Aminoglycoside Ototoxicity and the Medial Efferent System: I. Comparison of Acute and Chronic Gentamicin Treatments

Abstract

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 Vlllth 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.

Key Words:

• Ensemble background activity
• Medial olivocochlear efferent system
• Aminoglycoside antibiotics
• Gentamicin