Neuroepithelial endocrine and nervous system in the respiratory tract of Cynops pyrrhogaster with special reference to the distribution of nitric oxide synthase and serotonin.

The respiratory tract of urodeles harbours an intramural nerve network comprising an innervated system of neuroepithelial endocrine (NEE) cells. However, striking differences have been noted between phylogenetically closely related species. Zamboni- or formaldehyde-fixed whole-mount preparations and sections of the saclike lungs of a Japanese salamander, Cynops salamander, Cynops pyrrhogaster, have been investigated for the immunocytochemical detection of nitric oxide synthase (NOS), serotonin (5-HT), VIP, somatostatin, calcitonin, and bombesin; for the enzyme-cytochemical demonstration of NADPH diaphorase (NADPHd); and for formaldehyde-induced fluorescence. In addition, the ultrastructural morphology has been examined by using glutaraldehyde/osmium tetroxide fixed lung tissues. Ovoid 5-HT-immunoreactive (IR) NEE cells occur singly or grouped in the ciliomucous epithelium of the trachea and lungs of Cynops, and a few somatostatin-, calcitonin-, and bombesin-like IR NEE cells are also observed. These cells exhibit a characteristic neuroendocrine morphology as seen with the electron microscope. In addition, large numbers of 5-HT-IR interstitial cells, with round to oval cell bodies and two or three long, slender, sometimes branching processes, are located preferentially along large blood vessels in the connective tissue capsule of the lung and trachea. Immunoelectronmicroscopy shows that 5-HT is localized over large dense granules in the cell bodies and processes of these interstitial cells. NOS-like immunoreactivity occurs in a nerve plexus composed of thick nerve bundles and nerve cells, and in a fine varicose nerve network that originates at least partly from intrapulmonary NOS-containing nerve cells. VIP-like immunoreactivity appears to be colocalized with NOS in the latter network. All NOS-positive nerve fibres in the lungs of Cynops pyrrhogaster and Ambystoma mexicanum stain for NADPHd. It is concluded that the pulmonary NEE cells observed in Cynops pyrrhogaster are similar to those described in other vertebrate species and that the 5-HT-IR interstitial cells resemble mast cells. In addition, nitric oxide is likely to be a bioactive substance involved in nonadrenergic, noncholinergic inhibitory neurotransmission in the pulmonary nervous system of urodeles, where it may be colocalized with VIP.