Differential susceptibility to gentamicin ototoxicity between albino and pigmented guinea pigs

Summating Potential as Marker of Intracochlear Position in Bipolar Electrocochleography

OBJECTIVES

Cochlear implantation criteria include subjects with residual low-frequency hearing. To minimize implantation trauma and to avoid unwanted interactions of electric- and acoustic stimuli, it is often recommended to stop cochlear implantation before the cochlear implant (CI) reaches the cochlear partition with residual hearing, as determined by an audiogram. For this purpose, the implant can be used to record acoustically evoked signals during implantation, including cochlear compound action potentials (CAP), cochlear microphonics (CMs), and summating potentials (SPs). The former two have previously been used to monitor residual hearing in clinical settings.

DESIGN

In the present study we investigated the use of intracochlear, bipolar SP recordings to determine the exact cochlear position of the contacts of implanted CIs in guinea pig cochleae (n = 13). Polarity reversals of SPs were used as a functional marker of intracochlear position. Micro computed tomography (µCT) imaging and a modified Greenwood function were used to determine the cochleotopic positions of the contacts in the cochlea. These anatomical reconstructions were used to validate the SP-based position estimates.

RESULTS

The precision of the SP-based position estimation was on average within ± 0.37 octaves and was not impaired by moderate hearing loss caused by noise exposure after implantation. It is important to note that acute hearing impairment did not reduce the precision of the method. The cochleotopic position of CI accounted for ~70% of the variability of SP polarity reversals. Outliers in the dataset were associated with lateral CI positions. Last, we propose a simplified method to avoid implantation in functioning parts of the cochlea by approaching a predefined frequency region using bipolar SP recordings through a CI.

CONCLUSIONS

Bipolar SP recordings provide reliable information on electrode position in the cochlea. The position estimate remains reliable after moderate hearing loss. The technique presented here could be applied during CI surgery to monitor the CI approach to a predefined frequency region.

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.

Application of Mouse Models to Research in Hearing and Balance

Laboratory mice (Mus musculus) have become the major model species for inner ear research. The major uses of mice include gene discovery, characterization, and confirmation. Every application of mice is founded on assumptions about what mice represent and how the information gained may be generalized. A host of successes support the continued use of mice to understand hearing and balance. Depending on the research question, however, some mouse models and research designs will be more appropriate than others. Here, we recount some of the history and successes of the use of mice in hearing and vestibular studies and offer guidelines to those considering how to apply mouse models.

Assessment of nutrient supplement to reduce gentamicin-induced ototoxicity

Gentamicin is an aminoglycoside antibiotic used to treat gram-negative bacterial infections. Treatment with this antibiotic carries the potential for adverse side effects, including ototoxicity and nephrotoxicity. Ototoxic effects are at least in part a consequence of oxidative stress, and various antioxidants have been used to attenuate gentamicin-induced hair cell death and hearing loss. Here, a combination of nutrients previously shown to reduce oxidative stress in the hair cells and attenuate hearing loss after other insults was evaluated for potential protection against gentamicin-induced ototoxicity. Guinea pigs were maintained on a nutritionally complete standard laboratory animal diet or a diet supplemented with β-carotene, vitamins C and E, and magnesium. Three diets with iterative increases in nutrient levels were screened; the final diet selected for study use was one that produced statistically reliable increases in plasma levels of vitamins C and E and magnesium. In two separate studies, significant decreases in gentamicin-induced hearing loss at frequencies including 12 kHz and below were observed, with less benefit at the higher frequencies. Consistent with the functional protection, robust protection of both the inner and outer hair cell populations was observed, with protection largely in the upper half of the cochlea. Protection was independently assessed in two different laboratories, using two different strains of guinea pigs. Additional in vitro tests did not reveal any decrease in antimicrobial activity with nutrient additives. Currently, there are no FDA-approved treatments for the prevention of gentamicin-induced ototoxicity. The current data provide a rationale for continued investigations regarding translation to human patients.

