Behavioural evidence for recovery of auditory function in guinea pigs following kanamycin administration

Mechanisms and Impact of Aminoglycoside-Induced Vestibular Deficits

PURPOSE

Acquired vestibulotoxicity from hospital-prescribed medications such as aminoglycoside antibiotics affects as many as 40,000 people each year in North America. However, there are no current federally approved drugs to prevent or treat the debilitating and permanent loss of vestibular function caused by bactericidal aminoglycoside antibiotics. This review will cover our current understanding of the impact of, and mechanisms underlying, aminoglycoside-induced vestibulotoxicity and highlight the gaps in our knowledge that remain.

CONCLUSIONS

Aminoglycoside-induced vestibular deficits have long-term impacts on patients across the lifespan. Additionally, the prevalence of aminoglycoside-induced vestibulotoxicity appears to be greater than cochleotoxicity. Thus, monitoring for vestibulotoxicity should be independent of auditory monitoring and encompass patients of all ages from young children to older adults before, during, and after aminoglycoside therapy.

Systemic toll-like receptor 9 agonist CpG oligodeoxynucleotides exacerbates aminoglycoside ototoxicity

The Toll-like receptor (TLR) signaling pathway is the key regulator of the innate immune system in response to systemic infection. Several studies have reported that the systemic TLR4 agonist lipopolysaccharide exacerbates aminoglycoside ototoxicity, but the influence of virus-associated TLR7 and TLR9 signaling cascades on the cochlea is unclear. The present study aimed to investigate the auditory effects of systemic TLR7 and TLR9 agonists during chronic kanamycin treatment. CBA/CaJ mice received the TLR7 agonist gardiquimod or TLR9 agonist CpG oligodeoxynucleotides (ODN) one day before kanamycin injection and on the 5th and 10th days during a 14-day course of kanamycin treatment. We observed that systemic gardiquimod or CpG ODN alone did not affect the baseline auditory brainstem response (ABR) threshold. Three weeks after kanamycin treatment, gardiquimod did not significantly change ABR threshold shifts, whereas CpG ODN significantly increased kanamycin-induced ABR threshold shifts. Furthermore, outer hair cell (OHC) evaluation revealed that CpG ODN reduced distortion product otoacoustic emission amplitudes and increased kanamycin-induced OHC loss. CpG ODN significantly elevated cochlear Irf-7, Tnf-α, Il-1, and Il-6 transcript levels. In addition, an increased number of Iba-1⁺ cells, which represented activated macrophages, was observed in the cochlea treated with CpG ODN. Our results indicated that systemic CpG ODN exacerbated kanamycin-induced ototoxicity and increased cochlear inflammation. This study implies that patients with underlying virus infection may experience more severe aminoglycoside-induced hearing loss if it occurs.

Mechanisms of Aminoglycoside- and Cisplatin-Induced Ototoxicity

Purpose This review article summarizes our current understanding of the mechanisms underlying acquired hearing loss from hospital-prescribed medications that affects as many as 1 million people each year in Western Europe and North America. Yet, there are currently no federally approved drugs to prevent or treat the debilitating and permanent hearing loss caused by the life-saving platinum-based anticancer drugs or the bactericidal aminoglycoside antibiotics. Hearing loss has long-term impacts on quality-of-life measures, especially in young children and older adults. This review article also highlights some of the current knowledge gaps regarding iatrogenic causes of hearing loss. Conclusion Further research is urgently needed to further refine clinical practice and better ameliorate iatrogenic drug-induced hearing loss.

Aminoglycoside- and Cisplatin-Induced Ototoxicity: Mechanisms and Otoprotective Strategies

Ototoxicity refers to damage of inner ear structures (i.e., the cochlea and vestibule) and their function (hearing and balance) following exposure to specific in-hospital medications (i.e., aminoglycoside antibiotics, platinum-based drugs), as well as a variety of environmental or occupational exposures (e.g., metals and solvents). This review provides a narrative derived from relevant papers describing factors contributing to (or increasing the risk of) aminoglycoside and cisplatin-induced ototoxicity. We also review current strategies to protect against ototoxicity induced by these indispensable pharmacotherapeutic treatments for life-threatening infections and solid tumors. We end by highlighting several interventional strategies that are currently in development, as well as the diverse challenges that still need to be overcome to prevent drug-induced hearing loss.

