Ototoxicity of kanamycin in developing rats: relationship with the onset of the auditory function

Hair cell uptake of gentamicin in the developing mouse utricle

Intratympanic injection of gentamicin has proven to be an effective therapy for intractable vestibular dysfunction. However, most studies to date have focused on the cochlea, so little is known about the distribution and uptake of gentamicin by the counterpart of the auditory system, specifically vestibular hair cells (HCs). Here, with a combination of in vivo and in vitro approaches, we used a gentamicin-Texas Red (GTTR) conjugate to investigate the mechanisms of gentamicin vestibulotoxicity in the developing mammalian utricular HCs. In vivo, GTTR fluorescence was concentrated in the apical cytoplasm and the cellular membrane of neonatal utricular HCs, but scarce in the nucleus of HCs and supporting cells. Quantitative analysis showed the GTTR uptake by striolar HCs was significantly higher than that in the extrastriola. In addition, the GTTR fluorescence intensity in the striola was increased gradually from 1 to 8 days, peaking at 8-9 days postnatally. In vitro, utricle explants were incubated with GTTR and candidate uptake conduits, including mechanotransduction (MET) channels and endocytosis in the HC, were inhibited separately. GTTR uptake by HCs could be inhibited by quinine, a blocker of MET channels, under both normal and stressed conditions. Meanwhile, endocytic inhibition only reduced GTTR uptake in the CoCl₂ hypoxia model. In sum, the maturation of MET channels mediated uptake of GTTR into vestibular HCs. Under stressed conditions, MET channels play a pronounced role, manifested by channel-dependent stress enhanced GTTR permeation, while endocytosis participates in GTTR entry in a more selective manner.

The effect of the aquatic contaminants bisphenol-A and PCB-95 on the zebrafish lateral line

Environmental toxicants such as bisphenol-A (BPA) and polychlorinated biphenyls (PCBs) are prevalent in our water supply, soil, and many food products and can profoundly affect the central nervous system. Both BPA and PCBs can disrupt endocrine signaling, which is important for auditory development and function, but the effect of these toxicants on the auditory periphery is not understood. In this study we investigated the effect of PCB-95 and BPA on lateral line development, function, and regeneration in larval zebrafish. The lateral line is a system of mechanosensory hair cells on the exterior of the fish that are homologous to the hair cells located in the mammalian inner ear. We found that PCB-95 had no effect on lateral line development or hair cell survival. BPA also did not affect lateral line development, but instead had a significant effect on both hair cell survival and regeneration. BPA-induced hair cell loss is both dose- and time-dependent, with concentrations of 1 μM or higher killing lateral line hair cells during a 24h exposure period. Pharmacologic manipulation experiments suggest that BPA kills hair cells via activation of oxidative stress pathways, similar to prior reports of BPA toxicity in other tissues. We also observed that hair cells killed with neomycin, a known ototoxin, failed to regenerate normally when BPA was present, suggesting that BPA in aquatic environments could impede innate regenerative responses in fishes. Collectively, these data demonstrate that BPA can have detrimental effects on sensory systems, both in aquatic life and perhaps in terrestrial organisms, including humans.

Nestin expression and reactive phenomena in the mouse cochlea after kanamycin ototoxicity

Following injury to the adult mammalian cochlea, hair cells cannot be spontaneously replaced. Nonetheless, the postnatal cochlea contains progenitor cells, distinguished by the expression of nestin, which are able to proliferate and form neurospheres in vitro. Such resident progenitors might be endowed with reparative potential. However, to date little is known about their behaviour in situ following hair cell injury. Using adult mice and ex vivo cochlear cultures, we sought to determine whether: (i) resident cochlear progenitors respond to kanamycin ototoxicity and compensate for it; and (ii) the reparative potential of cochlear progenitors can be stimulated by the addition of growth factors. Morphological changes of cochlear tissue, expression of nestin mRNA and protein and cell proliferation were investigated in these models. Our observations show that ototoxic injury has modest effects on nestin expression and cell proliferation. On the other hand, the addition of growth factors to the injured cochlear explants induced the appearance of nestin-positive cells in the supporting cell area of the organ of Corti. The vast majority of nestin-expressing cells, however, were not proliferating. Growth factors also had a robust stimulatory effect on axonal sprouting and the proliferative response, which was more pronounced in injured cochleae. On the whole, our findings indicate that nestin expression after kanamycin ototoxicity is related to tissue reactivity rather than activation of resident progenitors attempting to replace the lost receptors. In addition, administration of growth factors significantly enhances tissue remodelling, suggesting that cochlear repair may be promoted by the exogenous application of regeneration-promoting substances.

