Leptin excites proopiomelanocortin neurons via activation of TRPC channels

Estradiol elicits distinct firing patterns in arcuate nucleus kisspeptin neurons of females through altering ion channel conductances

Hypothalamic kisspeptin (Kiss1) neurons are vital for pubertal development and reproduction. Arcuate nucleus Kiss1 (Kiss1 ARH ) neurons are responsible for the pulsatile release of Gonadotropin-releasing Hormone (GnRH). In females, the behavior of Kiss1 ARH neurons, expressing Kiss1, Neurokinin B (NKB), and Dynorphin (Dyn), varies throughout the ovarian cycle. Studies indicate that 17β-estradiol (E2) reduces peptide expression but increases Vglut2 mRNA and glutamate neurotransmission in these neurons, suggesting a shift from peptidergic to glutamatergic signaling. To investigate this shift, we combined transcriptomics, electrophysiology, and mathematical modeling. Our results demonstrate that E2 treatment upregulates the mRNA expression of voltage-activated calcium channels, elevating the whole-cell calcium current and that contribute to high-frequency burst firing. Additionally, E2 treatment decreased the mRNA levels of Canonical Transient Receptor Potential (TPRC) 5 and G protein-coupled K + (GIRK) channels. When TRPC5 channels in Kiss1 ARH neurons were deleted using CRISPR, the slow excitatory postsynaptic potential (sEPSP) was eliminated. Our data enabled us to formulate a biophysically realistic mathematical model of the Kiss1 ARH neuron, suggesting that E2 modifies ionic conductances in Kiss1 ARH neurons, enabling the transition from high frequency synchronous firing through NKB-driven activation of TRPC5 channels to a short bursting mode facilitating glutamate release. In a low E2 milieu, synchronous firing of Kiss1 ARH neurons drives pulsatile release of GnRH, while the transition to burst firing with high, preovulatory levels of E2 would facilitate the GnRH surge through its glutamatergic synaptic connection to preoptic Kiss1 neurons.

Loss of transient receptor potential channel 5 causes obesity and postpartum depression

Hypothalamic neural circuits regulate instinctive behaviors such as food seeking, the fight/flight response, socialization, and maternal care. Here, we identified microdeletions on chromosome Xq23 disrupting the brain-expressed transient receptor potential (TRP) channel 5 (TRPC5). This family of channels detects sensory stimuli and converts them into electrical signals interpretable by the brain. Male TRPC5 deletion carriers exhibited food seeking, obesity, anxiety, and autism, which were recapitulated in knockin male mice harboring a human loss-of-function TRPC5 mutation. Women carrying TRPC5 deletions had severe postpartum depression. As mothers, female knockin mice exhibited anhedonia and depression-like behavior with impaired care of offspring. Deletion of Trpc5 from oxytocin neurons in the hypothalamic paraventricular nucleus caused obesity in both sexes and postpartum depressive behavior in females, while Trpc5 overexpression in oxytocin neurons in knock-in mice reversed these phenotypes. We demonstrate that TRPC5 plays a pivotal role in mediating innate human behaviors fundamental to survival, including food seeking and maternal care.

Do POMC neurons have a sweet tooth for leptin? Special issue: Role of nutrients in nervous control of energy balance

Coordinated detection of changes in metabolic state by the nervous system is fundamental for survival. Hypothalamic pro-opiomelanocortin (POMC) neurons play a critical role in integrating metabolic signals, including leptin levels. They also coordinate adaptative responses and thus represent an important relay in the regulation of energy balance. Despite a plethora of work documenting the effects of individual hormones, nutrients, and neuropeptides on POMC neurons, the importance for crosstalk and additive effects between such signaling molecules is still underexplored. The ability of the metabolic state and the concentrations of nutrients, such as glucose, to influence leptin's effects on POMC neurons appears critical for understanding the function and complexity of this regulatory network. Here, we summarize the current knowledge on the effects of leptin on POMC neuron electrical excitability and discuss factors potentially contributing to variability in these effects, with a particular focus on the mouse models that have been developed and the importance of extracellular glucose levels. This review highlights the importance of the metabolic "environment" for determining hypothalamic neuronal responsiveness to metabolic cues and for determining the fundamental effects of leptin on the activity of hypothalamic POMC neurons.

Normotensive metabolic syndrome in Transient Receptor Potential Canonical Channel type 1 Trpc1-/- mice

Metabolic syndrome has become a global epidemic, affecting all developed countries and communities with growing economies. Worldwide, increasing efforts have been directed at curbing this growing problem. Mice deleted of the gene encoding Type 1 Transient Receptor Potential Canonical Channel (Trpc1) were found to weigh heavier than controls. They had fasting hyperglycemia and impaired glucose tolerance compared with wild-type controls. Beyond 1 year of age, plasma triglyceride level in Trpc1-/- mice was elevated. Plasma cholesterol levels tended to be higher than in controls. The livers of Trpc1-/- mice were heavier, richer in triglyceride, and more echogenic than those of controls on ultrasound evaluation. Hematocrit was lower in Trpc1-/- mice of both genders beginning at the second to third months of age in the absence of bleeding or hemolysis. Measured by the indirect tail-cuff method or by the direct arterial cannulation, blood pressures in null mice were lower than controls. We conclude that TRPC1 gene regulates body metabolism and that except for hypertension, phenotypes of mice after deletion of the Trpc1 gene resemble mice with metabolic syndrome, suggesting that this could be a good experimental model for future investigation of the pathogenesis and management of this disorder.

Canonical transient receptor potential channels and hypothalamic control of homeostatic functions

Recent molecular biological and electrophysiological studies have identified multiple transient receptor potential (TRP) channels in hypothalamic neurons as critical modulators of homeostatic functions. In particular, the canonical transient receptor potential channels (TRPCs) are expressed in hypothalamic neurons that are vital for the control of fertility and energy homeostasis. Classical neurotransmitters such as serotonin and glutamate and peptide neurotransmitters such as kisspeptin, neurokinin B and pituitary adenylyl cyclase-activating polypeptide signal through their cognate G protein-coupled receptors to activate TPRC 4, 5 channels, which are essentially ligand-gated calcium channels. In addition to neurotransmitters, circulating hormones like insulin and leptin signal through insulin receptor (InsR) and leptin receptor (LRb), respectively, to activate TRPC 5 channels in hypothalamic arcuate nucleus pro-opiomelanocortin (POMC) and kisspeptin (arcuate Kiss1 [Kiss1ARH]) neurons to have profound physiological (excitatory) effects. Besides its overt depolarizing effects, TRPC channels conduct calcium ions into the cytoplasm, which has a plethora of downstream effects. Moreover, not only the expression of Trpc5 mRNA but also the coupling of receptors to TRPC 5 channel opening are regulated in different physiological states. In particular, the mRNA expression of Trpc5 is highly regulated in kisspeptin neurons by circulating estrogens, which ultimately dictates the firing pattern of kisspeptin neurons. In obesity states, InsRs are "uncoupled" from opening TRPC 5 channels in POMC neurons, rendering them less excitable. Therefore, in this review, we will focus on the critical role of TRPC 5 channels in regulating the excitability of Kiss1ARH and POMC neurons in different physiological and pathological states.

Leptin excites basolateral amygdala principal neurons and reduces food intake by LepRb-JAK2-PI3K-dependent depression of GIRK channels

Leptin is an adipocyte-derived hormone that modulates food intake, energy balance, neuroendocrine status, thermogenesis, and cognition. Whereas a high density of leptin receptors has been detected in the basolateral amygdala (BLA) neurons, the physiological functions of leptin in the BLA have not been determined yet. We found that application of leptin excited BLA principal neurons by activation of the long form leptin receptor, LepRb. The LepRb-elicited excitation of BLA neurons was mediated by depression of the G protein-activated inwardly rectifying potassium (GIRK) channels. Janus Kinase 2 (JAK2) and phosphoinositide 3-kinase (PI3K) were required for leptin-induced excitation of BLA neurons and depression of GIRK channels. Microinjection of leptin into the BLA reduced food intake via activation of LepRb, JAK2, and PI3K. Our results may provide a cellular and molecular mechanism to explain the physiological roles of leptin in vivo.

Central Channelopathies in Obesity

As obesity has raised heightening awareness, researchers have attempted to identify potential targets that can be treated for therapeutic intervention. Focusing on the central nervous system (CNS), the key organ in maintaining energy balance, a plethora of ion channels that are expressed in the CNS have been inspected and determined through manipulation in different hypothalamic neural subpopulations for their roles in fine-tuning neuronal activity on energy state alterations, possibly acting as metabolic sensors. However, a remaining gap persists between human clinical investigations and mouse studies. Despite having delineated the pathways and mechanisms of how the mouse study-identified ion channels modulate energy homeostasis, only a few targets overlap with the obesity-related risk genes extracted from human genome-wide association studies. Here, we present the most recently discovered CNS-specific metabolism-correlated ion channels using reverse and forward genetics approaches in mice and humans, respectively, in the hope of illuminating the prospects for future therapeutic development.

