Effects of surgical lesions on choline acetyltransferase activity in the cat cochlea

Although it is well established that the choline acetyltransferase (ChAT, the enzyme for acetylcholine synthesis) in the mammalian cochlea is associated with its olivocochlear innervation, the distribution of this innervation in the cochlea varies somewhat among mammalian species. The quantitative distribution of ChAT activity in the cochlea has been reported for guinea pigs and rats. The present study reports the distribution of ChAT activity within the organ of Corti among the three turns of the cat cochlea and the effects of removing olivocochlear innervation either by a lateral cut aimed to totally transect the left olivocochlear bundle or a more medial cut additionally damaging the superior olivary complex on the same side. Similarly to results for guinea pig and rat, the distribution of ChAT activity in the cat outer hair cell region showed a decrease from base to apex, but, unlike in the guinea pig and rat, the cat inner hair cell region did not. As in the rat, little ChAT activity was measured in the outer supporting cell region. As previously reported for whole cat cochlea and for rat cochlear regions, transection of the olivocochlear bundle resulted in almost total loss of ChAT activity in the hair cell regions of the cat cochlea. Lesions of the superior olivary complex resulted in loss of ChAT activity in the inner hair cell region of all cochlear turns only on the lesion side but bilateral losses in the outer hair cell region of all turns. The results are consistent with previous evidence that virtually all cholinergic synapses in the mammalian cochlea are associated with its olivocochlear innervation, that the olivocochlear innervation to the inner hair cell region is predominantly ipsilateral, and that the olivocochlear innervation to the outer hair cells is bilateral.

Cisplatin and oxaliplatin are toxic to cochlear outer hair cells and both target thioredoxin reductase in organ of Corti cultures

Conclusion

Inhibition of thioredoxin reductase (TrxR) may be a contributing factor in cisplatin-induced ototoxicity. Direct exposure of organ of Corti to cisplatin and oxaliplatin gives equal loss of hair cells.

Objectives

Platinum-containing drugs are known to target the anti-oxidant selenoprotein TrxR in cancer cells. Two such anti-cancer, platinum-containing drugs, cisplatin and oxaliplatin, have different side effects. Only cisplatin induces hearing loss, i.e. has an ototoxic side effect that is not seen after treatment with oxaliplatin. The objective of this study was to evaluate if TrxR is a target in the cochlea. Loss of outer hair cells was also compared when cisplatin and oxaliplatin were administered directly to the organ of Corti.

Methods

Organ of Corti cell culture was used for direct exposure to cisplatin and oxaliplatin. Hair cells were evaluated and the level of TrxR was assessed. Immunohistochemical staining for TrxR was performed. An animal model was used to evaluate the effect on TrxR after treatment with cisplatin and oxaliplatin in vivo.

Results

Direct exposure of cochlear organotypic cultures to either cisplatin or oxaliplatin induced comparable levels of outer hair cell loss and inhibition of TrxR, demonstrating that both drugs are similarly ototoxic provided that the cochlea becomes directly exposed.

Blocking caspase-3-dependent pathway preserves hair cells from salicylate-induced apoptosis in the guinea pig cochlea

In the present study, we aim to explore whether the caspase-3-dependent pathway is involved in the apoptotic cell death that occurs in the hair cells (HCs) of guinea pig cochlea following a salicylate treatment. Guinea pigs received sodium salicylate (Na-SA), at a dose of 200 mg·kg(-1)·d(-1) i.p., as a vehicle for 5 consecutive days. In some experiments, N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone (zDEVD-FMK), a specific apoptosis inhibitor, was directly applied into the cochlea via the round window niche (RWN) prior to salicylate treatment for determination of caspase-3 activation. Alterations in auditory function were evaluated with auditory brainstem responses (ABR) thresholds. Caspase-3 activity was determined by measuring the proteolytic cleavage product of caspase-3 (N-terminated peptide substrate). DNA fragmentation within the nuclei was examined with a terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) method. Ultrastructure variation in the target cell was assessed by electron microscopy (EM). Salicylate treatment initiated an obvious elevation in ABR thresholds with a maximum average shift of 60 dB sound pressure level (SPL), and caused significant apoptosis in both inner (IHCs) and outer (OHCs) hair cells resulted from an evident increasing in immunoreactivity to caspase-3 protease. Transmission electron microscopy (TEM) displayed chromatin condensation and nucleus margination accompanied by cell body shrinkage in the OHCs, but not in the IHCs. Scanning electron microscopy (SEM) showed breakdown, fusion, and loss in the stereociliary bundles at the apex of OHCs rather than IHCs. zDEVD-FMK pretreatment prior to salicylate injection substantially attenuated an expression of the apoptotic protease and protected HCs against apoptotic death, followed by a moderate relief in the thresholds of ABR, an alleviation in the submicroscopic structure was also identified. In particular, disorientation and insertion in the hair bundles at the apex of OHCs was exhibited though no classic apoptotic change found. The above changes were either prevented or significantly attenuated by zDEVD-FMK. These findings indicate that salicylate could damage cochlear hair cells via inducing apoptosis associated with caspase-3 activation.

