Effects of streptomycin and amiloride on the function of the pigeon inner ear

In vitro, streptomycin and amiloride block the mechano-electrical transduction channels in isolated auditory hair cells of mammals and birds. The present in vive experiments investigate their action in the intact pigeon inner ear. Streptomycin dissolved in artificial endolymph at concentrations between 1.0 and 6.2 mmol/l was applied into the scala media. After a bolus injection (0.09 to 0.36 μl) the endocochlear potential (EP) was elevated from 6.8 to 7.9 mV. Similarly applied bolt of 0.09 to 0.55 μl of amiloride in concentrations of 12 mmol/l did not significantly elevate EP (8.7 versus 9.4 mV). Compound action potential (CAP) frequency threshold curves (FTCs) were elevated by both streptomycin and amiloride. The threshold elevation increased with frequency. With streptomycin injection, a complete loss of CAPs was found at frequencies above 1.2 kHz. With amiloride, the loss in sensitivity was 40 dB for the same frequency range. Single fibre recordings showed elevated thresholds and decreased sharpness of tuning with both drugs. With streptomycin, the evoked activity was affected regardless of stimulation frequencies (above or below the fibre's characteristic frequency). Amiloride, however, preferentially elevated thresholds at frequencies above characteristic frequency. Thus amiloride seems to interfere with hair cell tuning mechanisms. Spontaneous activity decreased by about 30%, following the application of streptomycin. In contrast, injections of amiloride into the scala media were followed by an increase in spontaneous discharge rates. When the drugs were applied into scala tympani, streptomycin reduced CAP amplitudes within 2 min. Amiloride produced no effect if applied into the scala tympani.