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

Microglial depletion rescues spatial memory impairment caused by LPS administration in adult mice

Recent studies have highlighted the importance of microglia, the resident macrophages in the brain, in regulating cognitive functions such as learning and memory in both healthy and diseased states. However, there are conflicting results and the underlying mechanisms are not fully understood. In this study, we examined the effect of depleting adult microglia on spatial learning and memory under both physiological conditions and lipopolysaccharide (LPS)-induced neuroinflammation. Our results revealed that microglial depletion by PLX5622 caused mild spatial memory impairment in mice under physiological conditions; however, it prevented memory deficits induced by systemic LPS insult. Inactivating microglia through minocycline administration replicated the protective effect of microglial depletion on LPS-induced memory impairment. Furthermore, our study showed that PLX5622 treatment suppressed LPS-induced neuroinflammation, microglial activation, and synaptic dysfunction. These results strengthen the evidence for the involvement of microglial immunoactivation in LPS-induced synaptic and cognitive malfunctions. They also suggest that targeting microglia may be a potential approach to treating neuroinflammation-associated cognitive dysfunction seen in neurodegenerative diseases.

Increased GHS-R1a expression in the hippocampus impairs memory encoding and contributes to AD-associated memory deficits

Growth hormone secretagogue receptor 1a (GHS-R1a), also known as the ghrelin receptor, is an important nutrient sensor and metabolic regulator in both humans and rodents. Increased GHS-R1a expression is observed in the hippocampus of both Alzheimer's disease (AD) patients and AD model mice. However, the causal relationship between GHS-R1a elevation in the hippocampus and AD memory deficits remains uncertain. Here, we find that increasing GHS-R1a expression in dCA1 pyramidal neurons impairs hippocampus-dependent memory formation, which is abolished by local administration of the endogenous antagonist LEAP2. GHS-R1a elevation in dCA1 pyramidal neurons suppresses excitability and blocks memory allocation in these neurons. Chemogenetic activation of those high GHS-R1a neurons during training rescues GHS-R1a overexpression-induced memory impairment. Moreover, we demonstrate that increasing GHS-R1a expression in dCA1 pyramidal neurons hampers these neurons' ability to encode spatial memory and reduces engram size in the dCA1 region. Finally, we show that GHS-R1a deletion mitigates spatial memory deficits in APP/PS1 mice with increased GHS-R1a expression in the hippocampus. Our findings reveal a negative, causal relationship between hippocampal GHS-R1a expression and memory encoding, and suggest that blocking the abnormal increase in GHS-R1a activity/expression may be a promising approach to improve memory and treat cognitive decline in AD.

Overexpression of Motopsin, an Extracellular Serine Protease Related to Intellectual Disability, Promotes Adult Neurogenesis and Neuronal Responsiveness in the Dentate Gyrus

Motopsin, a serine protease encoded by PRSS12, is secreted by neuronal cells into the synaptic clefts in an activity-dependent manner, where it induces synaptogenesis by modulating Na+/K+-ATPase activity. In humans, motopsin deficiency leads to severe intellectual disability and, in mice, it disturbs spatial memory and social behavior. In this study, we investigated mice that overexpressed motopsin in the forebrain using the Tet-Off system (DTG-OE mice). The elevated agrin cleavage or the reduced Na+/K+-ATPase activity was not detected. However, motopsin overexpression led to a reduction in spine density in hippocampal CA1 basal dendrites. While motopsin overexpression decreased the ratio of mature mushroom spines in the DG, it increased the ratio of immature thin spines in CA1 apical dendrites. Female DTG-OE mice showed elevated locomotor activity in their home cages. DTG-OE mice showed aberrant behaviors, such as delayed latency to the target hole in the Barnes maze test and prolonged duration of sniffing objects in the novel object recognition test (NOR), although they retained memory comparable to that of TRE-motopsin littermates, which normally express motopsin. After NOR, c-Fos-positive cells increased in the dentate gyrus (DG) of DTG-OE mice compared with that of DTG-SO littermates, in which motopsin overexpression was suppressed by the administration of doxycycline, and TRE-motopsin littermates. Notably, the numbers of doublecortin- and 5-bromo-2'-deoxyuridine-labeled cells significantly increased in the DG of DTG-OE mice, suggesting increased adult neurogenesis. Importantly, our results revealed a new function in addition to modulating neuronal responsiveness and spine morphology in the DG: the regulation of neurogenesis.

Ghrelin for Neuroprotection in Post-Cardiac Arrest Coma: A Randomized Clinical Trial

Importance

Out-of-hospital cardiac arrest survival rates have markedly risen in the last decades, but neurological outcome only improved marginally. Despite research on more than 20 neuroprotective strategies involving patients in comas after cardiac arrest, none have demonstrated unequivocal evidence of efficacy; however, treatment with acyl-ghrelin has shown improved functional and histological brain recovery in experimental models of cardiac arrest and was safe in a wide variety of human study populations.

Objective

To determine safety and potential efficacy of intravenous acyl-ghrelin to improve neurological outcome in patients in a coma after cardiac arrest.

Design, Setting, and Participants

A phase 2, double-blind, placebo-controlled, multicenter, randomized clinical trial, Ghrelin Treatment of Comatose Patients After Cardiac Arrest: A Clinical Trial to Promote Cerebral Recovery (GRECO), was conducted between January 18, 2019, and October 17, 2022. Adult patients 18 years or older who were in a comatose state after cardiac arrest were assessed for eligibility; patients were from 3 intensive care units in the Netherlands. Expected death within 48 hours or unfeasibility of treatment initiation within 12 hours were exclusion criteria.

Interventions

Patients were randomized to receive intravenous acyl-ghrelin, 600 μg (intervention group), or placebo (control group) within 12 hours after cardiac arrest, continued for 7 days, twice daily, in addition to standard care.

Main Outcomes and Measures

Primary outcome was the score on the Cerebral Performance Categories (CPC) scale at 6 months. Safety outcomes included any serious adverse events. Secondary outcomes were mortality and neuron-specific enolase (NSE) levels on days 1 and 3.

Results

A total of 783 adult patients in a coma after cardiac arrest were assessed for eligibility, and 160 patients (median [IQR] age, 68 [57-75] years; 120 male [75%]) were enrolled. A total of 81 patients (51%) were assigned to the intervention group, and 79 (49%) were assigned to the control group. The common odds ratio (OR) for any CPC improvement in the intervention group was 1.78 (95% CI, 0.98-3.22; P = .06). This was consistent over all CPC categories. Mean (SD) NSE levels on day 1 after cardiac arrest were significantly lower in the intervention group (34 [6] μg/L vs 56 [13] μg/L; P = .04) and on day 3 (28 [6] μg/L vs 52 [14] μg/L; P = .08). Serious adverse events were comparable in incidence and type between the groups. Mortality was 37% (30 of 81) in the intervention group vs 51% (40 of 79) in the control group (absolute risk reduction, 14%; 95% CI, -2% to 29%; P = .08).

Conclusions and Relevance

In patients in a coma after cardiac arrest, intravenous treatment with acyl-ghrelin was safe and potentially effective to improve neurological outcome. Phase 3 trials are needed for conclusive evidence.

