Aminoglycoside antibiotics impair calcium entry but not viability and motility in isolated cochlear outer hair cells

GHz Ultrasonic Chip-Scale Device Induces Ion Channel Stimulation in Human Neural Cells

Emergent trends in the device development for neural prosthetics have focused on establishing stimulus localization, improving longevity through immune compatibility, reducing energy re-quirements, and embedding active control in the devices. Ultrasound stimulation can single-handedly address several of these challenges. Ultrasonic stimulus of neurons has been studied extensively from 100 kHz to 10 MHz, with high penetration but less localization. In this paper, a chip-scale device consisting of piezoelectric Aluminum Nitride ultrasonic transducers was engineered to deliver gigahertz (GHz) ultrasonic stimulus to the human neural cells. These devices provide a path towards complementary metal oxide semiconductor (CMOS) integration towards fully controllable neural devices. At GHz frequencies, ultrasonic wavelengths in water are a few microns and have an absorption depth of 10-20 µm. This confinement of energy can be used to control stimulation volume within a single neuron. This paper is the first proof-of-concept study to demonstrate that GHz ultrasound can stimulate neurons in vitro. By utilizing optical calcium imaging, which records calcium ion flux indicating occurrence of an action potential, this paper demonstrates that an application of a nontoxic dosage of GHz ultrasonic waves [Formula: see text] caused an average normalized fluorescence intensity recordings >1.40 for the calcium transients. Electrical effects due to chip-scale ultrasound delivery was discounted as the sole mechanism in stimulation, with effects tested at α = 0.01 statistical significance amongst all intensities and con-trol groups. Ionic transients recorded optically were confirmed to be mediated by ion channels and experimental data suggests an insignificant thermal contributions to stimulation, with a predicted increase of 0.03 ^oC for [Formula: see text] This paper paves the experimental framework to further explore chip-scale axon and neuron specific neural stimulation, with future applications in neural prosthetics, chip scale neural engineering, and extensions to different tissue and cell types.

Pasireotide protects mammalian cochlear hair cells from gentamicin ototoxicity by activating the PI3K-Akt pathway

Gentamicin is a widely used antibiotic for the treatment of gram-negative bacterial infections; however, its use often results in significant and permanent hearing loss. Hearing loss resulting from hair cell (HC) degeneration affects millions of people worldwide, and one major cause is the loss of sensory HCs in the inner ear due to aminoglycoside exposure. Strategies to overcome the apparently irreversible loss of HCs in mammals are crucial for hearing protection. Here, we report that the somatostatin analog pasireotide protects mouse cochlear HCs from gentamicin damage using a well-established in vitro gentamicin-induced HC loss model and that the otoprotective effects of pasireotide are due to Akt up-regulation via the PI3K–Akt signal pathway activation. We demonstrate active caspase signal in organ of Corti (OC) explants exposed to gentamicin and show that pasireotide treatment activates survival genes, reduces caspase signal, and increases HC survival. The neuropeptide somatostatin and its selective analogs have provided neuroprotection by activating five somatostatin receptor (SSTR1–SSTR5) subtypes. Pasireotide has a high affinity for SSTR2 and SSTR5, and the addition of SSTR2- and SSTR5-specific antagonists leads to a loss of protection. The otoprotective effects of pasireotide were also observed in a gentamicin-injured animal model. In vivo studies have shown that 13 days of subcutaneous pasireotide application prevents gentamicin-induced HC death and permanent hearing loss in mice. Auditory brainstem response analysis confirmed the protective effect of pasireotide, and we found a significant threshold shift at all measured frequencies (4, 8, 16, 24, and 32 kHz). Together, these findings indicate that pasireotide is a novel otoprotective peptide acting via the PI3K–Akt pathway and may be of therapeutic value for HC protection from ototoxic insults.

Intracellular free Ca2+signals antibiotic exposure in cyanobacteria

Intracellular free Ca²⁺, [Ca²⁺]_i, is a key element of the cellular response to many abiotic and biotic stresses.

Pasireotide prevents nuclear factor of activated T cells nuclear translocation and acts as a protective agent in aminoglycoside-induced auditory hair cell loss

Hearing impairment is a global health problem with a high socioeconomic impact. Damage to auditory hair cells (HCs) in the inner ear as a result of aging, disease, trauma, or toxicity, underlies the majority of cases of sensorineural hearing loss. Previously we demonstrated that the Ca²⁺ -sensitive neuropeptide, somatostatin (SST), and an analog, octreotide, protect HCs from gentamicin-induced cell death in vitro. Aminoglycosides such as gentamicin trigger a calcium ion influx (Ca²⁺ ) that activates pro-apoptotic signaling cascades in HCs. SST binding to the G-protein-coupled receptors (SSTR1-SSTR5) that are directly linked to voltage-dependent Ca²⁺ channels inhibits Ca²⁺ channel activity and associated downstream events. Here, we report that the SST analog pasireotide, a high affinity ligand to SSTRs 1-3, and 5, with a longer half-life than octreotide, prevents gentamicin-induced HC death in the mouse organ of Corti (OC). Explant experiments using OCs derived from SSTR1 and SSTR1and 2 knockout mice, revealed that SSTR2 mediates pasireotide's anti-apoptotic effects. Mechanistically, pasireotide prevented a nuclear translocation of the Ca²⁺ -sensitive transcription factor, nuclear factor of activated T cells (NFAT), which is ordinarily provoked by gentamicin in OC explants. Direct inhibition of NFAT with 11R-VIVIT also prevented the gentamicin-dependent nuclear translocation of NFAT and apoptosis. Both pasireotide and 11R-VIVIT partially reversed the effects of gentamicin on the expression of downstream survival targets (NMDA receptor and the regulatory subunit of phosphatidylinositol-4,5-bisphosphate 3-kinase, PI3K). These data suggest that SST analogs antagonize aminoglycoside-induced cell death in an NFAT-dependent fashion. SST analogs and NFAT inhibitors may therefore offer new therapeutic possibilities for the treatment of hearing loss.

