Caroline J. Small

Gut hormone PYY3-36 physiologically inhibits food intake

Food intake is regulated by the hypothalamus, including the melanocortin and neuropeptide Y (NPY) systems in the arcuate nucleus. The NPY Y2 receptor (Y2R), a putative inhibitory presynaptic receptor, is highly expressed on NPY neurons in the arcuate nucleus, which is accessible to peripheral hormones. Peptide YY3-36 (PYY3-36), a Y2R agonist, is released from the gastrointestinal tract postprandially in proportion to the calorie content of a meal. Here we show that peripheral injection of PYY3-36 in rats inhibits food intake and reduces weight gain. PYY3-36 also inhibits food intake in mice but not in Y2r-null mice, which suggests that the anorectic effect requires the Y2R. Peripheral administration of PYY3-36 increases c-Fos immunoreactivity in the arcuate nucleus and decreases hypothalamic Npy messenger RNA. Intra-arcuate injection of PYY3-36 inhibits food intake. PYY3-36 also inhibits electrical activity of NPY nerve terminals, thus activating adjacent pro-opiomelanocortin (POMC) neurons. In humans, infusion of normal postprandial concentrations of PYY3-36 significantly decreases appetite and reduces food intake by 33% over 24u2009h. Thus, postprandial elevation of PYY3-36 may act through the arcuate nucleus Y2R to inhibit feeding in a gut–hypothalamic pathway.

AMP-activated Protein Kinase Plays a Role in the Control of Food Intake

AMP-activated protein kinase (AMPK) is the downstream component of a protein kinase cascade that acts as an intracellular energy sensor maintaining the energy balance within the cell. The finding that leptin and adiponectin activate AMPK to alter metabolic pathways in muscle and liver provides direct evidence for this role in peripheral tissues. The hypothalamus is a key regulator of food intake and energy balance, coordinating body adiposity and nutritional state in response to peripheral hormones, such as leptin, peptide YY-(3–36), and ghrelin. To date the hormonal regulation of AMPK in the hypothalamus, or its potential role in the control of food intake, have not been reported. Here we demonstrate that counter-regulatory hormones involved in appetite control regulate AMPK activity and that pharmacological activation of AMPK in the hypothalamus increases food intake. In vivo administration of leptin, which leads to a reduction in food intake, decreases hypothalamic AMPK activity. By contrast, injection of ghrelin in vivo, which increases food intake, stimulates AMPK activity in the hypothalamus. Consistent with the effect of ghrelin, injection of 5-amino-4-imidazole carboxamide riboside, a pharmacological activator of AMPK, into either the third cerebral ventricle or directly into the paraventricular nucleus of the hypothalamus significantly increased food intake. These results suggest that AMPK is regulated in the hypothalamus by hormones which regulate food intake. Furthermore, direct pharmacological activation of AMPK in the hypothalamus is sufficient to increase food intake. These findings demonstrate that AMPK plays a role in the regulation of feeding and identify AMPK as a novel target for anti-obesity drugs.

The inhibitory effects of peripheral administration of peptide YY(3-36) and glucagon-like peptide-1 on food intake are attenuated by ablation of the vagal-brainstem-hypothalamic pathway.

The vagus nerve forms a neuro-anatomical link between the gastrointestinal tract and the brain. A number of gastrointestinal hormones, including cholecystokinin and ghrelin, require an intact vagal-brainstem-hypothalamic pathway to affect CNS feeding circuits. We have shown that the effects of peripheral administration of both peptide YY(3-36) (PYY(3-36)) and glucagon-like peptide-1 (GLP-1) on food intake and activation of hypothalamic arcuate feeding neurones are abolished following either bilateral sub-diaphragmatic total truncal vagotomy or brainstem-hypothalamic pathway transectioning in rodents. These findings suggest that the vagal-brainstem-hypothalamic pathway may also play a role in the effects of circulating PYY(3-36) and GLP-1 on food intake.

Repeated intracerebroventricular administration of glucagon-like peptide-1-(7-36) amide or exendin-(9-39) alters body weight in the rat.