Examination of calcium-binding protein expression in the inner ear of wild-type, heterozygous and homozygous pituitary adenylate cyclase-activating polypeptide (PACAP)-knockout mice in kanamycin-induced ototoxicity

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide with diverse biological effects. It also occurs and exerts protective effects in sensory organs; however, little is known about its effects in the auditory system. Recently, we have shown that PACAP protects cochlear cells against oxidative-stress-induced apoptosis and homozygous PACAP-deficient animals show stronger expression of Ca(2+)-binding proteins in the hair cells of the inner ear, but there are no data about the consequences of the lack of endogenous PACAP in different ototoxic insults such as aminoglycoside-induced toxicity. In this study, we examined the effect of kanamycin treatment on Ca(2+)-binding protein expression in hair cells of wild-type, heterozygous and homozygous PACAP-deficient mice. We treated 5-day-old mice with kanamycin, and 2 days later, we examined the Ca(2+)-binding protein expression of the hair cells with immunohistochemistry. We found stronger expression of Ca(2+)-binding proteins in the hair cells of control heterozygous and homozygous PACAP-deficient mice compared with wild-type animals. Kanamycin induced a significant increase in Ca(2+)-binding protein expression in wild-type and heterozygous PACAP-deficient mice, but the baseline higher expression in homozygous PACAP-deficient mice did not show further changes after the treatment. Elevated endolymphatic Ca(2+) is deleterious for the cochlear function, against which the high concentration of Ca(2+)-buffers in hair cells may protect. Meanwhile, the increased immunoreactivity of Ca(2+)-binding proteins in the absence of PACAP provide further evidence for the important protective role of PACAP in ototoxicity, but further investigations are necessary to examine the exact role of endogenous PACAP in ototoxic insults.

Functional mechanotransduction is required for cisplatin-induced hair cell death in the zebrafish lateral line

Cisplatin, one of the most commonly used anticancer drugs, is known to cause inner ear hair cell damage and hearing loss. Despite much investigation into mechanisms of cisplatin-induced hair cell death, little is known about the mechanism whereby cisplatin is selectively toxic to hair cells. Using hair cells of the zebrafish lateral line, we found that chemical inhibition of mechanotransduction with quinine and EGTA protected against cisplatin-induced hair cell death. Furthermore, we found that the zebrafish mutants mariner (myo7aa) and sputnik (cad23) that lack functional mechanotransduction were resistant to cisplatin-induced hair cell death. Using a fluorescent analog of cisplatin, we found that chemical or genetic inhibition of mechanotransduction prevented its uptake. These findings demonstrate that cisplatin-induced hair cell death is dependent on functional mechanotransduction in the zebrafish lateral line.

Decreased Immunoreactivities of the Chloride Transporters, KCC2 and NKCC1, in the Lateral Superior Olive Neurons of Kanamycin-treated Rats

Objectives

From our previous study about the weak expressions of potassium-chloride (KCC2) and sodium-potassium-2 chloride (NKCC1) co-transporters in the lateral superior olive (LSO) in circling mice, we hypothesized that partially damaged cochlea of circling mice might be a cause of the weak expressions of KCC2 or NKCC1. To test this possibility, we reproduced the altered expressions of KCC2 and NKCC1 in the LSO of rats, whose cochleae were partially destroyed with kanamycin.

Methods

Rat pups were treated with kanamycin from postnatal (P)3 to P8 (700 mg/kg, subcutaneous injection, twice a day) and sacrificed for immunohistochemical analysis, scanning electron microscope (SEM) and auditory brain stem response.

Results

The SEM study revealed partially missing hair cells in P9 rats treated with kanamycin, and the hearing threshold was elevated to 63.8±2.5 dB SPL (4 ears) at P16. Both KCC2 and NKCC1 immunoreactivities were more prominent in control rats on P16. On 9 paired slices, the mean densities of NKCC1 immunoreactivities were 118.0±1.0 (control) and 112.2±1.2 (kanamycin treated), whereas those of KCC2 were 115.7±1.5 (control) and 112.0±0.8 (kanamycin treated).

Conclusion

We concluded that weak expressions of KCC2 and NKCC1 in circling mice were due to partial destruction of cochleae.

New developments in aminoglycoside therapy and ototoxicity

After almost seven decades in clinical use, aminoglycoside antibiotics still remain indispensible drugs for acute infections and specific indications such as tuberculosis or the containment of pseudomonas bacteria in patients with cystic fibrosis. The review will describe the pathology and pathophysiology of aminoglycoside-induced auditory and vestibular toxicity in humans and experimental animals and explore contemporary views of the mechanisms of cell death. It will also outline the current state of protective therapy and recent advances in the development of aminoglycoside derivatives with low toxicity profiles for antimicrobial treatment and for stop-codon suppression in the attenuation of genetic disorders.

