Pulsatile cerebrospinal fluid and plasma ghrelin in relation to growth hormone secretion and food intake in the sheep

The low ratio of ghrelin in plasma and cerebrospinal fluid might be beneficial to sleep

OBJECTIVE

Ghrelin is physiologically important for maintaining sleep rhythm. Cigarette smoking has been demonstrated to significantly increase the risk of insufficient sleep by regulating ghrelin at the central and peripheral levels. No research has been published to study the relationship between active smoking and sleep via ghrelin level in cerebrospinal fluid (CSF).

METHODS

A total of 139 Chinese males were recruited and divided into active smokers (n = 77) and non-smokers (n = 62). The levels of CSF and plasma ghrelin were measured. The Pittsburgh Sleep Quality Index (PSQI) was used to evaluate sleep.

RESULTS

Non-smokers had lower PSQI scores (1.71 ± 1.93) than active smokers (3.70 ± 1.78). Non-smokers have significantly lower plasma ghrelin levels and lower plasma/CSF ghrelin ratio but higher CSF ghrelin than active smokers. Among non-smokers, plasma ghrelin levels were not correlated with PSQI scores (all p > 0.05), CSF ghrelin levels were positively correlated with PSQI scores (r = 0.309, p = 0.019), and the plasma/CSF ghrelin ratio was negatively correlated with PSQI scores (r = -0.346, p = 0.008).

CONCLUSIONS

This study is the first to reveal the relationship between cigarette smoking, high CSF ghrelin levels and insufficient sleep, suggesting that maintaining a normal plasma/CSF ghrelin ratio may be the physiological mechanism of healthy sleep, and the insufficient sleep population must quit smoking.

Therapeutic potential of GHSR-1A antagonism in alcohol dependence, a review

Growth hormone secretagogue receptor type 1A (GHSR-1A) is a functional receptor of orexigenic peptide ghrelin and is highly expressed in mesolimbic dopaminergic systems that regulate incentive value of artificial reward in substance abuse. Interestingly, GHSR-1A has also shown ligand-independent constitutive activity. Alcohol use disorder (AUD) is one of the growing concerns worldwide as it involves complex neuro-psycho-endocrinological interactions. Positive correlation of acylated ghrelin and alcohol-induced human brain response in the right and left ventral striatum are evident. In the last decade, the beneficial effects of ghrelin receptor (GHSR-1A) antagonism to suppress artificial reward circuitries and induce self-control for alcohol consumption have drawn significant attention from researchers. In this updated review, we summarize the available recent preclinical, clinical, and experimental data to discuss functional, molecular actions of central ghrelin-GHSR-1A signaling in different craving levels for alcohol as well as to promote "GHSR-1A antagonism" as one of the potential therapies in early abstinence.

An Overview of Appetite-Regulatory Peptides in Addiction Processes; From Bench to Bed Side

There is a substantial need for new pharmacological treatments of addiction, and appetite-regulatory peptides are implied as possible candidates. Appetite regulation is complex and involves anorexigenic hormones such as glucagon-like peptide-1 (GLP-1) and amylin, and orexigenic peptides like ghrelin and all are well-known for their effects on feeding behaviors. This overview will summarize more recent physiological aspects of these peptides, demonstrating that they modulate various aspects of addiction processes. Findings from preclinical, genetic, and experimental clinical studies exploring the association between appetite-regulatory peptides and the acute or chronic effects of addictive drugs will be introduced. Short or long-acting GLP-1 receptor agonists independently attenuate the acute rewarding properties of addictive drugs or reduce the chronic aspects of drugs. Genetic variation of the GLP-1 system is associated with alcohol use disorder. Also, the amylin pathway modulates the acute and chronic behavioral responses to addictive drugs. Ghrelin has been shown to activate reward-related behaviors. Moreover, ghrelin enhances, whereas pharmacological or genetic suppression of the ghrelin receptor attenuates the responses to various addictive drugs. Genetic studies and experimental clinical studies further support the associations between ghrelin and addiction processes. Further studies should explore the mechanisms modulating the ability of appetite-regulatory peptides to reduce addiction, and the effects of combination therapies or different diets on substance use are warranted. In summary, these studies provide evidence that appetite-regulatory peptides modulate reward and addiction processes, and deserve to be investigated as potential treatment target for addiction.

Targeting the Ghrelin Receptor as a Novel Therapeutic Option for Epilepsy

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

Circulating ghrelin crosses the blood-cerebrospinal fluid barrier via growth hormone secretagogue receptor dependent and independent mechanisms

Ghrelin is a peptide hormone mainly secreted from gastrointestinal tract that acts via the growth hormone secretagogue receptor (GHSR), which is highly expressed in the brain. Strikingly, the accessibility of ghrelin to the brain seems to be limited and restricted to few brain areas. Previous studies in mice have shown that ghrelin can access the brain via the blood-cerebrospinal fluid (CSF) barrier, an interface constituted by the choroid plexus and the hypothalamic tanycytes. Here, we performed a variety of in vivo and in vitro studies to test the hypothesis that the transport of ghrelin across the blood-CSF barrier occurs in a GHSR-dependent manner. In vivo, we found that the uptake of systemically administered fluorescent ghrelin in the choroid plexus epithelial (CPE) cells and in hypothalamic tanycytes depends on the presence of GHSR. Also, we detected lower levels of CSF ghrelin after a systemic ghrelin injection in GHSR-deficient mice, as compared to WT mice. In vitro, the internalization of fluorescent ghrelin was reduced in explants of choroid plexus from GHSR-deficient mice, and unaffected in primary cultures of hypothalamic tanycytes derived from GHSR-deficient mice. Finally, we found that the GHSR mRNA is detected in a pool of CPE cells, but is nearly undetectable in hypothalamic tanycytes with current approaches. Thus, our results suggest that circulating ghrelin crosses the blood-CSF barrier mainly by a mechanism that involves the GHSR, and also possibly via a GHSR-independent mechanism.

Brain accessibility delineates the central effects of circulating ghrelin

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

The actions of ghrelin in the paraventricular nucleus: energy balance and neuroendocrine implications

Ghrelin is a peptide mainly produced and secreted by the stomach. Since its discovery, the impact of ghrelin on the regulation of food intake has been the most studied function of this hormone; however, ghrelin affects a wide range of physiological systems, many of which are controlled by the hypothalamic paraventricular nucleus (PVN). Several pathways may mediate the effects of ghrelin on PVN neurons, such as direct or indirect effects mediated by circumventricular organs and/or the arcuate nucleus. The ghrelin receptor is expressed in PVN neurons, and the peripheral or intracerebroventricular administration of ghrelin affects PVN neuronal activity. Intra-PVN application of ghrelin increases food intake and decreases fat oxidation, which chronically contribute to the increased adiposity. Additionally, ghrelin modulates the neuroendocrine axes controlled by the PVN, increasing the release of vasopressin and oxytocin by magnocellular neurons and corticotropin-releasing hormone by neuroendocrine parvocellular neurons, while possibly inhibiting the release of thyrotropin-releasing hormone. Thus, the PVN is an important target for the actions of ghrelin. Our review discusses the mechanisms of ghrelin actions in the PVN, and its potential implications for energy balance, neuroendocrine, and integrative physiological control.

