Synaptic interaction between hypocretin (orexin) and neuropeptide Y cells in the rodent and primate hypothalamus: a novel circuit implicated in metabolic and endocrine regulations

Acupuncture for obesity and related diseases: Insight for regulating neural circuit

Obesity is defined as abnormal or excessive fat accumulation that may impair health. Obesity is associated with numerous pathological changes including insulin resistance, fatty liver, hyperlipidemias, and other obesity-related diseases. These comorbidities comprise a significant public health threat. Existing anti-obesity drugs have been limited by side effects that include depression, suicidal thoughts, cardiovascular complications and stroke. Acupuncture treatment has been shown to be effective for treating obesity and obesity-related conditions, while avoiding side effects. However, the mechanisms of acupuncture in treating obesity-related diseases, especially its effect on neural circuits, are not well understood. A growing body of research has studied acupuncture's effects on the endocrine system and other mechanisms related to the regulation of neural circuits. In this article, recent research that was relevant to the use of acupuncture to treat obesity and obesity-related diseases through the neuroendocrine system, as well as some neural circuits involved, was summarized. Based on this, acupuncture's potential ability to regulate neural circuits and its mechanisms of action in the endocrine system were reviewed, leading to a deeper mechanistic understanding of acupuncture's effects and providing insight and direction for future research about obesity. Please cite this article as: Jiang LY, Tian J, Yang YN, Jia SH, Shu Q. Acupuncture for obesity and related diseases: insight for regulating neural circuit. J Integr Med. 2024; 22(2): 93-101.

The Orexin/Hypocretin System, the Peptidergic Regulator of Vigilance, Orchestrates Adaptation to Stress

The orexin/hypocretin neuropeptide family has emerged as a focal point of neuroscientific research following the discovery that this family plays a crucial role in a variety of physiological and behavioral processes. These neuropeptides serve as powerful neuromodulators, intricately shaping autonomic, endocrine, and behavioral responses across species. Notably, they serve as master regulators of vigilance and stress responses; however, their roles in food intake, metabolism, and thermoregulation appear complementary and warrant further investigation. This narrative review provides a journey through the evolution of our understanding of the orexin system, from its initial discovery to the promising progress made in developing orexin derivatives. It goes beyond conventional boundaries, striving to synthesize the multifaceted activities of orexins. Special emphasis is placed on domains such as stress response, fear, anxiety, and learning, in which the authors have contributed to the literature with original publications. This paper also overviews the advancement of orexin pharmacology, which has already yielded some promising successes, particularly in the treatment of sleep disorders.

Molecularly stratified hypothalamic astrocytes are cellular foci for obesity

Astrocytes safeguard the homeostasis of the central nervous system1,2. Despite their prominent morphological plasticity under conditions that challenge the brain's adaptive capacity3-5, the classification of astrocytes, and relating their molecular make-up to spatially devolved neuronal operations that specify behavior or metabolism, remained mostly futile6,7. Although it seems unexpected in the era of single-cell biology, the lack of a major advance in stratifying astrocytes under physiological conditions rests on the incompatibility of 'neurocentric' algorithms that rely on stable developmental endpoints, lifelong transcriptional, neurotransmitter, and neuropeptide signatures for classification6-8 with the dynamic functional states, anatomic allocation, and allostatic plasticity of astrocytes1. Simplistically, therefore, astrocytes are still grouped as 'resting' vs. 'reactive', the latter referring to pathological states marked by various inducible genes3,9,10. Here, we introduced a machine learning-based feature recognition algorithm that benefits from the cumulative power of published single-cell RNA-seq data on astrocytes as a reference map to stepwise eliminate pleiotropic and inducible cellular features. For the healthy hypothalamus, this walk-back approach revealed gene regulatory networks (GRNs) that specified subsets of astrocytes, and could be used as landmarking tools for their anatomical assignment. The core molecular censuses retained by astrocyte subsets were sufficient to stratify them by allostatic competence, chiefly their signaling and metabolic interplay with neurons. Particularly, we found differentially expressed mitochondrial genes in insulin-sensing astrocytes and demonstrated their reciprocal signaling with neurons that work antagonistically within the food intake circuitry. As a proof-of-concept, we showed that disrupting Mfn2 expression in astrocytes reduced their ability to support dynamic circuit reorganization, a time-locked feature of satiety in the hypothalamus, thus leading to obesity in mice. Overall, our results suggest that astrocytes in the healthy brain are fundamentally more heterogeneous than previously thought and topologically mirror the specificity of local neurocircuits.

Synaptic plasticity and the role of astrocytes in central metabolic circuits

Neural circuits in the brain, primarily in the hypothalamus, are paramount to the homeostatic control of feeding and energy utilization. They integrate hunger, satiety, and body adiposity cues from the periphery and mediate the appropriate behavioral and physiological responses to satisfy the energy demands of the animal. Notably, perturbations in central homeostatic circuits have been linked to the etiology of excessive feeding and obesity. Considering the ever-changing energy requirements of the animal and required adaptations, it is not surprising that brain-feeding circuits remain plastic in adulthood and are subject to changes in synaptic strength as a consequence of nutritional status. Indeed, synapse density, probability of presynaptic transmitter release, and postsynaptic responses in hypothalamic energy balance centers are tailored to behavioral and physiological responses required to sustain survival. Mounting evidence supports key roles of astrocytes facilitating some of this plasticity. Here we discuss these synaptic plasticity mechanisms and the emerging roles of astrocytes influencing energy and glucose balance control in health and disease. This article is categorized under: Cancer > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology.

Timing Matters: The Interplay between Early Mealtime, Circadian Rhythms, Gene Expression, Circadian Hormones, and Metabolism-A Narrative Review

Achieving synchronization between the central and peripheral body clocks is essential for ensuring optimal metabolic function. Meal timing is an emerging field of research that investigates the influence of eating patterns on our circadian rhythm, metabolism, and overall health. This narrative review examines the relationship between meal timing, circadian rhythm, clock genes, circadian hormones, and metabolic function. It analyzes the existing literature and experimental data to explore the connection between mealtime, circadian rhythms, and metabolic processes. The available evidence highlights the importance of aligning mealtime with the body's natural rhythms to promote metabolic health and prevent metabolic disorders. Specifically, studies show that consuming meals later in the day is associated with an elevated prevalence of metabolic disorders, while early time-restricted eating, such as having an early breakfast and an earlier dinner, improves levels of glucose in the blood and substrate oxidation. Circadian hormones, including cortisol and melatonin, interact with mealtimes and play vital roles in regulating metabolic processes. Cortisol, aligned with dawn in diurnal mammals, activates energy reserves, stimulates appetite, influences clock gene expression, and synchronizes peripheral clocks. Consuming meals during periods of elevated melatonin levels, specifically during the circadian night, has been correlated with potential implications for glucose tolerance. Understanding the mechanisms of central and peripheral clock synchronization, including genetics, interactions with chronotype, sleep duration, and hormonal changes, provides valuable insights for optimizing dietary strategies and timing. This knowledge contributes to improved overall health and well-being by aligning mealtime with the body's natural circadian rhythm.

