The Role of the Ghrelin System in the Metabolic Responses to Exercise

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PROJECT SUMMARY Exercise is often highly effective for prevention of weight gain, for prevention of weight regain after weight loss, and for weight loss, however the hormonal and neuronal mediators that influence the complicated and highly variable exercise-associated changes in appetite and food intake and the exercise-associated effects on energy expenditure and related physiology such as exercise endurance, are poorly understood. In the current proposal, we aim to investigate the role of the hormone ghrelin and its receptor, the growth hormone secretagogue receptor (GHSR), in mediating the effects of exercise on food intake as well as in mediating exercise endurance. The proposal follows up on recently published findings in mice demonstrating that exercise transiently raises circulating levels of the hormone ghrelin and that without the action of this increased ghrelin, post-exercise food intake and exercise endurance are both markedly diminished. The proposal uses a unique collection of genetically-engineered mouse models to investigate the mechanisms regulating exercise- induced ghrelin release and the neuronal sites and other mechanisms through and by which ghrelin and its receptor mediate metabolic responses to exercise and exercise endurance. In particular, we will determine if ghrelin-dependent or ghrelin-independent (constitutive) signaling via GHSRs is required for the metabolic responses to exercise by evaluating the effects of ghrelin deletion and a GHSR mutation (which eliminates GHSR constitutive activity) on food intake, energy expenditure, neuronal excitability, and exercise endurance. We will use a mouse model lacking β1-adrenergic receptors exclusively in ghrelin cells to determine if activation of these ghrelin cell β1-adrenergic receptors, as induced by the sympathoadrenal system, is required for the ghrelin response to exercise. We also will use mice lacking GHSRs selectively from arcuate hypothalamic AgRP neurons or ventromedial hypothalamic SF1 neurons and mice with chemogenetically- inhibited mediobasal hypothalamic GHSR-expressing neurons to determine if those populations of hypothalamic neurons mediate the exercise-associated effects of the ghrelin system on metabolism and exercise endurance. Our studies will provide fundamental insight into the metabolic effects of exercise, exercise endurance, arcuate hypothalamic AgRP and POMC neuronal excitability, ghrelin cell physiology, and ghrelin action. Ultimately, we hope that studying the role of the ghrelin system in the metabolic responses to exercise may allow us to determine and therapeutically harness the mechanisms by which exercise promotes whole-body health.