Comparison of different aminoglycoside antibiotic treatments to refine ototoxicity studies in adult mice. Lab Anim. 2010;44:124-131. [PMID: 19858169 DOI: 10.1258/la.2009.009046]

Murine cochlear damage models in the context of hair cell regeneration research

Understanding the complex pathologies associated with hearing loss is a significant motivation for conducting inner ear research. Lifelong exposure to loud noise, ototoxic drugs, genetic diversity, sex, and aging collectively contribute to human hearing loss. Replicating this pathology in research animals is challenging because hearing impairment has varied causes and different manifestations. A central aspect, however, is the loss of sensory hair cells and the inability of the mammalian cochlea to replace them. Researching therapeutic strategies to rekindle regenerative cochlear capacity, therefore, requires the generation of animal models in which cochlear hair cells are eliminated. This review discusses different approaches to ablate cochlear hair cells in adult mice. We inventoried the cochlear cyto- and histo-pathology caused by acoustic overstimulation, systemic and locally applied drugs, and various genetic tools. The focus is not to prescribe a perfect damage model but to highlight the limitations and advantages of existing approaches and identify areas for further refinement of damage models for use in regenerative studies.

Macrophage depletion attenuates degeneration of spiral ganglion neurons in kanamycin-induced unilateral hearing loss model

Pathological conditions in cochlea, such as ototoxicity, acoustic trauma, and age-related cochlear degeneration, induce cell death in the organ of Corti and degeneration of the spiral ganglion neurons (SGNs). Although macrophages play an essential role after cochlear injury, its role in the SGNs is limitedly understood. We analyzed the status of macrophage activation and neuronal damage in the spiral ganglion after kanamycin-induced unilateral hearing loss in mice. The number of ionized calcium-binding adapter molecule 1 (Iba1)-positive macrophages increased 3 days after unilateral kanamycin injection. Macrophages showed larger cell bodies, suggesting activation status. Interestingly, the number of activating transcription factor 3 (ATF3)-positive-neurons, an indicator of early neuronal damage, also increased at the same timing. In the later stages, the number of macrophages decreased, and the cell bodies became smaller, although the number of neuronal deaths increased. To understand their role in neuronal damage, macrophages were depleted via intraperitoneal injection of clodronate liposome 24 h after kanamycin injection. Macrophage depletion decreased the number of ATF3-positive neurons at day 3 and neuronal death at day 28 in the spiral ganglion following kanamycin injection. Our results suggest that suppression of inflammation by clodronate at early timing can protect spiral ganglion damage following cochlear insult.

Preclinical prospects of investigational agents for hearing loss treatment

INTRODUCTION

: Hearing loss has a high prevalence, with aging, noise exposure, ototoxic drug therapies, and genetic mutations being some of the leading causes of hearing loss. Health conditions such as cardiovascular disease and diabetes are associated with hearing loss, perhaps due to shared vascular pathology in the ear and in other tissues.

AREAS COVERED

: Issues in the design of preclinical research preclude the ability to make comparisons regarding the relative efficacy of different drugs of interest for possible hearing loss prevention or hearing restoration. This has not slowed the advancement of candidate therapeutics into human clinical testing. There is a robust pipeline with drugs that have different mechanisms of action providing diverse candidate therapies and opportunities for combination therapies to be considered.

EXPERT OPINION

: Much of the preclinical research literature lacks standard study design elements such as dose response testing, and lack of standardization of test protocols significantly limits conclusions regarding relative efficacy. Nonetheless, the many positive results to date have supported translation of preclinical efforts into clinical trials assessing potential human benefits. Approval of the first hearing loss prevention therapeutic is a major success, providing a pathway for other drugs to follow.

The vestibular calyceal junction is dismantled following subchronic streptomycin in rats and sensory epithelium stress in humans

Hair cell (HC) loss by epithelial extrusion has been described to occur in the rodent vestibular system during chronic 3,3'-iminodipropionitrile (IDPN) ototoxicity. This is preceded by dismantlement of the calyceal junction in the contact between type I HC (HCI) and calyx afferent terminals. Here, we evaluated whether these phenomena have wider significance. First, we studied rats receiving seven different doses of streptomycin, ranging from 100 to 800 mg/kg/day, for 3-8 weeks. Streptomycin caused loss of vestibular function associated with partial loss of HCI and decreased expression of contactin-associated protein (CASPR1), denoting calyceal junction dismantlement, in the calyces encasing the surviving HCI. Additional molecular and ultrastructural data supported the conclusion that HC-calyx detachment precede HCI loss by extrusion. Animals allowed to survive after the treatment showed functional recuperation and rebuilding of the calyceal junction. Second, we evaluated human sensory epithelia obtained during therapeutic labyrinthectomies and trans-labyrinthine tumour excisions. Some samples showed abnormal CASPR1 label strongly suggestive of calyceal junction dismantlement. Therefore, reversible dismantlement of the vestibular calyceal junction may be a common response triggered by chronic stress, including ototoxic stress, before HCI loss. This may partly explain clinical observations of reversion in function loss after aminoglycoside exposure.

Neomycin-Induced Deafness in Neonatal Mice

BACKGROUND

Hearing impairment is a rising public health issue, and current therapeutics fail to restore normal auditory sensation. Animal models are essential to a better understanding of the pathophysiology of deafness and developing therapeutics to restore hearing.

NEW METHODS

Wild-type CBA/CaJ neonatal mice P2-5 were used in this study. Neomycin suspension (500nl of 50 or 100mg/ml) was micro-injected into the endolymphatic space. Cochlear morphology was examined 3 and 7 days after injection; hair cell (HC) loss, supporting cell morphology, and neurite denervation pattern were assessed with whole-mounts. At 2 and 4 weeks post-injection, the spiral ganglion neuron (SGN) density was analyzed with cryostat sections. Audiometric responses were measured with auditory brain response (ABR) at 4 weeks.

RESULTS

Rapid and complete degeneration of the inner and outer HCs occurred as early as 3 days post-injection. Subsequently, time- and dose-dependent degeneration patterns were observed along the axis of the cochlear membranous labyrinth forming a flat epithelium. Likewise, the SGN histology demonstrated significant cell density reduction at 2 and 4 weeks. The ABR threshold measurements confirmed profound deafness at 4 weeks.

COMPARISON WITH EXISTING METHODS

Compared to previously described local and systemic aminoglycoside injections, this method provides a reliable, robust, and rapid deafening model with a single infusion of neomycin in neonatal mice. This model also allows for investigating the effects of inner ear damage during auditory maturation.

CONCLUSIONS

A single injection of neomycin into the endolymphatic space induces robust HC loss and denervation in neonatal mice.

