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

Nucleus accumbens ghrelin signaling controls anxiety-like behavioral response to acute stress

BACKGROUND

Anxiety disorders are one of the most common mental disorders. Ghrelin is a critical orexigenic brain-gut peptide that regulates food intake and metabolism. Recently, the ghrelin system has attracted more attention for its crucial roles in psychiatric disorders, including depression and anxiety. However, the underlying neural mechanisms involved have not been fully investigated.

METHODS

In the present study, the effect and underlying mechanism of ghrelin signaling in the nucleus accumbens (NAc) core on anxiety-like behaviors were examined in normal and acute stress rats, by using immunofluorescence, qRT-PCR, neuropharmacology, molecular manipulation and behavioral tests.

RESULTS

We reported that injection of ghrelin into the NAc core caused significant anxiolytic effects. Ghrelin receptor growth hormone secretagogue receptor (GHSR) is highly localized and expressed in the NAc core neurons. Antagonism of GHSR blocked the ghrelin-induced anxiolytic effects. Moreover, molecular knockdown of GHSR induced anxiogenic effects. Furthermore, injection of ghrelin or overexpression of GHSR in the NAc core reduced acute restraint stress-induced anxiogenic effects.

CONCLUSIONS

This study demonstrates that ghrelin and its receptor GHSR in the NAc core are actively involved in modulating anxiety induced by acute stress, and raises an opportunity to treat anxiety disorders by targeting ghrelin signaling system.

Ghrelin is associated with an elevated mood after an overnight fast in depression

BACKGROUND

Major depressive disorder (MDD) comprises subtypes with distinct symptom profiles. For example, patients with melancholic and atypical MDD differ in the direction of appetite and body weight changes as well as mood reactivity. Despite reported links to altered energy metabolism, the role of circulating neuropeptides from the gut in modulating such symptoms remains largely elusive.

METHODS

We collected data from 103 participants, including 52 patients with MDD and 51 healthy control participants (HCP). After an overnight fast, we measured plasma levels of (acyl and des-acyl) ghrelin and participants reported their current metabolic and mood states using visual analog scales (VAS). Furthermore, they completed symptom-related questionnaires (i.e., STAI-T).

RESULTS

Patients with atypical versus melancholic MDD reported less negative affect (p = 0.025). Higher levels of acyl ghrelin (corrected for BMI) were associated with improved mood (p = 0.012), specifically in patients with MDD. These associations of ghrelin were not mood-item specific and exceeded correlations with trait markers of negative affectivity. In contrast to associations with mood state, higher levels of ghrelin were not associated with increased hunger per se or changes in appetite in patients with MDD.

LIMITATIONS

The study is limited by the cross-sectional design without an intervention.

CONCLUSIONS

Our results reveal potentially mood-enhancing effects of ghrelin in fasting individuals that exceed associations with metabolic state ratings. These associations with circulating neuropeptides might help explain anti-depressive effects of fasting interventions and could complement conventional treatments in patients with melancholic MDD.

Ghrelin/GHSR signaling in the lateral septum ameliorates chronic stress-induced depressive-like behaviors

Ghrelin is a gastrointestinal hormone on feeding and metabolism regulation, and acts through its receptor-growth hormone secretagogue receptor (GHSR), which is widely distributed throughout the central nervous system. Recent studies have suggested that ghrelin plays an important role in the regulation of depression, but the underlying mechanisms remain uncertain. Lateral septum (LS) is a critical brain region in modulating depression. Therefore, we investigated the role of ghrelin/GHSR signaling in the LS on the depressive-like behaviors of mice under conditions of chronic stress by using behavioral tests, neuropharmacology, and molecular biology techniques. We found that infusion of ghrelin into the LS produced antidepressant-like responses in mice. Activation of LS GABAergic neurons was involved in the antidepressant effect of ghrelin. Importantly, GHSR was highly expressed and distributed in the LS neurons. Blockade of GHSR in the LS reversed the ghrelin-induced antidepressant-like effects. Molecular knockdown of GHSR in the LS induced depressive-like symptoms in mice. Furthermore, administration of ghrelin into the LS alleviated depressive-like behaviors induced by chronic social defeat stress (CSDS). Consistent with the neuropharmacological results, overexpression of GHSR in the LS reversed CSDS-induced depressive-like behaviors. Our findings clarify a key role for ghrelin/GHSR signaling in the regulation of chronic stress-induced depressive-like behaviors, which could provide new strategies for the treatment of depression.

Ghrelin as a Biomarker of "Immunometabolic Depression" and Its Connection with Dysbiosis

Ghrelin, a gastrointestinal peptide, is an endogenous ligand of growth hormone secretagogue receptor 1a (GHSR1a), which is mainly produced by X/A-like cells in the intestinal mucosa. Beyond its initial description as a growth hormone (GH) secretagogue stimulator of appetite, ghrelin has been revealed to have a wide range of physiological effects, for example, the modulation of inflammation; the improvement of cardiac performance; the modulation of stress, anxiety, taste sensation, and reward-seeking behavior; and the regulation of glucose metabolism and thermogenesis. Ghrelin secretion is altered in depressive disorders and metabolic syndrome, which frequently co-occur, but it is still unknown how these modifications relate to the physiopathology of these disorders. This review highlights the increasing amount of research establishing the close relationship between ghrelin, nutrition, microbiota, and disorders such as depression and metabolic syndrome, and it evaluates the ghrelinergic system as a potential target for the development of effective pharmacotherapies.

Childhood sexual abuse is associated with higher total ghrelin serum levels in adulthood: results from a large, population-based study

Ghrelin is an orexigenic peptide hormone synthesized in times of stress and hunger and alterations of the ghrelin system following acute stressors could be repeatedly shown in humans. However, little data exists on long-term effects of trauma on the ghrelin system. We aimed to investigate the influence of childhood trauma on total ghrelin serum levels in a large, population-based study. Total serum ghrelin was measured in 1666 participants of a population-based cross-sectional study ('LIFE study'). The Childhood Trauma Screener (CTS) was used for the assessment of childhood trauma in the final sample (n = 1086; mean age: 57.10 ± 16.23 years; 632 males, 454 females). Multiple linear regression analyses and generalized linear models were chosen to examine the association between childhood trauma and total serum ghrelin concentrations. Childhood sexual abuse went along with significantly higher ghrelin serum levels in the total sample (β = 0.114, t = 3.958; p = 0.00008) and in women (β = 0.142, t = 3.115; p = 0.002), but not in men (β = 0.055; t = 1.388; p = 0.166). Women with severe emotional neglect in the childhood had higher ghrelin levels than those without (odds ratio = 1.204; p = 0.018). For the CTS Sum Score and other CTS sub-scale scores, no significant association with ghrelin serum levels was found. Our study is the first to show associations between childhood sexual trauma and total ghrelin levels in adults in a large, community-based sample. Our results should initiate further research of the role of ghrelin in human stress response in prospective study designs.

Potential of Heterogeneous Compounds as Antidepressants: A Narrative Review

Depression is a globally widespread disorder caused by a complicated interplay of social, psychological, and biological factors. Approximately 280 million people are suffering from depression worldwide. Traditional frontline antidepressants targeting monoamine neurotransmitters show unsatisfactory effects. The development and application of novel antidepressants for dissimilar targets are on the agenda. This review characterizes the antidepressant effects of multiple endogenous compounds and/or their targets to provide new insight into the working mechanism of antidepressants. We also discuss perspectives and challenges for the generation of novel antidepressants.