Screening Mammalian Cochlear Hair Cells to Identify Critical Processes in Aminoglycoside-Mediated Damage

There is considerable interest in discovering drugs with the potential to protect inner ear hair cells (HCs) from damage. One means of discovery is to screen compound libraries. Excellent screening protocols have been developed employing cell lines derived from the cochlea and zebrafish larvae. However, these do not address the differentiated mammalian hair cell. We have developed a screening method employing micro-explants of the mammalian organ of Corti (oC) to identify compounds with the ability to influence aminoglycoside-induced HC loss. The assay is based on short segments of the neonatal mouse oC, containing ~80 HCs which selectively express green fluorescent protein (GFP). This allows the screening of hundreds of potential protectants in an assay that includes both inner and outer HCs. This review article describes various screening methods, including the micro-explant assay. In addition, two micro-explant screening studies in which antioxidant and kinase inhibitor libraries were evaluated are reviewed. The results from these screens are related to current models of HC damage and protection.

Aminoglycoside-Induced Cochleotoxicity: A Review

Aminoglycoside antibiotics are used as prophylaxis, or urgent treatment, for many life-threatening bacterial infections, including tuberculosis, sepsis, respiratory infections in cystic fibrosis, complex urinary tract infections and endocarditis. Although aminoglycosides are clinically-essential antibiotics, the mechanisms underlying their selective toxicity to the kidney and inner ear continue to be unraveled despite more than 70 years of investigation. The following mechanisms each contribute to aminoglycoside-induced toxicity after systemic administration: (1) drug trafficking across endothelial and epithelial barrier layers; (2) sensory cell uptake of these drugs; and (3) disruption of intracellular physiological pathways. Specific factors can increase the risk of drug-induced toxicity, including sustained exposure to higher levels of ambient sound, and selected therapeutic agents such as loop diuretics and glycopeptides. Serious bacterial infections (requiring life-saving aminoglycoside treatment) induce systemic inflammatory responses that also potentiate the degree of ototoxicity and permanent hearing loss. We discuss prospective clinical strategies to protect auditory and vestibular function from aminoglycoside ototoxicity, including reduced cochlear or sensory cell uptake of aminoglycosides, and otoprotection by ameliorating intracellular cytotoxicity.

Endotoxemia-mediated inflammation potentiates aminoglycoside-induced ototoxicity

The ototoxic aminoglycoside antibiotics are essential to treat severe bacterial infections, particularly in neonatal intensive care units. Using a bacterial lipopolysaccharide (LPS) experimental model of sepsis, we tested whether LPS-mediated inflammation potentiates cochlear uptake of aminoglycosides and permanent hearing loss in mice. Using confocal microscopy and enzyme-linked immunosorbent assays, we found that low-dose LPS (endotoxemia) greatly increased cochlear concentrations of aminoglycosides and resulted in vasodilation of cochlear capillaries without inducing paracellular flux across the blood-labyrinth barrier (BLB) or elevating serum concentrations of the drug. Additionally, endotoxemia increased expression of both serum and cochlear inflammatory markers. These LPS-induced changes, classically mediated by Toll-like receptor 4 (TLR4), were attenuated in TLR4-hyporesponsive mice. Multiday dosing with aminoglycosides during chronic endotoxemia induced greater hearing threshold shifts and sensory cell loss compared to mice without endotoxemia. Thus, endotoxemia-mediated inflammation enhanced aminoglycoside trafficking across the BLB and potentiated aminoglycoside-induced ototoxicity. These data indicate that patients with severe infections are at greater risk of aminoglycoside-induced hearing loss than previously recognized.

