Changes in electrovestibular brainstem responses after aminoglycoside intoxication in guinea pigs.

OBJECTIVE: To verify the usefulness of short-latency vestibular responses evoked by a combination of round window electrical stimulation and sinusoidal rotation (electrovestibular brainstem responses; EVBRs) as a new monitoring tool of the vestibular function in animal experiments. METHODS: EVBRs were obtained before, during, and after treatment with aminoglycosides, along with compound action potential (CAP) audiograms. The changes in EVBRs were compared with morphological changes observed by scanning electron microscopy. RESULTS: EVBR amplitudes did not change in the group of guinea pigs treated with amikacin, but markedly decreased in the streptomycin and gentamicin- treated groups. CAP audiograms indicated a significant threshold elevation in the amikacin group, a moderate elevation in the gentamicin group, and no change in the streptomycin group. Under scanning electron microscopy, the loss of the sensory hair cells observed in the cristae ampullares was slight to moderate in the amikacin group, moderate to severe in the streptomycin group, and severe in the gentamicin group. CONCLUSION: EVBRs reflect overall pathological changes undergone by vestibular hair cells, and support the vestibular specificity of EVBRs.

Acute and chronic effects of aminoglycosides on cochlear hair cells.

The first detectable effect on the auditory system after a single high-dose injection of an aminoglycosidic antibiotic (AA) like gentamicin (GM) is the reversible blockade of medial efferent function, probably via blockade of calcium channels at the base of the outer hair cells (OHC). The kinetics of this effect are compatible with that of the molecule in perilymph. In the course of chronic treatment with lower doses, however, ototoxicity develops only after several days of treatment. Still GM can be observed inside the OHCs as soon as 24 hours after the first injection, and will be still present in some OHCs as long as 11 months after a chronic, nonototoxic 6-day treatment. In vitro, the short-term viability of isolated OHCs is not affected by exposure to AAs, but their transduction channels and their response to acetylcholine are reversibly blocked. However, developing organs of Corti in culture are highly and rapidly affected by exposure to AAs. Yet during direct intracochlear perilymphatic perfusion of GM, 2-mM solutions are not ototoxic, and with perfusion with a 20-mM solution ototoxicity develops only after several days of perfusion. From these various observations one can describe some aspects of the mechanisms of ototoxicity of AAs, from their access to perilymph and endolymph, to penetration in the hair cells, likely via endocytosis at their apical pole, and intracellular cytotoxic events.

Aminoglycoside ototoxicity and the medial efferent system: I. Comparison of acute and chronic gentamicin treatments.

Recently our laboratory has demonstrated, in the guinea pig (GP), that an intramuscular (i.m.) injection of a high dose of gentamicin (GM) (150 mg/kg), can reversibly block the contralateral efferent suppression of ipsilateral cochlear activity. The aims of the present study were: (1) to investigate this effect with lower doses of GM; and (2) to find out whether this effect could constitute an anticipatory sign of ototoxicity during a chronic GM treatment (60 mg/kg i.m., 10 days). The function of the medial olivocochlear efferent system (MOES) was tested by recording the VIIIth nerve ensemble background activity (EBA) without and with contralateral low level (55 dB SPL) broadband noise stimulation. The results show a dose-dependent effect of GM on contralateral suppression, as the dose of 120 mg/kg induced a smaller blockade of the MOES, compared to 150 mg/kg, and no blockade was observed with lower doses. During the ten-day treatment no significant changes in the EBA without acoustic stimulation, nor in contralateral efferent suppression were detected. GPs monitored over several weeks after the treatment showed progressive reduction of the EBA without contralateral stimulation parallel to reduced suppression coefficients of the EBA, and CAP threshold elevations, denoting impaired cochlear function. Thus, this study demonstrated that a chronic treatment with 60 mg/kg of GM, although ototoxic, does not affect the contralateral efferent suppression, at least before the development of ototoxicity.

Aminoglycoside ototoxicity and the medial efferent system: II. Comparison of acute effects of different antibiotics.

Gentamicin (GM) has been shown to reversibly reduce the ability of contralateral noise to suppress ipsilateral cochlear activity, in a dose-dependent manner. However, during chronic administration of lower doses (60 mg/kg) the involvement of medial efferents could not be demonstrated. The purposes of the present study were to determine whether other aminoglycosides would display the same acute effects as GM and whether there was any correlation between their specificity and degree of cochlear and vestibular toxicity and their potency of blockade of the medial efferent system. Thus, we observed changes in ipsilateral ensemble background activity (EBA) of the VIIIth nerve without and with contralateral low level (55 dB SPL) broadband noise stimulation, in awake guinea pigs (GPs), before and after one single high-dose intramuscular injection of different aminoglycoside antibiotics (AAs) (gentamicin, amikacin, neomycin, netilmicin, streptomycin, tobramycin). For comparison, the effects of strychnine, a known antagonist of the efferent transmission and of cisplatin, an antineoplastic agent with cochleotoxic properties were also studied. Netilmicin displayed blocking properties similar to GM, although less pronounced, while amikacin and neomycin had no effect on medial efferent function. With tobramycin and streptomycin a decrease in suppression was usually associated with a reduction of the EBA measured without acoustic stimulation. However, with cisplatin, suppression was still effective when EBA was severely decreased. We could not observe specific effects of strychnine on medial efferent function. In conclusion, no correlation was found between specificity and degree of AA ototoxicity and their action on the medial efferent system.

