Quantitative approaches to the ototoxicity problem

Auditory brainstem response as a possible objective indicator for salicylate-induced tinnitus in rats

The auditory response to an acoustic stimulus will usually be suppressed, or masked, by a preceding sound. Here, we show that forward acoustic masking at a high frequency can boost the auditory brainstem response (ABR) in rats injected with a high dose of sodium salicylate (NaSal), a tinnitus inducer. The forward narrow band noise caused a decrease in the amplitude of the ABR to a probe tone burst in normal rats, but caused an unexpected increase in the amplitude at 16 kHz in rats treated with NaSal (300 mg/kg). The observed effect could be manifested in normal rats presented with a background tone added to the masker and the probe, suggesting an underlying mechanism associated with tinnitus. We hypothesize that in NaSal-treated rats, tinnitus can "internally" mask the ABR in a similar way as an external background sound does and the "unmasking" effect of forward masking can result in a rebound of the otherwise suppressed ABR. Our study raises the possibility of using the ABR as an objective indicator for NaSal-induced tinnitus in animals. This article is part of a Special Issue entitled: Tinnitus Neuroscience.

Too much of a good thing: long-term treatment with salicylate strengthens outer hair cell function but impairs auditory neural activity

Aspirin has been extensively used in clinical settings. Its side effects on auditory function, including hearing loss and tinnitus, are considered as temporary. A recent promising finding is that chronic treatment with high-dose salicylate (the active ingredient of aspirin) for several weeks enhances expression of the outer hair cell (OHC) motor protein (prestin), resulting in strengthened OHC electromotility and enhanced distortion product otoacoustic emissions (DPOAE). To follow up on these observations, we carried out two studies, one planned study of age-related hearing loss restoration and a second unrelated study of salicylate-induced tinnitus. Rats of different strains and ages were injected with salicylate at a dose of 200 mg/kg/day for 5 days per week for 3 weeks or at higher dose levels (250-350 mg/kg/day) for 4 days per week for 2 weeks. Unexpectedly, while an enhanced or sustained DPOAE was seen, permanent reductions in the amplitude of the cochlear compound action potential (CAP) and the auditory brainstem response (ABR) were often observed after the chronic salicylate treatment. The mechanisms underlying these unexpected, permanent salicylate-induced reductions in neural activity are discussed.

Auditory sensori-neural alterations induced by salicylate

Early after the development of aspirin, almost 150 years ago, its auditory toxicity has been associated with high doses employed in the treatment of chronic inflammatory diseases. Tinnitus, loss of absolute acoustic sensitivity and alterations of perceived sounds are the three auditory alterations described by human subjects after ingestion of large doses of salicylate. They develop over the initials days of treatment but may then level off, fluctuate or decrease, and are reversible within a few days of cessation of treatment. They may also occur within hours of ingestion of an extremely large dose. Individual subjects vary notably as to their susceptibility to salicylate-induced auditory toxicity. Tinnitus may be the first subjective symptom, and is often described as a continuous high pitch sound of mild loudness. The hearing loss is slight to moderate, bilaterally symmetrical and affects all frequencies with often a predominance at the high frequencies. Alterations of perceived sounds include broadening of frequency filtering, alterations in temporal detection, deterioration of speech understanding and hypersensitivity to noise. Behavioral conditioning of animals provides evidence for mild and reversible hearing loss and tinnitus, similar to those observed in humans. Anatomical examinations revealed significant alterations only at outer hair cell lateral membrane. Electrophysiological investigations showed no change in endocochlear resting potential, and small changes in the compound sensory potentials, cochlear microphonic and summating potential, at low acoustic levels. Measures of cochlear mechanical responses to sounds indicated a clear loss of absolute sensitivity and an associated broadening of frequency filtering, both of a magnitude similar to audiometric alterations in humans, but at extremely high salicylate levels. Otoacoustic emissions demonstrated changes in the mechano-sensory functioning of the cochlea in the form of decrease of spontaneous emissions and reduced nonlinearities. In vitro measures of isolated outer hair cells showed reduction of their fast motile responses which are thought to be at the origin of cochlear absolute sensitivity and associated fine filtering. Acoustically evoked neural responses from the eighth nerve to the auditory cortex showed reversible and mild losses of absolute sensitivity and associated broadening of frequency filtering. There is no evidence of a direct alteration of cochlear efferent innervation. Evidence was obtained for decreases in cochlear blood supply under control of autonomous innervation. Spontaneous neural activity of the auditory nerve revealed increases in firings and/or in underlying temporal synchronies. Similar effects were found at the inferior colliculus, mostly at the external nucleus, and at the cortex, mostly at the anterior and less at the secondary auditory cortex but not at the primary auditory cortex. These changes in spontaneous activity might underlie tinnitus as they affect mostly neural elements coding high frequencies, can occur without a loss of sensitivity, are dose dependent, develop progressively, and are reversible. Biochemical cochlear alterations are poorly known. Modifications of oxydative phosphorylation does not seem to occur, involvement of inhibition of prostaglandin synthesis appears controversial but could underlie changes in blood supply. Other biochemical alterations certainly also occur at outer hair cells and at afferent nerve fibers but remain unknown.

