Lipopolysaccharide-induced expression of inducible nitric oxide synthase in the guinea pig organ of Corti

Prenatal lipopolysaccharide exposure induces anxiety-like behaviour in male mouse offspring and aberrant glial differentiation of embryonic neural stem cells

BACKGROUND

Prenatal infection has been implicated in the development of neuropsychiatric disorders in children. We hypothesised that exposure to lipopolysaccharide during prenatal development could induce anxiety-like behaviour and sensorineural hearing loss in offspring, as well as disrupt neural differentiation during embryonic neural development.

METHODS

We simulated prenatal infection in FVB mice and mouse embryonic stem cell (ESC) lines, specifically 46C and E14Tg2a, through lipopolysaccharide treatment. Gene expression profiling analyses and behavioural tests were utilized to study the effects of lipopolysaccharide on the offspring and alterations in toll-like receptor (TLR) 2-positive and TLR4-positive cells during neural differentiation in the ESCs.

RESULTS

Exposure to lipopolysaccharide (25 µg/kg) on gestation day 9 resulted in anxiety-like behaviour specifically in male offspring, while no effects were detected in female offspring. We also found significant increases in the expression of GFAP and CNPase, as well as higher numbers of GFAP + astrocytes and O4+ oligodendrocytes in the prefrontal cortex of male offspring. Furthermore, increased scores for genes related to oligodendrocyte and lipid metabolism, particularly ApoE, were observed in the prefrontal cortex regions. Upon exposure to lipopolysaccharide during the ESC-to-neural stem cell (NSC) transition, Tuj1, Map2, Gfap, O4, and Oligo2 mRNA levels increased in the differentiated neural cells on day 14. In vitro experiments demonstrated that lipopolysaccharide exposure induced inflammatory responses, as evidenced by increased expression of IL1b and ApoB mRNA.

CONCLUSIONS

Our findings suggest that prenatal infection at different stages of neural differentiation may result in distinct disturbances in neural differentiation during ESC-NSC transitions. Furthermore, early prenatal challenges with lipopolysaccharide selectively induce anxiety-like behaviour in male offspring. This behaviour may be attributed to the abnormal differentiation of astrocytes and oligodendrocytes in the brain, potentially mediated by ApoB/E signalling pathways in response to inflammatory stimuli.

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.

Hearing impairment, cochlear morphology, and peroxynitrite (ONOO(-)) formation in adult and aging NOS II knockout mice

CONCLUSION

Nitric oxide synthase (NOS) II induction is a protective mechanism against age-related degeneration of the cochlea.

OBJECTIVES

An induction of NOS II has been described in different inner ear pathologies. The objective was to examine the role of NOS II in age-related degeneration of the cochlea.

METHODS

The hearing ability in adult and aging NOS II knockout mice (KO) and their wildtype (WT) littermates was explored via auditory brainstem response (ABR) measurements. Inner ear morphological differences were studied with scanning electron microscopy (SEM). Immunohistochemistry was used to examine the induction of NOS II in the inner ear of aging WT mice. Expression of nitrotyrosin, a marker protein for the reactive oxygen species peroxynitrite, was compared between KO and WT mice using immunohistochemistry.

RESULTS

Adult KO mice exhibited a mild hearing impairment. WT mice showed an induction of NOS II after 6 months of age. Age-related hearing deterioration was accelerated in KO mice, which was accompanied by increased nitrotyrosin formation and outer hair cell loss.

Changes in Cochlear Blood Flow Due to Endotoxin-Induced Otitis Media

The purpose of this study was to elucidate the influence of otitis media on blood flow in the lateral wall of the cochlea by means of a model of endotoxin-induced otitis media. The cochlear blood flow (CBF) following lipopolysaccharide inoculation into the middle ear cavities of rats was measured by laser-Doppler flowmetry and compared with that of untreated ears. After this evaluation, the influence on CBF of concomitant use of a nitric oxide synthase inhibitor was also investigated. The first day after inoculation, the CBF of treated ears decreased significantly. This decrease recovered gradually between the 7th and 14th days. With concomitant use of a nitric oxide synthase inhibitor, the decrease in CBF was prevented to some extent. The results showed a functional influence upon CBF by endotoxin-induced otitis media. The significance of prophylactic use of the drug is also discussed in regard to the effect on CBF following otitis media.

