![]() Many neuropeptides are cosynthesized as a cohort of closely related peptide isoforms that derive from a single propeptide precursor. ![]() In this role these seven peptides appear to be functionally redundant. Together, these results indicate that the products of the Drosophila FMRFamide gene function as neurohormones to modulate the strength of contraction at the larval neuromuscular junction. At 0.1 μ m, DPKQDFMRFamide increased the amplitude of the excitatory junctional current to 151% of baseline within 3 min. One peptide was tested and found to enhance synaptic transmission. When the peptides were presented as mixtures predicted by their stoichiometric ratios in the dFMRFamide propeptide, the effects were additive, and there were no detectable higher-order interactions among them. At higher concentrations, twitch tension remained elevated for 5–10 min or more after wash-out of the peptide. At concentrations <0.1 μ m, the responses to each of the seven excitatory peptides followed a time course that matched the fluctuations of the peptide concentration in the bath. Each had a threshold concentration near 1 n m, and the EC 50 for each peptide was ∼40 n m. The seven active peptides showed similar dose–response curves. The effects of one peptide, DPKQDFMRFamide, were unchanged after the onset of metamorphosis. Their targets were distributed widely throughout the somatic musculature. Seven of the peptides strongly enhanced twitch tension, and one of the peptides was inactive. We analyzed the effects of eight of these peptides on nerve-stimulated contraction (twitch tension) of Drosophila larval body-wall muscles. ![]() These peptides are expressed by neurosecretory cells and may be released into the blood to act as neurohormones. The Drosophila FMRFamide gene encodes multiple FMRFamide-related peptides.
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