Serum ghrelin is positively associated with physiological anxiety but negatively associated with pathological anxiety in humans: Data from a large community-based study

The orexigenic hormone ghrelin is being increasingly recognized as a stress hormone being involved in anxiety regulation. In animals, ghrelin effects on, and responses to acute stress differed from those in chronic stress, an animal model for anxiety and depression. In humans, elevated ghrelin levels were reported in pathological anxiety (e.g. panic disorder). However, no reports exist on physiological anxiety in mentally healthy subjects. In addition, reports on generalized anxiety symptoms, both in mentally healthy subjects (e.g. worrying) or in adult patients, are lacking. Total serum ghrelin was determined in 1666 subjects of a population-based cross-sectional study ('LIFE'). The 7-item Generalized Anxiety Disorder Scale (GAD-7), detecting also other anxiety disorders, was administered. For multiple linear regression analyses, 1091 subjects were finally included. Serum ghrelin and GAD-7 scores were positively but not significantly associated in the total group (ß=0.00025, standardized β = 0.039, 95%CI: -0.00006;0.0006;p = 0.144), in subjects with no more than mild anxiety, there was a significant positive association (GAD-7 ≤9: n = 1061, 97.25%, β = 0.00032; standardized β = 0.060; 95%CI: 0.000023;0.00062;p = 0.036). In contrast, there was a negative association in subjects with anxiety symptoms above the GAD-7 cut-off (GAD-7 ≥10: n = 30, 2.75%, ß=-0.003, standardized β = -0.462; 95% CI:-0.006;0.0001;p = 0.045). Ghrelin levels were only numerically (p = 0.23) higher in subjects with clinically relevant anxiety symptoms (963.5 ± 399.6 pg/ml; mean±SD) than in those without (901.0 ± 416.4 pg/ml). In conclusion, the positive association between ghrelin and no more than mild anxiety is an initial indication for a role for ghrelin in the regulation of physiological anxiety in humans. This association and the opposed association in pathological anxiety resemble findings in animals showing diverging ghrelin effects in acute and chronic stress.

Association Between Self-rating Depression Scores and Total Ghrelin and Adipokine Serum Levels in a Large Population-Based Sample

Background

Ghrelin and the adipokines leptin and adiponectin have been suggested to be involved in mood and anxiety regulation and to be altered in affective disorders. However, studies investigating the association between ghrelin, leptin and adiponectin and depressive symptomatology are scarce but might contribute to a better understanding of their involvement in mood regulation. We thus aimed investigating the association between depressive symptomatology and total ghrelin as well as leptin and adiponectin serum levels in a large population-based sample.

Methods

Total serum ghrelin, adiponectin and leptin levels were determined in 1666 subjects of a population-based cross-sectional study ("LIFE"). The Center for Epidemiological Studies Depression Scale (CES-D) and the Inventory of Depressive Symptoms - Self Rating (IDS-SR) were administered. Multiple linear regression analyses were conducted to examine the association between total serum ghrelin, leptin and adiponectin and the intensity of depressive symptoms.

Results

In the total sample (n = 1,092), neither ghrelin nor leptin or adiponectin serum levels showed a significant association with CES-D or IDS-SR sum scores (N = 1,092) or in depressed/non-depressed subjects. Leptin serum levels showed a significantly positive association with IDS-SR sum scores in elderly men (≥60 years; β = 0.122, 95% CI: 0.009; 0.236; p = 0.035).

Conclusion

Our study suggests that peripheral levels of ghrelin and adipokines in a cross-sectional study design might not be sufficient to measure their involvement in depression, suggesting that associations are more complex and multi-layered.

New Aspects of Corpus Luteum Regulation in Physiological and Pathological Conditions: Involvement of Adipokines and Neuropeptides

The corpus luteum is a small gland of great importance because its proper functioning determines not only the appropriate course of the estrous/menstrual cycle and embryo implantation, but also the subsequent maintenance of pregnancy. Among the well-known regulators of luteal tissue functions, increasing attention is focused on the role of neuropeptides and adipose tissue hormones—adipokines. Growing evidence points to the expression of these factors in the corpus luteum of women and different animal species, and their involvement in corpus luteum formation, endocrine function, angiogenesis, cells proliferation, apoptosis, and finally, regression. In the present review, we summarize the current knowledge about the expression and role of adipokines, such as adiponectin, leptin, apelin, vaspin, visfatin, chemerin, and neuropeptides like ghrelin, orexins, kisspeptin, and phoenixin in the physiological regulation of the corpus luteum function, as well as their potential involvement in pathologies affecting the luteal cells that disrupt the estrous cycle.

Ghrelin Is Effective on Passive Avoidance Memory by Altering the Expression of NMDAR and HTR1a Genes in the Hippocampus of Male Wistar Rats

Background

Memory-dependent psychological behaviors have an important role in life. Memory strengthening in adulthood to prevent its defects in aging is a significant issue. The ghrelin endogenous hormone improves memory by targeting glutamatergic and serotonergic circuits. Also, citicoline, a memory strengthening drug in aging, is not recommended to adults due to its side effects. The current study aims to test that ghrelin treatment, like citicoline, would improve passive avoidance memory via expression of the genes encoding the N-methyl-D-aspartate receptor (NMDAR1) and the serotonin receptor 1A (HTR1α) involved in this process.

Methods

Five groups of adult male rats received (1) saline (as control), (2) 0.5 mg/kg citicoline, or (3-5) 0.3, 1.5, and 3 nmol/μl ghrelin). The rats received the drugs via intra-hippocampal injection. Passive avoidance memory was determined using a shuttle box device. The latency to enter the dark chamber before (IL) and after (RL) injection and the total duration of the animal's presence in the light compartment (TLC) were evaluated. Then, the gene expression rates of NMDAR1 and HTR1α were measured by the Real-Time PCR.

Results

Ghrelin and citicoline had some similar and significant effects on passive avoidance memory, and both increased NMDAR1 and decreased HTR1α expression.

Conclusion

Ghrelin, like citicoline, improves passive avoidance learning by altering the NMDAR1 and HTR1α expression in the hippocampus.

Ghrelin receptor antagonist attenuated sickness behavior and activation of HPA-axis induced by immunological challenge in male rats

AIMS

During illnesses caused by infectious diseases, a suite of brain-mediated responses called sickness syndrome occurs, triggering behavioral and physiological changes. This study investigated whether ghrelin modulates sickness syndrome induced by systemic administration of lipopolysaccharide (LPS).

MAIN METHODS

Male Wistar rats were pretreated with vehicle or [D-lys3]-GHRP-6, a ghrelin receptor GHS-R1 antagonist (20 nmol, i.c.v), 30 min before injection of LPS (200 μg/kg, i.p.) or sterile saline. We investigated the behavioral effects in male rats after LPS administration by screening for depressive-like behavior, locomotor activity alterations, and corticosterone release. Changes in body temperature were measured using a biotelemetry probe preimplanted in the peritoneal cavity to evaluate the effect of ghrelin on the thermoregulatory response during immunological challenge.

KEY FINDINGS

Pretreatment with [D-lys3]-GHRP-6 blunted most of the assessed parameters related to sickness syndrome, including social withdrawal, anhedonia, depressive-like behavior, and anorexia, reduced the activation of the HPA axis, but did not alter LPS-induced fever.

SIGNIFICANCE

Our findings suggest that ghrelin centrally mediates the sickness behavior and activation of HPA, as a ghrelin receptor antagonist attenuates social withdrawal, anhedonia, depressive-like behavior, anorexia, and HPA activation in response to LPS.

