Vestibular semicircular canal epithelium of the rat in culture on filter support: polarity and barrier properties

The inner ear of mammals contains the vestibular apparatus which is involved in the maintenance of posture and balance. The tubular structure of the apparatus is bathed by the potassium-rich endolymph and sodium-rich perilymph in the luminal and abluminal compartments, respectively. The luminal compartment is lined by a continuous epithelium with islets of receptor organs, which separates the luminal from the abluminal compartment. The present work focuses on the epithelium, without the receptor organs, and shows that it can be reconstituted in culture. The epithelium from 4-day-old Wistar rats was grown on microporous membranes. High transepithelial electrical resistances (4000-6000 Omega.cm2) were achieved after 4-8 days in culture. The epithelium was characterized by the presence of cytokeratin, ZO-1 protein, occludin, and the presence of tight junctions and kinocilia. The transepithelial resistance of the cell monolayer withstood endolymph/perilymph dual bathing when the apical pole of the cells was in contact with endolymph, but collapsed in the reverse configuration. Weak but statistically highly significant basal to apical rubidium (86Rb) transport was observed. These findings show that this epithelium maintains its in vivo polarity and could enhance the potassium composition of endolymph up to maturity. This new culture model, in which dual bathing is possible, should enable further in vitro studies of the sensory vestibular epithelia.

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.

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).

Histopathology of the peripheral vestibular system in small vestibular schwannomas

Gadolinium-enhanced magnetic resonance imaging can be used to detect small vestibular schwannomas/acoustic neuromas. Early detection raises the question of the necessity of their surgical removal. Do all tumors induce lesions in the vestibule and to what extent? We thus investigated the ultrastructure of peripheral vestibular systems in grades I and II schwannomas. Vestibular tissues were fixed as soon as they were removed during the resection of tumors, by the translabyrinthine approach, and then processed for transmission electron microscope observations. In neurosensory epithelia, hair cells lost stereocilia, whereas cuticular plates disaggregated. The cytoplasm of hair cells degenerated in either a dense or vacuolated manner, and cytoplasmic blisters extended into the endolymph. In some cases, supporting cells extended processes covering the apical surface of hair cells. Nerve fibers massively disappeared from epithelia, only few nerve fibers contacted type I and type II hair cells, and both afferent and efferent terminals were abnormal. In vestibular nerves, axons degenerated, and myelin sheaths disaggregated. Glycogen was present in both intracellular and extracellular spaces. Luse bodies associated with collagen bundles were found between fibers. Scarpa ganglion neurons contained lysosomes/lipofuscin granules and vacuoles. Tumor cells were found in both the ganglion and the vestibular nerve. Thus small tumors induce extensive degeneration of vestibular tissues. The various hallmarks of schwannomas are already present in small acoustic neuromas. Moreover, different types of degeneration of hair cells and neurons were observed, together with the covering of hair cells by supporting cells and the accumulation of glycogen in the vestibular nerve.

Effects of ototoxins on quinuclidinyl benzylate binding in the rat cochlea

Ototoxins inhibit the muscarinic receptor-activated inositol phosphate synthesis in the rat cochlea. In order to study this inhibitory mechanism, we investigated the effects of the ototoxins ethacrynate, cisplatin, HgCl2 and neomycin on [3H]quinuclidinyl benzylate binding to muscarinic receptors in adult and 12-day-old rat cochleas. The results are similar whatever the age: at concentrations that inhibit the inositol phosphate synthesis, ethacrynate is without effect. Neomycin only reduces [3H]quinuclidinyl benzylate binding at concentrations in the millimolar range. Cisplatin and HgCl2 block the binding in a dose-dependent way. These results suggest that the block of the transduction system by cisplatin and HgCl2 is due to direct interactions with muscarinic binding sites. Moreover, considering these data together with previous results, ethacrynate and neomycin may affect the phosphoinositide signalling pathway at targets including phosphoinositides and G proteins.

