Electrophysiological and morphological evaluation of the acute ototoxicity of sodium nitroprusside

Effects of Calcitonin-Gene-Related-Peptide on Auditory Nerve Activity

Calcitonin-gene-related peptide (CGRP) is a lateral olivocochlear (LOC) efferent neurotransmitter. Depression of sound-driven auditory brainstem response amplitude in CGRP-null mice suggests the potential for endogenous CGRP release to upregulate spontaneous and/or sound-driven auditory nerve (AN) activity. We chronically infused CGRP into the guinea pig cochlea and evaluated changes in AN activity as well as outer hair cell (OHC) function. The amplitude of both round window noise (a measure of ensemble spontaneous activity) and the synchronous whole-nerve response to sound (compound action potential, CAP) were enhanced. Lack of change in both onset adaptation and steady state amplitude of sound-evoked distortion product otoacoustic emission (DPOAE) responses indicated CGRP had no effect on OHCs, suggesting the origin of the observed changes was neural. Combined with results from the CGRP-null mice, these results appear to confirm that endogenous CGRP enhances auditory nerve activity when released by the LOC neurons. However, infusion of the CGRP receptor antagonist CGRP (8–37) did not reliably influence spontaneous or sound-driven AN activity, or OHC function, results that contrast with the decreased ABR amplitude measured in CGRP-null mice.

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.

Effects of perilymphatic pressure, sodium nitroprusside, and bupivacaine on cochlear fluid pH of guinea pigs

CONCLUSIONS

Hydrostatic positive pressure and vasoconstrictor acidified the cochlear fluids, whereas the vasodilator made the fluids alkaline. CBF might play a role in regulating cochlea fluid pH.

OBJECTIVES

Cochlea fluid pH is highly dependent on the HCO3(-)/CO2 buffer system. Cochlear blood flow (CBF) supplies O2 and removes CO2. It is speculated that cochlear blood flow changes might affect the balance of the HCO3(-)/CO2 buffer system in the cochlea. It is known that the elevation of inner ear pressure decreases the CBF, and local application of vasodilating or vasoconstricting agents directly to the cochlea changes the CBF. The purpose of this study was to elucidate the effect of positive hydrostatic inner ear pressure and application of a vasodilator and vasoconstrictor of cochlear vessels on the pH of the endolymph and perilymph.

METHODS

The authors performed animal physiological experiments on 30 guinea pigs. Hydrostatic positive pressure was infused through a glass capillary tube inserted into the scala tympani of the basal turn. The vasodilator, nitric oxide donor (sodium nitroprusside; SNP), and the vasoconstrictor, bupivacaine, were placed topically onto the round window of the guinea pig cochlea. Endolymph pH (pHe) and endocochlear potential (EP) were monitored by double-barreled ion-selective microelectrodes in the second turn of the guinea pig cochlea. During the topical application study, scala vestibuli perilymph pH (pHv) was also measured simultaneously in the second turn.

RESULTS

The application of hydrostatic positive pressure caused a decrease in pHe and EP. Positive perilymphatic pressure caused the endolymph to become acidic pressure-dependently. Application of 3.0% SNP evoked an increase in both the pHe and pHv, following by a gradual recovery to baseline levels. On the other hand, 0.5% bupivacaine caused a decrease in both the pHe and pHv. The EP during topical application showed slight, non-significant changes.

Protective effect of omega-3 fatty acid against mercury chloride intoxication in mice

The aim of this study was to investigate the protective effect of omega-3 fatty acid in HgCI2 toxicity in mice. Levels of malondialdehyde (MDA), reduced glutathione (GSH), nitric oxide (NO) and total sialic acid (TSA), and histopathological changes in selected organs were evaluated. Twenty-eight mice were equally divided into 4 groups, namely Groups I-IV. Group I animals received intraperitoneal (ip) injection of physiological saline solution; Group II animals received ip injection of 0.4mg/kg/day HgCI2; Group III animals received ip injection of 0.4mg/kg/day HgCI2 in addition to subcutaneous (sc) injection of 0.5g/kg/day omega-3 fatty acid; and Group IV animals received sc injection of 0.5g/kg/day omega-3 fatty acid. All treatments lasted 7 days. The levels of MDA, NO and TSA were significantly higher in Group II and lower in Groups III and IV as compared to the Group I. GSH level was the highest in Group IV. In histopathology, severe degeneration in liver and kidney was observed in Group II animals. These degrading changes were seen to be reduced greatly in Group III animals. The results suggested that omega-3 fatty acid might attenuate HgCI2-induced toxicity by improving antioxidant status and acute phase response in mice.

