Effects of albumin-conjugated PYY on food intake: the respective roles of the circumventricular organs and vagus nerve

Neurocircuitry underlying the actions of glucagon-like peptide 1 and peptide YY3-36 in the suppression of food, drug-seeking, and anxiogenesis

Obesity is a critical health condition worldwide that increases the risks of comorbid chronic diseases, but it can be managed with weight loss. However, conventional interventions relying on diet and exercise are inadequate for achieving and maintaining weight loss, thus there is significant market interest for pharmaceutical anti-obesity agents. For decades, receptor agonists for the gut peptide glucagon-like peptide 1 (GLP-1) featured prominently in anti-obesity medications by suppressing appetite and food reward to elicit rapid weight loss. As the neurocircuitry underlying food motivation overlaps with that for drugs of abuse, GLP-1 receptor agonism has also been shown to decrease substance use and relapse, thus its therapeutic potential may extend beyond weight management to treat addictions. However, as prolonged use of anti-obesity drugs may increase the risk of mood-related disorders like anxiety and depression, and individuals taking GLP-1-based medication commonly report feeling demotivated, the long-term safety of such drugs is an ongoing concern. Interestingly, current research now focuses on dual agonist approaches that include GLP-1 receptor agonism to enable synergistic effects on weight loss or associated functions. GLP-1 is secreted from the same intestinal cells as the anorectic gut peptide, Peptide YY3-36 (PYY3-36), thus this review assessed the therapeutic potential and underlying neural circuits targeted by PYY3-36 when administered independently or in combination with GLP-1 to curb the appetite for food or drugs of abuse like opiates, alcohol, and nicotine. Additionally, we also reviewed animal and human studies to assess the impact, if any, for GLP-1 and/or PYY3-36 on mood-related behaviors in relation to anxiety and depression. As dual agonists targeting GLP-1 and PYY3-36 may produce synergistic effects, they can be effective at lower doses and offer an alternative approach for therapeutic benefits while mitigating undesirable side effects.

Derivatization with fatty acids in peptide and protein drug discovery

Peptides and proteins are widely used to treat a range of medical conditions; however, they often have to be injected and their effects are short-lived. These shortcomings of the native structure can be addressed by molecular engineering, but this is a complex undertaking. A molecular engineering technology initially applied to insulin - and which has now been successfully applied to several biopharmaceuticals - entails the derivatization of peptides and proteins with fatty acids. Various protraction mechanisms are enabled by the specific characteristics and positions of the attached fatty acid. Furthermore, the technology can ensure a long half-life following oral administration of peptide drugs, can alter the distribution of peptides and may hold potential for tissue targeting. Due to the inherent safety and well-defined chemical nature of the fatty acids, this technology provides a versatile approach to peptide and protein drug discovery.

The hypothalamus for whole-body physiology: from metabolism to aging

Obesity and aging are two important epidemic factors for metabolic syndrome and many other health issues, which contribute to devastating diseases such as cardiovascular diseases, stroke and cancers. The brain plays a central role in controlling metabolic physiology in that it integrates information from other metabolic organs, sends regulatory projections and orchestrates the whole-body function. Emerging studies suggest that brain dysfunction in sensing various internal cues or processing external cues may have profound effects on metabolic and other physiological functions. This review highlights brain dysfunction linked to genetic mutations, sex, brain inflammation, microbiota, stress as causes for whole-body pathophysiology, arguing brain dysfunction as a root cause for the epidemic of aging and obesity-related disorders. We also speculate key issues that need to be addressed on how to reveal relevant brain dysfunction that underlines the development of these disorders and diseases in order to develop new treatment strategies against these health problems.

Integrative Hedonic and Homeostatic Food Intake Regulation by the Central Nervous System: Insights from Neuroimaging

Food intake regulation in humans is a complex process controlled by the dynamic interaction of homeostatic and hedonic systems. Homeostatic regulation is controlled by appetitive signals from the gut, adipose tissue, and the vagus nerve, while conscious and unconscious reward processes orchestrate hedonic regulation. On the one hand, sight, smell, taste, and texture perception deliver potent food-related feedback to the central nervous system (CNS) and influence brain areas related to food reward. On the other hand, macronutrient composition stimulates the release of appetite signals from the gut, which are translated in the CNS into unconscious reward processes. This multi-level regulation process of food intake shapes and regulates human ingestive behavior. Identifying the interface between hormones, neurotransmitters, and brain areas is critical to advance our understanding of conditions like obesity and develop better therapeutical interventions. Neuroimaging studies allow us to take a glance into the central nervous system (CNS) while these processes take place. This review focuses on the available neuroimaging evidence to describe this interaction between the homeostatic and hedonic components in human food intake regulation.

