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

New antidepressant mechanism of Yueju Pill: Increasing ghrelin level by inhibiting gastric mTOR/S6K signaling pathway and sensitizing hippocampal GHS-R

Background and aim

Yueju Pill (YJ) not only has good antidepressant effect but also can effectively treat digestive system diseases. However,it remains unclear whether the mechanism of antidepressant action of YJ is related to the peripheral digestive system. The purpose of this study was to elucidate the antidepressant mechanism of YJ on ghrelin level based on gastric mTOR/S6K signal pathway and sensitized hippocampal Ghrelin/GHS-R system in CUMS mice.

Experimental procedure

The depression model was induced by chronic unpredictable mild stress (CUMS) and social isolation. The antidepressant effect of YJ was observed by behavioral experiment and hemodynamic experiments. Ghrelin levels in in hippocampus and blood were measured by Elisa kit, and the mRNA of ghrelin in mice stomach was measured by Real-time Quantitative PCR (RT-qPCR). The activation level of gastric mTOR/S6K signal pathway was detected by Western Blot (WB). Rapamycin (Rapa) and L-Leucine (L-Leu) were used to verify the effects of YJ on the synthesis and release of ghrelin. The activity of GHS-R in hippocampus was observed by immunofluorescence. Hippocampal neuronal damage was evaluated by HE staining and Nissl staining. The level of central neurotransmitter was measured by liquid chromatograph mass spectrometer (LC-MS).

Results and conclusion

YJ ameliorates CUMS-induced depressive-like behavior by inhibiting the gastric mTOR/S6K signaling pathway and increasing GHR expression in the mouse stomach. However, these effects of YJ could be resisted by L-Leu (a mTOR receptor agonist). Further studies have shown that YJ can sensitize the Ghrelin/GHS-R system in the hippocampus, with significant neuroprotective effects, and is also involved in regulating the levels of key neurotransmitters (5-hydroxytryptamine, Dopamine and γ-aminobutyric acid) in depressive-like states.

Ghrelin/GHSR System in Depressive Disorder: Pathologic Roles and Therapeutic Implications

Depression is the most common chronic mental illness and is characterized by low mood, insomnia, and affective disorders. However, its pathologic mechanisms remain unclear. Numerous studies have suggested that the ghrelin/GHSR system may be involved in the pathophysiologic process of depression. Ghrelin plays a dual role in experimental animals, increasing depressed behavior and decreasing anxiety. By combining several neuropeptides and traditional neurotransmitter systems to construct neural networks, this hormone modifies signals connected to depression. The present review focuses on the role of ghrelin in neuritogenesis, astrocyte protection, inflammatory factor production, and endocrine disruption in depression. Furthermore, ghrelin/GHSR can activate multiple signaling pathways, including cAMP/CREB/BDNF, PI3K/Akt, Jak2/STAT3, and p38-MAPK, to produce antidepressant effects, given which it is expected to become a potential therapeutic target for the treatment of depression.

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.

Neuronal ablation of GHSR mitigates diet-induced depression and memory impairment via AMPK-autophagy signaling-mediated inflammation

Obesity is associated with chronic inflammation in the central nervous system (CNS), and neuroinflammation has been shown to have detrimental effects on mood and cognition. The growth hormone secretagogue receptor (GHSR), the biologically relevant receptor of the orexigenic hormone ghrelin, is primarily expressed in the brain. Our previous study showed that neuronal GHSR deletion prevents high-fat diet-induced obesity (DIO). Here, we investigated the effect of neuronal GHSR deletion on emotional and cognitive functions in DIO. The neuron-specific GHSR-deficient mice exhibited reduced depression and improved spatial memory compared to littermate controls under DIO. We further examined the cortex and hippocampus, the major regions regulating cognitive and emotional behaviors, and found that the neuronal deletion of GHSR reduced DIO-induced neuroinflammation by suppressing proinflammatory chemokines/cytokines and decreasing microglial activation. Furthermore, our data showed that neuronal GHSR deletion suppresses neuroinflammation by downregulating AMPK-autophagy signaling in neurons. In conclusion, our data reveal that neuronal GHSR inhibition protects against DIO-induced depressive-like behavior and spatial cognitive dysfunction, at least in part, through AMPK-autophagy signaling-mediated neuroinflammation.

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.

