Scheduled feeding results in adipogenesis and increased acylated ghrelin

The intersection between ghrelin, metabolism and circadian rhythms

Despite the growing popular interest in sleep and diet, many gaps exist in our scientific understanding of the interaction between circadian rhythms and metabolism. In this Review, we explore a promising, bidirectional role for ghrelin in mediating this interaction. Ghrelin both influences and is influenced by central and peripheral circadian systems. Specifically, we focus on how ghrelin impacts outputs of circadian rhythm, including neuronal activity, circulating growth hormone levels, locomotor activity and eating behaviour. We also consider the effects of circadian rhythms on ghrelin expression and the consequences of disrupted circadian patterns, such as shift work and jet lag, on ghrelin secretion. Our Review is aimed at both the casual reader interested in gaining more insight into the scientific context surrounding the trending topics of sleep and metabolism, as well as experienced scientists in the fields of ghrelin and circadian biology seeking inspiration and a comprehensive overview of how these fields are related.

Chronic food restriction in mice and increased systemic ghrelin induce preference for running wheel activity

OBJECTIVES

In eating disorders, particularly anorexia nervosa (AN), patients exhibit intense physical activity which is inappropriate regarding food restriction and chronic undernutrition, and exacerbates weight loss and energy deprivation. Rodent models of food restriction exhibit increased running wheel activity in the food anticipation period, also known as Food Anticipatory Activity (FAA). FAA probably has various physiological and/or neurobiological origins. Plasma concentrations of the orexigenic hormone ghrelin are, for example, increased during FAA. We hypothesize that the drive for physical activity in chronic food restriction is triggered by metabolic factors but also relies on motivational aspects that we aim to decipher in this study.

METHODS

Young female C57Bl6/J mice were exposed to a paradigm based on a progressive 50% quantitative food restriction alone (FR) or associated with running wheel activity (Food Restriction Wheel: FRW) in their home-cage during 15 days. We measured preference for running wheel in a three-chamber apparatus in which animals could choose to explore either a known running wheel or a novel object. Testing took place either during resting or during FAA. We calculated the time spent in each compartment and the activity in running wheels. After progressive refeeding over 10 days, mice were tested again when refed. Plasma levels of both ghrelin isoforms were measured with selective immunoassays.

RESULTS

When tested during FAA period, food restricted mice displayed increased preference for the running wheel compared to ad libitum fed controls. Both FR and FRW mice exhibited increased running time and distance in the wheel and running distance was correlated with ghrelin levels. Similar preference and behavior were found when testing took place during the resting period. Animals housed without an active wheel also exhibited active running. Progressive refeeding resulted in body weight restoration, a decrease in FAA and completely abolished preference for the running wheel. Refed animals displayed similar behavior as ad libitum fed controls.

CONCLUSIONS

These data provide evidence that food restriction-induced physical activity is closely correlated with metabolic adaptations to nutritional status implicating ghrelin in the quantity of physical activity.

Effect of Growth Hormone Secretagogue Receptor Deletion on Growth, Pulsatile Growth Hormone Secretion, and Meal Pattern in Male and Female Mice

INTRODUCTION

While the vast majority of research investigating the role of ghrelin or its receptor, GHS-R1a, in growth, feeding, and metabolism has been conducted in male rodents, very little is known about sex differences in this system. Furthermore, the role of GHS-R1a signaling in the control of pulsatile GH secretion and its link with growth or metabolic parameters has never been characterized.

METHODS

We assessed the sex-specific contribution of GHS-R1a signaling in the activity of the GH/IGF-1 axis, metabolic parameters, and feeding behavior in adolescent (5-6 weeks old) or adult (10-19 weeks old) GHS-R KO (Ghsr-/-) and WT (Ghsr+/+) male and female mice.

RESULTS

Adult Ghsr-/- male and female mice displayed deficits in weight and linear growth that were correlated with reduced GH pituitary contents in males only. GHS-R1a deletion was associated with reduced meal frequency and increased meal intervals, as well as reduced hypothalamic GHRH and NPY mRNA in males, not females. In adult, GH release from Ghsr-/- mice pituitary explants ex vivo was reduced independently of the sex. However, in vivo pulsatile GH secretion decreased in adult but not adolescent Ghsr-/- females, while in males, GHS-R1a deletion was associated with reduction in pulsatile GH secretion during adolescence exclusively. In males, linear growth did not correlate with pulsatile GH secretion, but rather with ApEn, a measure that reflects irregularity of the rhythmic secretion. Fat mass, plasma leptin concentrations, or ambulatory activity did not predict differences in GH secretion.

