Deconstruction of a neural circuit for hunger

Targeting the T-type calcium channel Cav3.2 in GABAergic arcuate nucleus neurons to treat obesity

OBJECTIVE

Cav3.2, a T-type low voltage-activated calcium channel widely expressed throughout the central nervous system, plays a vital role in neuronal excitability and various physiological functions. However, the effects of Cav3.2 on energy homeostasis remain unclear. Here, we examined the role of Cav3.2 expressed by hypothalamic GABAergic neurons in the regulation of food intake and body weight in mice and explored the underlying mechanisms.

METHODS

Male congenital Cana1h (the gene coding for Cav3.2) global knockout (Cav3.2KO) mice and their wild type (WT) littermates were first used for metabolic phenotyping studies. By using the CRISPR-Cas9 technique, Cav3.2 was selectively deleted from GABAergic neurons in the arcuate nucleus of the hypothalamus (ARH) by specifically overexpressing Cas9 protein and Cav3.2-targeting sgRNAs in ARH Vgat (Vgat^ARH) neurons. These male mutants (Cav3.2KO-Vgat^ARH) were used to determine whether Cav3.2 expressed by Vgat^ARH neurons is required for the proper regulation of energy balance. Subsequently, we used an electrophysiological patch-clamp recording in ex vivo brain slices to explore the impact of Cav3.2KO on the cellular excitability of Vgat^ARH neurons.

RESULTS

Male Cav3.2KO mice had significantly lower food intake than their WT littermate controls when fed with either a normal chow diet (NCD) or a high-fat diet (HFD). This hypophagia phenotype was associated with increased energy expenditure and decreased fat mass, lean mass, and total body weight. Selective deletion of Cav3.2 in Vgat^ARH neurons resulted in similar feeding inhibition and lean phenotype without changing energy expenditure. These data provides an intrinsic mechanism to support the previous finding on ARH non-AgRP GABA neurons in regulating diet-induced obesity. Lastly, we found that naringenin extract, a predominant flavanone found in various fruits and herbs and known to act on Cav3.2, decreased the firing activity of Vgat^ARH neurons and reduced food intake and body weight. These naringenin-induced inhibitions were fully blocked in Cav3.2KO-Vgat^ARH mice.

CONCLUSION

Our results identified Cav3.2 expressed by Vgat^ARH neurons as an essential intrinsic modulator for food intake and energy homeostasis, which is a potential therapeutic target in the treatment of obesity.

A hypothalamomedullary network for physiological responses to environmental stresses

Various environmental stressors, such as extreme temperatures (hot and cold), pathogens, predators and insufficient food, can threaten life. Remarkable progress has recently been made in understanding the central circuit mechanisms of physiological responses to such stressors. A hypothalamomedullary neural pathway from the dorsomedial hypothalamus (DMH) to the rostral medullary raphe region (rMR) regulates sympathetic outflows to effector organs for homeostasis. Thermal and infection stress inputs to the preoptic area dynamically alter the DMH → rMR transmission to elicit thermoregulatory, febrile and cardiovascular responses. Psychological stress signalling from a ventromedial prefrontal cortical area to the DMH drives sympathetic and behavioural responses for stress coping, representing a psychosomatic connection from the corticolimbic emotion circuit to the autonomic and somatic motor systems. Under starvation stress, medullary reticular neurons activated by hunger signalling from the hypothalamus suppress thermogenic drive from the rMR for energy saving and prime mastication to promote food intake. This Perspective presents a combined neural network for environmental stress responses, providing insights into the central circuit mechanism for the integrative regulation of systemic organs.

Do oxytocin neurones affect feeding?

There has been a long history of research on the effects of oxytocin on feeding behaviour. The classic-held view is that the neurohormone is anorexigenic at least in rodents, although the data for humans are not so clear cut. Likewise, a physiological role for oxytocin is disputed. Thus, although pharmacological, anatomical and physiological data suggest oxytocin may have a function in satiety signalling, this view is not supported by the latest research using the genetic recording and manipulation of oxytocin neurones. Here, we avoid a discussion of the pharmacological effects of oxytocin and examine evidence, from both sides of the argument, concerning whether the endogenous oxytocin system has a role in the regulation of normal feeding.

Astrocyte Gliotransmission in the Regulation of Systemic Metabolism

Normal brain function highly relies on the appropriate functioning of astrocytes. These glial cells are strategically situated between blood vessels and neurons, provide significant substrate support to neuronal demand, and are sensitive to neuronal activity and energy-related molecules. Astrocytes respond to many metabolic conditions and regulate a wide array of physiological processes, including cerebral vascular remodeling, glucose sensing, feeding, and circadian rhythms for the control of systemic metabolism and behavior-related responses. This regulation ultimately elicits counterregulatory mechanisms in order to couple whole-body energy availability with brain function. Therefore, understanding the role of astrocyte crosstalk with neighboring cells via the release of molecules, e.g., gliotransmitters, into the parenchyma in response to metabolic and neuronal cues is of fundamental relevance to elucidate the distinct roles of these glial cells in the neuroendocrine control of metabolism. Here, we review the mechanisms underlying astrocyte-released gliotransmitters that have been reported to be crucial for maintaining homeostatic regulation of systemic metabolism.

Endogenous cannabinoids are required for MC4R-mediated control of energy homeostasis

Hypothalamic regulation of feeding and energy expenditure is a fundamental and evolutionarily conserved neurophysiological process critical for survival. Dysregulation of these processes, due to environmental or genetic causes, can lead to a variety of pathological conditions ranging from obesity to anorexia. Melanocortins and endogenous cannabinoids (eCBs) have been implicated in the regulation of feeding and energy homeostasis; however, the interaction between these signaling systems is poorly understood. Here, we show that the eCB 2-arachidonoylglycerol (2-AG) regulates the activity of melanocortin 4 receptor (MC4R) cells in the paraventricular nucleus of the hypothalamus (PVN^MC4R) via inhibition of afferent GABAergic drive. Furthermore, the tonicity of eCBs signaling is inversely proportional to energy state, and mice with impaired 2-AG synthesis within MC4R neurons weigh less, are hypophagic, exhibit increased energy expenditure, and are resistant to diet-induced obesity. These mice also exhibit MC4R agonist insensitivity, suggesting that the energy state-dependent, 2-AG-mediated suppression of GABA input modulates PVN^MC4R neuron activity to effectively respond to the MC4R natural ligands to regulate energy homeostasis. Furthermore, post-developmental disruption of PVN 2-AG synthesis results in hypophagia and death. These findings illustrate a functional interaction at the cellular level between two fundamental regulators of energy homeostasis, the melanocortin and eCB signaling pathways in the hypothalamic feeding circuitry.

AgRP neuronal activity across feeding-related behaviours

AgRP neurons trigger one of the most potent orexigenic responses and are both necessary and sufficient for feeding. Recent technical advances for monitoring in vivo neuronal activity have revisited a previously well-established model of AgRP neurons' feeding regulatory effects. Our current understanding of AgRP neurons has increased in complexity and revealed a fine-tuned regulation of their activity dynamics across the whole sequence of feeding-related behaviours. This review focuses on recent studies that refined and re-evaluated our understanding of the regulatory principles and behavioural effects of AgRP circuits. We aim to cover major discoveries on the dynamic regulation of AgRP neuronal activity by exteroceptive and interoceptive food-related cues, their pleiotropic effects in feeding and whole-body homeostasis, and the associated AgRP circuits. The function and regulation of AgRP neuron will be sequentially discussed across the temporal series of behavioural and physiological changes occurring during the appetitive (food craving and foraging), the anticipatory (discovery of food-predicting cues), and the consummatory/post-ingestive phase of feeding (calorie ingestion).