Hair cell toxicology: With the help of a little fish

Hearing or balance loss are disabling conditions that have a serious impact in those suffering them, especially when they appear in children. Their ultimate cause is frequently the loss of function of mechanosensory hair cells in the inner ear. Hair cells can be damaged by environmental insults, like noise or chemical agents, known as ototoxins. Two of the most common ototoxins are life-saving medications: cisplatin against solid tumors, and aminoglycoside antibiotics to treat infections. However, due to their localization inside the temporal bone, hair cells are difficult to study in mammals. As an alternative animal model, zebrafish larvae have hair cells similar to those in mammals, some of which are located in a fish specific organ on the surface of the skin, the lateral line. This makes them easy to observe in vivo and readily accessible for ototoxins or otoprotective substances. These features have made possible advances in the study of the mechanisms mediating ototoxicity or identifying new potential ototoxins. Most importantly, the small size of the zebrafish larvae has allowed screening thousands of molecules searching for otoprotective agents in a scale that would be highly impractical in rodent models. The positive hits found can then start the long road to reach clinical settings to prevent hearing or balance loss.

Induced Pluripotent Stem Cells, a Stepping Stone to In Vitro Human Models of Hearing Loss

Hearing loss is the most prevalent sensorineural impairment in humans. Yet despite very active research, no effective therapy other than the cochlear implant has reached the clinic. Main reasons for this failure are the multifactorial nature of the disorder, its heterogeneity, and a late onset that hinders the identification of etiological factors. Another problem is the lack of human samples such that practically all the work has been conducted on animals. Although highly valuable data have been obtained from such models, there is the risk that inter-species differences exist that may compromise the relevance of the gathered data. Human-based models are therefore direly needed. The irruption of human induced pluripotent stem cell technologies in the field of hearing research offers the possibility to generate an array of otic cell models of human origin; these may enable the identification of guiding signalling cues during inner ear development and of the mechanisms that lead from genetic alterations to pathology. These models will also be extremely valuable when conducting ototoxicity analyses and when exploring new avenues towards regeneration in the inner ear. This review summarises some of the work that has already been conducted with these cells and contemplates future possibilities.

Identifying targets to prevent aminoglycoside ototoxicity

Aminoglycosides are potent antibiotics that are commonly prescribed worldwide. Their use carries significant risks of ototoxicity by directly causing inner ear hair cell degeneration. Despite their ototoxic side effects, there are currently no approved antidotes. Here we review recent advances in our understanding of aminoglycoside ototoxicity, mechanisms of drug transport, and promising sites for intervention to prevent ototoxicity.

Aminoglycoside treatment alters hearing-related genes and depicts behavioral defects in Drosophila

The hearing organ of Drosophila is present within the second segment of antennae. The hearing organ of Drosophila (Johnston's organ [JO]) shares much structural, developmental, and functional similarity with the vertebrate hearing organ (Organ of Corti). JO is evolving as a potential model system to examine the hearing-associated defects in vertebrates. In the vertebrates, aminoglycosides like gentamicin, kanamycin, and neomycin have been known to cause defects in the hearing organ. However, a complete mechanism of toxicity is not known. Taking the evolutionary conservation into account the current study aims to test various concentrations of aminoglycoside on the model organism, Drosophila melanogaster. The current study uses the oral route to check the toxicity of various aminoglycosides at different concentrations (50, 100, 150, 200, and 250 μg ml^{- 1} ). In Drosophila, many foreign particles enter the body through the gut via food. The aminoglycoside treated third instar larvae show defective crawling and sound avoidance behavior. The adult flies release lower amounts of acetylcholine esterase and higher amounts of reactive oxygen species than control untreated animals, accompanied by defective climbing and aggressive behavior. All these behavioral defects are further confirmed by the altered expression level of hearing genes such as nompC, inactive, nanchung, pyrexia. All the behavioral and genetic defects are reported as a readout of aminoglycoside toxicity.

Effect of Oral Allylnitrile Administration on Cochlear Functioning in Mice Following Comparison of Different Anesthetics for Hearing Assessment

Background: Allylnitrile is a compound found in cruciferous vegetables and has the same lethality and toxic effects as the other nitriles. In 2013, a viable allylnitrile ototoxicity mouse model was established. The toxicity of allylnitrile was limited through inhibition of CYP2E1 with trans-1,2-dichloroethylene (TDCE). The allylnitrile intoxication model has been extensively tested in the 129S1 mouse strain for vestibular function, which showed significant HC loss in the vestibular organ accompanied by severe behavioral abnormalities. However, the effect of allylnitrile on auditory function remains to be evaluated. Commonly used anesthetics to conduct hearing measurements are isoflurane and ketamine/xylazine anesthesia but the effect of these anesthetics on hearing assessment is still unknown. In this study we will evaluate the otovestibular effects of oral allylnitrile administration in mice. In addition, we will compare the influence of isoflurane and ketamine/xylazine anesthesia on hearing thresholds.

Methods and Materials: Fourteen Coch+/– CBACa mice were randomly allocated into an allylnitrile (n = 8) and a control group (n = 6). Baseline measurements were done with isoflurane and 1 week later under ketamine/xylazine anesthesia. After baseline audiovestibular measurements, mice were co-administered with a single dose of allylnitrile and, to reduce systemic toxicity, three intraperitoneal injections of TDCE were given. Hearing loss was evaluated by recordings of auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE). Specific behavioral test batteries for vestibular function were used to assess alterations in vestibular function.

Results: Hearing thresholds were significantly elevated when using isoflurane anesthesia compared to ketamine/xylazine anesthesia for all frequencies of the ABR and the mid-to-high frequencies in DPOAE. Allylnitrile-treated mice lacked detectable ABR thresholds at each frequency tested, while DPOAE thresholds were significantly elevated in the low-frequency region of the cochlea and completely lacking in the mid-to high frequency region. Vestibular function was not affected by allylnitrile administration.

Conclusion: Isoflurane anesthesia has a negative confounding effect on the measurement of hearing thresholds in mice. A single oral dose of allylnitrile induced hearing loss but did not significantly alter vestibular function in mice. This is the first study to show that administration of allylnitrile can cause a complete loss of hearing function in mice.