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.

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 mediates TNF-α-induced apoptosis in the auditory cell line

OBJECTIVES/HYPOTHESIS

Tumor necrosis factor-alpha (TNF-α) is released in a variety of pathological states in the inner ear. Inducible nitric oxide synthase (iNOS) can be induced by cytokines and other inflammatory factors, and is generally thought to be associated with inflammation and other pathological processes in the cochlea. The purpose of the present study was to reveal that TNF-α could induce apoptosis in the auditory cell line and to investigate the role of nitric oxide (NO) in TNF-α-induced auditory cell death.

STUDY DESIGN

Experimental study.

METHODS

UB-OC1 cells and zebrafish were exposed to TNF-α. Flow cytometry, terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling (TUNEL) assay, assay of mitochondrial membrane potential (MMP), and electron microscopy were used to show that TNF-α could induce apoptosis. Western blot was used to measure iNOS expression and mitogen-activated protein kinase pathway.

RESULTS

Flow cytometric analysis, TUNEL assay, MMP, and electron microscopy all demonstrated that TNF-α could induce apoptosis in UB-OC1 cells. TNF-α significantly increased NO generation and iNOS expression. Pretreatment with iNOS blocker NG-methyl-L-arginine (NMA) attenuated TNF-α-induced cell death and caspase-3 activation. Also, TNF-α treatment increased p-p38 and p-ERK, and pretreatment of NMA reduced this increased expression of p-p38 and p-ERK.

CONCLUSIONS

TNF-α can induce apoptosis in the auditory cell line, and NO production in response to TNF-α is essential for apoptosis.

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.

Hyperbaric oxygen upregulates cochlear constitutive nitric oxide synthase

Background

Hyperbaric oxygen therapy (HBOT) is a known adjuvant for treating ischemia-related inner ear diseases. Controversies still exist in the role of HBOT in cochlear diseases. Few studies to date have investigated the cellular changes that occur in inner ears after HBOT. Nitric oxide, which is synthesized by nitric oxide synthase (NOS), is an important signaling molecule in cochlear physiology and pathology. Here we investigated the effects of hyperbaric oxygen on eardrum morphology, cochlear function and expression of NOS isoforms in cochlear substructures after repetitive HBOT in guinea pigs.

Results

Minor changes in the eardrum were observed after repetitive HBOT, which did not result in a significant hearing threshold shift by tone burst auditory brainstem responses. A differential effect of HBOT on the expression of NOS isoforms was identified. Upregulation of constitutive NOS (nNOS and eNOS) was found in the substructures of the cochlea after HBOT, but inducible NOS was not found in normal or HBOT animals, as shown by immunohistochemistry. There was no obvious DNA fragmentation present in this HBOT animal model.

Conclusions

The present evidence indicates that the customary HBOT protocol may increase constitutive NOS expression but such upregulation did not cause cell death in the treated cochlea. The cochlear morphology and auditory function are consequently not changed through the protocol.

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.

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.

Protective effect of edaravone against the ototoxicity of Pseudomonas aeruginosa exotoxin A

CONCLUSION

Our findings suggest that edaravone can protect against cochlear damage caused by Pseudomonas aeruginosa exotoxin A (PaExoA).

OBJECTIVE

To analyze the protective effect of a free radical scavenger, edaravone, against the ototoxicity resulting from exposure of the middle ear to PaExoA.

MATERIAL AND METHODS

In nine groups of albino rats the following solutions were instilled either via the tympanic membrane into the round window niche [intratympanically (i.t.)] or intravenously (i.v.): edaravone (i.v.); edaravone (i.t.); PaExoA (i.t.) + edaravone (i.t.; simultaneously); PaExoA (i.t.) + edaravone (i.t.; 1 h after); PaExoA (i.t.) + edaravone (i.t.; 24 h after); PaExoA (i.t.) + edaravone (i.v.; simultaneously); PaExoA (i.t.) + edaravone (i.v.; 1 h after); PaExoA (i.t.) + edaravone (i.v.; 24 h after); PaExoA (i.t.) + saline (i.v.). Frequency-specific (2-20 kHz) auditory brainstem responses were measured to determine hearing thresholds before and 2, 5 and 10 days after instillation.

