Rescuing Noise-Induced Permanent Threshold Shift by D-Methionine in Guinea Pigs

Abstract

Objectives:

This study examined the effectiveness of D-methionine (D-met) in rescuing a noise-induced permanent threshold shift (PTS) and cochlear biochemistry following noise exposure.

Methods:

One hour after being exposed to continuous white noise at 105 ± 2 dB SPL for 6 hours, guinea pigs were treated 5 times at 12-hour intervals with 200, 400, or 600 mg/kg D-met or sterile 0.9% saline by intraperitoneal injection. Auditory brainstem responses were examined before and 14 days after noise exposure. Six guinea pigs with normal hearing that were not exposed to noise served as control animals. The level of D-met in rescuing noise-induced hearing loss and the changes in cochlear biochemistry, including membranous Na+, K+-ATPase, and Ca2+-ATPase, lipid peroxidation, and nitric oxide, were evaluated.

Results:

Hearing acuity had no significant differences before and 14 days after noise exposure for the animals treated with 600 mg/kg D-met, compared with the control animals, whereas significant differences occurred among the animals in the other treatment groups. The level of rescue from noise-induced PTS following treatment with D-met was dose-dependent. No significant differences in the mean enzyme activities were observed between the controls and the D-met 600 mg/kg treatment group. The attenuation of the noise-induced decreases in the enzyme activities was dose-dependent. Likewise, D-met-dose-dependent decreases in oxidative stress were observed in the D-met treated groups.

Conclusions:

Treatment with D-met significantly rescued noise-induced PTS in a dose-dependent manner. Significant attenuation of increased oxidative stress and decreased ATPase activities were concurrent with the D-met-mediated improvements in noise-induced auditory dysfunction.