Hypothalamic fatty acid metabolism: a housekeeping pathway that regulates food intake

Brain lipid sensing and the neural control of energy balance

The central nervous system continuously detects circulating concentrations of lipids such as fatty acids and troglycerides. Once information has been detected, the central nervous system can in turn participate in the control of energy balance and blood sugar levels and in particular regulate the secretion and action of insulin. Neurons capable of detecting circulating lipid variations are located in the hypothalamus and in other regions such as the nucleus accumbens, the striatum or the hippocampus. An excess of lipids will have deleterious effects and may induce central lipotoxicity, in particular following local production of ceramides and the appearance of neuroinflammation which may lead to metabolic diseases such as obesity and type 2 diabetes.

Maternal obesity increases hypothalamic miR-505-5p expression in mouse offspring leading to altered fatty acid sensing and increased intake of high-fat food

In utero exposure to maternal obesity programs increased obesity risk. Animal models show that programmed offspring obesity is preceded by hyperphagia, but the mechanisms that mediate these changes are unknown. Using a mouse model of maternal obesity, we observed increased intake of a high-fat diet (HFD) in offspring of obese mothers that precedes the development of obesity. Through small RNA sequencing, we identified programmed overexpression of hypothalamic miR-505-5p that is established in the fetus, lasts to adulthood and is maintained in hypothalamic neural progenitor cells cultured in vitro. Metabolic hormones and long-chain fatty acids associated with obesity increase miR-505-5p expression in hypothalamic neurons in vitro. We demonstrate that targets of miR-505-5p are enriched in fatty acid metabolism pathways and overexpression of miR-505-5p decreased neuronal fatty acid metabolism in vitro. miR-505-5p targets are associated with increased BMI in human genetic studies. Intra-cerebroventricular injection of miR-505-5p in wild-type mice increased HFD intake, mimicking the phenotype observed in offspring exposed to maternal obesity. Conversely, maternal exercise intervention in an obese mouse pregnancy rescued the programmed increase of hypothalamic miR-505-5p in offspring of obese dams and reduced HFD intake to control offspring levels. This study identifies a novel mechanism by which maternal obesity programs obesity in offspring via increased intake of high-fat foods.

Increased Lipogenesis Is Important for Hexavalent Chromium-Transformed Lung Cells and Xenograft Tumor Growth

Hexavalent chromium, Cr(VI), is a known carcinogen and environmental health concern. It has been established that reactive oxygen species, genomic instability, and DNA damage repair deficiency are important contributors to the Cr(VI)-induced carcinogenesis mechanism. However, some hallmarks of cancer remain under-researched regarding the mechanism behind Cr(VI)-induced carcinogenesis. Increased lipogenesis is important to carcinogenesis and tumorigenesis in multiple types of cancers, yet the role increased lipogenesis has in Cr(VI) carcinogenesis is unclear. We report here that Cr(VI)-induced transformation of three human lung cell lines (BEAS-2B, BEP2D, and WTHBF-6) resulted in increased lipogenesis (palmitic acid levels), and Cr(VI)-transformed cells had an increased expression of key lipogenesis proteins (ATP citrate lyase [ACLY], acetyl-CoA carboxylase [ACC1], and fatty acid synthase [FASN]). We also determined that the Cr(VI)-transformed cells did not exhibit an increase in fatty acid oxidation or lipid droplets compared to their passage-matched control cells. Additionally, we observed increases in ACLY, ACC1, and FASN in lung tumor tissue compared with normal-adjacent lung tissue (in chromate workers that died of chromate-induced tumors). Next, using a known FASN inhibitor (C75), we treated Cr(VI)-transformed BEAS-2B with this inhibitor and measured cell growth, FASN protein expression, and growth in soft agar. We observed that FASN inhibition results in a decreased protein expression, decreased cell growth, and the inhibition of colony growth in soft agar. Next, using shRNA to knock down the FASN protein in Cr(VI)-transformed BEAS-2B cells, we saw a decrease in FASN protein expression and a loss of the xenograft tumor development of Cr(VI)-transformed BEAS-2B cells. These results demonstrate that FASN is important for Cr(VI)-transformed cell growth and cancer properties. In conclusion, these data show that Cr(VI)-transformation in vitro caused an increase in lipogenesis, and that this increase is vital for Cr(VI)-transformed cells.

Predisposition of the Common MC4R rs17782313 Female Carriers to Elevated Obesity and Interaction with Eating Habits

The global rise in obesity is attributed to genetic predisposition interaction with an obesogenic environment. Melanocortin 4 receptor (MC4R) rs17782313 polymorphism has been linked to common obesity with varying influence across different populations. MC4R is a crucial player in the leptin proopiomelanocortin pathway that regulates weight hemostasis. We aimed to study MC4R rs17782313 and its interaction with eating behaviors on obesity predisposition in the Israeli population. Adults' (n = 5785, >18 y) genotype and anthropometric and demographic data were analyzed using logistic regression models adjusting for age, sex, T1DM, and T2DM. MC4R rs17782313 significantly predisposes to elevated obesity risk under the recessive and additive models (OR = 1.38, 95% CI: 1.1-1.72, p = 0.005 and OR = 1.1, 95% CI: 1.01-1.2, p = 0.03, respectively) adjusted for confounders (age, sex, T1DM, and T2DM). Stratification by sex demonstrated that carrying the common MC4R rs17782313 is significantly associated with an elevated predisposition to obesity under the recessive model among females only (OR = 1.41, 95% CI: 1.09-1.82, p = 0.01), with an average of 0.85 BMI increment compared with wild type and one risk allele carriers. MC4R rs17782313 significantly interacted with several eating behaviors to enhance the risk of obesity. Our findings demonstrate that MC4R rs17782313 homozygous female carriers are significantly predisposed to obesity amplified by eating behaviors.

Targeting carnitine palmitoyltransferase 1 isoforms in the hypothalamus: A promising strategy to regulate energy balance

Tackling the growing incidence and prevalence of obesity urgently requires uncovering new molecular pathways with therapeutic potential. The brain, and in particular the hypothalamus, is a major integrator of metabolic signals from peripheral tissues that regulate functions such as feeding behavior and energy expenditure. In obesity, hypothalamic capacity to sense nutritional status and regulate these functions is altered. An emerging line of research is that hypothalamic lipid metabolism plays a critical role in regulating energy balance. Here, we focus on the carnitine palmitoyltransferase 1 (CPT1) enzyme family responsible for long-chain fatty acid metabolism. The evidence suggests that two of its isoforms expressed in the brain, CPT1A and CPT1C, play a crucial role in hypothalamic lipid metabolism, and their promise as targets in food intake and bodyweight management is currently being intensively investigated. In this review we describe and discuss the metabolic actions and potential up- and downstream effectors of hypothalamic CPT1 isoforms, and posit the need to develop innovative nanomedicine platforms for selective targeting of CPT1 and related nutrient sensors in specific brain areas as potential next-generation therapy to treat obesity.

CPT1A in AgRP neurons is required for sex-dependent regulation of feeding and thirst

BACKGROUND

Fatty acid metabolism in the hypothalamus has an important role in food intake, but its specific role in AgRP neurons is poorly understood. Here, we examined whether carnitinea palmitoyltransferase 1A (CPT1A), a key enzyme in mitochondrial fatty acid oxidation, affects energy balance.

