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 con tralateral noise to suppress ipsilateral cochlear activity, in a dose- dependent manner. However, during chronic administration of lower dose s (60 mg/kg) the involvement of medial efferents could not be demonstr ated. The purposes of the present study were to determine whether othe r aminoglycosides would display the same acute effects as GM and wheth er there was any correlation between their specificity and degree of c ochlear and vestibular toxicity and their potency of blockade of the m edial efferent system. Thus, we observed changes in ipsilateral ensemb le background activity (EBA) of the VIIIth nerve without and with cont ralateral 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 compari son, the effects of strychnine, a known antagonist of the efferent tra nsmission 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 strepto mycin a decrease in suppression was usually associated with a reductio n of the EBA measured without acoustic stimulation. However, with cisp latin, suppression was still effective when EBA was severely decreased . We could not observe specific effects of strychnine on medial effere nt function. In conclusion, no correlation was found between specifici ty and degree of AA ototoxicity and their action on the medial efferen t system.