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.

Exploring the Therapeutic Effect of Neurotrophins and Neuropeptides in Neurodegenerative Diseases: at a Glance

Neurotrophins and neuropeptides are the essential regulators of peripheral nociceptive nerves that help to induce, sensitize, and maintain pain. Neuropeptide has a neuroprotective impact as it increases trophic support, regulates calcium homeostasis, and reduces excitotoxicity and neuroinflammation. In contrast, neurotrophins target neurons afflicted by ischemia, epilepsy, depression, and eating disorders, among other neuropsychiatric conditions. Neurotrophins are reported to inhibit neuronal death. Strategies maintained for "brain-derived neurotrophic factor (BDNF) therapies" are to upregulate BDNF levels using the delivery of protein and genes or compounds that target BDNF production and boosting BDNF signals by expanding with BDNF mimetics. This review discusses the mechanisms of neurotrophins and neuropeptides against acute neural damage as well as highlighting neuropeptides as a potential therapeutic agent against Parkinson's disease (PD), Huntington's disease (HD), Alzheimer's disease (AD), and Machado-Joseph disease (MJD), the signaling pathways affected by neurotrophins and their receptors in both standard and diseased CNS systems, and future perspectives that can lead to the potent application of neurotrophins and neuropeptides in neurodegenerative diseases (NDs).

Estradiol-dependent hypocretinergic/orexinergic behaviors throughout the estrous cycle

RATIONALE

The female menstrual or estrous cycle and its associated fluctuations in circulating estradiol (E2), progesterone, and other gonadal hormones alter orexin or hypocretin peptide production and receptor activity. Depending on the estrous cycle phase, the transcription of prepro-orexin mRNA, post-translational modification of orexin peptide, and abundance of orexin receptors change in a brain region-specific manner. The most dramatic changes occur in the hypothalamus, which is considered the starting point of the hypothalamic-pituitary-gonadal axis as well as the hub of orexin-producing neurons. Thus, hypothalamus-regulated behaviors, including arousal, feeding, reward processing, and the stress response depend on coordinated efforts between E2, progesterone, and the orexin system. Given the rise of orexin therapeutics for various neuropsychiatric conditions including insomnia and affective disorders, it is important to delineate the behavioral outcomes of this drug class in both sexes, as well as within different time points of the female reproductive cycle.

OBJECTIVES

Summarize how the menstrual or estrous cycle affects orexin system functionality in animal models in order to predict how orexin pharmacotherapies exert varying degrees of behavioral effects across the dynamic hormonal milieu.

Plasticity of the hypocretinergic/orexinergic system after a chronic treatment with suvorexant in rats. Role of the hypocretinergic/orexinergic receptor 1 as an autoreceptor

The hypothalamic hypocretinergic/orexinergic (Hcrt/Ox) system is involved in many physiological and pathophysiological processes. Malfunction of Hcrt/Ox transmission results in narcolepsy, a sleep disease caused in humans by progressive neurodegeneration of hypothalamic neurons containing Hcrt/Ox. To explore the Hcrt/Ox system plasticity we systemically administered suvorexant (a dual Hcrt/Ox receptor antagonist) in rats to chronically block Hcrt/Ox transmission without damaging Hcrt/Ox cells. Three groups of eight rats (four males and four females) received daily i.p. injections of suvorexant (10 or 30 mg/kg) or vehicle (DMSO) over a period of 7 days in which the body weight was monitored. After the treatments cerebrospinal fluid (CSF) Hcrt1/OxA concentration was measured by ELISA, and hypothalamic Hcrt/OxR1 and Hcrt/OxR2 levels by western blot. The systemic blockade of the Hcrt/Ox transmission with the suvorexant high dose produced a significant increase in body weight at the end of the treatment, and a significant decrease in CSF Hcrt1/OxA levels, both features typical in human narcolepsy type 1. Besides, a significant overexpression of hypothalamic Hcrt/OxR1 occurred. For the Hcrt/OxR2 two very close bands were detected, but they did not show significant changes with the treatment. Thus, the plastic changes observed in the Hcrt/Ox system after the chronic blockade of its transmission were a decrease in CSF Hcrt1/OXA levels and an overexpression of hypothalamic Hcrt/OxR1. These findings support an autoregulatory role of Hcrt/OxR1 within the hypothalamus, which would induce the synthesis/release of Hcrt/Ox, but also decrease its own availability at the plasma membrane after binding Hcrt1/OxA to preserve Hcrt/Ox system homeostasis.

Ovariectomy induces abdominal fat accumulation by improving gonadotropin-releasing hormone secretion in mouse

The ovariectomy would induce the occurrence of obesity, but its regulatory mechanism is not clear. This study aimed to elucidate the regulation on fat accumulation for ovariectomy in mouse. In the current study, the abdominal fat mass dramatically increased in OVX mice compared with sham mice at eighth week after ovariectomy, accompanied with the higher GnRH level in blood and abdominal fat tissue. Also, a decrease of the abdominal fat mass was occurred in OVX mice with a GnRH-antagonist injection. Furthermore, the results in vivo and in vitro confirmed that GnRH promoted the transition of G1/S phase by upregulating CCND1 and CCNE1 mRNA levels by the mediation of GnRHR via the PKA-CREB pathway. Meanwhile, the higher FSH secretion was induced by increase GnRH and accelerate fat deposition in abdominal fat tissue. Our findings are the first to elucidate the effect mechanism of ovariectomy on obesity in mouse. GnRH stimulates fat accumulation in adipocytes via PKA-CREB pathway by directly promoting cell proliferation for driving the cell cycle and simultaneously accelerating differentiation for improving the FSH secretion.