The effect of fatty diacid acylation of human PYY3-36 on Y2 receptor potency and half-life in minipigs

Peptides are notoriously known to display very short in vivo half-lives often measured in minutes which in many cases greatly reduces or eliminates sufficient in vivo efficacy. To obtain long half-lives allowing for up to once-weekly dosing regimen, fatty acid acylation (lipidation) have been used to non-covalently associate the peptide to serum albumin thus serving as a circulating depot. This approach is generally considered in the scientific and patent community as a standard approach to protract almost any given peptide. However, it is not trivial to prolong the half-life of peptides by lipidation and still maintain high potency and good formulation properties. Here we show that attaching a fatty acid to the obesity-drug relevant peptide PYY_3-36 is not sufficient for long pharmacokinetics (PK), since the position in the backbone, but also type of fatty acid and linker strongly influences PK and potency. Furthermore, understanding the proteolytic stability of the backbone is key to obtain long half-lives by lipidation, since backbone cleavage still occurs while associated to albumin. Having identified a PYY analogue with a sufficient half-life, we show that in combination with a GLP-1 analogue, liraglutide, additional weight loss can be achieved in the obese minipig model.

Trefoil Factor Family Member 2: From a High-Fat-Induced Gene to a Potential Obesity Therapy Target

Obesity has its epidemiological patterns continuously increasing. With controlling both diet and exercise being the main approaches to manage the energy metabolism balance, a high-fat (HF) diet is of particular importance. Indeed, lipids have a low satiety potential but a high caloric density. Thus, focusing on pharmacologically targetable pathways remains an approach with promising therapeutic potential. Within this context, trefoil factor family member 2 (Tff2) has been characterized as specifically induced by HF diet rather than low-fat diet. TFF2 has also been linked to diverse neurological mechanisms and metabolic patterns suggesting its role in energy balance. The hypothesis is that TFF2 would be a HF diet-induced signal that regulates metabolism with a focus on lipids. Within this review, we put the spotlight on key findings highlighting this line of thought. Importantly, the hypothetical mechanisms pointed highlight TFF2 as an important contributor to obesity development via increasing lipids intestinal absorption and anabolism. Therefore, an outlook for future experimental activities and evaluation of the therapeutic potential of TFF2 inhibition is given. Indeed, its knockdown or downregulation would contribute to an antiobesity phenotype. We believe this work represents an addition to our understanding of the lipidic molecular implications in obesity, which will contribute to develop therapies aiming to manage the lipidic metabolic pathways including the absorption, storage and metabolism via targeting TFF2-related pathways. We briefly discuss important relevant concepts for both basic and clinical researchers.

The subfornical organ and organum vasculosum of the lamina terminalis: Critical roles in cardiovascular regulation and the control of fluid balance

In this chapter, we review the extensive literature describing the roles of the subfornical organ (SFO), the organum vasculosum of the terminalis (OVLT), and the median preoptic nucleus (MnPO), comprising the lamina terminalis, in cardiovascular regulation and the control of fluid balance. We present this information in the context of both historical and technological developments which can effectively be overlaid upon each other. We describe intrinsic anatomy and connectivity and then discuss early work which described how circulating angiotensin II acts at the SFO to stimulate drinking and increase blood pressure. Extensive studies using direct administration and lesion approaches to highlight the roles of all regions of the lamina terminalis are then discussed. At the cellular level we describe c-Fos and electrophysiological work, which has highlighted an extensive group of circulating hormones which appear to influence the activity of specific neurons in the SFO, OVLT, and MnPO. We highlight optogenetic studies that have begun to unravel the complexities of circuitries underlying physiological outcomes, especially those related to different components of drinking. Finally, we describe the somewhat limited human literature supporting conclusions that these structures play similar and potentially important roles in human physiology.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

Impact of Gut and Metabolic Hormones on Feeding Reward

Ingestion of food activates a cascade of endocrine responses (thereby reflecting a contemporaneous feeding status) that include the release of hormones from the gastrointestinal (GI) tract, such as cholecystokinin (CCK), glucagonlike peptide YY (PYY), peptide PP, and oleoylethanolamide, as well as suppression of ghrelin secretion. The pancreas and adipose tissue, on the other hand, release hormones that serve as a measure of the current metabolic state or the long-term energy stores, that is, insulin, leptin, and adiponectin. It is well known and intuitively understandable that these hormones target either directly (by crossing the blood-brain barrier) or indirectly (e.g., via vagal input) the "homeostatic" brainstem-hypothalamic pathways involved in the regulation of appetite. The current article focuses on yet another target of the metabolic and GI hormones that is critical in inducing changes in food intake, namely, the reward system. We discuss the physiological basis of this functional interaction, its importance in the control of appetite, and the impact that disruption of this crosstalk has on energy intake in select physiological and pathophysiological states. We conclude that metabolic and GI hormones have a capacity to strengthen or weaken a response of the reward system to a given food, and thus, they are fundamental in ensuring that feeding reward is plastic and dependent on the energy status of the organism. © 2021 American Physiological Society. Compr Physiol 11:1425-1447, 2021.

