Hindbrain ghrelin receptor signaling is sufficient to maintain fasting glucose

Abstract

The neuronal coordination of metabolic homeostasis requires the integration of hormonal signals with multiple interrelated central neuronal circuits to produce appropriate levels of food intake, energy expenditure and fuel availability. Ghrelin, a peripherally produced peptide hormone, circulates at high concentrations during nutrient scarcity. Ghrelin promotes food intake, an action lost in ghrelin receptor null mice and also helps maintain fasting blood glucose levels, ensuring an adequate supply of nutrients to the central nervous system. To better understand mechanisms of ghrelin action, we have examined the roles of ghrelin receptor (GHSR) expression in the mouse hindbrain. Notably, selective hindbrain ghrelin receptor expression was not sufficient to restore ghrelin-stimulated food intake. In contrast, the lowered fasting blood glucose levels observed in ghrelin receptor-deficient mice were returned to wild-type levels by selective re-expression of the ghrelin receptor in the hindbrain. Our results demonstrate the distributed nature of the neurons mediating ghrelin action.

Figure 1. GHSR mRNA expression is restored selectively in the hindbrain in GHSR-null/Phox2b mice.

Coronal sections of a representative wild-type mouse brain demonstrating expression in the nucleus of the solitary tract, area postrema and vagus motor nerve (A, NTS, AP, DMV), nucleus ambiguus (B, Amb), 7th nerve (C, nVII) and the arcuate nucleus of the hypothalamus (D, ARH). Coronal sections of a representative GHSR-null/Phox2b mouse brain demonstrating re-activated GHSR expression in all hindbrain nuclei (E-G) but not in the ARH (H).

Figure 2. Phox2b-Cre-mediated GHSR re-expression does not restore CNS c-fos induction.

Ghrelin (2 µg/g s.c.) induces robust c-fos expression in the arcuate nucleus (A) and the paraventricular nucleus (B) of the hypothalamus and in the area postrema and nucleus of the solitary tract of the hindbrain (C) in wild-type mice. Saline does not induce c-fos in the brains of wild-type mice (D-F). Ghrelin administration fails to induce c-fos expression in GHSR-null (G-I) and GHSR-null/Phox2b mice (J−L). (n = 3) 3v, third ventricle. cc, central canal.

Figure 3. Phox2b-Cre-mediated GHSR re-expression fails to normalize ghrelin-stimulated feeding but restores fasting glucose levels.

Ghrelin (2 µg/g s.c.) potently induces 2-hr food intake when compared to saline injection in wild-type but not in GHSR-null mice or in GHSR null/Phox2b cre mice. (n = 7, * = P<0.05 One-way ANOVA with Tukey’s post-hoc test) (A). Hindbrain-selective GHSR expression restores fasting blood glucose to that of wild type. (n = 25, * = P<0.05 1-way ANOVA with Tukey’s post-hoc test C).