Coordination of metabolism, arousal, and reward by orexin/hypocretin neurons

Orexin/hypocretin neurons located in the lateral hypothalamus play a critical role in the maintenance of arousal and contribute to the regulation of multiple homeostatic and behavioral processes. In this issue of the JCI, Tan and Hang et al. report that feeding a high-fat diet to mice compromised the function of the orexin system, leading to impairments in reward-seeking and active coping mechanisms. The researchers observed changes at the cellular and circuit levels suggesting that reduced excitability of orexin neurons affects behavior through induction of a hypoarousal state.

Diverging metabolic programmes and behaviours during states of starvation, protein malnutrition, and cachexia

BACKGROUND

Our evolutionary history is defined, in part, by our ability to survive times of nutrient scarcity. The outcomes of the metabolic and behavioural adaptations during starvation are highly efficient macronutrient allocation, minimization of energy expenditure, and maximized odds of finding food. However, in different contexts, caloric deprivation is met with vastly different physiologic and behavioural responses, which challenge the primacy of energy homeostasis.

METHODS

We conducted a literature review of scientific studies in humans, laboratory animals, and non-laboratory animals that evaluated the physiologic, metabolic, and behavioural responses to fasting, starvation, protein-deficient or essential amino acid-deficient diets, and cachexia. Studies that investigated the changes in ingestive behaviour, locomotor activity, resting metabolic rate, and tissue catabolism were selected as the focus of discussion.

RESULTS

Whereas starvation responses prioritize energy balance, both protein malnutrition and cachexia present existential threats that induce unique adaptive programmes, which can exacerbate the caloric insufficiency of undernutrition. We compare and contrast the behavioural and metabolic responses and elucidate the mechanistic pathways that drive state-dependent alterations in energy seeking and partitioning.

CONCLUSIONS

The evolution of energetically inefficient metabolic and behavioural responses to protein malnutrition and cachexia reveal a hierarchy of metabolic priorities governed by discrete regulatory networks.

The dynamic regulation of appetitive behavior through lateral hypothalamic orexin and melanin concentrating hormone expressing cells

The lateral hypothalamic area (LHA) is a heterogeneous brain structure extensively studied for its potent role in regulating energy balance. The anatomical and molecular diversity of the LHA permits the orchestration of responses to energy sensing cues from the brain and periphery. Two of the primary cell populations within the LHA associated with integration of this information are Orexin (ORX) and Melanin Concentrating Hormone (MCH). While both of these non-overlapping populations exhibit orexigenic properties, the activities of these two systems support feeding behavior through contrasting mechanisms. We describe the anatomical and functional properties as well as interaction with other neuropeptides and brain reward and hedonic systems. Specific outputs relating to arousal, food seeking, feeding, and metabolism are coordinated through these mechanisms. We then discuss how both the ORX and MCH systems harmonize in a divergent yet overall cooperative manner to orchestrate feeding behavior through transitions between various appetitive states, and thus offer novel insights into LHA allostatic control of appetite.

Ultra-sparse Connectivity within the Lateral Hypothalamus

The lateral hypothalamic area (LH) is a vital controller of arousal, feeding, and metabolism [1, 2], which integrates external and internal sensory information. Whereas sensory and whole-body output properties of LH cell populations have received much interest, their intrinsic synaptic organization has remained largely unstudied. Local inhibitory and excitatory connections could help integrate and filter sensory information and mutually inhibitory connections [3] could allow coordinating activity between LH cell types, some of which have mutually exclusive behavioral effects, such as LH VGLUT2 and VGAT neurons [4-7] and orexin- (ORX) and melanin-concentrating hormone (MCH) neurons [8-10]. However, classical Golgi staining studies did not find interneurons with locally ramifying axons in the LH [11, 12], and nearby subthalamic and thalamic areas lack local synaptic connectivity [13, 14]. Studies with optogenetic circuit mapping within the LH have demonstrated only a minority of connections when a large pool of presynaptic neurons was activated [15-19]. Because multiple patch clamp has not been used to study LH connectivity, aside from a limited dataset of MCH neurons where no connections were discovered [15], we used quadruple whole-cell recordings to screen connectivity within the LH with standard methodology we previously used in the neocortex [20-22]. Finding a lack of local connectivity, we used optogenetic circuit mapping to study the strength of LH optogenetic responses and network oscillations, which were consistent with ultra-sparse intrinsic connectivity within the LH. These results suggest that input from other brain structures is decisive for selecting active populations in the LH.

Orexin neurons and inhibitory Agrp→orexin circuits guide spatial exploration in mice

KEY POINTS

Photoinhibition of endogenous activity of lateral hypothalamic orexin neurons causes place preference and reduces innate avoidance Endogenous activity of orexin neurons correlates with place preference Mediobasal hypothalamic Agrp neurons inhibit orexin neurons via GABA, and chemogenetic suppression of Agrp neurons increases avoidance in an orexin receptor-dependent manner.

ABSTRACT

Hypothalamic orexin/hypocretin neurons integrate multiple sensory cues and project brain-wide to orchestrate diverse innate behaviours. Their loss impairs many context-appropriate actions, but the motivational characteristics of orexin cell activity remain unclear. We and others previously approached this question by artificial orexin stimulation, which could induce either rewarding (positive valence) or aversive (negative valence) brain activity. It is unknown to what extent such approaches replicate natural/endogenous orexin signals, which rapidly fluctuate during wakefulness. Here we took an alternative approach, focusing on observing and silencing natural orexin cell signals associated with a fundamental innate behaviour, self-paced spatial exploration. We found that mice are more likely to stay in places paired with orexin cell optosilencing. The orexin cell optosilencing also reduced avoidance of places that mice find innately aversive. Correspondingly, calcium recordings revealed that orexin cell activity rapidly reduced upon exiting the innately aversive places. Furthermore, we provide optogenetic evidence for an inhibitory GABAergic Agrp→orexin hypothalamic neurocircuit, and find that Agrp cell suppression increases innate avoidance behaviour, consistent with orexin disinhibition. These results imply that exploration may be motivated and oriented by a need to reduce aversive orexin cell activity, and suggest a hypothalamic circuit for fine-tuning orexin signals to changing ethological priorities.

