Orexigenic action of peripheral ghrelin is mediated by neuropeptide Y and agouti-related protein

Hunger signalling in the olfactory bulb primes exploration, food-seeking and peripheral metabolism

OBJECTIVE

Although the metabolic state of an organism affects olfactory function, the precise mechanisms and their impact on behavior and metabolism remain unknown. Here, we assess whether ghrelin receptors (GHSRs) in the olfactory bulb (OB) increase olfactory function and influence foraging behaviors and metabolism.

METHODS

We performed a detailed behavioural and metabolic analysis in mice lacking GHSRs in the OB (OBGHSR deletion). We also analsyed OB scRNA-seq and spatial transcriptomic datasets to assess GHSR+ cells in the main and accessory olfactory bulbs, as well as the anterior olfactory nucleus.

RESULTS

OBGHSR deletion affected olfactory discrimination and habituation to both food and non-food odors. Anxiety-like and depression-like behaviors were significantly greater after OBGHSR deletion, whereas exploratory behavior was reduced, with the greatest effect under fasted conditions. OBGHSR deletion impacted feeding behavior as evidenced by altered bout number and duration, as well as buried food-seeking. OBGHSR deletion increased body weight and fat mass, spared fat utilisation on a chow diet and impaired glucose metabolism indicating metabolic dysfunction. Cross referenced analysis of OB scRNA-seq and spatial transcriptomic datasets revealed GHSR+ glutamate neurons in the main and accessory olfactory bulbs, as well as the anterior olfactory nucleus. Ablation of glutamate neurons in the OB reduced ghrelin-induced food finding and phenocopied results seen after OBGHSR deletion.

CONCLUSIONS

OBGHSRs help to maintain olfactory function, particularly during hunger, and facilitate behavioral adaptations that optimise food-seeking in anxiogenic environments, priming metabolic pathways in preparation for food consumption.

Hypothalamic hormone deficiency enables physiological anorexia in ground squirrels during hibernation

Mammalian hibernators survive prolonged periods of cold and resource scarcity by temporarily modulating normal physiological functions, but the mechanisms underlying these adaptations are poorly understood. The hibernation cycle of thirteen-lined ground squirrels (Ictidomys tridecemlineatus) lasts for 5-7 months and comprises weeks of hypometabolic, hypothermic torpor interspersed with 24-48-h periods of an active-like interbout arousal (IBA) state. We show that ground squirrels, who endure the entire hibernation season without food, have negligible hunger during IBAs. These squirrels exhibit reversible inhibition of the hypothalamic feeding center, such that hypothalamic arcuate nucleus neurons exhibit reduced sensitivity to the orexigenic and anorexigenic effects of ghrelin and leptin, respectively. However, hypothalamic infusion of thyroid hormone during an IBA is sufficient to rescue hibernation anorexia. Our results reveal that thyroid hormone deficiency underlies hibernation anorexia and demonstrate the functional flexibility of the hypothalamic feeding center.

Genetic or pharmacological GHSR blockade has sexually dimorphic effects in rodents on a high-fat diet

The stomach-derived hormone ghrelin regulates essential physiological functions. The ghrelin receptor (GHSR) has ligand-independent actions; therefore, GHSR gene deletion may be a reasonable approach to investigate the role of this system in feeding behaviors and diet-induced obesity (DIO). Here, we investigate the effects of a long-term (12-month) high-fat (HFD) versus regular diet on obesity-related measures in global GHSR-KO and wild-type (WT) Wistar male and female rats. Our main findings are that the GHSR gene deletion protects against DIO and decreases food intake during HFD in male but not in female rats. GHSR gene deletion increases thermogenesis and brain glucose uptake in male rats and modifies the effects of HFD on brain glucose metabolism in a sex-specific manner, as assessed with small animal positron emission tomography. We use RNA-sequencing to show that GHSR-KO rats have upregulated expression of genes responsible for fat oxidation in brown adipose tissue. Central administration of a novel GHSR inverse agonist, PF-5190457, attenuates ghrelin-induced food intake, but only in male, not in female mice. HFD-induced binge-like eating is reduced by inverse agonism in both sexes. Our results support GHSR as a promising target for new pharmacotherapies for obesity.

A brainstem to hypothalamic arcuate nucleus GABAergic circuit drives feeding

The obesity epidemic is principally driven by the consumption of more calories than the body requires. It is therefore essential that the mechanisms underpinning feeding behavior are defined. Neurons within the brainstem dorsal vagal complex (DVC) receive direct information from the digestive system and project to second-order regions in the brain to regulate food intake. Although γ-aminobutyric acid is expressed in the DVC (GABADVC), its function in this region has not been defined. In order to discover the unique gene expression signature of GABADVC cells, we used single-nucleus RNA sequencing (Nuc-seq), and this revealed 19 separate clusters. We next probed the function of GABADVC cells and discovered that the selective activation of GABADVC neurons significantly controls food intake and body weight. Optogenetic interrogation of GABADVC circuitry identified GABADVC → hypothalamic arcuate nucleus (ARC) projections as appetite suppressive without creating aversion. Electrophysiological analysis revealed that GABADVC → ARC stimulation inhibits hunger-promoting neuropeptide Y (NPY) neurons via GABA release. Adopting an intersectional genetics strategy, we clarify that the GABADVC → ARC circuit curbs food intake. These data identify GABADVC as a new modulator of feeding behavior and body weight and a controller of orexigenic NPY neuron activity, thereby providing insight into the neural underpinnings of obesity.

Hypothalamic hormone deficiency enables physiological anorexia

Mammalian hibernators survive prolonged periods of cold and resource scarcity by temporarily modulating normal physiological functions, but the mechanisms underlying these adaptations are poorly understood. The hibernation cycle of thirteen-lined ground squirrels (Ictidomys tridecemlineatus) lasts for 5-7 months and comprises weeks of hypometabolic, hypothermic torpor interspersed with 24-48-hour periods of an active-like interbout arousal (IBA) state. We show that ground squirrels, who endure the entire hibernation season without food, have negligible hunger during IBAs. These squirrels exhibit reversible inhibition of the hypothalamic feeding center, such that hypothalamic arcuate nucleus neurons exhibit reduced sensitivity to the orexigenic and anorexigenic effects of ghrelin and leptin, respectively. However, hypothalamic infusion of thyroid hormone during an IBA is sufficient to rescue hibernation anorexia. Our results reveal that thyroid hormone deficiency underlies hibernation anorexia and demonstrate the functional flexibility of the hypothalamic feeding center.

Regulation of peripheral tissue substrate metabolism by the gut-derived hormone ghrelin

Ghrelin increases in the circulation prior to entrained mealtimes, with the acylated (AG) form functioning to stimulate food intake and growth hormone release. Acutely, AG induces whole-body insulin resistance, potentially to maintain glycemia between meals. Alternatively, chronic administration of both AG and the unacylated isoform of ghrelin (unAG) is associated with improved skeletal muscle insulin sensitivity as well as reduced intramuscular lipids and inflammation. This may be due to effects on lipid metabolism, with ghrelin promoting storage of fat in adipose and liver while stimulating oxidation in skeletal muscle, preventing ectopic lipid accumulation. This is of specific relevance in the handling of meal-derived lipids, as ghrelin rises preprandially with effects persisting for 2-3 h following exposure in skeletal muscle, coinciding with elevated plasma FFAs. We hypothesize that ghrelin acts as a preparatory signal for incoming lipids, as well as a regulatory hormone for their use and storage. The effects of ghrelin on skeletal muscle are lost with high fat diet feeding and physical inactivity, potentially being implicated in the pathogenesis of metabolic disease. This review summarizes the metabolic effects of both ghrelin isoforms on peripheral tissues including the pancreas, adipose, liver, and skeletal muscle. Additionally, we speculate on the physiological relevance of these effects in vivo and suggest that ghrelin may be a key regulatory hormone for nutrient handling in the postprandial state.