Dynamic, sex- and diet-specific pleiotropism in the PAC1 receptor-mediated regulation of arcuate proopiomelanocortin and Neuropeptide Y/Agouti related peptide neuronal excitability by anorexigenic ventromedial nucleus PACAP neurons

This study furthers the investigation of how pituitary adenylate cyclase activating polypeptide (PACAP) and the PAC1 receptor (PAC1R) regulate the homeostatic energy balance circuitry. We hypothesized that apoptotic ablation of PACAP neurones in the hypothalamic ventromedial nucleus (VMN) would affect both energy intake and energy expenditure. We also hypothesized that selective PAC1R knockdown would impair the PACAP-induced excitation in anorexigenic proopiomelanocortin (POMC) neurones and inhibition of orexigenic neuropeptide Y (NPY)/agouti-related peptide (AgRP) neurones in the hypothalamic arcuate nucleus (ARC). The results show CASPASE-3-induced ablation of VMN PACAP neurones leads to increased energy intake and meal frequency as well as decreased energy expenditure in lean animals. The effects were more robust in obese males, whereas we saw the opposite effects in obese females. We then utilized visualized whole-cell patch clamp recordings in hypothalamic slices. PAC1R knockdown in POMC neurones diminishes the PACAP-induced depolarization, increase in firing, decreases in energy intake and meal size, as well as increases in CO2 production and O2 consumption. Similarly, the lack of expression of the PAC1R in NPY/AgRP neurones greatly attenuates the PACAP-induced hyperpolarization, suppression of firing, decreases in energy intake and meal frequency, as well as increases in energy expenditure. The PACAP response in NPY/AgRP neurones switched from predominantly inhibitory to excitatory in fasted animals. Finally, the anorexigenic effect of PACAP was potentiated when oestradiol was injected into the ARC in ovariectomized females. This study demonstrates the critical role of anorexigenic VMN PACAP neurones and the PAC1R in exciting POMC and inhibiting NPY/AgRP neurons to control homeostatic feeding.

The Unrestrained Overeating Behavior and Clinical Perspective

Obesity-related co-morbidities decrease life quality, reduce working ability, and lead to early death. In the adult population, eating addiction manifests with excessive food consumption and the unrestrained overeating behavior, which is associated with increased risk of morbidity and mortality and defined as the binge eating disorder (BED). This hedonic intake is correlated with fat preference and the total amount of dietary fat consumption is the most potent risk factor for weight gain. Long-term BED leads to greater sensitivity to the rewarding effects of palatable foods and results in obesity fatefully. Increased plasma concentrations of non-esterified free fatty acids and lipid-overloaded hypertrophic adipocytes may cause insulin resistance. In addition to dietary intake of high-fat diet, sedentary lifestyle leads to increased storage of triglycerides not only in adipose tissue but also ectopically in other tissues. Lipid-induced apoptosis, ceramide accumulation, reactive oxygen species overproduction, endoplasmic reticulum stress, and mitochondrial dysfunction play role in the pathogenesis of lipotoxicity. Food addiction and BED originate from complex action of dopaminergic, opioid, and cannabinoid systems. BED may also be associated with both obesity and major depressive disorder. For preventing morbidity and mortality, as well as decreasing the impact of obesity-related comorbidities in appropriately selected patients, opiate receptor antagonists and antidepressant combination are recommended. Pharmacotherapy alongside behavioral management improves quality of life and reduces the obesity risk; however, the number of licensed drugs is very few. Thus, stereotactic treatment is recommended to break down the refractory obesity and binge eating in obese patient. As recent applications in the field of non-invasive neuromodulation, transcranial magnetic stimulation and transcranial direct current stimulation are thought to be important in image-guided deep brain stimulation in humans. Chronic overnutrition most likely provides repetitive and persistent signals that up-regulate inhibitor of nuclear factor kappa B (NF-κB) kinase beta subunit/NF-κB (IKKβ/NF-κB) in the hypothalamus before the onset of obesity. However, how the mechanisms of high-fat diet-induced peripheral signals affect the hypothalamic arcuate nucleus remain largely unknown.

Electrophysiological Comparison of Definitive Pro-opiomelanocortin Neurons in the Arcuate Nucleus and the Retrochiasmatic Area of Male and Female Mice

Pro-opiomelanocortin (POMC)-expressing neurons in the arcuate nucleus of the hypothalamus (ARC) are considered a major site of leptin action. Due to increasing evidence that POMC neurons are highly heterogeneous and indications that the conventional molecular tools to study their functions have important limitations, a reassessment of leptin's effects on definitive POMC neurons is needed. POMC neurons are also expressed in the retrochiasmatic area (RCA), where their function is poorly understood. Furthermore, the response of POMC neurons to leptin in females is largely unknown. Therefore, the present study aimed to determine the differences in leptin responsiveness of POMC neurons in the ARC and the RCA using a mouse model allowing adult-inducible fluorescent labeling. We performed whole-cell patch clamp electrophysiology on 154 POMC neurons from male and female mice. We confirmed and extended the model by which leptin depolarizes POMC neurons, in both the ARC and the RCA. Furthermore, we characterized the electrophysiological properties of an underappreciated subpopulation representing ∼10% of hypothalamic POMC neurons that are inhibited by leptin. We also provide evidence that sex does not appear to be a major determinant of basal properties and leptin responsiveness of POMC neurons, but that females are overall less responsive to leptin compared to males.

Neural stem cell self-renewal stimulation by store-operated calcium entries in adult mouse area postrema: influence of leptin

Neural stem cells (NSCs) persist in specific brain germinative niches and sustain neurogenesis throughout life in adult mammals. In addition to the two major stem cell niches in the subventricular zone and the hippocampal dentate gyrus, the area postrema located in the brainstem has been identified as a neurogenic zone as well. NSCs are regulated by signals from the microenvironment that adjust stem cell response to the needs of the organism. Evidence accumulated over the past decade indicates that Ca²⁺ channels play pivotal functions in NSC maintenance. In this study, we explored in area postrema NSCs the presence and roles of a subset of Ca²⁺ channels, the store-operated Ca²⁺ channels (SOCs) that have the capacity to transduce extracellular signals into Ca²⁺ signals. Our data show that NSCs derived from the area postrema express TRPC1 and Orai1, known to form SOCs, as well as their activator STIM1. Ca²⁺ imaging indicated that NSCs exhibit store-operated Ca²⁺ entries (SOCEs). Pharmacological blockade of SOCEs with SKF-96365, YM-58483 (also known as BTP2) or GSK-7975A resulted in decreased NSC proliferation and self-renewal, indicating a major role for SOCs in maintaining NSC activity within the area postrema. Furthermore, our results show that leptin, an adipose tissue-derived hormone whose ability to control energy homeostasis is dependent on the area postrema, decreased SOCEs and reduced self-renewal of NSCs in the area postrema. As aberrant SOC function has been linked to an increasing number of diseases, including brain disorders, our study opens new perspectives for NSCs in brain pathophysiology.

GLP-1 and nicotine combination therapy engages hypothalamic and mesolimbic pathways to reverse obesity

Glucagon-like peptide-1 receptor (GLP-1R) agonists promote nicotine avoidance. Here, we show that the crosstalk between GLP-1 and nicotine extends beyond effects on nicotine self-administration and can be exploited pharmacologically to amplify the anti-obesity effects of both signals. Accordingly, combined treatment with nicotine and the GLP-1R agonist, liraglutide, inhibits food intake and increases energy expenditure to lower body weight in obese mice. Co-treatment with nicotine and liraglutide gives rise to neuronal activity in multiple brain regions, and we demonstrate that GLP-1R agonism increases excitability of hypothalamic proopiomelanocortin (POMC) neurons and dopaminergic neurons in the ventral tegmental area (VTA). Further, using a genetically encoded dopamine sensor, we reveal that liraglutide suppresses nicotine-induced dopamine release in the nucleus accumbens in freely behaving mice. These data support the pursuit of GLP-1R-based therapies for nicotine dependence and encourage further evaluation of combined treatment with GLP-1R agonists and nicotinic receptor agonists for weight loss.

CRISPR/SaCas9 mutagenesis of stromal interaction molecule 1 in proopiomelanocortin neurons increases glutamatergic excitability and protects against diet-induced obesity

OBJECTIVE

Proopiomelanocortin (POMC) neurons are the key anorexigenic hypothalamic neuron for integrating metabolic cues to generate the appropriate output for maintaining energy homeostasis and express the requisite channels as a perfect synaptic integrator in this role. Similar to the metabolic hormones leptin and insulin, glutamate also excites POMC neurons via group I metabotropic glutamate receptors (mGluR1 and 5, mGluR1/5) that activate Transient Receptor Potential Canonical (TRPC 5) Channels to cause depolarization. A key modulator of TRPC 5 channel activity is stromal interaction molecule 1 (STIM1), which is involved in recruitment of TRPC 5 channels from receptor-operated to store-operated calcium entry following depletion of calcium from the endoplasmic reticulum.

