Obesity medication lorcaserin activates brainstem GLP-1 neurons to reduce food intake and augments GLP-1 receptor agonist induced appetite suppression

GIPR agonism and antagonism decrease body weight and food intake via different mechanisms in male mice

Agonists and antagonists of the glucose-dependent insulinotropic polypeptide receptor (GIPR) enhance body weight loss induced by glucagon-like peptide-1 receptor (GLP-1R) agonism. However, while GIPR agonism decreases body weight and food intake in a GLP-1R-independent manner via GABAergic GIPR+ neurons, it remains unclear whether GIPR antagonism affects energy metabolism via a similar mechanism. Here we show that the body weight and food intake effects of GIPR antagonism are eliminated in mice with global loss of either Gipr or Glp-1r but are preserved in mice with loss of Gipr in either GABAergic neurons of the central nervous system or peripherin-expressing neurons of the peripheral nervous system. Single-nucleus RNA-sequencing shows opposing effects of GIPR agonism and antagonism in the dorsal vagal complex, with antagonism, but not agonism, closely resembling GLP-1R signalling. Additionally, GIPR antagonism and GLP-1R agonism both regulate genes implicated in synaptic plasticity. Collectively, we show that GIPR agonism and antagonism decrease body weight via different mechanisms, with GIPR antagonism, unlike agonism, depending on functional GLP-1R signalling.

Targeting central pathway of Glucose-Dependent Insulinotropic Polypeptide, Glucagon and Glucagon-like Peptide-1 for metabolic regulation in obesity and type 2 diabetes

Obesity and type 2 diabetes are significant public health challenges that greatly impact global well-being. The development of effective therapeutic strategies has become more and more concentrated on the central nervous system and metabolic regulation. The primary pharmaceutical interventions for the treatment of obesity and uncontrolled hyperglycemia are now generally considered to be incretin-based anti-diabetic treatments, particularly glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide receptor agonists. This is a result of their substantial influence on the central nervous system and the consequent effects on energy balance and glucose regulation. It is increasingly crucial to understand the neural pathways of these pharmaceuticals. The purpose of this review is to compile and present the most recent central pathways regarding glucagon-like peptide-1, glucose-dependent insulinotropic polypeptide and glucagon receptors, with a particular emphasis on central metabolic regulation.

Serotonin signaling to regulate energy metabolism: a gut microbiota perspective

Serotonin is one of the most potent gastrointestinal, peripheral, and neuronal signaling molecules and plays a key role in regulating energy metabolism. Accumulating evidence has shown the complex interplay between gut microbiota and host energy metabolism. In this review, we summarize recent findings on the role of gut microbiota in serotonin metabolism and discuss the complicated mechanisms by which serotonin, working in conjunction with the gut microbiota, affects total body energy metabolism in the host. Gut microbiota affects serotonin synthesis, storage, release, transport, and catabolism. In addition, serotonin plays an indispensable role in regulating host energy homeostasis through organ crosstalk and microbe-host communication, particularly with a wide array of serotonergic effects mediated by diverse serotonin receptors with unique tissue specificity. This fresh perspective will help broaden the understanding of serotonergic signaling in modulating energy metabolism, thus shedding light on the design of innovative serotonin-targeting strategies to treat metabolic diseases.

Advance in peptide-based drug development: delivery platforms, therapeutics and vaccines

The successful approval of peptide-based drugs can be attributed to a collaborative effort across multiple disciplines. The integration of novel drug design and synthesis techniques, display library technology, delivery systems, bioengineering advancements, and artificial intelligence have significantly expedited the development of groundbreaking peptide-based drugs, effectively addressing the obstacles associated with their character, such as the rapid clearance and degradation, necessitating subcutaneous injection leading to increasing patient discomfort, and ultimately advancing translational research efforts. Peptides are presently employed in the management and diagnosis of a diverse array of medical conditions, such as diabetes mellitus, weight loss, oncology, and rare diseases, and are additionally garnering interest in facilitating targeted drug delivery platforms and the advancement of peptide-based vaccines. This paper provides an overview of the present market and clinical trial progress of peptide-based therapeutics, delivery platforms, and vaccines. It examines the key areas of research in peptide-based drug development through a literature analysis and emphasizes the structural modification principles of peptide-based drugs, as well as the recent advancements in screening, design, and delivery technologies. The accelerated advancement in the development of novel peptide-based therapeutics, including peptide-drug complexes, new peptide-based vaccines, and innovative peptide-based diagnostic reagents, has the potential to promote the era of precise customization of disease therapeutic schedule.

