Local Corticotropin-Releasing Factor Signaling in the Hypothalamic Paraventricular Nucleus.

Corticotropin-releasing factor (CRF) neurons in the hypothalamic paraventricular nucleus (PVN) initiate hypothalamic-pituitary-adrenal axis activity through the release of CRF into the portal system as part of a coordinated neuroendocrine, autonomic, and behavioral response to stress. The recent discovery of neurons expressing CRF receptor type 1 (CRFR1), the primary receptor for CRF, adjacent to CRF neurons within the PVN, suggests that CRF also signals within the hypothalamus to coordinate aspects of the stress response. Here, we characterize the electrophysiological and molecular properties of PVN-CRFR1 neurons and interrogate their monosynaptic connectivity using rabies virus-based tracing and optogenetic circuit mapping in male and female mice. We provide evidence that CRF neurons in the PVN form synapses on neighboring CRFR1 neurons and activate them by releasing CRF. CRFR1 neurons receive the majority of monosynaptic input from within the hypothalamus, mainly from the PVN itself. Locally, CRFR1 neurons make GABAergic synapses on parvocellular and magnocellular cells within the PVN. CRFR1 neurons resident in the PVN also make long-range glutamatergic synapses in autonomic nuclei such as the nucleus of the solitary tract. Selective ablation of PVN-CRFR1 neurons in male mice elevates corticosterone release during a stress response and slows the decrease in circulating corticosterone levels after the cessation of stress. Our experiments provide evidence for a novel intra-PVN neural circuit that is activated by local CRF release and coordinates autonomic and endocrine function during stress responses. SIGNIFICANCE STATEMENT The hypothalamic paraventricular nucleus (PVN) coordinates concomitant changes in autonomic and neuroendocrine function to organize the response to stress. This manuscript maps intra-PVN circuitry that signals via CRF, delineates CRF receptor type 1 neuron synaptic targets both within the PVN and at distal targets, and establishes the role of this microcircuit in regulating hypothalamic-pituitary-adrenal axis activity.