Ethacrynic acid-induced hearing loss in guinea pigs

The effects of intracochlear and systemic furosemide on the properties of single cochlear nerve fibres in the cat

1. Tuning properties and spontaneous discharge rate of single cochlear fibres in the anaesthetized cat were determined during short- and long-term poisoning of the cochlea by locally and systemically applied furosemide.2. With intra-arterial administration of furosemide, short-term reversible elevation occurred of the low threshold sharply tuned ;tip' segment of the frequency threshold (;tuning') curve (f.t.c.) by up to 40 db, without substantial changes in the threshold of the low frequency ;tail' segment of the f.t.c. These changes could occur in part without changes in the spontaneous activity and entirely without changes in the maximal evoked activity. These effects were observed in all fibres examined, the characteristic frequencies of which ranged from 3.5 to 31 kHz.3. Intracochlear administration of furosemide in 0.9 mM concentrations produced similar changes, but these were not reversible.4. The changes correlated with the depression of the amplitude of the gross cochlear action potential. The cochlear microphonic potential, however, was either unchanged, or only slightly reduced.5. In long-term furosemide poisoning of the cochlea, fibres with anomalous response properties were found alongside fibres having normal tuning. The former exhibited either reduced excitability of the low threshold tip segment, or a tip segment attenuated in both excitability and threshold.6. It is concluded that the selective effects of furosemide on the tip segment of cochlear fibre f.t.c.s offer further evidence for a physiologically vulnerable ;second filter' in the cochlea. The selective influence of the furosemide on the low threshold tip segment provides support for the hypothesis that the normal f.t.c. is generated by two largely independent processes: one vulnerable, low threshold and sharply tuned, and the other less vulnerable, but high threshold and more broadly tuned.7. The findings, obtained with an agent known to produce reversible impairment of hearing in man, provide direct physiological evidence in support of the hypothesis that in sensorineural hearing loss of cochlear origin the frequency selectivity of cochlear nerve fibres is impaired.

Application of loop diuretics for treatment of sensorineural hearing impairment. Experimental and clinical study

In a series of animal experiments on guinea pigs and rats, conducted in an attempt to find a better solution for the treatment of sensorineural hearing impairment and tinnitus-known as diseases almost impossible to heal-it was revealed electrophysiologically, biochemically and electronmicroscopically that loop diuretics, referred to as ototoxic drugs, cause an increased amount of the therapeutic preparation to transfer to the inner ear fluid. It is concluded, on the basis of the encouraging results of these experiments, that combined medication with metabolic activator, steroid hormone and loop diuretics is significantly more effective in improving the symptoms of patients with inner ear deafness and tinnitus.

Chronic effects of loop diuretics on the guinea-pig cochlea

Groups of pigmented guinea pigs (weighing 480 to 650 g at the start of the experiment) were given daily intraperitoneal injections of 16 mg/kg bumetanide or 10, 20 or 40 mg/kg furosemide over a period of 4 weeks. Two weeks following the last diuretic dose the animals were anaesthetized with urethane and the responses of single auditory nerve fibres to pure-tone and click stimuli were recorded. The tuning of individual units, as measured by Q10 dB values of frequency threshold curves, tended to be more affected than threshold at characteristic frequency by both loop diuretics. Light microscope examination of treated cochleas suggested greater effect of the drugs on the organ of Corti than on stria vascularis.

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.

The quantification of kanamycin ototoxicity in the rat using conditioned tone discrimination

Twelve male Lister hooded rats were conditioned to discriminate an 8 kHz tone (56.5 +/- 0.5 dB re 0.0002 dyne cm-2) and were subsequently injected subcutaneously with kanamycin (400 mg kg-1 day-1) for 28 days; during this time and for five weeks after dosage was stopped, the nature and extent of the resultant hearing deficits were studied. The animals' Preyer reflex thresholds were determined at intervals during the study. Only one rat was unaffected by the kanamycin dosage. The onset of hearing impairment (reduced discrimination performance), which was gradual in some rats and sudden in others, generally occurred during the fourth week of kanamycin dosage although the earliest onset was towards the end of the second week. In most animals the hearing impairment progressed after kanamycin was stopped and in one rat there was a latency between the end of drug dosage and onset of hearing impairment. Hearing impairment was irreversible in five rats. One rat, whose impairment was slight, recovered normal hearing. Some rats showed a reduced discrimination performance at a time when their Preyer reflex threshold showed no elevation suggesting that kanamycin, at least initially, caused a threshold elevation rather than reduced sensitivity to intense sounds.