Ghrelin stimulates locomotor activity and accumbal dopamine-overflow via central cholinergic systems in mice: implications for its involvement in brain reward

Diet-induced obesity attenuates fasting-induced hyperphagia

Obesity impairs arcuate (ARC) neuropeptide Y (NPY)/agouti-releated peptide (AgRP) neuronal function and renders these homeostatic neurones unresponsive to the orexigenic hormone ghrelin. In the present study, we investigated the effect of diet-induced obesity (DIO) on feeding behaviour, ARC neuronal activation and mRNA expression following another orexigenic stimulus, an overnight fast. We show that 9 weeks of high-fat feeding attenuates fasting-induced hyperphagia by suppressing ARC neuronal activation and hypothalamic NPY/AgRP mRNA expression. Thus, the lack of appropriate feeding responses in DIO mice to a fast is caused by failure ARC neurones to recognise and/or respond to orexigenic cues. We propose that fasting-induced hyperphagia is regulated not by homeostatic control of appetite in DIO mice, but rather by changes in the reward circuitry.

Ghrelin mediates stress-induced food-reward behavior in mice

The popular media and personal anecdotes are rich with examples of stress-induced eating of calorically dense "comfort foods." Such behavioral reactions likely contribute to the increased prevalence of obesity in humans experiencing chronic stress or atypical depression. However, the molecular substrates and neurocircuits controlling the complex behaviors responsible for stress-based eating remain mostly unknown, and few animal models have been described for probing the mechanisms orchestrating this response. Here, we describe a system in which food-reward behavior, assessed using a conditioned place preference (CPP) task, is monitored in mice after exposure to chronic social defeat stress (CSDS), a model of prolonged psychosocial stress, featuring aspects of major depression and posttraumatic stress disorder. Under this regime, CSDS increased both CPP for and intake of high-fat diet, and stress-induced food-reward behavior was dependent on signaling by the peptide hormone ghrelin. Also, signaling specifically in catecholaminergic neurons mediated not only ghrelin's orexigenic, antidepressant-like, and food-reward behavioral effects, but also was sufficient to mediate stress-induced food-reward behavior. Thus, this mouse model has allowed us to ascribe a role for ghrelin-engaged catecholaminergic neurons in stress-induced eating.

The role of the central ghrelin system in reward from food and chemical drugs

Here we review recent advances that identify a role for the central ghrelin signalling system in reward from both natural rewards (such as food) and artificial rewards (that include alcohol and drugs of abuse). Whereas ghrelin emerged as a stomach-derived hormone involved in energy balance, hunger and meal initiation via hypothalamic circuits, it now seems clear that it also has a role in motivated reward-driven behaviours via activation of the so-called "cholinergic-dopaminergic reward link". This reward link comprises a dopamine projection from the ventral tegmental area (VTA) to the nucleus accumbens together with a cholinergic input, arising primarily from the laterodorsal tegmental area. Ghrelin administration into the VTA or LDTg activates the "cholinergic-dopaminergic" reward link, suggesting that ghrelin may increase the incentive value of motivated behaviours such as reward-seeking behaviour ("wanting" or "incentive motivation"). Further, direct injection of ghrelin into the brain ventricles or into the VTA increases the consumption of rewarding foods as well as alcohol in mice and rats. Studies in rodents show beneficial effects of ghrelin receptor (GHS-R1A) antagonists to suppress the intake of palatable food, to reduce preference for caloric foods, to suppress food reward and motivated behaviour for food. They have also been shown to reduce alcohol consumption, suppress reward induced by alcohol, cocaine and amphetamine. Furthermore, variations in the GHS-R1A and pro-ghrelin genes have been associated with high alcohol consumption, smoking and increased weight gain in alcohol dependent individuals as well as with bulimia nervosa and obesity. Thus, the central ghrelin signalling system interfaces neurobiological circuits involved in reward from food as well as chemical drugs; agents that directly or indirectly suppress this system emerge as potential candidate drugs for suppressing problematic over-eating that leads to obesity as well as for the treatment of substance use disorder.

Reduced locomotor responses to cocaine in ghrelin-deficient mice

Ghrelin, an orexigenic hormone produced by the stomach, increases food intake and enhances the locomotor and rewarding effects of cocaine. Consistent with these behavioral effects, ghrelin increases dopamine cell activity in the mesolimbic system resulting in elevated levels of dopamine release and turnover in target regions such as the ventral striatum. In the current study, we examined the psychostimulant effects of acute and daily cocaine in mice with targeted deletion of the ghrelin gene (ghrelin knockout; KO) and that of their wild-type (WT) littermates. We hypothesized that ghrelin-KO mice would be hyporesponsive to the effects of cocaine as reflected in attenuated locomotor activity following both acute and chronic injections, and that this would be correlated with striatal dopamine and dopamine metabolite concentrations. Results show that the locomotor stimulating effect of cocaine (10 mg/kg) was decreased in ghrelin-KO mice as compared with their WT littermates. In addition, repeated daily injection of cocaine resulted in gradual increases in locomotor activity in WT mice, an effect that was attenuated in ghrelin-KO mice. These behavioral effects were correlated with changes in dopamine utilization in the striatum of WT mice that were not seen in ghrelin-KO mice unless these were pretreated with ghrelin. These data suggest that ghrelin is important for normal function of the mesolimbic dopaminergic system, potentially modulating both dopamine release and reuptake.

Role for insulin signaling in catecholaminergic neurons in control of energy homeostasis

Dopaminergic midbrain neurons integrate signals on food palatability and food-associated reward into the complex control of energy homeostasis. To define the role of insulin receptor (IR) signaling in this circuitry, we inactivated IR signaling in tyrosine hydroxylase (Th)-expressing cells of mice (IR(ΔTh)). IR inactivation in Th-expressing cells of mice resulted in increased body weight, increased fat mass, and hyperphagia. While insulin acutely stimulated firing frequency in 50% of dopaminergic VTA/SN neurons, this response was abolished in IR(ΔTh) mice. Moreover, these mice exhibited an altered response to cocaine under food-restricted conditions. Taken together, these data provide in vivo evidence for a critical role of insulin signaling in catecholaminergic neurons to control food intake and energy homeostasis.

