Ghrelin increases anxiety-like behavior and memory retention in rats

Recent Developments in Ghrelin Receptor Ligands

The 28-amino acid peptide ghrelin is a neuroendocrine hormone synthesized primarily in the stomach. It stimulates growth hormone secretion and appetite, thus promoting food intake and body-weight gain. The pharmacological properties of this peptide are mediated by the growth hormone secretagogue receptor type 1a (GHS-R1a). Given its wide spectrum of biological activities, it is evident that the discovery of ghrelin and its receptor has opened up many perspectives in the fields of neuroendocrine and metabolic research and has had an influence on such fields of internal medicine as gastroenterology, oncology, and cardiology. It is therefore increasingly likely that synthetic, peptidyl, and nonpeptidyl GHS-R1a ligands, acting as agonists, partial agonists, antagonists, or inverse agonists, could have both clinical and therapeutic potential. This review summarizes the various types of GHS-R1a ligands that have been described in the literature and discusses the recent progress made in this research area.

Endogenous acyl ghrelin is involved in mediating spontaneous phase III-like contractions of the rat stomach

In humans and dogs, it is known that motilin regulates phase III contractions of migrating motor complex (MMC) in the fasted state. In rats, however, motilin and its receptor have not been found, and administration of motilin failed to induce any phase III-like contractions. Ghrelin was discovered as the endogenous ligand for the growth hormone secretagogue receptor (GHS-R) from the rat stomach. Ghrelin promotes gastric premature phase III (phase III-like contractions) in the fasted state in rats. We hypothesized that endogenous ghrelin regulates spontaneous phase III-like contractions in rats. Strain gauge transducer was sutured on the antrum and a catheter was inserted into the jugular vein. We studied the effects of i.v. administration of ghrelin and a GHS-R antagonist on gastric phase III-like contractions in conscious rats. Plasma level of ghrelin was measured by a radioimmunoassay. Ghrelin augmented spontaneous phase III-like contractions and a GHS-R antagonist significantly attenuated the occurrence of spontaneous phase III-like contractions. During the phase I period, plasma ghrelin level increased to its peak then returned to basal level, subsequently phase III-like contractions were observed. These results suggest that endogenous ghrelin regulates gastric phase III-like contractions in rats.

Fasting in the American marten (Martes americana): a physiological model of the adaptations of a lean-bodied animal

The American marten (Martes americana) is a boreal forest marten with low body adiposity throughout the year. The aim of this study was to investigate the adaptations of this lean-bodied species to fasting for an ecologically relevant duration (48 h) by exposing eight farm-bred animals to total food deprivation with seven control animals. Selected morphological and hematological parameters, plasma and serum biochemistry, endocrinological variables and liver and white adipose tissue (WAT) enzyme activities were determined. After 48 h without food, the marten were within phase II of fasting with depleted liver and muscle glycogen stores, but with active lipid mobilization indicated by the high lipase activities in several WAT depots. The plasma ghrelin concentrations were higher due to food deprivation, possibly increasing appetite and enhancing foraging behavior. The lower plasma insulin and higher cortisol concentrations could mediate augmented lipolysis and the lower triiodothyronine levels could suppress the metabolic rate. Fasting did not affect the plasma levels of stress-associated catecholamines or variables indicating tissue damage. In general, the adaptations to short-term fasting exhibited some differences compared to the related farm-bred American mink (Mustela vison), an example of which was the better ability of the marten to hydrolyze lipids despite its significantly lower initial fat mass.

Anorexia in hemodialysis patients: the possible role of des-acyl ghrelin

BACKGROUND

Anorexia is frequently found in end-stage renal disease and is a reliable predictor of morbidity and mortality in hemodialysis (HD) patients. The pathogenesis of anorexia is complex and the appetite-modulating hormone ghrelin could be involved. Two forms of circulating ghrelin have been described: acylated ghrelin (<10% of circulating ghrelin) which promotes food intake, and des-acyl ghrelin which induces a negative energy balance. The aim of this cross-sectional study is to clarify whether anorexia and body weight change in HD patients relate to plasma des-acyl ghrelin levels.

METHODS

34 HD patients and 15 healthy controls were studied. The presence of anorexia was assessed by a questionnaire. Serum des-acyl ghrelin was measured in HD patients and in 15 body mass index-, sex- and age-matched controls by ELISA. Energy intake was assessed by a 3-day dietary diary, and fat-free mass (FFM) was evaluated by body impedance analysis. Data have been statistically analyzed and are presented as mean +/- SD.

RESULTS

14 patients (41%) were found to be anorexic, and 20 patients (59%) non-anorexic. Energy intake (kcal/day) was significantly lower in anorexic than in non-anorexic patients (1,682 +/- 241 vs. 1,972.50 +/- 490; p < 0.05). FFM (%) was lower in anorexic than in non-anorexic patients (65.8 +/- 4.4 vs. 70.9 +/- 8.7; p = 0.05). Plasma des-acyl ghrelin levels (fmol/ml) were significantly higher in HD patients than in controls (214.88 +/- 154.24 vs. 128.93 +/- 51.07; p < 0.05), and in anorexic HD patients than in non-anorexic (301.7 +/- 162.4 vs. 159.1 +/- 115.5; p < 0.01).

CONCLUSION

Anorexia is highly prevalent among HD patients and des-acyl ghrelin could be involved in its pathogenesis.

