Localization of some neurotransmitters during developments in hydroidomedusae.

Neurotransmission in Cnidarians is known to occur mainly by peptidergic neuronal pathways; the role of cholinergic neurotransmission, that in other animals this involves both neurotransmission and developmental regulation, has never been shown with certainty. This histochemical-immunohistochemical study was performed, in different developmental stages of some hydroidomedusan species (characterized by different reproductive and developmental patterns) to investigate the presence and distribution of cholinesterase (ChE) activity, acetylcholine receptors, biogenic amines, and molecules immunologically related to secretory peptides (FMRF amide and VIP). The results showed that ChE (revealed by all the kinds of activities: AChE, PrChE, BuChE) is mainly present in developing or 'juvenile' structures. Biogenic amines are revealed by aldehyde-induced fluorescence at every stage studied, while peptide-like immunoreactivities are mainly present in differentiated structures. These findings support the hypothesis that all the chemical messengers here investigated arose very early in evolution, although, in some cases, with different functions. For most of the investigated species, similarities in the distribution pattern of markers generally reflect similarities in life cycle, rather than phylogenetic affinities.