Uptake of amikacin by hair cells of the guinea pig cochlea and vestibule and ototoxicity: comparison with gentamicin

Regeneration of Hair Cells from Endogenous Otic Progenitors in the Adult Mammalian Cochlea: Understanding Its Origins and Future Directions

Sensorineural hearing loss is caused by damage to sensory hair cells and/or spiral ganglion neurons. In non-mammalian species, hair cell regeneration after damage is observed, even in adulthood. Although the neonatal mammalian cochlea carries regenerative potential, the adult cochlea cannot regenerate lost hair cells. The survival of supporting cells with regenerative potential after cochlear trauma in adults is promising for promoting hair cell regeneration through therapeutic approaches. Targeting these cells by manipulating key signaling pathways that control mammalian cochlear development and non-mammalian hair cell regeneration could lead to regeneration of hair cells in the mammalian cochlea. This review discusses the pathways involved in the development of the cochlea and the impact that trauma has on the regenerative capacity of the endogenous progenitor cells. Furthermore, it discusses the effects of manipulating key signaling pathways targeting supporting cells with progenitor potential to promote hair cell regeneration and translates these findings to the human situation. To improve hearing recovery after hearing loss in adults, we propose a combined approach targeting (1) the endogenous progenitor cells by manipulating signaling pathways (Wnt, Notch, Shh, FGF and BMP/TGFβ signaling pathways), (2) by manipulating epigenetic control, and (3) by applying neurotrophic treatments to promote reinnervation.

LGR5-Positive Supporting Cells Survive Ototoxic Trauma in the Adult Mouse Cochlea

Sensorineural hearing loss is mainly caused by irreversible damage to sensory hair cells (HCs). A subgroup of supporting cells (SCs) in the cochlea express leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5), a marker for tissue-resident stem cells. LGR5+ SCs could be used as an endogenous source of stem cells for regeneration of HCs to treat hearing loss. Here, we report long-term presence of LGR5+ SCs in the mature adult cochlea and survival of LGR5+ SCs after severe ototoxic trauma characterized by partial loss of inner HCs and complete loss of outer HCs. Surviving LGR5+ SCs (confirmed by GFP expression) were located in the third row of Deiters' cells. We observed a change in the intracellular localization of GFP, from the nucleus in normal-hearing to cytoplasm and membrane in deafened mice. These data suggests that the adult mammalian cochlea possesses properties essential for regeneration even after severe ototoxic trauma.

Gentamicin Applied to the Oval Window Suppresses Vestibular Function in Guinea Pigs

Intratympanic gentamicin therapy is widely used clinically to treat the debilitating symptoms of Ménière's disease. Cochleotoxicity is an undesirable potential side effect of the treatment and the risk of hearing loss increases proportionately with gentamicin concentration in the cochlea. It has recently been shown that gentamicin is readily absorbed through the oval window in guinea pigs. The present study uses quantitative functional measures of vestibular and cochlea function to investigate the efficacy of treating the vestibule by applying a small volume of gentamicin onto the stapes footplate in guinea pigs. Vestibular and cochlea function were assessed by recording short latency vestibular evoked potentials in response to linear head acceleration and changes in hearing threshold, respectively, 1 and 2 weeks following treatment. Histopathology was analyzed in the crista ampullaris of the posterior semi-circular canal and utricular macula in the vestibule, and in the basal and second turns of the cochlea. In animals receiving gentamicin on the stapes footplate, vestibular responses were significantly suppressed by 72.7 % 2 weeks after treatment with no significant loss of hearing. This suggests that the vestibule can be treated directly by applying gentamicin onto the stapes footplate.

Amikacin Concentrations Predictive of Ototoxicity in Multidrug-Resistant Tuberculosis Patients

Aminoglycosides, such as amikacin, are used to treat multidrug-resistant tuberculosis. However, ototoxicity is a common problem and is monitored using peak and trough amikacin concentrations based on World Health Organization recommendations. Our objective was to identify clinical factors predictive of ototoxicity using an agnostic machine learning method. We used classification and regression tree (CART) analyses to identify clinical factors, including amikacin concentration thresholds that predicted audiometry-confirmed ototoxicity among 28 multidrug-resistant pulmonary tuberculosis patients in Botswana. Amikacin concentrations were measured for all patients. The quantitative relationship between predictive factors and the probability of ototoxicity were then identified using probit analyses. The primary predictors of ototoxicity on CART analyses were cumulative days of therapy, followed by cumulative area under the concentration-time curve (AUC), which improved on the primary predictor by 87%. The area under the receiver operating curve was 0.97 on the test set. Peak and trough were not predictors in any tree. When algorithms were forced to pick peak and trough as primary predictors, the area under the receiver operating curve fell to 0.46. Probit analysis revealed that the probability of ototoxicity increased sharply starting after 6 months of therapy to near maximum at 9 months. A 10% probability of ototoxicity occurred with a threshold cumulative AUC of 87,232 days · mg · h/liter, while that of 20% occurred at 120,000 days · mg · h/liter. Thus, cumulative amikacin AUC and duration of therapy, and not peak and trough concentrations, should be used as the primary decision-making parameters to minimize the likelihood of ototoxicity in multidrug-resistant tuberculosis.