The alcohol-induced locomotor stimulation and accumbal dopamine release is suppressed in ghrelin knockout mice

Ghrelin, the first endogenous ligand for the type 1A growth hormone secretagogue receptor (GHS-R1A), plays a role in energy balance, feeding behavior, and reward. Previously, we showed that pharmacologic and genetic suppression of the GHS-R1A attenuates the alcohol-induced stimulation, accumbal dopamine release, and conditioned place preference as well as alcohol consumption in mice, implying that the GHS-R1A is required for alcohol reward. The present study further elucidates the role of ghrelin for alcohol-induced dopamine release in nucleus accumbens and locomotor stimulation by means of ghrelin knockout mice. We found that the ability of alcohol to increase accumbal dopamine release in wild-type mice is not observed in ghrelin knockout mice. Furthermore, alcohol induced a locomotor stimulation in the wild-type mice and ghrelin knockout mice; however, the locomotor stimulation in homozygote mice was significantly lower than in the wild-type mice. The present series of experiments suggest that endogenous ghrelin may be required for the ability of alcohol to activate the mesolimbic dopamine system.

Lifestyle factors and ghrelin: critical review and implications for weight loss maintenance

Ghrelin, the only known appetite-stimulating hormone in humans, may be one factor involved in increased appetite, cravings and food intake following weight loss. Innovative strategies for suppressing ghrelin and decreasing appetite during weight loss maintenance are needed. Recent research has highlighted relationships between ghrelin, stress and lifestyle factors. The purposes of the current review are to (i) describe the current status of knowledge about ghrelin and lifestyle factors; (ii) critically examine research in this area, highlighting inconsistencies and methodological issues and (iii) discuss future directions and implications for obesity treatment. Based on Literature search using PsycINFO and Medline databases, we reviewed experimental studies on relationships between ghrelin, stress, exercise and sleep. Ghrelin levels are positively related to stress hormones, and stress management interventions including exercise and sleep may help to reduce acylated ghrelin and corresponding appetite. Behavioural interventions may offer a practical, cost-effective alternative for reducing or stabilizing ghrelin levels after initial weight loss. Adding behavioural techniques designed to reduce ghrelin to traditional weight loss maintenance protocols may help individuals to maintain weight loss. Future directions for investigating relationships between ghrelin and behavioural factors, examining the efficacy of behavioural programmes in reducing ghrelin and improving weight loss maintenance are discussed.

Acute and chronic suppression of the central ghrelin signaling system reveals a role in food anticipatory activity

Using the rodent activity-based anorexia (ABA) model that mimics clinical features of anorexia nervosa that include food restriction-induced hyperlocomotion, we found that plasma ghrelin levels are highly associated with food anticipatory behaviour, measured by running wheel activity in rats. Furthermore, we showed that ghrelin receptor (GHS-R1A) knockout mice do not anticipate food when exposed to the ABA model, unlike their wild type littermate controls. Likewise, food anticipatory activity in the ABA model was suppressed by a GHS-R1A antagonist administered either by acute central (ICV) injection to rats or by chronic peripheral treatment to mice. Interestingly, the GHS-R1A antagonist did not alter food intake in any of these models. Therefore, we hypothesize that suppression of the central ghrelin signaling system via GHS-R1A provides an interesting therapeutic target to treat hyperactivity in patients suffering from anorexia nervosa.

Effects of 18-methoxycoronaridine on ghrelin-induced increases in sucrose intake and accumbal dopamine overflow in female rats

RATIONALE

18-methoxycoronaridine (18-MC), a selective antagonist of α3β4 nicotinic receptors, has been previously shown, in rats, to reduce the self-administration of several drugs of abuse, reduce operant responding for sucrose, and prevent the development of sucrose-induced obesity. It has become increasingly apparent that there is a significant overlap between the systems regulating drug reward and food intake, therefore, we investigated whether 18-MC might modulate the effects of ghrelin, one of several orexigenic peptides recently implicated in both feeding and drug reward.

OBJECTIVES

In female Sprague-Dawley rats, we determined whether acute 18-MC treatment would reduce both ghrelin-induced increases in sucrose intake and ghrelin-elicited increases in accumbal dopamine levels.

RESULTS

Pretreatment with 18-MC (20 mg/kg, i.p.), given prior to the administration of ghrelin (1 μg, lateral ventricle), blocked ghrelin-induced increases in sucrose (5%) intake in a two-bottle open access paradigm. Using in vivo microdialysis, 18-MC (both 20 and 40 mg/kg) prevented ghrelin (2 μg, intraventral tegmental area)-induced increases in extracellular dopamine in the nucleus accumbens. 18-MC had no effect on deposition of fat or on serum levels of glucose, triglycerides, and cholesterol in ghrelin-treated rats.

CONCLUSIONS

The present results suggest that one potential mechanism by which 18-MC exerts its effects on palatable food consumption is via modulation of ghrelin's effects.

Ghrelin, peptide YY and their hypothalamic targets differentially regulate spontaneous physical activity

Recent studies suggest that spontaneous physical activity (SPA) may be under the non-conscious control of neuroendocrine circuits that are known to control food intake. To further elucidate endocrine gut-brain communication as a component of such circuitry, we here analyzed long-term and acute effects of the gastrointestinal hormones ghrelin and PYY 3-36 as well as their hypothalamic neuropeptide targets NPY, AgRP and POMC (alpha-MSH), on locomotor activity and home cage behaviors in rats. For the analysis of SPA, we used an automated infrared beam break activity measuring system, combined with a novel automated video-based behavior analysis system (HomeCageScan (HCS)). Chronic (one-month) peripheral infusion of ghrelin potently increased body weight and fat mass in rats. Such positive energy balance was intriguingly not due to an overall increased caloric ingestion, but was predominantly associated with a decrease in SPA. Chronic intracerebroventricular infusion (7 days) of ghrelin corroborated the decrease in SPA and suggested a centrally mediated mechanism. Central administration of AgRP and NPY increased food intake as expected. AgRP administration led to a delayed decrease in SPA, while NPY acutely (but transiently) increased SPA. Behavioral dissection using HCS corroborated the observed acute and transient increases of food intake and SPA by central NPY infusion. Acute central administration of alpha-MSH rapidly decreased food intake but did not change SPA. Central administration of the NPY receptor agonist PYY 3-36 transiently increased SPA. Our data suggest that the control of spontaneous physical activity by gut hormones or their neuropeptide targets may represent an important mechanistic component of energy balance regulation.