Gut-brain axis and addictive disorders: A review with focus on alcohol and drugs of abuse

Due to the limited efficacy of existing medications for addictive disorders including alcohol use disorder (AUD), the need for additional medications is substantial. Potential new medications for addiction can be identified through investigation of the neurochemical substrates mediating the ability of drugs of abuse such as alcohol to activate the mesolimbic dopamine system. Interestingly, recent studies implicate neuropeptides of the gut-brain axis as modulators of reward and addiction processes. The present review therefore summarizes the current studies investigating the ability of the gut-brain peptides ghrelin, glucagon-like peptide-1 (GLP-1), amylin and neuromedin U (NMU) to modulate alcohol- and drug-related behaviors in rodents and humans. Extensive literature demonstrates that ghrelin, the only known orexigenic neuropeptide to date, enhances reward as well as the intake of alcohol, and other drugs of abuse, while ghrelin receptor antagonism has the opposite effects. On the other hand, the anorexigenic peptides GLP-1, amylin and NMU independently inhibits reward from alcohol and drugs of abuse in rodents. Collectively, these rodent and human studies imply that central ghrelin, GLP-1, amylin and NMU signaling may contribute to addiction processes. Therefore, the need for randomized clinical trials investigating the effects of agents targeting these aforementioned systems on drug/alcohol use is substantial.

Clarifying the Ghrelin System's Ability to Regulate Feeding Behaviours Despite Enigmatic Spatial Separation of the GHSR and Its Endogenous Ligand

Ghrelin is a hormone predominantly produced in and secreted from the stomach. Ghrelin is involved in many physiological processes including feeding, the stress response, and in modulating learning, memory and motivational processes. Ghrelin does this by binding to its receptor, the growth hormone secretagogue receptor (GHSR), a receptor found in relatively high concentrations in hypothalamic and mesolimbic brain regions. While the feeding and metabolic effects of ghrelin can be explained by the effects of this hormone on regions of the brain that have a more permeable blood brain barrier (BBB), ghrelin produced within the periphery demonstrates a limited ability to reach extrahypothalamic regions where GHSRs are expressed. Therefore, one of the most pressing unanswered questions plaguing ghrelin research is how GHSRs, distributed in brain regions protected by the BBB, are activated despite ghrelin’s predominant peripheral production and poor ability to transverse the BBB. This manuscript will describe how peripheral ghrelin activates central GHSRs to encourage feeding, and how central ghrelin synthesis and ghrelin independent activation of GHSRs may also contribute to the modulation of feeding behaviours.

Development of Fluorinated Non-Peptidic Ghrelin Receptor Ligands for Potential Use in Molecular Imaging

The ghrelin receptor (GhrR) is a widely investigated target in several diseases. However, the current knowledge of its role and distribution in the brain is limited. Recently, the small and non-peptidic compound (S)-6-(4-bromo-2-fluorophenoxy)-3-((1-isopropylpiperidin-3-yl)methyl)-2-methylpyrido[3,2-d]pyrimidin-4(3H)-one ((S)-9) has been described as a GhrR ligand with high binding affinity. Here, we describe the synthesis of fluorinated derivatives, the in vitro evaluation of their potency as partial agonists and selectivity at GhrRs, and their physicochemical properties. These results identified compounds (S)-9, (R)-9, and (S)-16 as suitable parent molecules for ¹⁸F-labeled positron emission tomography (PET) radiotracers to enable future investigation of GhrR in the brain.

Is Ghrelin Synthesized in the Central Nervous System?

Ghrelin is an octanoylated peptide that acts via its specific receptor, the growth hormone secretagogue receptor type 1a (GHSR-1a), and regulates a vast variety of physiological functions. It is well established that ghrelin is predominantly synthesized by a distinct population of endocrine cells located within the gastric oxyntic mucosa. In addition, some studies have reported that ghrelin could also be synthesized in some brain regions, such as the hypothalamus. However, evidences of neuronal production of ghrelin have been inconsistent and, as a consequence, it is still as a matter of debate if ghrelin can be centrally produced. Here, we provide a comprehensive review and discussion of the data supporting, or not, the notion that the mammalian central nervous system can synthetize ghrelin. We conclude that no irrefutable and reproducible evidence exists supporting the notion that ghrelin is synthetized, at physiologically relevant levels, in the central nervous system of adult mammals.

Seasonal and parity effects on ghrelin levels throughout the estrous cycle in dairy cows

In dairy cows, heat stress depresses appetite, leading to decreased food intake, a negative energy balance, and modifies ghrelin levels. Ghrelin is a gut-brain peptide with two major forms: acylated, with an O-n-octanoylated serine in position 3, and nonacylated. To date, the effect of heat stress and estrous cycle on ghrelin secretion in dairy cows has not been studied. We characterized ghrelin secretion during the estrous cycle in each, the winter and the summer seasons. We further examined the effects of parity on ghrelin secretion. Blood was collected from 10 primiparous or multiparous Israeli-Holstein dairy cows throughout the estrous cycle, in both, the hot and cold seasons. The levels of acylated and total ghrelin were measured in the blood samples. We found that both acylated and total ghrelin levels during heat stress were lower than their respective levels in the winter in both, primiparous and multiparous cows. No differences in acylated and total ghrelin levels were found between primiparous and multiparous cows in both seasons. We further found that in multiparous but not primiparous cows acylated ghrelin secretion oscillated during the estrous cycle in both seasons. Its levels peaked on the last days of the first follicular wave and on the days before and during ovulation. Interestingly, we found that elevated acylated ghrelin levels correlated with conception success and increased total ghrelin levels were associated with successful conception from first insemination. Our data is the first to demonstrate seasonal variation in ghrelin secretion. This study provides evidence for the yet unfamiliar link between heat stress, ghrelin and fertility. Increased circulating acylated ghrelin may contribute to improved fertility in dairy cows. It further raises the possibility of a link between ghrelin levels and successful inseminations. Further research is required to determine the effects of ghrelin on dairy cow performance.