Effect of Ginkgo biloba extract on endotoxin-induced labyrinthitis

OBJECTIVE

There are no reports on the therapeutic effect of Ginkgo biloba extract (GBE) on otitis media-induced labyrinthitis. The present study examined whether GBE can protect against cochlear damage induced by intratympanic instillation of lipopolysaccharide (LPS)-induced labyrinthitis.

MATERIALS AND METHODS

Experiments were performed in 20 healthy young male guinea pigs. The control group (n=10) received an intratympanic instillation of LPS (20 μl, 3mg/ml). The experimental group (n=10) received intratympanic instillation of LPS immediately after instillation of GBE (10mg/kg) and then experimental groups received GBE (100mg/kg) by intraperitoneal injection every day for 3 days. Instillation of LPS or LPS immediately after GBE was done in the right ear; the untreated left ear was considered normal. Physiological and morphological changes were evaluated.

RESULTS

Statistical analysis of treatment of GBE revealed significantly less hearing loss than LPS group (p<0.05). The ratio of the value of cochlear blood flow (CBF) compared to untreated left side was significantly higher in the GBE treated group than in the LPS-treated group (p<0.05). This result indicated the recovery of CBF in GBE treated group compared to LPS treated group. In the LPS group, scanning electron microscopy revealed hair cell damage with edema. Missing stereocilia in the third layer of the outer hair cell was revealed. However, both the inner hair cells and the outer hair cells had normal appearance in the GBE group. LPS group showed that cochlear Evans blue extravasation was increased strongly in the stria vascularis, spiral limbus, and in the spiral ligament compared with the GBE treated group.

CONCLUSION

GBE significantly minimizes cochlear damage against LPS-induced otitis media with labyrinthitis in a guinea pig model. GBE has potential as an adjunctive therapy to antibiotics in the treatment of acute otitis media with complicated labyrinthitis.

Response of cochlear blood flow to prostaglandin E1 applied topically to the round window

CONCLUSIONS

The increase in cochlear blood flow (CBF) after administration of prostaglandin E1 (PGE1) to the round window depends on increased blood flow through the anterior inferior cerebellar artery (AICA).

OBJECTIVES

To evaluate the response of CBF to PGE1 applied topically to the round window, and to investigate the origin of blood flow changes after this topical application.

MATERIAL AND METHODS

The response of CBF to topically applied PGE1 was measured by placing the tip of a laser Doppler probe on the bony wall of the basal turn of the cochlea after the middle ear mucosa over the cochlea had been removed in guinea pigs and rats. In rats, the CBF response to PGE1 administration was investigated after occlusion of the AICA or stapedial artery.

RESULTS

CBF increased following PGE1 administration in both guinea pigs and rats. In rats, CBF increased from 100% to 132%+/-10% (mean+/-SD) after the topical application of 0.5 microl of a 0.014% PGE1 solution. CBF decreased after occlusion of the AICA or stapedial artery but did not increase after PGE1 administration during occlusion of the AICA. The CBF response to PGE1 administration was similar before and after occlusion of the stapedial artery.

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.