Hypothalamic AMP-Activated Protein Kinase as a Whole-Body Energy Sensor and Regulator

5´-Adenosine monophosphate (AMP)-activated protein kinase (AMPK), a cellular energy sensor, is an essential enzyme that helps cells maintain stable energy levels during metabolic stress. The hypothalamus is pivotal in regulating energy balance within the body. Certain neurons in the hypothalamus are sensitive to fluctuations in food availability and energy stores, triggering adaptive responses to preserve systemic energy equilibrium. AMPK, expressed in these hypothalamic neurons, is instrumental in these regulatory processes. Hypothalamic AMPK activity is modulated by key metabolic hormones. Anorexigenic hormones, including leptin, insulin, and glucagon-like peptide 1, suppress hypothalamic AMPK activity, whereas the hunger hormone ghrelin activates it. These hormonal influences on hypothalamic AMPK activity are central to their roles in controlling food consumption and energy expenditure. Additionally, hypothalamic AMPK activity responds to variations in glucose concentrations. It becomes active during hypoglycemia but is deactivated when glucose is introduced directly into the hypothalamus. These shifts in AMPK activity within hypothalamic neurons are critical for maintaining glucose balance. Considering the vital function of hypothalamic AMPK in the regulation of overall energy and glucose balance, developing chemical agents that target the hypothalamus to modulate AMPK activity presents a promising therapeutic approach for metabolic conditions such as obesity and type 2 diabetes mellitus.

Ghrelin signalling in AgRP neurons links metabolic state to the sensory regulation of AgRP neural activity

OBJECTIVE

The sensory detection of food and food cues suppresses Agouti related peptide (AgRP) neuronal activity prior to consumption with greatest suppression occurring in response to highly caloric food or interoceptive energy need. However, the interoceptive mechanisms priming an appropriate AgRP neural response to external sensory information of food availability remain unexplored. Since hunger increases plasma ghrelin, we hypothesized that ghrelin receptor (GHSR) signalling on AgRP neurons is a key interoceptive mechanism integrating energy need with external sensory cues predicting caloric availability.

METHODS

We used in vivo photometry to measure the effects of ghrelin administration or fasting on AgRP neural activity with GCaMP6s and dopamine release in the nucleus accumbens with GRAB-DA in mice lacking ghrelin receptors in AgRP neurons.

RESULTS

The deletion of GHSR on AgRP neurons prevented ghrelin-induced food intake, motivation and AgRP activity. The presentation of food (peanut butter pellet) or a wooden dowel suppressed AgRP activity in fasted WT but not mice lacking GHSRs in AgRP neurons. Similarly, peanut butter and a wooden dowel increased dopamine release in the nucleus accumbens after ip ghrelin injection in WT but not mice lacking GHSRs in AgRP neurons. No difference in dopamine release was observed in fasted mice. Finally, ip ghrelin administration did not directly increase dopamine neural activity in the ventral tegmental area.

CONCLUSIONS

Our results suggest that AgRP GHSRs integrate an interoceptive state of energy need with external sensory information to produce an optimal change in AgRP neural activity. Thus, ghrelin signalling on AgRP neurons is more than just a feedback signal to increase AgRP activity during hunger.

Defining Predictors of Weight Loss Response to Lorcaserin

CONTEXT

Individual responses to weight loss (WL) medications vary widely and prediction of response remains elusive.

OBJECTIVE

We investigated biomarkers associated with use of lorcaserin (LOR), a 5HT2cR agonist that targets proopiomelanocortin (POMC) neurons that regulate energy and glucose homeostasis, to identify predictors of clinical efficacy.

METHODS

Thirty individuals with obesity were treated with 7 days of placebo and LOR in a randomized crossover study. Nineteen participants continued on LOR for 6 months. Cerebrospinal fluid (CSF) POMC peptide measurements were used to identify potential biomarkers that predict WL. Insulin, leptin, and food intake during a meal were also studied.

RESULTS

LOR induced a significant decrease in CSF levels of the POMC prohormone and an increase in its processed peptide β-endorphin after 7 days; β-endorphin/POMC increased by 30% (P < .001). This was accompanied by a substantial decrease in insulin, glucose, and homeostasis model assessment of insulin resistance before WL. Changes in CSF POMC peptides persisted after WL (6.9%) at 6 months that were distinct from prior reports after diet alone. Changes in POMC, food intake, or other hormones did not predict WL. However, baseline CSF POMC correlated negatively with WL (P = .07) and a cutoff level of CSF POMC was identified that predicted more than 10% WL.

CONCLUSION

Our results provide evidence that LOR affects the brain melanocortin system in humans and that effectiveness is increased in individuals with lower melanocortin activity. Furthermore, early changes in CSF POMC parallel WL-independent improvements in glycemic indexes. Thus, assessment of melanocortin activity could provide a way to personalize pharmacotherapy of obesity with 5HT2cR agonists.

CaMK1D signalling in AgRP neurons promotes ghrelin-mediated food intake

Hypothalamic AgRP/NPY neurons are key players in the control of feeding behaviour. Ghrelin, a major orexigenic hormone, activates AgRP/NPY neurons to stimulate food intake and adiposity. However, cell-autonomous ghrelin-dependent signalling mechanisms in AgRP/NPY neurons remain poorly defined. Here we show that calcium/calmodulin-dependent protein kinase ID (CaMK1D), a genetic hot spot in type 2 diabetes, is activated upon ghrelin stimulation and acts in AgRP/NPY neurons to mediate ghrelin-dependent food intake. Global Camk1d-knockout male mice are resistant to ghrelin, gain less body weight and are protected against high-fat-diet-induced obesity. Deletion of Camk1d in AgRP/NPY, but not in POMC, neurons is sufficient to recapitulate above phenotypes. In response to ghrelin, lack of CaMK1D attenuates phosphorylation of CREB and CREB-dependent expression of the orexigenic neuropeptides AgRP/NPY in fibre projections to the paraventricular nucleus (PVN). Hence, CaMK1D links ghrelin action to transcriptional control of orexigenic neuropeptide availability in AgRP neurons.

How gut hormones shape reward: A systematic review of the role of ghrelin and GLP-1 in human fMRI

The gastrointestinal hormones ghrelin and glucagon-like peptide-1 (GLP-1) have opposite secretion patterns, as well as opposite effects on metabolism and food intake. Beyond their role in energy homeostasis, gastrointestinal hormones have also been suggested to modulate the reward system. However, the potential of ghrelin and GLP-1 to modulate reward responses in humans has not been systematically reviewed before. To evaluate the convergence of published results, we first conduct a multi-level kernel density meta-analysis of studies reporting a positive association of ghrelin (N_comb = 353, 18 contrasts) and a negative association of GLP-1 (N_comb = 258, 12 contrasts) and reward responses measured using task functional magnetic resonance imaging (fMRI). Second, we complement the meta-analysis using a systematic literature review, focusing on distinct reward phases and applications in clinical populations that may account for variability across studies. In line with preclinical research, we find that ghrelin increases reward responses across studies in key nodes of the motivational circuit, such as the nucleus accumbens, pallidum, putamen, substantia nigra, ventral tegmental area, and the dorsal mid insula. In contrast, for GLP-1, we did not find sufficient convergence in support of reduced reward responses. Instead, our systematic review identifies potential differences of GLP-1 on anticipatory versus consummatory reward responses. Based on a systematic synthesis of available findings, we conclude that there is considerable support for the neuromodulatory potential of gut-based circulating peptides on reward responses. To unlock their potential for clinical applications, it may be useful for future studies to move beyond anticipated rewards to cover other reward facets.