METHODS

We used a single adeno-associated viral (AAV) vector containing a recombinase-dependent Staphylococcus aureus Cas9 (SaCas) and a single guide RNA (sgRNA) to mutate Stim1 in POMC^Cre neurons in male mice, verified by qPCR of Stim1 mRNA expression in single POMC neurons. Whole-cell patch clamp experiments were conducted to validate the effects of Stim1 mutagenesis. Body weight and food intake were measured in male mice to assess disruptions in energy balance.

RESULTS

Reduced Stim1 expression augmented the efficacy of the mGluR1/5 agonist 3, 5-Dihydroxyphenylglycine (DHPG) to depolarize POMC neurons via a Gα_q-coupled signaling pathway, which is an essential part of excitatory glutamatergic input in regulating energy homeostasis. The TRPC 5 channel blockers HC070 and Pico145 antagonized the excitatory effects of DHPG. As proof of principle, mutagenesis of Stim1 in POMC neurons reduced food intake, attenuated weight gain, reduced body fat and fat pad mass in mice fed a high fat diet.

CONCLUSIONS

Using CRISPR technology we have uncovered a critical role of STIM1 in modulating glutamatergic activation of TRPC 5 channels in POMC neurons, which ultimately is important for maintaining energy balance.

Socs3 ablation in kisspeptin cells partially prevents lipopolysaccharide-induced body weight loss

Many cytokines have been proposed to regulate reproduction due to their actions on hypothalamic kisspeptin cells, the main modulators of gonadotropin-releasing hormone (GnRH) neurons. Hormones such as leptin, prolactin and growth hormone are good examples of cytokines that lead to Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway activation, consequently exerting effects in kisspeptin neurons. Different studies have investigated how specific components of the JAK/STAT signaling pathway affect the functions of kisspeptin cells, but the role of the suppressor of cytokine signaling 3 (SOCS3) in mediating cytokine actions in kisspeptin cells remains unknown. Cre-Loxp technology was used in the present study to ablate Socs3 expression in kisspeptin cells (Kiss1/Socs3-KO). Then, male and female control and Kiss1/Socs3-KO mice were evaluated for sexual maturation, energy homeostasis features, and fertility. It was found that hypothalamic Kiss1 mRNA expression is significantly downregulated in Kiss1/Socs3-KO mice. Despite reduced hypothalamic Kiss1 mRNA content, these mice did not present any sexual maturation or fertility impairments. Additionally, body weight gain, leptin sensitivity and glucose homeostasis were similar to control mice. Interestingly, Kiss1/Socs3-KO mice were partially protected against lipopolysaccharide (LPS)-induced body weight loss. Our results suggest that Socs3 ablation in kisspeptin cells partially prevents the sickness behavior induced by LPS, suggesting that kisspeptin cells can modulate energy metabolism in mice in certain situations.

Leptin signaling and leptin resistance

With the prevalence of obesity and associated comorbidities, studies aimed at revealing mechanisms that regulate energy homeostasis have gained increasing interest. In 1994, the cloning of leptin was a milestone in metabolic research. As an adipocytokine, leptin governs food intake and energy homeostasis through leptin receptors (LepR) in the brain. The failure of increased leptin levels to suppress feeding and elevate energy expenditure is referred to as leptin resistance, which encompasses complex pathophysiological processes. Within the brain, LepR-expressing neurons are distributed in hypothalamus and other brain areas, and each population of the LepR-expressing neurons may mediate particular aspects of leptin effects. In LepR-expressing neurons, the binding of leptin to LepR initiates multiple signaling cascades including janus kinase (JAK)-signal transducers and activators of transcription (STAT) phosphatidylinositol 3-kinase (PI3K)-protein kinase B (AKT), extracellular regulated protein kinase (ERK), and AMP-activated protein kinase (AMPK) signaling, etc., mediating leptin actions. These findings place leptin at the intersection of metabolic and neuroendocrine regulations, and render leptin a key target for treating obesity and associated comorbidities. This review highlights the main discoveries that shaped the field of leptin for better understanding of the mechanism governing metabolic homeostasis, and guides the development of safe and effective interventions to treat obesity and associated diseases.

Sexual Dimorphism in Adipose-Hypothalamic Crosstalk and the Contribution of Aryl Hydrocarbon Receptor to Regulate Energy Homeostasis

There are fundamental sex differences in the regulation of energy homeostasis. Better understanding of the underlying mechanisms of energy balance that account for this asymmetry will assist in developing sex-specific therapies for sexually dimorphic diseases such as obesity. Multiple organs, including the hypothalamus and adipose tissue, play vital roles in the regulation of energy homeostasis, which are regulated differently in males and females. Various neuronal populations, particularly within the hypothalamus, such as arcuate nucleus (ARC), can sense nutrient content of the body by the help of peripheral hormones such leptin, derived from adipocytes, to regulate energy homeostasis. This review summarizes how adipose tissue crosstalk with homeostatic network control systems in the brain, which includes energy regulatory regions and the hypothalamic–pituitary axis, contribute to energy regulation in a sex-specific manner. Moreover, development of obesity is contingent upon diet and environmental factors. Substances from diet and environmental contaminants can exert insidious effects on energy metabolism, acting peripherally through the aryl hydrocarbon receptor (AhR). Developmental AhR activation can impart permanent alterations of neuronal development that can manifest a number of sex-specific physiological changes, which sometimes become evident only in adulthood. AhR is currently being investigated as a potential target for treating obesity. The consensus is that impaired function of the receptor protects from obesity in mice. AhR also modulates sex steroid receptors, and hence, one of the objectives of this review is to explain why investigating sex differences while examining this receptor is crucial. Overall, this review summarizes sex differences in the regulation of energy homeostasis imparted by the adipose–hypothalamic axis and examines how this axis can be affected by xenobiotics that signal through AhR.

CX3CL1 Action on Microglia Protects from Diet-Induced Obesity by Restoring POMC Neuronal Excitability and Melanocortin System Activity Impaired by High-Fat Diet Feeding

Both hypothalamic microglial inflammation and melanocortin pathway dysfunction contribute to diet-induced obesity (DIO) pathogenesis. Previous studies involving models of altered microglial signaling demonstrate altered DIO susceptibility with corresponding POMC neuron cytological changes, suggesting a link between microglia and the melanocortin system. We addressed this hypothesis using the specific microglial silencing molecule, CX3CL1 (fractalkine), to determine whether reducing hypothalamic microglial activation can restore POMC/melanocortin signaling to protect against DIO. We performed metabolic analyses in high fat diet (HFD)-fed mice with targeted viral overexpression of CX3CL1 in the hypothalamus. Electrophysiologic recording in hypothalamic slices from POMC-MAPT-GFP mice was used to determine the effects of HFD feeding and microglial silencing via minocycline or CX3CL1 on GFP-labeled POMC neurons. Finally, mice with hypothalamic overexpression of CX3CL1 received central treatment with the melanocortin receptor antagonist SHU9119 to determine whether melanocortin signaling is required for the metabolic benefits of CX3CL1. Hypothalamic overexpression of CX3CL1 increased leptin sensitivity and POMC gene expression, while reducing weight gain in animals fed an HFD. In electrophysiological recordings from hypothalamic slice preparations, HFD feeding was associated with reduced POMC neuron excitability and increased amplitude of inhibitory postsynaptic currents. Microglial silencing using minocycline or CX3CL1 treatment reversed these HFD-induced changes in POMC neuron electrophysiologic properties. Correspondingly, blockade of melanocortin receptor signaling in vivo prevented both the acute and chronic reduction in food intake and body weight mediated by CX3CL1. Our results show that suppressing microglial activation during HFD feeding reduces DIO susceptibility via a mechanism involving increased POMC neuron excitability and melanocortin signaling.

The PACAP Paradox: Dynamic and Surprisingly Pleiotropic Actions in the Central Regulation of Energy Homeostasis

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP), a pleiotropic neuropeptide, is widely distributed throughout the body. The abundance of PACAP expression in the central and peripheral nervous systems, and years of accompanying experimental evidence, indicates that PACAP plays crucial roles in diverse biological processes ranging from autonomic regulation to neuroprotection. In addition, PACAP is also abundantly expressed in the hypothalamic areas like the ventromedial and arcuate nuclei (VMN and ARC, respectively), as well as other brain regions such as the nucleus accumbens (NAc), bed nucleus of stria terminalis (BNST), and ventral tegmental area (VTA) - suggesting that PACAP is capable of regulating energy homeostasis via both the homeostatic and hedonic energy balance circuitries. The evidence gathered over the years has increased our appreciation for its function in controlling energy balance. Therefore, this review aims to further probe how the pleiotropic actions of PACAP in regulating energy homeostasis is influenced by sex and dynamic changes in energy status. We start with a general overview of energy homeostasis, and then introduce the integral components of the homeostatic and hedonic energy balance circuitries. Next, we discuss sex differences inherent to the regulation of energy homeostasis via these two circuitries, as well as the activational effects of sex steroid hormones that bring about these intrinsic disparities between males and females. Finally, we explore the multifaceted role of PACAP in regulating homeostatic and hedonic feeding through its actions in regions like the NAc, BNST, and in particular the ARC, VMN and VTA that occur in sex- and energy status-dependent ways.

