Nitric oxide distribution and production in the guinea pig cochlea

Regulation of Endothelial Nitric Oxide Synthase in the Reticular Lamina of the Organ of Corti by a Nitric Oxide Donor

In the vertebrate cochlea, the reticular lamina seals the organ of Corti against the endolymph filled scala media. After noise exposure, fast alterations in the endothelial nitric oxide synthase (eNOS) expression level were identified in this cochlear structure. Minor amounts of nitric oxide (NO) produced by eNOS or applied by NO donors such as S-nitroso-N-acetyl-penicillamine (SNAP) might protect this vulnerable part of the organ of Corti, on the line of gap junctions of supporting cells and cochlear microcirculation. In n=5 anesthetized guinea pigs, SNAP was intravenously applied in two concentrations. Six untreated animals served as controls. The cochleae were removed and prepared for immunoelectron microscopy using specific gold-labeled anti-eNOS antibodies. The density of the gold particles was quantified for seven cellular regions in the reticular lamina at the ultrastructural level. Following SNAP application, a significant increase in eNOS expression (+176%) was detected compared with controls (p=0.012). The increase occurred mainly in actin-rich cuticular structures and the prominent microtubules bundles. Correlation analysis revealed three clear and five moderate cellular associations for controls, whereas only one clear and one moderate after SNAP application. Thus, application of the NO donor SNAP resulted in an increase in eNOS expression in distinct regions of the reticular lamina.

The mechanoelectrical transducer channel is not required for regulation of cochlear blood flow during loud sound exposure in mice

The mammalian cochlea possesses unique acoustic sensitivity due to a mechanoelectrical ‘amplifier’, which requires the metabolic support of the cochlear lateral wall. Loud sound exposure sufficient to induce permanent hearing damage causes cochlear blood flow reduction, which may contribute to hearing loss. However, sensory epithelium involvement in the cochlear blood flow regulation pathway is not fully described. We hypothesize that genetic manipulation of the mechanoelectrical transducer complex will abolish sound induced cochlear blood flow regulation. We used salsa mice, a Chd23 mutant with no mechanoelectrical transduction, and deafness before p56. Using optical coherence tomography angiography, we measured the cochlear blood flow of salsa and wild-type mice in response to loud sound (120 dB SPL, 30 minutes low-pass filtered noise). An expected sound induced decrease in cochlear blood flow occurred in CBA/CaJ mice, but surprisingly the same sound protocol induced cochlear blood flow increases in salsa mice. Blood flow did not change in the contralateral ear. Disruption of the sympathetic nervous system partially abolished the observed wild-type blood flow decrease but not the salsa increase. Therefore sympathetic activation contributes to sound induced reduction of cochlear blood flow. Additionally a local, non-sensory pathway, potentially therapeutically targetable, must exist for cochlear blood flow regulation.

Distribution and change of peroxynitrite in the guinea pig cochlea following noise exposure

Nitric oxide (NO)-mediated pathology depends on the formation of reactive intermediates, such as the peroxynitrite (ONOO⁻). ONOO⁻ can nitrate free tyrosine and tyrosine residues of proteins. Therefore, increases in tyrosine nitration reflect the amount of ONOO⁻ produced by oxidative stress. The distribution of 3-nitrotyrosine (3-NT), an ONOO⁻ marker, in the organ of corti and the cochlear lateral wall tissue from the guinea pig were examined using fluorescence immunohistochemistry. The immunoactivity of 3-NT in the normal guinea pig was compared with animals exposed to 122dBA broadband noise, 4 h/day, for 2 consecutive days. In the normal animals, 3-NT immunoreactivity was found in the outer hair cells (OHCs), inner hair cells (IHCs), pillar cells (PCs), spiral ganglion cells (SPCs) and the marginal cells of stria vascularis in the lateral wall. Sound exposure increased the 3-NT signal in all of the cells and resulted in extensive outer hair cell loss. A quantitative analysis of the 3-NT change in OHCs and marginal cells of lateral wall showed that immunolabeling was significant (P<0.01, n=10) in the noise exposure group compared with that of the control group. Anti-3-NT and propidium iodide double labeling showed that 3-NT was distributed mainly in the apical end of OHCs. In addition, 3-NT was distributed outside of the nucleus of the OHCs and marginal cells. In conclusion, the data indicate that noise exposure leads to a significant production of ONOO⁻ in the cochlear lateral wall and organ of corti. This is consistent with the known increase of NO production by loud sound stress and suggests that NO-derived free radicals participate in the cochlear pathophysiology of noise-induced hearing loss.