A Novel Mouse Model of Aminoglycoside-Induced Hyperacusis and Tinnitus

Aminoglycosides (AG) such as amikacin are commonly used in cystic fibrosis patients with opportunistic pulmonary infections including multi-drug resistant mycobacterium tuberculous and non-tuberculous mycobacterium. Unfortunately, this class of drugs is known to cause peripheral damage to the cochlea leading to hearing loss that can fluctuate and become permanent over time or multiple exposures. However, whether amikacin can lead to central auditory dysfunction like hyperacusis (increased sensitivity to sound) or tinnitus (perception of sound in the absence of acoustic stimulation) is not well-described in the literature. Thus, an animal model needs to be developed that documents these side effects in order to develop therapeutic solutions to reduce AG-induced auditory dysfunction. Here we present pioneer work in mice which demonstrates that amikacin can lead to fluctuating behavioral evidence of hyperacusis and tinnitus as assessed by the acoustic startle reflex. Additionally, electrophysiological assessments of hearing via auditory brainstem response demonstrate increased central activity in the auditory brainstem. These data together suggest that peripheral AG-induced dysfunction can lead to central hyperactivity and possible behavioral manifestations of hyperacusis and tinnitus. Importantly, we demonstrate that ebselen, a novel investigational drug that acts as both an antioxidant and anti-inflammatory, can mitigate AG-induced hyperacusis.

Intravenously delivered aminoglycoside antibiotics, tobramycin and amikacin, are not ototoxic in mice

Many drugs on the World Health Organization's list of critical medicines are ototoxic, destroying sensory hair cells within the ear. These drugs preserve life, but patients can experience side effects including permanent hearing loss and vestibular dysfunction. Aminoglycoside ototoxicity was first recognised 80 years ago. However, no preventative treatments have been developed. In order to develop such treatments, we must identify the factors driving hair cell death. In vivo, studies of cell death are typically conducted using mouse models. However, a robust model of aminoglycoside ototoxicity does not exist. Previous studies testing aminoglycoside delivery via intraperitoneal or subcutaneous injection have produced variable ototoxic effects in the mouse. As a result, surgical drug delivery to the rodent ear is often used to achieve ototoxicity. However, this technique does not accurately model clinical practice. In the clinic, aminoglycosides are administered to humans intravenously (i.v.). However, repeated i.v. delivery has not been reported in the mouse. This study evaluated whether repeated i.v. administration of amikacin or tobramycin would induce hearing loss. Daily i.v. injections over a two-week period were well tolerated and transient low frequency hearing loss was observed in the aminoglycoside treatment groups. However, the hearing changes observed did not mimic the high frequency patterns of hearing loss observed in humans. Our results indicate that the i.v. delivery of tobramycin or amikacin is not an effective technique for inducing ototoxicity in mice. This result is consistent with previously published reports indicating that the mouse cochlea is resistant to systemically delivered aminoglycoside ototoxicity.

Functional and morphological analysis of different aminoglycoside treatment regimens inducing hearing loss in mice

Aminoglycoside ototoxicity is common in clinical practice but reliable protective agents currently do not exist. Aminoglycoside regimens causing ototoxicity in different laboratory animals are under investigation. The assessment method used most commonly to determine auditory effects is the auditory brainstem response (ABR). Distortion product otoacoustic emissions (DPOAE) have been used less frequently. A precise recommendation on the specific method to assess peripheral auditory function before and after aminoglycoside toxicity in mice does not exist. In order to evaluate various mouse models for ototoxic injury caused by various aminoglycoside regimens, there is a need for performing preliminary tests in small cohorts before large experiments. The aim of our study was to investigate different aminoglycoside regimens that cause substantial ototoxic damage in vivo. Aminoglycosides are safe and produce a detectable hearing threshold shift in a small cohort of mice that can be used as a model for preliminary tests. Different ototoxic regimens were assessed by ABR and DPOAE measurements pre- and post-treatment. Further, the sensory cell loss was quantified by counting hair cells in the cochlea. It was revealed that an ototoxic regimen with kanamycin twice daily for 15 consecutive days is safe, well tolerated and produces an early significant hearing threshold shift detected by DPOAE in a small cohort of mice. The study compared ABR and DPOAE in mentioned regimens for the first time and illustrated that DPOAE is well suited for detecting hearing threshold shifts in high frequencies before ABR threshold shifts occur in accordance with predominating outer hair cell damage mainly in the basal turn of the cochlea.

Deficit of mitogen-activated protein kinase phosphatase 1 (DUSP1) accelerates progressive hearing loss

Mitogen-activated protein kinases (MAPK) such as p38 and the c-Jun N-terminal kinases (JNKs) are activated during the cellular response to stress signals. Their activity is regulated by the MAPK-phosphatase 1 (DUSP1), a key component of the anti-inflammatory response. Stress kinases are well-described elements of the response to otic injury and the otoprotective potential of JNK inhibitors is being tested in clinical trials. By contrast, there are no studies exploring the role of DUSP1 in hearing and hearing loss. Here we show that Dusp1 expression is age-regulated in the mouse cochlea. Dusp1 gene knock-out caused premature progressive hearing loss, as confirmed by auditory evoked responses in Dusp1^-/- mice. Hearing loss correlated with cell death in hair cells, degeneration of spiral neurons and increased macrophage infiltration. Dusp1^-/- mouse cochleae showed imbalanced redox status and dysregulated expression of cytokines. These data suggest that DUSP1 is essential for cochlear homeostasis in the response to stress during ageing.

Ciprofloxacin and Rifampin Dual Antibiotic-Loaded Biopolymer Chitosan Sponge for Bacterial Inhibition

Complex extremity wounds in Wounded Warriors can become contaminated with microbes, which may cause clinical outcomes resulting in amputation, morbidity, or even fatality. Local delivery of multiple or broad-spectrum antibiotics allows practicing clinicians treatment solutions that may inhibit biofilm formation. Propagation of vancomycin-resistant Staphylococcus aureus is also a growing concern. The development of vancomycin-resistant S. aureus has become a critical challenge in nosocomial infection prevention in the USA, but to date has seen little occurrence in osteomyelitis. As an alternative, locally delivered ciprofloxacin and rifampin were investigated in a preclinical model for the prevention of biofilm in complex extremity wounds with implanted fixation device. In vitro assays demonstrated ciprofloxacin and rifampin possess an additive effect against Gram-negative Pseudomonas aeruginosa and were actively eluted from a chitosan sponge based local delivery system. In an in vivo orthopedic hardware-associated polymicrobial model (S. aureus and Escherichia coli) the combination was able to achieve complete clearance of both bacterial strains. E. coli was detected in bone of untreated animals, but did not form biofilm on wires. Results reveal the clinical potential of antibiotic-loaded chitosan sponges to inhibit infection through tailored antibiotic selection at desired concentrations with efficacy towards biofilm inhibition.