METHODS

To obtain Cpt1aKO mice and their control littermates, Cpt1a^(flox/flox) mice were crossed with tamoxifen-inducible AgRP^CreERT2 mice. Food intake and body weight were analyzed weekly in both males and females. At 12 weeks of age, metabolic flexibility was determined by ghrelin-induced food intake and fasting-refeeding satiety tests. Energy expenditure was analyzed by calorimetric system and thermogenic activity of brown adipose tissue. To study fluid balance the analysis of urine and water intake volumes; osmolality of urine and plasma; as well as serum levels of angiotensin and components of RAAS (renin-angiotensin-aldosterone system) were measured. At the central level, changes in AgRP neurons were determined by: (1) analyzing specific AgRP gene expression in RiboTag-Cpt1aKO mice obtained by crossing Cpt1aKO mice with RiboTag mice; (2) measuring presynaptic terminal formation in the AgRP neurons with the injection of the AAV1-EF1a-DIO-synaptophysin-GFP in the arcuate nucleus of the hypothalamus; (3) analyzing AgRP neuronal viability and spine formations by the injection AAV9-EF1a-DIO-mCherry in the arcuate nucleus of the hypothalamus; (4) analyzing in situ the specific AgRP mitochondria in the ZsGreen-Cpt1aKO obtained by breeding ZsGreen mice with Cpt1aKO mice. Two-way ANOVA analyses were performed to determine the contributions of the effect of lack of CPT1A in AgRP neurons in the sex.

RESULTS

Changes in food intake were just seen in male Cpt1aKO mice while only female Cpt1aKO mice increased energy expenditure. The lack of Cpt1a in the AgRP neurons enhanced brown adipose tissue activity, mainly in females, and induced a substantial reduction in fat deposits and body weight. Strikingly, both male and female Cpt1aKO mice showed polydipsia and polyuria, with more reduced serum vasopressin levels in females and without osmolality alterations, indicating a direct involvement of Cpt1a in AgRP neurons in fluid balance. AgRP neurons from Cpt1aKO mice showed a sex-dependent gene expression pattern, reduced mitochondria and decreased presynaptic innervation to the paraventricular nucleus, without neuronal viability alterations.

CONCLUSIONS

Our results highlight that fatty acid metabolism and CPT1A in AgRP neurons show marked sex differences and play a relevant role in the neuronal processes necessary for the maintenance of whole-body fluid and energy balance.

Hepatic Energy Metabolism under the Local Control of the Thyroid Hormone System

The energy homeostasis of the organism is orchestrated by a complex interplay of energy substrate shuttling, breakdown, storage, and distribution. Many of these processes are interconnected via the liver. Thyroid hormones (TH) are well known to provide signals for the regulation of energy homeostasis through direct gene regulation via their nuclear receptors acting as transcription factors. In this comprehensive review, we summarize the effects of nutritional intervention like fasting and diets on the TH system. In parallel, we detail direct effects of TH in liver metabolic pathways with regards to glucose, lipid, and cholesterol metabolism. This overview on hepatic effects of TH provides the basis for understanding the complex regulatory network and its translational potential with regards to currently discussed treatment options of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) involving TH mimetics.

Symposium review: Fueling appetite: Nutrient metabolism and the control of feed intake

Conceptual models developed over the past century describe 2 key constraints to feed intake (FI) of healthy animals: gut capacity and metabolic demand. Evidence that greater energy demands (e.g., greater milk production) drive a corresponding increase in caloric intake led to the dominant concept that animals "eat to energy requirements." Although this model provides reasonable initial estimates of FI, it lacks a proposed physiological basis for the control system, does not consider nutrient constraints beyond energy, and fails to explain differential energy intake responses to different fuels. To address these gaps, research has focused on mechanisms for sensing nutrient availability and providing feedback to hypothalamic centers that integrate signals to control feeding behavior. The elimination of FI response to certain nutrients by vagotomy suggests that peripheral tissues play a role in nutrient sensing. These findings and the central role of the liver in metabolic flux led to the development of the hepatic oxidation theory (HOT). According to the HOT, liver energy charge is the regulated variable that induces dietary intake changes and consequently affects whole-body energy balance. Evidence in support of HOT includes associations between hepatic energy charge and meal patterns, increased FI in response to phosphate trapping, and reduced FI in response to phosphate loading. In accordance with the HOT, infusion studies in dairy cattle have consistently demonstrated that providing fuels that either oxidize or stimulate oxidation in the liver decreases FI and energy intake to a greater extent than fuels that bypass the liver. Importantly, this holds true for glucose, which is readily oxidized by nerve cells, but is rarely taken up by the bovine liver. Although the brain integrates multiple signals including those related to gastric distention and illness, the HOT provides a physiological framework for understanding the dominant role the liver likely plays in sensing short-term energy status. Understanding this model provides insights into how to use or bypass the regulatory system to manage FI of animals.

Nanomedicine targeting brain lipid metabolism as a feasible approach for controlling the energy balance

Targeting brain lipid metabolism is a promising strategy to regulate the energy balance and fight metabolic diseases such as obesity. The development of stable platforms for selective delivery of drugs, particularly to the hypothalamus, is a challenge but a possible solution for these metabolic diseases. Attenuating fatty acid oxidation in the hypothalamus via CPT1A inhibition leads to satiety, but this target is difficult to reach in vivo with the current drugs. We propose using an advanced crosslinked polymeric micelle-type nanomedicine that can stably load the CPT1A inhibitor C75-CoA for in vivo control of the energy balance. Central administration of the nanomedicine induced a rapid attenuation of food intake and body weight in mice via regulation of appetite-related neuropeptides and neuronal activation of specific hypothalamic regions driving changes in the liver and adipose tissue. This nanomedicine targeting brain lipid metabolism was successful in the modulation of food intake and peripheral metabolism in mice.

Fatty acid-sensing mechanisms in the hypothalamus of European sea bass (Dicentrarchus labrax): The potential role of monounsaturated and polyunsaturated fatty acids

This study aimed to evaluate the hypothalamus fatty acid (FA)-sensing mechanisms response to different FA in European sea bass. For that purpose, fish (body weight of 90 g) were intraperitoneally (IP) injected (time 0 h) with five long-chain unsaturated fatty acids, namely, docosahexaenoic acid (DHA; C22:5n3); eicosapentaenoic acid (EPA; C20:4n3); α-linolenic (ALA; C18:3n3); linoleic acid (LA; C18:2n6) and oleic acid (OA; C18:1n9) at a dose of 300 μg kg^-1, or with 0.9% saline solution (control). Feed intake (FI) was recorded at 3, 6, and 24 h after the IP injection. One week later, fish were IP injected with the same FA, and the hypothalamus was collected 3 h after the IP injection for measurement of molecules related to FI regulation and FA-sensing mechanisms. Cumulative FI (g/kg/day) was not affected by treatments. However, compared to the control, FI increased with the OA treatment at 6 h after the IP injection. FI decreased with mealtime in the DHA and LA groups. Gene expression of orexigenic (npy/agrp) and anorexigenic (cart2/pomc1) neurons was not affected by the FA treatments. Attending the enzymes involved in the FA-sensing mechanisms activation, compared to the control carnitine palmitoyltransferase I (CPT1) and ATP citrate lyase (ACLY) activity were not affected by FA treatments. Contrarily the key enzymes of lipid metabolisms, malic enzyme and hydroxyacylCoA dehydrogenase was higher in fish that received the EPA and OA treatment, than fish treated to the control. Overall, the results of the present study indicate that gene expression of orexigenic and anorexigenic neurons was not affected at 3 h after IP injection with different FA. However, the activity of key enzymes of lipid metabolism was differently affected by circulating FA, indicating that FA-sensing mechanisms respond to different FA. Further studies are required involving different sampling times to further characterize the response of FA-sensing mechanisms to FA. These findings may be of relevance to the aquaculture industry in an era where alternative lipid sources are being increasingly used.

