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

Ghrelin-reactive immunoglobulins and anxiety, depression and stress-induced cortisol response in adolescents. The TRAILS study

BACKGROUND

Ghrelin, a hunger hormone, has been implicated in the regulation of stress-response, anxiety and depression. Ghrelin-reactive immunoglobulins (Ig) were recently identified in healthy and obese humans showing abilities to increase ghrelin's stability and orexigenic effects. Here we studied if ghrelin-reactive Ig are associated with anxiety and depression and with the stress-induced cortisol response in a general population of adolescents. Furthermore, to test the possible infectious origin of ghrelin-reactive Ig, their levels were compared with serum IgG against common viruses.

METHODS

We measured ghrelin-reactive IgM, IgG and IgA in serum samples of 1199 adolescents from the Dutch TRAILS study and tested their associations with 1) anxiety and depression symptoms assessed with the Youth Self-Report, 2) stress-induced salivary cortisol levels and 3) IgG against human herpesvirus 1, 2, 4 and 6 and Influenza A and B viruses.

RESULTS

Ghrelin-reactive IgM and IgG correlated positively with levels of antibodies against Influenza A virus. Ghrelin-reactive IgM correlated negatively with antibodies against Influenza B virus. Ghrelin-reactive IgM correlated positively with anxiety scores in girls and ghrelin-reactive IgG correlated with stress-induced cortisol secretion, but these associations were weak and not significant after correction for multiple testing.

CONCLUSION

These data indicate that production of ghrelin-reactive autoantibodies could be influenced by viral infections. Serum levels of ghrelin-reactive autoantibodies probably do not play a role in regulating anxiety, depression and the stress-response in adolescents from the general population.

Ghrelin signalling on food reward: a salient link between the gut and the mesolimbic system

'Hunger is the best spice' is an old and wise saying that acknowledges the fact that almost any food tastes better when we are hungry. The neurobiological underpinnings of this lore include activation of the brain's reward system and the stimulation of this system by the hunger-promoting hormone ghrelin. Ghrelin is produced largely from the stomach and levels are higher preprandially. The ghrelin receptor is expressed in many brain areas important for feeding control, including not only the hypothalamic nuclei involved in energy balance regulation, but also reward-linked areas such as the ventral tegmental area. By targeting the mesoaccumbal dopamine neurones of the ventral tegmental area, ghrelin recruits pathways important for food reward-related behaviours that show overlap with but are also distinct from those important for food intake. We review a variety of studies that support the notion that ghrelin signalling at the level of the mesolimbic system is one of the key molecular substrates that provides a physiological signal connecting gut and reward pathways.

Mood disorders: A potential link between ghrelin and leptin on human body?

Leptin and ghrelin are two hormones associated with multiple physiological functions, especially energy balance. Leptin is an adipocyte-secreted hormone discovered in 1950 and ghrelin which was found in 1999, is a peptide hormone produced and secreted in the stomach. A number of previous studies showed that these hormones could be associated with different types of mood disorders. The results of previous studies, nevertheless, are confounded by diverse sample selection and different methodologies. A search for related articles in the PubMed database was attempted. The search covered studies, reports, reviews and editorials published in the last ten years. Older references served as auxiliary sources for comparison purposes. However, due to the different results of the studies, there is a need for more investigation in order to establish the exact biochemical mechanisms that are responsible for these diseases and ghrelin’s and leptin’s effects on mood.

Ghrelin in psychiatric disorders - A review

Ghrelin is a 28-amino-acid peptide hormone, first described in 1999 and broadly expressed in the organism. As the only known orexigenic hormone secreted in the periphery, it increases hunger and appetite, promoting food intake. Ghrelin has also been shown to be involved in various physiological processes being regulated in the central nervous system such as sleep, mood, memory and reward. Accordingly, it has been implicated in a series of psychiatric disorders, making it subject of increasing investigation, with knowledge rapidly accumulating. This review aims at providing a concise yet comprehensive overview of the role of ghrelin in psychiatric disorders. Ghrelin was consistently shown to exert neuroprotective and memory-enhancing effects and alleviated psychopathology in animal models of dementia. Few human studies show a disruption of the ghrelin system in dementia. It was also shown to play a crucial role in the pathophysiology of addictive disorders, promoting drug reward, enhancing drug seeking behavior and increasing craving in both animals and humans. Ghrelin's exact role in depression and anxiety is still being debated, as it was shown to both promote and alleviate depressive and anxiety-behavior in animal studies, with an overweight of evidence suggesting antidepressant effects. Not surprisingly, the ghrelin system is also implicated in eating disorders, however its exact role remains to be elucidated. Its widespread involvement has made the ghrelin system a promising target for future therapies, with encouraging findings in recent literature.

