Specific activities of antioxidative enzymes in the cochlea of guinea pigs at different stages of development

Roles of Bak and Sirt3 in Paraquat-Induced Cochlear Hair Cell Damage

Paraquat, a superoxide generator, can damage the cochlea causing an ototoxic hearing loss. The purpose of the study was to determine if deletion of Bak, a pro-apoptotic gene, would reduce paraquat ototoxicity or if deletion of Sirt3, which delays age-related hearing loss under caloric restriction, would increase paraquat ototoxicity. We tested these two hypotheses by treating postnatal day 3 cochlear cultures from Bak^±, Bak^-/-, Sirt3^±, Sirt3^-/-, and WT mice with paraquat and compared the results to a standard rat model of paraquat ototoxicity. Paraquat damaged nerve fibers and dose-dependently destroyed rat outer hair cells (OHCs) and inner hair cells (IHCs). Rat hair cell loss began in the base of the cochlea with a 10 μM dose and as the dose increased from 50 to 500 μM, the hair cell loss increased near the base of the cochlea and spread toward the apex of the cochlea. Rat OHC losses were consistently greater than IHC losses. Unexpectedly, in all mouse genotypes, paraquat-induced hair cell lesions were maximal near the apex of the cochlea and minimal near the base. This unusual damage gradient is opposite to that seen in paraquat-treated rats and in mice and rats treated with other ototoxic drugs. However, paraquat always induced greater OHC loss than IHC loss in all mouse strains. Contrary to our hypothesis, Bak deficient mice were more vulnerable to paraquat ototoxicity than WT mice (Bak^-/- > Bak^± > WT), suggesting that Bak plays a protective role against hair cell stress. Also, contrary to expectation, Sirt3-deficient mice did not differ significantly from WT mice, possibly due to the fact that Sirt3 was not experimentally upregulated in Sirt3-expressing mice prior to paraquat treatment. Our results show for the first time a gradient of ototoxic damage in mice that is greater in the apex than the base of the cochlea.

Neurotrophic and antioxidant effects of silymarin comparable to 4-methylcatechol in protection against gentamicin-induced ototoxicity in guinea pigs

BACKGROUND

Despite that gentamicin is a very effective aminoglycoside, its potential ototoxicity which is of irreversible nature makes a challenge and limitation for its use. This study was designed to investigate possible neurotrophic and antioxidant effects of silymarin comparable to 4-methylcatechol in protection against gentamicin-induced ototoxicity.

METHODS AND RESULTS

Twenty pigmented guinea pigs were divided into four equal groups, where group I served as normal control group. The other groups received gentamicin (120 mg/kg/day, ip) for 19 days where group II given vehicle of 1% CMC, group III and group IV were pre-treated 2h before gentamicin by 4-methylcatechol (10 μg/kg, ip) and silymarin (100mg/kg, oral gavage), respectively. The main findings indicated that silymarin exhibited restoration of nerve growth factor (NGF) levels and increased tropomyosin-related kinase receptors-A (Trk-A) m-RNA expression in cochlear tissue and preservation of hair cells of organ of Corti by scanning electron microscopy (SEM) with significant decrease in auditory brainstem response (ABR) threshold compared to 4-methylcatechol. Only silymarin caused significant amelioration in oxidative stress state by reducing malondialdehyde (MDA) levels and increasing catalase activity.

CONCLUSIONS

Silymarin exerts superiority over 4-methylcatechol when recommended as protective agent against gentamicin ototoxicity based on its efficient neurotrophic and antioxidant activities.

