Hypothalamic Mechanisms of Food Intake in Birds

Abstract

Appetite is a complex behavior which can be influenced by factors within the animal's body as well as the environment around it. Internal factors include hormonal and nutrient concentrations found in the blood stream and subsequent neuropeptide and neurotransmitter signaling in the hypothalamus. External factors, such as high ambient temperature (HAT), can indirectly affect appetite regulation through other neuroendocrine systems such as the hypothalamo-pituitary-adrenal (HPA) axis. Understanding the physiological responses to endogenous factors and HAT exposure in birds will have implications in both the agricultural and biomedical fields. Thus, the purpose of this dissertation research was to explore the hypothalamic molecular mechanisms associated with food intake in broiler type chickens and Japanese quail, and the effect of HAT exposure on food intake in broiler type chickens. Broiler type chickens have undergone intense artificial selection for traits that promote rapid growth, consequently driving them to consume feed incessantly. It is hypothesized that broiler type chickens lack a mechanism that signals satiety, causing them to eat significantly more than layer type chickens. Selection for rapid growth of meat (muscle tissue) has made the broiler more susceptible to the deficits associated with HAT exposure, as animals composed primarily of muscle dissipate less heat while also producing more heat than those composed of fat. The Japanese quail have undergone relatively minor artificial selection compared to the chicken, suggesting that use of this model may provide insight into the mechanisms of appetite regulation in wild-type bird species. This research involved administrating appetite associated factors into the avian brain via an intracerebroventricular (ICV) injection including gastrin releasing peptide, vasoactive intestinal polypeptide, neuropeptide AF, and prostaglandin D2. Additionally, I explored the effects of HAT on food intake, and on the efficacy of several ICV administered appetite associated factors including neuropeptide Y, corticotropin releasing factor and α-melanocyte stimulating hormone. After treatment administration, I measured changes in food intake and behavior, activation of hypothalamic nuclei including the arcuate nucleus, dorsomedial nucleus, lateral hypothalamus, paraventricular nucleus, and the ventromedial nucleus, and the nucleus of the hippocampal commissure. I then measured changes in gene expression in both whole hypothalamic samples and specific hypothalamic nuclei. The data from non-HAT associated studies provided information on the hypothalamic nuclei which respond to the various appetite associated factors and the molecular mechanisms mediating changes in appetite. The data from the HAT study provided information on the hypothalamic nuclei involved in the avian response to HAT exposure, and the molecular mechanisms involved in the effect on food intake. Overall, these data provide insight on the mechanisms associated with short-term regulation of appetite, and pathways associated with stress and food intake.