Evidence that phosphoinositides mediate motility in cochlear outer hair cells

Detection without deflection? A hypothesis for direct sensing of sound pressure by hair cells

It is widely thought that organisms detect sound by sensing the deflection of hair-like projections, the stereocilia, at the apex of hair cells. In the case of mammals, the standard interpretation is that hair cells in the cochlea respond to deflection of stereocilia induced by motion generated by a hydrodynamic travelling wave. But in the light of persistent anomalies, an alternative hypothesis seems to have some merit: that sensing cells (in particular the outer hair cells) may, at least at low intensities, be reacting to a different stimulus - the rapid pressure wave that sweeps through the cochlear fluids at the speed of sound in water. This would explain why fast responses are sometimes seen before the peak of the travelling wave. Yet how could cells directly sense fluid pressure? Here, a model is constructed of the outer hair cell as a pressure vessel able to sense pressure variations across its cuticular pore, and this 'fontanelle' model, based on the sensing action of the basal body at this compliant spot, could explain the observed anomalies. Moreover, the fontanelle model can be applied to a wide range of other organisms, suggesting that direct pressure detection is a general mode of sensing complementary to stereociliar displacement.

Muscarinic acetylcholine receptor subtype expression in avian vestibular hair cells, nerve terminals and ganglion cells

Muscarinic acetylcholine receptors (mAChRs) are widely expressed in the CNS and peripheral nervous system and play an important role in modulating the cell activity and function. We have shown that the cholinergic agonist carbachol reduces the pigeon's inwardly rectifying potassium channel (pKir2.1) ionic currents in native vestibular hair cells. We have cloned and sequenced pigeon mAChR subtypes M2-M5 and we have studied the expression of all five mAChR subtypes (M1-M5) in the pigeon vestibular end organs (semicircular canal ampullary cristae and utricular maculae), vestibular nerve fibers and the vestibular (Scarpa's) ganglion using tissue immunohistochemistry (IH), dissociated single cell immunocytochemistry (IC) and Western blotting (WB). We found that vestibular hair cells, nerve fibers and ganglion cells each expressed all five (M1-M5) mAChR subtypes. Two of the three odd-numbered mAChRs (M1, M5) were present on the hair cell cilia, supporting cells and nerve terminals. And all three odd numbered mAChRs (M1, M3 and M5) were expressed on cuticular plates, myelin sheaths and Schwann cells. Even-numbered mAChRs were seen on the nerve terminals. M2 was also shown on the cuticular plates and supporting cells. Vestibular efferent fibers and terminals were not identified in our studies. Results from WB of the dissociated vestibular epithelia, nerve fibers and vestibular ganglia were consistent with the results from IH and IC. Our findings suggest that there is considerable co-expression of the subtypes on the neural elements of the labyrinth. Further electrophysiological and pharmacological studies should delineate the mechanisms of action of muscarinic acetylcholine receptors on structures in the labyrinth.

The significance of the calcium signal in the outer hair cells and its possible role in tinnitus of cochlear origin

Finely tuned changes in intracellular Ca(2+) concentration modulate a variety of cellular functions in eukaryotic cells. The cytosolic Ca(2+) concentration is also tightly controlled in the outer hair cells (OHCs), the highly specialized receptor and effector cells in the mammalian auditory epithelium, which are responsible for high sensitivity and sharp frequency discrimination in hearing. OHCs possess a complex system of transporters, pumps, exchangers, channels and binding proteins to develop and to halt the regulatory Ca(2+) signal. The crucial role of elevated intracellular Ca(2+) concentration in OHCs is to increase the efficacy of the electromechanical (electromotile) feedback via remodeling of the cortical cytoskeleton. Anomalies in the Ca(2+) signaling pathway may lead to hypersensitivity of the cochlear amplifier and subsequently trigger tinnitus of cochlear origin. This review describes the dynamics of Ca(2+) signaling in the OHCs and a model that may convey a putative mechanism of development of subjective idiopathic cochlear tinnitus.

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.

Expression of pituitary adenylate cyclase-activating polypeptide mRNA in the cochlea of rats

Expression of mRNA for pituitary adenylate cyclase-activating polypeptide (PACAP) was detected in the cochlea of rats using a reverse transcription-polymerase chain reaction and in situ hybridization. Examination of in situ hybridization demonstrated that cells in the spiral ganglion, and marginal cells in the stria vascularis expressed mRNA for PACAP. These findings suggest that PACAP may play an important role in regulating cochlear functions.

Distribution of immunophilin FKBP-12 protein and mRNA within the mammalian cochlea and cochlear nucleus

Immunophilin FK binding protein-12 (FKBP-12), the soluble receptor for the immunosuppressant drug FK506, is involved in a number of neuronal activities including increased nerve regeneration in the peripheral nervous system and enhanced recovery in animal models of neurodegenerative diseases. In addition, FKBP-12 is tightly bound to the calcium release channel ryanodine receptor and physiologically interacts with the inositol 1,4,5-trisphosphate receptor. In nearly all cell types, release of intracellular Ca(2+) and subsequent second messenger signaling involves activation of these ion channels. We determined the distribution of FKBP-12 within the mammalian cochlea and dorsal cochlear nucleus (DCN) in order to gain insight into Ca(2+) regulation within the cochlea and to possibly identify potential cellular targets for neuroimmunophilin ligands that may prove useful in protection and recovery following ototoxic insult. FKBP-12 protein and mRNA were found to be abundant throughout rat and guinea pig cochlea and DCN.

Immunohistochemical localization of phospholipase C isozymes in mature and developing gerbil cochlea

The possibility that phospholipase C contributes to intracellular signaling in the cochlea was investigated by immunostaining for eight different isoforms of the enzyme. In the mature gerbil cochlea, expression of the isozymes varied widely among different cell types. The phospholipase C-beta1 isoform was detected in inner and outer hair cells, and spiral ganglion neurons where it may participate in regulating Ca(2+) flux. The beta3 isozyme was expressed in epithelial cells thought to mediate lateral and medial circulation of potassium. The beta2 isozyme was present in border, inner phalangeal and Hensen cells, the stria vascularis, and suprastrial and supralimbal fibrocytes where it also may be involved in regulating ion transport activities. The phospholipase C-gamma isozymes were expressed in supporting cells, the stria vascularis, and certain fibrocytes where they possibly participate in activating tyrosine kinase and modulating ion conductances. The delta2 isoform was found in pillar, outer sulcus and strial marginal cells as well as spiral ganglion neurons and their radial processes. Documentation of changes in the expression pattern of phospholipase C isoforms during postnatal development and knowledge of their distribution in several positive control tissues provided further data for speculation about the biologic significance of the cochlear reactivity. The results demonstrate a wide diversity of isozyme distribution in the cochlea and suggest that the enzymes affect activities of various cochlear cell types in different ways.