Cobalt Chloride Treatment Used to Ablate the Lateral Line System Also Impairs the Olfactory System in Three Freshwater Fishes

Fishes use multimodal signals during both inter- and intra-sexual displays to convey information about their sex, reproductive state, and social status. These complex behavioral displays can include visual, auditory, olfactory, tactile, and hydrodynamic signals, and the relative role of each sensory channel in these complex multi-sensory interactions is a common focus of neuroethology. The mechanosensory lateral line system of fishes detects near-body water movements and is implicated in a variety of behaviors including schooling, rheotaxis, social communication, and prey detection. Cobalt chloride is commonly used to chemically ablate lateral line neuromasts, thereby eliminating water-movement cues to test for mechanosensory-mediated behavioral functions. However, cobalt acts as a nonspecific calcium channel antagonist and could potentially disrupt function of all superficially located sensory receptor cells, including those for chemosensing. Here, we examined whether CoCl2 treatment used to ablate the lateral line system also impairs olfaction in three freshwater fishes, the African cichlid fish Astatotilapia burtoni, goldfish Carassius auratus, and the Mexican blind cavefish Astyanax mexicanus. To examine the impact of CoCl2 on the activity of peripheral receptors, we quantified DASPEI fluorescence intensity of the olfactory epithelium from fish exposed to control and CoCl2 solutions. In addition, we examined brain activation in olfactory processing regions of A. burtoni immersed in either control or cobalt solutions. All three species exposed to CoCl2 had decreased DASPEI staining of the olfactory epithelium, and in A. burtoni, cobalt treatment caused reduced neural activation in olfactory processing regions of the brain. To our knowledge this is the first empirical evidence demonstrating that the same CoCl2 treatment used to ablate the lateral line system also impairs olfactory function. These data have important implications for the use of CoCl2 in future research and suggest that previous studies using CoCl2 should be reinterpreted in the context of both impaired mechanoreception and olfaction.

Systemic Delivery and Biodistribution of Cisplatin in Vivo

Cisplatin is widely used to treat a variety of cancers. However, ototoxicity and nephrotoxicity remain serious side effects of cisplatin-based chemotherapy. In order to inform the study of cisplatin's off-target effects, a new drug-fluorophore conjugate was synthesized that exhibited utility as a tracer to determine the cellular uptake and in vivo distribution of cisplatin. This probe will serve as a useful tool to facilitate investigations into the kinetics and biodistribution of cisplatin and its associated side effects in preclinical models after systemic administration.

Rhodamine analogues for molecular ruler applications

A series of geometrically well-defined cationic fluorophores were designed based on molecular mechanics. They contain biaryl linkers to impart rigidity preventing intramolecular folding between a conjugated biomolecule and fluorophore. All probes have absorption and emission maxima within 20 nm from Texas Red, as predicted by TDDFT calculations and validated experimentally.

Gentamicin blocks the ACh-induced BK current in guinea pig type II vestibular hair cells by competing with Ca²⁺ at the L-type calcium channel

Type II vestibular hair cells (VHCs II) contain big-conductance Ca²⁺-dependent K⁺ channels (BK) and l-type calcium channels. Our previous studies in guinea pig VHCs II indicated that acetylcholine (ACh) evoked the BK current by triggering the influx of Ca²⁺ ions through l-type Ca²⁺ channels, which was mediated by M2 muscarinic ACh receptor (mAChRs). Aminoglycoside antibiotics, such as gentamicin (GM), are known to have vestibulotoxicity, including damaging effects on the efferent nerve endings on VHCs II. This study used the whole-cell patch clamp technique to determine whether GM affects the vestibular efferent system at postsynaptic M2-mAChRs or the membrane ion channels. We found that GM could block the ACh-induced BK current and that inhibition was reversible, voltage-independent, and dose-dependent with an IC₅₀ value of 36.3 ± 7.8 μM. Increasing the ACh concentration had little influence on GM blocking effect, but increasing the extracellular Ca²⁺ concentration ([Ca²⁺]_o) could antagonize it. Moreover, 50 μM GM potently blocked Ca²⁺ currents activated by (−)-Bay-K8644, but did not block BK currents induced by NS1619. These observations indicate that GM most likely blocks the M2 mAChR-mediated response by competing with Ca²⁺ at the l-type calcium channel. These results provide insights into the vestibulotoxicity of aminoglycoside antibiotics on mammalian VHCs II.

Somatostatin receptor types 1 and 2 in the developing mammalian cochlea

The neuropeptide somatostatin (SST) exerts several important physiological actions in the adult central nervous system through interactions with membrane-bound receptors. Transient expression of SST and its receptors has been described in several brain areas during early ontogeny. It is therefore believed that SST may play a role in neural maturation. The present study provides the first evidence for the developmental expression of SST receptors in the mammalian cochlea, emphasizing their possible roles in cochlear maturation. In the developing mouse cochlea, cells immunoreactive to somatostatin receptor 1 (SSTR1) and somatostatin receptor 2 (SSTR2) were located in the embryonic cochlear duct on Kolliker's organ as early as embryonic day (E) 14 (E14). At E17, the expression of both receptors was high and already located at the hair cells and supporting cells along the length of the cochlear duct, which have become arranged into the characteristic pattern for the organ of Corti (OC) at this stage. At birth, SSTR1- and SSTR2-containing cells were only localized in the OC. In general, immunoreactivity for both receptors increased in the mouse cochlea from postnatal day (P) 0 (P0) to P10; the majority of immunostained cells were inner hair cells, outer hair cells, and supporting cells. Finally, a peak in the mRNA and protein expression of both receptors is present near the time when they respond to physiological hearing (i.e., hearing of airborne sound) at P14. At P21, SSTR1 and SSTR2 levels decrease dramatically. A similar developmental pattern was observed for SSTR1 and SSTR2 mRNA, suggesting that the expression of the SSTR1 and SSTR2 genes is controlled at the transcriptional level throughout development. In addition, we observed reduced levels of phospho-Akt and total Akt in SSTR1 knockout and SSTR1/SSTR2 double-knockout mice compared with wild-type mice. We know from previous studies that Akt is involved in hair cell survival. Taken together, the dynamic nature of SSTR1 and SSTR2 expression at a time of major developmental changes in the cochlea suggests that SSTR1 and SSTR2 (and possibly other members of this family) are involved in the maturation of the mammalian cochlea.

