Ghrelin increases the rewarding value of high-fat diet in an orexin-dependent manner

Background
Ghrelin is a potent orexigenic hormone that likely impacts eating via several mechanisms. Here, we hypothesized that ghrelin can regulate extra-homeostatic, hedonic aspects of eating behavior.

Methods
In the current study, we assessed the effects of different pharmacological, physiological and genetic models of increased ghrelin and/or ghrelin signaling blockade on two classic behavioral tests of reward behavior: conditioned place preference (CPP) and operant conditioning.

Results
Using both CPP and operant conditioning, we found that ghrelin enhanced the rewarding value of high-fat diet (HFD) when administered to ad lib-fed mice. Conversely, wild-type mice treated with ghrelin receptor antagonist and ghrelin receptor-null mice both failed to show CPP to HFD normally observed under calorie restriction. Interestingly, neither pharmacologic nor genetic blockade of ghrelin signaling inhibited the body weight homeostasis-related, compensatory hyperphagia associated with chronic calorie restriction. Also, ghrelin's effects on HFD reward were blocked in orexin-deficient mice and wild-type mice treated with an orexin 1 receptor antagonist.

Conclusions
Our results demonstrate an obligatory role for ghrelin in certain rewarding aspects of eating that is separate from eating associated with body weight homeostasis and that requires the presence of intact orexin signaling.