New concepts of the central control of reproduction, integrating influence of stress, metabolic state, and season

Gonadotropin releasing hormone is the primary driver of reproductive function and pulsatile GnRH secretion from the brain causes the synthesis and secretion of LH and FSH from the pituitary gland. Recent work has revealed that the secretion of GnRH is controlled at the level of the GnRH secretory terminals in the median eminence. At this level, projections of kisspeptin cells from the arcuate nucleus of the hypothalamus are seen to be closely associated with fibers and terminals of GnRH cells. Direct application of kisspeptin into the median eminence causes release of GnRH. The kisspeptin cells are activated at the time of a natural "pulse" secretion of GnRH, as reflected in the secretion of LH. This appears to be due to input to the kisspeptin cells from glutamatergic cells in the basal hypothalamus, indicating that more than 1 neural element is involved in the secretion of GnRH. Because the GnRH secretory terminals are outside the blood-brain barrier, factors such as kisspeptin may be administered systemically to cause GnRH secretion; this offers opportunities for manipulation of the reproductive axis using factors that do not cross the blood-brain barrier. In particular, kisspeptin or analogs of the same may be used to activate reproduction in the nonbreeding season of domestic animals. Another brain peptide that influences reproductive function is gonadotropin inhibitory hormone (GnIH). Work in sheep shows that this peptide acts on GnRH neuronal perikarya, but projections to the median eminence also allow secretion into the hypophysial portal blood and action of GnIH on pituitary gonadotropes. GnIH cells are upregulated in anestrus, and infusion of GnIH can block the ovulatory surge in GnRH and/or LH secretion. Metabolic status may also affect the secretion of reproduction, and this could involve action of gut peptides and leptin. Neuropeptide Y and Y-receptor ligands have a negative impact on reproduction, and Neuropeptide Y production is markedly increased in negative energy balance; this may be the cause of lowered GnRH and gonadotropin secretion in this state. There is a complex interaction between appetite-regulating peptide neurons and kisspeptin neurons that enables the former to regulate the latter both positively and negatively. In terms of how GnRH secretion is reduced during stress, recent data indicate that GnIH cells are integrally involved, with increased input to the GnRH cells. The secretion of GnIH into the portal blood is not increased during stress, so the negative effect is most likely effected at the level of GnRH neuronal cell bodies.

Hippocampal Dopamine/DRD1 Signaling Dependent on the Ghrelin Receptor

The ghrelin receptor (GHSR1a) and dopamine receptor-1 (DRD1) are coexpressed in hippocampal neurons, yet ghrelin is undetectable in the hippocampus; therefore, we sought a function for apo-GHSR1a. Real-time single-molecule analysis on hippocampal neurons revealed dimerization between apo-GHSR1a and DRD1 that is enhanced by DRD1 agonism. In addition, proximity measurements support formation of preassembled apo-GHSR1a:DRD1:Gαq heteromeric complexes in hippocampal neurons. Activation by a DRD1 agonist produced non-canonical signal transduction via Gαq-PLC-IP3-Ca(2+) at the expense of canonical DRD1 Gαs cAMP signaling to result in CaMKII activation, glutamate receptor exocytosis, synaptic reorganization, and expression of early markers of hippocampal synaptic plasticity. Remarkably, this pathway is blocked by genetic or pharmacological inactivation of GHSR1a. In mice, GHSR1a inactivation inhibits DRD1-mediated hippocampal behavior and memory. Our findings identify a previously unrecognized mechanism essential for DRD1 initiation of hippocampal synaptic plasticity that is dependent on GHSR1a, and independent of cAMP signaling.

A ghrelin receptor (GHS-R1A) antagonist attenuates the rewarding properties of morphine and increases opioid peptide levels in reward areas in mice

Gut-brain hormones such as ghrelin have recently been suggested to have a role in reward regulation. Ghrelin was traditionally known to regulate food intake and body weight homoeostasis. In addition, recent work has pin-pointed that this peptide has a novel role in drug-induced reward, including morphine-induced increase in the extracellular levels of accumbal dopamine in rats. Herein the effect of the ghrelin receptor (GHS-R1A) antagonist, JMV2959, on morphine-induced activation of the mesolimbic dopamine system was investigated in mice. In addition, the effects of JMV2959 administration on opioid peptide levels in reward related areas were investigated. In the present series of experiment we showed that peripheral JMV2959 administration, at a dose with no effect per se, attenuates the ability of morphine to cause locomotor stimulation, increase the extracellular levels of accumbal dopamine and to condition a place preference in mice. JMV2959 administration significantly increased tissue levels of Met-enkephalin-Arg(6)Phe(7) in the ventral tegmental area, dynorphin B in hippocampus and Leu-enkephalin-Arg(6) in striatum. We therefore hypothesise that JMV2959 prevents morphine-induced reward via stimulation of delta receptor active peptides in striatum and ventral tegmental areas. In addition, hippocampal peptides that activate kappa receptor may be involved in JMV2959׳s ability to regulate memory formation of reward. Given that development of drug addiction depends, at least in part, of the effects of addictive drugs on the mesolimbic dopamine system the present data suggest that GHS-R1A antagonists deserve to be elucidated as novel treatment strategies of opioid addiction.

Impact of visual, olfactory, and auditory cues on circulating concentrations of ghrelin in wethers

Ghrelin is a hormone that stimulates feed intake and regulates energy homeostasis. A link has been observed in sheep, in which simulated feedings at scheduled meal times resulted in an increase in ghrelin concentrations. The present study sought to characterize the effect of feeding cues outside of scheduled meal times on circulating ghrelin concentrations in sheep. Katahdin wethers (age 201 ± 4.9 d; weight 35 ± 1.2 kg) were not offered feed (CONT; = 5), offered 275 g of feed (FED; = 5), or fitted with a muzzle and offered 275 g of feed (SHAM; = 5) during the sampling period, which began 2.5 h after normally scheduled daily feeding time. Blood samples were collected via jugular catheter every 15 min for 2.5 h. Feed was offered for 15 min 0.5 h after the start of blood sampling. The CONT samples were collected on d 1, and FED and SHAM samples were collected on d 2. The active ghrelin present in the plasma was then analyzed by RIA. After the Shapiro-Wilk W goodness of fit test demonstrated that 1 SHAM wether was an outlier and it was removed, data were tested for effect of treatment (FED, SHAM, or CONT), time, and treatment × time interaction using procedures for repeated measures with JMP Software (SAS Inst. Inc., Cary, NC). There was no treatment or time effect ( > 0.05); however, there was a treatment × time interaction on plasma ghrelin concentrations ( = 0.0028) such that ghrelin concentrations in SHAM wethers were greater than in CONT wethers 15, 60, and 90 min after feeding, whereas ghrelin concentrations in SHAM wethers were greater than those in FED wethers 30, 60, 90, and 120 min after feeding ( < 0.05). Within the SHAM treatment, ghrelin concentrations were greater at 15 min than at -30 min. Moreover, ghrelin concentrations within the FED treatment were greater at -30 min than at 30, 45, 60, 90, 105, and 120 min and at -15 min than at 15 through 120 min. The area under the curve representing circulating concentrations of ghrelin in CONT, FED, and SHAM treatments, determined using the trapezoidal method, yielded a treatment effect with a tendency toward significance ( = 0.0866). These results indicate plasma ghrelin concentrations in scheduled meal-fed wethers are elevated following visual, olfactory, and auditory feeding cues outside of scheduled feeding times.