Odors associated with neonatal experiences with the dam have enhanced anxiolytic effects in rat

Odor stimuli are widely reported to promote relaxation and reduce anxiety in humans and rodents. However, it remains unclear if this anxiolytic efficacy can be further enhanced by association with positive experiences. Therefore, we compared the effects of a novel odor to a familiar odor previously paired with a positive experience on anxiety-like behaviors in rats. One group of Wistar-Imamichi female and male pups was exposed to an odor stimulus with their dams during postnatal days (PNDs) 8-12, whereas another control group was exposed to perfused air during the same period. Starting on PND 42, all animals were examined in the open field test (OFT) and elevated plus maze (EPM) test during exposure to scent-free air (vehicle), a novel odor, or the positive-familiar odor from postnatal exposure. In the EPM, female rats entered open arms with all 4 paws (complete entry) more frequently and spent more time on open arms during exposure to the positive-familiar odor than during exposure to air or a novel odor, whereas partial open arm entries with forepaws only were increased during exposure to both novel and positive-familiar odors compared to air. In contrast, male rats demonstrated no significant increase in open arm activity during positive-familiar odor exposure, but did show equally reduced grooming frequency during novel and familiar-positive odor exposure in the OFT. Exposure to positive-familiar odors may be an effective and safe method for anxiety reduction, especially in females.

The dorsal raphe nucleus in the control of energy balance

Energy balance is orchestrated by an extended network of highly interconnected nuclei across the central nervous system. While much is known about the hypothalamic circuits regulating energy homeostasis, the 'extra-hypothalamic' circuits involved are relatively poorly understood. In this review, we focus on the brainstem's dorsal raphe nucleus (DRN), integrating decades of research linking this structure to the physiologic and behavioral responses that maintain proper energy stores. DRN neurons sense and respond to interoceptive and exteroceptive cues related to energy imbalance and in turn induce appropriate alterations in energy intake and expenditure. The DRN is also molecularly differentiable, with different populations playing distinct and often opposing roles in controlling energy balance. These populations are integrated into the extended circuit known to regulate energy balance. Overall, this review summarizes the key evidence demonstrating an important role for the DRN in regulating energy balance.

Potential gut-brain mechanisms behind adverse mental health outcomes of bariatric surgery

Bariatric surgery induces sustained weight loss and metabolic benefits via notable effects on the gut-brain axis that lead to alterations in the neuroendocrine regulation of appetite and glycaemia. However, in a subset of patients, bariatric surgery is associated with adverse effects on mental health, including increased risk of suicide or self-harm as well as the emergence of depression and substance use disorders. The contributing factors behind these adverse effects are not well understood. Accumulating evidence indicates that there are important links between gut-derived hormones, microbial and bile acid profiles, and disorders of mood and substance use, which warrant further exploration in the context of changes in gut-brain signalling after bariatric surgery. Understanding the basis of these adverse effects is essential in order to optimize the health and well-being of people undergoing treatment for obesity.

Preliminary evidence of acylated ghrelin association with depression severity in postmenopausal women

We have previously shown increased depression and anxiety scores in postmenopausal overweight women, when compared to overweight premenopausal women. The mechanisms responsible for these alterations are not understood. Although ghrelin involvement in mood modulation has been suggested, its role is still ambiguous and has not been evaluated in postmenopause. Here we investigated the association of ghrelin with depression and anxiety symptoms in postmenopausal women. Fifty-five postmenopausal women with depression symptoms, who were not in use of hormones or antidepressants, were included in the study. Depression symptoms were evaluated by Beck's Depression Inventory (BDI) and Patient Health Questionnaire-9 (PHQ-9) and anxiety symptoms were evaluated by Beck's Anxiety Inventory (BAI). Women were allocated into three groups, according to BDI classification of mild, moderate, or severe depression symptoms. Anthropometric, biochemical and hormonal parameters were analyzed. Total and acylated ghrelin levels were higher in the severe depression than in the mild depression group. Multivariate regression analyses showed positive associations of BDI scores with acylated ghrelin and BMI, and of PHQ-9 scores with acylated ghrelin and homeostasis model assessment of insulin resistance (HOMA-IR). BAI scores associated positively with waist-to-hip ratio. To the best of our knowledge, this is the first demonstration of an association between acylated ghrelin and the severity of depression symptoms in postmenopausal women. This association may reflect either a physiological response aimed at fighting against depression symptoms or a causal factor of this mental disorder.

The Ghrelin/Growth Hormone Secretagogue Receptor System Is Involved in the Rapid and Sustained Antidepressant-Like Effect of Paeoniflorin

Major depressive disorder (MDD) is a debilitating mental illness affecting people worldwide. Although significant progress has been made in the development of therapeutic agents to treat this condition, fewer than half of all patients respond to currently available antidepressants, highlighting the urgent need for the development of new classes of antidepressant drugs. Here, we found that paeoniflorin (PF) produced rapid and sustained antidepressant-like effects in multiple mouse models of depression, including the forced swimming test and exposure to chronic mild stress (CMS). Moreover, PF decreased the bodyweight of mice without affecting food intake and glucose homeostasis, and also reduced the plasma levels of total ghrelin and the expression of ghrelin O-acyltransferase in the stomach; however, the plasma levels of ghrelin and the ghrelin/total ghrelin ratio were unaffected. Furthermore, PF significantly increased the expression of growth hormone secretagogue receptor 1 alpha (GHSR1α, encoded by the Ghsr gene) in the intestine, whereas the levels of GHSR1α in the brain were only marginally downregulated following subchronic PF treatment. Finally, the genetic deletion of Ghsr attenuated the antidepressant-like effects of PF in mice exposed to CMS. These results suggested that increased GHSR1α expression in the intestine mediates the antidepressant-like effects of PF. Understanding peripheral ghrelin/GHSR signaling may provide new insights for the screening of antidepressant drugs that produce fast-acting and sustained effects.

Biased Ghrelin Receptor Signaling and the Dopaminergic System as Potential Targets for Metabolic and Psychological Symptoms of Anorexia Nervosa

Anorexia Nervosa (AN) is a complex disease that impairs the metabolic, mental and physiological health of affected individuals in a severe and sometimes lethal way. Many of the common symptoms in AN patients, such as reduced food intake, anxiety, impaired gut motility or overexercising are connected to both the orexigenic gut hormone ghrelin and the dopaminergic system. Targeting the ghrelin receptor (GhrR) to treat AN seems a promising possibility in current research. However, GhrR signaling is highly complex. First, the GhrR can activate four known intracellular pathways Gαq, Gαi/o, Gα12/13 and the recruitment of β-arrestin. Biased signaling provides the possibility to activate or inhibit only one or a subset of the intracellular pathways of a pleiotropic receptor. This allows specific targeting of physiological functions without adverse effects. Currently little is known on how biased signaling could specifically modulate GhrR effects. Second, GhrR signaling has been shown to be interconnected with the dopaminergic system, particularly in the context of AN symptoms. This review highlights that a biased agonist for the GhrR may be a promising target for the treatment of AN, however extensive and systematic translational studies are still needed and the connection to the dopaminergic system has to be taken into account.

