EFFECTS OF ANNETOCIN, AN OXYTOCIN-RELATED PEPTIDE ISOLATED FROM THE EARTHWORM EISENIA-FOETIDA, AND SOME PUTATIVE NEUROTRANSMITTERS ON GUT MOTILITY OF THE EARTHWORM

Annetocin, an oxytocin-related peptide recently isolated from the lumb ricid earthworm Eisenia foetida, and putative transmitter substances w ere examined for their effects on rhythmic, spontaneous contractions o f isolated gut preparations of the earthworm. Significant, dose-depend ent effects of the following substances were observed: acetylcholine ( ACh), gamma-aminobutyric acid (GABA), and dopamine were excitatory, wh ile serotonin (5-HT) and octopamine were inhibitory; Annetocin, oxytoc in, and vasotocin stimulated spontaneous contraction of the earthworm gut, annetocin being approximately 10-fold more potent than oxytocin o r vasotocin. However, arginine-vasopressin (Arg-vasopressin), lysine-v asopressin (Lys-vasopressin), tocinoic acid (N-terminal hexapeptide fr agment of oxytocin), and MSH release-inhibiting factor (MIF; C-termina l tripeptide fragment of oxytocin) did not show any effect on the eart hworm gut motility. On the other hand, oxytocin, vasotocin, Arg-vasopr essin, Lys-vasopressin, and tocinoic acid caused spontaneous contracti ons of isolated rat uterine preparations, where the potency was in thi s order, while annetocin and MIF exerted no oxytocic activity on the u terus. Dose-response relationship of the effects of annetocin and its related peptides on the annelid and mammalian systems shows that amino acid residue at the third position of these peptides is important for exertion of excitatory action on the smooth muscle systems. The resul ts in the present study suggest that receptors for annetocin and for G ABA on the earthworm gut, unlike those for ACh, desensitize during con tinuous exposure to these substances. (C) 1995 Wiley-Liss, Inc.