Modulation of feeding behavior and metabolism by dynorphin

The neuronal regulation of metabolic and behavioral responses to different diets and feeding regimens is an important research area. Herein, we investigated if the opioid peptide dynorphin modulates feeding behavior and metabolism. Mice lacking dynorphin peptides (KO) were exposed to either a normal diet (ND) or a high-fat diet (HFD) for a period of 12 weeks. Additionally, mice had either time-restricted (TR) or ad libitum (AL) access to food. Body weight, food intake and blood glucose levels were monitored throughout the 12-week feeding schedule. Brain samples were analyzed by immunohistochemistry to detect changes in the expression levels of hypothalamic peptides. As expected, animals on HFD or having AL access to food gained more weight than mice on ND or having TR access. Unexpectedly, KO females on TR HFD as well as KO males on AL ND or AL HFD demonstrated a significantly increased body weight gain compared to the respective WT groups. The calorie intake differed only marginally between the genotypes: a significant difference was present in the female ND AL group, where dynorphin KO mice ate more than WT mice. Although female KO mice on a TR feeding regimen consumed a similar amount of food as WT controls, they displayed significantly higher levels of blood glucose. We observed significantly reduced levels of hypothalamic orexigenic peptides neuropeptide Y (NPY) and orexin-A in KO mice. This decrease became particularly pronounced in the HFD groups and under AL condition. The kappa opiod receptor (KOR) levels were higher after HFD compared to ND feeding in the ventral pallidum of WT mice. We hypothesize that HFD enhances dynorphin signaling in this hedonic center to maintain energy homeostasis, therefore KO mice have a more pronounced phenotype in the HFD condition due to the lack of it. Our data suggest that dynorphin modulates metabolic changes associated with TR feeding regimen and HFD consumption. We conclude that the lack of dynorphin causes uncoupling between energy intake and body weight gain in mice; KO mice maintained on HFD become overweight despite their normal food intake. Thus, using kappa opioid receptor agonists against obesity could be considered as a potential treatment strategy.

Hormones as adaptive control systems in juvenile fish

Growth is an important theme in biology. Physiologists often relate growth rates to hormonal control of essential processes. Ecologists often study growth as a function of gradients or combinations of environmental factors. Fewer studies have investigated the combined effects of environmental and hormonal control on growth. Here, we present an evolutionary optimization model of fish growth that combines internal regulation of growth by hormone levels with the external influence of food availability and predation risk. The model finds a dynamic hormone profile that optimizes fish growth and survival up to 30 cm, and we use the probability of reaching this milestone as a proxy for fitness. The complex web of interrelated hormones and other signalling molecules is simplified to three functions represented by growth hormone, thyroid hormone and orexin. By studying a range from poor to rich environments, we find that the level of food availability in the environment results in different evolutionarily optimal strategies of hormone levels. With more food available, higher levels of hormones are optimal, resulting in higher food intake, standard metabolism and growth. By using this fitness-based approach we also find a consequence of evolutionary optimization of survival on optimal hormone use. Where foraging is risky, the thyroid hormone can be used strategically to increase metabolic potential and the chance of escaping from predators. By comparing model results to empirical observations, many mechanisms can be recognized, for instance a change in pace-of-life due to resource availability, and reduced emphasis on reserves in more stable environments.This article has an associated First Person interview with the first author of the paper.

Effect of cannabinoid-serotonin interactions in the regulation of neuropeptide Y1 receptors expression in rats: the role of CB1 and 5-HT2C receptor

Neuropeptide Y (NPY) is involved in a diversity of critical functions such as circadian rhythms, energy homeostasis, and appetite regulation in the hypothalamus. It has identified as a crucial participant in adjusting energy intake and energy storage as fat via central neuropeptide Y1 receptor (NPY1R), leading to obesity and metabolic disorders. The present study was expected to investigate the interaction between 2-AG (CB1R agonist), m-CPP (5HT2CR agonist), SB-242084 (5HT2CR antagonist), and SR-141716A (CB1R antagonist) by mediating through the NPY1R for treating or preventing obesity, metabolic disorders, and other abnormalities. The expression level of NPY1R mRNA has studied on the rat brain by real-time quantitative PCR assay. Based on our findings, intracerebroventricular (ICV) injection of combined 2-AG (1 μg) + m-CPP (2.5 μg) has antagonistic interaction in the expression of the NPY1R gene (P < 0.001). Moreover, the ICV co-injection of SB-242084 (3 μg) + SR-141716A (1 μg) has antagonistic interaction in the NPY1R gene expression (P < 0.001). Co-administration of 2-AG (1 μg) + SB-242084 (3 μg) amplified NPY1R gene expression (P < 0.001), while the ICV co-injection of m-CPP (2.5 μg) + SR-141716A (1 μg) decreased NPY1R gene expression in the hypothalamus (P < 0.001). These results revealed the interference in cannabinoid and serotonergic systems via CB1 and 5HT2C receptors in the expression of NPY1R mRNA in the hypothalamic area of rats.

Neuropeptide Y Signaling in the Lateral Hypothalamus Modulates Diet Component Selection and is Dysregulated in a Model of Diet-Induced Obesity

The preclinical multicomponent free-choice high-fat high-sucrose (fcHFHS) diet has strong validity to model diet-induced obesity (DIO) and associated maladaptive molecular changes in the central nervous system. fcHFHS-induced obese rats demonstrate increased sensitivity to intracerebroventricular infusion of the orexigenic Neuropeptide Y (NPY). The brain region-specific effects of NPY signaling on fcHFHS diet component selection are not completely understood. For example, fcHFHS-fed rats have increased intake of chow and fat following intracerebroventricular NPY infusion, whereas NPY administration in the nucleus accumbens, a key hub of the reward circuitry, specifically increases fat intake. Here, we investigated whether NPY infusion in the lateral hypothalamic area (LHA), which is crucially involved in the regulation of intake, regulates fcHFHS component selection, and if LHA NPY receptor subtypes 1 or 5 (NPYR1/5) are involved. Male Wistar rats were fed a chow or fcHFHS diet for at least seven days, and received intra-LHA vehicle or NPY infusions in a cross-over design. Diet component intake was measured two hours later. Separate experimental designs were used to test the efficacy of NPY1R- or NPY5R antagonism to prevent the orexigenic effects of intra-LHA NPY. Intra-LHA NPY increased caloric intake in chow- and fcHFHS-fed rats. This effect was mediated specifically by chow intake in fcHFHS-fed rats. The orexigenic effects of intra-LHA NPY were prevented by NPY1R and NPY5R antagonism in chow-fed rats, but only by NPY5R antagonism in fcHFHS-fed rats. Thus, NPY signaling has brain region-specific effects on fcHFHS component selection and LHA NPYR sensitivity is dysregulated during consumption of a fcHFHS diet.