Mechanisms Involved in Ototoxicity

The modern era of evidence-based ototoxicity emerged in the 1940s following the discovery of aminoglycosides and their ototoxic side effects. New classes of ototoxins have been identified in subsequent decades, notably loop diuretics, antineoplastic drugs, and metal chelators. Ototoxic drugs are frequently nephrotoxic, as both organs regulate fluid and ion composition. The mechanisms of ototoxicity are as diverse as the pharmacological properties of each ototoxin, though the generation of toxic levels of reactive oxygen species appears to be a common denominator. As mechanisms of cytotoxicity for each ototoxin continue to be elucidated, a new frontier in ototoxicity is emerging: How do ototoxins cross the blood-labyrinth barrier that tightly regulates the composition of the inner ear fluids? Increased knowledge of the mechanisms by which systemic ototoxins are trafficked across the blood-labyrinth barrier into the inner ear is critical to developing new pharmacotherapeutic agents that target the blood-labyrinth barrier to prevent trafficking of ototoxic drugs and their cytotoxic sequelae.

Mechanisms of rapid sensory hair-cell death following co-administration of gentamicin and ethacrynic acid

Concurrent administration of a high dose of gentamicin (GM; 125mg/kg IM) and ethacrynic acid (EA; 40mg/kg IV) results in rapid destruction of virtually all cochlear hair cells; however, the cell death signaling pathways underlying this rapid form of hair-cell degeneration are unclear. To elucidate the mechanisms underlying GM/EA-mediated cell death, several key cell death markers were assessed in the chinchilla cochlea during the early stages of degeneration. In the middle and basal turns of the cochlea, massive hair-cell loss including destruction of the stereocilia and cuticular plate occurred 12h after GM/EA treatment. Condensation and fragmentation of outer hair-cell nuclei, morphological features of apoptosis, were first observed 5-6h post-treatment in the basal turn of the cochlea. Metabolic function, reflected by succinate dehydrogenase histochemistry and mitochondrial staining, decreased significantly in the basal turn 4h following GM/EA treatment; these early changes were accompanied by the release of cytochrome c from the mitochondria into the cytosol and intense expression of initiator caspase-9 and effector caspase-3. GM/EA failed to induce expression of extrinsic initiator caspase-8. These results suggest that the rapid loss of hair cells following GM/EA treatment involves cell death pathways mediated by mitochondrial dysfunction leading to the release of cytochrome c, activation of initiator caspase-9 and effector caspase-3.

Synergistic ototoxicity due to noise exposure and aminoglycoside antibiotics

Acoustic exposure to high intensity and/or prolonged noise causes temporary or permanent threshold shifts in auditory perception, reflected by reversible or irreversible damage in the cochlea. Aminoglycoside antibiotics, used for treating or preventing life-threatening bacterial infections, also induce cytotoxicity in the cochlea. Combined noise and aminoglycoside exposure, particularly in neonatal intensive care units, can lead to auditory threshold shifts greater than simple summation of the two insults. The synergistic toxicity of acoustic exposure and aminoglycoside antibiotics is not limited to simultaneous exposures. Prior acoustic insult which does not result in permanent threshold shifts potentiates aminoglycoside ototoxicity. In addition, exposure to subdamaging doses of aminoglycosides aggravates noise-induced cochlear damage. The mechanisms by which aminoglycosides cause auditory dysfunction are still being unraveled, but likely include the following: 1) penetration into the endolymphatic fluid of the scala media, 2) permeation of nonselective cation channels on the apical surface of hair cells, and 3) generation of toxic reactive oxygen species and interference with other cellular pathways. Here we discuss the effect of combined noise and aminoglycoside exposure to identify pivotal synergistic events that can potentiate ototoxicity, in addition to a current understanding of aminoglycoside trafficking within the cochlea. Preventing the ototoxic synergy of noise and aminoglycosides is best achieved by using non-ototoxic bactericidal drugs, and by attenuating perceived noise intensity when life-saving aminoglycoside therapy is required.

Activation of cell death pathways in the inner ear of the aging CBA/J mouse

We have previously demonstrated that oxidative stress increases in the inner ear of aging CBA/J mice and might contribute to the loss of function of the sensory system. We now investigate the activation of cell death pathways in the cochleae of these animals. Middle-aged (12 months) and old (18-26 months) mice with hearing deficits displayed outer hair cell nuclei with apoptotic and, to a lesser extent, necrotic features. Both intrinsic and extrinsic cell death pathways were activated by translocation or post-translational modification of proteins in the aging cochlea as compared to young (3 months) animals. Cytosolic cytochrome c increased, formed a complex with, and activated caspase 9. Endonuclease G translocated to the nuclei of aging outer hair cells suggesting its function as an apoptotic DNase. The cleaved (and hence active) forms of calpain I and calpain II increased while active cathepsin D was transiently elevated in middle-aged but not old animals. Finally, increases in the phosphorylation of p38 MAPK and JNK implicated the additional involvement of the MAPK pathway. The results suggest that multiple cell death pathways, all potentially linked to oxidative stress, are activated in hair cells of the auditory organ in aging mice.

Metabolic imaging of the organ of corti--a window on cochlea bioenergetics

Hair cell loss is a major cause of sensorineural hearing loss. We have developed a method to examine metabolic events in hair cells in response to stimuli known to cause hair cell loss, such as acoustic trauma and aminoglycoside administration. The method employs two-photon excitation of the metabolic intermediate, reduced nicotinamide adenine dinucleotide (NADH), in hair cell mitochondria in an explanted mouse cochlea. Using this method, we show evidence that the aminoglycoside gentamicin selectively affects the level of mitochondrial NADH in outer hair cells, but not inner hair cells, within minutes of administration.

A systemic gentamicin pathway across the stria vascularis

The mechanism(s) by which systemically-administered aminoglycosides enter the cochlea remain poorly understood. To elucidate which mechanisms may be involved, we co-administered different molar ratios of gentamicin and fluorescent gentamicin (GTTR) to mice in three different regimens: (1) gentamicin (150, 300 or 600mg/kg) containing a constant 300:1 molar ratio of gentamicin:GTTR; (2) 300mg/kg gentamicin containing a variable molar ratio of gentamicin:GTTR (150:1-600:1), or (3) an increasing dose of gentamicin (150-900mg/kg), each dose containing 1.7mg/kg GTTR. Three hours later, cochleae were fixed and examined by confocal microscopy. First, increasing doses of a constant molar ratio of gentamicin:GTTR, resulted in increasing intensities of GTTR fluorescence in hair cells and strial tissues. Second, a fixed gentamicin dose with increasing molar dilution of GTTR led to decreasing GTTR fluorescence in hair cells and strial tissues. Third, a fixed GTTR dose with increasing molar dilution by gentamicin led to decreased GTTR uptake in hair cells and marginal cells, but not intra-strial tissues and capillaries. Thus, only hair cell and marginal cell uptake of GTTR is competitively inhibited by gentamicin, suggesting that a regulatable barrier for gentamicin entry into endolymph exists at the interface between marginal cells, the intra-strial space and intermediate cells.

