The influence of injected AC and DC currents on spontaneous otoacoustic emissions in the bobtail lizard

Spontaneous otoacoustic emissions (SOAEs) are accepted as being indicators of an active process in the inner ear and have been observed in all groups of land vertebrates. Previous studies have indicated that there may be more than one cellular substrate driving the active processes in different groups of vertebrates. To investigate the cellular substrate in non-mammals and to relate the findings to studies in mammalian ears, we injected AC and DC currents of amplitudes generally below 5 μA into scala media of the bobtail skink Tiliqua rugosa and recorded their influences on SOAEs. Injected AC current has been shown previously to induce emissions [electrically induced otoacoustic emissions (EEOAEs)] in mammals and lizards. AC current in the appropriate frequency range entrained SOAEs but was relatively ineffective at increasing their magnitude compared with the dependence on current strength of EEOAEs generated at non-SOAE frequencies. The effects of DC current were polarity-dependent and changed over time with prolonged stimulation. Whereas positive current caused a small increase in SOAE frequency and a fall in magnitudes, negative DC current induced a decrease in SOAE frequency of more than 30%, accompanied by changes in level that were strongly time-dependent. All changes were slowly reversible. The effects observed in this species of lizard strongly resemble those observed in frogs and chinchillas. The similarity is interesting in view of the putatively different cellular mechanisms driving the active process in mammals and non-mammals.