The interplay between hypothalamic and brainstem nuclei in homeostatic control of energy balance

Energy balance and body weight are tightly regulated by homeostatic and hedonic systems of the brain. These systems are ultimately finely tuned by hypothalamic and extrahypothalamic neurocircuitry that modulate feeding and the appetite signalling cascade. The hypothalamus has been extensively researched and its role in homeostatic regulation of energy balance is well established. Later on, evidence indicated that extrahypothalamic signalling also has a critical role in regulation of body mass across the lifespan. One of these brain regions was the brainstem and specifically the dorsal vagal complex (DVC), which comprises of the area postrema (AP), nucleus of the solitary tract (NTS) and dorsal motor complex of the vagus (DMV). These brain stem nuclei were shown to also finely tune feeding behaviour through catecholaminergic, glutamatergic, and GABAergic signals. Moreover, these nuclei also receive afferent signals from the viscera through the gut, as well as humoral input from the bloodstream. Therefore, these brain stem nuclei are deemed as the port of entry where initial appetite-related signals are first conveyed and then modulated to the forebrain to hypothalamic and extrahypothalamic regions such as the arcuate nucleus (ARC) and parabrachial nucleus (PBN). Understanding the intricate interactions and projections between hypothalamic and brainstem nuclei is instrumental to comprehend energy balance regulation as a whole and to potentially address metabolic conditions such as diabetes and obesity. Further research in this area may lead to the development of targeted pharmacological and lifestyle intervention strategies that could lead to mitigation of metabolic disorders and/or promote a healthier body mass across the life span.

The metabolic and cardiovascular effects of amphetamine are partially mediated by the central melanocortin system

Amphetamine (AMPH) exerts metabolic and cardiovascular effects. The central melanocortin system is a key regulator of both metabolic and cardiovascular functions. Here, we show that the melanocortin system partially mediates AMPH-induced anorexia, energy expenditure, tachycardia, and hypertension. AMPH increased α-melanocyte stimulating hormone (αMSH) secretion from the hypothalamus, elevated blood pressure and heart rate (HR), increased brown adipose tissue (BAT) thermogenesis, and reduced both food intake (FI) and body weight (BW). In melanocortin 4 receptor-deficient (MC4R knockout [KO]) mice, metabolic and cardiovascular effects of AMPH were significantly attenuated. Antagonism of serotonergic and noradrenergic neurotransmitter systems attenuated AMPH-induced αMSH secretion as well as AMPH-induced metabolic and cardiovascular effects. We propose that AMPH increases serotonergic activation of proopiomelanocortin (POMC) neurons and reduces the noradrenergic inhibition of POMC neurons, thereby disinhibiting them. Together, these presynaptic mechanisms result in increased POMC activity, increased αMSH secretion, and increased activation of MC4R pathways that regulate both the metabolic and cardiovascular systems.

GLP-1 and the Neurobiology of Eating Control: Recent Advances

Obesity is now considered a chronic relapsing progressive disease, associated with increased all-cause mortality that scales with body weight, affecting more than 1 billion people worldwide. Excess body fat is strongly associated with excess energy intake, and most successful anti-obesity medications (AOMs) counter this positive energy balance through the suppression of eating to drive weight loss. Historically, AOMs have been characterized by modest weight loss and side effects which are compliance-limiting, and in some cases life-threatening. However, the field of obesity pharmacotherapy has now entered a new era of AOMs based on analogues of the gut hormone and neuropeptide glucagon-like peptide-1 (GLP-1). The latest versions of these drugs elicit unprecedented levels of weight loss in clinical trials, which are now starting to be substantiated in real-world usage. Notably, these drugs reduce weight primarily by reducing energy intake, via activation of the GLP-1 receptor on multiple sites of action primarily in the central nervous system, although the most relevant sites of action, and the neural circuits recruited remain contentious. Here we provide a targeted synthesis of recent developments in the field of GLP-1 neurobiology, highlighting studies which have advanced our understanding of how GLP-1 signaling modulates eating, and identify open questions and future challenges we believe still need to be addressed to aid the prevention and/or treatment of obesity.