Ghrelin effects expression of several genes associated with depression-like behavior

Ghrelin (Ghr) is an orexigenic peptide that is being investigated for its potential role in development of anxiety-like behavior and modulation of depressive-like symptoms induced by bilateral olfactory bulbectomy (OB) in rodents. Olfactory bulbectomy is an animal model useful to study of depression and Ghr could be an alternative therapeutic tool in depression therapy. We studied the effects of intracerebroventricular (i.c.v.) Ghr administration on the expression of hypothalamic genes related to depression and mood (delta opioid receptor (DOR), mu opioid receptor (MOR) and kappa opioid receptor (KOR), lutropin-choriogonadotropic hormone receptor (LHCGR), serotonin transporter (SERT), interleukin 1 beta (IL-1b), vasopressin (AVP) and corticotrophin releasing hormone (CRH)) in OB animals, as well as changes in plasma levels of AVP, CRH and adenocorticotropic hormone (ACTH). We found that acute Ghr 0.3 nmol/μl administration increases gene expression of DOR, SERT and LHCGR in OB mice and decreased expression of IL-1b, suggesting that these genes could be involved in the antidepressant-like effects of Ghr. In addition, OB animals exhibit high AVP gene expression and elevated plasma concentrations of AVP and ACTH and acute Ghr 0.3 nmol/μl administration reduces AVP gene expression and the concentration of these hormones, suggesting that peptide-effects on depressive-like behavior could be mediated at least in part via AVP. In conclusion, this study provides new evidence about genes, receptors and hormones involved in the antidepressant mechanism/s induced by Ghr in OB animals.

Activation of urocortin 1 and ghrelin signaling in the basolateral amygdala induces anxiogenesis

Prior anatomical and functional studies have demonstrated the importance of the basolateral region of the amygdala in the regulation of anxiogenic and anxiolytic behaviors. In the present report we investigated the anxiety-inducing effects of the corticotropin-releasing hormone-related peptide urocortin 1 (Ucn1) and the gut-brain peptide ghrelin. Both peptides were injected directly into the basolateral amygdala of male Sprague-Dawley rats and performance in the elevated plus maze was assessed. Ghrelin was administered at doses of 3-300 pmol and Ucn1 at doses of 0.01-1.0 pmol. Separate groups of rats were pretreated with Ucn1 before ghrelin treatment. In all experiments each test was performed as a single trial per animal. Results indicated that both ghrelin and Ucn1 elicited an increase in anxiogenic behavior. Moreover, Ucn1 pretreament potentiated the anxiogenic action of ghrelin. Overall these findings provide support for an integrated role of ghrelin and urocortin signaling within the basolateral amygdala in the expression of anxiogenesis.

Serum ghrelin and leptin levels in patients with depression and the effects of treatment

OBJECTIVE

Ghrelin and leptin, appetite-regulating hormones, play a role in mood regulation. Current data about the relation between leptin/ghrelin and depression are still controversial. This study aimed to investigate serum leptin and ghrelin levels in patients with depression and the effects of treatment on these levels.

METHODS

Serum ghrelin and leptin levels were measured before and after treatment with antidepressant drugs and/or electroconvulsive therapy in 28 patients with depression and once in 21 healthy controls.

RESULTS

Serum ghrelin levels of the patients were high in the pre-treatment. After the treatment, ghrelin levels were not different from those of the controls. We found no difference in serum levels of leptin between the patients and controls and no change with treatment. body mass index of the patients increased after the treatment especially in the drug-treated group.

CONCLUSION

The present study found increased serum ghrelin levels in depressive patients and normalization with improving of depression but no alteration in leptin levels.

Role of appetite-regulating peptides in the pathophysiology of addiction: implications for pharmacotherapy

