Cisplatin blocks depolarization-induced calcium entry in isolated cochlear outer hair cells

Cisplatin modulates voltage gated channel currents of dorsal root ganglion neurons of rats

The anticancer drug cis-diammindichloroplatin (CDDP, cisplatin) causes severe side effects like peripheral sensitive neuropathy. The toxicity of CDDP has been linked to changes in intracellular calcium homeostasis ([Ca2+]i). Voltage activated calcium channel currents (ICa(V)) are important for the regulation of [Ca2+]i; therefore, this study was designed to examine the effect of CDDP on ICa(V) in comparison to voltage activated potassium (IK(V)) and sodium (INa(V)) channel currents using the whole cell patch clamp method on dorsal root ganglion neurons of rats. In small neurons (<or=Ø20 microm) CDDP reduced peak and sustained ICa(V) concentration dependently (1-100 microM). The IC50 was 23.9+/-4.5 microM (+/-S.D.) for the peak current with a Hill-coefficient of 0.6+/-0.1 and 38.8+/-6.1 microM for the sustained current (Hill-coefficient: 0.7+/-0.1). IK(V) were reduced by 20.9+/-4.8% (10 microM) and INa(V) were only reduced by 9.2%+/-7.2% (10 microM). ICa(V) of large neurons (>or=Ø25 microm) were less sensitive to CDDP. The peak ICa(V) was reduced by 14.1+/-2.3% and IK(V) by 12.8+/-3.4% (100 microM). The sensitivity of INa(V) in large neurons to CDDP was not different compared to small neurons. We conclude that the reduction of ICa(V) in small cells may be responsible for the neurotoxic side effects CDDP causes in sensory neurons.

Protective effect of T-type calcium channel blocker flunarizine on cisplatin-induced death of auditory cells

Changes in intracellular Ca2+ level are involved in a number of intracellular events, including triggering of apoptosis. The role of intracellular calcium mobilization in cisplatin-induced hair cell death, however, is still unknown. In this study, the effect of calcium channel blocker flunarizine (Sibelium), which is used to prescribe for vertigo and tinnitus, on cisplatin-induced hair cell death was investigated in a cochlear organ of Corti-derived cell line, HEI-OC1, and the neonatal (P2) rat organ of Corti explant. Cisplatin induced apoptotic cell death showing nuclear fragmentation, DNA ladder, and TUNEL positive in both HEI-OC1 and primary organ of Corti explant. Flunarizine significantly inhibited the cisplatin-induced apoptosis. Unexpectedly, flunarizine increased the intracellular calcium ([Ca2+]i) levels of HEI-OC1. However, the protective effect of flunarizine against cisplatin was not mediated by modulation of intracellular calcium level. Treatment of cisplatin resulted in ROS generation and lipid peroxidation in HEI-OC1. Flunarizine did not attenuate ROS production but inhibited lipid peroxidation and mitochondrial permeability transition in cisplatin-treated cells. This result suggests that the protective mechanism of flunarizine on cisplatin-induced cytotoxicity is associated with direct inhibition of lipid peroxidation and mitochondrial permeability transition.

Carboplatin-induced early cochlear lesion in chinchillas

Carboplatin preferentially damages inner hair cells (IHC) and type I spiral ganglion neurons (SGNs) in the chinchilla; however, the temporal sequence of events leading to the destruction of these structures is poorly understood. To better understand the mechanisms leading up to the destruction of IHCs and type I SGNs, we measured the activity in single auditory nerve fibers for the first 8 h following carboplatin treatment and also monitored the development of histopathologies in SGNs and IHCs using a dose of carboplatin that killed approximately 50% of the IHCs. The spontaneous discharge rate (SDR) showed a slight increase around 3 h post carboplatin followed by a significant decline at 4-5 h. The saturation driven discharge rate (DDR) showed a significant increase 1-5 h post carboplatin. These physiological changes were associated with the formation of small vacuoles in type I afferent terminals and proximal nerve fibers 1-6 h post carboplatin; signs of IHC damage were first observed around 24-48 h. Thus, the neurotoxic effects of carboplatin occur approximately a day before the IHCs are damaged. The large fluctuations in SDR and DDR that occur several hours after carboplatin treatment are most likely due to the neurotoxic effects of carboplatin.