Trial Registration

Clinicaltrialsregister.eu: EUCTR2018-000005-23-NL.

The 5:2 diet does not increase adult hippocampal neurogenesis or enhance spatial memory in mice

New neurones are generated throughout life in the mammalian brain in a process known as adult hippocampal neurogenesis (AHN). Since this phenomenon grants a high degree of neuroplasticity influencing learning and memory, identifying factors that regulate AHN may be important for ameliorating age-related cognitive decline. Calorie restriction (CR) has been shown to enhance AHN and improve memory, mediated by the stomach hormone, ghrelin. Intermittent fasting (IF), a dietary strategy offering more flexibility than conventional CR, has also been shown to promote aspects of AHN. The 5:2 diet is a popular form of IF; however, its effects on AHN are not well characterised. To address this, we quantified AHN in adolescent and adult wild-type and ghrelin-receptor-deficient mice following 6 weeks on a 5:2 diet. We report an age-related decline in neurogenic processes. However, the 5:2 diet does not increase AHN nor enhance memory performance, suggesting that this specific form of IF is ineffective in promoting brain plasticity to support learning.

Ghrelin/GHS-R1A antagonism in memory test and its effects on central molecular signaling involved in addiction in rats

Central ghrelin signaling seems to play important role in addiction as well as memory processing. Antagonism of the growth hormone secretagogue receptor (GHS-R1A) has been recently proposed as a promising tool for the unsatisfactory drug addiction therapy. However, molecular aspects of GHS-R1A involvement in specific brain regions remain unclear. The present study demonstrated for the first time that acute as well as subchronic (4 days) administration of the experimental GHS-R1A antagonist JMV2959 in usual intraperitoneal doses including 3 mg/kg, had no influence on memory functions tested in the Morris Water Maze in rats as well as no significant effects on the molecular markers linked with memory processing in selected brain areas in rats, specifically on the β-actin, c-Fos, two forms of the calcium/calmodulin-dependent protein kinase II (CaMKII, p-CaMKII) and the cAMP-response element binding protein (CREB, p-CREB), within the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), dorsal striatum, and hippocampus (HIPP). Furthermore, following the methamphetamine intravenous self-administration in rats, the 3 mg/kg JMV2959 pretreatment significantly reduced or prevented the methamphetamine-induced significant decrease of hippocampal β-actin and c-Fos as well as it prevented the significant decrease of CREB in the NAC and mPFC. These results imply, that the GHS-R1A antagonist/JMV2959 might reduce/prevent some of the memory-linked molecular changes elicited by methamphetamine addiction within brain structures associated with memory (HIPP), reward (NAc), and motivation (mPFC), which may contribute to the previously observed significant JMV2959-induced reduction of the methamphetamine self-administration and drug-seeking behavior in the same animals. Further research is necessary to corroborate these results.

Ghrelin activates the mesolimbic dopamine system via nitric oxide associated mechanisms in the ventral tegmental area

Besides enhanced feeding, the orexigenic peptide ghrelin activates the mesolimbic dopamine system to cause reward as measured by locomotor stimulation, dopamine release in nucleus accumbens shell (NAcS), and conditioned place preference. Although the ventral tegmental area (VTA) appears to be a central brain region for this ghrelin-reward, the underlying mechanisms within this area are unknown. The findings that the gaseous neurotransmitter nitric oxide (NO) modulate the ghrelin enhanced feeding, led us to hypothesize that ghrelin increases NO levels in the VTA, and thereby stimulates reward-related behaviors. We initially demonstrated that inhibition of NO synthesis blocked the ghrelin-induced activation of the mesolimbic dopamine system. We then established that antagonism of downstream signaling of NO in the VTA, namely sGC, prevents the ability of ghrelin to stimulate the mesolimbic dopamine system. The association of ghrelin to NO was further strengthened by in vivo electrochemical recordings showing that ghrelin enhances the NO release in the VTA. Besides a GABA_B -receptor agonist, known to reduce NO and cGMP, blocks the stimulatory properties of ghrelin. The present series of experiments reveal that ablated NO signaling, through pharmacologically inhibiting the production of NO and/or cGMP, prevents the ability of ghrelin to induced reward-related behaviors.

Effects of ghrelin on sulfite induced changes in lipid peroxidation, spatial memory, and locomotor activity in rats

BACKGROUND

Humans are constantly exposed to sulfites and their derivatives, both endogenously and exogenously. Recent studies have shown that sulfite and its derivatives can cause oxidative stress. . Ghrelin has been reported to possess antioxidant properties and stimulates neurogenesis in hippocampal progenitor cells. This study aimed to investigate the effects of ghrelin on sulfite-induced changes in hippocampal oxidative status, spatial learning and locomotor activity in rats.

METHODS

Forty male albino Wistar rats were randomized into four groups as follows; Group 1: Control (C); Group 2: Sodium metabisulfite (Na2S2O5) treated (S); Group 3: Ghrelin treated (G); Group 4: Na2S2O5 + Ghrelin treated (SG). Sodium metabisulfite (100 mg/kg/day) was given by gastric gavage, and ghrelin (20 µg/kg/day) was administered intraperitoneally for 5 weeks. Thiobarbituric acid reactive substances (TBARS) were measured through fluorometric method. The spatial memory and locomotor activity of the rats were evaluated by Y-maze test.

RESULTS

Y-maze results revealed an enhancement of short-term spatial learning and memory in S and SG groups compared to C group. TBARS levels were increased significantly in S group with respect to C group. The increase in TBARS levels induced by sulfite was completely prevented by ghrelin in SG group.

CONCLUSION

We suggest that systemic ghrelin administration might ameliorate ingested sodium metabisulfite-induced hippocampal oxidative damage without providing any changes in spatial learning, memory and locomotion. Further investigation concerning the mechanism of ghrelin action in hippocampus might provide valuable information for developing new therapeutic approaches to attenuate oxidative stress in hippocampal tissue.

Virus-mediated GHS-R1a expression in the basolateral amygdala blocks extinction of conditioned taste aversion memory in rats

Ghrelin is an orexigenic gastric hormone that promotes feeding behaviors and regulating energy homeostasis in both humans and rodents. Our previous studies have shown that ghrelin, when locally infused into the basolateral amygdala (BLA), blocks both acquisition and extinction of conditioned taste aversion (CTA) memory in rats. In this study, we further investigated the effect of virus-mediated overexpression of ghrelin receptor growth hormone secretagogue receptor 1a (GHS-R1a) in BLA pyramidal neurons on CTA memory processes. We found that upregulation of GHS-R1a expression in BLA pyramidal neurons repressed CTA extinction while it had no effect on CTA acquisition. In addition, we reported that local infusion of the endogenous GHS-R1a antagonist, liver-expressed antimicrobial peptide 2 (LEAP2), in the BLA abolished the inhibitory effect of increased GHS-R1a on CTA memory extinction. Those findings provide new supportive evidence that ghrelin/GHS-R1a signaling in the BLA circuit shapes emotional memory processes.

