Localization of nitric oxide synthase I and III in the cochlea.

Nitric oxide synthase (NOS) isoforms I and III were localized in the guinea pig cochlea by indirect immunohistochemistry using frozen sections and paraffin sections. NOS I staining was observed in the cytoplasm of outer hair cells, in nerve cell somata and fibers of the spiral ganglion, and in axonal profiles of the spiral lamina next to the base of inner hair cells. In addition, lining cells of the inner sulcus and limbus, and cells of the spiral ligament stained for NOS I but vascular walls remained unstained. NOS III reactivity was seen in the cytoplasm of outer and inner hair cell, in lining cells of the limbus, and on the endolymphatic surface of marginal cells. Staining for NOS III of spiral ganglion perikarya showed varying intensity. Endothelial cells of cochlear glomeruli reacted for NOS III. NOS III in vascular endothelial cells implies regulatory effects of nitric oxide (NO) on vascular wall tonus and cochlear blood supply. NOS I in cochlear neurons indicates these cells as possible sources for NO during neuronal activity. Activated neurons may provide NO that adjusts cochlear perfusion to neuronal activity. Finally, NO that is liberated from hair cells or afferent synaptic terminals may act as an inhibitor on N-methyl-D-aspartate (NMDA) receptors (negative feed-back inhibition).