Cholinergic activation of settlement in Ciona intestinalis metamorphosing larvae.

During metamorphosis, ascidian tadpoles settle on appropriate substrates, by secreting adhesive substances from the papillae. The papillae were shown by different authors to possess neural structures, probably for substrate recognition. In free-swimming tadpoles, AChE activity was localized by histochemical methods at the cell surface of the adhesive papillae, suggesting the possible presence of a cholinergic system in these structures. In vivo experiments showed that the treatment with AChCl caused dose-dependent enhancement of metamorphosis, shortening of the exploratory period of the larvae, and secretion within a few minutes of sConA-binding glycoconjugates in the tunica, beginning from the adhesive papillae.