PYY(3-36) and exendin-4 reduce food intake and activate neuronal circuits in a synergistic manner in mice

Peptide YY(3-36) ((PYY(3-36)) and glucagon like peptide 1 (GLP-1) in combination reduce food intake and body weight in an additive or synergistic manner in animal models and in humans. Nevertheless, the mechanisms behind are not completely understood. The present study aims to investigate the effect of combining PYY(3-36) and the GLP-1 receptor agonist exendin-4 (Ex4) by examining acute food intake and global neuronal activation as measured by c-fos in C57BL/6 J mice. An additive reduction in food intake was found 1.5 h after s.c dosing with the combination of PYY(3-36) (200 μg/kg) and Ex4 (2.5 μg/kg). This was associated with a synergistic enhancement of c-fos reactivity in central amygdalar nucleus (CeA), rostral part of the mediobasal arcuate nucleus (ARH), supratrigeminal nucleus (SUT), lateral parabrachial nucleus (PB), area postrema (AP) and nucleus tractus solitarius (NTS) compared to vehicle, PYY(3-36) and Ex4 individually dosed mice. The regions activated by Ex4 individually and PYY(3-36) and Ex4 in combination resembled each other, but the combination group had a significantly stronger c-fos response. Twenty-five brain areas were activated by PYY(3-36) and Ex4 in combination compared to vehicle versus nine brain areas by Ex4 individually. No significant increase in c-fos reactivity was found by PYY(3-36) compared to vehicle dosed mice. The neuronal activation of ARH and the AP/NTS to PB to CeA pathway is important for appetite regulation while SUT has not previously been reported in the regulation of energy balance. As PYY(3-36) and Ex4 act on different neurons leading to recruitment of different signalling pathways within and to the brain, an interaction of these pathways may contribute to their additive/synergistic action. Thus, PYY(3-36) boosts the effect of Ex4 possibly by inducing less inhibition of neuronal activity leading to an enhanced neuronal activity induced by Ex4.

Emerging therapeutic potential for peptide YY for obesity-diabetes

The vast majority of research to date on the gut hormone Peptide YY (PYY) has focused on appetite suppression and body weight regulation effects. These biological actions are believed to occur through interaction of PYY with hypothalamic Y₂ receptors. However, more recent studies have added additional knowledge to understanding of the physiological, and potential therapeutic, roles of PYY beyond obesity alone. Thus, PYY has now been shown to impart improvements in pancreatic beta-cell survival and function, with obvious benefits for diabetes. This effect has been linked mainly to binding and activation of Y₁ receptors by PYY, but more evidence is still required in this regard. Given the potential therapeutic promise of PYY-derived compounds, and complexity of receptor interactions, it is important to fully understand the complete biological action profile of PYY. Therefore, the current review aims to compile, evaluate and summarise current knowledge on PYY, with particular emphasis on obesity and diabetes treatment, and the importance of specific Y receptor interactions for this.

Ghrelin, CCK, GLP-1, and PYY(3-36): Secretory Controls and Physiological Roles in Eating and Glycemia in Health, Obesity, and After RYGB

The efficacy of Roux-en-Y gastric-bypass (RYGB) and other bariatric surgeries in the management of obesity and type 2 diabetes mellitus and novel developments in gastrointestinal (GI) endocrinology have renewed interest in the roles of GI hormones in the control of eating, meal-related glycemia, and obesity. Here we review the nutrient-sensing mechanisms that control the secretion of four of these hormones, ghrelin, cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1), and peptide tyrosine tyrosine [PYY(3-36)], and their contributions to the controls of GI motor function, food intake, and meal-related increases in glycemia in healthy-weight and obese persons, as well as in RYGB patients. Their physiological roles as classical endocrine and as locally acting signals are discussed. Gastric emptying, the detection of specific digestive products by small intestinal enteroendocrine cells, and synergistic interactions among different GI loci all contribute to the secretion of ghrelin, CCK, GLP-1, and PYY(3-36). While CCK has been fully established as an endogenous endocrine control of eating in healthy-weight persons, the roles of all four hormones in eating in obese persons and following RYGB are uncertain. Similarly, only GLP-1 clearly contributes to the endocrine control of meal-related glycemia. It is likely that local signaling is involved in these hormones' actions, but methods to determine the physiological status of local signaling effects are lacking. Further research and fresh approaches are required to better understand ghrelin, CCK, GLP-1, and PYY(3-36) physiology; their roles in obesity and bariatric surgery; and their therapeutic potentials.