Characterization of muscarinic binding sites in the adult and developing rat cochlea

The maturation of the cholinergic innervation of the rat cochlea is associated with a transient increase in the muscarinic-receptor activated inositol phosphate synthesis. In order to investigate the mechanisms involved in this transient enhancement of the inositol phosphate response, the binding properties of the cochlear muscarinic receptors were studied during rat cochlear development. Incubating the membranes from 4-day-old, 12-day-old and adult cochleas with [3H]quinuclidinyl benzylate indicates that their respective, mean concentrations of cholinoceptors are 454 +/- 51 (+/- SEM), 39 +/- 2 and 42 +/- 3 fmol/mg of protein. The dissociation constants at equilibrium are 207 +/- 80, 42 +/- 7 and 28 +/- 3 pM for the binding sites of the 4-day-old, 12-day-old and adult cochleas, respectively. Pharmacological characterization of the binding, using selective antagonists, shows that M3 cholinoceptors are expressed in developing and adult cochleas. The data demonstrate that changes in muscarinic receptor affinity and number do not correlate with the previously observed peak of the inositol phosphate metabolism. The transient enhanced inositol phosphate response is therefore not due to changes in cholinoceptors, but probably due to alterations involving the intrinsic activity of the phospholipase C and/or the efficacy of coupling of the transduction system.

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.

Anticholinergic effects of strychnine in the cochlea do not involve muscarinic receptors

Central control of cochlear function is mediated by the cholinergic (medial) efferent system and both muscarinic and nicotinic acetylcholine receptors are thought to be present on outer hair cells. All the physiological effects of acetylcholine in the cochlea are blocked by strychnine and we therefore investigated whether strychnine interacts with muscarinic receptors in the cochlea. The effects of strychnine on both (3H)-quinuclidinyl benzylate binding and atropine sensitive carbachol-induced (3H)-inositol phosphate formation were examined. Strychnine (1 to 50 microM) has no effect on either quinuclidinyl benzylate binding or carbachol (1 mM)-induced inositol phosphate synthesis. Moreover, strychnine does not change basal inositol phosphate metabolism. These data indicate that muscarinic receptors are not sensitive to strychnine at concentrations which are known to block the effects of acetylcholine on outer hair cells.

Sulphhydryl-modifying reagents alter ototoxin block of muscarinic receptor-linked phosphoinositide turnover in the cochlea

In the 12-day-old rat cochlea, the synthesis of inositol phosphates (IPs) can be activated via M3 cholinoceptors. This stimulation is blocked by ototoxins (mercury, ethacrynate, cisplatin, neomycin), drugs with side effects that lead to damage of hair cells and strial cells. As these toxic effects can be reversed in vivo by thiol molecules, we investigated whether modifications of thiol compounds could be involved in ototoxin-induced inhibition of the IP turnover in the cochlea. For this purpose, we assessed whether the sulphhydryl-modifying reagents N-ethylmaleimide and cadmium modify the carbachol-stimulated formation of IPs in the 12-day-old rat cochlea. Both molecules inhibit the carbachol effect on a dose-dependent way without altering the basal metabolism of IPs. As cadmium may block some calcium channels, the effect of verapamil, another calcium channel antagonist, was tested. Verapamil (1-50 microM) does not alter carbachol-evoked IP formation, suggesting that the inhibitory effect of cadmium is not due to a calcium influx block. Binding experiments with the muscarinic ligand quinuclidinyl benzylate (QNB) showed that the sulphhydryl-modifying reagents do not displace QNB from binding sites. Combining ototoxins and reagents shows that N-ethylmaleimide acts synergistically with all ototoxins but ethacrynate while cadmium does so only with mercury. Both N-ethylmaleimide and cadmium have additive effects with ethacrynate. As a supplement, disulphide bond-modifying agents do not alter the carbachol-enhanced metabolism of IPs. These results suggest that molecules having thiol-modifying properties inhibit the carbachol-induced turnover of IPs without acting at the muscarinic sites.(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.