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.

Nitrosoglutathione suppresses cochlear potentials and DPOAEs but not outer hair cell currents or voltage-dependent capacitance

Biochemical and pharmacological evidence support a role for nitric oxide (NO) and glutathione (GSH) in the cochlea. GSH combines with NO in tissue to form nitrosoglutathione (GSNO) that can act as a storage form for GSH and NO. Therefore, we tested GSNO on sound-evoked responses of the cochlea (cochlear microphonic, CM; summating potential, SP; compound action potential, CAP; cubic distortion product otoacoustic emission, DPOAE), on the endocochlear potential (EP), on isolated outer hair cell (OHC) currents and voltage-dependent capacitance, and on Deiters' cell currents. In vivo application of GSNO in increasing concentrations reversibly reduced low-intensity sound-evoked CAP, SP and DPOAEs starting at about 1 mM (CAP) and 3.3 mM (SP, DPOAE). However, even at 10 mM, GSNO had little effect on the EP. In vitro, salicylate (10 mM) but not GSNO (3 and 10 mM) suppressed the early capacitative transients of OHCs. GSNO (3 and 10 mM) had no effect on the whole cell currents of OHCs or Deiters' cells. Results show that GSNO suppresses cochlear function. This suppression may be due to an effect of GSNO on the cochlear amplifier. The actions of GSNO were different from those of other NO donors; therefore, the effects of GSNO may not be mediated by NO. The mechanisms underlying GSNO effects seem to be different from those of salicylate.

A characteristic of aspirin-induced hearing loss in auditory brainstem response of conscious rats

The acute effects of aspirin on auditory functions were examined electrophysiologically in conscious rats with chronically implanted electrodes for auditory brainstem response (ABR) recording. A single intravenous injection of aspirin at a dose of 225 mg/kg caused a reduction in the amplitude of the ABR P1 wave evoked by a 2 kHz tone pip 1 and 24 hr after dosing at almost all sound intensity levels, while the P1 amplitude at 4 kHz was reduced mainly 1 hr after dosing, and the P1 amplitude at 8 kHz was not significantly affected at middle and high intensities even 1 hr after dosing. The audiogram obtained from the P1 amplitude showed a significant increase in the sound threshold 1 and 24 hr after dosing at 2 kHz, and 1 hr after dosing at 4 kHz, but not at 8 kHz. The peak latency of the P1 wave was also prolonged. Furthermore, reduction of the P2 and P4 wave amplitude and prolongation of the P1-P2 and P2-P4 interpeak latency were also observed at 2 kHz but not a 4 or 8 kHz. These results suggest that the rat auditory function for low frequency is vulnerable to the effects of aspirin. This paradigm, i.e., frequency selectivity, n rats may be useful to further assess the different outer hair cells along the cochlear duct and provide additional evidence for the mechanism(s) or site underlying aspirin ototoxicity.

Intracochlear salicylate reduces low-intensity acoustic and cochlear microphonic distortion products

Salicylate is well-known to produce reversible hearing loss and tinnitus. The site and mechanism of salicylate's ototoxic actions, however, remain unresolved. Recent experiments demonstrating primarily low-intensity effects on cochlear afferent outflow and effects on otoacoustic emissions (OAEs) suggest that salicylate acts to compromise active, energy-enhancing processes within the cochlea (i.e., the active process). We tested this hypothesis by examining the effect of salicylate on distortion product emissions. Distortion product responses to two-tone stimulation were monitored in the guinea pig before, during, and after intracochlear administration of increasing concentrations of salicylate (0.6-5 mM). These responses were recorded as acoustic signals in the ear canal spectrum (ADP), and as present in the cochlear microphonic (CM) recorded from a wire in basal turn scala vestibuli (CMDP). We also recorded the CM response to a single tone. Cochlear perfusion of salicylate resulted in a dose-responsive reduction in ADPs that was greater for low intensities of stimulation. CMDPs also demonstrated a concentration-dependent reduction at low intensities, but were increased slightly, though not significantly, by salicylate when elicited by high intensity primaries. CM was essentially unchanged by intracochlear salicylate. These results are consistent with an action of salicylate that involves the outer hair cells (OHCs) and are in harmony with the hypothesis that salicylate may selectively compromise the active process.

