Vulnerability and adaptation of distortion product otoacoustic emissions to endocochlear potential variation

Abstract

The endocochlear potential (EP) was reversibly decreased in adult gerbils by the intraperitoneal injection of furosemide, while cochlear functioning was monitored by measurement of distortion production otoacoustic emissions (DPE) at a range of stimulus intensities. Stimulus frequencies for DPEs were f1 = 6.8 and f2 = 8 kHz (f2/f1 = 1.18). Emissions monitored in the ear canal and scala media were 2f1-f2, 3f1-2f2, 2f2-f1, and f2-f1. Typically, the EP decreased smoothly, reached a minimum one-half hour after injection, then recovered slowly over several hours. Emissions at 2f1-f2 and 3f1-2f2 at low stimulus levels were particularly vulnerable to the change in EP. These vulnerable emissions showed characteristic trajectories in which the amplitudes changed little with the initial EP decrease, then dropped sharply as the EP continued to decrease. However, the amplitudes then began to recover even before the EP reached minimum, and recovered completely while the EP remained subnormal. The trajectories of the other odd order emissions were similar, but lacked the abrupt decrease. The variation of the even order (f2-f1) component was completely different, but appeared related to the odd order trajectories in a complex fashion. During the initial decrease for the vulnerable components, the decrease in emission amplitude (in dB) was found to be proportional to the square of the change in EP (in mV). The recovery with a subnormal EP was interpreted as an adaptive effect with a time constant of about 15 min.

Variations in OHC-generated voltage and DPOAEs with low EP

“…Similar to the finding of Mills et al [5], after i.p. furosemide was administered, EP decreased quickly, reached its minimum within 20 ~ 40 min, and slowly recovered over ~ 140 min (see Fig.1a for an example).…”

“…The time course varied somewhat between animals. The EP has not fully recovered in our preparations to date, and lack of full EP recovery is common in the literature, for example [5]. Fig.1c shows, the voltage response showed cochlear amplification.…”

“…Furosemide is a loop diuretic that has ototoxic effects, primarily on the stria vascularis, and reversibly decreases EP. A study by Mills and colleagues [5] using furosemide to reversibly reduce EP, showed that distortion product otoacoustic emissions (DPOAEs) recovered before EP. This suggested that cochlear amplification may be able to adjust to a lower EP.…”

Influence of Ketamine–Xylazine Anaesthesia on Cubic and Quadratic High-Frequency Distortion-Product Otoacoustic Emissions

“…Endocochlear potential decrease is expected to induce a strong decrease of cubic DPOAE at levels close to hearing threshold (Mills et al 1993;Rübsamen et al 1995). Endocochlear potential related changes of quadratic DPOAE are not directly related to the change in cubic DPOAE level (Mills et al 1993). Endocochlear potential change affects cochlear thresholds more strongly at higher sound frequencies than at lower frequencies, which presumably is related to blockage of the cochlear amplifier whose gain may increase with increasing sound frequency towards the cochlear base (Schmiedt et al 2002).…”

“…This could affect cochlear sound transduction, if reduction of metabolic energy supply would change the endocochlear potential. Endocochlear potential decrease is expected to induce a strong decrease of cubic DPOAE at levels close to hearing threshold (Mills et al 1993;Rübsamen et al 1995). Endocochlear potential related changes of quadratic DPOAE are not directly related to the change in cubic DPOAE level (Mills et al 1993).…”