Taurine modulates calcium influx through L-type voltage-gated calcium channels in isolated cochlear outer hair cells in guinea pigs

Taurine has been proposed to play a role in calcium modulation. To explore the effect of taurine on intracellular calcium homeostasis of isolated cochlear outer hair cells and on the gentamycin-induced inhibition of calcium influx evoked by high K(+) depolarization, we employed fluo-3 imaging of intracellular calcium ([Ca(2+)](i)) via confocal laser scanning microscopy to measure real-time changes of [Ca(2+)](i). We found that the sole application of taurine (5, 10, 20 mM) induced a transient [Ca(2+)](i) increase in a concentration-dependent manner, which was inhibited either by the application of an L-type calcium-channel blocker nifedipine or a calcium-free medium. Pre-incubation with 1mM gentamicin induced inhibition of [C(a)(2+)](i) elevation evoked by high K(+). Short-term (10 min) exposure with a high level of taurine (20 mM) prevented this inhibition. These results indicated that taurine at a high concentration was able to promote calcium influx through L-type calcium channels in isolated outer hair cells and antagonize gentamycin-induced inhibition of calcium elevation evoked by high K(+) by its calcium homeostatic effect.

Understanding drug ototoxicity: molecular insights for prevention and clinical management

Ototoxicity is a trait shared by aminoglycoside and macrolide antibiotics, loop diuretics, platinum-based chemotherapeutic agents, some NSAIDs and antimalarial medications. Because their benefits in combating certain life-threatening diseases often outweigh the risks, the use of these ototoxic drugs cannot simply be avoided. In this review, the authors discuss some of the most frequently used ototoxic drugs and what is currently known about the cell and molecular mechanisms underlying their noxious effects. The authors also provide suggestions for the clinical management of ototoxic medications, including ototoxic detection and drug monitoring. Understanding the mechanisms of drug ototoxicity may lead to new strategies for preventing and curing drug-induced hearing loss, as well as developing new pharmacological drugs with less toxic side effects.

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.

Uptake of fluorescent gentamicin by vertebrate sensory cells in vivo

Aminoglycoside uptake in the inner ear remains poorly understood. We subcutaneously injected a fluorescently-conjugated aminoglycoside, gentamicin-Texas Red (GTTR), to investigate the in vivo uptake of GTTR in the inner ear of several vertebrates, and in various murine sensory cells using confocal microscopy. In bullfrogs, GTTR uptake was prominent in mature hair cells, but not in immature hair cells. Avian hair cells accrued GTTR more rapidly at the base of the basilar papilla. GTTR was associated with the hair bundle; and, in guinea pigs and mice, somatic GTTR fluorescence was initially diffuse before punctate (endosomal) fluorescence could be observed. A baso-apical gradient of intracellular GTTR uptake in guinea pig cochleae could only be detected at early time points (<3h). In 21-28 day mice, cochlear GTTR uptake was greatly reduced compared to guinea pigs, 6-day-old mice, or mice treated with ethacrynic acid. In mice, GTTR was also rapidly taken up, and retained, in the kidney, dorsal root and trigeminal ganglia. In linguinal and vibrissal tissues rapid GTTR uptake cleared over a period of several days. The preferential uptake of GTTR by mature saccular, and proximal hair cells resembles the pattern of aminoglycoside-induced hair cell death in bullfrogs and chicks. Differences in the degree of GTTR uptake in hair cells of different species suggests variation in serum levels, clearance rates from serum, and/or the developmental and functional integrity of the blood-labyrinth barrier. GTTR uptake by hair cells in vivo suggests that GTTR has potential to elucidate aminoglycoside transport mechanisms into the inner ear, and as a bio-tracer for in vivo pharmacokinetic studies.

Effects of electrical stimulation of the inferior colliculus on 2f1-f2 distortion product otoacoustic emissions in anesthetized guinea pigs

The effects of electrical stimulation of the inferior colliculus (IC) on the activation of olivocochlear nerve fibers were investigated in guinea pigs in which the 2f1-f2 distortion product otoacoustic emissions (DPOAE) were recorded. Animals were anesthetized with ketamine (33 mg/kg) and xylazine (6.6 mg/kg). Bipolar electrical stimulation of the IC by a train of pulses with currents less than the threshold for evoking muscle twitches resulted in a small depression of the DPOAE amplitude by 0.1-2 dB. The maximal effect was observed when the stimulating electrodes were located in the rostro-medial or ventral parts of the IC. The suppression of electrically evoked DPOAE was similar to the DPOAE suppression produced by acoustical stimulation of the contralateral ear by a broad-band noise. Suppression of DPOAE amplitude in response to both acoustical and electrical stimulation was abolished 1-2 h after a single intramuscular injection of gentamicin (210-250 mg/kg). The results indicate that electrical stimulation of the IC can activate the efferent system and produce DPOAE changes by similar mechanisms as does acoustical stimulation of the contralateral ear.

Involvement of the mitochondrial permeability transition in gentamicin ototoxicity

Aminoglycosides may induce irreversible hearing loss in both animals and humans. In order to study the nature and mechanisms underlying gentamicin-induced cell death in the inner ear, the cochlear neurosensory epithelia were dissected from guinea pigs and incubated with 0.5-10 mM gentamicin. Concentration-dependent loss of cell viability was detected by the inability of damaged cells to exclude propidium iodide. Outer hair cells were most sensitive towards gentamicin toxicity, followed by inner hair cells whereas Deiters and Hensen cells were not affected by the gentamicin concentrations used. The iron chelators 2,2'-dipyridyl and deferoxamine provided partial protection against gentamicin-induced hair cell death while the calcium chelator Quin-2 AM had no effect. Gentamicin (0.5-1 mM) induced condensation of chromatin typical for apoptosis. Using the fluorescent dye tetramethyl-rhodamine methyl ester and laser scanning microscopy we could visualize a loss of the mitochondrial membrane potential in damaged outer hair cells about 1 h before cell death occurred. Cyclosporin A, an inhibitor of the mitochondrial permeability pore, provided partial protection against gentamicin toxicity. This strongly suggests an involvement of the mitochondrial permeability transition in gentamicin-induced apoptosis.