Bioactive compounds regulate appetite through the melanocortin system: a review

Obesity, a significant health crisis, arises from an imbalance between energy intake and expenditure. Enhancing appetite regulation has garnered substantial attention from researchers as a novel and effective strategy for weight management. The melanocortin system, situated in the hypothalamus, is recognized as a critical node in the regulation of appetite. It integrates long-term and short-term hormone signals from the periphery as well as nutrients, forming a complex network of interacting feedback mechanisms with the gut-brain axis, significantly contributing to the regulation of energy homeostasis. Appetite regulation by bioactive compounds has been a focus of intensive research due to their favorable safety profiles and easy accessibility. These bioactive compounds, derived from a variety of plant and animal sources, modulate the melanocortin system and influence appetite and energy homeostasis through multiple pathways: central nervous system, peripheral hormones, and intestinal microbiota. Here, we review the anatomy, function, and receptors of the melanocortin system, outline the long-term and short-term regulatory hormones that act on the melanocortin system, and discuss the bioactive compounds and their mechanisms of action that exert a regulatory effect on appetite by targeting the melanocortin system. This review contributes to a better understanding of how bioactive compounds regulate appetite via the melanocortin system, thereby providing nutritional references for citizens' dietary preferences.

The 5-HT-related gut-brain axis in obesity

AIMS

The incidence of obesity increases annually. It is closely related to the occurrence of cardiovascular diseases, malignant tumors, etc., and has become a major global health problem. 5-hydroxytryptamine (5-HT), a multifunctional monoamine neurotransmitter, is dispersed throughout the central nervous system and digestive tract. It is intimately related to the mechanism of obesity.

MATERIALS AND METHODS

PubMed, Web of Science and Embase were carefully searched. We collected articles that are closely related to 5-HT, the gut-brain axis, and obesity.

KEY FINGDINGS

The gut microbiota not only influences nutrient metabolism but also centrally meditates appetite and mood regulation. The gut-brain axis, a system connecting the gut and the brain, is known to participate in two-way communication between the gut flora and the central nervous system.

SIGNIFICANCE

There have been few reports on whether peripheral and central 5-HT interact bidirectionally via the gut-brain axis and jointly play a role in the pathogenesis of obesity. In this review, we summarize the rationale for the contribution of the 5-HT-related gut-brain axis to the development of obesity and explore feasible signaling pathways, which elucidates new targets for preventing and treating obesity.

The cost-effectiveness analysis of semaglutide for the treatment of adult and adolescent patients with overweight and obesity: a systematic review

PURPOSE

The present study aimed to systematically collect and synthesize available cost-effectiveness studies of semaglutide in patients with obesity or overweight in comparison with other interventions.

METHODS

We comprehensively searched multiple electronic databases to identify relevant literature. Studies were selected based on inclusion and exclusion criteria. The quality of studies was appraised using the "Consolidated Health Economic Evaluation Reporting Standards" (CHEERS) tool. This study is conducted and reported following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement.

RESULTS

Out of a total of 252 items, after review, 32 articles were fully reviewed, and, finally, 7 studies met inclusion and exclusion criteria. The discount rate was in the range of 1.5-3.5%. Studies included showed semaglutide offered more QALYs than anti-obesity drugs but because of higher cost, in some cases, ICER exceeds the willingness to pay threshold. Results show that semaglutide creates higher total cost compared to conventional interventions in patients with class I, II, and III obesities. Results show that in patients with class I obesity (BMI 33) lifestyle intervention (LI), endoscopic sleeve gastroplasty (ESG), Sleeve gastrectomy (SG), and semaglutide create $124,195; $126,732; $139,971; and $370,776, respectively.