Secretion of ghrelin from rat stomach ghrelin cells in response to local microinfusion of candidate messenger compounds: a microdialysis study

Ghrelin is produced by A-like cells (ghrelin cells) in the mucosa of the acid-producing part of the stomach. The mobilization of ghrelin is stimulated by nutritional deficiency and suppressed by nutritional abundance. In an attempt to identify neurotransmitters and regulatory peptides that may contribute to the physiological, nutrient-related regulation of ghrelin secretion, we challenged the ghrelin cells in situ with a wide variety of candidate messengers, including known neurotransmitters (e.g. acetylcholine, catecholamines), candidate neurotransmitters (e.g. neuropeptides), local tissue hormones (e.g. serotonin, histamine, bradykinin, endothelin), circulating gut hormones (e.g. gastrin, CCK, GIP, neurotensin, PYY, secretin) and other circulating hormones/regulatory peptides (e.g. calcitonin, glucagon, insulin, PTH). Microdialysis probes were placed in the submucosa of the acid-producing part of the rat stomach. Three days later, the putative messenger compounds were administered via the microdialysis probe (reverse microdialysis) at a screening dose of 0.1 mmol l(-1) for regulatory peptides and 0.1 and 1 mmol l(-1) for amines and amino acids. The rats were awake during the experiments. The resulting microdialysate ghrelin concentration was monitored continuously for 3 h (radioimmunoassay), thereby revealing stimulators or inhibitors of ghrelin secretion. Dose-response curves were constructed for each candidate messenger that significantly (p<0.05) affected ghrelin mobilization at the screening dose. Peptides that showed a (non-significant) tendency to affect ghrelin release at the screening dose were also given at a dose of 0.3 or 1 mmol l(-1). Adrenaline, noradrenaline, endothelin and secretin stimulated ghrelin release, while somatostatin and GRP inhibited. Whether these agents act directly or indirectly on the ghrelin cells remains to be investigated. All other candidate messengers were without measurable effects, including acetylcholine, serotonin, histamine, GABA, aspartic acid, glutamic acid, glycine, VIP, PACAP, CGRP, substance P, NPY, PYY, PP, gastrin, CCK, GIP, insulin, glucagon, GLP and glucose.

Selective serotonin reuptake inhibitor (fluoxetine) decreases the effects of ghrelin on memory retention and food intake

Ghrelin (Ghr) is an appetite stimulating hormone that is produced peripherally, by the stomach, and centrally as well. Previous investigations show that Ghr increases food intake and memory retention in rats, and that extra-hypothalamic structures, such as the hippocampus, participate in these effects. In the present work we analyzed the effect on food intake and memory retention induced by Ghr after serotonin (5-HT) availability modification at the serotoninergic synapses. Animals only treated with a selective serotonin reuptake inhibitor (SSRI), fluoxetine (FLU) 5 mg/kg or clomipramine (CLO) 2.5 and 5 mg/kg, showed a significant reduction in both food intake and memory retention. On the contrary, Ghr administration induces a significant increase in food intake and a dose-dependent increase in short and long term memory retention. When the animals were treated with FLU prior to Ghr injection, the food intake induced, as well as the expression of short and long term memory retention, was decreased. In conclusion, evidence presented in this paper suggests that the effects of Ghr on both feeding and memory retention in extra-hypothalamic structures such as the hippocampus, could depend on the availability of 5-HT.

Physiological, pathological and potential therapeutic roles of ghrelin

Ghrelin, a hormone that is produced mainly by the stomach, was identified originally as the endogenous ligand of the growth hormone secretagogue (GHS) receptor. Ghrelin might also be synthesized in other organs, where it might have autocrine or paracrine effects. GHS receptors are present in tissues other than the hypothalamus and pituitary, which indicates that ghrelin has other effects in addition to stimulating the release of growth hormone. Recently, it has been suggested that ghrelin might be involved in the pathogenesis of many diseases and be a therapeutic target in these diseases. Here, we provide an overview of the physiological effects of ghrelin and of its pathological and potential therapeutic roles.

Correlation between serum ghrelin levels and cocaine-seeking behaviour triggered by cocaine-associated conditioned stimuli in rats

Ghrelin is a brain-gut peptide with growth hormone-releasing and appetite-inducing activities. A growing body of evidence suggests that ghrelin may affect the central reward system and modulate the activity of the mesolimbic system. Recent clinical studies also showed a significant positive correlation between plasma ghrelin levels and craving in alcoholics. Accordingly, the present study investigated the potential role of serum ghrelin levels in the reinstatement of cocaine-seeking behaviour triggered by cocaine-associated cues. In addition, serum corticosterone levels were determined in the light of evidence suggesting that corticosterone plays a modulatory role in cocaine-seeking behaviour. Male Lister Hooded rats under a restricted diet regime were first trained to intravenously self-administer cocaine under a fixed ratio-1 schedule of reinforcement. Conditioned stimuli (CS: tone and cue-light on for 5 seconds) were presented contingently with cocaine delivery. Once a stable baseline of cocaine self-administration was observed, lever presses were extinguished to less than 30% of baseline rates by removing both cocaine and CS. Reinstatement of responding was then induced by re-exposure to cocaine-associated CS. Blood samples for the enzyme immunoassay determination of serum ghrelin and the radioimmunoassay determination of serum corticosterone levels were collected 30 minutes before the beginning of reinstatement sessions. Rats significantly reinstated their responding when exposed to CS. A positive and significant correlation was observed between ghrelin levels (r = 0.64; P < 0.05), but not corticosterone (r = 0.37; NS), and the increased active lever presses only in animals exposed to CS. These findings suggest a potential role of ghrelin in the modulation of cue-triggered reinstatement of cocaine-seeking behaviour.

Ghrelin in neuroendocrine organs and tumours

Ghrelin is a 28 amino-acid hormone with multiple functions. It is predominantly produced by the stomach but has also been detected in other organs, including the small intestine, pancreas, hypothalamus and pituitary, as well as in the immune system and almost every other normal human tissue examined. It is also present in neuroendocrine tumours, pituitary adenomas, endocrine tumours of the pancreas, breast tumours, and thyroid and medullary thyroid carcinomas. Ghrelin is a brain-gut peptide with growth hormone-releasing and appetite-inducing activities, and is the endogenous ligand of the G protein-coupled growth hormone secretagogue receptor (GHS-R). In this review we comprehensively summarize the available data regarding (a) the expression of ghrelin and the GHS-R in normal endocrine tissues and in pituitary adenomas and neuroendocrine tumours, (b) the levels of circulating ghrelin in patients with pituitary adenomas and neuroendocrine tumours and (c) the effects of ghrelin administration in these patients on the levels of other hormones and on the rate of proliferation of the tumour. It is clear that ghrelin has many more functions and is involved in many more processes than was initially postulated, and its endocrine, paracrine and autocrine effects play a role in its physiological and pathophysiological functions.