Eating in mice with gastric bypass surgery causes exaggerated activation of brainstem anorexia circuit

Background/Objective

Obesity and metabolic diseases are at an alarming level globally and increasingly affect children and adolescents. Gastric bypass and other bariatric surgeries have proven remarkably successful and are increasingly performed worldwide. Reduced desire to eat and changes in eating behavior and food choice account for most of the initial weight loss and diabetes remission after surgery, but the underlying mechanisms of altered gut-brain communication are unknown.

Subjects/Methods

To explore the potential involvement of a powerful brainstem anorexia pathway centered around the lateral parabrachial nucleus (lPBN) we measured meal-induced neuronal activation by means of c-Fos immunohistochemistry in a new high-fat diet-induced obese mouse model of Roux-en-Y gastric bypass (RYGB) at 10 and 40 days after RYGB or sham surgery.

Results

Voluntary ingestion of a meal 10 days after RYGB, but not after sham surgery, strongly and selectively activates calcitonin gene-related peptide neurons in the external lPBN as well as neurons in the nucleus tractus solitaries, area postrema, and medial amygdala. At 40 days after surgery, meal-induced activation in all these areas was greatly diminished and did not reach statistical significance.

Conclusions

The neural activation pattern and dynamics suggest a role of the brainstem anorexia pathway in the early effects of RYGB on meal size and food intake that may lead to adaptive neural and behavioral changes involved in the control of food intake and body weight at a lower level. However, selective inhibition of this pathway will be required for a more causal implication.

Physiological roles for the subfornical organ: a dynamic transcriptome shaped by autonomic state

The subfornical organ (SFO) is a circumventricular organ recognized for its ability to sense and integrate hydromineral and hormonal circulating fluid balance signals, information which is transmitted to central autonomic nuclei to which SFO neurons project. While the role of SFO was once synonymous with physiological responses to osmotic, volumetric and cardiovascular challenge, recent data suggest that SFO neurons also sense and integrate information from circulating signals of metabolic status. Using microarrays, we have confirmed the expression of receptors already described in the SFO, and identified many novel transcripts expressed in this circumventricular organ including receptors for many of the critical circulating energy balance signals such as adiponectin, apelin, endocannabinoids, leptin, insulin and peptide YY. This transcriptome analysis also identified SFO transcripts, the expressions of which are significantly changed by either 72 h dehydration, or 48 h starvation, compared to fed and euhydrated controls. Expression and potential roles for many of these targets are yet to be confirmed and elucidated. Subsequent validation of data for adiponectin and leptin receptors confirmed that receptors for both are expressed in the SFO, that discrete populations of neurons in this tissue are functionally responsive to these adipokines, and that such responsiveness is regulated by physiological state. Thus, transcriptomic analysis offers great promise for understanding the integrative complexity of these physiological systems, especially with development of technologies allowing description of the entire transcriptome of single, carefully phenotyped, SFO neurons. These data will ultimately elucidate mechanisms through which these uniquely positioned neurons respond to and integrate complex circulating signals.

Central nervous system control of gastrointestinal motility and secretion and modulation of gastrointestinal functions

Although the gastrointestinal (GI) tract possesses intrinsic neural plexuses that allow a significant degree of autonomy over GI functions, the central nervous system (CNS) provides extrinsic neural inputs that regulate, modulate, and control these functions. While the intestines are capable of functioning in the absence of extrinsic inputs, the stomach and esophagus are much more dependent upon extrinsic neural inputs, particularly from parasympathetic and sympathetic pathways. The sympathetic nervous system exerts a predominantly inhibitory effect upon GI muscle and provides a tonic inhibitory influence over mucosal secretion while, at the same time, regulates GI blood flow via neurally mediated vasoconstriction. The parasympathetic nervous system, in contrast, exerts both excitatory and inhibitory control over gastric and intestinal tone and motility. Although GI functions are controlled by the autonomic nervous system and occur, by and large, independently of conscious perception, it is clear that the higher CNS centers influence homeostatic control as well as cognitive and behavioral functions. This review will describe the basic neural circuitry of extrinsic inputs to the GI tract as well as the major CNS nuclei that innervate and modulate the activity of these pathways. The role of CNS-centered reflexes in the regulation of GI functions will be discussed as will modulation of these reflexes under both physiological and pathophysiological conditions. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide these answers.

