Altered expression of inducible nitric oxide synthase (iNOS) in the cochlea

Phosphodiesterase-5 inhibitors and hearing impairment: a disproportionality analysis using the US food and drug administration adverse event reporting system

BACKGROUND

This study aimed to investigate the association between phosphodiesterase-5 inhibitors (PDE-5i) and hearing impairment adverse events (HIAEs) while providing an overview of the characteristics of drug-related HIAEs.

RESEARCH DESIGN AND METHODS

We conducted a detailed pharmacovigilance analysis using data from the US Food and Drug Administration Adverse Event Reporting System (FAERS) database, covering 2004 to 2022. By calculating the reporting odds ratio (ROR) and the information component (IC), we identified signals that indicate the association between PDE-5i use and HIAEs.

RESULTS

Among the 191,398 reports related to PDE-5i, we identified 2,608 cases of HIAEs. Signals were observed for both PDE-5i monotherapy and polytherapy, indicating combinations of drugs. Avanafil exhibited the strongest signal (ROR: 4.35, 95% CI: 2.56-7.41, IC: 2.09, 95% CI: 0.10-3.51), while vardenafil showed the weakest signal (ROR: 2.69, 95% CI: 2.21-3.28, IC: 1.14, 95% CI: 0.74-2.04). Sildenafil had the highest reported cases (ROR: 3.03, 95% CI: 2.82-3.24, IC: 1.57, 95% CI: 1.34-1.80).

CONCLUSION

These findings highlight a significant correlation between PDE-5i use and HIAEs, emphasizing the need for careful evaluation in clinical practice and providing appropriate guidance to patients before initiating treatment.

GelMA/PEDOT:PSS Composite Conductive Hydrogel-Based Generation and Protection of Cochlear Hair Cells through Multiple Signaling Pathways

Recent advances in cochlear implantology are exemplified by novel functional strategies such as bimodal electroacoustic stimulation, in which the patient has intact low-frequency hearing and profound high-frequency hearing pre-operatively. Therefore, the synergistic restoration of dysfunctional cochlear hair cells and the protection of hair cells from ototoxic insults have become a persistent target pursued for this hybrid system. In this study, we developed a composite GelMA/PEDOT:PSS conductive hydrogel that is suitable as a coating for the cochlear implant electrode for the potential local delivery of otoregenerative and otoprotective drugs. Various material characterization methods (e.g., 1H NMR spectroscopy, FT-IR, EIS, and SEM), experimental models (e.g., murine cochlear organoid and aminoglycoside-induced ototoxic HEI-OC1 cellular model), and biological analyses (e.g., confocal laser scanning microscopy, real time qPCR, flow cytometry, and bioinformatic sequencing) were used. The results demonstrated decent material properties of the hydrogel, such as mechanical (e.g., high tensile stress and Young's modulus), electrochemical (e.g., low impedance and high conductivity), biocompatibility (e.g., satisfactory cochlear cell interaction and free of systemic toxicity), and biosafety (e.g., minimal hemolysis and cell death) features. In addition, the CDR medicinal cocktail sustainably released by the hydrogel not only promoted the expansion of the cochlear stem cells but also boosted the trans-differentiation from cochlear supporting cells into hair cells. Furthermore, hydrogel-based drug delivery protected the hair cells from oxidative stress and various forms of programmed cell death (e.g., apoptosis and ferroptosis). Finally, using large-scale sequencing, we enriched a complex network of signaling pathways that are potentially downstream to various metabolic processes and abundant metabolites. In conclusion, we present a conductive hydrogel-based local delivery of bifunctional drug cocktails, thereby serving as a potential solution to intracochlear therapy of bimodal auditory rehabilitation and diseases beyond.

Cochlear implantation impairs intracochlear microcirculation and counteracts iNOS induction in guinea pigs

Introduction

Preservation of residual hearing remains a great challenge during cochlear implantation. Cochlear implant (CI) electrode array insertion induces changes in the microvasculature as well as nitric oxide (NO)-dependent vessel dysfunction which have been identified as possible mediators of residual hearing loss after cochlear implantation.

Methods

A total of 24 guinea pigs were randomized to receive either a CI (n = 12) or a sham procedure (sham) by performing a cochleostomy without electrode array insertion (n = 12). The hearing threshold was determined using frequency-specific compound action potentials. To gain visual access to the stria vascularis, a microscopic window was created in the osseous cochlear lateral wall. Cochlear blood flow (CBF) and cochlear microvascular permeability (CMP) were evaluated immediately after treatment, as well as after 1 and 2 h, respectively. Finally, cochleae were resected for subsequent immunohistochemical analysis of the iNOS expression.

Results

The sham control group showed no change in mean CBF after 1 h (104.2 ± 0.7%) and 2 h (100.8 ± 3.6%) compared to baseline. In contrast, cochlear implantation resulted in a significant continuous decrease in CBF after 1 h (78.8 ± 8.1%, p < 0.001) and 2 h (60.6 ± 11.3%, p < 0.001). Additionally, the CI group exhibited a significantly increased CMP (+44.9% compared to baseline, p < 0.0001) and a significant increase in median hearing threshold (20.4 vs. 2.5 dB SPL, p = 0.0009) compared to sham after 2 h. Intriguingly, the CI group showed significantly lower iNOS-expression levels in the organ of Corti (329.5 vs. 54.33 AU, p = 0.0003), stria vascularis (596.7 vs. 48.51 AU, p < 0.0001), interdental cells (564.0 vs. 109.1 AU, p = 0.0003) and limbus fibrocytes (119.4 vs. 18.69 AU, p = 0.0286).

Conclusion

Mechanical and NO-dependent microvascular dysfunction seem to play a pivotal role in residual hearing loss after CI electrode array insertion. This may be facilitated by the implantation associated decrease in iNOS expression. Therefore, stabilization of cochlear microcirculation could be a therapeutic strategy to preserve residual hearing.

Cochlear Immune Response in Presbyacusis: a Focus on Dysregulation of Macrophage Activity

Age-related hearing loss, or presbyacusis, is a prominent chronic degenerative disorder that affects many older people. Based on presbyacusis pathology, the degeneration occurs in both sensory and non-sensory cells, along with changes in the cochlear microenvironment. The progression of age-related neurodegenerative diseases is associated with an altered microenvironment that reflects chronic inflammatory signaling. Under these conditions, resident and recruited immune cells, such as microglia/macrophages, have aberrant activity that contributes to chronic neuroinflammation and neural cell degeneration. Recently, researchers identified and characterized macrophages in human cochleae (including those from older donors). Along with the age-related changes in cochlear macrophages in animal models, these studies revealed that macrophages, an underappreciated group of immune cells, may play a critical role in maintaining the functional integrity of the cochlea. Although several studies deciphered the molecular mechanisms that regulate microglia/macrophage dysfunction in multiple neurodegenerative diseases, limited studies have assessed the mechanisms underlying macrophage dysfunction in aged cochleae. In this review, we highlight the age-related changes in cochlear macrophage activities in mouse and human temporal bones. We focus on how complement dysregulation and the nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 3 inflammasome could affect macrophage activity in the aged peripheral auditory system. By understanding the molecular mechanisms that underlie these regulatory systems, we may uncover therapeutic strategies to treat presbyacusis and other forms of sensorineural hearing loss.

