MCH Neurons Regulate Permeability of the Median Eminence Barrier

The interactions between energy homeostasis and neurovascular plasticity

Food intake and energy expenditure are sensed and processed by multiple brain centres to uphold energy homeostasis. Evidence from the past decade points to the brain vasculature as a new critical player in regulating energy balance that functions in close association with the local neuronal networks. Nutritional imbalances alter many properties of the neurovascular system (such as neurovascular coupling and blood-brain barrier permeability), thus suggesting a bidirectional link between the nutritional milieu and neurovascular health. Increasing numbers of people are consuming a Western diet (comprising ultra-processed food with high-fat and high-sugar content) and have a sedentary lifestyle, with these factors contributing to the current obesity epidemic. Emerging pharmacological interventions (for example, glucagon-like peptide 1 receptor agonists) successfully trigger weight loss. However, whether these approaches can reverse the detrimental effects of long-term exposure to the Western diet (such as neurovascular uncoupling, neuroinflammation and blood-brain barrier disruption) and maintain stable body weight in the long-term needs to be clarified in addition to possible adverse effects. Lifestyle interventions revert the nutritional trigger for obesity and positively affect our overall health, including the cardiovascular system. This Perspective examines how lifestyle interventions affect the neurovascular system and neuronal networks.

Glutamatergic signaling from melanin-concentrating hormone-producing neurons: A requirement for memory regulation, but not for metabolism control

Melanin-concentrating hormone-producing neurons (MCH neurons), found mainly in the lateral hypothalamus and surrounding areas, play essential roles in various brain functions, including sleep and wakefulness, reward, metabolism, learning, and memory. These neurons coexpress several neurotransmitters and act as glutamatergic neurons. The contribution of glutamate from MCH neurons to memory- and metabolism-related functions has not been fully investigated. In a mouse model, we conditionally knocked out Slc17a6 gene, which encodes for vesicular glutamate transporter 2 (vGlut2), in the MCH neurons exclusively by using two different methods: the Cre recombinase/loxP system and in vivo genome editing using CRISPR/Cas9. Then, we evaluated several aspects of memory and measured metabolic rates using indirect calorimetry. We found that mice with MCH neuron-exclusive vGlut2 ablation had higher discrimination ratios between novel and familiar stimuli for novel object recognition, object location, and three-chamber tests. In contrast, there was no significant change in body weight, food intake, oxygen consumption, respiratory quotient, or locomotor activity. These findings suggest that glutamatergic signaling from MCH neurons is required to regulate memory, but its role in regulating metabolic rate is negligible.

Modeling hypothalamic pathophysiology in vitro for metabolic, circadian, and sleep disorders

The hypothalamus, a small and intricate brain structure, orchestrates numerous neuroendocrine functions through specialized neurons and nuclei. Disruption of this complex circuitry can result in various diseases, including metabolic, circadian, and sleep disorders. Advances in in vitro models and their integration with new technologies have significantly benefited research on hypothalamic function and pathophysiology. We explore existing in vitro hypothalamic models and address their challenges and limitations as well as translational findings. We also highlight how collaborative efforts among multidisciplinary teams are essential to develop relevant and translational experimental models capable of replicating intricate neural circuits and neuroendocrine pathways, thereby advancing our understanding of therapeutic targets and drug discovery in hypothalamus-related disorders.

Cerebrospinal Fluid from Patients After Craniotomy with the Appearance of Interleukin-6 Storm Can Activate Microglia to Damage the Hypothalamic Neurons in Mice

We monitored CSF (cerebrospinal fluid) for Th1/Th2 inflammatory cytokines in a patient with unexplained postoperative disturbance of consciousness after craniotomy and found that the level of IL-6 (interleukin-6) concentrations was extremely high, meeting the traditional criteria for an inflammatory cytokine storm. Subsequently, the cerebrospinal fluid specimens of several patients were tested, and it was found that IL-6 levels were increased in different degrees after craniotomy. Previous studies have focused more on mild and long-term IL-6 elevation, but less on the effects of this short-term IL-6 inflammatory cytokine storm. Cerebrospinal fluid rich in IL-6 may play a significant role in patients after craniotomy. The objective is to explore the degree of IL-6 elevation and the incidence of IL-6 inflammatory cytokine storm in patients after craniotomy, as well as the effect of IL-6 elevation on the brain. In this study, the levels and clinical manifestations of inflammatory factors in cerebrospinal fluid after craniotomy were statistically classified, and the underlying mechanisms were discussed preliminarily. CSF specimens of patients after craniotomy were collected, IL-6 level was measured at 1, 5, and 10 days after operation, and cognitive function was analyzed at 1, 10, and 180 days after surgery. Craniotomy mouse model, cerebrospinal fluid of patients with the appearance of IL-6 storm after craniotomy, and IL-6 at the same concentration stimulation model were established. Behavioral tests, fluorescence in situ hybridization (FISH), pathological means, western blot, and ELISA (enzyme-linked immune-sorbent assay) were performed for verification. CSF from patients after craniotomy caused disturbance of consciousness in mice, affected neuronal damage in the hypothalamus, activation of microglia in the hypothalamus, and decreased expression of barrier proteins in the hypothalamus and brain. The large amount of interleukin-6 in CSF after craniotomy was found to be mainly derived from astrocytes. The IL-6 level in CSF after craniotomy correlated inversely with patients' performance in MoCA test. High levels of IL-6 in the cerebrospinal fluid derived from astrocytes after craniotomy may lead to disruption of the brain-cerebrospinal fluid barrier, most notably around the hypothalamus, which might result in inflammatory activation of microglia to damage the hypothalamic neurons and impaired cognitive function/more gradual cognitive repairment in patients after craniotomy with the appearance of IL-6 storm.

