[d-Lys3]-GHRP-6, a ghrelin receptor antagonist, reveals CRF signaling maintains glucose in calorie-restricted mice via the sympathetic-ghrelin pathway.
Background
During periods of severe calorie restriction (CR), the body must activate compensatory mechanisms to prevent hypoglycemia and maintain energy homeostasis. The ghrelin-growth hormone axis is a key player in this process. Ghrelin, a peptide primarily produced in the stomach, is known to stimulate appetite and growth hormone release, but its role in glucose maintenance under duress is critical. The sympathetic nervous system up-regulates ghrelin secretion in negative energy states. Separately, central corticotrophin-releasing factor receptor (CRF-R) signaling is a primary driver of the stress response and is known to stimulate sympathetic tone. This study investigates the missing link: whether this central CRF-R stress signaling directly drives the ghrelin-mediated pathway to defend plasma glucose levels during severe energy deficit, a crucial survival mechanism.
Study Design
The study used a mouse model to investigate the pathway. First, researchers administered intracerebroventricular (i.c.v.) injections of urocorin-1 and urocorin-2 (natural CRF-R ligands) to assess the direct impact on plasma ghrelin. Next, they established a 60% calorie restriction (CR) model in mice. In these CR mice, they administered [d-Lys3]-GHRP-6 (a ghrelin receptor antagonist), atenolol (a β1 adrenergic receptor antagonist), or alpha helical CRH (a non-selective CRF-R antagonist) to block different points in the proposed pathway. The primary endpoint was the measurement of plasma glucose concentrations, with rescue experiments involving co-administration of ghrelin to confirm the mechanism.
Results
Central activation of corticotrophin-releasing factor receptors directly stimulated ghrelin release. Intracerebroventricular injections of urocorin-1 and urocorin-2, ligands for CRF-R1 and CRF-R2 respectively, elevated plasma ghrelin concentrations. This effect was mediated by the sympathetic nervous system, as the ghrelin elevation from urocorin-1 was cancelled by co-administration of atenolol, a β1 adrenergic antagonist. In the 60% calorie-restricted mouse model, blocking the ghrelin receptor had a direct impact on blood sugar maintenance.
Administration of the ghrelin receptor antagonist [d-Lys3]-GHRP-6 in calorie-restricted mice reduced plasma glucose concentration more significantly compared to vehicle-treated mice. Similarly, blocking sympathetic signaling with atenolol or central CRF signaling with alpha helical CRH also significantly reduced plasma glucose. Crucially, these glucose drops were rescued by the co-administration of ghrelin, confirming that the glucose-maintaining effect of the central CRF-sympathetic axis is dependent on downstream ghrelin action.
Key Findings
- Administration of the ghrelin receptor antagonist [d-Lys3]-GHRP-6 significantly reduced plasma glucose in 60% calorie-restricted mice compared to vehicle.
- Central injection of CRF receptor antagonist alpha helical CRH significantly lowered plasma glucose in CR mice.
- The glucose reduction caused by the CRF antagonist was rescued by co-administration of ghrelin, maintaining plasma glucose levels.
- The ghrelin increase from the CRF-R1 agonist urocorin-1 was completely cancelled by atenolol (a β1 adrenergic antagonist).
Why It Matters
This research clarifies a specific neuro-hormonal survival circuit that maintains blood glucose during severe energy deficit. For individuals undergoing aggressive dieting, fasting, or experiencing conditions mimicking severe calorie restriction, this highlights the body's reliance on the ghrelin system, orchestrated by central stress pathways (CRF). Blocking ghrelin signaling, for example with an antagonist like [d-Lys3]-GHRP-6, could dangerously impair the ability to maintain glycemic control under these conditions. This finding has implications for the safety of therapies that might target the ghrelin receptor, suggesting that their use should be carefully considered in populations at risk of hypoglycemia. It also provides a mechanistic rationale for why managing central stress (CRF signaling) might be important for metabolic stability during periods of fasting or caloric restriction, linking the brain's stress response directly to peripheral energy regulation.