Food intake and appetite are regulated by various circulating hormones including ghrelin and glucagon-like-peptide 1 (GLP-1). Ghrelin, mainly released from the stomach, increases food intake, induces appetite, enhances adiposity as well as releases growth hormone. Hypothalamic "ghrelin receptors" (GHS-R1A) have a critical role in food intake regulation, but GHS-R1A are also expressed in reward related areas. GLP-1 is produced in the intestinal mucosa as well as in the hindbrain in response to nutrient ingestion. This gut-brain hormone reduces food intake as well as regulates glucose homeostasis, foremost via GLP-1 receptors in hypothalamus and brain stem. However, GLP-1 receptors are expressed in areas intimately associated with reward regulation. Given that regulation of food and drug intake share common neurobiological substrates, the possibility that ghrelin and GLP-1 play an important role in reward regulation should be considered. Indeed, this leading article describes that the orexigenic peptide ghrelin activates the cholinergic-dopaminergic reward link, an important part of the reward systems in the brain associated with reinforcement and thereby increases the incentive salience for motivated behaviors via this system. We also review the role of ghrelin signaling for reward induced by alcohol and addictive drugs from a preclinical, clinical and human genetic perspective. In addition, the recent findings showing that GLP-1 controls reward induced by alcohol, amphetamine, cocaine and nicotine in rodents are overviewed herein. Finally, the role of several other appetite regulatory hormones for reward and addiction is briefly discussed. Collectively, these data suggest that ghrelin and GLP-1 receptors may be novel targets for development of pharmacological treatments of alcohol and drug dependence.

Ghrelin-Derived Peptides: A Link between Appetite/Reward, GH Axis, and Psychiatric Disorders?

Psychiatric disorders are often associated with metabolic and hormonal alterations, including obesity, diabetes, metabolic syndrome as well as modifications in several biological rhythms including appetite, stress, sleep-wake cycles, and secretion of their corresponding endocrine regulators. Among the gastrointestinal hormones that regulate appetite and adapt the metabolism in response to nutritional, hedonic, and emotional dysfunctions, at the interface between endocrine, metabolic, and psychiatric disorders, ghrelin plays a unique role as the only one increasing appetite. The secretion of ghrelin is altered in several psychiatric disorders (anorexia, schizophrenia) as well as in metabolic disorders (obesity) and in animal models in response to emotional triggers (psychological stress …) but the relationship between these modifications and the physiopathology of psychiatric disorders remains unclear. Recently, a large literature showed that this key metabolic/endocrine regulator is involved in stress and reward-oriented behaviors and regulates anxiety and mood. In addition, preproghrelin is a complex prohormone but the roles of the other ghrelin-derived peptides, thought to act as functional ghrelin antagonists, are largely unknown. Altered ghrelin secretion and/or signaling in psychiatric diseases are thought to participate in altered appetite, hedonic response and reward. Whether this can contribute to the mechanism responsible for the development of the disease or can help to minimize some symptoms associated with these psychiatric disorders is discussed in the present review. We will thus describe (1) the biological actions of ghrelin and ghrelin-derived peptides on food and drugs reward, anxiety and depression, and the physiological consequences of ghrelin invalidation on these parameters, (2) how ghrelin and ghrelin-derived peptides are regulated in animal models of psychiatric diseases and in human psychiatric disorders in relation with the GH axis.

Ghrelin administration enhances neurogenesis but impairs spatial learning and memory in adult mice

Ghrelin, an orexigenic brain-gut hormone promoting feeding and regulating energy metabolism in human and rodents, was reported to enhance both adult neurogenesis and hippocampus-dependent memory formation. However, it is still unclear whether ghrelin-induced hippocampus neurogenesis is responsible for its memory improvement. Using 5-bromo-2' deoxyuridien (BrdU) to birth-date newborn neurons and c-Fos expression to identify dentate gyrus (DG) neurons involved in memory processes, we checked here the effect of ghrelin treatment on adult neurogenesis and cognitive behaviors in mice. We further examined the possible effect of ghrelin on the recruitment of new neurons into the spatial memory traces in intact mice. We found that systemic ghrelin treatment (80μg/kg, ip injection once daily for 8days) stimulated neurogenesis in the adult hippocampus, but had no effect on spatial memory formation. Consistently, it did not affect the incorporation of newborn neurons into the spatial memory circuits. On the contrary, local infusion of ghrelin (8ng/0.5μl into CA1 region of the hippocampus) impaired spatial memory formation, but did not affect adult neurogenesis. Our results thus suggested that ghrelin plays distinct roles in modulating adult neurogenesis and the memory acquisition in the hippocampus, the two processes may not be correlated and may be mediated by different mechanisms.