Local Delivery of Amikacin and Vancomycin from Chitosan Sponges Prevent Polymicrobial Implant-Associated Biofilm

Military personnel have high risk for infection, particularly those with combat-related extremity trauma. Administration of multiple or broad-spectrum antibiotics provides clinicians with a strategy for preventing biofilm-based medical device infections. Selection of effective antibiotic combinations based on common pathogens may be used to improve chitosan wound dressing sponge-based local antibiotic delivery systems. In vitro assays in this study demonstrate that vancomycin and amikacin have a synergistic relationship against a strain of osteomyelitis-producing Gram-positive Staphylococcus aureus, although an indifferent relationship was observed against Gram-negative Pseudomonas aeruginosa. In an in vivo model of orthopedic hardware-associated polymicrobial (S. aureus and Escherichia coli) biofilm, chitosan sponges loaded with a combination of vancomycin and amikacin at 5 mg/mL each showed a greater percentage of complete clearance, 50%, than either antibiotic alone, 8.33%. Doubling the loading concentration of the combination achieved a complete clearance rate of 100%, a four log-fold reduction of S. aureus on the wire and a six log-fold reduction in bone. E. coli was detected in bone of untreated animals but did not form biofilm on wires. Results demonstrate the clinical potential of chitosan sponges to prevent infection and illustrates antibiotic selection and loading concentrations necessary for effective biofilm prevention.

Antibiotics and the nervous system: More than just the microbes?

The use of antibiotics has recently risen to prominence in neuroscience due to their potential value in studying the microbiota-gut-brain axis. In this context they have been largely employed to illustrate the many influences of the gut microbiota on brain function and behaviour. Much of this research is bolstered by the abnormal behaviour seen in germ-free animals and other well-controlled experiments. However, this literature has largely failed to consider the neuroactive potential of antibiotics themselves, independent from, or in addition to, their microbicidal effects. This is problematic, as clinical as well as experimental literature, largely neglected through the past decade, has clearly demonstrated that broad classes of antibiotics are neuroactive or neurotoxic. This is true even for some antibiotics that are widely regarded as not absorbed in the intestinal tract, and is especially concerning when considering the highly-concentrated and widely-ranging doses that have been used. In this review we will critically survey the clinical and experimental evidence that antibiotics may influence a variety of nervous system functions, from the enteric nervous system through to the brain and resultant behaviour. We will discuss substantial evidence which clearly suggests neuro-activity or -toxicity by most classes of antibiotics. We will conclude that, while evidence for the microbiota-gut-brain axis remains strong, clinical and experimental studies which employ antibiotics to probe it must consider this potential confound.

Calyx junction dismantlement and synaptic uncoupling precede hair cell extrusion in the vestibular sensory epithelium during sub-chronic 3,3'-iminodipropionitrile ototoxicity in the mouse

The cellular and molecular events that precede hair cell (HC) loss in the vestibular epithelium during chronic ototoxic exposure have not been widely studied. To select a study model, we compared the effects of sub-chronic exposure to different concentrations of 3,3'-iminodipropionitrile (IDPN) in the drinking water of two strains of mice and of both sexes. In subsequent experiments, male 129S1/SvImJ mice were exposed to 30 mM IDPN for 5 or 8 weeks; animals were euthanized at the end of the exposure or after a washout period of 13 weeks. In behavioral tests, IDPN mice showed progressive vestibular dysfunction followed by recovery during washout. In severely affected animals, light and electron microscopy observations of the vestibular epithelia revealed HC extrusion towards the endolymphatic cavity. Comparison of functional and ultrastructural data indicated that animals with fully reversible dysfunction did not have significant HC loss or stereociliary damage, but reversible dismantlement of the calyceal junctions that characterize the contact between type I HCs (HCI) and their calyx afferents. Immunofluorescent analysis revealed the loss of calyx junction proteins, Caspr1 and Tenascin-C, during exposure and their recovery during washout. Synaptic uncoupling was also recorded, with loss of pre-synaptic Ribeye and post-synaptic GluA2 puncta, and differential reversibility among the three different kinds of synaptic contacts existing in the epithelium. qRT-PCR analyses demonstrated that some of these changes are at least in part explained by gene expression modifications. We concluded that calyx junction dismantlement and synaptic uncoupling are early events in the mouse vestibular sensory epithelium during sub-chronic IDPN ototoxicity.

A Simple Model for Inducing Optimal Increase of SDF-1 with Aminoglycoside Ototoxicity

Objectives

As a homing factor of stem cell, stromal derived factor-1 (SDF-1) is important for the regenerative research in ototoxicity. Mice models with aminoglycoside ototoxicity have been widely used to study the regeneration capacity of MSCs in repair of cochlear injury. We developed a mouse model with maximal increase in SDF-1 levels in the inner ear, according to the “one-shot” doses of kanamycin and furosemide.

Methods

C57BL/6 mice had kanamycin (420, 550, and 600 mg/kg) dissolved in PBS, followed by an intraperitoneal injection of furosemide (130 mg/kg). The injuries of inner ear were measured with hearing thresholds, histology, and outer hair cell counts at 0, 3, 5, 7, 10, and 14 days before the sacrifice. The levels of SDF-1 in the inner ear were tested by real-time RT-PCR and immunohistochemistry.

Results

There were a significant reduction in hearing thresholds and a maximal increase of SDF-1 levels in the furosemide 130 mg/kg + kanamycin 550 mg/kg group, but severe hearing deterioration over time was observed in the furosemide 130 mg/kg + kanamycin 600 mg/kg group and four mice were dead. SDF-1 was detected mostly in the stria vascularis and organ of Corti showing the highest increase in expression.

Conclusion

We observed optimal induction of the stem cell homing factor in the newly generated aminoglycoside-induced ototoxicity mouse model using a “one-shot” protocol. This study regarding high SDF-1 levels in our mouse model of ototoxicity would play a major role in the development of therapeutic agents using MSC homing.

FoxO3a plays a key role in the protective effects of pomegranate peel extract against amikacin-induced ototoxicity

The use of amikacin (AMK) in present treatment strategies results in severe ototoxicity; however, the underlying molecular mechanisms of this toxicity remain unclear. In this study, we investigated the effectiveness of orally administered pomegranate peel extract (PPE), a strong antioxidant, as a protective agent against AMK-induced ototoxicity. To this end, PPE was orally administered to adult BALB/c mice for 5 days, and the mice were then concurrently treated with AMK (500 mg/kg/day for 15 consecutive days). Auditory threshold shifts induced by AMK were significantly attenuated. The results of immunohistochemical staining and western blot analysis revealed that PPE exerted its protective effects by by downregulating the phosphorylation of Forkhead box O3a (FoxO3a), an important transcription factor which is involved in the responses to oxidative stress. The results also showed that PPE treatment inhibited mitogen-activated protein kinase phosphorylation, prevented the activation of pro-apoptotic protein caspase-3, decreased the levels of apoptosis-inducing Bax protein, and increased the levels of the anti-apoptotic mediator, Bcl-2, induced by AMK in the mouse cochlea. Taken together, our experimental findings suggest that phosphorylated FoxO3a mediates AMK-induced apoptosis in BALB/c mice cochlea. PPE effectively attenuated oxidative stress and ototoxicity by regulating FoxO3a, and may thus prove to be beneficial in protecting auditory cells from ototoxic drugs.