Involvement of ghrelin in the regulation of swallowing motor activity in an arterially perfused rat preparation

Ghrelin, a peripheral peptide produced in the stomach, is involved in the neural networks that control food intake. Alterations in motor components, such as swallowing, are believed to be significant in the regulation food intake by orexigenic signals. However, there has been no detailed investigation of the relationship between ghrelin and swallowing activities induced in motor nerves innervating the pharyngeal and laryngeal muscles. In this study, we examined the effects of ghrelin administration on swallowing motor activity in arterially perfused rats. Injection of distilled water (0.5 ml) into the oral cavity or electrical stimulation of the superior laryngeal nerve evoked swallowing motor activity in the cervical vagus nerve. Administration of ghrelin (6 nM), but not des-acylated ghrelin (6 nM), into the perfusate increased the peak burst amplitude and burst duration, and shortened the first burst interval of water injection-induced swallowing. These ghrelin-induced changes in swallowing motor activity were blocked by the administration of JMV2959 (6 µM), a growth hormone secretagogue receptor antagonist. In preparations in which the hypothalamus was removed, ghrelin had no effect on swallowing motor activity. Furthermore, ghrelin-induced changes were counteracted by the administration of BIBO3304 (1 µM) or L-152,804 (1 µM), antagonists of neuropeptide Y Y1 and Y5 receptors, respectively, which are essential for ghrelin-induced enhancement of food intake. Ghrelin also increased the peak burst amplitude and burst duration of the swallowing motor activity evoked by electrical stimulation of the superior laryngeal nerve, although the effects of ghrelin on the number of swallowing bursts and burst intervals varied with stimulus intensity. These results suggest that ghrelin enhances the magnitude and frequency of bursts of swallowing motor activity by acting via the hypothalamic neural network, and that neuropeptide Y Y1 and Y5 receptors are involved in this enhancement.

Food-induced dopamine signaling in AgRP neurons promotes feeding

Obesity comorbidities such as diabetes and cardiovascular disease are pressing public health concerns. Overconsumption of calories leads to weight gain; however, neural mechanisms underlying excessive food consumption are poorly understood. Here, we demonstrate that dopamine receptor D1 (Drd1) expressed in the agouti-related peptide/neuropeptide Y (AgRP/NPY) neurons of the arcuate hypothalamus is required for appropriate responses to a high-fat diet (HFD). Stimulation of Drd1 and AgRP/NPY co-expressing arcuate neurons is sufficient to induce voracious feeding. Delivery of a HFD after food deprivation acutely induces dopamine (DA) release in the ARC, whereas animals that lack Drd1 expression in ARC^AgRP/NPY neurons (Drd1^AgRP-KO) exhibit attenuated foraging and refeeding of HFD. These results define a role for the DA input to the ARC that encodes acute responses to food and position Drd1 signaling in the ARC^AgRP/NPY neurons as an integrator of the hedonic and homeostatic neuronal feeding circuits.

Effects of dietary eucommia ulmoides leaf extract on growth performance, expression of feeding-related genes, activities of digestive enzymes, antioxidant capacity, immunity and cytokines expression of large yellow croaker (Larimichthys crocea) larvae

A 30-d feeding trial was conducted to investigate effects of dietary eucommia ulmoides leaf extract (ELE) on growth performance, activities of digestive enzymes, antioxidant capacity, immunity, expression of inflammatory factors and feeding-related genes of large yellow croaker larvae. Five micro-diets were formulated with supplementation of 0 g kg^-1 (the control), 5 g kg^-1 (0·5 %), 10 g kg^-1 (1·0 %) and 20 g kg^-1 (2·0 %) of ELE, respectively. Results showed that the best growth performance was found in larvae fed the diet with 1·0 % ELE. Furthermore, ELE supplementation significantly increased the npy expression at 1·0 % dosage, while increased ghrelin in larvae at 0·5 % dosages. The activity of leucine aminopeptidase in larvae fed the diet with 1·0 % ELE was significantly higher than the control, while alkaline phosphatase was significantly upregulated in larvae fed the diet with 2·0 % ELE. A clear increase in total antioxidant capacity in larvae fed the diet with 1·0 % ELE was observed, whereas catalase activity was significantly higher in 1·0 % and 2·0 % ELE supplementation compared with the control. Larvae fed the diet with 1·0 % ELE had a significantly higher activities of lysozyme, total nitric oxide synthase and nitric oxide content than the control. Moreover, transcriptional levels of cox-2, il-1β and il-6 were remarkably downregulated by the supplementation of 0·5-1·0 % ELE. This study demonstrated that the supplementation of 1·0 % ELE in diet could increase the growth performance of large yellow croaker larvae probably by promoting expression of feeding-related genes, enhancing antioxidant capacity and immunity and inhibiting expression of inflammatory factors.

The Circadian Axis and Cardiometabolic Syndrome

Circadian rhythm refers to the daily physiologically fluctuating patterns of systemic processes that occur within a circa 24-hour timeframe, independently of external factors. There is evidence that in time, external and internal cycle misalignment leads to severe health consequences, resulting in the development of cardiometabolic disturbances. Desynchronized hormonal fluctuations along with daily specific macronutrient utilization patterns are also discussed, which by consequence, are all predictors of metabolic syndrome. The aim of this paper is to provide insight on the circadian clock’s organization throughout the human body and to explain the underlying genetic background. By understanding these well-established molecular mechanisms and processes, we believe this paper will provide accuracy regarding the importance of the circadian clock’s integrity and will highlight its role in the etiopathology of cardiometabolic syndrome.

A nexus of lipid and O-Glcnac metabolism in physiology and disease

Although traditionally considered a glucose metabolism-associated modification, the O-linked β-N-Acetylglucosamine (O-GlcNAc) regulatory system interacts extensively with lipids and is required to maintain lipid homeostasis. The enzymes of O-GlcNAc cycling have molecular properties consistent with those expected of broad-spectrum environmental sensors. By direct protein-protein interactions and catalytic modification, O-GlcNAc cycling enzymes may provide both acute and long-term adaptation to stress and other environmental stimuli such as nutrient availability. Depending on the cell type, hyperlipidemia potentiates or depresses O-GlcNAc levels, sometimes biphasically, through a diversity of unique mechanisms that target UDP-GlcNAc synthesis and the availability, activity and substrate selectivity of the glycosylation enzymes, O-GlcNAc Transferase (OGT) and O-GlcNAcase (OGA). At the same time, OGT activity in multiple tissues has been implicated in the homeostatic regulation of systemic lipid uptake, storage and release. Hyperlipidemic patterns of O-GlcNAcylation in these cells are consistent with both transient physiological adaptation and feedback uninhibited obesogenic and metabolic dysregulation. In this review, we summarize the numerous interconnections between lipid and O-GlcNAc metabolism. These links provide insights into how the O-GlcNAc regulatory system may contribute to lipid-associated diseases including obesity and metabolic syndrome.

