Pharmacological profile of the 5-hydroxytryptamine receptor of Fasciola hepatica body wall muscle.

5-HT is a candidate for the excitatory transmitter at the neuromuscular junction in trematodes including Fasciola hepatica. This study has determined the response of a muscle strip preparation from Fasciola hepatica to 5-HT and a range of agonists that distinguish between the vertebrate receptor 5-HT subtypes. 5-HT increased the resting tone and the rhythmic activity of the muscle strip. Of the 19 compounds tested, only 10 had an effect similar to 5-HT and all but 2 of these were tryptamine compounds. 5-HT was more potent than tryptamine whilst 4-OH-tryptamine had no effect, suggesting that the response is mediated by a 5-HT rather than a tryptamine receptor. 5-Fluorotryptamine and 5-carboxyamidotryptamine were the most potent agonists. 8-OH-DPAT also mimicked the effect of 5-HT, though less potently. Assuming that these agents elicit their response through a common receptor, this suggests the presence of a 5-HT receptor with similar properties in terms of agonist recognition as the vertebrate 5-HT1 class of receptor involved in controlling Fasciola muscle motility.