Translational bypass of nonsense mutations in zebrafish rep1, pax2.1 and lamb1 highlights a viable therapeutic option for untreatable genetic eye disease

The extensive molecular genetic heterogeneity seen with inherited eye disease is a major barrier to the development of gene-based therapeutics. The underlying molecular pathology in a considerable proportion of these diseases however are nonsense mutations leading to premature termination codons. A therapeutic intervention targeted at this abnormality would therefore potentially be relevant to a wide range of inherited eye diseases. We have taken advantage of the ability of aminoglycoside drugs to suppress such nonsense mutations and partially restore full-length, functional protein in a zebrafish model of choroideraemia (chm(ru848); juvenile chorio-retinal degeneration) and in two models of ocular coloboma (noi(tu29a) and gup(m189); congenital optic fissure closure defects). In vitro cell-based assays showed significant readthrough with two drugs, gentamicin and paromomycin, which was confirmed by western blot and in vitro prenylation assays. The presence of either aminoglycoside during zebrafish development in vivo showed remarkable prevention of mutant ocular phenotypes in each model and a reduction in multisystemic defects leading to a 1.5-1.7-fold increase in survival. We also identified a significant reduction in abnormal cell death shown by TUNEL assay. To test the hypothesis that optic fissure closure was apoptosis-dependent, the anti-apoptotic agents, curcumin and zVAD-fmk, were tested in gup(m189) embryos. Both drugs were found to reduce the size of the coloboma, providing molecular evidence that cell death is required for optic fissure remodelling. These findings draw attention to the value of zebrafish models of eye disease as useful preclinical drug screening tools in studies to identify molecular mechanisms amenable to therapeutic intervention.

The role of PKCzeta in amikacin-induced apoptosis in the cochlea: prevention by aspirin

Aminoglycoside antibiotics are ototoxic, inducing irreversible sensorineural hearing loss mediated by oxidative and excitotoxic stresses. The NF-kappaB pathway is involved in the response to aminoglycoside damage in the cochlea. However, the molecular mechanisms of this ototoxicity remain unclear. We investigated the expression of PKCzeta, a key regulator of NF-kappaB activation, in response to aminoglycoside treatment. Amikacin induced PKCzeta cleavage and nuclear translocation. These events were concomitant with chromatin condensation and paralleled the decrease in NF-kappaB (p65) levels in the nucleus. Amikacin also induced the nuclear translocation of apoptotic inducing factor (AIF). Prior treatment with aspirin prevented PKCzeta cleavage and nuclear translocation. Thus, aspirin counteracts the early effects of amikacin, thereby protecting hair cells and spiral ganglion neurons. These results demonstrate that PKCzeta acts as sentinel connecting specific survival pathways to mediate cellular responses to amikacin ototoxicity.

Lateral line hair cell maturation is a determinant of aminoglycoside susceptibility in zebrafish (Danio rerio)

Developmental differences in hair cell susceptibility to aminoglycoside-induced cell death has been observed in multiple species. Increased sensitivity to aminoglycosides has been temporally correlated with the onset of mechanotransduction-dependent activity. We have used in vivo fluorescent vital dye markers to further investigate the determinants of aminoglycoside induced hair cell death in the lateral line of zebrafish (Danio rerio). Labeling hair cells of the lateral line in vivo with the dyes FM 1-43, To-Pro-3, and Yo-Pro-1 served as reliable indicators of hair cell viability. Results indicate that hair cell maturation is a determinant of developmental differences in susceptibility. The age dependent differences in susceptibility to aminoglycosides are independent of the onset of mechanotransduction-dependent activity as measured by FM 1-43 uptake and independent of hair cell ability to take up fluorescently conjugated aminoglycosides.

Developmental differences in susceptibility to neomycin-induced hair cell death in the lateral line neuromasts of zebrafish (Danio rerio)

Mechanosensory hair cells of the inner ear are susceptible to death when exposed to a variety of drugs including aminoglycoside antibiotics. During avian and mammalian development, there is a period of relative insensitivity to aminoglycoside-induced hair cell death. This study was designed to test the hypothesis that zebrafish (Danio rerio) have developmental differences in sensitivity to aminoglycoside-induced hair cell death in the lateral line neuromasts. Larval zebrafish of various ages were exposed to several concentrations of neomycin, and their hair cells were examined using the potentiometric vital dye, DASPEI. Results indicate that zebrafish larvae aged 4 days post-fertilization are relatively insensitive to aminoglycoside-induced hair cell death compared to older fish. Thus zebrafish hair cells show developmental differences in sensitivity to aminoglycoside-induced death similar to those reported for inner ear hair cells of birds and mammals.

Influence of auditory deprivation upon the tonopic organization in the inferior colliculus: a Fos immunocytochemical study in the rat

The frequency organization in the inferior colliculus of neonatally-deafened rats was investigated using electrical stimulation of the cochlea and immunoreactivity for Fos as a marker of neuronal activity. An electrode implanted either at the base or at the apex of the right cochlea delivered a unique 45-min stimulation at two different level intensities and at two time points, i.e. either at 4 weeks or at 4 months. In 4-week-old rats stimulated at 5x threshold, a site-for-site organization was observed since basal or apical stimulation induced a strong labelling in the ventro-medial or in the dorsolateral part of the left inferior colliculus, respectively. In 4-month-old rats, stimulation of the base induced an extremely weak Fos labelling without any specific location in the left inferior colliculus while stimulation of the apex induced a diffuse labelling with two discrete bands being distinguishable in the left inferior colliculus. In 4-week-old rats stimulated at 15x threshold, basal stimulation elicited a diffuse Fos-like immunoreactivity in the left inferior colliculus while apical stimulation yielded a response restricted to the dorsal part of the left inferior colliculus. In 4-month-old rats, no response was detected in the left inferior colliculus after stimulation of the basal part of the cochlea. Stimulation of the apex could still induce a labelling in the dorsolateral left inferior colliculus. Thus, the inferior colliculus exhibits an adult-like tonotopic organization early on independently of any acoustic stimulation. Prolonged absence of auditory input dramatically alters this organization in the inferior colliculus, especially for high frequencies. From a clinical standpoint, these results could argue for early implantation in deaf children.

