Evidence for proctolin-like and RFamide-like neuropeptides associated with the hindgut of the crayfish Procambarus clarkii

Immunohistochemical staining revealed proctolin-like immunoreactivity in nerve endings associated with the hindgut of the crayfish Procambarus clarkii. Proctolin-like bioactivity, detected using both locust oviducts and crayfish hindguts for bioassays, co-eluted with authentic proctolin through five consecutive reversed-phase high-performance liquid chromatography (RP-HPLC) systems. This strongly suggests that proctolin (or a peptide very similar to it) is contained in nerve endings on the crayfish hindgut. RFamide-like immunoreactivity (RFLI) was extracted from the hindguts and intestinal nerves of crayfish and separated using RP-HPLC. Initial separation on a C18 column gave a broad peak of RFLI, and these fractions were bioactive on the crayfish hindgut. Subsequently, RFLI was separated on two additional RP-HPLC systems. The predominant peak could be distinguished from FMRFamide and several known FMRFamide-like peptides on the basis of elution times. Partial sequence analysis indicated the presence of a decapeptide having some sequence homology with leucomyosuppressin and SchistoFLRFamide. These results support earlier evidence that extended RFamide peptides may function as neurotransmitters or neuromodulators on the crayfish hindgut, and suggest a similar role for proctolin or a closely related peptide.