Signaling through the ghrelin receptor modulates hippocampal function and meal anticipation in mice

The ability to predict a particular meal is achieved in part by learned associations with stimuli that predict nutrient availability. Ghrelin is an orexigenic peptide produced by both the gut and brain that rises before anticipated meals and it has been suggested that pre-prandial ghrelin increases may act as a signal to predict meal delivery. Here, we used wild type and ghrelin receptor deficient mice to test the hypothesis that ghrelin signaling is necessary for the processing of emotionally relevant stimuli, spatial learning and habituated feeding responses. We tested spatial and fear-related memory with the Morris water maze and step through passive avoidance tests, respectively and utilized food anticipatory activity to monitor habituated feeding responses following two weeks of a meal feeding paradigm. Our results indicate that ghrelin signaling modulates spatial memory performance and is necessary for the development of food anticipatory activity. Collectively, these results suggest that ghrelin receptor signaling is necessary for adaptations in the anticipatory responses that accompany restricted feeding.

Implications from addiction research towards the understanding and treatment of obesity

Recent research indicates similarities between obesity and addictive disorders on both the phenomenological and neurobiological level. In particular, neuroendocrine and imaging studies suggest a close link between the homeostatic regulation of appetite on the on hand, and motivation and reward expectancy on the other. In addition, findings from neuropsychological studies additionally demonstrate alterations of cognitive function in both obesity and addictive disorders that possibly contribute to a lack of control in resisting consumption. In this review, recent findings on overlapping neurobiological and phenomenological pathways are summarized and the impact with regard to new treatment approaches for obesity is discussed.

Peripheral ghrelin stimulates feeding behavior and positive energy balance in a sciurid hibernator

Hibernators exhibit a robust circannual cycle of body mass gain and loss primarily mediated by food intake, but the pathways controlling food intake in these animals have not been fully elucidated. Ghrelin is an orexigenic hormone that increases feeding in all mammals studied so far, but has not until recently been studied in hibernators. In other mammals, ghrelin stimulates feeding through phosphorylation and activation of AMP-activated protein kinase (AMPK). Activation of AMPK phosphorylates and deactivates acetyl Co-A carboxylase (ACC), a committed step in fatty acid synthesis. In order to determine the effects of exogenous ghrelin on food intake and metabolic factors (i.e. non-esterified fatty acids (NEFAs), and hypothalamic AMPK and ACC) in hibernators, ghrelin was peripherally injected into ground squirrels in all four seasons. Changes in food intake and body mass were recorded over a 2-6 hour period post injections, and squirrels were euthanized. Brains and blood were removed, and Western blots were performed to determine changes in phosphorylation of hypothalamic AMPK and ACC. A colorimetric assay was used to determine changes in concentration of serum NEFAs. We found that food intake, body mass, and locomotor activity significantly increased with ghrelin injections versus saline-injected controls, even in animals injected during their aphagic winter season. Injected ghrelin was correlated with increased phosphorylation of AMPK, but didn't have an effect on ACC in winter. Ghrelin-injected animals also had increased levels of serum NEFAs compared with saline controls. This study is the first to show an effect of injected ghrelin on a hibernator.

The ghrelin signalling system is involved in the consumption of sweets

The gastric-derived orexigenic peptide ghrelin affects brain circuits involved in energy balance as well as in reward. Indeed, ghrelin activates an important reward circuit involved in natural- as well as drug-induced reward, the cholinergic-dopaminergic reward link. It has been hypothesized that there is a common reward mechanism for alcohol and sweet substances in both animals and humans. Alcohol dependent individuals have higher craving for sweets than do healthy controls and the hedonic response to sweet taste may, at least in part, depend on genetic factors. Rat selectively bred for high sucrose intake have higher alcohol consumption than non-sucrose preferring rats and vice versa. In the present study a group of alcohol-consuming individuals selected from a population cohort was investigated for genetic variants of the ghrelin signalling system in relation to both their alcohol and sucrose consumption. Moreover, the effects of GHS-R1A antagonism on voluntary sucrose-intake and operant self-administration, as well as saccharin intake were investigated in preclinical studies using rodents. The effects of peripheral grelin administration on sucrose intake were also examined. Here we found associations with the ghrelin gene haplotypes and increased sucrose consumption, and a trend for the same association was seen in the high alcohol consumers. The preclinical data show that a GHS-R1A antagonist reduces the intake and self-administration of sucrose in rats as well as saccharin intake in mice. Further, ghrelin increases the intake of sucrose in rats. Collectively, our data provide a clear indication that the GHS-R1A antagonists reduces and ghrelin increases the intake of rewarding substances and hence, the central ghrelin signalling system provides a novel target for the development of drug strategies to treat addictive behaviours.