Administration of antisense DNA for ghrelin causes an antidepressant and anxiolytic response in rats

RATIONALE

Ghrelin is a peptide of 28 amino acids found in mammals that increases the release of growth hormone, food intake, and body weight.

OBJECTIVES

We investigated the relationship between ghrelin and the states of anxiety and depression by giving rats either antisense DNA for ghrelin, scrambled DNA or vehicle into the lateral ventricle of rats.

RESULTS

In forced swimming tests, rats that received antisense DNA decreased the length of time that they were immobile in the water. Ghrelin antisense oligonucleotides produced an anxiolytic-like effects in the elevated plus maze test, black and white test, or conditioned fear tests. Treatment with antisense DNA for ghrelin significantly decreased rat body weight. No significant effect on general locomotor activity was seen.

CONCLUSIONS

These results suggest that administration of antisense DNA for ghrelin causes an antidepressant and anxiolytic response in rats.

Obestatin improves memory performance and causes anxiolytic effects in rats

Obestatin is a peptide hormone that is derived from the same polypeptide precursor (preprogrelin) as ghrelin, but it acts in opposing way on ingestive behavior. Our previous studies showed that ghrelin affects memory and anxiety. Here, we studied the possible effects of icv obestatin injection in rats upon memory retention (using two different paradigms), anxiety like behavior (plus maze test), and food intake. Obestatin induces an increase in the percentage of open arms entries (Obestatin 3.0nmol/rat: 61.74+/-1.81), and percentage of time spent in open arms (Obestatin 3.0nmol/rat: 72.07+/-4.21) in relation to the control (33.31+/-1.54; 25.82+/-1.68), indicating an anxiolytic effect. The two doses of obestatin increased latency time in a step down test and the percentage time of novel object exploration, suggesting that the peptide influences learning and memory processes that involve the hippocampus and the amygdala. This report provides evidence indicating that obestatin effects are functionally opposite on anxiety and hunger to the ghrelin effects, while both these related peptides increase memory retention.

Interleukin-1 beta-induced anorexia is reversed by ghrelin

Interleukins, in particular interleukin-1beta (IL-1beta), reduce food intake after peripheral and central administration, which suggests that they contribute to anorexia during various infectious, neoplastic, and autoimmune diseases. On the other hand, ghrelin stimulates food intake by acting on the central nervous system (CNS) and is considered an important regulator of food intake in both rodents and humans. In the present study, we investigated if ghrelin could reverse IL-1beta-induced anorexia. Intracerebroventricular (i.c.v.) injection of 15, 30 or 45 ng/microl of IL-1beta caused significant suppression of food intake in 20 h fasting animals. This effect lasted for a 24h period. Ghrelin (0.15 nmol or 1.5 nmol/microl) produced a significant increase in cumulative food intake in normally fed animals. However, it did not alter food intake in 20 h fasting animals. Central administration of ghrelin reduced the anorexic effect of IL-1beta (15 ng/microl). The effect was observed 30 min after injection and lasted for the next 24h. This study provides evidence that ghrelin is an orexigenic peptide capable of antagonizing IL-1beta-induced anorexia.

Ghrelin microinjection into forebrain sites induces wakefulness and feeding in rats

Ghrelin, a gut-brain peptide, is best known for its role in the stimulation of feeding and growth hormone release. In the brain, orexin, neuropeptide Y (NPY), and ghrelin are parts of a food intake regulatory circuit. Orexin and NPY are also implicated in maintaining wakefulness. Previous experiments in our laboratory revealed that intracerebroventricular injections of ghrelin induce wakefulness in rats. To further elucidate the possible role of ghrelin in the regulation of arousal, we studied the effects of microinjections of ghrelin into hypothalamic sites, which are implicated in the regulation of feeding and sleep, such as the lateral hypothalamus (LH), medial preoptic area (MPA), and paraventricular nucleus (PVN) on sleep in rats. Sleep responses, motor activity, and food intake after central administration of 0.04, 0.2, or 1 mug (12, 60, or 300 pmol) ghrelin were recorded. Microinjections of ghrelin into the LH had strong wakefulness-promoting effects lasting for 2 h. Wakefulness was also stimulated by ghrelin injection into the MPA and PVN; the effects were confined to the first hour after the injection. Ghrelin's non-rapid-eye-movement sleep-suppressive effect was accompanied by attenuation in the electroencephalographic (EEG) slow-wave activity and changes in the EEG power spectrum. Food consumption was significantly stimulated after microinjections of ghrelin into each hypothalamic site. Together, these results are consistent with the hypothesis that forebrain ghrelinergic mechanisms play a role in the regulation of vigilance, possibly through activating the components of the food intake- and arousal-promoting network formed by orexin and NPY.