Divergence of noise vulnerability in cochleae of young CBA/J and CBA/CaJ mice

CBA/CaJ and CBA/J inbred mouse strains appear relatively resistant to age- and noise-related cochlear pathology, and constitute the predominant 'good hearing' control strains in mouse studies of hearing and deafness. These strains have often been treated as nearly equivalent in their hearing characteristics, and have even been mixed in some studies. Nevertheless, we recently showed that their trajectories with regard to age-associated cochlear pathology diverge after one year of age (Ohlemiller et al., 2010a). We also recently reported that they show quite different susceptibility to cochlear noise injury during the 'sensitive period' of heightened vulnerability to noise common to many models, CBA/J being far more vulnerable than CBA/CaJ (Fernandez et al., 2010 J. Assoc. Res. Otolaryngol. 11:235-244). Here we explore this relation in a side-by-side comparison of the effect of varying noise exposure duration in young (6 week) and older (6 month) CBA/J and CBA/CaJ mice, and in F1 hybrids formed from these. Both the extent of permanent noise-induced threshold shifts (NIPTS) and the probability of a defined NIPTS were determined as exposure to intense broadband noise (4-45 kHz, 110 dB SPL) increased by factors of two from 7 s to 4 h. At 6 months of age the two strains appeared very similar by both measures. At 6 weeks of age, however, both the extent and probability of NIPTS grew much more rapidly with noise duration in CBA/J than in CBA/CaJ. The 'threshold' exposure duration for NIPTS was <1.0 min in CBA/J versus >4.0 min in CBA/CaJ. F1 hybrid mice showed both NIPTS and hair cell loss similar to that in CBA/J. This suggests that dominant-acting alleles at unknown loci distinguish CBA/J from CBA/CaJ. These loci have novel effects on hearing phenotype, as they come into play only during the sensitive period, and may encode factors that demarcate this period. Since the cochlear cells whose fragility defines the early window appear to be hair cells, these loci may principally impact the mechanical or metabolic resiliency of hair cells or the organ of Corti.

Migration of cochlear lateral wall cells

The role of apoptosis and proliferation in maintenance of cochlear lateral wall cells was examined. The methods employed for detection of apoptosis were the Hoechst fluorescence stain and TUNEL (TdT-mediated dUTP-biotin nick-end-labeling) assay, and proliferations were 5-bromo-2'-deoxyuridine (BrdU) incorporation and presence of the proliferating cell nuclear antigen. The incidence of apoptosis in the strial marginal cell was 50% greater (32.9+/-3.7%) than strial intermediate and basal cells but similar to spiral ligament cells. Although division of marginal strial cells was rarely detected, a significant number of proliferating cells in the remaining stria vascularis and spiral ligament were observed. These data implied that replacement of marginal cells arose elsewhere and could be followed by a BrdU-deoxythymidine pulse-chase study. At 2 h post injection, nuclear BrdU in marginal cells was not detected; however, by 24 h post injection, 20-25% of marginal cell nuclei were BrdU-positive. These observations are consistent with the hypothesis that marginal cells were replaced by underlying cells. Cell migration appears to be an important mechanism for preserving the function and structure of the stria vascularis.

Susceptibility to noise exposure during postnatal development in rats

Susceptibility to noise-induced hearing loss was studied during maturation in 20 female pigmented rats (strain Long-Evans). Young rats, 3, 4, 5 and 6-7 weeks old, were exposed for 1 h to a broad-band noise with an intensity of 120 dB SPL. The thresholds and amplitudes of middle latency responses (MLR) recorded from electrodes implanted on the surface of the auditory cortex were analyzed before and after noise exposure. The results were compared with data from our previous study, in which the effects of broad-band noise exposure on MLR were investigated in adult rats [Syka, J. and Rybalko, N. (2000) Hear. Res. 139, 59-68]. The hearing thresholds of 3-7 week old rats before noise exposure were within the normal adult range. Noise exposure in young rats produced an adult-like pattern with an elevation of hearing thresholds. One-two weeks post-exposure a recovery of MLR thresholds was observed, though full recovery only occurred in the low frequency range. Recovery of hearing thresholds in the high frequency range depended on the age of the animal at the time of exposure. In all animals aged less that 6-7 weeks, exposure resulted in a permanent threshold shift in the range of 4-32 kHz. The mean values of permanent threshold shifts at 16 kHz (the frequency of maximal hearing loss) were 53.0+/-4.5, 47.6+/-9.6, 37.5+/-7.5 and 27+/-10 dB for rats exposed at 3, 4, 5 and 6-7 weeks of age, respectively. Similar to adult rats, young rats exposed to noise exhibited an enhancement of MLR amplitudes. This amplitude enhancement was more pronounced in the high frequency range. In several rats exposed at 3-5 weeks of age, the recovery period to normal amplitudes was substantially prolonged and lasted 4-8 weeks in comparison with 1-2 weeks in adult rats. These results demonstrate a greater susceptibility to noise exposure in rats during the first 5 postnatal weeks.