Oxytocin and Food Intake Control: Neural, Behavioral, and Signaling Mechanisms

The neuropeptide oxytocin is produced in the paraventricular hypothalamic nucleus and the supraoptic nucleus of the hypothalamus. In addition to its extensively studied influence on social behavior and reproductive function, central oxytocin signaling potently reduces food intake in both humans and animal models and has potential therapeutic use for obesity treatment. In this review, we highlight rodent model research that illuminates various neural, behavioral, and signaling mechanisms through which oxytocin’s anorexigenic effects occur. The research supports a framework through which oxytocin reduces food intake via amplification of within-meal physiological satiation signals rather than by altering between-meal interoceptive hunger and satiety states. We also emphasize the distributed neural sites of action for oxytocin’s effects on food intake and review evidence supporting the notion that central oxytocin is communicated throughout the brain, at least in part, through humoral-like volume transmission. Finally, we highlight mechanisms through which oxytocin interacts with various energy balance-associated neuropeptide and endocrine systems (e.g., agouti-related peptide, melanin-concentrating hormone, leptin), as well as the behavioral mechanisms through which oxytocin inhibits food intake, including effects on nutrient-specific ingestion, meal size control, food reward-motivated responses, and competing motivations.

Targeting Histamine and Histamine Receptors for the Precise Regulation of Feeding

Histamine has long been accepted as an anorexigenic agent. However, lines of evidence have suggested that the roles of histamine in feeding behaviors are much more complex than previously thought, being involved in satiety, satiation, feeding motivation, feeding circadian rhythm, and taste perception and memory. The functional diversity of histamine makes it a viable target for clinical management of obesity and other feeding-related disorders. Here, we update the current knowledge about the functions of histamine in feeding and summarize the underlying molecular and neural circuit mechanisms. Finally, we review the main clinical studies about the impacts of histamine-related compounds on weight control and discuss insights into future research on the roles of histamine in feeding. Despite the recent progress in histamine research, the histaminergic feeding circuits are poorly understood, and it is also worth verifying the functions of histamine receptors in a more spatiotemporally specific manner.

Endothelial peroxynitrite causes disturbance of neuronal oscillations by targeting caspase-1 in the arcuate nucleus

Severe anorexia limits the clinical application of cisplatin, and even leads to the discontinuation of treatment. However, the mechanisms underlying cisplatin-induced anorexia are unknown. Herein, we demonstrated that cisplatin could affect neuronal gamma oscillations and induce abnormal neuronal theta-gamma phase-amplitude coupling in the arcuate nucleus (Arc) of the hypothalamus, and these findings were associated with significantly decreased food intake and weight loss in mice. Chemogenetic activation of AgRP neurons in the Arc reversed the cisplatin-induced food intake reduction in mice. We further demonstrated that endothelial peroxynitrite (ONOO⁻) formation in the Arc induced nitrosative stress following cisplatin treatment via a previously uncharacterized pathway involving neuronal caspase-1 activation. Strikingly, treatment with the ONOO⁻ scavenger uric acid (UA) reversed the reduced action potential (AP) frequency of AgRP neurons and increased the AP frequency of POMC neurons induced by SIN1, a donor of ONOO⁻, in the Arc, as determined by whole-cell patch-clamp electrophysiological recording. Consistent with these findings, UA treatment effectively alleviated cisplatin-induced dysfunction of neuronal oscillations and neuronal theta-gamma phase-amplitude coupling in the Arc of mice. Taken together, these results suggest, for the first time, that targeting the overproduction of endothelial ONOO⁻ can regulate cisplatin-induced neurotoxicity through neuronal caspase-1, and thereby serve as a potential therapeutic approach to alleviate chemotherapy-induced anorexia and weight loss.

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Endothelial ONOO^– induced the abnormal neuronal oscillations following cisplatin treatment through caspase-1 in the Arc.

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ONOO^– scavenger UA could attenuate cisplatin-induced neurotoxicity and caspase-1 activation in the Arc.

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Targeting endothelial ONOO^– provided a promising approach to alleviate chemotherapy-induced anorexia and weight loss.

Rolling out physical exercise and energy homeostasis: Focus on hypothalamic circuitries

Energy balance is the fine regulation of energy expenditure and energy intake. Negative energy balance causes body weight loss, while positive energy balance promotes weight gain. Modern societies offer a maladapted way of life, where easy access to palatable foods and the lack of opportunities to perform physical activity are considered the roots of the obesity pandemic. Physical exercise increases energy expenditure and, consequently, is supposed to promote weight loss. Paradoxically, physical exercise acutely drives anorexigenic-like effects, but the mechanisms are still poorly understood. Using an evolutionary background, this review aims to highlight the potential involvement of the melanocortin system and other hypothalamic neural circuitries regulating energy balance during and after physical exercise. The physiological significance of these changes will be explored, and possible signalling agents will be addressed. The knowledge discussed here might be important for clarifying obesity aetiology as well as new therapeutic approaches for body weight loss.

Ghrelin treatment induces rapid and delayed increments of food intake: a heuristic model to explain ghrelin's orexigenic effects

Ghrelin is a stomach-derived peptide hormone with salient roles in the regulation of energy balance and metabolism. Notably, ghrelin is recognized as the most powerful known circulating orexigenic hormone. Here, we systematically investigated the effects of ghrelin on energy homeostasis and found that ghrelin primarily induces a biphasic effect on food intake that has indirect consequences on energy expenditure and nutrient partitioning. We also found that ghrelin-induced biphasic effect on food intake requires the integrity of Agouti-related peptide/neuropeptide Y-producing neurons of the hypothalamic arcuate nucleus, which seem to display a long-lasting activation after a single systemic injection of ghrelin. Finally, we found that different autonomic, hormonal and metabolic satiation signals transiently counteract ghrelin-induced food intake. Based on our observations, we propose a heuristic model to describe how the orexigenic effect of ghrelin and the anorectic food intake-induced rebound sculpt a timely constrain feeding response to ghrelin.

Anti-Obesity Medication Use in Children and Adolescents with Prader-Willi Syndrome: Case Review and Literature Search

(1) Background: children with Prader-Willi syndrome (PWS) have high obesity rates due to hyperphagia and decreased metabolic rates. Although anti-obesity medications (AOMs) are prescribed to this population, there are no consensus guidelines on acceptability, safety, and efficacy. We present literature review and case series on AOMs in youth with PWS. (2) Methods: we performed PubMed review from January 2000 to April 2021 utilizing keywords: "Prader-Willi syndrome" or "PWS" and "medication" including: topiramate, metformin, phentermine, liraglutide, orlistat, oxytocin, semaglutide, naltrexone-bupropion. For our case series, patients were identified through retrospective chart reviews from a multi-disciplinary PWS clinic. Eligibility criteria: age ≤ 18 years, genetically confirmed PWS, AOM use for at least 16 weeks, and recent anthropometric data. (3) Results: a literature search yielded 14 articles (3 topiramate, 1 metformin, 4 liraglutide, 5 oxytocin, 1 naltrexone-bupropion). All studies reported improved hyperphagia with variable BMI effects. Ten adolescents met case series eligibility (mean age 13.2 ± 2.6 years, 40% female; AOMs: 6 metformin, 5 topiramate, 2 semaglutide, 3 liraglutide). After AOM course, 60% had decreased or stable BMI z-score. No significant side effects. (4) Conclusions: results suggest AOMs may be useful for weight management in youth with PWS. Additional studies are required to validate findings and support AOM treatment guidelines.

Targeting the Oxytocinergic System: A Possible Pharmacological Strategy for the Treatment of Inflammation Occurring in Different Chronic Diseases

Unresolved inflammation represents a central feature of different human pathologies including neuropsychiatric, cardiovascular, and metabolic diseases. The epidemiologic relevance of such disorders justifies the increasing interest in further understanding the mechanisms underpinning the inflammatory process occurring in such chronic diseases to provide potential novel pharmacological approaches. The most common and effective therapies for controlling inflammation are glucocorticoids; however, a variety of other molecules have been demonstrated to have an anti-inflammatory potential, including neuropeptides. In recent years, the oxytocinergic system has seen an explosion of scientific studies, demonstrating its potential to contribute to a variety of physiological processes including inflammation. Therefore, the aim of the present review was to understand the role of oxytocin in the modulation of inflammation occurring in different chronic diseases. The criterion we used to select the diseases was based on the emerging literature showing a putative involvement of the oxytocinergic system in inflammatory processes in a variety of pathologies including neurological, gastrointestinal and cardiovascular disorders, diabetes and obesity. The evidence reviewed here supports a beneficial role of oxytocin in the control of both peripheral and central inflammatory response happening in the aforementioned pathologies. Although future studies are necessary to elucidate the mechanistic details underlying such regulation, this review supports the idea that the modulation of the endogenous oxytocinergic system might represent a new potential pharmacological approach for the treatment of inflammation.

