Disturbance of regulation of sodium by cis-diamminedichloroplatinum in perilymph of the guinea pig cochlea

Principles of calcite dissolution in human and artificial otoconia

Human otoconia provide mechanical stimuli to deflect hair cells of the vestibular sensory epithelium for purposes of detecting linear acceleration and head tilts. During lifetime, the volume and number of otoconia are gradually reduced. In a process of degeneration morphological changes occur. Structural changes in human otoconia are assumed to cause vertigo and balance disorders such as benign paroxysmal positional vertigo (BPPV). The aim of this study was to investigate the main principles of morphological changes in human otoconia in dissolution experiments by exposure to hydrochloric acid, EDTA, demineralized water and completely purified water respectively. For comparison reasons artificial (biomimetic) otoconia (calcite gelatin nanocomposits) and natural calcite were used. Morphological changes were detected in time steps by the use of environmental scanning electron microscopy (ESEM). Under in vitro conditions three main dissolution mechanisms were identified as causing characteristic morphological changes of the specimen under consideration: pH drops in the acidic range, complex formation with calcium ions and changes of ion concentrations in the vicinity of otoconia. Shifts in pH cause a more uniform reduction of otoconia size (isotropic dissolution) whereas complexation reactions and changes of the ionic concentrations within the surrounding medium bring about preferred attacks at specific areas (anisotropic dissolution) of human and artificial otoconia. Owing to successive reduction of material, all the dissolution mechanisms finally produce fragments and remnants of otoconia. It can be assumed that the organic component of otoconia is not significantly attacked under the given conditions. Artificial otoconia serve as a suitable model system mimicking chemical attacks on biogenic specimens. The underlying principles of calcite dissolution under in vitro conditions may play a role in otoconia degeneration processes such as BPPV.

The cochlear targets of cisplatin: an electrophysiological and morphological time-sequence study

Cisplatin ototoxicity has at least three major targets in the cochlea: the stria vascularis, the organ of Corti, and the spiral ganglion. This study aims to differentiate between these three targets. In particular, we address the question of whether the effects at the level of the organ of Corti and spiral ganglion are mutually dependent or whether they develop in parallel. This question was approached by studying the ototoxic effects while they develop electrophysiologically and comparing these to earlier presented histological data [Van Ruijven et al., 2004. Hear. Res. 197, 44-54]. Guinea pigs were treated with intraperitoneal injections of cisplatin at a dose of 2 mg/kg/day for either 4, 6, or 8 consecutive days. This time sequence has not revealed any evidence of one ototoxic process triggering another. Therefore, we have to stay with the conclusion of Van Ruijven et al. (2004) that both processes run in parallel.

Increases in spontaneous neural activity in the hamster dorsal cochlear nucleus following cisplatin treatment: a possible basis for cisplatin-induced tinnitus

Recent investigations in the hamster have implicated increased spontaneous activity (SA) in the dorsal cochlear nucleus (DCN) as a contributing factor in the etiology of tinnitus induced by intense sound exposure. It might therefore be expected that increased SA would also develop in the DCN of hamsters treated with cisplatin, another cause of tinnitus. We tested this hypothesis by measuring the effects of cisplatin on SA in the DCN. Adult hamsters were divided into three groups, each receiving five injections of cisplatin at one of the following doses: 3 mg/kg, 2.25 mg/kg, or 1.5 mg/kg. Each group had corresponding controls receiving injections of isotonic saline. The effects of cisplatin were studied electrophysiologically 1 month after treatment by recording multiunit SA on the surface of the DCN. Measurements of SA were obtained in three rows of 13-15 locations spaced roughly 100 microm apart and spanning the length of the DCN along the tonotopic axis. Effects of cisplatin were evaluated by comparing plots of mean SA vs. tonotopic locus for cisplatin-treated groups with those of their corresponding untreated control groups. The results demonstrated a consistently higher level of SA in cisplatin-treated groups than in untreated controls. Whereas the highest rates of mean SA in control groups were between 10 and 15 events/s, the highest mean spontaneous rates in cisplatin-treated groups were between 25 and 38 events/s. The cisplatin-induced hyperactivity was greatest in the medial half of the DCN corresponding to the high frequency portion of the tonotopic range. These results suggest that cisplatin treatment is an effective inducer of hyperactivity in the DCN. This hyperactivity may be an important neural correlate of cisplatin-induced tinnitus.

