Stress Plasticity of CRH Neurons - Summary Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis and altered circulating glucocorticoid levels are closely linked to the development of psychiatric illness, and males and females show differential sensitivity to acute stress and susceptibility to mental health disorders. Corticotropin-releasing hormone (CRH) neurons of the hypothalamic paraventricular nucleus (PVN) control activation of the HPA axis and direct stress and associated behaviors. Changes in CRH neuron synaptic circuits result in altered HPA activity and glucocorticoid secretion, which cause changes in physiological homeostasis and behavioral outputs. Brainstem noradrenergic circuits control CRH neuron activity and HPA activation in response to interoceptive stress exposure, such as immune challenge, while limbic circuits activate CRH neurons in response to psychological stress. Yet, little is known about the sex-dependent input-output organization of CRH neuronal circuits in the hypothalamus and how they regulate somatic vs. psychological stress activation of the HPA axis and sex- specific stress behaviors. Our previous studies in male mice revealed novel mechanisms of somatic stress activation of the HPA axis via noradrenergic stimulation of a CRH neuronal-glial circuit and dendritic retrograde volume transmission that stimulates local synaptic circuits in the PVN. We also found a stress-induced glucocorticoid feedback suppression of the somatic stress activation of the HPA axis in male mice via desensitization of the CRH neurons to norepinephrine (NE) due to rapid glucocorticoid-induced trafficking of 1 adrenoreceptors out of the membrane. This rapid glucocorticoid effect is specific to somatic stress activation of the HPA axis. Recent preliminary findings indicate that males and females respond differently to NE, but still little is known about the sex-dependent stress regulation of the CRH neurons and their afferent and efferent circuit organization. Here, we will use a combination of ex vivo patch clamp recordings and fluorescence imaging of CRH neurons, activity-dependent tagging and monosynaptic retrograde tracing of CRH neuronal circuits in vivo, and stress-associated behavioral assessment to test the hypothesis that discrete somatic and psychological stress circuits activate distinct CRH neuron populations, which in turn project to different areas of the brain and drive different acute stress behaviors. Specific Aim 1 will expand on findings from the previous funding period to determine the sex-dependent mechanisms of NE activation of PVN CRH neurons. Specific Aim 2 will determine whether CRH neuron subpopulations in the PVN are activated by discrete afferent circuits driven by somatic vs. psychological stressors. And finally, Specific Aim 3 will extend this CRH neuron circuit analysis to determine the downstream targets of the somatic and psychological stress- activated subpopulations of CRH neurons and their respective impacts on stress-related behaviors. Together, these studies will fill a critical gap in our understanding of the circuit organization of hypothalamic CRH neurons important for stress modality-specific regulation of the HPA axis and stress-associated behaviors.
