Abstract
The overproduction of reactive oxygen species (ROS) and reactive nitrogen species (RNS) has been known to contribute to the pathogenesis of noise-induced hearing loss. In this study, we discovered that in BALB/c mice pretreatment with methylene blue (MB) for 4 consecutive days significantly protected against cochlear injury by intense broad-band noise for 3 h. It decreased both compound threshold shift and permanent threshold shift and, further, reduced outer hair cell death in the cochlea. MB also reduced ROS and RNS formation after noise exposure. Furthermore, it protected against rotenone- and antimycin A-induced cell death and also reversed ATP generation in the in vitro UB-OC1 cell system. Likewise, MB effectively attenuated the noise-induced impairment of complex IV activity in the cochlea. In addition, it increased the neurotrophin-3 (NT-3) level, which could affect the synaptic connections between hair cells and spiral ganglion neurons in the noise-exposed cochlea, and also promoted the conservation of both efferent and afferent nerve terminals on the outer and inner hair cells. These findings suggest that the amelioration of impaired mitochondrial electron transport and the potentiation of NT-3 expression by treatment with MB have a significant therapeutic value in preventing ROS-mediated sensorineural hearing loss.