Brain access of incretins and incretin receptor agonists to their central targets relevant for appetite suppression and weight loss

New incretin-based pharmacotherapies provide efficient and safe therapeutic options to curb appetite and produce weight loss in patients with obesity. Delivered systemically, these molecules produce pleiotropic metabolic benefits, but the target sites mediating their weight-suppressive action are located within the brain. Recent research has increased our understanding of the neural circuits and behavioral mechanisms involved in the anorectic and metabolic consequences of glucagon-like peptide 1 (GLP-1)-based weight loss strategies, yet little is known about how these drugs access their functional targets in the brain to produce sustained weight loss. The majority of brain cells expressing incretin receptors are located behind the blood-brain barrier, shielded from the circulation and fluctuations in the availability of peripheral signals, which is a major challenge for the development of CNS-targeted therapeutic peptides. GLP-1 receptor (GLP-1R) agonists with increased half-life and enhanced therapeutic benefit do not cross the blood-brain barrier, yet they manage to access discrete brain sites relevant to the regulation of energy homeostasis. In this review, we give a brief overview of the different routes for peptide hormones to access the brain. We then examine the evidence informing the routes employed by incretins and incretin receptor agonists to access brain targets relevant for their appetite and weight-suppressive actions. We highlight existing controversies and suggest future directions to further establish the functionally relevant access routes for GLP-1-based weight loss compounds, which might guide the development and selection of the future generation of incretin receptor polypharmacologies.

Neuroanatomical, electrophysiological, and morphological characterization of melanin-concentrating hormone cells coexpressing cocaine- and amphetamine-regulated transcript

Melanin-concentrating hormone (MCH) cells in the hypothalamus regulate fundamental physiological functions like energy balance, sleep, and reproduction. This diversity may be ascribed to the neurochemical heterogeneity among MCH cells. One prominent subpopulation of MCH cells coexpresses cocaine- and amphetamine-regulated transcript (CART), and as MCH and CART can have opposing actions, MCH/CART+ and MCH/CART- cells may differentially modulate behavioral outcomes. However, it is not known if there are differences in the cellular properties underlying their functional differences; thus, we compared the neuroanatomical, electrophysiological, and morphological properties of MCH cells in male and female Mch-cre;L10-Egfp reporter mice. Half of MCH cells expressed CART and were most prominent in the medial hypothalamus. Whole-cell patch-clamp recordings revealed differences in their passive and active membrane properties in a sex-dependent manner. Female MCH/CART+ cells had lower input resistances, but male cells largely differed in their firing properties. All MCH cells increased firing when stimulated, but their firing frequency decreases with sustained stimulation. MCH/CART+ cells showed stronger spike rate adaptation than MCH/CART- cells. The kinetics of excitatory events at MCH cells also differed by cell type, as the rising rate of excitatory events was slower at MCH/CART+ cells. By reconstructing the dendritic arborization of our recorded cells, we found no sex differences, but male MCH/CART+ cells had less dendritic length and fewer branch points. Overall, distinctions in topographical division and cellular properties between MCH cells add to their heterogeneity and help elucidate their response to stimuli or effect on modulating their respective neural networks.

SEMA6A drives GnRH neuron-dependent puberty onset by tuning median eminence vascular permeability

Innervation of the hypothalamic median eminence by Gonadotropin-Releasing Hormone (GnRH) neurons is vital to ensure puberty onset and successful reproduction. However, the molecular and cellular mechanisms underlying median eminence development and pubertal timing are incompletely understood. Here we show that Semaphorin-6A is strongly expressed by median eminence-resident oligodendrocytes positioned adjacent to GnRH neuron projections and fenestrated capillaries, and that Semaphorin-6A is required for GnRH neuron innervation and puberty onset. In vitro and in vivo experiments reveal an unexpected function for Semaphorin-6A, via its receptor Plexin-A2, in the control of median eminence vascular permeability to maintain neuroendocrine homeostasis. To support the significance of these findings in humans, we identify patients with delayed puberty carrying a novel pathogenic variant of SEMA6A. In all, our data reveal a role for Semaphorin-6A in regulating GnRH neuron patterning by tuning the median eminence vascular barrier and thereby controlling puberty onset.

CRH neurons in the lateral hypothalamic area regulate feeding behavior of mice

Food cues serve as pivotal triggers for eliciting physiological responses that subsequently influence food consumption. The magnitude of response induced by these cues stands as a critical determinant in the context of obesity risk. Nonetheless, the underlying neural mechanism that underpins how cues associated with edible food potentiate feeding behaviors remains uncertain. In this study, we revealed that corticotropin-releasing hormone (CRH)-expressing neurons in the lateral hypothalamic area played a crucial role in promoting consummatory behaviors in mice, shedding light on this intricate process. By employing an array of diverse assays, we initially established the activation of these neurons during feeding. Manipulations using optogenetic and chemogenetic assays revealed that their activation amplified appetite and promoted feeding behaviors, whereas inhibition decreased them. Additionally, our investigation identified downstream targets, including the ventral tegmental area, and underscored the pivotal involvement of the CRH neuropeptide itself in orchestrating this regulatory network. This research casts a clarifying light on the neural mechanism underlying the augmentation of appetite and the facilitation of feeding behaviors in response to food cues. VIDEO ABSTRACT.

Hypothalamic MCH Neurons: From Feeding to Cognitive Control

Modern neuroscience is progressively elucidating that the classic view positing distinct brain regions responsible for survival, emotion, and cognitive functions is outdated. The hypothalamus demonstrates the interdependence of these roles, as it is traditionally known for fundamental survival functions like energy and electrolyte balance, but is now recognized to also play a crucial role in emotional and cognitive processes. This review focuses on lateral hypothalamic melanin-concentrating hormone (MCH) neurons, producing the neuropeptide MCH-a relatively understudied neuronal population with integrative functions related to homeostatic regulation and motivated behaviors, with widespread inputs and outputs throughout the entire central nervous system. Here, we review early findings and recent literature outlining their role in the regulation of energy balance, sleep, learning, and memory processes.

Orexin and MCH neurons: regulators of sleep and metabolism

Sleep-wake and fasting-feeding are tightly coupled behavioral states that require coordination between several brain regions. The mammalian lateral hypothalamus (LH) is a functionally and anatomically complex brain region harboring heterogeneous cell populations that regulate sleep, feeding, and energy metabolism. Significant attempts were made to understand the cellular and circuit bases of LH actions. Rapid advancements in genetic and electrophysiological manipulation help to understand the role of discrete LH cell populations. The opposing action of LH orexin/hypocretin and melanin-concentrating hormone (MCH) neurons on metabolic sensing and sleep-wake regulation make them the candidate to explore in detail. This review surveys the molecular, genetic, and neuronal components of orexin and MCH signaling in the regulation of sleep and metabolism.