Central Regulation of Brown Fat Thermogenesis in Response to Saturated or Unsaturated Long-Chain Fatty Acids

Sensing of long-chain fatty acids (LCFA) in the hypothalamus modulates energy balance, and its disruption leads to obesity. To date, the effects of saturated or unsaturated LCFA on hypothalamic-brown adipose tissue (BAT) axis and the underlying mechanisms have remained largely unclear. Our aim was to characterize the main molecular pathways involved in the hypothalamic regulation of BAT thermogenesis in response to LCFA with different lengths and degrees of saturation. One-week administration of high-fat diet enriched in monounsaturated FA led to higher BAT thermogenesis compared to a saturated FA-enriched diet. Intracerebroventricular infusion of oleic and linoleic acids upregulated thermogenesis markers and temperature in brown fat of mice, and triggered neuronal activation of paraventricular (PaV), ventromedial (VMH) and arcuate (ARC) hypothalamic nuclei, which was not found with saturated FAs. The neuron-specific protein carnitine palmitoyltransferase 1-C (CPT1C) was a crucial effector of oleic acid since the FA action was blunted in CPT1C-KO mice. Moreover, changes in the AMPK/ACC/malonyl-CoA pathway and fatty acid synthase expression were evoked by oleic acid. Altogether, central infusion of unsaturated but not saturated LCFA increases BAT thermogenesis through CPT1C-mediated sensing of FA metabolism shift, which in turn drive melanocortin system activation. These findings add new insight into neuronal circuitries activated by LCFA to drive thermogenesis.

Fatty Acid Synthase: Structure, Function, and Regulation

Fatty acid (FA) biosynthesis plays a central role in the metabolism of living cells as building blocks of biological membranes, energy reserves of the cell, and precursors to second messenger molecules. In keeping with its central metabolic role, FA biosynthesis impacts several cellular functions and its misfunction is linked to disease, such as cancer, obesity, and non-alcoholic fatty liver disease. Cellular FA biosynthesis is conducted by fatty acid synthases (FAS). All FAS enzymes catalyze similar biosynthetic reactions, but the functional architectures adopted by these cellular catalysts can differ substantially. This variability in FAS structure amongst various organisms and the essential role played by FA biosynthetic pathways makes this metabolic route a valuable target for the development of antibiotics. Beyond cellular FA biosynthesis, the quest for renewable energy sources has piqued interest in FA biosynthetic pathway engineering to generate biofuels and fatty acid derived chemicals. For these applications, based on FA biosynthetic pathways, to succeed, detailed metabolic, functional and structural insights into FAS are required, along with an intimate knowledge into the regulation of FAS. In this review, we summarize our present knowledge about the functional, structural, and regulatory aspects of FAS.

Memory regulation in feeding habit transformation to dead prey fish of Chinese perch (Siniperca chuatsi)

Memory drove a critical process of feeding habit transformation in Chinese perch when they re-trained to eat dead prey fish. To investigate the regulatory mechanism of cAMP-response element-binding protein (CREB) signaling pathway on the memory of Chinese perch during feeding habit transformation, the phosphorylation levels of upstream signal proteins of CREB between the control group (trained once) and the experimental group (trained twice) were measured. The results illustrated that the re-training was correlated to phosphorylation of extracellular regulated protein kinase (ERK1/2) and calcium/calmodulin-dependent protein kinase II (CaMKII), and dephosphorylation of protein kinase A (PKA) of Chinese perch. Inhibition of ERK1/2-CREB pathway decreased the mRNA levels of memory-related genes ((fos-related antigen 2 (fra2), CCAAT enhancer-binding protein delta (c/ebpb), immediate-early gene zif268 (zif268), proto-oncogenes c-fos (c-fox) and synaptotagmin-IV (sytIV)) and mRNA levels of appetite-related genes (agouti-related peptide (agrp) and ghrelin), and activation of PP1-CREB pathway increased the phosphorylated levels of CREB, the mRNA levels of memory-related genes (fra2, c/ebpb, zif268, and c-fox), and the mRNA levels of appetite-related genes (pro-opiomelanocortin (pomc) and leptin) in primary brain cells of Chinese perch. The memory in Chinese perch feeding habit transformation was associated with the ERK1/2-CREB and PP1-CREB pathways, which could regulate the transcription of memory-related genes and appetite-related genes.

MECHANISMS IN ENDOCRINOLOGY: The gut-brain axis: regulating energy balance independent of food intake

Obesity is a global pandemic with a large health and economic burden worldwide. Bodyweight is regulated by the ability of the CNS, and especially the hypothalamus, to orchestrate the function of peripheral organs that play a key role in metabolism. Gut hormones play a fundamental role in the regulation of energy balance, as they modulate not only feeding behavior but also energy expenditure and nutrient partitioning. This review examines the recent discoveries about hormones produced in the stomach and gut, which have been reported to regulate food intake and energy expenditure in preclinical models. Some of these hormones act on the hypothalamus to modulate thermogenesis and adiposity in a food intake-independent fashion. Finally, the association of these gut hormones to eating, energy expenditure, and weight loss after bariatric surgery in humans is discussed.

Integration of Nutrient Sensing in Fish Hypothalamus

The knowledge regarding hypothalamic integration of metabolic and endocrine signaling resulting in regulation of food intake is scarce in fish. Available studies pointed to a network in which the activation of the nutrient-sensing (glucose, fatty acid, and amino acid) systems would result in AMP-activated protein kinase (AMPK) inhibition and activation of protein kinase B (Akt) and mechanistic target of rapamycin (mTOR). Changes in these signaling pathways would control phosphorylation of transcription factors cAMP response-element binding protein (CREB), forkhead box01 (FoxO1), and brain homeobox transcription factor (BSX) leading to food intake inhibition through changes in the expression of neuropeptide Y (NPY), agouti-related peptide (AgRP), pro-opio melanocortin (POMC), and cocaine and amphetamine-related transcript (CART). The present mini-review summarizes information on the topic and identifies gaps for future research.

Nutrient regulation of somatic growth in teleost fish. The interaction between somatic growth, feeding and metabolism

This review covers the current knowledge on the regulation of the somatic growth axis and its interaction with metabolism and feeding regulation. The main endocrine and neuroendocrine factors regulating both the growth axis and feeding behavior will be briefly summarized. Recently discovered neuropeptides and peptide hormones will be mentioned in relation to feeding control as well as growth hormone regulation. In addition, the influence of nutrient and nutrient sensing mechanisms on growth axis will be highlighted. We expect that in this process gaps of knowledge will be exposed, stimulating future research in those areas.

The long-chain fatty acid receptors FFA1 and FFA4 are involved in food intake regulation in fish brain

We hypothesized that the free fatty acid receptors FFA1 and FFA4 might be involved in the anorectic response observed in fish after rising levels of long-chain fatty acids (LCFAs) such as oleate. In one experiment we demonstrated that intracerebroventricular (i.c.v.) treatment of rainbow trout with FFA1 and FFA4 agonists elicited an anorectic response 2, 6 and 24 h after treatment. In a second experiment, the same i.c.v. treatment resulted after 2 h in an enhancement in the mRNA abundance of anorexigenic neuropeptides pomca1 and cartpt and a decrease in the values of orexigenic peptides npy and agrp1 These changes occurred in parallel with those observed in the mRNA abundance and/or protein levels of the transcription factors Creb, Bsx and FoxO1, protein levels and phosphorylation status of Ampkα and Akt, and mRNA abundance of plcb1 and itrp3 Finally, we assessed in a third experiment the response of all these parameters after 2 h of i.c.v. treatment with oleate (the endogenous ligand of both free fatty acid receptors) alone or in the presence of FFA1 and FFA4 antagonists. Most effects of oleate disappeared in the presence of FFA1 and FFA4 antagonists. The evidence obtained supports the involvement of FFA1 and FFA4 in fatty acid sensing in fish brain, and thus involvement in food intake regulation through mechanisms not exactly comparable (differential response of neuropeptides and cellular signalling) to those known in mammals.

Nitenpyram disturbs gut microbiota and influences metabolic homeostasis and immunity in honey bee (Apis mellifera L.)

Recently, environmental risk and toxicity of neonicotinoid insecticides to honey bees have attracted extensive attention. However, toxicological understanding of neonicotinoid insecticides on gut microbiota is limited. In the present study, honey bees (Apis mellifera L.) were exposed to a series of nitenpyram for 14 days. Results indicated that nitenpyram exposure decreased the survival and food consumption of honey bees. Furthermore, 16S rRNA gene sequencing revealed that nitenpyram caused significant alterations in the relative abundance of several key gut microbiotas, which contribute to metabolic homeostasis and immunity. Using high-throughput RNA-Seq transcriptomic analysis, we identified a total of 526 differentially expressed genes (DEGs) that were significantly altered between nitenpyram-treated and control honey bee gut, including several genes related to metabolic, detoxification and immunity. In addition, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed nitenpyram affected several biological processes, of which most were related to metabolism. Collectively, our study demonstrates that the dysbiosis of gut microbiota in honey bee caused by nitenpyram may influence metabolic homeostasis and immunity of bees, and further decrease food consumption and survival of bees.

