Effects of noise, hypothermia and barbiturate on cochlear electrical activity.

Pentobarbital has been reported as both increasing and decreasing the effects of noise trauma on the inner ear. In the C57BL/6 laboratory mouse, the effects depend upon which electrical events are measured and whether or not the hypothermic effects of pentobarbital are counteracted. Pentobarbital-induced hypothermia per se increased the latency of both the high-intensity-threshold, short-latency (H) and low-intensity-threshold, long-latency (L) waves by approximately 60 mus per degrees C decrease, had no significant effect on H-wave amplitude, but caused a severe reduction of the L-wave amplitude. As little as 1 min of 120 dB SPL high-frequency noise can temporarily reduce the cochlear microphonic (CM) amplitude, abolish the H and reduce the amplitude of the L waves of the click-evoked, volume-conducted gross auditory nerve action potential (AP). 1 week later, CMs were more depressed in subjects which had been anesthetized during 5 min noise exposure, regardless of body temperature during the exposure. The long-term decrement of the L-wave amplitude did not consistently differ as a function of anesthetic state or body temperature at exposure. However, hypothermia protected from, while barbiturate increased the effects of noise stress on the H wave. It was hypothesized that metabolic factors differentially affect the various electrical generators in the inner ear.