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.

[Possible molecular mechanisms of spontaneous remission in sudden idiopathic hearing loss]

According to current knowledge, it must be assumed that temporary idiopathic hearing loss and its spontaneous remission are based on mechanical and/or pathological alterations in the inner ear. The causal mechanisms might be based on inter-individual variations. Induced by dose-dependent activators, temporary as well as permanent damage might occur. Sudden hearing loss may be initiated by an increase in the local nitric oxide (NO) concentration. Spontaneous remission, i.e. functional restoration, can be explained by a local decrease in the NO concentration. In this context, regulatory systems such as the gap-junction system, blood vessels or synapses might be affected. In addition, alterations in the hormone level of estrogen and mineralocorticoids, as well as cellular glutathione and vitamin levels, might lead to temporary alterations in the inner ear. Recent experimental findings indicate a role for the shuttle protein Survivin in the spontaneous remission of sudden hearing loss.

Physiopathology of the cochlear microcirculation

Normal blood supply to the cochlea is critically important for establishing the endocochlear potential and sustaining production of endolymph. Abnormal cochlear microcirculation has long been considered an etiologic factor in noise-induced hearing loss, age-related hearing loss (presbycusis), sudden hearing loss or vestibular function, and Meniere's disease. Knowledge of the mechanisms underlying the pathophysiology of cochlear microcirculation is of fundamental clinical importance. A better understanding of cochlear blood flow (CoBF) will enable more effective management of hearing disorders resulting from aberrant blood flow. This review focuses on recent discoveries and findings related to the physiopathology of the cochlear microvasculature.

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.

Modulation of hair cell efferents

Outer hair cells (OHCs) amplify the sound-evoked motion of the basilar membrane to enhance acoustic sensitivity and frequency selectivity. Medial olivocochlear (MOC) efferents inhibit OHCs to reduce the sound-evoked response of cochlear afferent neurons. OHC inhibition occurs through the activation of postsynaptic α9α10 nicotinic receptors tightly coupled to calcium-dependent SK2 channels that hyperpolarize the hair cell. MOC neurons are cholinergic but a number of other neurotransmitters and neuromodulators have been proposed to participate in efferent transmission, with emerging evidence for both pre- and postsynaptic effects. Cochlear inhibition in vivo is maximized by repetitive activation of the efferents, reflecting facilitation and summation of transmitter release onto outer hair cells. This review summarizes recent studies on cellular and molecular mechanisms of cholinergic inhibition and the regulation of those molecular components, in particular the involvement of intracellular calcium. Facilitation at the efferent synapse is compared in a variety of animals, as well as other possible mechanisms of modulation of ACh release. These results suggest that short-term plasticity contributes to effective cholinergic inhibition of hair cells.

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.

Visualization and contractile activity of cochlear pericytes in the capillaries of the spiral ligament

Pericytes, mural cells located on microvessels, are considered to play an important role in the formation of the vasculature and the regulation of local blood flow in some organs. Little is known about the physiology of cochlear pericytes. In order to investigate the function of cochlear pericytes, we developed a method to visualize cochlear pericytes using diaminofluorescein-2 diacetate (DAF-2DA) and intravital fluorescence microscopy. This method can permit the study of the effect of vasoactive agents on pericytes under the in vivo and normal physiological condition. The specificity of the labeling method was verified by the immunofluorescence labeling of pericyte maker proteins such as desmin, neural proteoglycan (NG2), and thymocyte differentiation antigen 1 (Thy-1). Superfused K(+) and Ca(2+) to the cochlear lateral wall resulted in localized constriction of capillaries at pericyte locations both in vivo and in vitro, while there was no obvious change in cochlear capillary diameters with application of the adrenergic neurotransmitter noradrenaline. The method could be an effective way to visualize cochlear pericytes and microvessels and study lateral wall vascular physiology. Moreover, we demonstrate for the first time that cochlear pericytes have contractility, which may be important for regulation of cochlear blood flow.

The cochlear pericytes

OBJECTIVES

Cochlear pericytes are not well characterized. The aim of this study was to further advance the characterization of cochlear pericyte location and distribution, with particular focus on pericyte-related proteins on the capillaries of the cochlear lateral wall that are functionally integral to structure, contraction, and gap junction transport.