A Neural basis for Octanoic acid regulation of energy balance

Objectives

Nutrient sensing by hypothalamic neurons is critical for the regulation of food intake and energy expenditure. We aimed to identify long- and medium-chain fatty acid species transported into the brain, their effects on energy balance, and the mechanisms by which they regulate activity of hypothalamic neurons.

Methods

Simultaneous blood and cerebrospinal fluid (CSF) sampling was undertaken in rats and metabolic analyses using radiolabeled fatty acid tracers were performed on mice. Electrophysiological recording techniques were used to investigate signaling mechanisms underlying fatty acid-induced changes in activity of pro-opiomelanocortin (POMC) neurons.

Results

Medium-chain fatty acid (MCFA) octanoic acid (C8:0), unlike long-chain fatty acids, was rapidly transported into the hypothalamus of mice and almost exclusively oxidized, causing rapid, transient reductions in food intake and increased energy expenditure. Octanoic acid differentially regulates the excitability of POMC neurons, activating these neurons directly via GPR40 and inducing inhibition via an indirect non-synaptic, purine, and adenosine receptor-dependent mechanism.

Conclusions

MCFA octanoic acid is a central signaling nutrient that targets POMC neurons via distinct direct and indirect signal transduction pathways to instigate changes in energy status. These results could explain the beneficial health effects that accompany MCFA consumption.

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Octanoic acid (C8:0) is rapidly transported from blood to the cerebrospinal fluid.

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Octanoic acid rapidly reduces food intake and increases energy expenditure.

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Octanoic acid targets POMC neurons through direct and indirect signaling pathways.

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Activation of POMC neurons occurs directly through GPR40.

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Inhibition occurs through a nonsynaptic, purine and adenosine receptor-dependent mechanism.

Neurophysiological and cognitive changes in pregnancy

The hormonal fluctuations in pregnancy drive a wide range of adaptive changes in the maternal brain. These range from specific neurophysiological changes in the patterns of activity of individual neuronal populations, through to complete modification of circuit characteristics leading to fundamental changes in behavior. From a neurologic perspective, the key hormone changes are those of the sex steroids, estradiol and progesterone, secreted first from the ovary and then from the placenta, the adrenal glucocorticoid cortisol, as well as the anterior pituitary peptide hormone prolactin and its pregnancy-specific homolog placental lactogen. All of these hormones are markedly elevated during pregnancy and cross the blood-brain barrier to exert actions on neuronal populations through receptors expressed in specific regions. Many of the hormone-induced changes are in autonomic or homeostatic systems. For example, patterns of oxytocin and prolactin secretion are dramatically altered to support novel physiological functions. Appetite is increased and feedback responses to metabolic hormones such as leptin and insulin are suppressed to promote a positive energy balance. Fundamental physiological systems such as glucose homeostasis and thermoregulation are modified to optimize conditions for fetal development. In addition to these largely autonomic changes, there are also changes in mood, behavior, and higher processes such as cognition. This chapter summarizes the hormonal changes associated with pregnancy and reviews how these changes impact on brain function, drawing on examples from animal research, as well as available information about human pregnancy.

Effect of long-chain saturated and unsaturated fatty acids on hypothalamic fatty acid sensing in Chinese perch (Siniperca chuatsi)

In order to evaluate fatty acid (FA) sensing systems based on binding to FAT/CD36 in hypothalamus of Chinese perch (Siniperca chuatsi) and its sensitivity to FAs with the same chain length and different unsaturation levels. The effects of Stearate (SA; C18:0), oleate (OA; C18:1 n-9), linoleic acid (LA; C18:2 n-6), and α-linolenic acid (ALA; C18:3 n-3) on hypothalamic FA sensing were evaluated by intracerebroventricular (ICV) administration. Food intake was assessed after 2, 4, 6, 8 and 12 h. Gene expression associated with FA sensing mechanism such as cd36, pparα and srebp1c, and neuropeptides controlling appetite such as pomca, cart, agrp2 and npy were assessed after 6 h. The ICV treatment of OA, LA and ALA activated FAT/CD36 and PPARα, rather than SA, and modulated gene expression levels of hypothalamic neuropeptides associated with appetite. And then, OA, LA and ALA inhibited food intake, which was consistent with the activation of hypothalamus FA sensing. Our data indicated some mechanisms of the hypothalamic FA sensing systems also existed in Chinese perch. It's worth noting that polyunsaturated fatty acids (PUFA) could also activate hypothalamic FA sensing mechanisms in Chinese perch. The unsaturation of FA appears to be extremely important for FA sensing mechanisms, since no major influences in Chinese perch after SA treatment. Our findings will contribute to the study of long-chain FAs sensing mechanisms in fish hypothalamus and highlight the importance of PUFAs in fish species.

Synthesis of new Carnitine Palmitoyltransferase I inhibitors derivatives of C75

Carnitine Palmitoyltransferase (CPT1) is an enzyme that catalyzes the transport of fatty acids from the cytosol into the mitochondria. CPT1 inhibition in the hypothalamus increases fatty acid levels, which produces an increased expression of anorexigenic neuropeptides, a sign of satiety. C75 acts as an antiobesity predrug. In vivo C75, is converted into C75-CoA adduct, which is a potent inhibitor of CPT1 and produces a loss of appetite and body weight. In this work, we present three new derivatives of C75, where the carboxylic group is replaced by a carnitine unit, malonic group, and a hydroxyl group with changes from trans to cis relative stereochemistry. Our results suggest that introducing a bigger group than carboxylic in β position or cis relative configuration of the lactone leads to a decrease of CPT1 inhibitory activity.

iTRAQ-Based Proteomics to Reveal the Mechanism of Hypothalamus in Kidney-Yin Deficiency Rats Induced by Levothyroxine

Kidney-yin deficiency syndrome (KYDS) is a typical syndrome encountered in traditional Chinese medicine (TCM) and is characterized by impaired lipid and glucose homeostasis. The hypothalamus acts as an important regulatory organ by controlling lipid and glucose metabolism in the body. Therefore, proteins in the hypothalamus could play important roles in KYDS development; however, the mechanisms responsible for KYDS remain unclear. Herein, iTRAQ-based proteomics was performed to analyze the protein expression in the hypothalamus of KYDS rats induced by levothyroxine (L-T₄). Results revealed a total of 44 downregulated and 18 upregulated proteins in KYDS group relative to the control group. Gene Ontology (GO) analysis revealed that the differently expressed proteins (DEPs) were related to single-organism metabolism process under the biological process (BP), extracellular region part and organelle under the cellular component (CC), and oxidoreductase activity under the molecular function (MF). Kyoto Encyclopedia of Gene and Genomes (KEGG) analysis showed that fatty acid degradation and pyruvate metabolism participated in the metabolism regulation in KYDS rats. RT-PCR validation of five distinctly expressed proteins related to the two pathways was consistent with the results of proteomics analysis. Taken together, the inhibition of fatty acid degradation and pyruvate metabolism in hypothalamus could potentially cause the dysfunction of the lipid and glucose metabolism in KYDS rats. This current study identified some novel potential biomarkers of KYDS and provided the basis for further research of KYDS.