An efficient strategy for establishing a model of sensorineural deafness in rats

Ototoxic drugs can be used to produce a loss of cochlear hair cells to create animal models of deafness. However, to the best of our knowledge, there is no report on the establishment of a rat deafness model through the combined application of aminoglycosides and loop diuretics. The aim of this study was to use single or combined administration of furosemide and kanamycin sulfate to establish rat models of deafness. The rats received intravenous injections of different doses of furosemide and/or intramuscular injections of kanamycin sulfate. The auditory brainstem response was measured to determine the hearing threshold after drug application. Immunocytochemistry and confocal microscopy were performed to evaluate inner ear morphology. In the group receiving combined administration of furosemide and kanamycin, the auditory brainstem response threshold showed significant elevation 3 days after administration, higher than that produced by furosemide or kanamycin alone. The hair cells showed varying degrees of injury, from the apical turn to the basal turn of the cochlea and from the outer hair cells to the inner hair cells. The spiral ganglion cells maintained a normal morphology during the first week after the hair cells completely disappeared, and then gradually degenerated. After 2 months, the majority of spiral ganglion cells disappeared, but a few remained. These findings demonstrate that the combined administration of furosemide and kanamycin has a synergistic ototoxic effect, and that these drugs can produce hair cell loss and hearing loss in rats. These findings suggest that even in patients with severe deafness, electronic cochlear implants may partially restore hearing.

An Evaluation of the Protective Effects of Thymoquinone on Amikacin-Induced Ototoxicity in Rats

Objectives

In this study we investigated the probable protective effects of thymoquinone on amikacin-induced ototoxicity in rats.

Methods

Thirty-two healthy rats were divided into four groups (amikacin, amikacin+thymoquinone, thymoquinone, and no treatment). Thymoquinone was fed to the rats via oral gavage in a dose of 40 mg/kg/day throughout the study period of 14 days. Amikacin was given by the intramuscular route in a dose of 600 mg/kg/day. Audiological assessment was conducted by the distortion product otoacoustic emission (DPOAE) and auditory brainstem response (ABR) tests, administered to all rats at the beginning of the study, and also on days 7 and 15. Biochemical parameters were calculated at the termination of the study to evaluate the oxidative status.

Results

There were significant decreases in DPOAE values and significant increases in ABR thresholds of the amikacin group on days 7 and 15, as compared to the amikacin+thymoquinone group. While ABR thresholds of the amikacin group increased significantly on days 7 and 15 as compared to their initial values, there were no significant differences between the initial and the 7th and 15th day values of ABR thresholds in the amikacin+thymoquinone group. Total oxidant status and oxidative stress index values of the amikacin+thymoquinone group were significantly lower than those of the amikacin group. Total antioxidant status values of the amikacin+thymoquinone group were significantly higher than those of the amikacin group.

Conclusion

Our study has demonstrated that the ototoxic effect brought forth by amikacin could be overcome with the concurrent use of thymoquinone.

Novel in vivo imaging analysis of an inner ear drug delivery system: Drug availability in inner ear following different dose of systemic drug injections

Systemic application of drugs is commonly used in clinical situations. Some of these drugs are ototoxic. Since there are few studies on in vivo monitoring of drug delivery dynamics, the time course or bioavailability of drugs in the inner ear of live animals following systemic drug application remains unknown. For instance, it is unknown whether the volume of a drug delivered systemically correlates with its inner ear pharmacokinetics. We previously established a new in vivo imaging system to monitor drug delivery in live mice. In the present study, we used this system to compare drug concentration in the inner ear over time after systemic drug injections. We used transgenic GFAP-Luc mice that harbor a firefly luciferase gene expression cassette regulated by 12 kb of murine GFAP promoter and human beta-globin intron 2. Luciferin delivered into the inner ear of these mice reacts with luciferase, and the resulting signals are detected in GFAP-expressing cells in the cochlear nerve. Thus, we assessed in the inner ear the intensity and duration of luciferin/luciferase signals after systemic injections of different volumes of luciferin. An IVIS(®) imaging system was used to observe signals, and these signals were compared to the drug dynamics of luciferin delivered through subcutaneous (sc) injections. The volume of sc-injected drug correlated significantly with photon counts measured in the inner ear. Photons were detected almost immediately after injection, peaking 20 min after injection. Drug concentration did not affect inner ear signals. Luciferin injected systemically appeared in the inner ear between highest and lowest concentration. Drug volume is an important parameter to know if the inner ear requires a higher level of the drug. We observed that it is the volume of a drug-not its concentration-that is the important factor. Indeed, the more volume of a drug injected systemically increased the concentration of that drug in the inner ear. This study provides a better understanding of in vivo drug delivery dynamics measured in the inner ear. Further studies will show whether a high dosage of drug is effective or not.

Transient alteration of the vestibular calyceal junction and synapse in response to chronic ototoxic insult in rats

Ototoxicity is known to cause permanent loss of vestibule function through degeneration of sensory hair cells (HCs). However, functional recovery has been reported during washout after chronic ototoxicity, although the mechanisms underlying this reversible dysfunction are unknown. Here, we study this question in rats chronically exposed to the ototoxic compound 3,3′-iminodipropionitrile (IDPN). Pronounced alterations in vestibular function appeared before significant loss of HCs or stereociliary coalescence became evident by ultrastructural analyses. This early dysfunction was fully reversible if the exposure was terminated promptly. In cristae and utricles, the distinct junctions formed between type I HCs (HCI) and calyx endings were completely dismantled at these early stages of reversible dysfunction, and completely rebuilt during washout. Immunohistochemical observations revealed loss and recovery of the junction proteins CASPR1 and tenascin-C and RT-PCR indicated that their loss was not due to decreased gene expression. KCNQ4 was mislocalized during intoxication and recovered control-like localization after washout. At early stages of the intoxication, the calyces could be classified as showing intact or lost junctions, indicating that calyceal junction dismantlement is triggered on a calyx-by-calyx basis. Chronic toxicity also altered the presence of ribeye, PSD-95 and GluA2 puncta in the calyces. These synaptic alterations varied between the two types of calyx endings (formed by calyx-only or dimorphic afferents) and some persisted at the end of the washout period. The present data reveal new forms of plasticity of the calyx endings in adult mammals, including a robust capacity for rebuilding the calyceal junction. These findings contribute to a better understanding of the phenomena involved in progressive vestibular dysfunction and its potential recovery during and after ototoxic exposure.