DISCUSSION/CONCLUSION

These results point to a sex-dependent dimorphic effect of GHS-R1a signaling to modulate pulsatile GH secretion and meal pattern in mice with different compensatory mechanisms occurring in the hypothalamus of adult males and females after GHS-R1a deletion. Altogether, we show that GHS-R1a signaling plays a more critical role in the regulation of pulsatile GH secretion during adolescence in males and adulthood in females.

A new understanding of GHSR1a--independent of ghrelin activation

Growth hormone secretagogue receptor 1a (GHSR1a), a member of the G protein-coupled receptor (GPCR) family, is a functional receptor of ghrelin. The expression levels and activities of GHSR1a are affected by various factors. In past years, it has been found that the ghrelin-GHSR1a system can perform biological functions such as anti-inflammation, anti-apoptosis, and anti-oxidative stress. In addition to mediating the effect of ghrelin, GHSR1a also has abnormally high constitutive activity; that is, it can still transmit intracellular signals without activation of the ghrelin ligand. This constitutive activity affects brain functions, growth and development of the body; therefore, it has profound impacts on neurodegenerative diseases and some other age-related diseases. In addition, GHSR1a can also form homodimers or heterodimers with other GPCRs, affecting the release of neurotransmitters, appetite regulation, cell proliferation and insulin release. Therefore, further understanding of the constitutive activities and dimerization of GHSR1a will enable us to better clarify the characteristics of GHSR1a and provide more therapeutic targets for drug development. Here, we focus on the roles of GHSR1a in various biological functions and provide a comprehensive summary of the current research on GHSR1a to provide broader therapeutic prospects for age-related disease treatment.

Ghrelin receptor in agouti-related peptide neurones regulates metabolic adaptation to calorie restriction

Ghrelin is a gut hormone that signals to the hypothalamus to stimulate growth hormone release, increase food intake and promote fat deposition. The ghrelin receptor, also known as growth hormone secretagogue receptor (GHS-R), is highly expressed in the brain, with the highest expression in agouti-related peptide (AgRP) neurones in the hypothalamus. Compelling evidence indicates that ghrelin serves as a survival hormone with respect to maintaining blood glucose and body weight during nutritional deficiencies. Recent studies have demonstrated that AgRP neurones are involved in metabolic and behavioural adaptation to an energy deficit to improve survival. In the present study, we used a neuronal subtype-specific GHS-R knockout mouse (AgRP-Cre;Ghsr^f/f ) to investigate the role of GHS-R in hypothalamic AgRP neurones in metabolic and behavioural adaptation to hypocaloric restricted feeding. We subjected the mice to a restricted feeding regimen of 40% mild calorie restriction (CR), with one-quarter of food allotment given in the beginning of the light cycle and three-quarters given at the beginning of the dark cycle, to mimic normal mouse intake pattern. The CR-fed AgRP-Cre;Ghsr^f/f mice exhibited reductions in body weight, fat mass and blood glucose. Metabolic profiling of these CR-fed AgRP-Cre;Ghsr^f/f mice showed a trend toward reduced basal metabolic rate, significantly reduced core body temperature and a decreased expression of thermogenic genes in brown adipose tissue. This suggests a metabolic reset to a lower threshold. Significantly increased physical activity, a trend toward increased food anticipatory behaviour and altered fuel preferences were also observed in these mice. In addition, these CR-fed AgRP-Cre;Ghsr^f/f mice exhibited a decreased counter-regulatory response, showing impaired hepatic glucose production. Lastly, hypothalamic gene expression in AgRP-Cre;Ghsr^f/f mice revealed increased AgRP expression and a decreased expression of genes in β-oxidation pathways. In summary, our data suggest that GHS-R in AgRP neurones is a key component of the neurocircuitry involved in metabolic adaptation to calorie restriction.

Metabolic and neuroendocrine adaptations to undernutrition in anorexia nervosa: from a clinical to a basic research point of view

The exact mechanisms linking metabolic and neuroendocrine adaptations to undernutrition and the pathophysiology of anorexia nervosa (AN) are not fully understood. AN is a psychiatric disorder of complex etiology characterized by extreme starvation while the disease is progressing into a chronic state. Metabolic and endocrine alterations associated to this disorder are part of a powerful response to maintain whole body energy homeostasis. But these modifications may also contribute to associated neuropsychiatric symptoms (reward abnormalities, anxiety, depression) and thus participate to sustain the disease. The current review presents data with both a clinical and basic research point of view on the role of nutritional and energy sensors with neuroendocrine actions in the pathophysiology of the disease, as they modulate metabolic responses, reproductive functions, stress responses as well as physical activity. While clinical data present a full description of changes occurring in AN, animal models that integrate either spontaneous genetic mutations or experimentally-induced food restriction with hyperactivity and/or social stress recapitulate the main metabolic and endocrine alterations of AN and provide mechanistic information between undernutrition state and symptoms of the disease. Further progress on the central and peripheral mechanism involved in the pathophysiology of eating disorders partly relies on the development and/or refinement of existing animal models to include recently identified genetic traits and better mimic the complex and multifactorial dimensions of the disease.

