Physiological actions of putative neuropeptides in the nematodes Ascaris suum and Caenorhabditis elegans

The effect of the neuropeptides on the body wall muscle of A. suum was determined using an in vitro preparation of the dorsal muscle strip. The direct effect of the peptide and is modulation of the response to acetylcholine (ACh) were examined. ACh, 10 μM, elicited a rapid and reversible contraction. Prior application of FLPs (FMRFa-like peptides) modified the ACh response. FLP1B (AGSDPNFLRFa) and FLP13A (APEASPFIRFa) both reduced the ACh contraction while FLP3A (SAEPFGTMRFa) and FLP21 (GLGPRPLRFa) potentiated the ACh response. FLP21 also induced a muscle contraction. None of the NLPs (neuropeptides-like proteins) tested had a direct effect on the muscle but all three reduced the ACh contraction. To investigate the effect of neuropeptides in C. elegans more than 30 FLPs (encoded by 23 flp genes) were tested for biological activity on a semi-intact preparation of C. elegans pharynx. Eleven flp genes encoded peptides which inhibited pharyngeal activity, while eight flp genes encoded peptides which were excitatory. One nlp gene encoded a peptide which had a weak excitatory action. Three of the potent peptides (FLP13, APEASPFIRFa, inhibitory; FLP17A, KSAFVRFa, excitatory; FLP17B, KSQYIRFa, excitatory) are encoded by flp genes expressed in pharyngeal motorneurones and therefore have a high probability of acting through specific receptors directly on the pharyngeal muscle. The two other potent peptides FLP8 (AF1, KNEFIRFa, excitatory) and FLP11A (AMRNALVRFa, inhibitory) are only expressed in extra pharyngeal neurones and are therefore likely to act either indirectly or as neurohormones. Overall this suggests that the peptidergic regulation of C. elegans pharyngeal activity is a complex phenomenon.