Vitamin B12 conjugation of peptide-YY(3-36) decreases food intake compared to native peptide-YY(3-36) upon subcutaneous administration in male rats

Challenges to peptide-based therapies include rapid clearance, ready degradation by hydrolysis/proteolysis, and poor intestinal uptake and/or a need for blood brain barrier transport. This work evaluates the efficacy of conjugation of vitamin B12 (B12) on sc administered peptide tyrosine tyrosine (PYY)(3-36) function. In the current experiments, a B12-PYY(3-36) conjugate was tested against native PYY(3-36), and an inactive conjugate B12-PYYC36 (null control) in vitro and in vivo. In vitro experiments demonstrated similar agonism for the neuropeptide Y2 receptor by the B12-PYY(3-36) conjugate (EC50 26.5 nM) compared with native PYY(3-36) (EC50 16.0 nM), with the null control having an EC50 of 1.8 μM. In vivo experiments were performed in young adult male Sprague Dawley rats (9 wk). Daily treatments were delivered sc in five 1-hour pulses, each pulse delivering 5-10 nmol/kg, by implanted microinfusion pumps. Increases in hindbrain Fos expression were comparable 90 minutes after B12-PYY(3-36) or PYY3-36 injection relative to saline or B12-PYYC36. Food intake was reduced during a 5-day treatment for both B12-PYY(3-36)- (24%, P = .001) and PYY(3-36)-(13%, P = .008) treated groups relative to baseline. In addition, reduction of food intake after the three dark cycle treatment pulses was more consistent with B12-PYY(3-36) treatment (-26%, -29%, -27%) compared with the PYY(3-36) treatment (-3%, -21%, -16%), and B12-PYY(3-36) generated a significantly longer inhibition of food intake vs. PYY(3-36) treatment after the first two pulses (P = .041 and P = .036, respectively). These findings demonstrate a stronger, more consistent, and longer inhibition of food intake after the pulses of B12-PYY(3-36) conjugate compared with the native PYY(3-36).

Appetite and body weight regulation after bariatric surgery

Bariatric surgery continues to be remarkably efficient in treating obesity and type 2 diabetes mellitus and a debate has started whether it should remain the last resort only or also be used for the prevention of metabolic diseases. Intense research efforts in humans and rodent models are underway to identify the critical mechanisms underlying the beneficial effects with a view towards non-surgical treatment options. This non-systematic review summarizes and interprets some of this literature, with an emphasis on changes in the controls of appetite. Contrary to earlier views, surgery-induced reduction of energy intake and subsequent weight loss appear to be the main drivers for rapid improvements of glycaemic control. The mechanisms responsible for suppression of appetite, particularly in the face of the large weight loss, are not well understood. Although a number of changes in food choice, taste functions, hedonic evaluation, motivation and self-control have been documented in both humans and rodents after surgery, their importance and relative contribution to diminished appetite has not yet been demonstrated. Furthermore, none of the major candidate mechanisms postulated in mediating surgery-induced changes from the gut and other organs to the brain, such as gut hormones and sensory neuronal pathways, have been confirmed yet. Future research efforts should focus on interventional rather than descriptive approaches in both humans and rodent models.

Role of capsaicin-sensitive peripheral sensory neurons in anorexic responses to intravenous infusions of cholecystokinin, peptide YY-(3-36), and glucagon-like peptide-1 in rats

Cholecystokinin (CCK)-induced suppression of feeding is mediated by vagal sensory neurons that are destroyed by the neurotoxin capsaicin (CAP). Here we determined whether CAP-sensitive neurons mediate anorexic responses to intravenous infusions of gut hormones peptide YY-(3-36) [PYY-(3-36)] and glucagon-like peptide-1 (GLP-1). Rats received three intraperitoneal injections of CAP or vehicle (VEH) in 24 h. After recovery, non-food-deprived rats received at dark onset a 3-h intravenous infusion of CCK-8 (5, 17 pmol·kg⁻¹·min⁻¹), PYY-(3-36) (5, 17, 50 pmol·kg⁻¹·min⁻¹), or GLP-1 (17, 50 pmol·kg⁻¹·min⁻¹). CCK-8 was much less effective in reducing food intake in CAP vs. VEH rats. CCK-8 at 5 and 17 pmol·kg⁻¹·min⁻¹ reduced food intake during the 3-h infusion period by 39 and 71% in VEH rats and 7 and 18% in CAP rats. In contrast, PYY-(3-36) and GLP-1 were similarly effective in reducing food intake in VEH and CAP rats. PYY-(3-36) at 5, 17, and 50 pmol·kg⁻¹·min⁻¹ reduced food intake during the 3-h infusion period by 15, 33, and 70% in VEH rats and 13, 30, and 33% in CAP rats. GLP-1 at 17 and 50 pmol·kg⁻¹·min⁻¹ reduced food intake during the 3-h infusion period by 48 and 60% in VEH rats and 30 and 52% in CAP rats. These results suggest that anorexic responses to PYY-(3-36) and GLP-1 are not primarily mediated by the CAP-sensitive peripheral sensory neurons (presumably vagal) that mediate CCK-8-induced anorexia.