Recognition of Modified Conditioning Sounds by Competitively Trained Guinea Pigs

The guinea pig (GP) is an often-used species in hearing research. However, behavioral studies are rare, especially in the context of sound recognition, because of difficulties in training these animals. We examined sound recognition in a social competitive setting in order to examine whether this setting could be used as an easy model. Two starved GPs were placed in the same training arena and compelled to compete for food after hearing a conditioning sound (CS), which was a repeat of almost identical sound segments. Through a 2-week intensive training, animals were trained to demonstrate a set of distinct behaviors solely to the CS. Then, each of them was subjected to generalization tests for recognition of sounds that had been modified from the CS in spectral, fine temporal and tempo (i.e., intersegment interval, ISI) dimensions. Results showed that they discriminated between the CS and band-rejected test sounds but had no preference for a particular frequency range for the recognition. In contrast, sounds modified in the fine temporal domain were largely perceived to be in the same category as the CS, except for the test sound generated by fully reversing the CS in time. Animals also discriminated sounds played at different tempos. Test sounds with ISIs shorter than that of the multi-segment CS were discriminated from the CS, while test sounds with ISIs longer than that of the CS segments were not. For the shorter ISIs, most animals initiated apparently positive food-access behavior as they did in response to the CS, but discontinued it during the sound-on period probably because of later recognition of tempo. Interestingly, the population range and mean of the delay time before animals initiated the food-access behavior were very similar among different ISI test sounds. This study, for the first time, demonstrates a wide aspect of sound discrimination abilities of the GP and will provide a way to examine tempo perception mechanisms using this animal species.

Age-related hearing impairment and the triad of acquired hearing loss

Understanding underlying pathological mechanisms is prerequisite for a sensible design of protective therapies against hearing loss. The triad of age-related, noise-generated, and drug-induced hearing loss displays intriguing similarities in some cellular responses of cochlear sensory cells such as a potential involvement of reactive oxygen species (ROS) and apoptotic and necrotic cell death. On the other hand, detailed studies have revealed that molecular pathways are considerably complex and, importantly, it has become clear that pharmacological protection successful against one form of hearing loss will not necessarily protect against another. This review will summarize pathological and pathophysiological features of age-related hearing impairment (ARHI) in human and animal models and address selected aspects of the commonality (or lack thereof) of cellular responses in ARHI to drugs and noise.

Insights into elevated distortion product otoacoustic emissions in sickle cell disease: Comparisons of hydroxyurea-treated and non-treated young children

Distortion product otoacoustic emissions (DPOAEs) were examined in 15 normal-hearing African-American children between the ages of 6 and 14 years with homozygous sickle cell disease (SCD), who were on a regimen of hydroxyurea (HDU), a drug that reduces inflammatory processes and symptoms of SCD; a matched group of 15 African-American children with homozygous SCD not on HDU; and 15 African-American children with normal hemoglobin. DPOAEs were evoked by 13 primary tone pairs with f2 frequencies ranging from 1000 to 4500 Hz. Increased DPOAE amplitudes, believed to be a precursor of eventual hearing loss, were evident in children with SCD who were not receiving HDU. Those taking HDU had DPOAE amplitudes similar to normal controls. These findings suggest that HDU, in addition to reducing symptoms of SCD, may play a role in inhibiting or preventing cochlear pathology and hearing loss in individuals with SCD.

Survival of bundleless hair cells and subsequent bundle replacement in the bullfrog's saccule

Our senses of hearing and balance depend upon hair cells, the sensory receptors of the inner ear. Millions of people suffer from hearing and balance deficits caused by damage to hair cells as a result of exposure to noise, aminoglycoside antibiotics, and antitumor drugs. In some species such damage can be reversed through the production of new cells. This proliferative response is limited in mammals but it has been hypothesized that damaged hair cells might survive and undergo intracellular repair. We examined the fate of bullfrog saccular hair cells after exposure to a low dose of the aminoglycoside antibiotic gentamicin to determine whether hair cells could survive such treatment and subsequently be repaired. In organ cultures of the bullfrog saccule a combination of time-lapse video microscopy, two-photon microscopy, electron microscopy, and immunocytochemistry showed that hair cells can lose their hair bundle and survive as bundleless cells for at least 1 week. Time-lapse and electron microscopy revealed stages in the separation of the bundle from the cell body. Scanning electron microscopy (SEM) of cultures fixed 2, 4, and 7 days after antibiotic treatment showed that numerous new hair bundles were produced between 4 and 7 days of culture. Further examination revealed hair cells with small repaired hair bundles alongside damaged remnants of larger surviving bundles. The results indicate that sensory hair cells can undergo intracellular self-repair in the absence of mitosis, offering new possibilities for functional hair cell recovery and an explanation for non-proliferative recovery.