Permanent threshold shift caused by acute cochlear mitochondrial dysfunction is primarily mediated by degeneration of the lateral wall of the cochlea

Mitochondrial dysfunction in the cochlea is thought to be an important cause of sensorineural hearing loss. Recently, we have established a novel rat model with acute hearing impairment caused by exposure to the mitochondrial toxin 3-nitropropionic acid (3-NP) to analyze the mechanism of cochlear mitochondrial dysfunction. Both permanent and temporary threshold shifts were observed in this model depending on the amount of 3-NP used to induce hearing impairment. In this study, we demonstrate cochlear morphological changes in the permanent threshold shift model. Marked degeneration was detected in type 2 fibrocytes in the spiral prominence, type 4 fibrocytes in the spiral ligament, marginal cells and intermediate cells in the stria vascularis 3 h after 3-NP administration; these changes were progressive for at least 14 days. Less prominent degeneration was detected in type 1 and type 3 fibrocytes in the spiral ligament. These results indicate that permanent threshold shift caused by acute cochlear mitochondrial dysfunction is primarily mediated by cellular degeneration in the lateral wall of the cochlea, and suggest that therapy of cochlear hearing loss due to acute energy failure may be achieved through protection and regeneration of the cochlear lateral wall.

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.

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.

Etiology, pathophysiology of symptoms, and pathogenesis of Meniere's disease

Endolymphatic hydrops is the pathologic feature associated with Meniere's disease. The development of endolymphatic hydrops appears to arise from multifactorial inheritance with alteration of endolymphatic homeostasis. Various factors associated with the phenomenon of hydrops include functional or anatomic obstruction of endolymphatic flow, malabsorption of endolymph, genetic anomalies, vasodilation, allergy, viral infection, and autoimmunity.

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.

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.

Effects of nitric oxide on normal and ischemic cochlea of the guinea pig

To determine whether nitric oxide (NO)/peroxynitrite plays any role in neurodestruction observed in ischemic cochlea of the guinea pig, the effects of NO donors like S-nitrosocysteine (S-NC) and nitroglycerin (NTG), peroxynitrite generators like 3-morpholinosydnonimine (SIN-1), peroxynitrite inhibitors like superoxide dismutase plus catalase (SOD/Cat), as well as NOS inhibitors like N(G)-nitro-l-arginine methyl ether (L-NAME), were tested on normal and ischemic cochleae. Various concentrations of S-NC and SIN-1 were introduced into the perilymphatic space of normal guinea pig cochlea. Quantitative scanning electron microscopy of inner and outer hair cells was carried out 2 days later. To determine the level of NO in the cochlea after 20 to 120 min of ischemia, nitrites/nitrates in the perilymph were measured. The effects of NO on the ischemic cochlea were tested by infusion of SOD/Cat, L-NAME, S-NC, and NTG into the perilymphatic space just before decapitation. Introduction of fixative into the cochlea was delayed for 15 min to investigate the effects of the chemicals on nerve endings at the base of inner hair cells. The results showed that the level of nitrites/nitrates tended to decline with increasing time of ischemia. There was no significant hair cell loss in the cochleae treated with SIN-1 or S-NC. At 15 min after ischemia, most of the nerve endings at the base of the inner hair cells were protected from damage when 1 mM S-NC or NTG was infused into the perilymph. Taken together, the results indicate that NO/peroxynitrite is unlikely to be involved in the neurodestruction in the ischemic cochlea. In fact, exogenous NO may have a neural protective effect.

Role of nitric oxide in focal microcirculation disorder of guinea pig cochlea

This study was designed to evaluate the role of endogenous nitric oxide (NO) in focal microcirculation disorder of the guinea pig cochlea. Focal microcirculation disorder was induced by a photochemical reaction at the lateral wall of the second cochlear turn. Saline or N omega-nitro-L-arginine methyl ester (L-NAME) was administered before the onset of photochemical reaction. Cochlear blood flow (CBF) was measured at the focal lesion (ischemic core), 1 mm from the lesion in the apical and basal direction (ischemic border zone) by using a novel non-contact laser blood flowmeter. NO synthase activities were measured by radioenzymeassay. In the saline pretreatment group, CBF was significantly decreased to 58.8+/-4.4% of the baseline at the ischemic core 30 min after the onset of photochemical reaction (P<0.01), while CBF showed no significant change at the ischemic border zone. In the L-NAME pretreatment group, CBF was significantly decreased not only at the focal lesion (48.3+/-6.5%, P<0.01), but also at the ischemic border zone (apical, 49.3+/-2.3%, P<0.05; basal, 58.7+/-7.1%, P<0.05, respectively). NO synthase III activity of cochlea was increased significantly (P<0.01) 15 min after microcirculation disorder. These findings suggest that formation of endogenous NO plays a key role in the maintenance of CBF in acute focal cochlear microcirculation disorder.