Ghrelin Acylation-A Post-Translational Tuning Mechanism Regulating Adult Hippocampal Neurogenesis

Adult hippocampal neurogenesis—the generation of new functional neurones in the adult brain—is impaired in aging and many neurodegenerative disorders. We recently showed that the acylated version of the gut hormone ghrelin (acyl-ghrelin) stimulates adult hippocampal neurogenesis while the unacylated form of ghrelin inhibits it, thus demonstrating a previously unknown function of unacyl-ghrelin in modulating hippocampal plasticity. Analysis of plasma samples from Parkinson’s disease patients with dementia demonstrated a reduced acyl-ghrelin:unacyl-ghrelin ratio compared to both healthy controls and cognitively intact Parkinson’s disease patients. These data, from mouse and human studies, suggest that restoring acyl-ghrelin signalling may promote the activation of pathways to support memory function. In this short review, we discuss the evidence for ghrelin’s role in regulating adult hippocampal neurogenesis and the enzymes involved in ghrelin acylation and de-acylation as targets to treat mood-related disorders and dementia.

Ghrelin mediated hippocampal neurogenesis

The stomach hormone, ghrelin, which is released during food restriction, provides a link between circulating energy state and adaptive brain function. The maintenance of such homeostatic systems is essential for an organism to survive and thrive, and accumulating evidence points to ghrelin being a key regulator of adult hippocampal neurogenesis and memory function. Aberrant neurogenesis is linked to cognitive decline in aging and neurodegeneration. Therefore, identifying endogenous metabolic factors that regulate new adult-born neuron formation is an important objective in understanding the link between nutritional status and CNS function. Here, we review current developments in our understanding of ghrelin's role in regulating neurogenesis and memory function.

Ghrelin mediated regulation of neurosynaptic transmitters in depressive disorders

Ghrelin is a peptide released by the endocrine cells of the stomach and the neurons in the arcuate nucleus of the hypothalamus. It modulates both peripheral and central functions. Although ghrelin has emerged as a potent stimulator of growth hormone release and as an orexigenic neuropeptide, the wealth of literature suggests its involvement in the pathophysiology of affective disorders including depression. Ghrelin exhibits a dual role through the advancement and reduction of depressive behavior with nervousness in the experimental animals. It modulates depression-related signals by forming neuronal networks with various neuropeptides and classical neurotransmitter systems. The present review emphasizes the integration and signaling of ghrelin with other neuromodulatory systems concerning depressive disorders. The role of ghrelin in the regulation of neurosynaptic transmission and depressive illnesses implies that the ghrelin system modulation can yield promising antidepressive therapies.

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Ghrelin is the orexigenic type of neuropeptide.

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It binds with the growth hormone secretagogue receptor (GHSR).

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GHSR is ubiquitously present in the various brain regions.

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Ghrelin is involved in the regulation of depression-related behavior.

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The review focuses on the neurotransmission and signaling of ghrelin in neuropsychiatric and depressive disorders.

Targeting the Ghrelin Receptor as a Novel Therapeutic Option for Epilepsy

Epilepsy is a neurological disease affecting more than 50 million individuals worldwide. Notwithstanding the availability of a broad array of antiseizure drugs (ASDs), 30% of patients suffer from pharmacoresistant epilepsy. This highlights the urgent need for novel therapeutic options, preferably with an emphasis on new targets, since “me too” drugs have been shown to be of no avail. One of the appealing novel targets for ASDs is the ghrelin receptor (ghrelin-R). In epilepsy patients, alterations in the plasma levels of its endogenous ligand, ghrelin, have been described, and various ghrelin-R ligands are anticonvulsant in preclinical seizure and epilepsy models. Up until now, the exact mechanism-of-action of ghrelin-R-mediated anticonvulsant effects has remained poorly understood and is further complicated by multiple downstream signaling pathways and the heteromerization properties of the receptor. This review compiles current knowledge, and discusses the potential mechanisms-of-action of the anticonvulsant effects mediated by the ghrelin-R.

The effect of ghrelin injection in the CA1 region of hippocampus on the MK801- induced memory impairment in wistar rats

N-Methyl-D-Aspartate (NMDA) receptors are critically involved in the learning and memory formation and dizocilpine (MK-801) is an antagonist of NMDA receptor. Ghrelin plays a crucial role in learning and memory processes. The present study was conducted to the evaluation of ghrelin effect on passive avoidance memory impairment induced by MK801. In this experimental study, 24 male wistar rats were randomly distributed into 3 groups of 8 each. Passive avoidance tests of animals were evaluated using Shuttle Box apparatus. One week after the surgery, ghrelin (3 nmol) was injected intra-hippocampally, 5 min before the MK-801administration. MK-801 (0.15 mg/kg) was injected intraperitoneally (i.p.), 10 min before the test session. Pre-test injection of MK-801 significantly decreased STL (step through latency) at 24 h and 48 h (P < 0.001) and 10 days (P < 0.01) and increased TDC (time spent in dark compartment) at 24 h, 48 h and 10 days (P < 0.001) after training in comparison with control group. Pre-test injection of ghrelin + MK-801 significantly increased STL at 24 h (P < 0.01), 48 h and 10 days (P < 0.001) and decreased TDC at 24 h, 48 h and 10 days (P < 0.001) after training in comparison with MK-801 received group. It is concluded that pre-test injection of MK-801 impaired passive avoidance memory. Administration of ghrelin before MK-801 ameliorated memory impairment induced by MK-801. It is assumed that this compensative effect of ghrelin was mediated by NMDA receptor.

Acylated Ghrelin as a Multi-Targeted Therapy for Alzheimer's and Parkinson's Disease

Much thought has been given to the impact of Amyloid Beta, Tau and Alpha-Synuclein in the development of Alzheimer's disease (AD) and Parkinson's disease (PD), yet the clinical failures of the recent decades indicate that there are further pathological mechanisms at work. Indeed, besides amyloids, AD and PD are characterized by the culminative interplay of oxidative stress, mitochondrial dysfunction and hyperfission, defective autophagy and mitophagy, systemic inflammation, BBB and vascular damage, demyelination, cerebral insulin resistance, the loss of dopamine production in PD, impaired neurogenesis and, of course, widespread axonal, synaptic and neuronal degeneration that leads to cognitive and motor impediments. Interestingly, the acylated form of the hormone ghrelin has shown the potential to ameliorate the latter pathologic changes, although some studies indicate a few complications that need to be considered in the long-term administration of the hormone. As such, this review will illustrate the wide-ranging neuroprotective properties of acylated ghrelin and critically evaluate the hormone's therapeutic benefits for the treatment of AD and PD.

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.

(Neuro) Peptides, Physical Activity, and Cognition

Regular physical activity (PA) improves cognitive functions, prevents brain atrophy, and delays the onset of cognitive decline, dementia, and Alzheimer’s disease. Presently, there are no specific recommendations for PA producing positive effects on brain health and little is known on its mediators. PA affects production and release of several peptides secreted from peripheral and central tissues, targeting receptors located in the central nervous system (CNS). This review will provide a summary of the current knowledge on the association between PA and cognition with a focus on the role of (neuro)peptides. For the review we define peptides as molecules with less than 100 amino acids and exclude myokines. Tachykinins, somatostatin, and opioid peptides were excluded from this review since they were not affected by PA. There is evidence suggesting that PA increases peripheral insulin growth factor 1 (IGF-1) levels and elevated serum IGF-1 levels are associated with improved cognitive performance. It is therefore likely that IGF-1 plays a role in PA induced improvement of cognition. Other neuropeptides such as neuropeptide Y (NPY), ghrelin, galanin, and vasoactive intestinal peptide (VIP) could mediate the beneficial effects of PA on cognition, but the current literature regarding these (neuro)peptides is limited.