Glucagon-like peptide-1 (GLP-1) signalling in the brain: From neural circuits and metabolism to therapeutics

Glucagon‐like‐peptide‐1 (GLP‐1) derived from gut enteroendocrine cells and a discrete population of neurons in the caudal medulla acts through humoral and neural pathways to regulate satiety, gastric motility and pancreatic endocrine function. These physiological attributes contribute to GLP‐1 having a potent therapeutic action in glycaemic regulation and chronic weight management. In this review, we provide an overview of the neural circuits targeted by endogenous versus exogenous GLP‐1 and related drugs. We also highlight candidate subpopulations of neurons and cellular mechanisms responsible for the acute and chronic effects of GLP‐1 and GLP‐1 receptor agonists on energy balance and glucose metabolism. Finally, we present potential future directions to translate these findings towards the development of effective therapies for treatment of metabolic disease.

Developmental genes controlling neural circuit formation are expressed in the early postnatal hypothalamus and cellular lining of the third ventricle

The arcuate nucleus of the hypothalamus is central in the regulation of body weight homeostasis through its ability to sense peripheral metabolic signals and relay them, through neural circuits, to other brain areas, ultimately affecting physiological and behavioural changes. The early postnatal development of these neural circuits is critical for normal body weight homeostasis, such that perturbations during this critical period can lead to obesity. The role for peripheral regulators of body weight homeostasis, including leptin, insulin and ghrelin, in this postnatal development is well described, yet some of the fundamental processes underpinning axonal and dendritic growth remain unclear. Here, we hypothesised that molecules known to regulate axonal and dendritic growth processes in other areas of the developing brain would be expressed in the postnatal arcuate nucleus and/or target nuclei where they would function to mediate the development of this circuitry. Using state-of-the-art RNAscope^® technology, we have revealed the expression patterns of genes encoding Dcc/Netrin-1, Robo1/Slit1 and Fzd5/Wnt5a receptor/ligand pairs in the early postnatal mouse hypothalamus. We found that individual genes had unique expression patterns across developmental time in the arcuate nucleus, paraventricular nucleus of the hypothalamus, ventromedial nucleus of the hypothalamus, dorsomedial nucleus of the hypothalamus, median eminence and, somewhat unexpectedly, the third ventricle epithelium. These observations indicate a number of new molecular players in the development of neural circuits regulating body weight homeostasis, as well as novel molecular markers of tanycyte heterogeneity.

AgRP neurons: Regulators of feeding, energy expenditure, and behavior

Neurons in the hypothalamic arcuate nucleus (ARC) that express agouti-related peptide (AgRP) govern a critical aspect of survival: the drive to eat. Equally important to survival is the timing at which food is consumed-seeking or eating food to alleviate hunger in the face of a more pressing threat, like the risk of predation, is clearly maladaptive. To ensure optimal prioritization of behaviors within a given environment, therefore, AgRP neurons must integrate signals of internal need states with contextual environmental cues. In this state-of-the-art review, we highlight recent advances that extend our understanding of AgRP neurons, including the neural circuits they engage to regulate feeding, energy expenditure, and behavior. We also discuss key findings that illustrate how both classical feedback and anticipatory feedforward signals regulate this neuronal population and how the integration of these signals may be disrupted in states of energy excess. Finally, we examine both technical and conceptual challenges facing the field moving forward.

Dopamine receptor 2 downregulation and brain-derived neurotrophic factor upregulation in the paraventricular nucleus are correlated with brown adipose tissue thermogenesis in rats with bilateral substantia nigra lesions

The thermogenesis resulting from brown adipose tissue (BAT)-induced energy consumption is an important method of energy regulation. It has been reported that brain-derived neurotrophic factor (BDNF)-positive neurons in the paraventricular nucleus (PVN) can regulate adaptive thermogenesis in interscapular brown adipose tissue (IBAT), but the upstream regulatory mechanism is still unclear. Our previous studies have found that a large number of dopamine (DA) receptors (DRs) are expressed on BDNF-positive neurons in the PVN and that the substantia nigra (SN) can directly project to the PVN (forming the SN-PVN pathway). Therefore, we speculate that DA in the SN can regulate the expression of BDNF via DRs and then affect IBAT thermogenesis. In this study, bilateral SN lesions were induced in rats with 6-hydroxydopamine (6-OHDA), and the altered expression of DRs and BDNF in the PVN and the metabolic changes in IBAT were studied via double immunofluorescence and western blotting. The results showed that BDNF-positive neurons in the PVN expressed DR 1 (D1) and DR 2 (D2) and were surrounded by a large number of tyrosine hydroxylase (TH)-positive nerve fibers. Compared with the control group, the 6-OHDA group exhibited significantly fewer TH-positive neurons and significantly lower TH expression in the SN, but body weight, IBAT weight and food consumption did not differ between the groups. In the PVN, BDNF expression was upregulated in the 6-OHDA group, while D2 and TH expression was downregulated. In IBAT, the expression of uncoupling protein-1 (UCP-1), phosphorylated hormone-sensitive lipase (p-HSL), TH and β3-adrenergic receptor (β3-AR) was increased, while the expression of fatty acid synthase (FAS) was decreased. The IBAT cell diameter was also decreased in the 6-OHDA group. The results suggest that the SN-PVN pathway may be an upstream neural pathway that can affect BDNF expression in the PVN and that DRs may mediate its regulatory effects. This study expands our understanding of the relationship between DA and obesity.

Identification of novel GPCR partners of the central melanocortin signaling

OBJECTIVE

Homo- or heterodimerization of G protein-coupled receptors (GPCRs) generally affects the normal functioning of these receptors and mediates the responses to a variety of physiological stimuli in vivo. It is well known that melanocortin-3 receptor (MC3R) and melanocortin-4 receptor (MC4R) are key regulators of appetite and energy homeostasis in the central nervous system. However, the GPCR partners of MC3R and MC4R are not well understood. Our objective is to analyze single cell RNA-seq datasets of the hypothalamus to explore and identify novel GPCR partners of MC3R and MC4R and examine the pharmacological effect on the downstream signal transduction and membrane translocation of melanocortin receptors.

METHODS

We conducted an integrative analysis of multiple single cell RNA-seq datasets to reveal the expression pattern and correlation of GPCR families in the mouse hypothalamus. The emerging GPCRs with important metabolic functions were selected for cloning and co-immunoprecipitation validation. The positive GPCR partners were then tested for the pharmacological activation, competitive binding assay and surface translocation ELISA experiments.

RESULTS

Based on the expression pattern of GPCRs and their function enrichment results, we narrowed down the range of potential GPCR interaction with MC3R and MC4R for further confirmation. Co-immunoprecipitation assay verified 23 and 32 novel GPCR partners that interacted with MC3R and MC4R in vitro. The presence of these GPCR partners exhibited different effects in the physiological regulation and signal transduction of MC3R and MC4R.

CONCLUSIONS

This work represented the first large-scale screen for the functional GPCR complex of central melanocortin receptors and defined a composite metabolic regulatory GPCR network of the hypothalamic nucleuses.