Exposure to low levels of jet-propulsion fuel impairs brainstem encoding of stimulus intensity

Jet propulsion fuel-8 (JP-8) is a kerosene-based fuel that is used in military jets. The U.S. Armed Services and North Atlantic Treaty Organization countries adopted JP-8 as a standard fuel source and the U.S. military alone consumes more than 2.5 billion gallons annually. Preliminary epidemiologic data suggested that JP-8 may interact with noise to induce hearing loss, and animal studies revealed damage to presynaptic sensory cells in the cochlea. In the current study, Long-Evans rats were divided into four experimental groups: control, noise only, JP-8 only, and JP-8 + noise. A subototoxic level of JP-8 was used alone or in combination with a nondamaging level of noise. Functional and structural assays of the presynaptic sensory cells combined with neurophysiologic studies of the cochlear nerve revealed that peripheral auditory function was not affected by individual exposures and there was no effect when the exposures were combined. However, the central auditory nervous system exhibited impaired brainstem encoding of stimulus intensity. These findings may represent important and major shifts in the theoretical framework that governs current understanding of jet fuel and/or jet fuel + noise-induced ototoxicity. From an epidemiologic perspective, results indicate that jet fuel exposure may exert consequences on auditory function that may be more widespread and insidious than what was previously shown. It is possible that a large population of military personnel who are suffering from the effects of jet fuel exposure may be misidentified because they would exhibit normal hearing thresholds but harbor a "hidden" brainstem dysfunction.

Viability of Human Melanocytes HEMa-LP Exposed to Amikacin and Kanamycin

Aminoglycosides, such as amikacin and kanamycin, are powerful broad-spectrum antibiotics used for the treatment of many bacterial infections. The widely used aminoglycosides have the unfortunate side effects of targeting sensory hair cells of the inner ear, so that treatment often results in permanent hair cell loss. The aim of the study was to evaluate the influence of incubation time and drug concentration on viability of melanocytes cultured in the presence of amikacin or kanamycin. The normal human melanocytes HEMa-LP and the different concentrations of amikacin (0.075, 0.75 and 7.5 mmol/l) and kanamycin (0.06, 0.6 and 6.0 mmol/l), were used. The estimations were performed after 24, 48 and 72 h. The observed decrease in melanocytes viability may be an explanation for the mechanisms involved in aminoglycosides toxicity on pigmented tissues during high-dose and/or long-term therapy.

Modulation of melanogenesis and antioxidant defense system in melanocytes by amikacin

Amikacin is principally used to treat infections caused by microorganisms resistant to other aminoglycosides. Ototoxicity is one of the side effects of amikacin, but the causative mechanism of damage to the ear has not been fully established. Thus, the aim of this work was to examine the impact of amikacin on the melanogenesis and antioxidant defense system in cultured human normal melanocytes (HEMa-LP). Amikacin induced the concentration - dependent loss in melanocytes viability. The value of EC50 was determined to be ~7.5 mM. The analyzed antibiotic inhibited melanin biosynthesis in concentration-dependent manner. Increasing the amikacin concentration also resulted in a decrease in cellular tyrosinase activity. To study the antioxidant defense system in melanocytes, the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) in cells exposed to amikacin were determined. Significant changes in cellular antioxidant enzymes activities were observed. Modulation of melanogenesis and the antioxidant status of melanocytes resulting from the use of amikacin in vitro may explain a potential role of melanin and melanocytes in the mechanisms of aminoglycosides ototoxic effects in vivo.

Object recognition testing: rodent species, strains, housing conditions, and estrous cycle

The object recognition task (ORT) allows assessing learning and memory processes in rodents. In this study, two areas in which knowledge about the ORT could be extended were addressed; i.e. generality to species and strains, and intervening variables including housing and estrous cycle. Regarding generality to species and strains, the ORT performance of golden hamsters was assessed. The hamsters showed sufficient exploration times, object recognition performance, and a retention-interval dependent decline similar to rats and mice. Subsequently, we tested three mouse strains which have not been described before in the ORT; i.e. OF1, NMRI, and SJL mice. OF1 and NMRI strains performed equally well, whereas the SJL strain showed low exploration times and no memory retention. Therefore, the SJL strain is unsuited for ORT experiments using a 1h retention interval and a fixed (3 min) trial duration. Furthermore, the sensitivity to a pharmacological memory deficit model (scopolamine) was tested in three rat strains. Each strain showed a dose dependent relationship, but the least effective dose of scopolamine differed among the three strains, the effect being greater in the order of Wistar, Long-Evans, Hooded Lister rats. Finally, to investigate potential intervening variables in the ORT, the effects of housing conditions and estrous cycle were investigated with rats. Single housing resulted in absolute higher performance than social housing. Furthermore, females in pro-estrus/estrus showed better performance compared to females in met-estrus/di-estrus. Taken together, object recognition appears to be a common ability of rodent species, but different strains have different memory capacities and sensitivities to scopolamine, individual housing leads to higher performance, and performance of females is dependent on the estrous cycle phase. Thus, rodent species, strain, housing, and estrous cycle should be taken into consideration in ORT studies.