Central nervous system glucagon-like peptide-1-(7-36) amide (GLP-1) administration has been reported to acutely reduce food intake in the rat. We here report that repeated intracerebroventricular (i.c.v.) injection of GLP-1 or the GLP-1 receptor antagonist, exendin-(9-39), affects food intake and body weight. Daily i.c.v. injection of 3 nmol GLP-1 to schedule-fed rats for 6 days caused a reduction in food intake and a decrease in body weight of 16 +/- 5 g (P < 0.02 compared with saline-injected controls). Daily i.c.v. administration of 30 nmol exendin-(9-39) to schedule-fed rats for 3 days caused an increase in food intake and increased body weight by 7 +/- 2 g (P < 0.02 compared with saline-injected controls). Twice daily i.c.v. injections of 30 nmol exendin-(9-39) with 2.4 nmol neuropeptide Y to ad libitum-fed rats for 8 days increased food intake and increased body weight by 28 +/- 4 g compared with 14 +/- 3 g in neuropeptide Y-injected controls (P < 0.02). There was no evidence of tachyphylaxis in response to i.c.v. GLP-1 or exendin-(9-39). GLP-1 may thus be involved in the regulation of body weight in the rat.Central nervous system glucagon-like peptide-1-(7–36) amide (GLP-1) administration has been reported to acutely reduce food intake in the rat. We here report that repeated intracerebroventricular (icv) injection of GLP-1 or the GLP-1 receptor antagonist, exendin-(9–39), affects food intake and body weight. Daily icv injection of 3 nmol GLP-1 to schedule-fed rats for 6 days caused a reduction in food intake and a decrease in body weight of 16 ± 5 g (P < 0.02 compared with saline-injected controls). Daily icv administration of 30 nmol exendin-(9–39) to schedule-fed rats for 3 days caused an increase in food intake and increased body weight by 7 ± 2 g (P < 0.02 compared with saline-injected controls). Twice daily icv injections of 30 nmol exendin-(9–39) with 2.4 nmol neuropeptide Y to ad libitum-fed rats for 8 days increased food intake and increased body weight by 28 ± 4 g compared with 14 ± 3 g in neuropeptide Y-injected controls (P < 0.02). There was no evidence of tachyphylaxis in response to icv GLP-1 o...

Gastrointestinal hormones regulating appetite

The role of gastrointestinal hormones in the regulation of appetite is reviewed. The gastrointestinal tract is the largest endocrine organ in the body. Gut hormones function to optimize the process of digestion and absorption of nutrients by the gut. In this capacity, their local effects on gastrointestinal motility and secretion have been well characterized. By altering the rate at which nutrients are delivered to compartments of the alimentary canal, the control of food intake arguably constitutes another point at which intervention may promote efficient digestion and nutrient uptake. In recent decades, gut hormones have come to occupy a central place in the complex neuroendocrine interactions that underlie the regulation of energy balance. Many gut peptides have been shown to influence energy intake. The most well studied in this regard are cholecystokinin (CCK), pancreatic polypeptide, peptide YY, glucagon-like peptide-1 (GLP-1), oxyntomodulin and ghrelin. With the exception of ghrelin, these hormones act to increase satiety and decrease food intake. The mechanisms by which gut hormones modify feeding are the subject of ongoing investigation. Local effects such as the inhibition of gastric emptying might contribute to the decrease in energy intake. Activation of mechanoreceptors as a result of gastric distension may inhibit further food intake via neural reflex arcs. Circulating gut hormones have also been shown to act directly on neurons in hypothalamic and brainstem centres of appetite control. The median eminence and area postrema are characterized by a deficiency of the blood–brain barrier. Some investigators argue that this renders neighbouring structures, such as the arcuate nucleus of the hypothalamus and the nucleus of the tractus solitarius in the brainstem, susceptible to influence by circulating factors. Extensive reciprocal connections exist between these areas and the hypothalamic paraventricular nucleus and other energy-regulating centres of the central nervous system. In this way, hormonal signals from the gut may be translated into the subjective sensation of satiety. Moreover, the importance of the brain–gut axis in the control of food intake is reflected in the dual role exhibited by many gut peptides as both hormones and neurotransmitters. Peptides such as CCK and GLP-1 are expressed in neurons projecting both into and out of areas of the central nervous system critical to energy balance. The global increase in the incidence of obesity and the associated burden of morbidity has imparted greater urgency to understanding the processes of appetite control. Appetite regulation offers an integrated model of a brain–gut axis comprising both endocrine and neurological systems. As physiological mediators of satiety, gut hormones offer an attractive therapeutic target in the treatment of obesity.

