The Change In The Cochlea Active Process Induced By Moderate High Level Broad-band Noise In Guinea Pig And The Study Of Mechanism

Excessive noise is a pervasive occupational hazard with many adverse effects, including physical function and psychological disease, is the most common causative agent for adult-onset hearing loss. However, the occupational noise-induced hearing loss (NIHL) is more serious in the developing and undeveloped countries because of the necessity of the development of industrialization and urbanization, with or without the pervasion of NIHL health education, without the use of hearing conservation method and the requirement to prevent poverty. It is important to early detection and early intervention of NIHL.’The best health policies are those based on scientific evidence". According to the result of longitudinal and cross study for NIHL and noise and the country state of politics, economy and culture,85 dB(A) or 90 dB(A) is the limited noise level in most countries. However, most workers are exposed to moderate high noise level with 85-95 dB(A), which is larger than the limited level. So, it is necessary to investigate the occurrence and development of hearing loss cause by noise with the level of 85-95 dB(A) and the effect of noise to hearing function. Studies on animals are necessary for science study. Many factors can be controlled in animal studies. The active process is provided for hearing sensitivity and sharp frequency selection. Acoustic overstimulation can affect the active process. Otoacoustic emissions (OAEs) is the byproduct of the active process. It provides us a noninvasive method to study active process in vivo. OAEs was interfered when the animal was anesthetized during the measurement. Now, we focused on the effect of broad-band noise with moderate high level on the active process and the underling mechanism. We divided the study into two parts:Part one the effect of broad-band noise with moderate high level on the active process in guinea pig 87、90 and 93 dB(A) broad-band noise was used in our study. Distortion product otoacoustic emissions (DPOAE) was measured before and after the noise exposure for 8 hours or before, during and after the noise exposure with 8 hours on/16hours off for 14 days. The guinea pigs without noise exposure were also measured for DPOAE in 14 days. The result was broad-band noise decreased the cochlea active process. The decrease first appeared at high frequencies, but later also at lower frequencies, and was exacerbated by increasing noise exposure level and exposure times. During the repeated noise exposure, the active process was decreased in the first days, than it reached an asymptotic or plateau level without continuous decrease with the increasing exposure times. The change in active process is bipolar. It can only be found in special frequency and can be saturation. The decrease in active process was robust after 93 dB(A) broad-band noise exposure.Part two:the effect of 90 dB(A) broad-band noise on ion level in cochlea perilymph It is believed that the ionic equilibrium plays important role in hearing function. Ion chromatography was used to simultaneously determine the level of ions in the same cochlea perilymph. Single exposure to 90 dB(A) broad-band noise reduced DPOAE amplitude at high frequency, but the effect was reversible. Repeated exposure to 90 dB(A) broad-band noise reduced DPOAE amplitude at 8 kHz and enhanced DPOAE amplitude at 3 kHz. The level of Na+, Cl", Ca2+ and Mg2+ in cochlea perilymph was not changed after single exposure to 90 dB(A) broad-band noise, but, the level of Ca2+ and Mg2+ was increased after repeated to 90 dB(A) broad-band noise. Moreover, there was negative correlation between the change in DPOAE amplitude at 3kHz and Mg2+ concentration in perilymph after repeated noise exposure. Thus, the elevation of Mg2+ concentration and its effect to physical function in cochlea may be one of the cause of the enhancement of the active process after 90 dB(A) broad-band noise repeated exposure.As a result, the change in the active process after noise exposure in our study paralleled to the occurrence, development of NIHL, so it is believed to the important cause for NIHL. In addition, the noise level and exposure module in our study is reasonable and can be used in the continuous studies for the investigation of pathology and physical mechanisms and prevention method of NIHL.90 dB(A) may be the "critical level" for guinea pigs under this condition in our study. The enhancement of DPOAE amplitude may be a "warning signal" and it means that the animal was developed to tinnitus.