Abstract
Regulation of food intake and energy expenditure is crucial to maintain a stable body weight. Excessive energy intake combined with inadequate energy expenditure leads to obesity, which is co-morbid with many devastating diseases such as cancer, cardiovascular diseases, and infertility. The hypothalamus plays a pivotal role in the regulation of energy balance. It integrates signals from both the external environment and internal milieu to modulate feeding and energy expenditure to maintain energy homeostasis. Neuropeptides are critical molecular mediators underlying many important energy homeostatic functions of the hypothalamus. Thus, a detailed knowledge of the role of hypothalamic peptides and their receptors in controlling feeding, metabolism, and energy expenditure is essential to understand the causes of obesity and related metabolic disorders. Urocortin 3 (Ucn 3) is a member of the corticotropin-releasing factor (CRF) family of peptides. The CRF family plays a critical role in coordinating aspects of the stress response including stimulation of the hypothalamic-pituitary-adrenal axis, behavioral arousal, and energy adaptation through suppression of feeding and promotion of energy mobilization. Centrally-injected Ucn 3 suppresses feeding, elevates blood glucose concentration and body temperature, and stimulates the hypothalamic-pituitary-adrenal axis, indicating that central Ucn 3 may be involved in regulating energy homeostasis and the stress response. Cell bodies of neurons expressing Ucn 3 are located in the hypothalamus and medial amygdala. Direct injection of Ucn 3 into the ventromedial hypothalamus (VMH) has been shown to potently suppress feeding and rapidly elevate blood glucose levels. Anatomical studies revealed that Ucn 3 neurons in the anterior parvicellular part of the paraventricular nucleus of the hypothalamus (PVHap) provide the major Ucn 3 afferent input into the VMH, therefore it is conceivable that the Ucn 3 PVHap-VMH pathway may play a critical role in modulating energy homeostasis in response to stress. Currently, little is known about the Ucn 3 neurons in the PVHap. In my thesis studies, I combined functional and anatomical approaches to characterize these neurons in great detail. I first showed that acute stress rapidly stimulates Ucn 3 expression in the PVHap. A functional neuroanatomical tracing study identified a number of brain areas that provide stress-activated input into the PVHap area. In the forebrain, the bed nucleus of the stria terminalis, lateral septal nucleus, the medial amygdala, and a number of nuclei in the hypothalamus including the VMH, the arcuate nucleus, the posterior nucleus, and the ventral premammillary nucleus provide stress-activated input into the PVHap. In the brainstem, stress-sensitive input originates from the periaqueductal gray, the nucleus of the solitary tract, and the ventrolateral medulla. These areas are potentially important in mediating the stress-induced activation of Ucn 3 neurons in the PVHap. I then determined that Ucn 3 neurons in the PVHap do not express oxytocin, CRF, or vasopressin, major neuropeptides expressed in the PVH that have been shown to play an important role in regulating energy balance. A conditional viral tracing study then confirmed that Ucn 3 neurons in the PVHap project prominently to the VMH, an area important for feeding and sympathetic outflow. I also determined that the PVHap Ucn 3 cells project to the external zone of the median eminence where neuropeptides are released from nerve terminals into the portal blood system to influence release of hormones from the pituitary gland, raising the possibility that PVHap Ucn 3 cells may be involved in neuroendocrine regulation of pituitary function. Finally, I attempted to test the hypothesis that enhanced Ucn 3 input into the VMH from the PVHap can recapitulate energy adaptation induced by stress. I found that enhanced Ucn 3 expression in the PVHap resulted in suppression of basal feeding and elevation of circulating glycerol, indicative of enhanced lipolysis. In conclusion, these studies thoroughly characterized Ucn 3 neurons in the PVHap and provide significant insight into the hypothalamic Ucn 3 neurocircuit in regulating stress-associated energy adaptation.