Subcutaneous Ghrelin Enhances Acute Food Intake in Malnourished Patients Who Receive Maintenance Peritoneal Dialysis: A Randomized, Placebo-Controlled Trial

Anorexia and malnutrition confer significant morbidity and mortality to patients with end-stage kidney disease but are resistant to therapy. The aim of this study was to determine whether subcutaneous administration of ghrelin, an appetite-stimulating gut hormone, could enhance food intake in patients who are receiving maintenance peritoneal dialysis and have evidence of malnutrition. The principal outcome measure was energy intake during a measured study meal. Secondary outcome measures were BP and heart rate and 3-d food intake after intervention. Nine peritoneal dialysis patients with mild to moderate malnutrition (mean serum albumin 28.6 +/- 5.0 g/L, total cholesterol 4.4 +/- 0.6 mmol/L, subjective global assessment score of 5.7 +/- 1.7) were given subcutaneous ghrelin (3.6 nmol/kg) and saline placebo in a randomized, double-blind, crossover protocol. Administration of subcutaneous ghrelin significantly increased the group mean absolute energy intake, compared with placebo, during the study meal (690 +/- 190 versus 440 +/- 250 kcal; P = 0.0062). When expressed as proportional energy increase for each individual, ghrelin administration resulted in immediate doubling of energy intake (204 +/- 120 versus 100%; P = 0.0319). Administration of ghrelin maintained a nonsignificant increase in energy intake over 24 h after intervention (2009 +/- 669 versus 1579 +/- 330 kcal) and was not followed by subsequent underswing (1790 +/- 370 versus 1670 +/- 530 and 1880 +/- 390 versus 1830 +/- 530 kcal on days 2 and 3, respectively). Ghrelin administration resulted in a significant fall in mean arterial BP (P = 0.0030 by ANOVA). There were no significant adverse events during the study. Subcutaneous ghrelin administration enhances short-term food intake in dialysis patients with mild to moderate malnutrition.

Actions of cocaine- and amphetamine-regulated transcript (CART) peptide on regulation of appetite and hypothalamo-pituitary axes in vitro and in vivo in male rats

Cocaine- and amphetamine-regulated transcript (CART) and CART peptide are abundant in hypothalamic nuclei controlling anterior pituitary function. Intracerebroventricular (ICV) injection of CART peptide results in neuronal activation in the paraventricular nucleus (PVN), rich in corticotrophin-releasing factor (CRH) and thyrotrophin-releasing factor (TRH) immunoreactive neurons. The aims of this study were three-fold. Firstly, to examine the effects of CART peptide on hypothalamic releasing factors in vitro, secondly, to examine the effect of ICV injection of CART peptide on plasma pituitary hormones and finally to examine the effect of PVN injection of CART peptide on food intake and circulating pituitary hormones. CART(55-102) (100 nM) peptide significantly stimulated the release of CRH, TRH and neuropeptide Y from hypothalamic explants but significantly reduced alpha melanocyte stimulating hormone release in vitro. Following ICV injection of 0.2 nmol CART(55-102), a dose which significantly reduces food intake, plasma prolactin (PRL), growth hormone (GH) and adrenocorticotrophin hormone (ACTH) and corticosterone increased significantly. Following PVN injection of CART(55-102), food intake was significantly reduced only at 0.2 and 0.6 nmol. However, PVN injection of 0.02 nmol CART(55-102) produced a significant increase in plasma ACTH. ICV injection of CART peptide significantly reduces food intake. Unlike many anorexigenic peptides, there is no increased sensitivity to PVN injection of CART(55-102). In contrast, both ICV and PVN injection of CART(55-102) significantly increased plasma ACTH and release of hypothalamic CRH is significantly increased by CART peptide in vitro. This suggests that CART peptide may play a role in the control of pituitary function and in particular the hypothalamo-pituitary adrenal axis.