Distribution and projections of nitric oxide synthase neurons in the rodent superior olivary complex

The superior olivary complex (SOC), a group of interrelated brainstem nuclei, sends efferents to a variety of neuronal structures including the cochlea and the inferior colliculus. The present review describes data obtained from rodents providing evidence that the gaseous, short-living neuroactive substance nitric oxide (NO) is produced in the SOC. The NO-synthesizing enzyme neuronal NO-synthase (nNOS) has been localized by means of several methods including histochemistry and immunohistochemistry. Perikarya containing nNOS were found in several nuclei of the SOC. Their largest numbers and percentages of total cells were observed in the medial nucleus of the trapezoid body. Stained terminals were observed mainly in the lateral superior olivary nucleus and in the superior paraolivary nucleus. While retrograde neuronal tracing identified a considerable number of nNOS-immunoreactive neurons as to be part of the olivo-cochlear pathway, the projection patterns of other nNOS-immunoreactive SOC cell groups remain to be investigated. We also review other putative sources of cochlear NO, and discuss the possible role of NO in the lower auditory brainstem and organ of Corti with regard to physiological and pathophysiological mechanisms.

The diagnosis and management of hypertensive crises

Severe hypertension is a common clinical problem in the United States, encountered in various clinical settings. Although various terms have been applied to severe hypertension, such as hypertensive crises, emergencies, or urgencies, they are all characterized by acute elevations in BP that may be associated with end-organ damage (hypertensive crisis). The immediate reduction of BP is only required in patients with acute end-organ damage. Hypertension associated with cerebral infarction or intracerebral hemorrhage only rarely requires treatment. While nitroprusside is commonly used to treat severe hypertension, it is an extremely toxic drug that should only be used in rare circumstances. Furthermore, the short-acting calcium channel blocker nifedipine is associated with significant morbidity and should be avoided. Today, a wide range of pharmacologic alternatives are available to the practitioner to control severe hypertension. This article reviews some of the current concepts and common misconceptions in the management of patients with acutely elevated BP.

Expression of inducible nitric oxide synthase (iNOS/NOS II) in the hydropic cochlea of guinea pigs

Immunohistochemical investigations of the guinea pig cochlea, using a specific antibody to the inducible isoform of NO synthase (iNOS/NOS II), have been performed 3 weeks after closure of the right endolymphatic duct (n=7). Endolymphatic hydrops, the morphological substrate of Meniere's disease, became evident by distension of the Reissner's membrane. iNOS expression could be noted in endothelium, spiral ganglion cells, in nerve fibers, in supporting cells of the organ of Corti and cells of the spiral ligament. Temporal bones of non-operated controls (n=6) as well as of sham-operated animals (n=3) did not show structures positive to iNOS. These findings imply that iNOS-generated NO could be involved in the pathophysiology of cochlear dysfunction in Meniere's disease.

Inhibition of inducible nitric oxide synthase lowers the cochlear damage by lipopolysaccharide in guinea pigs

Endotoxin-treated cochleas of the guinea pig were examined electrophysiologically and immunohistochemically concerning the expression of inducible nitric oxide synthase (iNOS/NOS II). One mg of bacterial lipopolysaccharide (LPS, 5 mg/ml) or mixed solution of 1 mg of LPS plus 1 mg of N(G)-nitro-L-arginine methyl ester (L-NAME, 5 mg/ml) (L-NAME/LPS) was injected into the middle ear of guinea pigs transtympanically. The electrocochleograms were measured prior to, immediately and 48 h after the injection. Immunohistological studies for iNOS followed after fixation, embedding and sectioning of the temporal bones. The threshold and amplitude of the compound action potential (CAP) became significantly worse in the LPS treated group. In contrast, the changes of the threshold and amplitude of CAP were decreased in the L-NAME/LPS group. iNOS was expressed in the stria vascularis, the spiral ligament, the organ of Corti and the spiral ganglion in the LPS group. These immunoreactivities in the L-NAME/LPS group were less intense than that in the LPS group. These results indicate that LPS has an ototoxic effect on the cochlea and that this effect could be mediated by iNOS produced high nitric oxide under inflammatory conditions.