Salicylate, mefenamate, meclofenamate, and quinine on cochlear potentials

The perilymphatic spaces of guinea pig cochleae were perfused with artificial perilymph, with and without drug, at a rate of 2.5 microliters/minute for 10 minutes. The compound action potential of the auditory nerve, cochlear microphonics, and the summating potential evoked by 10 kHz tone bursts of varying intensities were recorded from a wire inserted in the basal turn scala vestibuli. The endocochlear potential was recorded from the scala media. Sodium salicylate (1.25 to 10 mmol/L) reduced the magnitude of the compound action potential evoked by low-sound intensities without affecting the compound action potential evoked by high-sound intensities. Sodium salicylate also reduced cochlear microphonics and had no effect on summating potential. Cochlear perfusions of prostaglandin synthesis inhibitors, mefenamate (200 mumol/L), and meclofenamate (200 mumol/L), had no effect on the cochlear potentials. Quinine (10 to 100 mumol/L) reduced the compound action potential input-output function in a parallel fashion rather than selectively affecting the low-intensity compound action potential. Quinine (100 mumol/L) reduced cochlear microphonics and summating potential. Neither quinine (100 mumol/L) nor salicylate (5 mmol/L) affected endocochlear potential. These results suggest that salicylate-induced hearing loss is not caused by either antagonism of the hair cell transmitter or cyclooxygenase inhibition, nor is it caused by the same mechanism that causes quinine-induced hearing loss.

Comparative actions of salicylate on the amphibian lateral line and guinea pig cochlea

1. Salicylate actions on afferent nerve activity in the Xenopus lateral line and on cochlear potentials in guinea pig were investigated. 2. In the lateral line, salicylate (0.3-2.5 mM) suppressed spontaneous activity, water motion evoked excitation and responses to L-glutamate (1-2 mM) and kainate (10-20 microM). 3. In the guinea pig, salicylate (0.6-10 mM) suppressed the compound action potential (CAP) and increased N1 latency at low but not high sound intensities. 4. In the lateral line salicylate action may involve an antagonism of the hair-cell transmitter on the afferent nerve. 5. In the cochlea salicylate may suppress the active process or cochlear amplifier.

The pharmacology of ototoxic drugs

This article briefly reviews the nature of the toxic effects of drugs on the inner ear and the incidence of ototoxic side effects in man. There follows a more detailed discussion of the most important groups of ototoxic drugs which are identified as the aminoglycoside antibiotics, the "loop" diuretics, quinine and chloroquine, the salicylates and some antitumour drugs. Attention is drawn to the synergistic interaction between aminoglycoside antibiotics and "loop" diuretics and the predisposition to ototoxicity if the drugs are given to subjects with renal impairment. The comparative ototoxicological potential of individual aminoglycosides is discussed and their toxic effects on the kidney and the neuromuscular junction summarized. The importance of an understanding of the pharmacokinetics of aminoglycosides both in relation to toxicity and the rational control of therapy is emphasized.

Noxious effects upon cochlear metabolism

The influence of various toxic substances and of drugs with ototoxic side effects upon energy generation, energy utilization, and membrane processes of the cochlea were studied. None of the drugs tested interfered with energy generation to as great an extent as did anoxia or cyanide and 2,4-dinitrophenol. Ouabain produced a pronounced interference with energy utilization of the stria vascularis. The "loop" diuretics ethacrynic acid and furosemide produced a reduction of energy utilization of a lesser degree than did ouabain. The "loop" diuretics do not seem to exert their toxic action upon strial Na+K+-ATPase, but may act by interfering with strial adenylate cyclase. Aminoglycoside antibiotics and diuretic and nondiuretic mercurials seem to exert their primary noxious action upon cochlear function by interfering with membrane processes of the structures bounding the cochlear duct.

Effects of anoxia and ethacrynic acid upon ampullar endolymphatic potential and upon high energy phosphates in ampullar wall

The ampullar endolymphatic potential (AEP) was studied in the guinea pig during ischemia and asphyxia and following systemic application of ethacrynic acid. In addition the specialized and nonspecialized portions of the ampullar wall were analyzed for ATP and P-creatine at different conditions of metabolic interference. Under control conditions the AEP amounted to + 4.6 +/- 1.2 mV. In both types of hypoxia the decline of the AEP proceeded on a much slower time scale than that of the cochlear endolymphatic potential (CEP), and the maximum negativity reached was considerably less. Quantitative analysis of both types of ampullar wall tissue indicated a much slower decline in hypoxia of ATP levels than in the stria vascularis. Changes in P-creatine levels were considerably more rapid. The AEP became reduced and changed polarity also by intoxication with ethacrynic acid (EA), but higher dosages (above 70 mg/kg) were necessary than for effects upon the CEP and much longer time periods were required for attainment of maximum negativity. The maximum negativity of the AEP was significantly greater at a dosage of 100 mg/kg of EA than during ischemia. At the point of maximum depression of the AEP P-creatine levels in both types of ampullar tissue were unchanged, but ATP levels were significantly reduced in the specialized portions of ampullar wall.