that synchronizes central and peripheral organ clocks to orchestrate physiological functions. Recently, molecular clock machinery has been identified in the cochlea unravelling the potential involvement in the circadian regulation of auditory functions. Here, we present background information on the circadian system and review the recent findings that introduce circadian rhythms to the auditory field. Understanding the mechanisms by which circadian rhythms regulate auditory function will provide fundamental knowledge on the signalling networks that control vulnerability and resilience to auditory insults.

Differential effects of pannexins on noise-induced hearing loss

Hearing loss, including noise-induced hearing loss, is highly prevalent and severely hinders an individual's quality of life, yet many of the mechanisms that cause hearing loss are unknown. The pannexin (Panx) channel proteins, Panx1 and Panx3, are regionally expressed in many cell types along the auditory pathway, and mice lacking Panx1 in specific cells of the inner ear exhibit hearing loss, suggesting a vital role for Panxs in hearing. We proposed that Panx1 and/or Panx3 null mice would exhibit severe hearing loss and increased susceptibility to noise-induced hearing loss. Using the auditory brainstem response, we surprisingly found that Panx1^-/- and Panx3^-/- mice did not harbor hearing or cochlear nerve deficits. Furthermore, while Panx1^-/- mice displayed no protection against loud noise-induced hearing loss, Panx3^-/- mice exhibited enhanced 16- and 24-kHz hearing recovery 7 days after a loud noise exposure (NE; 12 kHz tone, 115 dB sound pressure level, 1 h). Interestingly, Cx26, Cx30, Cx43, and Panx2 were up-regulated in Panx3^-/- mice compared with wild-type and/or Panx1^-/- mice, and assessment of the auditory tract revealed morphological changes in the middle ear bones of Panx3^-/- mice. It is unclear if these changes alone are sufficient to provide protection against loud noise-induced hearing loss. Contrary to what we expected, these data suggest that Panx1 and Panx3 are not essential for baseline hearing in mice tested, but the therapeutic targeting of Panx3 may prove protective against mid-high-frequency hearing loss caused by loud NE.

Repair of human periodontal bone defects by autologous grafting stem cells derived from inflammatory dental pulp tissues

BACKGROUND

Recently, stem cells derived from inflammatory dental pulp tissues (DPSCs-IPs) have demonstrated regenerative potential, but the real effect remains to be examined. This pilot study attempted to isolate DPSCs-IPs from two patients and to evaluate the feasibility and the effect of reconstructing periodontal intrabone defects in each patient.

METHODS

DPSCs-IPs were harvested from two patients with periodontal intrabone defects with their approval. After discussing the biological characteristics of DPSCs-IPs in each patient, DPSCs-IPs were loaded onto the scaffold material β-tricalcium phosphate and engrafted into the periodontal defect area in the root furcation. After 1, 3, and 9 months, the outcome was evaluated by clinical assessment and radiological study. Furthermore, new samples were collected and the biological characteristics of DPSCs-IPs were further studied compared with normal dental pulp stem cells. The primary cell culture success rate, cell viability, cell cycle analysis, and proliferation index were used to describe the growth state of DPSCs-IPs. In-vitro differentiation ability detection was used to further discuss the stem cell characteristics of DPSCs-IPs.

RESULTS

As expected, DPSCs-IPs were able to engraft and had an effect of regeneration of new bones to repair periodontal defects 9 months after surgical reconstruction. Although the success rate of primary cell culture and growth status was slightly inhibited, DPSCs-IPs expressed comparable levels of stem cell markers as well as retaining their multidifferentiation ability.

CONCLUSIONS

We developed a standard procedure that is potentially safe and technological for clinical periodontal treatment using human autologous DPSCs-IPs.

TRIAL REGISTRATION

According to the editorial policies, the present study is a purely observational study, so trial registration is not required.

Spontaneous Cannabinoid Receptor 2 (CB2) Expression in the Cochlea of Adult Albino Rat and Its Up-Regulation after Cisplatin Treatment

We provide evidence for the presence of cannabinoid CB₂ receptors in some cellular types of the cochlea of the adult albino rat. Cannabinoids and their receptors are increasingly being studied because of their high potential for clinical use. As a hyperspecialized portion of the peripheral nervous system, study of the expression and function of cannabinoid receptors in the hearing organ is of high interest. Stria vascularis and inner hair cells express CB₂ receptor, as well as neurites and cell bodies of the spiral ganglion. Cellular types such as supporting cells and outer hair cells, in which the expression of other types of functional receptors has been reported, do not significantly express CB₂ receptors in this study. An up-regulation of CB₂ gene expression was detected after an ototoxic event such as cisplatin treatment, probably due to pro-inflammatory events triggered by the drug. That fact suggests promising potential of CB₂ receptor as a therapeutic target for new treatments to palliate cisplatin-induced hearing loss and other ototoxic events which triggers inflammatory pathways.