Influence of 4'-O-Glycoside Constitution and Configuration on Ribosomal Selectivity of Paromomycin

A series of 20 4'-O-glycosides of the aminoglycoside antibiotic paromomycin were synthesized and evaluated for their ability to inhibit protein synthesis by bacterial, mitochondrial and cytosolic ribosomes. Target selectivity, i.e., inhibition of the bacterial ribosome over eukaryotic mitochondrial and cytosolic ribosomes, which is predictive of antibacterial activity with reduced ototoxicity and systemic toxicity, was greater for the equatorial than for the axial pyranosides, and greater for the d-pentopyranosides than for the l-pentopyranosides and d-hexopyranosides. In particular, 4'-O-β-d-xylopyranosyl paromomycin shows antibacterioribosomal activity comparable to that of paromomycin, but is significantly more selective showing considerably reduced affinity for the cytosolic ribosome and for the A1555G mutant mitochondrial ribosome associated with hypersusceptibility to drug-induced ototoxicity. Compound antibacterioribosomal activity correlates with antibacterial activity, and the ribosomally more active compounds show activity against Escherichia coli, Klebsiella pneumonia, Enterobacter cloacae, Acinetobacter baumannii, and methicillin-resistant Staphylococcus aureus (MRSA). The paromomycin glycosides retain activity against clinical strains of MRSA that are resistant to paromomycin, which is demonstrated to be a consequence of 4'-O-glycosylation blocking the action of 4'-aminoglycoside nucleotidyl transferases by the use of recombinant E. coli carrying the specific resistance determinant.

Hearing threshold measurements of five stranded short-finned pilot whales (Globicephala macrorhynchus)

On May 5, 2011, 23 short-finned pilot whales, Globicephala macrorhynchus, were stranded along the coastline near Cudjoe Key, FL. Five animals (two adult females, two juvenile females, and an adult male) were transported to a rehabilitation facility in Key Largo, FL. Auditory evoked potentials (AEPs) were recorded in response to amplitude modulated tone pips modulated at 1000 Hz. AEP thresholds were determined at 10, 20, 40, 80, and 120 kHz for the four females. However, the adult male was euthanized prior to testing. Short-finned pilot whales had peak sensitivity at lower frequencies than other odontocetes such as bottlenose dolphins. Greatest sensitivity was around 40 kHz for all whales, while thresholds for the two adult females were 25-61 dB higher at 80 kHz than the juveniles. Click evoked potentials were similar between the four whales and comparable to other echolocating odontocetes. Click evoked potential data from a fifth short-finned pilot whale that had stranded in Curacao showed no response. These findings add to the limited database of pilot whale (short- and long-finned) hearing studies, of which there are only two others [Schlundt et al. (2011). J. Acoust. Soc. Am. 129, 1111-1116 and Pacini et al. (2010). J. Exp. Biol. 213, 3138-3143].

Amikacin-induced Fin Reduction is Mediated by Autophagy

Despite its medical use, little is known about the mechanisms underlying amikacin-induced embryotoxicity, including fin reduction, in zebrafish. In this study, we examined the expression of well-known autophagy markers mTOR (target of rapamycin), atg10 (autophagy-related gene), atg12 and LC3 (mammalian homolog of Atg8) in amikacin-treated zebrafish embryos. Our results indicated that the mRNA expression level of atg12 in the amikacin-treated group was significantly increased by 1.5-fold (p<0.05) compared with the corresponding mock control group, while the expression levels of atg10 and mTOR were significantly decreased by 0.74-fold (p<0.05) and 0.58-fold (p<0.05), respectively. Western blot analysis revealed that LC3 protein expression was induced by amikacin. Taken together, these data suggest that amikacin-induced fin reduction is mediated by fin cell autophagy.