Mediation of the behavioral, endocrine and thermoregulatory actions of ghrelin

The action of ghrelin on telemetrically recorded motor activity and the transmission of the effects of this neuropeptide on spontaneous and exploratory motor activity and some related endocrine and homeostatic parameters were investigated. Different doses (0.5-5 microg) of ghrelin administered intracerebroventricularly caused significant increases in both square crossing and rearing activity in the "open-field" apparatus, while only the dose of 5 microg evoked a significant increase in the spontaneous locomotor activity recorded by telemetry. Ghrelin also induced significant increases in corticosterone release and core temperature. To determine the transmission of these neuroendocrine actions, the rats were pretreated with different antagonists, such as a corticotropin-releasing hormone (CRH) antagonist (alpha-helical CRH(9-41)), the nitric oxide synthase inhibitor Nomega-nitro-L-arginine-methyl ester (L-NAME), haloperidol, cyproheptadine or the cyclooxygenase inhibitor noraminophenazone (NAP). The open-field and biotelemetric observations revealed that the motor responses were diminished by pretreatment with the CRH antagonist and haloperidol. In the case of HPA (hypothalamic pituitary adrenal) activation, only cyproheptadine pretreatment proved effective; haloperidol and L-NAME did not modify the corticosterone response. NAP had only a transient, while cyproheptadine elicited a more permanent impact on the hyperthermic response evoked by ghrelin; the other antagonists proved to be ineffective. The present data suggest that both CRH release and dopaminergic transmission may be involved in the ghrelin-evoked behavioral responses. On the other hand, ghrelin appears to have an impact on the HPA response via a serotonergic pathway and on the hyperthermic response via a cyclooxygenase and a serotonergic pathway.

Peripheral ghrelin participates in the glucostatic signaling mediated by the ventromedial and lateral hypothalamus neurons

Employing immunohistochemistry techniques, we examined the c-fos expression in different hypothalamic areas, when plasma glucose levels were modified by the administration of insulin and 2-deoxyglucose (2-DG) respectively. Subsequently, the hypoglycemia produced by an injection of insulin significantly increased feeding concomitant to higher c-fos expression in the arcuate nucleus (ARC), paraventricular nucleus (PVN), dorsomedial hypothalamus (DMH) and lateral hypothalamus (LH), while no statistical changes in the ventromedial hypothalamus (VMH) were found. Also, the glucopenia induced by 2-DG administration produced similar stimulatory effects on appetite and the neuronal activity affecting all the hypothalamic areas studied, including the VMH. The peripheral blockade of the orexigenic hormone ghrelin with a specific antibody (AGA) significantly decreased food intake as induced from acute hypoglycemia and glucopenia. Curiously, the conjoint AGA and insulin or 2-DG administration produced a differential effect on the hypothalamic neurons analyzed, by increasing the number of c-fos positive neurons in the ARC, PVN and DMH, but not in the VMH and LH. This outcome suggests an interactive effect of the glucostatic pathways involving these two areas with the ghrelin signaling.

Central nervous system and other effects

Amylin enhanced the uptake of certain amino acids, crossed the blood-brain barrier, and increased body temperature. The physiological significance of these responses is currently unclear. An effect of peripherally injected amylin to enhance weakly trained memory fitted with similar effects of other gastrointestinal peptide hormones. Centrally administered amylin reduced locomotor and exploratory behavior. Amylin administered alone was analgesic when administered peripherally, via a non-opiate pathway. When administered in combination with opiates, there was an opiate-sparing synergy.

Ghrelin and the growth hormone secretagogue receptor constitute a novel autocrine pathway in astrocytoma motility

Originally thought of as a stomach-derived endocrine peptide acting via its receptors in the central nervous system to stimulate food intake and growth hormone expression, ghrelin and its receptor (growth hormone secretagogue receptor (GHS-R)) are widely expressed in a number of organ systems, including cancer cells. However, the direct functional role of ghrelin and its receptor in tumors of central nervous system origin remains to be defined. Here, we demonstrate that the human astrocytoma cell lines U-118, U-87, CCF-STTG1, and SW1088 express 6-, 11-, 15-, and 29-fold higher levels of GHS-R compared with primary normal human astrocytes. The ligation of GHS-R by ghrelin on these cells resulted in an increase in intracellular calcium mobilization, protein kinase C activation, actin polymerization, matrix metalloproteinase-2 activity, and astrocytoma motility. In addition, ghrelin led to actin polymerization and membrane ruffling on cells, with the specific co-localization of the small GTPase Rac1 with GHS-R on the leading edge of the astrocytoma cells and imparting the tumor cells with a motile phenotype. Disruption of the endogenous ghrelin/GHS-R pathway by RNA interference resulted in diminished motility, matrix metalloproteinase activity, and Rac expression, whereas tumor cells stably overexpressing GHS-R exhibited increased cell motility. The relevance of ghrelin and GHS-R expression was verified in clinically relevant tissues from 20 patients with oligodendrogliomas and grade II-IV astrocytomas. Analysis of a central nervous system tumor tissue microarray revealed that strong GHS-R and ghrelin expression was significantly more common in high grade tumors compared with low grade ones. Together, these findings suggest a novel role for the ghrelin/GHS-R axis in astrocytoma cell migration and invasiveness of cancers of central nervous system origin.

Acute psychological stress raises plasma ghrelin in the rat

Ghrelin is produced by the A-like cells of the stomach and mobilized by food deprivation. It was reported recently that acute psychological stress increases ghrelin gene expression in rat oxyntic mucosa. The aim of this study was to examine the effect of such stress on circulating ghrelin levels. To this end, we measured plasma ghrelin in Wistar Kyoto (WKY) rats (a high-anxiety strain) and Sprague-Dawley (SPD) rats (a low-anxiety strain), exposed to water avoidance stress for 60 min. Blood was collected before and after the stress. Acute stress increased the plasma ACTH concentration approximately 5-fold (p<0.01) in both strains of rats, while plasma ghrelin increased by 85% (p<0.01) in the SPD rats and by 40% (p<0.001) in the WKY rats. Ghrelin levels after acute stress were higher (p<0.05) in the SPD rats than in the WKY rats. Sham stress did not affect plasma ghrelin. We conclude that acute psychological stress mobilizes ghrelin and that the SPD rats respond with a higher plasma ghrelin concentration than the WKY rats.

