EXPERIMENTAL-STUDY ON THE EFFECTS OF GENTAMICIN INJECTION ON THE GUINEA-PIG - ELECTROPHYSIOLOGICAL STUDIES

The inner ear distortion induced by gentamicin (GM), a type of aminogl ycoside antibiotic, was examined in guinea-pis. Previous studies which investigated the function of the eighth cranial nerve and organ of Co rti using cochlear microphonics (CM) and compound action potential (CA P) reported ototoxicity following experimental exposure to GM. In this report, the effects of GM on the cochlea and the eighth cranial nerve were investigated systematically by measuring CM, CAP, summating pote ntials induced by 90 dB (SPL) tone burst, endocochlear potential (EP) and K+ ion concentration of the endolymph. Guinea-pigs were treated wi th a daily intramuscular injection of GM (60 mg in 1.5 cc) at a dose o f 60 mg/kg during different treatment periods. The maximum output volt age of AP was decreased by injection of GM (60 mg/kg x 24). A decrease in the CM maximum output voltage and the elevation of CM pseudothresh old were typically seen after GM injection at a test frequency of 8 kH z and a decrease in CAP was also observed. Changes in EP during 3 min anoxia were observed, in particular a decrease in the absolute value o f the negative potential. The endolymph K+ ion concentration remained unchanged. These findings indicate that the diffusion potentials decre ased at the same time as reduction of maximum output voltage in CM ind uced by GM injection.