Effects of gentamicin and pH on [Ca2+]i in apical and basal outer hair cells from guinea pigs

Aminoglycosides are widely used antibiotics and frequently produce acute ototoxicity. In this study we attempted to comparatively investigate the effects of gentamicin on Ca2+ influx of apical and basal outer hair cells (OHCs) isolated from guinea-pig cochlea. Since the solution of gentamicin sulfate salt is acidic (pH 3.1-3.3), we also explored the effect of external acidification on Ca2+ influx. By means of fura-2 microspectrofluorimetry, we measured the intracellular calcium concentration ([Ca2+]i) of OHCs bathed in Hanks' balanced salt solution (pH 7.40) during either a resting state or high K+-induced depolarization. Our results show that at the resting state, the baseline [Ca2+]i in apical OHCs (94+/-2.0 nM) was slightly lower than that in basal OHCs (101.1+/-2.4 nM). By contrast, the increase in [Ca2+]i evoked by high K+ depolarization in apical OHCs was about two-fold greater than that in basal OHCs. Nifedipine (30 microM) abolished the increased [Ca2+]i in both types of OHCs, suggesting that Ca2+ influx was mainly through L-type Ca2+ channels of OHCs. While gentamicin and extracellular acidification (pH 7.14) can separately attenuate this increase in [Ca2+]i in both types of OHCs, their suppressive effects are additive in basal OHCs, but not in apical OHCs. The implications of these findings are that: (1) apical and basal OHCs behave differently in response to depolarization-increased [Ca2+]i, and (2) basal OHCs are more vulnerable to the impairment of Ca2+ entry during depolarization by a combination of gentamicin and extracellular acidification, which is correlated with the clinical observation that ototoxicity of aminoglycosides at the basal coil of OHCs is more severe than that at the apical coils. Moreover, the possibility that extracellular acidification may enhance the acute ototoxic effects of aminoglycosides should be considered especially in topical applications.

Thimerosal-induced Ca2+ mobilization in isolated guinea pig cochlear outer hair cells

Intracellular calcium mobilization of isolated guinea pig cochlear outer hair cells (OHCs) was investigated using thimerosal, a -SH group oxidizing agent, and fura-2 fluorescence ratio imaging microscopy. In the presence of thimerosal, intracellular Ca2+ concentrations ([Ca2+]i) of OHCs were elevated in a dose-dependent manner. Even in Ca(2+)-free medium, Ca2+ response was still induced. The effects of thimerosal on [Ca2+]i were completely blocked and reversed by dithiothreiotol (DTT). Neither 1-100 microM ryanodine nor 5-20 mM caffeine altered the effects of thimerosal. Pretreatment with pertussis toxin (PTX) for 30 min did not affect the thimerosal-induced increase in [Ca2+]i. The increase in [Ca2+]i when Ca2+ was added during thimerosal application in Ca(2+)-free medium was almost completely blocked by 500 microM LaCl3, while nifedipine did not inhibit further increase in [Ca2+]i caused by thimerosal. Thus, oxidation of the -SH group of the OHC membrane can induce a Ca2+ release from intracellular Ca2+ stores, which are ryanodine- and caffeine-insensitive, and Ca2+ influx through non-specific Ca2+ channels, but not the nifedipine-sensitive Ca2+ channels. The possible oxidation of -SH group gated Ca2+ channels in OHCs is worthy of further study.

Effects of chronic lithium on ototoxicity induced by gentamicin and amikacin in guinea-pigs

The effects of chronic lithium co-therapy on the expression of gentamicin and amikacin ototoxicity were tested in guinea-pigs. Intramuscular injection of different doses of gentamicin (5, 10 mg/kg/day) and amikacin (150, 300 mg/kg/day) for three weeks, induced hearing loss consistent with the established pattern of aminoglycoside ototoxicity. Lithium salts remains one of the most widely used treatment for depressive illness. Administration of lithium chloride (600 mg/l, 35 days) in drinking water changed auditory brainstem response in a time-dependent manner. Pretreatment of animals with lithium chloride after seven days induced significant alterations in wave latency and interval. The present study assesses the protective effects of chronic lithium on gentamicin-induced ototoxicity in guinea pig. The results suggest that duration of lithium administration may be involved in auditory brainstem response changes and the observations could be accounted for, at least partially, by lithium- and aminoglycosides-induced perturbations of the phosphoinositide cascade within the inner ear.

Efferent-mediated protection of the cochlear base from acoustic overexposure by low doses of lithium

Many studies on anaesthetized animals and a few on awake animals have suggested that the cholinergic olivocochlear efferent feedback to outer hair cells can participate in the protection of the cochlea from acoustic overexposure. Lithium is known to stimulate acetylcholine synthesis and release in the brain and it is likely to act similarly at the level of the cochlear efferent synapses. We demonstrate here that, in the awake guinea-pig with a chronically implanted electrode on the round window of the cochlea, the temporary threshold shift induced by 1 minute exposure to different pure tones at around 90 dB sound pressure level (SPL) was reduced by as much as 40 dB, when exposure occurred after lithium treatment. The protection effect was not observed in anaesthetized animals. The effect was seen across the test frequency range of 6.4-12.5 kHz, suggesting that both 'fast' and 'slow' efferent effects are likely to be mediated by acetylcholine. Together our results provide new evidence that the olivocochlear efferents can provide a more efficient protection from acoustic overexposure when animals are awake.

Effect of endoplasmic Ca2+-ATPase inhibitors on cochlear potentials in the guinea-pig

The effect of endoplasmic Ca2+-ATPase inhibitors on cochlear potentials was examined in the guinea-pig. Perilymphatic perfusion with thapsigargin (10[-6] M) produced a significant decrease in the amplitudes of cochlear microphonics, negative summating potential and compound action potential, and a significant prolongation of N1 latency with no change in the endocochlear potential. These changes were all dose dependent. Another endoplasmic Ca2+-ATPase inhibitor, cyclopiazonic acid (10[-5] M), produced the same effects as thapsigargin on cochlear potentials. These results suggest that endoplasmic Ca2+-ATPase inhibitors may have inhibitory functions on cochlear potentials.

Inhibition of calcium-dependent motility of cochlear outer hair cells by the protein kinase inhibitor, ML-9

The calcium ionophore ionomycin has been shown to induce length increases of guinea pig outer hair cells (Dulon et al., 1990). We have demonstrated that these length increases can be inhibited by a 30 min preincubation of the cells with the protein kinase inhibitor ML-9. At either 30 or 60 s after ionomycin application, the effect of ML-9 was dose-dependent with a half maximal response at approximately 0.3 microM. No effect on cell length was detected after 30 min incubation with 0.5 and 5 microM ML-9 alone. However, with 50 and 500 microM ML-9, significant contraction in cell length was observed. 50 microM ML-9 did not interfere with the ability of ionomycin to elevate fluorescence of the calcium indicator Fluo-3, nor did it alter the ability of cells to exclude propidium iodide from their nuclei. Treatment with 500 microM ML-9 resulted in impaired cell morphology. The data support the hypothesis that protein kinase activity regulates calcium-dependent processes that affect shape changes of outer hair cells. They are consistent with the involvement of the calcium/calmodulin-dependent enzyme, myosin light chain kinase, a known target of ML-9, but do not preclude the possibility of another intracellular target for ML-9.