Ventromedial arcuate nucleus communicates peripheral metabolic information to the suprachiasmatic nucleus

The arcuate nucleus (ARC) is crucial for the maintenance of energy homeostasis as an integrator of long- and short-term hunger and satiety signals. The expression of receptors for metabolic hormones, such as insulin, leptin, and ghrelin, allows ARC to sense information from the periphery and signal it to the central nervous system. The ventromedial ARC (vmARC) mainly comprises orexigenic neuropeptide agouti-related peptide and neuropeptide Y neurons, which are sensitive to circulating signals. To investigate neural connections of vmARC within the central nervous system, we injected the neuronal tracer cholera toxin B into vmARC. Due to variation of injection sites, tracer was also injected into the subependymal layer of the median eminence (seME), which showed similar projection patterns as the vmARC. We propose that the vmARC forms a complex with the seME, their reciprocal connections with viscerosensory areas in brain stem, and other circumventricular organs, suggesting the exchange of metabolic and circulating information. For the first time, the vmARC-seME was shown to have reciprocal interaction with the suprachiasmatic nucleus (SCN). Activation of vmARC neurons by systemic administration of the ghrelin mimetic GH-releasing peptide-6 combined with SCN tracing showed vmARC neurons to transmit feeding related signals to the SCN. The functionality of this pathway was demonstrated by systemic injection of GH-releasing peptide-6, which induced Fos in the vmARC and resulted in a reduction of about 40% of early daytime Fos immunoreactivity in the SCN. This observation suggests an anatomical and functional pathway for peripheral hormonal feedback to the hypothalamus, which may serve to modulate the activity of the SCN.

Ghrelin, the same peptide for different functions: player or bystander?

Ghrelin is a recently discovered brain-gut peptide with two main physiological actions: growth hormone secretagogue activity and food intake inducer. Although its production is prevalently gastric, ghrelin is widely expressed in several tissues, where it might therefore act as a paracrine or autocrine factor. It is becoming clear that ghrelin is much more than a simple growth hormone secretagogue. In addition to its formerly envisaged role, ghrelin has other activities including stimulation of pituitary hormones secretion, modulation of food intake and control of energy metabolism, regulation of gastric and pancreatic activity, and cardiovascular and hemodynamic activities. In addition, modulation of cartilage and bone homeostasis, sleep and behavioral influences, and modulation of the immune system, as well as effects on cell proliferation, are other relevant actions of ghrelin. Thus, the peptide appears to be an important component of an integrated multifaceted regulatory system. In this review, we summarize several aspects of ghrelin biology and attempt to inform the reader with information regarding unexpected functions of this gastric peptide.

Ghrelin and immunity: a young player in an old field

There is increasing evidence of the coupling of immune status to the metabolic system. The communication between the state of systemic and cellular energy balance to immune compartment is mediated via a complex array of cytokines, hormones and neuropeptides. Ghrelin, a recently described orexigenic peptide hormone, is predominantly produced by the stomach and functions as a positive regulator of the somatotropic axis and a peripheral signal of negative energy balance. Apart from its well-studied metabolic effects, ghrelin also exerts multiple regulatory effects on several other organ systems including the cardiovascular, central nervous and immune systems. Here, we summarize the growing evidence of ghrelin as a significant player in the regulation of inflammation and the immune function and the potential therapeutic targeting of ghrelin or its receptor, the growth hormone secretagogue receptor (GHS-R), in various inflammatory and cachexic disease states.

Hormonal and metabolic responses to acute ghrelin administration in patients with bulimia nervosa

Ghrelin is generally influenced by energy balance status and is inversely associated with body mass index (BMI), being reduced in simple obesity, notable exception being Prader Willi syndrome, and elevated in several conditions of undernutrition, including anorexia nervosa (AN). Interestingly, ghrelin levels have also been found elevated in patients with bulimia nervosa (BN) in spite of normal BMI. In humans, intravenous (iv) ghrelin administration induces endocrine (increase in GH, PRL, ACTH and cortisol) and metabolic (increase in glucose and decrease in insulin) effects as well as an increase in appetite and food intake. In AN, ghrelin administration surprisingly leads to a decreased GH response and absence of glycemic variations but normal PRL, ACTH and insulin response. This pattern would reflect a decrease in sensitivity to ghrelin or, alternatively, the metabolic status of AN. To further clarify the function of ghrelin in eating disorders, the endocrine and metabolic response to acute iv ghrelin (1.0 microg/kg) was studied in seven young women with purging BN (BW, BMI, mean+/-SEM: 20.3+/-0.5 kg/m2). Circulating total ghrelin levels were also measured. The results in BW were compared to those recorded in a group of nine healthy women (HW; BMI 22.3+/-2.5 kg/m2). The GH response to ghrelin in BW overlapped with that in HW. Ghrelin administration also led to a similar increase in PRL, ACTH, cortisol and glucose levels in the two groups. Insulin levels were not significantly modified by ghrelin administration in either group. The overlapping endocrine and metabolic response to ghrelin in the two groups occurred with regard to circulating total ghrelin levels which were higher in BW than in HW. In conclusion, BN, a condition of ghrelin hypersecretion, is connoted by a normal endocrine and metabolic response to exogenous ghrelin administration.

Des-acyl ghrelin acts by CRF type 2 receptors to disrupt fasted stomach motility in conscious rats

BACKGROUND & AIMS

Although it has been shown that des-acyl ghrelin decreases food intake and gastric emptying, no previous studies have examined the effects of des-acyl ghrelin on physiologic fed and fasted motor activity in the gastrointestinal tract.

METHODS

We examined the effects of intraperitoneal (IP) administration of des-acyl ghrelin on food intake and the effects of intracerebroventricular (ICV) or intravenous (IV) administration of des-acyl ghrelin on gastroduodenal motility using freely moving conscious rat models. The brain nuclei responding to these effects were examined by c- fos immunohistochemistry of the brain sections.

