Comparative chemical anatomy of the brain: concepts and methods.

The study of neuropeptides represents an appropriate playground for comparative and evolutionary research. Comparative analysis can give insight into the conservative pattern of intercellular transmission molecules, possibly bound both to some evolutionary antiquity and to cellular constraints. In the same time it can teach us how modulation has occurred at molecular, cellular, multicellular levels in order to give the species-specific functional organization. Using some examples from vertebrate central neurons system (CNS) immunocytochemical analyses, the results so far obtained suggest the rise of a new comparative chemical neuroanatomy. The rationale of "what" and "why" we are comparing is, however, needed in order to understand constancy, heterogeneity or else trends toward complexity in the distribution of neuropeptides.