Ghrelin Causes a Decline in GABA Release by Reducing Fatty Acid Oxidation in Cortex

Lipid metabolism, specifically fatty acid oxidation (FAO) mediated by carnitine palmitoyltransferase (CPT) 1A, has been described to be an important actor of ghrelin action in hypothalamus. However, it is not known whether CPT1A and FAO mediate the effect of ghrelin on the cortex. Here, we show that ghrelin produces a differential effect on CPT1 activity and γ-aminobutyric acid (GABA) metabolism in the hypothalamus and cortex of mice. In the hypothalamus, ghrelin enhances CPT1A activity while GABA transaminase (GABAT) activity, a key enzyme in GABA shunt metabolism, is unaltered. However, in cortex CPT1A activity and GABAT activity are reduced after ghrelin treatment. Furthermore, in primary cortical neurons, ghrelin reduces GABA release through a CPT1A reduction. By using CPT1A floxed mice, we have observed that genetic ablation of CPT1A recapitulates the effect of ghrelin on GABA release in cortical neurons, inducing reductions in mitochondrial oxygen consumption, cell content of citrate and α-ketoglutarate, and GABA shunt enzyme activity. Taken together, these observations indicate that ghrelin-induced changes in CPT1A activity modulate mitochondrial function, yielding changes in GABA metabolism. This evidence suggests that the action of ghrelin on GABA release is region specific within the brain, providing a basis for differential effects of ghrelin in the central nervous system.

Impact of estrogens and estrogen receptor-α in brain lipid metabolism

Estrogens and their receptors play key roles in regulating body weight, energy expenditure, and metabolic homeostasis. It is known that lack of estrogens promotes increased food intake and induces the expansion of adipose tissues, for which much is known. An area of estrogenic research that has received less attention is the role of estrogens and their receptors in influencing intermediary lipid metabolism in organs such as the brain. In this review, we highlight the actions of estrogens and their receptors in regulating their impact on modulating fatty acid content, utilization, and oxidation through their direct impact on intracellular signaling cascades within the central nervous system.

Central regulation of food intake in fish: an evolutionary perspective

Evidence indicates that central regulation of food intake is well conserved along the vertebrate lineage, at least between teleost fish and mammals. However, several differences arise in the comparison between both groups. In this review, we describe similarities and differences between teleost fish and mammals on an evolutionary perspective. We focussed on the existing knowledge of specific fish features conditioning food intake, anatomical homologies and analogies between both groups as well as the main signalling pathways of neuroendocrine and metabolic nature involved in the homeostatic and hedonic central regulation of food intake.

Astrocytic metabolic switch is a novel etiology for Cocaine and HIV-1 Tat-mediated neurotoxicity

Calcium (Ca2+) dynamics and oxidative signaling control mitochondrial bioenergetics in the central nervous system, where astrocytes are a major energy source for neurons. Cocaine use exacerbates HIV-associated neurocognitive disorders, but little is known about disruptions in astrocyte metabolism in this context. Our data show that the HIV protein Tat and cocaine induce a metabolic switch from glucose to fatty acid oxidation in astrocytes, thereby limiting lactate transport to neurons. Mechanistic analyses revealed increased Mitochondrial Ca2+ Uniporter (MCU)-mediated Ca2+ uptake in astrocytes exposed to Tat and cocaine due to oxidation of MCU. Since our data suggest that mitochondrial oxidation is dependent in part on MCU-mediated Ca2+ uptake, we targeted MCU to restore glycolysis in astrocytes to normalize extracellular lactate levels. Knocking down MCU in astrocytes prior to Tat and cocaine exposure prevented metabolic switching and protected neurons. These findings identify a novel molecular mechanism underlying neuropathogenesis in HIV and cocaine use.

The effect of fasting on the appetite-associated factors and energy sensors expression in the hypothalamus of different TI broilers

Tonic immobility (TI) is a behaviour related to fear and stress response. Birds can exhibit a short (STI) or long (LTI) tonic immobility phenotype on the basis in TI duration. In this study, the differences in the hypothalamic appetite-associated factors and energy sensor gene expression between STI and LTI broilers were evaluated under free feed access or 16-h fasting. The results showed that the concentrations of cholesterol, high density lipoprotein-cholesterol, non-esterified fatty acid, malonaldehyde, lactate dehydrogenase, creatine kinase, alanine aminotransferase and aspartate aminotransferase in plasma were significantly increased (P < 0.05) in fasting broilers compared with broilers fed ad libitum, whereas plasma glucose, triglyceride, and total antioxidant capacity concentrations were decreased (P < 0.05). With respect to TI, however, only low density lipoprotein-cholesterol and glutathione peroxidase concentrations in plasma showed significant differences between LTI and STI broilers, with higher concentrations in LTI compared with STI. Real-time PCR results showed that only NPY mRNA expression demonstrated a tendency to increase in STI broilers compared with LTI (P = 0.095). Fasting downregulated SREBP-1 and its target gene FAS but upregulated CPT1 in the hypothalamus. Additionally, levels of hypothalamic p-GR and p-AMPK protein expression decreased after fasting. These results indicate that a 16-h fasting results in a negative energy status, and is accompanied with changes in expression of hypothalamic energy sensor and appetite-associated factors.

Palmitate Induces an Anti-Inflammatory Response in Immortalized Microglial BV-2 and IMG Cell Lines that Decreases TNFα Levels in mHypoE-46 Hypothalamic Neurons in Co-Culture

BACKGROUND AND OBJECTIVES

Elevated levels of saturated fatty acids (SFA) induce a state of neuroinflammation in the hypothalamus. It has been suggested that microglia sense palmitate, a prevalent circulating SFA, and act as mediators of this inflammatory process by communicating with neurons, particularly those involved in appetite regulation. In this study, we examined the inflammatory response to palmitate in immortalized microglial cell lines, BV-2 and IMG, and the subsequent effects on inflammatory gene expression in a model of NPY/AgRP neurons, mHypoE-46.

METHODS

The BV-2 cells were treated with 50 µM palmitate for 4 and 24 h, and the transcriptional regulation of markers for inflammation and cellular stress was assessed using an RT2 Profiler PCR Array. Select genes were verified with qRT-PCR. The BV-2 and IMG cells were then co-cultured using 1.0-µm cell culture inserts with an immortalized hypothalamic cell line, mHypoE-46, to investigate potential intercellular communication between microglia and neurons.

RESULTS

We found that palmitate increased the mRNA levels of specific inflammatory genes, and a general anti-inflammatory profile was revealed in the microglia cells. The mRNA changes in TNFα at 4 and 24 h in BV-2 cells were abrogated with the toll-like receptor 4 (TLR4) inhibitor, TAK-242, indicating the involvement of TLR4. Co-culture of mHypoE-46 neurons with microglia pre-treated with palmitate resulted in repression of TNFα expression in the hypothalamic neurons. As palmitate significantly increased IL-13 expression in microglia, the effect of this cytokine was tested in mHypoE-46 neurons. The addition of IL-13 to neuronal cultures normalized the palmitate-mediated increase in IL-6 and AgRP expression, suggesting that microglia may protect surrounding neurons, at least in part, through the release of IL-13.

CONCLUSIONS

These results suggest a potential anti-inflammatory role of microglia towards the palmitate-induced neuroinflammation, and potentially energy homeostasis, in hypothalamic neurons.

Brain Ceramide Metabolism in the Control of Energy Balance

The regulation of energy balance by the central nervous system (CNS) is a key actor of energy homeostasis in mammals, and deregulations of the fine mechanisms of nutrient sensing in the brain could lead to several metabolic diseases such as obesity and type 2 diabetes (T2D). Indeed, while neuronal activity primarily relies on glucose (lactate, pyruvate), the brain expresses at high level enzymes responsible for the transport, utilization and storage of lipids. It has been demonstrated that discrete neuronal networks in the hypothalamus have the ability to detect variation of circulating long chain fatty acids (FA) to regulate food intake and peripheral glucose metabolism. During a chronic lipid excess situation, this physiological lipid sensing is impaired contributing to type 2 diabetes in predisposed subjects. Recently, different studies suggested that ceramides levels could be involved in the regulation of energy balance in both hypothalamic and extra-hypothalamic areas. Moreover, under lipotoxic conditions, these ceramides could play a role in the dysregulation of glucose homeostasis. In this review we aimed at describing the potential role of ceramides metabolism in the brain in the physiological and pathophysiological control of energy balance.