Mechanistic insight into high-fat diet-induced metabolic inflammation in the arcuate nucleus of the hypothalamus

A high-fat diet (HFD) is linked with cytokines production by non-neuronal cells within the hypothalamus, which mediates metabolic inflammation. These cytokines then activate different inflammatory mediators in the arcuate nucleus of the hypothalamus (ARC), a primary hypothalamic area accommodating proopiomelanocortin (POMC) and agouti-related peptide (AGRP) neurons, first-order neurons that sense and integrate peripheral metabolic signals and then respond accordingly. These mediators, such as inhibitor of κB kinase-β (IKKβ), suppression of cytokine signaling 3 (SOCS3), c-Jun N-terminal kinases (JNKs), protein kinase C (PKC), etc., cause insulin and leptin resistance in POMC and AGRP neurons and support obesity and related metabolic complications. On the other hand, inhibition of these mediators has been shown to counteract the impaired metabolism. Therefore, it is important to discuss the contribution of neuronal and non-neuronal cells in HFD-induced hypothalamic inflammation. Furthermore, understanding few other questions, such as the diets causing hypothalamic inflammation, the gender disparity in response to HFD feeding, and how hypothalamic inflammation affects ARC neurons to cause impaired metabolism, will be helpful for the development of therapeutic approaches to prevent or treat HFD-induced obesity.

Chemogenetic approaches to unravel circuit wiring and related behavior after spinal cord injury

A critical shortcoming of the central nervous system is its limited ability to repair injured nerve connections. Trying to overcome this limitation is not only relevant to understand basic neurobiological principles but also holds great promise to advance therapeutic strategies related, in particular, to spinal cord injury (SCI). With barely any SCI patients re-gaining complete neurological function, there is a high need to understand how we could target and improve spinal plasticity to re-establish neuronal connections into a functional network. The development of chemogenetic tools has proven to be of great value to understand functional circuit wiring before and after injury and to correlate novel circuit formation with behavioral outcomes. This review covers commonly used chemogenetic approaches based on metabotropic receptors and their use to improve our understanding of circuit wiring following spinal cord injury.

Kamikihito, a traditional Japanese Kampo medicine, increases the secretion of oxytocin in rats with acute stress

ETHNOPHARMACOLOGICAL RELEVANCE

Kamikihito (KKT) is a Kampo medicine that is prescribed in Japan for the treatment of anemia, insomnia and mental anxiety in Japan. However, its precise mechanism of action remains unclear.

AIM OF THE STUDY

This study aimed to evaluate the possible antistress effect of KKT in rats with acute stress and the contribution of oxytocin to the process.

MATERIALS AND METHODS

Acute immobilization stress (AIS; for 90 min) was used to assess the effect of KKT on acute stress. Male Wistar rats were orally treated with KKT. Parameters of stress were evaluated, and concentrations of oxytocin in plasma and cerebrospinal fluid (CSF) were measured.

RESULTS

AIS-induced defecation and fecal weight were significantly decreased because of treatment with KKT. The plasma levels of stress-related hormones following AIS were investigated. The pre-administration of KKT significantly increased adrenocorticotrophic hormone (ACTH) and corticosterone (CORT) levels following AIS. Conversely, there was no significant change in the plasma oxytocin level. Microdialysis and hydrophilic interaction liquid chromatography-tandem mass spectrometry (HILIC-MS/MS) were used to monitor the oxytocin secretion in CSF. Oxytocin level increased during AIS following the treatment of KKT. At 30 min after AIS, the level remained higher than before AIS. Furthermore, using an open field test, the locomotion (exploratory behavior) immediately after AIS was examined. The total traveled distance decreased after AIS; however, the decrease was significantly inhibited by the treatment of KKT. However, the effect of KKT was obstructed by the pre-administration of the oxytocin receptor antagonist.

CONCLUSIONS

These results suggest that KKT has antistress activity and increased oxytocin secretion may be a mechanism underlying this phenomenon.

Neuropeptide Y suppresses thermogenic and cardiovascular sympathetic nerve activity via Y1 receptors in the paraventricular nucleus and dorsomedial hypothalamus

In hungry animals, neuropeptide Y (NPY) neurones in the arcuate nucleus (ArcN) are activated to suppress energy expenditure, in part by decreasing brown adipose tissue sympathetic nerve activity (BAT SNA); however, the NPY receptor subtype and brain neurocircuitry are unclear. In the present study, we investigated the inhibition of BAT SNA by exogenous and endogenous NPY via binding to Y1 receptors (NPY1R) in the hypothalamic paraventricular nucleus (PVN) and dorsomedial hypothalamus (DMH), in anaesthetised male rats. Downstream projections of PVN/DMH NPY1R-expressing neurones were identified using male Npy1r-cre mice and localised unilateral DMH or PVN injections of an adeno-associated virus, which allows for the cre-dependent expression of a fluorescent protein (mCherry) in the cell bodies, axon fibres and nerve terminals of NPY1R-containing neurones. Nanoinjections of NPY into the DMH of cooled rats decreased BAT SNA, as well as mean arterial pressure (MAP) and heart rate (HR), and these responses were reversed by subsequent injection of the selective NPY1R antagonist, BIBO3304. In warmed rats, with little to no BAT SNA, bilateral nanoinjections of BIBO3304 into the DMH or PVN increased BAT SNA, MAP and HR. DMH NPY1R-expressing neurones projected heavily to the raphe pallidus (RPa), which houses BAT presympathetic neurones, as well as the PVN. In anaesthetised mice, DMH BIBO3304 increased splanchnic SNA, MAP and HR, all of which were reversed by nonselective blockade of the PVN with muscimol, suggesting that DMH-to-PVN connections are involved in this DMH BIBO3304 disinhibition. PVN Y1R expressing neurones also projected to the RPa, as well as to the nucleus tractus solitarius. We conclude that NPY tonically released in the DMH and PVN suppresses BAT SNA, MAP and HR via Y1R. Downstream neuropathways for BAT SNA may utilise direct projections to the RPa. Release of tonic NPY inhibition of BAT SNA may contribute to feeding- and diet-induced thermogenesis.

The Effects of Oxytocin on Appetite Regulation, Food Intake and Metabolism in Humans

The hypothalamic peptide oxytocin and its receptor are involved in a range of physiological processes, including parturition, lactation, cell growth, wound healing, and social behavior. More recently, increasing evidence has established the effects of oxytocin on food intake, energy expenditure, and peripheral metabolism. In this review, we provide a comprehensive description of the central oxytocinergic system in which oxytocin acts to shape eating behavior and metabolism. Next, we discuss the peripheral beneficial effects oxytocin exerts on key metabolic organs, including suppression of visceral adipose tissue inflammation, skeletal muscle regeneration, and bone tissue mineralization. A brief summary of oxytocin actions learned from animal models is presented, showing that weight loss induced by chronic oxytocin treatment is related not only to its anorexigenic effects, but also to the resulting increase in energy expenditure and lipolysis. Following an in-depth discussion on the technical challenges related to endogenous oxytocin measurements in humans, we synthesize data related to the association between endogenous oxytocin levels, weight status, metabolic syndrome, and bone health. We then review clinical trials showing that in humans, acute oxytocin administration reduces food intake, attenuates fMRI activation of food motivation brain areas, and increases activation of self-control brain regions. Further strengthening the role of oxytocin in appetite regulation, we review conditions of hypothalamic insult and certain genetic pathologies associated with oxytocin depletion that present with hyperphagia, extreme weight gain, and poor metabolic profile. Intranasal oxytocin is currently being evaluated in human clinical trials to learn whether oxytocin-based therapeutics can be used to treat obesity and its associated sequela. At the end of this review, we address the fundamental challenges that remain in translating this line of research to clinical care.

Clinical applicability of optogenetic gene regulation

The field of optogenetics is rapidly growing in relevance and number of developed tools. Among other things, the optogenetic repertoire includes light-responsive ion channels and methods for gene regulation. This review will be confined to the optogenetic control of gene expression in mammalian cells as suitable models for clinical applications. Here optogenetic gene regulation might offer an excellent method for spatially and timely regulated gene and protein expression in cell therapeutic approaches. Well-known systems for gene regulation, such as the LOV-, CRY2/CIB-, PhyB/PIF-systems, as well as other, in mammalian cells not yet fully established systems, will be described. Advantages and disadvantages with regard to clinical applications are outlined in detail. Among the many unanswered questions concerning the application of optogenetics, we discuss items such as the use of exogenous chromophores and their effects on the biology of the cells and methods for a gentle, but effective gene transfection method for optogenetic tools for in vivo applications.

