Uncovering Neural Circuitry Involved in Fear

Fear is an emotional response that helps animals survive in the face of danger. Fear responses are characterized by specific behaviors, such as immobilization, and physiological changes mediated by the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis is activated by the release of corticotropin releasing hormone (CRH) from  CRH neurons in the paraventricular nucleus (PVN) of the hypothalamus. CRH leads to secretion of ACTH from the pituitary and, subsequently, stress hormones from the adrenal cortex. Previous studies have linked dysregulation of the HPA axis to a number of human stress-related pathologies, including major depressive disorder. Thus, precise regulation of the HPA axis is important for appropriate fear responses and possibly mental health. The mechanisms and neural circuits that regulate the HPA axis are still unclear. To obtain anatomical and molecular insight into regulatory mechanisms in the HPA axis, CRH neurons were injected with conditional “tracer/reporter” viruses that travel retrogradely through chains of connected neurons to infect neurons upstream to CRH neurons. The virus-infected neurons are being analyzed for their locations and markers of excitatory or inhibitory neurons. We are using immunofluorescence to detect virus-infected neurons and in situ hybridization to detect markers of glutamatergic or GABAergic neurons. We have thus far identified several brain areas that contain inhibitory neurons upstream of CRH neurons. The results obtained from this study could give us a better understanding of the neural circuitry regulating the HPA axis and eventually lead to more effective therapeutic modalities for psychiatric disorders.