Pivotal role of actin depolymerization in the regulation of cochlear outer hair cell motility

Cochlear outer hair cells undergo reversible changes in shape when externally stimulated. This response, known as OHC motility, is a central component of the cochlear amplifier, the mechanism responsible for the high sensitivity of mammalian hearing. We report that actin depolymerization, as regulated by activation/inhibition of LIMK/cofilin-mediated pathways, has a pivotal role in OHC motility. LIMK-mediated cofilin phosphorylation, which inhibits the actin depolymerizing activity of this protein, increases both electromotile amplitude and total length of guinea pig OHCs. In contrast, a decrease in cofilin phosphorylation reduces both OHC electromotile amplitude and OHC length. Experiments with acetylcholine and lysophosphatidic acid indicate that the effects of these agents on OHC motility are associated with regulation of cofilin phosphorylation via different signaling cascades. On the other hand, nonlinear capacitance measurements confirmed that all observed changes in OHC motile response were independent of the performance of the motor protein prestin. Altogether, these results strongly support the hypothesis that the cytoskeleton has a major role in the regulation of OHC motility, and identify actin depolymerization as a key process for modulating cochlear amplification.

Ebselen treatment reduces noise induced hearing loss via the mimicry and induction of glutathione peroxidase

Previous studies indicate that noise induced hearing loss (NIHL) involves a decrease in glutathione peroxidase (GPx) activity and a subsequent loss of outer hair cells (OHC). However, the cellular localization of this GPx decrease and the link to OHC loss are still poorly understood. In this report, we examined the cellular localization of GPx (GPx1, GPx 3 and GPx 4) in F-344 rat before and after noise exposure and after oral treatment with ebselen, a small molecule mimic of GPx activity. Results indicate that GPx1 is the major isoform within the cochlea and is highly expressed in cells of the organ of Corti, spiral ganglia, stria vascularis, and spiral ligament. Within 5h of noise exposure (4h at 113 dB, 4-16 kHz), significant OHC loss was already apparent in regions coincident with the 8-16 kHz region of the cochlea. In addition, the stria vascularis exhibited significant edema or swelling and a decrease in GPx1 immunoreactivity or fluorescent intensity. Treatment with ebselen (4 mg/kg p.o.) before and immediately after noise exposure reduced both OHC loss and the swelling of the stria vascularis typically observed within 5h post-noise exposure. Interestingly, GPx1 levels increased in the stria vascularis after noise and ebselen treatment vs noise and vehicle-only treatment, and exceeded baseline no noise control levels. These data indicate that ebselen acts to prevent the acute loss of OHCs and reduces the acute swelling of the stria vascularis by two potential mechanisms: one, as a ROS/RNS scavenger through its intrinsic GPx activity, and two, as a stimulator of GPx1 expression or activity. This latter mechanism may be due to the preservation of endogenous GPx1 from ROS/RNS induced degradation and/or the stimulation of GPx1 expression or activity.