Clarifying the Ghrelin System's Ability to Regulate Feeding Behaviours Despite Enigmatic Spatial Separation of the GHSR and Its Endogenous Ligand

Ghrelin is a hormone predominantly produced in and secreted from the stomach. Ghrelin is involved in many physiological processes including feeding, the stress response, and in modulating learning, memory and motivational processes. Ghrelin does this by binding to its receptor, the growth hormone secretagogue receptor (GHSR), a receptor found in relatively high concentrations in hypothalamic and mesolimbic brain regions. While the feeding and metabolic effects of ghrelin can be explained by the effects of this hormone on regions of the brain that have a more permeable blood brain barrier (BBB), ghrelin produced within the periphery demonstrates a limited ability to reach extrahypothalamic regions where GHSRs are expressed. Therefore, one of the most pressing unanswered questions plaguing ghrelin research is how GHSRs, distributed in brain regions protected by the BBB, are activated despite ghrelin’s predominant peripheral production and poor ability to transverse the BBB. This manuscript will describe how peripheral ghrelin activates central GHSRs to encourage feeding, and how central ghrelin synthesis and ghrelin independent activation of GHSRs may also contribute to the modulation of feeding behaviours.

[Ghrelin: a gastric hormone at the crossroad between growth and appetite regulation]

Ghrelin is a 28 amino acid peptide hormone synthesized within the gastrointestinal tract. Initially identified as the endogenous ligand of the GHS-R1a (Growth Hormone Secretagogue Receptor 1a), ghrelin is a powerful stimulator of growth hormone (GH) secretion. At the crossroad between nutrition, growth and long-term energy metabolism, ghrelin also plays a unique role as the first identified gastric hormone increasing appetite and adiposity. However, the role of the ghrelin/GHS-R system in the physiology of growth, feeding behaviour and energy homeostasis needs to be better understood. Utilization of pharmacological tools and complementary animal models with deficiency in preproghrelin, ghrelin-O-acyl-transferase (GOAT - the enzyme that acylates ghrelin -) or GHS-R in situations of chronic undernutrition or high fat diet gives a more precise overview of the role of ghrelin in the pathophysiology of eating and metabolic disorders.

Increased Eating Frequency Is Associated with Lower Obesity Risk, But Higher Energy Intake in Adults: A Meta-Analysis

Body weight is regulated by energy intake which occurs several times a day in humans. In this meta-analysis, we evaluated whether eating frequency (EF) is associated with obesity risk and energy intake in adults without any dietary restriction. Experimental and observational studies published before July 2015 were selected through English-language literature searches in several databases. These studies reported the association between EF and obesity risk (odd ratios, ORs) in adults who were not in dietary restriction. R software was used to perform statistical analyses. Ten cross-sectional studies, consisting of 65,742 participants, were included in this analysis. ORs were considered as effect size for the analysis about the effect of EF on obesity risk. Results showed that the increase of EF was associated with 0.83 time lower odds of obesity (i.e., OR = 0.83, 95% confidence intervals (CI) 0.70-0.99, p = 0.040). Analysis about the effect of EF on differences in participants' energy intake revealed that increased EF was associated with higher energy intake (β = 125.36, 95% CI 21.76-228.97, p = 0.017). We conclude that increased EF may lead to lower obesity risk but higher energy intake. Clinical trials are warranted to confirm these results and to assess the clinical practice applicability.

Ghrelin: A link between memory and ingestive behavior

Feeding is a highly complex behavior that is influenced by learned associations between external and internal cues. The type of excessive feeding behavior contributing to obesity onset and metabolic deficit may be based, in part, on conditioned appetitive and ingestive behaviors that occur in response to environmental and/or interoceptive cues associated with palatable food. Therefore, there is a critical need to understand the neurobiology underlying learned aspects of feeding behavior. The stomach-derived "hunger" hormone, ghrelin, stimulates appetite and food intake and may function as an important biological substrate linking mnemonic processes with feeding control. The current review highlights data supporting a role for ghrelin in mediating the cognitive and neurobiological mechanisms that underlie conditioned feeding behavior. We discuss the role of learning and memory on food intake control (with a particular focus on hippocampal-dependent memory processes) and provide an overview of conditioned cephalic endocrine responses. A neurobiological framework is provided through which conditioned cephalic ghrelin secretion signals in neurons in the hippocampus, which then engage orexigenic neural circuitry in the lateral hypothalamus to express learned feeding behavior.