Intra-cochlear administration of dexamethasone attenuates aminoglycoside ototoxicity in the guinea pig

This study demonstrates the attenuation of aminoglycoside ototoxicity by cochlear infusion of dexamethasone (Dex) using a microcannulation-osmotic pump delivery system. The results indicate that treating the cochlea with Dex both before and after kanamycin administration was more effective in preventing ototoxicity than Dex treatment only after kanamycin administration. A concentration of 1 ng/ml Dex showed the greatest protective effect on both kanamycin-induced threshold shift of the auditory brainstem response and outer hair cell survival. These results show that the Dex treatment attenuates both functional and structural damage of the inner ear from aminoglycoside toxicity.

Leupeptin protects cochlear and vestibular hair cells from gentamicin ototoxicity

Calpains, a family of calcium-activated proteases that breakdown proteins, kinases, phosphatases and transcription factors, can promote cell death. Since leupeptin, a calpain inhibitor, protected against hair cell loss from acoustic overstimulation, we hypothesized that it might protect cochlear and vestibular hair cells against gentamicin (GM) ototoxicity. To test this hypothesis, mouse organotypic cultures from the cochlea, maculae of the utricle and the crista of the semicircular canal (P1-P3) were treated with different doses of GM (0.1-3 mM) alone or in the presence of leupeptin (0.1-3 mM). The percentage of outer hair cells (OHCs) and inner hair cells (IHCs) decreased with increasing doses of GM between 0.1 and 3 mM. The addition of 1 mM of leupeptin significantly reduced GM-induced damage to IHCs and OHCs; this protective effect was dose-dependent. GM also significantly reduced hair cell density in the crista and utricle in a dose-dependent manner between 0.1 and 3 mM. The addition of 1 mM of leupeptin significantly reduced hair cell loss in the crista and utricle for GM concentrations between 0.1 and 3 mM. These results suggest that one of the early steps in GM ototoxicity may involve calcium-activated proteases that lead to the demise of cochlear and vestibular hair cells.

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

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

Objective method for differentiating between drug-induced vestibulotoxicity and cochleotoxicity

HYPOTHESIS

An objective direct method is proposed to differentiate between drug-induced functional vestibulotoxicity and cochleotoxicity.

BACKGROUND

Many substances are ototoxic. Although there are objective methods to directly evaluate functional cochlear toxicity (auditory nerve brainstem responses [ABR]), it is more difficult to assess direct functional ototoxicity to the various vestibular end organs.

METHODS

Short-latency vestibular evoked potentials (VsEP) from different vestibular end organs and ABR, were used to assess functional impairment of the vestibular and cochlear end organs caused by daily injections of the aminoglycoside amikacin (known to be preferentially cochleotoxic) in guinea pigs.

RESULTS

There was no significant change in the various VsEPs. whereas ABR thresholds were elevated, confirming the selective functional cochleotoxicity previously reported, as evaluated by other (mainly nondirect) methods.

CONCLUSION

This study demonstrates the feasibility in general of using short-latency evoked potentials to evaluate functional cochleotoxicity and vestibulotoxicity of ototoxic drugs and to differentiate between them.

Simultaneous monitoring of slow cell motility and calcium signals of the guinea pig outer hair cells

'Slow' motility (shape changes over seconds to minutes) of the mammalian cochlear outer hair cell (OHC) could play a protection role from intense sound pressure and is associated with elevation of the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). In the present work, a new approach was elaborated using fluorescent imaging for continuous monitoring of both [Ca(2+)](i) changes and slow motility of OHCs employing the Ca(2+) fluorescent indicator Fura-2. Whole OHC fluorescence and that of cell segments were analyzed to discriminate between fluorescence changes caused by [Ca(2+)](i) rise and those related to change of the cell shape. The reliability of the method was examined by simultaneous monitoring of [Ca(2+)](i) and OHC length changes induced by change of buffer osmolarity or by increase of KCl concentration. The method revealed that the time course of [Ca(2+)](i) increase and rate of cell shortening often do not coincide. It was also observed that [Ca(2+)](i) increased in 70 mM KCl more slowly than the rate of KCl delivery to OHCs. The comparison of the time courses of [Ca(2+)](i) elevation, induced by increase of K(+)/Na(+) ratio and by substitution of Na(+) with N-methyl-D-glucamine(+), indicated that the relatively slow kinetics of [Ca(2+)](i) increase in the OHC is partially attributed to regulation of Ca(2+) homeostasis by the Na(+)/Ca(2+) exchanger.

Gentamicin blocks ACh-evoked K+ current in guinea-pig outer hair cells by impairing Ca2+ entry at the cholinergic receptor

Aminoglycoside antibiotics such as gentamicin are known to block the medial olivocochlear efferent system. In order to determine whether this inhibition takes place at the postsynaptic cholinergic receptors in outer hair cells (OHCs), we studied the effects of these polycationic molecules on cholinergic currents evoked in isolated guinea-pig OHCs. The cholinergic response of OHCs involves nicotinic-like receptors (nAChRs) permeable to Ca2+ ions that activate nearby Ca2+-sensitive K+ channels (KCa(ACh) channels). The extracellular application of gentamicin and neomycin reversibly blocked ACh-evoked K+ current (IK(ACh)) with IC50 values of 5.5 and 3.2 microM, respectively. The results showed that the blocking mechanism of IK(ACh) was due to inhibition of Ca2+ influx via nAChRs. Our study also provides interesting insights into the functional coupling between nAChRs and KCa(ACh) channels in OHCs. By directly recording the cation current flowing through nAChRs (In(ACh)) using an intracellular solution containing 10 mM BAPTA, we measured an EC50 near 110 microM for ACh-evoked In(ACh). This EC50 for ACh is one order of magnitude higher than that measured indirectly on IK(ACh). This reveals a rather low affinity of ACh for its receptor but a very efficient coupling between nAChRs and KCa(ACh) channels. We also show that a high external Ca2+ concentration reverts the gentamicin inhibition of IK(ACh) and that gentamicin directly alters the cation current flowing through the nAChRs of OHCs. We propose that gentamicin acts as a non-competitive cholinergic blocker by displacing Ca2+ from specific binding sites at the nAChRs. This block of the nAChRs at the level of the postsynaptic membrane in OHCs could explain the inhibitory effect of gentamicin reported on the crossed medial olivocochlear efferent system in vivo.