Vital staining of the hearing organ: Visualization of cellular structure with confocal microscopy

Cells inside the intact organ of Corti were labelled with fluorescent probes reflecting various aspects of structure and function. The dyes were introduced into the perilymphatic space by perfusion of the scala tympani of the temporal bone from the guinea-pig maintained in isolation. The dyes were able to diffuse through the basilar membrane and into the organ of Corti where they were spontaneously absorbed by the sensory and supporting cells. Confocal microscopic observation was made through an opening in the apex of the cochlea. A number of different dyes were used; a carbocyanine dye which stains mitochondria; two styryl dyes which are absorbed by the cell membranes and calcein, a cytoplasmic marker that fluoresces in vital cells. Extracellular space was stained by a cell-impermeant Dextran fluorescein. The most striking finding was that the membrane dyes preferentially stained the sensory cells and neural elements whereas the staining of the supporting cells was faint. The cytoplasmic dye in general stained sensory and supporting cells to the same extent. By tilting the organ, a view could be obtained from the side like a radial section through the organ. Outer and inner hair cells with their sensory hairs, nerve fibres and nerve endings, especially under the inner hair cells, could be seen in profile. Introduction of a high molecular weight Dextran into the endolymphatic space outlined the tectorial membrane which was seen in negative contrast. The simultaneous perfusion with a membrane dye stained the hair cells and their sensory hairs. Merging of the two images gave the possibility to examine, in the living tissue, the cilia to tectorial membrane relationship. Of general interest is the finding that the membrane dyes preferentially stained the sensory and neural elements of the nervous system, represented here by the hair cells and nerve fibres of the inner ear.

P2 purinoceptor of the globular substance in the otoconial membrane of the guinea pig inner ear

The biological characteristics of the globular substance, a precursor of otoconia, are unclear. In the present study, the ATP-induced internal free Ca2+ concentration ([Ca2+]i) changes of the globular substance and the ATP distribution in the vestibular organ were investigated using a Ca2+ indicator, fluo 3, and an adenine nucleotide-specific fluorochrome, quinacrine, by means of confocal laser scanning microscopy. [Ca2+]i showed a rapid and dose-dependent increase in response to ATP with a 50% effective concentration (EC50) of 16.7 microM. This reaction was independent of external Ca2+, indicating the presence of an internal Ca2+ reservoir. Neither adenosine, alpha, beta-methylene-ATP, 3'-O-(4-benzoylbenzoyl)-ATP, ADP, nor UTP evoked this reaction, whereas 2-methylthio-ATP induced an increase of [Ca2+]i with an EC50 of 14.4 microM. Moreover, P2 antagonists, reactive blue 2 and suramin, and a phospholipase C inhibitor, U-73122, inhibited the ATP-induced [Ca2+]i increase. These findings indicate the presence of a P2Y purinoceptor on the globular substance. In addition, granular fluorescence was observed in the quinacrine-stained macular sensory epithelium, indicating the presence of ATP-containing granules in this tissue. These results suggest that a paracrine mechanism involving ATP may exist in the macula and that this mechanism regulates the biological behavior of the globular substance.

Calmodulin-dependent protein kinases mediate calcium-induced slow motility of mammalian outer hair cells

Cochlear outer hair cells in vitro respond to elevation of intracellular calcium with slow shape changes over seconds to minutes ('slow motility'). This process is blocked by general calmodulin antagonists suggesting the participation of calcium/calmodulin-dependent enzymatic reactions. The present study proposes a mechanism for these reactions. Length changes of outer hair cells isolated from the guinea pig cochlea were induced by exposure to the calcium ionophore ionomycin. ATP levels remained unaffected by this treatment ruling out depletion of ATP (by activation of calcium-dependent ATPases) as a cause of the observed shape changes. Involvement of protein kinases was suggested by the inhibition of shape changes by K252a, a broad-spectrum inhibitor of protein kinase activity. Furthermore, the inhibitors ML-7 and ML-9 blocked the shape changes at concentrations compatible with inhibition of myosin light chain kinase (MLCK). KN-62, an inhibitor of Ca2+/calmodulin-dependent protein kinase II (CaMKII), also attenuated the length changes. Inhibitors with selectivity for cyclic nucleotide-dependent protein kinases (H-89, staurosporine) were tested to assess potential additional contributions by such enzymes. The dose dependence of their action supported the notion that the most likely mechanism of slow motility involves phosphorylation reactions catalyzed by MLCK or CaMKII or both.

Reversible hearing impairment induced by lithium in the guinea pig

Lithium salts remain one of the most widely used treatments for depressive illness. The mechanisms involved probably include reduction in free inositol. Visual perceptive disturbances can be a side effect of the treatment. We report here for the first time that chronic lithium treatment in the guinea pig induces a predominantly low frequency hearing loss and, in the longer term, loss of sensitivity is observed across the whole audiogram. The changes are reversed when treatment is arrested. The observations could be accounted for, at least partially, by a lithium-induced perturbation of the phosphoinositide cascade within the inner ear.

The cellular localization of the neuropeptides substance P, neurokinin A, calcitonin gene-related peptide and neuropeptide Y in guinea-pig vestibular sensory organs: a high-resolution confocal microscopy study

Four neuropeptides, substance P, neurokinin A, calcitonin gene-related peptide and neuropeptide Y, were detected by radioimmunoassay in guinea-pig vestibular end-organs. High-resolution confocal microscopy visualization of immunofluorescence staining was used to determine the cellular localization of these peptides. Substance P- and neurokinin A-like immunoreactivities were found to co-exist in afferent fibers innervating the peripheral regions of both the utricular and ampullar sensory organs. The immunoreactivity was more concentrated in the distal ends of the calyceal-shaped nerve endings that innervate type I sensory cells. While in the guinea-pig, nerve calyces and type I cells are distributed in both the central and peripheral regions of the sensory epithelia, immunoreactive calyces were found only in the peripheral regions. Calcitonin gene-related peptide-like immunoreactivity was localized in small bouton endings situated at the level of the base of the hair cells. These boutons were in a position to make axosomatic contacts with type II sensory cells and axodendritic contacts with afferent nerve endings. Calcitonin gene-related peptide immunoreactivity co-existed with choline acetyltransferase immunoreactivity. The localization and shape of these boutons identified them as the axonal endings of efferent vestibular fibers. Neuropeptide Y-like immunoreactivity was not observed in the actual sensory epithelium but in the underlying connective tissue, where it was located in varicose fibers along blood vessels. The synaptic position of the tachykinins is clearly distinct from that of calcitonin gene-related peptide. This segregation distinguishes the vestibular end-organs from most peripheral tissues where these peptides are co-localized. The tachykinin-immunoreactive afferent fibers are postsynaptic to the hair cells. If, as in somatic sensory endings, these fibers can be triggered to release the neuropeptides by an axon reflex type of activation, then the tachykinins could interfere directly with the function of type I and type II vestibular hair cells. Calcitonin gene-related peptide co-exists with acetylcholine in the efferent axonal endings that are presynaptic to type II hair cells and to afferent fibers. Calcitonin gene-related peptide can thus interfere by direct synaptic action with type II hair cells only. It may also regulate the activity of the tachykinin-containing afferents.

