In the anterior byssus retractor muscle (ABRM) of Mytilus, low concentrations of the molluscan neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide) relax ACh-induced catch tension, whereas high concentrations cause contraction. In the present studies the actions of a FMRFamide-analog Trp-Nle-Arg-Phe-NH2 (W-Nle-RFamide) on the ABRM were investigated in order to compare them with those of FMRFamide.
The actions of W-Nle-RFamide were qualitatively similar to those of FMRFamide, but catch-relaxing activity of the former peptide was found to be 10-30 times more potent than that of the latter.
Cytochemical studies showed that FMRFamide-like immunoreactivity was localized to nerve branches in the ABRM and also to neurons in the pedal ganglion.
A peptide which structurally resembles FMRFamide may be involved in the mechanism of the control of relaxation of catch tension in the ABRM of Mytilus.