Altered expression of inducible nitric oxide synthase (iNOS) in the cochlea

Using immunohistochemistry and Western blot, the expression of inducible nitric oxide synthase (iNOS) in the lateral wall and organ of Corti was examined in normal (unstimulated) and stimulated mice and guinea pigs. The stimuli were: (1) injection of bacterial lipopolysaccharide (LPS, 5 mg/ml) into the middle ear through the tympanic membrane and (2) exposure to a 110 dB SPL (A-weighted) broadband noise, 3 h/day, for three consecutive days. For the unstimulated condition, weak iNOS expression was found in the vascular endothelium, marginal cells, nerve fibers, stereocilia of hair cells and Hensen’s cells of the organ of Corti. More intense iNOS fluorescence signals were observed in cochlear tissues (particularly in hair cells and stria vascularis marginal cells) in animals exposed to loud sound or treated with LPS. Although the precise roles of iNOS expression in normal cochlear function have yet to be determined, enhanced iNOS expression following noise exposure and LPS suggests its participation in cochlear pathophysiology, including noise- and inflammatory factor-induced hearing loss.