The formation and maintenance of social relationships increases nonapeptide mRNA in zebra finches of both sexes.

The nonapeptides oxytocin and vasopressin are believed to be involved in affiliation across species, but converging evidence is lacking. In monogamous zebra finches, oxytocin antagonists decrease pairing. The goal of the present study was to test if this relationship is bidirectional. We predicted that pairing would increase mesotocin (MT) and vasotocin (VT) mRNA (avian homologues of oxytocin and vasopressin) at nonapeptide neurosecretory sites: the paraventrivular nucleus (PVN) of the hypothalamus, which contains both MT and VT, and the bed nucleus of the stria terminalis (BSTm), which contains VT. To test our hypotheses, zebra finches of both sexes paired for 48 hr or 2 weeks in a naturalistic choice paradigm. Birds that did not pair or were not given the opportunity to pair were included as control groups. Pairing increased VT and MT mRNA in the PVN compared to both control groups. In the BSTm, animals paired for 48 hr had more cells expressing VT mRNA than did animals not given the opportunity to pair, and males had higher VT mRNA than did females. In males, singing primarily explained variations in MT mRNA, as well as VT mRNA in both the PVN and BSTm. In females, pairing behaviors explained variations in MT, but did not explain variations in VT in either brain region. Our results provide evidence that the relationship between nonapeptides and pairing are bidirectional and may be sexually differentiated.