Evolution of the Neurohypophysial Hormones, with Reference to Amphibians

Abstract

THE neurohypophysial hormones of two European species of amphibians, the frog Rana esculenta^1,2 and the toad Bufo bufo³, have been chemically identified. One of these, vasotocin (Arg₈-oxytocin), is identical to the hormone found in birds⁴ or in bony fish^5–7. The other, mesotocin (Ile₈-oxytocin), has a structure intermediate between that of isotocin (Ser₄-Ile₈-oxytocin) found in all the groups of bony fishes, and that of oxytocin found in mammals. It may be presumed that mesotocin represents a transition molecule in the evolution of the oxytocic principles (for a review of this subject see ref. 8). If this opinion is well founded, then mesotocin must be widely distributed among amphibians and could exist in certain reptiles. Indeed, mesotocin has been recently isolated from a snake, the viper Vipera aspis⁹. Besides, on the basis of pharmacological data, mesotocin is believed to exist in an African anuran^10,11, in several species of the urodeles as well as in the lungfishes¹⁰. Other biological evidence suggests, however, that the leopard frog Rana pipiens has oxytocin and not mesotocin¹². Unlike the other amphibians chemically studied up to now, R. pipiens lives on the American continent and it might reflect a particular form of evolution. We have isolated the oxytocin-like hormone of this species to make a comparison.