Ablation of Nampt in AgRP neurons leads to neurodegeneration and impairs fasting- and ghrelin-mediated food intake

Agouti-related protein (AgRP) neurons in the arcuate nucleus of the hypothalamus regulates food intake and whole-body metabolism. NAD⁺ regulates multiple cellular processes controlling energy metabolism. Yet, its role in hypothalamic AgRP neurons to control food intake is poorly understood. Here, we aimed to assess whether genetic deletion of nicotinamide phosphoribosyltransferase (Nampt), a rate-limiting enzyme in NAD⁺ production, affects AgRP neuronal function to impact whole-body metabolism and food intake. Metabolic parameters during fed and fasted states, and upon systemic ghrelin and leptin administration were studied in AgRP-specific Nampt knockout (ARNKO) mice. We monitored neuropeptide expression levels and density of AgRP neurons in ARNKO mice from embryonic to adult age. NPY cells were used to determine effects of NAMPT inhibition on neuronal viability, energy status, and oxidative stress in vitro. In these cells, NAD⁺ depletion reduced ATP levels, increased oxidative stress, and promoted cell death. Agrp expression in the hypothalamus of ARNKO mice gradually decreased after weaning due to progressive AgRP neuron degeneration. Adult ARNKO mice had normal glucose and insulin tolerance, but exhibited an elevated respiratory exchange ratio (RER) when fasted. Remarkably, fasting-induced food intake was unaffected in ARNKO mice when evaluated in metabolic cages, but fasting- and ghrelin-induced feeding and body weight gain decreased in ARNKO mice when evaluated outside metabolic cages. Collectively, deletion of Nampt in AgRP neurons causes progressive neurodegeneration and impairs fasting and ghrelin responses in a context-dependent manner. Our data highlight an essential role of Nampt in AgRP neuron function and viability.

The melanocortin-3 receptor is a pharmacological target for the regulation of anorexia

Ablation of hypothalamic AgRP (Agouti-related protein) neurons is known to lead to fatal anorexia, whereas their activation stimulates voracious feeding and suppresses other motivational states including fear and anxiety. Despite the critical role of AgRP neurons in bidirectionally controlling feeding, there are currently no therapeutics available specifically targeting this circuitry. The melanocortin-3 receptor (MC3R) is expressed in multiple brain regions and exhibits sexual dimorphism of expression in some of those regions in both mice and humans. MC3R deletion produced multiple forms of sexually dimorphic anorexia that resembled aspects of human anorexia nervosa. However, there was no sexual dimorphism in the expression of MC3R in AgRP neurons, 97% of which expressed MC3R. Chemogenetic manipulation of arcuate MC3R neurons and pharmacologic manipulation of MC3R each exerted potent bidirectional regulation over feeding behavior in male and female mice, whereas global ablation of MC3R-expressing cells produced fatal anorexia. Pharmacological effects of MC3R compounds on feeding were dependent on intact AgRP circuitry in the mice. Thus, the dominant effect of MC3R appears to be the regulation of the AgRP circuitry in both male and female mice, with sexually dimorphic sites playing specialized and subordinate roles in feeding behavior. Therefore, MC3R is a potential therapeutic target for disorders characterized by anorexia, as well as a potential target for weight loss therapeutics.

Neuropeptide Y and Metabolism Syndrome: An Update on Perspectives of Clinical Therapeutic Intervention Strategies

Through the past decade of research, the pathogenic mechanisms underlying metabolic syndrome have been suggested to involve not only the peripheral tissues, but also central metabolic regulation imbalances. The hypothalamus, and the arcuate nucleus in particular, is the control center for metabolic homeostasis and energy balance. Neuropeptide Y neurons are particularly abundantly expressed in the arcuate of the hypothalamus, where the blood-brain barrier is weak, such as to critically integrate peripheral metabolic signals with the brain center. Herein, focusing on metabolic syndrome, this manuscript aims to provide an overview of the regulatory effects of Neuropeptide Y on metabolic syndrome and discuss clinical intervention strategy perspectives for neurometabolic disease.

Hypothalamic control of interoceptive hunger

While energy balance is critical to survival, many factors influence food intake beyond caloric need or "hunger." Despite this, some neurons that drive feeding in mice are routinely referred to as "hunger neurons," whereas others are not. To understand how specific hypothalamic circuits control interoceptive hunger, we trained mice to discriminate fasted from sated periods. We then manipulated three hypothalamic neuronal populations with well-known effects on feeding while mice performed this task. While activation of ARC^AGRP neurons in sated mice caused mice to report being food-restricted, LH^VGAT neuron activation or LH^VGLUT2 neuron inhibition did not. In contrast, LH^VGAT neuron inhibition or LH^VGLUT2 neuron activation in fasted mice attenuated natural hunger, whereas ARC^AGRP neuron inhibition did not. Each neuronal population evoked distinct effects on food consumption and reward. After satiety- or sickness-induced devaluation, ARC^AGRP neurons drove calorie-specific feeding, while LH^VGAT neurons drove calorie-indiscriminate food intake. Our data support a role for ARC^AGRP neurons in homeostatic feeding and implicate them in driving a hunger-like internal state that directs behavior toward caloric food sources. Moreover, manipulations of LH circuits did not evoke hunger-like effects in sated mice, suggesting that they may govern feeding more related to reward, compulsion, or generalized consumption than to energy balance, but also that these LH circuits can be powerful negative appetite modulators in fasted mice. This study highlights the complexity of hypothalamic feeding regulation and can be used as a framework to characterize how other neuronal circuits affect hunger and identify potential therapeutic targets for eating disorders.

Reciprocal control of obesity and anxiety-depressive disorder via a GABA and serotonin neural circuit

The high comorbidity between obesity and mental disorders, such as depression and anxiety, often exacerbates metabolic and neurological symptoms significantly. However, neural mechanisms that underlie reciprocal control of feeding and mental states are largely elusive. Here we report that melanocortin 4 receptor (MC4R) neurons located in the dorsal bed nucleus of the stria terminus (dBNST) engage in the regulation of mentally associated weight gain by receiving GABAergic projections from hypothalamic AgRP neurons onto α5-containing GABAA receptors and serotonergic afferents onto 5-HT3 receptors. Chronic treatment with a high-fat diet (HFD) significantly blunts the hyperexcitability of AgRP neurons in response to not only hunger but also anxiety and depression-like stimuli. Such HFD-mediated desensitization reduces GABAergic outputs from AgRP neurons to downstream MC4RdBNST neurons, resulting in severe mental dysregulation. Genetic enhancement of the GABAAR-α5 or suppression of the 5-HT3R within the MC4RdBNST neurons not only abolishes HFD-induced anxiety and depression but also robustly reduces body weight by suppression of food intake. To gain further translational insights, we revealed that combined treatment of zonisamide (enhancing the GABAAR-α5 signaling) and granisetron (a selective 5-HT3R antagonist) alleviates mental dysfunction and yields a robust reversal of diet-induced obesity by reducing total calorie intake and altering food preference towards a healthy low-fat diet. Our results unveil a neural mechanism for reciprocal control of appetite and mental states, which culminates in a novel zonisamide-granisetron cocktail therapy for potential tackling the psychosis-obesity comorbidity.

