Inhibitory effect of ghrelin on food intake is mediated by the corticotropin-releasing factor system in neonatal chicks

It is known that, in rats, central and peripheral ghrelin increases food intake mainly through activation of neuropeptide Y (NPY) neurons. In contrast, intracerebroventricular (ICV) injection of ghrelin inhibits food intake in neonatal chicks. We examined the mechanism governing this inhibitory effect in chicks. The ICV injection of ghrelin or corticotropin-releasing factor (CRF), which also inhibits feeding and causes hyperactivity in chicks. Thus, we examined the interaction of ghrelin with CRF and the hypothalamo-pituitary-adrenal (HPA) axis. The ICV injection of ghrelin increased plasma corticosterone levels in a dose-dependent or a time-dependent manner. Co-injection of a CRF receptor antagonist, astressin, attenuated ghrelin-induced plasma corticosterone increase and anorexia. In addition, we also investigated the effect of ghrelin on NPY-induced food intake and on expression of hypothalamic NPY mRNA. Co-injection of ghrelin with NPY inhibited NPY-induced increase in food intake, and the ICV injection of ghrelin did not change NPY mRNA expression. These results indicate that central ghrelin does not interact with NPY as seen in rodents, but instead inhibits food intake by interacting with the endogenous CRF and its receptor.     

Reserpine-induced immunocytochemical change of neuropeptide Y in the hypothalamic arcuate nucleus

The effect of reserpine on neuropeptide Y immunoreactive (NPY-IR) neurons in the rat hypothalamic arcuate nucleus was examined by immunocytochemical techniques. Although only NPY-IR fibers and terminals were distributed in this nucleus in untreated and saline treated rats, single treatment of reserpine (10 mg/kg, i.p.) visualized abundant NPY-IR neuronal cell bodies: the increase began at 12 h of postinjection, reached its maximal level at 48 h, and returned to its normal level at 96 h. Pretreatment of nialamide, a monoamine oxidase inhibitor, prevented these acute reserpine-induced changes, suggesting reserpine acts on NPY neurons through monoaminergic mechanism. Chronic treatment of haloperidol (5 mg/kg, once daily for 5 days) a dopamine receptor antagonist, could induce the similar increase of NPY immunoreactivity. However, interruption of adrenergic and serotonergic neurotransmissions by chronic treatment of propranorol and methysergide, or chemical lesions of ascending noradrenergic and serotononergic pathways by 6-hydroxydopamine and 5,6-dihydroxytryptamine, could not induce any immunoreactive increase of NPY in arcuate neurons. These findings strongly suggest that reserpine-induced NPY increase occurs through dopaminergic afferents in hypothalamic arcuate neurons. Special issue dedicated to Dr. Kinya Kuriyama.     

Neuropeptide Y: A potent inducer of consummatory behavior in rats

Neuropeptide Y (NPY) is a 36 amino acid peptide with potent cardiovascular effects. In the present study, intraventricular injection of NPY was shown to markedly stimulate feeding and drinking during the illuminated period of the light/dark cycle, a time when rats ingest small amounts of food. It also enhanced nocturnal food and water intake following a 24 hour period of food deprivation and during nocturnal feeding. The NPY induction of food intake was suppressed by the opiate antagonist, naloxone, and by the dopamine antagonist, haloperidol. Phentolamine, an alpha adrenergic antagonist, failed to suppress NPY-induced feeding. Based on the maximum quantity of food which was ingested following central administration of NPY, this peptide appears to represent one of the most potent stimulators of feeding yet to be described.     

Central administration of neuropeptide Y induces wakefulness in rats

Neuropeptide Y (NPY) is a well-characterized neuromodulator in the central nervous system, primarily implicated in the regulation of feeding. NPY, orexins, and ghrelin form a hypothalamic food intake regulatory circuit. Orexin and ghrelin are also implicated in sleep-wake regulation. In the present experiments, we studied the sleep-modulating effects of central administration of NPY in rats. Rats received intracerebroventricular injection of physiological saline or three different doses of NPY (0.4, 2, and 10 microg in a volume of 4 microl) at light onset. Another group of rats received bilateral microinjection of saline or 2 microg NPY in the lateral hypothalamus in a volume of 0.2 microl. Sleep-wake activity and motor activity were recorded for 23 h. Food intake after the control and treatment injections was also measured on separate days. Intracerebroventricular and lateral hypothalamic administration of NPY suppressed non-rapid-eye-movement sleep and rapid-eye-movement sleep in rats during the first hour after the injection and also induced changes in electroencephalogram delta power spectra. NPY stimulated food intake in the first hour after both routes of administration. Data are consistent with the hypothesis that NPY has a role in the integration of feeding, metabolism, and sleep regulation.     

