Nitric oxide synthase localized in a subpopulation of vestibular efferents with NADPH diaphorase histochemistry and nitric oxide synthase immunohistochemistry

Efferent innervation of the vestibular labyrinth is known to be cholinergic. More recent studies have also demonstrated the presence of the neuropeptide calcitonin gene-related peptide in this system. Nitric oxide is one of a new class of neurotransmitters, the gaseous transmitters. It acts as a second messenger and neurotransmitter in diverse physiological systems. We decided to investigate the anatomical distribution of the synthetic enzyme for nitric oxide, nitric oxide synthase (NOS), to clarify the role of nitric oxide in the vestibular periphery. NADPH diaphorase histochemical and NOS I immunohistochemical studies were done in the adult chinchilla and rat vestibular brainstem; diaphorase histochemistry was done in the chinchilla periphery. Retrograde tracing studies to verify the presence of NOS in brainstem efferent neurons were performed in young chinchillas. Our light microscopic results show that NOS I, as defined mainly by the presence of NADPH diaphorase, is present in a subpopulation of both brainstem efferent neurons and peripheral vestibular efferent boutons. Our ultrastructural results confirm these findings in the periphery. NADPH diaphorase is also present in a subpopulation of type I hair cells, suggesting that nitric oxide might be produced in and act locally upon these cells and other elements in the sensory epithelium. A hypothesis about how nitric oxide is produced in the vestibular periphery and how it may interact with other elements in the vestibular sensory apparatus is presented in the discussion.