Fast neurotransmitter identity of MCH neurons: Do contents depend on context?

Hypothalamic melanin-concentrating hormone (MCH) neurons participate in many fundamental neuroendocrine processes. While some of their effects can be attributed to MCH itself, others appear to depend on co-released neurotransmitters. Historically, the subject of fast neurotransmitter co-release from MCH neurons has been contentious, with data to support MCH neurons releasing GABA, glutamate, both, and neither. Rather than assuming a position in that debate, this review considers the evidence for all sides and presents an alternative explanation: neurochemical identity, including classical neurotransmitter content, is subject to change. With an emphasis on the variability of experimental details, we posit that MCH neurons may release GABA and/or glutamate at different points according to environmental and contextual factors. Through the lens of the MCH system, we offer evidence that the field of neuroendocrinology would benefit from a more nuanced and dynamic interpretation of neurotransmitter identity.

Neuronal primary cilia integrate peripheral signals with metabolic drives

Neuronal primary cilia have recently emerged as important contributors to the central regulation of energy homeostasis. As non-motile, microtubule-based organelles, primary cilia serve as signaling antennae for metabolic status. The impairment of ciliary structure or function can produce ciliopathies for which obesity is a hallmark phenotype and global ablation of cilia induces non-syndromic adiposity in mouse models. This organelle is not only a hub for metabolic signaling, but also for catecholamine neuromodulation that shapes neuronal circuitry in response to sensory input. The objective of this review is to highlight current research investigating the mechanisms of primary cilium-regulated metabolic drives for maintaining energy homeostasis.

Bitter taste cells in the ventricular walls of the murine brain regulate glucose homeostasis

The median eminence (ME) is a circumventricular organ at the base of the brain that controls body homeostasis. Tanycytes are its specialized glial cells that constitute the ventricular walls and regulate different physiological states, however individual signaling pathways in these cells are incompletely understood. Here, we identify a functional tanycyte subpopulation that expresses key taste transduction genes including bitter taste receptors, the G protein gustducin and the gustatory ion channel TRPM5 (M5). M5 tanycytes have access to blood-borne cues via processes extended towards diaphragmed endothelial fenestrations in the ME and mediate bidirectional communication between the cerebrospinal fluid and blood. This subpopulation responds to metabolic signals including leptin and other hormonal cues and is transcriptionally reprogrammed upon fasting. Acute M5 tanycyte activation induces insulin secretion and acute diphtheria toxin-mediated M5 tanycyte depletion results in impaired glucose tolerance in diet-induced obese mice. We provide a cellular and molecular framework that defines how bitter taste cells in the ME integrate chemosensation with metabolism.

Maternal obesity damages the median eminence blood-brain barrier structure and function in the progeny: the beneficial impact of cross-fostering by lean mothers

Maternal obesity is an important risk factor for obesity, cardiovascular, and metabolic diseases in the offspring. Studies have shown that it leads to hypothalamic inflammation in the progeny, affecting the function of neurons regulating food intake and energy expenditure. In adult mice fed a high-fat diet, one of the hypothalamic abnormalities that contribute to the development of obesity is the damage of the blood-brain barrier (BBB) at the median eminence-arcuate nucleus (ME-ARC) interface; however, how the hypothalamic BBB is affected in the offspring of obese mothers requires further investigation. Here, we used confocal and transmission electron microscopy, transcript expression analysis, glucose tolerance testing, and a cross-fostering intervention to determine the impact of maternal obesity and breastfeeding on BBB integrity at the ME-ARC interface. The offspring of obese mothers were born smaller; conversely, at weaning, they presented larger body mass and glucose intolerance. In addition, maternal obesity-induced structural and functional damage of the offspring's ME-ARC BBB. By a cross-fostering intervention, some of the defects in barrier integrity and metabolism seen during development in an obesogenic diet were recovered. The offspring of obese dams breastfed by lean dams presented a reduction of body mass and glucose intolerance as compared to the offspring continuously exposed to an obesogenic environment during intrauterine and perinatal life; this was accompanied by partial recovery of the anatomical structure of the ME-ARC interface, and by the normalization of transcript expression of genes coding for hypothalamic neurotransmitters involved in energy balance and BBB integrity. Thus, maternal obesity promotes structural and functional damage of the hypothalamic BBB, which is, in part, reverted by lactation by lean mothers.NEW & NOTEWORTHY Maternal dietary habits directly influence offspring health. In this study, we aimed at determining the impact of maternal obesity on BBB integrity. We show that DIO offspring presented a leakier ME-BBB, accompanied by changes in the expression of transcripts encoding for endothelial and tanycytic proteins, as well as of hypothalamic neuropeptides. Breastfeeding in lean dams was sufficient to protect the offspring from ME-BBB disruption, providing a preventive strategy of nutritional intervention during early life.

Acute changes in systemic glycemia gate access and action of GLP-1R agonist on brain structures controlling energy homeostasis

Therapies based on glucagon-like peptide-1 (GLP-1) long-acting analogs and insulin are often used in the treatment of metabolic diseases. Both insulin and GLP-1 receptors are expressed in metabolically relevant brain regions, suggesting a cooperative action. However, the mechanisms underlying the synergistic actions of insulin and GLP-1R agonists remain elusive. In this study, we show that insulin-induced hypoglycemia enhances GLP-1R agonists entry in hypothalamic and area, leading to enhanced whole-body fat oxidation. Mechanistically, this phenomenon relies on the release of tanycyctic vascular endothelial growth factor A, which is selectively impaired after calorie-rich diet exposure. In humans, low blood glucose also correlates with enhanced blood-to-brain passage of insulin, suggesting that blood glucose gates the passage other energy-related signals in the brain. This study implies that the preventing hyperglycemia is important to harnessing the full benefit of GLP-1R agonist entry in the brain and action onto lipid mobilization and body weight loss.