Expression of the gene encoding the ghrelin receptor in rats selected for differential alcohol preference

The mechanisms involved in alcohol use disorder, a chronic relapsing brain disorder, are complex and involve various signalling systems in the brain. Recently, the orexigenic peptide ghrelin was shown to be required for alcohol-induced reward, an effect mediated via ghrelin receptors, GHS-R1A, at the level of the cholinergic-dopaminergic reward link. Moreover, ghrelin increases and GHR-R1A antagonists reduce moderate alcohol consumption in mice, and a single nucleotide polymorphism in the GHS-R1A gene has been associated with high alcohol consumption in humans. Therefore, GHS-R1A gene expression and alcohol intake were investigated in high, AA (Alko, Alcohol), versus low, ANA (Alko, Non-Alcohol), alcohol consuming rats as well as in Wistar rats. In the AA and ANA rats plasma ghrelin levels were also measured. GHS-R1A gene expression was increased in AA compared to ANA rats in nucleus accumbens, ventral tegmental area, amygdala, prefrontal cortex and hippocampus. A similar trend was observed in the ventral tegmental area of Wistar rats consuming high amounts of alcohol. Furthermore, the AA rats had significantly smaller reduction of plasma ghrelin levels over time, after several weeks of alcohol exposure, than had the ANA rats. The present study provides further evidence for that the ghrelin signalling system, in particular at the level of the mesocortocolimbic dopamine system, is involved in alcohol consumption, and thus possibly contributes to alcohol use disorder. Therefore the GHS-R1A may constitute a novel candidate for development of new treatment strategies for alcohol dependence.

Reward mechanisms in obesity: new insights and future directions

Food is consumed in order to maintain energy balance at homeostatic levels. In addition, palatable food is also consumed for its hedonic properties independent of energy status. Such reward-related consumption can result in caloric intake exceeding requirements and is considered a major culprit in the rapidly increasing rates of obesity in developed countries. Compared with homeostatic mechanisms of feeding, much less is known about how hedonic systems in brain influence food intake. Intriguingly, excessive consumption of palatable food can trigger neuroadaptive responses in brain reward circuitries similar to drugs of abuse. Furthermore, similar genetic vulnerabilities in brain reward systems can increase predisposition to drug addiction and obesity. Here, recent advances in our understanding of the brain circuitries that regulate hedonic aspects of feeding behavior will be reviewed. Also, emerging evidence suggesting that obesity and drug addiction may share common hedonic mechanisms will also be considered.

Ghrelin directly targets the ventral tegmental area to increase food motivation

Ghrelin, a circulating orexigenic stomach-derived hormone, has recently been implicated in extra-homeostatic feeding, increasing food reward and food-motivated behavior. The precise target site(s) for ghrelin's effects on food reward have yet to be elucidated. The neurocircuitry underpinning food-motivated behavior involves, in particular, the dopamine cells of the ventral tegmental area (VTA) that project to the nucleus accumbens (NAcc). Ghrelin stimulation in both of these mesolimbic reward areas increases chow intake. Here we sought to determine if ghrelin acts directly within these mesolimbic reward areas to increase food reward/motivation in studies that combine feeding behavior, pharmacology, and neuroanatomy. We found that motivated behavior for a sucrose reward, assessed in an operant conditioning paradigm in rats, was increased when ghrelin was microinjected directly into the VTA but not into the NAcc. By contrast, ghrelin administration to both areas increased the free feeding of chow. Importantly, in a state of overnight food restriction, where endogenous levels of ghrelin are increased, ghrelin receptor (GHS-R1A) blockade in the VTA was sufficient to decrease the motivation to work for a sugar reward. Blockade of the GHS-R1A in VTA or NAcc was not sufficient to reduce fasting-induced chow hyperphagia. Taken together our data identify the VTA but not the NAcc as a direct, necessary, and sufficient target site for ghrelin's action on food motivation.

Central administration of ghrelin alters emotional responses in rats: behavioural, electrophysiological and molecular evidence

The orexigenic and pro-obesity hormone ghrelin targets key hypothalamic and mesolimbic circuits involved in energy balance, appetite and reward. Given that such circuits are closely integrated with those regulating mood and cognition, we sought to determine whether chronic (>2 weeks) CNS exposure to ghrelin alters anxiety- and depression-like behaviour in rats as well as some physiological correlates. Rats bearing chronically implanted i.c.v. catheters were treated with ghrelin (10 μg/d) or vehicle for 4 weeks. Tests used to assess anxiety- and depression-like behaviour were undertaken during weeks 3-4 of the infusion. These revealed an increase in anxiety- and depression-like behaviour in the ghrelin-treated rats relative to controls. At the end of the 4-week infusion, brains were removed and the amygdala dissected for subsequent qPCR analysis that revealed changes in expression of a number of genes representing key systems implicated in these behavioural changes. Finally, given the key role of the dorsal raphe serotonin system in emotional reactivity, we examined the electrophysiological response of dorsal raphe neurons after a ghrelin challenge, and found mainly inhibitory responses in this region. We demonstrate that the central ghrelin signalling system is involved in emotional reactivity in rats, eliciting pro-anxiety and pro-depression effects and have begun to explore novel target systems for ghrelin that may be of importance for these effects.

Relationship between gut and sepsis: Role of ghrelin

Ghrelin is a growth hormone secretagogue produced by the gut, and is expressed in the hypothalamus and other tissues as well. Ghrelin not only plays an important role in the regulation of appetite, energy balance and glucose homeostasis, but also shows anti-bacterial activity, suppresses pro-inflammatory cytokine production and restores gut barrier function. In experimental animals, ghrelin has shown significant beneficial actions in preventing mortality from sepsis. In the critically ill, corticosteroid insufficiency as a result of dysfunction of the hypothalamic-pituitary-adrenal axis is known to occur. It is therefore possible that both gut and hypothalamus play an important role in the pathogenesis of sepsis by virtue of their ability to produce ghrelin, which, in turn, could be a protective phenomenon to suppress inflammation. It remains to be seen whether ghrelin and its analogues are of benefit in treating patients with sepsis.

