Peripheral leptin and ghrelin receptors are regulated in a tissue-specific manner in activity-based anorexia

Abstract

The aim of this research was to investigate the effect of long-term exposure to low leptin and high ghrelin levels, inherent to activity-based anorexia (ABA), on peripheral metabolism-implicated tissues such as muscle and fat depots. For this purpose, rats under ABA were submitted to a global study which included the characterization of body weight and composition change, the evaluation of leptin and ghrelin levels as well as their receptors expression at peripheral level. Our results confirm that feeding restriction to 1 h per day, and particularly the combination of this fasting regime with exercise (ABA), significantly reduces fat mass, decreases leptin circulating levels, increases ghrelin levels strikingly and enhances insulin sensitivity. By direct in vitro assays, we show that visceral and gonadal fat participate more than subcutaneous fat in the hypoleptinemia of these animals. The study of ghrelin (GHS-R1a) and leptin (LEPR) receptors at peripheral level exhibits a tissue-specific expression pattern. Concretely, oxidative-soleus type of muscle appears to be more susceptible to ghrelin and leptin circulating levels than glycolytic-gastrocnemius type under exercise and food restriction situations. In relation to adipose tissue, chronic hyperghrelinemia induces GHS-R1a expression on visceral and subcutaneous fat which might suggest the prevention of lipid loss. On the other hand, only subcutaneous fat express the active long form of LEPR compared to visceral and gonadal fat under low leptin levels in ABA animals. All together, these findings indicate tissue-specific mechanisms for the control of energy homeostasis in response to nutrient and energy availability.

Introduction

Obesity and human eating disorders such as anorexia nervosa (AN) and bulimia have become a major problem in industrialized countries [23]. Eating disorders are a group of severe psychiatric disorders that affect mainly young women and can have grave consequences on their lives and their families. These illnesses are characterized by an altered eating behavior with desire for slimness that can cause serious physical and psychological symptoms and death [23]. Worryingly, the outcome in these disorders remains still poor and available treatments have little if any effect [9].

The control of energy intake and body weight is characterized by a complex interaction between peripheral organs involved in energy homeostasis such as adipose and muscle tissues, thyroid, liver and gastrointestinal tract, and the central nervous system (CNS). Peripheral tissues inform to the hypothalamic appetite regulating centers about the nutritional and metabolic status through several circulating signaling factors being glucose, insulin, leptin and ghrelin part of the most relevant [4]. Leptin is secreted by adipocytes and plays an important role in regulating food intake, energy expenditure and adiposity. It reflects total body adipose tissue mass, and in situations of negative and positive energy balance, the variations of plasma leptin concentration function as a sensor of energy balance influencing the efferent energy regulation pathways [16]. The receptors for leptin (LEPR) are expressed principally in afferent satiety centers at the central nervous system of hypothalamus and in peripheral organs such as adipose tissues, skeletal muscles, pancreatic beta-cells and liver, indicating the autocrine and paracrine role of leptin in energy regulation [1], [17]. On the other hand, ghrelin is mainly produced by the stomach but also by many other tissues [12]. In addition to its ability to stimulate GH release, ghrelin shows many other functions such as stimulating appetite and food intake, increasing fat mass deposition and weight gain, regulating gastric motility and acid secretion, modulating cell proliferation, and influencing glucose and lipid metabolism. The widespread distribution of ghrelin and its receptor (GHS-R) expression in central and peripheral tissues is consistent with all these actions [8], [14]. The deregulation of all these mechanisms would be responsible for obesity and other disorders like anorexia.

The use of animal models simulating human eating disorders have been proved to mimic human food intake alterations and their usefulness for research is now recognized [11]. Activity-based anorexia (ABA) is a well established animal model used to study different aspects of anorexia nervosa (AN) and situations of undernutrition plus increased activity [31]. This rodent AN model has been described as the most promising animal model able to mimic AN leading to pathological weight loss [11]. In this model, anorexia is experimentally induced by restricting food intake to one daily feeding period (1–3 h) and giving free access to running wheels during the remaining of the day (21–23 h). Animals under this situation exhibit a reduction in food intake and an increase in wheel activity, progressive body weight diminution, disruption of the oestrous cycle, hypothermia, and will eventually die of emaciation [6], [31].

Despite all the issues mentioned above, there is a need for a deeper characterization of these animal models to improve our knowledge about food related disorders. Although there are several descriptive investigations about the effect of different factors, such as nutritional status and physical exercise on peripheral signals implicated in homeostasis regulation, there is not a clear picture yet. The study of the hormonal changes in animal models under food restriction plus exercise would increase our insight on energy homeostasis mechanisms. This may allow not just a better understanding and treatment of anorexia nervosa patients, but it may be also valuable for opposite situations such obesity. In the present work we undertook a detailed study of food restriction, exercise and ultimately anorexia on rat models analyzing leptin and ghrelin mechanism of action at peripheral level. The presented results suggest a tissue-specific mechanism for the control of energy homeostasis in response to nutrient and energy availability, and a failure of the adaptive feeding response in extreme situations such as activity-based anorexia.