Adult neurogenesis of the median eminence contributes to structural reconstruction and recovery of body fluid metabolism in hypothalamic self-repair after pituitary stalk lesion

Body fluid homeostasis is critical to survival. The integrity of the hypothalamo-neurohypophysial system (HNS) is an important basis of the precise regulation of body fluid metabolism and arginine vasopressin (AVP) hormone release. Clinically, some patients with central diabetes insipidus (CDI) due to HNS lesions can experience recovery compensation of body fluid metabolism. However, whether the hypothalamus has the potential for structural plasticity and self-repair under pathological conditions remains unclear. Here, we report the repair and reconstruction of a new neurohypophysis-like structure in the hypothalamic median eminence (ME) after pituitary stalk electrical lesion (PEL). We show that activated and proliferating adult neural progenitor cells differentiate into new mature neurons, which then integrate with remodeled AVP fibers to reconstruct the local AVP hormone release neural circuit in the ME after PEL. We found that the transcription factor of NK2 homeobox 1 (NKX2.1) and the sonic hedgehog signaling pathway, mediated by NKX2.1, are the key regulators of adult hypothalamic neurogenesis. Taken together, our study provides evidence that adult ME neurogenesis is involved in the structural reconstruction of the AVP release circuit and eventually restores body fluid metabolic homeostasis during hypothalamic self-repair.

Signaling Pathways in Neurovascular Development

During development, the central nervous system (CNS) vasculature grows to precisely meet the metabolic demands of neurons and glia. In addition, the vast majority of the CNS vasculature acquires a unique set of molecular and cellular properties-collectively referred to as the blood-brain barrier-that minimize passive diffusion of molecules between the blood and the CNS parenchyma. Both of these processes are controlled by signals emanating from neurons and glia. In this review, we describe the nature and mechanisms-of-action of these signals, with an emphasis on vascular endothelial growth factor (VEGF) and beta-catenin (canonical Wnt) signaling, the two best-understood systems that regulate CNS vascular development. We highlight foundational discoveries, interactions between different signaling systems, the integration of genetic and cell biological studies, advances that are of clinical relevance, and questions for future research.

Glial cells as integrators of peripheral and central signals in the regulation of energy homeostasis

Communication between the periphery and the brain is key for maintaining energy homeostasis. To do so, peripheral signals from the circulation reach the brain via the circumventricular organs (CVOs), which are characterized by fenestrated vessels lacking the protective blood-brain barrier (BBB). Glial cells, by virtue of their plasticity and their ideal location at the interface of blood vessels and neurons, participate in the integration and transmission of peripheral information to neuronal networks in the brain for the neuroendocrine control of whole-body metabolism. Metabolic diseases, such as obesity and type 2 diabetes, can disrupt the brain-to-periphery communication mediated by glial cells, highlighting the relevance of these cell types in the pathophysiology of such complications. An improved understanding of how glial cells integrate and respond to metabolic and humoral signals has become a priority for the discovery of promising therapeutic strategies to treat metabolic disorders. This Review highlights the role of glial cells in the exchange of metabolic signals between the periphery and the brain that are relevant for the regulation of whole-body energy homeostasis.

Historical and current perspectives on blood endothelial cell heterogeneity in the brain

Dynamic brain activity requires timely communications between the brain parenchyma and circulating blood. Brain-blood communication is facilitated by intricate networks of brain vasculature, which display striking heterogeneity in structure and function. This vascular cell heterogeneity in the brain is fundamental to mediating diverse brain functions and has long been recognized. However, the molecular basis of this biological phenomenon has only recently begun to be elucidated. Over the past century, various animal species and in vitro systems have contributed to the accumulation of our fundamental and phylogenetic knowledge about brain vasculature, collectively advancing this research field. Historically, dye tracer and microscopic observations have provided valuable insights into the anatomical and functional properties of vasculature across the brain, and these techniques remain an important approach. Additionally, recent advances in molecular genetics and omics technologies have revealed significant molecular heterogeneity within brain endothelial and perivascular cell types. The combination of these conventional and modern approaches has enabled us to identify phenotypic differences between healthy and abnormal conditions at the single-cell level. Accordingly, our understanding of brain vascular cell states during physiological, pathological, and aging processes has rapidly expanded. In this review, we summarize major historical advances and current knowledge on blood endothelial cell heterogeneity in the brain, and discuss important unsolved questions in the field.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Arcuate Nucleus-Dependent Regulation of Metabolism-Pathways to Obesity and Diabetes Mellitus

The central nervous system (CNS) receives information from afferent neurons, circulating hormones, and absorbed nutrients and integrates this information to orchestrate the actions of the neuroendocrine and autonomic nervous systems in maintaining systemic metabolic homeostasis. Particularly the arcuate nucleus of the hypothalamus (ARC) is of pivotal importance for primary sensing of adiposity signals, such as leptin and insulin, and circulating nutrients, such as glucose. Importantly, energy state-sensing neurons in the ARC not only regulate feeding but at the same time control multiple physiological functions, such as glucose homeostasis, blood pressure, and innate immune responses. These findings have defined them as master regulators, which adapt integrative physiology to the energy state of the organism. The disruption of this fine-tuned control leads to an imbalance between energy intake and expenditure as well as deregulation of peripheral metabolism. Improving our understanding of the cellular, molecular, and functional basis of this regulatory principle in the CNS could set the stage for developing novel therapeutic strategies for the treatment of obesity and metabolic syndrome. In this review, we summarize novel insights with a particular emphasis on ARC neurocircuitries regulating food intake and glucose homeostasis and sensing factors that inform the brain of the organismal energy status.

A single-cell atlas of the normal and malformed human brain vasculature

Cerebrovascular diseases are a leading cause of death and neurologic disability. Further understanding of disease mechanisms and therapeutic strategies requires a deeper knowledge of cerebrovascular cells in humans. We profiled transcriptomes of 181,388 cells to define a cell atlas of the adult human cerebrovasculature, including endothelial cell molecular signatures with arteriovenous segmentation and expanded perivascular cell diversity. By leveraging this reference, we investigated cellular and molecular perturbations in brain arteriovenous malformations, which are a leading cause of stroke in young people, and identified pathologic endothelial transformations with abnormal vascular patterning and the ontology of vascularly derived inflammation. We illustrate the interplay between vascular and immune cells that contributes to brain hemorrhage and catalog opportunities for targeting angiogenic and inflammatory programs in vascular malformations.