Inhibition of Cochlear HMGB1 Expression Attenuates Oxidative Stress and Inflammation in an Experimental Murine Model of Noise-Induced Hearing Loss

Noise-induced hearing loss (NIHL) is a common inner ear disease but has complex pathological mechanisms, one of which is increased oxidative stress in the cochlea. The high-mobility group box 1 (HMGB1) protein acts as an inflammatory mediator and shows different activities with redox modifications linked to the generation of reactive oxygen species (ROS). We aimed to investigate whether manipulation of cochlear HMGB1 during noise exposure could prevent noise-induced oxidative stress and hearing loss. Sixty CBA/CaJ mice were divided into two groups. An intraperitoneal injection of anti-HMGB1 antibodies was administered to the experimental group; the control group was injected with saline. Thirty minutes later, all mice were subjected to white noise exposure. Subsequent cochlear damage, including auditory threshold shifts, hair cell loss, expression of cochlear HMGB1, and free radical activity, was then evaluated. The levels of HMGB1 and 4-hydroxynonenal (4-HNE), as respective markers of reactive nitrogen species (RNS) and ROS formation, showed slight increases on post-exposure day 1 and achieved their highest levels on post-exposure day 4. After noise exposure, the antibody-treated mice showed markedly less ROS formation and lower expression of NADPH oxidase 4 (NOX4), nitrotyrosine, inducible nitric oxide synthase (iNOS), and intercellular adhesion molecule-1 (ICAM-1) than the saline-treated control mice. A significant amelioration was also observed in the threshold shifts of the auditory brainstem response and the loss of outer hair cells in the antibody-treated versus the saline-treated mice. Our results suggest that inhibition of HMGB1 by neutralization with anti-HMGB1 antibodies prior to noise exposure effectively attenuated oxidative stress and subsequent inflammation. This procedure could therefore have potential as a therapy for NIHL.

Treatment With Calcineurin Inhibitor FK506 Attenuates Noise-Induced Hearing Loss

Attenuation of noise-induced hair cell loss and noise-induced hearing loss (NIHL) by treatment with FK506 (tacrolimus), a calcineurin (CaN/PP2B) inhibitor used clinically as an immunosuppressant, has been previously reported, but the downstream mechanisms of FK506-attenuated NIHL remain unknown. Here we showed that CaN immunolabeling in outer hair cells (OHCs) and nuclear factor of activated T-cells isoform c4 (NFATc4/NFAT3) in OHC nuclei are significantly increased after moderate noise exposure in adult CBA/J mice. Consequently, treatment with FK506 significantly reduces moderate-noise-induced loss of OHCs and NIHL. Furthermore, induction of reactive oxygen species (ROS) by moderate noise was significantly diminished by treatment with FK506. In agreement with our previous finding that autophagy marker microtubule-associated protein light chain 3B (LC3B) does not change in OHCs under conditions of moderate-noise-induced permanent threshold shifts, treatment with FK506 increases LC3B immunolabeling in OHCs after exposure to moderate noise. Additionally, prevention of NIHL by treatment with FK506 was partially abolished by pretreatment with LC3B small interfering RNA. Taken together, these results indicate that attenuation of moderate-noise-induced OHC loss and hearing loss by FK506 treatment occurs not only via inhibition of CaN activity but also through inhibition of ROS and activation of autophagy.

RNA-seq Profiling and Co-expression Network Analysis of Long Noncoding RNAs and mRNAs Reveal Novel Pathogenesis of Noise-induced Hidden Hearing Loss

Noise-induced hidden hearing loss (NIHHL), one of the family of conditions described as noise-induced hearing loss (NIHL), is characterized by synaptopathy following moderate noise exposure that causes only temporary threshold elevation. Long noncoding RNAs (lncRNAs) mediate several essential regulatory functions in a wide range of biological processes and diseases, but their roles in NIHHL remain largely unknown. In order to determine the potential roles of these lncRNAs in the pathogenesis of NIHHL, we first evaluated their expression in NIHHL mice model and mapped possible regulatory functions and targets using RNA-sequencing (RNA-seq). In total, we identified 133 lncRNAs and 522 mRNAs that were significantly dysregulated in the NIHHL model. Gene Ontology (GO) showed that these lncRNAs were involved in multiple cell components and systems including synapses and the nervous and sensory systems. In addition, a lncRNA-mRNA network was constructed to identify core regulatory lncRNAs and transcription factors. KEGG analysis was also used to identify the potential pathways being affected in NIHHL. These analyses allowed us to identify the guanine nucleotide binding protein alpha stimulating (GNAS) gene as a key transcription factor and the adrenergic signaling pathway as a key pathway in the regulation of NIHHL pathogenesis. Our study is the first, to our knowledge, to isolate a lncRNA mediated regulatory pathway associated with NIHHL pathogenesis; these observations may provide fresh insight into the pathogenesis of NIHHL and may pave the way for therapeutic intervention in the future.

Early Alterations of Endothelial Nitric Oxide Synthase Expression Patterns in the Guinea Pig Cochlea After Noise Exposure

Constitutively expressed endothelial nitric oxide synthase (eNOS) is supposed to play a role in noise-induced nitric oxide (NO)-production. It is commonly known that intense noise exposure results in inducible NOS (iNOS) expression and increased NO-production, but knowledge about a contribution of the eNOS isoform is still lacking. Effects of noise exposure on eNOS immunolabeling were determined in male guinea pigs (n=24). For light microscopic analysis, 11 animals were exposed to 90 dB for 1 hr and 6 animals were used as controls. After exposure, eNOS immunostaining was performed on paraffin sections, and the staining intensities were quantified for 4 cochlear regions. For electron microscopic analysis, 2 animals were exposed for 2 hr to 90 dB and 5 animals were used as controls. The intensity of eNOS immunolabeling was found to be already comprehensively increased 1 hr after noise exposure to 90 dB. At the ultrastructural level, a clear increase in eNOS immunolabeling was found in microtubules-rich areas of cochlear cuticular structures. Hence, our findings indicate that the reticular lamina forming the endolymph-perilymph barrier at the apical side of the organ of Corti is involved in a fast intrinsic otoprotective mechanism of the cochlea.