Cochlear electrical stimulation: influence of age of implantation on Fos immunocytochemical reactions in inferior colliculi and dorsal cochlear nuclei of the rat

The influence of age at the time of implantation of a stimulating electrode unilaterally in the inner ear on central auditory pathways was investigated in rats deafened shortly after birth. Immunoreactivity for Fos served as a functional marker of neuronal activity. Electrodes were implanted in the left cochlea of rats aged 3 weeks or 4 months. Stimulation lasted 45 minutes, then rats were sacrificed and tissues processed for immunocytochemistry. The younger animals showed significantly more neurons with Fos immunoreactivity bilaterally in the dorsal cochlear nuclei (DCN) and inferior colliculi (IC) than the older rats or control animals with normal hearing receiving the same stimulation. Activity was more prominent in the left DCN and right IC. The results show that electrical stimulation of the inner ear is more effective in younger animals in eliciting gene expression associated with development of a functional network in the auditory pathways. This suggests that deaf children should be provided with cochlear implants as early as possible.

Expression of NMDA, AMPA and GABA(A) receptor subunit mRNAs in the rat auditory brainstem. I. Influence of early auditory deprivation

Impact of early post-natal deafening on auditory pathways was investigated in newborn rats deafened by daily amikacin injections from P7 to P16 inducing a complete destruction of the organ of Corti. The expression of mRNAs encoding N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) and gamma-aminobutyric acid type A (GABA(A)) receptor subunits was then studied by in situ hybridization in the dorsal and ventral cochlear nucleus and in the central nucleus of the inferior colliculus (CNIC). Early post-natal deafening decreased bilaterally the expression of mRNAs encoding NR1, NR2a, NR2b and flop isoforms of AMPA receptors. On the contrary, it increased the expression of mRNAs encoding some GABA(A) subunits (alpha1, beta1, gamma2) and flip isoforms of AMPA receptors. These changes were more pronounced in cochlear nuclei than in CNIC. They suggest that auditory sensation is essential in the normal development of central auditory pathways.

Aminoglycoside antibiotics

In the 50 years since their discovery, the aminoglycoside antibiotics have seen unprecedented use. Discovered in the 1940s, they were the long-sought remedy for tuberculosis and other serious bacterial infections. The side effects of renal and auditory toxicity, however, led to a decline of their use in most countries in the 1970s and 1980s. Nevertheless, today the aminoglycosides are still the most commonly used antibiotics worldwide thanks to the combination of their high efficacy with low cost. This review first summarizes the history, chemistry, antibacterial actions and acute side effects of the drugs. It then details the pathophysiology of aminoglycoside ototoxicity including experimental and clinical observations, risk factors and incidence. Pharmacokinetics, cellular actions and our current understanding of the underlying molecular mechanisms of ototoxicity are discussed at length. The review concludes with recent advances towards therapeutic intervention to prevent aminoglycoside ototoxicity.

Age-related sensitivity to cisplatin ototoxicity in gerbils

This study was undertaken to define the developmental period of maximal sensitivity to cisplatin ototoxicity in gerbils. Five groups were established based upon post-natal age (P) at exposure to cisplatin, P10 (n = 8), P14 (n = 8), P18 (n = 6), P22 (n = 7) and P42 (n = 7). Animals were given cisplatin, 1 mg/kg/day intraperitoneal for 4 days. In the first four groups, P10, P14, P18 and P22, distortion product otoacoustic emissions were measured at 45 days of age, when responses were expected to be developmentally stable. Distortion product grams and input-output functions were measured. There was a statistically significant difference only between P14 and P42 (P<0.01). There was a significant interaction of age and frequency in the P14 group only (P<0.01). A secondary analysis compared distortion product grams of P14 animals, exposed to cisplatin, and age-matched saline-treated animals (n = 6). There was a significant treatment effect. In summary, there was an effect of age on the cisplatin ototoxicity in gerbils. Also, there was an effect of the frequency on DPOAE levels in P14 gerbils. These data support the presence of a 'sensitive' period to cisplatin ototoxicity in gerbils.