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.

Acute changes in cochlear potentials due to cisplatin

The purpose of this study was to investigate how the hair cells and stria vascularis are affected at the onset of cisplatin ototoxicity. The effects on the endocochlear potential (EP) and the cochlear microphonics (CM) were observed simultaneously in two groups of adult chinchillas receiving as follows: (1) 5 microl of cisplatin (1 mg/ml) in normal saline, and (2) 5 microl of normal saline on the round window. The EP and the CM were recorded for 12-14 h after cisplatin application, and morphological changes were assessed using scanning electron microscopy. Both the EP and the CM amplitude demonstrated a profound reduction, and a very strong correlation was observed between these two values during this time period. Although the reduction of the EP and the CM was observed by 12-14 h, only very slight degeneration of outer hair cells was seen at that time. These data suggested that a reduction of the EP which was caused by the alteration of the stria vascularis might be primarily responsible for very early changes in cochlear function after topical cisplatin application, while later changes were the direct result of hair cell damage.

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.

Calpain inhibitors protect auditory sensory cells from hypoxia and neurotrophin-withdrawal induced apoptosis

Inhibitors of calpain have been shown to protect nerve growth factor (NGF)-deprived ciliary ganglion neurons and hypoxic cortical neurons. Calpains have been identified in the cochlea and are active during ischemic injury. Since apoptosis can be initiated by loss of neurotrophic support, hypoxia, and ototoxins (e.g., cisplatin, CDDP), the role of calpain inhibitors under these conditions was examined in auditory hair cells and neurons. Dissociated spiral ganglion neuron (SGN) cell cultures and organ of Corti explants from P3 rats were used to test the efficacy of calpain inhibitors as otoprotective molecules. Our results indicate that calpain inhibitor I, calpain inhibitor II, and leupeptin all provided significant protection of SGNs against neurotrophin-withdrawal and hypoxia-induced apoptosis. The increase in neuronal survival ranged from 2.16 to 2.31 times greater than in untreated neurotrophin-withdrawn SGN cell cultures. BOC-Asp(Ome)-Fluoromethyl Ketone (B-D-FMK), a general caspase inhibitor, increased neuronal survival 2.16 times more. Neuronal survival rates were from 1.88 to 2.27 times greater than in untreated, hypoxic neurons and hair cell survival rates were from 1.98 to 2.03 times greater than untreated, hypoxic organ of Corti explants. However, protection of auditory hair cells and neurons from CDDP-induced damage (10 and 6 micrograms/ml, respectively) was limited with any of these calpain inhibitors. Apoptotic pathways initiated by neurotrophin-deprivation and ototoxic stress (e.g., CDDP) have been shown to be different. Our results agree with this finding, with neurotrophin-withdrawal and hypoxia, but not CDDP damage-induced apoptosis being calpain-dependent.

Protection and spontaneous recovery from cisplatin-induced hearing loss

Cisplatin [cis-diamminechloroplatinum(II)] has proved itself as a potent antineoplastic agent. However, nephrotoxicity, neurotoxicity, gastrointestinal toxicity, myelosuppression, and ototoxicity interfere with its therapeutical efficacy. Forced diuresis reduces nephrotoxicity, effectively leaving neurotoxicity and ototoxicity as the major side effects of concern, and gastrointestinal toxicity and myelosuppression as the secondary side effects. So far, attempts to reduce these side effects by developing equally potent platinum analogs have been unsuccessful. Some success has been achieved, however, by co-treatment with protective agents. Nearly all these agents are sulfur- or sulfhydryl-containing compounds (thio compounds), known as antioxidants and potent heavy metal chelators. These thio compounds may provide protection from cisplatin toxicity either (1) by direct interaction between the cisplatin and the thio moiety, (2) by displacing platinum from its site of toxic action, (3) by preventing platinum from interfering with superoxide dismutase, or (4) by scavenging of cisplatin-induced free radicals. In particular the first two protective mechanisms bear the risk of reducing the antineoplastic activity of cisplatin. Since nephrotoxicity can be controlled effectively by forced diuresis, a more specific approach of coping with ototoxicity by focusing on protection at the sensorineural level was chosen. Being familiar with the neuro-protective and neurotrophic properties of ACTH-related neuropeptides, specifically against cisplatin-induced peripheral neuropathies, it was judged expedient to test for a possible otoprotective action of these neuropeptides. The results were positive, although tainted with high interanimal variability. When testing for the possibility that the neuropeptides would merely delay cisplatin-induced ototoxicity rather than reduce it, it was discovered in control series without neuropeptide co-treatment that the ear can recover spontaneously from cisplatin-induced hearing loss. This was found both electrophysiologically and in outer hair cell (OHC) counts. Although these preliminary findings require further investigation, they strongly suggest that spontaneous recovery of cochlear injury can occur in the mature mammalian cochlea. Moreover, the otoprotective action of the ACTH-related neuropeptides suggests that it may be possible to stimulate recovery from acute hearing loss using neuropeptides.

