Synaptic and non-synaptic effects of molluscan cardioexcitatory neuropeptides on locust skeletal muscle.

Neurally evoked contractions of the locust jumping muscle are potentiated by the molluscan cardioexcitatory peptide (FMRFamide) and related peptides. This effect is due to a synaptic and a non-synaptic action of the peptides: (1) excitatory synaptic potentials are increased; (2) excitation-contraction coupling becomes more efficient, as indicated by an increased strength of potassium contractures. Octopus heptapeptide (YGGFMRFamide), which contains the entire enkephalin structure (YGGFM), is 100 times more potent than FMRFamide. Structure-activity studies indicate that the observed actions are not mediated by opioid receptors but are dependent on the C-terminal structure of the peptides.