Ghrelin-induced Food Intake, but not GH Secretion, Requires the Expression of the GH Receptor in the Brain of Male Mice

Ghrelin stimulates both GH secretion and food intake. The orexigenic action of ghrelin is mainly mediated by neurons that coexpress agouti-related protein (AgRP) and neuropeptide Y (NPY) in the arcuate nucleus of the hypothalamus (ARH). GH also stimulates food intake and, importantly, ARHAgRP/NPY neurons express GH receptor (GHR). Thus, ghrelin-induced GH secretion may contribute to the orexigenic effect of ghrelin. Here, we investigated the response to ghrelin in male mice carrying GHR ablation specifically in neurons (brain GHR knockout [KO] mice) or exclusively in ARHAgRP/NPY neurons (AgRP GHR KO mice). Although brain GHR KO mice showed normal ghrelin-induced increase in plasma GH levels, these mutants lacked the expected orexigenic response to ghrelin. Additionally, brain GHR KO mice displayed reduced hypothalamic levels of Npy and Ghsr mRNA and did not elicit ghrelin-induced c-Fos expression in the ARH. Furthermore, brain GHR KO mice exhibited a prominent reduction in AgRP fiber density in the ARH and paraventricular nucleus of the hypothalamus (PVH). In contrast, AgRP GHR KO mice showed no changes in the hypothalamic Npy and Ghsr mRNAs and conserved ghrelin-induced food intake and c-Fos expression in the ARH. AgRP GHR KO mice displayed a reduced AgRP fiber density (~16%) in the PVH, but this reduction was less than that observed in brain GHR KO mice (~61%). Our findings indicate that GHR signaling in the brain is required for the orexigenic effect of ghrelin, independently of GH action on ARHAgRP/NPY neurons.

Hypothalamic primary cilium: A hub for metabolic homeostasis

Obesity is a global health problem that is associated with adverse consequences such as the development of metabolic disorders, including cardiovascular disease, neurodegenerative disorders, and type 2 diabetes. A major cause of obesity is metabolic imbalance, which results from insufficient physical activity and excess energy intake. Understanding the pathogenesis of obesity, as well as other metabolic disorders, is important in the development of methods for prevention and therapy. The coordination of energy balance takes place in the hypothalamus, a major brain region that maintains body homeostasis. The primary cilium is an organelle that has recently received attention because of its role in controlling energy balance in the hypothalamus. Defects in proteins required for ciliary function and formation, both in humans and in mice, have been shown to cause various metabolic disorders. In this review, we provide an overview of the critical functions of primary cilia, particularly in hypothalamic areas, and briefly summarize the studies on the primary roles of cilia in specific neurons relating to metabolic homeostasis.

Food cue regulation of AGRP hunger neurons guides learning

Agouti-related peptide (AGRP)-expressing neurons are activated by fasting-this causes hunger^1-4, an aversive state that motivates the seeking and consumption of food^5,6. Eating returns AGRP neuron activity towards baseline on three distinct timescales: rapidly and transiently following sensory detection of food cues^6-8, slowly and longer-lasting in response to nutrients in the gut^9,10, and even more slowly and permanently with restoration of energy balance^9,11. The rapid regulation by food cues is of particular interest as its neurobiological basis and purpose are unknown. Given that AGRP neuron activity is aversive⁶, the sensory cue-linked reductions in activity could function to guide behaviour. To evaluate this, we first identified the circuit mediating sensory cue inhibition and then selectively perturbed it to determine function. Here, we show that a lateral hypothalamic glutamatergic → dorsomedial hypothalamic GABAergic (γ-aminobutyric acid-producing)¹² → AGRP neuron circuit mediates this regulation. Interference with this circuit impairs food cue inhibition of AGRP neurons and, notably, greatly impairs learning of a sensory cue-initiated food-acquisition task. This is specific for food, as learning of an identical water-acquisition task is unaffected. We propose that decreases in aversive AGRP neuron activity⁶ mediated by this food-specific circuit increases the incentive salience¹³ of food cues, and thus facilitates the learning of food-acquisition tasks.

Food reward depends on TLR4 activation in dopaminergic neurons

The rising prevalence of obesity and being overweight is a worldwide health concern. Food reward dysregulation is the basic factor for the development of obesity. Dopamine (DA) neurons in the ventral tegmental area (VTA) play a vital role in food reward. Toll-like receptor 4 (TLR4) is a transmembrane pattern recognition receptor that can be activated by saturated fatty acids. Here, we show that the deletion of TLR4 specifically in DA neurons increases body weight, increases food intake, and decreases food reward. Conditional deletion of TLR4 also decreased the activity of DA neurons while suppressing the expression of tyrosine hydroxylase (TH) in the VTA, which regulates the concentration of DA in the nucleus accumbens (NAc) to affect food reward. Meanwhile, AAV-Cre-GFP mediated VTA-specific TLR4-deficient mice recapitulates food reward of DAT-TLR4-KO mice. Food reward could be rescued by re-expressing TLR4 in VTA DA neurons. Moreover, effects of intra-VTA infusion of lauric acid (a saturated fatty acid with 12 carbon) on food reward were abolished in mice lacking TLR4 in DA neurons. Our study demonstrates the critical role of TLR4 signaling in regulating the activity of VTA DA neurons and the normal function of the mesolimbic DA system that may contribute to food reward.

Hypothalamic GPCR Signaling Pathways in Cardiometabolic Control

Obesity is commonly associated with sympathetic overdrive, which is one of the major risk factors for the development of cardiovascular diseases, such as hypertension and heart failure. Over the past few decades, there has been a growing understanding of molecular mechanisms underlying obesity development with central origin; however, the relative contribution of these molecular changes to the regulation of cardiovascular function remains vague. A variety of G-protein coupled receptors (GPCRs) and their downstream signaling pathways activated in distinct hypothalamic neurons by different metabolic hormones, neuropeptides and monoamine neurotransmitters are crucial not only for the regulation of appetite and metabolic homeostasis but also for the sympathetic control of cardiovascular function. In this review, we will highlight the main GPCRs and associated hypothalamic nuclei that are important for both metabolic homeostasis and cardiovascular function. The potential downstream molecular mediators of these GPCRs will also be discussed.

Deciphering an AgRP-serotoninergic neural circuit in distinct control of energy metabolism from feeding

Contrasting to the established role of the hypothalamic agouti-related protein (AgRP) neurons in feeding regulation, the neural circuit and signaling mechanisms by which they control energy expenditure remains unclear. Here, we report that energy expenditure is regulated by a subgroup of AgRP neurons that send non-collateral projections to neurons within the dorsal lateral part of dorsal raphe nucleus (dlDRN) expressing the melanocortin 4 receptor (MC4R), which in turn innervate nearby serotonergic (5-HT) neurons. Genetic manipulations reveal a bi-directional control of energy expenditure by this circuit without affecting food intake. Fiber photometry and electrophysiological results indicate that the thermo-sensing MC4RdlDRN neurons integrate pre-synaptic AgRP signaling, thereby modulating the post-synaptic serotonergic pathway. Specifically, the MC4RdlDRN signaling elicits profound, bi-directional, regulation of body weight mainly through sympathetic outflow that reprograms mitochondrial bioenergetics within brown and beige fat while feeding remains intact. Together, we suggest that this AgRP neural circuit plays a unique role in persistent control of energy expenditure and body weight, hinting next-generation therapeutic approaches for obesity and metabolic disorders.

The melanocortin pathway and energy homeostasis: From discovery to obesity therapy

BACKGROUND

Over the past 20 years, insights from human and mouse genetics have illuminated the central role of the brain leptin-melanocortin pathway in controlling mammalian food intake, with genetic disruption resulting in extreme obesity, and more subtle polymorphic variations influencing the population distribution of body weight. At the end of 2020, the U.S. Food and Drug Administration (FDA) approved setmelanotide, a melanocortin 4 receptor agonist, for use in individuals with severe obesity due to either pro-opiomelanocortin (POMC), proprotein convertase subtilisin/kexin type 1 (PCSK1), or leptin receptor (LEPR) deficiency.

SCOPE OF REVIEW

Herein, we chart the melanocortin pathway's history, explore its pharmacology, genetics, and physiology, and describe how a neuropeptidergic circuit became an important druggable obesity target.

MAJOR CONCLUSIONS

Unravelling the genetics of the subset of severe obesity has revealed the importance of the melanocortin pathway in appetitive control; coupling this with studying the molecular pharmacology of compounds that bind melanocortin receptors has brought a new obesity drug to the market. This process provides a drug discovery template for complex disorders, which for setmelanotide took 25 years to transform from a single gene into an approved drug.