Co-administration of dopamine D1 and D2 agonists additively decreases daily food intake, body weight and hypothalamic neuropeptide Y level in rats

This study investigated whether co-administration of dopamine D1 and D2 agonists might additively inhibit the feeding effect and whether this effect was mediated by the action on hypothalamic neuropeptide Y (NPY). The D1 agonist SKF 38393 (SKF) and D2 agonists apomorphine (APO) or quinpirole (QNP) were administered, alone or in combination, to examine this possibility. In single administration, decreases of daily food intake were observed only in rats treated twice a day with a higher dose of SKF, APO or QNP. However, combined administration of D1 and D2 agonists, with each agent at a dose that alone did not induce anorexia in one daily treatment, exerted a significant effect. These results reveal that co-activation of D1 and D2 receptors can additively reduce daily food intake and body weight. The same treatment also decreased the level of hypothalamic NPY 24 h post-treatment. These results suggest an additive effect during combined activation of D1 and D2 receptor subtypes to decrease food intake and body weight that are mediated by the action of hypothalamic NPY. Similar to the effects seen in healthy rats, combined D1/D2 administration was also effective in the reduction of food intake in diabetic rats, revealing the efficiency of D1/D2 agonist in the improvement of hyperphasia in diabetic animals.     

Decreased orexigenic response to neuropeptide Y in rats with obstructive cholestasis

Neuropeptide Y (NPY) is a key factor in the neurochemical control of food intake, and obstructive cholestasis can be associated with disturbances in food intake. Our aim in this study was to determine whether obstructive cholestasis in the rat is associated with defective central responsiveness to NPY. Cholestasis was induced in rats by surgical bile duct resection. Rats with obstructive cholestasis exhibited a 20% reduction in food intake 2 days after laparotomy (compared with sham-resected controls) that had resolved by 4 days after surgery. Responsiveness to the orexigenic action of NPY was tested by measuring food intake after intracerebroventricular injection of NPY. In sham-resected rats, NPY infusion strikingly increased food intake, whereas bile duct-resected (BDR) rats showed a consistent significantly impaired feeding response to NPY at postlaparotomy days 2, 4, and 7. Separate experiments measured specific binding of [(3)H]NPY to hypothalamic receptors. Fos protein expression was measured in the hypothalamic paraventricular nucleus (PVN) as a marker of NPY-induced neuronal activation. The decreased orexigenic responsiveness to NPY was not caused by altered NPY binding at hypothalamic receptors or its ability to activate neurons in the PVN. Therefore, cholestatic rats demonstrate an attenuated NPY-induced orexigenic drive that occurs early after biliary obstruction, when cholestatic rats exhibit reduced food intake, and persists despite the return of food intake to normal levels and the presence of intact central NPY-related neuronal pathways.     

Effects of cyclo (Leu-Gly) on neurochemical indices of striatal dopaminergic supersensitivity induced by prolonged haloperidol treatment

The effects of a prolonged treatment with cyclo (Leu-Gly) and/or haloperidol on biochemical parameters indicative of striatal dopamine target cell supersensitivity have been investigated in the rat. When given acutely, cyclo (Leu-Gly) (2 mg/kg sc) did not affect striatal homovanillic acid, dihydroxyphenylacetic acid and acetylcholine levels both under basal conditions or after acute haloperidol (1 mg/kg ip) treatment. When given concomitantly with haloperidol (infused by means of osmotic minipumps at a rate of 2.5 μg/h sc) for 14 days, cyclo (Leu-Gly) (2 mg/kg sc once daily) failed to prevent the fall of striatal dopamine metabolites observed 2 days following withdrawal and the tolerance to the elevation of dopamine metabolites which occurs in response to challenge with the neuroleptic during withdrawal. Prolonged treatment with cyclo (Leu-Gly) also failed to affect the tolerance to the decrease of striatal acetylcholine levels which occurs under chronic haloperidol treatment. These data suggest that the mechanism whereby cyclo (Leu-Gly) inhibits the development of neuroleptic-induced dopaminergic supersensitivity does not involve an action of the peptide on nigro-striatal dopaminergic and striatal cholinergic neurons and is probably exerted distally to both dopaminergic and cholinergic synapses.     