Cisplatin ototoxicity in developing gerbils

This experimental study was undertaken to investigate the dose-related effect of cisplatin exposure in young gerbils (2 weeks of age) and explore the relationship between different methods used to monitor auditory function after exposure to cisplatin. Four groups of animals, including a control group, were used. The treatment groups, D1 (n = 6), D2 (n = 7) and D3 (n = 6), received one, two, and three doses of cisplatin (5 mg/kg/dose), respectively, at weekly intervals. Treated animals were first exposed to cisplatin at 2 weeks of age. Distortion product otoacoustic emissions (DPOAE) and auditory brainstem responses (ABR) were measured in treated and control animals at 6 weeks of age. The effects of dose and frequency on the DPOAE amplitude, as well as the relationship between the DPOAE and the ABR thresholds were analyzed. Animals in the D1 and D3 groups demonstrated significant elevation of DPOAE and ABR thresholds. Interestingly, animals in the D2 group demonstrated a bimodal distribution of DPOAE and ABR responses, with four animals severely affected and three not showing an effect. A tendency for a bimodal distribution of DPOAE and ABR responses was also observed in the D1 group, at frequencies below 8 kHz.

Pharmacokinetics of kanamycin in the developing rat

The developing rat is hypersensitive to aminoglycoside ototoxicity during the period of anatomical and functional development of the cochlea. Toxicity is expressed only after a few days of treatment when kanamycin is given during the most sensitive period for production of ototoxicity (postnatal days 11-20). In contrast, when the drug is administered after the 20th postnatal day, the same dose and duration of treatment do not produce an ototoxic effect. Only after prolonged treatment (e.g., > or = 20 days) is there an observed effect. We characterized the pharmacokinetics of kanamycin in the serum of 12- and 25-day-old rats and observed a greater than 2.5-fold increase in elimination half-life in the 12- versus 25-day-old rat. The longer duration half-life of kanamycin in younger rats may explain the hypersensitivity of immature mammals to aminoglycoside ototoxicity.

Sensitive developmental periods for kanamycin ototoxic effects on distortion-product otoacoustic emissions

The developing rat is hypersensitive to aminoglycoside toxicity, which is expressed early on as a destruction of outer hair cells (OHC). In the current study, distortion-product otoacoustic emissions (DPOAE), which specifically measure the micromechanical activity of OHCs, were used to assess functional effects of administering a regimen of kanamycin to three groups of neonatal rats representing discrete postnatal developmental periods. In this manner, pigmented rats were treated at postnatal days 1-10, 11-20, and 21-30. A series of input-output (I/O) functions obtained for the 2f1-f2 DPOAE during the post-treatment period indicated that detection thresholds were significantly elevated for the animals treated on postnatal days 1-10 and 11-20, with the greatest elevations observed at the higher test frequencies.

Onset of ototoxicity in the cat is related to onset of auditory function

Cats are altricial mammals; they are born deaf and undergo rapid maturation of the auditory periphery late in the first and throughout the 2nd week of life. Previous studies, using multiple aminoglycoside administration over several days or weeks, have indicated that there is a reduction in the degree of ototoxicity in young animals provided the drug is administered prior to the onset of auditory function. In order to provide a more precise relationship between the degree of ototoxicity and auditory development, we used a single administration of Kanamycin (KA) and the loop diuretic ethacrynic acid (EA), as the co-administration of these drugs is known to produce a rapid and profound hearing loss in adult animals. Thirty kittens were administered with KA and EA at ages that varied from 2 to 16 days after birth (DAB) using a fixed dose per kilogram body weight sufficient to profoundly deafen adult animals. All animals made an uneventful recovery from the procedure. At 26 DAB, tone-pip-evoked auditory brainstem responses (ABR) were recorded from each animal in order to establish the extent of the hearing loss. The degree of hearing loss was compared with normal ABR audiograms recorded from 6 age-matched control animals. All animals treated with KA/EA at 9 DAB or older had a profound hearing loss similar to adult animals. Animals treated between 2 and 8 DAB exhibited severe high-frequency hearing losses. The extent of the loss was correlated with age (r = 0.63) and body weight (r = 0.72) such that hearing loss tended to spread towards lower frequencies as age and/or weight increased. All animals exhibited bilaterally symmetrical hearing losses which remained relatively stable over monitoring periods of up to 6 months following the drug treatment. These findings imply that the onset of ototoxicity is related, at least in part, to the onset of auditory function in the kitten. The rapid onset of deafness following this procedure makes it a useful technique in the study of both ototoxicity and cochlear development.

Ototoxicity in developing mammals

Developing mammals are more sensitive to noise, chemical and drug-induced ototoxicity than adults, with maximum sensitivity occurring during periods of anatomical and functional maturation of the cochlea. Normal physiological development of resting potentials (the endocochlear potential) and sound-evoked potentials including cochlear microphonics, summating potentials, compound action potentials, auditory brainstem responses and more recently distortion-product otoacoustic emissions have been characterized in several species including rats, mice, kittens, gerbils and guinea pigs. All of these responses are significantly impaired following acoustic trauma and/or exposure to a variety of ototoxic agents including aminoglycoside antibiotics, loop diuretics, antithyroid and antitumor drugs (alpha-difluoromethylornithine) and excitatory amino acids. Coupled with physiological and anatomical development is the maturation of specific biochemical pathways, which may be vulnerable targets of environmental noise and chemicals, excitatory amino acids and therapeutic drugs with ototoxic potentials.