A radical demise. Toxins and trauma share common pathways in hair cell death

The pathologic similarities noted after ototoxic and/or traumatic injury to the cochlea as well as the key features of the cochlea that make it susceptible to reactive oxygen species (ROS) damage are reviewed. Recent evidence linking ROS to cochlear damage associated with both ototoxins and/or trauma are presented. Mechanisms of generation of ROS in the cochlea and how these metabolites damage the cochlea and impair function are also reviewed. Finally, examples of novel therapeutic strategies to prevent and reverse hearing loss due to noise and/or ototoxins are presented to illustrate the clinical relevance of these new findings.

Cisplatin ototoxicity and the possibly protective effect of alpha-melanocyte stimulating hormone

It is known that adrenocorticotrophic hormone (ACTH)-derived peptides, the so-called melanocortins, can reduce cisplatin-induced neurotoxicity. Recently, our group has found that cisplatin-induced ototoxicity can also be reduced or prevented by treatment with the synthetic melanocortin-like peptide, ORG 2766 (Hamers et al., 1994; De Groot et al., 1997). The present study was designed to investigate the possibly ameliorating effects of the physiologically more relevant naturally occurring neuropeptide alpha-melanocyte stimulating hormone (alpha-MSH) upon cisplatin ototoxicity and to compare its protective effects to those of ORG 2766. For eight consecutive days guinea pigs were treated with cisplatin at a concentration of either 1.5 mg/kg/day or 2 mg/kg/day. Animals were co-treated with either alpha-MSH (75 microg/kg/day), ORG 2766 (75 microg/kg/day), or a sham injection containing physiological saline. Electrocochleography and hair cell counts were performed. Treatment with 1.5 mg/kg/day cisplatin resulted in a large variability of the morphological and electrophysiological data, a variability that might have masked possible effects of ORG 2766 and alpha-MSH. Treatment with 2 mg/kg/day cisplatin caused less variable, severe reductions in the compound action potentials and cochlear microphonics combined with basal and middle-turn outer hair cell loss in five out of six animals. However, in the alpha-MSH co-treated groups, two out of six animals could be classified as normal, two animals as moderately affected and two animals as severely affected. In the ORG 2766 co-treated group we found three animals that were not affected and three animals that were severely affected. We conclude that the protective effects of alpha-MSH and ORG 2766 co-treatment are comparable and that alpha-MSH might be clinically useful in protecting against cisplatin-induced ototoxicity.

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.

Ethics of cochlear implantation in young children: a review and reply from a Deaf-World perspective

This article examines ethical dilemmas related to cochlear implant surgery in children. These dilemmas arise from the existence of a linguistic and cultural minority called the Deaf World. Organizations of culturally Deaf adults in the United States and abroad, as well as the World Federation of the Deaf, have, on ethical grounds, strongly criticized the practice of cochlear implant surgery in children. Three ethical dilemmas are examined. (1) The surgery is of unproven value for the main significant benefit sought, language acquisition, whereas the psychological, social, and linguistic risks have not been assessed. Thus the surgery appears to be innovative, but innovative surgery on children is ethically problematic. (2) It is now widely recognized that the signed languages of the world are full-fledged natural languages, and the communities that speak those languages have distinct social organizations and cultures. Deaf culture values lead to a different assessment of pediatric cochlear implant surgery than do mainstream (hearing) values, and both sets of values have standing. (3) The fields of otology and audiology want to provide cochlear implants to Deaf children but also, their leaders say, want to protect Deaf culture; those appear to be conflicting goals in principle because, if there were perfect implants, the ranks of the Deaf World would diminish.

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.