Activation of muscarinic cholinergic receptors stimulates inositol phosphates synthesis in the developing avian cochlear duct

We previously reported that the inositol phosphates (IPs) synthesis is induced by muscarinic agonists in the rat cochlea and that this stimulation is maximal at postnatal day 12. This peak response is concomitant with the onset of the efferent synaptogenesis at the outer hair cell level. Whether the correlation between this neuronal plasticity and the enhanced IPs formation is unique to the rat or a general feature of the developing vertebrate cochlea is not known. To examine this question, we measured, in the presence of LiCl, the accumulation of (3H)-IPs induced by carbachol, in the developing chick cochlear duct during a period ranging from embryonic day (E) 8 to post-hatching day (P) 20. Carbachol (1 mM) causes a significant increase of IPs formation relative to basal values at all ages. This IPs accumulation is maximal at E8 (1854% of the basal level), then, rapidly decreases until P13 when it reaches a steady-state level of 294% of the basal level. Strikingly, this gradual decline in IPs formation is interrupted between E15 and E19, by a transient increase in IPs synthesis. This rise peaks at E16 with a stimulation value of 757% of the control level. This maximal stimulation is inhibited by atropine in a dose-dependent manner, as is the case at E9, suggesting the involvement of muscarinic receptors. Interestingly, the occurrence of the peak response is concomitant with the plastic events associated with the maturation of the efferent innervation of the cochlear duct. Thus, these results suggest that there may be a correlation between cochlear plasticity and enhanced IPs synthesis, which is not species-specific.(ABSTRACT TRUNCATED AT 250 WORDS)

Appearance and distribution of the 275 kD hair-cell antigen during development of the avian inner ear

The 275 kD hair-cell antigen (HCA) is a protein that was originally identified using immunological techniques in the inner ears of early hatchling and adult chickens. The HCA is specifically associated with the apical surface of sensory hair cells; in the vestibular system the antigen is distributed over the entire stereocilia bundle, but in the auditory system it only extends a short distance up the shafts of the stereocilia. The objectives of this study were to ascertain when the HCA is first expressed during inner ear development, to compare the temporal and spatial patterns of HCA expression with those of neurite ingrowth, and to determine how the distribution of the antigen observed in the auditory system arises during development. Serial sections of otocysts from embryonic day (ED) 4 to ED7.5 (stages 24 to 32) were stained with a monoclonal antibody to the HCA and polyclonal antibodies to the neuron-glial cell adhesion molecule in order to analyse patterns of HCA expression and neurite ingrowth. Nerve fibres are first observed in the anterior pole of the otocyst at ED4.5 (stage 24), and in the evaginating basilar papilla by ED5 (stage 26). The HCA first appears within the vestibular system in the anterior pole of the otocyst at ED5 (stage 26), and within the auditory system in the distal end of the basilar papilla at ED6.5 (stage 29). Serial section analysis indicates that expression of the HCA is always limited to areas of the epithelium where nerve fibres are found, although the delay between the onset of innervation and the onset of HCA expression varies from one region of the otocyst to another. The growth of stereocilia bundles in the auditory system was studied from ED10 to 2 days after hatching in sections from the medial to distal regions of the basilar papilla double labelled with rhodamine phalloidin and monoclonal anti-HCA. At ED12 the stereocilia bundles are 1.7 microns high and the staining observed with both phalloidin and the antibody extend to the same maximum height above the apical surface of the hair cell. The maximum height that anti-HCA staining extends up the stereocilia bundle remains almost constant between ED12 and postnatal day 2, but between ED15 and ED18 the stereocilia bundle grows rapidly in height, with a membrane domain lacking the HCA forming at the distal ends of the stereocilia.(ABSTRACT TRUNCATED AT 400 WORDS)

Angiotensin receptors from rat liver, brain and pituitary gland. Expression of two subtypes in Xenopus oocytes

Xenopus laevis oocytes were used to express angiotensin receptors encoded by mRNAs extracted from rat liver, adenohypophysis and brain. Groups of ten mRNA-injected oocytes were loaded with 45Ca2+ and the responsiveness to angiotensin II (A II) and related molecules tested by monitoring 45Ca2+ outflux. A II and angiotensin III (A III) induced a marked and transient increase in 45Ca2+ outflux from mRNA, but not from control, water-injected, oocytes. The increase over basal value of 45Ca2+ outflux during a 5 min application period of A II or A III was used as a response index. Observed responses were of high magnitude, reproducible and dose-dependent. For these reasons, mRNA-injected oocytes constitute a valuable system for investigating the pharmacological properties of angiotensin receptors from tissues of different origin under experimental conditions which eliminate tissue-specific interference which might be encountered in classical binding studies on acellular preparations. We demonstrate a fairly good parallelism between the relative potencies of A I, A II and A III in eliciting an increase in 45Ca2+ outflux from liver and adenohypophyseal mRNA-injected oocytes and the relative affinities of these peptides for binding to liver or adenohypophyseal membranes (A II greater than A III much greater than A I). The predominant receptor subtype expressed by brain mRNA discriminated very poorly between A II and A III, whereas angiotensin receptors expressed by liver or adenohypophyseal mRNA discriminated between AII and AIII very efficiently.