Obesity, Neuroinflammation, and Reproductive Function

The increasing occurrence of obesity has become a significant public health concern. Individuals with obesity have higher prevalence of heart disease, stroke, osteoarthritis, diabetes, and reproductive disorders. Reproductive problems include menstrual irregularities, pregnancy complications, and infertility due to anovulation, in women, and lower testosterone and diminished sperm count, in men. In particular, women with obesity have reduced levels of both gonadotropin hormones, and, in obese men, lower testosterone is accompanied by diminished LH. Taken together, these findings indicate central dysregulation of the hypothalamic-pituitary-gonadal axis, specifically at the level of the GnRH neuron function, which is the final brain output for the regulation of reproduction. Obesity is a state of hyperinsulinemia, hyperlipidemia, hyperleptinemia, and chronic inflammation. Herein, we review recent advances in our understanding of how these metabolic and immune changes affect hypothalamic function and regulation of GnRH neurons. In the latter part, we focus on neuroinflammation as a major consequence of obesity and discuss findings that reveal that GnRH neurons are uniquely positioned to respond to inflammatory changes.

Central somatostatin signaling and regulation of food intake

The discovery of somatostatin (SST) in the hypothalamus implicated the peptide in the inhibition of growth hormone release. However, as observed for numerous neuropeptides, SST was neither restricted to this one brain site nor to this one function. Subsequent studies established a widespread but specific expression of SST in the central nervous system of rodents and humans along with the expression patterns of five receptors (sst_1-5 ). Among biological actions, the activation of central SST signaling induced a robust stimulation of food and water intake, which is mediated by the sst₂ as assessed using selective sst agonists. The past years have witnessed the identification of brain SST circuitries involved using chemogenetic and optogenetic approaches and further established a physiological orexigenic role of brain SST signaling. The present review will discuss these recent findings.

Orexin-A signaling in the paraventricular nucleus promote gastric acid secretion and gastric motility through the activation neuropeptide Y Y1 receptors and modulated by the hypothalamic lateral area

OBJECTIVE

Abnormal gastric acid secretion and gastric dyskinesia are common gastroenterological ailments. Our study aims to investigate the effect of orexin-A in the paraventricular nucleus (PVN) gastric motility and gastric acid secretion.

METHODS

The source of orexin-A neuronal projections to the PVN were explored by retrograde tracing and fluorescence immunohistochemistry experiments. Neuronal discharge recordings of single cells were taken within the PVN. Gastric motility was recorded using a force transducer implanted into the stomach, and gastric acid secretion measured through a pyloric catheter.

RESULTS

Orexin-A-positive neuronal projections from LHA to PVN were found. Administration of orexin-A to PVN activated the firing of 63.2% NPY-excited/GD-excitatory (GD-E) neurons but suppressed the firing of 55.9% NPY-inhibited/GD-inhibitory (GD-I) neurons, promoted gastric motility and gastric acid secretion in a dose-dependent manner. Responses produced by orexin-A could be partially blocked by Y₁ receptor antagonist GR-231118; Electrical stimulation to the the hypothalamic lateral area (LHA) altered NPY-sensitive/GD neuronal activity in the PVN, stimulated gastric motility and gastric acid secretion. Additionally, these effects induced by LHA electrical stimulation were blocked by administration of the OX1R antagonist SB-334867 to the PVN.

CONCLUSION

Orexin-A from LHA neurons act on the PVN to enhance gastric motility and gastric acid secretion, with Y₁ receptor signaling playing a critical role.

Functional characterization of an orexin neuropeptide in amphioxus reveals an ancient origin of orexin/orexin receptor system in chordate

Amphioxus belongs to the subphylum cephalochordata, an extant representative of the most basal chordates, whose regulation of endocrine system remains ambiguous. Here we clearly demonstrated the existence of a functional orexin neuropeptide in amphioxus, which is able to interact with orexin receptor, activate both PKC and PKA pathways, decrease leptin expression, and stimulate lipogenesis. We also showed the transcription level of amphioxus orexin was affected by fasting or temperature, indicating a role of this gene in the regulation of energy balance. In addition, the expression of the amphioxus orexin was detected at cerebral vesicle, which has been proposed to be a homolog of the vertebrate brain. These data collectively suggest that a functional orexin neuropeptide has already emerged in amphioxus, which provide insights into the evolutionary origin of orexin in chordate and the functional homology between the cerebral vesicle and vertebrate brain.

Orexins/Hypocretins: Key Regulators of Energy Homeostasis

Originally described to be involved in feeding regulation, orexins/hypocretins are now also considered as major regulatory actors of numerous biological processes, such as pain, sleep, cardiovascular function, neuroendocrine regulation, and energy expenditure. Therefore, they constitute one of the most pleiotropic families of hypothalamic neuropeptides. Although their orexigenic effect is well documented, orexins/hypocretins also exert central effects on energy expenditure, notably on the brown adipose tissue (BAT) thermogenesis. A better comprehension of the underlying mechanisms and potential interactions with other hypothalamic molecular pathways involved in the modulation of food intake and thermogenesis, such as AMP-activated protein kinase (AMPK) and endoplasmic reticulum (ER) stress, is essential to determine the exact implication and pathophysiological relevance of orexins/hypocretins on the control of energy balance. Here, we will review the actions of orexins on energy balance, with special focus on feeding and brown fat function.

Role of astrocytes, microglia, and tanycytes in brain control of systemic metabolism

Astrocytes, microglia, and tanycytes play active roles in the regulation of hypothalamic feeding circuits. These non-neuronal cells are crucial in determining the functional interactions of specific neuronal subpopulations involved in the control of metabolism. Recent advances in biology, optics, genetics, and pharmacology have resulted in the emergence of novel and highly sophisticated approaches for studying hypothalamic neuronal-glial networks. Here we summarize the progress in the field and argue that glial-neuronal interactions provide a core hub integrating food-related cues, interoceptive signals, and internal states to adapt a complex set of physiological responses operating on different timescales to finely tune behavior and metabolism according to metabolic status. This expanding knowledge helps to redefine our understanding of the physiology of food intake and energy metabolism.

Orexin-A signaling in the paraventricular nucleus modulates spontaneous firing of glucose-sensitive neurons and promotes food intake via the NPY pathway in rats

Understanding the mechanisms regulating feeding is crucial to unraveling the pathogenesis of obesity. The study primary explored the effects of orexin-A and neuropeptide Y (NPY) signaling in the hypothalamic paraventricular nucleus (PVN) on feeding and glucose-sensitive (GS) neuron activity in rats. Microinjection of orexin-A into the PVN promoted feeding and modulated the spontaneous firing of GS neurons. Those effects were eliminated by pre-injection of the orexin-A receptor-1 (OX1R) antagonist SB-334867 and weaken by the NPY-1 receptor (NPY-1R) antagonist BMS-193885. After orexin-A administration into the PVN, the number of c-fos cells in the arcuate nucleus (ARC) was significantly higher than that in the group receiving normal saline. Furthermore, most cells exhibited co-expression of NPY and c-fos, indicating activation of NPY neurons in the ARC by PVN-administered orexin-A, which might be involved in feeding regulation. These findings indicate that orexin-A and NPY signaling in the PVN are essential to regulating GS neuronal excitability and feeding in rats.

