Binding and Signaling Properties of the Leu8 and Pro8 Isoforms of Oxytocin at Vasopressin V1a Receptors from Primate Species Expressing Leu8 or Pro8 Oxytocin
Authors: Toews Myron L , Schulte Nancy A , Taylor Jack H , Mustoe Aaryn C , French Jeffrey A 
Published: June 2021

Among New World monkeys, there is a remarkable correlation between multiple aspects of social behavior and their expression of an oxytocin (OT) variant with proline at the eighth position (Pro8OT) vs. the common mammalian OT ligand that has leucine at position 8 (Leu8OT). In light of evidence for OT involvement in normative and dysfunctional social behavior, a better understanding of the impact of OT ligand variation on receptor binding and signaling could provide insights into the pharmacological modulation of sociality. We previously showed that binding and calcium signaling at oxytocin receptors (OTRs) from four primate species were not different for Leu8 vs. Pro8 (Taylor JH et al., J Pharmacol Exp Ther, 367:101–107, 2018). OT also binds and activates vasopressin V1a receptors (V1aRs), whose preferred ligand is the closely related nonapeptide hormone arginine vasopressin (AVP). Thus the binding and signaling of Leu8OT and Pro8OT at V1aRs from Leu8OT‐ vs. Pro8OT‐expressing species was examined. V1aRs from marmoset (a Pro8OT species), macaque (a Leu8OT species), and human (a Leu8OT species) were expressed in CHO cells. Competition binding assays with intact cells on ice were performed using 125I‐OVTA. IC50 values in nM for Leu8OT, Pro8OT, and arginine vasopressin (AVP), respectively, were 220, 150, and 0.78 for marmoset; 28, 21, and 1.1 for macaque; and 16, 8, and 0.63 for human. Pro8OT thus has a slightly higher affinity than Leu8OT at all three receptors. Both OT isoforms exhibited about 20‐fold lower affinity than AVP for the macaque and human V1aRs but a nearly 200‐fold lower affinity for the marmoset V1aR. Calcium signaling downstream of V1aRs was assessed with FlexStation‐II assays. EC50 values in nM for Leu8OT, Pro8OT, and AVP, respectively, were 8.4, 6.7, and 0.06 for marmoset; 21, 10, and 0.03 for macaque; and 145, 110, and 2.1 for human. Thus the potency for Pro8OT is slightly higher than for Leu8OT, and the potencies for both OT isoforms are 50–100 times lower than for AVP, at all three V1aRs. These signaling potency differences are similar to the binding affinity differences for the OT isoforms at each receptor. The maximal calcium response to Leu8OT was about 80% of that for Pro8OT and AVP at the marmoset and macaque receptors; in contrast, the maximal Leu8OT and Pro8OT responses were only about 20% of the response to AVP for the human receptor, indicating that both OT isoforms are relatively weak partial agonists for the human V1aR. This difference in partial agonism at human V1aRs compared to marmoset and macaque V1aRs for both OT isoforms is intriguing. However, neither differences in V1aR ligand binding or calcium signaling provide an obvious explanation for the different behaviors of Pro8OT‐ versus Leu8OT‐expressing species, similar to our previous data showing minimal differences between the OT isoforms for binding and signaling at OTRs. Differential signaling to other downstream responses merits further investigation.Support or Funding InformationSupported by NIH grant 1‐R01‐HD089147 to JAF.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.