Immunocytochemical demonstration of 5-hydroxytryptamine (serotonin) and vertebrate neuropeptides in the nervous system of excysted cysticercoid larvae of the rat tapeworm, Hymenolepis diminuta (Cestoda, Cyclophyllidea).

The localisation and distribution of 5-hydroxytryptamine (5-HT, or serotonin) and a number of vertebrate neuropeptides in the nervous system of excysted (0-24 h) cysticercoid larvae of Hymenolepis diminuta were determined by an indirect immunofluorescence technique in whole-mount preparations. In the central nervous system, cell bodies and nerve fibres immunoreactive to 5-HT are present in the main commissure, lateral and rostellar ganglia, and the longitudinal nerve cords and their connectives. In the peripheral nervous system, immunoreactive nerve fibres occur in a poorly developed nerve plexus within each sucker. Among the vertebrate peptides tested, antisera to pancreatic polypeptide (PP), polypeptide YY (PYY), peptide histidine isoleucine (PHI) and gastrin-releasing peptide (GRP) gave positive results. Immunoreactivity to PP and PYY paralleled that of 5-HT, with greater numbers of cell bodies present in the different locations within the scolex nervous system, and the sucker plexus being more prominent. The number of PP-reactive cells in the lateral ganglia and main lateral, longitudinal nerve cords increased over the 24-h period in culture. Results with antisera of different specificities to PP and PYY suggest that the immunoreactivity may be due to a peptide with closer structural affinity to PYY than to PP. Immunoreactivity to PHI is restricted to the main lateral nerve cords in the body of 0-h worms, extending into the median nerve cords by 12 h and 24 h. Immunoreactivity to GRP became evident after 12 h in culture and was confined to the longitudinal nerve cords, in particular the median nerve cords. The results are discussed in relation to the proposed transmitter and regulatory roles of 5-hydroxytryptamine and the neuropeptides.