Mediation of the behavioral, endocrine and thermoregulatory actions of ghrelin

The Orexin/Hypocretin System, the Peptidergic Regulator of Vigilance, Orchestrates Adaptation to Stress

The orexin/hypocretin neuropeptide family has emerged as a focal point of neuroscientific research following the discovery that this family plays a crucial role in a variety of physiological and behavioral processes. These neuropeptides serve as powerful neuromodulators, intricately shaping autonomic, endocrine, and behavioral responses across species. Notably, they serve as master regulators of vigilance and stress responses; however, their roles in food intake, metabolism, and thermoregulation appear complementary and warrant further investigation. This narrative review provides a journey through the evolution of our understanding of the orexin system, from its initial discovery to the promising progress made in developing orexin derivatives. It goes beyond conventional boundaries, striving to synthesize the multifaceted activities of orexins. Special emphasis is placed on domains such as stress response, fear, anxiety, and learning, in which the authors have contributed to the literature with original publications. This paper also overviews the advancement of orexin pharmacology, which has already yielded some promising successes, particularly in the treatment of sleep disorders.

Ghrelin Amplifies the Nicotine-Induced Release of Dopamine in the Bed Nucleus of Stria Terminalis (BNST)

Ghrelin is an orexigenic neuropeptide that is known for stimulating the release of growth hormone (GH) and appetite. In addition, ghrelin has been implicated in addiction to drugs such as nicotine. Nicotine is the principal psychoactive component in tobacco and is responsible for the reward sensation produced by smoking. In our previous in vitro superfusion studies, it was demonstrated that ghrelin and nicotine stimulate equally the dopamine release in the rat amygdala, and ghrelin amplifies the nicotine-induced dopamine release in the rat striatum. However, less attention was paid to the actions of ghrelin and nicotine in the bed nucleus of the stria terminalis (BNST). Therefore, in the present study, nicotine and ghrelin were superfused to the BNST of male Wistar rats, and the dopamine release from the BNST was measured in vitro. In order to determine which receptors mediate these effects, mecamylamine, a non-selective nicotinic acetylcholine receptor (nAchR) antagonist, and GHRP-6, a selective growth hormone secretagogue receptor (GHS-R1A) antagonist, were also superfused to the rat BNST. Nicotine significantly increased the release of dopamine, and this effect was significantly inhibited by mecamylamine. Ghrelin increased dopamine release even more significantly than nicotine did, and this effect was significantly inhibited by GHRP-6. Moreover, when administered together, ghrelin significantly amplified the nicotine-induced release of dopamine in the BNST, and this additive effect was reversed partly by mecamylamine and partly by GHRP-6. Therefore, the present study provides a new base of evidence for the involvement of ghrelin in dopamine signaling implicated in nicotine addiction.

Perioperative neurocognitive dysfunction: thinking from the gut?

With the aging of the world population, and improvements in medical and health technologies, there are increasing numbers of elderly patients undergoing anaesthesia and surgery. Perioperative neurocognitive dysfunction has gradually attracted increasing attention from academics. Very recently, 6 well-known journals jointly recommended that the term perioperative neurocognitive dysfunction (defined according to the Diagnostic and Statistical Manual of Mental Disorders, fifth edition) should be adopted to improve the quality and consistency of academic communications. Perioperative neurocognitive dysfunction currently includes preoperatively diagnosed cognitive decline, postoperative delirium, delayed neurocognitive recovery, and postoperative cognitive dysfunction. Increasing evidence shows that the gut microbiota plays a pivotal role in neuropsychiatric diseases, and in central nervous system functions via the microbiota-gut-brain axis. We recently reported that abnormalities in the composition of the gut microbiota might underlie the mechanisms of postoperative cognitive dysfunction and postoperative delirium, suggesting a critical role for the gut microbiota in perioperative neurocognitive dysfunction. This article therefore reviewed recent findings on the linkage between the gut microbiota and the underlying mechanisms of perioperative neurocognitive dysfunction.