The Hypothalamic Mechanisms of the Hypophysiotropic Action of Ghrelin

Ghrelin is an endogenous ligand for the growth hormone secretagogue (GHS) receptor, expressed in the hypothalamus and pituitary. Ghrelin, like synthetic GHSs, stimulates food intake and growth hormone (GH) release following systemic or intracerebroventricular administration. In addition to GH stimulation, ghrelin and synthetic GHSs are reported to stimulate the hypothalamo-pituitary-adrenal (HPA) axis in vivo. The aims of this study were to elucidate the hypothalamic mechanisms of the hypophysiotropic actions of ghrelin in vitro and to assess the relative contribution of hypothalamic and systemic actions of ghrelin on the HPA axis in vivo. Ghrelin (100 and 1,000 nM) stimulated significant release of GH-releasing hormone (GHRH) from hypothalamic explants (100 nM: 39.4 ± 8.3 vs. basal 18.3 ± 3.5 fmol/explant, n = 49, p < 0.05) but did not affect either basal or 28 mM KCl-stimulated somatostatin release. Ghrelin (10, 100 and 1,000 nM) stimulated the release of both corticotropin-releasing hormone (CRH) (100 nM: 6.0 ± 0.8 vs. basal 4.2 ± 0.5 pmol/explant, n = 49, p < 0.05) and arginine vasopressin (AVP) (100 nM: 49.2 ± 5.9 vs. basal 35.0 ± 3.3 fmol/explant, n = 48, p < 0.05), whilst ghrelin (100 and 1,000 nM) also stimulated the release of neuropeptide Y (NPY) (100 nM: 111.4 ± 25.0 vs. basal 54.4 ± 9.0 fmol/explant, n = 26, p < 0.05) from hypothalamic explants in vitro. The HPA axis was stimulated in vivo following acute intracerebroventricular administration of ghrelin 2 nmol [adrenocorticotropic hormone (ACTH) 38.2 ± 3.9 vs. saline 18.2 ± 2.0 pg/ml, p < 0.01; corticosterone 310.1 ± 32.8 ng/ml vs. saline 167.4 ± 40.7 ng/ml, p < 0.05], but not following intraperitoneal administration of ghrelin 30 nmol, suggesting a hypothalamic site of action. These data suggest that the mechanisms of GH and ACTH regulation by ghrelin may include hypothalamic release of GHRH, CRH, AVP and NPY.

Blockade of the neuropeptide Y Y2 receptor with the specific antagonist BIIE0246 attenuates the effect of endogenous and exogenous peptide YY(3-36) on food intake.

The gastrointestinal-derived hormone peptide YY (PYY) is released from intestinal L-cells post-prandially in proportion to calorie intake, and modulates food intake. Peripheral administration of PYY((3-36)) reduces food intake and body weight in rodents and suppresses appetite and food intake in humans. PYY((3-36)) is hypothesised to inhibit food intake via activation of the auto-inhibitory pre-synaptic neuropeptide Y (NPY) Y2 receptor (Y2R) present on arcuate (ARC) NPY neurons. We aimed to investigate the feeding effect of PYY((3-36)) following blockade of ARC Y2R, using the specific receptor antagonist BIIE0246, in the rat. We found that pre-treatment with BIIE0246 (1 nmol) into the ARC attenuated the reduction in feeding observed following intraperitoneal injection of PYY((3-36)) (7.5 nmol/kg) (0-1 h food intake: BIIE0246/PYY((3-36)): 3.8 +/- 0.4 g; vs. Vehicle/PYY((3-36)): 2.7 +/- 0.2 g; P < 0.05). We found plasma PYY levels to be maximal at 120 min post-initiation of feeding. On investigation of the endogenous role of the Y2R, we found that ARC administration of BIIE0246 alone significantly increased feeding in satiated rats compared to vehicle-injected controls (0-1 h food intake: BIIE0246: 4.1 +/- 0.7 g; vs. vehicle: 1.7 +/- 0.7 g; P < 0.05), suggesting that Y2R antagonism disinhibits the NPY neuron thus stimulating feeding in otherwise satiated rats. These studies suggest that the Y2R plays an important role in post-prandial satiety and provide further insight into the mechanisms of action of PYY((3-36)).

Leptin interacts with glucagon-like peptide-1 neurons to reduce food intake and body weight in rodents

The adipose tissue hormone, leptin, and the neuropeptide glucagon‐like peptide‐1 (7–36) amide (GLP‐1) both reduce food intake and body weight in rodents. Using dual in situ hybridization, long isoform leptin receptor (OB‐Rb) was localized to GLP‐1 neurons originating in the nucleus of the solitary tract. ICV injection of the specific GLP‐1 receptor antagonist, exendin(9–39), at the onset of dark phase, did not affect feeding in saline pre‐treated controls, but blocked the reduction in food intake and body weight of leptin pre‐treated rats. These findings suggest that GLP‐1 neurons are a potential target for leptin in its control of feeding.