ABSTRACT
Recently, a thorough, comparative analysis of catecholamine (CA) systems in the brain and spinal cord of vertebrates was done (1). The results of that study have forced us to reconsider our view on the organization of CA systems. Evidence has been provided for the existence of extensive, putatively CA cell groups that do not fit within the A1A17/C1-C3 classification as proposed by Hökfelt and colleagues (2). Moreover, in contrast with the classical idea that the evolution of CA systems is marked by an increase in complexity going from anamniotes (fishes, amphibians) to amniotes (reptiles, birds, mammals), it is now evident that the brains of anamniotes contain CA cell groups, of which the counterparts in amniotes have lost the capacity to produce CAs. In addition, it has become clear that a segmental approach in studying the organization of CA systems is preferable. Such an approach has been fruitful in the comparison of dopamine (DA) cell groups among vertebrates, in particular between amniotes and anamniotes. In that same review data were brought together about the distribution of receptors and CA fibers as well as about developmental aspects. From these data it is obvious that there is a good match between CA fibers and receptors, but at many places volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of CA cell groups, in particular those in the retina and olfactory bulb.