Central dopaminergic circuitry controlling food intake and reward: implications for the regulation of obesity

Prevalence of obesity in the general population has increased in the past 15 years from 15% to 35%. With increasing obesity, the coincident medical and social consequences are becoming more alarming. Control over food intake is crucial for the maintenance of body weight and represents an important target for the treatment of obesity. Central nervous system mechanisms responsible for control of food intake have evolved to sense the nutrient and energy levels in the organism and to coordinate appropriate responses to adjust energy intake and expenditure. This homeostatic system is crucial for maintenance of stable body weight over long periods of time of uneven energy availability. However, not only the caloric and nutritional value of food but also hedonic and emotional aspects of feeding affect food intake. In modern society, the increased availability of highly palatable and rewarding (fat, sweet) food can significantly affect homeostatic balance, resulting in dysregulated food intake. This review will focus on the role of hypothalamic and mesolimbic/mesocortical dopaminergic (DA) circuitry in coding homeostatic and hedonic signals for the regulation of food intake and maintenance of caloric balance. The interaction of dopamine with peripheral and central indices of nutritional status (e.g., leptin, ghrelin, neuropeptide Y), and the susceptibility of the dopamine system to prenatal insults will be discussed. Additionally, the importance of alterations in dopamine signaling that occur coincidently with obesity will be addressed.

Glutamatergic regulation of ghrelin-induced activation of the mesolimbic dopamine system

Recently, we demonstrated that the central ghrelin signalling system, involving the ghrelin receptor (GHS-R1A), is important for alcohol reinforcement. Ghrelin targets a key mesolimbic circuit involved in natural as well as drug-induced reinforcement, that includes a dopamine projection from the ventral tegmental area (VTA) to the nucleus accumbens. The aim of the present study was to determine whether it is possible to suppress ghrelin's effects on this mesolimbic dopaminergic pathway can be suppressed, by interrupting afferent inputs to the VTA dopaminergic cells, as shown previously for cholinergic afferents. Thus, the effects of pharmacological suppression of glutamatergic, orexin A and opioid neurotransmitter systems on ghrelin-induced activation of the mesolimbic dopamine system were investigated. We found in the present study that ghrelin-induced locomotor stimulation was attenuated by VTA administration of the N-methyl-D-aspartic acid receptor antagonist (AP5) but not by VTA administration of an orexin A receptor antagonist (SB334867) or by peripheral administration of an opioid receptor antagonist (naltrexone). Intra-VTA administration of AP5 also suppressed the ghrelin-induced dopamine release in the nucleus accumbens. Finally the effects of peripheral ghrelin on locomotor stimulation and accumbal dopamine release were blocked by intra-VTA administration of a GHS-R1A antagonist (BIM28163), indicating that GHS-R1A signalling within the VTA is required for the ghrelin-induced activation of the mesolimbic dopamine system. Given the clinical knowledge that hyperghrelinemia is associated with addictive behaviours (such as compulsive overeating and alcohol use disorder) our finding highlights a potential therapeutic strategy involving glutamatergic control of ghrelin action at the level of the mesolimbic dopamine system.

Systemic ghrelin and reward: effect of cholinergic blockade

AIMS

Ghrelin is one of the most potent orexigens known to date. Recent data suggested that ghrelin is involved in reward-mediated processes such as the rewarding value of food. Whereas the neuronal pathways by which ghrelin regulates energy balance are well described, those involved in ghrelin-induced reward are still confusing. Therefore, we attempted to delineate the involvement of physiological and pharmacological rises in plasma ghrelin in the modulation of food reward seeking behaviours, using the classical conditioned place preference (CPP) procedure in C57BL6J mice, as well as in mice lacking the ghrelin receptor (GHSR1a -/-). We also determined whether these effects on reward-related behaviours could be partly mediated by cholinergic pathways by pre-treating mice with mecamylamine.

RESULTS

Upon moderate caloric restriction, systemic ghrelin levels increased from 108 ± 21 to 148 ± 39 pg/ml in C57BL6J mice and from 111 ± 24 to 179 ± 41 pg/ml in GHSR1a-null mice. Short exposure to rewarding food elicited a strong CPP and stimulation of locomotor activity in GHSR1a wild-type and C57BL6J mice. Conversely, the GHSR1a -/- mice did not exhibit such a food CPP, despite a negative energy balance. Pharmacological rise in systemic ghrelin further increased the time spent in the food-paired side with a higher CPP score (+71%) and this effect was blunted after cholinergic blockade by mecamylamine.

CONCLUSIONS

The ghrelin receptor is obligatory to acquire a food-CPP. The level of plasma ghrelin during conditioning determines the strength of food-induced reward seeking behaviours. The cholinergic pathway partly mediates the further enhancement of food reward induced by pharmacological rises in plasma ghrelin, but not that induced by physiological increases in ghrelin.

Serum ghrelin is inversely associated with cognitive function in a sample of non-demented elderly

AIM

The orexigenic hormone, ghrelin, is linked to learning and memory in animal studies. No previous study has investigated whether cognition is related to ghrelin in the non-demented elderly.

METHODS

Thirty-five older adults underwent neuropsychological testing and fasting blood draw with subsequent serum ghrelin quantification.

RESULTS

Ghrelin was negatively correlated with several cognitive domains, including verbal memory, working memory, and naming.

CONCLUSION

Areas of cognition associated with ghrelin level were similar to the pattern of deficits observed in early Alzheimer's disease. Findings suggest a potential moderational role of ghrelin in pathological cognitive decline. Further work investigating mechanisms is needed.

Role of feeding-related pathways in alcohol dependence: A focus on sweet preference, NPY, and ghrelin

Converging research evidence suggests that alcohol and food-seeking behaviors share common neural pathways. There is preclinical and clinical evidence linking the consumption of sweets to alcohol intake in both animals and humans. In addition, a growing body of animal and human literature suggests the involvement of "feeding-related" peptides in alcohol-seeking behavior. In particular, both central and peripheral appetitive peptides have shown a possible role in alcohol dependence. The present mini-review will summarize the literature on the link between sweet preference and alcohol dependence, and on the role of feeding-related peptides in alcohol dependence. Specifically, in an attempt to narrow the field, the present mini-review will focus on 2 specific pathways, the central neuropeptide Y and the peripheral gut peptide ghrelin. Although more research is needed, data available suggest that studying feeding-related pathways in alcohol dependence may have theoretic, biologic, diagnostic, and therapeutic implications.