Association of skin color, race/ethnicity, and hearing loss among adults in the USA

Epidemiologic studies of hearing loss in adults have demonstrated that the odds of hearing loss are substantially lower in black than in white individuals. The basis of this association is unknown. We hypothesized that skin pigmentation as a marker of melanocytic functioning mediates this observed association and that skin pigmentation is associated with hearing loss independent of race/ethnicity. We analyzed cross-sectional data from 1,258 adults (20-59 years) in the 2003-2004 cycle of the National Health and Nutritional Examination Survey who had assessment of Fitzpatrick skin type and pure-tone audiometric testing. Audiometric thresholds in the worse hearing ear were used to calculate speech- (0.5-4 kHz) and high-frequency (3-8 kHz) pure-tone averages (PTA). Regression models were stratified by Fitzpatrick skin type or race/ethnicity to examine the association of each factor with hearing loss independent of the other. Models were adjusted for potential confounders (demographic, medical, and noise exposure covariates). Among all participants, race/ethnicity was associated with hearing thresholds (black participants with the best hearing followed by Hispanics and then white individuals), but these associations were not significant in analyses stratified by skin color. In contrast, in race-stratified analyses, darker-skinned Hispanics had better hearing than lighter-skinned Hispanics by an average of -2.5 dB hearing level (HL; 95% CI, -4.8 to -0.2) and -3.1 dB HL (95% CI, -5.3 to -0.8) for speech and high-frequency PTA, respectively. Associations between skin color and hearing loss were not significant in white and black participants. Our results demonstrate that skin pigmentation is independently associated with hearing loss in Hispanics and suggest that skin pigmentation as a marker of melanocytic functioning may mediate the strong association observed between race/ethnicity and hearing loss.

Melanin precursors prevent premature age-related and noise-induced hearing loss in albino mice

Strial melanocytes are required for normal development and correct functioning of the cochlea. Hearing deficits have been reported in albino individuals from different species, although melanin appears to be not essential for normal auditory function. We have analyzed the auditory brainstem responses (ABR) of two transgenic mice: YRT2, carrying the entire mouse tyrosinase (Tyr) gene expression-domain and undistinguishable from wild-type pigmented animals; and TyrTH, non-pigmented but ectopically expressing tyrosine hydroxylase (Th) in melanocytes, which generate the precursor metabolite, L-DOPA, but not melanin. We show that young albino mice present a higher prevalence of profound sensorineural deafness and a poorer recovery of auditory thresholds after noise-exposure than transgenic mice. Hearing loss was associated with absence of cochlear melanin or its precursor metabolites and latencies of the central auditory pathway were unaltered. In summary, albino mice show impaired hearing responses during ageing and after noise damage when compared to YRT2 and TyrTH transgenic mice, which do not show the albino-associated ABR alterations. These results demonstrate that melanin precursors, such as L-DOPA, have a protective role in the mammalian cochlea in age-related and noise-induced hearing loss.

Trafficking of systemic fluorescent gentamicin into the cochlea and hair cells

Aminoglycosides enter inner ear hair cells across their apical membranes via endocytosis, or through the mechanoelectrical transduction channels in vitro, suggesting that these drugs enter cochlear hair cells from endolymph to exert their cytotoxic effect. We used zebrafish to determine if fluorescently tagged gentamicin (GTTR) also enters hair cells via apically located calcium-sensitive cation channels and the cytotoxicity of GTTR to hair cells. We then examined the serum kinetics of GTTR following systemic injection in mice and which murine cochlear sites preferentially loaded with systemically administered GTTR over time by confocal microscopy. GTTR is taken up by, and is toxic to, wild-type zebrafish neuromast hair cells. Neuromast hair cell uptake of GTTR is attenuated by high concentrations of extracellular calcium or unconjugated gentamicin and is blocked in mariner mutant zebrafish, suggestive of entry via the apical mechanotransduction channel. In murine cochleae, GTTR is preferentially taken up by the stria vascularis compared to the spiral ligament, peaking 3 h after intra-peritoneal injection, following GTTR kinetics in serum. Strial marginal cells display greater intensity of GTTR fluorescence compared to intermediate and basal cells. Immunofluorescent detection of gentamicin in the cochlea also revealed widespread cellular labeling throughout the cochlea, with preferential labeling of marginal cells. Only GTTR fluorescence displayed increasing cytoplasmic intensity with increasing concentration, unlike the cytoplasmic intensity of fluorescence from immunolabeled gentamicin. These data suggest that systemically administered aminoglycosides are trafficked from strial capillaries into marginal cells and clear into endolymph. If so, this will facilitate electrophoretically driven aminoglycoside entry into hair cells from endolymph. Trans-strial trafficking of aminoglycosides from strial capillaries to marginal cells will be dependent on as-yet-unidentified mechanisms that convey these drugs across the intra-strial electrical barrier and into marginal cells.