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

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

Transient receptor potential cation channel 6 (TRPC6) deficiency leads to increased body weight and metabolic dysfunction

TRPC6, a member of the TRPC family, is expressed in the hypothalamus and modulates cell Ca²⁺ influx. However, the role of TRPC6 in controlling metabolic and cardiovascular functions under normal conditions has not been previously determined. Thus, the impacts of TRPC6 deletion on energy balance, metabolic and cardiovascular regulation as well as the anorexic responses to leptin and melanocortin 3/4 receptor (MC3/4R) activation were investigated in this study. Extensive cardiometabolic phenotyping was conducted in male and female TRPC6 knock out (KO) and control mice from 6 to 24 weeks of age to assess mechanisms by which TRPC6 influences regulation of energy balance and blood pressure (BP). We found that TRPC6 KO mice are heavier with greater adiposity, hyperphagic, and have reduced energy expenditure, impaired glucose tolerance, hyperinsulinemia, and increased liver fat compared to controls. TRPC6 KO mice also have smaller brains, reduced POMC mRNA levels in the hypothalamus, and impaired anorexic response to leptin but not to MC3/4R activation. BP and heart rate, assessed by telemetry, were similar in TRPC6 KO and control mice, and BP responses to air-jet stress were attenuated in TRPC6 KO mice despite increased body weight and metabolic disorders that normally raise BP and increase BP responses to stress. Our results provide evidence for a novel and important role of TRPC6 in controlling energy balance, adiposity, and glucose homeostasis, which suggests that normal TRPC6 function may be necessary to link weight gain and hyperleptinemia with BP responses to acute stress.

Hypothalamic Kisspeptin Neurons and the Control of Homeostasis

Hypothalamic kisspeptin (Kiss1) neurons provide indispensable excitatory transmission to gonadotropin-releasing hormone (GnRH) neurons for the coordinated release of gonadotropins, estrous cyclicity, and ovulation. But maintaining reproductive functions is metabolically demanding so there must be a coordination with multiple homeostatic functions, and it is apparent that Kiss1 neurons play that role. There are 2 distinct populations of hypothalamic Kiss1 neurons, namely arcuate nucleus (Kiss1ARH) neurons and anteroventral periventricular and periventricular nucleus (Kiss1AVPV/PeN) neurons in rodents, both of which excite GnRH neurons via kisspeptin release but are differentially regulated by ovarian steroids. Estradiol (E2) increases the expression of kisspeptin in Kiss1AVPV/PeN neurons but decreases its expression in Kiss1ARH neurons. Also, Kiss1ARH neurons coexpress glutamate and Kiss1AVPV/PeN neurons coexpress gamma aminobutyric acid (GABA), both of which are upregulated by E2 in females. Also, Kiss1ARH neurons express critical metabolic hormone receptors, and these neurons are excited by insulin and leptin during the fed state. Moreover, Kiss1ARH neurons project to and excite the anorexigenic proopiomelanocortin neurons but inhibit the orexigenic neuropeptide Y/Agouti-related peptide neurons, highlighting their role in regulating feeding behavior. Kiss1ARH and Kiss1AVPV/PeN neurons also project to the preautonomic paraventricular nucleus (satiety) neurons and the dorsomedial nucleus (energy expenditure) neurons to differentially regulate their function via glutamate and GABA release, respectively. Therefore, this review will address not only how Kiss1 neurons govern GnRH release, but how they control other homeostatic functions through their peptidergic, glutamatergic and GABAergic synaptic connections, providing further evidence that Kiss1 neurons are the key neurons coordinating energy states with reproduction.

Pituitary Adenylate Cyclase Activating Polypeptide Inhibits A10 Dopamine Neurons and Suppresses the Binge-like Consumption of Palatable Food

Pituitary adenylate cyclase-activating polypeptide (PACAP) binds to PACAP-specific (PAC1) receptors in multiple hypothalamic areas, especially those regulating energy balance. PACAP neurons in the ventromedial nucleus (VMN) exert anorexigenic effects within the homeostatic energy balance circuitry. Since PACAP can also reduce the consumption of palatable food, we tested the hypothesis that VMN PACAP neurons project to the ventral tegmental area (VTA) to inhibit A₁₀ dopamine neurons via PAC1 receptors and K_ATP channels, and thereby suppress binge-like consumption. We performed electrophysiological recordings in mesencephalic slices from male PACAP-Cre and tyrosine hydroxylase (TH)-Cre mice. Initially, we injected PACAP (30 pmol) into the VTA, where it suppressed binge intake in wildtype male but not female mice. Subsequent tract tracing studies uncovered projections of VMN PACAP neurons to the VTA. Optogenetic stimulation of VMN PACAP neurons in voltage clamp induced an outward current and increase in conductance in VTA neurons, and a hyperpolarization and decrease in firing in current clamp. These effects were markedly attenuated by the K_ATP channel blocker tolbutamide (100 μM) and PAC1 receptor antagonist PACAP_6-38 (200 nM). In recordings from A₁₀ dopamine neurons in TH-Cre mice, we replicated the outward current by perfusing PACAP_1-38 (100 nM). This response was again completely blocked by tolbutamide and PACAP_6-38, and associated with a hyperpolarization and decrease in firing. These findings demonstrate that PACAP activates PAC1 receptors and K_ATP channels to inhibit A₁₀ dopamine neurons and sex-dependently suppress binge-like consumption. Accordingly, they advance our understanding of how PACAP regulates energy homeostasis via the hedonic energy balance circuitry.

Deletion of Stim1 in Hypothalamic Arcuate Nucleus Kiss1 Neurons Potentiates Synchronous GCaMP Activity and Protects against Diet-Induced Obesity

Kisspeptin (Kiss1) neurons are essential for reproduction, but their role in the control of energy balance and other homeostatic functions remains unclear. High-frequency firing of hypothalamic arcuate Kiss1 (Kiss1^ARH) neurons releases kisspeptin into the median eminence, and neurokinin B (NKB) and dynorphin onto neighboring Kiss1^ARH neurons to generate a slow EPSP mediated by TRPC5 channels that entrains intermittent, synchronous firing of Kiss1^ARH neurons. High-frequency optogenetic stimulation of Kiss1^ARH neurons also releases glutamate to excite the anorexigenic proopiomelanocortin (POMC) neurons and inhibit the orexigenic neuropeptide Y/agouti-related peptide (AgRP) neurons via metabotropic glutamate receptors. At the molecular level, the endoplasmic reticulum (ER) calcium-sensing protein stromal interaction molecule 1 (STIM1) is critically involved in the regulation of neuronal Ca²⁺ signaling and neuronal excitability through its interaction with plasma membrane (PM) calcium (e.g., TRPC) channels. Therefore, we hypothesized that deletion of Stim1 in Kiss1^ARH neurons would increase neuronal excitability and their synchronous firing, which ultimately would affect energy homeostasis. Using optogenetics in combination with whole-cell recording and GCaMP6 imaging in slices, we discovered that deletion of Stim1 in Kiss1 neurons significantly increased the amplitude and duration of the slow EPSP and augmented synchronous [Ca²⁺]i oscillations in Kiss1^ARH neurons. Deletion of Stim1 in Kiss1^ARH neurons amplified the actions of NKB and protected ovariectomized female mice from developing obesity and glucose intolerance with high-fat dieting (HFD). Therefore, STIM1 appears to play a critical role in regulating synchronous firing of Kiss1^ARH neurons, which ultimately affects the coordination between energy homeostasis and reproduction.SIGNIFICANCE STATEMENT Hypothalamic arcuate kisspeptin (Kiss1^ARH) neurons are essential for stimulating the pulsatile release of gonadotropin-releasing hormone (GnRH) and maintaining fertility. However, Kiss1^ARH neurons appear to be a key player in coordinating energy balance with reproduction. The regulation of calcium channels and hence calcium signaling is critically dependent on the endoplasmic reticulum (ER) calcium-sensing protein stromal interaction molecule 1 (STIM1), which interacts with the plasma membrane (PM) calcium channels. We have conditionally deleted Stim1 in Kiss1^ARH neurons and found that it significantly increased the excitability of Kiss1^ARH neurons and protected ovariectomized female mice from developing obesity and glucose intolerance with high-fat dieting (HFD).