Pre- and postsynaptic M3 muscarinic receptor mRNAs in the rodent peripheral auditory system

The medial and lateral efferent innervations originate from distinct parts of the superior olivary complex. Both use acetylcholine, respectively, to modulate the activity of outer hair cells (OHC), and spiral ganglion neurons (SGN) which are postsynaptic to the inner hair cells (IHC). Besides predominantly activating nicotinic receptors, acetylcholine recognizes muscarinic M3 receptors, whose the role(s) and cellular localization(s) are not yet firmly established. We used reverse transcription and polymerase chain reaction to amplify the M3 receptor cDNA in the rat and guinea pig organ of Corti and spiral ganglion. Then, we localized the M3 receptor mRNAs in cochleas and superior olivary complex of both species. The M3 receptor cDNA was amplified from samples of brain, organ of Corti and spiral ganglion. Indeed, its corresponding mRNA was localized in SGNs, OHCs and IHCs. However, in the apical turns, OHCs were often found unlabeled. In the superior olivary complex, M3 mRNAs were colocalized with choline acetyltransferase mRNAs in neurons of the lateral superior olive and ventral nucleus of the trapezoid body. These results suggest that the M3 receptor-induced inositol phosphate formation described in previous studies [21] takes place in both postsynaptic (SGNs, OHCs) and presynaptic components of efferent cochlear synapses, and in cells that are not contacted by efferents in the adult cochlea (IHCs).

Modulation by purines of calcium-activated non-selective cation channels in the outer hair cells of the guinea-pig cochlea

1. The cell-attached and cell-free configurations of the patch-clamp technique were used to investigate whether external ATP and its derivatives modulate channel activity in outer hair cells freshly isolated from the guinea-pig cochlea. 2. Submicromolar concentrations of ATP stimulated a non-selective cation channel with a conductance of about 25 pS. The ATP-elicited stimulation was partly blocked by the membrane-permeant blocker 3',5-dichlorodiphenylamine-2-carboxylic acid (DCDPC), and mimicked by the calcium ionophore, ionomycin, suggesting that the channel activated by ATP is identical to a previously reported calcium-activated non-selective (CAN) cation channel. 3. The P2x agonist beta, gamma-methylene-ATP (beta, gamma-MeATP, 10 microM) and the P2Y agonist 2-methyl-thio-ATP (2-MeSATP, 1 microM) both activated CAN channels. The effect of ATP was inhibited by the P2 antagonist suramin but not by the P2Y antagonist Reactive Blue 2. These results suggest that both purinergic receptors are involved in the ATP-evoked response and that internal calcium acts as a second messenger for opening CAN channels. 4. In contrast, adenosine inhibited CAN channels. This effect was reproduced by the A2 agonist 5'-N-ethylcarboxyamidoadenosine (NECA) and the permeant cAMP analogue 8-bromo-adenosine 3',5'-cyclic monophosphate (8-Br-cAMP), but not by the A1 agonist N6-cyclo-hexyladenosine (CHA). CAN channels were also inhibited when the catalytic subunit of protein kinase A was applied internally on inside-out patches, suggesting that adenosine A2 receptor downregulates CAN channels via a cAMP-dependent phosphorylation.

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.

Chemical synaptic transmission in the cochlea

The last two decades have witnessed major progress in the understanding of cochlear mechanical functioning, and in the emergence of cochlear neurochemistry and neuropharmacology. Recent models describe active processes within the cochlea that amplify and sharpen the mechanical response to sound. Although it is widely accepted that outer hair cells (OHCs) contribute to these processes, the nature of the medial efferent influence on cochlear mechanics needs further clarification. Acetylcholine (ACh) is the major transmitter released onto OHCs during the stimulation of these efferents. The inhibitory influence of this system is mediated by post- and presynaptic nicontinic and muscarinic receptors and the role of other neuroactive substances [gamma-aminobutyric acid (GABA), calcitonin gene-related peptide (CGRP), adenosine 5'-triphosphate (ATP) or nitric oxide (NO)] remains to be determined. The inner hair cells (IHCs) that transduce the mechanical displacements into neural activity, release glutamate on receptor-activated channels of AMPA, kainate, and NMDA types. This synapse is in turn controlled and/or regulated by the lateral efferents containing a cocktail of neuroactive substances (ACh, GABA, dopamine, enkephalins, dynorphin, CGRP). This glutamatergic nature of the IHCs is responsible for the acute destruction of the nerve endings and subsequently for neuronal death, damage usually described in various cochlear diseases (noise-induced hearing losses, neural presbycusis and certain forms of sudden deafness or peripheral tinnitus). These pathologies also include a regrowth of new dendritic processes by surviving neurons up to IHCs. Understanding the subtle molecular mechanisms which underly the control of neuronal excitability, synaptic plasticity and neuronal death in cochlear function and disease is a very important issue for the development of future therapies.

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.

The effect of protein kinase C stimulator and inhibitor on cochlear potentials in the guinea pig

To determine a possible role of protein kinase C (PKC) in the cochlear, the effects of a PKC stimulator (phorbol-12-myristate-13-acetate; PMA), an inactive analogue of PKC stimulator (4 alpha-phorbol-12,13-didecanoate; 4 alpha-PDD) and a PKC inhibitor (D-sphingosine) on cochlear potentials were examined in the guinea pig. The perilymphatic perfusion with PMA (3 x 10(-6) M) produced an increase in compound action potential (CAP) amplitude and no change in N1 latency, the amplitudes of negative summating potential (-SP), cochlear microphonics (CM) and endocochlear potential (EP). The perfusion with 4 alpha-PDD (3 x 10(-6) M) did not change the sound-evoked cochlear potentials and the EP. The perfusion with D-sphingosine (10(-5) M) produced a decrease in CAP amplitude and no change in N1 latency and the amplitudes of -SP, CM and EP. The results suggest that PKC may be involved in the mechanism underlying the CAP generation.

Direct effects of reactive oxygen species on cochlear outer hair cell shape in vitro

Reactive oxygen species (ROS) have been implicated in the ototoxicity of various agents. This study examines the effects of superoxide anion (O2), hydroxyl radical (OH.) and hydrogen peroxide (H2O2), on isolated cochlear outer hair cell (OHC) morphology. OHCs were superfused with artificial perilymph (AP) or AP containing a specific ROS scavenger, and then with AP, ROS system or scavenger plus ROS system for 90 min. The generation of ROS as well as the scavenging properties of other agents were confirmed by specific biochemical assays. Control cells decreased 4.8% in mean length, and showed no obvious membrane damage. Generation of O2. or OH. resulted in high rates (85.7 and 42.9%, respectively) of bleb formation at the synaptic pole, and decreased (O2., 15.2%; OH., 17.3%) mean cell length. Length change and bleb formation rate were H2O2 concentration-dependent. 20 mM H2O2 led to 33.3% decreased mean cell length, and only 20% bleb formation; 0.1 mM H2O2 led to 83.3% bleb formation, with no length decrease. Superoxide dismutase, deferoxamine and catalase protected against O2., OH. and H2O2 effects, respectively. Bleb formation and diminished cell length likely represent differential lipid peroxidative outcomes at supra- and infranuclear membranes, and are consistent with effects of certain ototoxicants.