Kanamycin depletes cochlear polyamines in the developing rat

Developing mammals are more sensitive to aminoglycoside antibiotics and other ototoxic agents than adults, with maximum sensitivity occurring during the period of anatomic and functional maturation of the cochlea. For the aminoglycoside antibiotics, the hypersensitive period in rats occurs during the second and third postnatal weeks. Toxicity is initially expressed as outer hair cell (OHC) damage in the high-frequency, basal region of the cochlea. Distortion-product otoacoustic emissions (DPOAEs), physiologic measures of OHC function, are particularly sensitive to aminoglycoside exposure during the period of rapid cochlear physiologic development. Toxicity is characterized by increased DPOAE thresholds and decreased amplitudes. The mechanism of developmental sensitivity to aminoglycosides is unknown. A potential biochemical target of aminoglycosides is the ornithine decarboxylase (ODC)-polyamine pathway. ODC activity is elevated in the developing rat cochlea, aminoglycosides inhibit cochlear ODC in developing rats, and alpha-difluoromethylornithine (a specific ODC inhibitor) impairs development of cochlear function. In the present study we demonstrate an incomplete polyamine response to aminoglycoside damage, characterized by inhibition of the polyamines spermidine and spermine and accumulation of putrescine in the organ of Corti. Aminoglycoside inhibition of polyamine synthesis may mediate developmental ototoxic hypersensitivity by interfering with developmental and repair processes.

Insensitivity of the chicken embryo to the ototoxicity of aminoglycoside antibiotics and a loop diuretic

Guinea pigs are routinely used in the histological evaluation of the cochlea as a method of testing for ototoxicity, but the procedures are very time-consuming. Because the avian cochlea is easier to examine and newly hatched chicks are sensitive to the ototoxic effects of gentamicin, birds may be useful in testing for ototoxicity. The use of chicken embryos would be even better for testing, but whether or not chicken embryos are sensitive to ototoxicants is unknown. In an attempt to determine whether or not chicken embryos may be used instead of guinea pigs in screening tests for ototoxicity, aminoglycoside antibiotics and a loop diuretic, ethacrynic acid, were administered to chicken embryos. A maximum-tolerated dose of gentamicin, kanamycin, streptomycin, ethacrynic acid, or a combination of gentamicin and ethacrynic acid was administered to fertile eggs of White Leghorn chickens on incubation days 10-17. To compare the effect of route of exposure on ototoxicity, gentamicin was administered by injection into the allantoic space, yolk sac, and air cell as well as by submerging the egg in gentamicin solution. With the preferred air cell route the effects of the ototoxic drugs kanamycin, streptomycin, ethacrynic acid, and a combination of ethacrynic acid and gentamicin were compared. On incubation day 18, cochleas were removed from the chicken embryos. Serial sections of these avian cochleas were examined and hair cells were counted. No significant difference was seen between the number of hair cells in cochleas of control chicken embryos and those from chicken embryos treated with drugs. Therefore, the chicken embryo appears to be insensitive to the ototoxicity of aminoglycoside antibiotics and a loop diuretic.

The sensitivity to 3,3'-iminodipropionitrile differs for high- and midfrequency hearing loss in the developing rat

3,3'-Iminodipropionitrile (IDPN) has been demonstrated to produce a loss of hearing following both neonatal and adult exposures. Adult exposure induces a full spectrum hearing loss, whereas early postnatal exposure produces a high-frequency loss only. The purpose of this work was to delineate the period of development during which the rat becomes sensitive to the full ototoxic effects of IDPN. Primiparous Long Evans rats or their offspring were exposed to either saline or 300 mg/kg IDPN for three consecutive days. Ages of exposure were as follows: gestational days 15-17 or postnatal days (PND) 1-3, 5-7, 15-17, 20-22, 25-27, 30-32, 40-42, or 70-72. All animals were tested as adults for auditory thresholds to 5- and 40-kHz tones using reflex modification audiometry. Results demonstrate that adult-like susceptibility to IDPN was not reached until approximately PND 30-32. Early exposures (PND 5-22) to IDPN will induce a highfrequency selective hearing loss, sparing the lower frequency. Prenatal or early neonatal (PND 1-3) IDPN exposure resulted in a high degree of mortality (> 70%). The long period of time between the susceptible period for the high frequency (PND 5-7) and the lower frequency (PND 30-32) does not correspond to the basal to apical ontogenic profile of any one physiological or anatomical process. These data suggest either a unique site of action for IDPN in the cochlea or the possibility of two different mechanisms, one operating at early postnatal ages and one at later ages.

