Figure 1.

The oxytocin (OT) and vasopressin (VP) pathway includes the OT receptor (OTR) and the V1a receptor (V1aR). We hypothesize that in a context of perceived safety, OT predominately acts on the OTR, facilitating “immobility without fear,” including high levels of social engagement, social bonds, and social reward; these behaviors are at the heart of mammalian reproduction and “love.” VP and the V1aR are more ancient and probably become dominant under conditions of anxiety or trauma. In a context of anxiety or fear, OT may function primarily through effects on the V1aR; under these conditions both OT and VP may act, via the V1aR, to induce additional anxiety, social avoidance, defensiveness, aggression, and fear. We hypothesize that under extreme conditions, fear and the V1aR may dominate leaving the individual vulnerable to “immobility with fear,” which may lead to freezing and cognitive and emotional dissociation. These responses are mediated in part by interactive effects of OT and VP on the sympathetic nervous system and the parasympathetic nervous system, including the ventral vagal complex (necessary for social engagement) and the dorsal vagal complex (functioning to conserve energy and protect against shutting down in the face of trauma) (5). Other components of this adaptive system including the V1bR, and many other molecules or receptors, including those regulated by CRH, dopamine, opioids, GABA, and serotonin, play a role in the expression of social and defensive behaviors. The differential actions of OT and VP are dose, time, and brain-region dependent. The OT and V1a receptors are affected by genetics and epigenetic tuning, especially in early life.