PROJECT SUMMARY Responses to stressors are regulated by a network of limbic forebrain structures, whereas dysfunction in these neural systems following chronic conditions has been widely implicated in the pathogenesis of stress-related psychiatric illness. To date, there is virtually no information accounting for central effectors of chronic stress- induced endocrine and behavioral modifications, nor has there been any attempt to rescue normal functioning of key circuits after chronic stress. Work from our laboratory over the past funding period suggests that the medial prefrontal cortex provides top-down inhibitory control over hypothalamo-pituitary-adrenal (HPA) effector neurons in the paraventricular hypothalamic nucleus (PVH) during acute stress, via a disynaptic pathway involving GABAergic neurons in the bed nuclei of the stria terminalis (BST). However, no information is currently available regarding the neural circuit mechanisms in the genesis of exaggerated HPA responses upon subsequent exposure to novel challenges (i.e., sensitization). Additionally, recent preliminary data suggests that BST plays a broader role in coordinating both endocrine and behavioral coping responses during inescapable stress (e.g., tail suspension and forced swim), via dissociable pathways involving the PVH and periaqueductal gray area (PAG). Therefore, our objective in this renewal application is to manipulate these circuit elements to elucidate the mechanisms of chronic stress-induced HPA sensitization and increased passive coping responses to a subsequent novel challenge. These studies will combine optogenetics and neurophysiology to build on our existing strengths using anatomical and behavioral approaches, to manipulate putative stress modulatory networks in rats. In Aim 1, we will interrogate the divergent pathways from BST to PVH and PAG in mediating the maladaptive HPA and behavioral changes following chronic variable stress exposure. Aim 2 will examine whether diminished prefrontal control over descending pathways to BST and/ or PAG account for chronic stress-induced maladaptive HPA and behavioral alterations. This work will advance our thinking of how different features of stress responses are coordinated, and will provide a clearer picture of how dysfunction in modulatory brain circuits may lead to chronic stress-related dysfunction of multiple systems as is common in psychiatric illnesses such as depression.