Seasonal vascular plasticity in the mediobasal hypothalamus of the adult ewe

Sheep, like most seasonal mammals, exhibit a cyclic adaptive reproductive physiology that allows ewes to give birth to their progeny during the spring when environmental conditions are favorable to their survival. This process relies on the detection of day length (or photoperiod) and is associated with profound changes in cellular plasticity and gene expression in the hypothalamic-pituitary-gonadal axis, mechanisms that are suggested to participate in the seasonal adaptation of neuroendocrine circuits. Recently, pituitary vascular growth has been proposed as a seasonally regulated process in which the vascular endothelial growth factor A (VEGFA), a well-known angiogenic cytokine, is suspected to play a crucial role. However, whether this mechanism is restricted to the pituitary gland or also occurs in the mediobasal hypothalamus (MBH), a crucial contributor to the control of the reproductive function, remains unexplored. Using newly developed image analysis tools, we showed that the arcuate nucleus (ARH) of the MBH exhibits an enhanced vascular density during the long photoperiod or non-breeding season, associated with higher expression of VEGFA. In the median eminence (ME), a structure connecting the MBH to the pituitary gland, higher VEGFA, kinase insert domain receptor (KDR/VEGFR2) and plasmalemma vesicle-associated protein (PLVAP) gene expressions were detected during the long photoperiod. We also found that VEGFA and its receptor, VEGFR2, are expressed by neurons and tanycytes in both the ARH and ME. Altogether, these data show variations in the MBH vasculature according to seasons potentially through a VEGFA-dependent pathway, paving the way for future studies aiming to decipher the role of these changes in the hypothalamic control of seasonal reproduction.

Utilizing network pharmacology and experimental validation to explore the potential molecular mechanisms of BanXia-YiYiRen in treating insomnia

BanXia-YiYiRen (Pinellia Ternata and Coix Seed, BX-YYR) has been clinically proven to be an effective Chinese medicine compatible with the treatment of insomnia. However, the underlying mechanism of BX-YYR against insomnia remains unclear. This study aimed to explore the pharmacological mechanisms of BX-YYR in treating insomnia based on network pharmacology and experimental validation. The drug-disease targets were obtained using publicly available databases. The analysis revealed 21 active compounds and 101 potential targets of BX-YYR from the pharmacological database of Chinese medicine system and analysis platform (TCMSP) and 1020 related targets of insomnia from the GeneCards and Online Mendelian Inheritance in Man (OMIM) databases. Furthermore, 38 common targets of BX-YYR against insomnia were identified, and these common targets were used to construct a protein–protein interaction (PPI) network. The visual PPI network was constructed by Cytoscape software. The top three genes from PPI according to degree value are FOS, AKT1, and CASP3. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment were applied to reveal the potential targets and signaling pathways involved in BX-YYR against insomnia, especially the serotonergic pathway. In addition, molecular docking revealed that baicalein, beta-sitosterol, and stigmasterol displayed strong binding to AKT1, FOS, PRKCA, and VEGFA. Experimental study found that BX-YYR against insomnia might play a role in improving sleep by modulating the serotonergic pathway. In summary, our findings revealed the underlying mechanism of BX-YYR against insomnia and provided an objective basis for further experimental study and clinical application.

The Properties and Functions of Glial Cell Types of the Hypothalamic Median Eminence

The median eminence (ME) is part of the neuroendocrine system (NES) that functions as a crucial interface between the hypothalamus and pituitary gland. The ME contains many non-neuronal cell types, including oligodendrocytes, oligodendrocyte precursor cells (OPCs), tanycytes, astrocytes, pericytes, microglia and other immune cells, which may be involved in the regulation of NES function. For example, in mice, ablation of tanycytes (a special class of ependymal glia with stem cell-like functions) results in weight gain, feeding, insulin insensitivity and increased visceral adipose, consistent with the demonstrated ability of these cells to sense and transport both glucose and leptin, and to differentiate into neurons that control feeding and metabolism in the hypothalamus. To give a further example, OPCs in the ME of mice have been shown to rapidly respond to dietary signals, in turn controlling composition of the extracellular matrix in the ME, derived from oligodendrocyte-lineage cells, which may contribute to the previously described role of these cells in actively maintaining leptin-receptor-expressing dendrites in the ME. In this review, we explore and discuss recent advances such as these, that have developed our understanding of how the various cell types of the ME contribute to its function in the NES as the interface between the hypothalamus and pituitary gland. We also highlight avenues of future research which promise to uncover additional functions of the ME and the glia, stem and progenitor cells it contains.

Oral Bioavailability Enhancement of Melanin Concentrating Hormone, Development and In Vitro Pharmaceutical Assessment of Novel Delivery Systems

The rapid progress in biotechnology over the past few decades has accelerated the large-scale production of therapeutic peptides and proteins, making them available in medical practice. However, injections are the most common method of administration; these procedures might lead to inconvenience. Non-invasive medications, such as oral administration of bio-compounds, can reduce or eliminate pain and increase safety. The aim of this project was to develop and characterize novel melanin concentrating hormone (MCH) formulations for oral administration. As a drug delivery system, penetration enhancer combined alginate beads were formulated and characterized. The combination of alginate carriers with amphiphilic surfactants has not been described yet. Due to biosafety having high priority in the case of novel pharmaceutical formulations, the biocompatibility of selected auxiliary materials and their combinations was evaluated using different in vitro methods. Excipients were selected according to the performed toxicity measurements. Besides the cell viability tests, physical properties and complex bioavailability assessments were performed as well. Our results suggest that alginate beads are able to protect melanin concentrating hormones. It has been also demonstrated that penetration enhancer combined alginate beads might play a key role in bioavailability improvement. These formulations were found to be promising tools for oral peptide delivery. Applied excipients and the performed delivery systems are safe and highly tolerable; thus, they can improve patients’ experience and promote adherence.

