Glutathione-dependent antioxidant systems in the mammalian inner ear: effects of aging, ototoxic drugs and noise

Glutathione and glutathione-related enzymes protect against oxidative (free radical) cell injury. This study presents basic information on this antioxidant system in inner ear tissues and preliminary results of the influence of age, ototoxic drugs and noise. These conditions affect inner ear function, possibly through free radicals, and are therefore expected to affect cellular defense mechanisms. In 24-month old Fischer 344 rats, a standard model for aging, glutathione levels were significantly decreased in the auditory nerve by 86% as compared to 3-month old rats but remained unchanged in other cochlear tissues. In guinea pig, the common model for drug- and noise-induced trauma, glutathione levels in the cochlear sensory epithelium were about 8-fold higher (223 +/- 35 nmol glutathione/mg protein) than in the rat. Cochlear glutathione S-transferase and glutathione reductase activities were similar between the two species, whereas selenium-independent glutathione peroxidase was strikingly lower in guinea pig than in rat (9 +/- 3 nmol vs. 161 +/- 84 nmol glutathione converted/mg protein/min). Cisplatin treatment of guinea pigs (56 dB threshold shift at 18 kHz) significantly lowered cochlear glutathione levels by 65% and glutathione S-transferase activity by 44%. Gentamicin treatment (80 dB threshold shift at 18 kHz) and noise exposure (43 dB threshold shift at 18 kHz) did not affect glutathione at the tissue level. These results demonstrate species differences in cochlear glutathione and glutathione-related enzymes. The antioxidant system is sensitive towards environmental influences as seen for age and cisplatin. For gentamicin and noise trauma, whole tissue glutathione and enzyme levels do not correlate with functional damage. This indicates that glutathione homeostasis is largely maintained in the cochlea and that biochemical changes, if they occur under these conditions, may be limited to specific cells.

Firing properties of spherical bushy cells in the anteroventral cochlear nucleus of the gerbil

In gerbils, spherical bushy cells (SBCs) encode low frequency sound signals into a temporal firing pattern. To investigate the support for the timing in this temporal code, we characterized the membrane electrical properties of visually identified SBCs in brainstem slices. A brief depolarizing subthreshold transient potential (TP) triggered, with relatively invariant latency, a single spike at the onset of a response to depolarizing current pulses. The activation of a subthreshold Na+-conductance, sensitive to blockade with tetrodotoxin, and a high threshold Ca2+-conductance, sensitive to blockade with Co2+ or Cd2+, accelerated the rising phase and amplified the TP. A K+-conductance, sensitive to blockade by 4-aminopyridine (4-AP, 50 microM), shaped the decay of the TP. Following a single spike, voltage-gated activation of transient and sustained K+-conductances suppressed any tendency to repetitively discharge. A reduction in either K+-conductance due to application of 4-AP or tetraethylammonium (TEA, 10 mM), converted the single spike mode to repetitive firing during the depolarizing pulses. A persistent, tetrodotoxin-sensitive Na+-conductance amplified steady-state depolarizing responses. A hyperpolarization-activated conductance, greatly decreased by extracellular Cs+ (3 mM) but resistant to Ba2+ (up to 1 mM), filtered the responses to hyperpolarizing current inputs. A depolarized membrane potential promoted repetitive firing in SBCs. This state, expected in pathophysiological conditions, would corrupt the temporal code.

alpha-difluoromethylornithine ototoxicity. Chemoprevention clinical trial results

OBJECTIVES

To determine the effects of low-dose oral eflornithine hydrochloride (difluoromethylornithine [DFMO]) administration on hearing and to identify factors that influence those effects.

DESIGN

Combined data from 2 studies: a prospective, randomized phase 1 clinical trial of eflornithine (n = 26 subjects) and a prospective, randomized, placebo-controlled phase 2 clinical trial of eflornithine (n = 40 subjects).

SETTING

Ambulatory academic tertiary care referral center.