Absence of strial melanin coincides with age-associated marginal cell loss and endocochlear potential decline

Cochlear stria vascularis contains melanin-producing intermediate cells that play a critical role in the production of the endocochlear potential (EP) and in maintaining the high levels of K(+) that normally exist in scala media. The melanin produced by intermediate cells can be exported to the intrastrial space, where it may be taken up by strial marginal cells and basal cells. Because melanin can act as an antioxidant and metal chelator, evidence for its role in protecting the stria and organ of Corti against noise, ototoxins, and aging has long been sought. While some evidence supports a protective role of melanin against noise and ototoxins, no evidence yet presented has demonstrated a clear role for melanin in maintaining the EP during aging. We tested this by comparing basal turn EPs and a host of cochlear cellular metrics in aging C57BL/6 (B6) mice and C57BL/6-Tyr(c-2J) mice. The latter mice carry a naturally occurring inactivating mutation of the tyrosinase locus, and produce no strial melanin. Because these two strains are coisogenic, and because pigmented B6 mice show essentially no age-related EP decline, they provide an ideal test of importance of melanin in the aging stria. Pigmented and albino B6 mice showed identical rates of hearing loss and sensory cell loss. However, after two years of age, basal turn EPs significantly diverged, with 42% of albinos showing EPs below 100 mV versus only 18% of pigmented mice. The clearest anatomical correlate of this EP difference was significantly reduced strial thickness in the albinos that was highly correlated with loss of marginal cells. Combined with findings in human temporal bones, plus recent work in BALB/c mice and gerbils, the present findings point to a common etiology in strial presbycusis whereby EP reduction is principally linked to marginal cell loss or dysfunction. For any individual, genetic background, environmental influences, and stochastic events may work together to determine whether marginal cell density or function falls below some critical level, and thus whether EP decline occurs.

Cisplatininduzierte Hörstörungen bei Kindern in Abhängigkeit von der Pigmentierung der Iris

BACKGROUND

Cisplatin is commonly used as a chemotherapeutic agent in the treatment of solid tumors. Ototoxicity is an important side-effect. Melanin in the inner ear either plays an otoprotective role or has a negative influence on hearing. The concentration of cochlear melanin correlates with its concentration in the iris.

PATIENTS AND METHODS

We retrospectively examined 65 children (37 males, 28 females, average age 7.5 years) treated with cisplatin at the University Clinic of Muenster, Germany. We checked whether their eye color could be inferred from the prevalence and extent of cisplatin-induced hearing loss.

RESULTS

We found a hearing loss of >20 dB in 29 light-eyed and in 21 dark-eyed patients. Seven light-eyed and eight dark-eyed patients did not suffer from hearing impairment. Using the chi(2)-test on these four parameters, we found no significant connection between iris pigmentation and the prevalence or extent of hearing loss, although light-eyed children (80.6%) suffered more from hearing loss than dark-eyed children (72.4%). After the end of therapy with cisplatin, the prevalence of hearing loss was 83.3% in children up to 6 years and 71.4% in children older than 6 years. The average cumulative dose of cisplatin was 372 mg/m(2) of body surface in children with hearing loss, compared to 390 mg/m(2) in children without hearing loss.

CONCLUSION

We found no significant correlation between iris pigmentation (eye color) and hearing loss. Cisplatin-induced hearing loss occurs frequently and requires repeated monitoring.

Effect of melanin on netilmicin-induced inhibition of collagen biosynthesis in human skin fibroblasts

It is known that various drugs form complexes with melanins and that melanins are abundant constituents of the inner ear. In this study, we determined whether the aminoglycoside antibiotic, netilmicin, interacts with melanin and how this process affects collagen biosynthesis in cultured human skin fibroblasts. The obtained results indicate that netilmicin forms stable complexes with melanin characterized by the association constants K(1) approximately 10(6)M(-1) and K(2) approximately 10(3)M(-1). We have suggested that prolidase, an enzyme involved in collagen metabolism, may be one of the targets for aminoglycoside-induced inhibition of collagen biosynthesis. We found that netilmicin strongly induced inhibition of prolidase activity (IC(50)<5microM) and collagen biosynthesis (IC(50) approximately 10microM). At 10microM concentration of netilmicin, prolidase activity in human skin fibroblasts was inhibited by about 80% and DNA biosynthesis-only by about 25%. Melanin at 100microg/mL produced about 30% inhibition of collagen biosynthesis and about 30% inhibition of prolidase activity in cultured fibroblasts. However, the addition of melanin (100microg/mL) to netilmicin-treated cells (10microM) restored the prolidase activity in fibroblasts to almost 100% of control values and partially reversed the inhibitory action of the drug on collagen and DNA biosynthesis. The data suggest that the ability of netilmicin to form stable complexes with melanin may prevent its toxicity on prolidase activity and collagen biosynthesis.