RESULTS

PaExoA had penetrated from the middle ear into the cochlea and caused hearing loss. This impairment was blocked by intratympanic injection of edaravone when given simultaneously or 1 h after the first instillation of PaExoA, or by intravenous injection of edaravone when given simultaneously. There were significant differences in protective effect between the intratympanic and intravenous routes.

Pharmakotherapie des akuten Tinnitus

Hypoxia/ischemia may play an important role in the pathogenesis of sensorineural tinnitus due to the characteristics of the cochlear blood supply. In addition, hypoxia modulates molecular processes both in the acute and chronic forms of tinnitus. Transcription factor HIF-1 (hypoxia-inducible factor) may play a key role in the cells' adaptation to hypoxia and ischemia, while under hypoxic/ischemic conditions, HIF-1 induces changes in the gene expression which may contribute to the remodeling of particular structures within the cochlea. Disturbances in the cochlear blood supply may result in membrane changes, perineural edema, inflammation, disturbances in ion homeostasis and in the formation of reactive oxygen species. Thus, the pharmacotherapy of acute tinnitus may be aimed at the improvement of cochlear blood supply and the prevention of acute processes leading to cell damage. Pharmacotherapies with colloidal plasma substitutes, vasodilators, calcium antagonists, procaine, and cortisone have been described in the literature and are discussed here. Many of the pharmacological treatments have not been validated in double blind studies. Although it is impossible to deduce the cause of tinnitus from a drug's efficiency, there is some evidence that it can be effectively suppressed by improving blood supply, at least at certain stages. The aim is to achieve an improved pharmacotherapy by means of sophisticated diagnostic instruments for classifying particular types of tinnitus.

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.

Ischemic damage increases nitric oxide production via inducible nitric oxide synthase in the cochlea

The present study was designed to elucidate the dynamic changes of nitric oxide (NO) production in the perilymph and to investigate the immunostaining for inducible nitric oxide synthase (iNOS) in the cochlea for 7 days after transient cochlear ischemia. Moreover, aminoguanidine, which is a selective iNOS inhibitor, was administrated immediately following ischemia and every 24h thereafter for 7 days to investigate whether the production of NO is dependent on the iNOS pathway. Significant increases in the oxidative NO metabolites, nitrite (NO(2)(-)) and nitrate (NO(3)(-)), were measured on day 1 using an in vivo microdialysis and on-line high performance liquid chromatography (HPLC) system. The immunostaining for iNOS was strongly expressed on days 1 and 4 and returned to normal on day 7 after the ischemia. The administration of aminoguanidine reduced the oxidative NO metabolites on day 1 and suppressed the expression of iNOS. These findings suggest that transient ischemia causes a remarkable increase in NO production in the perilymph, which might be attributable to the iNOS pathway.

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.

Probable function of Boettcher cells based on results of morphological study: localization of nitric oxide synthase

Boettcher cells lie on the basilar membrane beneath Claudius cells. The cells are considered supporting cells for the organ of Corti, and present only in the lower turn of the cochlea, which responds to high-frequency sound. Boettcher cells interdigitate with each other, and project microvilli into the intercellular space. Their structural specialization suggests that Boettcher cells may play a significant role in the function of the cochlea. Nitric oxide synthase (NOS) has previously been detected in substructures of the cochlea. In the cochlea, it is believed that nitric oxide plays an important role in neurotransmission, blood flow regulation, and induction of cytotoxicity under pathological conditions. Findings concerning detection of NOS on Boettcher cells are rare. We demonstrated here the localization of NOS on Boettcher cells of the rat by immunohistochemistry using polyclonal antibody to NOS. On observation with the light microscope using DAB staining, positive immunostaining to NOS was observed in Boettcher cells. In immunoelectron micrographs, NOS was detected abundantly in the cytoplasm of the interdigitations. This suggests that the interdigitations may play significant roles by using NOS. It follows from this that the nitric oxide (NO) on Boettcher cells may influences neighboring Boettcher cells. The ultrastructure of Boettcher cells suggests that they may be active cells, which perform both secretory and absorptive functions.