Ototoxicity caused by aminoglycosides is ameliorated by melatonin without interfering with the antibiotic capacity of the drugs

The production of free radicals seems to be involved in the mechanisms of ototoxicity. Aminoglycosides produce ototoxicity, which can be determined through distortion product otoacoustic emissions (OAEs) that measure the activity of the outer hair cells of the organ of Corti. An ototoxic chart was obtained in rats using gentamicin or tobramycin. Together with this treatment, the animals ingested melatonin in the drinking water, or melatonin was injected subcutaneously or intramuscularly. The distortion product OAEs were determined over a prolonged period of time for each of the groups. The effect of melatonin on the antibiotic capacity of the aminoglycosides used was also studied. Antibiograms inoculated with Escherichia coli or Pseudomonas aeruginosa and treated with gentamicin or tobramycin in the presence or absence of melatonin at quantities from pharmacological to physiological doses were performed. The ototoxicity produced by gentamicin and tobramycin was maximal from days 3 to 5 post-treatment, returning to normal values in 2 wk. When melatonin was present, the recovery was at day 5 post-treatment, independently of the means of administration of the pineal product. The antibiograms showed that melatonin had no effect on the antibiotic capacity. It is concluded that the ototoxicity caused by gentamicin and tobramycin is ameliorated by melatonin and that the pineal hormone does not interfere with the antibiotic capacity of these antibiotics.

Maturation of cochlear glutathione-S-transferases correlates with the end of the sensitive period for ototoxicity

The developing mammalian cochlea is especially sensitive to chemical toxins. In rats, the period of increased sensitivity falls roughly between postnatal days (P) 8 and 28. One unexplored hypothesis for this 'sensitive period' is that young cochleas may have immature complements of detoxification enzymes. Glutathione-S-transferases (GSTs) are a family of detoxification enzymes which catalyze the conjugation of many xenobiotics to glutathione. Using high performance liquid chromatography (HPLC), we measured the concentrations of soluble GST isoforms in cochleas of developing Fischer 344 rats. At P1, the concentration of isoform rGSTP1 was 9 pmol/mg protein. That of the remaining isoforms studied was low, <2 pmol/mg protein, and, except for rGSTA3, remained so throughout the period of study. At P2, immunolabelling visualized rGSTP1 in the stria vascularis, Reissner's membrane, spiral limbus and organ of Corti. From P1 to P28, rGSTP1 increased to 15 pmol/mg protein and was detected additionally in satellite cells of the spiral ganglion and in the spiral ligament. From P7 to P28, rGSTA3 increased 8-fold (3-24 pmol/mg protein), became the predominant isoform in the adult organ and localized to pillar cells, the limbus and the spiral ligament. In the vestibule, rGSTP1 predominated, although rGSTA3 increased slightly over time. These observations suggest that biochemical immaturity in detoxification enzymes in the cochlea may contribute to the increased sensitivity to ototoxins during development and that differences in detoxification enzymes between cells in the cochlea and between inner ear organs may underlie differences in susceptibility to ototoxins.

