To investigate the evolutionary transition that has shaped the catecholaminergic systems of vertebrates, the organization of catecholamine-synthesizing neurons and the nature of the catecholamines were examined in the central nervous system of adult amphioxus (Branchiostoma lanceolatum), a cephalochordate. We isolated a gene transcript encoding tyrosine hydroxylase (TH), the limiting enzyme of catecholamine biosynthesis, and studied its distribution together with that of dopamine and serotonin. Dopamine and TH are found in the same neurons of which they are three separate populations. Two are located in the anterior brain, the first being dorsal and lying in a row and the second being more posterior and lateral. A third population comprising a few dorsal commissural neurons was found in the posterior brain. The anterior dopaminergic cells innervate the ventral commissure of the cephalic vesicle, the hindbrain, and the spinal cord. A serotonin-containing cell group is located in the same plane as the second dopaminergic cell population but is more caudal, marking the probable transition between anterior brain and hindbrain, as deduced from gene expression patterns. The overall distribution of dopaminergic and serotoninergic systems is similar in amphioxus and vertebrate central nervous system and could be an ancestral character of chordates. As assayed by high-performance liquid chromatography and electrochemical detection, significant amounts of dopamine and octopamine, but not of noradrenaline, are present in amphioxus head. This finding is consistent with data obtained from most prostomian species. We conclude that the noradrenergic system is probably an innovation of vertebrates that appeared along with the neural crest and specific hindbrain nuclei.
Copyright 2003 Wiley-Liss, Inc.