CONCLUSION

The current systematic review showed that semaglutide provides more QALYs and creates more costs in comparison with phentermine-topiramate, phentermine, and naltrexone-bupropion. Semaglutide may be cost-effective with substantial cost reduction. Semaglutide appears to be cost-effective versus diet and exercise (D&E) and liraglutide but it was not cost-effective versus sleeve gastrectomy, endoscopic sleeve gastroplasty, and gastric bypass.

Psychotropic drugs, eating behaviour and weight gain

PURPOSE OF REVIEW

Psychotropic drug related weight gain is a common side-effect of significant concern to both clinicians and patients. Recent studies and treatment guidelines strongly support taking preventive and early treatment approaches to psychotropic drug-related weight gain (PDWG). Arguably the main pathway that PDWG occurs is via changes in eating behaviour leading to increased caloric intake.

RECENT FINDINGS

Systematic reviews and meta-analyses have provided good data on the nature and prevalence of alterations in eating behaviour with psychotropic treatment including increased hunger, night eating and binge eating. These changes are unsurprisingly more prominent with agents like olanzapine and clozapine that have high propensity to cause weight gain.

SUMMARY

Altered eating behaviour can serve as an earlier measure of the risk of weight gain and can be examined easily in clinical practice. Detecting these changes can enable earlier action in terms of switching treatments and starting pharmacological and nonpharmacological preventive strategies.

A Light-Responsive Neural Circuit Suppresses Feeding

Light plays an essential role in a variety of physiological processes, including vision, mood, and glucose homeostasis. However, the intricate relationship between light and an animal's feeding behavior has remained elusive. Here, we found that light exposure suppresses food intake, whereas darkness amplifies it in male mice. Interestingly, this phenomenon extends its reach to diurnal male Nile grass rats and healthy humans. We further show that lateral habenula (LHb) neurons in mice respond to light exposure, which in turn activates 5-HT neurons in the dorsal Raphe nucleus (DRN). Activation of the LHb→5-HTDRN circuit in mice blunts darkness-induced hyperphagia, while inhibition of the circuit prevents light-induced anorexia. Together, we discovered a light-responsive neural circuit that relays the environmental light signals to regulate feeding behavior in mice.

Interaction of serotonin/GLP-1 circuitry in a dual preclinical model for psychiatric disorders and metabolic dysfunction

Isolation of rodents throughout adolescence is known to induce many behavioral abnormalities which resemble neuropsychiatric disorders. Separately, this paradigm has also been shown to induce long-term metabolic changes consistent with a pre-diabetic state. Here, we investigate changes in central serotonin (5-HT) and glucagon-like peptide 1 (GLP-1) neurobiology that dually accompany behavioral and metabolic outcomes following social isolation stress throughout adolescence. We find that adolescent-isolation mice exhibit elevated blood glucose levels, impaired peripheral insulin signaling, altered pancreatic function, and fattier body composition without changes in bodyweight. These mice further exhibited disruptions in sleep and enhanced nociception. Using bulk and spatial transcriptomic techniques, we observe broad changes in neural 5-HT, GLP-1, and appetitive circuits. We find 5-HT neurons of adolescent-isolation mice to be more excitable, transcribe fewer copies of Glp1r (mRNA; GLP-1 receptor), and demonstrate resistance to the inhibitory effects of the GLP-1R agonist semaglutide on action potential thresholds. Surprisingly, we find that administration of semaglutide, commonly prescribed to treat metabolic syndrome, induced deficits in social interaction in group-housed mice and rescued social deficits in isolated mice. Overall, we find that central 5-HT circuitry may simultaneously influence mental well-being and metabolic health in this model, via interactions with GLP-1 and proopiomelanocortin circuitry.

