Non-synaptically released oxytocin regulates social communication by acting on vasopressin V1a receptors

How neuropeptides act within the neural circuits that control social behavior is not well understood. While the prevailing view is that neuropeptides act through synaptic release and then activation of their canonical receptors on postsynaptic membranes, we investigated the role of a very different form of neuropeptide action in a neural circuit regulating social communication. Specifically, we tested the hypothesis that non-synaptically released oxytocin (OT) can act via the non-canonical receptors vasopressin V1a receptors (V1aR) to regulate social communication in Syrian hamsters. Scent marking, a key form of hamster social communication, can be enhanced by the α-melanocortin stimulating hormone (α-MSH), which stimulates OT but not arginine-vasopressin (AVP) release. Here, we employed hypothalamic injections of α-MSH and the α-MSH MC4R receptor antagonist MCL-0020 to determine the role of α-MSH in the expression of flank marking. To determine if these effects were intracellular calcium (iCa²⁺) dependent, hamsters were injected with AVP to induce flank marking and with the iCa²⁺ antagonist TMB-8 to test whether it was possible to block this behavioral effect. Further, a highly selective AVP V1a receptor (V1aR) antagonist and an OT receptor (OTR) antagonist were injected into the hypothalamus to investigate the receptor responsible for activating flank marking. Finally, we employed an in vitro hypothalamic slice preparation using "Sniffer cells" biosensors to confirm that α-MSH induced the release of OT but not AVP. First, we found that the in vivo hypothalamic injection of α-MSH increased odor-stimulated scent marking, whereas blockade of its receptor with MCL-0020 reduced this behavior. Hypothalamic infusion of the iCa²⁺ antagonist TMB-8 significantly reduced both AVP-induced and α-MSH-induced flank marking. Moreover, only the V1aR antagonist, and not the OTR antagonist, significantly decreased scent marking in response to hypothalamic infusion of α-MSH. Finally, biosensor recordings from hypothalamic slices confirmed that α-MSH stimulates OT, but not AVP, release. Together, these results demonstrate that α-MSH triggers non-synaptic OT release that regulates scent marking via V1aR activation, revealing a novel mechanism by which neuropeptides modulate social behavior.