Body mass variability in age-matched outbred male Swiss mice is associated to differential control of food intake by ghrelin

Swiss mice belong to an outbred strain of mice largely used as a model for experimental obesity induced by high fat diet (HFD). We have previously demonstrated that a given cohort of age-matched Swiss mice is hallmarked by heterogeneous changes in body weight when exposed to HFD. The reasons underlying such variability, however, are not completely understood. Therefore we aimed to clarify the mechanisms underlying the variability in spontaneous weight gain in age-matched male swiss mice. To achieve that, individuals in a cohort of age-matched male Swiss mice were categorized as prone to body mass gain (PBMG) and resistant to body mass gain (RBMG). PBMG animals had higher caloric intake and body mass gain. RBMG and PBMG mice had a similar reduction in food intake when challenged with leptin but only RBMG exhibited a drop in ghrelin concentrations after refeeding. PBMG also showed increased midbrain levels of ghrelin receptor (Ghsr) and Dopamine receptor d2 (Drd2) mRNAs upon refeeding. Pharmacological blockade of GHSR with JMV3002 failed to reduce food intake in PMBG mice as it did in RBMG. On the other hand, the response to JMV3002 seen in PBMG was hallmarked by singular transcriptional response in the midbrain characterized by a simultaneous increase in both tyrosine hydroxylase (Th) and Proopiomelanocortin (Pomc) expressions. In conclusion, our data show that differences in the expression of genes related to the reward system in the midbrain as well as in ghrelin concentrations in serum correlate with spontaneous variability in body mass and food intake seen in age-matched male Swiss mice.

Targeting appetite and satiety in diabetes and obesity, via G protein-coupled receptors

Type 2 diabetes and obesity have reached pandemic proportions throughout the world, so much so that the World Health Organisation coined the term "Globesity" to help encapsulate the magnitude of the problem. G protein-coupled receptors (GPCRs) are highly tractable drug targets due to their wide involvement in all aspects of physiology and pathophysiology, indeed, GPCRs are the targets of approximately 30% of the currently approved drugs. GPCRs are also broadly involved in key physiologies that underlie type 2 diabetes and obesity including feeding reward, appetite and satiety, regulation of blood glucose levels, energy homeostasis and adipose function. Despite this, only two GPCRs are the target of approved pharmaceuticals for treatment of type 2 diabetes and obesity. In this review we discuss the role of these, and select other candidate GPCRs, involved in various facets of type 2 diabetic or obese pathophysiology, how they might be targeted and the potential reasons why pharmaceuticals against these targets have not progressed to clinical use. Finally, we provide a perspective on the current development pipeline of anti-obesity drugs that target GPCRs.

The PACAP Paradox: Dynamic and Surprisingly Pleiotropic Actions in the Central Regulation of Energy Homeostasis

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP), a pleiotropic neuropeptide, is widely distributed throughout the body. The abundance of PACAP expression in the central and peripheral nervous systems, and years of accompanying experimental evidence, indicates that PACAP plays crucial roles in diverse biological processes ranging from autonomic regulation to neuroprotection. In addition, PACAP is also abundantly expressed in the hypothalamic areas like the ventromedial and arcuate nuclei (VMN and ARC, respectively), as well as other brain regions such as the nucleus accumbens (NAc), bed nucleus of stria terminalis (BNST), and ventral tegmental area (VTA) - suggesting that PACAP is capable of regulating energy homeostasis via both the homeostatic and hedonic energy balance circuitries. The evidence gathered over the years has increased our appreciation for its function in controlling energy balance. Therefore, this review aims to further probe how the pleiotropic actions of PACAP in regulating energy homeostasis is influenced by sex and dynamic changes in energy status. We start with a general overview of energy homeostasis, and then introduce the integral components of the homeostatic and hedonic energy balance circuitries. Next, we discuss sex differences inherent to the regulation of energy homeostasis via these two circuitries, as well as the activational effects of sex steroid hormones that bring about these intrinsic disparities between males and females. Finally, we explore the multifaceted role of PACAP in regulating homeostatic and hedonic feeding through its actions in regions like the NAc, BNST, and in particular the ARC, VMN and VTA that occur in sex- and energy status-dependent ways.

An insight into the multifunctional role of ghrelin and structure activity relationship studies of ghrelin receptor ligands with clinical trials

Ghrelin is a multifunctional gastrointestinal acylated peptide, primarily synthesized in the stomach and regulates the secretion of growth hormone and energy homeostasis. It plays a central role in modulating the diverse biological, physiological and pathological functions in vertebrates. The synthesis of ghrelin receptor ligands after the finding of growth hormone secretagogue developed from Met-enkephalin led to reveal the endogenous ligand ghrelin and the receptors. Subsequently, many peptides, small molecules and peptidomimetics focusing on the ghrelin receptor, GHS-R1a, were derived. In this review, the key features of ghrelin's structure, forms, its bio-physiological functions, pathological roles and therapeutic potential have been highlighted. A few peptidomimetics and pseudo peptide synthetic perspectives have also been discussed to make ghrelin receptor ligands, clinical trials and their success.

Arcuate Nucleus-Dependent Regulation of Metabolism-Pathways to Obesity and Diabetes Mellitus

The central nervous system (CNS) receives information from afferent neurons, circulating hormones, and absorbed nutrients and integrates this information to orchestrate the actions of the neuroendocrine and autonomic nervous systems in maintaining systemic metabolic homeostasis. Particularly the arcuate nucleus of the hypothalamus (ARC) is of pivotal importance for primary sensing of adiposity signals, such as leptin and insulin, and circulating nutrients, such as glucose. Importantly, energy state-sensing neurons in the ARC not only regulate feeding but at the same time control multiple physiological functions, such as glucose homeostasis, blood pressure, and innate immune responses. These findings have defined them as master regulators, which adapt integrative physiology to the energy state of the organism. The disruption of this fine-tuned control leads to an imbalance between energy intake and expenditure as well as deregulation of peripheral metabolism. Improving our understanding of the cellular, molecular, and functional basis of this regulatory principle in the CNS could set the stage for developing novel therapeutic strategies for the treatment of obesity and metabolic syndrome. In this review, we summarize novel insights with a particular emphasis on ARC neurocircuitries regulating food intake and glucose homeostasis and sensing factors that inform the brain of the organismal energy status.

Dietary fat: a potent microglial influencer

Poor nutrition, lack of exercise, and genetic predisposition all contribute to the growing epidemic of obesity. Overweight/obesity create an environment of chronic inflammation that leads to negative physiological and neurological outcomes, such as diabetes, cardiovascular disease, and anxiety/depression. While the whole body contributes to metabolic homeostasis, the neuroimmune system has recently emerged as a key regulator of metabolism. Microglia, the resident immune cells of the brain, respond both directly and indirectly to dietary fat, and the environment in which microglia develop contributes to their responsiveness later in life. Thus, high maternal weight during pregnancy may have consequences for microglial function in offspring. Here, we discuss the most recent findings on microglia signaling in overweight/obesity with a focus on perinatal programming.