Ion channels alterations in the forebrain of high-fat diet fed rats

Evidence suggests that transient receptor potential (TRP) ion channels dysfunction significantly contributes to the physiopathology of metabolic and neurological disorders. Dysregulation in functions and expression in genes encoding the TRP channels cause several inherited diseases in humans (the so-called 'TRP channelopathies'), which affect the cardiovascular, renal, skeletal, and nervous systems. This study aimed to evaluate the expression of ion channels in the forebrain of rats with diet-induced obesity (DIO). DIO rats were studied after 17 weeks under a hypercaloric diet (high-fat diet, HFD) and were compared to the control rats with a standard diet (CHOW). To determine the systemic effects of HFD exposure, we examined food intake, fat mass content, fasting glycemia, insulin levels, cholesterol, and triglycerides. qRT-PCR, Western blot, and immunochemistry analysis were performed in the frontal cortex (FC) and hippocampus (HIP). After 17 weeks of HFD, DIO rats increased their body weight significantly compared to the CHOW rats. In DIO rats, TRPC1 and TRPC6 were upregulated in the HIP, while they were downregulated in the FC. In the case of TRPM2 expression, instead was increased both in the HIP and in the FC. These could be related to the increase of proteins and nucleic acid oxidation. TRPV1 and TRPV2 gene expression showed no differences both in the FC and HIP. In general, qRT-PCR analyses were confirmed by Western blot analysis. Immunohistochemical procedures highlighted the expression of the channels in the cell body of neurons and axons, particularly for the TRPC1 and TRPC6. The alterations of TRP channel expression could be related to the activation of glial cells or the neurodegenerative process presented in the brain of the DIO rat highlighted with post synaptic protein (PSD 95) alterations. The availability of suitable animal models may be useful for studying possible pharmacological treatments to counter obesity-induced brain injury. The identified changes in DIO rats may represent the first insight to characterize the neuronal alterations occurring in obesity. Further investigations are necessary to characterize the role of TRP channels in the regulation of synaptic plasticity and obesity-related cognitive decline.

Genetic Deletion of KLHL1 Leads to Hyperexcitability in Hypothalamic POMC Neurons and Lack of Electrical Responses to Leptin

Kelch-like 1 (KLHL1) is a neuronal actin-binding protein that modulates voltage-gated calcium channels. The KLHL1 knockout (KO) model displays altered calcium channel expression in various brain regions. We analyzed the electrical behavior of hypothalamic POMC (proopiomelanocortin) neurons and their response to leptin. Leptin's effects on POMC neurons include enhanced gene expression, activation of the ERK1/2 pathway and increased electrical excitability. The latter is initiated by activation of the Jak2-PI3K-PLC pathway, which activates TRPC1/5 (Transient Receptor Potential Cation) channels that in turn recruit T-type channel activity resulting in increased excitability. Here we report over-expression of Ca_V3.1 T-type channels in the hypothalamus of KLHL1 KO mice increased T-type current density and enhanced POMC neuron basal excitability, rendering them electrically unresponsive to leptin. Electrical sensitivity to leptin was restored by partial blockade of T-type channels. The overexpression of hypothalamic T-type channels in POMC neurons may partially contribute to the obese and abnormal feeding phenotypes observed in KLHL1 KO mice.

Barbadin Potentiates Long-Term Effects of Lorcaserin on POMC Neurons and Weight Loss

Obesity is a serious global health problem because of its increasing prevalence and comorbidities, but its treatments are limited. The serotonin 2C receptor (5-HT2CR), a G-protein-coupled receptor, activates proopiomelanocortin (POMC) neurons in the arcuate nucleus of hypothalamus (ARH) to reduce appetite and weight gain. However, several 5-HT analogs targeting this receptor, e.g., lorcaserin (Lor), suffer from diminished efficacy to reduce weight after prolonged administration. Here, we show that barbadin (Bar), a novel β-arrestin/β2-adaptin inhibitor, can prevent 5-HT2CR internalization in cells and potentiate long-term effects of Lor to reduce appetite and body weight in male mice. Mechanistically, we demonstrate that Bar co-treatment can effectively maintain the sensitivity of the 5-HT2CR in POMCARH neurons, despite prolonged Lor exposure, thereby allowing these neurons to be activated through opening the transient receptor potential cation (TRPC) channels. Thus, our results prove the concept that inhibition of 5-HT2CR desensitization can be a valid strategy to improve the long-term weight loss effects of Lor or other 5-HT2CR agonists, and also provide an intellectual framework to develop effective long-term management of weight by targeting 5-HT2CR desensitization.SIGNIFICANCE STATEMENT By demonstrating that the combination of barbadin (Bar) with a G-protein-coupled receptor (GPCR) agonist can provide prolonged weight-lowering benefits in a preclinical setting, our work should call for additional efforts to validate Bar as a safe and effective medicine or to use Bar as a lead compound to develop more suitable compounds for obesity treatment. These results prove the concept that inhibition of serotonin 2C receptor (5-HT2CR) desensitization can be a valid strategy to improve the long-term weight loss effects of lorcaserin (Lor) or other 5-HT2CR agonists. Since GPCRs represent a major category as therapeutic targets for various human diseases and desensitization of GPCRs is a common issue, our work may provide a conceptual framework to enhance effects of a broad range of GPCR medicines.

TRPC1/5-Ca V 3 Complex Mediates Leptin-Induced Excitability in Hypothalamic Neurons

Leptin regulates hypothalamic POMC⁺ (pro-opiomelanocortin) neurons by inducing TRPC (Transient Receptor Potential Cation) channel-mediate membrane depolarization. The role of TRPC channels in POMC neuron excitability is clearly established; however, it remains unknown whether their activity alone is sufficient to trigger excitability. Here we show that the right-shift voltage induced by the leptin-induced TRPC channel-mediated depolarization of the resting membrane potential brings T-type channels into the active window current range, resulting in an increase of the steady state T-type calcium current from 40 to 70% resulting in increased intrinsic excitability of POMC neurons. We assessed the role and timing of T-type channels on excitability and leptin-induced depolarization in vitro in cultured mouse POMC neurons. The involvement of TRPC channels in the leptin-induced excitability of POMC neurons was corroborated by using the TRPC channel inhibitor 2APB, which precluded the effect of leptin. We demonstrate T-type currents are indispensable for both processes, as treatment with NNC-55-0396 prevented the membrane depolarization and rheobase changes induced by leptin. Furthermore, co-immunoprecipitation experiments suggest that TRPC1/5 channels and Ca_V3.1 and Ca_V3.2 channels co-exist in complex. The functional relevance of this complex was corroborated using intracellular Ca²⁺ chelators; intracellular BAPTA (but not EGTA) application was sufficient to preclude POMC neuron excitability. However, leptin-induced depolarization still occurred in the presence of either BAPTA or EGTA suggesting that the calcium entry necessary to self-activate the TRPC1/5 complex is not blocked by the presence of BAPTA in hypothalamic neurons. Our study establishes T-type channels as integral part of the signaling cascade induced by leptin, modulating POMC neuron excitability. Leptin activation of TRPC channels existing in a macromolecular complex with T-type channels recruits the latter by locally induced membrane depolarization, further depolarizing POMC neurons, triggering action potentials and excitability.

A neural basis for brain leptin action on reducing type 1 diabetic hyperglycemia

Central leptin action rescues type 1 diabetic (T1D) hyperglycemia; however, the underlying mechanism and the identity of mediating neurons remain elusive. Here, we show that leptin receptor (LepR)-expressing neurons in arcuate (LepRArc) are selectively activated in T1D. Activation of LepRArc neurons, Arc GABAergic (GABAArc) neurons, or arcuate AgRP neurons, is able to reverse the leptin's rescuing effect. Conversely, inhibition of GABAArc neurons, but not AgRP neurons, produces leptin-mimicking rescuing effects. Further, AgRP neuron function is not required for T1D hyperglycemia or leptin's rescuing effects. Finally, T1D LepRArc neurons show defective nutrient sensing and signs of cellular energy deprivation, which are both restored by leptin, whereas nutrient deprivation reverses the leptin action. Our results identify aberrant activation of LepRArc neurons owing to energy deprivation as the neural basis for T1D hyperglycemia and that leptin action is mediated by inhibiting LepRArc neurons through reversing energy deprivation.

Adaptive Changes in the Central Control of Energy Homeostasis Occur in Response to Variations in Energy Status

Energy homeostasis is regulated in coordinate fashion by the brain-gut axis, the homeostatic energy balance circuitry in the hypothalamus and the hedonic energy balance circuitry comprising the mesolimbcortical A10 dopamine pathway. Collectively, these systems convey and integrate information regarding nutrient status and the rewarding properties of ingested food, and formulate it into a behavioral response that attempts to balance fluctuations in consumption and food-seeking behavior. In this review we start with a functional overview of the homeostatic and hedonic energy balance circuitries; identifying the salient neural, hormonal and humoral components involved. We then delve into how the function of these circuits differs in males and females. Finally, we turn our attention to the ever-emerging roles of nociceptin/orphanin FQ (N/OFQ) and pituitary adenylate cyclase-activating polypeptide (PACAP)-two neuropeptides that have garnered increased recognition for their regulatory impact in energy homeostasis-to further probe how the imposed regulation of energy balance circuitry by these peptides is affected by sex and altered under positive (e.g., obesity) and negative (e.g., fasting) energy balance states. It is hoped that this work will impart a newfound appreciation for the intricate regulatory processes that govern energy homeostasis, as well as how recent insights into the N/OFQ and PACAP systems can be leveraged in the treatment of conditions ranging from obesity to anorexia.