Perilymphatic application of cisplatin over several days in albino guinea pigs: dose-dependency of electrophysiological and morphological effects

Cisplatin, at 0, 3, 30 or 300 microg/ml in saline, was applied to the scala tympani of the cochlea of guinea pigs via osmotic mini-pumps, operating at a pump rate of 0.5 microl/h. Electrocochleographic recordings were made from an implanted round window electrode. When an electrocochleographic criterion of ototoxicity was reached (40 dB loss in compound action potential (CAP) threshold at 8 kHz), or after 1 week if this criterion was not reached, the animals were sacrificed for light microscopy. A subgroup of animals had endocochlear potentials (EPs) measured prior to sacrifice. Hearing remained stable in the 0 microg/ml control group, but a sudden drop of auditory sensitivity across the whole frequency range was observed in all other groups. It took 1-5 days before the drop occurred, dependent on cisplatin concentration. CAP and cochlear microphonics were lost simultaneously. The EP was severely depressed in the affected animals, suggesting that cisplatin effects on the EP are primary. However, histology revealed an accompanying loss of outer hair cells, primarily in the basal turn. It is concluded that if cisplatin is given until ototoxicity becomes apparent electrophysiologically, then the cochlear pathology from intrascalar cisplatin administration resembles that from daily parenteral administration at 1.5-2.0 mg/kg. The cochlear pathology from the parenteral treatment was greater than that observed with 30 microg/ml pumps, and less than that from 300 microg/ml pumps.

Cisplatin-induced apoptotic cell death in Mongolian gerbil cochlea

Cisplatin is well known to cause cochleotoxicity. In order to determine the underlying mechanisms of cisplatin-induced cell death in the cochlea, we investigated the apoptotic changes and the expression of bcl-2 family proteins controlling apoptosis. Mongolian gerbils were administered 4 mg/kg/day cisplatin consecutively for 5 days. The cisplatin-treated animals showed a significant deterioration in the responses of both distortion product otoacoustic emissions and the endocochlear potential as compared with those of the age-matched controls, suggesting outer hair cell and stria vascularis dysfunction. The presence of DNA fragmentation revealed by a terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling method was recognized in the organ of Corti, the spiral ganglion, and the stria vascularis in the cisplatin-treated animals whereas almost negative results were obtained in the control animals. The nuclear morphology obtained by Hoechst 33342 staining revealed pyknotic and condensed nuclei, confirming the presence of the characteristic features of apoptosis. A significant increase and reduction in the number of bax- and bcl-2-positive cells, respectively, following cisplatin treatment was observed in the cells of the organ of Corti, the spiral ganglion, and the lateral wall. These findings suggest a critical role for bcl-2 family proteins in the regulation of apoptotic cell death induced by cisplatin. The underlying mechanisms of the cisplatin-induced cell death are discussed.

Global problem of drug-induced hearing loss

Clinically used drugs and chemical agents may potentially cause adverse effects to the human auditory and vestibular systems. Many of them, such as aminoglycosides and cisplatin, can play a critical role in the treatment of serious or life-threatening diseases; others, like loop diuretics or salycilates, offer such important therapeutical effects compared to the ototoxic side effects that the ototoxicity risk can be considered to be of minor importance. The problem of ototoxic side effects is more acute in developing countries, where highly effective and low-cost drugs are more easily prescribed without adequate monitoring. Medical awareness of doses, forms of administration, populations at risk, and possible synergism is necessary in order to develop appropriate care in the prescription of drugs with ototoxic side effects. Relatively recent issues such as risk-benefit analysis, patient-informed consent, and quality-of-life considerations, particularly when life expectancy can be low, are also to be considered. At present, a uniform method of monitoring for all potentially ototoxic therapeutics does not seem reasonable or practical. It is recommended, however, that individual auditory function be noted for a particular drug being employed. Protocols and exams should be easy, quick, sensitive, reliable, and as objective as possible. Benefits of audiological monitoring include the opportunity to change the patient's treatment course, improvement of patient and family awareness of the impact of hearing impairment, and timely prescription of amplification devices. Finally, particular attention should be paid to high-risk populations such as neonatal intensive care unit patients.