Potential role of ghrelin in the regulation of inflammation

Several diseases are caused or progress due to inflammation. In the past few years, accumulating evidence suggests that ghrelin, a gastric hormone of 28-amino acid residue length, exerts protective effects against inflammation by modulating the related pathways. This review focuses on ghrelin's anti-inflammatory and potential therapeutic effects in neurological, cardiovascular, respiratory, hepatic, gastrointestinal, and kidney disorders. Ghrelin significantly alleviates excessive inflammation and reduces damage to different target organs mainly by reducing the secretion of inflammatory cytokines, including interleukin-6 (IL-6), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α), and inhibiting the nuclear factor kappa-B (NF-κB) and NLRP3 inflammasome signaling pathways. Ghrelin also regulates inflammation and apoptosis through the p38 MAPK/c-Jun N-terminal kinase (JNK) signaling pathway; restores cerebral microvascular integrity, and attenuates vascular leakage. Ghrelin activates the phosphoInositide-3 kinase (PI3K)/protein kinase B (Akt) pathway and inhibits inflammatory responses in cardiovascular diseases and acute kidney injury. Some studies show that ghrelin exacerbates colonic and intestinal manifestations of colitis. Interestingly, some inflammatory states, such as non-alcoholic steatohepatitis, inflammatory bowel diseases, and chronic kidney disease, are often associated with high ghrelin levels. Thus, ghrelin may be a potential new therapeutic target for inflammation-related diseases.

"Sibling" battle or harmony: crosstalk between nesfatin-1 and ghrelin

Ghrelin was first identified as an endogenous ligand of the growth hormone secretagogue receptor (GHSR) in 1999, with the function of stimulating the release of growth hormone (GH), while nesfatin-1 was identified in 2006. Both peptides are secreted by the same kind of endocrine cells, X/A-like cells in the stomach. Compared with ghrelin, nesfatin-1 exerts opposite effects on energy metabolism, glucose metabolism, gastrointestinal functions and regulation of blood pressure, but exerts similar effects on anti-inflammation and neuroprotection. Up to now, nesfatin-1 remains as an orphan ligand because its receptor has not been identified. Several studies have shown the effects of nesfatin-1 are dependent on the receptor of ghrelin. We herein compare the effects of nesfatin-1 and ghrelin in several aspects and explore the possibility of their interactions.

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.

Expression of the growth hormone secretagogue receptor 1a (GHS-R1a) in the brain

The review presents data on the expression of growth hormone secretagogue receptor 1a (GHS-R1a) in the brain regions in model animals (zebrafish, rodents, primates), and in the human brain. Studies show widespread distribution of the receptor in the brain, which evidences the involvement of the receptor in many physiological processes. Using various organisms, data have been obtained regarding the participation of the GHS-R1a in the regulation of the anti- and pro-inflammatory response, proliferation, and apoptosis. It is known that the receptor plays an important role in eating behavior and is also involved in the pathogenetic mechanisms of drug addiction, obesity, and chronic alcohol consumption. Based on this, research is underway with the use of various therapeutic agents that can be used for the pharmacological correction of these conditions. This review also presents hypothetical pathways of intracellular signaling, in which GHS-R1a may participate. A complete understanding of these mechanisms has not yet been reached. The ghrelin intracellular signaling seem to be specific to brain region and, probably, also depend on the metabolic or stress status of the organism.

The ATP Level in the mPFC Mediates the Antidepressant Effect of Calorie Restriction

Food deprivation can rescue obesity and overweight-induced mood disorders, and promote mood performance in normal subjects. Animal studies and clinical research have revealed the antidepressant-like effect of calorie restriction, but little is known about the mechanism of calorie restriction-induced mood modification. Previous studies have found that astrocytes modulate depressive-like behaviors. Inositol 1,4,5-trisphosphate receptor type 2 (IP3R2) is the predominant isoform in mediating astrocyte Ca2+ signals and its genetic knockout mice are widely used to study astrocyte function in vivo. In this study, we showed that deletion of IP3R2 blocked the antidepressant-like effect induced by calorie restriction. In vivo microdialysis experiments demonstrated that calorie restriction induced an increase in ATP level in the medial prefrontal cortex (mPFC) in naïve mice but this effect disappeared in IP3R2-knockout mice, suggesting a role of astrocytic ATP in the calorie restriction-induced antidepressant effect. Further experiments showed that systemic administration and local infusion of ATP into the mPFC induced an antidepressant effect, whereas decreasing ATP by Apyrase in the mPFC blocked calorie restriction-induced antidepressant regulation. Together, these findings support a role for astrocytic ATP in the antidepressant-like effect caused by calorie restriction.