Role of Oxidative Stress and Ca2+ Signaling in Psychiatric Disorders

Psychiatric disorders are caused by complex and diverse factors, and numerous mechanisms have been proposed for the pathogenesis of these disorders. Accumulating evidence suggests that oxidative stress is one of the general factors involved in the pathogenesis/pathophysiology of major psychiatric disorders, including bipolar disorder, depression, anxiety disorder, and schizophrenia. Indeed, some clinical trials have shown improvement of the symptoms of these disorders by antioxidant supplementation. However, the molecular basis for the relationship between oxidative stress and the pathogenesis of psychiatric disorders remains largely unknown. In general, Ca2+ channels play central roles in neuronal functions, including neuronal excitability, neurotransmitter release, synaptic plasticity, and gene regulation, and genes that encode Ca2+ channels have been found to be associated with psychiatric disorders. Notably, a class of Ca2+-permeable transient receptor potential (TRP) cation channels is activated by changes in cellular redox status, whereby these TRP channels can link oxidative stress to Ca2+ signals. Given the unique characteristic of redox-sensitive TRP channels, these channels could be a target for delineating the pathogenesis or pathophysiology of psychiatric disorders. In this review, we summarize the outcomes of clinical trials for antioxidant treatment in patients with psychiatric disorders and the current insights into the physiological/pathological significance of redox-sensitive TRP channels in the light of neural functions, including behavioral phenotypes, and discuss the potential role of TRP channels in the pathogenesis of psychiatric disorders. Investigation of redox-sensitive TRP channels may lead to the development of novel therapeutic strategies for the treatment of psychiatric disorders.

The Role of Apelin/Apelin Receptor in Energy Metabolism and Water Homeostasis: A Comprehensive Narrative Review

The apelin receptor (APJ) is a member of the family A of G-protein-coupled receptors (GPCRs) and is involved in range of physiological and pathological functions, including fluid homeostasis, anxiety, and depression, as well as cardiovascular and metabolic disorders. APJ was classically described as a monomeric transmembrane receptor that forms a ternary complex together with its ligand and associated G proteins. More recently, increasing evidence indicates that APJ may interact with other GPCRs to form heterodimers, which may selectively modulate distinct intracellular signal transduction pathways. Besides, the apelin/APJ system plays important roles in the physiology and pathophysiology of several organs, including regulation of blood pressure, cardiac contractility, angiogenesis, metabolic balance, and cell proliferation, apoptosis, or inflammation. Additionally, the apelin/APJ system is widely expressed in the central nervous system, especially in neurons and oligodendrocytes. This article reviews the role of apelin/APJ in energy metabolism and water homeostasis. Compared with the traditional diuretics, apelin exerts a positive inotropic effect on the heart, while increases water excretion. Therefore, drugs targeting apelin/APJ system undoubtedly provide more therapeutic options for patients with congestive heart failure accompanied with hyponatremia. To provide more precise guidance for the development of clinical drugs, further in-depth studies are warranted on the metabolism and signaling pathways associated with apelin/APJ system.

Evaluation of VGF peptides as potential anti-obesity candidates in pre-clinical animal models

VGF is a peptide precursor expressed in neuroendocrine cells that is suggested to play a role in the regulation of energy homeostasis. VGF is proteolytically cleaved to yield multiple bioactive peptides. However, the specific actions of VGF-derived peptides on energy homeostasis remain unclear. The aim of the present work was to investigate the role of VGF-derived peptides in energy homeostasis and explore the pharmacological actions of VGF-derived peptides on body weight in preclinical animal models. VGF-derived peptides (NERP-1, NERP-2, PGH-NH₂, PGH-OH, NERP-4, TLQP-21, TLQP-30, TLQP-62, HHPD-41, AQEE-30, and LQEQ-19) were synthesized and screened for their ability to affect neuronal activity in vitro on hypothalamic brain slices and modulate food intake and energy expenditure after acute central administration in vivo. In addition, the effects of NERP-1, NERP-2, PGH-NH₂, TLQP-21, TLQP-62, and HHPD-41 on energy homeostasis were studied after chronic central infusion. NERP-1, PGH-NH₂, HHPD-41, and TLQP-62 increased the functional activity of hypothalamic neuronal networks. However, none of the peptides altered energy homeostasis after either acute or chronic ICV administration. The present data do not support the potential use of the tested VGF-derived peptides as novel anti-obesity drug candidates.

Contribution of TRPC Channels in Neuronal Excitotoxicity Associated With Neurodegenerative Disease and Ischemic Stroke

The seven canonical members of transient receptor potential (TRPC) proteins form cation channels that evoke membrane depolarization and intracellular calcium concentration ([Ca²⁺] _i ) rise, which are not only important for regulating cell function but their deregulation can also lead to cell damage. Recent studies have implicated complex roles of TRPC channels in neurodegenerative diseases including ischemic stroke. Brain ischemia reduces oxygen and glucose supply to neurons, i.e., Oxygen and Glucose Deprivation (OGD), resulting in [Ca²⁺] _i elevation, ion dyshomeostasis, and excitotoxicity, which are also common in many forms of neurodegenerative diseases. Although ionotropic glutamate receptors, e.g., N-methyl-D-aspartate receptors, are well established to play roles in excitotoxicity, the contribution of metabotropic glutamate receptors and their downstream effectors, i.e., TRPC channels, should not be neglected. Here, we summarize the current findings about contributions of TRPC channels in neurodegenerative diseases, with a focus on OGD-induced neuronal death and rodent models of cerebral ischemia/reperfusion. TRPC channels play both detrimental and protective roles to neurodegeneration depending on the TRPC subtype and specific pathological conditions involved. When illustrated the mechanisms by which TRPC channels are involved in neuronal survival or death seem differ greatly, implicating diverse and complex regulation. We provide our own data showing that TRPC1/C4/C5, especially TRPC4, may be generally detrimental in OGD and cerebral ischemia/reperfusion. We propose that although TRPC channels significantly contribute to ischemic neuronal death, detailed mechanisms and specific roles of TRPC subtypes in brain injury at different stages of ischemia/reperfusion and in different brain regions need to be carefully and systematically investigated.

Nociceptin/orphanin FQ neurons in the Arcuate Nucleus and Ventral Tegmental Area Act via Nociceptin Opioid Peptide Receptor Signaling to Inhibit Proopiomelanocortin and A10 Dopamine Neurons and Thereby Modulate Ingestion of Palatable Food

The neuropeptide nociceptin/orphanin FQ (N/OFQ) inhibits neuronal activity via its cognate nociceptin opioid peptide (NOP) receptor throughout the peripheral and central nervous systems, including those areas involved in the homeostatic and hedonic regulation of energy homeostasis. We thus tested the hypothesis that N/OFQ neurons in the hypothalamic arcuate nucleus (ARC) and ventral tegmental area (VTA) act via NOP receptor signaling to inhibit nearby anorexigenic proopiomelanocortin (POMC) and A10 dopamine neuronal excitability, respectively, and thereby modulate ingestion of palatable food. Electrophysiologic recordings were performed in slices prepared from transgenic male and ovariectomized (OVX) female N/OFQ-cre/enhanced green fluorescent protein-POMC, N/OFQ-cre and tyrosine hydroxylase-cre animals to see if optogenetically-stimulated peptide release from N/OFQ neurons could directly inhibit these neuronal populations. Binge-feeding behavioral experiments were also conducted where animals were exposed to a high-fat-diet (HFD) for one hour each day for five days and monitored for energy intake. Photostimulation of ARC and VTA N/OFQ neurons produces an outward current in POMC and A10 dopamine neurons receiving input from these cells. This is associated with a hyperpolarization and decreased firing. These features are also sex hormone- and diet-dependent; with estradiol-treated slices from OVX females being less sensitive, and obese males being more sensitive, to N/OFQ. Limited access to HFD causes a dramatic escalation in consumption, such that animals eat 25-45% of their daily intake during that one-hour exposure. Moreover, the NOP receptor-mediated regulation of these energy balance circuits are engaged, as N/OFQ injected directly into the VTA or ARC respectively diminishes or potentiates this binge-like increase in a manner heightened by diet-induced obesity or dampened by estradiol in females. Collectively, these findings provide key support for the idea that N/OFQ regulates appetitive behavior in sex-, site- and diet-specific ways, along with important insights into aberrant patterns of feeding behavior pertinent to the pathogenesis of food addiction.