Diagnostic Value of Acyl-Ghrelin in Type 2 Diabetic Patients with Non-alcoholic Fatty Liver Disease

BACKGROUND: Nonalcoholic fatty liver disease (NAFLD) has become the leading cause of chronic liver disease worldwide. Type 2 diabetes (T2D) is described as one of the most significant risk factor for developing NAFLD, non-alcoholic steatohepatitis, and advanced cirrhosis. Liver biopsy cannot be used routinely to diagnose NAFLD. Therefore, it is critically urgent to develop a simple non-invasive test. AIM: This study examined fasting Acyl-Ghrelin (AG) as a non-invasive biomarker to accurately diagnose NAFLD in diabetic patients. PATIENTS AND METHODS: Sixty-one patients with T2D were divided into a test group with NAFLD, and a control group without NAFLD. Secondary causes of fatty liver, chronic viral hepatitis, and drug-induced liver damage were excluded from the study. Anthropometric measurements, lipid profile, fasting blood sugar (FBS), liver enzyme activities, and fasting AG levels were collected. Data management and analysis were performed using statistical package for social sciences version 20. RESULTS: Fasting AG level (pg/ml) in the test group (56.1 ± 10.7) was increased, but not statically significant compared with the control group (37.8 ± 9.3), p > 0.05. However, significant metabolic changes were observed in body weight, waist circumference, FBS, alanine transaminase, and aspartate transaminase between test and control groups. The mean values in the test group are 93.2 ± 14.5, 115.4 ± 7.6, 144.2 ± 25.9, 21.1 ± 5.7, and 32.3 ± 2.1. While the mean values are 87.7 ± 7.3, 95 ± 3.8, 123.7 ± 20.7, 18.6 ± 5, and 20 ± 7, respectively, in the control group. CONCLUSIONS: While elevated AG levels alone were not significant, elevated AG levels plus other parameters of liver damage and obesity were associated with the diagnosis of NAFLD. However, more studies are needed to consider elevated AG as a diagnostic marker in NAFLD patients with T2D.

In Vivo Reductionist Approach Identifies miR-15a Protecting Mice From Obesity

Obesity is a growing medical and social problem worldwide. The control of energy homeostasis in the brain is achieved by various regions including the arcuate hypothalamic nucleus (ARH). The latter comprises a number of neuronal populations including the first order metabolic neurons, appetite-stimulating agouti-related peptide (AgRP) neurons and appetite-suppressing proopiomelanocortin (POMC) neurons. Using an in vivo reductionist approach and POMC^Cre-dependent CRISPR-Cas9, we demonstrate that miR-15a-5p protects from obesity. Moreover, we have identified Bace1, a gene previously linked to energy metabolism imbalance, as a direct target of miR-15a-5p. This work warrants further investigations of non-coding RNA-mediated regulation of energy homeostasis and might contribute to the development of novel therapeutic approaches to treat metabolic diseases.

Free triiodothyronine (T3) is negatively associated with fasting ghrelin serum levels in a population sample of euthyroid subjects

PURPOSE

Ghrelin is an orexigenic peptide hormone secreted in times of stress and hunger. It is deeply involved in the regulation of metabolism and energy homeostasis, promoting energy intake and inhibiting energy expenditure on a metabolic level. In this regard, it has in many ways antagonistic effect on the thyroid hormones, which increase metabolism and thus energy expenditure. While there is reasonable evidence of a negative association between ghrelin and hormones of the hypothalamic-pituitary-thyroid (HPT-) axis from studies in patients with thyroid dysfunction and small intervention studies, large-scale studies in healthy subjects are lacking. Therefore, we studied the relationship between total ghrelin serum levels and serum levels of the thyroid hormones in a large sample of euthyroid subjects.

METHODS

Total ghrelin, thyroid-stimulating hormone (TSH), free thyroxine (fT4) and free triiodothyronine (fT3) were determined after an overnight fast in 1666 subjects participating in a population-based cross-sectional study ('LIFE') including 10,000 adults. 1012 subjects were included in this analysis. Multiple linear regression analyses were performed.

RESULTS

FT3 was negatively associated with serum ghrelin; total sample: β = - 0.0001, p < 0.001; men: β = - 0.0002, p = 0.013; women: β = - 0.0001, p = 0.010, adjusted for age, BMI, alcohol consumption, serum levels of TSH and fT4 and smoking status. No associations were found between ghrelin serum levels and serum levels of fT4 or TSH.

CONCLUSION

This is to date the largest study investigating the relationship between total serum ghrelin and thyroid hormones. The results point to a complex interaction and should initiate further research.

Time-Restricted Feeding in Mice Prevents the Disruption of the Peripheral Circadian Clocks and Its Metabolic Impact during Chronic Jetlag

We used time-restricted feeding (TRF) to investigate whether microbial metabolites and the hunger hormone ghrelin can become the dominant entraining factor during chronic jetlag to prevent disruption of the master and peripheral clocks, in order to promote health. Therefore, hypothalamic clock gene and Agrp/Npy mRNA expression were measured in mice that were either chronically jetlagged and fed ad libitum, jetlagged and fed a TRF diet, or not jetlagged and fed a TRF diet. Fecal short-chain fatty acid (SCFA) concentrations, plasma ghrelin and corticosterone levels, and colonic clock gene mRNA expression were measured. Preventing the disruption of the food intake pattern during chronic jetlag using TRF restored the rhythmicity in hypothalamic clock gene mRNA expression of Reverbα but not of Arntl. TRF countered the changes in plasma ghrelin levels and in hypothalamic Npy mRNA expression induced by chronic jetlag, thereby reestablishing the food intake pattern. Increase in body mass induced by chronic jetlag was prevented. Alterations in diurnal fluctuations in fecal SCFAs during chronic jetlag were prevented thereby re-entraining the rhythmic expression of peripheral clock genes. In conclusion, TRF during chronodisruption re-entrains the rhythms in clock gene expression and signals from the gut that regulate food intake to normalize body homeostasis.

Ghrelin treatment induces rapid and delayed increments of food intake: a heuristic model to explain ghrelin's orexigenic effects

Ghrelin is a stomach-derived peptide hormone with salient roles in the regulation of energy balance and metabolism. Notably, ghrelin is recognized as the most powerful known circulating orexigenic hormone. Here, we systematically investigated the effects of ghrelin on energy homeostasis and found that ghrelin primarily induces a biphasic effect on food intake that has indirect consequences on energy expenditure and nutrient partitioning. We also found that ghrelin-induced biphasic effect on food intake requires the integrity of Agouti-related peptide/neuropeptide Y-producing neurons of the hypothalamic arcuate nucleus, which seem to display a long-lasting activation after a single systemic injection of ghrelin. Finally, we found that different autonomic, hormonal and metabolic satiation signals transiently counteract ghrelin-induced food intake. Based on our observations, we propose a heuristic model to describe how the orexigenic effect of ghrelin and the anorectic food intake-induced rebound sculpt a timely constrain feeding response to ghrelin.

Ghrelin Regulates Cyclooxygenase-2 Expression and Promotes Gastric Cancer Cell Progression

Aim

To research the molecular mechanism of ghrelin in apoptosis, migratory, and invasion of gastric cancer (GC) cells.

Methods

After GC AGS cells were handled with ghrelin (10^–8 M), cyclooxygenase-2 inhibitor NS398 (100 μM), and Akt inhibitor perifosine (10uM), the rates of apoptosis were detected by TUNEL assay and flow cytometry assay. We assessed the expressions of PI3K, p-Akt, and COX-2 proteins by making use of Western blot analysis. The cell migratory and invasion were detected by using wound-healing and transwell analysis.

Results

The migratory and invasion were increased in ghrelin-treated cells, while the rates of apoptosis were decreased. GC AGS cells treated with ghrelin showed an increase in protein expression of p-Akt, PI3K, and COX-2. After cells were treated with Akt inhibitor perifosine, the protein expression of p-Akt, PI3K, and COX-2 and the cell migratory, invasion, and apoptosis were partly recovered. After cells were treated with cyclooxygenase-2 inhibitor NS398, the protein expression of COX-2 and the cell migratory and invasion were decreased, while the rates of apoptosis were increased.