The caspase pathway in noise-induced apoptosis of the chinchilla cochlea

We previously reported that intense noise exposure causes outer hair cell (OHC) death primarily through apoptosis. Here we investigated the intracellular signal pathways associated with apoptotic OHC death. Chinchillas were exposed to a 4 kHz narrowband noise at 110 dB SPL for 1 h. After the noise exposure, the cochleas were examined for the activity of each of three caspases, including caspase-3, -8, or -9 with carboxyfluorescein-labeled fluoromethyl ketone (FMK)-peptide inhibitors. The cochleas were further examined for cytochrome c release from mitochondria by immunohistology and for DNA degradation by the TUNEL method. The results showed that the noise exposure triggered activation of caspase-3, an important mediator of apoptosis. The noise exposure also caused the activation of caspase-8 and caspase-9, each of which is associated with a distinct signaling pathway that leads to activation of caspase-3. Caspase activation occurred only in the apoptotic OHCs and not in the necrotic OHCs. These results indicate that multiple signaling pathways leading to caspase-3 activation take place simultaneously in the apoptotic OHCs. In addition to caspase activation, noise exposure caused the release of cytochrome c from mitochondria, resulting in a punctate fluorescence in the cytosol. In contrast to activation of caspases, the release of cytochrome c took place in both apoptotic and necrotic OHCs. Moreover, the release of cytochrome c in a subpopulation of OHCs took place early in the cell death process, prior to any outward signs of necrosis or apoptosis. These data suggest that in this subpopulation there exists a common step that is shared by cell death pathways before entering either necrosis or apoptosis. Lastly, use of the TUNEL assay in combination with PI labeling provides a more accurate discrimination between apoptosis and necrosis.

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.

Rescue of auditory hair cells from aminoglycoside toxicity by Clostridium difficile toxin B, an inhibitor of the small GTPases Rho/Rac/Cdc42

The hair cells (HCs) are the most vulnerable elements in the cochlea and damage to them is the most common cause of sensorineural hearing loss. Understanding the intracellular events that lead to the death of HCs is a key to developing protective strategies. Recently, it has been shown that the c-Jun-N-terminal kinase (JNK) pathway is activated in HCs in response to aminoglycosides (J. Neurosci. 20 (2000) 43). We have studied the upstream events leading to JNK activation in aminoglycoside toxicity in vitro. The small GTPases Rac and Cdc42 are well known upstream activators of JNK in other cell types. Clostridium difficile toxin B monoglucosylates all members of the Rho GTPase subfamily (Rho, Rac and Cdc42 isoforms) and inhibits GTP binding by steric interference (Nature 341 (1989) 209). Organ of Corti explants from p5 rat basal turns were maintained in tissue culture and treated with C. difficile toxin B for 12 h. They were then treated with toxin B plus gentamicin for 72 h. Significantly less HC death was observed compared to with gentamicin alone. Toxin B alone had no effect on HCs at the highest concentration used. Using antibodies against phospho-c-Jun, we observed background immunoreactivity in control explants, strong staining of outer hair cell nuclei in gentamicin treated explants, and weaker immunostaining in explants treated with gentamicin and C. difficile toxin B. We conclude that Rho family small GTPases play a role in aminoglycoside toxicity signaling as upstream activators of the JNK signaling pathway.

Involvement of apoptosis in progression of cochlear lesion following exposure to intense noise

It has been known for some time that noise-induced outer hair cell (OHC) death in the cochlea continues well after the termination of a noise exposure. However, the underlying mechanisms leading to the expansion of a cochlear lesion are not fully understood. Here we report involvement of the apoptotic pathway in the progression of OHC death in the chinchilla cochlea following exposure to a 4 kHz narrow band noise at 110 dB SPL for 1 h. Morphological examination of OHC nuclei revealed nuclear condensation and fragmentation, typical morphological features of apoptosis. OHC apoptosis developed asymmetrically toward the apical and basal parts of the cochleas following the noise exposure. Two days after the noise exposure, there was still active OHC pathology with condensed and fragmented nuclei in the basal part of the cochleas. Detection of caspase-3 activation, an intracellular marker for apoptosis, showed a spatial agreement between the apoptotic nuclei and activated caspase-3. These results clearly implicate the apoptotic pathway in the post-exposure progression of OHC demise.

Protection of outer hair cells from reperfusion injury by an iron chelator and a nitric oxide synthase inhibitor in the guinea pig cochlea

To examine whether an active process of the cochlea was injured by ischemia-reperfusion, time courses of distortion-product otoacoustic emissions (DPOAEs) were examined before, during and after 30 min cochlear ischemia using albino guinea pigs. DPOAEs decreased to the minimum level when the animals were subjected to ischemia. When the cochlea was recirculated, DPOAEs initially recovered with time until 20 min after the onset of reperfusion. However, thereafter the amplitude of DPOAEs gradually decreased toward the noise level. Administration of deferoxamine (an iron chelator) or N-nitro-L-arginine (a nitric oxide synthase inhibitor) ameliorated this decrease of DPOAEs during reperfusion and significantly increased the DPOAE amplitudes 60 min after the onset of reperfusion as compared with those in non-treated animals. These results suggest that cochlear reperfusion as well as ischemia injured the active process of the cochlea and that free radicals and nitric oxide play important roles in this injury.

