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1. In the radula protractor of the prosobranch mollusc Rapana thomasiana , both twitch contractions and acetylcholine contractions were markedly depressed or blocked by propantheline (10 −5 M) and strychnine (10 −5 M), but in the radula retractor, only acetylcholine contraction was markedly affected by the antagonists, 2. Glutamate contractions of both of the muscles were little or slightly affected by the drugs. 3. Twitch contraction of the protractor was slowly depressed when the muscle was immersed in concanavalin A (0.3 mg/ml), while that of the retractor was first potentiated and then slowly depressed in it. 4. In both of the muscles, glutamate contractions were markedly enhanced by the lectin, but acetylcholine contractions were not affected. 5. These results support the notion that the principal excitatory neurotransmitter in the protractor is acetylcholine, whereas that in the retractor is glutamate.

1. 1. In the radula protractor of the prosobranch mollusc Rapana thomasiana, the molluscan neuropeptide FMRFamide (≧ 10 −9 M ) 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 (≧ 10 −10 M ) , a FMRFamide-related neuropeptide, and the crustacean red pigment concentrating hormone (≧ 10 −8 M ) 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. 4. 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. 5. 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. 6. 6. These results suggest that the peptides act on the presynaptic sites in the protractor or retractor to increase release of excitatory neurotransmitter.