Localization of F-actin and fodrin along the organ of Corti in the chinchilla

The distribution of the two cytoskeletal proteins, filamentous actin (F-actin) and fodrin, was investigated along the organ of Corti of the chinchilla using laser scanning confocal fluorescence microscopy. High intensity labeling of F-actin was seen in outer and inner hair cells, including the stereocilia. High intensity staining was also seen for fodrin in outer and inner hair cells, but not in their stereocilia. Staining intensity of both proteins along the lateral cell wall of the outer hair cells appeared to be greater in the middle and basal cochlear turns than in the apical turn. Pillars and Deiters cells also exhibited high intensity labeling of F-actin. The lack of significant differences in the distribution of fodrin between outer and inner hair cells makes the role of this protein in the active processes still unclear. Comparison of the distribution of F-actin and fodrin in the chinchilla with those reported in the guinea pigs suggest possible species differences.

Effects of adenosine 5'-triphosphate and related agonists on cochlear function

Several lines of evidence implicate a neurotransmitter/modulator role for ATP in the cochlea. Most of the work supporting such a notion has been accomplished using in vitro preparations of sensory hair cells or other cochlear tissues. Little is known regarding the functional consequences of ATP receptor activation in vivo. In the present experiments, we tested ATP and related agonist analogs for their effects on sound-evoked responses of the cochlea (cochlear microphonic, CM; summating potential, SP; distortion product otoacoustic emissions, DPOAE) and auditory nerve (compound action potential, CAP) in vivo and on outer hair cell (OHC) currents and cell length in vitro. In vivo, local application of these compounds was associated with concentration- and intensity-dependent response alterations. The slowly-hydrolyzable P2y agonist, ATP-gamma-S, was clearly of greatest in vivo potency: At low to moderate stimulus intensities, micromolar concentrations of this drug reduced all responses, in particular CAP and DPOAEs, which fell to the level of the noise floor. At high intensities, response suppression was smaller and SP was increased. In vivo effects of ATP, ATP-alpha-S and 2-Me-S-ATP were qualitatively similar to, but smaller in magnitude and requiring higher concentrations than those observed for ATP-gamma-S. Adenosine was without significant effect on responses of the cochlea and auditory nerve. In vitro, effects of ATP-gamma-S and ATP were similar: both induced inward currents in OHCs held at -60 mV without producing observable (> 0.3 micron) changes in OHC length. Results suggest that endogenous ATP influences cochlear function through receptors at several sites in the cochlea. Results suggest further that these response alterations are mediated, at least in part, by receptors of the P2y subtype.

ATP-induced cytoplasmic [Ca2+] increases in isolated cochlear outer hair cells. Involved receptor and channel mechanisms

Outer hair cells (OHC) of the mammalian cochlea are thought to preprocess the sound signal by active movements, which can be induced by electrical or chemical stimulation, e.g. depolarization evoked by high [K+] or increased cytoplasmic [Ca2+]. Extracellular ATP has been found to induce cytoplasmic [Ca2+] increases in OHC but involved mechanisms have not been elucidated. Cytoplasmic [Ca2+] was measured in non-enzymatically isolated single OHC using Fura-2 microspectrometry. Results, using ATP/derivatives and other P2-purinergic receptor (P2R) ligands, as well as Ca(2+)-channel blockers and pertussis toxin, revealed several signal transduction pathways that increase cytoplasmic [Ca2+] in OHC: a P2-purinergic receptor (P2R)--G-protein--effector (phospholipase C or an ion channel) system and a voltage-dependent Ca2+ channel. Agonist potency studies denote a pattern analogous to that found in skeletal muscle, i.e. ATP-alpha-S > ATP = 2-methyl-S-ATP >> ADP > alpha,beta-methylene-ATP, but no activation by ADP beta F or UTP, leaving a choice of P2y or P2zR subtypes. The latter possibility gained strength from calculations showing that up to 8% of ATP may have formed the P2zR agonist ATP4- in the experimental medium. Experiments in Ca(2+)-free medium and with pertussis toxin revealed that the main Ca2+ source was intracellular. Pertussis toxin did not affect [Ca2+] increase induced by carbachol. Acetylcholine, administered a few seconds before ATP, did not affect total cytoplasmic [Ca2+] increases. Induced cytoplasmic [Ca2+] increases were high enough (> 500 nM at 50 microM ATP/derivatives) to hyperpolarize the OHC membrane by opening K(+)-channels and decreased little with time.(ABSTRACT TRUNCATED AT 250 WORDS)

Localisation of functional muscarinic receptors in the rat cochlea: evidence for efferent presynaptic autoreceptors

In the rat cochlea, the activation of muscarinic receptors stimulates the hydrolysis of phosphoinositides but the importance of this muscarinic effect is still unknown. In order to find out about the role of the muscarinic receptors in the cochlea, we examined their functional distribution within this organ. This was achieved by measuring the formation of [3H]inositol phosphates induced by carbachol (1 mM) in two regions of the cochlea: the modiolus and the organ of Corti. At both sites, carbachol enhanced the accumulation of inositol phosphates in an atropine-sensitive way. These stimulations were completely antagonised by 4-diphenylacetoxy-N-methyl piperidine methiodide (1 microM) but unchanged by pirenzepine (1 microM). In cochleas depleted of outer hair cells by a treatment with amikacin, the carbachol-induced formation of inositol phosphates is not altered with respect to control, undamaged cochleas. Conversely, when the medial cholinergic axons which form synapses with the outer hair cells are destroyed by the section of the crossed olivocochlear bundle the carbachol-stimulated inositol phosphates response is reduced by 35% in the organ of Corti. This section has no effect in the modiolus, despite the degeneration of some modiolar fibers. Our results show that functional muscarinic receptors are distributed both in the organ of Corti and in the modiolus. These two structures contain presumably the same class of cholinoceptor. The effects of selective destruction clearly demonstrate that a population of muscarinic receptors is located on presynaptic membranes at the level of the medial axon-outer hair cell contacts. They also point to spiral ganglion neurons and/or the Schwann cells as sites for the functional cholinoceptors in the modiolus.

Evidence for calcium-binding proteins and calcium-dependent regulatory proteins in sensory cells of the organ of Corti

Calcium is thought to play a major signaling role in outer hair cells to control metabolism, cytoskeletal integrity, cell shape and cell excitability. For this to happen, in resting cells the concentration of free calcium ions must be maintained at low levels so that focal increases can trigger specific events. In this paper, the localization of calcium, calcium-binding and calcium-dependent regulatory proteins in sensory cells from the guinea pig inner ear was demonstrated using immunocytochemical and histochemical techniques. We found the calcium buffer and/or calcium sensor proteins calmodulin, calbindin and calsequestrin predominantly in sensory cells and that when present, these proteins can be enriched in the outer hair cells. Calmodulin is found in the stereocilia, in the cuticular plate and in the cytoplasm and calbindin is found only in the cuticular plate and cytoplasm of both the inner and outer hair cells. The staining for these proteins in the outer hair cells is homogeneous, with no apparent compartmentalization along the lateral wall. Calsequestrin, thought to store and release calcium from membrane bound intracellular storage sites is found only in the cytoplasm of outer hair cells. There, it has a more punctuate staining pattern than does calmodulin or calbindin suggesting that it may be present in calciosomes rather than soluble in the cytoplasm. We did not detect caldesmon and S-100. Using the potassium pyroantimonate technique, we found precipitates containing calcium ions distributed throughout the cytoplasm of outer hair cells, with no evidence that the subsurface cisterns along the lateral wall act as calcium storage sites. Thus, calcium in resting cells is found in the cytoplasm along with calbindin and calmodulin and appears to have a punctate distribution consistent with a co-localization with calsequestrin. The implications of this distribution with respect to the slow shortening and elongation seen in outer hair cells are discussed.

