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

Abstract

The superficial extensor muscles of the crayfish abdomen were examined as a possible site for modulation by DF 2 (Asp-Arg-Asn-Phe-Leu-Arg-Phe-NH 2), 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. DF 2, 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. DF 2 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 DF 2 on contraction appear to represent direct effects on the muscle and not changes in release of chemical transmitter from nerve terminals. DF 2 did not alter resting membrane potential or input resistance in the muscle fibres. The effects of DF 2 on contraction were blocked by the Ca 2+ channel antagonists Mn 2+ Ni 2+ and Cd 2+ and nicardipine, and by replacing extracellular Ca 2+ with Mg 2+. This suggests that the peptide's effect may require an influx of extracellular Ca 2+ through dihydropyridine-sensitive Ca 2+ channels. The Ca 2+ channel antagonists also reduced muscle tonus on their own, suggesting that they may lower the intracellular calcium concentration. The peptide might act by enhancing Ca 2+ influx or by enhancing Ca 2+-dependent release of Ca 2+ ions from internal stores.