Project Summary/Abstract Psychological stress can induce activation of the hypothalamic-pituitary-adrenal (HPA) axis, impairing the function of multiple biological systems and posing a risk to mental and physical health. In contrast, positive social interactions, especially social support from deeply rooted social bonds, can ameliorate stress-induced mental, physiological, and behavioral deficits and improve an individual's overall well-being—a phenomenon known as social buffering [1, 2]. Such social buffering effects have been described in both human [3-5] and animal models [6, 7]. Although we have begun to understand the neuromechanisms underlying biobehavioral responses to psychological stress, little is known about the neuromechanisms by which social support buffers the stress response [1]. This is largely due to the difficulties inherent in studying neurobiological mechanisms in humans as well as a lack of appropriate animal models to assess the effects of social buffering. Recently, the socially monogamous prairie vole (Microtus ochrogaster) has emerged as an animal model to study the neurobiology of social behavior. In prairie voles, mating induces pair bonding, which is regulated by several neurochemicals including oxytocin (OT), vasopressin (AVP), corticotrophin releasing hormone (CRH), and gamma-aminobutyric acid (GABA) [8, 9]. Pair bonding reduces stress-induced anxiety-like behavior by attenuating the action of the HPA axis [10]. Interactions with the partner also promote the release of central OT [11], which plays a role in attenuating the biobehavioral response to stress in female voles [12]. Using this unique animal model, we propose, in Specific Aim 1, to examine how social buffering by a sibling cage mate or a bonding partner attenuates immobilization (IMO)-induced stress responses in male and female prairie voles. We will examine the effects of social buffering on (1) anxiety-like, depression-like, and affiliative behaviors, (2) circulating levels of corticosterone (CORT), (3) CRH, OT, AVP, GABA, and their receptors gene and protein expression in the paraventricular nucleus of the hypothalamus (PVN), and (4) neurochemical release in the PVN during IMO and social buffering. In Specific Aim 2, we will perform pharmacological manipulations with behavioral testing to examine the functional role of PVN OT, and its interactions with GABA, CRH and AVP, in the social buffering of the stress response. In Specific Aim 3, we will examine the neurochemical and physiological involvement of PVN neuromicrocircuitry in the regulation of social buffering. Data from this study will not only enhance our understanding of sex differences in the neurochemical regulation of social buffering of stress responses but also further establish a much needed animal model for such investigation.