Opioid effects on macrovascular dopamine release

Both alkaloid opiates and met-enkephalin occur in vertebrate chromaffin cells, where they affect catecholamine (CA) secretion. Since the large blood vessels of the eel and the rat release dopamine (DA) from as yet unidentified source(s), we studied the impact of alkaloid opiates and met-enkephalin on the secretion of DA from three macrovessels of the American eel (Anguilla rostrata) in a perifusion system. Codeine, morphine, and met-enkephalin increased the release of DA from both the ventral aorta and the caudal vein. The antagonist naloxone stimulated DA release from the caudal vein, but had no impact on release from the ventral aorta. Only codeine had a significant effect on DA release from the posterior cardinal vein. These findings show that the DA release from the macrovessels is sensitive to opioid substances, and they suggest that the antagonistic effects between alkaloid opiate and opioid peptide, seen in other systems, are absent in large blood vessels. Furthermore, the "unorthodox" stimulatory effect of naloxone in the caudal vein raises the question of as yet unidentified receptor and/or effector systems.