The number of preproghrelin mRNA expressing cells is increased in mice with activity-based anorexia

Plasma levels of ghrelin, an orexigenic peptide, are increased during conditions of chronic starvation, such as in patients with anorexia nervosa. However, it is not known whether such increase can be related to the number of preproghrelin mRNA-expressing cells in the stomach, and if chronic starvation may activate a tentative central ghrelin production. In this work, in situ hybridization technique was used to analyze the presence and number of preproghrelin mRNA-expressing cells in the stomach and the hypothalamus of mice with activity-based anorexia (ABA) induced by the combination of running wheel activity with progressive, during 10 days, feeding-time restriction (FTR) and compared with sedentary FTR, ABA pair-fed (PF) and ad libitum-fed control mice. All food-restricted mice lost more than 20% of body weight. Body weight loss was similar in ABA and PF mice, but it was more pronounced than in FTR mice. Food intake was also lower in ABA than in FTR mice. Preproghrelin mRNA-expressing cells in the stomach were increased proportionally to the body weight loss in all food-restricted groups with the highest number in ABA mice. No preproghrelin mRNA-producing cells were detectable in the hypothalamus of either control or food-restricted mice. Thus, the increased number of gastric preproghrelin mRNA-producing cells during chronic starvation proportionally to the body weight loss and reduced food intake may underlie increased plasma ghrelin. Hyperactivity-induced anorexia appears to further increase the number of preproghrelin mRNA-producing cells in the stomach. No evidence was found for ghrelin expression in the hypothalamus, not even in any of the present experimental models.

Behavioral and endocrine evaluation of the stressfulness of single-pen housing compared to group-housing and social isolation conditions

Regulation of neuroendocrine responses is often studied in animals housed indoors in individual contiguous pens. In sheep, these housing conditions are used to control the environment, facilitate biological sampling and limit social stress. However, this type of housing also prevents exploratory behaviors and could induce stereotypies, non-compliant with welfare and possibly associated with a state of stress. In this context, we investigated the impact of housing in a single-pen, with other familiar conspecifics, on emotional state by evaluating behavioral, hormonal and neuronal measures in adult ewes. We hypothesized that emotional state would be more negative in animals housed in a single-pen for one week (Pen) than in freely moving animals (Free) but less negative than in socially isolated subjects (Isol). We tested our hypothesis in ovariectomized ewes to avoid the interaction with sexual steroid variations. Our behavioral, endocrine and neuronal (Fos activation of the corticotropin-releasing hormone neurons in the paraventricular nucleus of the hypothalamus) measures confirmed that withdrawing familiar conspecifics was sufficient to induce strong stress responses in Isol ewes, but there was no indication that Pen ewes were stressed. However, the latter showed less mastication activity than Free ewes, probably due to limited accessibility to straw. The highest plasma prolactin levels were observed in Isol and Free animals, which might result from stress and physical activity, respectively. In Free ewes, plasma dopamine was low, consistent with its inhibitory control of prolactin. However, Isol animals had both high levels of prolactin and dopamine, suggesting a dysregulated balance in socially stressed ewes. As in other species, we suggest that the regulation of prolactin by dopamine varies with stress and/or social context. Overall, this study shows that the impact of housing conditions on different neuroendocrine systems should be considered more in the future.

Ghrelin in psychiatric disorders - A review

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

Divergent neuronal circuitries underlying acute orexigenic effects of peripheral or central ghrelin: critical role of brain accessibility

Ghrelin is an octanoylated peptide hormone that potently and rapidly increases food intake. The orexigenic action of ghrelin involves the hypothalamic arcuate nucleus (ARC), which is accessible to plasma ghrelin and expresses high levels of the ghrelin receptor. Local administration of ghrelin in a variety of other brain nuclei also increases food intake. It is currently unclear, however, whether these non-ARC ghrelin brain targets are impacted by physiological increases of plasma ghrelin. Thus, the present study aimed to clarify which ghrelin brain targets participate in the short-term orexigenic actions of ghrelin. First, c-Fos induction into mouse brains centrally or peripherally treated with ghrelin was analysed. It was confirmed that peripherally administered ghrelin dose-dependently increases food intake and mainly activates c-Fos in ARC neurones. By contrast, centrally administered ghrelin activates c-Fos in a larger number of brain nuclei. To determine which nuclei are directly accessible to ghrelin, mice were centrally or peripherally injected with a fluorescent ghrelin tracer. It was found that peripherally injected tracer mainly accesses the ARC, whereas centrally injected tracer reaches most brain areas known to express ghrelin receptors. Subsequently, the effects of ghrelin were tested in ARC-ablated mice and it was found that these mice failed to increase food intake in response to peripherally administered ghrelin but fully responded to centrally administered ghrelin. ARC-ablated mice showed patterns of ghrelin-induced c-Fos expression similar to those seen in control mice with the exception of the ARC, where no c-Fos was found. Thus, peripheral ghrelin mainly accesses the ARC, which is required for the orexigenic effects of the hormone. Central ghrelin accesses a variety of nuclei, which can mediate the orexigenic effects of the hormone, even in the absence of an intact ARC.

Nicotine administration in healthy non-smokers reduces appetite but does not alter plasma ghrelin

OBJECTIVE

We studied whether suppressed secretion of the orexigenic peptide ghrelin might be involved in the anorexigenic effects of nicotine.

METHODS

Fifty healthy non-smokers chewed gums containing 2 mg nicotine, or no nicotine in a double-blind randomised crossover design in two independent studies.

RESULTS

Plasma nonacylated ghrelin was not significantly affected by nicotine after 30 and 60 min. Increased blood pressure and decreased appetite ratings confirmed a biological nicotine effect.

CONCLUSIONS

These results do not support a key role of peripheral ghrelin secretion in weight changes related to smoking or smoking cessation, but do not rule out that central nervous system ghrelin is involved.

Increased plasma ghrelin suppresses insulin release in wethers fed with a high-protein diet

Ghrelin is a multifunctional peptide that promotes an increase of food intake and stimulates GH secretion. Ghrelin secretion is regulated by nutritional status and nutrients. Although a high-protein (HP) diet increases plasma ghrelin secretion in mammals, the mechanisms and the roles of the elevated ghrelin concentrations due to a HP diet have not been fully established. To clarify the roles of elevated acylated ghrelin upon intake of a HP diet, we investigated the regulation of ghrelin concentrations in plasma and tissues in wethers fed with either the HP diet or the control (CNT) diet for 14 days, and examined the action of the elevated plasma ghrelin by using a ghrelin-receptor antagonist. The HP diet gradually increased the plasma acylated-ghrelin concentrations, but the CNT diet did not. Although the GH concentrations did not vary significantly across the groups, an injection of ghrelin-receptor antagonist enhanced insulin levels in circulation in the HP diet group. In the fundus region of the stomach, the ghrelin levels did not differ between the HP and CNT diet groups, whereas ghrelin O-acyltransferase mRNA levels were higher in the group fed with HP diet than those of the CNT diet group were. These results indicate that the HP diet elevated the plasma ghrelin levels by increasing its synthesis; this elevation strongly suppresses the appearance of insulin in the circulation of wethers, but it is not involved in GH secretion. Overall, our findings indicate a role of endogenous ghrelin action in secretion of insulin, which acts as a regulator after the consumption of a HP diet.

