The goal of the present project is to gain an understanding of the role of the brain in the regulatory biology of stress responsiveness, focusing upon homeostatic mechanisms of glucocorticoid secretion. The ability to cope with stress is of fundamental importance to the survival of all organisms, and in mammals is known to involve the HPA axis. Inputs indicative of stress converge upon a chemically distinct collection of neurons located within the PVN; these neurons, which synthesize CRH and other secretagogues, initiate a cascade of events culminating in the adrenocorticotropic hormone (ACTH)-induced release of glucocorticoid hormones from the adrenals. Glucocorticoids have widespread and potent effects throughout the body, readying the organism for changes required for coping. Maintenance of appropriate levels of these steroid hormones is of critical importance to human health, as either too much or too little is deleterious. Dysregulation of the HPA axis has been linked to several forms of psychiatric illness, including major depression, and is thought to contribute to age-related neurodegeneration and cognitive decline. While the brain is involved in the moment-to-moment regulation of glucocorticoid secretion, the specific neural pathways which mediate stress effects remain to be firmly established. The present proposal will examine brain pathways hypothesized to mediate inhibitory and stimulatory components of the stress response, focusing upon PVN afferent systems that utilize the neurotransmitters GABA and glutamate, respectively. These studies will investigate the hypothesis that GABAergic and glutamatergic neuronal circuits regulate the HPA axis through effects exerted upon the stress-integrative CRH neurons of the PVN. The present studies will: (1) establish the roles of GABA and glutamate on the CRH neuron at the level of cellular activation, transcriptional regulation, and secretion, (2) uncover the precise origins of stress-responsive GABAergic and glutamatergic inputs to the PVN, testing their functional impact on multiple parameters of HPA activity, as well as effects in mediating steroid feedback inhibition, and (3) establish the ability of defined circuits to replay effects of limbic structures implicated in HPA activity. An integrated approach to this investigation will be taken, including combinations of pharmacological, biochemical, molecular biological and neuroanatomical methods. The results are expected to reveal brain pathways which serve as key elements of control over basal and stress-induced glucocorticoid secretion, elucidating mechanisms by which these steroid hormones are regulated.