Inhibition of caspases alleviates gentamicin-induced cochlear damage in guinea pigs

The efficacy of caspase inhibitors for protecting the cochlea was evaluated in an in vivo study using guinea pigs, as the animal model system. Gentamicin (12 mg/ml) was delivered via an osmotic pump into the cochlear perilymphatic space of guinea pigs at 0.5 microl/h for 14 days. Additional animals were given either z-Val-Ala-Asp (Ome)-fluoromethyl ketone (z-VAD-FMK) or z-Leu-Glu-His-Asp-FMK (z-LEHD-FMK), a general caspase inhibitor and a caspase 9 inhibitor, respectively, in addition to gentamicin. The elevation in auditory brain stem response thresholds, at 4, 7, and 14 days following gentamicin administration, were decreased in animals that received both z-VAD-FMK and z-LEHD-FMK. Cochlear sensory hair cells survived in greater numbers in animals that received caspase inhibitors in addition to gentamicin, whereas sensory hair cells in animals that received gentamicin only were severely damaged. These results suggest that auditory cell death induced by gentamicin is closely related to the activation of caspases in vivo.

Inner ear changes with intracochlear gentamicin administration in Guinea pigs

OBJECTIVES/HYPOTHESIS

Transtympanic administration of gentamicin is reported to be a useful treatment for vertigo in such conditions as Meniere's disease, and determining appropriate clinical dosage of gentamicin is difficult. The authors examined the relation between gentamicin dosages and inner ear function in guinea pigs.

STUDY DESIGN

This study is a basic science project designed to examine cochlear and vestibular function in animal models.

METHODS

Various concentrations of gentamicin solution were infused into the right inner ear of guinea pigs by osmotic pumps. Caloric nystagmus as a marker of vestibular function and the change in auditory brainstem response (ABR) threshold as a marker of cochlear function were observed.

RESULTS

After 14 days of treatment, high gentamicin concentrations of 40 mg/mL caused canal paralysis and a rapid shift in ABR threshold. Animals exposed to low gentamicin concentrations of 4 mg/mL showed no obvious change in either vestibular or cochlear function. Animals exposed to moderate gentamicin concentrations of 12 mg/mL showed a moderate shift in ABR threshold and caloric malfunction. Histopathological examination revealed that after 14 days of treatment with 40 mg/mL gentamicin, severe cytoplasmic damage occurred in both vestibular and cochlear end organs. In animals treated with 12 mg/mL gentamicin, hair cells remained in the cochlear third turn and ampulla of the lateral semicircular canal.

CONCLUSION

The authors established an animal model that showed the moderate damage of inner ear with moderate-dose gentamicin. The study results indicated that the appropriate administration of gentamicin could establish a stable effect on the inner ear. It may be important to select the protocol that delivers a stable dosage of gentamicin to treat patients with Meniere's disease safely and effectively.

Aminoglycoside ototoxicity in adult CBA, C57BL and BALB mice and the Sprague-Dawley rat

The availability of genetic information, transgenic and knock-out animals make the 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 kanamycin ototoxicity in adult CBA/J, C57BL/6 and BALB/c mouse strains and a comparison to Sprague-Dawley rats. Five-week-old mice were injected subcutaneously twice daily with 400-900 mg kanamycin base/kg body weight for 15 days. Kanamycin induced dose-dependent auditory threshold shifts of up to 70 dB at 24 kHz as measured by auditory brain stem-evoked responses. Vestibular function was also affected in all strains. The functional deficits were accompanied by hair cell loss in both cochlear and vestibular neurosensory epithelia. Concomitant administration of the antioxidant 2,3-dihydroxybenzoate significantly attenuated the kanamycin-induced threshold shifts. In adult male Sprague-Dawley rats, doses of 1 x 500 mg or 2 x 300 mg kanamycin base/kg body weight/day x 14 days induced threshold shifts of approximately 50 dB at 20 kHz. These were accompanied by loss of outer hair cells. The order of susceptibility, BALB>CBA>C57, was not due to differences in the pharmacokinetics of kanamycin. It also did not correlate with the presence of Ahl/Ahl2 genes which predispose C57 and BALB strains, respectively, to accelerated age-related hearing loss. Pigmentation, however, paralleled this rank order suggesting an influence of melanin on cochlear antioxidant status.