RESULTS

IP injection of des-acyl ghrelin decreased food intake, and this effect was not altered by capsaicin treatment. IP injection of des-acyl ghrelin enhanced c- fos expression in the arcuate and paraventricular nucleus but not in the nucleus of the solitary tract. Both ICV and IV injection of des-acyl ghrelin disrupted fasted motor activity in the antrum but not in the duodenum. Changes in gastric motility induced by IV injection of des-acyl ghrelin were completely antagonized by ICV injection of a selective corticotropin-releasing factor (CRF) 2 receptor antagonist; however, the CRF 1 receptor antagonist had no effects.

CONCLUSIONS

The results suggest that des-acyl ghrelin decreases food intake and disrupts the fasted motor activity of the antrum in freely moving conscious rats. Peripheral des-acyl ghrelin may induce this function by direct activation of brain receptor by crossing the blood-brain barrier but not by the activation of vagal afferent pathways. In the brain, CRF 2 receptor, but not CRF 1 receptor, is involved in this action.

Ghrelin: structure and function

Small synthetic molecules called growth hormone secretagogues (GHSs) stimulate the release of growth hormone (GH) from the pituitary. They act through the GHS-R, a G protein-coupled receptor whose ligand has only been discovered recently. Using a reverse pharmacology paradigm with a stable cell line expressing GHS-R, we purified an endogenous ligand for GHS-R from rat stomach and named it "ghrelin," after a word root ("ghre") in Proto-Indo-European languages meaning "grow." Ghrelin is a peptide hormone in which the third amino acid, usually a serine but in some species a threonine, is modified by a fatty acid; this modification is essential for ghrelin's activity. The discovery of ghrelin indicates that the release of GH from the pituitary might be regulated not only by hypothalamic GH-releasing hormone, but also by ghrelin derived from the stomach. In addition, ghrelin stimulates appetite by acting on the hypothalamic arcuate nucleus, a region known to control food intake. Ghrelin is orexigenic; it is secreted from the stomach and circulates in the bloodstream under fasting conditions, indicating that it transmits a hunger signal from the periphery to the central nervous system. Taking into account all these activities, ghrelin plays important roles for maintaining GH release and energy homeostasis in vertebrates.

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

Ghrelin: a link between eating disorders, obesity and reproduction

Ghrelin, a 28-amino acid acylated peptide predominantly produced by the stomach, displays strong GH-releasing activity mediated by the hypothalamic-pituitary GH secretagogues (GHS) receptors (GHS-R) which had been shown specific for a family of synthetic, orally active molecules known as GHS. However, ghrelin and GHS, acting on central and peripheral receptors, also exert other actions. These include influence on pituitary functions, orexigenic action, influence on exocrine and endocrine gastro-entero-pancreatic functions, cardiovascular and anti-proliferative effects. In particular, the effect of ghrelin in promoting food intake and modulating energy metabolism strongly suggested that ghrelin has a key role in managing the neuroendocrine and metabolic response to starvation and that could be involved in the pathogenesis and/or in the metabolic and neuro-hormonal alterations of obesity and eating disorders. Although specific alterations in ghrelin secretion and/or action in obesity and anorexia nervosa (AN) have already been reported, the possibility that ghrelin analogues acting as agonists or antagonists has clinical perspectives for treatment of eating disorders presently remains a dream.

Augmentation of cocaine hyperactivity in rats by systemic ghrelin

The feeding-relevant pathway by which food deprivation (FD) augments cocaine action is unknown. Systemic administration of the 28 amino acid acylated peptide ghrelin (1-10 nmol) increases food intake in rats and circulating levels of rat ghrelin are up-regulated by FD. The present experiment examined the impact of ghrelin or vehicle pretreatment on the locomotion and stereotypy induced by systemic cocaine hydrochloride. Male Sprague-Dawley rats were pretreated at -60 min with 0 or 5 nmol rat ghrelin (IP) and then injected (IP) at time 0 with 0, 2.5, 5.0, or 10.0 mg/kg cocaine. Locomotor activity was monitored over a 45-min post-cocaine period. Rats received the same ghrelin dose, but a different cocaine dose (in random order) on each of the four drug trials, with each drug trial separated by at least 2 days. Administration of 5 nmol ghrelin-0 mg/kg cocaine slightly increased locomotion relative to that of 0 nmol ghrelin-0 mg/kg cocaine. Cocaine increased locomotion as a function of dose in the 0 nmol ghrelin group, but the effect of cocaine was even greater when preceded by 5 nmol ghrelin. These results indicate that acute injection of ghrelin, at a feeding-relevant dose, augments the acute effects of cocaine on locomotion in rats.

Ghrelin: more than a natural GH secretagogue and/or an orexigenic factor

Ghrelin, an acylated peptide produced predominantly by the stomach, has been discovered to be a natural ligand of the growth hormone secretagogue receptor type 1a (GHS-R1a). Ghrelin has recently attracted considerable interest as a new orexigenic factor. However, ghrelin exerts several other neuroendocrine, metabolic and also nonendocrine actions that are explained by the widespread distribution of ghrelin and GHS-R expression. The likely existence of GHS-R subtypes and evidence that the neuroendocrine actions, but not all the other actions, of ghrelin depend on its acylation in serine-3 revealed a system whose complexity had not been completely explored by studying synthetic GHS. Ghrelin secretion is mainly regulated by metabolic signals and, in turn, the modulatory action of ghrelin on the control of food intake and energy metabolism seems to be among its most important biological actions. However, according to a recent study, ghrelin-null mice are neither anorectics nor dwarfs and this evidence clearly depicts a remarkable difference from leptin null mice. Nevertheless, the original and fascinating story of ghrelin, as well as its potential pathophysiological implications in endocrinology and internal medicine, is not definitively cancelled by these data as GHS-R1a null aged mice show significant alterations in body composition and growth, in glucose metabolism, cardiac function and contextual memory. Besides potential clinical implications for natural or synthetic ghrelin analogues acting as agonists or antagonists, there are several open questions awaiting an answer. How many ghrelin receptor subtypes exist? Is ghrelin 'the' or just 'a' GHS-R ligand? That is, are there other natural GHS-R ligands? Is there a functional balance between acylated and unacylated ghrelin forms, potentially with different actions? Within the next few years suitable answers to these questions will probably be found, making it possible to gain a better knowledge of ghrelin's potential clinical perspectives.