Molecular cloning and expression of ghrelin in the hypothalamus-pituitary-gastrointestinal tract axis of the Yak (Bos grunniens) in the Qinghai-Tibetan Plateau

Ghrelin is a very important brain-gut peptide that modulates appetite and energy metabolism in mammals. The yak is the only large mammal that can adapt to the cold temperatures and hypoxia conditions present in the Qinghai-Tibet Plateau. However, there are no reports on ghrelin molecular characterization and expression in the hypothalamus-pituitary-digestive tract axis of the yak to date. In this study, the coding region sequence of the yak ghrelin, containing a complete ORF (351) encoding for 117 amino acids, was cloned. Immunohistochemistry analysis of the yak samples showed that ghrelin-immunoreactive cells were expressed at the arcuate nucleus (ARC), the ventromedial nucleus (VMN), the dorsomedial nucleus (DMN) of the hypothalamus and also at the anterior pituitary. Ghrelin-positive cells were also present in approximately two thirds of the submucosa of the abomasum fundic gland and mucous layer of the duodenum intestinal gland. Ghrelin's mRNA highest expression occurred in the abomasum sample, followed by the duodenum, hypothalamus and lowest at the pituitary gland. The level of ghrelin mRNA measured in yak was higher than in cattle for all the tissues that were compared. The ghrelin protein and mRNA expression profiles were similar. These data imply that the high expression of ghrelin in the hypothalamus-pituitary-digestive tract axis of yak could aid adaptation to the extreme environment better than cattle, by improving appetite and fat accumulation, regulating body temperature and reducing energy consumption via regulating energy metabolism.

Neuroanatomical pathways underlying the effects of hypothalamo-hypophysial-adrenal hormones on exploratory activity

When injected via the intracerebroventricular route, corticosterone-releasing hormone (CRH) reduced exploration in the elevated plus-maze, the center region of the open-field, and the large chamber in the defensive withdrawal test. The anxiogenic action of CRH in the elevated plus-maze also occurred when infused in the basolateral amygdala, ventral hippocampus, lateral septum, bed nucleus of the stria terminalis, nucleus accumbens, periaqueductal grey, and medial frontal cortex. The anxiogenic action of CRH in the defensive withdrawal test was reproduced when injected in the locus coeruleus, while the amygdala, hippocampus, lateral septum, nucleus accumbens, and lateral globus pallidus contribute to center zone exploration in the open-field. In addition to elevated plus-maze and open-field tests, the amygdala appears as a target region for CRH-mediated anxiety in the elevated T-maze. Thus, the amygdala is the principal brain region identified with these three tests, and further research must identify the neural circuits underlying this form of anxiety.

Ghrelin O-acyltransferase (GOAT) and energy metabolism

Ghrelin O-acyltransferase (GOAT), a member of MBOATs family, is essential for octanoylation of ghrelin, which is required for active ghrelin to bind with and activate its receptor. GOAT is expressed mainly in the stomach, pancreas and hypothalamus. Levels of GOAT are altered by energy status. GOAT contains 11 transmembrane helices and one reentrant loop. Its invariant residue His-338 and conserved Asn-307 are located in the endoplasmic reticulum lumen and cytosol respectively. GOAT contributes to the regulation of food intake and energy expenditure, as well as glucose and lipids homeostasis. Deletion of GOAT blocks the acylation of ghrelin leading to subsequent impairment in energy homeostasis and survival when mice are challenged with high energy diet or severe caloric restriction. GO-CoA-Tat, a peptide GOAT inhibitor, attenuates acyl-ghrelin production and prevents weight gain induced by a medium-chain triglycerides-rich high fat diet. Further, GO-CoA-Tat increases glucose- induced insulin secretion. Overall, inhibition of GOAT is a novel strategy for treatment of obesity and related metabolic disorders.

Gut microbiota regulates key modulators of social behavior

Social behavior plays a pivotal role in the mental well-being of an individual. Continuous efforts in the past have led to advancements in the area of how the brain regulates emotion and cognition, while the understanding of human social behavior still remains eluded. A major breakthrough in understanding the etiology of neurological disorders is the recent insight on the role of the gut microbiota (GM). Human GM also referred to as the "forgotten organ" is home to 10(13-14) microorganisms, which is 10 times the number of cells present in the human body. In addition, the gut microbiome (total genome of GM) is 150 times greater as compared to the human genome. An emerging concept gaining worldwide focus and acceptance is that, this much big genome can potentially control human behavior and other biological functions. Herein we hypothesize on the basis of GM's ability to modify brain and behavior and that it can directly or indirectly control social behavior. This review focuses on the association of GM with various domains of social behavior like stress, cognition and anxiety.