The hypothalamic-pituitary-interrenal axis and the control of food intake in teleost fish

Although environmental, social and physical stressors have been shown to inhibit food intake and feeding behavior in fish, little is known about the mechanisms that mediate the appetite-suppressing effects of stress. Since the hypothalamic-pituitary-interrenal (HPI) axis is activated in response to most forms of stress in fish, components of this axis may be involved in mediating the food intake reductions elicited by stress. Recent investigations into the brain regulation of food intake in fish have identified several signals with orexigenic and anorexigenic properties. Among these appetite-regulating signals are related neuropeptides that can activate the HPI axis, namely corticotropin-releasing factor (CRF) and urotensin I (UI). Central injections of CRF or UI, or treatments that result in an increase in hypothalamic CRF and UI gene expression, can elicit dose-dependent decreases in food intake that can be reversed by pre-treatment with a CRF-receptor antagonist. Evidence also suggests that cortisol, the end product of HPI activation in most fishes (i.e. Osteichthyes), may be involved in the regulation of food intake. Overall, while elements of the HPI axis may mediate some of the appetite-suppressing effects of stress, it is undetermined how either CRF-related peptides, cortisol, or other elements of the stress response interact with the complex circuitry of the hypothalamic feeding center.     

Neuropeptide Y effect on food intake in broiler and layer chicks

Broiler chicks eat more food than layer chicks. In this study, we examined the involvement of orexigenic peptide neuropeptide Y (NPY) in the difference in food intake between broiler and layer chicks (Gallus gallus). First, we compared the hypothalamic mRNA levels of NPY and its receptors (Y1 and Y5 receptors) between these strains at 1, 2, 4, and 8 days of age. Daily food intake was significantly higher in broiler chicks than layer chicks after 2 days of age. However, the hypothalamic NPY mRNA level was significantly lower in broiler chicks than layer chicks except at 8 days of age. In addition, the mRNA levels of NPY receptors were also significantly lower in broiler chicks than layer chicks at 2 and 4 days of age (Y1 receptor) or 2 days of age (Y5 receptor). These results suggest that the differences in the expressions of hypothalamic NPY and its receptors do not cause the increase in food intake in broiler chicks. To compare the orexigenic effect of NPY between broiler and layer chicks, we next examined the effects of central administration of NPY on food intake in these strains. In both strains, central administration of NPY significantly increased food intake at 2, 4 and 8 days of age. All our findings demonstrated that the increase in food intake in broiler chicks is not accompanied with the over-expression of NPY or its receptor.     

Central mechanisms involved in the orexigenic actions of ghrelin

Ghrelin is a stomach hormone, secreted into the bloodstream, that initiates food intake by activating NPY/AgRP neurons in the hypothalamic acruate nucleus. This review focuses on recent evidence that details the mechanisms through which ghrelin activate receptors on NPY neurons and downstream signaling within NPY neurons. The downstream signaling involves a novel CaMKK-AMPK-CPT1-UCP2 pathway that enhances mitochondrial efficiency and buffers reactive oxygen species in order to maintain an appropriate firing response in NPY. Recent evidence that shows metabolic status affects ghrelin signaling in NPY is also described. In particular, ghrelin does not activate NPY neurons in diet-induced obese mice and ghrelin does not increase food intake. The potential mechanisms and implications of ghrelin resistance are discussed.     

Regulatory processes of hunger motivated behavior

While food intake and body weight are under homeostatic regulation, eating is a highly motivated and reinforced behavior that induces feelings of gratification and pleasure. The chemical senses (taste and odor) and their evaluation are essential to these functions. Brainstem and limbic glucose-monitoring (GM) neurons receiving neurochemical information from the periphery and from the local brain milieu are important controlling hunger motivation, and brain gut peptides have a modulatory role on this function. The hypothalamic and limbic forebrain areas are responsible for evaluation of reward quality and related emotions. They are innervated by the mesolimbic dopaminergic system (MLDS) and majority of GM neurons are also influenced by dopamine. Via dopamine release, the MLDS plays an essential role in rewarding-reinforcing processes of feeding and addiction. The GM network and the MLDS in the limbic system represent essential elements in the neural substrate of motivation.     

Hypothalamic NPY status during positive energy balance and the effects of the NPY antagonist, BW 1229U91, on the consumption of highly palatable energy-rich diet.