Are hungry sheep more pessimistic? The effects of food restriction on cognitive bias and the involvement of ghrelin in its regulation

Food restriction is considered to be a welfare issue in extensively reared animals. However, the effects of food restriction on the affective state, and its physiological regulation, are unknown. In Experiment 1, we aimed to assess the effects of increased plasma concentrations of acyl-ghrelin on judgement bias (an indicator of affective states) by fasting sheep for 24h or by ghrelin administration. In Experiment 2, we aimed to assess the effects of chronic food restriction on judgement bias and attention bias towards a food-related cue. For the judgement bias test, sheep were trained in an arena to approach a positive location cue associated with conspecifics and not approach a negative location cue associated with a dog. Three non-trained, non-reinforced ambiguous location cues were situated between the positive and negative locations. Attention bias towards a food-related cue was assessed by placing an empty food bucket against the wall of the arena halfway between the entry point and the positive location. In Experiment 1, sheep were divided into three treatments; 24h fast, ghrelin administration or control. Judgement bias, locomotor activity and plasma cortisol concentrations were assessed. The ghrelin treated group tended to express a more pessimistic bias compared to the control group (P<0.1), and plasma cortisol concentrations tended to be increased (P<0.1). In Experiment 2, sheep were subjected to a high feeding level (HF) or low feeding level (LF) for 7days. The LF group tended to show a more optimistic judgement bias (P<0.1). When the food-related cue was presented, LF ewes took longer to reach the positive location (P<0.001), spent longer with their head inside the bucket (P<0.001) and more time interacting with the bucket (P<0.01). This study provides preliminary evidence that food restriction alters judgement bias and attention bias towards a food-related cue which may indicate altered affective states of sheep.

Enteroendocrine hormones - central effects on behavior

A number of appetite-regulating gut hormones alter behaviors linked to reward, anxiety/mood, memory and cognitive function, although for some of these (notably GLP-1 and CCK) the endogenous signal may be CNS-derived. From a physiological perspective it seems likely that these hormones, whose secretion is altered by nutritional status and by bariatric weight loss surgery, orchestrate neurobiological effects that are integrated and linked to feeding/metabolic control. Consistent with a role in hunger and meal initiation, ghrelin increases motivated behavior for food and, when food is not readily available, decreases behaviors in anxiety tests that would otherwise hinder the animal from finding food. Of the many anorexigenic signals, GLP-1 and PYY have been linked to a suppressed reward function and CCK (and possibly GLP-1) to increased anxiety-like behavior.

Ghrelin and ghrelin receptor modulation of psychostimulant action

Ghrelin (GHR) is an orexigenic gut peptide that modulates multiple homeostatic functions including gastric emptying, anxiety, stress, memory, feeding, and reinforcement. GHR is known to bind and activate growth-hormone secretagogue receptors (termed GHR-Rs). Of interest to our laboratory has been the assessment of the impact of GHR modulation of the locomotor activation and reward/reinforcement properties of psychostimulants such as cocaine and nicotine. Systemic GHR infusions augment cocaine stimulated locomotion and conditioned place preference (CPP) in rats, as does food restriction (FR) which elevates plasma ghrelin levels. Ghrelin enhancement of psychostimulant function may occur owing to a direct action on mesolimbic dopamine function or may reflect an indirect action of ghrelin on glucocorticoid pathways. Genomic or pharmacological ablation of GHR-Rs attenuates the acute locomotor-enhancing effects of nicotine, cocaine, amphetamine and alcohol and blunts the CPP induced by food, alcohol, amphetamine and cocaine in mice. The stimulant nicotine can induce CPP and like amphetamine and cocaine, repeated administration of nicotine induces locomotor sensitization in rats. Inactivation of ghrelin circuit function in rats by injection of a ghrelin receptor antagonist (e.g., JMV 2959) diminishes the development of nicotine-induced locomotor sensitization. These results suggest a key permissive role for GHR-R activity for the induction of locomotor sensitization to nicotine. Our finding that GHR-R null rats exhibit diminished patterns of responding for intracranial self-stimulation complements an emerging literature implicating central GHR circuits in drug reward/reinforcement. Finally, antagonism of GHR-Rs may represent a smoking cessation modality that not only blocks nicotine-induced reward but that also may limit weight gain after smoking cessation.

Ghrelin modulates lateral amygdala neuronal firing and blocks acquisition for conditioned taste aversion

Ghrelin is an orexigenic brain-gut hormone promoting feeding and regulating energy metabolism in human and rodents. An increasing number of studies have reported that ghrelin and its identified receptor, the growth hormone secretagogue receptor 1a (GHS-R1a), produces remarkably wide and complex functions and biological effects on specific populations of neurons in central nervous system. In this study, we sought to explore the in vivo effects of acute ghrelin exposure on lateral amygdala (LA) neurons at the physiological and behavioral levels. In vivo extracellular single-unit recordings showed that ghrelin with the concentration of several nanomolars (nM) stimulated spontaneous firing of the LA neurons, an effect that was dose-dependent and could be blocked by co-application of a GHS-R1a antagonist D-Lys3-GHRP-6. We also found that D-Lys3-GHRP-6 inhibited spontaneous firing of the LA neurons in a dose-dependent manner, revealing that tonic GHS-R1a activity contributes to orchestrate the basal activity of the LA neurons. Behaviorally, we found that microinfusion of ghrelin (12 ng) into LA before training interfered with the acquisition of conditioned taste aversion (CTA) as tested at 24 h after conditioning. Pre-treatment with either purified IgG against GHS-R1a or GHS-R1a antagonist blocked ghrelin’s effect on CTA memory acquisition. Ghrelin (12 ng) had no effect on CTA memory consolidation or the expression of acquired CTA memory; neither did it affect the total liquid consumption of tested rats. Altogether, our data indicated that ghrelin locally infused into LA blocks acquisition of CTA and its modulation effects on neuronal firing may be involved in this process.