The effect of ghrelin and estradiol on mean concentration of thyroid hormones

BACKGROUND

Ghrelin is a novel peptide hormone that has GH releasing activity and also other endocrine and metabolic functions. It can also increase food intake and decrease T3 and T4 concentrations. Several parameters of hypothalamic-pituitary-thyroid (HPT) axis function are modulated by 17β-estradiol (E2).

OBJECTIVES

The purpose of this study was to investigate the effect of interactions between ghrelin and estradiol (injected via ICV route) on plasma T3 and T4 concentrations in female rats.

MATERIALS AND METHODS

Eighteen Wistar female rats (bodyweight, 200-250 g) were randomly divided into three groups. Group 1 received estradiol, Group 2 received ghrelin and Group 3 received ghrelin and estradiol. Plasma samples were used to assess T3 and T4 concentration by RIA.

RESULTS

The results indicated that ghrelin significantly decreased thyroid hormone concentrations, whereas estradiol increased these concentrations. The simultaneous injection of ghrelin and estradiol significantly decreased the inhibitory effect of ghrelin on thyroid hormone concentrations (P < 0.05).

CONCLUSIONS

According to the results of this study, both ghrelin and estradiol affect the concentration of thyroid hormone but in opposite directions. This difference might be due to different underlying hormonal mechanisms such as HPA and/or HPT axis melanocyte stimulating hormone (MSH) systems could be suggested.

The impact of microbiota on brain and behavior: mechanisms & therapeutic potential

There is increasing evidence that host-microbe interactions play a key role in maintaining homeostasis. Alterations in gut microbial composition is associated with marked changes in behaviors relevant to mood, pain and cognition, establishing the critical importance of the bi-directional pathway of communication between the microbiota and the brain in health and disease. Dysfunction of the microbiome-brain-gut axis has been implicated in stress-related disorders such as depression, anxiety and irritable bowel syndrome and neurodevelopmental disorders such as autism. Bacterial colonization of the gut is central to postnatal development and maturation of key systems that have the capacity to influence central nervous system (CNS) programming and signaling, including the immune and endocrine systems. Moreover, there is now expanding evidence for the view that enteric microbiota plays a role in early programming and later response to acute and chronic stress. This view is supported by studies in germ-free mice and in animals exposed to pathogenic bacterial infections, probiotic agents or antibiotics. Although communication between gut microbiota and the CNS are not fully elucidated, neural, hormonal, immune and metabolic pathways have been suggested. Thus, the concept of a microbiome-brain-gut axis is emerging, suggesting microbiota-modulating strategies may be a tractable therapeutic approach for developing novel treatments for CNS disorders.

Ghrelin gene products, receptors, and GOAT enzyme: biological and pathophysiological insight

Ghrelin is a 28-amino acid acylated hormone, highly expressed in the stomach, which binds to its cognate receptor (GHSR1a) to regulate a plethora of relevant biological processes, including food intake, energy balance, hormonal secretions, learning, inflammation, etc. However, ghrelin is, in fact, the most notorious component of a complex, intricate regulatory system comprised of a growing number of alternative peptides (e.g. obestatin, unacylated ghrelin, and In1-ghrelin, etc.), known (GHSRs) and, necessarily unknown receptors, as well as modifying enzymes (e.g. ghrelin-O-acyl-transferase), which interact among them as well as with other regulatory systems in order to tightly modulate key (patho)-physiological processes. This multiplicity of functions and versatility of the ghrelin system arise from a dual, genetic and functional, complexity. Importantly, a growing body of evidence suggests that dysregulation in some of the components of the ghrelin system can lead to or influence the development and/or progression of highly concerning pathologies such as endocrine-related tumors, inflammatory/cardiovascular diseases, and neurodegeneration, wherein these altered components could be used as diagnostic, prognostic, or therapeutic targets. In this context, the aim of this review is to integrate and comprehensively analyze the multiple components and functions of the ghrelin system described to date in order to define and understand its biological and (patho)-physiological significance.

