Relaxing and Inhibitory Actions of Pedal Ganglion Extracts on the Anterior Byssus Retractor Muscle of Mytilus

Pedal ganglion homogenates from the bivalve, Mytilus edulis, were subjected to gel filtration and the biological activities of the extracts were assayed on the ABRM of the mussel. The extracts resolved into a catch-relaxing peak and a contraction-inhibiting peak of activities.
Relaxation of ACh-induced catch tension in the ABRM by catch-relaxing peak was not affected by pretreatment of the muscle with 10^-5 M FMRFamide, suggesting that the active principle of the peak is not FMRFamide. The relaxation was blocked by 5 x 10^-4 M mersalyl, which suggests that the active substance is neither dopamine nor octopamine. The relaxation was markedly depressed after the muscle had been denervated by treating it with KCl-EGTA solution, suggesting that the substance is not serotonin and that it relaxes the catch tension acting on intramuscular relaxing nerve elements. The relaxing activity of the peak was destroyed by incubating it with a protease, subtilisin. Thus, the active substance in the peak seems to be a peptide which acts presynaptically to increase the release of relaxing transmitter serotonin.
The contraction-inhibiting peak also lost its activity when incubated with subtilisin, suggesting that the inhibitory substance in the peak is also a peptide. The substance inhibited not only phasic contraction by repetitive electrical stimulation but also ACh contraction and FMRFamide contraction, which suggests that it acts directly on muscle fibres to inhibit the contractions.