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

The extra-hypothalamic actions of ghrelin on neuronal function

Ghrelin is a peptide hormone produced and secreted in the stomach. Numerous studies over the past decade demonstrate its importance in food intake, body-weight regulation and glucose homeostasis. These effects are driven largely by the high expression of the ghrelin receptor (GHSR1a) in the hypothalamus. However, GHSR1a is also expressed in numerous extra-hypothalamic neuronal populations, suggesting that ghrelin has physiological functions besides those involved in metabolic functions. In this review, I focus on increasing evidence that ghrelin has important roles in extra-hypothalamic functions, including learning and memory, reward and motivation, anxiety and depression, and neuroprotection. Furthermore, I discuss how the recently demonstrated role of ghrelin in promoting survival during periods of caloric restriction could contribute to its inherent neuroprotective and neuromodulatory properties.

Ghrelin: new molecular pathways modulating appetite and adiposity

Ghrelin is a unique endogenous peptidic hormone regulating both hunger and adiposity. Many of the actions of ghrelin are modulated specifically by the central nervous system. A number of molecular events triggered via the activation of the ghrelin receptor (GHS-R1a), leading to increased levels of neuropeptide Y (NPY) and agoutirelated peptide (AgRP) and ultimately responsible for the orexigenic effect of ghrelin have been characterized. Moreover, the discovery of ghrelin O-acyltransferase (GOAT), the enzyme responsible for the octanoylation of ghrelin, provides a mechanism allowing specific targeting of the ghrelin/GHS-R1a system without affecting the role of des-acyl-ghrelin in other pathways involved in the regulation of energy balance. This review aims to summarize novel roles of ghrelin in energy balance, focusing particularly on both the newly identified neuronal pathways mediating the effects of ghrelin and on peripheral mechanisms leading to increased adiposity.

Ghrelin increases intake of rewarding food in rodents

We investigated whether ghrelin action at the level of the ventral tegmental area (VTA), a key node in the mesolimbic reward system, is important for the rewarding and motivational aspects of the consumption of rewarding/palatable food. Mice with a disrupted gene encoding the ghrelin receptor (GHS-R1A) and rats treated peripherally with a GHS-R1A antagonist both show suppressed intake of rewarding food in a free choice (chow/rewarding food) paradigm. Moreover, accumbal dopamine release induced by rewarding food was absent in GHS-R1A knockout mice. Acute bilateral intra-VTA administration of ghrelin increased 1-hour consumption of rewarding food but not standard chow. In comparison with sham rats, VTA-lesioned rats had normal intracerebroventricular ghrelin-induced chow intake, although both intake of and time spent exploring rewarding food was decreased. Finally, the ability of rewarding food to condition a place preference was suppressed by the GHS-R1A antagonist in rats. Our data support the hypothesis that central ghrelin signaling at the level of the VTA is important for the incentive value of rewarding food.

Ghrelin in central neurons

Ghrelin, an orexigenic peptide synthesized by endocrine cells of the gastric mucosa, is released in the bloodstream in response to a negative energetic status. Since discovery, the hypothalamus was identified as the main source of ghrelin in the CNS, and effects of the peptide have been mainly observed in this area of the brain. In recent years, an increasing number of studies have reported ghrelin synthesis and effects in specific populations of neurons also outside the hypothalamus. Thus, ghrelin activity has been described in midbrain, hindbrain, hippocampus, and spinal cord. The spectrum of functions and biological effects produced by the peptide on central neurons is remarkably wide and complex. It ranges from modulation of membrane excitability, to control of neurotransmitter release, neuronal gene expression, and neuronal survival and proliferation. There is not at present a general consensus concerning the source of ghrelin acting on central neurons. Whereas it is widely accepted that the hypothalamus represents the most important endogenous source of the hormone in CNS, the existence of extra-hypothalamic ghrelin-synthesizing neurons is still controversial. In addition, circulating ghrelin can theoretically be another natural ligand for central ghrelin receptors. This paper gives an overview on the distribution of ghrelin and its receptor across the CNS and critically analyses the data available so far as regarding the effects of ghrelin on central neurotransmission.

18-Methoxycoronaridine, a potential anti-obesity agent, does not produce a conditioned taste aversion in rats

18-Methoxycoronaridine (18-MC), a selective antagonist of alpha3beta4 nicotinic receptors, has been shown to reduce the self-administration of several drugs of abuse. Recently, this agent has also been shown to attenuate sucrose reward, decrease sucrose intake and prevent the development of sucrose-induced obesity in rats. The present experiments were designed to determine whether the latter effect was due to an 18-MC-induced conditioned taste aversion to sucrose. Both 18-MC (20mg/ kg, i.p.) and control agent, lithium chloride (100mg/kg, i.p.), reduced sucrose intake 24h after association with sucrose; however, only lithium chloride reduced sucrose intake 72h later. Consistent with previous data, 18-MC appears to have proactive effect for 24h and it does not induce a conditioned taste aversion.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Ghrelin in chronic kidney disease

Patients with chronic kidney disease (CKD) often exhibit symptoms of anorexia and cachexia, which are associated with decreased quality of life and increased mortality. Chronic inflammation may be an important mechanism for the development of anorexia, cachexia, renal osteodystrophy, and increased cardiovascular risk in CKD. Ghrelin is a gastric hormone. The biological effects of ghrelin are mediated through the growth hormone secretagogue receptor (GHSR). The salutary effects of ghrelin on food intake and meal appreciation suggest that ghrelin could be an effective treatment for anorexic CKD patients. In addition to its appetite-stimulating effects, ghrelin has been shown to possess anti-inflammatory properties. The known metabolic effects of ghrelin and the potential implications in CKD will be discussed in this review. The strength, shortcomings, and unanswered questions related to ghrelin treatment in CKD will be addressed.