PARTICIPANTS

Sixty-six volunteer subjects who had previously treated bladder, prostate, or colon cancer with no current evidence of neoplastic disease, or who were healthy individuals at increased risk for colon cancer, all without need of hearing amplification.

INTERVENTIONS

Subjects were randomized to receive oral eflornithine at daily doses between 0.5 and 3 g per square meter of body surface area (g/m2) for 6 to 12 months (phase 1 study) or randomized to receive placebo or eflornithine, 0.5 g/m2 for 12 months (phase 2 study).

OUTCOME MEASURES

Auditory thresholds were measured before, during, and after eflornithine administration. Auditory thresholds and threshold shifts were evaluated with regard to eflornithine dose, serologic variables, and demographic factors.

RESULTS

Predictable shifts in auditory thresholds occurred following administration of eflornithine. As the daily dose of eflornithine increased, the magnitude and incidence of threshold shift increased, and the time until onset of threshold shift decreased. Threshold changes were greater in the lower frequencies than in the higher frequencies. Subjects' sex, age, and renal function had no effect on eflornithine-associated threshold shifts. Threshold shifts were reversible after eflornithine treatment was discontinued.

CONCLUSIONS

Administration of eflornithine is associated with a predictable shift in auditory thresholds. The magnitude and incidence of threshold shift correlate with the daily eflornithine dose.

Modulation of outer hair cell compliance and force by agents that affect hearing

Force generated by outer hair cells is thought to be an essential source of mechanical input to the normal cochlea. Many disease processes in the inner ear act via outer hair cells. It is therefore plausible that such disease processes modulate outer hair cell force generation. The force generated by an isolated, electrically stimulated outer hair cell against a load may be represented by an intrinsic motor and a passive axial stiffness in series. Thus modulation of outer hair cell force generation may occur either by action on the motor or indirectly by an action on cell stiffness. In this study, the effects of agents that affect hearing on outer hair cell stiffness and force generation have been examined. Overstimulation and hypoosmotic challenge caused cells to decrease in length and increase in stiffness. The force generated by a constant voltage stimulus increased consequent to the stiffness increase. Hyperosmotic challenge elicited a stiffness decrease and a force decrease. In contrast, salicylate caused a decrease in force without stiffness change. The results suggest that outer hair cell force generation in vivo may be modulated in at least two ways.

R-phenylisopropyladenosine attenuates noise-induced hearing loss in the chinchilla

Reactive oxygen species, which are cytotoxic to living tissues, are thought to be partly responsible for noise-induced hearing loss. In this study R-phenylisopropyladenosine (R-PIA), a stable non-hydrolyzable adenosine analogue which has been found effective in upregulating antioxidant enzyme activity levels, was topologically applied to the round window of the right ears of chinchillas. Physiological saline was applied to the round window of the left ears (control). The animals were then exposed to a 4 kHz octave band noise at 105 dB SPL for 4 h. Inferior colliculus evoked potential thresholds and distortion product otoacoustic emissions (DPOAE) were measured and hair cell damage was documented. The mean threshold shifts immediately after the noise exposure were 70-90 dB at frequencies between 2 and 16 kHz. There were no significant differences in threshold shifts at this point between the R-PIA-treated and control ears. By 4 days after noise exposure, however, the R-PIA-treated ears showed 20-30 dB more recovery than saline-treated ears at frequencies between 4 and 16 kHz. More importantly, threshold measurements made 20 days after noise exposure showed 10-15 dB less permanent threshold shifts in R-PIA-treated ears. The amplitudes of DPOAE also recovered to a greater extent and outer hair cell losses were less severe in the R-PIA-treated ears. The results suggest that administration of R-PIA facilitates the recovery process of the outer hair cell after noise exposure.

