Involvement of nitric oxide synthase in the physiology and pathophysiology of facial nerve function and dysfunction

To date few reports have discussed the presence and function of nitric oxide (NO) in structures of the facial nerve. We performed nicotinamide adenine dinucleotide phosphate (NADPH-d)-diaphorase-histochemistry and immunohistochemistry on the intratemporal portion of the facial nerve, including the geniculate ganglion, of guinea pigs using specific antibodies to the three known isoforms of NO synthase and soluble guanylyl-cyclase (sGC). Normal facial nerves were compared to those treated intratympanically with bacterial lipopolysaccharides (LPS) and tumor necrosis factor-alpha (TNF-alpha). Both constitutive NOS isoforms and sGC could be detected in the bipolar ganglion cells of normal animals, while the inducible isoform (iNOS or NOS II) was not found. Endothelial NOS (NOS III) and sGC were present in blood vessels and were predominantly found in the perineurial sheath and less in the endoneurium. sGC could be detected in all fibers in a cross section of the facial nerve. LPS and TNF treatment led to the detection of iNOS in the perikaryia of the geniculate ganglion and the perineural sheath. These findings imply that NO may be involved in neurotransmission at least in the visceroafferent system. NO regulates vascular tone of nutrient blood vessels in the perineural sheath and endoneurium. The presence of sGC indicates that NO acts via its second messenger cGMP. NOS II expression may be a contributing factor to facial nerve palsy via two different mechanisms: NOS II-generated NO may lead to an overstimulation of the visceroefferent nerve fibers and motor fibers of the facial nerve. Dysregulation in facial nerve blood vessels could lead to edema and elevated pressure on the nerve within its osseous canal.