A new understanding of GHSR1a--independent of ghrelin activation

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

THE INTRIGUING LIGAND-DEPENDENT AND LIGAND-INDEPENDENT ACTIONS OF THE GROWTH HORMONE SECRETAGOGUE RECEPTOR ON REWARD-RELATED BEHAVIORS

The growth hormone secretagogue receptor (GHSR) is a G-protein-coupled receptor (GPCR) highly expressed in the brain, and also in some peripheral tissues. GHSR activity is evoked by the stomach-derived peptide hormone ghrelin and abrogated by the intestine-derived liver-expressed antimicrobial peptide 2 (LEAP2). In vitro, GHSR displays ligand-independent actions, including a high constitutive activity and an allosteric modulation of other GPCRs. Beyond its neuroendocrine and metabolic effects, cumulative evidence shows that GHSR regulates the activity of the mesocorticolimbic pathway and modulates complex reward-related behaviors towards different stimuli. Here, we review current evidence indicating that ligand-dependent and ligand-independent actions of GHSR enhance reward-related behaviors towards appetitive stimuli and drugs of abuse. We discuss putative neuronal networks and molecular mechanisms that GHSR would engage to modulate such reward-related behaviors. Finally, we briefly discuss imaging studies showing that ghrelin would also regulate reward processing in humans. Overall, we conclude that GHSR is a key regulator of the mesocorticolimbic pathway that influences its activity and, consequently, modulates reward-related behaviors via ligand-dependent and ligand-independent actions.

The Good, the Bad and the Unknown Aspects of Ghrelin in Stress Coping and Stress-Related Psychiatric Disorders

Ghrelin is a peptide hormone released by specialized X/A cells in the stomach and activated by acylation. Following its secretion, it binds to ghrelin receptors in the periphery to regulate energy balance, but it also acts on the central nervous system where it induces a potent orexigenic effect. Several types of stressors have been shown to stimulate ghrelin release in rodents, including nutritional stressors like food deprivation, but also physical and psychological stressors such as foot shocks, social defeat, forced immobilization or chronic unpredictable mild stress. The mechanism through which these stressors drive ghrelin release from the stomach lining remains unknown and, to date, the resulting consequences of ghrelin release for stress coping remain poorly understood. Indeed, ghrelin has been proposed to act as a stress hormone that reduces fear, anxiety- and depression-like behaviors in rodents but some studies suggest that ghrelin may - in contrast - promote such behaviors. In this review, we aim to provide a comprehensive overview of the literature on the role of the ghrelin system in stress coping. We discuss whether ghrelin release is more than a byproduct of disrupted energy homeostasis following stress exposure. Furthermore, we explore the notion that ghrelin receptor signaling in the brain may have effects independent of circulating ghrelin and in what way this might influence stress coping in rodents. Finally, we examine how the ghrelin system could be utilized as a therapeutic avenue in stress-related psychiatric disorders (with a focus on anxiety- and trauma-related disorders), for example to develop novel biomarkers for a better diagnosis or new interventions to tackle relapse or treatment resistance in patients.

Ghrelin and leptin levels in children with anxiety disorders

Background Anxiety disorders are common psychiatric disorders in childhood and an important health problem that is associated with the risk of serious mental, educational and economical problems. Researchers have mentioned many different mechanisms in the etiopathology of anxiety disorders. This study aimed to investigate ghrelin and leptin levels in children with anxiety disorders and thus to contribute to the clarification of anxiety in children. Methods Forty-three children aged 6-12 years with a diagnosis of the Anxiety Disorder according to DSM 5 and 21 healthy children age- and gender-matched to the study group were included. All the subjects were assessed with Kiddie Schedule for Affective Disorders and Schizophrenia Present and Lifetime Version (K-SADS-PL) and State-Trait Anxiety Inventory for Children (STAI-C) scale. Blood samples were obtained in the morning and serum ghrelin and leptin levels were measured with enzyme-linked immunosorbent assay (ELISA) kits. Results In the anxiety group the ghrelin levels were higher than the control group (p = 0.037) but there was no significant difference between the leptin levels (p = 0.430). Also, when the girls in the anxiety group and the girls in the control group were compared, ghrelin levels were higher in the anxiety group (p < 0.01). Conclusions These findings suggest that ghrelin may play a significant role in the etiologic mechanisms of anxiety disorders. However, more detailed studies are needed to explain the linkage between anxiety disorders and neuropeptides.

Ghrelin signalling within the rat nucleus accumbens and skilled reach foraging

Motivation alters behaviour in a complex manner and nucleus accumbens (NAc) shell has been implied as a key structure regulating such behaviour. Recent studies show that acute ghrelin signalling enhances motivation when assessed in a simple motor task. The aim of the present study was to define the role of ghrelin signalling on motivation in a more complex motor behaviour. Rats were tested in the Montoya staircase, an animal model of skilled reach foraging assessed by the number of sucrose pellets consumed. Electrophysiological recordings were conducted to explore the neurophysiological correlates of ghrelin signalling. The initial electrophysiological results displayed that ex vivo administration of ghrelin increased NAc shell output in brain slices from drug- and training-naïve rats. In rats with an acquired skilled reach performance, acute as well as repeated treatment with a ghrelin receptor (GHSR-1 A) antagonist (JMV2959) decreased the number of sucrose pellets consumed. Moreover, infusion of JMV2959 into NAc shell reduced this consumption. Sub-chronic, during ten days, JMV2959 treatment during training on the Montoya staircase reduced the number of pellets consumed, whereas ghrelin improved this behaviour. In addition, field potential and whole cell recordings were conducted in NAc shell of rats that had been treated with ghrelin or GHSR-1 A antagonist during training on the Montoya staircase. Sub-chronic administration of ghrelin during motor-skill learning selectively increased the frequency of inhibitory transmission in the NAc shell, resulting in a net suppression of accumbal output. Collectively these data suggest that ghrelin signalling in NAc shell enhances skilled reached foraging tentatively by increasing the motivation.

Ghrelin exhibited antidepressant and anxiolytic effect via the p38-MAPK signaling pathway in hippocampus

Ghrelin, a peptide derived from stomach, is an endogenous ligand for growth hormone secretagogue receptor (GHSR). So far, the exact role of ghrelin in depression and anxiety is still being debated. The p38 mitogen-activated protein kinase (p38-MAPK) is known to be activated in response to various stress stimuli. Thus, we hypothesize that ghrelin has an antidepressant effect, to which the p38-MAPK signaling pathway significantly contributes. To test this hypothesis, chronic social defeat stress (CSDS) was used as a model of depression. We employed the adeno-associated virus-mediated siRNA approach to down-regulate GHSR expression in the hippocampus of mice in vivo. Both ghrelin and the p38 inhibitor, SB203580, were administered to identify the effect of ghrelin on depressive-like behavior of stressed mice and to better assess the role of the p38-MAPK signaling pathway in this process. We found that CSDS activated the endogenous ghrelin-GHSR in hippocampal neurons, which possibly resulted in opposing the formation of depression- and anxiety-like behaviors in mice. Furthermore, the p38-MAPK signaling pathway had an important role in the antidepressant effect of ghrelin. Therefore, we conclude that ghrelin may reduce CSDS-induced depression- and anxiety-like behaviors via inhibiting the p38-MAPK signaling pathway in hippocampal neurons of mice.

Neuropeptides at the crossroad of fear and hunger: a special focus on neuropeptide Y

Survival in a natural environment forces an individual into constantly adapting purposive behavior. Specified interoceptive neurons monitor metabolic and physiological balance and activate dedicated brain circuits to satisfy essential needs, such as hunger, thirst, thermoregulation, fear, or anxiety. Neuropeptides are multifaceted, central components within such life‐sustaining programs. For instance, nutritional depletion results in a drop in glucose levels, release of hormones, and activation of hypothalamic and brainstem neurons. These neurons, in turn, release several neuropeptides that increase food‐seeking behavior and promote food intake. Similarly, internal and external threats activate neuronal pathways of avoidance and defensive behavior. Interestingly, specific nuclei of the hypothalamus and extended amygdala are activated by both hunger and fear. Here, we introduce the relevant neuropeptides and describe their function in feeding and emotional‐affective behaviors. We further highlight specific pathways and microcircuits, where neuropeptides may interact to identify prevailing homeostatic needs and direct respective compensatory behaviors. A specific focus will be on neuropeptide Y, since it is known for its pivotal role in metabolic and emotional pathways. We hypothesize that the orexigenic and anorexigenic properties of specific neuropeptides are related to their ability to inhibit fear and anxiety.

