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 bg 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 an d generate slow contractions that affect posture. DF2, at concentratio ns of 10(-8) M or higher, increased muscle tonus in isolated, unstimul ated neuromuscular preparations. In some preparations, the peptide als o induced small, arrhythmic contractions or increased the amplitude of such contractions if they were already present. The spontaneous contr actions were temperature-dependent and insensitive to 10(-7) M tetrodo toxin, indicating that they were myogenic. DF2 increased muscle tonus in the presence of tetrodotoxin and when nerve-evoked contractions wer e blocked using Joro spider toxin (JSTX), a glutamate receptor antagon ist. Thus, the effects of DF2 on contraction appear to represent direc t effects on the muscle and not changes in release of chemical transmi tter from nerve terminals. DF2 did not alter resting membrane potentia l or input resistance in the muscle fibres. The effects of DF2 on cont raction 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 extracell ular Ca2+ through dihydropyridine sensitive Ca2+ channels. The Ca2+ ch annel antagonists also reduced muscle tonus on their own, suggesting t hat they may lower the intracellular calcium concentration. The peptid e might act by enhancing Ca2+ influx or by enhancing Ca2+-dependent re lease of Ca2+ ions from internal stores. (C) 1997 Elsevier Science Inc .