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

The purpose of the investigation was to ascertain whether inoculation of bacterial lipopolysaccharide (LPS) into the cochlea of the guinea pig could elicit formation of inducible nitric oxide synthase (iNOS). Immunohistochemical study revealed that immunoreactivity to iNOS was seen below outer hair cells representing nerve fibers and synaptic nerve endings. iNOS-staining could also be observed in phalangeal dendrites of Deiter's cells pointing to the cuticular membrane, Hensen's cells and on stria vascularis 48 h after inoculation with LPS. Immunohistochemical investigation with a specific anti-nitrotyrosine antibody also revealed intense immunoreactivity identical to that of iNOS, suggesting formation of peroxynitrite in the organ of Corti by the reaction of NO with O(2)(-). On the basis of these findings, it can be concluded that NO together with O(2)(-), which form the more reactive peroxynitrite, are the most important pathogenic agents in LPS-induced damage of cochlea in the guinea pig.

Localization of soluble guanylate cyclase activity in the guinea pig inner ear

The aim of this study was to characterize the nitric oxide (NO) receptor soluble guanylate cyclase (sGC), to determine the cells targeted by NO and to elucidate the function of the NO/cGMP pathway in the inner ear. sGC activity in the inner ear was localized by immunohistochemical detection of NO-stimulated cGMP. Soluble guanylate cyclase activity in the cochlea was detected in the nerve endings underneath the outer and inner hair cells, supporting cells, stria vascularis and vessels. In the vestibular organs, sGC activity was detected in the cytoplasm of sensory cells, nerve fibres, dark cells and transitional cells and vessels. These findings suggest that the NO/cGMP pathway may be involved in regulatory processes in neurotransmission, blood flow and inner ear fluid homeostasis.

Nitric oxide synthase in identified olivocochlear projection neurons in rat and guinea pig

Nitric oxide (NO) is thought to be involved in the effects of amino acids at the level of cochlear hair cell afferents. Recently, the isoform of the NO-producing enzyme, neuronal NO synthase (nNOS), has been demonstrated in neuronal structures of the cochlea in rats and guinea pigs histochemically and immunohistochemically. To investigate the sources of cochlear NO, we injected Fluoro-Gold (FG) into the cochlea of rats and guinea pigs. Upon terminal uptake of the tracer and neuronal transport we observed FG in terminals at the base of inner (IHC) and outer hair cells (OHC) and in neurons of the spiral ganglion. Ganglion cells and terminals at the IHC were clearly nNOS-positive, while terminals at the OHC exhibited nNOS-immunoreactivity to a minor degree. The immunohistochemical investigation of the auditory brainstem showed that about one-fourth of the neurons of the superior olivary complex contained nNOS. The comparison with retrogradely labeled neurons showed that perikarya in the lateral superior olivary nucleus and, in particular, the medial nucleus of the trapezoid body were double-labeled. These results were similar in both, rat and guinea pig. Our data reveal that neurons of the superior olivary complex are likely to be additional sources of neuronal NOS in the cochlea.

Nitric oxide synthase in trigeminal ganglion cells projecting to the cochlea of rat and guinea pig

Nitric oxide (NO) influences electrophysiological and morphological parameters of the mammalian cochlea. Recently, the isoform of the NO-producing enzyme neuronal NO synthase (nNOS) has been demonstrated in spiral ganglion cells and olivocochlear neurons. The cochlea is also innervated by fibers stemming from the trigeminal ganglion (TG) and superior cervical ganglion (SCG). Whether these ganglion cells contain nNOS is not known yet. We therefore identified TG and SCG cells upon injection of Fluoro-Gold (FG) into the cochlea and retrograde neuronal transport of FG in rat and guinea pig. These ganglion cells were investigated for neuronal NOS immunohistochemically. Perikarya labeled by FG were found in the ipsilateral TG and SCG. In both species investigated, a considerable number of FG-labeled TG cells were also nNOS-immunoreactive whereas SCG cells were not. These data, demonstrating the existence of nNOS-containing TG cells that project to the cochlea, provide evidence that these neurons are further sources of nitric oxide in the cochlea.