Ghrelin and aggressive behaviours-Evidence from preclinical and human genetic studies

Aggressive behaviour is of crucial importance in the defence for limited resources including food and mates and involves central serotonin as well as dopamine signalling. As ghrelin modulates food intake and sexual behaviour we initially investigated the hypothesis that central ghrelin signalling regulates aggressive behaviour in the resident intruder paradigm in male mice. Moreover, interaction between ghrelin signalling and serotonergic, noradrenergic as well as dopaminergic neurotransmission in aggression was investigated. The relevance of ghrelin for human aggression per se as well as for aggression induced by alcohol was evaluated in a human genetic association study comprising young men (n = 784) from the normal population assessed for anti-social behaviours. The present study demonstrates that central ghrelin infusion, but not ghrelin administered systemically, increases aggression. Moreover aggressive behaviour is decreased by pharmacological suppression of the growth hormone secretagogue receptor-1 A (GHSR-1A) by JMV2959. As indicated by the ex vivo biochemical data serotonin, rather than dopamine or noradrenaline, in amygdala may have central roles for the ability of JMV2959 to reduce aggression. This link between central serotonin, GHSR-1A and aggression is further substantiated by the behavioural data showing that JMV2959 cannot decrease aggression following depletion of central serotonin signalling. The genetic association study demonstrates that males carrying the Leu72Leu genotype of the pre-pro-ghrelin gene and displaying hazardous alcohol use are more aggressive when compared to the group carrying the Met-allele. Collectively, this contributes to the identification of central ghrelin pathway as an important modulator in the onset of aggressive behaviours in male mice.

The protective effects of Ghrelin/GHSR on hippocampal neurogenesis in CUMS mice

Ghrelin is an orexigenic hormone that also plays an important role in mood disorders. Our previous studies demonstrated that ghrelin administration could protect against depression-like behaviors of chronic unpredictable mild stress (CUMS) in rodents. However, the mechanism related to the effect of ghrelin on CUMS mice has yet to be revealed. This article shows that ghrelin (5 nmol/kg/day for 2 weeks, i.p.) decreased depression-like behaviors induced by CUMS and increased hippocampal integrity (neurogenesis and spine density) measured via Ki67, 5-bromo-2-deoxyuridine (BrdU), doublecortin (DCX) labeling and Golgi-cox staining, which were decreased under CUMS. The behavioral phenotypes of Growth hormone secretagogue receptor (Ghsr)-null and wild type (WT) mice were evaluated under no stress condition and after CUMS exposure to determine the effect of Ghsr knockout on the behavioral phenotypes and stress susceptibility of mice. Ghsr-null mice exhibited depression-like behaviors under no stress condition. CUMS induced similar depression- and anxiety-like behavioral manifestations in both Ghsr-null and WT mice. A similar pattern of behavioral changes was observed after hippocampal GHSR knockdown. Additionally, both Ghsr knockout as well as CUMS exhibited deleterious effects on neurogenesis and spine density in the dentate gyrus (DG). Besides, CCK8 assay and 5-Ethynyl-2'-deoxyuridine (EdU) incorporation assay showed that ghrelin has a proliferative effect on primary cultured hippocampal neural stem cells (NSCs) and this proliferation was blocked by D-Lys3-GHRP-6 (DLS, the antagonist of GHSR, 100 μM) pretreatment. Ghrelin-induced proliferation is associated with the inhibition of G1 arrest, and this inhibition was blocked by LY294002 (specific inhibitor of PI3K, 20 μM). Furthermore, the in vivo data displayed that LY294002 (50 nmol, i.c.v.) can significantly block the antidepressant-like action of exogenous ghrelin treatment. All these results suggest that ghrelin/GHSR signaling maintains the integrity of hippocampus and has an inherent neuroprotective effect whether facing stress or not.

GHS-R1a Deficiency Alleviates Depression-Related Behaviors After Chronic Social Defeat Stress

Ghrelin is an important orexigenic hormone that regulates feeding, metabolism and glucose homeostasis in human and rodents. Ghrelin functions by binding to its receptor, the growth hormone secretagogue receptor 1a (GHS-R1a), which is widely expressed inside and outside of the brain. Recent studies suggested that acyl-ghrelin, the active form of ghrelin, is a persistent biomarker for chronic stress exposure. However, how ghrelin/GHS-R1a signaling contributes to stress responses and mood regulation remains uncertain. In this study, we applied the chronic social defeat stress (CSDS) paradigm to both GHS-R1a knock-out (Ghsr^-/-) mice and littermate control (Ghsr^+/+) mice, and then measured their depression- and anxiety-related behaviors. We found that Ghsr^+/+ mice, but not Ghsr^-/- mice, displayed apparent anxiety and depression after CSDS, while two groups mice showed identical behaviors at baseline, non-stress state. By screening the central and peripheral responses of Ghsr^-/- mice and Ghsr^+/+ mice to chronic stress, we found similar elevations of total ghrelin and adrenocorticotropic hormone (ACTH) in the serum of Ghsr^-/- mice and Ghsr^+/+ mice after CSDS, but decreased interleukin-6 (IL-6) in the serum of defeated Ghsr^-/- mice compared to defeated Ghsr^+/+ mice. We also found increased concentration of brain derived neurotropic factor (BDNF) in the hippocampus of Ghsr^-/- mice compared to Ghsr^+/+ mice after CSDS. The basal levels of ghrelin, ACTH, IL-6, and BDNF were not different between Ghsr^-/- mice and Ghsr^+/+ mice. Our findings thus suggested that the differential expressions of BDNF and IL-6 after CSDS may contribute to less anxiety and less despair observed in GHS-R1a-deficient mice than in WT control mice. Therefore, ghrelin/GHS-R1a signaling may play a pro-anxiety and pro-depression effect in response to chronic stress, while GHS-R1a deficiency may provide resistance to depressive symptoms of CSDS.

Intracerebroventricular Ghrelin Administration Increases Depressive-Like Behavior in Male Juvenile Rats

Major depressive disorder (MDD) is arguably the largest contributor to the global disease and disability burden, but very few treatment options exist for juvenile MDD patients. Ghrelin is the principal hunger-stimulating peptide, and it has also been shown to reduce depressive-like symptoms in adult rodents. We examined the effects of intracerebroventricular (icv) injection of ghrelin on depressive-like behavior. Moreover, we determined whether ghrelin increased neurogenesis in the hippocampus. Ghrelin (0.2-nM, 0.5-nM, and 1.0-nM) was administered acutely by icv injection to juvenile rats to determine the most effective dose (0.5-nM) by a validated feeding behavior test and using the forced swim test (FST) as an indicator of depressive-like behavior. 0.5-nM ghrelin was then administered icv against an artificial cerebrospinal fluid (aCSF) vehicle control to determine behavioral changes in the tail suspension test (TST) as an indicator of depressive-like behavior. Neurogenesis was investigated using a mitogenic paradigm, as well as a neurogenic paradigm to assess whether ghrelin altered neurogenesis. Newborn hippocampal cells were marked using 5′-bromo-2′-deoxyuridine (BrdU) administered intraperitoneally (ip) at either the end or the beginning of the experiment for the mitogenic and neurogenic paradigms, respectively. We found that ghrelin administration increased immobility time in the TST. Treatment with ghrelin did not change mitogenesis or neurogenesis. These results suggest that ghrelin administration does not have an antidepressant effect in juvenile rats. In contrast to adult rodents, ghrelin increases depressive-like behavior in male juvenile rats. These results highlight the need to better delineate differences in the neuropharmacology of depressive-like behavior between juvenile and adult rodents.