Interface between metabolic balance and reproduction in ruminants: focus on the hypothalamus and pituitary

This article is part of a Special Issue "Energy Balance". The interface between metabolic regulators and the reproductive system is reviewed with special reference to the sheep. Even though sheep are ruminants with particular metabolic characteristics, there is a broad consensus across species in the way that the reproductive system is influenced by metabolic state. An update on the neuroendocrinology of reproduction indicates the need to account for the way that kisspeptin provides major drive to gonadotropin releasing hormone (GnRH) neurons and also mediates the feedback effects of gonadal steroids. The way that kisspeptin function is influenced by appetite regulating peptides (ARP) is considered. Another newly recognised factor is gonadotropin inhibitory hormone (GnIH), which has a dual function in that it suppresses reproductive function whilst also acting as an orexigen. Our understanding of the regulation of food intake and energy expenditure has expanded exponentially in the last 3 decades and historical perspective is provided. The function of the regulatory factors and the hypothalamic cellular systems involved is reviewed with special reference to the sheep. Less is known of these systems in the cow, especially the dairy cow, in which a major fertility issue has emerged in parallel with selection for increased milk production. Other endocrine systems--the hypothalamo-pituitary-adrenal axis, the growth hormone (GH) axis and the thyroid hormones--are influenced by metabolic state and are relevant to the interface between metabolic function and reproduction. Special consideration is given to issues such as season and lactation, where the relationship between metabolic hormones and reproductive function is altered.

Apo-Ghrelin Receptor (apo-GHSR1a) Regulates Dopamine Signaling in the Brain

The orexigenic peptide hormone ghrelin is synthesized in the stomach and its receptor growth hormone secretagogue receptor (GHSR1a) is expressed mainly in the central nervous system (CNS). In this review, we confine our discussion to the physiological role of GHSR1a in the brain. Paradoxically, despite broad expression of GHSR1a in the CNS, other than trace amounts in the hypothalamus, ghrelin is undetectable in the brain. In our efforts to elucidate the function of the ligand-free ghrelin receptor (apo-GHSR1a), we identified subsets of neurons that co-express GHSR1a and dopamine receptors. In this review, we focus on interactions between apo-GHSR1a and dopamine-2 receptor (DRD2) and formation of GHSR1a:DRD2 heteromers in hypothalamic neurons that regulate appetite, and discuss implications for the treatment of Prader-Willi syndrome (PWS). GHSR1a antagonists of distinct chemical structures, a quinazolinone and a triazole, respectively, enhance and inhibit dopamine signaling through GHSR1a:DRD2 heteromers by an allosteric mechanism. This finding illustrates a potential strategy for designing the next generation of drugs for treating eating disorders as well as psychiatric disorders caused by abnormal dopamine signaling. Treatment with a GHSR1a antagonist that enhances dopamine/DRD2 activity in GHSR1a:DRD2 expressing hypothalamic neurons has the potential to inhibit the uncontrollable hyperphagia associated with PWS. DRD2 antagonists are prescribed for treating schizophrenia, but these block dopamine signaling in all DRD2 expressing neurons and are associated with adverse side effects, including enhanced appetite and excessive weight gain. A GHSR1a antagonist of structural class that allosterically blocks dopamine/DRD2 action in GHSR1a:DRD2 expressing neurons would have no effect on neurons expressing DRD2 alone; therefore, the side effects of DRD2 antagonists would potentially be reduced thereby enhancing patient compliance.

Structure and physiological actions of ghrelin

Ghrelin is a gastric peptide hormone, discovered as being the endogenous ligand of growth hormone secretagogue receptor. Ghrelin is a 28 amino acid peptide presenting a unique n-octanoylation modification on its serine in position 3, catalyzed by ghrelin O-acyl transferase. Ghrelin is mainly produced by a subset of stomach cells and also by the hypothalamus, the pituitary, and other tissues. Transcriptional, translational, and posttranslational processes generate ghrelin and ghrelin-related peptides. Homo- and heterodimers of growth hormone secretagogue receptor, and as yet unidentified receptors, are assumed to mediate the biological effects of acyl ghrelin and desacyl ghrelin, respectively. Ghrelin exerts wide physiological actions throughout the body, including growth hormone secretion, appetite and food intake, gastric secretion and gastrointestinal motility, glucose homeostasis, cardiovascular functions, anti-inflammatory functions, reproductive functions, and bone formation. This review focuses on presenting the current understanding of ghrelin and growth hormone secretagogue receptor biology, as well as the main physiological effects of ghrelin.

Are hungry sheep more pessimistic? The effects of food restriction on cognitive bias and the involvement of ghrelin in its regulation

Food restriction is considered to be a welfare issue in extensively reared animals. However, the effects of food restriction on the affective state, and its physiological regulation, are unknown. In Experiment 1, we aimed to assess the effects of increased plasma concentrations of acyl-ghrelin on judgement bias (an indicator of affective states) by fasting sheep for 24h or by ghrelin administration. In Experiment 2, we aimed to assess the effects of chronic food restriction on judgement bias and attention bias towards a food-related cue. For the judgement bias test, sheep were trained in an arena to approach a positive location cue associated with conspecifics and not approach a negative location cue associated with a dog. Three non-trained, non-reinforced ambiguous location cues were situated between the positive and negative locations. Attention bias towards a food-related cue was assessed by placing an empty food bucket against the wall of the arena halfway between the entry point and the positive location. In Experiment 1, sheep were divided into three treatments; 24h fast, ghrelin administration or control. Judgement bias, locomotor activity and plasma cortisol concentrations were assessed. The ghrelin treated group tended to express a more pessimistic bias compared to the control group (P<0.1), and plasma cortisol concentrations tended to be increased (P<0.1). In Experiment 2, sheep were subjected to a high feeding level (HF) or low feeding level (LF) for 7days. The LF group tended to show a more optimistic judgement bias (P<0.1). When the food-related cue was presented, LF ewes took longer to reach the positive location (P<0.001), spent longer with their head inside the bucket (P<0.001) and more time interacting with the bucket (P<0.01). This study provides preliminary evidence that food restriction alters judgement bias and attention bias towards a food-related cue which may indicate altered affective states of sheep.