Receptor-mediated release of inositol phosphates in the cochlear and vestibular sensory epithelia of the rat

Various neurotransmitters, hormones and other modulators involved in intercellular communication exert their biological action at receptors coupled to phospholipase C (PLC). This enzyme catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) to inositol 1,4,5-trisphosphate (InsP3) and 1,2-diacylglycerol (DG) which act as second messengers. In the organ of Corti of the guinea pig, the InsP3 second messenger system is linked to muscarinic cholinergic and P2y purinergic receptors. However, nothing is known about the InsP3 second messenger system in the vestibule. In this study, the receptor-mediated release of inositol phosphates (InsPs) in the vestibular sensory epithelia was compared to that in the cochlear sensory epithelia of Fischer-344 rats. After preincubation of the isolated intact tissues with myo-[3H]inositol, stimulation with the cholinergic agonist carbamylcholine or the P2 purinergic agonist ATP-gamma-S resulted in a concentration-dependent increase in the formation of [3H]InsPs in both epithelia. Similarly, the muscarinic cholinergic agonist muscarine enhanced InsPs release in both organs, while the nicotinic cholinergic agonist dimethylphenylpiperadinium (DMPP) was ineffective. The muscarinic cholinergic antagonist atropine completely suppressed the InsPs release induced by carbamylcholine, while the nicotinic cholinergic antagonist mecamylamine was ineffective. Potassium depolarization did not alter unstimulated or carbamylcholine-stimulated release of InsPs in either organ. In both tissues, the P2 purinergic agonist alpha,beta-methylene ATP also increased InsPs release, but the P1 purinergic agonist adenosine did not. These results extend our previous observations in the organ of Corti of the guinea pig to the rat and suggest a similar control of the InsP3 second messenger system in the vestibular sensory epithelia.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of the carbachol-evoked synthesis of inositol phosphates by ototoxic drugs in the rat cochlea

The ability of amikacin, neomycin, ethacrynate, mercuric chloride and cisplatin to alter the inositol phosphate (IP) signalling pathway was assessed in the 12-day-old rat cochlea, where the turnover of IPs is coupled to muscarinic receptors. This study was motivated by: (1) the demonstration of neomycin binding to phosphatidylinositol 4,5-biphosphate, the precursor of IPs, and (2) the fact that ototoxic drugs induce some common symptoms in outer hair cells. At concentrations below 1 mM, none of the compounds changed the control 3H-IP formation. Mercuric chloride, cisplatin and ethacrynate inhibited the carbachol-induced formation of IPs in a dose-dependent manner with IC50 values of 74,340 and 430 microM, respectively. The aminoglycosides were less efficient in reducing the carbachol-stimulated accumulation of IPs, since neither amikacin nor neomycin, both at 1 mM, had any significant effect. However, neomycin applied at 15 and 30 microM induced 29% and 43% of inhibition of the stimulated IP response. Finally, additive effects are obtained between some of the toxic drugs. The results suggest that a block of the IP transduction system, associated with the cholinergic efferent innervation of the organ of Corti, is a feature that may be involved in some types of ototoxicity. The inefficiency of aminoglycosides and the putative targets of the ototoxic agents are discussed.

Confocal laser microscopical images of calcium distribution and intracellular organelles in the outer hair cell isolated from the guinea pig cochlea

We report the use of a confocal laser fluorescence microscope to observe the distribution of cytosolic Ca2+ and the localization of intracellular organelles and cytoskeleton in the isolated outer hair cell (OHC). Membrane-bound Ca2+ stained by chlortetracycline was mainly seen in the subcuticular region, the infranuclear region, and the region adjacent to the lateral wall. In contrast, the central portion of the cytoplasm and nucleus were devoid of detectable fluorescence of membrane-associated Ca2+, but were relatively rich in free Ca2+. The cuticular plate showed a lack of both membrane-bound and free Ca2+. Fluorescent clusters of mitochondria and endoplasmic reticulum were predominantly seen in the infracuticular and infranuclear regions, and some were associated with the lateral wall. These two types of cytosolic organelles which fluoresced upon chlortetracycline treatment are therefore presumed to sequester calcium. The characteristic distribution of the endoplasmic reticulum was observed in coincidence with the infracuticular network of F-actin. The subsurface cistern, which was shown to be analogous to the endoplasmic reticulum in terms of its biological function, is likely to be the source of Ca2+ for the actin-mediated process.

Immunohistochemical localization of intracellular Ca-ATPase in outer hair cells, neurons and fibrocytes in the adult and developing inner ear

Intracellular isoforms of the enzyme Ca-ATPase were identified in the inner ear by immunostaining paraffin sections with a polyclonal antiserum against rabbit cardiac muscle Ca-ATPase. In the adult cochlea, intense staining was present at the lateral border of outer hair cells in regions corresponding with the distribution of the subsurface cisternal system. Other cell types containing high levels of Ca-ATPase were skeletal muscle fibers in the tensor tympani, vascular smooth muscle, spiral ganglion neurons and subpopulations of fibrocytes in the limbus, spiral ligament and underlying vestibular neurosensory epithelium. In neonatal gerbils, staining of tensor tympani muscle fibers was observed at 4 days after birth and approached adult levels by 8 days after birth. Ca-ATPase was first detected in other cell types between postnatal days 12 and 14 but immunostaining still remained well below the intensity seen in adults at 20 days after birth. The demonstration of abundant calcium pumps in the subsurface cisternae confirms the role of this organelle as an intracellular reservoir for Ca2+ in outer hair cells. The presence of high levels of Ca-ATPase in spiral ganglion neurons and in fibrocytes specialized for ion transport points to a role for the enzyme in regulating the activity of other cell types of importance to normal hearing.

Evoked otoacoustic emissions and pure tone threshold audiometry in patients receiving cisplatinum therapy

Eight children and young adults with cancer were evaluated serially using pure tone audiometry as well as registration of click-evoked otoacoustic emissions (EOAE) 1 day prior to therapy as well as after various numbers of doses of cisplatinum. A reduction of EOAE-amplitudes following cisplatinum therapy was observed in all patients. This reduction tended to recover after the end of cisplatinum administration. Since EOAE are believed to result from cochlear bio-mechanical processes, the reduced emissions are interpreted as signs of cochlear dysfunction. We conclude, that EOAE testing may be a simple, non-invasive method that may detect early, transient functional impairment of hearing due to ototoxic agents such as cisplatinum, even in children. Further controlled trials are needed.