The electroreceptor organ of the catfish, Ictalurus melas, as a model for cisplatin-induced ototoxicity

The ototoxic side-effects of the anti-cancer drug cisplatin (cis-diaminedichloroplatinum) have been widely investigated. However, the exact site of action remains unclear. In this study, the electroreceptor organ of the freshwater catfish Ictalurus melas is used as a model for examining the acute effects of cisplatin. The sensory cells in the electroreceptor organ are homologous to the inner hair cells in the cochlea of mammals. The effects of cisplatin administration can be investigated by in vivo recording of the spike trains from the electroreceptor organ primary afferents. Exposure of electroreceptor organs to 330 microM cisplatin for 1 h causes the spontaneous activity to drop, the overall sensitivity to diminish and the shape of the frequency characteristics to change. These effects persist in the week after administration. Control levels have returned at day 22. These results demonstrate an acute and, with considerable hysteresis, reversible cisplatin effect on the electroreceptor organs, which is to a large extent consistent with the cisplatin-induced effects in isolated hair cells in mammals. The time-course of the effect supports the hypotheses that ion channels are blocked immediately by cisplatin administration, and that cisplatin metabolites disturb enzymatic cellular processes.

Effect of 4-methylthiobenzoic acid on cisplatin-induced ototoxicity in the rat

The purpose of this study was to investigate the effectiveness of 4-methylthiobenzoic acid (MTBA) as a protection agent against cisplatin (CDDP)-induced changes in organ of Corti surface structure, compared to electrophysiological changes. Electrophysiological change was assessed using auditory brainstem response (ABR) and morphological changes were assessed using scanning electron microscopy (SEM). Male Wistar rats underwent pre-treatment ABRs in response to clicks, and tone bursts at 2, 4, 8, 16, and 32 kHz. The three groups of rats were injected as follows: (1) MTBA (250 mg/kg, i.p.), (2) CDDP (16 mg/kg, i.p.), (3) CDDP+MTBA (16 mg/kg, i.p. + 250 mg/kg, i.p.). Post-treatment ABRs were performed 3 days after drug administration and rats were sacrificed. Their cochleae were harvested and SEM was used to examine the surface of the organ of Corti, specifically the number of inner hair cells (IHCs) and outer hair cells (OHCs) in the apical, middle and basal turns of the cochlea. Animal weight was measured on the first and final days. There was a good correlation between ABR threshold changes and hair cell loss in the high frequency region of the cochlea (basal turn), while threshold changes in the lower test frequencies (middle turn) appeared to be the result of more subtle changes in the cochlea. MTBA provided effective protection against cisplatin-induced ABR threshold changes at all test frequencies as well as hair cell loss. MTBA also protected against body weight loss.

Experimental model of cisplatin ototoxicity in chinchillas

Cisplatin is an important antineoplastic agent. Its ototoxicity has been well defined, both in human and animal studies. However, animal models of systemic cisplatin administration have been complicated by multiple toxic effects. We studied cisplatin ototoxicity in an animal model involving topical application of cisplatin to the round-window membrane. Adult chinchillas were anesthetized with ketamine and pentobarbital, and auditory function was tested with the use of auditory brain-stem responses to various stimuli (clicks and 8-and 16-kHz tone bursts). Each animal was used as its own control. The middle-ear cavity was exposed through the bulla. In the experimental ear, a 25-microl solution of 0.25 mg cisplatin/1.0 ml normal saline solution was applied to the round-window membrane. In the control ear, 25 microl normal saline solution was applied to the round-window membrane. Follow-up auditory brain-stem response testing was conducted 7 days after treatment. A significant increase in threshold in the experimental ears was seen on comparison with the control ears. This finding suggests that application of cisplatin to the round-window membrane is a useful animal model in which to study cisplatin ototoxicity.

