Modulation of crayfish superficial extensor muscles by a FMRFamide-related neuropeptide

The superficial extensor muscles of the crayfish abdomen were examined as a possible site for modulation by DF₂ (Asp-Arg-Asn-Phe-Leu-Arg-Phe-NH₂), a FMRFamide-related neuropeptide found in crayfish pericardial organs (26). The superficial extensor muscles are of the tonic type and generate slow contractions that affect posture. DF₂, at concentrations of 10⁻⁸ M or higher, increased muscle tonus in isolated, unstimulated neuromuscular preparations. In some preparations, the peptide also induced small, arrhythmic contractions or increased the amplitude of such contractions if they were already present. The spontaneous contractions were temperature-dependent and insensitive to 10⁻⁷ M tetrodotoxin, indicating that they were myogenic. DF₂ increased muscle tonus in the presence of tetrodotoxin and when nerve-evoked contractions were blocked using Joro spider toxin (JSTX), a glutamate receptor antagonist. Thus, the effects of DF₂ on contraction appear to represent direct effects on the muscle and not changes in release of chemical transmitter from nerve terminals. DF₂ did not alter resting membrane potential or input resistance in the muscle fibres.

The effects of DF₂ on contraction were blocked by the Ca²⁺ channel antagonists Mn²⁺ Ni²⁺ and Cd²⁺ and nicardipine, and by replacing extracellular Ca²⁺ with Mg²⁺. This suggests that the peptide's effect may require an influx of extracellular Ca²⁺ through dihydropyridine-sensitive Ca²⁺ channels. The Ca²⁺ channel antagonists also reduced muscle tonus on their own, suggesting that they may lower the intracellular calcium concentration. The peptide might act by enhancing Ca²⁺ influx or by enhancing Ca²⁺-dependent release of Ca²⁺ ions from internal stores.