In vitro effects of hydrogen peroxide on the cochlear neurosensory epithelium of the guinea pig

Reactive oxygen species (ROS) have been postulated to be involved in drug ototoxicity and noise-induced hearing loss. Hydrogen peroxide (H(2)O(2))-induced cell damage in the inner ear was investigated using the neurosensory epithelium of a guinea pig cochlea. Hair cells and supporting cells of the epithelium incubated in Hanks' balanced salt solution were viable up to 6 h. After 2 h of treatment with 0.2 mM H(2)O(2) about 85% of the outer hair cells lost their viability. In contrast inner hair cells slowly began to die after 2 h of H(2)O(2) treatment. The Deiters cells and Hensen cells did not show any signs of damage in the presence of H(2)O(2). Nifedipine, a calcium channel blocker, Quin-2 AM, an intracellular calcium chelator, and 2,2'-dipyridyl, a membrane-permeable iron chelator, all provided partial protection against H(2)O(2)-induced outer hair cell death. The combination of both chelators showed an additional protective effect. The antioxidants N-acetylcysteine and glutathione-monoethyl ester completely protected against H(2)O(2) damage. These results suggest that calcium, iron, and thiol homeostasis play a crucial role in hair cell death caused by H(2)O(2).

Gentamicin blocks both fast and slow effects of olivocochlear activation in anesthetized guinea pigs

The medial olivocochlear (MOC) efferent system, which innervates cochlear outer hair cells, suppresses cochlear responses. MOC-mediated suppression includes both slow and fast components, with time courses differing by three orders of magnitude. Pharmacological studies in anesthetized guinea pigs suggest that both slow and fast effects on cochlear responses require an initial acetylcholine activation of alpha-9 nicotinic receptors on outer hair cells and that slow effects require additional intracellular events downstream from those mediating fast effects. Gentamicin, an aminoglycoside antibiotic, has been reported to block fast effects of sound-evoked OC activation following intramuscular injection in unanesthetized guinea pigs, without changing slow effects. In the present study, we show that electrically evoked fast and slow effects in the anesthetized guinea pig are both blocked by either intramuscular or intracochlear gentamicin, with similar time courses and/or dose-response curves. We suggest that sound-evoked slow effects in unanesthetized animals are fundamentally different from electrically evoked slow effects in anesthetized animals, and that the former may arise from effects of the lateral OC system.

Acute and chronic effects of aminoglycosides on cochlear hair cells

The first detectable effect on the auditory system after a single high-dose injection of an aminoglycosidic antibiotic (AA) like gentamicin (GM) is the reversible blockade of medial efferent function, probably via blockade of calcium channels at the base of the outer hair cells (OHC). The kinetics of this effect are compatible with that of the molecule in perilymph. In the course of chronic treatment with lower doses, however, ototoxicity develops only after several days of treatment. Still GM can be observed inside the OHCs as soon as 24 hours after the first injection, and will be still present in some OHCs as long as 11 months after a chronic, nonototoxic 6-day treatment. In vitro, the short-term viability of isolated OHCs is not affected by exposure to AAs, but their transduction channels and their response to acetylcholine are reversibly blocked. However, developing organs of Corti in culture are highly and rapidly affected by exposure to AAs. Yet during direct intracochlear perilymphatic perfusion of GM, 2-mM solutions are not ototoxic, and with perfusion with a 20-mM solution ototoxicity develops only after several days of perfusion. From these various observations one can describe some aspects of the mechanisms of ototoxicity of AAs, from their access to perilymph and endolymph, to penetration in the hair cells, likely via endocytosis at their apical pole, and intracellular cytotoxic events.

Capability of serum to convert streptomycin to cytotoxin in patients with aminoglycoside-induced hearing loss

Individual variations in sensitivity to the ototoxic effects of aminoglycoside antibiotics are well documented. Our research demonstrates that there is an apparent difference in serum from patients who are resistant or susceptible to aminoglycoside ototoxicity. In the first study, the cytotoxicity of sera from patients with and without hearing loss after various time periods following the discontinuation of aminoglycoside treatment was assayed using the isolated outer hair cell toxicity assay. The results indicate that sera from patients with hearing loss were significantly more toxic than sera from patients with normal hearing or minimal hearing loss. This toxicity may persist for up to 1 year after discontinuation of aminoglycoside therapy. In a second study, sera were obtained from patients who had received aminoglycoside therapy several years previously. None of these sera was toxic to isolated outer hair cells in vitro. Streptomycin was then incubated with the sera or a protein fraction isolated from sera, and the incubation mixtures were tested for toxicity. The percentage of damaged outer hair cells was significantly higher when streptomycin had been treated with sera or a serum protein fraction from patients with hearing loss (58+/-10% and 68+/-9%, respectively) than with sera or a serum protein fraction from a control group (10+/-5% and 17+/-4%, respectively). In addition, several incubation mixtures were analyzed using high performance liquid chromatography. A new chromatographic peak was only found in the incubations of streptomycin with serum protein from patients with hearing loss. The results suggest that sera from individuals sensitive to aminoglycoside antibiotics may metabolize these drugs to cytotoxins.

Once daily aminoglycoside therapy. Is it less toxic than multiple daily doses and how should it be monitored?