Functionally distinct POMC-expressing neuron subpopulations in hypothalamus revealed by intersectional targeting

Pro-opiomelanocortin (POMC)-expressing neurons in the arcuate nucleus of the hypothalamus represent key regulators of metabolic homeostasis. Electrophysiological and single-cell sequencing experiments have revealed a remarkable degree of heterogeneity of these neurons. However, the exact molecular basis and functional consequences of this heterogeneity have not yet been addressed. Here, we have developed new mouse models in which intersectional Cre/Dre-dependent recombination allowed for successful labeling, translational profiling and functional characterization of distinct POMC neurons expressing the leptin receptor (Lepr) and glucagon like peptide 1 receptor (Glp1r). Our experiments reveal that POMC^Lepr+ and POMC^Glp1r+ neurons represent largely nonoverlapping subpopulations with distinct basic electrophysiological properties. They exhibit a specific anatomical distribution within the arcuate nucleus and differentially express receptors for energy-state communicating hormones and neurotransmitters. Finally, we identify a differential ability of these subpopulations to suppress feeding. Collectively, we reveal a notably distinct functional microarchitecture of critical metabolism-regulatory neurons.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

AMPK mediates regulation of voltage-gated calcium channels by leptin in isolated neurons from arcuate nucleus

Neuronal control of the energy homeostasis requires the arcuate nucleus of the hypothalamus. This structure integrates peripheral and central signals concerning the energy state of the body. It comprises two populations of neurons releasing anorexigenic and orexigenic peptides, among others. Both populations are regulated by leptin, an anorexigenic hormone, released by white adipose tissue. Voltage-gated calcium entry is critical to promote neurotransmitter and hormone release. It is already known that calcium channel current is inhibited by leptin in orexigenic neurons. However, fine-tuning details of calcium channel regulation in arcuate nucleus by leptin remain to be elucidated. This work aimed to investigate whether 5' adenosine monophosphate-activated protein kinase (AMPK) underlies the leptin-induced inhibition of calcium channels. By using patch-clamping methods, immunocytochemical, and biochemical reagents, we recorded calcium channel currents in orexigenic neuropeptide Y neurons of the arcuate nucleus of rats. Consistently, leptin inhibition of the calcium channel current was not only prevented by AMPK inhibition with Compound C but also hampered with 5-aminoimidazole-4-carboxamide ribonucleoside. Furthermore, leptin selectively inhibited L-type calcium channel current amplitude without major changes in voltage dependence or current kinetics. These results support for the first time the key role of AMPK in the maintenance and regulation of voltage-gated calcium channels. Together, they advance our understanding of the regulation of calcium channels in the central nervous system and emerging questions concerning food intake and energy balance.NEW & NOTEWORTHY Our results readily support the hypothesis that AMPK is responsible for the maintenance of the calcium current and mediates the fine-tuning modulation of the leptin response. The novelty of these results strengthens the critical role of AMPK in the general energy balance and homeostasis.

A Leptin-Mediated Neural Mechanism Linking Breathing to Metabolism

Breathing is coupled to metabolism. Leptin, a peptide mainly secreted in proportion to adipose tissue mass, increases energy expenditure with a parallel increase in breathing. We demonstrate that optogenetic activation of LepRb neurons in the nucleus of the solitary tract (NTS) mimics the respiratory stimulation after systemic leptin administration. We show that leptin activates the sodium leak channel (NALCN), thereby depolarizing a subset of glutamatergic (VGluT2) LepRb NTS neurons expressing galanin. Mice with selective deletion of NALCN in LepRb neurons have increased breathing irregularity and central apneas. On a high-fat diet, these mice gain weight with an associated depression of minute ventilation and tidal volume, which are not detected in control littermates. Anatomical mapping reveals LepRb NTS-originating glutamatergic axon terminals in a brainstem inspiratory premotor region (rVRG) and dorsomedial hypothalamus. These findings directly link a defined subset of NTS LepRb cells to the matching of ventilation to energy balance.

Neurochemical Characterization of Brainstem Pro-Opiomelanocortin Cells

Genetic research has revealed pro-opiomelanocortin (POMC) to be a fundamental regulator of energy balance and body weight in mammals. Within the brain, POMC is primarily expressed in the arcuate nucleus of the hypothalamus (ARC), while a smaller population exists in the brainstem nucleus of the solitary tract (POMCNTS). We performed a neurochemical characterization of this understudied population of POMC cells using transgenic mice expressing green fluorescent protein (eGFP) under the control of a POMC promoter/enhancer (PomceGFP). Expression of endogenous Pomc mRNA in the nucleus of the solitary tract (NTS) PomceGFP cells was confirmed using fluorescence-activating cell sorting (FACS) followed by quantitative PCR. In situ hybridization histochemistry of endogenous Pomc mRNA and immunohistochemical analysis of eGFP revealed that POMC is primarily localized within the caudal NTS. Neurochemical analysis indicated that POMCNTS is not co-expressed with tyrosine hydroxylase (TH), glucagon-like peptide 1 (GLP-1), cholecystokinin (CCK), brain-derived neurotrophic factor (BDNF), nesfatin, nitric oxide synthase 1 (nNOS), seipin, or choline acetyltransferase (ChAT) cells, whereas 100% of POMCNTS is co-expressed with transcription factor paired-like homeobox2b (Phox2b). We observed that 20% of POMCNTS cells express receptors for adipocyte hormone leptin (LepRbs) using a PomceGFP:LepRbCre:tdTOM double-reporter line. Elevations in endogenous or exogenous leptin levels increased the in vivo activity (c-FOS) of a small subset of POMCNTS cells. Using ex vivo slice electrophysiology, we observed that this effect of leptin on POMCNTS cell activity is postsynaptic. These findings reveal that a subset of POMCNTS cells are responsive to both changes in energy status and the adipocyte hormone leptin, findings of relevance to the neurobiology of obesity.

3-Iodothyronamine and Derivatives: New Allies Against Metabolic Syndrome?

In the two decades since its discovery, a large body of evidence has amassed to highlight the potential of 3-iodothyronamine (T1AM) as an antiobesity drug, whose pleiotropic signaling actions profoundly impact energy metabolism. In the present review, we recapitulate the most relevant properties of T1AM, including its structural and functional relationship to thyroid hormone, its endogenous levels, molecular targets, as well as its genomic and non-genomic effects on metabolism elicited in experimental models after exogenous administration. The physiological and pathophysiological relevance of T1AM in the regulation of energy homeostasis and metabolism is also discussed, along with its potential therapeutic applications in metabolic disturbances. Finally, we examine a number of T1AM analogs that have been recently developed with the aim of designing novel pharmacological agents for the treatment of interlinked diseases, such as metabolic and neurodegenerative disorders, as well as additional synthetic tools that can be exploited to further explore T1AM-dependent mechanisms and the physiological roles of trace amine-associated receptor 1 (TAAR1)-mediated effects.

Arcuate Kisspeptin Neurons Coordinate Reproductive Activities with Metabolism

Hypothalamic control of fertility is the quintessential homeostatic function. However, fertility is metabolically demanding; so, there must be coordination between energy states and reproductive functions. Because gonadotropin-releasing hormone (GnRH) neurons are devoid of many of the critical metabolic hormone receptors for sensing nutrient levels, it has long been recognized that the sensing of energy stores had to be done by neurons presynaptic to GnRH neurons. Some of the obvious players have been the anorexigenic proopiomelanocortin (POMC) and orexigenic neuropeptide Y (NPY)/agouti-related peptide (AgRP) neurons, both of which are in close apposition to the median eminence, a circumventricular organ. Indeed, POMC and NPY/AgRP neurons are inversely regulated by glucose and metabolic hormones including insulin and leptin. However, their synaptic connections with GnRH neurons are sparse and/or GnRH neurons are lacking the postsynaptic receptors to mediate the appropriate physiological response. Kisspeptin neurons were discovered in the early part of this century and subsequently shown to project to and control GnRH neuronal excitability. In fact, more recently the arcuate kisspeptin neurons have been identified as the command neurons driving pulsatile release of GnRH. Subsequently, it was shown that arcuate kisspeptin neurons express not only steroid hormone receptors but also metabolic hormone receptors such that similar to POMC neurons, they are excited by insulin and leptin. Therefore, based on the premise that arcuate kisspeptin neurons are the key neurons coordinating energy states with reproduction, we will review not only how these vital neurons control pulsatile GnRH release but how they control energy homeostasis through their synaptic connections with POMC and NPY/AgRP neurons and ultimately how E2 can regulate their excitability.

Role of POMC and AgRP neuronal activities on glycaemia in mice

Leptin regulates both feeding and glycaemia primarily through its receptors expressed on agouti-related peptide (AgRP) and pro-opiomelanocortin-expressing (POMC) neurons; however, it is unknown whether activity of these neuronal populations mediates the regulation of these processes. To determine this, we injected Cre-dependent designer receptors exclusively activated by designer drugs (DREADD) viruses into the hypothalamus of normoglycaemic and diabetic AgRP-ires-cre and POMC-cre mice to chemogenetically activate or inhibit these neuronal populations. Despite robust changes in food intake, activation or inhibition of AgRP neurons did not affect glycaemia, while activation caused significant (P = 0.014) impairment in insulin sensitivity. Stimulation of AgRP neurons in diabetic mice reversed leptin’s ability to inhibit feeding but did not counter leptin’s ability to lower blood glucose levels. Notably, the inhibition of POMC neurons stimulated feeding while decreasing glucose levels in normoglycaemic mice. The findings suggest that leptin’s effects on feeding by AgRP neurons are mediated by changes in neuronal firing, while the control of glucose balance by these cells is independent of chemogenetic activation or inhibition. The firing-dependent glucose lowering mechanism within POMC neurons is a potential target for the development of novel anti-diabetic medicines.