GLAST Deficiency in Mice Exacerbates Gap Detection Deficits in a Model of Salicylate-Induced Tinnitus

Gap detection or gap pre-pulse inhibition of the acoustic startle (GPIAS) has been successfully used in rat and guinea pig models of tinnitus, yet this system has been proven to have low efficacy in CBA mice, with low basal GPIAS and subtle tinnitus-like effects. Here, we tested five mouse strains (CBA, BalbC, CD-1, C57BL/6 and 129sv) for pre-pulse inhibition (PPI) and gap detection with varying interstimulus intervals (ISI) and found that mice from a CBA genetic background had the poorest capacities of suppressing the startle response in the presence of a pre-pulse or a gap. CD-1 mice displayed variable responses throughout all ISI. Interestingly, C57BL/6, 129sv and BalbC showed efficient suppression with either pre-pulses or gaps with shorter ISI. The glutamate aspartate transporter (GLAST) is expressed in support cells from the cochlea and buffers the excess of glutamate. We hypothesized that loss of GLAST function could sensitize the ear to tinnitus-inducing agents, such as salicylate. Using shorter ISI to obtain a greater dynamic range to assess tinnitus-like effects, we found that disruption of gap detection by salicylate was exacerbated across various intensities of a 32-kHz narrow band noise gap carrier in GLAST knockout (KO) mice when compared to their wild-type (WT) littermates. Auditory brainstem responses (ABR) and distortion-product otoacoustic emission (DPOAE) were performed to evaluate the effects on hearing functions. Salicylate caused greater auditory threshold shifts (near 15 dB) in GLAST KO mice than in WT mice across all tested frequencies, despite similarly reduced DPOAE. Despite these changes, inhibition using broad-band gap carriers and 32 kHz pre-pulses were not affected. Our study suggests that GLAST deficiency could become a useful experimental model to decipher the mechanisms underlying drug-induced tinnitus. Future studies addressing the neurological correlates of tinnitus in this model could provide additional insights into the mechanisms of tinnitus.

Identification of a Circadian Clock in the Inferior Colliculus and Its Dysregulation by Noise Exposure

Circadian rhythms regulate bodily functions within 24 h and long-term disruptions in these rhythms can cause various diseases. Recently, the peripheral auditory organ, the cochlea, has been shown to contain a self-sustained circadian clock that regulates differential sensitivity to noise exposure throughout the day. Animals exposed to noise during the night are more vulnerable than when exposed during the day. However, whether other structures throughout the auditory pathway also possess a circadian clock remains unknown. Here, we focus on the inferior colliculus (IC), which plays an important role in noise-induced pathologies such as tinnitus, hyperacusis, and audiogenic seizures. Using PER2::LUC transgenic mice and real-time bioluminescence recordings, we revealed circadian oscillations of Period 2 protein in IC explants for up to 1 week. Clock genes (Cry1, Bmal1, Per1, Per2, Rev-erbα, and Dbp) displayed circadian molecular oscillations in the IC. Averaged expression levels of early-induced genes and clock genes during 24 h revealed differential responses to day or night noise exposure. Rev-erbα and Dbp genes were affected only by day noise exposure, whereas Per1 and Per2 were affected only by night noise exposure. However, the expression of Bdnf was affected by both day and night noise exposure, suggesting that plastic changes are unlikely to be involved in the differences in day or night noise sensitivity in the IC. These novel findings highlight the importance of circadian responses in the IC and emphasize the importance of circadian mechanisms for understanding central auditory function and disorders.

SIGNIFICANCE STATEMENT

Recent findings identified the presence of a circadian clock in the inner ear. Here, we present novel findings that neurons in the inferior colliculus (IC), a central auditory relay structure involved in sound processing, express a circadian clock as evidenced at both the mRNA and protein levels. Using a reporter mouse that expresses a luciferase protein coupled to the core clock protein PERIOD2 (PER2::LUC), we could observe spontaneous circadian oscillations in culture. Furthermore, we reveal that the mRNA profile of clock-related genes in the IC is altered differentially by day or night noise exposure. The identification of a clock in the IC is relevant for understanding the mechanisms underlying dysfunctions of the IC such as tinnitus, hyperacusis, or audiogenic seizures.