MATERIALS AND METHODS

Cochlear pericytes were identified by the immunofluorescence labeling of pericyte marker proteins, including alpha-smooth muscle actin (alpha-SMA), desmin, Thy-1, tropomyosin, and NG2, and by morphological identification, using fluorescence, electron, and differential interference contrast microscopy.

RESULTS

Pericytes were predominately found in the capillary network of the cochlear lateral wall, with considerable morphological heterogeneity across different types of microvessels. For example, pericytes on the vessels of the spiral ligament (V/SL) strongly expressed a gap junction protein, connexin 40, and were positive for alpha-SMA, tropomyosin, and desmin. In contrast, pericytes on the vessels of the stria vascularis (V/SV) were positive for desmin, and were negative for alpha-SMA and tropomyosin.

CONCLUSIONS

The capillary networks of the cochlear lateral wall comprise a rich population of pericytes. These pericytes are morphologically heterogeneous, with protein expression potentially indicative of function.

Hyposmotic stimulation-induced nitric oxide production in outer hair cells of the guinea pig cochlea

Nitric oxide (NO) production during hyposmotic stimulation in outer hair cells (OHCs) of the guinea pig cochlea was investigated using the NO sensitive dye DAF-2. Simultaneous measurement of the cell length and NO production showed rapid hyposmotic-induced cell swelling to precede NO production in OHCs. Hyposmotic stimulation failed to induce NO production in the Ca2+-free solution. L-NG-nitroarginine methyl ester (L-NAME), a non-specific NO synthase inhibitor and gadolinium, a stretch-activated channel blocker inhibited the hyposmotic stimulation-induced NO production whereas suramin, a P2 receptor antagonist did not. S-nitroso-N-acetylpenicillamine (SNAP), a NO donor inhibited the hyposmotic stimulation-induced increase in the intracellular Ca2+ concentrations ([Ca2+]i) while L-NAME enhanced it. 1H-[1,2,4]oxadiazole[4,3a]quinoxalin-1-one, an inhibitor of guanylate cyclase and KT5823, an inhibitor of cGMP-dependent protein kinase (PKG) mimicked effects of L-NAME on the Ca2+ response. Transient receptor potential vanilloid 4 (TRPV4), an osmo- and mechanosensitive channel was expressed in the OHCs by means of immunohistochemistry. 4alpha-phorbol 12,13-didecanoate, a TRPV4 synthetic activator, induced NO production in OHCs. These results suggest that hyposmotic stimulation can induce NO production by the [Ca2+]i increase, which is presumably mediated by the activation of TRPV4 in OHCs. NO conversely inhibits the Ca2+ response via the NO-cGMP-PKG pathway by a feedback mechanism.

Nitric oxide production from cultured spiral ligament fibrocytes: effects of corticosteroids

CONCLUSION

This is the first report of nitric oxide (NO) production by cultured spiral ligament (SL) fibrocytes. The data suggest that dexamethasone (Dex) suppresses NO production by SL fibrocytes. Given that SL fibrocytes play a role in cochlear fluid and ion homeostasis, glucocorticoids may suppress cochlear malfunction caused by NO production in SL fibrocytes.

OBJECTIVE

To investigate NO production by cultured SL fibrocytes and regulation of NO production by Dex in mouse secondary cell cultures.

MATERIALS AND METHODS

Cultured SL fibrocytes were stimulated with tumor necrosis factor-alpha (TNF-alpha), and NO production was visualized with membrane-permeable 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM diacetate). Inducible nitric oxide synthase (NOSII) messenger RNA (mRNA) was measured by reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

After TNF-alpha stimulation, the fluorescence intensity increased in a time-dependent manner. This increase in fluorescence intensity was suppressed by a nonspecific NOS inhibitor, N(G)-nitro L-arginine methyl ester (L-NAME), and Dex. After TNF-alpha stimulation, expression of NOSII mRNA was observed. Expression of NOSII mRNA was inhibited by Dex.

