The Orexigenic Effect of Ghrelin Is Mediated through Central Activation of the Endogenous Cannabinoid System

Abstract

IntroductionGhrelin and cannabinoids stimulate appetite, this effect possibly being mediated by the activation of hypothalamic AMP-activated protein kinase (AMPK), a key enzyme in appetite and metabolism regulation. The cannabinoid receptor type 1 (CB1) antagonist rimonabant can block the orexigenic effect of ghrelin. In this study, we have elucidated the mechanism of the putative ghrelin-cannabinoid interaction.MethodsThe effects of ghrelin and CB1 antagonist rimonabant in wild-type mice, and the effect of ghrelin in CB1-knockout animals, were studied on food intake, hypothalamic AMPK activity and endogenous cannabinoid content. In patch-clamp electrophysiology experiments the effect of ghrelin was assessed on the synaptic inputs in parvocellular neurons of the hypothalamic paraventricular nucleus, with or without the pre-administration of a CB1 antagonist or of cannabinoid synthesis inhibitors.Results and ConclusionsGhrelin did not induce an orexigenic effect in CB1-knockout mice. Correspondingly, both the genetic lack of CB1 and the pharmacological blockade of CB1 inhibited the effect of ghrelin on AMPK activity. Ghrelin increased the endocannabinoid content of the hypothalamus in wild-type mice and this effect was abolished by rimonabant pre-treatment, while no effect was observed in CB1-KO animals. Electrophysiology studies showed that ghrelin can inhibit the excitatory inputs on the parvocellular neurons of the paraventricular nucleus, and that this effect is abolished by administration of a CB1 antagonist or an inhibitor of the DAG lipase, the enzyme responsible for 2-AG synthesis. The effect is also lost in the presence of BAPTA, an intracellular calcium chelator, which inhibits endocannabinoid synthesis in the recorded parvocellular neuron and therefore blocks the retrograde signaling exerted by endocannabinoids. In summary, an intact cannabinoid signaling pathway is necessary for the stimulatory effects of ghrelin on AMPK activity and food intake, and for the inhibitory effect of ghrelin on paraventricular neurons.

Ghrelin in obesity and endocrine diseases

“…Hypothalamic AMPK has also been suggested as the mediator of the orexigenic effects of several other metabolic hormones and neuropeptides/modulators including cannabinoids (Kola et al, 2005) and glucocorticoids (Christ-Crain et al, 2008). Evidence that AMPK activity in the hypothalamus is also regulated by anorexigenic hormones: leptin (Andersson et al, 2004;Minokoshi et al, 2004) and insulin (Minokoshi et al, 2004), further demonstrates that hypothalamic AMPK is directly involved in appetite regulation.…”

“…We have shown that rat and mouse hypothalamic AMPK activity is stimulated by both central (intracerebroventricular) and peripheral (intraperitoneal) ghrelin administration (Kola et al, 2005;Kola et al, 2008). The central orexigenic effect of ghrelin is AMPK-dependent as compound C, an inhibitor of AMPK, completely prevented ghrelin's orexigenic effect, and adenoviruses harbouring α1/α2 dominant negative AMPK were able to impair the stimulatory effect of ghrelin on food intake (Lopez et al, 2008).…”

“…The central orexigenic effect of ghrelin is AMPK-dependent as compound C, an inhibitor of AMPK, completely prevented ghrelin's orexigenic effect, and adenoviruses harbouring α1/α2 dominant negative AMPK were able to impair the stimulatory effect of ghrelin on food intake (Lopez et al, 2008). The signalling pathway of ghrelin's orexigenic effect in the hypothalamus has been further elucidated with the identification of the endocannabinoid system (Kola et al, 2005;Kola et al, 2008) and calmodulin kinase kinase 2 (CaMKK2) (Anderson et al, 2008) as upstream regulators to AMPK activation. The fatty acid pathways (Lopez et al, 2008) and mitochondrial uncoupling protein 2 (UCP2) (Andrews et al, 2008), have been identified as novel downstream effectors.…”

The endocannabinoid system as a link between homoeostatic and hedonic pathways involved in energy balance regulation

“…Indeed, such a mechanism has been shown to be responsible for (1) the disinhibition of the release of the orexigenic neuropeptide, a melanin-concentrating hormone (MCH), from LH neurons through a retrograde inhibition of g-aminobutyric acid (GABA) release; 9 and (2) the inhibition of anorectic neurons in the PVN after a sequential activation of either ghrelin or 'fast' glucocorticoid receptors in these neurons (which likely produce the corticotropin-releasing hormone (CRH)), the biosynthesis and release of ECs, and subsequent retrograde CB 1 -mediated inhibition of glutamate release from parvocellular neurons innervating these neurons. 10,11 In some cases, 9,10 these retrograde orexigenic actions of ECs are under the negative control of post-synaptic leptin receptors that reduce EC biosynthesis by decreasing intracellular calcium. In fact, it had been shown earlier that hypothalamic EC levels are inhibited by an intravenous injection of leptin in rats, an effect that is likely responsible for the permanently elevated EC levels in the hypothalamus of obese rodents that lack leptin (ob/ob mice) or fully functional leptin receptors (db/db mice and fa/fa Zucker rats).…”

Ghrelin neutralization during fasting-refeeding cycle impairs the recuperation of body weight and alters hepatic energy metabolism

“…It has been demonstrated that administration of ghrelin to rats affects AMPK activity, although this effect is tissue-specific. In the hypothalamus, central and peripheral ghrelin treatments enhance protein levels of AMPK and this activation and alterations on lipid metabolism mediate the Page 17 of 37 A c c e p t e d M a n u s c r i p t 17 orexigenic effects of ghrelin (Andersson et al 2004;Kohno et al 2008;Kola et al 2005Kola et al , 2008Lopez et al 2008). In fed animals peripheral chronic and acute ghrelin treatments provoked AMPK activation in the heart, while in liver and adipose tissue it was inhibited and no effect was detected on skeletal muscle (Barazzoni et al 2005;Kola et al 2005).…”