Exogenous ghrelin administration increases alcohol self-administration and modulates brain functional activity in heavy-drinking alcohol-dependent individuals

Preclinical evidence suggests that ghrelin, a peptide synthesized by endocrine cells of the stomach and a key component of the gut-brain axis, is involved in alcohol seeking as it modulates both central reward and stress pathways. However, whether and how ghrelin administration may impact alcohol intake in humans is not clear. For, we believe, the first time, this was investigated in the present randomized, crossover, double-blind, placebo-controlled, human laboratory study. Participants were non-treatment-seeking alcohol-dependent heavy-drinking individuals. A 10-min loading dose of intravenous ghrelin/placebo (3 mcg kg^-1) followed by a continuous ghrelin/placebo infusion (16.9 ng/kg/min) was administered. During a progressive-ratio alcohol self-administration experiment, participants could press a button to receive intravenous alcohol using the Computerized Alcohol Infusion System. In another experiment, brain functional magnetic resonance imaging was conducted while participants performed a task to gain points for alcohol, food or no reward. Results showed that intravenous ghrelin, compared to placebo, significantly increased the number of alcohol infusions self-administered (percent change: 24.97±10.65, P=0.04, Cohen's d=0.74). Participants were also significantly faster to initiate alcohol self-administration when they received ghrelin, compared to placebo (P=0.03). The relationships between breath alcohol concentration and subjective effects of alcohol were also moderated by ghrelin administration. Neuroimaging data showed that ghrelin increased the alcohol-related signal in the amygdala (P=0.01) and modulated the food-related signal in the medial orbitofrontal cortex (P=0.01) and nucleus accumbens (P=0.08). These data indicate that ghrelin signaling affects alcohol seeking in humans and should be further investigated as a promising target for developing novel medications for alcohol use disorder.

Pharmacological Effects and Regulatory Mechanisms of Tobacco Smoking Effects on Food Intake and Weight Control

Beyond promoting smoking initiation and preventing smokers from quitting, nicotine can reduce food intake and body weight and thus is viewed as desirable by some smokers, especially many women. During the last several decades, the molecular mechanisms underlying the inverse correlation between smoking and body weight have been investigated extensively in both animals and humans. Nicotine's weight effects appear to result especially from the drug's stimulation of α3β4 nicotine acetylcholine receptors (nAChRs), which are located on pro-opiomelanocortin (POMC) neurons in the arcuate nucleus (ARC), leading to activation of the melanocortin circuit, which is associated with body weight. Further, α7- and α4β2-containing nAChRs have been implicated in weight control by nicotine. This review summarizes current understanding of the regulatory effects of nicotine on food intake and body weight according to the findings from pharmacological, molecular genetic, electrophysiological, and feeding studies on these appetite-regulating molecules, such as α3β4, α7, and α4β2 nAChRs; neuropeptide Y (NPY); POMC; melanocortin 4 receptor (MC4R); agouti-related peptide (AgRP); leptin, ghrelin, and protein YY (PYY).

Stress, Motivation, and the Gut-Brain Axis: A Focus on the Ghrelin System and Alcohol Use Disorder

Since its discovery, the gut hormone, ghrelin, has been implicated in diverse functional roles in the central nervous system. Central and peripheral interactions between ghrelin and other hormones, including the stress-response hormone cortisol, govern complex behavioral responses to external cues and internal states. By acting at ventral tegmental area dopaminergic projections and other areas involved in reward processing, ghrelin can induce both general and directed motivation for rewards, including craving for alcohol and other alcohol-seeking behaviors. Stress-induced increases in cortisol seem to increase ghrelin in the periphery, suggesting a pathway by which ghrelin influences how stressful life events trigger motivation for rewards. However, in some states, ghrelin may be protective against the anxiogenic effects of stressors. This critical review brings together a dynamic and growing literature, that is, at times inconsistent, on the relationships between ghrelin, central reward-motivation pathways, and central and peripheral stress responses, with a special focus on its emerging role in the context of alcohol use disorder.

Immune and metabolic alterations in first episode psychosis (FEP) patients

The molecular underpinnings associated to first episode psychosis (FEP) remains to be elucidated, but compelling evidence supported an association of FEP with blood alterations in biomarkers related to immune system, growth factors and metabolism regulators. Many of these studies have not been already confirmed in larger samples or have not considered the FEP diagnostic subgroups. In order to identify biochemical signatures of FEP, the serum levels of the growth factors BDNF and VEGF, the immune regulators IL-1RA, IL-6, IL-10 and IL-17, RANTES/CCL5, MIP-1b/CCL4, IL-8 and the metabolic regulators C-peptide, ghrelin, GIP, GLP-1, glucagon, insulin, leptin, PAI-1, resistin and visfatin were analysed in 260 subjects collected in the GET UP project. The results indicated an increase of MIP-1b/CCL4, VEGF, IL-6 and PAI-1, while IL-17, ghrelin, glucagon and GLP-1 were decreased in the whole sample of FEP patients (p < 0.01 for all markers except for PAI-1 p < 0.05). No differences were evidenced for these markers among the diagnostic groups that constitute the FEP sample, whereas IL-8 is increased only in patients with a diagnosis of affective psychosis. The principal component analysis (PCA) and variable importance analysis (VIA) indicated that MIP-1b/CCL4, ghrelin, glucagon, VEGF and GLP-1 were the variables mostly altered in FEP patients. On the contrary, none of the analysed markers nor a combination of them can discriminate between FEP diagnostic subgroups. These data evidence a profile of immune and metabolic alterations in FEP patients, providing new information on the molecular mechanism associated to the psychosis onset for the development of preventive strategies and innovative treatment targets.

Evaluating the Levels of Nesfatin-1 and Ghrelin Hormones in Patients with Moderate and Severe Major Depressive Disorders

OBJECTIVE

The goal of this study was to evaluate the importance of nesfatin-1, acylated and des-acylated ghrelin, which are known as energy regulatory hormones, in patients with moderate and severe major depression disorders (MDD).

METHODS

Thirty patients with a moderate degree of MDD and, 30 with a severe degree of MDD were used as participants in this study. Thirty subjects without depression were enrolled as a control group. The Hamilton Depression Rating Scale was used to classify the patients with MDD. Blood samples were taken after overnight fasting. The plasma nesfatin-1, acylated ghrelin and des-acylated ghrelin levels were measured using a commercially available enzyme-linked immunosorbent assay kit.

RESULTS

The nesfatin-1, the acylated ghrelin and the des-acylated ghrelin levels were found to be significantly higher in severe MDD (3.92±0.4 ng/mL; 88.56±4.1 pg/mL; 962.76±67 pg/mL) as compared to moderate MDD (2.91±0.5 ng/mL; 77.63±4.19 pg/mL; 631.16±35 pg/mL), or the control (1.01±0.3 ng/mL; 58.60±9.00 pg/mL; 543.13±62 pg/mL), respectively.

