Evolutionary ancient roles of serotonin: long-lasting regulation of activity and development.

Biogenic monoamines (catecholamines, indoleamines and histamine) are evolutionary old and important modulators of long-lasting changes in the functional state of cells. They are found in many protozoans and in almost all metazoans. Monoamines preserve their evolutionary old functions (first of all being intracellular signals and later hormones and growth factors) even in those animals in which they acquired the function of neurotransmitter. The older functions of serotonin, an important member of the family of indoleamines, are reviewed here. Described are: presence of serotonin in organisms at various phylogenetic levels; its role in embryonal, foetal and postnatal development, especially in the development of the central nervous system. It is concluded that in none of these functions serotonin is the only factor, but it is an ubiquitous and important modulator of a vast array of processes and functions taking part in development and plasticity.