Repeated allopregnanolone exposure induces weight gain in schedule fed rats on high fat diet

Ingestion of energy rich high fat diets is one of the determining factors associated with the obesity epidemic. Therefore, much can be learned from studies of obesity-related substances given to animals fed a high fat diet. The progesterone metabolite allopregnanolone is a potent positive modulator of the gamma-aminobutyric acid (GABA)A-receptor, and both allopregnanolone and GABA have been implicated in evoking hyperphagia. In this study, food intake and body weight gain were investigated during repeated allopregnanolone exposure. Male Wistar rats were studied when fed chow ad libitum, with chow access for 4h per day or with 45% high fat pellets for 4h per day. Rats on the high fat diet were separated into obesity prone and obesity resistant individuals. Subcutaneous injections of allopregnanolone were given once daily over five consecutive days. Repeated exposure to allopregnanolone lead to increased weight gain, significantly so in schedule fed rats on a high fat diet. The increased weight gain was correlated to an increased energy intake. Both obesity resistant and obesity prone rats responded to allopregnanolone with increased weight gain. Obesity resistant rats treated with allopregnanolone increased their energy intake and ate as much as vehicle treated obesity prone rats. Their weight gain was also increased to the level of obesity prone rats injected with just the vehicle carrier oil. Thus, it appears that allopregnanolone may be one of the endogenous factors involved in weight gain, especially when the diet is rich in fat.

Cognitive and neuronal systems underlying obesity

Since the late 1970s obesity prevalence and per capita food intake in the USA have increased dramatically. Understanding the mechanisms underlying the hyperphagia that drives obesity requires focus on the cognitive processes and neuronal systems controlling feeding that occurs in the absence of metabolic need (i.e., "non-homeostatic" intake). Given that a portion of the increased caloric intake per capita since the late 1970s is attributed to increased meal and snack frequency, and given the increased pervasiveness of environmental cues associated with energy dense, yet nutritionally depleted foods, there's a need to examine the mechanisms through which food-related cues stimulate excessive energy intake. Here, learning and memory principles and their underlying neuronal substrates are discussed with regard to stimulus-driven food intake and excessive energy consumption. Particular focus is given to the hippocampus, a brain structure that utilizes interoceptive cues relevant to energy status (e.g., neurohormonal signals such as leptin) to modulate stimulus-driven food procurement and consumption. This type of hippocampal-dependent modulatory control of feeding behavior is compromised by consumption of foods common to Western diets, including saturated fats and simple carbohydrates. The development of more effective treatments for obesity will benefit from a more complete understanding of the complex interaction between dietary, environmental, cognitive, and neurophysiological mechanisms contributing to excessive food intake.

Meal anticipatory rise in acylated ghrelin at dark onset is blunted after long-term fasting in rats

Ghrelin is a 28 amino acid acylated peptide originally characterised for its capacity to stimulate growth hormone secretion. Ghrelin is also an orexigenic and adipogenic hormone and is thought to be a signal to increase locomotor activity in anticipation of a scheduled meal. Although ghrelin is considered to be up-regulated during fasting, there are still conflicting data regarding the impact of starvation on ghrelin secretion. To test whether the secretory pattern of acylated ghrelin is altered during fasting, plasma levels were monitored every 20 min for 6 h in freely-behaving rats at the light/dark cycle transition, when animals initiate feeding and activity and use preferentially free fatty acids (FFA) as a source of energy. Rats were fed ad lib. or fasted at dark onset for 24, 48 or 72 h, with or without refeeding rate. The anticipatory rise in ghrelin levels, as well as home-cage activity at the onset of darkness, was significantly reduced from 48 h of fasting compared to ad lib. conditions. A delayed ghrelin peak, sensitive to renutrition, was observed in fasted animals. Although their motivation to eat appeared to be intact, rats fasted for 72 h showed the smallest compensatory refeeding rate after fasting, possibly reflecting altered gut function. Expression of agouti-related protein and neuropeptide Y, was significantly increased in 48- and 72-h fasted animals. Thus, following fasting, a blunted acylated ghrelin secretion at dark onset (i.e. a period when animals depend on FFA as a source of energy) is associated with reduced locomotor activity and refeeding and an up-regulation of anabolic neuropeptides. Such changes could be interpreted as compensatory mechanisms for helping to conserve energy under conditions where food is not available.