Aminoguanidine reduces cisplatin ototoxicity

Cisplatin is known to cause high-frequency neurosensory hearing loss. While reactive oxygen species have been shown to play a role, reactive nitrogen species have been implicated, but not proven to be involved, in cisplatin ototoxicity. The purpose of the present study was to investigate the role of nitric oxide (*NO) in cisplatin ototoxicity by administering aminoguanidine (AG), a relatively specific inhibitor of inducible nitric oxide synthase (iNOS), in conjunction with cisplatin. Rats were injected with cisplatin, AG, or both. Auditory brainstem evoked responses (ABR) were measured before and 3 days after cisplatin administration. The cochlear tissue was then assayed for *NO and malondialdehyde. Cisplatin alone caused significant ABR threshold shifts at all stimuli tested, whereas AG alone caused no shifts. There was a significant reduction in threshold shift for clicks and 16 kHz tone bursts (but not 32 kHz) when AG was given with cisplatin. The malondialdehyde concentration (but not the *NO concentration) in the AG/cisplatin group was significantly lower than that of the cisplatin group. This suggests that AG reduces cisplatin ototoxicity by directly scavenging hydroxyl radicals. The iNOS pathway may play a role in the generation of free radicals and hearing loss resulting from cisplatin administration, but this conclusion was not supported by our data.

A comparative study of intratympanic steroid and NO synthase inhibitor for treatment of cochlear lateral wall damage due to acute otitis media

We studied the damage to the cochlear lateral wall induced by otitis media and the therapeutic effects of intratympanic administration of steroid and nitric oxide (NO) synthase inhibitor. In Sprague-Dawley rats, right middle ear cavities were inoculated with lipopolysaccharide, followed after 30 min by intratympanic administration of dexamethasone, NOS-inhibitor or PBS. Twenty-four hours after lipopolysaccharide inoculation, cochlear blood flow was measured by laser-Doppler flowmetry. Prostaglandin E(1) was topically applied to the round window membrane of the right ear and changes in cochlear blood flow were calculated. Changes of cochlear blood flow were significantly different among the three groups. Increases in cochlear blood flow following PGE(1) application were higher in the group that received NOS-inhibitor. Electron microscopic examination revealed that changes in the stria vascularis were less severe in rats treated with dexamethasone or NOS-inhibitor. Our results show the effectiveness of intratympanic dexamethasone or NOS-inhibitor in treating cochlear lateral wall damage caused by otitis media.

Induction of heat shock protein 32 (Hsp32) in the rat cochlea following hyperthermia

The genes for heat shock proteins (Hsps) can be upregulated in response to cellular trauma, resulting in enhanced cell survival and protection. Hsp32, also known as heme oxygenase 1, catalyzes the degradation of heme to produce carbon monoxide and bilirubin, which play a variety of cytoprotective functions at physiological concentrations, and iron, which is rapidly sequestered by the iron-binding protein ferritin. In the present study we examined the expression and localization of Hsp32 in the rat cochlea after heat shock using semi-quantitative reverse transcription polymerase chain reaction (RT-PCR), Western blot, and immunocytochemistry. Low levels of constitutive Hsp32 expression were observed in the normal rat cochlea by RT-PCR and Western blot. Hsp32 mRNA (messenger RNA) was present at higher levels in a subfraction containing sensorineural epithelium and lateral wall than in a subfraction containing modiolus. Western blot revealed that Hsp32 protein levels increase in the rat cochlea following heat shock. Immunocytochemistry showed scattered staining of outer hair cells in the organ of Corti of normal untreated rats. Following heat shock Hsp32 is upregulated in outer hair cells and the cells of the stria vascularis. These results suggest a potential role for Hsp32 as a component of the oxidative stress response pathway in the rat cochlea.

Candidate???s Thesis: Platelet-Activating Factor???Induced Hearing Loss: Mediated by Nitric Oxide?

OBJECTIVES/HYPOTHESIS

Platelet-activating factor (PAF)in middle ear effusion is thought to induce hearing loss. The purpose of this study is to investigate the role of nitric oxide (NO) in the mechanism of PAF-induced hearing loss by studying the effects of PAF application on the round window membrane (RWM) with and without PAF-antagonist NO-blocker.