Localization of nitric oxide synthase isoforms in the human cochlea

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

Immuno-electron microscopic localization of the alpha(1) and beta(1)-subunits of soluble guanylyl cyclase in the guinea pig organ of corti

Guanylyl cyclases (GC) catalyze the formation of the intracellular signal molecule cyclic GMP from GTP. For some years it has been known that the heme-containing soluble guanylyl cyclase (sGC) is stimulated by NO and NO-containing compounds. The sGC enzyme consists of two subunits (alpha(1) and beta(1)). In the present study, the alpha(1) and beta(1)-subunits were identified in the guinea pig cochlea at the electron microscopic level using a post-embedding immuno-labeling procedure. Ultrathin sections of LR White embedded specimens were incubated with various concentrations of two rabbit polyclonal antibodies to the alpha(1)- and beta(1)-subunit, respectively. The immunoreactivity was visualized by a gold-labeled secondary antibody in an energy-filtering transmission electron microscope (EFTEM). Marked immunoreactivity for both antibodies was found in the inner and outer hair cells, with numerous gold particles at the border of the cuticular plates, associated with the cell nuclei or attached to electron-dense parts of the cytoplasm. In the pillar cells and apical Deiters cells, soluble guanylyl cyclase immunoreactivity was located at the rim of the cuticular plates and between the microtubuli bundles. Together with the recently identified nitric oxide synthase isoforms [Eur. Arch. Otorhinolaryngol. 254 (1997) 396; Eur. Arch. Otorhinolaryngol. 255 (1998) 483], the soluble guanylyl cyclase may be involved in signalling processes in the organ of Corti.

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.

Two types of voltage-dependent potassium channels in outer hair cells from the guinea pig cochlea

Cell-attached and cell-free configurations of the patch-clamp technique were used to investigate the conductive properties and regulation of the major K(+) channels in the basolateral membrane of outer hair cells freshly isolated from the guinea pig cochlea. There were two major voltage-dependent K(+) channels. A Ca(2+)-activated K(+) channel with a high conductance (220 pS, P(K)/P(Na) = 8) was found in almost 20% of the patches. The inside-out activity of the channel was increased by depolarizations above 0 mV and increasing the intracellular Ca(2+) concentration. External ATP or adenosine did not alter the cell-attached activity of the channel. The open probability of the excised channel remained stable for several minutes without rundown and was not altered by the catalytic subunit of protein kinase A (PKA) applied internally. The most frequent K(+) channel had a low conductance and a small outward rectification in symmetrical K(+) conditions (10 pS for inward currents and 20 pS for outward currents, P(K)/P(Na) = 28). It was found significantly more frequently in cell-attached and inside-out patches when the pipette contained 100 microM acetylcholine. It was not sensitive to internal Ca(2+), was inhibited by 4-aminopyridine, was activated by depolarization above -30 mV, and exhibited a rundown after excision. It also had a slow inactivation on ensemble-averaged sweeps in response to depolarizing pulses. The cell-attached activity of the channel was increased when adenosine was superfused outside the pipette. This effect also occurred with permeant analogs of cAMP and internally applied catalytic subunit of PKA. Both channels could control the cell membrane voltage of outer hair cells.

Cu/Zn SOD deficiency potentiates hearing loss and cochlear pathology in aged 129,CD-1 mice