Hair cell replacement in adult mouse utricles after targeted ablation of hair cells with diphtheria toxin

We developed a transgenic mouse to permit conditional and selective ablation of hair cells in the adult mouse utricle by inserting the human diphtheria toxin receptor (DTR) gene into the Pou4f3 gene, which encodes a hair cell-specific transcription factor. In adult wild-type mice, administration of diphtheria toxin (DT) caused no significant hair cell loss. In adult Pou4f3(+/DTR) mice, DT treatment reduced hair cell numbers to 6% of normal by 14 days post-DT. Remaining hair cells were located primarily in the lateral extrastriola. Over time, hair cell numbers increased in these regions, reaching 17% of untreated Pou4f3(+/DTR) mice by 60 days post-DT. Replacement hair cells were morphologically distinct, with multiple cytoplasmic processes, and displayed evidence for active mechanotransduction channels and synapses characteristic of type II hair cells. Three lines of evidence suggest replacement hair cells were derived via direct (nonmitotic) transdifferentiation of supporting cells: new hair cells did not incorporate BrdU, supporting cells upregulated the pro-hair cell gene Atoh1, and supporting cell numbers decreased over time. This study introduces a new method for efficient conditional hair cell ablation in adult mouse utricles and demonstrates that hair cells are spontaneously regenerated in vivo in regions where there may be ongoing hair cell turnover.

Fin reduction is a novel and unexpected teratogenic effect of amikacin-treated zebrafish embryos

We used zebrafish as a model to assess amikacin-induced embryotoxicity. We exposed zebrafish embryos to amikacin, using different amikacin doses (0-10 ppm), durations (12-48 h), and onsets (0, 24, 48 hpf). Amikacin-induced embryonic toxicity and reduced survival rate were found dependent on the exposure dose, duration and onset. Based on immunostaining with neuron-specific antibodies, amikacin reduced the number and size of zebrafish neuromasts. In addition, Amikacin caused pelvic, dorsal and anal fin defects in dose-dependent and duration-dependent manners. Proliferating cell nuclear antigen immunostaining revealed that amikacin-induced fin defects were not due to reduction of proliferating mesenchymal cells. TUNEL assay demonstrated that amikacin-induced fin defects might not associate with apoptosis. Therefore, further investigations are required to elucidate if other cell death pathways are involved in amikacin-induced fin defects.

Critical roles of transitional cells and Na/K-ATPase in the formation of vestibular endolymph

The mechanotransduction of vestibular sensory cells depends on the high endolymphatic potassium concentration ([K+]) maintained by a fine balance between K+ secretion and absorption by epithelial cells. Despite the crucial role of endolymph as an electrochemical motor for mechanotransduction, little is known about the processes that govern endolymph formation. To address these, we took advantage of an organotypic rodent model, which regenerates a genuine neonatal vestibular endolymphatic compartment, facilitating the determination of endolymphatic [K+] and transepithelial potential (Vt) during endolymph formation. While mature Vt levels are almost immediately achieved, K+ accumulates to reach a steady [K+] by day 5 in culture. Inhibition of sensory cell K+ efflux enhances [K+] regardless of the blocker used (FM1.43, amikacin, gentamicin, or gadolinium). Targeting K+ secretion with bumetanide partially and transiently reduces [K+], while ouabain application and Kcne1 deletion almost abolishes it. Immunofluorescence studies demonstrate that dark cells do not express Na-K-2Cl cotransporter 1 (the target of bumetanide) in cultured and young mouse utricles, while Na/K-ATPase (the target of ouabain) is found in dark cells and transitional cells. This global analysis of the involvement of endolymphatic homeostasis actors in the immature organ (1) confirms that KCNE1 channels are necessary for K+ secretion, (2) highlights Na/K-ATPase as the key endolymphatic K+ provider and shows that Na-K-2Cl cotransporter 1 has a limited impact on K+ influx, and (3) demonstrates that transitional cells are involved in K+ secretion in the early endolymphatic compartment.

