Large veins in the Atlantic cod (Gadus morhua) and the rainbow trout (Oncorhynchus mykiss) are innervated by neuropeptide-containing nerves

Previous studies have shown that fish veins are reactive to several hormone s known to exist in the fish circulation. Besides this humoral control, ano ther possible means of active regulation of venous return is by autonomic n ervous control of venous tone. This study therefore investigated the presen ce of a perivascular innervation of major veins in the Atlantic cod (Gadus morhua) and the rainbow trout (Oncorhynchus mykiss) using immunohistochemic al methods. Histological staining was employed to investigate the smooth mu scle distribution in the vessel walls. Vasoactive intestinal polypeptide- i mmunoreactive nerve fibers were found to be widespread in the venous system of G. morhua and O. mykiss, while pituitary adenylate cyclase-activating p olypeptide-immunoreactive fibers were demonstrated in the duct of Cuvier of both species. Fibers containing neurokinin A and/or substance P were found in the duct of Cuvier and the posterior cardinal vein of both species and in the hepatic portal vein of O. mykiss. Calcitonin-gene related peptide-im munoreactive fibers were present in the duct of Cuvier of both species and in the hepatic portal vein of O. mykiss. Galanin-immunoreactive fibers were found in the duct of Cuvier in O. mykiss and in the hepatic portal vein of both species. Co-existence of neuropeptides in the perivascular nerve fibe rs was investigated by double labelling. Vasoactive intestinal polypeptide/ pituitary adenylate cyclase-activating polypeptide-immunoreactive fibers we re found in both species. Vasoactive intestinal polypeptide/galanin-immunor eactive fibers and vasoactive intestinal polypeptide/calcitonin-gene relate d peptide-immunoreactive fibers were found in G. morhua but not in O. mykis s. This study gives further evidence for an active venoregulation by autono mic nerves in teleost fish.