A possible association between panic disorder and a polymorphism in the preproghrelingene

The aim of the study was to investigate whether polymorphisms in the preproghrelin gene are associated with anxiety disorders, such as panic disorder, in humans. Panic disorder is a severe anxiety disorder, characterized by sudden attacks of intense fear or anxiety in combination with somatic symptoms. The preproghrelin gene codes for two gut-derived circulating peptides that have been linked to anxiety-like behaviour in rodents: ghrelin (an orexigenic, pro-obesity hormone) and obestatin. In the present study, we genotyped three missense mutations in the preproghrelin gene in 215 patients suffering from panic disorder and in 451 controls. The A allele of the rs4684677 polymorphism was significantly associated with panic disorder, while there were no significant associations with the two other polymorphisms studied. We conclude that the rs4684677 (Gln90Leu) polymorphism in the preproghrelin gene may be associated with increased risk of panic disorder. It will be important to confirm these findings in additional panic disorder patient groups.

Ghrelin: central and peripheral implications in anorexia nervosa

Increasing clinical and therapeutic interest in the neurobiology of eating disorders reflects their dramatic impact on health. Chronic food restriction resulting in severe weight loss is a major symptom described in restrictive anorexia nervosa (AN) patients, and they also suffer from metabolic disturbances, infertility, osteopenia, and osteoporosis. Restrictive AN, mostly observed in young women, is the third largest cause of chronic illness in teenagers of industrialized countries. From a neurobiological perspective, AN-linked behaviors can be considered an adaptation that permits the endurance of reduced energy supply, involving central and/or peripheral reprograming. The severe weight loss observed in AN patients is accompanied by significant changes in hormones involved in energy balance, feeding behavior, and bone formation, all of which can be replicated in animals models. Increasing evidence suggests that AN could be an addictive behavior disorder, potentially linking defects in the reward mechanism with suppressed food intake, heightened physical activity, and mood disorder. Surprisingly, the plasma levels of ghrelin, an orexigenic hormone that drives food-motivated behavior, are increased. This increase in plasma ghrelin levels seems paradoxical in light of the restrained eating adopted by AN patients, and may rather result from an adaptation to the disease. The aim of this review is to describe the role played by ghrelin in AN focusing on its central vs. peripheral actions. In AN patients and in rodent AN models, chronic food restriction induces profound alterations in the « ghrelin » signaling that leads to the development of inappropriate behaviors like hyperactivity or addiction to food starvation and therefore a greater depletion in energy reserves. The question of a transient insensitivity to ghrelin and/or a potential metabolic reprograming is discussed in regard of new clinical treatments currently investigated.

Microinjection of Ghrelin into the Ventral Tegmental Area Potentiates Cocaine-Induced Conditioned Place Preference

Prior work has shown that systemic cocaine pretreatment augments cocaine conditioned place preference (CPP) in rats. In contrast, ghrelin receptor antagonism attenuates cocaine and amphetamine-induced CPP. In order to further investigate ghrelin’s role in dopamine-mediated reward, the present report examined whether pretreament with ghrelin, administered directly into the ventral tegmental area (VTA) of the midbrain, would potentiate the rewarding properties of cocaine as measured by CPP. Adult male Sprague-Dawley rats were given access to either side of the CPP chamber in order to determine initial side preferences. The rats were then restricted to either their non-preferred or preferred side over the course of conditioning which lasted for a total of 16 consecutive days. This was followed by a final test day to then reassess preference. On days where rats were confined to their non-preferred side, ghrelin (30 – 300 pmol) and cocaine (0.625 – 10 mg/kg IP) were administered immediately prior to the conditioning trial. On alternate days rats were treated with vehicle and placed into what was initially determined to be their preferred side. CPP was calculated as the difference in percentage of total time spent in the treatment-paired compartment during the post-conditioning session and the pre-conditioning session. Our results indicated that both cocaine and ghrelin elicited CPP and that ghrelin pre-treatment potentiated the effect of cocaine on place preference. Overall, these findings provide additional support for the argument that ghrelin signaling within the VTA enhances the rewarding effects of psychostimulant compounds.

