Abstract
This chapter discusses peptidergic neurons in the vertebrate spinal cord. The addresses four questions: (1) how the peptidergic neuron systems in the mammalian CNS evolved; (2) whether they were present in the earliest vertebrates, like many of the nonpeptide transmitter systems, or they appeared at later stages; (3) are the peptidergic systems similarly organized in all species, or do they differ; and (4) would they not only shed light on the evolution of the vertebrate CNS. In some instances, comparisons of peptide sequences among species have made it possible to deduce the mutational events during the evolution of a neuropeptide. However, in order to understand how a specific system of peptidergic neurons has evolved, it is necessary to compare its molecular features, anatomical organization, co-transmitters, target cells, and so on. Because of the anatomical differences between mammals and non-mammals, such comprehensive comparative analyses may be difficult to apply to certain CNS regions, such as the forebrain. In contrast, the spinal cord is well suited for this type of study, as its general outline can be recognized in all vertebrate classes, ranging from cyclostomes to primates. The organization, connectivity, and pharmacology of many spinal neuron systems have been thoroughly analyzed in several different vertebrates, thus providing a well-established framework for comparative studies. This chapter attempts to illustrate some trends in the evolution of spinal peptidergic neurons, by comparing the molecular and anatomical features of systems that have been characterized in mammals and in non-mammals.
Published Version
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