CONCLUSION

Although nesfatin-1 and ghrelin are known as adversely affecting the hormones involving the regulation of appetite and food intake, they all increase in depressive patients and are even associated with the severity of the disease. In clinical medicine, the evaluation of the role of nesfatin-1 and ghrelin in endocrine and neu-roendocrine regulation of major metabolic functions is an important key mechanism in solving numerous diseases associated with endocrine and neuroendocrine disturbance. Increased levels of nesfatin-1 and ghrelin may also be important criteria in describing the prognoses of the patients and the effectiveness of the treatments.

A cannabinoid receptor antagonist attenuates ghrelin-induced activation of the mesolimbic dopamine system in mice

Ghrelin has been attributed various physiological processes including food intake and reward regulation, through activation of the mesolimbic dopamine system. Reward modulation involves the mesolimbic dopamine system, consisting of the ventral tegmental area (VTA) dopamine neurons targeting nucleus accumbens (NAc), a system that ghrelin activates through VTA-dependent mechanisms. In the first study, we found that systemic intraperitoneal (ip) administration of rimonabant attenuated intracerebroventricular (icv) ghrelin's ability to cause locomotor stimulation and NAc dopamine release in mice. Ghrelin-induced (icv) chow intake was not altered by rimonabant administration (ip). Finally, we showed that bilateral VTA administration of rimonabant blocks the ability of intra-VTA administered ghrelin to increase locomotor activity, but does not affect food intake in mice. Collectively, these data indicate clear dissociation between regulation of food intake and activation of the mesolimbic dopamine system.

Chronic administration of parecoxib exerts anxiolytic-like and memory enhancing effects and modulates synaptophysin expression in mice

BACKGROUND

Previous studies have shown that cyclooxygenase-2, a key enzyme that converts arachidonic acid to prostaglandins, is involved in anxiety and cognitive processes, but few studies have investigated the effects of chronic administration of cyclooxygenase-2 inhibitors on anxiety, learning and memory under normal physiological conditions. The aim of the study was to investigate the effects of chronic administration of parecoxib, a cyclooxygenase-2 inhibitor, on anxiety behavior and memory performance under normal physiological conditions and to explore the possible neural mechanism underlying parecoxib-mediated effects.

METHODS

Adult male ICR mice were randomly divided into four groups: the control group and three parecoxib groups. Mice received normal saline or parecoxib (2.5, 5.0 or 10 mg/kg) intraperitoneal injection once a day for 21 days, respectively. Elevated plus-maze, novel object recognition and Y maze tests were conducted on day 23, 24 and 26, respectively. Four additional groups that received same drug treatment were used to measure synaptophysin protein levels by western blot and prostaglandin E2 (PGE2) levels by ELISA in the amygdala and hippocampus on day 26.

RESULTS

Chronic parecoxib exerted an anxiolytic-like effect in the plus-maze test test, and enhanced memory performance in the novel object recognition and Y maze tests. Western blot analysis showed that chronic parecoxib down-regulated synaptophysin levels in the amygdala and up-regulated synaptophysin levels in the hippocampus. ELISA assay showed that chronic parecoxib inhibited PGE2 in the hippocampus but not amygdala.

CONCLUSIONS

Chronic parecoxib exerts anxiolytic-like and memory enhancing effects, which might be mediated through differential modulation of synaptophysin and PGE2 in the amygdala and hippocampus.

Linking Stress and Infertility: A Novel Role for Ghrelin

Infertility affects a remarkable one in four couples in developing countries. Psychological stress is a ubiquitous facet of life, and although stress affects us all at some point, prolonged or unmanageable stress may become harmful for some individuals, negatively impacting on their health, including fertility. For instance, women who struggle to conceive are twice as likely to suffer from emotional distress than fertile women. Assisted reproductive technology treatments place an additional physical, emotional, and financial burden of stress, particularly on women, who are often exposed to invasive techniques associated with treatment. Stress-reduction interventions can reduce negative affect and in some cases to improve in vitro fertilization outcomes. Although it has been well-established that stress negatively affects fertility in animal models, human research remains inconsistent due to individual differences and methodological flaws. Attempts to isolate single causal links between stress and infertility have not yet been successful due to their multifaceted etiologies. In this review, we will discuss the current literature in the field of stress-induced reproductive dysfunction based on animal and human models, and introduce a recently unexplored link between stress and infertility, the gut-derived hormone, ghrelin. We also present evidence from recent seminal studies demonstrating that ghrelin has a principal role in the stress response and reward processing, as well as in regulating reproductive function, and that these roles are tightly interlinked. Collectively, these data support the hypothesis that stress may negatively impact upon fertility at least in part by stimulating a dysregulation in ghrelin signaling.

Ghrelin produces antidepressant-like effect in the estrogen deficient mice

Recent evidence shows that ghrelin plays an important role in depression. However, it was little known whether ghrelin produces antidepressant-like effect in the ovariectomized mice. The present study was aimed to investigate the antidepressant-like effects of the ghrelin in ovariectomized mice. In the forced swim test, ghrelin significantly decreased immobility time, reversing the “depressive-like” effect observed in ovariectomized mice, and this effect was reversed by the tamoxifen. In addition, immunohistochemical study indicated that ghrelin treatment reversed the reductions in c-Fos expression induced by ovariectomy. An estrogen antagonist tamoxifen also antagonized the effect of ghrelin on the c-Fos expression. Furthermore, the western blotting indicated that brain-derived neurotrophic factor (BDNF) in the hippocampus, but not phosphorylated cAMP response element-binding protein (pCREB)/CREB in the frontal cortex, were affected by ghrelin treatment. Ghrelin treatment significantly increased BrdU expression. Therefore, these findings suggest that ghrelin produces antidepressant-like effects in ovariectomized mice, and estrogen receptor may be involved in the antidepressant-like effects of the ghrelin.

Ghrelin alleviates anxiety- and depression-like behaviors induced by chronic unpredictable mild stress in rodents

As a regulator of food intake, ghrelin also plays a key role in mood disorders. Previous studies reported that acute ghrelin administration defends against depressive symptoms of chronic stress. However, the effects of long-term ghrelin on rodents under chronic stress hasn't been revealed. In this study, we found chronic peripheral administration of ghrelin (5nmol/kg/day for 2 weeks, i.p.) could alleviate anxiety- and depression-like behaviors induced by chronic unpredictable mild stress (CUMS). The depression-like behaviors were assessed by the forced swimming test (FST), and anxiety-like behaviors were assessed by the open field test (OFT) and the elevated plus maze test (EPM). Meanwhile, we observed that peripheral acylated ghrelin, together with gastral and hippocampal ghrelin prepropeptide mRNA level, were significantly up-regulated in CUMS mice. Besides, the increased protein level of growth hormone secretagogue receptor (GHSR) in hippocampus were also detected. These results suggested that the endogenous ghrelin/GHSR pathway activated by CUMS plays a role in homeostasis. Further results showed that central treatment of ghrelin (10μg/rat/day for 2 weeks, i.c.v.) or GHRP-6 (the agonist of GHSR, 10μg/rat/day for 2 weeks, i.c.v.) significantly alleviated the depression-like behaviors induced by CUMS in FST and sucrose preference test (SPT). Based on these results, we concluded that central GHSR is involved in the antidepressant-like effect of exogenous ghrelin treatment, and ghrelin/GHSR may have the inherent neuromodulatory properties against depressive symptoms.