Insulin signalling in tanycytes gates hypothalamic insulin uptake and regulation of AgRP neuron activity

Insulin acts on neurons and glial cells to regulate systemic glucose metabolism and feeding. However, the mechanisms of insulin access in discrete brain regions are incompletely defined. Here we show that insulin receptors in tanycytes, but not in brain endothelial cells, are required to regulate insulin access to the hypothalamic arcuate nucleus. Mice lacking insulin receptors in tanycytes (IR^∆Tan mice) exhibit systemic insulin resistance, while displaying normal food intake and energy expenditure. Tanycytic insulin receptors are also necessary for the orexigenic effects of ghrelin, but not for the anorexic effects of leptin. IR^∆Tan mice exhibit increased agouti-related peptide (AgRP) neuronal activity, while displaying blunted AgRP neuronal adaptations to feeding-related stimuli. Lastly, a highly palatable food decreases tanycytic and arcuate nucleus insulin signalling to levels comparable to those seen in IR^∆Tan mice. These changes are rooted in modifications of cellular stress responses and of mitochondrial protein quality control in tanycytes. Conclusively, we reveal a critical role of tanycyte insulin receptors in gating feeding-state-dependent regulation of AgRP neurons and systemic insulin sensitivity, and show that insulin resistance in tanycytes contributes to the pleiotropic manifestations of obesity-associated insulin resistance.

Multifaceted actions of melanin-concentrating hormone on mammalian energy homeostasis

Melanin-concentrating hormone (MCH) is a small cyclic peptide expressed in all mammals, mainly in the hypothalamus. MCH acts as a robust integrator of several physiological functions and has crucial roles in the regulation of sleep-wake rhythms, feeding behaviour and metabolism. MCH signalling has a very broad endocrine context and is involved in physiological functions and emotional states associated with metabolism, such as reproduction, anxiety, depression, sleep and circadian rhythms. MCH mediates its functions through two receptors (MCHR1 and MCHR2), of which only MCHR1 is common to all mammals. Owing to the wide variety of MCH downstream signalling pathways, MCHR1 agonists and antagonists have great potential as tools for the directed management of energy balance disorders and associated metabolic complications, and translational strategies using these compounds hold promise for the development of novel treatments for obesity. This Review provides an overview of the numerous roles of MCH in energy and glucose homeostasis, as well as in regulation of the mesolimbic dopaminergic circuits that encode the hedonic component of food intake.

Role of T cells during the cerebral infection with Trypanosoma brucei

The infection by Trypanosoma brucei brucei (T.b.b.), a protozoan parasite, is characterized by an early-systemic stage followed by a late stage in which parasites invade the brain parenchyma in a T cell-dependent manner.

Here we found that early after infection effector-memory T cells were predominant among brain T cells, whereas, during the encephalitic stage T cells acquired a tissue resident memory phenotype (T_RM) and expressed PD1. Both CD4 and CD8 T cells were independently redundant for the penetration of T.b.b. and other leukocytes into the brain parenchyma. The role of lymphoid cells during the T.b.b. infection was studied by comparing T- and B-cell deficient rag1^-/- and WT mice. Early after infection, parasites located in circumventricular organs, brain structures with increased vascular permeability, particularly in the median eminence (ME), paced closed to the sleep-wake regulatory arcuate nucleus of the hypothalamus (Arc). Whereas parasite levels in the ME were higher in rag1^-/- than in WT mice, leukocytes were instead reduced. Rag1^-/- infected mice showed increased levels of meca32 mRNA coding for a blood /hypothalamus endothelial molecule absent in the blood-brain-barrier (BBB). Both immune and metabolic transcripts were elevated in the ME/Arc of WT and rag1^-/- mice early after infection, except for ifng mRNA, which levels were only increased in WT mice. Finally, using a non-invasive sleep-wake cycle assessment method we proposed a putative role of lymphocytes in mediating sleep alterations during the infection with T.b.b.

Thus, the majority of T cells in the brain during the early stage of T.b.b. infection expressed an effector-memory phenotype while T_RM cells developed in the late stage of infection. T cells and parasites invade the ME/Arc altering the metabolic and inflammatory responses during the early stage of infection and modulating sleep disturbances.

Trypanosoma brucei (T.b.) causes an early systemic and a late encephalitic infection characterized by sleep alterations. In rodent models, brain invasion by T.b. brucei (T.b.b.) is strictly dependent on T cells. However, an in-depth characterization of T cell functions and phenotypes in the outcome of T.b.b. infection is still lacking.

Here we found that during the early stage of infection of mice, most brain T cells differentiated into memory cells, and acquired a tissue-resident memory phenotype during the encephalitic stage. CD4 and CD8 T cells were redundant for the invasion of other T cells and parasites into the brain. Early after infection T.b.b. and leukocytes invade different circumventricular organs (brain areas that lack a blood-brain barrier) including the median eminence (ME) located close to sleep-regulating arcuate nucleus (Arc). T.b.b. infection induced the expression of immune and metabolic molecules in this area. Lymphocytes modulated 1) the levels of invading parasites and leukocytes in the ME; 2) the structure of the blood/ hypothalamus interphase and 3) the expression of IFN-γ in the ME/Arc early after infection. Lymphocytes may also be involved in the regulation of sleep alterations observed in African trypanosomiasis.

Sensory Circumventricular Organs, Neuroendocrine Control, and Metabolic Regulation

The central nervous system is critical in metabolic regulation, and accumulating evidence points to a distributed network of brain regions involved in energy homeostasis. This is accomplished, in part, by integrating peripheral and central metabolic information and subsequently modulating neuroendocrine outputs through the paraventricular and supraoptic nucleus of the hypothalamus. However, these hypothalamic nuclei are generally protected by a blood-brain-barrier limiting their ability to directly sense circulating metabolic signals—pointing to possible involvement of upstream brain nuclei. In this regard, sensory circumventricular organs (CVOs), brain sites traditionally recognized in thirst/fluid and cardiovascular regulation, are emerging as potential sites through which circulating metabolic substances influence neuroendocrine control. The sensory CVOs, including the subfornical organ, organum vasculosum of the lamina terminalis, and area postrema, are located outside the blood-brain-barrier, possess cellular machinery to sense the metabolic interior milieu, and establish complex neural networks to hypothalamic neuroendocrine nuclei. Here, evidence for a potential role of sensory CVO-hypothalamic neuroendocrine networks in energy homeostasis is presented.

Ablation of glucokinase-expressing tanycytes impacts energy balance and increases adiposity in mice

Objectives

Glucokinase (GCK) is critical for glucosensing. In rats, GCK is expressed in hypothalamic tanycytes and appears to play an essential role in feeding behavior. In this study, we investigated the distribution of GCK-expressing tanycytes in mice and their role in the regulation of energy balance.