Summary: New forms of damage and repair have been identified in the vestibular sensory epithelium using a rat model of chronic ototoxicity and recovery that causes reversible vestibular dysfunction.

Selective deletion of cochlear hair cells causes rapid age-dependent changes in spiral ganglion and cochlear nucleus neurons

During nervous system development, critical periods are usually defined as early periods during which manipulations dramatically change neuronal structure or function, whereas the same manipulations in mature animals have little or no effect on the same property. Neurons in the ventral cochlear nucleus (CN) are dependent on excitatory afferent input for survival during a critical period of development. Cochlear removal in young mammals and birds results in rapid death of target neurons in the CN. Cochlear removal in older animals results in little or no neuron death. However, the extent to which hair-cell-specific afferent activity prevents neuronal death in the neonatal brain is unknown. We further explore this phenomenon using a new mouse model that allows temporal control of cochlear hair cell deletion. Hair cells express the human diphtheria toxin (DT) receptor behind the Pou4f3 promoter. Injections of DT resulted in nearly complete loss of organ of Corti hair cells within 1 week of injection regardless of the age of injection. Injection of DT did not influence surrounding supporting cells directly in the sensory epithelium or spiral ganglion neurons (SGNs). Loss of hair cells in neonates resulted in rapid and profound neuronal loss in the ventral CN, but not when hair cells were eliminated at a more mature age. In addition, normal survival of SGNs was dependent on hair cell integrity early in development and less so in mature animals. This defines a previously undocumented critical period for SGN survival.

Sensory hair cell death and regeneration in fishes

Sensory hair cells are specialized mechanotransductive receptors required for hearing and vestibular function. Loss of hair cells in humans and other mammals is permanent and causes reduced hearing and balance. In the early 1980’s, it was shown that hair cells continue to be added to the inner ear sensory epithelia in cartilaginous and bony fishes. Soon thereafter, hair cell regeneration was documented in the chick cochlea following acoustic trauma. Since then, research using chick and other avian models has led to great insights into hair cell death and regeneration. However, with the rise of the zebrafish as a model organism for studying disease and developmental processes, there has been an increased interest in studying sensory hair cell death and regeneration in its lateral line and inner ears. Advances derived from studies in zebrafish and other fish species include understanding the effect of ototoxins on hair cells and finding otoprotectants to mitigate ototoxin damage, the role of cellular proliferation vs. direct transdifferentiation during hair cell regeneration, and elucidating cellular pathways involved in the regeneration process. This review will summarize research on hair cell death and regeneration using fish models, indicate the potential strengths and weaknesses of these models, and discuss several emerging areas of future studies.

Aminoglycoside ototoxicity and hair cell ablation in the adult gerbil: A simple model to study hair cell loss and regeneration

The Mongolian gerbil, Meriones unguiculatus, has been widely employed as a model for studies of the inner ear. In spite of its established use for auditory research, no robust protocols to induce ototoxic hair cell damage have been developed for this species. In this paper, we demonstrate the development of an aminoglycoside-induced model of hair cell loss, using kanamycin potentiated by the loop diuretic furosemide. Interestingly, we show that the gerbil is relatively insensitive to gentamicin compared to kanamycin, and that bumetanide is ineffective in potentiating the ototoxicity of the drug. We also examine the pathology of the spiral ganglion after chronic, long-term hair cell damage. Remarkably, there is little or no neuronal loss following the ototoxic insult, even at 8 months post-damage. This is similar to the situation often seen in the human, where functioning neurons can persist even decades after hair cell loss, contrasting with the rapid, secondary degeneration found in rats, mice and other small mammals. We propose that the combination of these factors makes the gerbil a good model for ototoxic damage by induced hair cell loss.

Betahistine exacerbates amikacin ototoxicity

OBJECTIVE

Betahistine augments cochlear blood flow and is currently used as an efficient therapeutic agent. Amikacin is used in a wide range of areas, but its ototoxic effect continues to be problematic. This study investigates the effect of betahistine on amikacin-induced ototoxicity.

METHODS

Thirty-two healthy rats were randomized to 4 groups of 8 rats in each group (amikacin, amikacin+betahistine, betahistine, and no treatment). Amikacin was administered intramuscularly to groups 1 and 2 for 14 days. Betahistine was delivered by oral gavage to groups 2 and 3 for 21 days. Distortion-product otoacoustic emissions (DPOAE) and auditory brainstem response (ABR) tests were conducted on all rats.

RESULTS

There were significant decreases in the DPOAE levels and significant increases in the ABR thresholds of the amikacin and amikacin+betahistine groups on the 7th, 14th, and 21st days, as compared to their basal values. The DPOAE levels of the amikacin+betahistine group significantly decreased on days 7, 14, and 21, and the ABR thresholds significantly increased on the same days, as compared to the amikacin group.

CONCLUSION

Our study implies that amikacin's ototoxic effects are augmented by the concurrent use of betahistine. Experimental and clinical research, supported by histopathological studies, is needed to affirm our findings.

Protective effect of trimetazidine on amikacin-induced ototoxicity in rats

OBJECTIVE

Aminoglycoside antibiotics are known to have ototoxic effects and may induce sensorineural hearing loss. This study investigated the protective effect of trimetazidine, which has antioxidant and cytoprotective properties, against amikacin ototoxicity.

METHODS

Thirty-two male rats were divided into four groups - amikacin, amikacin + trimetazidine, trimetazidine, and control groups. Trimetazidine, 10 mg/kg per day, was given for 14 days by oral gavage. Amikacin, 600 mg/kg per day, was also given for 14 days, by the intramuscular route. Distortion product otoacoustic emission (DPOAE) and auditory brainstem response (ABR) tests were applied to the rats for hearing assessment. At the termination of the study, the biochemical parameters were calculated to evaluate the oxidative status.

RESULTS

The DPOAE values of the amikacin group were significantly lower on the 7th and 14th days than those of the trimetazidine + amikacin group and there was an increase in the ABR thresholds. The ABR thresholds for the amikacin group on the 7th and 14th days were significantly higher than the levels on the first day of the study, while there was no significant increase in those values in the trimetazidine + amikacin group. The total oxidant status (TOS) and oxidant status index (OSI) values of the amikacin group were significantly higher than those of the trimetazidine + amikacin group. The total antioxidant status (TAS) values of the amikacin group were lower than those of the trimetazidine + amikacin group.

CONCLUSIONS

The audiologic tests and biochemical parameters investigated in this study both point to the protective effect of trimetazidine against amikacin-induced ototoxicity.