Interactive effects of ghrelin and ketamine on forced swim performance: Implications for novel antidepressant strategies

The efficacy of ketamine to alleviate depressive symptoms has promoted a wealth of research exploring alternate therapeutic targets for depression. Given the caveats of ketamine treatment taken together with the increasingly greater emphasis on combinatorial therapeutic approaches to depression, we sought to asses whether the hypothalamic "hunger hormone", ghrelin, would augment the effects of ketamine. Indeed, ghrelin has recently been found to possess antidepressant potential and may be especially effective against the metabolic and feeding deficits observed with depression. Two studies were performed: 1. mice were given an intraperitoneal injection of ghrelin (80μg/kg) or saline, followed by a saline or a low or high dose of ketamine (5 or 10mg/kg) and 2. mice received 10mg/kg of ketamine together with saline or the ghrelin receptor antagonist JMV2959 (3 or 6mg/kg) and Forced Swim Test (FST) performance was assessed. In both studies, ketamine alone reduced FST immobility. Similarly, ghrelin alone reduced swim immobility suggesting an antidepressant-like response. However, ghrelin did not augment the impact of ketamine when co-administered and in fact, it appeared to antagonize its actions at the lower dose. As well, JMV2959 did not significantly influence FST performance. These data confirm the antidepressant-like effects of ketamine and further suggest that ghrelin might have similar properties. Yet, our results caution against combinatorial treatment with these agents, probably owing to unexpected allosteric or other antagonist actions.

Impact of [d-Lys(3)]-GHRP-6 and feeding status on hypothalamic ghrelin-induced stress activation

Ghrelin administration directly into hypothalamic nuclei, including the arcuate nucleus (ArcN) and the paraventricular nucleus (PVN), alters the expression of stress-related behaviors. In the present study we investigated the effect of feeding status on the ability of ghrelin to induce stress and anxiogenesis. Adult male Sprague Dawley rats were implanted with guide cannula targeting either the ArcN or PVN. In the first experiment we confirmed that ArcN and PVN ghrelin treatment produced anxiety-like behavior as measured using the elevated plus maze (EPM) paradigm. Ghrelin was administered during the early dark cycle. Immediately after microinjections rats were placed in the EPM for 5min. Both ArcN and PVN treatment reduced open arm exploration. The effect was attenuated by pretreatment with the ghrelin 1a receptor antagonist [d-Lys(3)]-GHRP-6. In a separate group of animals ghrelin was injected into either nucleus and rats were returned to their home cages for 60min with free access to food. An additional group of rats was returned to home cages with no food access. After 60min with or without food access all rats were tested in the EPM. Results indicated that food consumption just prior to EPM testing reversed the avoidance of the open arms of the EPM. In contrast, rats injected with ghrelin, placed in their home cage for 60min without food, and subsequently tested in the EPM, exhibited an increased avoidance of the open arms, consistent with stress activation. Overall, our findings demonstrate that ghrelin 1a receptor blockade and feeding status appear to impact the ability of ArcN and PVN ghrelin to elicit stress and anxiety-like behaviors.

Anxiolytic-Like Effects of Increased Ghrelin Receptor Signaling in the Amygdala

BACKGROUND

Besides the well-known effects of ghrelin on adiposity and food intake regulation, the ghrelin system has been shown to regulate aspects of behavior including anxiety and stress. However, the effect of virus-mediated overexpression of the ghrelin receptor in the amygdala has not previously been addressed directly.

METHODS

First, we examined the acute effect of peripheral ghrelin administration on anxiety- and depression-like behavior using the open field, elevated plus maze, forced swim, and tail suspension tests. Next, we examined the effect of peripheral ghrelin administration and ghrelin receptor deficiency on stress in a familiar and social environment using the Intellicage system. Importantly, we also used a novel approach to study ghrelin receptor signaling in the brain by overexpressing the ghrelin receptor in the amygdala. We examined the effect of ghrelin receptor overexpression on anxiety-related behavior before and after acute stress and measured the modulation of serotonin receptor expression.

RESULTS

We found that ghrelin caused an anxiolytic-like effect in both the open field and elevated plus maze tests. Additionally, it attenuated air-puff-induced stress in the social environment, while the opposite was shown in ghrelin receptor deficient mice. Finally, we found that overexpression of the ghrelin receptor in the basolateral division of the amygdala caused an anxiolytic-like effect and decreased the 5HT1a receptor expression.

CONCLUSIONS

Ghrelin administration and overexpression of the ghrelin receptor in the amygdala induces anxiolytic-like behavior. Since the ghrelin receptor has high constitutive activity, ligand-independent signaling in vivo may be important for the observed anxiolytic-like effects. The anxiolytic effects seem to be mediated independently from the HPA axis, potentially engaging the central serotonin system.

Early life social stress induced changes in depression and anxiety associated neural pathways which are correlated with impaired maternal care

Exposures to various types of early life stress can be robust predictors of the development of psychiatric disorders, including depression and anxiety. The objective of the current study was to investigate the roles of the translationally relevant targets of central vasopressin, oxytocin, ghrelin, orexin, glucocorticoid, and the brain-derived neurotrophic factor (BDNF) pathway in an early chronic social stress (ECSS) based rodent model of postpartum depression and anxiety. The present study reports novel changes in gene expression and extracellular signal related kinase (ERK) protein levels in the brains of ECSS exposed rat dams that display previously reported depressed maternal care and increased maternal anxiety. Decreases in oxytocin, orexin, and ERK proteins, increases in ghrelin receptor, glucocorticoid and mineralocorticoid receptor mRNA levels, and bidirectional changes in vasopressin underscore related work on the adverse long-term effects of early life stress on neural activity and plasticity, maternal behavior, responses to stress, and depression and anxiety-related behavior. The differences in gene and protein expression and robust correlations between expression and maternal care and anxiety support increased focus on these targets in animal and clinical studies of the adverse effects of early life stress, especially those focusing on depression and anxiety in mothers and the transgenerational effects of these disorders on offspring.

Midbrain raphe 5-HT1A receptor activation alters the effects of ghrelin on appetite and performance in the elevated plus maze

Prior research suggests that midbrain serotonergic signaling and hypothalamic ghrelinergic signaling both play critical roles in appetitive and emotional behaviors. In the present study, we investigated the effects of median raphe nucleus (MRN) somatodentritic 5-HT1A receptor activation on the feeding-stimulant and anxiogenic action of paraventricular nucleus (PVN) ghrelin. In an initial experiment, adult male Sprague-Dawley rats were injected with either ghrelin (200-800 pmol) into the PVN or 8-OH-DPAT (2.5-10 nmol), a 5-HT1A receptor agonist, into the MRN. Performance on the elevated plus maze (EPM) was then assessed. In separate rats, MRN 8-OH-DPAT (2.5-5 nmol) was administered 5 min prior to PVN injection of ghrelin (400 pmol) followed by EPM testing. The orexigenic effects of MRN 8-OH-DPAT (0.1-1.6 nmol) paired with PVN ghrelin (50 pmol) were also examined. When administered alone into the PVN, ghrelin significantly decreased the number of entries and time spent in the open arms of the EPM. This anxiogenic effect was blocked if rats were allowed to eat immediately after ghrelin administration and then tested in the plus maze. MRN injections of 8-OH-DPAT were anxiolytic, and when rats were pretreated with 8-OH-DPAT prior to ghrelin, the anxiogenic action of the peptide was attenuated. In contrast, MRN administration of 8-OH-DPAT potentiated the eating-stimulant effect of PVN ghrelin. Overall, our findings demonstrate that ghrelinergic and serotonergic circuits interact in the neural control of eating and anxiety-like behaviors, with 5-HT1A receptor mechanisms potentiating the orexigenic action of ghrelin while inhibiting ghrelin-induced anxiogenesis as measured via the EPM.

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.