A review on the effect of the photoperiod and melatonin on interactions between ghrelin and serotonin

Ghrelin and serotonin, which exhibit rhythmic secretion profiles under feeding/fasting conditions, are sensitive to increases and decreases in the day length and form a close web of interrelationships in the regulation of energy homeostasis. Ghrelin and serotonin are biochemically and functionally linked to the suprachiasmatic nucleus, which is a circadian pacemaker, and melatonin, which is an internal transducer of photic environmental changes. Ghrelin and serotonin might be candidates for integrating photic and nonphotic signals, such as light and food availability in the central nervous system. The mechanisms that convert a light signal into a variety of physiological and behavioral rhythms remain unknown. However, we know that the conversion of light signals is necessary to maximize an animal's chances of survival and reproduction.

The amygdala as a neurobiological target for ghrelin in rats: neuroanatomical, electrophysiological and behavioral evidence

Here, we sought to demonstrate that the orexigenic circulating hormone, ghrelin, is able to exert neurobiological effects (including those linked to feeding control) at the level of the amygdala, involving neuroanatomical, electrophysiological and behavioural studies. We found that ghrelin receptors (GHS-R) are densely expressed in several subnuclei of the amygdala, notably in ventrolateral (LaVL) and ventromedial (LaVM) parts of the lateral amygdaloid nucleus. Using whole-cell patch clamp electrophysiology to record from cells in the lateral amygdaloid nucleus, we found that ghrelin reduced the frequency of mEPSCs recorded from large pyramidal-like neurons, an effect that could be blocked by co-application of a ghrelin receptor antagonist. In ad libitum fed rats, intra-amygdala administration of ghrelin produced a large orexigenic response that lasted throughout the 4 hr of testing. Conversely, in hungry, fasted rats ghrelin receptor blockade in the amygdala significantly reduced food intake. Finally, we investigated a possible interaction between ghrelin's effects on feeding control and emotional reactivity exerted at the level of the amygdala. In rats allowed to feed during a 1-hour period between ghrelin injection and anxiety testing (elevated plus maze and open field), intra-amygdala ghrelin had no effect on anxiety-like behavior. By contrast, if the rats were not given access to food during this 1-hour period, a decrease in anxiety-like behavior was observed in both tests. Collectively, these data indicate that the amygdala is a valid target brain area for ghrelin where its neurobiological effects are important for food intake and for the suppression of emotional (anxiety-like) behaviors if food is not available.

Acute ghrelin administration reverses depressive-like behavior induced by bilateral olfactory bulbectomy in mice

This study aims to examine the antidepressant-like action of Ghrelin (Ghr), a hormone synthesized predominantly by gastrointestinal endocrine cells and released during periods of negative energy balance, in two behavioral models: tail suspension test (TST), a predictive model of antidepressant activity, and the olfactory bulbectomy (OB), an established animal model of depression. The reduction in the immobility time in the TST was the parameter used to assess antidepressant-like effect of Ghr. The depressive-like behavior in olfactory bulbectomized mice was inferred through the increase in the immobility time in the TST and the hyperlocomotor activity in the open-field test. Ghr produced antidepressant-like effect in TST (0.3 nmol/μl, i.c.v.), and reversed OB-induced depressive-like behavior. In conclusion, these results provide clear evidence that an acute administration of ghrelin produce antidepressant-like effect in the TST and OB.

Leptin levels are associated with decreased depressive symptoms in women across the weight spectrum, independent of body fat

OBJECTIVE

Leptin is anorexigenic, and levels are markedly decreased in women with low body weight and high in women with obesity. Ghrelin opposes leptin effects on appetite and is negatively associated with body mass index. These appetite-regulating hormones may have opposing effects on mood and stress pathways. Women with anorexia nervosa (AN), hypothalamic amenorrhoea (HA) and obesity are at increased risk of depression and anxiety. It is unknown whether dysregulation of leptin or ghrelin contributes to the development of depression and/or anxiety in these disorders. We investigated the relationship between leptin and ghrelin levels and symptoms of depression, anxiety and perceived stress in women across the weight spectrum.

DESIGN

Cross-sectional.

PATIENTS

64 women: 15 with AN, 12 normal-weight with HA, 17 overweight or obese (OB) and 20 normal-weight in good health (HC).