A metabolomic study on the anti-depressive effects of two active components from Chrysanthemum morifolium

Chrysanthemum morifolium (Chr) is a traditional Chinese medicine (TCM) that has been used in the treatment of inflammation-linked diseases for hundreds of years. Naringenin (Nar) and apigenin (Api) are the major active components in aqueous extracts of C. morifolium. The aim of our study was to clarify the roles of Chr, Nar and Api in ameliorating depression-like behaviour induced by corticosterone. First, the behavioural and biochemical indicators closely related to depression were examined to evaluate the therapeutic effects of Chr/Nar/Api on a depression model. Then, a metabolomics approach was utilized to screen for biomarkers and related pathways between a control group and Chr/Nar/Api groups. The comprehensive results revealed that Chr/Nar/Api exerted anti-depressant effects through interfering with tryptophan metabolism, arginine and prolinemetabolism, citrate cycle, niacin and niacinamide metabolism, phenylalanine metabolism, and alanine, aspartate and glutamate metabolism. The mechanism of Chr/Api/Nar in the treatment of depression was elucidated based on material and energy metabolism. Moreover, Nar could be used as a substitute for Chr for reversing depression-like behaviour, and Api was similar to a positive drug in terms of function on depression. The integrated metabolomics approach demonstrated here should be an effective method for interpreting the function of herbs from TCM and clarifying the mechanism of their components in future studies.

Expression of ghrelin or growth hormone secretagogue receptor in the brain of postpartum stress mice

Postpartum depression is one of the most common mental diseases that occur in women after childbirth; this disorder is extremely painful for women and represents a major burden on the society. Therefore, we designed this study to explore the possible material basis of the disease, and provide potential novel antidepressants therapy using a mouse model. We established a postpartum immobilization stress model. Maternal body weight changes and food intake were recorded for half a month after delivery, and levels of ghrelin and its receptor, growth hormone secretagogue receptor (GHSR) were measured. The mice in the immobilization stress group showed stress activity as well as low body weight and low feeding status. Ghrelin expression was elevated in blood whereas ghrelin or GHSR expression decreased in the hippocampus and prefrontal cortex of the immobilization stress mice, and the number of ghrelin-active and GHSR cells reduced.

Ghrelin attenuates depressive-like behavior, heart failure, and neuroinflammation in postmyocardial infarction rat model

Depression after myocardial infarction (MI) and chronic heart failure (CHF) is a common condition that is resistant to anti-depressive drugs. Ghrelin (a peptide hormone) shows dual protective effects on heart and brain. Whether ghrelin treatment attenuated depression after MI was investigated. Coronary artery occlusion was performed to induce MI and subsequent CHF in rats. Ghrelin (100 μg/kg in 0.5 ml of saline) or vehicle (0.5 ml of saline) was injected subcutaneously twice a day for 4 weeks. At week 5, all the animals underwent behavioral assessments including sucrose preference test (SPT), elevated plus maze test (EPM), and open field test (OFT). After cardiac function analysis, brain tissues were processed to determine inflammatory cytokines and microglial activations in hippocampus. Results showed that ghrelin substantially improved cardiac dysfunction, infarction size, and cardiac remodeling and modulated the release of inflammatory cytokines and the increase of Iba-1 positive microglia and glial fibrillary acidic protein-positive astrocytes in the CA1 area of hippocampus. Behavioral tests revealed that this treatment remarkably increased sucrose preference and mobile times and numbers. These findings provided evidence that peripheral ghrelin administration inhibits depression-like behavior and neuroinflammation and thus could be a new approach for the treatment of CHF-associated depression.

High-fat diet during adulthood interacts with prenatal stress, affecting both brain inflammatory and neuroendocrine markers in male rats

Prenatal stress (PNS) affects foetal programming and, through an interaction with subsequent challenges, can increase vulnerability to mood and metabolic disorders. We have previously shown that, following PNS, adult male rats are characterized by increased vulnerability to a metabolic stressor experienced at adulthood (8-week-high-fat diet-HFD). In this study, we specifically assessed whether PNS might interact with an adult metabolic challenge to induce an inflammatory phenotype. Changes in the expression levels of inflammatory (Il-1β, Tnf-α, Il-6) and of stress response mediators (Nr3c1, Fkbp5) as well as of mood and metabolic regulators (Bdnf, Ghs-R) were investigated in the hippocampus, prefrontal cortex and hypothalamus, brain regions involved in the pathogenesis of depression and prone to inflammation in response to stress. Overall, PNS reduced the expression of Bdnf and Tnf-α, while HFD administered at adulthood counteracted this effect suggesting that PNS impinges upon the same pathways regulating responses to a metabolic challenge at adulthood. Furthermore, HFD and PNS affected the expression of both Nr3c1 and Fkbp5, two neuroendocrine mediators involved in the response to stress, metabolic challenges and in the modulation of the emotional profile (as shown by the correlation between Fkbp5 and the time spent in the open arms of the elevated plus-maze). Overall, these results indicate that the same metabolic and neuroendocrine effectors engaged by PNS are affected by metabolic challenges at adulthood, providing some mechanistic insight into the well-known comorbidity between mood and metabolic disorders.