Apical phosphatidylserine externalization in auditory hair cells

In hair cells of the inner ear, phosphatidylserine (PS), detected with fluorescent annexin V labeling, was rapidly exposed on the external leaflet of apical plasma membranes upon dissection of the organ of Corti. PS externalization was unchanged by caspase inhibition, suggesting that externalization did not portend apoptosis or necrosis. Consistent with that conclusion, mitochondrial membrane potential and hair-cell nuclear structure remained normal during externalization. PS externalization was triggered by forskolin, which raises cAMP, and blocked by inhibitors of adenylyl cyclase. Blocking Na(+) influx by inhibiting the mechanoelectrical transduction channels and P2X ATP channels also inhibited external PS externalization. Diminished PS externalization was also seen in cells exposed to LY 294002, which blocks membrane recycling in hair cells by inhibiting phosphatidylinositol 3-kinase. These results indicate that PS exposure on the external leaflet, presumably requiring vesicular transport, results from elevation of intracellular cAMP, which can be triggered by Na(+) entry into hair cells.

Hyposmotic stimulation-induced nitric oxide production in outer hair cells of the guinea pig cochlea

Nitric oxide (NO) production during hyposmotic stimulation in outer hair cells (OHCs) of the guinea pig cochlea was investigated using the NO sensitive dye DAF-2. Simultaneous measurement of the cell length and NO production showed rapid hyposmotic-induced cell swelling to precede NO production in OHCs. Hyposmotic stimulation failed to induce NO production in the Ca(2+)-free solution. L-N(G)-nitroarginine methyl ester (L-NAME), a non-specific NO synthase inhibitor and gadolinium, a stretch-activated channel blocker inhibited the hyposmotic stimulation-induced NO production whereas suramin, a P2 receptor antagonist did not. S-nitroso-N-acetylpenicillamine (SNAP), a NO donor inhibited the hyposmotic stimulation-induced increase in the intracellular Ca(2+) concentrations ([Ca(2+)](i)) while L-NAME enhanced it. 1H-[1,2,4]oxadiazole[4,3a]quinoxalin-1-one, an inhibitor of guanylate cyclase and KT5823, an inhibitor of cGMP-dependent protein kinase (PKG) mimicked effects of L-NAME on the Ca(2+) response. Transient receptor potential vanilloid 4 (TRPV4), an osmo- and mechanosensitive channel was expressed in the OHCs by means of immunohistochemistry. 4alpha-phorbol 12,13-didecanoate, a TRPV4 synthetic activator, induced NO production in OHCs. These results suggest that hyposmotic stimulation can induce NO production by the [Ca(2+)](i) increase, which is presumably mediated by the activation of TRPV4 in OHCs. NO conversely inhibits the Ca(2+) response via the NO-cGMP-PKG pathway by a feedback mechanism.

Role of nitric oxide on ATP-induced Ca2+ signaling in outer hair cells of the guinea pig cochlea

Recently, a negative feedback effect of nitric oxide (NO) on the adenosine 5'-triphosphate (ATP)-induced Ca2+ response has been described in cochlear inner hair cells. We here investigated the role of NO on the ATP-induced Ca2+ response in outer hair cells (OHCs) of the guinea pig cochlea using the NO-sensitive dye DAF-2 and Ca2+ -sensitive dye fura-2. Extracellular ATP induced NO production in OHCs, which was inhibited by L-NG-nitroarginine methyl ester (L-NAME), a non-specific NO synthase (NOS) inhibitor, and suramin, a P2 receptor antagonist. ATP failed to induce NO production in the Ca2+ -free solution. S-nitroso-N-acetylpenicillamine (SNAP), a NO donor, enhanced the ATP-induced increase of the intracellular Ca2+ concentrations ([Ca2+]i), while L-NAME inhibited it. SNAP accelerated ATP-induced Mn2+ quenching in fura-2 fluorescence, while L-NAME suppressed it. 8-Bromoguanosine-cGMP, a membrane permeable analog of cGMP, mimicked the effects of SNAP. 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 inhibited the ATP-induced [Ca2+]i increase. Selective neuronal NOS inhibitors, namely either 7-nitro-indazole or 1-(2-trifluoromethylphenyl) imidazole, mimicked the effects of L-NAME regarding both ATP-induced Ca2+ response and NO production. Immunofluorescent staining of neuronal nitric oxide synthase (nNOS) in isolated OHCs showed the localization of nNOS in the apical region of OHCs. These results suggest that the ATP-induced Ca2+ influx via a direct action of P2X receptors may be the principal source for nNOS activity in the apical region of OHCs. Thereafter, NO can be produced while conversely enhancing the Ca2+ influx via the NO-cGMP-PKG pathway by a feedback mechanism.