Methods

In situ hybridization, reporter gene assay, and immunohistochemistry were used to assess GCK expression along the third ventricle in mice. To evaluate the impact of GCK-expressing tanycytes on arcuate neuron function and mouse physiology, Gck deletion along the ventricle was achieved using loxP/Cre recombinase technology in adult mice.

Results

GCK expression was low along the third ventricle, but detectable in tanycytes facing the ventromedial arcuate nucleus from bregma −1.5 to −2.2. Gck deletion induced the death of this tanycyte subgroup through the activation of the BAD signaling pathway. The ablation of GCK-expressing tanycytes affected different aspects of energy balance, leading to an increase in adiposity in mice. This phenotype was systematically associated with a defect in NPY neuron function. In contrast, the regulation of glucose homeostasis was mostly preserved, except for glucoprivic responses.

Conclusions

This study describes the role of GCK in tanycyte biology and highlights the impact of tanycyte loss on the regulation of energy balance.

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vmARH tanycytes express glucokinase.

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Glucokinase deletion in tanycytes induces cell death.

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Ablation of vmARH tanycytes alters energy balance and adiposity.

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Ablation of vmARH tanycytes alters NPY neuron function.

Brain Barriers and brain fluids research in 2020 and the fluids and barriers of the CNS thematic series on advances in in vitro modeling of the blood-brain barrier and neurovascular unit

This editorial discusses advances in brain barrier and brain fluid research in 2020. Topics include: the cerebral endothelium and the neurovascular unit; the choroid plexus; the meninges; cerebrospinal fluid and the glymphatic system; disease states impacting the brain barriers and brain fluids; drug delivery to the brain. This editorial also highlights the recently completed Fluids Barriers CNS thematic series entitled, Advances in in vitro modeling of the bloodbrain barrier and neurovascular unit. Such in vitro modeling is progressing rapidly.

Melanin-concentrating hormone-producing neurons in the hypothalamus regulate brown adipose tissue and thus contribute to energy expenditure

KEY POINTS

Melanin-concentrating hormone (MCH) neuron-ablated mice exhibit increased energy expenditure and reduced fat weight. Increased brown adipose tissue (BAT) activity and locomotor activity-independent energy expenditure contributed to body weight reduction in MCH neuron-ablated mice. MCH neurons send inhibitory input to the medullary raphe nucleus to modulate BAT activity.

ABSTRACT

Hypothalamic melanin-concentrating hormone (MCH) peptide robustly affects energy homeostasis. However, it is unclear whether and how MCH-producing neurons, which contain and release a variety of neuropeptides/transmitters, regulate energy expenditure in the central nervous system and peripheral tissues. We thus examined the regulation of energy expenditure by MCH neurons, focusing on interscapular brown adipose tissue (BAT) activity. MCH neuron-ablated mice exhibited reduced body weight, increased oxygen consumption, and increased BAT activity, which improved locomotor activity-independent energy expenditure. Trans-neuronal retrograde tracing with the recombinant pseudorabies virus revealed that MCH neurons innervate BAT via the sympathetic premotor region in the medullary raphe nucleus (MRN). MRN neurons were activated by MCH neuron ablation. Therefore, endogenous MCH neuron activity negatively modulates energy expenditure via BAT inhibition. MRN neurons might receive inhibitory input from MCH neurons to suppress BAT activity.

What is the physiological role of hypothalamic tanycytes in metabolism?

In vertebrates, the energy balance process is tightly controlled by complex neural circuits that sense metabolic signals and adjust food intake and energy expenditure in line with the physiological requirements of optimal conditions. Within neural networks controlling energy balance, tanycytes are peculiar ependymoglial cells that are nowadays recognized as multifunctional players in the metabolic hypothalamus. However, the physiological function of hypothalamic tanycytes remains unclear, creating a number of ambiguities in the field. Here, we review data accumulated over the years that demonstrate the physiological function of tanycytes in the maintenance of metabolic homeostasis, opening up new research avenues. The presumed involvement of tanycytes in the pathophysiology of metabolic disorders and age-related neurodegenerative diseases will be finally discussed.

Melanin-concentrating hormone and food intake control: Sites of action, peptide interactions, and appetition

Given the increased prevalence of obesity and its associated comorbidities, understanding the mechanisms through which the brain regulates energy balance is of critical importance. The neuropeptide melanin-concentrating hormone (MCH) is produced in the lateral hypothalamic area and the adjacent incerto-hypothalamic area and promotes both food intake and energy conservation, overall contributing to body weight gain. Decades of research into this system has provided insight into the neural pathways and mechanisms (behavioral and neurobiological) through which MCH stimulates food intake. Recent technological advancements that allow for selective manipulation of MCH neuron activity have elucidated novel mechanisms of action for the hyperphagic effects of MCH, implicating neural "volume" transmission in the cerebrospinal fluid and sex-specific effects of MCH on food intake control as understudied areas for future investigation. Highlighted here are historical and recent findings that illuminate the neurobiological mechanisms through which MCH promotes food intake, including the identification of various specific neural signaling pathways and interactions with other peptide systems. We conclude with a framework that the hyperphagic effects of MCH signaling are predominantly mediated through enhancement of an "appetition" process in which early postoral prandial signals promote further caloric consumption.

Application progress of RVG peptides to facilitate the delivery of therapeutic agents into the central nervous system

The incidence of central nervous system (CNS) diseases is increasing with the aging population. However, it remains challenging to deliver drugs into the CNS because of the existence of a blood-brain barrier (BBB). Notably, rabies virus glycoprotein (RVG) peptides have been developed as delivery ligands for CNS diseases. So far, massive RVG peptide modified carriers have been reported, such as liposomes, micelles, polymers, exosomes, dendrimers, and proteins. Moreover, these drug delivery systems can encapsulate almost all small molecules and macromolecule drugs, including siRNA, microRNAs, DNA, proteins, and other nanoparticles, to treat various CNS diseases with efficient and safe drugs. In this review, targeted delivery systems with RVG peptide modified carriers possessing favorable biocompatibility and delivery efficiency are summarized.