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.

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

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

Ghrelin/GHS-R1a signaling plays different roles in anxiety-related behaviors after acute and chronic caloric restriction

The brain-gut hormone ghrelin and its receptor GHS-R1a, the growth hormone secretagogue receptor 1a, regulates diverse functions of central nervous system including stress response and mood. Both acute and chronic caloric restrictions (CR) were reported to increase endogenous ghrelin level meanwhile regulate anxiety-related behaviors; however, the causal relationship between CR-induced ghrelin elevation and anxiety are not fully established. Here, we introduced an acute (24 h) and a chronic (10wks) CR procedure to both GHS-R1a KO (Ghsr^-/-) mice and WT (Ghsr^+/+) littermates, and compared their anxiety-related behaviors. We found that acute CR induced anxiolytic and anti-despairing behaviors in Ghsr^+/+ mice but not in Ghsr^-/- mice. Ad-libitum refeeding abolished the effect of acute CR on anxiety-related behaviors. In contrast, chronic CR for 10wks facilitated despair-like behavior meanwhile inhibited anxiety-like behavior in Ghsr^+/+ mice. GHS-R1a deficiency rescued despair-like behavior while did not affect anxiolytic response induced by chronic CR. In addition, we found elevated interleukin-6 (IL-6) in serum of Ghsr^+/+ mice after chronic CR, but not in Ghsr^-/- mice. Altogether, our findings indicated that acute CR and chronic CR have different impacts on anxiety-related behaviors, and the former is dependent on ghrelin/GHS-R1a signaling while the latter may not always be. In addition, our findings suggested that GHS-R1a-dependent elevation in serum IL-6 might contribute to increased despair-like behavior in chronic CR state.

Rodent models of post-traumatic stress disorder: behavioral assessment

Although the etiology and expression of psychiatric disorders are complex, mammals show biologically preserved behavioral and neurobiological responses to valent stimuli which underlie the use of rodent models of post-traumatic stress disorder (PTSD). PTSD is a complex phenotype that is difficult to model in rodents because it is diagnosed by patient interview and influenced by both environmental and genetic factors. However, given that PTSD results from traumatic experiences, rodent models can simulate stress induction and disorder development. By manipulating stress type, intensity, duration, and frequency, preclinical models reflect core PTSD phenotypes, measured through various behavioral assays. Paradigms precipitate the disorder by applying physical, social, and psychological stressors individually or in combination. This review discusses the methods used to trigger and evaluate PTSD-like phenotypes. It highlights studies employing each stress model and evaluates their translational efficacies against DSM-5, validity criteria, and criteria proposed by Yehuda and Antelman's commentary in 1993. This is intended to aid in paradigm selection by informing readers about rodent models, their benefits to the clinical community, challenges associated with the translational models, and opportunities for future work. To inform PTSD model validity and relevance to human psychopathology, we propose that models incorporate behavioral test batteries, individual differences, sex differences, strain and stock differences, early life stress effects, biomarkers, stringent success criteria for drug development, Research Domain Criteria, technological advances, and cross-species comparisons. We conclude that, despite the challenges, animal studies will be pivotal to advances in understanding PTSD and the neurobiology of stress.

Depressive-like state sensitizes 5-HT1A and 5-HT1B auto-receptors in the dorsal raphe nucleus sub-system

Dorsal raphe (DR) and median raphe (MR) 5-HT neurons are two distinct sub-systems known to be regulated by 5-HT_1A and 5-HT_1B auto-receptors. Whether the auto-receptors in each sub-system are functionally altered in depressive-like state remains unknown. The present study is aimed to study a specific circuit (DR-ventral hippocampus and MR-dorsal hippocampus) within each sub-system to investigate changes in receptor sensitivity in the pathogenesis of depression. A mouse model of depression was developed through the social defeat paradigm, and was then treated with fluoxetine (FLX). 5-HT_1A auto-receptor in the neuronal cell body (DR or MR) and 5-HT_1B auto-receptor in the axonal terminal (ventral or dorsal hippocampus) were directly targeted by local perfusion of antagonists (5-HT_1A: WAY100635; 5-HT_1B: GR127935) through reverse microdialysis. Time courses of dialysate 5-HT measured at the axonal terminal were subsequently determined for each circuit. At baseline, 5-HT_1A and 5-HT_1B antagonists dose-dependently increased dialysate 5-HT, with sub-circuit specificity. In the depressive-like state, greater increases in dialysate 5-HT were observed only in the DR-ventral hippocampus circuit following local delivery of both antagonists, which were then fully restored following the FLX treatment. In contrast, no changes were observed in the MR-dorsal hippocampus circuit. Our results demonstrate differential changes in sensitivities of 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus and MR-dorsal hippocampus circuits. 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus circuit are sensitized in the depressive-like state. Taken together, these results suggest that the DR sub-system maybe the neural substrate mediating depressive phenotypes.