Comparison of Five Agents in Protecting the Cochlea Against the Ototoxic Effects of Cisplatin in the Hamster

The purpose of this investigation was to study the ameliorating effects of four agents on cis-platin-induced ototoxicity. Hamsters were given a series of five cisplatin injections either alone or in combination with sodium thiosulfate (STS), diethyldihydrothiocarbamate (DDTC), and S-2(3-aminopropylamino) ethylphosphorothiolc acid (WR-2721), or fosfomycin. Ototoxicity was assessed anatomically by quantifying the extent of cochlear damage with the scanning electron microscope and physiologically with measures of the auditory brain stem response. When administered alone, cisplatin induced widespread loss of outer hair cells (OHCs) along much of the cochlea in the hamster, especially in the basal and middle turns, with an average survival of only 56% of the OHC population. In contrast, inner hair cells resisted cisplatin ototoxicity in the hamster. Thus the ameliorative effects of the different test agents were assessed by counting the number of surviving OHCs in each treatment group and comparing with cisplatin-treated controls. STS provided the most effective protection against the ototoxic effects of cisplatin, yielding 91% survival of OHCs. DDTC also reduced the ototoxic effects of cisplatin, yielding 68% survival of OHCs. Cotreatment with WR-2721 and fosfomycin yielded 45% and 52% OHC survival, respectively, and thus did not provide any chemoprotection. The results closely paralleled those based on auditory brain stem response recordings in that the magnitude of threshold shift was proportional to the amount of OHC loss; also, the amount of threshold shift at each frequency was in good agreement with the pattern of hair cell loss along the cochlear spiral. Thus both histologic and physiologic results suggest that STS and DDTC hold promise for ameliorating the ototoxic effects of cisplatin chemotherapy.

Chronic strychnine administration into the cochlea potentiates permanent threshold shift following noise exposure

To investigate whether elimination of the medial efferent system influences permanent threshold shift following noise exposure, we developed an animal model in which strychnine was chronically delivered into the cochlea via an osmotic pump. Pigmented female guinea pigs were allocated into three groups: group I was treated with strychnine (50 microM, 0.5 microl/h, 14 days) in the left ear and exposed to noise (105 dB SPL broadband, 3 h) 3 weeks after the cessation of the strychnine perfusion; group II received strychnine in the left ear but no noise exposure; group III was treated with Ringer's solution in the left ear and exposed to noise. Animals in group II developed no hearing loss after the strychnine perfusion. The operated ears in group I demonstrated greatest hearing threshold shift 3 h after noise exposure. Hearing recovered during 2 weeks after noise exposure in both operated and non-operated ears in groups I and III. Two weeks after noise exposure, the operated ears in group I showed significantly greater threshold shift at 12, 16, and 20 kHz compared to the operated ears in group III and non-operated ears in groups I and III. These findings suggest that chronic strychnine administration into the cochlea inactivates the medial efferents without changing hearing threshold and that the medial efferents help to protect against permanent threshold shift following noise exposure.

Protection by 4-methylthiobenzoic acid against cisplatin-induced ototoxicity: antioxidant system

This study was undertaken in order to determine the changes in auditory brainstem-evoked responses relationship with the changes in the levels of GSH, lipid peroxidation and antioxidant enzymes activity in cisplatin-induced ototoxicity and otoprotection by 4-methylthiobenzoic acid (MTBA). Male Wistar rats in different groups were treated as follows: 1) saline control; 2) cisplatin (16 mg/kg, intraperitoneally); 3) MTBA (250 mg/kg, intraperitoneally), and 4) cisplatin plus MTBA. Post-treatment auditory brainstem-evoked responses were performed after three days and the rats were sacrificed and cochleae harvested. The cochleae were analyzed for glutathione (GSH), antioxidant enzyme activity, and malondialdehyde levels. The cisplatin injected rats showed a threshold elevation of 31.9 +/- 16.0 dB above the pretreatment thresholds using click stimulus. Rats treated with MTBA plus cisplatin did not show significant elevation of hearing threshold. Cisplatin plus MTBA administration showed a higher levels of cochlear GSH (5.59 +/- 0.35 nmoles/mg protein) compared to cisplatin alone (4.46 +/- 0.13 nmoles/mg protein). Cisplatin treated rats showed a decrease in superoxide dismutase, catalase, glutathione peroxidase (GSH-peroxidase), and glutathione reductase (GSH-reductase) activities (57%, 83%, 78% and 58% of control). Cochlear superoxide dismutase, catalase and GSH-reductase activities and MDA levels were restored in the rats injected with cisplatin plus MTBA, compared to cisplatin alone. It is concluded that the protection conferred by MTBA against cisplatin ototoxicity is associated with sparing of the cochlear antioxidant system.