Tissue-Specific Effects of Leptin on Glucose and Lipid Metabolism

The discovery of leptin was intrinsically associated with its ability to regulate body weight. However, the effects of leptin are more far-reaching and include profound glucose-lowering and anti-lipogenic effects, independent of leptin's regulation of body weight. Regulation of glucose metabolism by leptin is mediated both centrally and via peripheral tissues and is influenced by the activation status of insulin signaling pathways. Ectopic fat accumulation is diminished by both central and peripheral leptin, an effect that is beneficial in obesity-associated disorders. The magnitude of leptin action depends upon the tissue, sex, and context being examined. Peripheral tissues that are of particular relevance include the endocrine pancreas, liver, skeletal muscle, adipose tissues, immune cells, and the cardiovascular system. As a result of its potent metabolic activity, leptin is used to control hyperglycemia in patients with lipodystrophy and is being explored as an adjunct to insulin in patients with type 1 diabetes. To fully understand the role of leptin in physiology and to maximize its therapeutic potential, the mechanisms of leptin action in these tissues needs to be further explored.

Tanycytes in the infundibular nucleus and median eminence and their role in the blood-brain barrier

The blood-brain barrier is generally attributed to endothelial cells. However, in circumventricular organs, such as the median eminence, tanycytes take over the barrier function. These ependymoglial cells form the wall of the third ventricle and send long extensions into the parenchyma to contact blood vessels and hypothalamic neurons. The shape and location of tanycytes put them in an ideal position to connect the periphery with central nervous compartments. In line with this, tanycytes control the transport of hormones and key metabolites in and out of the hypothalamus. They function as sensors of peripheral homeostasis for central regulatory networks. This chapter discusses current evidence that tanycytes play a key role in regulating glucose balance, food intake, endocrine axes, seasonal changes, reproductive function, and aging. The understanding of how tanycytes perform these diverse tasks is only just beginning to emerge and will probably lead to a more differentiated view of how the brain and the periphery interact.

Fibroblast Growth Factor-1 Activates Neurons in the Arcuate Nucleus and Dorsal Vagal Complex

Central administration of fibroblast growth factor-1 (FGF1) results in long-lasting resolution of hyperglycemia in various rodent models, but the pre- and postsynaptic mechanisms mediating the central effects of FGF1 are unknown. Here we utilize electrophysiology recordings from neuronal populations in the arcuate nucleus of the hypothalamus (ARH), nucleus of the solitary tract (NTS), and area postrema (AP) to investigate the mechanisms underlying FGF1 actions. While FGF1 did not alter membrane potential in ARH-NPY-GFP neurons, it reversibly depolarized 83% of ARH-POMC-EGFP neurons and decreased the frequency of inhibitory inputs onto ARH-POMC-EGFP neurons. This depolarizing effect persisted in the presence of FGF receptor (R) blocker FIIN1, but was blocked by pretreatment with the voltage-gated sodium channel (VGSC) blocker tetrodotoxin (TTX). Non-FGF1 subfamilies can activate vascular endothelial growth factor receptors (VEGFR). Surprisingly, the VEGFR inhibitors axitinib and BMS605541 blocked FGF1 effects on ARH-POMC-EGFP neurons. We also demonstrate that FGF1 induces c-Fos in the dorsal vagal complex, activates NTS-NPY-GFP neurons through a FGFR mediated pathway, and requires VGSCs to activate AP neurons. We conclude that FGF1 acts in multiple brain regions independent of FGFRs. These studies present anatomical and mechanistic pathways for the future investigation of the pharmacological and physiological role of FGF1 in metabolic processes.

The Diagnostic and Therapeutic Role of Leptin and Its Receptor ObR in Glioblastoma Multiforme

Simple Summary

Despite recent advances in molecular brain tumor therapies, glioblastoma multiforme remains a diagnostic and therapeutic challenge with, in most cases, unfavorable outcome. Leptin and related mediators of immune-metabolic traffic have attracted increased recognition in the past decade in brain tumor biology, in particular potential implications in the diagnosis and treatment of recurrent and newly diagnosed high and low grade gliomas. Randomized controlled trails are on the way to elaborate the role of leptin and its receptor ObR by targeting and using antidiabetic drugs known to interact with distinct pathways associated with leptin signaling. To date, most of the findings in clinical studies remain preliminary and of heterogenous character, although experimental studies have underpinned the relevance of leptin and ObR in the pathophysiology of brain tumors in general.

Abstract

Leptin has been recognized as a potential tumor growth promoter in various cancers including cranial tumor pathologies such as pituitary adenomas, meningiomas and gliomas. Despite recent advances in adjunctive therapy and the established surgical resection, chemo- and radiotherapy regimen, glioblastoma multiforme remains a particular diagnostic and therapeutic challenge among the intracranial tumor pathologies, with a poor long-term prognosis. Systemic inflammation and immune-metabolic signaling through diverse pathways are thought to impact the genesis and recurrence of brain tumors, and glioblastoma multiforme in particular. Among the various circulating mediators, leptin has gained especial diagnostic and therapeutic interest, although the precise relationship between leptin and glioblastoma biology remains largely unknown. In this narrative review (MEDLINE/OVID, SCOPUS, PubMed and manual searches of the bibliographies of known primary and review articles), we discuss the current literature using the following search terms: leptin, glioblastoma multiforme, carcinogenesis, immunometabolism, biomarkers, metformin, antidiabetic medication and metabolic disorders. An increasing body of experimental evidence implicates a relationship between the development and maintenance of gliomas (and brain tumors in general) with a dysregulated central and peripheral immune-metabolic network mediated by circulating adipokines, chemokines and cellular components, and in particular the leptin adipokine. In this review, we summarize the current evidence of the role of leptin in glioblastoma pathophysiology. In addition, we describe the status of alternative diagnostic tools and adjunctive therapeutics targeting leptin, leptin-receptors, antidiabetic drugs and associated pathways. Further experimental and clinical trials are needed to elucidate the mechanism of action and the value of immune-metabolism molecular phenotyping (central and peripheral) in order to develop novel adjunctive diagnostics and therapeutics for newly diagnosed and recurrent glioblastoma patients.

A Novel Neurovascular Liaison Governing the Blood-Brain Barrier

How circulating signals enter the median eminence to trigger homeostatic hypothalamic responses is not well understood. Jiang et al. describe a neural mechanism that increases endothelial fenestrations and enhances the hypothalamic response to the circulating hormone leptin, suggesting a novel way to regulate brain entry through vascular wall remodeling.