Temperature dependence of distortion-product otoacoustic emissions in tympanal organs of locusts.

Distortion-product otoacoustic emissions (DPOAEs) in tympanal organs of insects are vulnerable to manipulations that interfere with the animal's physiological state. Starting at a medium temperature, we raised and lowered the locust's body temperature within the range of 12 to 35°C by changing the temperature of the surrounding air, while recording DPOAEs. These experimental manipulations resulted in reversible amplitude changes of the 2f(1)-f(2) emission, which were dependent on stimulus frequency and level. Using low f(2) frequencies of up to 10 kHz, a temperature increase (median +8-9°C) led to an upward shift of DPOAE amplitudes of approximately +10 dB, whereas a temperature decrease (median -7°C) was followed by a reduction of DPOAE amplitudes by 3 to 5 dB. Both effects were only present in the range of the low-level component of DPOAE growth functions below L2 levels (levels of the f(2) stimulus) of approximately 30 dB SPL. DPOAEs evoked by higher stimulus levels as well as measurements using higher stimulation frequencies above 10 kHz remained unaffected by any temperature shifts. The Arrhenius activation energy was calculated from the -10 dB SPL thresholds (representing the low-level component) of growth functions, which had been measured with 8 and 10 kHz as f(2) frequencies and amounted to up to ~34 and 41 kJ mol(-1), respectively. Such activation energy values provide a hint that the dynein-tubulin system within the scolopidial receptors could play an essential part in the DPOAE generation in tympanal organs.