Differential vulnerability of basal and apical hair cells is based on intrinsic susceptibility to free radicals

The base of the cochlea is more vulnerable to trauma than the apex as seen in the pattern of hair cell damage by cisplatin or aminoglycosides. The differential vulnerability is maintained in organotypic cultures exposed directly to these drugs, suggesting there may be an intrinsic difference in sensitivity to damage along the cochlear spiral. We therefore investigated the survival capacity of isolated outer hair cells and strips dissected from different turns of the guinea pig organ of Corti in short-term culture. Cells were stained with fluorescent indicators of viable or dead cells, calcein-AM and ethidium homodimer. After 5 h at room temperature, up to 90% of outer hair cells from the apex survived, but less than 30% from the base. In contrast, basal inner hair cells remained viable, and supporting cells survived for at least 20 h. The difference in survival capacity between basal and apical outer hair cells coincided with a significantly lower level of the antioxidant glutathione in basal outer hair cells compared with apical outer hair cells. This suggested that basal outer hair cells may be more vulnerable to free-radical damage than apical outer hair cells. The survival of basal outer hair cells was significantly improved by addition of the radical scavengers n-acetyl cysteine, p-phenylenediamine, glutathione, mannitol or salicylate. The protection by antioxidants implies that the accelerated death of basal outer hair cells is due to free-radical damage. The results support an intrinsic susceptibility to free radicals that differs among cochlear cell populations. This differential provides a rational explanation for base-to-apex gradients observed in various forms of cochlear pathology.

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

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

Effects of prenatal alcohol exposure and aging on auditory function in the rat: preliminary results

This study investigated select aspects of peripheral and central auditory dysfunction, as well as the pathological effects of aging, In an animal model of fetal alcohol syndrome (FAS). Pregnant rats consumed liquid alcohol diets containing 0, 17.5, or 35% ethanol-derived calories, from gestation day 7 to parturition. A fourth group was untreated. Offspring of these mothers were tested for auditory and neurological function, using the auditory brainstem response at 6, 12, and 18 months of age. Some animals in the alcohol-exposed groups showed a peripheral auditory disorder in the form of congenital sensorineural hearing loss. This was correlated with punctate lesions and malformed stereocilia on the auditory sensory receptor cells of the inner ear. Alcohol-exposed animals also showed a central auditory processing disorder characterized by prolonged transmission of neural potentials along the brainstem portion of the auditory pathway. Animals in the highest dose group also showed an augmentation in the age-related deterioration of auditory acuity. Thus, increased peripheral and central auditory dysfunctions and pathological deterioration of auditory function in old age may be sequelae of FAS. Such morbidities have important implications for the long-term clinical assessment and management of FAS patients.

Differential effects of gentamicin on the distribution of cochlear function in albino and pigmented guinea pigs

It has been suggested that the high affinity of melanin pigment for aminoglycoside antibiotics may cause these drugs to bind preferentially to the pigmented inner ear, producing greater ototoxicity than in the amelanotic albino cochlea. However, evidence of greater ototoxicity in albinos has led to the hypothesis that melanin inhibits the toxicity of these drugs in the pigmented inner ear. On the other hand, ototoxicity in the pigmented animals may simply be delayed relative to the albinos, only to become equal or even more severe with time. The present study was conducted to determine whether a relatively low dose of gentamicin (68.5 mg/kg) would produce differential ototoxicity between albino and pigmented guinea pigs which would persist long after drug exposure had stopped. Nine pigmented and eight albino guinea pigs were given gentamicin sulfate for 14 consecutive days, and were then allowed a two-month recovery period before cochlear analysis; 11 pairs of saline-injected or untreated albino and pigmented guinea pigs served as controls. The results showed that the gentamicin-treated albinos had significantly elevated thresholds for the compound action potential from the auditory nerve (CAP), and significantly lower endocochlear potentials (EP) and cochlear microphonic (CM) input-output voltage functions when compared to their respective controls, or to either group of pigmented guinea pigs. The CAP in drug-treated pigmented animals did not differ significantly from controls, and the differences in EP and CM were marginally significant. The results indicate that the pigmented cochlea is less susceptible to gentamicin than the albino cochlea, and support the hypothesis that melanin may inhibit aminoglycoside ototoxicity in the pigmented inner ear.