Conclusion

Ghrelin regulates cell migration, invasion, and apoptosis in GC cells through targeting PI3K/Akt/COX-2. Ghrelin increases the expression of COX-2 in GC cells by targeting PI3K/Akt. Ghrelin is suggested to be one of the molecular targets in GC.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

The Gut-Brain Axis and Its Role in Controlling Eating Behavior in Intestinal Inflammation

Malnutrition represents a major problem in the clinical management of the inflammatory bowel disease (IBD). Presently, our understanding of the cross-link between eating behavior and intestinal inflammation is still in its infancy. Crohn's disease patients with active disease exhibit strong hedonic desires for food and emotional eating patterns possibly to ameliorate feelings of low mood, anxiety, and depression. Impulsivity traits seen in IBD patients may predispose them to palatable food intake as an immediate reward rather than concerns for future health. The upregulation of enteroendocrine cells (EEC) peptide response to food intake has been described in ileal inflammation, which may lead to alterations in gut-brain signaling with implications for appetite and eating behavior. In summary, a complex interplay of gut peptides, psychological, cognitive factors, disease-related symptoms, and inflammatory burden may ultimately govern eating behavior in intestinal inflammation.

Food, Nutrition, Physical Activity and Microbiota: Which Impact on Lung Cancer?

Lung cancer still represents the leading cause of cancer-related death, globally. Likewise, malnutrition and inactivity represent a major risk for loss of functional pulmonary capacities influencing overall lung cancer severity. Therefore, the adhesion to an appropriate health lifestyle is crucial in the management of lung cancer patients despite the subtype of cancer. This review aims to summarize the available knowledge about dietary approaches as well as physical activity as the major factors that decrease the risk towards lung cancer, and improve the response to therapies. We discuss the most significant dietary schemes positively associated to body composition and prognosis of lung cancer and the main molecular processes regulated by specific diet schemes, functional foods and physical activity, i.e., inflammation and oxidative stress. Finally, we report evidence demonstrating that dysbiosis of lung and/or gut microbiome, as well as their interconnection (the gut–lung axis), are strictly related to dietary patterns and regular physical activity playing a key role in lung cancer formation and progression, opening to the avenue of modulating the microbiome as coadjuvant therapy. Altogether, the evidence reported in this review highlights the necessity to consider non-pharmacological interventions (nutrition and physical activity) as effective adjunctive strategies in the management of lung cancer.

Association of Gut Hormones and Microbiota with Vascular Dysfunction in Obesity

In the past few decades, obesity has reached pandemic proportions. Obesity is among the main risk factors for cardiovascular diseases, since chronic fat accumulation leads to dysfunction in vascular endothelium and to a precocious arterial stiffness. So far, not all the mechanisms linking adipose tissue and vascular reactivity have been explained. Recently, novel findings reported interesting pathological link between endothelial dysfunction with gut hormones and gut microbiota and energy homeostasis. These findings suggest an active role of gut secretome in regulating the mediators of vascular function, such as nitric oxide (NO) and endothelin-1 (ET-1) that need to be further investigated. Moreover, a central role of brain has been suggested as a main player in the regulation of the different factors and hormones beyond these complex mechanisms. The aim of the present review is to discuss the state of the art in this field, by focusing on the processes leading to endothelial dysfunction mediated by obesity and metabolic diseases, such as insulin resistance. The role of perivascular adipose tissue (PVAT), gut hormones, gut microbiota dysbiosis, and the CNS function in controlling satiety have been considered. Further understanding the crosstalk between these complex mechanisms will allow us to better design novel strategies for the prevention of obesity and its complications.

A mathematical model of food intake

The metabolic, hormonal and psychological determinants of the feeding behavior in humans are numerous and complex. A plausible model of the initiation, continuation and cessation of meals taking into account the most relevant such determinants would be very useful in simulating food intake over hours to days, thus providing input into existing models of nutrient absorption and metabolism. In the present work, a meal model is proposed, incorporating stomach distension, glycemic variations, ghrelin dynamics, cultural habits and influences on the initiation and continuation of meals, reflecting a combination of hedonic and appetite components. Given a set of parameter values (portraying a single subject), the timing and size of meals are stochastic. The model parameters are calibrated so as to reflect established medical knowledge on data of food intake from the National Health and Nutrition Examination Survey (NHANES) database during years 2015 and 2016.

Chronic exposure to methylmercury disrupts ghrelin actions in C57BL/6J mice

Methylmercury (MeHg) is a neurotoxic pollutant widely present in the environment. Initial symptoms of MeHg may include loss of body weight. However, the mechanisms by which MeHg induces body weight changes have yet to be fully elucidated. Body weight is regulated by multiple mechanisms. Whereas multiple peripheral peptides lead to food intake cessation, ghrelin is the only recognized peripheral hormone that stimulates food intake. It exerts its action on Neuropeptide Y/Agouti-related peptide neurons in the hypothalamus. To test if MeHg affects ghrelin signaling C57BL/6J mice (males and females) were exposed to 5 ppm MeHg via drinking water during a month. On days 15 and 30 of MeHg exposure ghrelin was administered intraperitoneally and changes in body weight and food intake were recorded. In addition, changes in ghrelin-induced signaling pathways in hypothalamus were also analyzed. Here, we show that in males, MeHg enhanced ghrelin-induced body weight gain by activating the AMP-activated Kinase (AMPK)/Uncoupled protein 2 (UCP2) signaling pathway. In contrast, in females, MeHg inhibited ghrelin-induced mTOR signaling activation and decreased Npy mRNA expression, thus mitigating the ghrelin-induced weight gain. Combined, our novel results demonstrate, for the first time, that MeHg disrupts the physiological functions of ghrelin differently in males and females.

Zoladz J, Brzozowski T. Effect of Acute Sprint Exercise on Myokines and Food Intake Hormones in Young Healthy Men

Physical exercise is known to influence hormonal mediators of appetite, but the effect of short-term maximal intensity exercise on plasma levels of appetite hormones and cytokines has been little studied. We investigated the effect of a 30 s Wingate Test, followed by a postprandial period, on appetite sensations, food intake, and appetite hormones. Twenty-six physically active young males rated their subjective feelings of hunger, prospective food consumption, and fatigue on visual analogue scales at baseline, after exercise was completed, and during the postprandial period. Blood samples were obtained for the measurement of nesfatin-1, ghrelin, leptin, insulin, pancreatic polypeptide (PP), human growth factor (hGH) and cytokine interleukin-6 (IL-6), irisin and plasma lactate concentrations, at 30 min before exercise, immediately (210 s) after exercise, and 30 min following a meal and at corresponding times in control sedentary males without ad libitum meal intake, respectively. Appetite perceptions and food intake were decreased in response to exercise. Plasma levels of irisin, IL-6, lactate, nesfatin-1 and ghrelin was increased after exercise and then it was returned to postprandial/control period in both groups. A significant rise in plasma insulin, hGH and PP levels after exercise was observed while meal intake potentiated this response. In conclusion, an acute short-term fatiguing exercise can transiently suppress hunger sensations and food intake in humans. We postulate that this physiological response involves exercise-induced alterations in plasma hormones and the release of myokines such as irisin and IL-6, and supports the notion of existence of the skeletal muscle-brain-gut axis. Nevertheless, the detailed relationship between acute exercise releasing myokines, appetite sensations and impairment of this axis leading to several diseases should be further examined.