Recent Advances in the Therapeutic Potential of Carotenoids in Preventing and Managing Metabolic Disorders

Carotenoids constitute compounds of significant biological interest due to their multiple biological activities, such as antimicrobial, anticancer, antiadipogenic, antidiabetic, and antioxidant properties. Metabolic syndrome (MetS) comprehends a series of metabolic abnormalities (e.g., hypertension, obesity, and atherogenic dyslipidemia) that can affect children, adolescents, and the elderly. The treatment of MetS involves numerous medications, which, despite their efficacy, pose challenges due to prolonged use, high costs, and various side effects. Carotenoids and their derivatives have been proposed as alternative treatments to MetS because they reduce serum triglyceride concentrations, promote insulin response, inhibit adipogenesis, and downregulate angiotensin-converting enzyme activity. However, carotenoids are notably sensitive to pH, light exposure, and temperature. This review addresses the activity of carotenoids such as lycopene, lutein, fucoxanthin, astaxanthin, crocin, and β-carotene towards MetS. It includes a discussion of sources, extraction methods, and characterization techniques for analyzing carotenoids. Encapsulation approaches are critically reviewed as alternatives to prevent degradation and improve the biological performance of carotenoids. A brief overview of the physiopathology and epidemiology of the diseases, including MetS, is also provided.

Glucagon-Like Peptide-1: New Regulator in Lipid Metabolism

Glucagon-like peptide-1 (GLP-1) is a 30-amino acid peptide hormone that is mainly expressed in the intestine and hypothalamus. In recent years, basic and clinical studies have shown that GLP-1 is closely related to lipid metabolism, and it can participate in lipid metabolism by inhibiting fat synthesis, promoting fat differentiation, enhancing cholesterol metabolism, and promoting adipose browning. GLP-1 plays a key role in the occurrence and development of metabolic diseases such as obesity, nonalcoholic fatty liver disease, and atherosclerosis by regulating lipid metabolism. It is expected to become a new target for the treatment of metabolic disorders. The effects of GLP-1 and dual agonists on lipid metabolism also provide a more complete treatment plan for metabolic diseases. This article reviews the recent research progress of GLP-1 in lipid metabolism.

A brainstem to hypothalamic arcuate nucleus GABAergic circuit drives feeding

The obesity epidemic is principally driven by the consumption of more calories than the body requires. It is therefore essential that the mechanisms underpinning feeding behavior are defined. Neurons within the brainstem dorsal vagal complex (DVC) receive direct information from the digestive system and project to second-order regions in the brain to regulate food intake. Although γ-aminobutyric acid is expressed in the DVC (GABADVC), its function in this region has not been defined. In order to discover the unique gene expression signature of GABADVC cells, we used single-nucleus RNA sequencing (Nuc-seq), and this revealed 19 separate clusters. We next probed the function of GABADVC cells and discovered that the selective activation of GABADVC neurons significantly controls food intake and body weight. Optogenetic interrogation of GABADVC circuitry identified GABADVC → hypothalamic arcuate nucleus (ARC) projections as appetite suppressive without creating aversion. Electrophysiological analysis revealed that GABADVC → ARC stimulation inhibits hunger-promoting neuropeptide Y (NPY) neurons via GABA release. Adopting an intersectional genetics strategy, we clarify that the GABADVC → ARC circuit curbs food intake. These data identify GABADVC as a new modulator of feeding behavior and body weight and a controller of orexigenic NPY neuron activity, thereby providing insight into the neural underpinnings of obesity.

Lorcaserin: Worthy of Further Insights? Results from Recent Research

Lorcaserin is a 3-benzazepine that binds 5-HT2C serotonin receptors in the hypothalamus, where it mediates lack of hunger and/or satiety, and in the ventral tegmental area, the site of origin of the mesolimbic and mesocortical dopaminergic projections, which mediate pleasure and reward. The drug has been first developed for the treatment of obesity, where it has shown efficacy, and subsequently trialed to counter substance use (mostly cocaine, cannabis, opioids, and nicotine) and craving, but showed inconsistent effects. Since 2020, the US Food and Drug Administration obtained that the drug was voluntarily withdrawn from the US market on the grounds that its long-term use was found to be associated with a greater incidence of some types of cancer. Provided it can show to be free from cancerogenic effects, ongoing research suggests that lorcaserin may have therapeutic potential for a variety of disorders and conditions beyond obesity. Since 5-HT2C receptors are involved in many diversified physiological functions (mood, feeding, reproductive behavior, neuronal processes related to impulsiveness, and modulating reward-related mechanisms) this drug has the potential to treat different central nervous system conditions, such as depression and schizophrenia.