Vestibular damage in chronic ototoxicity: a mini-review

Ototoxicity is a major cause of the loss of hearing and balance in humans. Ototoxic compounds include pharmaceuticals such as aminoglycoside antibiotics, anti-malarial drugs, loop diuretics and chemotherapeutic platinum agents, and industrial chemicals including several solvents and nitriles. Human and rodent data indicate that the main target of toxicity is hair cells (HCs), which are the mechanosensory cells responsible for sensory transduction in both the auditory and the vestibular system. Nevertheless, the compounds may also affect the auditory and vestibular ganglion neurons. Exposure to ototoxic compounds has been found to cause HC apoptosis, HC necrosis, and damage to the afferent terminals, of differing severity depending on the ototoxicity model. One major pathway frequently involved in HC apoptosis is the c-jun N-terminal kinase (JNK) signaling pathway activated by reactive oxygen species, but other apoptotic pathways can also play a role in ototoxicity. Moreover, little is known about the effects of chronic low-dose exposure. In rodent vestibular epithelia, extrusion of live HCs from the sensory epithelium may be the predominant form of cell demise during chronic ototoxicity. In addition, greater involvement of the afferent terminals may occur, particularly the calyx units contacting type I vestibular HCs. As glutamate is the neurotransmitter in this synapse, excitotoxic phenomena may participate in afferent and ganglion neuron damage. Better knowledge of the events that take place in chronic ototoxicity is of great interest, as it will increase understanding of the sensory loss associated with chronic exposure and aging.

Cochlear implantation for chronic electrical stimulation in the mouse

The mouse is becoming an increasingly attractive model for auditory research due to the number of genetic deafness models available. These genetic models offer the researcher an array of congenital causes of hearing impairment, and are therefore of high clinical relevance. To date, the use of mice in cochlear implant research has not been possible due to the lack of an intracochlear electrode array and stimulator small enough for murine use, coupled with the difficulty of the surgery in this species. Here, we present a fully-implantable intracochlear electrode stimulator assembly designed for chronic implantation in the mouse. We describe the surgical approach for implantation, as well as presenting the first functional data obtained from intracochlear electrical stimulation in the mouse.

Reduced systemic toxicity and preserved vestibular toxicity following co-treatment with nitriles and CYP2E1 inhibitors: a mouse model for hair cell loss

Several nitriles, including allylnitrile and cis-crotononitrile, have been shown to be ototoxic and cause hair cell degeneration in the auditory and vestibular sensory epithelia of mice. However, these nitriles can also be lethal due in large part to the microsomal metabolic release of cyanide, which is mostly dependent on the activity of the 2E1 isoform of the cytochrome P450 (CYP2E1). In this study, we co-administered mice with a nitrile and, to reduce their lethal effects, a selective CYP2E1 inhibitor: diallylsulfide (DAS) or trans-1,2-dichloroethylene (TDCE). Both in female 129S1/SvImJ (129S1) mice co-treated with DAS and cis-crotononitrile and in male RjOrl:Swiss/CD-1 (Swiss) mice co-treated with TDCE and allylnitrile, the nitrile caused a dose-dependent loss of vestibular function, as assessed by a specific behavioral test battery, and of hair cells, as assessed by hair bundle counts using scanning electron microscopy. In the experiments, the CYP2E1 inhibitors provided significant protection against the lethal effects of the nitriles and did not diminish the vestibular toxicity as assessed by behavioral effects in comparison to animals receiving no inhibitor. Additional experiments using a single dose of allylnitrile demonstrated that TDCE does not cause hair cell loss on its own and does not modify the vestibular toxicity of the nitrile in either male or female 129S1 mice. In all the experiments, high vestibular dysfunction scores in the behavioral test battery predicted extensive to complete loss of hair cells in the utricles. This provides a means of selecting animals for subsequent studies of vestibular hair cell regeneration or replacement.

Reversible neurotoxicity of kanamycin on dorsal cochlear nucleus

The time course of aminoglycoside neurotoxic effect on cochlear nucleus is still obscure. We examined dynamic pathological changes of dorsal cochlear nucleus (DCN) and investigated whether apoptosis or autophagy was upregulated in the neurotoxic course of kanamycin on DCN after kanamycin treatment. Rats were treated with kanamycin sulfate/kg/day at a dose of 500mg by subcutaneous injection for 10 days. Dynamic pathological changes, neuron density and neuron apoptosis of the DCN were examined at 1, 7, 14, 28, 56, 70 and 140 days after kanamycin treatment. The expressions of JNK1, DAPK2, Bcl-2, p-Bcl-2, Caspase-3, LC3B and Beclin-1 were also detected. Under transmission electron microscopy, the mitochondrial swelling and focal vacuoles as well as endoplasmic reticulum dilation were progressively aggravated from 1 day to 14 days, and gradually recovered from 28 days to 140 days. Meanwhile, both autophagosomes and autolysosomes were increased from 1 day to 56 days. Only few neurons were positive to the TUNEL staining. Moreover, neither the expressions of caspase-3 and DAPK2 nor neurons density of DCN changed significantly. LC3-II was drastically increased at 7 days. Beclin-1 was upgraded at 1 and 7 days. P-Bcl-2 increased at 1, 7, 14 and 28 days. JNK1 increased at 7 days, and Bcl-2 was downgraded at 140 days. LC3-B positive neurons were increased at 1, 7 and 14 days. These data demonstrated that the neurons damage of the DCN caused by kanamycin was reversible and autophagy was upregulated in the neurotoxic course of kanamycin on DCN through JNK1-mediated phosphorylation of Bcl-2 pathway.

Effect of different gentamicin dose on the plasticity of the ribbon synapses in cochlear inner hair cells of C57BL/6J mice

Faithful information transfer at the hair cell afferent synapse requires synaptic transmission to be both reliable and temporally precise. The release of neurotransmitter must exhibit both rapid on and off kinetics to accurately follow acoustic stimuli with a periodicity of 1 ms or less. To ensure such remarkable temporal fidelity, the cochlear hair cell afferent synapse undoubtedly relies on unique cellular and molecular specializations. To study effects of different doses of gentamicin on the changes of synaptic ribbons of cochlear inner hair cells (IHCs) in mice, the availability of genetic information, transgenic and knock-out animals make the C57BL/6J mouse a primary model in biomedical research. Aminoglycoside ototoxicity, however, has rarely been studied in mature mice because they are considered highly resistant to the drugs. This study presents models for gentamicin ototoxicity in adult C57BL/6J mouse strains. Five-week-old mice were injected intraperitoneally once daily with 50-300 mg gentamicin base/kg body weight for 7 days. Higher doses of gentamicin appear to be associated with earlier hearing damage in C57BL/6J mice, although not necessarily with more severe damage. At 200 mg/kg, gentamicin appears to induce significant hearing damage while not significantly affect the animal's general condition. Therefore, 200 mg/kg may be an ideal dose for ototoxicity modeling in C57BL/6J mice using gentamicin. In the early period of different dose of gentamicin effect, when the number of hair cells had not changed, the number changes of IHC ribbon synapses had taken place. Through the number of ribbon synapses changing, IHCs increased or decreased connections with spiral ganglion nerves (SGNs). The ribbon synapses played a compensatory role for gentamicin ototoxicity, while this effect was not sufficient to maintain the normal threshold of hearing.