Copper/zinc superoxide dismutase (Cu/Zn SOD) is a first-line defense against free radical damage in the cochlea and other tissues. To determine whether deficiencies in Cu/Zn SOD increase age-related hearing loss and cochlear pathology, we collected auditory brainstem responses (ABRs) and determined cochlear hair cell loss in 13-month-old 129/CD-1 mice with (a) no measurable Cu/Zn SOD activity (homozygous knockout mice), (b) 50% reduction of Cu/Zn SOD (heterozygous knockout mice), and (c) normal levels of Cu/Zn SOD (wild-type mice). ABRs were obtained by using 4-, 8-, 16-, and 32-kHz tone bursts. Cochleas were harvested immediately after testing, and separate counts were made of inner and outer hair cells. Compared with wild-type mice, homozygous and heterozygous knockout mice exhibited significant threshold elevations and greater hair cell loss. Phenotypic variability was higher among heterozygous knockout mice than among wild-type or homozygous knockout mice. Separate groups of wild-type and homozygous knockout mice were examined for loss of spiral ganglion cells and eighth nerve fibers. At 13 months of age, both wild-type and knockout mice had significantly fewer nerve fibers than did 2-month-old wild-type mice, with significantly greater loss in aged knockout mice than in aged wild-type mice. Thirteen-month-old knockout mice also had a significant loss of spiral ganglion cells compared with 2-month-old wild-type mice. The results indicate that Cu/Zn SOD deficiencies increase the vulnerability of the cochlea to damage associated with normal aging, presumably through metabolic pathways involving the superoxide radical.

Effects of impulse noise on cortical response threshold and inner ear activity of succinic dehydrogenase and acetylcholinesterase in guinea pigs

The effects of impulse noise (firecrackers at 170 dB SPL, 1, 10, 20 rounds) on auditory cortical response threshold (CRT) and activity of succinic dehydrogenase (SDH) and acetylcholinesterase (AchE) in the inner ear were studied in 37 guinea pigs. The results showed that extent of damage in the cochlea was related to amount of exposure to the noise. Exposure to 10 rounds resulted in temporal threshold shift (TTS); to 20 rounds the result was permanent threshold shift (PTS). For the period when TTS existed, inverse correlation was noticed between enzyme activity change and CRT shift. The correlation could not be established when PTS was induced. The results suggest that the pathomechanism of PTS was more complex than that of TTS. The significance of the results is discussed.

[Abnormal augmentation of the evoked potential and morphological changes of guinea pig cochlea induced by cisplatin]

OBJECTIVE

To investigate the characteristic of cochlear microphone(CM), summating potential(SP), compound action potential(CAP) and the morphological changes of hair cells after selective damage to the inner ear by acute cochlear perfusion of cisplatin.

METHODS

Dynamic changes of CM, -SP and CAP were recorded by glass electrode from the third turn of the scala media guinea pig's cochlea before and during cisplatin perfusion of the whole cochlea for 2 hours.

RESULTS

It indicated that after one hour of the perfusion, the amplitude of CM, -SP and CAP decreased at stimulation intensity < or = 60 dB SPL, while the amplitude increased at stimulation intensity > or = 70 dB SPL, as compared with those before perfusion (the average of CM increased by 3.6 mV at 90 dB SPL, average of -SP increased by 1.6 mV at 120 dB SPL, average of CAP increased by 0.23 mV at 90 dB SPL). After two hours of perfusion, the amplitude of CM, -SP and CAP decreased in all the stimulation intensity. The succinic dehydrgenase (SDH) staining was decreased in OHCs, while that of IHC's remained normal. Transmission electron microscopic examination of organ of Corti showed morphological changes in OHCs, such as disappearance of nuclear chromosome, denature and reduction number, mitochondria while the structure of IHCs remained normal.

CONCLUSION

The abnormal augmentation phenomenon of CM and -SP may be due to the abnormal modulation of Ca2+ in IHCs and OHCs. The abnormal augmentation of CAP suggests that the suppression effects of OHCs and efferent neurotransmitter on IHCs and afferent neurotransmitter may be reduced after OHCs damage.

Ca(2+)-ATPases in the cochlear duct

Differing levels of the Ca(2+)-ATPase enzymes that reside on the plasma membrane (PM) and on the endoplasmic reticulum (ER) were identified in individual rat cochlear tissues by the use of a semi-quantitative enzyme-linked immunosorbent assay (ELISA). Unlike other studies, a specific antibody to PM Ca(2+)-ATPase was used to detect significantly greater levels (about 2x) of PM Ca(2+)-ATPase in the stria vascularis (SV) than that in the spiral ligament (SL) and organ of Corti (OC) tissues. Similarly, levels of ER Ca(2+)-ATPase were also significantly higher in the SV than in the SL and OC tissues. The presence of ER Ca(2+)-ATPase in the tissues of the SV has not been demonstrated previously. Given the importance of Ca2+ homeostasis in the inner ear, the statistically significantly higher densities of both PM and ER Ca(2+)-ATPase measured in the SV relative to the SL and OC regions would indicate tissue-specific responses to fluctuations in systemic and local Ca2+ concentrations.