MEASUREMENTS

Fasting serum leptin and plasma ghrelin levels were measured. Hamilton Rating Scales for Depression (HAM-D) and Anxiety (HAM-A) and the Perceived Stress Scale were administered.

RESULTS

Leptin levels were inversely associated with HAM-D, HAM-A and Perceived Stress scores. The negative relationships between leptin and severity of symptoms of both depression and anxiety remained significant after controlling for body fat or weight. There was no relationship between ghrelin and symptoms of depression or anxiety. Although ghrelin levels were positively associated with the degree of perceived stress, this relationship was not significant after controlling for body fat or weight.

CONCLUSIONS

Leptin may mediate depressive symptoms across the weight spectrum. Further investigation of the role of leptin in modulating mood will be important.

Ghrelin is an orexigenic peptide and elicits anxiety-like behaviors following administration into discrete regions of the hypothalamus

Previous evidence indicates that peripherally administered ghrelin significantly increases corticotropin releasing hormone (CRH) mRNA and serum corticosterone. In addition, intraventricular administration of ghrelin has been reported to elicit anxiety-like behaviors suggesting that the peptide plays a role in mediating neuroendocrine and behavioral responses to stress. In the present study, we characterized the orexigenic, metabolic, and anxiogenic actions of ghrelin following microinjection into the arcuate nucleus (ARN), paraventricular nucleus (PVN), perifornical hypothalamus (PFH), and ventromedial nucleus (VMN). To assess ghrelin's role in anxiogenic behavior, rats were injected with vehicle or 50-800pmol of ghrelin and then placed in an elevated plus maze (EPM) for 10min. Each test was performed as a single trial per animal. In separate behavioral testing we measured the induction of stereotypic behaviors. Doses of 200pmol or higher administered into the ARN and PVN elicited anxiety-like behaviors, including an increased avoidance of the open arms of the EPM. However, in the PFH and VMN, higher doses of ghrelin (400-800pmol) were required to induce anxiety. Ghrelin doses as low as 50pmol stimulated eating and altered energy substrate oxidation (respiratory quotient; RQ) when injected into the ARN and PVN. Injections into the PFH and VMN elicited more modest effects on eating and RQ at doses of 400pmol or greater. Our findings indicate that regions of the hypothalamus appear to be differentially sensitive and responsive to the feeding-stimulant, metabolic, and anxiogenic actions of ghrelin and that the ARN and PVN, in particular, exert a primary role in mediating these effects.

Neural substrates underlying interactions between appetite stress and reward

Neurobiological mechanisms that normally control food intake and energy expenditure can be overcome by environmental cues and by stress. Of particular importance is the influence of the mesolimbic reward pathway. In genetically susceptible individuals, problematic over-eating likely reflects a changing balance in the control exerted by homeostatic versus reward circuits that are strongly influenced by environmental factors such as stress. Both stress and activation of the reward pathway have been shown to increase food intake and promote a preference for palatable, high-energy foods. Recent research has focused on the important role of circulating and central neuropeptides that powerfully regulate the brain response to food cues. For example, ghrelin has a potent positive effect on the motivational aspects of food intake, and central oxytocin may be involved in satiety. Thus, the decision to eat, or indeed to over-eat, involves a complex integrated neurobiology that includes brain centres involved in energy balance, reward and stress and their regulation by metabolic and endocrine factors.

Ghrelin and food reward: the story of potential underlying substrates

The incidence of obesity is increasing at an alarming rate and this worldwide epidemic represents a significant decrease in life span and quality of life of a large part of the affected population. Therefore an understanding of mechanisms underlying food overconsumption and obesity development is urgent and essential to find potential treatments. Research investigating mechanisms underlying obesity and the control of food intake has recently experienced a major shift in focus, from the brain's hypothalamus to additional important neural circuits controlling emotion, cognition and motivated behavior. Among them, the mesolimbic system, and the changes in reward and motivated behavior for food, emerge as new promising treatment targets. Furthermore, there is also growing appreciation of the impact of peripheral hormones that signal nutrition status to the mesolimbic areas, and especially the only known circulating orexigenic hormone, ghrelin. This review article provides a synthesis of recent evidence concerning the impact of manipulation of ghrelin and its receptor on models of food reward/food motivation behavior and the mesolimbic circuitry. Particular attention is given to the potential neurocircuitry and neurotransmitter systems downstream of ghrelin's effects on food reward.

