Localization of nitric oxide synthase isoforms (NOS I, II and III) in the vestibular end organs of the guinea pig.

The localization of nitric oxide (NO) synthase (NOS) isoforms was investigated in the vestibular organ of the pigmented guinea pig by indirect immunohistochemistry. The cytoplasm of both type I and type II vestibular sensory cells as well as vestibular ganglion cells showed both NOS I and III immunoreactivity, whereas there was no reactivity in their nuclei and sensory hairs. The afferent nerve chalices were usually not stained. NOS III staining was also observed in the nerve fibers contacting type II cells and in the subepithelial tissue. The endothelial lining of the blood vessels displayed reactivity for NOS III. The cytoplasm of fluid transporting cells showed weak staining for NOS I and moderate staining for NOS III. Immunostaining for NOS II did not display any reactivity in general. These findings may suggest that NO is a mediator of neurotransmission in the vestibular system in sensory cells and ganglia. NO in the fluid transporting cells may play an important role for maintaining the endolymph and ion homeostasis, and NOS III in vascular endothelial cells implies regulatory effects of NO on vascular wall tonus and vestibular blood supply.