Paraventricular Calcitonin Receptor-Expressing Neurons Modulate Energy Homeostasis in Male Mice

The paraventricular nucleus of the hypothalamus (PVH) is a heterogeneous collection of neurons that play important roles in modulating feeding and energy expenditure. Abnormal development or ablation of the PVH results in hyperphagic obesity and defects in energy expenditure whereas selective activation of defined PVH neuronal populations can suppress feeding and may promote energy expenditure. Here, we characterize the contribution of calcitonin receptor-expressing PVH neurons (CalcRPVH) to energy balance control. We used Cre-dependent viral tools delivered stereotaxically to the PVH of CalcR2Acre mice to activate, silence, and trace CalcRPVH neurons and determine their contribution to body weight regulation. Immunohistochemistry of fluorescently-labeled CalcRPVH neurons demonstrates that CalcRPVH neurons are largely distinct from several PVH neuronal populations involved in energy homeostasis; these neurons project to regions of the hindbrain that are implicated in energy balance control, including the nucleus of the solitary tract and the parabrachial nucleus. Acute activation of CalcRPVH neurons suppresses feeding without appreciably augmenting energy expenditure, whereas their silencing leads to obesity that may be due in part due to loss of PVH melanocortin-4 receptor signaling. These data show that CalcRPVH neurons are an essential component of energy balance neurocircuitry and their function is important for body weight maintenance. A thorough understanding of the mechanisms by which CalcRPVH neurons modulate energy balance might identify novel therapeutic targets for the treatment and prevention of obesity.

Central nervous system regulation of organismal energy and glucose homeostasis

Growing evidence implicates the brain in the regulation of both immediate fuel availability (for example, circulating glucose) and long-term energy stores (that is, adipose tissue mass). Rather than viewing the adipose tissue and glucose control systems separately, we suggest that the brain systems that control them are components of a larger, highly integrated, 'fuel homeostasis' control system. This conceptual framework, along with new insights into the organization and function of distinct neuronal systems, provides a context within which to understand how metabolic homeostasis is achieved in both basal and postprandial states. We also review evidence that dysfunction of the central fuel homeostasis system contributes to the close association between obesity and type 2 diabetes, with the goal of identifying more effective treatment options for these common metabolic disorders.

Chemogenetics drives paradigm change in the investigation of behavioral circuits and neural mechanisms underlying drug action

Recent developments in chemogenetic approaches to the investigation of brain function have ushered in a paradigm change in the strategy for drug and behavior research and clinical drug-based medications. As the nature of the drug action is based on humoral regulation, it is a challenge to identify the neuronal mechanisms responsible for the expression of certain targeted behavior induced by drug application. The development of chemogenetic approaches has allowed researchers to control neural activities in targeted neurons through a toolbox, including engineered G protein-coupled receptors or ligand-gated ion channels together with exogenously inert synthetic ligands. This review provides a brief overview of the chemogenetics toolbox with an emphasis on the DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) technique used in rodent models, which is applicable to the investigation of how specific neural circuits regulate behavioral processes. The use of chemogenetics has had a significant impact on basic neuroscience for a better understanding of the relationships between brain activity and the expression of behaviors with cell- and circuit-specific orders. Furthermore, chemogenetics is potentially a useful tool to deconstruct the neuropathological mechanisms of mental diseases and its regulation by drug, and provide us with transformative therapeutics with medication. We also review recent findings in the use of chemogenetic techniques to uncover functional circuit connections of serotonergic neurons in rodent models.

Genistein during Development Alters Differentially the Expression of POMC in Male and Female Rats

Phytoestrogens are considered beneficial for health, but some studies have shown that they may cause adverse effects. This study investigated the effects of genistein administration during the second week of life on energy metabolism and on the circuits regulating food intake. Two different genistein doses, 10 or 50 µg/g, were administered to male and female rats from postnatal day (P) 6 to P13. Physiological parameters, such as body weight and caloric intake, were then analyzed at P90. Moreover, proopiomelanocortin (POMC) expression in the arcuate nucleus (Arc) and orexin expression in the dorsomedial hypothalamus (DMH), perifornical area (PF) and lateral hypothalamus (LH) were studied. Our results showed a delay in the emergence of sex differences in the body weight in the groups with higher genistein doses. Furthermore, a significant decrease in the number of POMC-immunoreactive (POMC-ir) cells in the Arc in the two groups of females treated with genistein was observed. In contrast, no alteration in orexin expression was detected in any of the structures analyzed in either males or females. In conclusion, genistein can modulate estradiol's programming actions on the hypothalamic feeding circuits differentially in male and female rats during development.

A distinct parabrachial-to-lateral hypothalamus circuit for motivational suppression of feeding by nociception

The motivation to eat is not only shaped by nutrition but also competed by external stimuli including pain. How the mouse hypothalamus, the feeding regulation center, integrates nociceptive inputs to modulate feeding is unclear. Within the key nociception relay center parabrachial nucleus (PBN), we demonstrated that neurons projecting to the lateral hypothalamus (^LHPBN) are nociceptive yet distinct from danger-encoding central amygdala-projecting (^CeAPBN) neurons. Activation of ^LHPBN strongly suppressed feeding by limiting eating frequency and also reduced motivation to work for food reward. Refined approach-avoidance paradigm revealed that suppression of ^LHPBN, but not ^CeAPBN, sustained motivation to obtain food. The effect of ^LHPBN neurons on feeding was reversed by suppressing downstream LH^VGluT2 neurons. Thus, distinct from a circuit for fear and escape responses, ^LHPBN neurons channel nociceptive signals to LH^VGluT2 neurons to suppress motivational drive for feeding. Our study provides a new perspective in understanding feeding regulation by external competing stimuli.

Flexible scaling and persistence of social vocal communication

Innate vocal sounds such as laughing, screaming or crying convey one's feelings to others. In many species, including humans, scaling the amplitude and duration of vocalizations is essential for effective social communication1-3. In mice, female scent triggers male mice to emit innate courtship ultrasonic vocalizations (USVs)4,5. However, whether mice flexibly scale their vocalizations and how neural circuits are structured to generate flexibility remain largely unknown. Here we identify mouse neurons from the lateral preoptic area (LPOA) that express oestrogen receptor 1 (LPOAESR1 neurons) and, when activated, elicit the complete repertoire of USV syllables emitted during natural courtship. Neural anatomy and functional data reveal a two-step, di-synaptic circuit motif in which primary long-range inhibitory LPOAESR1 neurons relieve a clamp of local periaqueductal grey (PAG) inhibition, enabling excitatory PAG USV-gating neurons to trigger vocalizations. We find that social context shapes a wide range of USV amplitudes and bout durations. This variability is absent when PAG neurons are stimulated directly; PAG-evoked vocalizations are time-locked to neural activity and stereotypically loud. By contrast, increasing the activity of LPOAESR1 neurons scales the amplitude of vocalizations, and delaying the recovery of the inhibition clamp prolongs USV bouts. Thus, the LPOA disinhibition motif contributes to flexible loudness and the duration and persistence of bouts, which are key aspects of effective vocal social communication.

The pleiotropic roles of leptin in metabolism, immunity, and cancer

The discovery of the archetypal adipocytokine leptin and how it regulates energy homeostasis have represented breakthroughs in our understanding of the endocrine function of the adipose tissue and the biological determinants of human obesity. Investigations on leptin have also been instrumental in identifying physio-pathological connections between metabolic regulation and multiple immunological functions. For example, the description of the promoting activities of leptin on inflammation and cell proliferation have recognized the detrimental effects of leptin in connecting dysmetabolic conditions with cancer and with onset and/or progression of autoimmune disease. Here we review the multiple biological functions and complex framework of operations of leptin, discussing why and how the pleiotropic activities of this adipocytokine still pose major hurdles in the development of effective leptin-based therapeutic opportunities for different clinical conditions.