Targeting dysregulation of redox homeostasis in noise-induced hearing loss: Oxidative stress and ROS signaling

Hearing loss caused by exposure to recreational and occupational noise remains a worldwide disabling condition and dysregulation of redox homeostasis is the hallmark of cochlear damage induced by noise exposure. In this review we discuss the dual function of ROS to both promote cell damage (oxidative stress) and cell adaptive responses (ROS signaling) in the cochlea undergoing a stressful condition such as noise exposure. We focus on animal models of noise-induced hearing loss (NIHL) and on the function of exogenous antioxidants to maintaining a physiological role of ROS signaling by distinguishing the effect of exogenous "direct" antioxidants (i.e. CoQ₁₀, NAC), that react with ROS to decrease oxidative stress, from the exogenous "indirect" antioxidants (i.e. nutraceutics and phenolic compounds) that can activate cellular redox enzymes through the Keap1-Nrf2-ARE pathway. The anti-inflammatory properties of Nrf2 signaling are discussed in relation to the ROS/inflammation interplay in noise exposure. Unveiling the mechanisms of ROS regulating redox-associated signaling pathways is essential in providing relevant targets for innovative and effective therapeutic strategies against NIHL.

Genetics of guanylyl cyclase pathways in the cochlea and their influence on hearing

Although hearing loss is the most common sensory deficit in Western societies, there are no successful pharmacological treatments for this disorder. Recent experiments have demonstrated that manipulation of intracellular cyclic guanosine monophosphate (cGMP) concentrations can have both beneficial and harmful effects on hearing. In this review, we will examine the role of cGMP as a key second messenger involved in many aspects of cochlear function and discuss the known functions of downstream effectors of cGMP in sound processing. The nitric oxide-stimulated soluble guanylyl cyclase system (sGC) and the two natriuretic peptide-stimulated particulate GCs (pGCs) will be more extensively covered because they have been studied most thoroughly. The cochlear GC systems are attractive targets for medical interventions that improve hearing while simultaneously representing an under investigated source of sensorineural hearing loss.

Oxidative Stress and Inflammation Induced by Environmental and Psychological Stressors: A Biomarker Perspective

SIGNIFICANCE

The environment can elicit biological responses such as oxidative stress (OS) and inflammation as a consequence of chemical, physical, or psychological changes. As population studies are essential for establishing these environment-organism interactions, biomarkers of OS or inflammation are critical in formulating mechanistic hypotheses. Recent Advances: By using examples of stress induced by various mechanisms, we focus on the biomarkers that have been used to assess OS and inflammation in these conditions. We discuss the difference between biomarkers that are the result of a chemical reaction (such as lipid peroxides or oxidized proteins that are a result of the reaction of molecules with reactive oxygen species) and those that represent the biological response to stress, such as the transcription factor NRF2 or inflammation and inflammatory cytokines.

CRITICAL ISSUES

The high-throughput and holistic approaches to biomarker discovery used extensively in large-scale molecular epidemiological exposome are also discussed in the context of human exposure to environmental stressors.

FUTURE DIRECTIONS

We propose to consider the role of biomarkers as signs and to distinguish between signs that are just indicators of biological processes and proxies that one can interact with and modify the disease process. Antioxid. Redox Signal. 28, 852-872.

The beneficial effect of Hangesha-shin-to (TJ-014) in gentamicin-induced hair cell loss in the rat cochlea

OBJECTIVE

Ototoxic damage caused by aminoglycosides (AG) leads to the loss of cochlear hair cells (HCs). In mammals, mature cochlear HCs are unable to regenerate, and their loss results in permanent hearing deficits. Our objective was to protect the inner ear from damage after an AG challenge. The generation of reactive oxygen species (ROS), one of the earliest events in the process of AG ototoxicity, is considered to play a key role in the initiation of HC death. We examined whether Hangesha-shin-to (TJ-014), a traditional Japanese Kampo medicine considered to be a potent antioxidant, protects HCs from gentamicin (GM)-induced damage.

METHODS

Organ of Corti explants removed from postnatal day 3-5 rats were maintained in tissue culture and exposed to 50μM GM for up to 48h. The effects of TJ-014 on GM-induced ototoxicity were assessed by HC counts and immunohistochemistry against cleaved caspase-3, 8-hydroxy-2'-deoxyguanosine (8-OHdG), and a probe reacting to mitochondrial function changes.

RESULTS

TJ-014 treatments significantly reduced GM-induced HC loss and immunoreactivities for cleaved caspase-3 and 8-OHdG; these effects were correlated with increasing TJ-014 concentrations. Moreover, TJ-014 protected the mitochondrial membrane potential from GM ototoxicity.

CONCLUSION

These findings indicate the potential of TJ-014 to prevent GM-induced cochlear damage involving ROS.

Noise-Induced Neural Degeneration and Therapeutic Effect of Antioxidant Drugs

The primary site of lesion induced by noise exposure is the hair cells in the organ of Corti and the primary neural degeneration occurs in synaptic terminals of cochlear nerve fibers and spiral ganglion cells. The cellular basis of noise-induced hearing loss is oxidative stress, which refers to a severe disruption in the balance between the production of free radicals and antioxidant defense system in the cochlea by excessive production of free radicals induced by noise exposure. Oxidative stress has been identified by a variety of biomarkers to label free radical activity which include four-hydroxy-2-nonenal, nitrotyrosine, and malondialdehyde, and inducible nitric oxide synthase, cytochrome-C, and cascade-3, 8, 9. Furthermore, oxidative stress is contributing to the necrotic and apoptotic cell deaths in the cochlea. To counteract the known mechanisms of pathogenesis and oxidative stress induced by noise exposure, a variety of antioxidant drugs including oxygen-based antioxidants such as N-acetyl-L-cystein and acetyl-L-carnitine and nitrone-based antioxidants such as phenyl-N-tert-butylnitrone (PBN), disufenton sodium, 4-hydroxy PBN, and 2, 4-disulfonyl PBN have been used in our laboratory. These antioxidant drugs were effective in preventing or treating noise-induced hearing loss. In combination with other antioxidants, antioxidant drugs showed a strong synergistic effect. Furthermore, successful use of antioxidant drugs depends on the optimal timing of treatment and the duration of treatment, which are highly related to the time window of free radical formation induced by noise exposure.