Aminoglycosides activate oxygen metabolites production in the cochlea of mature and developing rats

The ototoxicity of antibiotics, particularly of aminoglycosides, is a well-known undesirable side effect which may be based on a free radical mechanism. We studied the effect of different antibiotics in the production of reactive oxygen species in freshly isolated cochleas of mature and 2-10 weeks old developing rats. Phorbol myristate acetate induced the release of reactive oxygen species after a lag time close to 30 s and declined back to basal values in 10-20 min. The rate of reactive oxygen species production correlated inversely to the age in 2-10 weeks old rats. The study of a set of antibiotics showed that a very low concentration of gentamicin and streptomycin (10-100 ng/ml) enhanced the effect of phorbol myristate acetate. At the above-indicated concentrations, ciprofloxacin did not modify phorbol myristate acetate-induced activation. These results show the enhancement by aminoglycosides of reactive oxygen species production in cochlear tissues, particularly in developing rats.

The production of free oxygen radicals and nitric oxide in the rat cochlea

Rat cochleas were analysed for free oxygen radicals (FOR) and nitric oxide (NO) production by the chemiluminescent oxidation of luminol. 4Beta-phorbol-12beta-myristate-13alpha-acetate (PMA), a well-known agonist of protein kinase C, induced the release of FOR after a time lag close to 30 s and reverted to basal values in approximately 10 min. Sphingosine inhibited by nearly 50% the response to PMA, whereas staurosporine caused an inhibition of 100%. The incubation of rat cochleas with 0.5 mM arginine potentiated the chemiluminescent reaction induced by PMA causing an additional oxidation of luminol that was inhibited by the NO synthase inhibitor N-methyl-arginine (NMA). Our results show for the first time the presence in the cochlea of cell populations producing FOR and NO and the real time production following cell activation. This procedure may help to explain the mechanisms involved in ototoxicity, as in the case of streptomycin and gentamicin that enhanced PMA-dependent production of FOR and NO.

Changes in cochlear antioxidant enzyme activity after sound conditioning and noise exposure in the chinchilla

Exposure to low level noise prior to a high level exposure reduces noise-induced hearing loss in mammals. This phenomenon is known as sound conditioning or 'toughening'. Reactive oxygen intermediates have been implicated in noise-induced cochlear damage. To evaluate if in situ antioxidant processes may play a role in the toughening phenomenon initiated by low level noise exposure we analyzed glutathione reductase, gamma-glutamyl cysteine synthetase, and catalase in stria vascularis and organ of Corti fractions from cochleae of chinchillas exposed to a sound conditioning paradigm. Chinchillas were either (A) kept in quiet cages (control), (B) exposed to conditioning noise of a 0.5 kHz octave band (90 dB for 6 h/day for 10 days), (C) exposed to high level noise (105 dB for 4 h) or (D) exposed to conditioning noise (B) followed by exposure to the higher level noise (C). Each of the noise exposure conditions (B, C, D) induced changes in the levels of these three antioxidant enzymes. The enzyme-specific activity data for the four subject groups support the following two hypotheses. (1) Changes in glutathione reductase, gamma-glutamyl cysteine synthetase, and catalase play a role in attenuating hearing loss associated with sound conditioning followed by high level noise. (2) Hair cells in the organ of Corti are protected from noise-induced damage by increasing stria vascularis levels of catalase, a hydrogen peroxide scavenging enzyme, and of enzymes involved in maintaining glutathione in the reduced state. The model formulated by these hypotheses suggests that agents that protect or augment the glutathione system in the cochlea may be protective against noise-induced hearing loss.