After 50 years of clinical experience with the aminoglycoside agents, there is continuing debate over the most appropriate administration regimen for these drugs. In recent years, once daily administration has been used increasingly, in the hope of both improving efficacy and reducing toxicity. At least 30 controlled clinical trials have compared once versus conventional multiple daily administration. Efficacy was assessed in some, but not all, studies using clinical and/or bacteriological cure. Toxicity was generally determined using rather nonsensitive end-points such as measurement of serum creatinine for nephrotoxicity and clinically detectable hearing loss for ototoxicity. The results of individual clinical trials and subsequent meta-analyses have been variable. However, 5 of 9 meta-analyses found clinical efficacy to be significantly better with once daily administration, and in 3 of the 9 there were significantly less nephrotoxicity with once daily administration. The results were not significant for ototoxicity in any of the meta-analyses. There is debate about how therapeutic drug monitoring should be performed, and whether it is still required with once daily administration. Previous experience with the aminoglycosides, especially in patients with impaired drug clearance caused by renal impairment, suggests that monitoring is still prudent. Results from the once daily administration trials appear to support this. Various methods of monitoring and dose adjustment have been proposed. The most common is to measure a 24-hour trough concentration and to adjust the dose to maintain the trough concentration below a value of 2, 1 or 0.5 mg/L. However, this method allows for greater total aminoglycoside exposure than has been permitted with conventional dosages, increasing the likelihood of toxicity in patients with impaired aminoglycoside clearance. Other methods measure drug concentrations at a time-point or points within the dose interval (when the concentration is still measurable), and adjust the dose according to concentration-time curve nomograms or to a target area under the concentration-time curve. This allows the use of higher doses in those with high drug clearance. Furthermore, in patients with impaired clearance, drug exposure is limited to the same extent as, or less than, that with conventional multiple daily administration. To date no controlled trials have compared methods of dose-individualisation. In summary, in addition to a slight overall improvement in efficacy, once daily administration has resulted in a small reduction in nephrotoxicity. In the studies using more sensitive measures of toxicity, the differences in toxicity were greater, strengthening the case for once daily administration. Therapeutic drug monitoring is probably required with once daily administration. Methods which use mid-dosage interval concentrations to gauge drug exposure would seem to be preferable over trough concentration measurement.

Cochlear outer hair cell electromotility can provide force for both low and high intensity distortion product otoacoustic emissions

It is generally believed that the force for the otoacoustic emission (OAE) generation is provided by a mechanism of electromotility, observed in isolated cochlear outer hair cells (OHCs). OHC electromotility is resistant to several ototoxic reagents, it does not depend on ATP hydrolysis, but it can be blocked by specific sulfhydryl reagents: p-chloromercuriphenylsulfonic acid (pCMPS) and p-hydroxymercuriphenylsulfonic acid (pHMPS). We have used these reagents to test whether they also affect OAE. Application of pCMPS and pHMPS on the round window membrane of anesthetized guinea pigs produced a dose-dependent inhibition of the cubic (2F1-F2) distortion product OAE (DPOAE). The inhibition developed progressively from high to low frequencies, reflecting the diffusion of the drugs through the cochlear compartment. The effect of pCMPS and pHMPS was different from the effects of furosemide and lethal anoxia, which impair cochlear function but do not block OHC electromotility. pHMPS suppressed DPOAE completely at all sound intensities tested (45-80 dB SPL), whereas furosemide or lethal anoxia caused DPOAE to disappear at low-level stimulation (45-60 dB SPL) only. Our results suggest that the OHC electromotility might provide the force for DPOAE generation not only at low, but also at high stimulus intensities.

Inhibition of phosphoinositide metabolism or chelation of intracellular calcium blocks FSH-induced but not spontaneous meiotic resumption in mouse oocytes

Mammalian oocytes are arrested at the diplotene phase of the first meiotic division until ovulation. In the mouse, germinal vesicle breakdown (GVBD) and progression to metaphase II is thought to be triggered by a positive signal originating in the follicular cells following stimulation by the luteinizing hormone (LH) surge. Isolated, fully grown oocytes can also undergo spontaneous reinitiation of meiosis in vitro in the absence of gonadotrophin stimulation. To investigate the mechanism of meiotic resumption, inhibitors of phosphoinositide metabolism and an intracellular calcium chelator were used during maturation in vitro under different conditions. In a series of experiments, isolated cumulus cell-oocyte complexes (COCs) maintained in meiotic arrest by hypoxanthine were induced to resume meiosis by treatment with follicle-stimulating hormone (FSH). Under these conditions, both LiCl and neomycin, which inhibit phosphoinositide hydrolysis, produced a dose-dependent inhibitory effect on meiotic resumption. Similar results were obtained when FSH-induced meiotic resumption was observed in the presence of the acetoxymethyl ester form of 1, 2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA/AM), an intracellular calcium chelator. In hypoxanthine-arrested oocytes, GVBD induced by epidermal growth factor (EGF), which mimics FSH action in in vitro maturation, was also repressed by LiCl and neomycin. Conversely, meiotic resumption triggered by a pulse of 8-bromo-cyclic adenosine monophosphate (8-Br cAMP) was not affected by these two inhibitors. In experiments in which oocytes were cultured under conditions which permit spontaneous meiotic maturation, resumption of meiosis was not affected by either inhibition of phosphoinositide hydrolysis or chelation of intracellular calcium. Therefore, it appears that meiotic resumption induced by hormone stimulation requires activation of the phosphoinositide pathway and mobilization of intracellular calcium. In contrast, spontaneous maturation probably occurs through a different mechanism because it is not affected by inhibition of this signaling pathway.

Acute hearing loss following fractionated stereotactic radiosurgery for acoustic neuroma. Report of two cases

Two cases of acute hearing loss are reported following fractionated stereotactic radiosurgery for acoustic neuroma. Both patients had neurofibromatosis type 2 and were treated with a peripheral tumor dose of 21 Gy delivered in three fractions (7 Gy each) with a minimum interfraction interval of 10 hours. One patient who had previously undergone surgical resection of the treated tumor presented with only rudimentary hearing in the treated ear secondary to an abrupt decrease in hearing prior to treatment. That patient reported total loss of hearing before complete delivery of the third fraction. The second patient had moderately impaired hearing prior to treatment; however, within 10 hours after delivery of the final fraction, he lost all hearing. Both patients showed no improvement in response to glucocorticoid therapy. Possible explanations for this phenomenon are presented.

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.

