A NEMATODE FMRFAMIDE-LIKE PEPTIDE, SDPNFLRFAMIDE (PF1), RELAXES THE DORSAL MUSCLE STRIP PREPARATION OF ASCARIS-SUUM

PF1 (SDPNFLRFamide) is a FMRFamide-like peptide extracted from the fre e-living nematode Panagrellus redivivus. Here we show that this peptid e causes a hyperpolarization of somatic muscle cells of the parasitic nematode Ascaris suum and a relaxation of the somatic muscle strip pre paration. We have assessed whether or not the relaxation of Ascaris do rsal muscle strip by PF1 is due to (i) inhibition of the release of th e excitatory neuromuscular junction transmitter acetylcholine (ACh), ( ii) potentiation of the release of the inhibitory neuromuscular juncti on transmitter gamma-aminobutyric acid (GABA) or (iii) a direct inhibi tory action of the peptide on the muscle cells. Under the experimental conditions described here, tonic ACh release does not seem to be invo lved in determining the resting membrane potential or resting tone of the Ascaris dorsal muscle strip and thus inhibition of tonic ACh relea se is unlikely to explain the relaxation elicited bq the peptide. Furt hermore, PF1 (100 nM-1 mu M) inhibited the contraction of the muscle s trip elicited by bath application of ACh, suggesting either a direct i nhibitory action of the peptide on the muscle cells or a potentiation of GABA release. In electrophysiological experiments, the reversal pot ential for the PF1 hyperpolarization was not the same as that for GABA . Thus, PF1 hyperpolarizes Ascaris muscle by a mechanism that does not involve stimulation of GABA release from inhibitory pre-synaptic term inals.