Modulation of thresholds to acoustical and electrical stimulation of the intact ear in guinea pig by furosemide and noise.

Middle latency responses (MLR) to acoustical stimulation (AS) and to electrical stimulation (ES) of the intact inner ear were recorded in guinea pigs. ES threshold curve decreased in the frequency range 2-16 kHz with a slope 5.4 dB/octave. Immediately after 50 mg/kg intravenous injection of the furosemide, which resulted in a temporary suppression of the cochlear function, the ES thresholds increased and resembled thresholds found in gentamicin-treated animals. Whereas temporary threshold shift (TTS) at 1 kHz ES was negligible at this time, maximum TTS at 8 kHz and 20 kHz ES was limited to 27 dB and 37 dB resp. TTS to acoustical stimulation was larger than TTS to ES (in some cases exceeded 50 dB) and it was similar at all frequencies. Amplitude-intensity functions (AIF) to high-frequency ES stimuli (20 kHz) consisted of two parts--a flat part at low intensities and a steep part at high intensities of the ES. High-frequency noise exposure (third-octave band noise, centered at 16 kHz, intensity 105 dB for 1 h) reduced or abolished only the flat part of the AIF, the steep part, as well as the responses to low-frequency ES, were not substantially changed. TTS at high frequencies, elicited by the noise exposure, were similar for ES and AS. However, amplitudes of acoustically evoked MLR significantly increased after the noise exposure while MLR amplitudes to ES did not change. The results characterize the frequency-intensity domain of the electrophonic effect in the guinea pig and its changes after influencing the inner ear function by furosemide and noise.