Lipopolysaccharide-induced middle ear inflammation disrupts the cochlear intra-strial fluid-blood barrier through down-regulation of tight junction proteins

Middle ear infection (or inflammation) is the most common pathological condition that causes fluid to accumulate in the middle ear, disrupting cochlear homeostasis. Lipopolysaccharide, a product of bacteriolysis, activates macrophages and causes release of inflammatory cytokines. Many studies have shown that lipopolysaccharides cause functional and structural changes in the inner ear similar to that of inflammation. However, it is specifically not known how lipopolysaccharides affect the blood-labyrinth barrier in the stria vascularis (intra-strial fluid–blood barrier), nor what the underlying mechanisms are. In this study, we used a cell culture-based in vitro model and animal-based in vivo model, combined with immunohistochemistry and a vascular leakage assay, to investigate lipopolysaccharide effects on the integrity of the mouse intra-strial fluid–blood barrier. Our results show lipopolysaccharide-induced local infection significantly affects intra-strial fluid–blood barrier component cells. Pericytes and perivascular-resident macrophage-like melanocytes are particularly affected, and the morphological and functional changes in these cells are accompanied by substantial changes in barrier integrity. Significant vascular leakage is found in the lipopolysaccharide treated-animals. Consistent with the findings from the in vivo animal model, the permeability of the endothelial cell monolayer to FITC-albumin was significantly higher in the lipopolysaccharide-treated monolayer than in an untreated endothelial cell monolayer. Further study has shown the lipopolysaccharide-induced inflammation to have a major effect on the expression of tight junctions in the blood barrier. Lipopolysaccharide was also shown to cause high frequency hearing loss, corroborated by previous reports from other laboratories. Our findings show lipopolysaccharide-evoked middle ear infection disrupts inner ear fluid balance, and its particular effects on the intra-strial fluid–blood barrier, essential for cochlear homeostasis. The barrier is degraded as the expression of tight junction-associated proteins such as zona occludens 1, occludin, and vascular endothelial cadherin are down-regulated.

Attenuation of noise-induced hearing loss using methylene blue

The overproduction of reactive oxygen species (ROS) and reactive nitrogen species (RNS) has been known to contribute to the pathogenesis of noise-induced hearing loss. In this study, we discovered that in BALB/c mice pretreatment with methylene blue (MB) for 4 consecutive days significantly protected against cochlear injury by intense broad-band noise for 3 h. It decreased both compound threshold shift and permanent threshold shift and, further, reduced outer hair cell death in the cochlea. MB also reduced ROS and RNS formation after noise exposure. Furthermore, it protected against rotenone- and antimycin A-induced cell death and also reversed ATP generation in the in vitro UB-OC1 cell system. Likewise, MB effectively attenuated the noise-induced impairment of complex IV activity in the cochlea. In addition, it increased the neurotrophin-3 (NT-3) level, which could affect the synaptic connections between hair cells and spiral ganglion neurons in the noise-exposed cochlea, and also promoted the conservation of both efferent and afferent nerve terminals on the outer and inner hair cells. These findings suggest that the amelioration of impaired mitochondrial electron transport and the potentiation of NT-3 expression by treatment with MB have a significant therapeutic value in preventing ROS-mediated sensorineural hearing loss.

The protective effect of Salvia miltiorrhiza on gentamicin-induced ototoxicity

OBJECTIVE

The clinical use of aminoglycoside antibiotics is limited in most countries because of auditory toxicity side effects. However, their use is common in developing countries because they are inexpensive and convenient. Salvia miltiorrhiza extracts are used clinically in China for their antioxidant properties. We investigated the effect of a clinically approved injectable S. miltiorrhiza solution on inducible nitric oxide synthase (iNOS) generation induced by the aminoglycoside antibiotic gentamicin and an ototoxicity protective mechanism.

METHODS

Sixty adult guinea pigs were used in this study and divided into four groups. Auditory brainstem response (ABR) testing was performed before and after treatments and animals were sacrificed for morphological and immunostaining assays after determining threshold shifts in ABR. The cochleae were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to observe ultrastructural changes. In addition, hair cell loss, iNOS and caspase-3 expression, and apoptosis were measured.

RESULTS

The result showed that hearing loss, iNOS overexpression accompanied with disorganization in the cochlea, and terminal deoxynucleotidyl transferase- mediated dUTP- biotin nick end labeling (TUNEL)-stained positive cells in animals treated with gentamicin. However, pretreatment with S. miltiorrhiza (3g/kg/day for 10 days) decreased gentamicin-induced hearing loss, attenuated iNOS and caspase-3 expression, and decreased the number of apoptotic cells. Furthermore, it also reduced the ultrastructural damage due to ototoxicity as observed by SEM and TEM.

CONCLUSIONS

These findings indicate that S. miltiorrhiza protects against gentamicin-induced ototoxicity and could apply to the protection of ototoxicity.

Nitric oxide--a versatile key player in cochlear function and hearing disorders

Nitric oxide (NO) is a signaling molecule which can generally be formed by three nitric oxide synthases (NOS). Two of them, the endothelial nitric oxide synthase (eNOS) and the neural nitric oxide synthase (nNOS), are calcium/calmodulin-dependent and constitutively expressed in many cell types. Both isoforms are found in the vertebrate cochlea. The inducible nitric oxide synthase (iNOS) is independent of calcium and normally not detectable in the un-stimulated cochlea. In the inner ear, as in other tissues, NO was identified as a multitask molecule involved in various processes such as neurotransmission and neuromodulation. In addition, increasing evidence demonstrates that the NO-dependent processes of cell protection or, alternatively, cell destruction seem to depend, among other things, on changes in the local cochlear NO-concentration. These alterations can occur at the cellular level or within a distinct cell population both leading to an NO-imbalance within the hearing organ. This dysfunction can result in hearing loss or even in deafness. In cases of cochlear malfunction, regulatory systems such as the gap junction system, the blood vessels or the synaptic region might be affected temporarily or permanently by an altered NO-level. This review discusses potential cellular mechanisms how NO might contribute to different forms of hearing disorders. Approaches of NO-reduction are evaluated and the transfer of results obtained from experimental animal models to human medication is discussed.

Radix Astragali injection enhances recovery from acute acoustic trauma

CONCLUSION

The average recovery of hearing and cessation of tinnitus was significantly better after treatment with Radix Astragali (RA) than after non-treatment with RA. RA can be valuable adjuvant therapy for patients with acute acoustic trauma (AAT).

OBJECTIVES

AAT is one of the early indications for the use of RA. The reasons for administering RA to patients with AAT are based on experimental studies showing that noise exposure results in the formation of reactive oxygen species (ROS), which trigger metabolic damage to the organ of Corti. RA is a natural antioxidant. The aim of this study was to investigate the efficacy of RA in patients with AAT.

METHODS

We compared the recovery from hearing impairment and tinnitus in 40 ears treated with RA with 40 ears treated with non-RA. RA was given intravenously daily for 10 days. There were no significant differences in clinical or audiological data between RA and non-RA groups.

RESULTS

The average recovery of hearing at both high and speech frequencies was significantly better and tinnitus persisted less commonly in the RA group than in the non-RA group. Normal hearing at the end of the follow-up period was regained in 27 ears in the RA group and in 21 ears in the non-RA group (p < 0.01).

Fibro-vascular coupling in the control of cochlear blood flow

Background

Transduction of sound in the cochlea is metabolically demanding. The lateral wall and hair cells are critically vulnerable to hypoxia, especially at high sound levels, and tight control over cochlear blood flow (CBF) is a physiological necessity. Yet despite the importance of CBF for hearing, consensus on what mechanisms are involved has not been obtained.

