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Undeniable phenotypic similarities exist between patients suffering from Cushing’s syndrome (caused by excess circulating glucocorticoids (GCs)) and patients suffering from idiopathic obesity. Obese patients display normal circulating GC levels, but exhibit raised intracellular GC levels in several tissues. Intracellular GC levels are predominantly modulated by the enzyme 11β-hydroxysteroid dehydrogenase 1 (11βHSD1), which catalyses the conversion of inactive-GC (in rodents this is 11-dehydrocorticosterone) to active-GC (in rodents this is corticosterone). Glucocorticoids play a physiological role in the regulation of energy homeostasis, which is thought to be mediated via the hypothalamus; however the exact effects of GCs and their sites of action are unclear, partially due to the difficulty of manipulating GC levels in discrete hypothalamic nuclei. To overcome this, recombinant adeno-associated virus (rAAV) to overexpress 11βHSD1 (rAAV-11βHSD1) was stereotaxically injected into the arcuate nucleus (ARC) and the paraventricular nucleus (PVN) of male Wistar rats, to investigate the effects of a sustained increase in GC levels in these nuclei. Injection of rAAV-11βHSD1 successfully increased expression of 11βHSD1 mRNA in the hypothalamus, and significantly increased corticosterone levels in the target nuclei. In all experiments, circulating levels of corticosterone were unchanged, suggesting that HPA axis activity was unaffected by raised hypothalamic corticosterone levels. Animals injected with rAAV-11βHSD1 intra-ARC displayed a significant increase in cumulative food intake, bodyweight and white adipose tissue (WAT) compared to controls. Additionally, intra-ARC microinjection of rAAV-11βHSD1 reduced whole-body glucose tolerance and insulin sensitivity at the beginning and mid-point of the study period. Together, these results suggest that hypothalamic GCs play a physiological role in appetite regulation and glucose homeostasis, and that these effects are mediated via the ARC. Interestingly, effects on food intake and bodyweight were ameliorated when the experiment was repeated with animals fed a high-fat diet. Animals injected with rAAV-11βHSD1 intra-PVN displayed a significant increase in bodyweight gain compared to controls. This increase was not associated with altered food intake, suggesting that hypothalamic GCs play a physiological role in the regulation of energy expenditure, an effect mediated via the PVN. These experiments provide novel information about the role of hypothalamic GCs in energy homeostasis. They suggest that endogenous GCs exert nucleus-specific effects and act via alternate mechanisms in the hypothalamus to physiologically regulate food intake, glucose homeostasis, and energy expenditure. |