Analysis of muscarinic receptor subtypes in the mouse cochlea by means of the polymerase chain reaction

Total RNA was extracted with guanidine thiocyanate from the cochleas of 16-day-old CBAJ mice. The mRNA was purified from the total RNA using oligo-dT cellulose, and the mRNA was treated with DNase to degrade genomic DNA. After reverse transcription, resulting cDNA was amplified by polymerase chain reaction (PCR), using primers specific for the nucleotide sequences m1-m5, representing subtypes of muscarinic acetylcholine receptors. PCR products corresponding to subtypes m1, m3, and m5, but not to m2 and m4, were amplified. These results suggest that muscarinic acetylcholine receptors of these odd-numbered subtypes are expressed in the mammalian cochlea.

The tuned displacement response of the hearing organ is generated by the outer hair cells

The motile responses of the guinea-pig hearing organ in response to a tone applied to the ear were measured by laser interferometry. Two types of responses can be recorded: (i) a vibration at the frequency of the applied tone; and (ii) a displacement response consisting of a shift in the position of the organ surface. The purpose of this study is to characterize the displacement response. The results are as follows. There is a relationship between the frequency of highest sensitivity (best-frequency) of the displacement response and the site from which it is recorded. High best-frequencies are noticed at more basal locations, low best-frequencies towards the apex. The displacement response is more frequency-selective than the vibration response. The displacement response is observed within physiological sound pressure levels. Its sharpness is dependent on the stimulus intensity, it shows biological variability and can be manipulated by drugs that are known to modify the receptor potential of the sensory cells, or to interfere with outer hair cell motility. These results suggest that the displacement response is an important step in the transduction process in the mammalian hearing organ and that it is generated by the motile action of the outer hair cells.

Cholinergic regulation of the phosphoinositide second messenger system in the guinea pig organ of Corti

The effect of cholinergic agents on the phosphoinositide second messenger system was investigated in the cochlea of the adult guinea pig in vivo and in vitro. In vivo, phospholipids were labeled with [32P]-orthophosphate by perilymphatic perfusion and their hydrolysis assayed in 'chase' experiments with non-radioactive orthophosphate. Carbachol (1 mM) reduced the content of 32P-labeled phosphatidylinositol 4,5-bisphosphate in the organ of Corti from 31% to 21% of total 32P-lipids, indicating stimulated hydrolysis. The pharmacology of this effect was studied in detail in vitro via the release of inositol phosphates from phosphoinositides pre-labeled with 3H-inositol. Release was increased 2-fold by 1 mM carbachol, 1.6-fold by 1 mM muscarine, but was unaffected by dimethylphenylpiperazinium; the stimulation was blocked by 1 microM atropine but not mecamylamine. These responses indicate the coupling of phosphoinositides to a muscarinic receptor. Furthermore, stimulated inositol phosphate release was higher in the base of the organ of Corti than in the apex which correlates with the increased cholinergic efferent innervation of outer hair cells in the basal region. These results suggest that muscarinic-stimulated inositol phosphate release occurs at the level of the outer hair cell and thus may have an important modulatory role in auditory transduction.

Functional and morphological comparisons between cochlear outer hair cells and muscle tissues in the guinea-pig

The effect of polylysine on the motility of outer hair cells and various muscle types was compared. Poly-L-lysine and its stereoisomer, poly-D-lysine, inhibited acoustically induced length changes of isolated outer hair cells from the guinea-pig hearing organ. The frequency specific displacements of the hearing organ in response to a tone stimulus are also inhibited to polylysine (Brundin et al. 1991). Poly-L-lysine, and its stereoisomer, irreversibly attenuated motile responses to transmural stimulation of guinea-pig ileum, vas deferens and taenia coli in a dose dependent manner, but were without significant effect on motile responses in skeletal and heart muscle. L-lysine, D-lysine, and the negatively charged polyaminoacid poly-L-aspartate, were without significant effect on outer hair cell and smooth muscle motility. The inhibitory effect of polylysine in smooth muscle is a direct effect on the muscle cell since polylysine attenuated acetylcholine- and adenosine triphosphate-induced contractions in the ileum, and ATP- or noradrenaline-induced contractions in the vas deferens. Pillar structures, believed to be of importance to excitation contraction coupling, were compared. In heart and skeletal muscle the pillars span the gap between sarcoplasmic reticulum and T-tubuli, deeply recessed into the muscle cell. In smooth muscle and outer hair cell the pillars are in closer relation to the cell exterior. The length of the pillars of the outer hair cells exceeds by two times that of smooth and skeletal muscle. The susceptibility of outer hair cells and smooth muscle tissue to the positively charged polylysine may indicate similarities in membrane or channel composition.

Actin-binding and microtubule-associated proteins in the organ of Corti

Actin-binding and microtubule-associated proteins regulate microfilament and microtubule number, length, organization and location in cells. In freeze-dried preparations of the guinea pig cochlea, both actin and tubulin are found in the sensory and supporting cells of the organ of Corti. Fodrin (brain spectrin) co-localized with actin in the cuticular plates of both inner and outer hair cells and along the lateral wall of the outer hair cells. Alpha-actinin co-localized with actin in the cuticular plates of the hair cells and in the head and foot plates of the supporting cells. It was also found in the junctional regions between hair cells and supporting cells. Profilin co-localized with actin in the cuticular plates of the sensory hair cells. Myosin was detected only in the cuticular plates of the outer hair cells and in the supporting cells in the region facing endolymph. Gelsolin was found in the region of the nerve fibers. Tubulin is found in microtubules in all cells of the organ of Corti. In supporting cells, microtubules are bundled together with actin microfilaments and tropomyosin, as well as being present as individual microtubules arranged in networks. An intensely stained network of microtubules is found in both outer and inner sensory hair cells. The microtubules in the outer hair cells appear to course throughout the entire length of the cells, and based on their staining with antibodies to the tyrosinated form of tubulin they appear to be more dynamic structures than the microtubules in the supporting cells. The microtubule-associated protein MAP-2 is present only in outer hair cells within the organ of Corti and co-localizes with tubulin in these cells. No other MAPs (1,3,4,5) are present. Tau is found in the nerve fibers below both inner and outer hair cells and in the osseous spiral lamina. It is clear that the actin-binding and microtubule-associated proteins present in the cochlea co-localize with actin and tubulin and that they modulate microfilament and microtubule structure and function in a manner similar to that seen in other cell types. The location of some of these proteins in outer hair cells suggests a role for microfilaments and microtubules in outer hair cell motility.