Ginsenoside Rg1 fails to rescue PTSD-like behaviors in a mice model of single-prolonged stress

Previous studies reported that ginsenoside Rg1 (Rg1) exerts antidepressant-like effect in animal models of depression. However, its effect on post-traumatic stress disorder (PTSD) remains elusive; PTSD is a common and costly psychiatric condition with negative cognitive and affective dysfunctions, such as anxiety and depression. In this study, we evaluated the role of Rg1 in a validated mice model of PTSD induced by single-prolonged stress (SPS). Sertraline, one of the FDA-approved medications for PTSD was used as a positive control. Our results showed that SPS exposure led to increased anxiety-like and despair-like behaviors. SPS exposure also caused enhanced contextual fear memory and overgeneralization of learned fear. Sertraline significantly ameliorated those abnormal behaviors induced by SPS, while Rg1 did not. Meanwhile, we found that sertraline but not Rg1 blocked the suppressive effect of SPS on adult neurogenesis in the hippocampus. Consistently, we found that SPS elevated adrenocorticotropic hormone (ACTH) level in the serum, which was inhibited by sertraline but not Rg1. Our results thus demonstrate that Rg1 at a dose used to treat depression may not be effective to rescue behavioral deficits associated with PTSD.

Hippocampal growth hormone modulates relational memory and the dendritic spine density in CA1

Growth hormone (GH) deficiency is associated with cognitive decline which occur both in normal aging and in endocrine disorders. Several brain areas express receptors for GH although their functional role is unclear. To determine how GH affects the capacity for learning and memory by specific actions in one of the key areas, the hippocampus, we injected recombinant adeno-associated viruses (rAAVs) in male rats to express green fluorescent protein (GFP) combined with either GH, antagonizing GH (aGH), or no hormone, in the dorsal CA1. We found that aGH disrupted memory in the Morris water maze task, and that aGH treated animals needed more training to relearn a novel goal location. In a one-trial spontaneous location recognition test, the GH treated rats had better memory performance for object locations than the two other groups. Histological examinations revealed that GH increased the dendritic spine density on apical dendrites of CA1, while aGH reduced the spine density. GH increased the relative amount of immature spines, while aGH decreased the same amount. Our results imply that GH is a neuromodulator with strong influence over hippocampal plasticity and relational memory by mechanisms involving modulation of dendritic spines. The findings are significant to the increasing aging population and GH deficiency patients.

Ghrelin in Alzheimer's disease: Pathologic roles and therapeutic implications

Ghrelin, which has many important physiological roles, such as stimulating food intake, regulating energy homeostasis, and releasing insulin, has recently been studied for its roles in a diverse range of neurological disorders. Despite the several functions of ghrelin in the central nervous system, whether it works as a therapeutic agent for neurological dysfunction has been unclear. Altered levels and various roles of ghrelin have been reported in Alzheimer's disease (AD), which is characterized by the accumulation of misfolded proteins resulting in synaptic loss and cognitive decline. Interestingly, treatment with ghrelin or with the agonist of ghrelin receptor showed attenuation in several cases of AD-related pathology. These findings suggest the potential therapeutic implications of ghrelin in the pathogenesis of AD. In the present review, we summarized the roles of ghrelin in AD pathogenesis, amyloid beta (Aβ) homeostasis, tau hyperphosphorylation, neuroinflammation, mitochondrial deficit, synaptic dysfunction and cognitive impairment. The findings from this review suggest that ghrelin has a novel therapeutic potential for AD treatment. Thus, rigorously designed studies are needed to establish an effective AD-modifying strategy.

Ghrelin-Mediated Hippocampal Neurogenesis: Implications for Health and Disease

There is a close relationship between cognition and nutritional status, however, the mechanisms underlying this relationship require elucidation. The stomach hormone, ghrelin, which is released during food restriction, provides a link between circulating energy state and adaptive brain function. The maintenance of such homeostatic systems is essential for an organism to thrive and survive, and accumulating evidence points to ghrelin being key in promoting adult hippocampal neurogenesis and memory. Aberrant neurogenesis is linked to cognitive decline in ageing and neurodegeneration. Therefore, identifying endogenous metabolic factors that regulate new adult-born neurone formation is an important objective in understanding the link between nutritional status and central nervous system (CNS) function. Here, we review current developments in our understanding of ghrelin's role in regulating neurogenesis and memory function.

G protein-coupled estrogen receptor is involved in the neuroprotective effect of IGF-1 against MPTP/MPP+-induced dopaminergic neuronal injury

Insulin-like growth factor-1 (IGF-1), an endogenous peptide, exerts important role in brain development, neurogenesis and neuroprotection. There are accumulating evidence for the interaction of IGF-1 and 17β-estradiol systems. IGF-1/IGF-1 receptor (IGF-1R) signaling has been reported to regulate G-protein estrogen receptor (GPER) expression in cancer cells. Whether GPER is involved in the neuroprotective effect of IGF-1 against MPTP/MPP⁺-induced dopaminergic neuronal injury remains unclear. We showed that IGF-1 could improve MPTP-induced motor deficits and ameliorate the decreased contents of DA and its metabolites in striatum as well as the loss of TH-IR neurons in the substantia nigra (SN). IGF-1 pretreatment also reversed the changes of Bcl-2 and Bax protein expressions in SN in MPTP mice. These effects were abolished by IGF-1 receptor (IGF-1R) antagonist JB-1 or GPER antagonist G15 except the inhibitory effect of G15 on Bax protein expression. Moreover, IGF-1 pretreatment enhanced cell survival against MPP⁺-induced neurotoxicity in SH-SY5Y cells. IGF-1 exerted anti-apoptotic effects by restoring MPP⁺-induced changes of Bcl-2 and Bax protein expressions as well as mitochondria membrane potential. Co-treatment with JB-1 or G15 could block these effects. Furthermore, IGF-1 regulated the protein expression of GPER through activation of phosphatidylinositol 3-kinase (PI3-K) and mitogen-activated protein kinase (MAPK) signaling pathways. Overall, we show for the first time that GPER may contribute to the neuroprotective effects of IGF-1 against MPTP/MPP⁺-induced dopaminergic neuronal injury.

Short-term improvements in cognitive function following vertical sleeve gastrectomy and Roux-en Y gastric bypass: a direct comparison study

BACKGROUND

Cognitive deficits are observed in individuals with obesity. While bariatric surgery can reverse these deficits, it remains unclear whether surgery type differentially influences cognitive outcome. We compared the extent to which vertical sleeve gastrectomy (VSG) and Roux-en Y gastric bypass (RYGB) ameliorated cognitive impairments associated with obesity.