Aminoglycoside ototoxicity and the medial efferent system: II. Comparison of acute effects of different antibiotics

Gentamicin (GM) has been shown to reversibly reduce the ability of contralateral noise to suppress ipsilateral cochlear activity, in a dose-dependent manner. However, during chronic administration of lower doses (60 mg/kg) the involvement of medial efferents could not be demonstrated. The purposes of the present study were to determine whether other aminoglycosides would display the same acute effects as GM and whether there was any correlation between their specificity and degree of cochlear and vestibular toxicity and their potency of blockade of the medial efferent system. Thus, we observed changes in ipsilateral ensemble background activity (EBA) of the VIIIth nerve without and with contralateral low level (55 dB SPL) broadband noise stimulation, in awake guinea pigs (GPs), before and after one single high-dose intramuscular injection of different aminoglycoside antibiotics (AAs) (gentamicin, amikacin, neomycin, netilmicin, streptomycin, tobramycin). For comparison, the effects of strychnine, a known antagonist of the efferent transmission and of cisplatin, an antineoplastic agent with cochleotoxic properties were also studied. Netilmicin displayed blocking properties similar to GM, although less pronounced, while amikacin and neomycin had no effect on medial efferent function. With tobramycin and streptomycin a decrease in suppression was usually associated with a reduction of the EBA measured without acoustic stimulation. However, with cisplatin, suppression was still effective when EBA was severely decreased. We could not observe specific effects of strychnine on medial efferent function. In conclusion, no correlation was found between specificity and degree of AA ototoxicity and their action on the medial efferent system.

Co-administration of the neurotrophic ACTH(4-9) analogue, ORG 2766, may reduce the cochleotoxic effects of cisplatin

In this study the effect of the neurotrophic ACTH(4-9) analogue, ORG 2766, on cisplatin cochleotoxicity was investigated with both light- and transmission electron microscopy. Guinea pigs were treated with either cisplatin+ORG 2766 (n = 11) or cisplatin + physiological saline (n = 9). All animals treated with cisplatin + physiological saline showed complete loss of outer hair cells (OHC) and degeneration of the organ of Corti in the basal cochlear turns, while partial OHC loss was found in the middle and apical turns. The inner hair cells (IHC) and other cochlear tissues were not affected. Eight animals from the group treated with cisplatin + ORG 2766 demonstrated similar pathological changes, but to a lesser degree, especially in the middle turns. The three remaining animals demonstrated no cochlear alterations at all, light-microscopically, and only minor subcellular changes in the OHCs at the ultrastructural level. Electrophysiologically, these three animals showed normals compound action potential (CAP) amplitudes at stimulus frequencies from 0.5 to 16 kHz and normal cochlear microphonics (CM) in the frequency range from 0.5 to 8 kHz. The other animals treated with cisplatin + ORG 2766 showed a severe loss in their CAPs and CM, except for one showing intermediate loss. All animals from the group treated with cisplatin alone showed a severe loss in their CAPs and CM. Endolymphatic hydrops was present in all animals from the cisplatin- and the cisplatin + ORG 2766-treated groups. These data indicate that daily, concomitant administration of ORG 2766 may reduce OHC loss and subsequent degeneration of the organ of Corti in cisplatin-treated guinea pig cochleas.

