brain of vertebrates represents a field in neuroanatomy that has advanced rapidly. This relative modern research began in the 1970s with the discovery of enkephalins and other naturally occurring opioid peptides. Actually, the term "neuropeptide" was first used by De Wied et al. in 1971 in a Dutch journal and became widely adopted in 1974 . Thereafter, there has been intense interest in the neuropeptides as chemical messengers in the nervous system and rapid advances in this research field were made. With the development of molecular neurobiology techniques, a tremendous increase of new discoveries of neuropeptides and their associated cellular receptors has occurred, and some 50 different vertebrate neuropeptides are now known. All these neuroactive peptides are being studied with different experimental approaches to reveal their genes, receptors, distribution, biosynthesis, release, and functional significance.
Burbach and De Wied coined the first definition of the term neuropeptides as endogenous substances synthesized in nerve cells and involved in nervous system functions. However, a debate is currently maintained about the actual significance of a neuropeptide. Some authors prefer to consider a more common definition of a "neuropeptide," which would be that it is secreted from a neuron . However, there is a modern tendency to include under the term "neuropeptide" any peptide that has an action in the nervous system regardless of whether it is secreted by neural or non-neural tissue . Currently, the broader definition of a neuropeptide has been adopted by The International Neuropeptide Society. It includes those peptides, independently of whether they are secreted by neurons or non-neural cells, that express the same genetic information and undergo(es) identical processes of synthesis and transport, and of binding to similar families of receptors, in order to act on neural processes .
Neuropeptides are particularly suited to comparative and evolutionary studies since the structures of neuropeptides have been highly conserved during evolution . Nevertheless, they are often synthesized as multiple molecular variants and they may play different roles in different regions. The analysis of the distribution of neuropeptides in vertebrates and their phenotypic plasticity, especially during development, leads to an understanding of the basic neurochemical organization but at the same time, gives an appreciation of the wide diversity between species.
NEUROPEPTIDES IN AMPHIBIANS
Among vertebrates, amphibians constitute the class that marks a crucial point in evolution, i.e., the transition from an aquatic to a terrestrial lifestyle. Amphibians were generally chosen in comparative studies of vertebrate brain organization, because it was noticed AGUSTI ´N GONZA ´LEZ (Guest Editor