Effects of chronic social defeat on behavioral and neural correlates of sociality: Vasopressin, oxytocin and the vasopressinergic V1b receptor.

Chronic social stress in rodents produces behavioral and neuroendocrine patterns analogous to symptoms associated with psychopathologies in humans. Chronic social defeat in mice has been used to study the genetic and epigenetic precursors of stress-related social disorders. The neuropeptides arginine vasopressin (AVP) and oxytocin (OT) are released in central targets to modulate anti- and pro-social behaviors, respectively. AVP binds to V1a and V1b receptors (V1bRs) in discrete brain regions related to anxiety, depression and affiliative behaviors. Recent evidence suggests that V1bRs are involved in stress and anxiety and may be an attractive target for the treatment of associated disorders. In the present series of experiments, we aimed to evaluate the effects of chronic social defeat stress on: 1) anxiety-related behaviors in a social investigation paradigm and their potential modulation by an acute dose of SSR149415, a V1bR antagonist; 2) AVP and Fos protein levels in the paraventricular nucleus of the hypothalamus (PVN) and; 3) AVP- and OT-receptor (OTR) mRNA levels in brain regions associated with sociality. When compared to undefeated animals, socially defeated mice exhibited an anxiogenic behavioral profile towards a novel male conspecific, with SSR149415 partly attenuating these effects. Histochemistry using immunofluorescence showed defeat produced significant elevations of Fos and double labeling of AVP and Fos proteins in the paraventricular nucleus of the hypothalamus (PVN). SSR149415 attenuated the effects of defeat on Fos and AVP/Fos double labeling, consistent with an anxiolytic effect. Defeated mice showed elevated levels of OTR mRNA levels in the lateral septum (LS) in addition to increased V1bR and OTR mRNA in the medial amygdala (MeA). We suggest the involvement of V1bRs and OTRs in a circuit involving the PVN, MeA and LS in the effects of defeat on sociality. SSR149415 attenuated anxiogenesis in the social investigation model and both Fos and AVP/Fos labeling, suggesting V1bRs are an attractive target for the treatment of anxiety in general and disorders of sociality in particular.