The effect of neonatal handling on adult feeding behavior is not an anxiety‐like behavior

Brief periods of handling during the neonatal period have been shown to have profound and long-lasting physiological consequences. Previous studies performed in our laboratory have demonstrated that handling the pups during the neonatal period leads to increased sweet food ingestion in adult life. The objective of this study is to verify if this effect could be explained by the enhanced anxiety levels in these animals. Litters were divided in: (1) intact; (2) handled (10 min in an incubator/day) and (3) handled + tactile stimulation (10 min/day). Procedures were performed on days 1-10 after birth. When adults, rats were tested in the elevated plus maze apparatus, light dark exploration test and open field test. They were also tested for sweet food ingestion, being injected with 2 mg/kg diazepam or vehicle 60 min before the test. Handling and handling + tactile stimulation do not alter performance in the plus maze test, but handled rats presented more crossings in the light/dark exploration test and open field (two-way ANOVA). Females also spent more % time in the open arms in the plus maze and more time in the lit compartment in the light/dark test, presenting more crossings in both tests. Both treated rats (handled and handled + tactile stimulation groups) consumed more sweet food than intact ones (two-way ANOVA). When diazepam was injected prior to the measurement of sweet food ingestion, there was no effect of the drug. We suggest that handling during the neonatal period leads to plastic alterations in the central nervous system of these animals, causing an increased ingestion of palatable food in adult life, and this alteration does not express an anxiety-like behavior.

Learning at different satiation levels reveals parallel functions for the cAMP-protein kinase A cascade in formation of long-term memory

Learning and memory formation in intact animals is generally studied under defined parameters, including the control of feeding. We used associative olfactory conditioning of the proboscis extension response in honeybees to address effects of feeding status on processes of learning and memory formation. Comparing groups of animals with different but defined feeding status at the time of conditioning reveals new and characteristic features in memory formation. In animals fed 18 hr earlier, three-trial conditioning induces a stable memory that consists of different phases: a mid-term memory (MTM), translation-dependent early long-term memory (eLTM; 1-2 d), and a transcription-dependent late LTM (lLTM; > or =3 d). Additional feeding of a small amount of sucrose 4 hr before conditioning leads to a loss of all of these memory phases. Interestingly, the basal activity of the cAMP-dependent protein kinase A (PKA), a key player in LTM formation, differs in animals with different satiation levels. Pharmacological rescue of the low basal PKA activity in animals fed 4 hr before conditioning points to a specific function of cAMP-PKA cascade in mediating satiation-dependent memory formation. An increase in PKA activity during conditioning rescues only transcription-dependent lLTM; acquisition, MTM, and eLTM are still impaired. Thus, during conditioning, the cAMP-PKA cascade mediates the induction of the transcription-dependent lLTM, depending on the satiation level. This result provides the first evidence for a central and distinct function of the cAMP-PKA cascade connecting satiation level with learning.

Effects of peripheral ghrelin on the carbohydrate and lipid metabolism of the tundra vole (Microtus oeconomus)

Ghrelin is a novel orexigenic peptide hormone. In humans and rodents, it increases food intake and its levels are reduced in obesity but increased in fasting. It is an antagonistic signal to leptin informing the central nervous system about negative energy balance. The tundra vole (Microtus oeconomus) is an interesting model to study the effects of ghrelin, as it is poorly adapted to fasting. In this study, 10 male voles were injected with intraperitoneal ghrelin at 10 microg kg(-1)day(-1) for four days, while 10 males received sham injections. Additional five males were food deprived for 4 h with five males as fed controls. Exogenous ghrelin caused an expected elevation in the plasma ghrelin concentrations. Furthermore, the plasma glucose and high density lipoprotein cholesterol concentrations increased but the kidney and muscle glycogen contents decreased. The liver lipase and kidney glycogen phosphorylase activities increased at the same time. Food deprivation caused an increase in the plasma ghrelin concentrations. In voles, ghrelin may be a mediator to recruit body energy reserves during negative energy balance that would be detrimental to voles very rapidly if foraging does not prove to be successful.

Ghrelin, appetite, and gastric motility: the emerging role of the stomach as an endocrine organ

Recent progress in the field of energy homeostasis was triggered by the discovery of adipocyte hormone leptin and revealed a complex regulatory neuroendocrine network. A late addition is the novel stomach hormone ghrelin, which is an endogenous agonist at the growth hormone secretagogne receptor and is the motilin-related family of regulatory peptides. In addition to its ability to stimulate GH secretion and gastric motility, ghrelin stimulates appetite and induces a positive energy balance leading to body weight gain. Leptin and ghrelin are complementary, yet antagonistic, signals reflecting acute and chronic changes in energy balance, the effects of which are mediated by hypothalamic neuropeptides such as neuropeptide Y and agouti-related peptide. Endocrine and vagal afferent pathways are involved in these actions of ghrelin and leptin. Ghrelin is a novel neuroendocrine signal possessing a wide spectrum of biological activities that illustrates the importance of the stomach in providing input into the brain. Defective ghrelin signaling from the stomach could contribute to abnormalities in energy balance, growth, and associated gastrointestinal and neuroendocrine functions.