Disruption of Lipid Uptake in Astroglia Exacerbates Diet-Induced Obesity

Neuronal circuits in the brain help to control feeding behavior and systemic metabolism in response to afferent nutrient and hormonal signals. Although astrocytes have historically been assumed to have little relevance for such neuroendocrine control, we investigated whether lipid uptake via lipoprotein lipase (LPL) in astrocytes is required to centrally regulate energy homeostasis. Ex vivo studies with hypothalamus-derived astrocytes showed that LPL expression is upregulated by oleic acid, whereas it is decreased in response to palmitic acid or triglycerides. Likewise, astrocytic LPL deletion reduced the accumulation of lipid droplets in those glial cells. Consecutive in vivo studies showed that postnatal ablation of LPL in glial fibrillary acidic protein-expressing astrocytes induced exaggerated body weight gain and glucose intolerance in mice exposed to a high-fat diet. Intriguingly, astrocytic LPL deficiency also triggered increased ceramide content in the hypothalamus, which may contribute to hypothalamic insulin resistance. We conclude that hypothalamic LPL functions in astrocytes to ensure appropriately balanced nutrient sensing, ceramide distribution, body weight regulation, and glucose metabolism.

Hypothalamic pathways regulate the anorectic action of p-chloro-diphenyl diselenide in rats

Behavioral studies have suggested that (p-ClPhSe)₂ elicits an anorectic-like action in rats by inducing multiple effects such as satiety-enhancing effect, malaise and specific flavor; however, the molecular mechanisms underlying its anorexigenic action remain unclarified. Here, male Sprague-Dawley rats received acute and sub-chronic intraperitoneal treatments with (p-ClPhSe)₂; thereafter, in vivo and ex vivo analyses were carried out. The present study reveals that the reduction of food intake resulting from a single treatment with (p-ClPhSe)₂ (1mg/kg, i.p.) was associated with decreased hypothalamic levels of pro-melanin-concentrating hormone (pro-MCH) and orexin precursor. In addition, repeated administrations of (p-ClPhSe)₂ (10mg/kg; i.p.) for 7 days induced sustained food intake suppression, body weight loss and white fat reduction. Measurements of brown adipose tissue content and temperature as well as data obtained from a pair-fed group indicated that the effects of (p-ClPhSe)₂ on the body weight are closely related to its anorexigenic actions, ruling out the possibility of increased thermogenesis. Furthermore, (p-ClPhSe)₂ reduced the hypothalamic orexin precursor levels when repeatedly administered to rats. Sub-chronic treatment with (p-ClPhSe)₂ caused a decrease of serum triglyceride levels and down-regulation of hepatic cholesterol content. Therefore, the current study characterized the anorectic and reducing body weight actions of (p-ClPhSe)₂ in Sprague-Dawley rats. Besides, the set of results suggests that food intake suppressant effects triggered after (p-ClPhSe)₂ administration to rats are mainly related with the lower orexin levels in hypothalamus after acute and sub-chronic treatments.

Molecular mechanisms of appetite and obesity: a role for brain AMPK

Feeding behaviour and energy storage are both crucial aspects of survival. Thus, it is of fundamental importance to understand the molecular mechanisms regulating these basic processes. The AMP-activated protein kinase (AMPK) has been revealed as one of the key molecules modulating energy homoeostasis. Indeed, AMPK appears to be essential for translating nutritional and energy requirements into generation of an adequate neuronal response, particularly in two areas of the brain, the hypothalamus and the hindbrain. Failure of this physiological response can lead to energy imbalance, ultimately with extreme consequences, such as leanness or obesity. Here, we will review the data that put brain AMPK in the spotlight as a regulator of appetite.

Hypothalamic Integration of Metabolic, Endocrine, and Circadian Signals in Fish: Involvement in the Control of Food Intake

The regulation of food intake in fish is a complex process carried out through several different mechanisms in the central nervous system (CNS) with hypothalamus being the main regulatory center. As in mammals, a complex hypothalamic circuit including two populations of neurons: one co-expressing neuropeptide Y (NPY) and Agouti-related peptide (AgRP) and the second one population co-expressing pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) is involved in the integration of information relating to food intake control. The production and release of these peptides control food intake, and the production results from the integration of information of different nature such as levels of nutrients and hormones as well as circadian signals. The present review summarizes the knowledge and recent findings about the presence and functioning of these mechanisms in fish and their differences vs. the known mammalian model.

Involvement of CD36 in Modulating the Decrease of NPY and AgRP Induced by Acute Palmitic Acid Stimulation in N1E-115 Cells

Central nervous system (CNS) fatty acid sensing plays an important role in the regulation of food intake, and palmitic acid (PA) is the most important long chain fatty acid (LCFA) in the mammalian diet. To explore the effect of PA on central neuropeptide expression and the role of the cluster of the differentiation of 36 (CD36) in the process, N1E-115 cells were cultured with PA in the presence or absence of sulfosuccinimidyl-oleate (SSO), a CD36 inhibitor. Results showed that 10 μmol/L PA significantly reduced NPY and AgRP mRNA expression after 20 min of exposure, while the expression of CD36 was upregulated. The presence of SSO significantly attenuated the decrease of NPY and AgRP expression that was induced by PA alone, although no notable effect on PA- induced CD36 gene expression was observed. In conclusion, our study suggests the involvement of CD36 in the PA-induced decrease of NPY and AgRP in N1E-115 cells.

(-)-UB006: A new fatty acid synthase inhibitor and cytotoxic agent without anorexic side effects

C75 is a synthetic anticancer drug that inhibits fatty acid synthase (FAS) and shows a potent anorexigenic side effect. In order to find new cytotoxic compounds that do not impact food intake, we synthesized a new family of C75 derivatives. The most promising anticancer compound among them was UB006 ((4SR,5SR)-4-(hydroxymethyl)-3-methylene-5-octyldihydrofuran-2(3H)-one). The effects of this compound on cytotoxicity, food intake and body weight were studied in UB006 racemic mixture and in both its enantiomers separately. The results showed that both enantiomers inhibit FAS activity and have potent cytotoxic effects in several tumour cell lines, such as the ovarian cell cancer line OVCAR-3. The (-)-UB006 enantiomer's cytotoxic effect on OVCAR-3 was 40-fold higher than that of racemic C75, and 2- and 38-fold higher than that of the racemic mixture and its opposite enantiomer, respectively. This cytotoxic effect on the OVCAR-3 cell line involves mechanisms that reduce mitochondrial respiratory capacity and ATP production, DDIT4/REDD1 upregulation, mTOR activity inhibition, and caspase-3 activation, resulting in apoptosis. In addition, central and peripheral administration of (+)-UB006 or (-)-UB006 into rats and mice did not affect food intake or body weight. Altogether, our data support the discovery of a new potential anticancer compound (-)-UB006 that has no anorexigenic side effects.

Nutrient Sensing Systems in Fish: Impact on Food Intake Regulation and Energy Homeostasis

Evidence obtained in recent years in a few species, especially rainbow trout, supports the presence in fish of nutrient sensing mechanisms. Glucosensing capacity is present in central (hypothalamus and hindbrain) and peripheral [liver, Brockmann bodies (BB, main accumulation of pancreatic endocrine cells in several fish species), and intestine] locations whereas fatty acid sensors seem to be present in hypothalamus, liver and BB. Glucose and fatty acid sensing capacities relate to food intake regulation and metabolism in fish. Hypothalamus is as a signaling integratory center in a way that detection of increased levels of nutrients result in food intake inhibition through changes in the expression of anorexigenic and orexigenic neuropeptides. Moreover, central nutrient sensing modulates functions in the periphery since they elicit changes in hepatic metabolism as well as in hormone secretion to counter-regulate changes in nutrient levels detected in the CNS. At peripheral level, the direct nutrient detection in liver has a crucial role in homeostatic control of glucose and fatty acid whereas in BB and intestine nutrient sensing is probably involved in regulation of hormone secretion from endocrine cells.