Methodology/Principal Findings

We report on a local control mechanism for regulating inner ear blood flow involving fibrocyte signaling. Fibrocytes in the super-strial region are spatially distributed near pre-capillaries of the spiral ligament of the albino guinea pig cochlear lateral wall, as demonstrably shown in transmission electron microscope and confocal images. Immunohistochemical techniques reveal the inter-connected fibrocytes to be positive for Na+/K+ ATPase β1 and S100. The connected fibrocytes display more Ca²⁺ signaling than other cells in the cochlear lateral wall as indicated by fluorescence of a Ca²⁺ sensor, fluo-4. Elevation of Ca²⁺ in fibrocytes, induced by photolytic uncaging of the divalent ion chelator o-nitrophenyl EGTA, results in propagation of a Ca²⁺ signal to neighboring vascular cells and vasodilation in capillaries. Of more physiological significance, fibrocyte to vascular cell coupled signaling was found to mediate the sound stimulated increase in cochlear blood flow (CBF). Cyclooxygenase-1 (COX-1) was required for capillary dilation.

Conclusions/Significance

The findings provide the first evidence that signaling between fibrocytes and vascular cells modulates CBF and is a key mechanism for meeting the cellular metabolic demand of increased sound activity.

Protective effects of Salvia miltiorrhiza against cisplatin-induced ototoxicity in guinea pigs

OBJECTIVE

The aim of the study was to investigate the protective effects of Salvia miltiorrhiza (SM) against cisplatin-induced ototoxicity in guinea pigs.

METHODS

Thirty-nine guinea pigs were randomly divided into 3 groups. The first group (control group) received physiologic saline by intraperitoneal (i.p.) injection for 5 days. The second group (cisplatin group) was treated with cisplatin (2 mg/kg per day, i.p. injection) for 5 days. The third group (SM group) was given SM (8 g/kg per day, i.p. injection) for 2 days and then was given SM (8 g/kg per day, i.p. injection) and cisplatin (2 mg/kg per day, i.p. injection) for 5 days. Auditory brain stem response (ABR) and cochlea blood flow measurement were used to evaluate cochlea function. The structures of cochlea were observed by light microscope, scanning electron microscope, transmission electron microscope (TEM), and immunohistochemical examination.

RESULTS

Cisplatin could cause severe acoustic damages including significant elevation of ABR threshold, substantial losses of outer hair cells and inner hair cells, and severe damage on the stria vascularis and spiral ganglion cells (SGCs). Although in SM group, the increased tendency of threshold was milder than that in cisplatin group. The damages in cochlea and stria vascularis were also less severe than those in cisplatin group. The expression of induced nitric oxide synthase in the cochlea and SGC in SM group was lower than that in cisplatin group.

CONCLUSIONS

Salvia miltiorrhiza can significantly reduce the cisplatin-induced side effects.

Bone marrow cell recruitment mediated by inducible nitric oxide synthase/stromal cell-derived factor-1alpha signaling repairs the acoustically damaged cochlear blood-labyrinth barrier

Using a mouse model with noise-induced cochlear blood-labyrinth-barrier (CBLB) injury, we examined the effects of inducible nitric oxide synthase (iNOS) on the recruitment of bone marrow-derived cells (BMDCs) to the CBLB after acoustic injury. Lethally irradiated C57BL/6J and B6.129P2-Nos2(tm1Lau)/J mice were transplanted with GFP(+)-BMDCs from C57Bl/6-Tg (UBC GFP) mice. Four weeks after transplantation, we assessed the population of GFP(+)-BMDCs in the CBLB. Only small numbers of GFP(+)-BMDCs were found to infiltrate the area of the CBLB in the control recipient mice. However, robust GFP(+)-BMDC migration occurred in the area of the CBLB within the injured cochlea during the first week following acoustic trauma, and further BMDC accumulation was seen by 2 weeks posttrauma. After 4 weeks, the BMDCs were integrated into vessels. Local iNOS from perivascular resident macrophages was found to be important for BMDC infiltration, since mice deficient in iNOS (Inos(-/-)) and mice with iNOS that had been inhibited by 1400W displayed reduced BMDC infiltration. Stromal cell-derived factor-1α (SDF-1α) and its chemokine receptor 4 (CXCR4) were required for the iNOS-triggered recruitment. BMDC recruitment was significantly reduced by the inhibition of SDF-1α activity. Inhibition of the iNOS/SDF-1α signaling pathway reduced vascular repair as observed by reduced vascular density. Our study revealed an intrinsic signaling pathway of iNOS that mediates SDF-1α to promote GFP(+)-BMDC infiltration/targeting in cochlear vascular repair.

Potential role for lipopolysaccharide in congenital sensorineural hearing loss

Congenital sensorineural hearing loss (SNHL) is common. In the Western world, the incidence is 1-3 per 1000 live births. The aetiology encompasses genetic and non-genetic factors accounting for 55 % and 45 % of cases, respectively. Reports that describe the contribution of intrauterine infection to the occurrence of congenital SNHL are limited, and comparative analysis of the different pathogens is lacking. Lipopolysaccharide (LPS), a product of bacteriolysis, has been demonstrated to be associated with inner ear damage in experimental studies. To elucidate the potential role of this toxin in congenital SNHL and to identify the pathogenesis and transmission routes, we reviewed the literature. We speculate that different routes of exposure to LPS in utero may result in congenital inner ear damage.

[Protection of the cochlea by ascorbic acid in noise trauma]

BACKGROUND

The protective effect of ascorbic acid against noise-induced hearing loss and increased nitric oxide (NO) formation after noise exposure have already been demonstrated in animal models. However, the influence of ascorbic acid on noise-induced NO production within the cochlea is still unclear.

METHODS

Guinea pigs (n=48) were fed for 7 days with low [25 mg/kg bodyweight (bw)/day] and high (525 mg/kg bw/day) doses of ascorbic acid. Then half of the animals were exposed to noise (90 dB for 1 h). The hearing levels were recorded beforehand, on the 3rd and 7th days after feeding, and directly after noise exposure. Finally, the organ of Corti and the lateral wall were removed from the inner ear and incubated separately for 6 h in culture medium, and the nitrite content was determined in the supernatant.

RESULTS

Compared with low-dose feeding, feeding of high doses of ascorbic acid resulted in a reduction of hearing impairment of about 8 dB after noise exposure. A correlation between hearing improvement and decreased NO production was detectable for both cochlea regions but was more pronounced in the lateral wall.

CONCLUSION

A high dose of ascorbic acid lowers NO production in the inner ear, reduces hearing loss, and protects the cochlea from nitroactive stress.