Neuroendocrine control of appetite and metabolism

Body homeostasis is predominantly controlled by hormones secreted by endocrine organs. The central nervous system contains several important endocrine structures, including the hypothalamic-pituitary axis. Conventionally, neurohormones released by the hypothalamus and the pituitary gland (hypophysis) have received much attention owing to the unique functions of the end hormones released by their target peripheral organs (e.g., glucocorticoids released by the adrenal glands). Recent advances in mouse genetics have revealed several important metabolic functions of hypothalamic neurohormone-expressing cells, many of which are not readily explained by the action of the corresponding classical downstream hormones. Notably, the newly identified functions are better explained by the action of conventional neurotransmitters (e.g., glutamate and GABA) that constitute a neuronal circuit. In this review, we discuss the regulation of appetite and metabolism by hypothalamic neurohormone-expressing cells, with a focus on the distinct contributions of neurohormones and neurotransmitters released by these neurons.

The regulation of food intake by insulin in the central nervous system

Food intake and energy expenditure are regulated by peripheral signals providing feedback on nutrient status and adiposity to the central nervous system. One of these signals is the pancreatic hormone, insulin. Unlike peripheral administration of insulin, which often causes weight gain, central administration of insulin leads to a reduction in food intake and body weight when administered long-term. This is a result of feedback processes in regions of the brain that regulate food intake. Within the hypothalamus, the arcuate nucleus (ARC) contains subpopulations of neurones that produce orexinergic neuropeptides agouti-related peptide (AgRP)/neuropeptide Y (NPY) and anorexigenic neuropeptides, pro-opiomelanocortin (POMC)/cocaine- and amphetamine-regulated transcript (CART). Intracerebroventricular infusion of insulin down-regulates the expression of AgRP/NPY at the same time as up-regulating expression of POMC/CART. Recent evidence suggests that insulin activity within the amygdala may play an important role in regulating energy balance. Insulin infusion into the central nucleus of the amygdala (CeA) can decrease food intake, possibly by modulating activity of NPY and other neurone subpopulations. Insulin signalling within the CeA can also influence stress-induced obesity. Overall, it is evident that the CeA is a critical target for insulin signalling and the regulation of energy balance.

Insulin on the brain: The role of central insulin signalling in energy and glucose homeostasis

Insulin signals to the brain where it coordinates multiple physiological processes underlying energy and glucose homeostasis. This review explores where and how insulin interacts within the brain parenchyma, how brain insulin signalling functions to coordinate energy and glucose homeostasis and how this contributes to the pathogenesis of metabolic disease.

Central regulation of glucose metabolism in an insulin-dependent and -independent manner

The central nervous system (CNS) contributes significantly to glucose homeostasis. The available evidence indicates that insulin directly acts on the CNS, in particular the hypothalamus, to regulate hepatic glucose production, thereby controlling whole-body glucose metabolism. Additionally, insulin also acts on the brain to regulate food intake and fat metabolism, which may indirectly regulate glucose metabolism. Studies conducted over the last decade have found that the CNS can regulate glucose metabolism in an insulin-independent manner. Enhancement of central leptin signalling reverses hyperglycaemia in insulin-deficient rodents. Here, I review the mechanisms by which central insulin and leptin actions regulate glucose metabolism. Although clinical studies have shown that insulin treatment is currently indispensable for managing diabetes, unravelling the neuronal mechanisms underlying the central regulation of glucose metabolism will pave the way for the design of novel therapeutic drugs for diabetes.

Membrane and nuclear initiated estrogenic regulation of homeostasis

Reproduction and energy balance are inextricably linked in order to optimize the evolutionary fitness of an organism. With insufficient or excessive energy stores a female is liable to suffer complications during pregnancy and produce unhealthy or obesity-prone offspring. The quintessential function of the hypothalamus is to act as a bridge between the endocrine and nervous systems, coordinating fertility and autonomic functions. Across the female reproductive cycle various motivations wax and wane, following levels of ovarian hormones. Estrogens, more specifically 17β-estradiol (E2), coordinate a triumvirate of hypothalamic neurons within the arcuate nucleus (ARH) that govern the physiological underpinnings of these behavioral dynamics. Arising from a common progenitor pool of cells, this triumvirate is composed of the kisspeptin (Kiss1ARH), proopiomelanocortin (POMC), and neuropeptide Y/agouti-related peptide (AgRP) neurons. Although the excitability of these neuronal subpopulations is subject to genomic and rapid estrogenic regulation, kisspeptin neurons are the most sensitive, reflecting their integral function in female fertility. Based on the premise that E2 coordinates autonomic functions around reproduction, we will review the recent findings on the synaptic interactions between Kiss1, AgRP and POMC neurons and how the rapid membrane-initiated and intracellular signaling cascades activated by E2 in these neurons are critical for control of homeostatic functions supporting reproduction.

Projections from the dorsomedial division of the bed nucleus of the stria terminalis to hypothalamic nuclei in the mouse

As stressful environment is a potent modulator of feeding, we seek in the present work to decipher the neuroanatomical basis for an interplay between stress and feeding behaviors. For this, we combined anterograde and retrograde tracing with immunohistochemical approaches to investigate the patterns of projections between the dorsomedial division of the bed nucleus of the stria terminalis (BNST), well connected to the amygdala, and hypothalamic structures such as the paraventricular (PVH) and dorsomedial (DMH), the arcuate (ARH) nuclei and the lateral hypothalamic areas (LHA) known to control feeding and motivated behaviors. We particularly focused our study on afferences to proopiomelanocortin (POMC), agouti-related peptide (AgRP), melanin-concentrating-hormone (MCH) and orexin (ORX) neurons characteristics of the ARH and the LHA, respectively. We found light to intense innervation of all these hypothalamic nuclei. We particularly showed an innervation of POMC, AgRP, MCH and ORX neurons by the dorsomedial and dorsolateral divisions of the BNST. Therefore, these results lay the foundation for a better understanding of the neuroanatomical basis of the stress-related feeding behaviors.

Restriction of food intake by PPP1R17-expressing neurons in the DMH

We set out to identify novel neuronal populations controlling feeding behaviors. Therefore, we focused on identifying novel neuronal populations modulating food intake and body weight. In this article, we report that DMH^Ppp1r17 neurons are activated by increased food intake and that activating them results in decreased food intake and body weight, while inhibiting them leads to increased body weight and food intake. These data suggest that DMH^Ppp1r17 neurons restrict binges of eating. In addition to its basic science importance, these findings could have therapeutic applications, as they suggest that pharmacologic activation of PPP1R17 neurons could potentially reduce weight in settings of obesity and binge-like eating.

Leptin-deficient ob/ob mice eat voraciously, and their food intake is markedly reduced by leptin treatment. In order to identify potentially novel sites of leptin action, we used PhosphoTRAP to molecularly profile leptin-responsive neurons in the hypothalamus and brainstem. In addition to identifying several known leptin responsive populations, we found that neurons in the dorsomedial hypothalamus (DMH) of ob/ob mice expressing protein phosphatase 1 regulatory subunit 17 (PPP1R17) constitutively express cFos and that this is suppressed by leptin treatment. Because ob mice are hyperphagic, we hypothesized that activating PPP1R17 neurons would increase food intake. However, chemogenetic activation of PPP1R17 neurons decreased food intake and body weight of ob/ob mice while inhibition of PPP1R17 neurons increased them. Similarly, in a scheduled feeding protocol that elicits increased consumption, mice also ate more when PPP1R17 neurons were inhibited and ate less when they were activated. Finally, we found that pair-feeding of ob mice reduced cFos expression to a similar extent as leptin and that reducing the amount of food available during scheduled feeding in DMH^Ppp1r17 neurons also decreased cFos in DMH^Ppp1r17 neurons. Finally, these neurons do not express the leptin receptor, suggesting that the effect of leptin on these neurons is indirect and secondary to reduced food intake. In aggregate, these results show that PPP1R17 neurons in the DMH are activated by increased food intake and in turn restrict intake to limit overconsumption, suggesting that they function to constrain binges of eating.