Glutamate neurotoxicity in the developing rat cochlea: physiological and morphological approaches

The neurotoxic effects of exogenous glutamate were studied in the rat cochlea. Glutamate-treated rats (4 g/kg/day i.p., postnatal days 2-9) exhibited electrophysiologically-measured elevations in high frequency thresholds usually associated with hair cell loss in the basal region of the cochlea. While surface preparations of the organ of Corti revealed no loss of hair cells, there was a dramatic and selective reduction of neurons in the basal, high frequency-related portion of the spiral ganglion. This sensitivity of developing spiral ganglion cells to the neurotoxicity of glutamate is consistent with the hypothesis that glutamate or a structurally related substance is a neurotransmitter at afferent synapses of cochlear hair cells.

Cochlear cultures as a model system for studying aminoglycoside induced ototoxicity

Light and electron microscopy have been used to evaluate the effects of treating mouse cochlear cultures with the ototoxic aminoglycoside antibiotic neomycin sulphate at concentrations of 0.2 mM and greater for periods of up to 1 hour. Neomycin rapidly induces the formation of numerous, membrane filled blisters on the apical surfaces of the sensory hair cells. Such morphological damage is restricted to the hair cells, and is not observed on the surfaces of supporting cells within the organ of Corti. Hair cells in apical-coil cultures are less sensitive than those in basal-coil cultures, and, at any given point along the cochlea, outer hair cells appear to be more extensively damaged by neomycin than inner hair cells. These morphological effects of neomycin are considerably more severe when the drug is applied in calcium/magnesium free saline, and can be blocked by elevating the saline concentration of either calcium or magnesium. The effects can also be blocked by lowering the temperature to 4 degrees C, but not by either K+ depolarization or the lectin Concanavalin A. The potential value of this culture system as a model for studying aminoglycoside induced ototoxicity is discussed.

Ontogenetic features of audiogenic seizure susceptibility induced in immature rats by noise

Although numerous models are currently used for systematic study of the mechanisms of epileptogenesis in mature brain, few animal models have been developed that allow similar explorations in the developing nervous system. One experimental model of epilepsy supports a premise that perinatal experience can lead to eventual seizure susceptibility, however. Audiogenic seizure (AGS) susceptibility can be induced during a critical developmental period in normal mice by auditory deprivation and therefore by cochlear trauma. We studied the developmental parameters that affect success of both induction and testing of AGS-susceptibility in the rat. Intense high-frequency noise exposure was used as the traumatizing agent. The Wistar rat strain used is inherently seizure-resistant because in greater than 400 trials, untreated rats have never exhibited susceptibility at any age. Although single prolonged exposures to high-intensity noise were administered to groups of rats at ages between postnatal days (PNDs) 12 and 36, PND 14 was the age when exposure was most likely to result in eventual seizure susceptibility. Furthermore, duration of initial exposure on PND 14 determined the rate of susceptibility when measured 2 weeks later. Accordingly, we noted that single noise exposures at an intensity of 125 dB and ranging between 6 and 10 min in duration induced susceptibility in 100% of rats tested on PND 28; nonetheless, seizures among the rats exposed for 8 min were the most severe. Typically, these seizures began as wild running attacks and were followed by tonic/clonic convulsions.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-dependent changes in susceptibility to ototoxic hearing loss

Several species undergo age-dependent changes in susceptibility to noise-induced and drug-induced hearing loss. In this experiment 18-day-old mice were given 200 mg/kg kanamycin plus varying doses of bumetanide (20, 30 or 40 mg/kg). Thresholds of the scalp-recorded compound action potential of the auditory nerve indicated that diuretic doses of 30 and 40 mg/kg produced significant loss of hearing sensitivity. Thirty-eight-day-old mice were then treated with 200 mg/kg kanamycin plus 40 mg/kg bumetanide and demonstrated no loss of hearing sensitivity. These results confirm the existence of age-dependent changes in susceptibility to ototoxic hearing loss in mice. Furthermore, the treatment, consisting of two injections on the same day, offers promise as a means of defining the critical period of susceptibility in greater detail than has been possible with aminoglycosides alone.

Morphologic study of effects of kanamycin on the developing cochlea of the rat

The development of the rat inner ear starts about day 9 of a 22-day gestational period. The sensory and supporting cells are morphologically identifiable by day 18 of gestation; however, auditory function is not evident until day 9 postnatally. The organ of Corti attains adult maturity about day 16 postnatally. Morphologic and physiologic studies have shown that administration of sublethal doses of aminoglycosidic antibiotics, including kanamycin, damages the sensory and supporting cells of the inner ear in mammals. This results in the impairment of auditory function. This study was designed to determine the earliest stage in cochlear duct development at which kanamycin toxicity can be morphologically detected. A sublethal dose of kanamycin was administered to pregnant rats on days 10 through 20 of gestation and to neonates from days 1 to 8 or from days 8 to 16 postnatally. The inner ears were dissected and processed for light microscopic and ultrastructural observations. The cochlear ducts of fetuses exposed in utero to kanamycin and those of neonates exposed from days to 1 to 8 postnatally were unaffected. Kanamycin toxicity was evident in the cochlear ducts of neonates exposed from days 8 to 16 postnatally. The most severe damage occurred in the basal coil of the cochlea where in many instances sensory cells were totally absent. The sensory cells in the middle and apical coils exhibited abnormal morphology. These results suggest: (1) kanamycin does not have any permanent toxic effects on the rat cochlear duct during gestation and the first week of postnatal life. This provides a basis for further studies into the possible pharmacologic application of this finding in humans. (2) A correlation exists between the initiation of auditory function and the vulnerability of the organ of Corti to kanamycin toxicity.