Rapid and Prolonged Antidepressant-like Effect of Crocin Is Associated with GHSR-Mediated Hippocampal Plasticity-related Proteins in Mice Exposed to Prenatal Stress

Prenatal stress (PNS) has a prolonged and adverse effect on offspring, leading to a significantly increased vulnerability to developing depression in their later life. Traditional therapies have delayed onset and limited efficacy; thus, it remains an urgent need to find novel medications with fast-onset and high-efficacy potentials. Crocin, with its structure clearly examined, has shown antidepressant-like effects. However, few studies extensively investigated its effect especially in mice exposed to PNS. Using an established PNS model, we tested whether crocin could have a rapid and persistent antidepressant-like effect in PNS mice. Growth hormone secretagogue receptor (GHSR) and phosphoinositide 3-kinase (PI3K) inhibitors were used to test their effects in antidepressant-like effect of crocin. Hippocampal GHSR-PI3K signaling was examined both in PNS mice treated with a single dose of crocin and in combination of GHSR inhibitor. PNS mice showed depression-like behaviors at juvenile and adulthood, and crocin induced an instant and persistent antidepressant-like response in PNS mice in a dose-dependent manner. Moreover, crocin increased the expression of hippocampal synaptic plasticity-associated proteins through the restoration of GHSR-PI3K signaling. Inhibitions of both GHSR and PI3K abolished the effect of crocin in alleviating depressive-like behaviors. More importantly, GHSR inhibitor JMV2959 blocked the enhanced expression of hippocampal plasticity-related proteins induced by crocin. The present study demonstrated that crocin induced a fast-onset and prolonged antidepressant effect in PNS mice and suggested that GHSR-PI3K signaling may play a key role in crocin's effect at least partially by a restoration of hippocampal synaptic plasticity-associated proteins.

Effects of disrupted ghrelin receptor function on fear processing, anxiety and saccharin preference in mice

BACKGROUND

Obesity is a risk factor for stress-related mental disorders such as post-traumatic stress disorder. The underlying mechanism through which obesity affects mental health remains poorly understood but dysregulation of the ghrelin system may be involved. Stress increases plasma ghrelin levels, which stimulates food intake as a potential stress-coping mechanism. However, diet-induced obesity induces ghrelin resistance which in turn may have deleterious effects on stress-coping. In our study, we explored whether disruption of ghrelin receptor function though high-fat diet or genetic ablation affects fear processing, anxiety-like behavior and saccharin preference in mice.

METHODS

Adult male C57BL6/J mice were placed on a standard diet or high-fat diet for a total period of 8 weeks. We first established that high-fat diet exposure for 4 weeks elicits ghrelin resistance, evidenced by a blunted hyperphagic response following administration of a ghrelin receptor agonist. We then carried out an experiment in which we subjected mice to auditory fear conditioning after 4 weeks of diet exposure and evaluated effects on fear extinction, anxiety-like behavior and saccharin preference. To explore whether fear conditioning as such may influence the effect of diet exposure, we also subjected mice to auditory fear conditioning prior to diet onset and 4 weeks later we investigated auditory fear extinction, anxiety-like behavior and saccharin preference. In a final experiment, we further assessed lack of ghrelin receptor function by investigating auditory fear processing, anxiety-like behavior and saccharin preference in ghrelin receptor knockout mice and their wild-type littermates.

RESULTS

High-fat diet exposure had no significant effect on auditory fear conditioning and its subsequent extinction or on anxiety-like behavior but significantly lowered saccharin preference. Similarly, ghrelin receptor knockout mice did not differ significantly from their wild-type littermates for auditory fear processing or anxiety-like behavior but showed significantly lower saccharin preference compared to wild-type littermates.

CONCLUSION

Taken together, our data suggest that disruption of ghrelin receptor function per se does not affect fear or anxiety-like behavior but may decrease saccharin preference in mice.