Effects of intrahippocampal injection of ghrelin on spatial memory in PTZ-induced seizures in male rats

Ghrelin (gh) is a peptide hormone that may affect learning and memory. There is some evidence that ghrelin can have antiepileptic effects. So we decided to investigate the possible effects of ghrelin on spatial memory following PTZ-induced seizures in male rats. Ninety male rats were divided into 9 groups including control, saline, ghrelin (0.3, 1.5 or 3 nmol) and pentylenetetrazol (PTZ, 50 mg/kg, i.p.) plus saline or ghrelin (0.3, 1.5 or 3 nmol). All groups were trained in Morris water maze (MWM) for two consecutive days. Our results showed that ghrelin significantly improves spatial memory at the doses of 1.5 or 3 nmol (P<0.05) in normal rats. We also demonstrated the significant impairment of spatial memory in PTZ group (P<0.05). Intrahippocampal injection of ghrelin at the dose of 3 nmol significantly improved spatial memory in PTZ+gh group compared to PTZ group (P<0.05). These findings suggest that ghrelin as a neuropeptide can improve spatial memory in PTZ-treated rats.

Voices from within: gut microbes and the CNS

Recent advances in research have greatly increased our understanding of the importance of the gut microbiota. Bacterial colonization of the intestine is critical to the normal development of many aspects of physiology such as the immune and endocrine systems. It is emerging that the influence of the gut microbiota also extends to modulation of host neural development. Furthermore, the overall balance in composition of the microbiota, together with the influence of pivotal species that induce specific responses, can modulate adult neural function, peripherally and centrally. Effects of commensal gut bacteria in adult animals include protection from the central effects of infection and inflammation as well as modulation of normal behavioral responses. There is now robust evidence that gut bacteria influence the enteric nervous system, an effect that may contribute to afferent signaling to the brain. The vagus nerve has also emerged as an important means of communicating signals from gut bacteria to the CNS. Further understanding of the mechanisms underlying microbiome-gut-brain communication will provide us with new insight into the symbiotic relationship between gut microbiota and their mammalian hosts and help us identify the potential for microbial-based therapeutic strategies to aid in the treatment of mood disorders.

Central but not systemic administration of ghrelin induces wakefulness in mice

Ghrelin is a brain-gut peptide hormone widely known for its orexigenic and growth hormone-releasing activities. Findings from our and other laboratories indicate a role of ghrelin in sleep regulation. The effects of exogenous ghrelin on sleep-wake activity in mice are, however, unknown. The aim of the present study was to determine the sleep-modulating effects of ghrelin after central and systemic administrations in mice. Sleep-wake activity after intracerebroventricular (i.c.v.) administration of 0.2, 1 and 5 µg ghrelin and intraperitoneal injections of 40, 100, and 400 µg/kg ghrelin prior to light onset were determined in C57BL/6 mice. In addition, body temperature, motor activity and 1-hour food intake was measured after the systemic injections. Sleep effects of systemic ghrelin (40 and 400 µg/kg) injected before dark onset were also determined. I.c.v. injection of ghrelin increased wakefulness and suppressed non-rapid-eye-movement sleep and electroencephalographic slow-wave activity in the first hour after injections. Rapid-eye-movement sleep was decreased for 2-4 hours after each dose of ghrelin. Sytemic administration of ghrelin did not induce changes in sleep-wake activity in mice at dark or light onset. Motor activity and body temperature remained unaltered and food intake was significantly increased after systemic injections of ghrelin given prior the light period. These findings indicate that the activation of central, but not peripheral, ghrelin-sensitive mechanisms elicits arousal in mice. The results are consistent with the hypothesis that the activation of the hypothalamic neuronal circuit formed by ghrelin, orexin, and neuropeptide Y neurons triggers behavioral sequence characterized by increased wakefulness, motor activity and feeding in nocturnal rodents.

Ghrelin mediates anticipation to a palatable meal in rats

Food anticipatory activity (FAA) is displayed in rats when access to food is restricted to a specific time frame of their circadian phase, a behavior thought to reflect both hunger and the motivation to eat. Rats also display FAA in a feeding schedule with ad libitum access to normal chow, but limited availability of a palatable meal, which is thought to involve mainly motivational aspects. The orexigenic hormone ghrelin has been implicated in FAA in rodents with restricted access to chow. Because ghrelin plays an important role not only in the control of food intake, but also in reward, we sought to determine the role of ghrelin in anticipation to a palatable meal. Plasma ghrelin levels of non-restricted rats that anticipated chocolate correlated positively with FAA and were increased compared with chow-fed control rats. Furthermore, centrally injected ghrelin increased, whereas an antagonist of the ghrelin receptor decreased, the anticipation to chocolate. Therefore, we hypothesize that central ghrelin signaling is able to mediate the motivational drive to eat.

