Diet-induced obesity causes ghrelin resistance in arcuate NPY/AgRP neurons

The current obesity epidemic is a growing public health care concern across all ages and socioeconomic groups in developed and developing countries alike. To date, obesity research has focused on mechanisms that normally curb hyperphagia. We have taken the novel approach of studying the mechanisms that stimulate hyperphagia in the obese situation, by focussing our attention on the stomach-derived hormone ghrelin. Ghrelin is the only systemic orexigenic peptide, and promotes food intake and blood glucose under conditions of negative energy balance. As its role in obesity is not well understood, the research objective of this thesis was to characterise how the ghrelin system functions in diet-induced obese (DIO) mice. The premise was that if ghrelin contributes to the development and maintenance of obesity, then the ghrelin system presents a potential therapeutic target for the treatment of this disease. The first part of this thesis characterised ghrelin sensitivity in a model of DIO mice. We show for the first time that DIO mice are profoundly ghrelin resistant. We demonstrate that ghrelin resistance occurs in arcuate neuropeptide Y (NPY)/agouti-related peptide (AgRP) neurons as a consequence of a downregulation of the neuroendocrine ghrelin axis and impaired NPY/AgRP neuronal function. Together, these mechanisms prevent NPY/AgRP neurons from responding to ghrelin and initiating feeding behaviour. Importantly, failure of NPY/AgRP neurons to respond to ghrelin also extends to other orexigenic stimuli, such as fasting. We then established the timecourse over which this occurs, to identify mechanisms that contribute to ghrelin resistance. We show that ghrelin resistance develops after three weeks of high fat diet feeding, and is preceded by adipose tissue gain and increased plasma leptin. We hypothesised that leptin may impair ghrelin signalling in NPY/AgRP neurons. We show that leptin-deficient ob/ob mice are ghrelin sensitive in spite of obesity, and that leptin suppresses ghrelin-induced food intake in this model. vii This supports our hypothesis that increased circulating leptin in DIO mice suppresses ghrelin sensitivity in NPY/AgRP neurons. Further, after three weeks of high fat diet-feeding leptin signalling remains intact, though attenuated, in NPY/AgRP neurons. Thus, we have identified leptin as a potential mechanism by which DIO causes ghrelin resistance occurs in NPY/AgRP neurons. The final part of the thesis examines whether ghrelin resistance in ARC NPY/AgRP neurons is reversible. We show that diet-induced weight loss with calorie restriction increases plasma ghrelin along with hypothalamic NPY and AgRP mRNA expression and restores ghrelin sensitivity. This likely contributes to rebound weight gain commonly seen in human patients. Diet-induced weight loss without calorie restriction has milder effects; this diet paradigm does not increase orexigenic tone and only partially restores ghrelin sensitivity. Thus, losing weight without calorie restriction is likely better for the long-term maintenance of a lower body weight. The studies presented here highlight an emerging role for NPY/AgRP neuronal dysfunction in the pathophysiology of obesity. Ultimately, these studies further our understanding of how ghrelin regulates food intake in the obese and post-obese state and we hope that they will improve the clinical management of weight loss maintenance therapies.