Effect of aging on the expression of iNOS and cell death in the mouse cochlear spiral ganglion.

Abstract

The age-related induction of inducible nitric synthase (iNOS) and apoptotic cell death in spiral ganglion cells (SGCs) of ddy strain mice were studied with immunohistochemical method and TdT-mediated dUTP-biotin nick end labeling (TUNEL), respectively. A large amount of iNOS was expressed in SGCs of 18- to 24-month-old mice, but not in those of the mice less than 12 months of age. Moreover, these mice were accompanied by a great rise in auditory brainstem response threshold as well as a great decrease in the number of SGCs that seemed to be due to preceding cell death of the cells. However, we were unable to find apoptotic TUNEL-positive cells in the spiral ganglions. This was assumed to be due to a very short clearance time of the dead cell bodies of less than one hour. As has been known, NO produced by iNOS can implicate in causes for either protection of cells from peroxidation and cell death. In the present study, therefore, the steep augmentation of iNOS in the SGCs of senescent mice implies that the iNOS initially induced to protect SGCs from the cytotoxicity of cellular peroxidation eventually contribute to the cell death of SGCs themselves.