Two orcokinins and the novel octapeptide orcomyotropin in the hindgut of the crayfish Orconectes limosus: Identified myostimulatory neuropeptides originating together in neurones of the terminal abdominal ganglion

The tridecapeptides Asn(13)-orcokinin and Val(13)- orcokinin, two known mem bers of the orcokinin neuropeptide family native to crustaceans, and a nove l octapeptide, orcomyotropin, FDAFTTGFamide, have been identified from extr acts of hindguts of the crayfish Orconectes limosus using an isolated hindg ut contractility bioassay, high-performance liquid chromatography, microseq uencing and mass spectrometry. All three peptides display strong inotropic actions on crayfish hindguts. Orcomyotropin showed higher potency than the two orcokinins. Threshold concentration was approximately 5x10(-12)mol l(-1 ) versus 10(-10)mol l(-1) for the two orcokinins. An approximately fivefold increase in contraction amplitude was observed with 10(-9)mol l(-1) orcomy otropin and 10(-7)mol l(-1) of the orcokinins. Asn(13)- and Val(13)-orcokin in did not differ significantly with regard to their biological effects. Se mi-isolated crayfish hearts and locust oviducts did not respond to the thre e peptides, Immunocytochemistry using antisera against Asn(13)-orcokinin an d orcomyotropin showed that these neuropeptides are co-localized in approxi mately 80-90 neurones of the terminal abdominal ganglion that have been sho wn to innervate the entire hindgut muscularis via the intestinal nerve. The neurones form elaborate terminal branches preferentially on longitudinal h indgut muscles. Orcomyotropin is a novel crustacean member of the GF-amide family of myotropic and/or allatotropic neuropeptides from annelids, mollus cs and insects.