Analysis of brain nuclei accessible to ghrelin present in the cerebrospinal fluid

Ghrelin is a stomach-derived peptide hormone that acts in the brain to regulate many important physiological functions. Ghrelin receptor, named the growth hormone secretagogue receptor (GHSR), is present in many brain areas with or without obvious direct access to ghrelin circulating in the bloodstream. Ghrelin is also present in the cerebrospinal fluid (CSF) but the brain targets of CSF ghrelin are unclear. Here, we studied which brain areas are accessible to ghrelin present in the CSF. For this purpose, we centrally injected mice with fluorescein-labeled ghrelin (F-ghrelin) peptide tracer and then systematically mapped the distribution of F-ghrelin signal through the brain. Our results indicated that centrally injected F-ghrelin labels neurons in most of the brain areas where GHSR is present. Also, we detected F-ghrelin uptake in the ependymal cells of both wild-type and GHSR-null mice. We conclude that CSF ghrelin is able to reach most of brain areas expressing GHSR. Also, we propose that the accessibility of CSF ghrelin to the brain parenchyma occurs through the ependymal cells in a GHSR-independent manner.

Rapid sensing of circulating ghrelin by hypothalamic appetite-modifying neurons

To maintain homeostasis, hypothalamic neurons in the arcuate nucleus must dynamically sense and integrate a multitude of peripheral signals. Blood-borne molecules must therefore be able to circumvent the tightly sealed vasculature of the blood-brain barrier to rapidly access their target neurons. However, how information encoded by circulating appetite-modifying hormones is conveyed to central hypothalamic neurons remains largely unexplored. Using in vivo multiphoton microscopy together with fluorescently labeled ligands, we demonstrate that circulating ghrelin, a versatile regulator of energy expenditure and feeding behavior, rapidly binds neurons in the vicinity of fenestrated capillaries, and that the number of labeled cell bodies varies with feeding status. Thus, by virtue of its vascular connections, the hypothalamus is able to directly sense peripheral signals, modifying energy status accordingly.

A high-protein diet induces dissociation between plasma concentrations of growth hormone and ghrelin in wethers

High-carbohydrate or high-fat diets have been demonstrated to change ghrelin concentrations in plasma; however, there remains a need to clarify the effects of dietary protein on the interaction between circulating GH and ghrelin concentrations in the ruminant. In this study, we investigated the postprandial changes in plasma concentrations of GH and ghrelin and their interactions when wethers were fed either a high-protein (HP; 40% CP) or a low-protein (LP; 10% CP) diet for 2 wk. The wethers were divided into 2 groups and fed once a day for 2 wk in a randomized crossover design. Each diet contained the same level of ME. Blood was collected from the animals at specific times over 24 h to measure hormones and metabolites. Feeding once a day caused a prompt reduction in the GH and ghrelin concentrations regardless of the type of diet that the wethers consumed. The preprandial concentrations (P = 0.04), area under the curve (AUC; P = 0.04), and incremental AUC (iAUC; P = 0.06) for ghrelin in HP-fed wethers were or tended to be greater than those in LP-fed wethers although concentrations for GH were the same for both diets (P = 0.23). In addition, the time it took for the postprandial ghrelin concentrations to recover to the preprandial concentrations was greater in HP-fed wethers than in LP-fed wethers although this was not true for GH concentrations. Similarly, as for ghrelin, postprandial increase (P < 0.001) and AUC (P = 0.03) for insulin concentration was greater in the HP-fed wethers than in the LP-fed wethers. From these findings, we concluded that dietary proteins (or some other derived metabolites) may dissociate the interaction between plasma concentrations of GH and ghrelin in wethers.

Assessment of body cell mass at bedside in critically ill patients

Critical illness affects body composition profoundly, especially body cell mass (BCM). BCM loss reflects lean tissue wasting and could be a nutritional marker in critically ill patients. However, BCM assessment with usual isotopic or tracer methods is impractical in intensive care units (ICUs). We aimed to modelize the BCM of critically ill patients using variables available at bedside. Fat-free mass (FFM), bone mineral (Mo), and extracellular water (ECW) of 49 critically ill patients were measured prospectively by dual-energy X-ray absorptiometry and multifrequency bioimpedance. BCM was estimated according to the four-compartment cellular level: BCM = FFM - (ECW/0.98) - (0.73 × Mo). Variables that might influence the BCM were assessed, and multivariable analysis using fractional polynomials was conducted to determine the relations between BCM and these data. Bootstrap resampling was then used to estimate the most stable model predicting BCM. BCM was 22.7 ± 5.4 kg. The most frequent model included height (cm), leg circumference (cm), weight shift (Δ) between ICU admission and body composition assessment (kg), and trunk length (cm) as a linear function: BCM (kg) = 0.266 × height + 0.287 × leg circumference + 0.305 × Δweight - 0.406 × trunk length - 13.52. The fraction of variance explained by this model (adjusted r(2)) was 46%. Including bioelectrical impedance analysis variables in the model did not improve BCM prediction. In summary, our results suggest that BCM can be estimated at bedside, with an error lower than ±20% in 90% subjects, on the basis of static (height, trunk length), less stable (leg circumference), and dynamic biometric variables (Δweight) for critically ill patients.

Apo-ghrelin receptor forms heteromers with DRD2 in hypothalamic neurons and is essential for anorexigenic effects of DRD2 agonism

We identified subsets of neurons in the brain that coexpress the dopamine receptor subtype-2 (DRD2) and the ghrelin receptor (GHSR1a). Combination of FRET confocal microscopy and Tr-FRET established the presence of GHSR1a:DRD2 heteromers in hypothalamic neurons. To interrogate function, mice were treated with the selective DRD2 agonist cabergoline, which produced anorexia in wild-type and ghrelin⁻/⁻ mice; intriguingly, ghsr⁻/⁻ mice were refractory illustrating dependence on GHSR1a, but not ghrelin. Elucidation of mechanism showed that formation of GHSR1a:DRD2 heteromers allosterically modifies canonical DRD2 dopamine signaling resulting in Gβγ subunit-dependent mobilization of [Ca²⁺](i) independent of GHSR1a basal activity. By targeting the interaction between GHSR1a and DRD2 in wild-type mice with a highly selective GHSR1a antagonist (JMV2959) cabergoline-induced anorexia was blocked. Inhibiting dopamine signaling in subsets of neurons with a GHSR1a antagonist has profound therapeutic implications by providing enhanced selectivity because neurons expressing DRD2 alone would be unaffected.

