Oxytocin and Vasopressin Secretion: New Perspectives

Abstract

Six naturally occurring oxytocin-like and three vasopressin-like posterior pituitary hormones have been chemically characterized; all have nine amino-acid residues and six of these are consistent throughout. The posterior pituitary hormones of about 50 species have been fully characterized. This is a small sample, representing about 0.1% of all vertebrate species, but it has allowed a number of principles to be established (Acher, 1980). Most species have two posterior pituitary hormones of the oxytocin-vasopressin family; only the cyclostomes (lampreys) have one. This suggests that a gene duplication must have occurred before the evolution of fishes, some 450–500 million years ago. Point mutations then occurred separately within these two genes to give rise to two families of posterior pituitary hormones. The functions of these two hormones also underwent progressive change. The oxytocin-like peptides became linked primarily to reproduction and the vasopressin-like peptides to water and mineral metabolism. However, it remains impossible to determine the posterior pituitary peptides of extinct ancestral forms. A modern day North American opossum (Didelphis virginiana), often referred to as a living fossil, has been exposed to just as many years of evolution as any other mammalian species. One possible sequence for the derivation of the posterior pituitary hormones is presented in Figure 1. This is based on vasotocin, a hormone found in birds, reptiles, amphibia, fishes and cyclostomes.