Organophosphate Flame Retardants Excite Arcuate Melanocortin Circuitry and Increase Neuronal Sensitivity to Ghrelin in Adult Mice

Organophosphate flame retardants (OPFRs) are a class of chemicals that have become near ubiquitous in the modern environment. While OPFRs provide valuable protection against flammability of household items, they are increasingly implicated as an endocrine disrupting chemical (EDC). We previously reported that exposure to a mixture of OPFRs causes sex-dependent disruptions of energy homeostasis through alterations in ingestive behavior and activity in adult mice. Because feeding behavior and energy expenditure are largely coordinated by the hypothalamus, we hypothesized that OPFR disruption of energy homeostasis may occur through EDC action on melanocortin circuitry within the arcuate nucleus. To this end, we exposed male and female transgenic mice expressing green fluorescent protein in either neuropeptide Y (NPY) or proopiomelanocortin (POMC) neurons to a common mixture of OPFRs (triphenyl phosphate, tricresyl phosphate, and tris(1,3-dichloro-2-propyl)phosphate; each 1 mg/kg bodyweight/day) for 4 weeks. We then electrophysiologically examined neuronal properties using whole-cell patch clamp technique. OPFR exposure depolarized the resting membrane of NPY neurons and dampened a hyperpolarizing K+ current known as the M-current within the same neurons from female mice. These neurons were further demonstrated to have increased sensitivity to ghrelin excitation, which more potently reduced the M-current in OPFR-exposed females. POMC neurons from female mice exhibited elevated baseline excitability and are indicated in receiving greater excitatory synaptic input when exposed to OPFRs. Together, these data support a sex-selective effect of OPFRs to increase neuronal output from the melanocortin circuitry governing feeding behavior and energy expenditure, and give reason for further examination of OPFR impact on human health.

Dietary Allicin Improved the Survival and Growth of Large Yellow Croaker (Larimichthys crocea) Larvae via Promoting Intestinal Development, Alleviating Inflammation and Enhancing Appetite

A 30-day feeding experiment was conducted to investigate effects of dietary allicin on survival, growth, antioxidant capacity, innate immunity and expression of inflammatory and appetite related genes in large yellow croaker larvae. Four iso-nitrogenous (53% crude protein) and iso-lipidic (19% crude lipid) diets were formulated via supplementing graded levels of allicin (0.0 (the control), 0.005, 0.01, and 0.02% dry diet, respectively). Results showed that, among dietary treatments, larvae fed the diet with 0.005% allicin had the highest survival rate (SR) (P < 0.05), while larvae fed the diet with 0.01% allicin had the highest specific growth rate (SGR) (P < 0.05). Activities of α-amylase in both pancreatic (PS) and intestine segments (IS) of larvae fed the diet with 0.01% allicin were significantly lower than that in the control (P < 0.05). On the other hand, the supplementation of 0.01% allicin in diets significantly increased activities of alkaline phosphatase (AKP) and leucine aminopeptidase (LAP) in the intestinal brush border membrane (BBM) of larvae than the control (P < 0.05), indicating the promoting roles of allicin on fish larval intestinal development. Moreover, compared to the control, both the nitric oxide (NO) content and the activity of nitric oxide synthase (NOS) were significantly up-regulated in larvae fed the diet with 0.005% allicin, and catalase (CAT) were significantly upregulated in larvae fed the diet with 0.02% allicin (P < 0.05). Transcriptional levels of pro-inflammatory genes including cyclooxygenase-2 (cox-2), interleukin-1β (il-1β) and interleukin-6 (il-6) significantly decreased with increasing allicin, compared to the control. The expression of appetite genes including npy, ghrelin and leptin significantly increased with the prolonged fasting period, and dietary allicin supplementation significantly increased the transcriptional level of neuropeptide Y (npy) at 0.01%, while increased the transcriptional level of leptin in larvae at 0.02% dosages (P < 0.05). These results showed that the supplementation of 0.005% – 0.01% allicin in diets could improve the survival and growth of large yellow croaker larvae probably by promoting intestinal development, alleviating inflammation and enhancing appetite.

The emerging role of lactate as a mediator of exercise-induced appetite suppression

Lactate, a molecule originally considered metabolic waste, is now associated with a number of important physiological functions. Although the roles of lactate as a signaling molecule, fuel source, and gluconeogenic substrate have garnered significant attention in recent reviews, a relatively underexplored and emerging role of lactate is its control of energy intake (EI). To expand our understanding of the physiological roles of lactate, we present evidence from early infusion studies demonstrating the ability of lactate to suppress EI in both rodents and humans. We then discuss findings from recent human studies that have utilized exercise intensity and/or sodium bicarbonate supplementation to modulate endogenous lactate and examine its impact on appetite regulation. These studies consistently demonstrate that greater blood lactate accumulation is associated with greater suppression of the hunger hormone ghrelin and subjective appetite, thereby supporting a role of lactate in the control of EI. To stimulate future research investigating the role of lactate as an appetite-regulatory molecule, we also highlight potential underlying mechanisms explaining the appetite-suppressive effects of lactate using evidence from rodent and in vitro cellular models. Specifically, we discuss the ability of lactate to 1) inhibit the secretory function of ghrelin producing gastric cells, 2) modulate the signaling cascades that control hypothalamic neuropeptide expression/release, and 3) inhibit signaling through the ghrelin receptor in the hypothalamus. Unravelling the role of lactate as an appetite-regulatory molecule can shed important insight into the regulation of EI, thereby contributing to the development of interventions aimed at combatting overweight and obesity.

The Potential Roles of Ghrelin in Metabolic Syndrome and Secondary Symptoms of Alzheimer's Disease

Although the major causative factors of Alzheimer's disease (AD) are the accumulation of amyloid β and hyperphosphorylated tau, AD can also be caused by metabolic dysfunction. The major clinical symptom of AD is cognitive dysfunction. However, AD is also accompanied by various secondary symptoms such as depression, sleep-wake disturbances, and abnormal eating behaviors. Interestingly, the orexigenic hormone ghrelin has been suggested to have beneficial effects on AD-related metabolic syndrome and secondary symptoms. Ghrelin improves lipid distribution and alters insulin sensitivity, effects that are hypothesized to delay the progression of AD. Furthermore, ghrelin can relieve depression by enhancing the secretion of hormones such as serotonin, noradrenaline, and orexin. Moreover, ghrelin can upregulate the expression of neurotrophic factors such as brain-derived neurotrophic factor and modulate the release of proinflammatory cytokines such as tumor necrosis factor α and interleukin 1β. Ghrelin alleviates sleep-wake disturbances by increasing the levels of melatonin, melanin-concentrating hormone. Ghrelin reduces the risk of abnormal eating behaviors by increasing neuropeptide Y and γ-aminobutyric acid. In addition, ghrelin increases food intake by inhibiting fatty acid biosynthesis. However, despite the numerous studies on the role of ghrelin in the AD-related pathology and metabolic disorders, there are only a few studies that investigate the effects of ghrelin on secondary symptoms associated with AD. In this mini review, our purpose is to provide the insights of future study by organizing the previous studies for the role of ghrelin in AD-related pathology and metabolic disorders.