Localization of the NO/cGMP-pathway in the cochlea of guinea pigs

The presence of nitric oxide synthase (NOS) in substructures of the cochlea of guinea pigs is an issue of current focus. Moreover, information concerning the localization of cells effected by the NO/cGMP-pathway are rare. Paraffin sections of guinea pig cochlea were incubated with specific antibodies to the three known NOS isoforms, soluble guanylyl cyclase (sGC) and cyclic guanosine-monophosphate (cGMP), the second messenger system of NO. While detection of inducible iNOS failed in all cochlear structures, expression of endothelial eNOS was found in the spiral ligament, in the stria vascularis, in cells of the organ of Corti, in nerve fibers and in some perikaryia of the spiral ganglion. The cochlear nerve showed an accentuated affinity for immunostaining in distal, basal segments of the cochlea. Neuronal bNOS was found predominantly in the endosteum of the modiolus and cochlea and was less intensively present in all perikaryia of the spiral ganglion and in the spiral ligament. Supporting cells of the organ of Corti and cells in the limbus spiralis displayed only modest immunostaining, while bNOS was not found in outer and inner hair cells. NOS detection was accompanied by immunoreactivity to sGC and to cGMP. The presence of NOS and its second messenger system gives evidence for a possible involvement in neurotransmission, regulation of the cochlear amplifier and in homeostasis.

Ototoxicity of sodium nitroprusside is not due to nitric oxide

Sodium nitroprusside (SNP) has been used as a donor for nitric oxide (NO) to study the effects of NO on the mammalian cochlea. In the present study, we set out to determine whether NO was the chemical responsible for the ototoxic effects seen after the application of SNP at the round window membrane of the adult guinea pig cochlea. In the first instance, NO released from S-nitrosocysteine, a compound not related to cyanide, has no toxic effect on the hair cells of the cochlea. Light-exposed SNP that could no longer produce NO, light-exposed SNP to which acetylcysteine (ATC) or hydroxycobalamin (HCL) was added to eliminate cyanide, and freshly prepared SNP to which ATC or HCL was added were also tested. Six groups of animals consisting of three animals in each group were used. The single chemical or combination of chemicals stated above was soaked in a piece of gelfoam that was then applied to the round window membrane of the animal under ketamine-xylasine anesthesia. The animals were reanesthetized 3 days later and perfused for scanning electron microscopy and hair cell quantitative analysis. The results showed that, in animals given S-nitrosocysteine, no hair cell loss was noted, while light-exposed SNP led to severe hair cell damage similar to that seen after the administration of fresh SNP. In animals treated with the mixture of light-exposed SNP and ATC or HCL, or fresh SNP with ATC or HCL, ototoxicity was significantly attenuated. These results have convincingly demonstrated that NO at a certain level is not destructive to auditory hair cells and the hair cell loss observed after SNP application is most likely due to the cyanide released from the SNP instead of NO.

Expression of inducible nitric oxide synthase (iNOS/NOS II) in the cochlea of guinea pigs after intratympanical endotoxin-treatment

Since NO is believed to be involved in cochlear physiology, presence of the constitutive isoforms of nitric oxide synthase (NOS), and the target enzyme of NO, soluble guanylyl cyclase (sGC) in structures of the mammalian cochlea have been demonstrated. To date, no reports have been published regarding the detection of the inducible isoform (NOS II) in the cochlea. In order to show the capability of iNOS expression in cochlear tissue, a mixture of proinflammatory bacterial lipopolysaccharides (LPS) and tumor necrosis factor alpha (TNF-alpha) was injected into the tympanic cavity of guinea pigs, vs. saline-solution as control. Paraffin sections of LPS/TNF-alpha treated and saline-treated cochleae (6 h) were examined immunohistochemically with specific antibodies to neuronal, endothelial and inducible NOS and to sGC. Initiated expression of iNOS in the cochlea was observed in the wall of blood vessels of the spiral ligament (SL) and the modiolus, in supporting cells of the organ of Corti, in the limbus, in nerve fibers and in a part of the perikarya of the spiral ganglion after LPS/TNFalpha-treatment. iNOS was not detected in saline-treated control tissue. Expression of both constitutive NOS-isoforms (endothelial and neuronal NOS) and of sGC showed no significant differences in both experimental groups. Endothelial eNOS and neuronal bNOS were detected co-localized in ganglion cells, in nerve fibers, in cells of the SL and in supporting cells of the organ of Corti, but not in sensory cells. Strong labeling for bNOS became evident in the endosteum of the cochlea, while in the endothelium of blood vessels and in the epithelium of the limbus only eNOS could be labeled. sGC could be detected in SL, in supporting and sensory cells of the organ of Corti, in nerve fibers, ganglion cells, in the wall of blood vessels and in the limbus-epithelium. While small amounts of NO, generated by bNOS and eNOS, seem to support the cochlear blood flow and auditory function as well as neurotransmission, high amounts of iNOS-generated NO could have dysregulative and neurotoxic effects on the inner ear during bacterial and viral infections of the middle and inner ear.