STUDY DESIGN

Longitudinal study on randomized guinea pigs using PAF to induce hearing loss. METHODS Guinea pigs were divided into four groups: PBS, PAF, PAF-antagonist, and L-NAME. The PBS group received phosphate buffered saline (PBS) and the PAF groups received 10, 20, and 40 microg of PAF soaked into gelfoam and placed on the RWM. PAF-antagonist (WEB 2170) and NOS inhibitor NG-nitro-l-arginine-methylester (L-NAME) were injected intraperitoneally prior to PAF 20 microg application on the RWM. The following three tests were performed on each animal group: Hearing was tested with an auditory brainstem response (ABR) test over 24 hours. At the end of 24 hours, cochlear hair cells were examined by scanning electron microscopy (SEM) and immunohistochemistry was carried out on the cochlea to test the expression of inducible nitric oxide synthase (iNOS).

RESULTS

The PAF group developed significant elevation of ABR threshold and cochlear hair cell damage in the SEM group as compared with the PBS control group. The PAF-antagonist (WEB 2170) and the L-NAME groups did not show significant elevation of ABR threshold and cochlear hair cell damage compared with the group administered PAF 20 microg, but in the PAF-antagonist group, the elevation of ABR threshold was significant compared with that of the PBS control group, whereas it was not significant compared with the PBS group in the L-NAME group. Strong expression of iNOS on cochlea was observed in the PAF group and lighter expression was seen in PBS, WEB 2170, and L-NAME groups.

CONCLUSIONS

This study demonstrated that PAF placed on the RWM induced hearing loss and cochlear hair cell damage. The PAF-antagonists and L-NAME prevented the PAF-induced hearing loss and inhibited iNOS expression in the cochlea. These findings suggest that the PAF-induced hearing loss caused by cochlear hair cell damage may have been mediated by NO. PAF-antagonists and L-NAME may have future therapeutic implications in preventing sensorineural hearing loss associated with chronic otitis media. The results of this study have significant potential clinical application.

Upregulated iNOS and oxidative damage to the cochlear stria vascularis due to noise stress

Our previous work has revealed increased nitric oxide (NO) production in the cochlear perilymph following noise stress. However, it is not clear if the increase of NO is related to iNOS and whether NO-related oxidative stress can cause vascular tissue damage. In this study, iNOS immunoreactivity, NO production, and reactive oxygen species (ROS) in the lateral wall were examined in normal mice and compared with similar animals exposed to 120 dBA broadband noise, 3 h/day, for 2 consecutive days. In the normal animals, iNOS expression was not observed in the vascular endothelium of the stria vascularis and only weak iNOS immunoactivity was detected in the marginal cells. However, expression of iNOS in the wall of the blood vessels of stria vascularis and marginal cells was observed after loud sound stress (LSS). Relatively low levels of NO production and low ROS activity were detected in the stria vascularis in the unstimulated condition. In contrast, NO production was increased and ROS activity was elevated in the stria vascularis after LSS. These changes were attenuated by the iNOS inhibitor, GW 274150. To explore whether noise induces apoptotic processes in the stria vascularis, we examined morphological changes in endothelial- and marginal-cells. In vitro, annexin-V phosphatidylserine (PS) (to label and detect early evidence of apoptosis) was combined with propidium iodide (PI) (to probe plasma membrane integrity). PI alone was used in fixed tissues to detect later stage apoptotic cells by morphology of the nuclei. Following LSS, PS was expressed on cell surfaces of endothelial cells of blood vessels and marginal cells of the stria vascularis. Later stage apoptosis, characterized by irregular nuclei and condensation of nuclei, was also observed in these cells. The data indicate that increased iNOS expression and production of both NO and ROS following noise stress may lead to marginal cell pathology, and the dysfunction of cochlear microcirculation by inducing blood vessel wall damage.

