Defining how the agonist–receptor interaction differs from that of the antagonist–receptor and understanding the mechanisms of receptor activation are fundamental issues in cell signalling. The V1a vasopressin receptor (V1aR) is a member of a family of related G-protein coupled receptors that are activated by neurohypophysial peptide hormones, including vasopressin (AVP). It has recently been reported that anarginyl in the distalN-terminus of theV1a Riscritical for binding agonists but not antagonists