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.

[Comparison of different deafening strategies based on ototoxic drugs on mouse animals model]

AIM OF THE STUDY

To compare safety, reliability and usefulness of two deafening protocols on animal mouse model, based on aminoglycosides exposure

MATERIAL AND METHOD

Adults mice, Bulb/C, deafened with kanamycine 14 days treatment (group I), single kanamycin injection followed by etacrinic acid administration (group II) and control group. Hearing evaluation performed with ABR recordings on 6th day after drug exposure

RESULTS

Both protocols were not able to guarantee complete ablation of the inner ear in tested animals. Although short deafening strategy was more effective (83.33% deaf mice) it was combined with high rate of mortality during general anesthesia for hearing evaluation.

CONCLUSIONS

Variable outcomes in deafening mouse animal model implies the necessity of hearing evaluation every time prior to the pathophysiological as well as molecular studies. Mice exposed to severe oto- and nephrotoxic insult do not recover after anesthetic drug administration, thus harvesting inner ear tissues especially as the source of RNA should be performed immediately after ABR recordings.

Effects of p27Kip1- and p53- shRNAs on kanamycin damaged mouse cochlea

AIM: To study the effects of adeno-associated virus (AAV) delivered short hairpin RNAs (shRNAs) on adult CD-1 mouse cochlea damaged by aminoglycoside antibiotic kanamycin.

METHODS: Three different shRNAs were designed (p27 ^Kip1, p53 and p27 ^Kip1+p53) and tested in COS cells. A total of 20 adult CD-1 mice were used in the experiment. Mice were divided into five different groups (four animals/group) depending on the AAV-shRNA construct they received and whether they received kanamycin or not. Saline and AAV-EGFP injected animals were used as controls. All constructs were injected through the round window membrane (RWM) into the cochlea. Cochleae were harvested after 1 mo. Apoptosis was detected with Tunel labeling from paraffin-embedded cochlear tissue sections.

RESULTS: AAV2/2-p27 ^Kip1-shRNA and AAV2/2-p53-shRNA were tested in COS cells. Western blotting analysis confirmed that both constructs silenced their target genes effectively in the cell culture. AAV2/2-shRNA constructs were injected into the cochlea of CD-1 mice through the intact RWM. Cotransduction of individual AAV2/2-shRNAs with AAV2/2-EGFP resulted in EGFP expression in the organ of Corti. Kanamycin treatment had no effect on the expression pattern of the EGFP. AAV2/2-shRNA treated mice (either with p53 or p27^Kip1and p53 together) showed fewer apoptotic hair cells in the cochlea than the control group (P < 0.05; AAV2/2-p53-shRNA vs saline P = 0.00014; AAV2/2-p27+p53-shRNA vs saline P = 0.0011). AAV2/2-p27-shRNA injected cochleae showed no significant difference in the number of apoptotic cells when compared to the saline injected cochleae.

CONCLUSION: Silencing of p53 protein in the kanamycin treated ears may decrease cell death in the organ of Corti.

In vitro and in vivo models of drug ototoxicity: studying the mechanisms of a clinical problem

INTRODUCTION

Drug ototoxicity represents one of the main preventable causes of deafness. Ototoxicity is a trait shared by aminoglycoside and macrolide antibiotics, antimalarial medications, loop diuretics, platinum-based chemotherapeutic agents, some NSAIDs and most recently described, acetaminophen when abused with narcotic medication. These medications are prescribed despite their side effects, which includes inner ear toxicity, because they are life-saving drugs or there is a lack of better treatment.

AREAS COVERED

This review will discuss in vitro and in vivo models of ototoxicity highlighting recently published ototoxicity research. The reader will learn the strengths and limitations of different ototoxicity models and what molecular insights have been gained from their application. A better understanding of the cellular mechanisms of these ototoxins will help in the discovery of ways to prevent and treat hearing loss associated with ototoxic medications.

EXPERT OPINION

There are benefits to both in vitro and in vivo models of ototoxicity. Research of a particular medication and its ototoxic mechanisms should draw from several models, enabling a better answer to the clinical question of prevention and treatment of inner ear drug toxicity.

A comparative study of age-related hearing loss in wild type and insulin-like growth factor I deficient mice

Insulin-like growth factor-I (IGF-I) belongs to the family of insulin-related peptides that fulfils a key role during the late development of the nervous system. Human IGF1 mutations cause profound deafness, poor growth and mental retardation. Accordingly, Igf1^−/− null mice are dwarfs that have low survival rates, cochlear alterations and severe sensorineural deafness. Presbycusis (age-related hearing loss) is a common disorder associated with aging that causes social and cognitive problems. Aging is also associated with a decrease in circulating IGF-I levels and this reduction has been related to cognitive and brain alterations, although there is no information as yet regarding the relationship between presbycusis and IGF-I biodisponibility. Here we present a longitudinal study of wild type Igf1^+/+ and null Igf1^−/− mice from 2 to 12 months of age comparing the temporal progression of several parameters: hearing, brain morphology, cochlear cytoarchitecture, insulin-related factors and IGF gene expression and IGF-I serum levels. Complementary invasive and non-invasive techniques were used, including auditory brainstem-evoked response (ABR) recordings and in vivo MRI brain imaging. Igf1^−/− null mice presented profound deafness at all the ages studied, without any obvious worsening of hearing parameters with aging. Igf1^+/+ wild type mice suffered significant age-related hearing loss, their auditory thresholds and peak I latencies augmenting as they aged, in parallel with a decrease in the circulating levels of IGF-I. Accordingly, there was an age-related spiral ganglion degeneration in wild type mice that was not evident in the Igf1 null mice. However, the Igf1^−/− null mice in turn developed a prematurely aged stria vascularis reminiscent of the diabetic strial phenotype. Our data indicate that IGF-I is required for the correct development and maintenance of hearing, supporting the idea that IGF-I-based therapies could contribute to prevent or ameliorate age-related hearing loss.