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

Using immunohistochemistry and Western blot, the expression of inducible nitric oxide synthase (iNOS) in the lateral wall and organ of Corti was examined in normal (unstimulated) and stimulated mice and guinea pigs. The stimuli were: (1). injection of bacterial lipopolysaccharide (LPS, 5 mg/ml) into the middle ear through the tympanic membrane and (2). exposure to a 110 dB SPL (A-weighted) broadband noise, 3 h/day, for three consecutive days. For the unstimulated condition, weak iNOS expression was found in the vascular endothelium, marginal cells, nerve fibers, stereocilia of hair cells and Hensen's cells of the organ of Corti. More intense iNOS fluorescence signals were observed in cochlear tissues (particularly in hair cells and stria vascularis marginal cells) in animals exposed to loud sound or treated with LPS. Although the precise roles of iNOS expression in normal cochlear function have yet to be determined, enhanced iNOS expression following noise exposure and LPS suggests its participation in cochlear pathophysiology, including noise- and inflammatory factor-induced hearing loss.

Expression of Klotho protein in the inner ear

The Klotho mouse is a recently developed model that exhibits phenotypes resembling human aging. We used this model to investigate sensorineural hearing loss from the point of view that it may be considered an issue of aging. Using reverse transcription-polymerase chain reaction, Western blotting and immunohistochemical staining, we were able to confirm klotho gene transcription and protein synthesis in the kidney and inner ear. Klotho protein was mainly expressed in the stria vascularis and spiral ligament of the inner ear and in the distal convoluted tubule of the kidney, likely serving a common function in the two organs, i.e., modulating ion transport. The threshold for the auditory brainstem response was significantly higher in Klotho mice than in wild-type mice, and wave I latencies were prolonged. On the other hand, Klotho mice exhibited a normal distribution of I-IV interpeak intervals. No obvious morphological abnormalities were detected in Klotho mice, although no expression of Klotho protein was detected, and there was an apparent hearing disorder. Taken together, these findings suggest that by contributing to the maintenance ion homeostasis in the endolymph, Klotho protein serves as a key mediator of auditory function.

Nitric oxide in the inner ear

During the past year significant advances have been made in our understanding of the functional significance of nitric oxide (NO) in the inner ear. NO synthase and the NO production site have now been localized using immunohistochemistry and a new fluorescence indicator for NO. The functional significance of NO in the inner ear, in particular as a neurotransmitter, is becoming increasingly clear. Increasing evidence suggests that excessive NO production may play an essential role in inner ear disorders. The production of an inducible form of NO synthase may be closely related to this phenomenon. Based on the mechanisms of inner ear disorders, new pharmacological strategies for preventing or treating inner ear disorders have been suggested.

The electrochemical and fluorescence detection of nitric oxide in the cochlea and its increase following loud sound

A nitric oxide (NO)-selective sensor (tip diameter 30 microm) was inserted into the perilymph of the basal turn of the guinea pig cochlea. The basal level and stimulation-induced changes of NO were measured. The mean (+/-S.E.M.) basal level of NO was 273+/-42.9 nM. Following perilymphatic perfusion of the artificial perilymph containing NO synthase (NOS) substrate L-arginine (100 microM) combined with cofactor (6R)-5,6,7,8-tetrahydrobiopterin dihydrochloride (100 microM), a rapid and significant increase of NO to a mean concentration of 392+/-32.3 nM (P < 0.01, n = 10) was recorded. In contrast, a significant decrease of mean NO concentration to 180+/-32.7 nM (P < 0.01, n = 10) was observed following the perfusion of the NOS-inhibiting agent N(G)-nitro-L-arginine methyl ester (100 microM). No change in the NO concentration was found following the perfusion of either artificial perilymph or N(G)-monomethyl-D-arginine (100 microM) solution employed as controls. Broadband noise exposure (3 h/day at 120 dBA SPL) for three consecutive days produced an increase in NO concentration to 618+/-60.7 nM (P < 0.05, n = 10) in the perilymph. In addition, by using specific dyes for NO, 4,5-diaminofluoresceine diacetate and for the reactive oxygen species (ROS), dihydrorhodamine 1,2,3, the distribution of NO in the whole mounts of the organ of Corti and the production of ROS in vivo in the organ of Corti were investigated in both control (n = 5) and noise-exposed (n = 5) animals. The more intense NO and ROS fluorescence was observed in both the inner and outer hair cells in the noise-exposed groups. It is proposed that both the basal level and the increase in NO concentration following the addition of substrate (L-arginine) are produced by the constitutive NOS while the elevated NO and ROS following noise exposure indicate that NO may be involved in noise-induced hearing loss.