Ongoing proliferation of melanocytes in the stria vascularis of adult guinea pigs

The intermediate cells of the stria vascularis are melanocytes derived from the neural crest. These internalized pigment cells have been thought to be a static population, distinct from those found in the skin. However, this investigation demonstrates that the melanocytes of the adult stria vascularis undergo continuous replication. Cell proliferation was studied using [3H]-thymidine autoradiography and bromodeoxyuridine (BrdU) immunohistochemistry. Single or multiple injections of [3H]-thymidine within a six hour period labeled a mean of 9 intermediate cells. In pigmented guinea pigs, single daily injections of [3H]-thymidine for 2 or 4 days labeled a mean of 24 and 69 intermediate cells, respectively; Pigmented guinea pigs given BrdU once daily for 2 or 4 days labeled a mean of 38 and 75 intermediate cells. By contrast, albino littermates also given BrdU averaged only 23 and 42 labeled intermediate cells in the same 2 and 4 day experiments. The mean number of proliferating cells/mm of stria per 24 h was 1.54 in the pigmented animals and 0.88 in the albinos. Both the total number and density of labeled intermediate cells were significantly smaller in the albino than the pigmented guinea pigs. These results demonstrate that the melanocytes in the stria vascularis undergo continuous baseline mitosis, and at a rate comparable to the melanocytes of the skin. This surprising similarity promotes the speculation that the proliferative rate of the strial melanocytes may be influenced by some of the same factors known to affect replication and pigment production in the skin.

Effects of the interaction between ferric iron and L-dopa melanin on T1 and T2 relaxation times determined by magnetic resonance imaging

T1 and T2 relaxation times of agar phantoms containing L-dopa melanin and Fe3+ were measured under MRI conditions. Fe3+ shortened T1 and T2 relaxation times. Melanin influenced relaxation times only in the presence of Fe3+; thus, contrast in MR images of the basal ganglia may depend upon levels of both paramagnetic iron and neuromelanin.

Effects of tone burst frequency and intensity on the auditory brainstem response (ABR) from albino and pigmented rats

Young adult male Sprague-Dawley (SD) and Long-Evans (LE) rats were evaluated using the auditory brainstem response (ABR). ABRs were evoked by stimuli with intensities ranging from 15 to 100 dB peSPL. Stimuli were tone bursts of 2000, 4000 and 8000 Hz. As stimulus intensity decreased from 100 to 15 dB, the ABR peak latencies prolonged, interpeak latencies (IPLs) shortened and amplitudes decreased. As stimulus frequency decreased from 8000 to 2000 Hz, ABR latencies prolonged, amplitudes decreased and ABR thresholds increased. The longest IPLs were in response to the 4000 Hz tone bursts. SD rats had ABRs with shorter peak latencies, larger amplitudes and lower thresholds than LE rats. The IPLs usually did not show significant strain-dependent differences. Our observations on stimulus intensity and frequency are consistent with previous reports. Our observations also suggest that the SD (albino) rat has better auditory acuity than the LE (pigmented) rat over the frequency range of 2000 to 8000 Hz. This implies that previous concerns about the use of albino animals in audiological research are somewhat overstated.

Effect of isepamicin dosing scheme on concentration in cochlear tissue

To investigate the possible effect of the dosing scheme of aminoglycosides on their concentration in the cochlear tissue, we gave two groups of 12 guinea pigs subcutaneous doses of 45 mg of isepamicin (ca. 30 mg of active product) per kg of body weight daily for eight consecutive days. The first group received the drug by continuous infusion, while the second group received it by single daily injection. On the final day of administration, the animals were sacrificed and the cochlear tissue was removed. The tissues from the cochleas of pairs of guinea pigs were pooled. The isepamicin concentrations in the cochlear duct tissue (organ of Corti plus lateral wall) and the cochlear nerve tissue were determined separately. Hearing levels before and after treatment were assessed by means of frequency-specific auditory brain stem responses (ABR). The creatinine level in serum was determined on the last day of the administration. None of the animals in either group showed signs of renal insufficiency or of hearing impairment. The median isepamicin concentration in the cochlear duct was 2.40 micrograms/mg of protein after continuous administration and 2.50 micrograms/mg of protein after once-daily administration, compared with the concentration in the cochlear nerve, where it was 1.93 micrograms/mg of protein after continuous administration and 2.59 micrograms/mg of protein after once-daily administration. These differences are statistically insignificant. The results give evidence for linear uptake kinetics of isepamicin in the inner ear tissue and may be directly relevant to the clinical dosing of the drug.