The nitric oxide/cyclic GMP pathway: a potential major regulator of cochlear physiology

The nitric oxide (NO)/cyclic guanosine monophosphate (GMP) pathway is now recognized as a major regulatory system in cell physiology and tissue homeostasis. This pathway may control processes as diverse as muscle relaxation, gut peristalsis, neurotransmission and hormonal secretion. It is also involved in the development and function of sensory systems such as vision and olfaction. This review will detail the NO/cyclic GMP pathway, evaluate studies in the auditory system and discuss its potential participation in cochlear blood flow, supporting cell physiology and excitotoxicity.

Ototoxicity of sodium nitroprusside

Nitric oxide (NO) not only has normal physiological roles like vasodilation and neurotransmission in the living organism, it could also have possible neurodestructive effects under certain pathological conditions. The present study aimed to determine whether direct exposure of guinea pig cochlea to a NO donor like sodium nitroprusside (SNP), or a nitric oxide synthase (NOS) inhibitor like N(G)-nitro-L-arginine methyl ester (L-NAME), would cause damage to the auditory hair cells. A piece of gelfoam was placed on the round window of the right ear of adult albino guinea pigs. It was then soaked with 0.1 ml of SNP (3.4 microM), 0.1 ml of L-NAME (9.3 microM or 18.5 microM) or 0.1 ml of injection water, the vehicle used to dissolve the above chemicals. Twelve animals receiving SNP were perfused 1 day, 2, 3 and 7 days later, with three animals being used for each survival period. Six animals receiving L-NAME were allowed to survive up to 7 days before perfusion. Eight animals receiving injection water or 0.45% saline were used as controls. With the scanning electron microscope, the inner and outer hair cells were counted over a 1 mm length of the basilar membrane in each turn of every cochlea. The results showed that, in animals treated with L-NAME at both concentrations stated, no significant loss of either inner or outer hair cells was noted in any part of the cochlea studied. However, as early as 1 day after SNP treatment, a striking loss of inner and outer hair cells was observed in the three lower turns of the cochlea. Damage to the outer hair cells was extended to the apical turn with increasing survival period, but no significant loss of inner hair cells was evident in the apical turn at any of the survival periods studied. To rule out the possibility that the effects were due to the presence of cyanide, a metabolite of SNP, hydroxycobalamin was introduced into the scala tympani of three animals through a cannula-osmotic pump device during SNP treatment. There was no significant difference in the results between the groups with and without hydroxycobalamin infusion 7 days after SNP treatment. The present study suggests that an excessive production of NO in the inner ear could lead to extensive loss of hair cells.