Impaired wake-promoting mechanisms in ghrelin receptor-deficient mice

Ghrelin receptors are expressed by key components of the arousal system. Exogenous ghrelin induces behavioral activation, promotes wakefulness and stimulates eating. We hypothesized that ghrelin-sensitive mechanisms play a role in the arousal system. To test this, we investigated the responsiveness of ghrelin receptor knockout (KO) mice to two natural wake-promoting stimuli. Additionally, we assessed the integrity of their homeostatic sleep-promoting system using sleep deprivation. There was no significant difference in the spontaneous sleep-wake activity between ghrelin receptor KO and wild-type (WT) mice. WT mice mounted robust arousal responses to a novel environment and food deprivation. Wakefulness increased for 6 h after cage change accompanied by increases in body temperature and locomotor activity. Ghrelin receptor KO mice completely lacked the wake and body temperature responses to new environment. When subjected to 48 h food deprivation, WT mice showed marked increases in their waking time during the dark periods of both days. Ghrelin receptor KO mice failed to mount an arousal response on the first night and wake increases were attenuated on the second day. The responsiveness to sleep deprivation did not differ between the two genotypes. These results indicate that the ghrelin-receptive mechanisms play an essential role in the function of the arousal system but not in homeostatic sleep-promoting mechanisms.

The regulation of food intake in mammalian hibernators: a review

One of the most profound hallmarks of mammalian hibernation is the dramatic reduction in food intake during the winter months. Several species of hibernator completely cease food intake (aphagia) for nearly 7 months regardless of ambient temperature and in many cases, whether or not food is available to them. Food intake regulation has been studied in mammals that hibernate for over 50 years and still little is known about the physiological mechanisms that control this important behavior in hibernators. It is well known from lesion experiments in non-hibernators that the hypothalamus is the main brain region controlling food intake and therefore body mass. In hibernators, the regulation of food intake and body mass is presumably governed by a circannual rhythm since there is a clear seasonal rhythm to food intake: animals increase food intake in the summer and early autumn, food intake declines in autumn and actually ceases in winter in many species, and resumes again in spring as food becomes available in the environment. Changes in circulating hormones (e.g., leptin, insulin, and ghrelin), nutrients (glucose, and free fatty acids), and cellular enzymes such as AMP-activated protein kinase (AMPK) have been shown to determine the activity of neurons involved in the food intake pathway. Thus, it appears likely that the food intake pathway is controlled by a variety of inputs, but is also acted upon by upstream regulators that are presumably rhythmic in nature. Current research examining the molecular mechanisms and integration of environmental signals (e.g., temperature and light) with these molecular mechanisms will hopefully shed light on how animals can turn off food intake and survive without eating for months on end.

Peripheral ghrelin stimulates feeding behavior and positive energy balance in a sciurid hibernator

Hibernators exhibit a robust circannual cycle of body mass gain and loss primarily mediated by food intake, but the pathways controlling food intake in these animals have not been fully elucidated. Ghrelin is an orexigenic hormone that increases feeding in all mammals studied so far, but has not until recently been studied in hibernators. In other mammals, ghrelin stimulates feeding through phosphorylation and activation of AMP-activated protein kinase (AMPK). Activation of AMPK phosphorylates and deactivates acetyl Co-A carboxylase (ACC), a committed step in fatty acid synthesis. In order to determine the effects of exogenous ghrelin on food intake and metabolic factors (i.e. non-esterified fatty acids (NEFAs), and hypothalamic AMPK and ACC) in hibernators, ghrelin was peripherally injected into ground squirrels in all four seasons. Changes in food intake and body mass were recorded over a 2-6 hour period post injections, and squirrels were euthanized. Brains and blood were removed, and Western blots were performed to determine changes in phosphorylation of hypothalamic AMPK and ACC. A colorimetric assay was used to determine changes in concentration of serum NEFAs. We found that food intake, body mass, and locomotor activity significantly increased with ghrelin injections versus saline-injected controls, even in animals injected during their aphagic winter season. Injected ghrelin was correlated with increased phosphorylation of AMPK, but didn't have an effect on ACC in winter. Ghrelin-injected animals also had increased levels of serum NEFAs compared with saline controls. This study is the first to show an effect of injected ghrelin on a hibernator.