Glutamine synthetase and glutamate metabolism in the guinea pig cochlea

Glutamate is thought to act as a neurotransmitter of the sensory hair cells of the organ of Corti. Glutamine synthetase could be involved in a type of glutamate-glutamine cycle in the cochlea which could clear glutamate off the synaptic cleft and replenish the hair cell glutamate neurotransmitter store. Using both light and electron microscopic immunocytochemistry to localize this enzyme in the guinea pig cochlea, we have observed immunoreactive satellite glial cells surrounding parvalbumin-immunoreactive primary auditory neurons in the spiral ganglion. Glutamine synthetase was also detected in Schwann cells of the osseous spiral lamina which form the myelin sheath of nerve fibers. On the contrary, no immunoreactivity could be observed in the cochlear nerve and in the organ of Corti, although this organ contains structures able to take up glutamate. Although they confirm earlier works involving glutamine synthetase in the conversion of L-[3H]glutamate taken up by glial cells, our results suggest that the cochlear glutamate-glutamine cycle is not primarily involved in the recycling and replenishment of hair cell neurotransmitter glutamate. Alternatively, it is proposed that glutamine synthetase functions to limit the perilymphatic glutamate concentrations.

Development of intracellular Ca-ATPase in the gerbil outer hair cell lateral wall

The development of Ca-ATPase immunoreactivity in gerbil outer hair cells (OHCs), assayed by immunofluorescence and postembedding immunocytochemistry, is reported here. In the adult, a linear array of label is seen inside the lateral plasma membrane. The ultrastructural distribution of Ca-ATPase near the OHC lateral plasma membrane was examined using immunogold cytochemistry and showed this calcium pumping enzyme to be present throughout the subsurface cisternal complex (SSC), especially near the innermost layers. During development, Ca-ATPase immunoreactivity appeared in patches near the lateral plasma membrane of some OHCs of the third row by 12 days after birth (DAB). By 15-16 DAB, punctate immunoreactivity was detected in the second and first rows. At 20 DAB, immunostaining near OHC lateral plasma membrane was increased, but was less continuous than OHC staining in the adult cochlea. The appearance of Ca-ATPase in OHCs coincides with the onset of auditory function and isolated OHC motility in the gerbil. The ultrastructural demonstration of abundant sites of calcium pumps in the SSC supports a role for this structure in the intracellular storage of calcium. These findings suggest a possible role of Ca-ATPase and the SSC in the regulation of slow motility of OHCs which has been reported to depend on intracellular calcium concentration.

Subcellular distribution of protein kinase C in the living outer hair cell of the guinea pig cochlea

Immunohistochemical staining using isoform-specific antibodies and intracellular localization using fluorescent probes for protein kinase C (PKC) were evaluated in the cochlear outer hair cell (OHC). Among three isoforms of classic PKC, PKC alpha was selectively stained in the fixed OHC as well as inner hair cells under a surface preparation method. Two types of fluorescent probes to detect subcellular localization of PKC were observed with a confocal laser scanning microscopy in the present study, fim-1 diacetate which binds to the ATP-competitive catalytic domain of PKC and Bodipy FL C12-phorbol acetate which binds to specific site localized to the first cysteine-rich loop of the C1 region in the regulatory domain. High fluorescence intensity of both dyes was observed in subcuticular and subsynaptic regions, infracuticular network, and along the lateral wall. The displacement experiments to evaluate binding specificity were performed by incubating Bodipy FL C12-phorbol acetate in the presence of 10 microM phorbol 12-myritate 13-acetate (PMA) and the fluorescence was totally disappeared. For the acute treatment of phorbol ester, cells were preincubated with 1 microM PMA 30 min before loading with fim-1 diacetate. The brightest area in the plasma membrane became much larger as compared with untreated cells, which suggests a dramatic translocation of PKC to the plasma membrane. The biological functions involving PKC in the OHC are discussed.

