Serotonin Signalling in Flatworms: An Immunocytochemical Localisation of 5-HT(7) Type of Serotonin Receptors in Opisthorchis felineus and Hymenolepis diminuta

The study is dedicated to the investigation of serotonin (5-hydroxytryptamine, 5-HT) and 5-HT(7) type serotonin receptor of localisation in larvae of two parasitic flatworms Opisthorchis felineus (Rivolta, 1884) Blanchard, 1895 and Hymenolepis diminuta Rudolphi, 1819, performed using the immunocytochemical method and confocal laser scanning microscopy (CLSM). Using whole mount preparations and specific antibodies, a microscopic analysis of the spatial distribution of 5-HT(7)-immunoreactivity(-IR) was revealed in worm tissue. In metacercariae of O. felineus 5-HT(7)-IR was observed in the main nerve cords and in the head commissure connecting the head ganglia. The presence of 5-HT(7)-IR was also found in several structures located on the oral sucker. 5-HT(7)-IR was evident in the round glandular cells scattered throughout the larva body. In cysticercoids of H. diminuta immunostaining to 5-HT(7) was found in flame cells of the excretory system. Weak staining to 5-HT(7) was observed along the longitudinal and transverse muscle fibres comprising the body wall and musculature of suckers, in thin longitudinal nerve cords and a connective commissure of the central nervous system. Available publications on serotonin action in flatworms and serotonin receptors identification were reviewed. Own results and the published data indicate that the muscular structures of flatworms are deeply supplied by 5-HT(7)-IR elements. It suggests that the 5-HT(7) type receptor can mediate the serotonin action in the investigated species and is an important component of the flatworm motor control system. The study of the neurochemical basis of parasitic flatworms can play an important role in the solution of fundamental problems in early development of the nervous system and the evolution of neuronal signalling components.