Early High-Fat Diet Exposure Causes Dysregulation of the Orexin and Dopamine Neuronal Populations in Nonhuman Primates

Maternal obesity and consumption of a high-fat diet (HFD) during pregnancy has a negative impact on offspring, including an increased risk for the development of obesity in adolescence. The mechanism for this transferred metabolic risk is unclear, but many studies have focused on the brain due to its important role in appetite and body-weight regulation. Two main pathways regulate appetite in the brain; homeostatic regulation that occurs predominantly in hypothalamic circuits and hedonic regulation of feeding that occurs via dopaminergic pathways. The current proposal examined the impact of early HFD exposure on the dopaminergic control of hedonic feeding pathways in a translational nonhuman primate model. Japanese macaque offspring from mothers consuming a control (CTR) or HFD were weaned onto control or HFD at an average 8 months of age yielding four groups: maternal and post-weaning control diet (mCTRpCTR), maternal control diet and post-weaning HFD (mCTRpHFD), maternal HFD and post-weaning control diet (mHFDpCTR) and maternal and post-weaning HFD (mHFDpHFD). Brains from 13-month-old offspring were evaluated for expression of neuropeptides that regulate dopaminergic pathways including orexin, melanin-concentrating hormone (MCH) in the lateral hypothalamus (LH), and tyrosine hydroxylase expression in the ventral tegmental area (VTA). Orexin cell numbers in the LH were significantly increased in animals exposed to a post-weaning HFD, while no difference was observed for orexin mRNA content or MCH cell numbers. Orexin fiber projections to the rostral VTA were significantly reduced in mCTRpHFD, mHFDpCTR, and mHFDpHFD groups, but these differences were not significant in the caudal VTA. There was no difference in the percentage of dopamine neurons receiving close appositions from orexin fibers in either the rostral or caudal VTA, nor was there any difference between groups in the number of orexin contacts per TH cell. In conclusion, the current study finds that prolonged early exposure to HFD during the in utero and postnatal period causes alterations at several levels in the dopaminergic circuits regulating reward.

Orexin/hypocretin and dysregulated eating: Promotion of foraging behavior

At its discovery, orexin/hypocretin (OX) was hypothesized to promote food intake. Subsequently, with the identification of the participation of OX in numerous other phenomena, including arousal and drug seeking, this neuropeptide was proposed to be involved in highly motivated behaviors. The present review develops the hypothesis that the primary evolutionary function of OX is to promote foraging behavior, seeking for food under conditions of limited availability. Thus, it will first describe published literature on OX and homeostatic food intake, which shows that OX neurons are activated by conditions of food deprivation and in turn stimulate food intake. Next, it will present literature on excessive and binge-like food intake, which demonstrates that OX stimulates both intake and willingness to work for palatable food. Importantly, studies show that binge-like eating can be inhibited by OX antagonists at doses far lower than those required to suppress homeostatic intake (3 mg/kg vs. 30 mg/kg), suggesting that an OX-based pharmacotherapy, at the right dose, could specifically control dysregulated eating. Finally, the review will discuss the role of OX in foraging behavior, citing literature which shows that OX neurons, which are activated during the anticipation of food reward, can promote a number of phenomena involved in successful foraging, including food-anticipatory locomotor behavior, olfactory sensitivity, visual attention, spatial memory, and mastication. Thus, OX may promote homeostatic eating, as well as binge eating of palatable food, due to its ability to stimulate and coordinate the activities involved in foraging behavior.

The role of corticostriatal-hypothalamic neural circuits in feeding behaviour: implications for obesity

Emerging evidence from human imaging studies suggests that obese individuals have altered connectivity between the hypothalamus, the key brain region controlling energy homeostasis, and cortical regions involved in decision-making and reward processing. Historically, animal studies have demonstrated that the lateral hypothalamus is the key hypothalamic region involved in feeding and reward. The lateral hypothalamus is a heterogeneous structure comprised of several distinct types of neurons which are scattered throughout. In addition, the lateral hypothalamus receives inputs from a number of cortical brain regions suggesting that it is uniquely positioned to be a key integrator of cortical information and metabolic feedback. In this review, we summarize how human brain imaging can inform detailed animal studies to investigate neural pathways connecting cortical regions and the hypothalamus. Here, we discuss key cortical brain regions that are reciprocally connected to the lateral hypothalamus and are implicated in decision-making processes surrounding food.

Estradiol and the control of feeding behavior

This review lays out the evidence for the role of E2 in homeostatic and hedonic feeding across several species. While significant effort has been expended on homeostatic feeding research, more studies for hedonic feeding need to be conducted (i.e. are there increases in meal size and enhanced motivation to natural food rewards). By identifying the underlying neural circuitry involved, one can better delineate the mechanisms by which E2 influences feeding behavior. By utilizing more selective neural targeting techniques, such as optogenetics, significant progress can be made toward this goal. Together, behavioral and physiological techniques will help us to better understand neural deficits that can increase the risk for obesity in the absence of E2 (menopause) and aid in developing therapeutic strategies.

Feeding Behavior: Hypocretin/Orexin Neurons Act between Food Seeking and Eating

A report on the rapid change of activity of hypocretin/orexin cells in response to contact rather than digestion of food delivers new insights into the behavioral control of food intake and systemic energy expenditure.

Abnormal Growth and Feeding Behavior in Upper Airway Obstruction in Rats

Pediatric obstructive sleep apnea (OSA) is a syndrome manifesting with snoring and increased respiratory effort due to increased upper airway resistance. In addition to cause the abnormal sleep, this syndrome has been shown to elicit either growth retardation or metabolic syndrome and obesity. Treating OSA by adenotonsillectomy is usually associated with increased risk for obesity, despite near complete restoration of breathing and sleep. However, the underlying mechanism linking upper airways obstruction (AO) to persistent change in food intake, metabolism, and growth remains unclear. Rodent models have examined the impact of intermittent hypoxia on metabolism. However, an additional defining feature of OSA that is not related to intermittent hypoxia is enhanced respiratory loading leading to increased respiratory effort and abnormal sleep. The focus of this mini review is on recent evidence indicating the persistent abnormalities in endocrine regulation of feeding and growth that are not fully restored by the chronic upper AO removal in rats. Here, we highlight important aspects related to abnormal regulation of metabolism that are not related to intermittent hypoxia per se, in an animal model that mimics many of the clinical features of pediatric OSA. Our evidence from the AO model indicates that obstruction removal may not be sufficient to prevent the post-removal tendency for abnormal growth.