Ghrelin: integrative neuroendocrine peptide in health and disease

OBJECTIVE

Ghrelin is a novel gastric hormone recognized in 1999 as a mediator of growth hormone release. Since growth hormone is anabolic, an important function of ghrelin may be to coordinate energy needs with the growth process. Newly discovered biologic roles of ghrelin imply that it may have other important physiological functions as well. This is a review of recent clinically relevant, yet less well-known, physiologic actions of ghrelin.

SUMMARY BACKGROUND DATA

Ghrelin has profound orexigenic, adipogenic, and somatotrophic properties, increasing food intake and body weight. Secreted predominantly from the stomach, ghrelin is the natural ligand for the growth hormone secretagogue receptor in the pituitary gland, thus fulfilling criteria of a brain-gut peptide. The brain-gut axis is the effector of anabolism by regulating growth, feeding, and metabolism via vagal afferents mediating ghrelin signaling. However, the wide tissue distribution of ghrelin suggests that it may have other functions as well.

METHODS

Systematic literature review of all PubMed citations between 1999 and August 2003 focusing on clinically relevant biochemical and physiological characteristics of ghrelin.

RESULTS

Ghrelin is an important component of an integrated regulatory system of growth and metabolism acting via the vagus nerve, and is implicated in a variety of altered energy states such as obesity, eating disorders, neoplasia, and cachexia. It also enhances immune responses and potentially down-regulates anti-inflammatory molecules. Ghrelin's role as a brain-gut peptide emphasizes the significance of afferent vagal fibers as a major pathway to the brain, serving the purpose of maintaining physiologic homeostasis.

CONCLUSIONS

The discovery of ghrelin has increased our understanding of feeding regulation, nutritional homeostasis, and metabolic processes. Further characterization of ghrelin's functions will likely generate new pharmacological approaches to diagnose and treat different disease entities including those related to the over-nutrition of obesity and the catabolic response to surgical trauma.

Differential role of the hippocampus, amygdala, and dorsal raphe nucleus in regulating feeding, memory, and anxiety-like behavioral responses to ghrelin

Ghrelin is a peptide hormone produced and secreted from the stomach. Hypothalamic injection of the peptide increases food intake but it is not known if the peptide affects other brain regions. We measured several behavioral parameters such as anxiety (elevated plus maze), memory retention (step down test), and food intake after injections of different doses of the peptide in the hippocampus, amygdala, and dorsal raphe nucleus (DRN). The injection of ghrelin in the hippocampus and DRN significantly and dose dependently increased food intake in relation to controls rats, while injections into the amygdala did not affect the food intake. We also show for the first time that ghrelin clearly and dose dependently increases memory retention in the hippocampus, amygdala, and DRN. Moreover, ghrelin at different potencies induced anxiogenesis in these brain structures while the highest dose of 3 nmol/microl was effective in all of them. The comparison of sensitivity of each brain structure indicates a specific role of them for each of the behaviors studied. The results provide new insight in to the anatomical substrate and the functional role of extrahypothalamic ghrelin targets in the CNS.

Intraperitoneal injection of ghrelin induces Fos expression in the paraventricular nucleus of the hypothalamus in rats

Ghrelin is a 28-amino acid peptide hormone secreted from the stomach that acts as a gut-brain peptide with potent stimulatory effects on food intake. The aim of the present study was to investigate the effects of peripheral ghrelin (1 and 10 nmol/rat) injected intraperitoneally (i.p.) on food intake and neuronal activity in the hypothalamus and brain stem, as assessed by c-Fos-like-immunoreactivity (c-FLI), using a confocal laser scanning microscope (cLSM) as a sensitive microscopic technique to detect c-FLI-positive neurons. Cumulative food intake was significantly increased 5.3- and 3.7-fold for the 4-h period after i.p. injection of ghrelin at both doses. The number of c-FLI-positive neurons in the paraventricular nucleus of the hypothalamus (PVN) was significantly increased after peripheral administration of ghrelin (1 nmol i.p.; median: 41.8) compared with i.p. saline (median: 17.5). As described before, c-fos expression was increased in the arcuate nucleus of the hypothalamus (ARC). In the nucleus of the solitary tract (NTS) or the area postrema (AP), there was no significant change in the density of c-FLI-positive neurons. Our data suggest that an activation of the arcuate-paraventricular axis may be part of the brain circuits involved in the orexigenic effect of peripheral ghrelin.

An “eye” in the gut: the appendix as a sentinel sensory organ of the immune intelligence network

Neural systems are the traditional model of intelligence. Their complex interconnected network of wired neurons acquires, processes, and responds to environmental cues. We propose that the immune system is a parallel system of intelligence in which the gut, including the appendix, plays a prominent role in data acquisition. The immune system is essentially a virtual unwired network of interacting cells that acquires, processes, and responds to environmental data. The data is typically acquired by antigen-presenting cells (APCs) that gather antigenic information from the environment. The APCs chemically digest large antigens and deconstruct them into smaller data packets for sampling by other cells. The gut performs the same function on a larger scale. Morsels of environmental content that enter the gut are sequentially deconstructed by physical and chemical digestion. In addition to providing nutrients, the componentized contents offer environmental data to APCs in mucosa-associated lymphoid tissues (MALT) that relay the sampled information to the immune intelligence network. In this framework, positioning of the appendix immediately after the ileocecal valve is strategic: it is ideally positioned to sample environmental data in its maximally deconstructed state after small bowel digestion. For single-celled organisms, digestion of the environment has been the primary way to sample the surroundings. Prior to the emergence of complex sensory systems such as the eye, even multi-cellular organisms may have relied heavily on digestion to acquire environmental information. While the relative value of immune intelligence has diminished since the emergence of neural intelligence, organisms still use information from both systems in integrated fashion to respond appropriately to ecologic opportunities and challenges. Appendicitis may represent a momentary maladaptation in the evolutionary transition of sensory leadership from the gut to the eye. Relationships between immune dysfunctions and cognition are explored.