Extensive presence of serotonergic cells and fibers in the peripheral nervous system of lampreys.

Lampreys are suitable animal models for studying the evolution of the vertebrate nervous system because they belong to the earliest group of extant vertebrates, the agnathans. The general organization of the serotonergic central nervous system is well known in lampreys, but information about its peripheral organization is lacking. In the present study, high-performance liquid chromatography and immunohistochemistry with an antibody against serotonin were used to study the presence and peripheral distribution of serotonergic elements in the head of larval sea lampreys. Adult lampreys were also analyzed in order to compare the organization of the system in larval and postmetamorphic stages. Serotonergic cells were observed in the cranial nerve ganglia, the musculature, the taste buds, and in the gills. Serotonergic fibers were extensive in all tissues except the epidermis. Possible roles were proposed for the different cell types observed in relation to their location, structure, and innervation patterns. The serotonergic cells appear to be involved in the oxygen and taste chemoreception processes, the modulation of musculature contraction, and in the regulation of cilia beat frequency and mucus secretion in lampreys. The extensive presence of serotonergic elements in the peripheral nervous system of lampreys suggests a close relationship with the organization of this system in invertebrate groups. 2008 Wiley-Liss, Inc.