The sensitive period for ototoxicity of kanamycin in mice: morphological evidence

The ototoxicity of kanamycin was investigated in mice treated with the drug either from 6 to 9, 10 to 13, or 15 to 18 days of age. The results showed extensive damage to the outer-hair-cell (OHC) receptors of the cochlea in the 10-13-day group; some less severe damage to the OHC was found in the 6-9-day group, and little damage was observed in the 15-18 day group. The present finding is consistent with the view that the mouse cochlea is most vulnerable to the ototoxic effect of kanamycin during the period of its rapid development, i.e., between 10 and 13 days of age.

Aminoglycoside ototoxicity in the chick (Gallus domesticus) inner ear: I. The effects of kanamycin and netilmicin on the basilar papilla

A single dose (100 mg/kg of egg weight) of kanamycin or netilmicin was injected into the yolk sacs of 7-day-old chick (Gallus domesticus) embryos. Embryos were collected every 24 hours and processed for light and transmission electron microscopy. Morphologic study of the medial basilar papilla disclosed that both kanamycin and netilmicin are toxic to the hair cells in this region. Intoxication was manifested by an increased number of dense osmiophilic bodies, swollen mitochondria, agglomerated chromatin, and occasional disorganization of the kinocilium basal bodies. The cytologic changes observed in the hair cells of embryos injected with netilmicin and kanamycin were similar. However, mitochondrial damage was more severe in the chicks after kanamycin than after netilmicin injection. Some of the cytologic alterations described here are comparable to those already reported for aminoglycoside-intoxicated hair cells in several mammalian species. This study and previous work indicate that the chick embryo provides a satisfactory developmental model for testing ototoxicity of drugs in vivo.

Structure side-effect sorting of drugs. VI. Ototoxicities

From a literature survey, over 130 (about 7.8%) drugs and chemicals have been associated with ototoxicities. The major classes are basic aminoglycoside and other antibiotics, anti-inflammatory drugs, antimalarials, beta-blockers, antineoplastic agents, heavy metals, diuretics, some topical agents and various miscellaneous drugs. Possible mechanisms of action are presented and discussed. These include inhibition of protein synthesis, the glycolytic cycle, the TCA cycle, energy utilization, energy generation and the respiratory system within the mitochondria membrane of the hair cell, and also alteration of the permeability of the endolymphatic membrane or alteration of the excretion system for the basic aminoglycosides in the lateral wall of the membranous cochlea. The relative rank order of ototoxicity and reactivity toward mucopolysaccharides of five aminoglycosides is found to be related to the number of basic groups in each molecule.

Hyperthermia increases aminoglycoside ototoxicity

Similarities were noted between the nature of inner ear damage produced by loud sounds and by aminoglycoside antibiotics. Since body temperature affects cochlear function and influences the effects of noise on the ear, a similar effect was predicted for the aminoglycoside, kanamycin. By environmentally elevating the body temperatures of preweanling mice to approximately 1 degree C above that of the normal adult, kanamycin ototoxicity was increased (an average 20.4 dB threshold elevation, vs. 9.3 dB for kanamycin injected mice reared at room temperature). Hyperthermia per se had no influence on auditory thresholds. This may be of relevance to humans with fever who are treated with aminoglycoside antibiotics.

Comparative ototoxicity of four aminoglycosidic antibiotics during the critical period of cochlear development in the rat. A functional and structural study

The comparative ototoxicity of four aminoglycosides (amikacin, dibekacin, gentamicin and tobramycin) was evaluated in the rat during the critical period of cochlear development. Newborn rats received a daily subcutaneous injection of one of the four antibiotics for 8 consecutive days, starting on day 8 after birth (amikacin: 225 mg/kg, dibekacin: 60 mg/kg, gentamicin and tobramycin: 45 mg/kg). Evaluation of ototoxicity was assessed one month after the end of the treatment using cochlear recordings (action potential and cochlear microphonic), surface preparations and scanning electron microscopy. No functional or structural evidence of ototoxicity was found in animals treated with dibekacin. Tobramycin had a weak ototoxic effect characterized by a slight increase in N1 latency and moderate damage to OHC stereocilia (fusion and formation of giant cilia). Gentamicin-treated animals demonstrated more severe evidence of ototoxicity including increased thresholds for CM, and a higher incidence of missing hair cells and damage to OHC stereocilia. The maximal cochlear damage was observed in amikacin-treated animals: both AP and CM thresholds were severely impaired, cell counts and SEM showed extensive loss of hair cells.