Direct entry of gadolinium into the vestibule following intratympanic applications in Guinea pigs and the influence of cochlear implantation

Although intratympanic (IT) administration of drugs has gained wide clinical acceptance, the distribution of drugs in the inner ear following IT administration is not well established. Gadolinium (Gd) has been previously used as a marker in conjunction with magnetic resonance imaging (MRI) to visualize distribution in inner ear fluids in a qualitative manner. In the present study, we applied gadolinium chelated with diethylenetriamine penta-acetic acid (Gd-DTPA) to the round window niche of 12 guinea pigs using Seprapack(TM) (carboxlmethylcellulose-hyaluronic acid) pledgets which stabilized the fluid volume in the round window niche. Gd-DTPA distribution was monitored sequentially with time following application. Distribution in normal, unperforated ears was compared with ears that had undergone a cochleostomy in the basal turn of scala tympani and implantation with a silastic electrode. Results were quantified using image analysis software. In all animals, Gd-DTPA was seen in the lower basal scala tympani (ST), scala vestibuli (SV), and throughout the vestibule and semi-circular canals by 1 h after application. Although Gd-DTPA levels in ST were higher than those in the vestibule in a few ears, the majority showed higher Gd-DTPA levels in the vestibule than ST at both early and later time points. Quantitative computer simulations of the experiment, taking into account the larger volume of the vestibule compared to scala tympani, suggest most Gd-DTPA (up to 90%) entered the vestibule directly in the vicinity of the stapes rather than indirectly through the round window membrane and ST. Gd-DTPA levels were minimally affected by the implantation procedure after 1 h. Gd-DTPA levels in the basal turn of scala tympani were lower in implanted animals, but the difference compared to non-implanted ears did not reach statistical significance.

Live CT imaging of sound reception anatomy and hearing measurements in the pygmy killer whale, Feresa attenuata

In June 2008, two pygmy killer whales (Feresa attenuata) were stranded alive near Boca Grande, FL, USA, and were taken into rehabilitation. We used this opportunity to learn about the peripheral anatomy of the auditory system and hearing sensitivity of these rare toothed whales. Three-dimensional (3-D) reconstructions of head structures from X-ray computed tomography (CT) images revealed mandibles that were hollow, lacked a bony lamina medial to the pan bone and contained mandibular fat bodies that extended caudally and abutted the tympanoperiotic complex. Using auditory evoked potential (AEP) procedures, the modulation rate transfer function was determined. Maximum evoked potential responses occurred at modulation frequencies of 500 and 1000 Hz. The AEP-derived audiograms were U-shaped. The lowest hearing thresholds occurred between 20 and 60 kHz, with the best hearing sensitivity at 40 kHz. The auditory brainstem response (ABR) was composed of seven waves and resembled the ABR of the bottlenose and common dolphins. By changing electrode locations, creating 3-D reconstructions of the brain from CT images and measuring the amplitude of the ABR waves, we provided evidence that the neuroanatomical sources of ABR waves I, IV and VI were the auditory nerve, inferior colliculus and the medial geniculate body, respectively. The combination of AEP testing and CT imaging provided a new synthesis of methods for studying the auditory system of cetaceans.

Auditory evoked potentials in two short-finned pilot whales (Globicephala macrorhynchus)

The hearing sensitivities of two short-finned pilot whales (Globicephala macrorhynchus) were investigated by measuring auditory evoked potentials generated in response to clicks and sinusoidal amplitude modulated (SAM) tones. The first whale tested, an adult female, was a long-time resident at SeaWorld San Diego with a known health history. Click-evoked responses in this animal were similar to those measured in other echolocating odontocetes. Auditory thresholds were comparable to dolphins of similar age determined with similar evoked potential methods. The region of best sensitivity was near 40 kHz and the upper limit of functional hearing was between 80 and 100 kHz. The second whale tested, a juvenile male, was recently stranded and deemed non-releasable. Click-evoked potentials were not detected in this animal and testing with SAM tones suggested severe hearing loss above 10 kHz.

Sox2 and JAGGED1 expression in normal and drug-damaged adult mouse inner ear

Inner ear hair cells detect environmental signals associated with hearing, balance, and body orientation. In humans and other mammals, significant hair cell loss leads to irreversible hearing and balance deficits, whereas hair cell loss in nonmammalian vertebrates is repaired by the spontaneous generation of replacement hair cells. Research in mammalian hair cell regeneration is hampered by the lack of in vivo damage models for the adult mouse inner ear and the paucity of cell-type-specific markers for non-sensory cells within the sensory receptor epithelia. The present study delineates a protocol to drug damage the adult mouse auditory epithelium (organ of Corti) in situ and uses this protocol to investigate Sox2 and Jagged1 expression in damaged inner ear sensory epithelia. In other tissues, the transcription factor Sox2 and a ligand member of the Notch signaling pathway, Jagged1, are involved in regenerative processes. Both are involved in early inner ear development and are expressed in developing support cells, but little is known about their expressions in the adult. We describe a nonsurgical technique for inducing hair cell damage in adult mouse organ of Corti by a single high-dose injection of the aminoglycoside kanamycin followed by a single injection of the loop diuretic furosemide. This drug combination causes the rapid death of outer hair cells throughout the cochlea. Using immunocytochemical techniques, Sox2 is shown to be expressed specifically in support cells in normal adult mouse inner ear and is not affected by drug damage. Sox2 is absent from auditory hair cells, but is expressed in a subset of vestibular hair cells. Double-labeling experiments with Sox2 and calbindin suggest Sox2-positive hair cells are Type II. Jagged1 is also expressed in support cells in the adult ear and is not affected by drug damage. Sox2 and Jagged1 may be involved in the maintenance of support cells in adult mouse inner ear.