We have studied the hypothalamic activity of the neuropeptide Y (NPY) system in dietary-induced obese male Wistar rats and examined whether the NPY antagonist, BW1229U91, can inhibit the hyperphagia during positive energy balance associated with feeding rats an energy-rich, highly palatable diet. Rats given a highly palatable, high-fat diet became obese after 8 weeks and exhibited hyperinsulinemia and hyperleptinemia, as compared to lean rats fed on standard pellet laboratory diet. Hypothalamic NPY mRNA concentrations were significantly reduced by approximately 70% in dietary-obese rats compared with lean controls, and the former were hypersensitive to intracerebroventricular injections of NPY, possibly as a result of NPY receptor up-regulation. Intracerebroventricular injections of BW 1229U91, that inhibits food intake in starved rats, did not alter food intake in either control or obese rats fed either standard pellet diet or the highly palatable diet, respectively. We conclude that dietary-obese rats have underactive hypothalamic NPYergic neurons compared to lean controls, possibly as a result of increased plasma concentrations of leptin and/or insulin that directly inhibit the NPY neuronal activity. The lack of effect of BW1229U91 on the increased caloric intake of dietary-obese rats suggests that the hyperphagia is not NPY-driven and supports the data indicating reduced synaptic activity of the hypothalamic NPY system.     

Ghrelin indirectly activates hypophysiotropic CRF neurons in rodents

Ghrelin is a stomach-derived hormone that regulates food intake and neuroendocrine function by acting on its receptor, GHSR (Growth Hormone Secretagogue Receptor). Recent evidence indicates that a key function of ghrelin is to signal stress to the brain. It has been suggested that one of the potential stress-related ghrelin targets is the CRF (Corticotropin-Releasing Factor)-producing neurons of the hypothalamic paraventricular nucleus, which secrete the CRF neuropeptide into the median eminence and activate the hypothalamic-pituitary-adrenal axis. However, the neural circuits that mediate the ghrelin-induced activation of this neuroendocrine axis are mostly uncharacterized. In the current study, we characterized in vivo the mechanism by which ghrelin activates the hypophysiotropic CRF neurons in mice. We found that peripheral or intra-cerebro-ventricular administration of ghrelin strongly activates c-fos--a marker of cellular activation--in CRF-producing neurons. Also, ghrelin activates CRF gene expression in the paraventricular nucleus of the hypothalamus and the hypothalamic-pituitary-adrenal axis at peripheral level. Ghrelin administration directly into the paraventricular nucleus of the hypothalamus also induces c-fos within the CRF-producing neurons and the hypothalamic-pituitary-adrenal axis, without any significant effect on the food intake. Interestingly, dual-label immunohistochemical analysis and ghrelin binding studies failed to show GHSR expression in CRF neurons. Thus, we conclude that ghrelin activates hypophysiotropic CRF neurons, albeit indirectly.     

Third ventricular coinjection of subthreshold doses of NPY and AgRP stimulate food hoarding and intake and neural activation

We previously demonstrated that 3rd ventricular (3V) neuropeptide Y (NPY) or agouti-related protein (AgRP) injection potently stimulates food foraging/hoarding/intake in Siberian hamsters. Because NPY and AgRP are highly colocalized in arcuate nucleus neurons in this and other species, we tested whether subthreshold doses of NPY and AgRP coinjected into the 3V stimulates food foraging, hoarding, and intake, and/or neural activation [c-Fos immunoreactivity (c-Fos-ir)] in hamsters housed in a foraging/hoarding apparatus. In the behavioral experiment, each hamster received four 3V treatments by using subthreshold doses of NPY and AgRP for all behaviors: 1) NPY, 2) AgRP, 3) NPY+AgRP, and 4) saline with a 7-day washout period between treatments. Food foraging, intake, and hoarding were measured 1, 2, 4, and 24 h and 2 and 3 days postinjection. Only when NPY and AgRP were coinjected was food intake and hoarding increased. After identical treatment in separate animals, c-Fos-ir was assessed at 90 min and 14 h postinjection, times when food intake (0-1 h) and hoarding (4-24 h) were uniquely stimulated. c-Fos-ir was increased in several hypothalamic nuclei previously shown to be involved in ingestive behaviors and the central nucleus of the amygdala (CeA), but only in NPY+AgRP-treated animals (90 min and 14 h: magno- and parvocellular regions of the hypothalamic paraventricular nucleus and perifornical area; 14 h only: CeA and sub-zona incerta). These results suggest that NPY and AgRP interact to stimulate food hoarding and intake at distinct times, perhaps released as a cocktail naturally with food deprivation to stimulate these behaviors.     

