The actions of FMRFamide-like peptides on visceral and somatic muscles of the snail Helix aspersa

Phe-Met-Arg-Phe-NH2 (FMRFamide) and pyroGlu-Asp-Pro-Phe-Leu-Arg-Phe-NH2 (pQDPFLRFamide) occur in the ganglia and tissues of the snail, Helix aspersa. This report describes the effects of these two neuropeptides on five visceral organs or somatic muscles isolated from the snail (Table 1). The epiphallus, as well as the rest of the male reproductive tract, was contracted by both FMRFamide and pQDPFLRFamide, and the threshold was usually below 5 X 10(-9) mol l-1 (Fig. 1). Both Peptides also reduced the resting tone of the crop and decreased the force and frequency of its rhythmic activity; FMRFamide is about 10 times more potent (Fig. 4). In contrast, pQDPFLRFamide was about 100 times more potent than FMRFamide as a cardioexcitatory agent (Fig. 5). The actions of the Peptides on the pharyngeal and tentacle retractor muscles were markedly different: FMRFamide primarily contracted these muscles; and pQDPFLRFamide usually had no effect alone, but relaxed or diminished contractions induced by FMRFamide and acetylcholine (ACh) (Figs 6, 8, 9). Other analogues of FMRFamide were tested, but none was as effective a relaxing agent as pQDPFLRFamide. The effects of FMRFamide and pQDPFLRFamide on all of the preparations could be distinguished from those produced by ACh and 5-hydroxytryptamine (5-HT); thus the actions of the neuropeptides were not mediated by cholinergic or serotonergic neurones. The stimulation of the musculature in the male reproductive tract and the inhibition of motility of the digestive system by FMRFamide and pQDPFLRFamide implicate these Peptides in the control of reproductive behaviour. The effectiveness of pQDPFLRFamide in relaxing the retractor muscles and as a cardioexcitatory agent led to the hypothesis that this heptapeptide and FMRFamide, acting at distinct receptors, cooperate to regulate the excitability and contractility of the snail's musculature between the extremes of aestivation and active locomotion.