Frequency selectivity on aspirin-induced hearing loss in rats with auditory stimulus-induced conditioned suppression

The conditioned suppression technique was employed to examine the acute effects of aspirin on auditory function in rats. Lever pressing behavior for water reinforcement was suppressed in the presence of an auditory stimulus that had been previously paired with electric shocks. A single intravenous injection of aspirin at a dose of 225 mg/kg caused an erroneous lever pressing response in the broad sound intensities of 2 kHz tone stimulus during the conditioned stimulus period. A statistically significant increase in the threshold for 2 kHz was found 1 to 72 hr after dosing but not for 4, 8 and 10 kHz. These results suggest that the hearing for low sound frequency in rats is vulnerable to the effects of aspirin. This paradigm in rats may be useful to further assess the different outer hair cells along the cochlear duct and provide an additional evidence for the aspirin ototoxicity research.

Liver iron depletion and toxicity of the iron chelator deferiprone (L1, CP20) in the guinea pig

The use of the iron chelator deferiprone (L1, CP20, 1,2-dimethyl-3-hydroxypyrid-4-one) for the treatment of diseases of iron overload and other disorders is problematic and requires further evaluation. In this study the efficacy, toxicity and mechanism of action of orally administered L1 were investigated in the guinea pig using the carbonyl iron model of iron overload. In an acute trial, depletion of liver non-heme iron in drug-treated guinea pigs (normal iron status) was maximal (approximately 50% of control) after a single oral dose of L1 of 200 mg kg-1, suggesting a limited chelatable pool in normal tissue. There was no apparent toxicity up to 600 mg kg-1. In each of two sub-acute trials, normal and iron-loaded animals were fed L1 (300 mg kg-1 day-1) or placebo for six days. Final mortalities were 12/20 (L1) and 0/20 (placebo). Symptoms included weakness, weight loss and eye discharge. Iron-loaded as well as normal guinea pigs were affected, indicating that at this drug level iron loading was not protective. In a chronic trial guinea pigs received L1 (50 mg kg-1 day-1) or placebo for six days per week over eight months. Liver non-heme iron was reduced in animals iron-loaded prior to the trial. The increase in a wave latency (electroretinogram), the foci of hepatic, myocardial and musculo-skeletal necrosis, and the decrease in white blood cells in the drug--treated/normal diet group even at the low dose of 50 mg kg-1 day-1 suggests that L1 may be unsuitable for the treatment of diseases which do not involve Fe overload. However, the low level of pathology in animals treated with iron prior to the trial suggests that even a small degree of iron overload (two-fold after eight months) is protective at this drug level. We conclude that the relationship between drug dose and iron status is critical in avoiding toxicity and must be monitored rigorously as cellular iron is depleted.

[Effects of low concentration carbon monoxide on human physiological function]

Human volunteers were exposed to various low concentrations of carbon monoxide (CO) in a closed cabin. The results showed that 35 mg/m3 of CO caused slight subjective symptoms and reduction in contrast vision, operating efficiency and T-wave of ECG. At 80 mg/m3 and 115 mg/m3 the above changes were more severe and a rise in hearing threshold level was observed. It demonstrated that an inhibitory effect on the CNS and heart were caused by low concentrations of CO. According to the absorption curves for human exposed to CO, it is suggested that the sense effect level and vision effect level for CO are 7% COHb, and the hearing effect level is 9% COHb.