Involvement of nitric oxide generation in noise-induced temporary threshold shift in guinea pigs

The present study explored the role of endogenous nitric oxide (NO) in the temporary threshold shift caused by acoustic trauma. Guinea pigs were exposed to broadband white noise at a level of 105+/-2dB sound pressure level (SPL) for 10min, causing a temporary threshold shift (TTS). The guinea pigs were divided into six groups (N-1 to N-6) according to survival days after noise exposure (0, 1, 2, 3, 7, 28days). Auditory brainstem responses (ABR) were recorded before noise exposure, immediately after noise exposure and before sacrifice. Immediately after animals were sacrificed, the stria vascularis and the spiral ligament of the lateral wall of each individual cochlea were harvest as a unit and prepared for assay of NO. There was a significant correlation (P<0.001) between the NO concentration and final ABR threshold in the noise exposure groups. But the return of ABR threshold to pre-noise-exposed level is early than that of NO concentration. An average 16.2dB threshold shift was found immediately after noise exposure. The threshold returned to the pre-noise-exposed level on the second post-exposure day. Comparing to unexposed control animals, the NO concentration increased nearly threefold immediately following noise exposure and decreased to twofold when the hearing threshold had returned to the pre-noise-exposed level. On the seventh post-exposure day the NO concentration was not different from that in unexposed control animals. Those findings indicate that endogenous NO is generated in the noise-induced temporal threshold shift and its concentration is correlated with the hearing loss.

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.

Endothelial nitric oxide synthase upregulation in the cochlea of the guinea pig after intratympanic gentamicin injection

Single-shot transtympanic gentamicin therapy has become a popular treatment modality for Meniere's disease despite the known possible ototoxic properties of this drug. It was shown recently that NO production and iNOS were upregulated after gentamicin application, which was interpreted as a possible effect of ototoxicity. In this study we analyzed the expression of eNOS after gentamicin application to determine a possible correlation of this enzyme with gentamicin-induced ototoxicity. We compared eNOS expression in gentamicin-treated and non-treated guinea pigs in the second turn of the cochlea, an area corresponding to speech perception in humans. Gentamicin (4 mg) was injected intratympanically into the middle ear of guinea pigs ( n =3) and the reduction of the hearing threshold level was determined by recording acoustic-evoked potentials (AEP) before and 5 days after gentamicin application. Morphological alterations in the organ of Corti were analyzed by light and electron microscopy. Gold-labeled anti-eNOS antibodies were counted in eight different cell areas for quantification of eNOS expression. Seven animals were analyzed as controls. After gentamicin application, a deterioration of hearing level was observed varying from 10 to 30 dB. A high degree of vacuolization was identified in the third row of outer hair cells. At the subcellular level, the subsurface cisterns in outer hair cells were dissociated from the basolateral cell membrane, and the mitochondrial membranes were frequently damaged. Statistically significant upregulation of eNOS was observed in all cell types analyzed. Depending on the various cell types the amount of gold-labeled eNOS antibodies was 2.5 to 5.7 times higher after gentamicin application. We observed significant eNOS upregulation after gentamicin application in the cochlea, in conjunction with cellular damages and decreased hearing.

Acute effects of glucocorticoids on ATP-induced Ca2+ mobilization and nitric oxide production in cochlear spiral ganglion neurons

Rapid, non-genomic effects of glucocorticoids on extracellular adenosine 5'-triphosphate (ATP)-induced intracellular Ca(2+) concentration ([Ca(2+)](i)) changes and nitric oxide (NO) production were investigated in type I spiral ganglion neurons (SGNs) of the guinea-pig cochlea using the Ca(2+)-sensitive dye fura-2 and the NO-sensitive dye 4,5-diaminofluorescein (DAF-2). Pretreatment of SGNs with 1 microM dexamethasone for 10 min, a synthetic glucocorticoid hormone, enhanced the ATP-induced [Ca(2+)](i) increase in SGNs. RU 38486, a competitive glucocorticoid receptor antagonist eliminated the effects of dexamethasone on the ATP-induced [Ca(2+)](i) increase in SGNs. These acute effects of dexamethasone were dependent on the presence of extracellular Ca(2+), thereby suggesting that dexamethasone may rapidly enhance the Ca(2+) influx through the activation of ionotropic P2X receptors which may interact with glucocorticoid-mediated membrane receptors. Extracellular ATP increased the intensity of DAF-2 fluorescence, indicating NO production in SGNs. The ATP-induced NO production was mainly due to the Ca(2+) influx through the activation of P2 receptors. S-nitroso-N-acetylpenicillamine, a NO donor, enhanced the ATP-induced [Ca(2+)](i) increase in SGNs while L-N(G)-nitroarginine methyl ester (L-NAME), a NO synthesis inhibitor, inhibited it. Dexamethasone enhanced the ATP-induced NO production in SGNs. The augmentation of dexamethasone on ATP-induced NO production was abolished in the presence of l-NAME. It is concluded that the ATP-induced [Ca(2+)](i) increase induces NO production which enhances a [Ca(2+)](i) increase in SGNs by a positive-feedback mechanism. Dexamethasone enhances the ATP-induced [Ca(2+)](i) increase in SGNs which results in the augmentation of NO production. The present study suggests that NO may play an important role in auditory signal transduction. Our results also indicate that glucocorticoids may rapidly affect auditory neurotransmission due to a novel non-genomic mechanism.

