Oxytocin/vasopressin and gonadotropin‐releasing hormone from cephalopods to vertebrates

Recent advances in peptide search methods have revealed two peptide systems that have been conserved through metazoan evolution. Members of the oxytocin/vasopressin‐superfamily have been identified from protostomian and deuterostomian animals, indicating that the oxytocin/vasopressin hormonal system represents one of the most ancient systems. In most protostomian animals, a single member of the superfamily shares oxytocin‐like and vasopressin‐like actions. Co‐occurrence of two members has been discovered in modern cephalopods, octopus, and cuttlefish. We propose that cephalopods have developed two peptides in the molluscan evolutionary lineage like vertebrates have established two lineages in the oxytocin/vasopressin superfamily. The existence of gonadotropin‐releasing hormone (GnRH) in protostomian animals was initially suggested by immunohistochemical analysis using chordate GnRH antibodies. A peptide with structural features similar to those of chordate GnRHs was originally isolated from octopus, and an identical peptide has been characterized from squid and cuttlefish. Novel forms of GnRH‐like molecules from other molluscs, an annelid, arthropods, and nematodes demonstrate somewhat conserved structures at the N‐terminal regions; but structures of the C‐terminal regions critical to gonadotropin‐releasing activity are diverse. These findings may be important for the study of the molecular evolution of GnRH in protostomian animals.