Modulation of GABA release by 5-HT1B receptors: An interplay with AMPA-receptors and voltage-gated Ca2+ channels

Obesity has become a worldwide health challenge and commonly results from the intake of more calories than the body requires. The brain represents the master controller of food intake and as such has been the target of obesity medications. However, key mechanisms of druggable targets remain to be defined. Neurons within the arcuate nucleus of the hypothalamus co-expressing neuropeptide Y (NPY), agouti-related protein (AgRP) and GABA (NAG) are fundamental stimulators of hunger and food intake. NAG neurons also inhibit local satiety-promoting pro-opiomelanocortin (POMC) neurons. Agonists of the 1B subtype of metabotropic serotonin receptor (5-HT1BR) reduce food intake in part through the inhibition of hunger-promoting NAG neurons. We first confirmed that 5-HT1BR activation suppressed intake of a palatable Western diet in a mouse model of common dietary-induced obesity and genetically prone obesity. Next, we combined several electrophysiological approaches to analyse the effect of 5-HT1BRs in NAG neuron cell activity and GABA release. 5-HT1BR activation reduced NAG neuron action potential frequency and neurotransmitter release. We found that 5-HT1BR impact on GABA release from NAG neurons is mediated through voltage-gated Ca2+ channels with a critical input from glutamate receptors of AMPA subtype (AMPARs). As a fundamental outcome, this type of interplay provides an uncommon example of metabotropic action of AMPARs which regulates inhibitory signalling due to modulation of GABA release. As a translational outcome, our results provide a key mechanism through which 5-HT1BR drugs inhibit appetite-stimulating neurons within the brain to suppress food intake. This article is part of the Special Issue on "Ukrainian Neuroscience".

G protein-coupled receptors and obesity

G protein-coupled receptors (GPCRs) have emerged as important drug targets for various chronic diseases, including obesity and diabetes. Obesity is a complex chronic disease that requires long term management predisposing to type 2 diabetes, heart disease, and some cancers. The therapeutic landscape for GPCR as targets of anti-obesity medications has undergone significant changes with the approval of semaglutide, the first peptide glucagon like peptide 1 receptor agonist (GLP-1RA) achieving double digit weight loss (≥10%) and cardiovascular benefits. The enhanced weight loss, with the expected beneficial effect on obesity-related complications and reduction of major adverse cardiovascular events (MACE), has propelled the commercial opportunity for the obesity market leading to new players entering the space. Significant progress has been made on approaches targeting GPCRs such as single peptides that simultaneously activate GIP and/or GCGR in addition to GLP1, oral tablet formulation of GLP-1, small molecules nonpeptidic oral GLP1R and fixed-dose combination as well as add-on therapy for patients already treated with a GLP-1 agonist.

Mesenchymal-specific Alms1 knockout in mice recapitulates key metabolic features of Alström Syndrome

Background

Alström Syndrome (AS), a multi-system disease caused by mutations in the ALMS1 gene, includes obesity with disproportionately severe insulin resistant diabetes, dyslipidemia, and hepatosteatosis. How loss of ALMS1 causes this phenotype is poorly understood, but prior studies have circumstancially implicated impaired adipose tissue expandability. We set out to test this by comparing the metabolic effects of selective Alms1 knockout in mesenchymal cells including preadipocytes to those of global Alms1 knockout.

Methods

Global Alms1 knockout (KO) mice were generated by crossing floxed Alms1 and CAG-Cre mice. A Pdgfrα -Cre driver was used to abrogate Alms1 function selectively in mesenchymal stem cells (MSCs) and their descendants, including preadipocytes. We combined metabolic phenotyping of global and Pdgfrα + Alms1 -KO mice on a 45% fat diet with measurements of body composition and food intake, and histological analysis of metabolic tissues.

