Oxytocin-like peptide: a novel epitope colocalized with the FMRFamide-like peptide in the supernumerary neurons of the sex segmental ganglia of leeches--morphological and biochemical characterization; putative anti-diuretic function.

A large number of oxytocin (OT)-like neurons were detected in the sex segmental ganglia (SG5, SG6) of three species of leeches belonging to different orders: Theromyzon tessulatum, Hirudo medicinalis and Erpobdella octoculata. In this latter species, an epitope close to the vertebrate OT by its C-terminal part (MSH release inhibiting factor: MIF), localized in granules of a size diameter of ca 120 nm and colocalized with FMRFamide(FMRFa)-like material was demonstrated. With reverse phase-high performance liquid chromatography, evidence was given that the two epitopes (OT and FMRFa) colocalized in the same neurons were biochemically different. A titration of OT per SG indicated that the OT-like amount was considerably higher in sex SG than in non-sex SG (ca. 5 pmol vs. ca. 0.5 pmol). Moreover, at the level of sex SG, this amount was ca. 3-fold higher in immature leeches than in mature specimens. Injections of extracts of SG of E. octoculata and of fragments of OT (Tocinoic acid or MIF) to T. tessulatum, indicated that MIF (the epitope found in the sex SG) and sex SG have the same anti-diuretic effect on the leeches injected. These results pointed to an anti-diuretic role of the leech OT-like substance.