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Ghrelin's quick inhibition of androgen-dependent behaviors of male house mice (Mus musculus)

Ghrelin is a peptide hormone released by the stomach that stimulates hunger. Ghrelin also suppresses reproductive physiology by inhibiting the HPG axis. However, to our knowledge, our results are the first to demonstrate ghrelin's quick suppression of sex-hormone-regulated behaviors. In experiment 1, 2 orexigenic i.p. ghrelin injections (0.165 mg/kg and 0.33 mg/kg) suppressed male courtship behavior (ultrasonic calling to a female) and intermale aggression (latency to attack a stimulus male) 20 min following administration. Experiment 2 (examining only the 0.33 mg/kg dose ) replicated ghrelin's suppression of ultrasonic calling and intermale aggression; however, a third behavior, preference for volatile female odors (20 min following administration), was not significantly inhibited. In experiment 2, ghrelin treatment did not affect general locomotor activity (distance traveled 20 min following injection) or seminal vesicle weight (measured 5 days after completing ghrelin injections). We hypothesize that ghrelin's quick suppression of male aggression and ultrasonic mating calls was mediated through its effects on the brain (rather than indirectly through inhibition of the HPG axis).

Ghrelin's Roles in Stress, Mood, and Anxiety Regulation

Several studies suggest that the peptide hormone ghrelin mediates some of the usual behavioral responses to acute and chronic stress. Circulating ghrelin levels have been found to rise following stress. It has been proposed that this elevated ghrelin helps animals cope with stress by generating antidepressant-like behavioral adaptations, although another study suggests that decreasing CNS ghrelin expression has antidepressant-like effects. Ghrelin also seems to have effects on anxiety, although these have been shown to be alternatively anxiogenic or anxiolytic. The current review discusses our current understanding of ghrelin's roles in stress, mood, and anxiety.

Ghrelin, des-acyl ghrelin and nesfatin-1 in gastric X/A-like cells: role as regulators of food intake and body weight

Numerous peptides released from endocrine cells in the intestinal mucosa were established early on to be involved in the physiological regulation of food intake with a prominent role in termination of food ingestion when nutrients pass along the intestinal tract. Recently, peptides released from X/A-like endocrine cells of the gastric oxyntic mucosa were recognized as additional key players in the regulation of feeding and energy expenditure. Gastric X/A-like cells release the octanoylated peptide, ghrelin, the only known peripherally produced hormone stimulating food intake through interaction with growth hormone secretagogue 1a receptor (GHS-R1a). Additionally, non-octanoylated (des-acyl) ghrelin present in the circulation at higher levels than ghrelin is currently discussed as potential modulator of food intake by opposing ghrelin's action independent from GHS-R1a although the functional significance remains to be established. Obestatin, a ghrelin-associated peptide was initially reported as anorexigenic modulator of ghrelin's orexigenic action. However, subsequent reports did not support this contention. Interesting is the recent identification of nesfatin-1, a peptide derived from the nucleobindin2 gene prominently expressed in gastric X/A-like cells in different vesicles than ghrelin. Circulating nesfatin-1 levels vary with metabolic state and peripheral or central injection inhibits dark phase feeding in rodents. Overall, these data point to an important role of gastric X/A-like cells in food intake regulation through the expression of the orexigenic peptide ghrelin along with des-acyl ghrelin and nesfatin-1 capable of reducing food intake upon exogenous injection although their mechanisms of action and functional significance remain to be established.

Ghrelin receptor antagonism attenuates cocaine- and amphetamine-induced locomotor stimulation, accumbal dopamine release, and conditioned place preference

INTRODUCTION

Recently we demonstrated that genetic or pharmacological suppression of the central ghrelin signaling system, involving the growth hormone secretagogue receptor 1A (GHS-R1A), lead to a reduced reward profile from alcohol. As the target circuits for ghrelin in the brain include a mesolimbic reward pathway that is intimately associated with reward-seeking behaviour, we sought to determine whether the central ghrelin signaling system is required for reward from drugs of abuse other than alcohol, namely cocaine or amphetamine.

RESULTS

We found that amphetamine-as well as cocaine-induced locomotor stimulation and accumbal dopamine release were reduced in mice treated with a GHS-R1A antagonist. Moreover, the ability of these drugs to condition a place preference was also attenuated by the GHS-R1A antagonist.

CONCLUSIONS

Thus GHS-R1A appears to be required not only for alcohol-induced reward, but also for reward induced by psychostimulant drugs. Our data suggest that the central ghrelin signaling system constitutes a novel potential target for treatment of addictive behaviours such as drug dependence.

Appetite and reward

The tendency to engage in or maintain feeding behaviour is potently influenced by the rewarding properties of food. Affective and goal-directed behavioural responses for food have been assessed in response to various physiological, pharmacological and genetic manipulations to provide much insight into the neural mechanisms regulating motivation for food. In addition, several lines of evidence tie the actions of metabolic signals, neuropeptides and neurotransmitters to the modulation of the reward-relevant circuitry including midbrain dopamine neurons and corticolimbic nuclei that encode emotional and cognitive aspects of feeding. Along these lines, this review pulls together research describing the peripheral and central signalling molecules that modulate the rewarding effects of food and the underlying neural pathways.

Ghrelin promotes and protects nigrostriatal dopamine function via a UCP2-dependent mitochondrial mechanism

Ghrelin targets the hypothalamus to regulate food intake and adiposity. Endogenous ghrelin receptors [growth hormone secretagogue receptor (GHSR)] are also present in extrahypothalamic sites where they promote circuit activity associated with learning and memory, and reward seeking behavior. Here, we show that the substantia nigra pars compacta (SNpc), a brain region where dopamine (DA) cell degeneration leads to Parkinson's disease (PD), expresses GHSR. Ghrelin binds to SNpc cells, electrically activates SNpc DA neurons, increases tyrosine hydroxylase mRNA and increases DA concentration in the dorsal striatum. Exogenous ghrelin administration decreased SNpc DA cell loss and restricted striatal dopamine loss after 1-methyl-4-phenyl-1,2,5,6 tetrahydropyridine (MPTP) treatment. Genetic ablation of ghrelin or the ghrelin receptor (GHSR) increased SNpc DA cell loss and lowered striatal dopamine levels after MPTP treatment, an effect that was reversed by selective reactivation of GHSR in catecholaminergic neurons. Ghrelin-induced neuroprotection was dependent on the mitochondrial redox state via uncoupling protein 2 (UCP2)-dependent alterations in mitochondrial respiration, reactive oxygen species production, and biogenesis. Together, our data reveal that peripheral ghrelin plays an important role in the maintenance and protection of normal nigrostriatal dopamine function by activating UCP2-dependent mitochondrial mechanisms. These studies support ghrelin as a novel therapeutic strategy to combat neurodegeneration, loss of appetite and body weight associated with PD. Finally, we discuss the potential implications of these studies on the link between obesity and neurodegeneration.