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.

An investigation of the effects of ruthenium red, nitric oxide and endothelin-1 on infrared receptor activity in a crotaline snake

The infrared (IR) receptors in the pit organ of crotaline snakes are very sensitive to temperature. The vasculature of the pit organs, which is located in close proximity to IR-sensitive terminal nerve masses (IR receptors), is finer, flatter, and more convoluted than that of other sensory organs. Using extracellular recording in vivo from IR-sensitive primary afferent trigeminal ganglion (TG) neurons of the crotaline snake Trimeresurus flavoviridis, I studied the response to IR warming (24-25 degrees C) and to various chemicals: an exogenous vasoactive substance nitric oxide donor (sodium nitroprusside, SNP), endothelin-1 (ET-1), a transient receptor potential vanilloid (TRPV)1 agonist (capsaicin, CAP) and antagonist (capsazepine, CZP), and Ruthenium Red (RR), an antagonist of the TRPV family. IR-sensitive primary afferent TG neurons display regular background firing at 10-25 impulses per second at 24-25 degrees C. At this temperature, Ruthenium Red and endothelin-1 clearly suppressed the frequency of background firing, while sodium nitroprusside injected into the bloodstream significantly increased the frequency of discharges (P<0.01) and caused regular bursts of firing in IR-sensitive TG neurons. By contrast, capsaicin and capsazepine had no effect on the infrared responses. The possibility that these opposite responses result from their vasoactive effects on the unusual pit vasculature or from their chemical effects on the thermoreceptors of IR-sensitive nerve terminals in the pit organ, like those of the TRPV family, is discussed.

Disorders of cochlear blood flow

The cochlea is principally supplied from the inner ear artery (labyrinthine artery), which is usually a branch of the anterior inferior cerebellar artery. Cochlear blood flow is a function of cochlear perfusion pressure, which is calculated as the difference between mean arterial blood pressure and inner ear fluid pressure. Many otologic disorders such as noise-induced hearing loss, endolymphatic hydrops and presbycusis are suspected of being related to alterations in cochlear blood flow. However, the human cochlea is not easily accessible for investigation because this delicate sensory organ is hidden deep in the temporal bone. In patients with sensorineural hearing loss, magnetic resonance imaging, laser-Doppler flowmetry and ultrasonography have been used to investigate the status of cochlear blood flow. There have been many reports of hearing loss that were considered to be caused by blood flow disturbance in the cochlea. However, direct evidence of blood flow disturbance in the cochlea is still lacking in most of the cases.

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.

Rapid recovery from acoustic trauma: chicken soup, potato knish, or drug interaction?

OBJECTIVES

To describe the phenomenology and consider possible mechanisms mediating rapid and unexpected recovery from acoustic trauma after ingestion of a food substance (potato knish).

STUDY DESIGN

Single subject with repeated test measures.

SETTING

Regional Veteran's Administration Medical Center, tertiary care medical center.

METHODS

Pure-tone audiometry and distortion product otoacoustic emissions (DPOAEs) performed at 6 days, 21 days, and 1 year postexposure.

RESULTS

Medical treatment with corticosteriods and a diuretic alone failed to improve auditory function and related symptoms (tinnitus and aural fullness) over a 2-week period. Rapid recovery of auditory function (dramatic improvement in pure tone thresholds; reappearance of DPOAEs) and abatement of related symptoms directly followed physiologic reactions from ingesting a food substance.