Nitrones as therapeutics

Nitrones have the general chemical formula X-CH=NO-Y. They were first used to trap free radicals in chemical systems and then subsequently in biochemical systems. More recently several nitrones, including alpha-phenyl-tert-butylnitrone (PBN), have been shown to have potent biological activity in many experimental animal models. Many diseases of aging, including stroke, cancer development, Parkinson disease, and Alzheimer disease, are known to have enhanced levels of free radicals and oxidative stress. Some derivatives of PBN are significantly more potent than PBN and have undergone extensive commercial development for stroke. Recent research has shown that PBN-related nitrones also have anti-cancer activity in several experimental cancer models and have potential as therapeutics in some cancers. Also, in recent observations nitrones have been shown to act synergistically in combination with antioxidants in the prevention of acute acoustic-noise-induced hearing loss. The mechanistic basis of the potent biological activity of PBN-related nitrones is not known. Even though PBN-related nitrones do decrease oxidative stress and oxidative damage, their potent biological anti-inflammatory activity and their ability to alter cellular signaling processes cannot readily be explained by conventional notions of free radical trapping biochemistry. This review is focused on our studies and others in which the use of selected nitrones as novel therapeutics has been evaluated in experimental models in the context of free radical biochemical and cellular processes considered important in pathologic conditions and age-related diseases.

Expression of VEGF, iNOS, and eNOS is increased in cochlea of diabetic rat

CONCLUSION

The results of this study indicate that diabetes causes up-regulation of vascular endothelial growth factor (VEGF), inducible nitric oxide synthase (iNOS), and endothelial nitric oxide synthase (eNOS), which may be involved in the pathogenesis of cochlea functional loss.

OBJECTIVE

To investigate the underlying mechanisms that may be responsible for diabetic microangiopathy in the inner ear, we studied the expression of VEGF, iNOS, and eNOS in the streptozotocin (STZ)-induced diabetic rat cochlea.

MATERIALS AND METHODS

The immunofluorescence studies were performed by using FITC-labelled specific antibodies to VEGF, iNOS, and eNOS on paraffin sections of the cochlea. The expression levels of VEGF, iNOS, and eNOS were quantified by means of Western blot analysis of cochlea protein extracts. Evans blue (EB) was used to investigate blood-labyrinth barrier (BLB) permeability in the cochlea.

RESULTS

Increased cochlear expression of VEGF, iNOS, and eNOS was detected in the diabetic rat. Furthermore, increased permeability of BLB was evidenced by increased cochlear EB extravasation in the diabetic rat.

Nitric oxide and mitochondrial status in noise-induced hearing loss

The study investigated the distribution of nitric oxide (NO) within isolated outer hair cells (OHCs) from the cochlea, its relationship to mitochondria and its modulation of mitochondrial function. Using two fluorescent dyes--4,5-diamino-fluorescein diacetate (DAF-2DA), which detects NO, and tetramethyl rhodamine methyl ester (TMRM+), a mitochondrial membrane potential dye--it was found that a relatively greater amount of the DAF fluorescence in OHCs co-localized with mitochondria in comparison to DAF fluorescence in the cytosole. This study also observed reduced mitochondrial membrane potential of OHCs and increased DAF fluorescence following exposure of the cells to noise (120 dB SPL for 4 h) and to an exogenous NO donor, NOC-7 (>350 mm). Antibody label for nitrotyrosine was also increased, indicating NO-related formation of peroxynitrite in both mitochondria and the cytosol. The results suggest that NO may play an important physiological role in regulating OHC energy status and act as a potential agent in OHC pathology.

Intact blood-perilymph barrier in the rat after impulse noise trauma

CONCLUSION

The permeability of the blood-labyrinth barrier for radioactive mannitol was unchanged after impulse noise trauma. The present findings are contradictory to the theory of an increased permeability in the blood-labyrinth barrier as a result of extensive noise exposure.

OBJECTIVE

Noise trauma is reported to cause multiple effects on the cochlea including mechanical and metabolic damage. The aim of the study was to observe the effects of impulse noise on cochlear homeostasis.

MATERIALS AND METHODS

A well-established rat model was used for evaluation of the early effects of impulse noise trauma on the integrity of the blood-perilymph barrier. To evaluate whether a blood-perilymph barrier disruption contributes to cochlear injury after impulse noise, the paracellular transport of radioactive mannitol into scala vestibuli perilymph (PLV) and electrolyte concentration in perilymph were estimated. Thirteen animals exposed to synthesized impulses of 160 dB SPL peak value, at a rate of 100 pulses, were designed as the study group and 15 rats not exposed to noise were designed as the control group. After mannitol infusion each ear of the animal in the study group was separately exposed to impulse noise and PLV samples were taken during 2 h post-infusion. In the control group, corresponding PLV samples were taken after mannitol injection.

RESULTS

At 2 h after mannitol infusion there was no difference in PLV mannitol concentration in the study group and control group (21.5%+/-2.2 and 20.5%+/-2.1, respectively). Impulse noise had no effect on the electrochemical composition of PLV.

Nitric oxide synthase inhibitor reduces noise-induced cochlear damage in guinea pigs

CONCLUSION

The results obtained in this study indicate that NG-nitro-L-arginine methyl ester (L-NAME) protects cochlear damage from acoustic trauma through reducing the production of nitric oxide (NO).

OBJECTIVES

This study aimed to explore whether NO synthase inhibitor L-NAME could reduce cochlear damage in acoustic trauma.

MATERIALS AND METHODS

Seventy guinea pigs (300-350g) were divided randomly into four groups (n=20 in groups I, III, and IV; n=10 in group II). Two days consecutively and 30min before noise exposure (4kHz octave band, 115dB SPL 5h), subjects received an injection of 5ml saline/kg (groups I and III) or 10mg/kg L-NAME (groups II and IV). Sham-exposed guinea pigs were listed as groups I and II. Protection was assessed physiologically by the change in auditory brainstem response (ABR) threshold and histologically by survival of outer hair cells (OHCs). NO level of cochlear tissue was assayed 3days after noise exposure.

RESULTS

Group III showed significantly greater OHC loss, threshold shifts and NO level compared with group I and group IV. Compared with group III, noise-induced elevation in NO level in the cochlea was significantly attenuated by L-NAME (p<0.001).

Mechanisms of noise-induced hearing loss indicate multiple methods of prevention

Recent research has shown the essential role of reduced blood flow and free radical formation in the cochlea in noise-induced hearing loss (NIHL). The amount, distribution, and time course of free radical formation have been defined, including a clinically significant late formation 7-10 days following noise exposure, and one mechanism underlying noise-induced reduction in cochlear blood flow has finally been identified. These new insights have led to the formulation of new hypotheses regarding the molecular mechanisms of NIHL; and, from these, we have identified interventions that prevent NIHL, even with treatment onset delayed up to 3 days post-noise. It is essential to now assess the additive effects of agents intervening at different points in the cell death pathway to optimize treatment efficacy. Finding safe and effective interventions that attenuate NIHL will provide a compelling scientific rationale to justify human trials to eliminate this single major cause of acquired hearing loss.