Expression of plasma membrane Ca-ATPase in the adult and developing gerbil cochlea

The distribution of the plasma membrane Ca-ATPase (PMCA) was mapped in the adult and developing gerbil cochlea by immunostaining with a monoclonal antibody against the human erythrocyte PMCA. In the mature cochlea, intense immunoreactivity was present at the surface of stereocilia of both inner (IHC) and outer (OHC) hair cells. The basolateral plasma membrane of IHCs but not OHCs stained strongly whereas that of strial marginal cells and the epithelial cell layer of Reissner's membrane showed only weak reactivity. Nerve terminals underlying IHCs were also selectively stained. At birth, strong to moderate reactivity for PMCA was present in the basolateral plasma membrane of IHCs and OHCs, strial marginal cells, and epithelial cells lining the scala media surface of Reissner's membrane and in the neurolemma of spiral ganglion cells. Immunostaining in the basolateral plasmalemma of OHCs, strial marginal cells, and epithelial cells lining Reissner's membrane remained strong to moderate up to 14 days after birth when it diminished or disappeared entirely, suggesting a developmental role for PMCA activity in these sites. Expression of PMCA at the surface of IHC and OHC stereocilia was first observed at 10 days after birth and staining reached adult levels by 14 days after birth. The abundance of PMCA in the stereociliary plasma membrane of mature hair cells supports the suggested involvement of Ca2+ in regulating transduction and adaptation mechanisms.

Immunological identification of candidate proteins involved in regulating active shape changes of outer hair cells

By employing immunological methods, it has been demonstrated that myosin, myosin light chain (MLC) and myosin light chain kinase (MLCK) proteins in outer hair cells (OHC) are immunologically different from isoforms in platelets, smooth muscle and heart muscle, and are probably more related to isoforms found in red blood cells (RBC). Moreover, proteins related to band 3 protein (b3p) and protein 4.1 (p 4.1), ankyrin as well as fodrin and spectrin, but not glycophorin, have been identified in isolated OHCs. Both OHCs and RBC differ from other motile non-muscle cells in their lack of smooth muscle isoforms of actin, their common high levels of spectrin-, ankyrin- and band 3-like proteins, as well as the expression of the 80 kDa protein 4.1 isoform. The data support the notion that motility of OHC may be based upon regulation of the b3p/p 4.1/ankyrin complex, and thus may be reminiscent to the active shape changes in RBC.

Semiquantitative analysis by scanning electron microscopy of cochlear hair cell damage by ototoxic drugs

The ototoxicity of cisplatin and carboplatin in the organ of Corti of the guinea pig was evaluated semiquantitatively. Damage of the stereocilia of outer hair cells (OHCs) observed by scanning electron microscopy (SEM) was classified into normal, grade 1 (10-50% loss of stereocilia), grade 2 (less than 50% remaining stereocilia), or grade 3 (missing stereocilia). The OHCs observed by light microscopy (LM) were classified as remaining or missing cells. Fifty OHCs of each row in the middle part of each turn of the cochlea were counted (a total of 150 cells per turn). Guinea pigs were administered 5 mg/kg of cisplatin or 50 mg/kg of carboplatin intraperitoneally for three consecutive days. In groups 1 and 2, in which both cochlea were fixed in 2.5% glutaraldehyde and 1% osmium tetroxide (OsO4) and observed by SEM, the percentages of damage of the OHC stereocilia were similar in each cochlear turn bilaterally. In group 3, the right cochleae were fixed in OsO4 and observed by phase contrast microscopy as surface preparations. Left cochleae were submitted for SEM observation. Missing and grade 3 cells were observed at similar percentages in each row of each turn. In group 4, succinate dehydrogenase staining was performed in the right cochleae and observed by LM. The degree of damage in the right cochleae was compared with that of the left cochleae which was observed by SEM. On average, the mean numbers of missing cells and cells showing grade 3 damage were similar in each row of each turn. From these similarities of evaluation of ototoxicity at LM and SEM levels, it was concluded that semiquantitative analysis by SEM only is appropriate for the assessment of ototoxicity.