Bioelectrical cochlear noise and its contralateral suppression: relation to background activity of the eighth nerve and effects of sedation and anesthesia.

The bioelectrical activity of the cochlea, without any ipsilateral acoustic stimulation, was recorded in awake guinea pigs (GPs) between electrodes chronically implanted at the round window (RW) and the skull. Measuring its power in the band centered around 1.0 kHz (0.5-2.5 kHz) provided an indirect measure of the ensemble background (EBA) activity of the eighth nerve. Contralateral white-noise (CLWN) stimulation reduced this EBA, presumably by activation of medial olivocochlear fibers. The aim of the investigation was to validate measurements of EBA and of its contralateral suppression in order to study the medial efferent function. The first goal was to find the best conditions for recording the EBA in the absence of ipsilateral stimulation and for studying its suppression by contralateral acoustic stimulation, which implies that no noise was generated by the experimental animal. Thus recordings were compared in normal, awake GPs and in GPs under sedation with xylazine, anesthetized with a combination of xylazine and ketamine, and with and without temperature regulation. In order to monitor the effects of sedation and anesthesia, the recordings were analyzed not only in the 0.5- to 2.5-kHz frequency band but also in the other frequency bands, 5-50 Hz, 50-150 Hz, and 150-500 Hz, which presumably include general central and neuromuscular contributions. The results show that sedation with xylazine accompanied by regulation of body temperature does not affect the EBA value nor its contralateral suppression. Nevertheless, anesthesia should be avoided, even with control of body temperature. The second goal of this study was to identify the specific cochlear contribution to the raw RW signal. Thus recordings were performed in normal and deafened animals and analyzed in the frequency band 0.5-2.5 kHz and also in the other frequency bands of 5-50 Hz, 50-150 Hz, and 150-500 Hz. The results indicate that most of the cochlear activity lies in the frequency band 0.5-2.5 kHz, with also some minor contribution coming from the 150- to 500-Hz band. Analysis and comparison of power values in the different conditions indicate that specific cochlear EBA power was about 60 microV2. From a commonly accepted mean background discharge rate of 50 spikes/s (sp/s), the EBA power without CLWN should have been around 4.4 microV2 if the fibers' activity was random. This difference suggests that there is probably some degree of synchrony between individual fibers. There was a reduction of approximately 45% during CLWN stimulation. This suppression might correspond to a reduction in both discharge rate and synchrony of the fibers.

Electrical and physiological changes during short-term and chronic electrical stimulation of the normal cochlea.

In the electrical stimulation (ES) of auditory pathways, the type of stimulus and the electrode/tissue interface are critical parameters for the safety and efficacy of the protocol. In this study the influence of alternate pulses, applied between round window and vertex electrodes in chronically implanted guinea pigs, and maintained during 1 and 25 daily periods of 2 h (short-term and long-term experiments, respectively), was investigated. ES consisted of monophasic current pulses of +/- 70 microA and 300 (micro)s in duration at a rate of 167/s, with alternate polarity. Compound Action Potential (CAP) audiograms, amplitudes and latencies of click-evoked CAPs, amplitudes and latencies of electrically-evoked auditory responses (EARs), and electrode impedances, were measured periodically outside or during the ES periods. Short-term ES induced no change in CAP thresholds, amplitude and latency in response to clicks at 80 dB above normal threshold, but induced a slight latency increase and amplitude decrease of the EAR, correlated with an exponential decay of the electrode impedance. On a long-term basis, CAP audiograms and latencies did not change significantly, whereas CAP amplitudes and electrode impedances increased, in correlation with each other. In control guinea pigs receiving no ES, the same CAP amplitude and impedance changes were observed over the same long-term period. The EAR and CAP changes can be explained by a variation of the electrical impedance of the electrode/tissue interface. This is possibly due to a change in electrolytes around the electrode under the influence of the ES for the short-term variation, and to an electrode encapsulation by fibrous tissue independent of the ES for the long-term change. In itself, and under the conditions of this experiment, the ES demonstrated no adverse effects on the auditory function and can be safely used for inner-ear exploration.