Effects of a Nitric Oxide Synthase Type II Inhibitor on Compound Action Potential Thresholds in Experimental Endolymphatic Hydrops

HYPOTHESIS

Nitric oxide (NO) is likely to be synthesized by nitric oxide synthase Type II (NOS II) action and may partake in the origin of changes of compound action potential (CAP) threshold observed in guinea pigs with induced endolymphatic hydrops. This study aimed to assess the action of a NOS II inhibitor on CAP thresholds in these experimental samples.

BACKGROUND

In guinea pigs with experimental endolymphatic hydrops, there are lesions on the cochlea and progressive increase of CAP threshold. NOS II was found in the cochlea of this animal model, and it was inferred that NO can contribute by such alterations.

METHODS

The animals were divided into two groups, in which eight received an intake of a NOS II inhibitor, aminoguanidine, and another eight served as a control group. During 16 weeks, CAP thresholds at 1,000, 2,000, 4,000 and 6,000 on electrocochleography were compared between the groups.

RESULTS

The group that had an intake of aminoguanidine showed a lower increase on CAP thresholds at 2,000 (p < 0.05) and 6,000 Hz (p < 0.05) at the 12th postoperative week, and at 1,000 (p < 0.05), 2,000 (p < 0.001), 4,000 (p < 0.001), > and 6,000 Hz (p < 0.001) at the 16th week.

CONCLUSION

We conclude that NOS II inhibitor reduced the elevation of CAP thresholds in experimentally induced endolymphatic hydrops.

Experimental endolymphatic hydrops under action of a type II nitric oxide synthase inhibitor: otoacoustic emissions evaluation and electrocochleography

In experimental endolymphatic hydrops distortion-products otoacoustic emission (dpoae) amplitudes decrease and there is elevation on electrocochleographic thresholds. Some authors found type ii nitric oxide synthase (nos ii) expression in hydropic cochleas and they suggest nitric oxide (no) may be involved in endolymphatic hydrops pathogenesis. The aim of this study was to evaluate the action of a nos ii inhibitor on dpoae and electrocochleography in experimental endolymphatic hydrops. Material and methods: endolymphatic hydrops was induced in 16 guinea pigs by obliterating the endolymphatic duct and sac in the right ear. They were divided in two groups: eigth guinea pigs under the action of aminoguanidine, a nos ii inhibitor and eigth control guinea pigs. We compared dpoae amplitudes at geometric means of frequencies 1062, 2187, 4375 and 7000 hz, compound action potential threshold at 1000, 2000, 4000 and 6000 hz and summating potential to action potential (sp/ap) ratio between the groups during the postoperative observation period of 16 weeks. Results: there were no significant changes in the dpoae amplitudes and in the sp/ap ratio. The group that received aminoguanidine had a lower degree of threshold increase at 2000 (p<0.05) And 6000 hz (p<0.05) In 12th postoperative week and at 1000 (p<0.05), 2000 (P<0.001), 4000 (P<0.001) And 6000 hz (p<0.001) At 16th postoperative week. Conclusions: nos ii inhibitor decreased the electrocochleography threshold elevation on experimental endolymphatic hydrops.

Nuclear factor-kappa B nuclear translocation in the cochlea of mice following acoustic overstimulation

There is increasing evidence to suggest that the expression of many molecules in the lateral wall of the cochlea plays an important role in noise-induced stress responses. In this study, activation of the nuclear transcription factor nuclear factor-kappa B (NF-kappaB) was investigated in the cochlea of mice treated with intense noise exposure (4 kHz, octave band, 124 dB, for 2 h). The present noise exposure led to remarkable auditory brainstem response threshold shifts and cochlear damage on surface preparations. To assess the effects of noise exposure on NF-kappaB/DNA binding activity in the cochlea, we prepared nuclear extracts from the cochlea at different time points after noise exposure and carried out an electrophoretic mobility shift assay using a probe specific to NF-kappaB. NF-kappaB/DNA binding was significantly enhanced in the cochlea 2-6 h after noise exposure and returned to basal levels after 12 h. Supershift analysis using antibodies against p65 and p50 proteins, which are components of NF-kappaB, demonstrated that enhancement of NF-kappaB/DNA binding was at least in part due to nuclear translocation of p65. An immunohistochemical study also showed that nuclear translocation of both p65 and p50 was observed in the lateral wall after noise exposure and that there may be a possible close association between p65 and enhanced inducible nitric oxide synthase expression. These results suggest that NF-kappaB may have a detrimental role in the response to acoustic overstimulation in the cochlea of mice.

Gentamicin induced nitric oxide-related oxidative damages on vestibular afferents in the guinea pig

Gentamicin is a well-known ototoxic aminoglycoside. However, the mechanism underlying this ototoxicity remains unclear. One of the mechanisms which may be responsible for this ototoxicity is excitotoxic damage to hair cells. The overstimulation of the N-methyl-d-aspartate (NMDA) receptors increases the production of nitric oxide (NO), which induces oxidative stress on hair cells. In order to determine the mechanism underlying this excitotoxicity, we treated guinea pigs with gentamicin by placing gentamicin (0.5 mg) pellets into a round window niche. After the sacrifice of the animals, which occurred at 3, 7 and 14 days after the treatment, the numbers of hair cells in the animals were counted with a scanning electron microscope. We then performed immunostaining using neuronal nitric oxide synthase (nNOS), inducible NOS (iNOS) and nitrotyrosine antibodies. The number of hair cells in the animals was found to decrease significantly after 7 days. nNOS and iNOS expression levels were observed to have increased 3 days after treatment. Nitrotyrosine was expressed primarily at the calyceal afferents of the type I hair cells 3 days after treatment. Terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) staining revealed positive hair cells 3 days after treatment. Our results suggest that inner ear treatment with gentamicin may upregulate nNOS and iNOS to induce oxidative stress in the calyceal afferents of type I hair cells, via nitric oxide overproduction.

Endothelial nitric oxide synthase upregulation in the guinea pig organ of Corti after acute noise trauma

Endothelial nitric oxide synthase (eNOS) upregulation was identified 60 h after acute noise trauma in morphologically intact cells of the reticular lamina in the organ of Corti of the guinea pig in the second turn of the cochlea. Using gold-coupled anti-eNOS antibodies and electron microscopy, it was shown that eNOS expression was upregulated in all cell areas and cell types except inner hair cells. Furthermore, eNOS was found in the organelle-free cytoplasm and in mitochondria of various cell types. The density of eNOS in mitochondria was considerably higher compared with the surrounding cytoplasm. Since eNOS activity is regulated by calcium, the eNOS detection was combined with calcium precipitation, a method for visualizing intracellular Ca2+ distribution. After acute noise trauma, intracellular Ca2+ was increased in all cell types and cell areas except in outer hair cells. Comparing the distribution patterns of eNOS and calcium, significantly elevated levels (P < 0.0001) of eNOS were detected within a 100 nm radius near calcium precipitates in all cuticular structures as well as microtubule-rich regions and Deiters' cells near Hensen cells. The observed colocalization lends support to the postulated mechanism of eNOS activation by Ca2+. eNOS upregulation after acute noise trauma might therefore be part of an induced stress response. The eNOS upregulation in cell areas with numerous microtubule- and actin-rich structures is discussed with respect to possible cytoskeleton-dependent processes in eNOS regulation.