The effect of intravenous lidocaine injection on hearing thresholds

The effect of intravenous injection of lidocaine on hearing thresholds was studied in normal subjects. Continuous and intermittent tones at 1, 4 and 8 kHz were used as stimuli and the threshold change with lidocaine injection was measured using a self-recording audiometer (Békésy audiometer). Both increases and decreases in the threshold were observed. The former occurred more frequently than the latter. In cases of a threshold increase, lidocaine injection exhibited a frequency specific effect; the higher the frequency, the more often the threshold was increased by lidocaine injection. There was no significant difference in threshold changes between continuous and intermittent tones. The present results suggest that lidocaine may act on the inner ear hair cells.

A case report of fluctuant sensorineural hearing loss after hepatitis B vaccination

Hepatitis B vaccine is known to induce accidents and side-effects. We report a case of unilateral fluctuant sensorineural hearing loss with tinnitus after a series of hepatitis B vaccinations. Tinnitus regressed and hearing thresholds were normalized 6 months later. The authors discuss the potential causality of hepatitis B vaccine in the occurrence of fluctuant sensorineural hearing loss and propose a physiopathological hypothesis to explain this unusual case.

[Intravenous gentamicin therapy in bilateral Menière disease]

BACKGROUND

The treatment of severe forms of bilateral Menière's disease remains an especially challenging task. Similar problems also occur in debilitating Menière's disease in the only hearing ear. The intramuscular titration therapy with streptomycin has been the means of choice since 1984 to minimize the risk of total hearing loss in cases of severe bilateral disease.

METHOD

Since 1989 we have treated six out of 21 cases of bilateral Menière's disease by intravenous application of 2 x 120 mg gentamicin in Ringer's solution for several days. Additionally we reported on two cases in 1988. Only minor amounts of gentamicin were applied to sedate the function of both vestibular organs while avoiding damage to the cochlea.

RESULTS

In two cases hearing approved approximately about 10 dB, in two cases hearing remained stable, and in two cases hearing worsened about 10 dB. Five of six patients showed minor excitability in caloric tests on both sides, they did not complain of vertigo attacks one to five years after therapy.

CONCLUSION

Given that only very small amounts of gentamicin are applied to sedate the function of the vestibular organ while causing almost no damage to the cochlea, this method seems to be an excellent means for treatment of bilateral Menière's disease. Patients do not experience severe problems with equilibrium afterwards, and the treatment can be repeated as often as necessary.

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

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

Combined effects of a simultaneous exposure to noise and toluene on hearing function

To study the combined effects of noise and toluene on auditory function, three experimental groups of Long-Evans adult rats were used. The first group was exposed to toluene (2000 ppm, 6 h/day, 5 days/week, 4 weeks), the second group to an octave band of noise centered at 8 kHz (92 dB SPL), and the last group to a simultaneous exposure to toluene and noise. Auditory function was tested by recording brainstem (inferior colliculus) auditory-evoked potentials. The auditory deficit induced by the combined exposure exceeded the summated losses caused by toluene alone and by noise alone within the range (2-32 kHz) of test frequencies. The nature of the cochlear damage induced by noise alone (injured stereocilia) or by toluene alone (outer hair cells loss) is different.

Tympanic membrane displacement analysis after glycerol intake: effects on intracranial and intracochlear fluid pressure

BACKGROUND

Intracranial pressure is transmitted to the perilymph of the cochlea via the cochlear aqueduct and to the endolymph via Reissner's membrane. The tympanic membrane displacement analyzer (TDA) is a new system that may provide a useful, noninvasive, indirect method to identify intracranial and intracochlear pressure changes.