Orexin/Hypocretin System: Role in Food and Drug Overconsumption

The neuropeptide orexin/hypocretin (OX), while largely transcribed within the hypothalamus, is released throughout the brain to affect complex behaviors. Primarily through the hypothalamus itself, OX homeostatically regulates adaptive behaviors needed for survival, including food intake, sleep-wake regulation, mating, and maternal behavior. However, through extrahypothalamic limbic brain regions, OX promotes seeking and intake of rewarding substances of abuse, like palatable food, alcohol, nicotine, and cocaine. This neuropeptide, in turn, is stimulated by the intake of or early life exposure to these substances, forming a nonhomeostatic, positive feedback loop. The specific OX receptor involved in these behaviors, whether adaptive behavior or substance seeking and intake, is dependent on the particular brain region that contributes to them. Thus, we propose that, while the primary function of OX is to maintain arousal for the performance of adaptive behaviors, this neuropeptide system is readily co-opted by rewarding substances that involve positive feedback, ultimately promoting their abuse.

Lateral Hypothalamic Control of the Ventral Tegmental Area: Reward Evaluation and the Driving of Motivated Behavior

The lateral hypothalamus (LH) plays an important role in many motivated behaviors, sleep-wake states, food intake, drug-seeking, energy balance, etc. It is also home to a heterogeneous population of neurons that express and co-express multiple neuropeptides including hypocretin (Hcrt), melanin-concentrating hormone (MCH), cocaine- and amphetamine-regulated transcript (CART) and neurotensin (NT). These neurons project widely throughout the brain to areas such as the locus coeruleus, the bed nucleus of the stria terminalis, the amygdala and the ventral tegmental area (VTA). Lateral hypothalamic projections to the VTA are believed to be important for driving behavior due to the involvement of dopaminergic reward circuitry. The purpose of this article is to review current knowledge regarding the lateral hypothalamic connections to the VTA and the role they play in driving these behaviors.

Arcuate nucleus neuropeptide coexpression and connections to gonadotrophin-releasing hormone neurones in the female rhesus macaque

The underlying hypothalamic neurocircuitry by which metabolism and feeding regulates reproductive function has been well-studied in the rodent; however, recent data have demonstrated significant neuroanatomical differences in the human brain. The present study had three objectives, centred on arcuate nucleus neuropeptides regulating feeding and reproduction: (i) to characterise coexpression patterns in the female nonhuman primate; (ii) to establish whether these neuronal populations make potential contacts with gonadotophin-releasing hormone (GnRH) neurones; and (iii) to determine whether these contacts differ between the low and high GnRH-releasing states of pre-puberty and adulthood, respectively. Female nonhuman primates have several coexpression patterns of hypothalamic neuropeptides that differ from those reported in rodents. Cocaine- and amphetamine-regulated transcript (CART) is not coexpressed with pro-opiomelanocortin but instead with neuropeptide Y (NPY). CART is also expressed in a subpopulation of kisspeptin cells in the nonhuman primate, similar to observations in humans but diverging from findings in rodents. Very few GnRH-expressing neurones received close appositions from double-labelled kisspeptin/CART fibres; however, both single-labelled kisspeptin and CART fibres were in frequent apposition with GnRH neurones, with no differences between prepubertal and adult animals. NPY/agouti-related peptide (AgRP) coexpressing fibres contacted significantly more GnRH neurones in prepubertal animals than adults, consistent with increased NPY and AgRP mRNA observed in prepubertal animals. The findings of the present study detail significant differences in arcuate nucleus neuropeptide coexpression in the monkey compared to the rodent and are consistent with the hypothesis that arcuate nucleus NPY/AgRP neurones play an inhibitory role in controlling GnRH neuronal regulation in the prepubertal primate.

Hypocretin/Orexin Peptides Excite Rat Neuroendocrine Dopamine Neurons through Orexin 2 Receptor-Mediated Activation of a Mixed Cation Current

Hypocretin/Orexin (H/O) neurons of the lateral hypothalamus are compelling modulator candidates for the chronobiology of neuroendocrine output and, as a consequence, hormone release from the anterior pituitary. Here we investigate the effects of H/O peptides upon tuberoinfundibular dopamine (TIDA) neurons – cells which control, via inhibition, the pituitary secretion of prolactin. In whole cell recordings performed in male rat hypothalamic slices, application of H/O-A, as well as H/O-B, excited oscillating TIDA neurons, inducing a reversible depolarising switch from phasic to tonic discharge. The H/O-induced inward current underpinning this effect was post-synaptic (as it endured in the presence of tetrodotoxin), appeared to be carried by a Na⁺-dependent transient receptor potential-like channel (as it was blocked by 2-APB and was diminished by removal of extracellular Na⁺), and was a consequence of OX2R receptor activation (as it was blocked by the OX2R receptor antagonist TCS OX2 29, but not the OX1R receptor antagonist SB 334867). Application of the hormone, melatonin, failed to alter TIDA membrane potential or oscillatory activity. This first description of the electrophysiological effects of H/Os upon the TIDA network identifies cellular mechanisms that may contribute to the circadian rhythmicity of prolactin secretion.

Involvement of Novel Feeding-Related Peptides in Neuroendocrine Response to Stress

Various stressors are known to cause eating disorders. However, it is not known in detail about the neural network and molecular mechanism that are involved in the stress-induced changes of feeding behavior in the central nervous system. Many novel feeding-regulated peptides such as orexins/hypocretins and ghrelin have been discovered since the discovery of leptin derived from adipocytes as a product of the ob gene. These novel peptides were identified as endogenous ligands of orphan G protein-coupled receptors. The accumulating evidence reveals that these peptides may be involved in stress responses via the central nervous system, as well as feeding behavior. The possible involvement of novel feeding-related peptides in neuroendocrine responses to stress is reviewed here.

