1. In the radula protractor of the

1. In the radula protractor of the prosobranch mollusc Rapana thomasiana, the molluscan neuropeptide FMRFamide View the MathML source potentiated twitch contractions in response to a train of shortduration (0.5–1.0 msec) pulses of stimulation, but in the radula retractor, the peptide did not affect the contractions.
2.
2. FLRFamide View the MathML source, a FMRFamide-related neuropeptide, and the crustacean red pigment concentrating hormone View the MathML source potentiated twitch contractions of the retractor evoked by the same kind of stimulation, whereas they did not markedly change the evoked contractions of the protractor.
3.
3. In both of the muscles, these peptides did not markedly change the contractions in response to acetylcholine and glutamate, which have been suggested to be the principal excitatory neurotransmitters in the protractor and retractor, respectively.
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4. In the presence of the acetylcholine-antagonist propantheline (10−5 M) or high Mg2+ (80 mM Mg2+), FMRFamide hardly changed twitch contractions of the protractor in response to a train of long-duration (3–5 msec) pulses of stimulation.
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5. In the presence of high Mg2+, FLRFamide did not markedly change twitch contractions of the retractor in response to the same kind of stimulation, and the crustacean peptide hardly changed the contractions.
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6. These results suggest that the peptides act on the presynaptic sites in the protractor or retractor to increase release of excitatory neurotransmitter.