Localization of soluble guanylate cyclase activity in the guinea pig cochlea suggests involvement in regulation of blood flow and supporting cell physiology

Although the nitric oxide/cGMP pathway has many important roles in biology, studies of this system in the mammalian cochlea have focused on the first enzyme in the pathway, nitric oxide synthase (NOS). However, characterization of the NO receptor, soluble guanylate cyclase (sGC), is crucial to determine the cells targeted by NO and to develop rational hypotheses of the function of this pathway in auditory processing. In this study we characterized guinea pig cochlear sGC by determining its enzymatic activity and cellular localization. In cytosolic fractions of auditory nerve, lateral wall tissues, and cochlear neuroepithelium, addition of NO donors resulted in three- to 15-fold increases in cGMP formation. NO-stimulated sGC activity was not detected in particulate fractions. We also localized cochlear sGC activity through immunocytochemical detection of NO-stimulated cGMP. sGC activity was detected in Hensen's and Deiters' cells of the organ of Corti, as well as in vascular pericytes surrounding small capillaries in the lateral wall tissues and sensory neuroepithelium. sGC activity was not observed in sensory cells. Using NADPH-diaphorase histochemistry, NOS was localized to pillar cells and nerve fibers underlying hair cells. These results indicate that the NO/cGMP pathway may influence diverse elements of the auditory system, including cochlear blood flow and supporting cell physiology.

Endothelin-A receptors mediate vasoconstriction of capillaries in the spiral ligament

The purpose of this study was to determine whether endothelin-1 (ET-1), endothelin-2 (ET-2) or endothelin-3 (ET-3) alter the vascular diameter of capillaries in the spiral ligament. Changes in vascular tone were measured in capillaries from the isolated spiral ligament in vitro. Capillaries were occluded on one end and opened on the other end. Red blood cells trapped in the capillaries served as markers for a luminal volume defined by the red cell itself, the capillary wall and the occluder. Movement of the red cell toward the open end was taken as evidence for vasoconstriction and movement of the red cell toward the occluder was taken as evidence for vasodilation. The inner diameter of the capillaries was 7.0 microm and decreased maximally by a factor of 0.8 in response to ET-1 and ET-2 (both 10(-8) M). Vasoconstriction induced by ET-1 and ET-2 was concentration-dependent in the range between 10(-12) and 10(-8) M whereas ET-3 (10(-8) M) had no effect. The EC50s for ET-1 and ET-2 were 1.2 x 10(-10) M and 1.4 x 10(-9) M, respectively. Thus, the potency order was ET-1 > ET-2 >> ET-3. Vasoconstriction induced by ET-1 and ET-2 was completely inhibited by the competitive antagonist 10(-6) M BQ-123 (cyclic D-Asp-L-Pro-D-Val-L-Leu-D-Trp). Vasoconstriction induced by ET-1 or ET-2 continued for more than 1 min after removal of agonist from the perfusate. Rapid vasodilation of capillaries preconstricted by ET-1 was observed in response to 10(-3) M sodium nitroprusside. Sodium nitroprusside, however, had no significant effect on the vascular diameter of resting capillaries. These results demonstrate that capillaries in the spiral ligament can constrict and the endothelin-mediated vasoconstriction occurs via ET(A) receptors.

Nitrosoglutathione suppresses cochlear potentials and DPOAEs but not outer hair cell currents or voltage-dependent capacitance

Biochemical and pharmacological evidence support a role for nitric oxide (NO) and glutathione (GSH) in the cochlea. GSH combines with NO in tissue to form nitrosoglutathione (GSNO) that can act as a storage form for GSH and NO. Therefore, we tested GSNO on sound-evoked responses of the cochlea (cochlear microphonic, CM; summating potential, SP; compound action potential, CAP; cubic distortion product otoacoustic emission, DPOAE), on the endocochlear potential (EP), on isolated outer hair cell (OHC) currents and voltage-dependent capacitance, and on Deiters' cell currents. In vivo application of GSNO in increasing concentrations reversibly reduced low-intensity sound-evoked CAP, SP and DPOAEs starting at about 1 mM (CAP) and 3.3 mM (SP, DPOAE). However, even at 10 mM, GSNO had little effect on the EP. In vitro, salicylate (10 mM) but not GSNO (3 and 10 mM) suppressed the early capacitative transients of OHCs. GSNO (3 and 10 mM) had no effect on the whole cell currents of OHCs or Deiters' cells. Results show that GSNO suppresses cochlear function. This suppression may be due to an effect of GSNO on the cochlear amplifier. The actions of GSNO were different from those of other NO donors; therefore, the effects of GSNO may not be mediated by NO. The mechanisms underlying GSNO effects seem to be different from those of salicylate.