Mechanisms of hypothalamic regulation of food intake in birds

Energy homeostasis is essential for survival across all vertebrate species and involves a multitude of physiological systems that are regulated by both central and peripheral neural signaling. The hypothalamus is responsible for integrating and processing these signals and thus is regarded as the regulatory center for balancing energy homeostasis. Eating disorders, such as compulsive eating behavior associated with obesity, and anorexia, are significant public health concerns worldwide. Thus, studying appetite regulation is necessary to provide novel information for the design of solutions for health concerns that stem from altered energy intake. Such information is also relevant for improving chicken health and productivity in an agricultural setting. The objective of this dissertation research was to determine the hypothalamic mechanisms underlying appetite regulation in birds. In Experiment 1, the Virginia lines of chickens were used to elucidate the mechanisms underlying stress-induced anorexia. These chickens have been selected for low (LWS) or high (HWS) body weight at 56 days of age and have different severities of anorexia and obesity, respectively. Chicks were subjected to a combination of thermal and nutritional stress after hatch and hypothalamic nuclei, including the lateral hypothalamus (LH), paraventricular nucleus (PVN), ventromedial hypothalamus (VMH), and arcuate nucleus (ARC), were collected 5 days later. Real-time PCR was used to measure the mRNA abundance of appetite-associated neuropeptides and receptors in each nucleus. The results showed that the two lines displayed distinct gene expression profiles in response to stress. In particular, the PVN of the LWS was significantly affected by stress, and expression of several anorexigenic factors was up-regulated including corticotropin-releasing factor (CRF), CRF receptor sub-types 1 and 2 (CRFR1 and CRFR2, respectively), melanocortin receptor 4, and urocortin 3, suggesting that stress-induced anorexia in the LWS may result from overriding anorexigenic signaling in the PVN, primarily through CRF signaling. This CRF signaling-associated hypothesis was further supported by results showing that the original phenotypes were restored when the LWS chicks were treated with astressin (CRF receptor antagonist) before exposure to stress. In Experiments 2 and 3, we attempted to determine the mechanisms of CRF's anorexigenic effect in chickens and Japanese quail. We administered CRF by intracerebroventricular (ICV) injection and the hypothalamus was collected 1 hour later for molecular analyses. Results showed that CRF exerted a similar inhibitory effect on food intake in these two bird species, however the hypothalamic mechanisms underlying this anorexigenic effect were different. ICV injection of CRF increased c-Fos expression in the PVN, VMH, dorsomedial nucleus (DMN), and ARC in chicks while it only affected the PVN and LH in quail. Hypothalamic gene expression results suggested that CRF decreased neuropeptide Y receptor sub-type 1 (NPYR1) in chicks while it increased proopiomelanocortin (POMC), MC4R, CRF, and CRFR2 in quail. These results suggested that the anorexigenic effect of CRF may involve a dampened neuropeptide Y (NPY) system in chicks whereas it is associated with activated CRF and melanocortin systems in quail. At the nucleus level in chicks, CRF injection decreased NPY system-associated gene expression (ARC and DMN) and increased CRF (ARC and PVN) and mesotocin (MT) (VMH)-associated mRNAs, suggesting that orexigenic signaling through NPY was overridden by the heightened anorexigenic tone through CRF and MT, which led to the inhibition of food intake. In Experiments 4 and 5, we used the same experimental design as for CRF studies to determine the hypothalamic mechanisms of the anorexigenic effects of neuropeptide K (NPK) and adrenomedullin (AM) in Japanese quail. Results from Experiment 4 showed that NPK injection activated the ARC and PVN, which was associated with increased mRNAs for a group of anorexigenic factors including CRF, UCN3, cocaine and amphetamine-regulated transcript (CART), and POMC, and decreased expression of several orexigenic factors, such as NPY and agouti-related peptide (AgRP). In Experiment 5, ICV injection of AM activated the ARC, the nucleus in which POMC and CART mRNAs were increased. In conclusion, these experiments revealed novel hypothalamic mechanisms underlying stress or exogenous neuropeptide-induced anorexia in birds and may provide insights on understanding appetite regulation from evolutionary, agricultural, and biomedical perspectives.