METHODS

Female participants approved for VSG (N = 18) or RYGB (N = 18) were administered cognitive measures spanning the domains of attention [Hopkins Verbal Learning Test (HVLT) Trial 1 and Letter Number Sequencing], processing speed [Stroop Color Trial, Symbol Digit Modalities Test, and Trail Making Part A], memory [HVLT Retained and HVLT Discrimination Index], and executive functioning (Stroop Color Word Trials and Trail Making Part B-A) prior to surgery and at 2 weeks and 3 months following surgery. Scores for each cognitive domain were calculated and compared between surgical cohorts using repeated measures analyses of variance.

RESULTS

Significant weight loss was observed 2 weeks and 3 months following RYGB and VSG and was accompanied by improvements in processing speed and executive functioning. Patients who received RYGB also experienced improved attention as early as 2 weeks, which persisted at 3 months. This was not observed in individuals who underwent VSG. No changes in memory were observed from baseline measures in either group.

CONCLUSIONS

This is the first report of cognitive improvements following VSG and the first direct comparison of cognitive improvements following RYGB and VSG. Short-term improvements in specific domains of cognitive function are observed at the beginning of the active weight loss phase following bariatric surgery that persisted to 3 months. The anatomical distinction between the two surgeries and resulting differential metabolic profiles may be responsible for the improvements in attention observed following RYGB but not following VSG.

Calorie restriction activates new adult born olfactory-bulb neurones in a ghrelin-dependent manner but acyl-ghrelin does not enhance subventricular zone neurogenesis

The ageing and degenerating brain show deficits in neural stem/progenitor cell (NSPC) plasticity that are accompanied by impairments in olfactory discrimination. Emerging evidence suggests that the gut hormone ghrelin plays an important role in protecting neurones, promoting synaptic plasticity and increasing hippocampal neurogenesis in the adult brain. In the present study, we investigated the role of ghrelin with respect to modulating adult subventricular zone (SVZ) NSPCs that give rise to new olfactory bulb (OB) neurones. We characterised the expression of the ghrelin receptor, growth hormone secretagogue receptor (GHSR), using an immunohistochemical approach in GHSR-eGFP reporter mice to show that GHSR is expressed in several regions, including the OB but not in the SVZ of the lateral ventricle. These data suggest that acyl-ghrelin does not mediate a direct effect on NSPC in the SVZ. Consistent with these findings, treatment with acyl-ghrelin or genetic silencing of GHSR did not alter NSPC proliferation within the SVZ. Similarly, using a bromodeoxyuridine pulse-chase approach, we show that peripheral treatment of adult rats with acyl-ghrelin did not increase the number of new adult-born neurones in the granule cell layer of the OB. These data demonstrate that acyl-ghrelin does not increase adult OB neurogenesis. Finally, we investigated whether elevating ghrelin indirectly, via calorie restriction (CR), regulated the activity of new adult-born cells in the OB. Overnight CR induced c-Fos expression in new adult-born OB cells but not in developmentally born cells, whereas neuronal activity was absent following re-feeding. These effects were not present in ghrelin^-/- mice, suggesting that adult-born cells are uniquely sensitive to changes in ghrelin mediated by fasting and re-feeding. In summary, ghrelin does not promote neurogenesis in the SVZ and OB; however, new adult-born OB cells are activated by CR in a ghrelin-dependent manner.

Brain accessibility delineates the central effects of circulating ghrelin

Ghrelin is a hormone produced in the gastrointestinal tract that acts via the growth hormone secretagogue receptor. In the central nervous system, ghrelin signalling is able to recruit different neuronal targets that regulate the behavioural, neuroendocrine, metabolic and autonomic effects of the hormone. Notably, several studies using radioactive or fluorescent variants of ghrelin have found that the accessibility of circulating ghrelin into the mouse brain is both strikingly low and restricted to some specific brain areas. A variety of studies addressing central effects of systemically injected ghrelin in mice have also provided indirect evidence that the accessibility of plasma ghrelin into the brain is limited. Here, we review these previous observations and discuss the putative pathways that would allow plasma ghrelin to gain access into the brain together with their physiological implications. Additionally, we discuss some potential features regarding the accessibility of plasma ghrelin into the human brain based on the observations reported by studies that investigate the consequences of ghrelin administration to humans.

Regulation of Memory Function by Feeding-Relevant Biological Systems: Following the Breadcrumbs to the Hippocampus

The hippocampus (HPC) controls fundamental learning and memory processes, including memory for visuospatial navigation (spatial memory) and flexible memory for facts and autobiographical events (declarative memory). Emerging evidence reveals that hippocampal-dependent memory function is regulated by various peripheral biological systems that are traditionally known for their roles in appetite and body weight regulation. Here, we argue that these effects are consistent with a framework that it is evolutionarily advantageous to encode and recall critical features surrounding feeding behavior, including the spatial location of a food source, social factors, post-absorptive processing, and other episodic elements of a meal. We review evidence that gut-to-brain communication from the vagus nerve and from feeding-relevant endocrine systems, including ghrelin, insulin, leptin, and glucagon-like peptide-1 (GLP-1), promote hippocampal-dependent spatial and declarative memory via neurotrophic and neurogenic mechanisms. The collective literature reviewed herein supports a model in which various stages of feeding behavior and hippocampal-dependent memory function are closely linked.

Growth hormone secretagogue receptor constitutive activity impairs voltage-gated calcium channel-dependent inhibitory neurotransmission in hippocampal neurons

KEY POINTS

Presynaptic Ca_V 2 voltage-gated calcium channels link action potentials arriving at the presynaptic terminal to neurotransmitter release. Hence, their regulation is essential to fine tune brain circuitry. Ca_V 2 channels are highly sensitive to G protein-coupled receptor (GPCR) modulation. Our previous data indicated that growth hormone secretagogue receptor (GHSR) constitutive activity impairs Ca_V 2 channels by decreasing their surface density. We present compelling support for the impact of Ca_V 2.2 channel inhibition by agonist-independent GHSR activity exclusively on GABA release in hippocampal cultures. We found that this selectivity arises from a high reliance of GABA release on Ca_V 2.2 rather than on Ca_V 2.1 channels. Our data provide new information on the effects of the ghrelin-GHSR system on synaptic transmission, suggesting a putative physiological role of the constitutive signalling of a GPCR that is expressed at high levels in brain areas with restricted access to its natural agonist.

ABSTRACT

Growth hormone secretagogue receptor (GHSR) displays high constitutive activity, independent of its endogenous ligand, ghrelin. Unlike ghrelin-induced GHSR activity, the physiological role of GHSR constitutive activity and the mechanisms that underlie GHSR neuronal modulation remain elusive. We previously demonstrated that GHSR constitutive activity modulates presynaptic Ca_V 2 voltage-gated calcium channels. Here we postulate that GHSR constitutive activity-mediated modulation of Ca_V 2 channels could be relevant in the hippocampus since this brain area has high GHSR expression but restricted access to ghrelin. We performed whole-cell patch-clamp in hippocampal primary cultures from E16- to E18-day-old C57BL6 wild-type and GHSR-deficient mice after manipulating GHSR expression with lentiviral transduction. We found that GHSR constitutive activity impairs Ca_V 2.1 and Ca_V 2.2 native calcium currents and that Ca_V 2.2 basal impairment leads to a decrease in GABA but not glutamate release. We postulated that this selective effect is related to a higher Ca_V 2.2 over Ca_V 2.1 contribution to GABA release (∼40% for Ca_V 2.2 in wild-type vs. ∼20% in wild-type GHSR-overexpressing cultures). This effect of GHSR constitutive activity is conserved in hippocampal brain slices, where GHSR constitutive activity reduces local GABAergic transmission of the granule cell layer (intra-granule cell inhibitory postsynaptic current (IPSC) size ∼-67 pA in wild-type vs. ∼-100 pA in GHSR-deficient mice), whereas the glutamatergic output from the dentate gyrus to CA3 remains unchanged. In summary, we found that GHSR constitutive activity impairs IPSCs both in hippocampal primary cultures and in brain slices through a Ca_V 2-dependent mechanism without affecting glutamatergic transmission.