Similar pharmacokinetics and differential ototoxicity after administration with cisplatin and transplatin in guinea pigs

Transplatin is a transisomer of cisplatin. Although cisplatin exhibits strong ototoxicity, there is no report concerning the ototoxicity of transplatin. To evaluate differences in pharmacokinetics and ototoxicity, cisplatin (7.5 mg/kg) and transplatin (30 mg/kg) were administered to guinea pigs twice at an interval of 5 days. The N1 threshold of the compound action potential was significantly elevated after administration of cisplatin. Cochleogram of the cisplatin-treated group showed severe losses of outer hair cells essentially at the basal and second turns. Transplatin, however, did not induce any detectable functional or morphological changes. Furthermore, blood urea nitrogen and creatinine levels in serum were not elevated after administration of transplatin, whereas cisplatin showed strong nephrotoxicity. The serum and perilymphatic concentrations of platinum up to 24 h after administering an equimolar dose of cis- or transplatin (7.5 mg/kg) were almost similar. As has been reported by many investigators, transplatin has no anti-tumor activity because stereochemical limitations preclude transplatin from forming intra- and interstrand closs-links with nuclear or mitochondrial DNA. From these results, it was concluded that the stereochemical structure of the platinum compound and steric interaction with target molecules such as mitochondrial DNA in the cochlear outer hair cells might be important to the ototoxic mechanism of platinum compounds.

Cisplatin metabolites and their toxicity on isolated cochlear outer hair cells in vitro

The exact mechanism of ototoxicity by cisplatin remains obscure. In the present study, the toxicity of cisplatin metabolites was evaluated using isolated outer hair cells of guinea pig cochlea. Cisplatin metabolites were produced by reaction of cisplatin and S9 fraction from guinea pig liver after administration with phenobarbital. The viability of the outer hair cells did not decrease after the incubation in non-metabolized cisplatin solutions or S9 fraction only, demonstrating the absence of toxic effect from non-metabolized cisplatin and the absence of intrinsic toxic substances in S9 fraction. However, metabolized cisplatin reacted with S9 fraction at 37 degrees C significantly decreased the viability. Cisplatin reacted with heat-treated S9 fraction did not produce a cytotoxic effect, suggesting an enzymatic basis for the formation of metabolites. When NADP was omitted, outer hair cells remained viable, suggesting the significant role of oxygenases in the metabolism of cisplatin. The existence of a toxic metabolites could account for the poor correlation between drug peak level and pathologic damage to the inner ear, and a delay in the onset of ototoxicity could represent the time necessary for induction of the metabolizing enzymes or for reaching a critical point in toxicity by metabolites.

Transplatin blocks voltage-dependent calcium current in isolated cochlear outer hair cells but is not ototoxic in vivo

The effects of cisplatin (cis-DDP) and transplatin (trans-DDP) on the voltage-dependent calcium current (ICa) of outer hair cells isolated from the guinea pig cochlea were compared using the whole-cell patch-clamp technique. Both cis-DDP and trans-DDP blocked ICa in a concentration-dependent manner. Trans-DDP did not show ototoxicity or nephrotoxicity in vivo. From these results, it was suggested that the effect of cis-DDP on ICa does not directly lead to cell death.

Comparative entry of carboplatin and sucrose in endolymph in the rat cochlea

The permeability of the perilymphatic-endolymphatic interface for carboplatin was determined after lateral cerebral ventricle infusion of radioactive carboplatin (cis-diamine[1,1-cyclobutane-1-14C-dicarboxylate]platinum) to rats. [14C]sucrose, a similar weight molecule was used for comparison of the kinetics in the inner ear fluids. 14C-radioactivity was measured in perilymph and endolymph. The rate of elimination of the tracers from perilymph was equivalent indicating no difference in transport across the blood-perilymph barrier. The transport from perilymph to endolymph was very restricted for both substances. The present study indicates that the ototoxic effect of carboplatin cannot be explained by a specific endolymphatic transport mechanism.

The effects of terbium on the accumulation of cisplatin in human ovarian cancer cells

In this investigation, we report a relationship between the terbium (Tb3+) binding protein and the accumulation of cisplatin in human ovarian cancer cells. The number of Tb3+ binding sites in cisplatin-resistant C13+ cells is significantly greater by 79% than those in cisplatin-sensitive 2008 cells. Exposure to Tb3+ also increased the cellular accumulation of cisplatin. The accumulation of cisplatin as a function of the Tb3+ concentration in the C13+ cells (0.85%/microM Tb3+) was significantly greater than the accumulation of cisplatin in 2008 cells with respect to Tb3+ (0.46%/microM Tb3+). The number of Tb3+ binding sites in revertant RH4 cells was similar to that in 2008 cells. The RH4 cells were less sensitive to the stimulatory effects of Tb3+ than the C13+ cells. Our results show that the Tb3+ binding protein correlates with cisplatin resistance, and the receptor binding of Tb3+ increases the accumulation of cisplatin in cisplatin-resistant cells.