Determination of hair cell degeneration and hair cell death in neomycin treated cultures of the neonatal rat cochlea

The spatial-temporal course of hair cell degeneration and hair cell death was examined in the mammalian cochlea following aminoglycoside treatment. Organotypic cultures were established from postnatal rats (P3) and treated with 1 mM neomycin sulfate for 12-48 h and analyzed using a live/dead assay under epifluorescence microscopy. Live hair cells were labeled with calcein, a probe whose fluorescence and cellular retention depends upon intracellular esterase activity and cell-membrane integrity, respectively. Hair cell death was determined by ethidium homodimer-1, a probe that can enter cells with compromised cell membranes only. Inside the cell it binds to DNA. Hair cell morphology was also examined using phalloidin labeling, scanning electron microscopy and semi-thin section analysis. Results showed that hair cell degeneration and hair cell death occurred in a time dependent gradient from base to apex. After 48 h of neomycin treatment, most apical hair cells survived while most basal hair cells died. Calcein labeling provides a sensitive functional assay for measuring hair cell survival.

Cisplatin-induced ototoxicity; electrophysiological evidence of spontaneous recovery in the albino guinea pig

For 8 days albino guinea pigs (n = 48) were treated with cisplatin (cis-diamminedichloroplatinum(II), 1.5 mg/kg body weight/day). Compound action potentials (CAP), cochlear microphonics (CM) and summating potentials (SP) were recorded from the apical surface of the cochlea in response to tone bursts ranging in frequency from 0.5 to 16 kHz. The recordings were collected in different groups of animals, 1 day, 1 week, 2, 4, 8 and 16 weeks after cisplatin treatment, respectively. One day after the 8-day treatment we found frequency-dependent loss in the amplitudes of the three cochlear potentials, with the larger losses occurring at the higher frequencies. In terms of threshold shift the losses were larger for the CAP than for the hair cell-related potentials SP and CM. A salient improvement in both CAP and CM amplitude occurred over the next 8 weeks. Also, the SP showed improvement. These results indicate that guinea pig cochlear transduction recovers spontaneously after cisplatin injury. Recovery of the hair cell-related potentials suggests that recovery occurs already at the hair cell level. The question whether this recovery originates with the formation of new hair cells or with repair of damaged hair cells should be answered on the basis of subsequent morphological investigations.

Effect of unilateral partial cochlear lesions in adult cats on the representation of lesioned and unlesioned cochleas in primary auditory cortex

We examined the effect of unilateral restricted cochlear lesions in adult cats on the topographic representations ("maps") of the lesioned and unlesioned cochleas in the primary auditory cortex (AI) contralateral to the lesioned cochlea. Frequency (tonotopic) maps were derived by conventional multineuron mapping procedures in anesthetized animals. In confirmation of a study in adult guinea pigs (Robertson and Irvine [1989] J. Comp. Neurol. 282:456-471), we found that 2-11 months after the unilateral cochlear lesion the map of the lesioned cochlea in the contralateral AI was altered so that the AI region in which frequencies with lesion-induced elevations in cochlear neural sensitivity would have been represented was occupied by an enlarged representation of lesion-edge frequencies (i.e., frequencies adjacent to those with elevated cochlear neural sensitivity). Along the tonotopic axis of AI the total representation of lesion-edge frequencies could extend up to approximately 2.6 mm rostal to the area of normal representation of these frequencies. There was no topographic order within this enlarged representation. Examination of threshold sensitivity at the characteristic frequency (CF, frequency to which the neurons were most sensitive) in the reorganized regions of the map of the lesioned cochlea established that the changes in the map reflected a plastic reorganization rather than simply reflecting the residue of prelesion input. In contrast to the change in the map of the lesioned contralateral cochlea, the map of the unlesioned ipsilateral cochlea did not differ from those in normal animals. Thus, in contrast to the normal very good congruency between ipsilateral and contralateral AI maps, in the lesioned animals ipsilateral and contralateral maps differed in the region of AI in which there had been a reorganization of the map of the lesioned cochlea. Outside the region of contralateral map reorganization, ipsilateral and contralateral AI maps remained congruent within normal limits. The difference between the two maps in the region of contralateral map reorganization suggested, in light of the physiology of binaural interactions in the auditory pathway, that the cortical reorganization reflected subcortical changes. Finally, response properties of neuronal clusters within the reorganized map of the lesioned cochlea were compared to normative data with respect to threshold sensitivity at CF, the size of frequency "response areas," and response latencies. In the majority of cases, CF thresholds were similar to normative data. The frequency "response areas" were slightly less sharply tuned than normal, but not significantly. Response latencies were significantly shorter than normal in three animals and significantly longer in one animal.