Eat and Death: Chronic Over-Eating

Obesity-related co-morbidities decrease life quality, reduce working ability and lead to early death. The total amount of dietary fat consumption may be the most potent food-related risk factor for weight gain. In this respect, dietary intake of high-caloric, high-fat diets due to chronic over-eating and sedentary lifestyle lead to increased storage of triglycerides not only in adipose tissue but also ectopically in other tissues . Increased plasma concentrations of non-esterified free fatty acids and lipid-overloaded hypertrophic adipocytes may cause insulin resistance in an inflammation-independent manner. Even in the absence of metabolic disorders, mismatch between fatty acid uptake and utilization leads to the accumulation of toxic lipid species resulting in organ dysfunction. Lipid-induced apoptosis, ceramide accumulation, reactive oxygen species overproduction, endoplasmic reticulum stress, and mitochondrial dysfunction may play role in the pathogenesis of lipotoxicity. The hypothalamus senses availability of circulating levels of glucose, lipids and amino acids, thereby modifies feeding according to the levels of those molecules. However, the hypothalamus is also similarly vulnerable to lipotoxicity as the other ectopic lipid accumulated tissues. Chronic overnutrition most likely provides repetitive and persistent signals that up-regulate inhibitor of nuclear factor kappa B kinase beta subunit/nuclear factor kappa B (IKKβ/NF-κB) in the hypothalamus before the onset of obesity. However, the mechanisms by which high-fat diet induced peripheral signals affect the hypothalamic arcuate nucleus remain largely unknown. In this chapter, besides lipids and leptin, the role of glucose and insulin on specialized fuel-sensing neurons of hypothalamic neuronal circuits has been debated.

Fatty Acid Regulation of Voltage- and Ligand-Gated Ion Channel Function

Free fatty acids (FFA) are essential components of the cell, where they play a key role in lipid and carbohydrate metabolism, and most particularly in cell membranes, where they are central actors in shaping the physicochemical properties of the lipid bilayer and the cellular adaptation to the environment. FFA are continuously being produced and degraded, and a feedback regulatory function has been attributed to their turnover. The massive increase observed under some pathological conditions, especially in brain, has been interpreted as a protective mechanism possibly operative on ion channels, which in some cases is of stimulatory nature and in other cases inhibitory. Here we discuss the correlation between the structure of FFA and their ability to modulate protein function, evaluating the influence of saturation/unsaturation, number of double bonds, and cis vs. trans isomerism. We further focus on the mechanisms of FFA modulation operating on voltage-gated and ligand-gated ion channel function, contrasting the still conflicting evidence on direct vs. indirect mechanisms of action.

Ceramides are involved in the regulation of food intake in rainbow trout (Oncorhynchus mykiss)

We hypothesize that ceramides are involved in the regulation of food intake in fish. Therefore, we assessed in rainbow trout (Oncorhynchus mykiss) the effects of intracerebroventricular treatment with C6:0 ceramide on food intake. In a second experiment, we assessed the effects in brain areas of ceramide treatment on neuropeptide expression, fatty acid-sensing systems, and cellular signaling pathways. Ceramide treatment induced a decrease in food intake, a response opposed to the orexigenic effect described in mammals, which can be related to enhanced mRNA abundance of cocaine and amphetamine-related transcript and proopiomelanocortin and decreased mRNA abundance of Agouti-related protein and neuropeptide Y. Fatty acid-sensing systems appear to be inactivated by ceramide treatment. The mRNA abundance of integrative sensors AMPK and sirtuin 1, and the phosphorylation status of cellular signaling pathways dependent on protein kinase B, AMPK, mammalian target of rapamycin (mTOR), and forkhead box protein O1 (FoxO1) are generally activated by ceramide treatment. However, there are differences between hypothalamus and hindbrain in the phosphorylation status of AMPK (decreased in hypothalamus and increased in hindbrain), mTOR (decreased in hypothalamus and increased in hindbrain), and FoxO1 (increased in hypothalamus and decreased in hindbrain) to ceramide treatment. The results suggest that ceramides are involved in the regulation of food intake in rainbow trout through mechanisms comparable to those characterized previously in mammals in some cases.

Hypothalamic AMPK: a canonical regulator of whole-body energy balance

AMP-activated protein kinase (AMPK) has a major role in the modulation of energy balance. AMPK is activated in conditions of low energy, increasing energy production and reducing energy consumption. The AMPK pathway is a canonical route regulating energy homeostasis by integrating peripheral signals, such as hormones and metabolites, with neuronal networks. Current evidence has implicated AMPK in the hypothalamus and hindbrain with feeding, brown adipose tissue thermogenesis and browning of white adipose tissue, through modulation of the sympathetic nervous system, as well as glucose homeostasis. Interestingly, several potential antiobesity and/or antidiabetic agents, some of which are currently in clinical use such as metformin and liraglutide, exert some of their actions by acting on AMPK. Furthermore, the orexigenic and weight-gain effects of commonly used antipsychotic drugs are also mediated by hypothalamic AMPK. Overall, this evidence suggests that hypothalamic AMPK signalling is an interesting target for drug development, but is this approach feasible? In this Review we discuss the current understanding of hypothalamic AMPK and its role in the central regulation of energy balance and metabolism.

Sexually dimorphic brain fatty acid composition in low and high fat diet-fed mice

OBJECTIVE

In this study, we analyzed the fatty acid profile of brains and plasma from male and female mice fed chow or a western-style high fat diet (WD) for 16 weeks to determine if males and females process fatty acids differently. Based on the differences in fatty acids observed in vivo, we performed in vitro experiments on N43 hypothalamic neuronal cells to begin to elucidate how the fatty acid milieu may impact brain inflammation.

METHODS

Using a comprehensive mass spectrometry fatty acid analysis, which includes a profile for 52 different fatty acid isomers, we assayed the plasma and brain fatty acid composition of age-matched male and female mice maintained on chow or a WD. Additionally, using the same techniques, we determined the fatty acid composition of N43 hypothalamic cells following exposure to palmitic and linoleic acid, alone or in combination.

RESULTS

Our data demonstrate there is a sexual dimorphism in brain fatty acid content both following the consumption of the chow diet, as well as the WD, with males having an increased percentage of saturated fatty acids and reductions in ω6-polyunsaturated fatty acids when compared to females. Interestingly, we did not observe a sexual dimorphism in fatty acid content in the plasma of the same mice. Furthermore, exposure of N43 cells to the ω6-PUFA linoleic acid, which is higher in female brains when compared to males, reduces palmitic acid-induced inflammation.

CONCLUSIONS

Our data suggest male and female brains, and not plasma, differ in their fatty acid profile. This is the first time, to our knowledge, lipidomic analyses has been used to directly test the hypothesis there is a sexual dimorphism in brain and plasma fatty acid composition following consumption of the chow diet, as well as following exposure to the WD.

Intracerebroventricular ghrelin treatment affects lipid metabolism in liver of rainbow trout (Oncorhynchus mykiss)

We aimed to elucidate in rainbow trout (Oncorhynchus mykiss) the effects of central ghrelin (GHRL) treatment on the regulation of liver lipid metabolism, and the possible modulatory effect of central GHRL treatment on the simultaneous effects of raised levels of oleate. Thus, we injected intracerebroventricularly (ICV) rainbow trout GHRL in the presence or absence of oleate and evaluated in liver variables related to lipid metabolism. Oleate treatment elicited in liver of rainbow trout decreased lipogenesis and increased oxidative capacity in agreement with previous studies. Moreover, as demonstrated for the first time in fish in the present study, GHRL also acts centrally modulating lipid metabolism in liver, resulting in increased potential for lipogenesis and decreased potential for fatty acid oxidation, i.e. the converse effects to those elicited by central oleate treatment. The simultaneous treatment of GHRL and oleate confirmed these counteractive effects. Thus, the nutrient sensing mechanisms present in hypothalamus, particularly those involved in sensing of fatty acid, are involved in the control of liver energy metabolism in fish, and this control is modulated by the central action of GHRL. These results give support to the notion of hypothalamus as an integrative place for the regulation of peripheral energy metabolism in fish.