Brain-derived neurotrophic factor and nitric oxide synthase inhibitor protect the vestibular organ against gentamicin ototoxicity

In order to find a way to develop a new treatment for inner ear disorders, the effects of a nitric oxide synthase (NOS) inhibitor [N-nitro-L-arginine methylester (L-NAME)] and a neurotrophin [brain-derived neurotrophic factor (BDNF)] were investigated. The effect of L-NAME and BDNF on gentamicin-induced vestibular hair cell damage was investigated by using the in vitro LIVE/DEAD system. Both L-NAME and BDNF individually reduced the vestibular hair cell damage caused by gentamicin but the combination of L-NAME and BDNF was more successful in preventing damage. It is therefore suggested that treatment with a combination of an NOS inhibitor and a neurotrophin will help us to treat inner ear disorders.

Ischemia-reperfusion injury of the cochlea: effects of an iron chelator and nitric oxide synthase inhibitors

Release of free iron from cellular stores and activation of nitric oxide synthase (NOS) has been implicated in a wide variety of cochlear injuries. In order to evaluate the effects of deferoxamine (a iron chelator), 3-bromo-7-nitroindazole (a relatively selective neuronal NOS (nNOS) inhibitor) or aminoguanidine (a relatively selective inducible NOS (iNOS) inhibitor) on the post-ischemic cochlear dysfunction, albino guinea pigs were subjected to 30 min ischemia, and the threshold shifts of the compound action potential (CAP) from pre-ischemic values were compared with those of control animals 4 h after the onset of reperfusion. A statistically significant reduction in the post-ischemic CAP threshold shift was observed in the animals treated with deferoxamine or 3-bromo-7-nitroindazole. However, aminoguanidine did not affect the post-ischemic CAP threshold shift. These results suggest that free iron and nNOS play deleterious roles in the cochlear injury induced by transient ischemia.

Reactive nitrogen species contribute to blood-labyrinth barrier disruption in suppurative labyrinthitis complicating experimental pneumococcal meningitis in the rat

Sensorineural hearing damage is a frequent complication of bacterial meningitis, affecting as many as 30% of survivors of pneumococcal meningitis. There is a substantial body of evidence that oxidants, such as reactive nitrogen species (RNS), are central mediators of brain damage in experimental bacterial meningitis. In the present study, we investigated whether RNS also contribute to the pathophysiology of suppurative labyrinthitis in our well-established rat model of pneumococcal meningitis. In all infected rats, but not in uninfected controls, we observed suppurative labyrinthitis. Cochlear inflammation was accompanied by severe blood-labyrinth barrier (BLB) disruption as evidenced by increased Evans Blue extravasation. Furthermore, increased cochlear expression of endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS) was detected by immunohistochemistry. Colocalization of iNOS and tyrosine nitration (a marker of RNS attack) indicated that nitric oxide (NO) produced by iNOS contributes to oxidative cochlear damage through the action of RNS. To determine the pathophysiological role of RNS in BLB disruption, rats were treated with peroxynitrite scavengers (MnTBAP and uric acid, UA). Six h after adjunctive treatment with 300 mg/kg i.p. UA or 15 mg/kg i.p. MnTBAP+100 mg/kg i.p. ceftriaxone, BLB disruption was significantly reduced compared with that in infected animals treated only with ceftriaxone. Therefore, we conclude that RNS are involved in the breaching of the BLB during meningogenic pneumococcal labyrinthitis.

Tyrosine nitration: localisation, quantification, consequences for protein function and signal transduction

The nitration of free tyrosine or protein tyrosine residues generates 3-nitrotyrosine the detection of which has been utilised as a footprint for the in vivo formation of peroxynitrite and other reactive nitrogen species. The detection of 3-nitrotyrosine by analytical and immunological techniques has established that tyrosine nitration occurs under physiological conditions and levels increase in most disease states. This review provides an updated, comprehensive and detailed summary of the tissue, cellular and specific protein localisation of 3-nitrotyrosine and its quantification. The potential consequences of nitration to protein function and the pathogenesis of disease are also examined together with the possible effects of protein nitration on signal transduction pathways and on the metabolism of proteins.

