Immunohistochemical study of the innervation of pulmonary vessels and smooth muscles in the respiratory tract of two frog species

The innervation of the respiratory tract of amphibians is still poorly understood.
Therefore, the respiratory tracts of the frogs Rana esculenta and Discoglossus pictus
have been investigated in order to describe non-adrenergic non-cholinergic (NANC)
and adrenergic innervation, and the localization of neuromediators that are possibly
involved. Immunohistochemical staining of many bioactive substances was found in
neuroepithelial cells of the buccopharynx, larynx, lung septa, nerves and neurons
throughout the airway system. The findings indicate the occurrence of vasoactive
intestinal peptide (VIP)-immunopositive nerve fibers in fibromuscular septa and the
vasculature, nitrergic innervation of the large pulmonary veins showing a plexus of
nNOS-immunopositive nerve fibers that also innervate the lung wall and the
localization of neuronal nitric oxide synthase (nNOS) in neurons in the lung wall. In
addition, laryngeal blood vessels and small arteries in the wall of septa that form
capillary networks are supplied by enkephalin-immunopositive nerve terminals. We
conclude that the airway system of the two frog species studied is innervated by a
parasympathetic NANC system. Adrenergic innervation was also found that was
immunostained for tyrosine hydroxylase. Adrenergic fibers were mainly present in
muscles in septal edges, arteries present in septa and the wall of the lung. It is
suggested that nNOS-positive and leu-enkephalin-positive neurons mediate vasodilation
via the release of NO, but the nature of the NANC innervation remains obscure.
Despite the many pharmacological studies of the lungs of amphibians, the
physiological role of pulmonary autonomic innervation remains poorly understood.