Choline-acetyltransferase-like immunoreactivity in the organ of Corti of the rat during postnatal development

The mammalian cochlea receives efferent innervation from neurons located in the superior olivary complex. This efferent olivocochlear innervation is divided in two separate systems, lateral and medial, which mainly innervate afferent dendrites connected to inner hair cells and the cell body of outer hair cells, respectively. Besides other substances, lateral and medial efferent terminals of the adult cochlea use acetylcholine (ACh) as a neurotransmitter. In this study, we have used immunocytochemistry to detect the presence of choline acetyltransferase (ChAT), the synthesizing enzyme of ACh, in efferent olivocochlear terminals during the development of the rat. The appearance and distribution of immunoreactivity to ChAT has been studied in developing rat cochleas from birth (postnatal day 1, P1) to adulthood. Attention was paid to the temporal relationships between the expression of ChAT, the presence of other putative neuroactive substances, the onset of hearing and other developmental phenomena. Our results indicate that ChAT-like immunoreactivity is already present at birth (P1) in the region of inner hair cells, that it appears at P3 in the outer hair cell area and that it reaches an adult pattern of distribution by P15. ACh may thus be present early in the developing cochlea, before the onset of hearing, as it also occurs with other putative transmitters/modulators such as enkephalins, CGRP or GABA. It is suggested that ACh could be involved in the modulation of sound-evoked potentials as soon as they appear, and in the regulation of other developmental phenomena such as neurite outgrowth or synaptogenesis.

AChE-staining of type II ganglion cells, processes and terminals in the cochlea of the mustached bat

There have been a number of reports showing that ganglion cells of sensory neurons may be stained by traditional acetylcholinesterase (AChE) histochemical techniques commonly used to demonstrate efferent nerve fibers and terminals. AChE-staining has been described for cell bodies in the vestibular and spiral ganglia; staining of peripheral and central processes, however, is rare and the presence of reaction product in afferent terminals has not been reported. The outer hair cells of mustached bats, Pteronotus parnellii, differ from those of most mammals in that they typically have a single, large efferent terminal surrounded by 5-7 small, afferent terminals. In this animal an AChE-positive reaction was found not only in efferent fibers and terminals but also in type II ganglion cells, their peripheral and central processes and in outer hair cell terminals. The stained cell bodies were smaller than the unstained type I ganglion cells and they were much fewer in number. The processes of the stained cells could be followed from the soma. The central processes were dispersed throughout the VIIIth nerve trunk. Stained peripheral processes were evident in the osseous spiral lamina, floor of the tunnel of Corti and first space of Nuel and in the outer spiral plexus along the sides of the outer phalangeal (Deiters') cells. AChE-stained afferent terminals were easy to identify after transection of the crossed olivocochlear bundle (COCB) and subsequent degeneration of large efferent terminals. These results are of interest in that assessments of efferent nerve histochemistry after COCB transection need to recognize the potential contribution of AChE reaction product in afferent terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutathione S-transferases in the organ of Corti of the rat: enzymatic activity, subunit composition and immunohistochemical localization

Glutathione S-transferases (GSTs), a family of ubiquitous cytosolic isozymes, catalyze the detoxification of electrophilic substrates with reduced glutathione and participate in intracellular binding and transport of lipophilic substances. This study measured GST activity biochemically in the inner ear of the rat; determined the isozyme profile by Western blotting; and identified, immunohistochemically, the distribution of the mu and pi class GSTs in the organ of Corti. GST enzymatic activity in inner ear tissues ranged from 117 to 348 nmoles glutathione converted/min/mg protein, values somewhat higher than those found in brain (130) and much lower than in liver (1011). Of the GST isoforms, the pi class (identified by antibodies against the Yp subunit) was most prominent, the mu class (Yb1 subunit) clearly evident while the alpha class (Y(a) subunit) was barely detectable on Western blots. Immunocytochemical analysis showed differential distribution of the Yb1 and Yp subunits. The Yb1 subunit was present in the sensory cells, while supporting cells were not specifically stained. At the subcellular level, the isozyme was localized in the apical zones of inner (IHCs) and outer hair cells (OHCs) close to the cuticular plate. The extent of staining, however, varied between OHCs and IHCs. In the OHCs, staining appeared in discrete spots in the apical areas only, whereas in IHCs staining extended further towards the center of the cells. The Yp subunit was mainly localized to Deiters cell processes and pillar cells. Both Yb1 and Yp colocalized with tubulin-specific antibody. The functional significance of GST in the cochlear receptor cells is speculative. However, a role analogous to that in other tissues (detoxification, prostaglandin synthesis) can be assumed. In addition, an association of GST with the microtubule system is possible based on immunohistochemical colocalization with tubulin.