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In order to test the role of nitric oxide in flatworms, Mesocestoides vogae tetrathyridia were incubated together with L-arginine, which is the substrate for nitric oxide synthesis, or with NG-nitro-L-arginine, which is an irreversible inhibitor of nitric oxide synthase. Normally, tetrathyridia attach to each other with the aid of their suckers, forming clusters. The rate of cluster formation was followed during the incubations. L-Arginine stimulated, and NG-nitro-L-arginine clearly inhibited, the cluster formation. This is the first time that an effect of nitric oxide has been observed in a flatworm. In addition, the pattern of the NADPH-diaphorase histochemical reaction in the nervous system and the pattern of F-actin filaments in the musculature stained with TRITC-labelled phalloidin were studied. NADPH-d staining occurred in the brain and the main nerve cords but also followed the muscle fibres stained with phalloidin. The pattern of the NADPH-d reaction was compared with that of 5-HT immunoreactivity. The implications of the results are discussed in relation to the background of data on neuronal signal substances in M. vogae.

The ultrastructure of six type of putative sense receptors including three nonciliate (Types I-III) and three ciliate (Types IV-VI) types found in the tegument of the scolex of adult Proteocephalus exiguus has been described for the first time. Type I has a simple tegumental sensory ending, containing one electron-dense collar, circular septate desmosome, electron-lucent vesicles and microtubules. Type II is represented by a receptor with a large rootlet. Type III is a receptor with a very long cross-striated rootlet, with two electron-dense collars and septate junction. The ciliated receptors differ in the length of the cilium, in the number of electron-dense collars and in the shape of sensory bulb. A comparison of structural features of receptors in different systematic group within Platyhelminthes is made.

Neuropeptides and serotonin in the cestode, Proteocephalus exiguus: an immunocytochemical study. International Journal for Parasitology 25: 673-682. Neuropeptide F (NPF), RFamide and serotonin (5-HT) immunoreactivities have been detected in the nervous system of P. exiguus procercoids and adults, using an indirect immunocytochemical technique in conjunction with confocal scanning laser microscopy. The peptidergic nervous system of the procercoid is well developed, with two brain ganglia, three pairs of longitudinal nerve cords, transverse ring commissures and nerves in the suckers, all showing NPF-immunostaining. Strong NPF- and RF-immunostaining was observed in the CNS and PNS of the adult worm. The distribution patterns of the two neuropeptides were similar. Immunoreactivity for 5-HT was found only in the CNS.