Treatment of Ménière's Disease by Low-Dosage Intratympanic Gentamicin Application: Effect on Otolith Function

OBJECTIVES

The intratympanic application of a low dosage of gentamicin is increasingly favored as treatment for Ménière's disease. While posttreatment observations have confirmed a long-term success of the therapy of vertigo attacks, clear differences in the posttreatment recovery interval can be observed. In addition to differences in central-vestibular compensation, the degree of peripheral vestibular damage, i.e., to the saccule, utricle, and semicircular canal ampullae, varies among patients. This study provides comprehensive pre- and posttreatment results from unilateral functional tests of the individual vestibular receptors and of the cochlea in patients with Ménière's disease.

STUDY DESIGN

Prospective clinical study.

METHODS

Nineteen patients with unilateral Ménière's disease were treated by intratympanic application of gentamicin by injection of 0.3 mL (12 mg) through the tympanic membrane under local anesthesia. Tests were performed immediately previous to treatment and subsequently in the periods 4 to 8 weeks and 12 to 16 weeks after treatment. Unilateral saccular function was tested by means of acoustic-click, vestibular-evoked myogenic potentials (VEMP), and unilateral utricular function by subjective visual vertical (SVV) during unilateral centrifugation. Bithermal caloric testing was performed to assess unilateral semicircular canal function.

RESULTS

Prior to gentamicin treatment, the caloric response from the diseased ear was normal in 3 patients, below normal in 14 patients, and in 2 cases almost completely absent. VEMP responses could be recorded bilaterally in 13 patients; while in 6, no VEMPs could be measured from the diseased ear. Utricular function measured by SVV estimation was found to be normal in 11 patients and marginally abnormal in 2 patients. In six cases, the SVV was clearly underestimated during centrifugation of the diseased side. The posttreatment findings demonstrate that VEMPs were absent in all treated patients, and the caloric response was abnormally low in all but one case. In contrast, only 12 of 19 patients produced abnormal SVV responses.

CONCLUSION

The results demonstrate that incremental, intratympanic application of gentamicin effectively eliminates semicircular canal and saccular function. In contrast, utricular function appears to be maintained in 30 to 40% of cases.

Glomerular nephrotoxicity of aminoglycosides

Aminoglycoside antibiotics are the most commonly used antibiotics worldwide in the treatment of Gram-negative bacterial infections. However, aminoglycosides induce nephrotoxicity in 10-20% of therapeutic courses. Aminoglycoside-induced nephrotoxicity is characterized by slow rises in serum creatinine, tubular necrosis and marked decreases in glomerular filtration rate and in the ultrafiltration coefficient. Regulation of the ultrafiltration coefficient depends on the activity of intraglomerular mesangial cells. The mechanisms responsible for tubular nephrotoxicity of aminoglycosides have been intensively reviewed previously, but glomerular toxicity has received less attention. The purpose of this review is to critically assess the published literature regarding the toxic mechanisms of action of aminoglycosides on renal glomeruli and mesangial cells. The main goal of this review is to provide an actualized and mechanistic vision of pathways involved in glomerular toxic effects of aminoglycosides.

Gentamicin abolishes all cochlear effects of electrical stimulation of the inferior colliculus

Electrical stimulation of the inferior colliculus (IC) has been shown to result in suppression of cochlear output, due to activation of the medial olivocochlear system. This auditory efferent system originates in the brainstem and terminates on the outer hair cells in the cochlea. Recently, excitatory effects of IC stimulation have also been reported, both on cochlear gross potentials and on primary auditory afferents. It has been hypothesized that this excitation is due to co-activation of the lateral olivocochlear system, which synapses on the primary auditory afferent fibres contacting the inner hair cells. If stimulation of the IC leads to the activation of both the medial and lateral olivocochlear system, resulting in a mixture of inhibitory and excitatory effects in the cochlea, then removal of the inhibitory effects, by blocking the medial system, should lead to more pronounced excitatory effects out in the periphery. To investigate this hypothesis, we recorded the effect of IC stimulation on cochlear gross potentials as well as on single auditory primary afferents in guinea pigs following block of the medial olivocochlear system with gentamicin. We found that administration of gentamicin, whether intraperitoneally or by intracochlear perfusion, blocked all effects of IC stimulation, whether inhibitory or excitatory. These data strongly suggest that all effects observed after IC stimulation, both inhibitory as well as excitatory, are due to the activation of the medial olivocochlear system.