Peptide YY: more than just an appetite regulator

Replenishment of beta cell mass is a key aim of novel therapeutic interventions for diabetes, and the implementation of new strategies will be aided by understanding the mechanisms employed to regulate beta cell mass under normal physiological conditions. We have recently identified a new role for the gut hormone peptide YY (PYY) and the neuropeptide Y (NPY) receptor systems in the control of beta cell survival. PYY is perhaps best known for its role in regulating appetite and body weight, but its production by islet cells, the presence of NPY receptors on islets and the demonstration that Y1 activation causes proliferation of beta cells and protects them from apoptosis, suggest a role for this peptide in modulating beta cell mass. This review introduces PYY and its potential role in glucose homeostasis, then focuses on evidence supporting the concept that PYY and NPY receptors are exciting new targets for the preservation of beta cells.

GLP-1 receptor signaling is not required for reduced body weight after RYGB in rodents

Exaggerated GLP-1 and PYY secretion is thought to be a major mechanism in the reduced food intake and body weight after Roux-en-Y gastric bypass surgery. Here, we use complementary pharmacological and genetic loss-of-function approaches to test the role of increased signaling by these gut hormones in high-fat diet-induced obese rodents. Chronic brain infusion of a supramaximal dose of the selective GLP-1 receptor antagonist exendin-9-39 into the lateral cerebral ventricle significantly increased food intake and body weight in both RYGB and sham-operated rats, suggesting that, while contributing to the physiological control of food intake and body weight, central GLP-1 receptor signaling tone is not the critical mechanism uniquely responsible for the body weight-lowering effects of RYGB. Central infusion of the selective Y2R-antagonist BIIE0246 had no effect in either group, suggesting that it is not critical for the effects of RYGB on body weight under the conditions tested. In a recently established mouse model of RYGB that closely mimics surgery and weight loss dynamics in humans, obese GLP-1R-deficient mice lost the same amount of body weight and fat mass and maintained similarly lower body weight compared with wild-type mice. Together, the results surprisingly provide no support for important individual roles of either gut hormone in the specific mechanisms by which RYGB rats settle at a lower body weight. It is likely that the beneficial effects of bariatric surgeries are expressed through complex mechanisms that require combination approaches for their identification.

Preparation and characterization of albumin conjugates of a truncated peptide YY analogue for half-life extension

Recombinant human serum albumin (HSA) conjugates of a 15-amino-acid truncated peptide YY (PYY) analogue were prepared using three heterobifunctional linkers [succinimidyl 4-[N-maleimidomethyl]cyclohexane-1-carboxylate (SMCC), 6-maleimidohexanoic acid N-hydroxysuccinimide ester (MHS), and N-[γ-maleimidobutyryloxy]sulfosuccinimide ester (GMBS)] in 2 synthetic steps involving (1) reaction of succinimidyl ester on linker with ε-amine of Lys2 on the peptide and (2) reaction of maleimide on peptide linker with free thiol of Cysteine 34 (Cys34) on albumin. In-process controls using ESI LC-MS were used to follow reactions and identify reaction products. Proteolytic digests of the conjugate revealed that peptide conjugation occurs at Cys34 on HSA. Conjugates were assayed in cell-based assays to determine potency at the human Y2-receptor, and selectivity at the human Y1-, Y4-, and Y5-receptors using a calcium flux assay. All three conjugates assayed were selective agonists of the Y2-receptor, and displayed nanomolar potencies. MCC and MH conjugates were selected for acute PK/PD studies in DIO mice. Significant reduction in food intake was observed with the MH conjugate, which lasted for 24 h at the 10 mg (or 4 μmol)/kg dose. While the MCC conjugate exhibited greater potency in vitro, it was slightly less effective than the MH conjugate in vivo with respect to reduction in food intake. Both conjugates were significantly less active than the peptide coupled to a 30 kDa PEG. The observed T1/2 (8-9 h) for both conjugates was significantly lower than that observed for the PEGylated peptide (∼25 h). These results suggest that, as compared with the unmodified and PEGylated peptide, the extended circulation half-life of albumin conjugates is mediated through uptake and recirculation by FcRn, and allometric scaling methods are necessary to account for interspecies variation in pharmacokinetic properties.