Ghrelin neutralization during fasting-refeeding cycle impairs the recuperation of body weight and alters hepatic energy metabolism

Ghrelin, a hormone whose levels increase during food deprivation, plays a pivotal role in the regulation of food intake, energy metabolism and storage, as well as in insulin sensitivity. Here, we investigated the effects of acyl-ghrelin neutralization with the acyl-ghrelin-binding compound NOX-B11(2) during the fasting-refeeding cycle. Our data demonstrate that ghrelin neutralization with NOX-B11(2) impairs recuperation of lost body weight after food deprivation. Analysis of enzymes involved in glucose and lipid metabolism in liver of fed, fasted and refed rats revealed that neutralization of acyl-ghrelin resulted in minor decreases in the enzymes of glycolytic and lipogenic pathways during fasting. However, during refeeding these enzymes as well as glycogen levels recovered more slowly when acyl-ghrelin was blocked. The high levels of ghrelin in response to food deprivation may contribute to an adequate decrease in hepatic glycolytic and lipogenic enzymes and aid in the recovery of body weight and energetic reserves once food becomes available after the fasting period.

Persistence of hormonal and metabolic rhythms during fasting in 7- to 9-day-old rabbits entrained by nursing during the night

Rabbit does nurse their litter once every 24h during the night. We hypothesized that corticosterone, ghrelin, leptin, and metabolites such as glucose, liver glycogen, and free fatty acids could be affected in the pups by the time at which does nurse them. Therefore, we measured these parameters in pups nursed at 02:00 h (nighttime for the doe) to compare them with results from a previous study where does nursed at 10:00 h, during daytime. From postnatal day 7, pups were sacrificed either just before their scheduled time of nursing or at 4, 8, 12, 16, or 20 h after nursing (n=6 at each time point); additional pups were sacrificed at 4h intervals between 48 and 72 h after nursing to study the persistence of oscillations during fasting. All pups developed locomotor anticipatory activity to nursing. Corticosterone, ghrelin, and free fatty acids exhibited a rhythm that persisted in fasted pups. Glucose concentrations were lower in fasted than in nursed pups, and glycogen was only detected in nursed subjects. Leptin values were stable and low in nursed subjects but increased significantly in fasted subjects up to 72 h after the expected nursing time. The rhythm of ghrelin persisted during fasting, contrary to our previous findings in pups nursed during daytime (i.e., outside the natural time of nursing for this species). Therefore, in 7-day-old rabbit pups, night nursing is a strong zeitgeber for corticosterone, ghrelin, free fatty acids, and energy metabolites but not for leptin.

The rabbit pup, a natural model of nursing-anticipatory activity

Mother rabbits nurse their young once a day with circadian periodicity. Nursing bouts are brief (ca. 3 min) and occur inside the maternal burrow. Despite this limited contact mother rabbits and their pups are tuned to each other to ensure that the capacities of each party are used efficiently to ensure the weaning of a healthy litter. In this review we present behavioral, metabolic and hormonal correlates of this phenomenon in mother rabbits and their pups. Research is revealing that the circadian rhythm of locomotion shifts in parallel to the timing of nursing in both parties. In pups corticosterone has a circadian rhythm with highest levels at the time of nursing. Other metabolic and hormonal parameters follow an exogenous or endogenous rhythm which is affected by the time of nursing. In the brain, clock genes and their proteins (e.g. Per1) are differentially expressed in specific brain regions (e.g. suprachiasmatic nucleus, paraventricular nucleus) in relation to providing or ingesting milk in mothers and young, respectively. These findings suggest that circadian activities are modulated, in the mothers, by suckling stimulation and, in the young, by the ingestion of milk and/or the perception of the mammary pheromone. In conclusion, the rabbit pup is an extraordinary model for studying the entraining by a single daily food pulse with minimal manipulations. The mother offers the possibility of studying nursing as a non-photic synchronizer, also with minimal manipulation, as suckling stimulation from the litter occurs only once daily.

Neuroregulation of nonexercise activity thermogenesis and obesity resistance

High levels of spontaneous physical activity in lean people and the nonexercise activity thermogenesis (NEAT) derived from that activity appear to protect lean people from obesity during caloric challenge, while obesity in humans is characterized by dramatically reduced spontaneous physical activity. We have similarly demonstrated that obesity-resistant rats have significantly greater spontaneous physical activity than obesity-prone rats, and that spontaneous physical activity predicts body weight gain. Although the energetic cost of activity varies between types of activity and may be regulated, individual level of spontaneous physical activity is important in determining propensity for obesity. We review the current status of knowledge about the brain mechanisms involved in controlling the level of spontaneous physical activity and the NEAT so generated. Focus is on potential neural mediators of spontaneous physical activity and NEAT, including orexin A (also known as hypocretin 1), agouti-related protein, ghrelin, and neuromedin U, in addition to brief mention of neuropeptide Y, corticotrophin releasing hormone, cholecystokinin, estrogen, leptin, and dopamine effects on spontaneous physical activity. We further review evidence that strain differences in orexin stimulation pathways for spontaneous physical activity and NEAT appear to track with the body weight phenotype, thus providing a potential mechanistic explanation for reduced activity and weight gain.