Reduced expression of sialoglycoconjugates in the outer hair cell glycocalyx after systemic aminoglycoside administration

In this study we investigated the effect of systemic aminoglycoside administration on the expression of sialoglycoconjugates in the outer hair cell (OHC) glycocalyx of the adult guinea pig. Sialoglycoconjugates were visualized by means of ultrastructural lectin cytochemistry, using Limax flavus agglutinin (LFA) and wheat germ agglutinin (WGA) as probes. Labelling densities were determined for the apical membranes (including the stereocilia and stereociliary cross-links) and basolateral membranes of OHCs in the respective (basal, middle and apical) cochlear turns from animals that had been treated with gentamicin or neomycin for 5 or 15 consecutive days. Our results indicate that: (1) sialoglycoconjugate expression in the OHC glycocalyx demonstrates an intracochlear gradient decreasing towards the apical turn; (2) OHCs demonstrate a polarity in sialoglycoconjugate expression, in that the basolateral membranes contain more sialoglycoconjugates per surface area than the apical membranes; (3) aminoglycoside administration results in reduced expression of sialoglycoconjugates in the OHC glycocalyx; in this respect, basal-turn OHCs are more susceptible than those in the middle and apical turns; (4) reduction in sialoglycoconjugate expression after aminoglycoside administration is more prominent in the basolateral membranes; and (5) the difference in ototoxic potencies between gentamicin and neomycin is not reflected at the level of sialoglycoconjugate expression. The present data support our earlier hypothesis that aminoglycosides, already at an early phase of intoxication, interfere with the function of the endoplasmic reticulum and/or the Golgi apparatus, implying that these organelles play a crucial role in the initial phase of aminoglycoside-induced OHC degeneration.

Pure tone audiograms and possible aminoglycoside-induced hearing loss in belugas (Delphinapterus leucas)

A behavioral response paradigm was used to measure pure-tone hearing sensitivities in two belugas (Delphinapterus leucas). Tests were conducted over a 20-month period at the Point Defiance Zoo and Aquarium, in Tacoma, WA. Subjects were two males, aged 8-10 and 9-11 during the course of the study. Subjects were born in an oceanarium and had been housed together for all of their lives. Hearing thresholds were measured using a modified up/down staircase procedure and acoustic response paradigm where subjects were trained to produce audible responses to test tones and to remain quiet otherwise. Test frequencies ranged from approximately 2 to 130 kHz. Best sensitivities ranged from approximately 40 to 50 dB re 1 microPa at 50-80 kHz and 30-35 kHz for the two subjects. Although both subjects possessed traditional "U-shaped" mammalian audiograms, one subject exhibited significant high-frequency hearing loss above 37 kHz compared to previously published data for belugas. Hearing loss in this subject was estimated to approach 90 dB for frequencies above 50 kHz. Similar ages, ancestry, and environmental conditions between subjects, but a history of ototoxic drug administration in only one subject, suggest that the observed hearing loss was a result of the aminoglycoside antibiotic amikacin.

Critical signaling events during the aminoglycoside-induced death of sensory hair cells in vitro

Sensory hair cells undergo apoptosis following exposure to aminoglycoside antibiotics. In neurons, apoptosis is associated with a transient increase in intracellular Ca2+, phosphorylation of the transcription factor c-Jun, and the release of cytochrome c from mitochondria into the cytosol, which along with other cofactors results in the activation of caspases. To examine the possible role of these events in the survival and death of the sensory receptors of the inner ear, we examined the effects of neomycin treatment on cytoplasmic calcium, activation of c-Jun-N-Terminal kinases (JNKs), cytochrome c release, and caspase-3 activation in cultured vestibular hair cells. Increased numbers of phospho-c-Jun-labeled hair cells (a downstream indicator of JNK activation) were observed at 3-12 h after neomycin treatment, whereas increased numbers of cells with cytoplasmic cytochrome c were observed at 12-18 h following the onset of neomycin treatment. This was followed by an increase in the number of cells that contained activated caspase-3 and displayed pyknotic nuclei. Treatment with the general caspase inhibitor BAF did not affect the release of cytochrome c and the number of p-c-Jun-labeled cells, but reduced the number of cells with activated caspase-3 and pyknotic nuclei. In contrast, treatment with CEP-11004, an indirect inhibitor of the JNK signaling pathway, promoted hair cell survival following neomycin treatment and reduced the number of cells with phosphorylated JNK and c-Jun, cytoplasmic cytochrome c, and activated caspase 3. These results suggest that JNK activation occurs upstream of the release of cytochrome c and that cytochrome c release precedes caspase activation. Cytochrome c release and JNK activation were also preceded by large changes in cytoplasmic calcium. Cytoplasmic calcium increases may be causally related to the release of cytochrome c, and may also be a potential pathway for activation of JNK in hair cells.

