In humans there are three main arginine vasopressin receptors; AVPR1A, 1B and 2. AVPR1A (Thibonnier et al. 1994) and AVPR1B (Sugimoto et al. 1994) act as receptors for the signal peptide Arg-vasopressin (AVP(20-28)), a cleaved peptide from the precursor AVP protein (Mohr et al. 1985, Sausville et al. 1985) and are expressed mainly in the brain. The 1A and 1B forms use G protein alpha q/11 subunits as their second messenger system.