Involvement of the nitric oxide-cyclic GMP pathway and neuronal nitric oxide synthase in ATP-induced Ca2+ signalling in cochlear inner hair cells

We recently demonstrated that extracellular adenosine 5'-triphosphate (ATP) induced nitric oxide (NO) production in the inner hair cells (IHCs) of the guinea pig cochlea, which inhibited the ATP-induced increase in the intracellular Ca(2+) concentrations ([Ca(2+)](i)) by a feedback mechanism [Shen, J., Harada, N. & Yamashita, T. (2003) Neurosci. Lett., 337, 135-138]. We herein investigated the role of the NO-cGMP pathway and neuronal NO synthase (nNOS) in the ATP-induced Ca(2+) signalling in IHCs using the Ca(2+)-sensitive dye fura-2 and the NO-sensitive dye DAF-2. Fura-2 fluorescence-quenching experiments with Mn(2+) showed that ATP triggered a Mn(2+) influx. L-N(G)-nitroarginine methyl ester (L-NAME), a nonspecific NOS inhibitor, accelerated the ATP-induced Mn(2+) influx while S-nitroso-N-acetylpenicillamine (SNAP), a NO donor, suppressed it. 1H-[1,2,4]oxadiazole[4,3-a] quinoxalin-1-one, an inhibitor of guanylate cyclase, and KT5823, an inhibitor of cGMP-dependent protein kinase, enhanced the ATP-induced [Ca(2+)](i) increase. 8-Bromoguanosine-cGMP, a membrane-permeant analogue of cGMP mimicked the effects of SNAP. Moreover, the effects of 7-nitroindazole, a selective nNOS inhibitor, mimicked the effects of L-NAME regarding both the enhancement of the ATP-induced Ca(2+) response and the attenuation of NO production. Immunofluorescent staining of nNOS using a single IHC revealed that nNOS was distributed throughout the IHCs, but enriched in the apical region of the IHCs as shown by intense staining. In conclusion, the ATP-induced Ca(2+) influx may be the principal source for nNOS activity, which may interact with P2X receptors in the apical region of IHCs. Thereafter, NO can be produced and conversely inhibits the Ca(2+) influx via the NO-cGMP-PKG pathway by a feedback mechanism.

Lipopolysaccharide-induced up-regulation of activated macrophages in the degenerating taste system

Unilateral chorda tympani (CT) nerve section and maintenance on a sodium-restricted diet leads to a rapid decrease in neurophysiological taste responses to sodium in the contralateral, intact CT nerve. Up-regulation of immune function with lipopolysaccharide (LPS; 100 microg i.p.) induces a recovery of normal sodium taste responses, suggesting that the sodium-deficient diet is immunosuppressive. In fact, there is a bilateral increase in the number of lingual, activated macrophages in control-fed rats receiving CT nerve section that does not occur in sodium-deficient rats after sectioning. In the current study, we hypothesized that the LPS-induced recovery of normal taste function in sodium-deficient rats is based on an increase in the activated macrophage response to denervation. Rats receiving a unilateral CT nerve section, a sodium-restricted diet, and/or an injection of LPS (100 microg; i.p.) were overdosed with pentobarbital at day 2 postsectioning, and tongues were rapidly dissected and frozen. Cryosections were then immunohistochemically stained to determine the percentage of ED1 staining for activated macrophages or the number of alphabeta or gammadelta T cells. Activated macrophage levels were significantly increased in sodium-restricted rats that received LPS following unilateral CT nerve section, supporting our hypothesis. These novel findings suggest that LPS overcomes the immunosuppression induced by the sodium-restricted diet and also indicate that the immune system plays a role in regulating taste function after neural injury.

Ebselen prevents noise-induced excitotoxicity and temporary threshold shift

This investigation tested the hypothesis that a noise-induced temporary threshold shift (TTS) can be attenuated by a peroxynitrite scavenger, ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one). Guinea pigs received an oral dose of the vehicle or 10 mg/kg ebselen 1h before exposure to 115 dB SPL 4-kHz octave band noise for 3 h. In controls, auditory brainstem response (ABR) thresholds increased by 25-45 dB immediately after noise and returned to pre-exposure baseline thresholds 7 days later. Ebselen eliminated this ABR threshold shift following noise exposure. In controls, swelling of the afferent dendrites beneath the inner hair cells was evident immediately after noise, whereas ebselen significantly reduced this pathology. These findings suggest that scavenging peroxynitrite can attenuate noise-induced excitotoxicity and, thereby, TTS.

Basal nitric oxide production contributes to membrane potential and vasotone regulation of guinea pig in vitro spiral modiolar artery

Nitric oxide (NO) is a potent vasodilating agent implicated in cochlear blood flow regulation. We recently demonstrated that exogenously applied NO donor DPTA-NONOate hyperpolarizes both endothelial and smooth muscle cells of in vitro spiral modiolar artery (SMA) via activation of ATP-sensitive K+ channels (K(ATP)). Also, NO was detected in the SMA cells by NO indicator dye in the in vitro basal condition. Using intracellular recording techniques, electrochemical NO-sensing measurement, and a vaso-diameter video tracking method, we investigated the basal release of NO from the in vitro SMA and its role in the vascular function. We found that (1) 300 microM L-NAME, a NO synthase inhibitor, and 3 microM glipizide caused a depolarization of approximately 4.5 and approximately 3.2 mV, respectively, in cells with a resting potential less negative than -60 mV; (2) NO sensor in the close vicinity of the SMA detected a NO concentration of approximately 50 nM that was suppressed by L-NAME and enhanced by L-arginine (1-1000 microM); (3) NO donor DPTA-NONOate (0.1-30 microM) applications produced about 8-245 nM of NO in the recording bath. These data indicate a NO concentration-hyperpolarization relation, with an EC50 of 22 nM. (4) Finally, L-NAME but not glipizide produced a 4.8% reduction in SMA diameter (approximately 50 microm) in the majority of SMAs, whereas NONOate (10 microM) always caused a dilation. Both the induced constriction and dilation were not significantly affected by 3 microM glipizide. We conclude that a significant amount of NO (> 50 nM) is tonically released from the in vitro SMA, which is above the EC50 for activation of K(ATP), and thus contributes to the membrane polarization. The basal release of NO also contributes to vasotone relaxation, but the K(ATP) activation appears to play little role in the relaxation of the in vitro SMA.