Developmental effects of ghrelin

Ghrelin is a pleiotropic hormone that was originally described as promoting feeding and stimulating growth hormone release in adults. A growing body of evidence suggests that ghrelin may also exert developmental and organizational effects during perinatal life. The perinatal actions of ghrelin include the regulation of early developmental events such as blastocyst development and perinatal growth. Moreover, alterations in perinatal ghrelin levels result in structural differences in various peripheral organs, such as the pancreas and gastrointestinal tract. Recent data have also suggested that ghrelin acts on appetite-related brain centers in early life. Together, these observations indicate that exposure to factors that alter how ghrelin impacts development may induce lasting effects on physiological regulation.

Ghrelin, appetite regulation, and food reward: interaction with chronic stress

Obesity has become one of the leading causes of illness and mortality in the developed world. Preclinical and clinical data provide compelling evidence for ghrelin as a relevant regulator of appetite, food intake, and energy homeostasis. In addition, ghrelin has recently emerged as one of the major contributing factors to reward-driven feeding that can override the state of satiation. The corticotropin-releasing-factor system is also directly implicated in the regulation of energy balance and may participate in the pathophysiology of obesity and eating disorders. This paper focuses on the role of ghrelin in the regulation of appetite, on its possible role as a hedonic signal involved in food reward, and on its interaction with the corticotropin-releasing-factor system and chronic stress.

Ghrelin postsynaptically depolarizes dorsal raphe neurons in rats in vitro

Ghrelin promotes growth hormone (GH) secretion and feeding. Recent studies further showed that ghrelin displayed a defending effect against the depressive-like symptoms and affected sleep in animals and humans. Serotonergic system is considered to be implicated in feeding, depression and other mood disorders, and sleep. The dorsal raphe nucleus (DRN) utilizes serotonin (5-HT) as its major neurotransmitter and expresses GH secretagogue receptors (GHS-Rs). Therefore, the present study was carried out to examine the electrophysiological effect of ghrelin on rat DRN neurons in vitro and determine the ionic mechanism involved. Whole-cell recording revealed that ghrelin depolarized DRN neurons dose-dependently in tetrodotoxin-containing artificial cerebrospinal fluid (TTX ACSF). Pretreatment with [D-Lys(3)]-GHRP-6, a selective antagonist for GHS-Rs, antagonized the ghrelin-induced depolarization. The depolarization was significantly reduced in a low-Na(+) TTX ACSF and in a high-K(+) TTX ACSF and was abolished in the combination of both ACSFs, suggesting that the ghrelin-induced depolarization is mediated by a dual ionic mechanism including an increase in nonselective cationic conductance and a decrease in K(+) conductance. The experiments on the reversal potential also supported an involvement of the dual ionic mechanism in the ghrelin-induced depolarization. On the basis of their electrophysiological and pharmacological properties, approximately 80% of DRN neurons were classified as putative 5-HT-containing neurons and ghrelin depolarized 75% of them. These results suggest that DRN neurons, especially 5-HT-containing neurons, might be involved in the neural mechanisms through which ghrelin participates in the development and/or regulation of feeding behavior, sleep-wake states and depressive-like symptoms.

Gastrectomy alters emotional reactivity in rats: neurobiological mechanisms

Gastrectomy (Gsx) is associated with altered emotional function and a predisposition to depression/anxiety disorders. Here we investigated the effects of Gsx on emotional reactivity in rats and explored the underlying neurobiological mechanisms. Gsx- and sham-operated rats were exposed to behavioural tests that explore anxiety- and depression-like behaviour (open field, black and white box, elevated plus maze, social interaction, forced swim) as well as memory (object recognition). The potential neurobiological mechanisms underlying these differences were explored by measuring (i) turnover of candidate neurotransmitter systems in the nucleus accumbens, (ii) hippocampal neurogenesis by BrdU labelling or by analysis of candidate genes involved in neuronal growth and (iii) changes in mRNA expression of candidate genes in dissected hippocampal and amygdala tissue. Data from individual behavioural tests as well as from multivariate analysis revealed differing emotional reactivity between Gsx- and sham-operated rats. Gsx rats showed reduced emotional reactivity in a new environment and decreased depression-like behaviour. Accumbal serotonin and dopamine turnover were both reduced in Gsx rats. Gsx also led to a memory deficit, although hippocampal neurogenesis was unaffected. Of the many candidate genes studied by real-time RT-PCR, we highlight a Gsx-associated decrease in expression of Egr-1, a transcription factor linked to neural plasticity and cognition, in the hippocampus and amygdala. Thus, Gsx induces an alteration of emotional reactivity and a memory/cognitive deficit that is associated with reduced turnover of serotonin and dopamine in the nucleus accumbens and decreased expression of Egr-1 in the hippocampus and amygdala.