Integration of satiety signals by the central nervous system

Individual meals are products of a complex interaction of signals related to both short-term and long-term availability of energy stores. In addition to maintaining the metabolic demands of the individual in the short term, levels of energy intake must also maintain and defend body weight over longer periods. To accomplish this, satiety pathways are regulated by a sophisticated network of endocrine and neuroendocrine pathways. Higher brain centers modulate meal size through descending inputs to caudal brainstem regions responsible for the motor pattern generators associated with ingestion. Gastric and intestinal signals interact with central nervous system pathways to terminate food intake. These inputs can be modified as a function of internal metabolic signals, external environmental influences, and learning to regulate meal size.

Role of peptide YY(3-36) in the satiety produced by gastric delivery of macronutrients in rats

Peptide YY(3-36) [PYY(3-36)] is postulated to act as a hormonal signal from gut to brain to inhibit food intake. PYY(3-36) potently reduces food intake when administered systemically or into the brain. If action of endogenous PYY(3-36) is necessary for normal satiation to occur, then pharmacological blockade of its receptors should increase food intake. Here, we determined the effects of iv infusion of Y1, Y2, and Y5 receptor antagonists (BIBP 3226, BIIE 0246, CGP 71683) during the first 3 h of the dark period on food intake in non-food-deprived rats. Our results showed that 1) Y2 receptor blockade reversed the anorexic response to iv infusion of PYY(3-36) but did not increase food intake when administered alone; 2) Y1 and Y5 receptor antagonists neither attenuated PYY(3-36)-induced anorexia nor altered food intake when given alone; and 3) Y2 receptor blockade attenuated anorexic responses to gastric infusions of casein hydrolysate and long-chain triglycerides, but not maltodextrin. Previous work showed that Y2 antagonist BIIE 0246 does not penetrate the blood-brain barrier. Together, these results support the hypothesis that gut PYY(3-36) action at Y2 receptors peripheral to the blood brain barrier plays an essential role in mediating satiety responses to gastric delivery of protein and long-chain triglycerides, but not polysaccharide.

The interaction of amylin with other hormones in the control of eating

Twenty years of research established amylin as an important control of energy homeostasis. Amylin controls nutrient and energy fluxes by reducing energy intake, by modulating nutrient utilization via an inhibition of postprandial glucagon secretion and by increasing energy disposal via a prevention of compensatory decreases of energy expenditure in weight reduced individuals. Like many other gastrointestinal hormones, amylin is secreted in response to meals and it reduces eating by promoting meal-ending satiation. Not surprisingly, amylin interacts with many of these hormones to control eating. These interactions seem to occur at different levels because amylin seems to mediate the eating inhibitory effect of some of these gastrointestinal hormones, and the combination of some of these hormones seems to lead to a stronger reduction in eating than single hormones alone. Amylin's effect on eating is thought to be mediated by a stimulation of specific amylin receptors in the area postrema. Secondary brain sites that were defined to mediate amylin action - and hence potential additional sites of interaction with other hormones - include the nucleus of the solitary tract, the lateral parabrachial nucleus, the lateral hypothalamic area and other hypothalamic nuclei. The focus of this review is to summarize the current knowledge of amylin interactions in the control of eating. In most cases, these interactions have only been studied at a descriptive rather than a mechanistic level and despite the clear knowledge on primary sites of amylin action, the interaction sites between amylin and other hormones are often unknown.

Possible mechanisms of circulating PYY-induced satiation in male rats

Peptide tyrosine-tyrosine (PYY) is implicated in eating control, but the site(s) and mechanism(s) of its action remain uncertain. We tested acute effects of intrameal hepatic portal vein (HPV) PYY(3-36) infusions on eating in adult, male rats and measured HPV and jugular vein (JV) plasma levels of PYY in response to a solid, mixed-nutrient meal. We also examined the effects of HPV PYY(3-36) infusions on JV plasma levels, flavor acceptance, and neuronal activation. Intrameal HPV PYY(3-36) infusions [1 and 3 nmol/kg body weight (BW)] selectively reduced (P < 0.05) ongoing meal size. HPV PYY levels increased (P < 0.05) during a chow (12.5 kcal) or an isocaloric high-fat meal. JV PYY levels were generally lower than HPV levels but also increased in response to the chow meal. HPV PYY(3-36) infusion (1 nmol/kg BW) caused a greater increase in JV PYY than a meal, but neither 1 nor 3 nmol/kg BW PYY(3-36) caused conditioned flavor avoidance. HPV PYY(3-36) (1 nmol/kg BW) increased the number of c-Fos-expressing cells in the nucleus tractus solitarii, the hypothalamic arcuate and paraventricular nuclei, the central area of the amygdala, and the nucleus accumbens but not in the area postrema and parabrachial nucleus. These data show that HPV infusions of PYY(3-36) inhibit eating in rats without causing avoidance, and they identify some brain areas that might be involved. Endogenous PYY may induce satiation by acting directly in the brain, but further studies should examine whether PYY(3-36) administrations that mimic the meal-induced increase in plasma PYY are sufficient to inhibit eating.