In vivo characterization of high Basal signaling from the ghrelin receptor

The receptor for the orexigenic peptide, ghrelin, is one of the most constitutively active 7TM receptors known, as demonstrated under in vitro conditions. Change in expression of a constitutively active receptor is associated with change in signaling independent of the endogenous ligand. In the following study, we found that the expression of the ghrelin receptor in the hypothalamus was up-regulated approximately 2-fold in rats both during 48-h fasting and by streptozotocin-induced hyperphagia. In a separate experiment, to probe for the effect of the high basal signaling of the ghrelin receptor in vivo, we used intracerebroventricular administration by osmotic pumps of a peptide [D-Arg(1), D-Phe(5), D-Trp(7,9), Leu(11)]-substance P. This peptide selectively displays inverse agonism at the ghrelin receptor as compared with an inactive control peptide with just a single amino acid substitution. Food intake and body weight were significantly decreased in the group of rats treated with the inverse agonist, as compared with the groups treated with the control peptide or the vehicle. In the hypothalamus, the expression of neuropeptide Y and uncoupling protein 2 was decreased by the inverse agonist. In a hypothalamic cell line that endogenously expresses the ghrelin receptor, we observed high basal activity of the cAMP response element binding protein, an important signaling transduction pathway for appetite regulation. The activation was further increased by ghrelin administration and decreased by administration of the inverse agonist. It is suggested that the high constitutive signaling activity is important for the in vivo function of the ghrelin receptor in the control of food intake and body weight.

Assessing the role of the growth hormone secretagogue receptor in motivational learning and food intake

The orexigenic neuropeptide ghrelin is an endogeneous ligand for the growth hormone secretagogue receptor (GHS-R). This orexigen is expressed in both the periphery and in the central system, including portions of mesolimbic dopaminergic circuitry that play a role in affective behaviors. Here we examined pharmacological antagonism of GHS-R in motivational incentive learning, as reflected in Pavlovian-to-instrumental transfer (PIT). Furthermore, it is currently unclear whether the previous effects of ghrelin on food intake are mediated by pre- and/or postingestive influences on ingestive behavior. Thus, the authors also conducted detailed analyses of the temporal dynamics of sucrose licking. Mice received low (50 nmol), moderate (100 nmol), and high (200 nmol) intraperitoneal injections of the GHS-R antagonist GHRP-6 [D-Lys3] prior to subsequent transfer and sucrose consumption tests. Low and moderate doses led to an augmentation of PIT, while high dose injections led to generalized performance deficits. In addition, moderate and high doses of the antagonist resulted in reductions in sucrose intake by reducing palatability of the sucrose. These results suggest dissociable functions of GHS-R in its influence over motivational learning and ingestive behavior.

On the central mechanism underlying ghrelin's chronic pro-obesity effects in rats: new insights from studies exploiting a potent ghrelin receptor antagonist

In the present study, we explore the central nervous system mechanism underlying the chronic central effects of ghrelin with respect to increasing body weight and body fat. Specifically, using a recently developed ghrelin receptor antagonist, GHS-R1A (JMV2959), we investigate the role of GHS-R1A in mediating the effects of ghrelin on energy balance and on hypothalamic gene expression. As expected, in adult male rats, chronic central treatment with ghrelin for 14 days, when compared to vehicle-treated control rats, resulted in an increased body weight, lean mass and fat mass (assessed by dual X-ray absorptiometry), dissected white fat pad weight, cumulative food intake, food efficiency, respiratory exchange ratio and a decrease of energy expenditure. Co-administration of the ghrelin receptor antagonist JMV2959 suppressed/blocked the majority of these effects, with the notable exception of ghrelin-induced food intake and food efficiency. The hypothesis emerging from these data, namely that GHS-R1A mediates the chronic effects of ghrelin on fat accumulation, at least partly independent of food intake, is discussed in light of the accompanying data regarding the hypothalamic genes coding for peptides and receptors involved in energy balance regulation, which were found to have altered expression in these studies.

Ghrelin and dopamine: new insights on the peripheral regulation of appetite

A review is provided of current evidence supporting the actions of the stomach-derived peptide ghrelin on ventral tegmental area (VTA) dopamine cells to increase food intake and other appetitive behaviours. Ghrelin is a 28 amino-acid peptide that was first identified as an endogenous ligand to growth hormone secretagogue receptors (GHS-R). In addition to the hypothalamus and brain stem, GHS-R message and protein are distributed throughout the brain, with high expression being detected in regions associated with goal directed behaviour. Of these, the VTA shows relatively high levels of mRNA transcript and protein. Interestingly, ghrelin infusions into the VTA increase food intake dramatically, and stimulate dopamine release from the VTA. Moreover, VTA dopamine neurones increase their activity in response to ghrelin in slice preparations, suggesting that ghrelin increases food intake by modulating the activity of dopaminergic neurones in the VTA. On the basis of these data as well as the fact that VTA dopamine cells respond to other metabolic hormones such as insulin and leptin, it is proposed that VTA dopamine cells, similar to cells in the mediobasal hypothalamus, are first-order sensory neurones that regulate appetitive behaviour in response to metabolic and nutritional signals.