Growth hormone secretagogue receptor (GHS-R1a) knockout mice exhibit improved spatial memory and deficits in contextual memory

Although the hormone ghrelin is best known for its stimulatory effect on appetite and regulation of growth hormone release, it is also reported to have beneficial effects on learning and memory formation in mice. Nevertheless, controversy exists about whether endogenous ghrelin acts on its receptors in extra-hypothalamic areas of the brain. The ghrelin receptor (GHS-R1a) is co-expressed in neurons that express dopamine receptor type-1 (DRD1a) and type-2 (DRD2), and we have shown that a subset of GHS-R1a, which are not occupied by the agonist (apo-GHSR1a), heterodimerize with these two receptors to regulate dopamine signaling in vitro and in vivo. To determine the consequences of ghsr ablation on brain function, congenic ghsr -/- mice on the C57BL6/J background were subjected to a battery of behavioral tests. We show that the ghsr -/- mice exhibit normal balance, movement, coordination, and pain sensation, outperform ghsr +/+ mice in the Morris water maze, but show deficits in contextual fear conditioning.

Nanobiology and physiology of growth hormone secretion

Growth hormone (GH) secretion is controlled by hypothalamic releasing hormones from the median eminence together with hormones and neuropeptides produced by peripheral organs. Secretion of GH involves movement of secretory vesicles along microtubules, transient ‘docking’ with the porosome in the cell membrane and subsequent release of GH. Release of GH is stimulated by GH releasing hormone (GHRH) and inhibited by somatostatin (SRIF). Ghrelin may be functioning to stimulate GH release from somatotropes acting via the GH secretagogue (GHS) receptor (GHSR). However, recent physiological studies militate against this. In addition, ghrelin does influence GH release acting within the hypothalamus. Release of GH from the somatotropes involves the GH-containing secretory granules moving close to the cell surface followed by transitory fusion of the secretory granules with the porosomes located in multiple secretory pits in the cell membrane. Other peptides/proteins can influence GH secretion, particularly in species of non-mammalian vertebrates.

Ghrelin-mediated sympathoinhibition and suppression of inflammation in sepsis

Sepsis, a systemic inflammatory response to infection, continues to carry a high mortality despite advances in critical care medicine. Elevated sympathetic nerve activity in sepsis has been shown to contribute to early hepatocellular dysfunction and subsequently multiple organ failure, resulting in a poor prognosis, especially in the elderly. Thus, suppression of sympathetic nerve activity represents a novel therapeutic option for sepsis. Ghrelin is a 28-amino acid peptide shown to inhibit sympathetic nerve activity and inflammation in animal models of tissue injury. Age-related ghrelin hyporesponsiveness has also been shown to exacerbate sepsis. However, the mechanistic relationship between ghrelin-mediated sympathoinhibition and suppression of inflammation remains poorly understood. This review assesses the therapeutic potential of ghrelin in sepsis in the context of the neuroanatomical and molecular basis of ghrelin-mediated suppression of inflammation through inhibition of central sympathetic outflow.

Ghrelin-mediated appetite regulation in the central nervous system

The gut hormone and neuropeptide ghrelin was initially identified in the periphery as a compound released in the bloodstream in response to a negative energetic status. In the central nervous system (CNS), ghrelin mainly acts on the hypothalamus and the limbic system, with its best-known biological role being the regulation of appetitive functions. Recent research has shown that ghrelin is not an indispensable factor in the regulation of food intake. However, it plays a key role in the metabolic changes of lipids, mainly those involving hypothalamic NOS, AMPK, CaMKK2, CPT1 and UCP2 proteins. Ghrelin participates in the regulation of memory processes and the feeling of pleasure resulting from eating, both of which are metabolism-dependent and may be essential for the successful achievement of adaptive appetitive behavior. Ghrelin exerts its biological effect through a complicated network of neuroendocrine links, including the melanocortin and endocannabinoid systems. The activity of ghrelin is connected with circadian and annual fluctuations, which depend on seasons and food availability.

Physiological roles revealed by ghrelin and ghrelin receptor deficient mice

Ghrelin is a hormone made in the stomach and known primarily for its growth hormone releasing and orexigenic properties. Nevertheless, ghrelin through its receptor, the GHS-R1a, has been shown to exert many roles including regulation of glucose homeostasis, memory & learning, food addiction and neuroprotection. Furthermore, ghrelin could promote overall health and longevity by acting directly in the immune system and promoting an extended antigen repertoire. The development of mice lacking either ghrelin (ghrelin-/-) or its receptor (ghsr-/-) have provided a valuable tool for determining the relevance of ghrelin and its receptor in these multiple and diverse roles. In this review, we summarize the most important findings and lessons learned from the ghrelin-/- and ghsr-/- mice.

Investigation of the presence of ghrelin in the central nervous system of the rat and mouse

Ghrelin and ghrelin receptor agonist have effects on central neurons in many locations, including the hypothalamus, caudal brain stem, and spinal cord. However, descriptions of the distributions of ghrelin-like immunoreactivity in the CNS in published work are inconsistent. We have used three well-characterized anti-ghrelin antibodies, an antibody to the unacylated form of ghrelin, and a ghrelin peptide assay in rats, mice, ghrelin knockout mice, and ghrelin receptor reporter mice to re-evaluate ghrelin presence in the rodent CNS. The stomach served as a positive control. All antibodies were effective in revealing gastric endocrine cells. However, no specific staining could be found in the brain or spinal cord. Concentrations of antibody 10 to 30 times those effective in the stomach bound to nerve cells in rat and mouse brain, but this binding was not reduced by absorbing concentrations of ghrelin peptide, or by use of ghrelin gene knockout mice. Concentrations of ghrelin-like peptide, detected by enzyme-linked immunosorbent assay in extracts of hypothalamus, were 1% of gastric concentrations. Ghrelin receptor-expressing neurons had no adjacent ghrelin immunoreactive terminals. It is concluded that there are insignificant amounts of authentic ghrelin in neurons in the mouse or rat CNS and that ghrelin receptor-expressing neurons do not receive synaptic inputs from ghrelin-immunoreactive nerve terminals in these species.

Endocrine factors in the hypothalamic regulation of food intake in females: a review of the physiological roles and interactions of ghrelin, leptin, thyroid hormones, oestrogen and insulin

Controlling energy homeostasis involves modulating the desire to eat and regulating energy expenditure. The controlling machinery includes a complex interplay of hormones secreted at various peripheral endocrine endpoints, such as the gastrointestinal tract, the adipose tissue, thyroid gland and thyroid hormone-exporting organs, the ovary and the pancreas, and, last but not least, the brain itself. The peripheral hormones that are the focus of the present review (ghrelin, leptin, thyroid hormones, oestrogen and insulin) play integrated regulatory roles in and provide feedback information on the nutritional and energetic status of the body. As peripheral signals, these hormones modulate central pathways in the brain, including the hypothalamus, to influence food intake, energy expenditure and to maintain energy homeostasis. Since the growth of the literature on the role of various hormones in the regulation of energy homeostasis shows a remarkable and dynamic expansion, it is now becoming increasingly difficult to understand the individual and interactive roles of hormonal mechanisms in their true complexity. Therefore, our goal is to review, in the context of general physiology, the roles of the five best-known peripheral trophic hormones (ghrelin, leptin, thyroid hormones, oestrogen and insulin, respectively) and discuss their interactions in the hypothalamic regulation of food intake.