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

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

Quantitative differences in endolymphatic calcium and endocochlear potential between pigmented and albino guinea pigs

A number of previous studies have suggested that melanin may play a role in Ca2+ homeostasis of endolymph. In the present study, endolymph Ca2+ levels and endocochlear potential (EP) were measured in all four cochlear turns of pigmented or albino guinea pigs. Auditory sensitivity was also evaluated using cochlear action potential (AP) thresholds. In pigmented animals we found that endolymph Ca2+ tended to increase from base to apex of the cochlea, while EP systematically decreased towards the apex. In contrast, no significant Ca2+ gradient was found in albinos and the EP decline was far less. As a result, the apical turn of albino animals had significantly lower Ca2+ and significantly higher EP than in pigmented animals. AP thresholds pooled across all test frequencies were significantly lower in albino animals although no differences at individual frequencies reached significance. Even after correction for EP differences, the endolymph Ca2+ levels in albino animals were significantly lower than in pigmented ones. These results confirm that there are significant physiologic differences between pigmented and albino animals, which are a likely consequence of the absence of melanin in the albino cochlea. They are consistent with the involvement of melanin in the active transport of Ca2+ into endolymph.

Correspondence between middle frequency auditory loss in vivo and outer hair cell shortening in vitro

The aromatic hydrocarbon, toluene, has been reported to disrupt auditory system function both in occupational epidemiological and in laboratory animal investigations. This agent, along with several other organic solvents, impairs hearing preferentially at middle frequencies - a finding that distinguishes these agents from the traditional high frequency impairment observed with ototoxic drugs such as aminoglycoside antibiotics and cisplatin. Prior investigations performed in vivo have identified the outer hair cell as a probable target for toluene exposure. The purpose of this investigation was to determine directly whether outer hair cells isolated from the guinea pig cochlea show morphological alterations consistent with the toxic response seen in physiological studies with toluene exposure. The effect of toluene superfusion on outer hair cell shortening was assessed for cells harvested from different locations within the cochlea. Control studies included assessment of cell shortening among outer hair cells exposed to trimethyltin and cells exposed to benzene. Trimethyltin disrupts high frequency hearing preferentially and benzene does not produce hearing loss in vivo. Toluene at a concentration of 100 microM produced a marked shortening of outer hair cells although the effect was significantly greater among cells isolated from the apical half of the cochlea than from the basal half of the cochlea. By contrast, trimethyltin at the same concentration produced a preferential shortening among outer hair cells from the base of the cochlea. Benzene (100 microM) did not disrupt outer hair cell length of cells harvested from the apex. The results indicate that intrinsic features of outer hair cells contribute significantly to the site of ototoxic impairment observed in vivo for toluene.

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

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

A study on temperature dependent vestibular potential: effect of long lasting thermal stimulus and aminoglycoside agent

Effect of temperature changes on the vestibular receptor was studied using isolated posterior semicircular canals (PSC) of bull frogs. Cupula was removed from the crista. PSC was placed in the inner compartment of the double chamber. Warm or cool water was filled in the outer compartment to change the temperature of the inner compartment. Changes of the spontaneous discharge were recorded from the ampullary nerve. When cooled, the discharge temporarily increased, followed by a gradual decrease. When warmed, it temporarily decreased and then increased, forming response curves of a mirror image. After addition of streptomycin, the temperature dependent response disappeared. These results suggest that the semicircular canal receptor is activated by direct temperature effects.

Toluene disrupts outer hair cell morphometry and intracellular calcium homeostasis in cochlear cells of guinea pigs

The aromatic hydrocarbon, toluene, has been demonstrated to disrupt auditory system function both in occupational epidemiological and in laboratory animal investigations. This agent, along with several other organic solvents, impairs hearing preferentially at middle frequencies-a finding that distinguishes these agents from the traditional high-frequency impairment observed with ototoxic drugs such as aminoglycoside antibiotics and cisplatin. Prior investigations have identified the outer hair cell as a probable target for toluene exposure, although studies designed to evaluate spiral ganglion cell impairment have not been reported. The purpose of this investigation was to determine directly whether outer hair cells isolated from the guinea pig cochlea show morphological alterations consistent with a toxic response to toluene exposure. Since slow adjustments of outer hair cell length can result from alteration in free intracellular calcium concentration, the effect of toluene on calcium homeostasis was monitored in both outer hair cells and spiral ganglion cells. A dose-response relationship was observed in the extent of outer hair cell shortening produced by toluene with a significant shortening observed at concentrations of 100 microM and higher. By contrast, the nonototoxic solvent, benzene, produced little shortening at 100 microM to 1 mM concentrations. Studies of calcium homeostasis conducted using the fluorescent probe, Fura-2, showed that toluene enhanced free intracellular calcium levels of both outer hair cells and spiral ganglion cells within 5 min of exposure at concentrations of 30 microM and higher. Intracellular calcium levels were elevated only slightly following benzene administration at 1 mM, but not at lower concentrations. Cells cultured in artificial perilymph nominally containing no calcium and those to which EGTA was added still showed a maximal increase in intracellular calcium level when treated with toluene. These data indicate that the elevation in free intracellular calcium levels produced by toluene results from release of calcium from intracellular stores.

Gentamicin ototoxicity induced apoptosis of the vestibular hair cells of guinea pigs

To clarify mechanisms of inner ear cell death induced by aminoglycosides, we used an in situ nick-end labelling method to examine guinea pig vestibular epithelia after chronic systemic treatments with gentamicin to produce apoptosis. Such changes occurred in damaged hair cells, suggesting that this process may be crucial for subsequent repair and cell regeneration.

Distribution of Ca2+ channels on cochlear outer hair cells revealed by fluorescent dihydropyridines

Physiological evidence has shown that cochlear outer hair cells (OHC) possess L-type voltage-dependent Ca2+ channels through which Ca2+ enters the OHC during depolarization. Their subcellular distribution has, however, remained unclear. In this study, the distribution of L-type Ca2+ channels on the basolateral plasma membrane of OHC has been demonstrated by the use of a laser scanning confocal microscope (LSCM) and a fluorescent probe DMBODIPY-DHP. The fluorescent staining pattern on the basolateral wall is nonuniform, suggesting a heterogeneous distribution of the channels in the plasma membrane. Direct imaging of intracellular Ca2+ visualized in real time by means of the LSCM and the fluorescent Ca2+ probe fluo 3 revealed temporal and spatial integration of Ca2+ movements and Ca2+ channel distribution. Exposure to high-K+ solution induced heterogeneity in the subcellular increase in the intracellular Ca2+ concentration. These results suggest that the heterogeneous distribution of L-type Ca2+ channels on the basolateral membrane might induce heterogeneous intracellular Ca2+ distribution during electrical activity in the OHC.

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.