Involvement of the GHSR in the developmental programming and metabolic disturbances induced by maternal undernutrition

The mismatch between maternal undernutrition and adequate nutrition after birth increases the risk of developing metabolic diseases. We aimed to investigate whether the hyperghrelinemia during maternal undernourishment rewires the hypothalamic development of the offspring and contributes to the conversion to an obese phenotype when fed a high-fat diet (HFD). Pregnant C57BL/6 J, wild type (WT) and ghrelin receptor (GHSR)^-/- mice were assigned to either a normal nourished (NN) group, or an undernutrition (UN) (30% food restricted) group. All pups were fostered by NN Swiss mice. After weaning, pups were fed a normal diet, followed by a HFD from week 9. Plasma ghrelin levels peaked at postnatal day 15 (P15) in both C57BL/6 J UN and NN pups. Hypothalamic Ghsr mRNA expression was upregulated at P15 in UN pups compared to NN pups and inhibited agouti-related peptide (AgRP) projections. Adequate lactation increased body weight of UN WT but not of GHSR^-/- pups compared to NN littermates. After weaning with a HFD, body weight and food intake was higher in WT UN pups but lower in GHSR^-/- UN pups than in NN controls. The GHSR prevented a decrease in ambulatory activity and oxygen consumption in UN offspring during ad libitum feeding. Maternal undernutrition triggers developmental changes in the hypothalamus in utero which were further affected by adequate feeding after birth during the postnatal period by affecting GHSR signaling. The GHSR contributes to the hyperphagia and the increase in body weight when maternal undernutrition is followed by an obesity prone life environment.

Ghrelin Receptor Stimulation of the Lateral Parabrachial Nucleus in Rats Increases Food Intake but not Food Motivation

OBJECTIVE

The lateral parabrachial nucleus (lPBN) in the brainstem has emerged as a key area involved in feeding control that is targeted by several circulating anorexigenic hormones. Here, the objective was to determine whether the lPBN is also a relevant site for the orexigenic hormone ghrelin, inspired by studies in mice and rats showing that there is an abundance of ghrelin receptors in this area.

METHODS

This study first explored whether iPBN cells respond to ghrelin involving Fos mapping and electrophysiological studies in rats. Next, rats were injected acutely with ghrelin, a ghrelin receptor antagonist, or vehicle into the lPBN to investigate feeding-linked behaviors.

RESULTS

Curiously, ghrelin injection (intracerebroventricular or intravenous) increased Fos protein expression in the lPBN yet the predominant electrophysiological response was inhibitory. Intra-lPBN ghrelin injection increased chow or high-fat diet intake, whereas the antagonist decreased chow intake only. In a choice paradigm, intra-lPBN ghrelin increased intake of chow but not lard or sucrose. Intra-lPBN ghrelin did not alter progressive ratio lever pressing for sucrose or conditioned place preference for chocolate.

CONCLUSIONS

The lPBN is a novel locus from which ghrelin can alter consummatory behaviors (food intake and choice) but not appetitive behaviors (food reward and motivation).

Variation analysis of Ghrelin gene in Chinese patients with obesity, having polycystic ovarian syndrome

Association of single-nucleotide polymorphisms (SNPs) of the ghrelin gene with polycystic ovary syndrome (PCOS)is unclear. However, their correlation with PCOS-related obesity has been observed. The objective of this study was to evaluate the effects of ghrelin gene SNPs on PCOS-related obesity in Chinese women. The full-length sequence of the ghrelin gene was determined to explore the relationship of the SNPs with PCOS-related obesity in Chinese women. The gene was sequenced, including all exons, introns and exon-intron boundaries in 230 Han Chinese women with PCOS and 162 normal women. Significant genotypic and allelic differences were observed between the obese PCOS group and obese control group at rs35681 locus (p = .013 and .017). The genotypic analysis of obese and non-obese people in the PCOS group showed that the proportion of A allele in the obese PCOS group (10.9%) was higher than that of the G allele (3.6%). This study revealed that ghrelin rs35681 might be related to the occurrence of obesity associated with PCOS, and allele A was found to increase the risk of obesity in PCOS.

An overview of ghrelin O-acyltransferase inhibitors: a literature and patent review for 2010-2019

INTRODUCTION

The peptide hormone ghrelin regulates physiological processes associated with energy homeostasis such as appetite, insulin signaling, glucose metabolism, and adiposity. Ghrelin has also been implicated in a growing number of neurological pathways involved in stress response and addiction behavior. For ghrelin to bind the growth hormone secretagogue receptor 1a (GHS-R1a) and activate signaling, the hormone must first be octanoylated on a specific serine side chain. This key transformation is performed by the enzyme ghrelin O-acyltransferase (GOAT), and therefore GOAT inhibitors may be useful in treating disorders related to ghrelin signaling such as diabetes, obesity, and related metabolic syndromes.

AREAS COVERED

This report covers ghrelin and GOAT as potential therapeutic targets and summarizes work on GOAT inhibitors through the end of 2019, highlighting recent successes with both peptidomimetics and small molecule GOAT inhibitors as potent modulators of GOAT-catalyzed ghrelin octanoylation.

EXPERT OPINION

A growing body of biochemical and structural knowledge regarding the ghrelin/GOAT system now enables multiple avenues for identifying and optimizing GOAT inhibitors. We are at the beginning of a new era with increased opportunities for leveraging ghrelin and GOAT in the understanding and treatment of multiple health conditions including diabetes, obesity, and addiction.

LEAP2 changes with body mass and food intake in humans and mice

Acyl-ghrelin administration increases food intake, body weight, and blood glucose. In contrast, mice lacking ghrelin or ghrelin receptors (GHSRs) exhibit life-threatening hypoglycemia during starvation-like conditions, but do not consistently exhibit overt metabolic phenotypes when given ad libitum food access. These results, and findings of ghrelin resistance in obese states, imply nutritional state dependence of ghrelin's metabolic actions. Here, we hypothesized that liver-enriched antimicrobial peptide-2 (LEAP2), a recently characterized endogenous GHSR antagonist, blunts ghrelin action during obese states and postprandially. To test this hypothesis, we determined changes in plasma LEAP2 and acyl-ghrelin due to fasting, eating, obesity, Roux-en-Y gastric bypass (RYGB), vertical sleeve gastrectomy (VSG), oral glucose administration, and type 1 diabetes mellitus (T1DM) using humans and/or mice. Our results suggest that plasma LEAP2 is regulated by metabolic status: its levels increased with body mass and blood glucose and decreased with fasting, RYGB, and in postprandial states following VSG. These changes were mostly opposite of those of acyl-ghrelin. Furthermore, using electrophysiology, we showed that LEAP2 both hyperpolarizes and prevents acyl-ghrelin from activating arcuate NPY neurons. We predict that the plasma LEAP2/acyl-ghrelin molar ratio may be a key determinant modulating acyl-ghrelin activity in response to body mass, feeding status, and blood glucose.

The Physiological Role of Ghrelin in the Regulation of Energy and Glucose Homeostasis

Ghrelin is a peptide hormone that is primarily released from the stomach. It is best known for its role in appetite initiation. However, evidence also suggests that ghrelin may play a much wider role in energy homeostasis and glucose metabolism. It is known that exogenous ghrelin exerts an orexigenic signal via growth hormone secretagogue receptors in the arcuate nucleus of the hypothalamus. However, blocking ghrelin signalling in the arcuate nucleus does not decrease feeding. Evidence now proposes that an alternative pathway for ghrelin’s action is via the vagus nerve. Furthermore, it has been suggested that ghrelin signalling is an important physiological regulator of body adiposity and energy storage. Ghrelin also seems to be important in controlling glucose metabolism through action in the pancreatic islets of Langerhans, representing a promising novel therapeutic target in diabetes treatment. Despite these findings, further research in humans is required before ghrelin can be indicated as a therapeutic target in obesity or diabetes. This review summarises the current literature concerning ghrelin’s physiological roles in energy and glucose homeostasis.