The Neurocognitive Effects of Ghrelin-induced Signaling on the Hippocampus: A Promising Approach to Alzheimer's Disease

The communication between the gastrointestinal tract and the central nervous system (CNS) allows for certain peptide hormones to influence neurocognitive function. Ghrelin, also known as the 'hunger hormone,' has the unique ability to enter the CNS and interact with the growth hormone secretagogue receptor (GHS-R) within the hippocampus. Upon interaction with ghrelin, a conformational change in the receptor causes an increase in transcription factors to foster a wide array of physiologic changes in response to caloric deprivation. With the GHS-R in a relatively high concentration within the hippocampus, ghrelin can promote memory, spatial, learning, and behavioral effects. In fact, ghrelin appears to also have a neuroprotective and neuromodulatory response once active within the hippocampal dentate gyrus. Through the GHS-R, higher levels of ghrelin may alter cognitive circuitry and offer a possible link to the treatment of some pathologies implicated in neurological dysfunction. Alzheimer's disease (AD) is already becoming a significant target for ghrelin neuroreceptor therapy. In such experimental models, ghrelin has been shown to combat this degenerative process by eliciting an ameliorative and regenerative response. Although trials and research are still ongoing, further studies are indicated as early research into this adjuvant therapy is promising. The research team explored the effects of ghrelin by reviewing the downstream signaling modifications of ghrelin's interaction with a specific CNS receptor, the GHS-R. Although the GHS-R is found in multiple locations within the CNS, the team investigated the role of the GHS-R within the hippocampus to focus solely on the neurocognitive implications of ghrelin. The team noted which signaling pathways in particular that ghrelin initiated and used this approach to determine whether ghrelin may have any therapeutic benefits. The team explored the possible therapeutic indications of ghrelin by looking at studies conducted with a specific neurodegenerative disease known to target the hippocampus.

Adolescent Isolation Interacts With DISC1 Point Mutation to Impair Adult Social Memory and Synaptic Functions in the Hippocampus

Disrupted-in-schizophrenia 1 (DISC1) is a strong candidate susceptibility gene for a spectrum of neuropsychiatric diseases including schizophrenia, bipolar disorder and major depression, all of which are thought to result from interactions between gene mutations and environmental risk factors such as influenza, trauma and stress. Adolescence is a key period susceptible to stress and stress-related mental illnesses. In a previous study, we found that although DISC1 L100P point mutation mice shows object recognition deficits, their sociability and social memory are relatively normal. Therefore, in this article, we investigated whether the interaction between adolescent stress and DISC1 L100P point mutation affects adult social memory, and we explored the underlying mechanisms. We found that adolescent stress (isolation from 5 weeks to 8 weeks of age) specifically impaired social memory of adult DISC1 L100P mice but not that of WT littermates, which could be rescued by administration of atypical antipsychotic drug clozapine. On the other hand, it did not induce anxiety or depression in adult mice. Adolescent isolation exacerbated adult neurogenesis deficits in the hippocampus of DISC1 L100P mice, while it had no effect on WT mice. In addition, we found that adolescent isolation led to long lasting changes in synaptic transmission and plasticity in the hippocampal circuits, some of which are specific for DISC1 L100P mice. In summary, we identified here the specific interaction between genetic mutation (DISC1 L100P) and adolescence social stress that damages synaptic function and social memory in adult hippocampal circuits.

Highlights–

Adolescent isolation (from 5 weeks to 8 weeks of age) impairs adult social memory when combined with DISC1 L100P point mutation.

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Adolescent isolation exacerbates adult neurogenesis deficit in the hippocampus of L100P mice but has no similar effect on WT mice.

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Adolescent isolation causes long lasting changes in synaptic transmission and plasticity of the hippocampal network in DISC1 L100P mice.

Food consumption increases cell proliferation in the python brain

Pythons are model organisms for investigating physiological responses to food intake. While systemic growth in response to food consumption is well documented, what occurs in the brain is currently unexplored. In this study, male ball pythons (Python regius) were used to test the hypothesis that food consumption stimulates cell proliferation in the brain. We used 5-bromo-12'-deoxyuridine (BrdU) as a cell-birth marker to quantify and compare cell proliferation in the brain of fasted snakes and those at 2 and 6 days after a meal. Throughout the telencephalon, cell proliferation was significantly increased in the 6 day group, with no difference between the 2 day group and controls. Systemic postprandial plasticity occurs quickly after a meal is ingested, during the period of active digestion; however, the brain displays a surge of cell proliferation after most digestion and absorption is complete.

Gastrointestinal hormones in regulation of memory

The connection between the gastrointestinal hormones and the brain has been established many years ago. This relation is termed the gut-brain axis (GBA). The GBA is a bidirectional communication which not only regulates gastrointestinal homeostasis but is also linked with higher emotional and cognitive functions. Hypothalamus plays a critical role in the regulation of energy metabolism, nutrient partitioning and control of feeding behaviors. Various gut hormones are released inside the gastrointestinal tract on food intake. These hormones act peripherally and influence the different responses of the tissues to the food intake, but do also have effects on the brain. The hypothalamus, in turn, integrates visceral function with limbic system structures such as hippocampus, amygdala, and cerebral cortex. The hippocampus has been known for its involvement in the cognitive function and the modulation of synaptic plasticity. This review aims to establish the role of various gut hormones in learning and memory, through the interaction of various receptors in the hippocampus. Understanding their role in memory can also aid in finding novel therapeutic strategies for the treatment of the neurological disorders associated with memory dysfunctions.

Local injection of d-lys-3-GHRP-6 in the rat amygdala, dentate gyrus or ventral tegmental area impairs memory consolidation

It is well known that the hormone ghrelin affects learning and memory in different experimental models of learning. Though, the effect of antagonism of ghrelin receptor type 1a (GHS-R1a) in various regions of the brain and on different stages of learning has not been examined. In this study the effect of injection of a GHS-R1a selective antagonist (d-Lys-3-GHRP-6) into the basolateral amygdala, dentate gyrus or ventral tegmental area was examined on memory consolidation in the passive avoidance task. Adult male Wistar rats weighing 230-280g were used. Animals underwent stereotaxic surgery and cannulated in their amygdala, dentate gyrus or ventral tegmental area. One week after surgery, the rats received different doses of d-Lys-3-GHRP-6 (0.08, 0.8, and 8nM), immediately after training. The control groups received solvent of the drug. Twenty four hours later in the test day, memory retrieval was assessed. In all groups, post-training injection of d-Lys-3-GHRP-6 decreased step-through latency and increased entries into the dark compartment and time spent in the dark compartment, significantly and in a dose-dependent manner. The results indicate that antagonism of the GHS-R1a in the rat amygdala, dentate gyrus or ventral tegmental area impairs memory consolidation and show that the ghrelin signaling has a widespread influence on cognitive performance.