Localization of nitric oxide synthase isoforms in the human cochlea

The location of nitric oxide (NO) in the structures of the cochlea is a topical issue. Nitric oxide synthase (NOS) has been detected previously in mammalian cochleae, but information on its presence in the human cochlea is still sparse. The location of NOS isoforms I, II and III in substructures of the human cochlea was studied by immunohistochemistry (fluorescein isothiocyanate technique) using monoclonal antibodies to NOS I, II and III. NOS I was the predominant isoform and staining could be observed in cells of the spiral ganglion (SG), in nerve fibres and in the outer hair cells (OHC). Furthermore, the supporting cells of the organ of Corti and the stria vascularis showed a fluorescent reaction to NOS I. Staining for NOS III was less intense and was located in the OHC, supporting cells and SG cells, while the stria vascularis remained unstained. By contrast, NOS II showed weak staining in a few neuron fibres only. The results imply that NO in the human cochlea could act as a neurotransmitter/neuromodulator at the level of neural cells and may be involved in the physiology of the supporting cells and stria vascularis. Moreover, because NO is both a mediator of excitotoxicity and a non-specifically toxic radical, it may also play a role in neurotoxicity of the human cochlea.

Direct evidence of nitric oxide production in the guinea pig organ of Corti

Production of nitric oxide (NO) in the organ of Corti of the guinea pig was investigated using the new fluorescence indicator 4,5-diaminofluorescein diacetate for direct detection of NO. The organ of Corti, lateral wall of the cochlea and isolated outer and inner hair cells were examined to locate NO production sites. The fluorescence intensities were augmented by stimulation with L-arginine or glutamate, and significantly increased after inoculation with lipopolysaccharide. This is the first direct evidence of NO production in the cochlea. NO may play an important role in the physiology of the organ of Corti and may also be involved in hearing disorders.

Nitric oxide distribution and production in the guinea pig cochlea

Production sites and distribution of nitric oxide (NO) were detected in cochlear lateral wall tissue, the organ of Corti and in isolated outer hair cells (OHCs) from the guinea pig using the fluorescent dye, 4,5-diaminofluorescein diacetate. Fluorescent signal, indicating the presence of NO, was found in the afferent nerves and their putative endings near inner hair cells (IHCs) and putative efferent nerve endings near OHCs, the IHCs and OHCs, the endothelial cells of blood vessels of the spiral ligament, the stria vascularis, and the spiral blood vessels of the basilar membrane. An increased NO signal was observed following exposure to the substrate for NO, L-arginine, while exposure to NO synthase inhibitors resulted in a decrease in NO signal. Observation of OHCs at the subcellular level revealed differentially strong fluorescent signals at the locations of cuticular plate, the subcuticular plate region, the infranuclear region, and the region adjacent to the lateral wall. The findings indicate the presence of NO in the cochlea and suggest that NO may play an important role in both regulating vascular tone and mediating neurotransmission in guinea pig cochlea.

Pharmacological models for inner ear therapy with emphasis on nitric oxide

Nitric oxide (NO)-mediated neurotoxicity may be an appropriate pathophysiological model with which to explain a variety of inner ear diseases characterized by acute or progressive hearing loss, tinnitus and vertigo. The localization of NO synthase (NOS) isoforms was examined in the inner ear of the pigmented guinea pig after intratympanic injection of 1 mg lipopolysaccharide (LPS) or 5 mg gentamicin (GM) using an immunohistochemical method, revealing the expression of NOS II in the inner ear. Production of NO in the isolated organ of Corti and utricle or in the isolated vestibular and cochlear hair cells after stimulation with L-arginine, glutamate, GM and LPS was investigated using the fluorescence indicator 4,5-diaminofluorescein diacetate. The fluorescence intensity of the sensory cells was augmented by stimulation with L-arginine, glutamate, GM and LPS. A significant increase in NO production was also noted in the LPS-treated animals. These findings imply that NO from constitutive NOS may mediate ototoxicity in the early phase, whereas NO from NOS II may contribute to the late phase of tissue damage in the inner ear. Based on this hypothesis, reduction of glutamatergic excitotoxicity and inhibition of NOS, scavenging superoxide and scavenging peroxynitrite are thought to attenuate NO-mediated otoneurotoxicity.