METHODS

We performed measurements in 20 persons with normal hearing and in 29 patients with various unilateral diseases of the inner ear such as sudden hearing loss or Meniere's disease. Another series of measurements were done in 15 normal-hearing persons and in 15 patients with Meniere's disease before and after oral ingestion of glycerol.

RESULTS

In the first series of measurements, no significant differences in the tympanic membrane displacement were found between normal-hearing individuals and patients with unilateral diseases of the inner ear. In the second series, in all individuals a temporary, relative decrease in intracochlear pressure was found after oral ingestion of glycerol but without any significant difference between the two groups.

CONCLUSIONS

These results revealed that the TDA is apparently not useful in the evaluation of patients having Meniere's disease.

Ototoxic effects of supradose cisplatin with sodium thiosulfate neutralization in patients with head and neck cancer

OBJECTIVE

To assess the incidence and magnitude of ototoxicity in patients undergoing an experimental targeted chemoradiation protocol incorporating extremely high-dose intra-arterial cisplatin therapy with systemic sodium thiosulfate neutralization for the treatment of advanced carcinomas of the head and neck.

DESIGN

Inception cohort study.

SETTING

University-based, tertiary care referral center for advanced head and neck malignant disease.

PATIENTS

The first 70 patients with advanced carcinomas of the head and neck consecutively entered in the protocol.

INTERVENTION

Patients received up to 4 weekly courses of intra-arterial cisplatin (150 mg/m2 per infusion), together with systemic sodium thiosulfate and external beam radiation (68-70 Gy). Audiometric analysis was performed before the initiation of therapy, and subsequent to the second and fourth cisplatin infusions.

MAIN OUTCOME MEASURES

Audiometric thresholds. Ototoxicity was defined as an increase in pure-tone threshold of 15 dB at 1 frequency or 10 dB at 3 frequencies, between 250 and 4000 Hz.

RESULTS

The incidence of ototoxicity was 25% at 150 mg/m2, 50% at 300 mg/m2, 64% at 450 mg/m2, and 60% at 600 mg/m2. Hearing at frequencies of 2000 Hz or less was minimally or not affected. Previous hearing loss did not appear to affect the incidence of ototoxicity. A plateau of hearing loss at 60-dB hearing level, as noted by other authors, was not observed. There were no cases of debilitating tinnitus or of vestibular loss.

CONCLUSIONS

Ototoxicity did occur but was largely confirmed to the higher frequencies. Hearing losses resulting from this chemoradiation protocol were not sufficiently severe to alter its application.

Up-regulation of adenosine receptors in the cochlea by cisplatin

In a previous study, we have demonstrated the presence of two adenosine receptor (AR) subtypes, namely A1 and A3AR, in the chinchilla cochlea. One or both of these receptors couple to activation of antioxidant enzymes, with resulting decreases in lipid peroxidation. The chemotherapeutic agent, cisplatin, was shown to produce ototoxicity within a few days of administration presumably by generating reactive oxygen species (ROS) and thereby increasing lipid peroxidation. In this study, we focused on whether lipid peroxidation induces hearing loss by assessing the cochlear antioxidant defense system over a shorter time period (24 h) following cisplatin administration. Cisplatin was administered to anesthetized chinchillas by round window membrane application and hearing loss was determined by compound action potential (CAP) and endocochlear potential (EP) 24 and 72 h post-treatment. Elevations in CAP thresholds in response to click and to 2, 4, 8 and 16 kHz tones and decreases in EP were obtained within 24 h of cisplatin treatment. These changes persisted for at least up to 72 h. Measurements of antioxidant enzymes indicate no change in the activities of superoxide dismutase, catalase or glutathione peroxidase, either 24 or 72 h following cisplatin treatment. The levels of malondialdehyde obtained at these time points were equivalent to those obtained from the controls. Furthermore, no difference in cochlear morphology was detectable by scanning electron microscopy at the basal, middle or apical turns of the cochlea within 24 h. By 72 h, however, losses in both inner and outer hair cells were observed in the basal and middle turns of the cochlea. A major finding of this study is that exposure to cisplatin led to a 5-fold up-regulation of [125I]N6-2-[4-amino-3-phenyl]ethyladenosine binding in the cochlea within 24 h, reflecting increases in expression of AR(s) in this tissue. These data indicate a dissociation between cisplatin acute (within 24 h) ototoxicity and lipid peroxidation. Furthermore, up-regulation of AR(s) may represent a rapid compensatory mechanism by the cochlea to counter the toxic effects of increased ROS generated by cisplatin.