Bowels control brain: gut hormones and obesity

Peptide hormones are released from the gastrointestinal tract in response to nutrients and communicate information regarding the current state of energy balance to the brain. These hormones regulate appetite, energy expenditure and glucose homeostasis. They can act either via the circulation at target peripheral tissues, by activation of the vagus nerve or by acting on key brain regions implicated in energy homeostasis such as the hypothalamus and brainstem. This review gives an overview of the main gut hormones implicated in the regulation of food intake and how some of these are being targeted to develop anti obesity treatments.

The receptive function of hypothalamic and brainstem centres to hormonal and nutrient signals affecting energy balance

The hypothalamic arcuate nucleus (ARC) and the area postrema (AP) represent targets for hormonal and metabolic signals involved in energy homoeostasis, e.g. glucose, amylin, insulin, leptin, peptide YY (PYY), glucagon-like peptide 1 (GLP-1) and ghrelin. Orexigenic neuropeptide Y expressing ARC neurons are activated by food deprivation and inhibited by feeding in a nutrient-dependent manner. PYY and leptin also reverse or prevent fasting-induced activation of the ARC. Interestingly, hypothalamic responses to fasting are blunted in different models of obesity (e.g. diet-induced obesity (DIO) or late-onset obesity). The AP also responds to feeding-related signals. The pancreatic hormone amylin acts via the AP to control energy intake. Amylin-sensitive AP neurons are also glucose-responsive. Furthermore, diet-derived protein attenuates amylin responsiveness suggesting a modulation of AP sensitivity by macronutrient supply. This review gives an overview of the receptive function of the ARC and the AP to hormonal and nutritional stimuli involved in the control of energy balance and the possible implications in the context of obesity. Collectively, there is consistency between the neurophysiological actions of these stimuli and their effects on energy homoeostasis under experimental conditions. However, surprisingly little progress has been made in the development of effective pharmacological approaches against obesity. A promising way to improve effectiveness involves combination treatments (e.g. amylin/leptin agonists). Hormonal alterations (e.g. GLP-1 and PYY) are also considered to mediate body weight loss observed in obese patients receiving bariatric surgery. The effects of hormonal and nutritional signals and their interactions might hold the potential to develop poly-mechanistic therapeutic strategies against obesity.

The role of the gut/brain axis in modulating food intake

Peptide hormones released from the gastrointestinal tract communicate information about the current state of energy balance to the brain. These hormones regulate appetite and energy expenditure via the vagus nerve or by acting on key brain regions implicated in energy homeostasis such as the hypothalamus and brainstem. This review gives an overview of the main gut hormones implicated in the regulation of food intake. Research in this area has provided novel targets for the pharmacological treatment of obesity. This article is part of a Special Issue entitled 'Central Control Food Intake'

Area postrema lesions attenuate LiCl-induced c-Fos expression correlated with conditioned taste aversion learning

Lesions of the area postrema (AP) block many of the behavioral and physiological effects of lithium chloride (LiCl) in rats, including formation of conditioned taste aversions (CTAs). Systemic administration of LiCl induces c-Fos immunoreactivity in several brain regions, including the AP, nucleus of the solitary tract (NTS), lateral parabrachial nucleus (latPBN), supraoptic nucleus (SON), paraventricular nucleus (PVN), and central nucleus of the amygdala (CeA). To determine which of these brain regions may be activated in parallel with the acquisition of LiCl-induced CTAs, we disrupted CTA learning in rats by ablating the AP and then quantified c-Fos-positive cells in these brain regions in sham- and AP-lesioned rats 1 h following LiCl or saline injection. Significant c-Fos induction after LiCl was observed in the CeA and SON of AP-lesioned rats, demonstrating activation independent of an intact AP. LiCl-induced c-Fos was significantly attenuated in the NTS, latPBN, PVN and CeA of AP-lesioned rats, suggesting that these regions are dependent on AP activation. Almost all of the lesioned rats showed some damage to the subpostremal NTS, and some rats also had damage to the dorsal motor nucleus of the vagus; this collateral damage in the brainstem may have contributed to the deficits in c-Fos response. Because c-Fos induction in several regions was correlated with magnitude of CTA acquisition, these regions are implicated in the central mediation of lithium effects during CTA learning.