Turn-specific and pigment-dependent differences in the stria vascularis of normal and gentamicin-treated albino and pigmented guinea pigs

The aims of the present study were to determine which structures in the stria vascularis (SV) may depend upon the presence of pigmented melanocytes both for normal morphology and for the expression of gentamicin ototoxicity in the inner ear. These pigment-dependent influences were inferred through comparisons of the SV in pigmented guinea pigs and in albinos containing nonpigmented melanocytes. Results were obtained from 6 albino and 8 pigmented guinea pigs given gentamicin, and from 3 albino and 3 pigmented control animals not receiving the drug. One-month old animals received gentamicin daily (100 mg/kg) for 14 days and recovered for an additional 14 days before being prepared for electron microscopy. The SV from each of the 4 cochlear turns was analyzed using stereological point counting procedures. In control animals, differences were found in the higher cochlear turns, where volume density for the marginal cells in albinos was abnormally large (turns 3 and 4), while the volume density for intermediate cells (melanocytes) was abnormally small (turn 3). Cell volume estimates for the intermediate cells were significantly smaller in the albino than pigmented control animals in the higher cochlear turns, indicating that functional abnormalities may be found in the albino cochlea. In animals exposed to gentamicin, marginal cell volume density was reduced significantly in turn 4 of albinos, but not in any region of the pigmented inner ears. Radial area of SV and estimates of the absolute volumes for marginal cells in albinos given gentamicin also were significantly reduced in turn 1 compared to their controls; such differences were not observed in the pigmented animals. The results indicate that marginal cell size is significantly reduced in albino but not pigmented animals 14 days after gentamicin exposure, and further suggest a role of pigmented melanocytes in ameliorating gentamicin-induced cochlear damage.

Are the differential effects of chloral hydrate on hooded rats vs. albino rats due to pigmentation or strain differences?

Effects of chloral hydrate anesthesia on EEG power spectra and VEP components were examined as a function of both pigmentation and strain differences in rats. Ten albino Westenberg Long Evans rats (WLE A) were compared to ten pigmented Westenberg Long Evans rats (WLE P), and to ten Wistar albino (Wis A) rats. Albino rats required less chloral hydrate to reach a deep level of anesthesia than pigmented rats. Wistar rats remained anesthetized longer than WLE rats. During deep levels of anesthesia, the lowest EEG frequency band contained more power in Wistar rats than in WLE rats. During moderate levels of anesthesia, frequencies less than 4 Hz lost power while frequencies greater than 13 Hz gained across all rats. Wistar rats had more power in the frequencies less than 8 Hz than did WLE rats; pigmented rats had more power in the frequencies greater than 13 Hz than did albinos. VEP component latencies of pigmented rats were shorter than albinos. Component amplitudes were not significantly different between groups.

Potentiation of cochlear hair cell loss by acoustic stimulus and gentamicin in the guinea pig

We explored the possibility of synergism between a pure-tone stimulus and gentamicin in causing cochlear injury by analyzing hair cell loss. Guinea pigs receiving daily injections of gentamicin (200 mg/kg body wt.) for 1 week were exposed to a 2 kHz tone (95 dB SPL, 2 hours daily). Surface preparations of the spiral organ were studied by phase contrast microscopy, and the extent of hair cell loss in the entire organ of Corti was recorded in cytocochleograms. Gentamicin by itself was slightly ototoxic, damaging the innermost row of outer hair cells, whereas exposure to sound alone caused no hair cell loss. Combined antibiotic and acoustic exposure produced extensive cochlear damage. A few animals showed massive hair cell degeneration and collapse of the organ of Corti, except in the apical turn. The site of damage was possibly determined by the frequency of the sound stimulus. Thus, an intermittent tonal stimulus such as that used in the present experiment can be harmless by itself, but causes injury to cochlear hair cells in guinea pigs when administered in combination with gentamicin.

Phantom auditory perception (tinnitus): mechanisms of generation and perception

Phantom auditory perception--tinnitus--is a symptom of many pathologies. Although there are a number of theories postulating certain mechanisms of its generation, none have been proven yet. This paper analyses the phenomenon of tinnitus from the point of view of general neurophysiology. Existing theories and their extrapolation are presented, together with some new potential mechanisms of tinnitus generation, encompassing the involvement of calcium and calcium channels in cochlear function, with implications for malfunction and aging of the auditory and vestibular systems. It is hypothesized that most tinnitus results from the perception of abnormal activity, defined as activity which cannot be induced by any combination of external sounds. Moreover, it is hypothesized that signal recognition and classification circuits, working on holographic or neuronal network-like representation, are involved in the perception of tinnitus and are subject to plastic modification. Furthermore, it is proposed that all levels of the nervous system, to varying degrees, are involved in tinnitus manifestation. These concepts are used to unravel the inexplicable, unique features of tinnitus and its masking. Some clinical implications of these theories are suggested.