Evaluation of mTOR signaling pathway proteins in rat gastric mucosa exposed to sulfite and ghrelin

BACKGROUND/AIMS

Mammalian target of rapamycin (mTOR) signaling serves as a central regulator of cell growth, proliferation, and survival. In this study, we planned to evaluate the expressions of mTOR signaling constituents (p-p70S6K, p-mTOR, and p-Tuberin) in rat gastric mucosa and to compare the results in sulfite- and sulfite+ghrelin-exposed groups.

MATERIALS AND METHODS

Rats were divided into three groups: the control group (C), the sodium metabisulfite (Na2S2O5) (S) group, and sulfite+ghrelin (SG) group. Sodium metabisulfite at 100 mg/kg/day was administered via gavage, and ghrelin at 20 μg/kg/day was administered intraperitoneally for 35 days. We have used immunohistochemistry for mTOR signaling pathway components.

RESULTS

There were no significant differences for p-p70S6K and p-mTOR expression among the C, S, and SG groups. Tuberin expression was significantly increased in the S group compared to the C group. Furthermore, tuberin expression was found to be significantly decreased in the SG group.

CONCLUSION

This study is the first one in the literature that shows the expression of mTOR signaling proteins in gastric mucosa of rats exposed to sulfite and ghrelin. Furthermore, it demonstrates that ghrelin treatment reduces p-Tuberin expression induced by ingested sulfite.

NO involvement in the inhibition of ghrelin on voltage-dependent potassium currents in rat hippocampal cells

Ghrelin is a peptide hormone that plays an important role in promoting appetite, regulating distribution and rate of use of energy, cognition, and mood disorders, but the relevant neural mechanisms of these function are still not clear. In this study, we examined the effect of ghrelin on voltage-dependent potassium (K⁺) currents in hippocampal cells of 1-3 days SD rats by whole-cell patch-clamp technique, and discussed whether NO was involved in this process. The results showed that ghrelin significantly inhibited the voltage-dependent K⁺ currents in hippocampal cells, and the inhibitory effect was more significant when l-arginine was co-administered. In contrast, N-nitro- l-arginine methyl ester increased the ghrelin inhibited K⁺ currents and attenuated the inhibitory effect of ghrelin. While d-arginine (D-AA) showed no significant impact on the ghrelin-induced decrease in K⁺ current. These results show that ghrelin may play a physiological role by inhibiting hippocampal voltage dependent K⁺ currents, and the NO pathway may be involved in this process.

Preclinical evidence of ghrelin as a therapeutic target in epilepsy

Ghrelin, an orexigenic peptide synthesized by endocrine cells of the gastric mucosa, plays a major role in inhibiting seizures. However, the underlying mechanism of ghrelin's anticonvulsant action is still unclear. Nowadays, there are considerable evidences showing that ghrelin is implicated in various neurophysiological processes, including learning and memory, neuroprotection, neurogenesis, and inflammatory effects. In this review, we will summarize the effects of ghrelin on epilepsy. It may provide a comprehensive picture of the role of ghrelin in epilepsy.

Neurogenic Effects of Ghrelin on the Hippocampus

Mammalian neurogenesis continues throughout adulthood in the subventricular zone of the lateral ventricle and in the subgranular zone of the dentate gyrus in the hippocampus. It is well known that hippocampal neurogenesis is essential in mediating hippocampus-dependent learning and memory. Ghrelin, a peptide hormone mainly synthesized in the stomach, has been shown to play a major role in the regulation of energy metabolism. A plethora of evidence indicates that ghrelin can also exert important effects on neurogenesis in the hippocampus of the adult brain. The aim of this review is to discuss the current role of ghrelin on the in vivo and in vitro regulation of neurogenesis in the adult hippocampus. We will also discuss the possible role of ghrelin in dietary restriction-induced hippocampal neurogenesis and the link between ghrelin-induced hippocampal neurogenesis and cognitive functions.

Disrupted-in-schizophrenia1 (DISC1) L100P mutation alters synaptic transmission and plasticity in the hippocampus and causes recognition memory deficits

Disrupted-in-schizophrenia 1(DISC1) is a promising candidate susceptibility gene for a spectrum of psychiatric illnesses that share cognitive impairments in common, including schizophrenia, bipolar disorder and major depression. Here we report that DISC1 L100P homozygous mutant shows normal anxiety- and depression-like behavior, but impaired object recognition which is prevented by administration of atypical antipsychotic drug clozapine. Ca²⁺ image analysis reveals suppression of glutamate-evoked elevation of cytoplasmic [Ca²⁺] in L100P hippocampal slices. L100P mutant slices exhibit decreased excitatory synaptic transmission (sEPSCs and mEPSCs) in dentate gyrus (DG) and impaired long-term potentiation in the CA1 region of the hippocampus. L100P mutation does not alter proteins expression of the excitatory synaptic markers, PSD95 and synapsin-1; neither does it changes dendrites morphology of primary cultured hippocampal neurons. Our findings suggest that the existence of abnormal synaptic transmission and plasticity in hippocampal network may disrupt declarative information processing and contribute to recognition deficits in DISC1 L100P mutant mice.

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.

Potential effects of current drug therapies on cognitive impairment in patients with type 2 diabetes

Type 2 diabetes mellitus is a complex metabolic disease that can cause serious damage to various organs. Among the best-known complications, an important role is played by cognitive impairment. Impairment of cognitive functioning has been reported both in type 1 and 2 diabetes mellitus. While this comorbidity has long been known, no major advances have been achieved in clinical research; it is clear that appropriate control of blood glucose levels represents the best current (although unsatisfactory) approach in the prevention of cognitive impairment. We have focused our attention on the possible effect on the brain of antidiabetic drugs, despite their effects on blood glucose levels, giving a brief rationale on the mechanisms (e.g. GLP-1, BDNF, ghrelin) that might be involved. Indeed, GLP-1 agonists are currently clinically studied in other neurodegenerative diseases (i.e. Parkinson's and Alzheimer's disease); furthermore, also other antidiabetic drugs have proven efficacy in preclinical studies. Overall, promising results are already available and finding new intervention strategies represents a current need in this field of research.

The Role of Hypothalamic Neuropeptides in Neurogenesis and Neuritogenesis

The hypothalamus is a source of neural progenitor cells which give rise to different populations of specialized and differentiated cells during brain development. Newly formed neurons in the hypothalamus can synthesize and release various neuropeptides. Although term neuropeptide recently undergoes redefinition, small-size hypothalamic neuropeptides remain major signaling molecules mediating short- and long-term effects on brain development. They represent important factors in neurite growth and formation of neural circuits. There is evidence suggesting that the newly generated hypothalamic neurons may be involved in regulation of metabolism, energy balance, body weight, and social behavior as well. Here we review recent data on the role of hypothalamic neuropeptides in adult neurogenesis and neuritogenesis with special emphasis on the development of food intake and social behavior related brain circuits.