Growth Hormone Secretion: Molecular and Cellular Mechanisms and In Vivo Approaches

Growth hormone (GH) release is under the direct control of hypothalamic releasing hormones, some being also produced peripherally. The role of these hypothalamic factors has been understood by in vitro studies together with such in vivo approaches as stalk sectioning. Secretion of GH is stimulated by GH-releasing hormone (GHRH) and ghrelin (acting via the GH secretagogue [GHS] receptor [GHSR]), and inhibited by somatostatin (SRIF). Other peptides/proteins influence GH secretion, at least in some species. The cellular mechanism by which the releasing hormones affect GH secretion from the somatotrope requires specific signal transduction systems (cAMP and/or calcium influx and/or mobilization of intracellular calcium) and/or tyrosine kinase(s) and/or nitric oxide (NO)/cGMP. At the subcellular level, GH release (at least in response to GHS) is accomplished by the following. The GH-containing secretory granules are moved close to the cell surface. There is then transient fusion of the secretory granules with the fusion pores in the multiple secretory pits in the somatotrope cell surface.

The wake-promoting drug modafinil stimulates specific hypothalamic circuits to promote adaptive stress responses in an animal model of PTSD

Pharmacotherapeutic intervention during traumatic memory consolidation has been suggested to alleviate or even prevent the development of posttraumatic stress disorder (PTSD). We recently reported that, in a controlled, prospective animal model, depriving rats of sleep following stress exposure prevents the development of a PTSD-like phenotype. Here, we report that administering the wake-promoting drug modafinil to rats in the aftermath of a stressogenic experience has a similar prophylactic effect, as it significantly reduces the prevalence of PTSD-like phenotype. Moreover, we show that the therapeutic value of modafinil appears to stem from its ability to stimulate a specific circuit within the hypothalamus, which ties together the neuropeptide Y, the orexin system and the HPA axis, to promote adaptive stress responses. The study not only confirms the value of sleep prevention and identifies the mechanism of action of a potential prophylactic treatment after traumatic exposure, but also contributes to understanding mechanisms underlying the shift towards adaptive behavioral response.

Estrogen-Induced Hypothalamic Synaptic Plasticity and Pituitary Sensitization in the Control of the Estrogen-Induced Gonadotrophin Surge

Proper gonadal function requires coordinated (feedback) interactions between the gonads, adenohypophysis, and brain: the gonads elaborate sex steroids (progestins, androgens, and estrogens) and proteins (inhibin-activin family) during gamete development. In both sexes, the brain-pituitary gonadotrophin-regulating interaction is coordinated by estradiol through its opposing actions on pituitary gonadotrophs (sensitization of the response to gonadotrophin-releasing hormone [GnRH]) versus hypothalamic neurons (inhibition of GnRH secretion). This dynamic tension between the gonadotrophs and the GnRH cells in the brain regulates the circulating gonadotrophins and is termed reciprocal/negative feedback. In females, reciprocal/negative feedback dominates approximately 90% of the ovarian cycle. In a spectacular exception, the dynamic tension is broken during the surge of circulating estrogen that marks follicle and oocyte(s) maturation. The cause is an estradiol-induced disinhibition of the GnRH neurons that releases GnRH secretion to the highly sensitized pituitary gonadotrophs that in turn release the gonadotrophin surge (the estrogen-induced gonadotrophin surge [EIGS], also known as positive feedback). Studies during the past 4 decades have shown this disinhibition to result from estrogen-induced synaptic plasticity (EISP), including a reversible approximately 50% loss in arcuate nucleus synapses. The disinhibited GnRH secretion occurs during maximal gonadotroph sensitization and results in the EIGS. Specific immunoneutralization of estradiol blocks the EISP and EIGS. The EISP is accompanied by increases in insulinlike growth factor 1, polysialylated neural cell adhesion molecule, and ezrin, 3 proteins that the authors believe are the links between estrogen-induced astroglial extension and the EISP that releases GnRH secretion at the moment of maximal sensitization of the pituitary gonadotrophs. The result is the paradoxical surge of gonadotrophins at the peak of ovarian estrogen secretion and the triggering of ovulation. This enhanced understanding of the mechanics of gonadotrophin control clarifies elements of the involved feedback loops and opens the way to a better understanding of the neurobiology of reproduction.

Lateral hypothalamic circuits for feeding and reward

In experiments conducted over 60 years ago, the lateral hypothalamic area (LHA) was identified as a critical neuroanatomical substrate for motivated behavior. Electrical stimulation of the LHA induces voracious feeding even in well-fed animals. In the absence of food, animals will work tirelessly, often lever-pressing thousands of times per hour, for electrical stimulation at the same site that provokes feeding, drinking and other species-typical motivated behaviors. Here we review the classic findings from electrical stimulation studies and integrate them with more recent work that has used contemporary circuit-based approaches to study the LHA. We identify specific anatomically and molecularly defined LHA elements that integrate diverse information arising from cortical, extended amygdala and basal forebrain networks to ultimately generate a highly specified and invigorated behavioral state conveyed via LHA projections to downstream reward and feeding-specific circuits.

Preferential development of neuropeptide Y/GABA circuit in hypothalamic arcuate nucleus in postnatal rats

The hypothalamus, which plays a critical role in regulation of energy homeostasis, is formed during the perinatal period and thus vulnerable to fetal/newborn environmental conditions. We investigated synaptogenesis and neurotransmission of neurons in arcuate nucleus of the hypothalamus (ARH) during the postnatal period using immunohistochemical and electrophysiological methods. Our results show that the density of neuropeptide Y (NPY) fibers increases abruptly after the second postnatal week. NPY and proopiomelanocortin (POMC) immunoreactive fibers/varicosities puncta are mutually juxtaposed to perikarya of both neurons with increasing NPY and decreasing POMC apposition until the third postnatal week. The frequencies of spontaneous GABAergic inhibitory and glutamatergic excitatory postsynaptic currents (sIPSC and sEPSC) increase with age, with action potential dependent sIPSCs predominant during first postnatal week and sEPSCs thereafter. The presynaptic function of ARH synapses appears to reach adult levels around the age of weaning, while the postsynaptic receptors are still undergoing modification, evidenced by changes of frequencies, amplitudes and deactivation kinetics of PSCs. The number of NPY fibers juxtaposed to NPY neurons is correlated with the frequency of postsynaptic currents, suggesting that NPY/GABA release may facilitate maturation of synapses on their innervated neurons. Our results indicate that a neural circuit in ARH with a stronger NPY/GABAergic tone undergoes significant development during the postnatal period, which may be important for the maturation and/or remodeling of ARH neural circuits.