Ghrelin modulates hypothalamic fatty acid-sensing and control of food intake in rainbow trout

There is no information available on fish as far as the possible effects of ghrelin on hypothalamic fatty acid metabolism and the response of fatty acid-sensing systems, which are involved in the control of food intake. Therefore, we assessed in rainbow trout the response of food intake, hypothalamic fatty acid-sensing mechanisms and expression of neuropeptides involved in the control of food intake to the central treatment of ghrelin in the presence or absence of a long-chain fatty acid such as oleate. We observed that the orexigenic actions of ghrelin in rainbow trout are associated with changes in fatty acid metabolism in the hypothalamus and an inhibition of fatty acid-sensing mechanisms, which ultimately lead to changes in the expression of anorexigenic and orexigenic peptides resulting in increased orexigenic potential and food intake. Moreover, the response to increased levels of oleate of hypothalamic fatty acid-sensing systems (activation), expression of neuropeptides (enhanced anorexigenic potential) and food intake (decrease) were counteracted by the simultaneous treatment with ghrelin. These changes provide evidence for the first time in fish of a possible modulatory role of ghrelin on the metabolic regulation by fatty acid of food intake occurring in the hypothalamus.

Extrinsic ghrelin in the paraventricular nucleus increases small intestinal motility in rats by activating central growth hormone secretagogue and enteric cholinergic receptors

BACKGROUND/OBJECTIVES

Ghrelin is a brain-gut peptide that regulates gastrointestinal (GI) motility. We hypothesized that the excitatory effect of ghrelin on the paraventricular nucleus (PVN) increases GI motility by activating the central growth hormone secretagogue receptor (GHSR) and central neuropeptide Y (NPY) signaling pathways, leading to increased enteric cholinergic activity.

METHODS

Thirty-six male Sprague Dawley rats were maintained on duodenal catheterization and PVN cannulation. Small intestinal transit (SIT) was observed and rats were divided as follows: experimental animals received ghrelin injections in the PVN (0.03, 0.08, or 0.24 nM); 1 nM GHSR antagonist D-Lys3-GHRP6 alone; 1nM D-Lys3-GHRP6 before ghrelin injection in the PVN, respectively. Electrophysiologic parameters of the interdigestive myoelectric complex (IMC) were examined by administration of 0.24 nM ghrelin in the PVN after small intestinal electrode implantation and PVN cannulation. GI cholinergic pathway activation was analyzed after intravenous atropine administration. The involvement of central NPY signaling was evaluated by injecting an anti-NPY immunoglobulin (IgG) in the PVN. Neuronal expression of c-Fos in the brain and GI tract was examined using immunohistochemistry.

RESULTS

Injection of ghrelin in the PVN dose-dependently accelerated SIT, and this excitatory effect was competitively inhibited by a GHSR antagonist. The excitatory effect of ghrelin on IMC activity was diminished by GHSR antagonism and NPY neutralization, as well as by blockade of peripheral muscarinic acetylcholine receptors. Extrinsic ghrelin significantly upregulated c-Fos expression in the PVN and other central nuclei, as well as in the enteric nervous plexuses of the stomach, duodenum, and proximal colon. The ghrelin-induced upregulation of central and enteric c-Fos expression was also dependent on central GHSR activation.

CONCLUSIONS

Ghrelin positively regulates GI motility by exciting both central and enteric neurons, including those of the PVN, by activating GHSR and NPY pathways, and peripheral muscarinic acetylcholine receptors.

Hypothalamic-autonomic control of energy homeostasis

Regulation of energy homeostasis is tightly controlled by the central nervous system (CNS). Several key areas such as the hypothalamus and brainstem receive and integrate signals conveying energy status from the periphery, such as leptin, thyroid hormones, and insulin, ultimately leading to modulation of food intake, energy expenditure (EE), and peripheral metabolism. The autonomic nervous system (ANS) plays a key role in the response to such signals, innervating peripheral metabolic tissues, including brown and white adipose tissue (BAT and WAT), liver, pancreas, and skeletal muscle. The ANS consists of two parts, the sympathetic and parasympathetic nervous systems (SNS and PSNS). The SNS regulates BAT thermogenesis and EE, controlled by central areas such as the preoptic area (POA) and the ventromedial, dorsomedial, and arcuate hypothalamic nuclei (VMH, DMH, and ARC). The SNS also regulates lipid metabolism in WAT, controlled by the lateral hypothalamic area (LHA), VMH, and ARC. Control of hepatic glucose production and pancreatic insulin secretion also involves the LHA, VMH, and ARC as well as the dorsal vagal complex (DVC), via splanchnic sympathetic and the vagal parasympathetic nerves. Muscle glucose uptake is also controlled by the SNS via hypothalamic nuclei such as the VMH. There is recent evidence of novel pathways connecting the CNS and ANS. These include the hypothalamic AMP-activated protein kinase-SNS-BAT axis which has been demonstrated to be a key modulator of thermogenesis. In this review, we summarize current knowledge of the role of the ANS in the modulation of energy balance.

Hypothalamic fatty acid sensing in Senegalese sole (Solea senegalensis): response to long-chain saturated, monounsaturated, and polyunsaturated (n-3) fatty acids

We assessed the presence of fatty acid (FA)-sensing mechanisms in hypothalamus of Senegalese sole (Solea senegalensis) and investigated their sensitivity to FA chain length and/or level of unsaturation. Stearate (SA, saturated FA), oleate (OA, monounsaturated FA of the same chain length), α-linolenate [ALA, a n-3 polyunsaturated fatty acid (PUFA) of the same chain length], and eicosapentanoate (EPA, a n-3 PUFA of a larger chain length) were injected intraperitoneally. Parameters related to FA sensing and neuropeptide expression in the hypothalamus were assessed after 3 h and changes in accumulated food intake after 4, 24, and 48 h. Three FA sensing systems characterized in rainbow trout were also found in Senegalese sole and were activated by OA in a way similar to that previously characterized in rainbow trout and mammals. These hypothalamic FA sensing systems were also activated by ALA, differing from mammals, where n-3 PUFAs do not seem to activate FA sensors. This might suggest additional roles and highlights the importance of n-3 PUFA in fish diets, especially in marine species. The activation of FA sensing seems to be partially dependent on acyl chain length and degree of saturation, as no major changes were observed after treating fish with SA or EPA. The activation of FA sensing systems by OA and ALA, but not SA or EPA, is further reflected in the expression of hypothalamic neuropeptides involved in the control of food intake. Both OA and ALA enhanced anorexigenic capacity compatible with the activation of FA sensing systems.

Effects of insulin treatment on the response to oleate and octanoate of food intake and fatty acid-sensing systems in rainbow trout

We hypothesized that food intake and the response of fatty acid (FA)-sensing systems in hypothalamus, liver, and Brockmann bodies of rainbow trout to raised levels of oleate (OL) or octanoate (OCT) is modified by insulin treatment. To assess this hypothesis, 15 fish per group received intraperitoneally 10-mL/kg injection of saline solution alone (control), or containing insulin (2-mg bovine insulin/kg body mass), OL (300 μg/kg), OCT (300 μg/kg), insulin + OL, or insulin + OCT to be sampled 6 h later to assess parameters related to FA sensing. Our results suggest that the modulatory role of insulin on the responses of hypothalamic FA-sensing systems to changes in circulating levels of OL or OCT was of minor importance in contrast to the mammalian model. However, this is in contrast with the effects observed in another experiment assessing changes in food intake after similar treatments because insulin treatment enhanced the anorectic effects of FA alone, and the effect was especially relevant (P < 0.001) for OCT, in contrast with the mammalian model where this FA is not inducing an anorectic response. In liver and Brockmann bodies, insulin treatment enhanced the responses to OL or OCT treatment in parameters related to FA sensing. Therefore, we provide for the first time in fish, and in a non-mammalian vertebrate, evidence for the modulation of FA-sensing systems by insulin.