We recently reviewed the status of peptide and nonpeptide agonists and antagonists for the V1a, V1b and V2 receptors for arginine vasopressin (AVP) and the oxytocin receptor for oxytocin (OT). In the present review, we update the status of peptides and nonpeptides as: (i) research tools and (ii) therapeutic agents. We also present our recent findings on the design of fluorescent ligands for V1b receptor localisation and for OT receptor dimerisation. We note the exciting discoveries regarding two novel naturally occurring analogues of OT. Recent reports of a selective VP V1a agonist and a selective OT agonist point to the continued therapeutic potential of peptides in this field. To date, only two nonpeptides, the V2/V1a antagonist, conivaptan and the V2 antagonist tolvaptan have received Food and Drug Administration approval for clinical use. The development of nonpeptide AVP V1a, V1b and V2 antagonists and OT agonists and antagonists has recently been abandoned by Merck, Sanofi and Pfizer. A promising OT antagonist, Retosiban, developed at Glaxo SmithKline is currently in a Phase II clinical trial for the prevention of premature labour. A number of the nonpeptide ligands that were not successful in clinical trials are proving to be valuable as research tools. Peptide agonists and antagonists continue to be very widely used as research tools in this field. In this regard, we present receptor data on some of the most widely used peptide and nonpeptide ligands, as a guide for their use, especially with regard to receptor selectivity and species differences.

G protein–coupled receptor (GPCR) oligomers have been proposed to play critical roles in cell signaling, but confirmation of their existence in a native context remains elusive, as no direct interactions between receptors have been reported. To demonstrate their presence in native tissues, we developed a time-resolved FRET strategy that is based on receptor labeling with selective fluorescent ligands. Specific FRET signals were observed with four different receptors expressed in cell lines, consistent with their dimeric or oligomeric nature in these transfected cells. More notably, the comparison between FRET signals measured with sets of fluorescent agonists and antagonists was consistent with an asymmetric relationship of the two protomers in an activated GPCR dimer. Finally, we applied the strategy to native tissues and succeeded in demonstrating the presence of oxytocin receptor dimers and/or oligomers in mammary gland.

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Using a three-dimensional model of G protein-coupled receptors (GPCR), we have previously succeeded in docking the neurohypophysial hormone argininevasopressin (AVP) into the Vla receptor. According to this model, the hormone is completely embedded in the transmembrane part of the receptor. Only the side chain of the Arg residue at position 8 projects outside the transmembrane core of the receptor and possibly interacts with a Tyr residue located in the first extracellular loop at position 115. Residue 8 varies in the two natural neurohypophysial hormones, AVP and oxytocin (OT); similarly, different residues are present at position 115 in the different members of the AVP/ OT receptor family. Here we show that Arg8 is crucial for high affinity binding of AVP to the rat Vla receptor. Moreover, when Tyrl15 is replaced by an Asp and a Phe, the amino acids naturally occurring in the V2 and in the OT receptor subtypes, the agonist selectivity of the Vla receptor switches accordingly. Our results indicate that the interaction between peptide residue 8 and the receptor residue at position 115 is not only crucial for agonist high affinity binding but also for receptor selectivity.