The ototoxicity of trichloroethylene: extrapolation and relevance of high-concentration, short-duration animal exposure data

Inhalation exposure to high concentrations of 1,1, 2-trichloroethylene (TCE) has been shown to damage hearing in the mid-frequency range in the rat. The present study directly evaluated the adequacy of high-concentration, short-term exposures to TCE for predicting the neurotoxicity produced by longer duration exposures. Adult male Long-Evans rats (n = 10-12 per group) were exposed to TCE via inhalation (whole body) in 1-m3 stainless steel flow-through chambers for 6 hr/day, 5 days/week. The following exposures were used: 1 day (4000-8000 ppm), 1 week (1000-4000 ppm), 4 weeks (800-3200 ppm), and 13 weeks (800-3200 ppm). Air-only exposed animals served as controls. Auditory thresholds were determined for a 16-kHz tone 3-5 weeks after exposure using reflex modification audiometry. Results replicated previous findings of a hearing loss at 16 kHz for all exposure durations. The dB15 concentrations (concentration that increases thresholds by 15 dB) for 16-kHz thresholds were 6218, 2992, 2592, and 2160 ppm for the 1-day, 1-week, 4-week and 13-week exposures, respectively. These data demonstrate that the ototoxicity of TCE was less than that predicted by a strict concentration x time relationship. These data also demonstrate that simple models of extrapolation (i.e., C x t = k, Haber's Law) overestimate the potency of TCE when extrapolating from short-duration to longer-duration exposures. Furthermore, these data suggest that, relative to ambient or occupational exposures, the ototoxicity of TCE in the rat is a high-concentration effect.

[Toxic inner ear damage in topical treatment of psoriasis with salicylates]

BACKGROUND

Whereas cochlear impairment after intravenous ingestions of salicylates is well known, reports of cochlear symptoms after topical application are quite rare. The extent of the ototoxic salicylate impact is increased by diabetes, renal insufficiency, and alcoholism.

PATIENT

This study presents a case report of a female patient who suffered a repeated, symmetric, pancochlear, reversible inner ear impairment after two treatments with salicylate containing ointment for psoriasis. The correlation of the salicylate therapy with the observed inner ear lesions is obvious due to the close interval between these incidences and to the audiologic criteria typical for salicylate intoxication.

CONCLUSION

Audiologic controls should be carried out during extended local application of salicylate containing ointment.

Distortion-product otoacoustic emissions and glycerol on the guinea pig hydropic ear

OBJECTIVE

The aim of the present work was to investigate the therapeutic effects of glycerol on the distortion-product otoacoustic emissions in guinea pigs in which experimental endolymphatic hydrops had been surgically induced.

METHODS

Thirty albino guinea pigs were used. The experimental protocol considered three groups of guinea pigs (10 animals each). Group 1 received no drug treatment, while group 2 and 3 were given glycerol orally 0.75 and 0.50 g/kg of body weight once a day for 24 days.

RESULTS

The animals treated with glycerol showed an improvement of the distortion-product emission responses in the middle frequencies. This effect was not observed in the higher frequency region. This effect was evident 7 days after glycerol administration in the guinea pigs treated with a dose of 0.75 g/kg. The 0.50 g/kg dosage gave evidence of ameliorating effects 14 days after treatment.

CONCLUSION

Glycerol given orally resulted in an improvement of distortion-product otoacoustic emissions in the guinea pig. Further studies are necessary before the effect of such a treatment can be assessed in humans.