Results

Global Alms1 KO caused hyperphagia, obesity, insulin resistance, dyslipidaemia, and fatty liver. Pdgfrα - cre driven KO of Alms1 (MSC KO) recapitulated insulin resistance, fatty liver, and dyslipidaemia in both sexes. Other phenotypes were sexually dimorphic: increased fat mass was only present in female Alms1 MSC KO mice. Hyperphagia was not evident in male Alms1 MSC KO mice, but was found in MSC KO females, despite no neuronal Pdgfr α expression.

Conclusions

Mesenchymal deletion of Alms1 recapitulates the metabolic features of AS, including severe fatty liver. This confirms a key role for Alms1 in the adipose lineage, where its loss is sufficient to cause systemic metabolic effects and damage to remote organs. AS should be regarded as a forme fruste of lipodystrophy. Therapies should prioritise targeting positive energy balance.

Unraveling the serotonin saga: from discovery to weight regulation and beyond - a comprehensive scientific review

The prevalence of obesity is rapidly increasing worldwide, while the development of effective obesity therapies lags behind. Although new therapeutic targets to alleviate obesity are identified every day, and drug efficacy is improving, adverse side effects and increased health risks remain serious issues facing the weight-loss industry. Serotonin, also known as 5-HT, has been extensively studied in relation to appetite reduction and weight loss. As a result, dozens of upstream and downstream neural targets of 5-HT have been identified, revealing a multitude of neural circuits involved in mediating the anorexigenic effect of 5-HT. Despite the rise and fall of several 5-HT therapeutics in recent decades, the future of 5-HT as a therapeutic target for weight-loss therapy looks promising. This review focuses on the history of serotonin, the state of current central serotonin research, previous serotonergic therapies, and the future of serotonin for treating individuals with obesity.

Neurochemical Basis of Inter-Organ Crosstalk in Health and Obesity: Focus on the Hypothalamus and the Brainstem

Precise neural regulation is required for maintenance of energy homeostasis. Essential to this are the hypothalamic and brainstem nuclei which are located adjacent and supra-adjacent to the circumventricular organs. They comprise multiple distinct neuronal populations which receive inputs not only from other brain regions, but also from circulating signals such as hormones, nutrients, metabolites and postprandial signals. Hence, they are ideally placed to exert a multi-tier control over metabolism. The neuronal sub-populations present in these key metabolically relevant nuclei regulate various facets of energy balance which includes appetite/satiety control, substrate utilization by peripheral organs and glucose homeostasis. In situations of heightened energy demand or excess, they maintain energy homeostasis by restoring the balance between energy intake and expenditure. While research on the metabolic role of the central nervous system has progressed rapidly, the neural circuitry and molecular mechanisms involved in regulating distinct metabolic functions have only gained traction in the last few decades. The focus of this review is to provide an updated summary of the mechanisms by which the various neuronal subpopulations, mainly located in the hypothalamus and the brainstem, regulate key metabolic functions.

RISING STARS: Targeting G protein-coupled receptors to regulate energy homeostasis

G protein-coupled receptors (GPCRs) have a critical role in energy homeostasis, contributing to food intake, energy expenditure and glycaemic control. Dysregulation of energy expenditure can lead to metabolic syndrome (abdominal obesity, elevated plasma triglyceride, LDL cholesterol and glucose, and high blood pressure), which is associated with an increased risk of developing obesity, diabetes mellitus, non-alcoholic fatty liver disease and cardiovascular complications. As the prevalence of these chronic diseases continues to rise worldwide, there is an increased need to understand the molecular mechanisms by which energy expenditure is regulated to facilitate the development of effective therapeutic strategies to treat and prevent these conditions. In recent years, drugs targeting GPCRs have been the focus of efforts to improve treatments for type-2 diabetes and obesity, with GLP-1R agonists a particular success. In this review, we focus on nine GPCRs with roles in energy homeostasis that are current and emerging targets to treat obesity and diabetes. We discuss findings from pre-clinical models and clinical trials of drugs targeting these receptors and challenges that must be overcome before these drugs can be routinely used in clinics. We also describe new insights into how these receptors signal, including how accessory proteins, biased signalling, and complex spatial signalling could provide unique opportunities to develop more efficacious therapies with fewer side effects. Finally, we describe how combined therapies, in which multiple GPCRs are targeted, may improve clinical outcomes and reduce off-target effects.