Expression and function of adenosine receptors in the chinchilla cochlea

Previous studies indicate the presence of adenosine receptors in the cochlear tissues obtained from different animals. This study was initiated to determine the subtypes of adenosine receptor (AR) present in the chinchilla cochlea and to assess their function. Radioligand binding studies demonstrate the presence of both the A1AR and A3AR in membranes prepared from the cochlea, using the radioligands [3H]DPCPX and [125I]APNEA. Estimates of the number (Bmax) of A1AR and A1AR plus A3AR by saturation curves were 118 +/- 13 and 417 +/- 120 fmol/mg, respectively, with the respective equilibrium dissociation constants (Kd) averaging 2.7 +/- 0.2 and 26.3 +/- 13.8 nM. No significant number of A2aAR were detected using [3H]CGS21680. The nonhydrolyzable adenosine analog R-phenylisopropyladenosine (R-PIA, 1 microM) elicited a small but significant degree of inhibition of forskolin-stimulated adenylyl cyclase activity (10.4 +/- 2.5%) in cochlear membrane preparations, which was insensitive to blockade by theophylline (100 microM). Furthermore, R-PIA elicited an increase in inositol 1,4,5-trisphosphate production in dissociated cell preparations obtained from the cochlea. No significant effect of R-PIA was observed on auditory measures such as auditory brainstem evoked response, cochlear action potential and endocochlear potential following round window application. However, round window application of R-PIA elicited significant increases in the activities of antioxidant enzymes such as superoxide dismutase and glutathione peroxidase and significantly reduced the levels of malondialdehyde, a marker of lipid peroxidation. These results suggest a potential cytoprotective role of adenosine in the cochlea against oxidative damage.

Immunohistochemical localization of manganese superoxide dismutase in rat vestibular dark cell regions

A modified immunoglobulin peroxidase bridge sequence method was used to detect the localization of manganese superoxide dismutase (MnSOD), a superoxide radical (O2-) scavenging enzyme locating in mitochondrial matrix, in the vestibular labyrinth of pigmented rats. Strong positive MnSOD immunostaining was demonstrated in the dark cell regions of the ampullae, utricle, and common crus. The result provides for the first time direct evidence demonstrating the existence of mitochondrial O2- scavengers in the vestibular labyrinth and illustrates that the specific sites for vestibular MnSOD immunolocalization are the dark cell regions. This site specificity of MnSOD immunolocalization suggests that dark cell regions may possess high metabolic activity and may encounter constant threat from O2-. We assume MnSOD is needed in protecting some physiologic functions of the dark cell regions. Cell types showing negative MnSOD immunostaining may conceivably be relatively vulnerable to acute O2- damage.

Localization of Cu/Zn-SOD and Mn-SOD in the rat cochlea

Distributions of the copper/zinc-superoxide dismutase (Cu/Zn-SOD) and the manganese superoxide dismutase (Mn-SOD) in the rat cochlea were studied by immunohistochemistry with specific polyclonal antibodies to Mn-SOD and Cu/Zn-SOD. Coexpression of Mn-SOD and Cu/Zn-SOD was observed in the cochlear tissues with differing immunoactivity intensities. The presence of Cu/Zn- and Mn-SOD implies the existence of a defense system against free oxygen radical (FOR) injury in cochlear tissues.

Detection and regulation of Cu/Zn-SOD and Mn-SOD in rat cochlear tissues

Relative levels of copper/zinc-superoxide dismutase (Cu/Zn-SOD) and manganese-superoxide dismutase (Mn-SOD) in individual cochlear tissues were detected by the use of an enzyme-linked immunosorbent assay (ELISA). A heterogeneous distribution of Cu/Zn-SOD was observed in the individual tissues of control animals: high levels were measured in the stria vascularis (SV), intermediate levels of enzyme were measured in the spiral ligament (SL), and low levels were measured in the organ of Corti region (OC); collectively, these levels were not statistically significant (P = 0.0645). Levels of Mn-SOD in individual tissues of the control group were statistically significant (P < 0.05): high levels were measured in the SV, medium levels were detected in the SL, and low levels were identified in the OC. Following the administration of methylprednisolone (MP), a significant reduction of Cu/Zn-SOD in the SV (P < 0.05) and a non-significant, but noticeable, increase (> 30%) of Mn-SOD in the OC were observed. These results indicate that levels of SOD are tissue specific and that SOD is subject to glucorticoid regulation.

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.