Caspase inhibitors promote vestibular hair cell survival and function after aminoglycoside treatment in vivo

The sensory hair cells of the inner ear undergo apoptosis after acoustic trauma or aminoglycoside antibiotic treatment, causing permanent auditory and vestibular deficits in humans. Previous studies have demonstrated a role for caspase activation in hair cell death and ototoxic injury that can be reduced by concurrent treatment with caspase inhibitors in vitro. In this study, we examined the protective effects of caspase inhibition on hair cell death in vivo after systemic injections of aminoglycosides. In one series of experiments, chickens were implanted with osmotic pumps that administrated the pan-caspase inhibitor z-Val-Ala-Asp(Ome)-fluoromethylketone (zVAD) into inner ear fluids. One day after the surgery, the animals received a 5 d course of treatment with streptomycin, a vestibulotoxic aminoglycoside. Direct infusion of zVAD into the vestibule significantly increased hair cell survival after streptomycin treatment. A second series of experiments determined whether rescued hair cells could function as sensory receptors. Animals treated with streptomycin displayed vestibular system impairment as measured by a greatly reduced vestibulo-ocular response (VOR). In contrast, animals that received concurrent systemic administration of zVAD with streptomycin had both significantly greater hair cell survival and significantly increased VOR responses, as compared with animals treated with streptomycin alone. These findings suggest that inhibiting the activation of caspases promotes the survival of hair cells and protects against vestibular function deficits after aminoglycoside treatment.

Block of the alpha9 nicotinic receptor by ototoxic aminoglycosides

In the present study, we report that the alpha9 nicotinic acetylcholine receptor (nAChR) expressed in Xenopus laevis oocytes is reversibly blocked by aminoglycoside antibiotics. The aminoglycosides tested blocked the alpha9 nAChR in a concentration-dependent manner with the following rank order of potency: neomycin>gentamicin>streptomycin>amikacin>kanamycin. The antagonistic effect of gentamicin was not overcome by increasing the concentration of acetylcholine (ACh), indicative of a non-competitive type of block. Blockage of ACh-evoked currents by gentamicin was found to be voltage-dependent, being more potent at hyperpolarized than at depolarized holding potentials. Furthermore, gentamicin blockage was dependent upon the extracellular Ca(2+) concentration, shown by the fact that increments in extracellular Ca(2+) significantly reduced the potency of this aminoglycoside to block the alpha9 nAChR. Possible mechanisms of blockage by the aminoglycosides are discussed. The present results suggest that the initial reversible actions of aminoglycosides at the organ of Corti, such as the elimination of the olivocochlear efferent function, are due in part to the interaction with the native alpha9-containing cholinergic receptor of the outer hair cells.

Vinpocetine protects from aminoglycoside antibiotic-induced hearing loss in guinea pig in vivo

The principal objective of this study is to explore the hypothesis that a blockade of Na(+) channels can prevent some of the mechanisms involved in ototoxicity. For this purpose, the potential action of the voltage sensitive Na(+) channel antagonist, vinpocetine, on the ototoxicity induced by the representative aminoglycoside antibiotic, amikacin, in guinea pigs was tested for almost half a year. Amikacin (450 mg/kg) administered daily (i.m.) for 5 days increases the thresholds of the auditory brainstem response (ABR) to the two frequencies tested (4 and 8 kHz). These threshold increases are permanent or at least long-lived, as after 40 days they are already established and are maintained until the end of the experiment (160 days after the antibiotic administration). Amikacin decreases the amplitude of ABR waves, particularly P1, and after 160 days increases the latency of ABR waves, particularly at the higher frequency tested (8 kHz). When the above amikacin regimen is followed by a daily (i.p.) vinpocetine (2 mg/kg) administration for 13 days the increase in ABR threshold and latency caused by amikacin alone is prevented. Moreover, the animals treated with amikacin alone show a decreased weight gain and a remarkable increased mortality in comparison with the group of animals post-treated with vinpocetine. We hope that the multiple beneficial effects exerted by the Na(+) channel blocker, vinpocetine, against aminoglycoside antibiotics-induced side effects could help to solve the serious limitations of the use of this type of antibiotic.