Early overnutrition alters synaptic signaling and induces leptin resistance in arcuate proopiomelanocortin neurons

Early overnutrition disrupts leptin sensitivity and the development of hypothalamic pathways involved in the regulation of metabolism and feeding behavior. While previous studies have largely focused on the development of neuronal projections, few studies have examined the impact of early nutrition on hypothalamic synaptic physiology. In this study we characterized the synaptic development of proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARH), their sensitivity to leptin, and the impact of early overnutrition on the development of these neurons. Electrophysiology recordings were performed in mouse ARH brain slices containing POMC-EGFP neurons from postnatal age (P) 7-9 through adulthood. We determined that pre- and postsynaptic components of inhibitory inputs increased throughout the first 3 weeks of the postnatal period, which coincided with a decreased membrane potential in POMC neurons. We then examined whether chronic postnatal overnutrition (CPO) altered these synaptic connections. CPO mice exhibited increased body weight and circulating leptin levels, as described previously. POMC neurons in CPO mice had an increase in post-synaptic inhibitory currents compared to controls at 2 weeks of age, but this effect reversed by the third week. In control mice we observed heterogenous effects of leptin on POMC neurons in early life that transitioned to predominantly stimulatory actions in adulthood. However, postnatal overfeeding resulted in POMC neurons becoming leptin-resistant which persisted into adulthood. These studies suggest that postnatal overfeeding alters the postsynaptic development of POMC neurons and induces long-lasting leptin resistance in ARH-POMC neurons.

Mechanisms for Sex Differences in Energy Homeostasis

Sex differences exist in the regulation of energy homeostasis. Better understanding of the underlying mechanisms for sexual dimorphism in energy balance may facilitate development of gender-specific therapies for human diseases, e.g. obesity. Multiple organs, including the brain, liver, fat and muscle, play important roles in the regulations of feeding behavior, energy expenditure and physical activity, which therefore contribute to the maintenance of energy balance. It has been increasingly appreciated that this multi-organ system is under different regulations in male vs. female animals. Much of effort has been focused on roles of sex hormones (including androgens, estrogens and progesterone) and sex chromosomes in this sex-specific regulation of energy balance. Emerging evidence also indicates that other factors (not sex hormones/receptors and not encoded by the sex chromosomes) exist to regulate energy homeostasis differentially in males vs. females. In this review, we summarize factors and signals that have been shown to regulate energy homeostasis in a sexually dimorphic fashion and propose a framework where these factors and signals may be integrated to mediate sex differences in energy homeostasis.

Role of Thermo-Sensitive Transient Receptor Potential Channels in Brown Adipose Tissue

Brown and beige adipocytes are a major site of mammalian non-shivering thermogenesis and energy dissipation. Obesity is caused by an imbalance between energy intake and expenditure and has become a worldwide health problem. Therefore modulation of thermogenesis in brown and beige adipocytes could be an important application for body weight control and obesity prevention. Over the last few decades, the involvement of thermo-sensitive transient receptor potential (TRP) channels (including TRPV1, TRPV2, TRPV3, TRPV4, TRPM4, TRPM8, TRPC5, and TRPA1) in energy metabolism and adipogenesis in adipocytes has been extensively explored. In this review, we summarize the expression, function, and pathological/physiological contributions of these TRP channels and discuss their potential as future therapeutic targets for preventing and combating human obesity and obesity-related metabolic disorders.

Calcium Channel CaV2.3 Subunits Regulate Hepatic Glucose Production by Modulating Leptin-Induced Excitation of Arcuate Pro-opiomelanocortin Neurons

Leptin acts on hypothalamic pro-opiomelanocortin (POMC) neurons to regulate glucose homeostasis, but the precise mechanisms remain unclear. Here, we demonstrate that leptin-induced depolarization of POMC neurons is associated with the augmentation of a voltage-gated calcium (CaV) conductance with the properties of the "R-type" channel. Knockdown of the pore-forming subunit of the R-type (CaV2.3 or Cacna1e) conductance in hypothalamic POMC neurons prevented sustained leptin-induced depolarization. In vivo POMC-specific Cacna1e knockdown increased hepatic glucose production and insulin resistance, while body weight, feeding, or leptin-induced suppression of food intake were not changed. These findings link Cacna1e function to leptin-mediated POMC neuron excitability and glucose homeostasis and may provide a target for the treatment of diabetes.

TRPCing around the hypothalamus

All of the canonical transient receptor potential channels (TRPC) with the exception of TRPC 2 are expressed in hypothalamic neurons and are involved in multiple homeostatic functions. Although the metabotropic glutamate receptors have been shown to be coupled to TRPC channel activation in cortical and sub-cortical brain regions, in the hypothalamus multiple amine and peptidergic G protein-coupled receptors (GPCRs) and growth factor/cytokine receptors are linked to activation of TRPC channels that are vital for reproduction, temperature regulation, arousal and energy homeostasis. In addition to the neurotransmitters, circulating hormones like insulin and leptin through their cognate receptors activate TRPC channels in POMC neurons. Many of the post-synaptic effects of the neurotransmitters and hormones are regulated in different physiological states by expression of TRPC channels in the post-synaptic neurons. Therefore, TRPC channels are key targets not only for neurotransmitters but circulating hormones in their vital role to control multiple hypothalamic functions, which is the focus of this review.

Insulin regulates POMC neuronal plasticity to control glucose metabolism

Hypothalamic neurons respond to nutritional cues by altering gene expression and neuronal excitability. The mechanisms that control such adaptive processes remain unclear. Here we define populations of POMC neurons in mice that are activated or inhibited by insulin and thereby repress or inhibit hepatic glucose production (HGP). The proportion of POMC neurons activated by insulin was dependent on the regulation of insulin receptor signaling by the phosphatase TCPTP, which is increased by fasting, degraded after feeding and elevated in diet-induced obesity. TCPTP-deficiency enhanced insulin signaling and the proportion of POMC neurons activated by insulin to repress HGP. Elevated TCPTP in POMC neurons in obesity and/or after fasting repressed insulin signaling, the activation of POMC neurons by insulin and the insulin-induced and POMC-mediated repression of HGP. Our findings define a molecular mechanism for integrating POMC neural responses with feeding to control glucose metabolism.

3-Iodothyronamine Activates a Set of Membrane Proteins in Murine Hypothalamic Cell Lines

3-Iodothyronamine (3-T1AM) is an endogenous thyroid hormone metabolite. The profound pharmacological effects of 3-T1AM on energy metabolism and thermal homeostasis have raised interest to elucidate its signaling properties in tissues that pertain to metabolic regulation and thermogenesis. Previous studies identified G protein-coupled receptors (GPCRs) and transient receptor potential channels (TRPs) as targets of 3-T1AM in different cell types. These two superfamilies of membrane proteins are largely expressed in tissue which influences energy balance and metabolism. As the first indication that 3-T1AM virtually modulates the function of the neurons in hypothalamus, we observed that intraperitoneal administration of 50 mg/kg bodyweight of 3-T1AM significantly increased the c-FOS activation in the paraventricular nucleus (PVN) of C57BL/6 mice. To elucidate the underlying mechanism behind this 3-T1AM-induced signalosome, we used three different murine hypothalamic cell lines, which are all known to express PVN markers, GT1-7, mHypoE-N39 (N39) and mHypoE-N41 (N41). Various aminergic GPCRs, which are the known targets of 3-T1AM, as well as numerous members of TRP channel superfamily, are expressed in these cell lines. Effects of 3-T1AM on activation of GPCRs were tested for the two major signaling pathways, the action of Gαs/adenylyl cyclase and Gi/o. Here, we demonstrated that this thyroid hormone metabolite has no significant effect on Gi/o signaling and only a minor effect on the Gαs/adenylyl cyclase pathway, despite the expression of known GPCR targets of 3-T1AM. Next, to test for other potential mechanisms involved in 3-T1AM-induced c-FOS activation in PVN, we evaluated the effect of 3-T1AM on the intracellular Ca2+ concentration and whole-cell currents. The fluorescence-optic measurements showed a significant increase of intracellular Ca2+ concentration in the three cell lines in the presence of 10 μM 3-T1AM. Furthermore, this thyroid hormone metabolite led to an increase of whole-cell currents in N41 cells. Interestingly, the TRPM8 selective inhibitor (10 μM AMTB) reduced the 3-T1AM stimulatory effects on cytosolic Ca2+ and whole-cell currents. Our results suggest that the profound pharmacological effects of 3-T1AM on selected brain nuclei of murine hypothalamus, which are known to be involved in energy metabolism and thermoregulation, might be partially attributable to TRP channel activation in hypothalamic cells.