The characterisation of Serotonin receptors in the parasitic nematode Ascaris suum

5-Hydroxytryptamine (5-HT, serotonin) receptors in the parasitic nematode A. suum regulate vital functions and may be novel targets for anthelmintics. The effects of 5-HT on feeding and locomotive behaviour were compared using the isolated pharyngeal muscle preparation and the dorsal muscle preparation. The second messenger pathways (cAMP, cGMP, IP3) in both pharyngeal and somatic muscle were determined. Complementary DNA library screening and PCR were used to try and identify the genes for these 5-HT receptors.5-HT stimulated pharyngeal pumping frequency in a concentration-dependent manner (EC50 = 44.4. ± 0.1 μM, n = 6; mean ± s.e. mean). The tension increase in the dorsal muscle strip in response to 30 μM ACh was significantly inhibited by 5-HT (1 mM) and was reversible.5-HT stimulated cAMP production in the somatic muscle in a concentration-dependent manner (basal = 1721 ± 134, 5-HT 100 μM = 2851 ± 212, 5-HT 1 mM = 4518 ± 1234; pmol mg-1 protein, mean ± s.e. mean) whereas in the pharyngeal muscle 5-HT did not stimulate cAMP synthesis (basal = 1540 ± 232, 5-HT 100μM = 1526 ± 84; pmol mg-1 protein; mean ± s.e. mean). Incubation of the pharyngeal muscle with 5-HT (1 mM) resulted in a 3 fold increase in the level of IP3 above basal levels.A 300 bp fragment was isolated from a A. suum larval cDNA library using a fragment from H. contortus and displayed homology to β-adrenoceptors.There is therefore functional evidence for at least two 5-HT receptor subtypes in this nematode. A number of approaches were used to attempt to isolate cDNAs for these receptors from A. suum. However, these receptors are likely to be rare transcripts, and from the screening of both the larval and neuronal libraries, only one fragment showing any homology to a G protein-coupled receptor was isolated.