Phospholipid metabolism in the cochlea: differences between base and apex

Phospholipid-derived second messenger systems are one of the primary means for the transduction of extracellular signals to intracellular effector sites. We have investigated the longitudinal distribution of phospholipid metabolism in the guinea pig cochlea because of increasing evidence that the apex and base process auditory signals differently. Phospholipid metabolism was assayed by measuring the incorporation of radioactive phosphate (32Pi) into lipids of the organ of Corti and the lateral wall tissues (stria vascularis and spiral ligament). 32P-labeling of total phospholipids was higher in the apex than the base, and individual phospholipids exhibited a tissue-specific base/apex distribution. Phosphatidylinositol was the most abundant of the labeled lipids in all tissues except the basal lateral wall, where phosphatidylinositol and phosphatidylcholine were labeled to a similar extent. Experiments on the availability of [32P]-ATP and other non-lipid substrates (inositol, choline, and cytidine) suggested that the base/apex distribution of phospholipid metabolism is based on differences in enzymatic activities. Additional evidence for this is an increased hydrolysis of phosphoinositides in the apex. The base/apex distribution of lipid metabolism suggests that physiological and pathological mechanisms involving phospholipids differ between the turns of the cochlea.

Monoclonal antibodies to inner ear antigens: II. Antigens expressed in sensory cell stereocilia

To develop biological reagents for investigating structure-function relationships in the organ of Corti, we have raised monoclonal antibodies, (MAb) to inner ear tissues. Our first series of antibodies prepared after intrasplenic immunization of mice with guinea pig tissues, identified antigens restricted to supporting cell structures, but no hair cell specific antibodies were developed [Zajic et al., Hear. Res. 52, 59-72, 1991]. In this report we describe the isolation, binding specificity and initial characterization of the stereocilia-binding monoclonal antibodies, KHRI-4, and KHRI-5. Mice were immunized with avian, amphibian and mammalian sensory hair cell-containing tissues and antibodies were screened for selective binding to cochlear extracts in ELISA. In the inner ear, KHRI-4 and KHRI-5 bind specifically to stereocilia in both avian and mammalian cochlear and vestibular tissue preparations using immunofluorescence and immunoperoxidase assays. In other tissues only certain cells of mesothelial origin, such as smooth muscle in gut and the arteriolar vasculature, were stained by KHRI-4 indicating that the antigenic structure defined by this antibody has limited distribution. KHRI-5 binding could be detected in other tissues only at high antibody concentrations suggesting that the gene product identified by this antibody is also weakly expressed in other cell lineages. Western blot analysis showed that KHRI-4 and -5 detect different protein complexes. KHRI-4 identifies an antigenic structure common to gut, cochlea, vestibular tissue and cultured fibroblasts consisting of a approximately 195 and a 230 kDa heterodimer designated p195/230. KHRI-5 binds to a prominent approximately 200-210 kDa band in Western blots of cochlear tissues, gut and fibroblasts. In immunoprecipitation experiments, KHRI-5 precipitated three proteins of Mr approximately 200-210, 230 and 260 kDa indicating that the approximately 200-210 kDa protein carrying the epitope for this antibody is a member of a heterotrimer complex. Our results show that these protein complexes are structural components of stereocilia and that the same proteins are arrayed in conjunction with the actin stress fibers of cultured mesothelial cells. Thus, they are likely to be important for maintaining the actin structure of stereocilia essential to transduction in sensory hair cells.

Protein phosphorylation in the organ of Corti: differential regulation by second messengers between base and apex

Major aspects of cellular physiology are regulated by the phosphorylation state of proteins through the action of protein kinases and protein phosphatases. Phosphorylation of proteins by endogenous protein kinase activity was assayed in homogenates from guinea pig inner ear tissues with [gamma-32P] ATP. Phosphoproteins showed distinct distributions in organ of Corti, lateral wall and spiral ganglion. In the organ of Corti, several protein kinase activities were distinguished by their activation by appropriate agonists: protein kinase C, calmodulin-dependent protein kinases and cyclic nucleotide-dependent protein kinases. Twelve putative substrates for these kinases were identified in organ of Corti on the basis of increased 32P-incorporation with addition of lipids, calmodulin, and cyclic nucleotides, respectively. In addition, differences in phosphorylation were observed between the base and apex of the organ of Corti. 32P-incorporation into proteins of molecular weights between 45 and 100 kDa was significantly higher in apical tissue than in tissue from the base. In contrast, phosphate incorporation into proteins of around 29 kDa was much lower in apical tissues than in basal tissues. Furthermore, labeling of both the high and low molecular weight proteins from the apex but not the base markedly increased in response to calcium. These data indicate the presence of differential modes of regulation that may underlie structural and functional gradients along the sensory epithelium.

A nicotinic acetylcholine receptor-like alpha-bungarotoxin-binding site on outer hair cells

Acetylcholine (ACh) appears to be the major neurotransmitter liberated from olivocochlear efferents terminating on outer hair cells (OHC). Recently, cholinergic receptor epitopes were visualized at the basal pole of the OHCs. To evaluate the ACh receptor type at OHC we performed binding studies with [125I]-labelled alpha-bungarotoxin (alpha-bgtx), a close to irreversibly acting blocker of the nicotinic acetylcholine receptor (nAChR) of skeletal muscle and of electrocytes of Torpedo and Electrophorus. An irreversible and saturable binding (80 nM) of the radiolabelled compound to OHCs was observed. The number of alpha-bgtx sensitive binding sites present on each OHC was calculated to be about 2 X 10(-17) mol/OHC, which would amount to about 10(7) binding sites/cell. Preincubation with the reversibly acting cholinergic ligands, carbamylcholine (1 mM), nicotine (0.1 mM) and d-tubocurarine (1-100 microM) was found to inhibit alpha-bgtx binding to a varying degree. Atropine (0.05 mM), a muscarinic antagonist, had no influence on the binding of alpha-bgtx to OHCs. [3H]-QNB, a specific marker and antagonist for muscarinic AChR, and [125I]-kappa-toxin, known to react with neuronal and ganglionic nAChR, showed no specific binding to OHCs. The data indicate that a peripheral type nAChR is present on OHCs mediating ACh-induced modulation of the biomechanics of the cochlea by influencing OHC motility.

Mechanically induced length changes of isolated outer hair cells are metabolically dependent

Isolated outer hair cells from the organ of Corti of the guinea pig have been shown to change length in response to a mechanical stimulus in the form of a tone burst at a fixed frequency of 200 Hz (Canlon et al., 1988). In the present study, the threshold of movement for individual outer hair cells is related to the original length of the cell such that long cells are more sensitive than short cells for all cochlear locations studied. Length changes could be elicited when the stimulus was projected at any site along the longitudinal axis of the plasma membrane. Length changes were not elicited when the stereocilia were stimulated directly. These mechanically-induced length changes were found to be metabolically dependent. In the presence of either sodium cyanide or 2,4-dinitrophenol, the motile response of outer hair cells was completely blocked within 30 min. When the extracellular pH was altered in a graded fashion, the motile response decreased gradually. Furthermore, 3 microM poly-L-lysine or poly-D-lysine of different molecular weights were also effective in blocking the motile response, whereas the negatively charged polyaminoacid, poly-L-aspartate, was not effective. Fluorescently-labelled poly-lysine demonstrated that the plasma membrane, stereocilia, and nucleus were the most intensely stained structures of the outer hair cells. It is suggested that the passive influx of poly-lysine is responsible for the inhibition of the motile response. Finally, the finding that the bidirectional motile response of isolated outer hair cells induced by mechanical stimulation is dependent on the metabolic state of the cell distinguishes this type of motility from the electrically induced outer hair cell shape changes.