Vasopressin docking and transmembrane topology of the human V1 vascular receptor (V1R). A model of arginine vasopressin (AVP), as bound to the human V1R, is depicted. Vasopressin is shown in ball-and-stick representation and the receptor is shown in ribbons. The intracellular loops of the receptor are labeled il1, il2, and il3, and the extracellular loops are labeled el1, el2, and el3. The transmembrane segments are labeled H1–H7. Reprinted from Thibonnier M, Coles P, Thibonnier A, Shoham M: Molecular pharmacology and modeling of vasopressin receptors. Prog Brain Res 2002, 139:179–196. © 2002, with permission from Elsevier .