Caffeine rapidly decreases potassium conductance of dissociated outer hair cells of guinea-pig cochlea

The effects of caffeine on the outer hair cells (OHCs) freshly dissociated from guinea-pig cochlea were investigated with the whole-cell patch-clamp technique, in both the conventional and the nystatin perforated patch-clamp configurations under voltage-clamp condition. Application of caffeine (> 1 mM for 10-30 s) induced an inward current (Icaffeine) with decrease of conductance in a dose-dependent manner at a holding potential (VH) of -60 mV. The reversal potential of Icaffeine (Ecaffeine) was close to the K+ equilibrium potential. The Icaffeine was not affected by Ca(2+)-free external solution. The internal perfusion of the Ca2+ chelator BAPTA had no effect on Icaffeine. The Icaffeine was not modulated by the external application of H-8 or staurosporine and by the internal perfusion of GDP-beta S. The amplitude of Icaffeine was the largest at the basal region of OHCs when caffeine was locally applied by the 'puffer' method. These results suggest that caffeine induces a decrease in membrane potassium conductance of the OHCs mainly at the basal region without mediating the intracellular signaling pathway.

Physiological and toxicological changes in the skin resulting from the action and interaction of metal ions

The human environment contains more than 50 metal or metalloid elements. At least 15 are recognized as trace elements, with zinc, calcium, copper, magnesium, and iron having specific roles in skin morphogenesis and function. The present review focuses on the presumed role of metal ions in the skin, their competition for carrier proteins, and membrane receptors. Evidence presented shows that the balance of trace metal ions is critical for normal skin and repair mechanisms following injury. Xenobiotic ions can impair this balance, leading to pathological change. The skin acts as an organ of elimination of excess trace metals and xenobiotic ions from the body, but mechanisms of voidance vary for different metals. Metal ions are an important cause of allergies, and evidence is presented to show that the majority of metals or metal compounds can induce allergic changes. Except for chromium and nickel, which are among the most common human allergens, animal models have provided little information. At least cadmium, thorium, lead, chromium, nickel, beryllium, and arsenic and proven or putative carcinogens in animals or humans on the basis of cytological or epidemiological evidence. However, only arsenic exhibits a clear predilection for the skin. Other metals such as gold can induce subcutaneous sarcoma following injection, but the relevance of this observation in terms of human occupational risk is discounted.

Cisplatin blocks voltage-dependent calcium current in dissociated outer hair cells of guinea-pig cochlea

The effect of the ototoxic molecule cisplatin (cis-DDP) on the voltage-dependent Ca2+ channel in dissociated outer hair cells (OHCs) of guinea-pig cochlea was investigated using a whole-cell patch-clamp technique. Cis-DDP had antagonistic effect on the Ca2+ channel and reversibly suppressed the Ca2+ current in a concentration-dependent manner. These results suggested that one of the ototoxic mechanisms of cis-DDP is involved in the inhibition of the Ca2+ channel in OHCs.

Cisplatin blocks mechano-electric transducer current in chick cochlear hair cells

The effects of cisplatin (cis-dichlorodiammine platinum II, CDDP) on the mechano-electrical transduction (MET) current were investigated with a whole-cell patch-electrode voltage clamp technique in dissociated cochlear hair cells of chicks. CDDP blocked the MET channel in a dose- and voltage-dependent manner. At -50 mV, CDDP blocked the MET channel with a Hill coefficient of approximately 2 and a dissociation constant (KD) of 1.5 x 10(-3) M. The kinetics of CDDP blockade consist of a voltage-independent and a voltage-dependent component.

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.