CONCLUSIONS

Rapid recovery from acoustic trauma was temporally correlated with urodynamic and cardiovascular reactions from ingesting food containing sulfite preservative, a substance to which the individual was allergic. Factors that may have contributed to recovery of function include massive diuresis, increased heart rate, release of biochemical mediators, mediator-induced vasodilatation, and changes in vascular or cell membrane permeability. Establishing relationships that lead to recovery of function from acoustic trauma may facilitate research and aid in the development of new treatment options for this condition.

The effect of L-arginine on slow motility of mammalian outer hair cell

The effect of L-arginine on the slow motility of mammalian cochlear outer hair cells was studied in this experiment. L-Arginine (3 mM) but not D-arginine (3 mM) or other amino acids (L-aspartate or L-glutamate) induced length increases of guinea pig outer hair cell. Similarly, the membrane-permeant cGMP analogues, 8-(4-chlorophenylthio)guanosine 3':5'-cyclic monophosphate (1 mM) or 8-bromo-guanosine 3':5'-cyclic monophosphate (1 mM) induced length increases of guinea pig outer hair cells. These length increases induced by L-arginine can be attenuated by a 30 min preincubation of the cells with the nitric oxide synthase inhibitors N(G)-nitro-L-arginine methyl ester hydrochloride (3 mM) or 7-nitroindazole (1 mM). Comparing the effects of L-arginine and ionomycin on cell length and intracellular calcium change in outer hair cells, both L-arginine and ionomycin were able to induce the elongation of outer hair cells but L-arginine did not change the fluorescence intensity of Fluo-3. Preincubation of the cells with EGTA (3 mM) for 40 min to reduce the extracellular calcium concentration did not influence the effect of L-arginine. This experiment demonstrated that nitric oxide/cGMP pathway involvement in regulating the slow motility of mammalian outer hair cells cannot be ruled out. The effect of L-arginine is independent of extracellular calcium concentration.

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.

The demonstration of nitric oxide in cochlear blood vessels in vivo and in vitro: the role of endothelial nitric oxide in venular permeability

The objectives of the current study were to investigate the distribution and production of NO in cochlear blood vessels, and to assess whether the inhibition of basal NO production leads to vascular protein leakage of the cochlear microvasculature. Using the fluorescent dye 4,5-diaminofluorescein diacetate, NO was detected, both in vitro and in vivo, in the endothelial cells of various cochlear blood vessels, including the spiral modiolar artery, the vessel of the basilar membrane and the vessels of the spiral osseous lamina. Vessel leakage was assessed using intravital fluorescence microscopy following systemic infusion of fluorescein isothiocyanate-labeled bovine albumin. Local perfusion of the cochlear basal turn with either Ringer's solution or Ringer's containing an inactive nitric oxide synthase inhibitor (100 microM) produced minimal protein leakage. Perfusion with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (100 microM) produced significantly enhanced vascular protein leakage. The findings demonstrate the presence of endothelial NO in the cochlear blood vessels and suggest that NO protects cochlear venules against excessive vascular leakage.

Possible roles of nitric oxide in the physiology and pathophysiology of the mammalian cochlea

Nitric oxide (NO) has been implicated as a mediator of vasodilation and neurotransmission in the mammalian cochlea. This is demonstrated by the presence of nitric oxide synthase (NOS) and nitric oxide (NO) in the blood vessels and the organ of Corti in the cochlea. It is not certain if the neurons in the spiral ganglion produce NO since no fluorescent signal could be detected by 4,5-diaminofluorescein diacetate (DAF-2DA), a fluorescent indicator of NO. To determine if NO/peroxynitrite plays any role in neurodestruction observed in ischemic cochlea of the guinea pig, the effects of NO donors, such as S-nitrosocysteine (S-NC) and nitroglycerine (NTG); peroxynitrite generators, such as 3-morpholinosydnonimine (SIN-1); peroxynitrite inhibitors, such as superoxide dismutase plus catalase (SOD/Cat); and NOS inhibitors, such as N(G)-nitro-L-arginine methyl ether (L-NAME) were tested on normal and ischemic cochleae. The level of NO in the cochlea after 20 to 120 minutes of ischemia was indicated by measurement of nitrites/nitrates in the perilymph. The evidence gathered from these experiments indicates that NO or peroxynitrite is not necessarily destructive to auditory hair cells, and in fact, exogenous NO may protect neural structures in the cochlea from damage under ischemic conditions.

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.