Effects of selective inner hair cell loss on auditory nerve fiber threshold, tuning and spontaneous and driven discharge rate

Current theories assume that the outer hair cells (OHC) are responsible for the sharp tuning and exquisite sensitivity of the ear whereas inner hair cells (IHC) are mainly responsible for transmitting acoustic information to the central nervous system. To further evaluate this model, we used a single (38 mg/kg) or double dose (38 mg/kg, 2 times) of carboplatin to produce a moderate (20-28%) or severe (60-95%) IHC loss while sparing a large proportion of the OHCs. The surviving OHCs were functionally intact as indicated by normal cochlear microphonic (CM) potentials and distortion product otoacoustic emissions (DPOAE). Single-unit responses were recorded from auditory nerve fibers to determine the effects of the moderate or severe IHC loss on the output of the surviving IHCs. Most neurons that responded to sound in the single-dose group had normal or near-normal thresholds and normal tuning. Relatively few neurons in the double-dose group responded to sound because of the severe IHC loss. The neurons that did respond to sound had narrow tuning curves. Some neurons in the double-dose group also had thresholds that were within the normal range, but most had thresholds that were elevated a mild-to-moderate degree. These results indicate that intact IHCs can retain relatively normal sensitivity and tuning despite massive IHC loss in surrounding regions of the cochlea. However, the spontaneous and driven discharge rates of neurons in the carboplatin-treated animals were significantly lower than normal. These changes could conceivably be due to sublethal damage to surviving IHCs or to postsynaptic dysfunction in the auditory nerve.

Nimodipine at a dose that slows ABR latencies does not protect the ear against noise

We tested the hypothesis that nimodipine, a dihydropyridine reported to increase blood flow, block calcium and potassium channels, and reduce ischemic damage, would alleviate noise-induced hearing loss. Young C57B1/6J mice were exposed to wide-band noise (2 min, 120 dB SPL), with ABR thresholds (4-50 kHz) determined before noise exposure, and from 1 h to 2 weeks afterwards. One group (n = 7) received nimodipine (30 mg/kg/day) in daily peanut butter food supplements beginning 24 h before exposure; the other group (n = 6) received peanut butter alone. In the pretest nimodipine significantly increased the latency of Wave P1 of the ABR (mean difference: 0.16 ms; P < 0.02), showing that calcium blockade depressed sensorineural efficiency, but ABR thresholds were not affected. Noise exposure produced a severe threshold loss that partially recovered in the first week after exposure, and then suffered a slight but significant loss in the second week. These effects were seen equally in both groups: nimodipine did not reduce the severity of the immediate hearing loss following noise exposure, nor did it benefit recovery.

Effects of lifetime lead exposure in monkeys on detection of pure tones

Increased detection thresholds for pure tones were observed in a large cohort of children exposed to lead environmentally while smaller studies in lead-exposed workers have reported conflicting results on assessments of auditory function. Pure tone detection thresholds were determined in a group of monkeys (Macaca fascicularis) dosed with 2 mg/kg/day of lead from birth through testing at 13 years of age. Blood lead concentrations were stable at about 30 micrograms/dl until monkeys were 10-11 years of age at which time they increased to between 50 and 70 micrograms/dl. Five age- and rearing-matched monkeys served as controls. Detection thresholds were determined at six frequencies between 0.125 and 31.5 kHz. Ear phones were fit over both ears, and thresholds were determined for each ear separately. The monkey signaled detection of the tone by breaking contact with a stainless steel bar. Three lead-exposed monkeys exhibited normal pure tone detection functions. Three monkeys had thresholds outside of the control range at some frequencies; there was a tendency for higher frequencies to be differentially more affected. These findings are consistent with reports of elevated pure tone thresholds in humans exposed to lead developmentally, although the effect is smaller than might have been predicted given the concurrent blood lead concentrations of these monkeys.