 The oxytocin-like peptides, which differ in positions 4 and/or 8, include oxytocin (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2, with arginine-8 of AVT changed to leucine), mesotocin (arginine-8 changed to isoleucine), and isotocin (glutamine-4 changed to serine and arginine-8 changed to isoleucine); they differ from the vasopressin-like peptides in having a neutral amino acid in place of a basic amino acid at position 8.Oxytocin occurs in placental mammals; mesotocin occurs in amphibians, reptiles, and birds, and isotocin occurs in fishes.AVT is synthesized as a preprohormone that includes a second peptide, neurophysin VT (neurophysins are carrier proteins that are secreted along with their passenger hormones); intracellular proteolytic processing generates the mature peptides.