Apelin and the proopiomelanocortin system: a new regulatory pathway of hypothalamic α-MSH release

Neuronal networks originating in the hypothalamic arcuate nucleus (Arc) play a fundamental role in controlling energy balance. In the Arc, neuropeptide Y (NPY)-producing neurons stimulate food intake, whereas neurons releasing the proopiomelanocortin (POMC)-derived peptide α-melanocyte-stimulating hormone (α-MSH) strongly decrease food intake. There is growing evidence to suggest that apelin and its receptor may play a role in the central control of food intake, and both are concentrated in the Arc. We investigated the presence of apelin and its receptor in Arc NPY- and POMC-containing neurons and the effects of apelin on α-MSH release in the hypothalamus. We showed, by immunofluorescence and confocal microscopy, that apelin-immunoreactive (IR) neuronal cell bodies were distributed throughout the rostrocaudal extent of the Arc and that apelin was strongly colocalized with POMC, but weakly colocalized with NPY. However, there were numerous NPY-IR nerve fibers close to the apelin-IR neuronal cell bodies. By combining in situ hybridization with immunohistochemistry, we demonstrated the presence of apelin receptor mRNA in Arc POMC neurons. Moreover, using a perifusion technique for hypothalamic explants, we demonstrated that apelin-17 (K17F) increased α-MSH release, suggesting that apelin released somato-dendritically or axonally from POMC neurons may stimulate α-MSH release in an autocrine manner. Consistent with these data, hypothalamic apelin levels were found to be higher in obese db/db mice and fa/fa Zucker rats than in wild-type animals. These findings support the hypothesis that central apelin is involved in regulating body weight and feeding behavior through the direct stimulation of α-MSH release.     

Effects of dopamine agonists and antagonists on gastric acid secretion and stress responses in rats

The dopamine agonists and promoters bromocriptine, bupropion, and p-hydroxymethylphenidate (a peripherally acting methylphenidate analog) reduced basal gastric acid secretion in rats, while the dopamine antagonists haloperidol, pimozide and metoclopramide augmented gastric acid output. Stress ulcer formation and plasma corticosterone levels were markedly reduced by l-dopa given either intraperitoneally or intracerebroventricularly as well as by intraperitoneally administered p-hydroxymethylphenidate. Domperidone, a peripheral dopamine receptor blocker, produced variable effects on stress responses, indicating a wider spectrum of action than hitherto realized for this compound. The results strongly support a role for both central and peripheral dopaminergic activity in reducing the pathological consequences of exposure to stress.     

Brain‐derived neurotrophic factor,corticotropin‐releasing factor,and hypothalamic neuronal histamine interact to regulate feeding behavior

Brain‐derived neurotrophic factor (BDNF), corticotropin‐releasing factor (CRF), and hypothalamic neuronal histamine are anorexigenic substances within the hypothalamus. This study examined the interactions among BDNF, CRF, and histamine during the regulation of feeding behavior in rodents. Food intake was measured after treatment with BDNF, α‐fluoromethyl histidine (FMH; a specific suicide inhibitor of histidine decarboxylase that depletes hypothalamic neuronal histamine), or CRF antagonist. We measured food intake in wild‐type mice and mice with targeted disruption of the histamine H1 receptor (H1KO mice) after central BDNF infusion. Furthermore, we investigated CRF content and histamine turnover in the hypothalamus after BDNF treatment, and conversely, BDNF content in the hypothalamus after histamine treatment. We used immunohistochemical staining for histamine H1 receptors (H1‐R) in BDNF neurons. BDNF‐induced feeding suppression was partially attenuated in rats pre‐treated with FMH or a CRF antagonist, and in H1KO mice. BDNF treatment increased CRF content and histamine turnover in the hypothalamus. Histamine increased BDNF content in the hypothalamus. Immunohistochemical analysis revealed that H1‐Rs were expressed on BDNF neurons in the ventromedial nucleus of the hypothalamus. These results indicate that CRF and hypothalamic neuronal histamine mediate the suppressive effects of BDNF on feeding behavior and body weight.     

Bilateral neural transections at the level of mesencephalon increase food intake and reduce latency to onset of feeding in response to neuropeptide Y

Administration of neuropeptide Y (NPY) into the IIIrd ventricle of the rat brain induces robust ingestive behavior with a latency to onset of feeding (LOF) ranging from 12 to 20 min. Since substantial amounts of NPY found in hypothalamic sites that mediate the control of feeding behavior originate from the brain stem, we studied the effects of NPY on LOF and food intake in male and female rats after bilateral severing of brain stem NPY input to the hypothalamus at the level of the mesencephalon. NPY in doses of 117 pmol significantly increased food intake and decreased LOF in both male and female transected rats. Higher doses of 470 pmol NPY decreased only the LOF in transected rats as compared to sham control rats. Additionally, 117 pmol NPY in transected rats elicited food consumption equivalent to that produced by 470 pmol NPY in control rats. These studies show that decreases in NPY levels found in the paraventricular nucleus and neighboring hypothalamic sites as a result of these neural transections may render rats hyperresponsive to NPY, presumably due to denervation-induced hypersensitivity in these sites.