Triennial Growth Symposium: neural regulation of feed intake: modification by hormones, fasting, and disease

Appetite is a complex process that results from the integration of multiple signals at the hypothalamus. The hypothalamus receives neural signals; hormonal signals such as leptin, cholecystokinin, and ghrelin; and nutrient signals such as glucose, FFA, AA, and VFA. This effect is processed by a specific sequence of neurotransmitters beginning with the arcuate nucleus and orexigenic cells containing neuropeptide Y or agouti-related protein and anorexigenic cells containing proopiomelanocortin (yielding the neurotransmitter α-melanocyte-stimulating hormone) or cells expressing cocaine amphetamine-related transcript. These so-called first-order neurons act on second-order orexigenic neurons (containing either melanin-concentrating hormone or orexin) or act on anorexigenic neurons (e.g., expressing corticotropin-releasing hormone) to alter feed intake. In addition, satiety signals from the liver and gastrointestinal tract signal through the vagus nerve to the nucleus tractus solitarius to cause meal termination, and in combination with the hypothalamus, integrate the various signals to determine the feeding response. The activities of these neuronal pathways are also influenced by numerous factors such as nutrients, fasting, and disease to modify appetite and hence affect growth and reproduction. This review will begin with the central nervous system pathways and then discuss the ways in which hormones and metabolites may alter the process to affect feed intake with emphasis on farm animals.

Insights into the mechanism by which kisspeptin stimulates a preovulatory LH surge and ovulation in seasonally acyclic ewes: potential role of estradiol

We have previously demonstrated that a constant intravenous infusion of kisspeptin (Kp) for 48 h in anestrous ewes induces a preovulatory luteinizing hormone (LH) surge followed by ovulation in approximately 75% of animals. The mechanisms underlying this effect are unknown. In this study, we investigated whether Kp-induced preovulatory LH surges in anestrous ewes were the result of the general activation of the whole gonadotropic axis or of the direct activation of central GnRH neurons required for the GnRH/LH surge. In the first experiment, a constant iv infusion of ovine kisspeptin 10 (Kp; 15.2 nmol/h) was given to 11 seasonally acyclic ewes over 43 h. Blood samples were taken every 10 min for 15 h, starting 5h before the infusion, and then hourly until the end of the infusion. We found that the infusion of Kp induced a well-synchronized LH surge (around 22 h after the start of the Kp infusion) in 82% of the animals. In all ewes with an LH surge, there was an immediate but transient increase in the plasma concentrations of LH, follicle-stimulating hormone (FSH), and growth hormone (GH) at the start of the Kp infusion. Mean (+/- SEM) concentrations for the 5-h periods preceding and following the start of the Kp infusion were, respectively, 0.33 +/- 0.09 vs 2.83 +/- 0.49 ng/mL (P = 0.004) for LH, 0.43 +/- 0.05 vs 0.55 +/- 0.03 ng/mL (P = 0.015) for FSH, and 9.34 +/- 1.01 vs 11.51 +/- 0.92 ng/mL (P = 0.004) for GH. In the first experiment, surges of LH were observed only in ewes that also had a sustained rise in plasma concentrations of estradiol (E(2)) in response to Kp. Therefore, a second experiment was undertaken to determine the minimum duration of Kp infusion necessary to induce such a pronounced and prolonged increase in plasma E(2) concentration. Kisspeptin (15.2 nmol/h) was infused for 6, 12, or 24h in seasonally acyclic ewes (N = 8), and blood samples were collected hourly for 28 h (beginning 5h before the start of infusion), then every 2h for the following 22 h. Kisspeptin infused for 24h induced LH surges in 75% of animals, and this percentage decreased with the duration of the infusion (12h = 50%; 6h = 12.5%). The plasma concentration of E(2) was greater in ewes with an LH surge compared to those without LH surges; mean (+/- SEM) concentrations for the 5-h period following the Kp infusion were, respectively, 2.23 +/- 0.16 vs 1.27 +/- 0.13 pg/mL (P < 0.001). Collectively, our results strongly suggest that the systemic delivery of Kp induced LH surges by activating E(2)-positive feedback on gonadotropin secretion in acyclic ewes.

Ghrelin: central nervous system sites of action in regulation of energy balance

Ghrelin, a peptide hormone secreted by the stomach, has been shown to regulate energy homeostasis by modulating electrical activity of neurons in the central nervous system (CNS). Like many circulating satiety signals, ghrelin is a peptide hormone and is unable to cross the blood-brain barrier without a transport mechanism. In this review, we address the notion that the arcuate nucleus of the hypothalamus is the only site in the CNS that detects circulating ghrelin to trigger orexigenic responses. We consider the roles of a specialized group of CNS structures called the sensory circumventricular organs (CVOs), which are not protected by the blood-brain barrier. These areas include the subfornical organ and the area postrema and are already well known to be key areas for detection of other circulating hormones such as angiotensin II, cholecystokinin, and amylin. A growing body of evidence indicates a key role for the sensory CVOs in the regulation of energy homeostasis.

Low doses of estradiol partly inhibit release of GH in sheep without affecting basal levels

Estradiol increases basal growth hormone (GH) concentrations in sheep and cattle. This study sought to determine the effects of estradiol on GH-releasing hormone (GRH)-stimulated GH release in sheep. Growth hormone secretory characteristics, the GH response to GRH, and steady-state GH mRNA concentrations were determined in castrated male lambs treated with 2 different doses of estradiol 17-beta for a 28-d experimental period. Although no differences between treatments in mean GH, basal GH, or GH pulse number were observed after 28 d of estradiol treatment, GH pulse amplitude was greater (P < 0.05) in the 2.00-cm implant-treated animals than in the control and 0.75-cm implant group. The effect of estradiol treatment on GRH-stimulated GH release revealed differences between the control and estradiol-treated animals (P < 0.05). The 15-min GH responses to 0.075 microg/kg hGRH in the control, 0.75-cm, and 2.00-cm implant groups, respectively, were 76 +/- 10, 22.6 +/- 2.1, and 43.6 +/- 15.0 ng/mL. Growth hormone mRNA content was determined for pituitary glands from the different treatment groups, and no differences in steady-state GH mRNA levels were observed. There were no differences in the mean plasma concentrations of IGF-I, cortisol, T(3), or T(4) from weekly samples. Growth hormone release from cultured ovine pituitary cells from control sheep was not affected by estradiol after 72 h or in a subsequent 3-h incubation with estradiol combined with GRH. These data suggest that estradiol has differing actions on basal and GRH-stimulated GH concentrations in plasma, but the increase in pulse amplitude does not represent an increased pituitary sensitivity to GRH.