Jason J. Radley, Ph.D. - US grants

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. PUBLIC HEALTH RELEVANCE: While it is widely hypothesized that dysregulation of brain systems controlling stress hormone function may be important for understanding the pathophysiology of major depression and other stress-related disease states, their underlying mechanisms have remained elusive. This proposal will (1) utilize animal studies to significantly enhance our understanding of the organization of brain circuitry regulating the stress response, and (2), will help to identify novel anatomical targets and cellular processes underlying the development of depression- related endocrine changes. In doing so, it is hoped that this information will help in the development of more effective treatments of stress-related psychiatric and systemic illnesses.

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.

Project Summary Optimal functioning of the medial prefrontal cortex (mPFC) relies on synaptic connections made onto dendritic spines in pyramidal neurons. Prefrontal dysfunction resulting from chronic stress and stress-related psychiatric illnesses are each linked to decreases in dendritic spine number and shape alterations in this cortical region. Work from our laboratory and others has shown that chronic stress and elevated glucocorticoids, the end products of the hypothalamo-pituitary-adrenal stress axis activation, induce structural deficits marked by dendritic spine loss in mPFC and impaired prefrontal cognitive functions. While some progress has been made in elucidating the biochemical and genetic components underlying disrupted prefrontal plasticity in animal models of psychiatric illnesses, more work is needed to identify the cellular mechanisms accounting for these changes to help develop targets for therapeutic intervention. In this regard, recent consideration has been given to the idea that mitochondrial deficiencies may contribute to chronic stress-induced cognitive impairments and the pathogenesis of human psychiatric disorders. A-kinase anchoring protein 1 (AKAP1) is a mitochondrial scaffolding protein that recruits protein kinase A (PKA) to the outer mitochondrial membrane leading to phosphorylation and inactivation of the mitochondrial fission protein, dynamin-related protein 1 (Drp1). We have previously shown that AKAP1 increases mitochondrial membrane potential, an indicator of the cell's ability to generate ATP by oxidative phosphorylation, whereas deletion of AKAP1 leads to mitochondrial fission and dendritic spine loss in cortical neurons. These observations have culminated in the novel hypothesis that chronic stress-induced alterations in prefrontal structural and functional plasticity are mediated by diminished AKAP1 signaling in mPFC neurons. Therefore, the goal of this exploratory/developmental R21 is to examine the role of AKAP1 in chronic stress-induced dendritic spine loss and functional compromise in mPFC neurons, thus developing a data set for a future R01. In aim 1, we will interrogate whether CVS's adverse effects on prefrontal dendritic spine structure are accompanied by AKAP1 loss, Drp1 dephosphorylation/activation, and mitochondrial fragmentation. Aim 2 will use AKAP1 KO mice and lentiviral delivery of wild type and PKA-binding deficient AKAP1 to identify the signaling mechanisms accounting for disruption of prefrontal mitochondrial, dendritic spine, and behavioral alterations. The long-term goal of this line of research is to elucidate the key mechanisms linking mitochondrial dynamics to stress-related prefrontal dysfunction, as this is a common underlying feature of stress-related psychiatric disorders such as major depressive illness.

Project Summary Responses to stress 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 function after chronic stress. Work in the field implicates the medial prefrontal cortex in providing 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, our preliminary data suggest that BST plays a broader role in coordinating both endocrine and behavioral coping responses during a variety of challenges (e.g., tail suspension, forced swim, shock probe defensive burying tests), via dissociable pathways involving the PVH and periaqueductal gray area (PAG). Therefore, our objective in this proposal is to manipulate these circuit elements to elucidate the mechanisms of chronic stress-induced HPA sensitization and shift toward passive behavioral responses to subsequent challenges. These studies will combine optogenetics and neurophysiology to build on our existing strengths in 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.