PHYSIOLOGICAL-EFFECTS OF 2 FMRFAMIDE-RELATED PEPTIDES FROM THE CRAYFISH PROCAMBARUS-CLARKII

The present study examined the effects of two recently identified neur opeptides on crayfish hearts and on neuromuscular junctions of the cra yfish deep abdominal extensor muscles, The two peptides, referred to a s NF1 (Asn-Arg-Asn-Phe-Leu-Arg-Phe-NH2) and DF2 (Asp-Arg-Asn-Phe-Leu-A rg-Phe-NH2), increased the rate and amplitude of spontaneous cardiac c ontractions and increased the amplitude of excitatory junctional poten tials (EJPs) in the deep extensors, Both effects were dose-dependent, but threshold and EC(50) values for the cardiac effects were at least 10 times lower than for the deep extensor effects, The heart responded equally well to three sequential applications of peptide in any given preparation, but the responses of the deep extensors appeared to decl ine with successive peptide applications, The results support the hypo thesis that these two neuropeptides act as neurohormones to modulate t he neuromuscular systems in crayfish. Quantal synaptic current recordi ngs from the deep extensor muscles indicate that both peptides increas e the number of quanta of transmitter released from synaptic terminals , Neither peptide elicited a measurable change in the size of quantal synaptic currents, NF1 caused a small increase in muscle cell input re sistance, while DF2 did not alter input resistance. These data suggest that DF2 increases EJP amplitudes primarily by increasing transmitter release, while the increase elicited by NF1 appears to involve presyn aptic and postsynaptic mechanisms.