Circuit and cellular mechanisms of chronic stress-induced HPA axis hyperactivity

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DESCRIPTION (provided by applicant): It is widely appreciated that dysregulation of brain systems controlling stress hormone function may underlie the development of major depression, and other stress-related disease states. The medial prefrontal cortex (mPFC) and hippocampal formation (HF) are known to negatively regulate the hypothalamo-pituitary-adrenal (HPA) axis, and their dysfunction is implicated in the mood and neuroendocrine disturbances in depression and in animal studies of chronic stress. Over the years, progress in developing interventions that target neuroendocrine systems has been lacking, due to the inability to unravel the complex neurocircuitry and mechanisms underlying the withdrawal of mPFC and HF restraining influences following chronic stress. Our recent identification of a discrete GABAergic cell group in the anterior subdivision of the bed nucleus of the stria terminalis (aBST), serves as a disynaptic relay between limbic cortical and the paraventricular hypothalamic nucleus (PVH), and is capable of integrating these stress-inhibitory influences over HPA output during acute emotional stress, allows us to directly examine the role of this novel pathway in controlling depression-related neuroendocrine changes. We hypothesize that chronic stress-induced neuroplasticity (i.e., dendritic/axonal retraction, synapse/terminal loss) in mPFC leads to a disruption in their normal restraining influences on the HPA axis, via decreasing their innervation of key disynaptic inhibitory relays (e.g., involving aBST) to PVH. Aim 1 will assess the involvement of this new circuit implicated in HPA axis modifications following chronic variable stress exposure, and immunotoxin ablation of key GABAergic relays in these circuits will test the involvement of this pathway in chronic stress-induced endocrine alterations. Aim 2 will utilize high- resolution microscopy, digital reconstructions, and stereology for the assessment of structural plasticity (dendritic, spine density, axon terminal alterations) in mPFC/HF neurons, specific to the circuitry implicated in HPA axis modifications, following chronic variable stress. Aim 3 will test whether blockade of GC receptors in mPFC/HF prevents circuit-specific neuroplasticity, and, in turn prevent HPA axis hyperactivity, following chronic variable stress. These studies are expected to (a) define a basis for inhibitory circuit disruptions implicated in the withdrawal of HPA restraining influences following chronic variable stress, and (b) to assess the relationship between limbic cortical neuroplasticity and the endocrine abnormalities associated with chronic stress. It is hoped that identification of novel neuroanatomical targets and underlying cellular processes in this proposal will inform the search for more effective treatments for stress-related psychiatric and systemic disorders.