Safety, tolerability and pharmacokinetics of intravenous ghrelin for cancer-related anorexia/cachexia: a randomised, placebo-controlled, double-blind, double-crossover study

Twenty-one adult patients were randomised to receive ghrelin on days 1 and 8 and placebo on days 4 and 11 or vice versa, given intravenously over a 60-min period before lunch: 10 received 2 μg kg⁻¹ (lower-dose) ghrelin; 11 received 8 μg kg⁻¹ (upper-dose) ghrelin. Active and total ghrelin, growth hormone (GH), and insulin-like growth factor 1 levels were monitored at baseline (4–5 days before day 1), during treatment days, and at end of study (day 17/18). Drug-related adverse events (assessed by NCI-CTC-toxicity criteria and cardiac examination) did not differ between ghrelin and placebo. No grade 3/4 toxicity or stimulation of tumour growth was observed. The peak increase of GH, a biological marker of ghrelin action, was 25 ng ml⁻¹ with lower-dose and 42 ng ml⁻¹ with upper-dose ghrelin. Morning fasting total ghrelin levels were higher (P<0.05) for upper-dose patients at end of study (3580 pg ml⁻¹) than at baseline (990 pg ml⁻¹). Insulin-like growth factor 1 levels did not change. At day 8, 81% of patients preferred ghrelin to placebo as against 63% at the end of study. Nutritional intake and eating-related symptoms, measured to explore preliminary efficacy, did not differ between ghrelin and placebo. Ghrelin is well tolerated and safe in patients with advanced cancer. For safety, tolerance, and patients' preference for treatment, no difference was observed between the lower- and upper-dose group.

Neuropeptidergic mediators of spontaneous physical activity and non-exercise activity thermogenesis

Lean individuals have high levels of spontaneous physical activity (SPA) and the energy expenditure derived from that activity, termed non-exercise activity thermogenesis or NEAT, appears to protect them from obesity. Conversely, obesity in different human populations is characterized by low levels of SPA and NEAT. Like in humans, elevated SPA in rats appears to protect against obesity: obesity-resistant rats have significantly greater SPA and NEAT than obesity-prone rats. We review the literature on brain mechanisms important in mediating SPA and NEAT. The focus is on neuropeptides, including cholecystokinin, corticotropin-releasing hormone (also known as corticotropin-releasing factor), neuromedin U, neuropeptide Y, leptin, agouti-related protein, orexin-A (also known as hypocretin-1), and ghrelin. We also review information regarding interactions between these neuropeptides and dopamine, a neurotransmitter important in mediating motor function. Finally, we present evidence that elevated signaling of pathways mediating SPA and NEAT may protect against weight gain and obesity.

Intraventricular ghrelin activates oxytocin neurons: implications in feeding behavior

Ghrelin affects behavioral and physiological responses, such as feeding or the activity of the HPA axis. Distribution of its receptor in central sites involved in neuroendocrine control, including the hypothalamic paraventricular nucleus, indicates that interplay with multiple neuropeptidergic systems underlies ghrelin's actions. We report that intracerebroventricular ghrelin increases c-Fos immunoreactivity of oxytocin neurons in magno and parvocellular portions of the paraventricular nucleus. The orexigenic response to ghrelin administered at the dose that activates oxytocin neurons can be further elevated by pretreatment with a selective oxytocin receptor antagonist. Our data suggest that oxytocin may be responsible for the mediation of some effects induced by ghrelin. Modifications in the activity of the oxytocin system may alter some of these effects.

