Cholinergic compounds inhibit FMRFamide-induced contractions in dispersed muscle fibres isolated from adult Schistosoma mansoni. Acetylcholine (ACh) was the most effective cholinergic agonist tested with an EC50 < 100 nM. Less effective were propionylcholine and arecoline with EC50 < 1 μM and butyrylcholine and carbachol with EC50 < 10μM. Choline, muscarine, pilocarpine, nicotine, DMPP (1,1-dimethylphenylpiperazine ) and levamisole were all ineffective. Amongst tested antagonists, d-tubocurarine (100μM), mecamylamine (1 mM), atropine (1 mM), scopolamine (1 mM) and quinuclidinyl benzilate (10 μM) were all ineffective. Bicuculline, picrotoxin and strychnine were also ineffective. However α-bungarotoxin, at 100 nM, was able to block the inhibitory ACh effect. From these data it appears that the cholinergic receptor on the schistosome muscle fibres may be of the nicotinic type, but that its pharmacology is different from that of nicotinic receptors of vertebrates as well as of nematodes or a variety of other invertebrates.