Central acylated ghrelin improves memory function and hippocampal AMPK activation and partly reverses the impairment of energy and glucose metabolism in rats infused with β-amyloid

Ghrelin is a gastric hormone released during the fasting state that targets the hypothalamus where it induces hunger; however, emerging evidence suggests it may also affect memory function. We examined the effect of central acylated-ghrelin and DES-acetylated ghrelin (native ghrelin) on memory function and glucose metabolism in an experimentally induced Alzheimer's disease (AD) rat model. AD rats were divided into 3 groups and Non-AD rats were used as a normal-control group. Each rat in the AD groups had intracerebroventricular (ICV) infusion of β-amyloid (25-35; 16.8nmol/day) into the lateral ventricle for 3 days, and then the pumps were changed to infuse either acylated-ghrelin (0.2nmol/h; AD-G), DES-acylated ghrelin (0.2nmol/h; AD-DES-G), or saline (control; AD-C) for 3 weeks. The Non-AD group had ICV infusion of β-amyloid (35-25) which does not deposit in the hippocampus. During the next 3 weeks memory function, food intake, body weight gain, body fat composition, and glucose metabolism were measured. AD-C exhibited greater β-amyloid deposition compared to Non-AD-C, and AD-G suppressed the increased β-amyloid deposition and potentiated the phosphorylation AMPK. In addition, AD-G increased the phosphorylation GSK and decreased the phosphorylation of Tau in comparison to AD-C and AD-DES-G. Cognitive function, measured by passive avoidance and water maze tests, was much lower in AD-C than Non-AD-C whereas AD-G but not AD-DES-G prevented the decrease (p<0.021). Body weight gain was lower in AD-C group than Non-AD-C group without changing epididymal fat mass. AD-G reversed the decrease in body weight which was due to increased energy intake and decreased energy expenditure. The AD-G group exhibited a decrease in the second part of serum glucose levels during an oral glucose tolerance test (OGTT) compared to the AD-C and AD-DES-G group (p<0.009). However, area under the curve of insulin during the first part of OGTT was higher in AD-DES-G than other groups, whereas during the second part it was suppressed in AD-G as much as Non-AD. In conclusion, central acylated ghrelin in rats prevented the deterioration of memory function, and energy and glucose metabolisms were partially improved, possibly due to less β-amyloid accumulation. This research suggests that interventions such as intermittent fasting to facilitate sustained elevations of acyl-ghrelin should be investigated for cognitive and metabolic benefits, especially in person with early symptoms of memory impairment.

Ghrelin in psychiatric disorders - A review

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

Potential ghrelin-mediated benefits and risks of hydrogen water

Molecular hydrogen (H2) can scavenge hydroxyl radical and diminish the toxicity of peroxynitrite; hence, it has interesting potential for antioxidant protection. Recently, a number of studies have explored the utility of inhaled hydrogen gas, or of hydrogen-saturated water, administered parenterally or orally, in rodent models of pathology and in clinical trials, oftentimes with very positive outcomes. The efficacy of orally ingested hydrogen-rich water (HW) has been particularly surprising, given that only transient and rather small increments in plasma hydrogen can be achieved by this method. A recent study in mice has discovered that orally administered HW provokes increased gastric production of the orexic hormone ghrelin, and that this ghrelin mediates the favorable impact of HW on a mouse model of Parkinson's disease. The possibility that most of the benefits observed with HW in experimental studies are mediated by ghrelin merits consideration. Ghrelin is well known to function as an appetite stimulant and secretagogue for growth hormone, but it influences physiological function throughout the body via interaction with the widely express GHS-R1a receptor. Rodent and, to a more limited extent, clinical studies establish that ghrelin has versatile neuroprotective and cognitive enhancing activity, favorably impacts vascular health, exerts anti-inflammatory activity useful in autoimmune disorders, and is markedly hepatoprotective. The stimulatory impact of ghrelin on GH-IGF-I activity, while potentially beneficial in sarcopenia or cachectic disorders, does raise concerns regarding the long-term impact of ghrelin up-regulation on cancer risk. The impact of ingesting HW water on ghrelin production in humans needs to be evaluated; if HW does up-regulate ghrelin in humans, it may have versatile potential for prevention and control of a number of health disorders.

The orexigenic hormone acyl-ghrelin increases adult hippocampal neurogenesis and enhances pattern separation

An important link exists between intact metabolic processes and normal cognitive functioning; however, the underlying mechanisms remain unknown. There is accumulating evidence that the gut hormone ghrelin, an orexigenic peptide that is elevated during calorie restriction (CR) and known primarily for stimulating growth hormone release, has important extra-hypothalamic functions, such as enhancing synaptic plasticity and hippocampal neurogenesis. The present study was designed to evaluate the long-term effects of elevating acyl-ghrelin levels, albeit within the physiological range, on the number of new adult born neurons in the dentate gyrus (DG) and performance on the Spontaneous Location Recognition (SLR) task, previously shown to be DG-dependent and sensitive to manipulations of plasticity mechanisms and cell proliferation. The results revealed that peripheral treatment of rats with acyl-ghrelin enhanced both adult hippocampal neurogenesis and performance on SLR when measured 8-10 days after the end of acyl-ghrelin treatment. Our data show that systemic administration of physiological levels of acyl-ghrelin can produce long-lasting improvements in spatial memory that persist following the end of treatment. As ghrelin is potentially involved in regulating the relationship between metabolic and cognitive dysfunction in ageing and neurodegenerative disease, elucidating the underlying mechanisms holds promise for identifying novel therapeutic targets and modifiable lifestyle factors that may have beneficial effects on the brain.

Unexpected improvements of spatial learning and memory abilities in chronic rotenone intoxicated mice

The liposoluble insecticide rotenone is commonly used as a mitochondrial complex I inhibitor to replicate Parkinson's disease (PD) pathological features. However, there was no assessment of the spatial learning and memory abilities in chronic rotenone-induced PD models. In the present study, by rotarod test and Thioflavine T staining, we first noted the impairment of motor coordination in rotenone-treated group for 3 months, as well as alpha-synuclein inclusions in the nigral dopaminergic neurons in C57BL/6 mice with intragastrical delivery of rotenone (5 mg/Kg) for 3 months rather than 1 month. We then evaluated spatial learning and memory abilities by Morris water maze task in this model. The results showed escape latency reduced in rotenone-intoxicated mice for 3 months, indicating an improvement of learning ability. However, it was delayed slightly but not significantly in rotenone-intoxicated mice for 1 month. Similarly, we demonstrated that spatial memory ability was enhanced in 3-month-treatment group, but impaired in 1-month-treatment group. There were no proliferating cell nuclear antigen and doublecortin positive cells in the hippocampus by double immunofluorescent staining, indicating the absence of hippocampal neurogenesis in rotenone-intoxicated mice. These results suggest that spatial learning and memory abilities are disturbed in chronic rotenone-intoxicated PD model.