Carbachol-induced inositol phosphate formation during rat cochlea development

The age related-intensity developmental pattern of the phosphoinositide breakdown, which leads to the formation of intracellular second messengers, was investigated in rat cochleas by measuring the accumulation of inositol phosphates induced by carbachol in the presence of LiCl. The accumulation of the phosphoinositide metabolites elicited by this muscarinic agonist is very low at post-natal day 1 and particularly large during the period between post-natal days 8 and 14 with a peak around day 12. In the 25-day-old rat cochlea, carbachol induced a 2-fold increase in inositol phosphates (IPs) accumulation, with respect to the basal control level. The apparent affinities of the carbachol-induced IPs responses are 49.6, 31.6 and 36.7 microM in cochleas of 12-, 16- and 25-day-old rats, respectively, thus suggesting that the specific developmental changes are rather due to a modification in the number of muscarinic cholinergic receptors than to alterations of the apparent affinity of carbachol for its receptors. This developmental pattern of carbachol-elicited IPs accumulation reveals a striking time coincidence with both the efferent synaptogenesis at the outer hair cells (OHCs) level and the period of increased sensitivity of OHCs to aminoglycoside toxicity. Phosphoinositide breakdown may, consequently, play a role in the maturation of OHCs and their efferent supply. In addition, the remaining IPs response measured at 25 post-natal days indicates that muscarinic agonist-mediated IPs metabolism also occurs in mature cochlea, and might be involved in the regulation of OHCs motility.

A M3 muscarinic receptor coupled to inositol phosphate formation in the rat cochlea?

Various neuroactive substances, including excitatory and inhibitory amino acids, biogenic amines and neuropeptides, were tested for their ability to stimulate the inositol phosphate (IPs) cascade in the presence of lithium in the rat cochlea. Among them, only the muscarinic agonists (carbachol and oxotremorine M) were able to stimulate the IPs formation in 12-day-old rat cochleas. The carbachol-elicited IPs formation was inhibited by muscarinic antagonists with the following relative order of potency: atropine greater than 4-DAMP much greater than pirenzepine greater than methoctramine = AF-DX 116. This pharmacological profile suggests that the activation of the M3 muscarinic receptor subtype is responsible for the increase in IPs synthesis in the rat cochlea. However, an interaction with a m5 receptor subtype could not be completely excluded. The unusual link of only one receptor subtype with the phosphoinositide breakdown in the cochlea, as opposed to the usual existence of several receptors coupled to this transduction system in other organs such as the brain, suggest a unique role for muscarinic agonists in the cochlea.

Identification of a 275-kD protein associated with the apical surfaces of sensory hair cells in the avian inner ear

Immunological techniques have been used to generate both polyclonal and monoclonal antibodies specific for the apical ends of sensory hair cells in the avian inner ear. The hair cell antigen recognized by these antibodies is soluble in nonionic detergent, behaves on sucrose gradients primarily as a 16S particle, and, after immunoprecipitation, migrates as a polypeptide with a relative molecular mass of 275 kD on 5% SDS gels under reducing conditions. The antigen can be detected with scanning immunoelectron microscopy on the apical surface of the cell and on the stereocilia bundle but not on the kinocilium. Double label studies indicate that the entire stereocilia bundle is stained in the lagena macula (a vestibular organ), whereas in the basilar papilla (an auditory organ) only the proximal region of the stereocilia bundle nearest to the apical surface is stained. The monoclonal anti-hair cell antibodies do not stain brain, tongue, lung, liver, heart, crop, gizzard, small intestine, skeletal muscle, feather, skin, or eye tissues but do specifically stain renal corpuscles in the kidney. Experiments using organotypic cultures of the embryonic lagena macula indicate that the antibodies cause a significant increase in the steady-state stiffness of the stereocilia bundle but do not inhibit mechanotransduction. The antibodies should provide a suitable marker and/or tool for the purification of the apical sensory membrane of the hair cell.