[Some neurological and psychiatric complications of the disorders of the hypothalamo-hypophyseal system]

Connection between the central nervous system and the endocrine system is extremely complex. The hypothalamus serves as a crucial centre for the integration and coordination of autonomic functions by neuronal and hormonal pathways. It plays a central role in the homeostatic regulation of internal physiological conditions. It controls growth and reproduction, stress reactions, and determines rhythmicity, periodicity and timing of physiological processes. Beside its well-known functions, antidiuretic hormone has a role in social behavior as it enhances aggression via vasopressin receptor 1A. Oxitocin is affected in the formation of maternal behavior, and in other social interactions, like the pair bounding, as well as in analgesia and pain modulation. The corticotrop-releasing hormone acts as a neurotransmitter, it has a special role in stress-behavior, anxiety, and depression, and it blocks deep sleeping. Among the neurotransmitters and neuropeptids of the hypothalamus, serotonin, norepinephrine, GABA, cholecystokinin, neuropeptide-Y, Agouti-related protein, alpha-MSH and ghrelin have essential importance in the eating disorders. The levels of leptin and galanin determine whether formation of anabolic or catabolic neurotransmitters should take place. In the thermoregulation the central thermoreceptors play role, and suprachiasmatic nucleus is responsible for circadian rhythm, through "timing genes". The diseases of the hypothalamus cause most frequently bulimia or anorexia, hypersomnia, impotency, and attacks of anxiety. The most common expansive process of the hypothalamus is craniopharyngioma. The lack or diminution of vasopressin causes diabetes insipidus, while inappropriate antidiuretic hormone secretion induces Schwartz-Barter syndrome. Fröhlich-, Kleine-Levin- or Prader-Willi syndromes have characteristic neuropsychiatric features. The main psychiatric symptom of hypopituitarism is a combination of dementia and delirium. The most characteristic neurological sign of pituitary adenoma is the visual field defect. Carpal tunnel syndrome, obstructive sleeping apnoe and headache are typical neurological features in somatotrop adenomas.

Ghrelin, the peripheral hunger hormone

In the current review we summarize the available data concerning the gastric hormone ghrelin and its receptor. Ghrelin stimulates short-term food intake and long-term body weight regulation via its adipogenic and diabetogenic effects. Ghrelin stimulates gastric emptying, and these effects could be explored from a therapeutic point of view. Ghrelin levels change profoundly in anorexia, in states of insulin resistance, in obesity, and after bariatric surgery, suggesting that this is an important hormone in body weight regulation.

Heterogeneity of ghrelin/growth hormone secretagogue receptors. Toward the understanding of the molecular identity of novel ghrelin/GHS receptors

Ghrelin is a gastric polypeptide displaying strong GH-releasing activity by activation of the type 1a GH secretagogue receptor (GHS-R1a) located in the hypothalamus-pituitary axis. GHS-R1a is a G-protein-coupled receptor that, upon the binding of ghrelin or synthetic peptidyl and non-peptidyl ghrelin-mimetic agents known as GHS, preferentially couples to G(q), ultimately leading to increased intracellular calcium content. Beside the potent GH-releasing action, ghrelin and GHS influence food intake, gut motility, sleep, memory and behavior, glucose and lipid metabolism, cardiovascular performances, cell proliferation, immunological responses and reproduction. A growing body of evidence suggests that the cloned GHS-R1a alone cannot be the responsible for all these effects. The cloned GHS-R1b splice variant is apparently non-ghrelin/GHS-responsive, despite demonstration of expression in neoplastic tissues responsive to ghrelin not expressing GHS-R1a; GHS-R1a homologues sensitive to ghrelin are capable of interaction with GHS-R1b, forming heterodimeric species. Furthermore, GHS-R1a-deficient mice do not show evident abnormalities in growth and diet-induced obesity, suggesting the involvement of another receptor. Additional evidence of the existence of another receptor is that ghrelin and GHS do not always share the same biological activities and activate a variety of intracellular signalling systems besides G(q). The biological actions on the heart, adipose tissue, pancreas, cancer cells and brain shared by ghrelin and the non-acylated form of ghrelin (des-octanoyl ghrelin), which does not bind GHS-R1a, represent the best evidence for the existence of a still unknown, functionally active binding site for this family of molecules. Finally, located in the heart and blood vessels is the scavenger receptor CD36, involved in the endocytosis of the pro-atherogenic oxidized low-density lipoproteins, which is a pharmacologically and structurally distinct receptor for peptidyl GHS and not for ghrelin. This review highlights the most recently discovered features of GHS-R1a and the emerging evidence for a novel group of receptors that are not of the GHS1a type; these appear involved in the transduction of the multiple levels of information provided by GHS and ghrelin.

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.