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.

A method for the induction of a cochlea-specific auditory deprivation in the gerbil (Meriones unguiculatus)

The neurophysiological effects of early electrical stimulation on the development and neural plasticity of the central auditory system in prelingually deafened children with cochlear implants are still unknown. Many of these basic questions can be answered systematically only in animal experiments. Meriones unguiculatus is a well-established animal model in hearing research. Deafening is produced by a single intracochlear application of an ototoxic aminoglycoside antibiotic (neomycin sulfate) on the 14th day after birth (DAB), i.e. before the late natural onset of hearing on the 16th DAB. A single application of the antibiotic abolishes auditory brainstem responses (ABR) to clicks completely and reduces sensitivity to low frequency tonebursts by 50 dB SPL. Scanning electron microscopy results show a destruction of the stereocilia of the inner and outer hair cells of the basal and medial cochlear turn and a reduction of those in the apical turn. Our method avoids a systemic application of antibiotics and can be used in studies dealing with the consequences of different forms of auditory deprivation, neuronal compensation processes or with ontogenetic studies and chronic electrostimulation in an animal model.

Lysosomal targeting and accumulation of aminoglycoside antibiotics in sensory hair cells

Our recent study demonstrated that aminoglycoside antibiotics are taken up into sensory hair cells of the inner ear by receptor-mediated endocytosis (E. Hashino, M. Shero, Endocytosis of aminoglycoside antibiotics in sensory hair cells, Brain Res. 704 (1995) 135-140). To elucidate the intracellular trafficking pathway of aminoglycosides following endocytotic uptake, we administered kanamycin to neonatal chicks for 1 or 5 days (400 mg/kg/day) and determined the location of kanamycin within the hair cells at various time points using immunogold electron microscopy. Quantitative and qualitative analysis of immunogold staining revealed that: (1) kanamycin was primarily localized in vesicles beneath the cuticular plate 27 h postinjection; (2) the number of vesicles per hair cell and the number of gold particles per vesicle increased over time; (3) individual vesicles tended to increase in size over time, presumably due to aggregation of smaller vesicles; and (4) in pathological hair cells, immunogold was dispersed throughout the entire subcellular region. Light microscopic observations of the basilar papilla stained with the same antibody confirmed the temporal changes in the kanamycin distribution. Moreover, results obtained from acid phosphatase cytochemistry indicated that vesicles accumulating kanamycin were mainly lysosomes. These results suggest that internalized aminoglycosides are transported via vesicular traffic into lysosomes where they accumulate over time and lead to disruption of lysosomes. The time of diffusion of kanamycin was closely related to the time of cell death, suggesting that lysosomal rupture could be a direct trigger for the hair cell degeneration.

Regenerated hair cells exhibit a transient resistance to aminoglycoside toxicity

Recent studies have demonstrated that sensory hair cells in the avian inner ear are reproduced by cell proliferation in response to the death of the original hair cell population. The regenerated hair cells appear to construct functional synaptic contacts, thereby transmitting acoustic signals to the peripheral nervous system. One of the most extraordinary, but overlooked characteristics of these regenerated hair cells, is their ability to survive in a highly ototoxic environment. Here, we report that hair cells regenerated after kanamycin induced hair cell loss can survive for a substantially longer time period than their predecessors during prolonged exposure to aminoglycoside antibiotics. The prolonged survival, however, belongs solely to the immature status of regenerated hair cells. Once the regenerated hair cells reach morphological maturation, they become vulnerable to aminoglycoside toxicity. Immunohistochemical evaluation of kanamycin suggested that kanamycin may be taken up into hair cells via a receptor-mediated endocytosis at their apical surfaces. By contrast, kanamycin was rarely incorporated into the cytoplasm of the regenerated hair cells. These results suggest that the process of a receptor-mediated transmembrane transport at the apical surface of hair cells is developmentally regulated, and that the lack of some of the assembly involved in the transmembrane transport could be responsible for the inhibition of aminoglycoside uptake, leading immature hair cells to be aminoglycoside resistant.