Differential effects of hypothalamic long-chain fatty acid infusions on suppression of hepatic glucose production

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.

Yin-Yang control of energy balance by lipids in the hypothalamus: The endocannabinoids vs bile acids case

Obesity is a chronic and debilitating disorder that originates from alterations in energy-sensing brain circuits controlling body weight gain and food intake. The dysregulated syntheses and actions of lipid mediators in the hypothalamus induce weight gain and overfeeding, but the molecular and cellular underpinnings of these alterations remain elusive. In response to changes in the nutritional status, different lipid sensing pathways in the hypothalamus direct body energy needs in a Yin-Yang model. Endocannabinoids orchestrate the crosstalk between hypothalamic circuits and the sympathetic nervous system to promote food intake and energy accumulation during fasting, whereas bile acids act on the same top-down axis to reduce energy intake and possibly storage after the meal. In obesity, the bioavailability and downstream cellular actions of endocannabinoids and bile acids are altered in hypothalamic neurons involved in body weight and metabolic control. Thus, the onset and progression of this disease might result from an imbalance in hypothalamic sensing of multiple lipid signals, which are possibly integrated by common molecular nodes. In this viewpoint, we discuss a possible model that explains how bile acids and endocannabinoids may exert their effects on energy balance regulation via interconnected mechanisms at the level of the hypothalamic neuronal circuits. Therefore, we propose a new conceptual framework for understanding and treating central mechanisms of maladaptive lipid action in obesity.

Fatty Acid Sensing in the Gastrointestinal Tract of Rainbow Trout: Different to Mammalian Model?

It is well established in mammals that the gastrointestinal tract (GIT) senses the luminal presence of nutrients and responds to such information by releasing signaling molecules that ultimately regulate feeding. However, gut nutrient sensing mechanisms are poorly known in fish. This research characterized fatty acid (FA) sensing mechanisms in the GIT of a fish species with great interest in aquaculture: the rainbow trout (Oncorhynchus mykiss). Main results showed that: (i) the trout GIT has mRNAs encoding numerous key FA transporters characterized in mammals (FA transporter CD36 -FAT/CD36-, FA transport protein 4 -FATP4-, and monocarboxylate transporter isoform-1 -MCT-1-) and receptors (several free FA receptor -Ffar- isoforms, and G protein-coupled receptors 84 and 119 -Gpr84 and Gpr119-), and (ii) intragastrically-administered FAs differing in their length and degree of unsaturation (i.e., medium-chain (octanoate), long-chain (oleate), long-chain polyunsaturated (α-linolenate), and short-chain (butyrate) FAs) exert a differential modulation of the gastrointestinal abundance of mRNAs encoding the identified transporters and receptors and intracellular signaling elements, as well as gastrointestinal appetite-regulatory hormone mRNAs and proteins. Together, results from this study offer the first set of evidence supporting the existence of FA sensing mechanisms n the fish GIT. Additionally, we detected several differences in FA sensing mechanisms of rainbow trout vs. mammals, which may suggest evolutionary divergence between fish and mammals.

Reduced Albumin Concentration Predicts Weight Gain and Higher Ad Libitum Energy Intake in Humans

OBJECTIVE

Circulating albumin is negatively associated with adiposity but whether it is associated with increased energy intake, lower energy expenditure or weight gain has not been examined.

METHODS

In study 1 (n=238; 146 men), we evaluated whether fasting albumin concentration was associated with 24-h energy expenditure and ad libitum energy intake. In study 2 (n=325;167 men), we evaluated the association between plasma albumin and change in weight and body composition.

RESULTS

After adjustment for known determinants of energy intake lower plasma albumin concentration was associated with greater total daily energy intake (β= 89.8 kcal/day per 0.1 g/dl difference in plasma albumin, p=0.0047). No associations were observed between plasma albumin concentrations and 24-h energy expenditure or 24-h respiratory quotient (p>0.2). Over 6 years, volunteers gained on average 7.5 ± 11.7 kg (p<0.0001). Lower albumin concentrations were associated with greater weight [β=3.53 kg, p=0.039 (adjusted for age, sex, follow up time), CI 0.16 to 6.21 per 1 g/dl difference albumin concentration] and fat mass (β=2.3 kg, p=0.022), respectively, but not with changes in fat free mass (p=0.06).

CONCLUSIONS

Lower albumin concentrations were associated with increased ad libitum food intake and weight gain, indicating albumin as a marker of energy intake regulation.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov, identifiers NCT00340132, NCT00342732.

DHA reduces hypothalamic inflammation and improves central leptin signaling in mice

AIMS

Anti-obesity effects and improved leptin sensitivity from n-3 polyunsaturated fatty acids (n-3 PUFAs) have been reported in diet-induced obese animals. This study sought to determine the beneficial central effects and mechanism of docosahexaenoic acid (DHA, 22:6 n-3) in high-fat (HF) diet fed mice.

MAIN METHODS

Male C57BL/6J mice were given HF diet with or without intracerebroventricular (icv) injection of docosahexaenoic acid (DHA, 22:6 n-3) for two days. Central leptin sensitivity, hypothalamic inflammation, leptin signaling molecules and tyrosine hydroxylase (TH) were examined by central leptin sensitivity test and Western blot. Furthermore, the expression of hepatic genes involved in lipid metabolism was examined by RT-PCR.

KEY FINDINGS

We found that icv administration of DHA not only reduced energy intake and body weight gain but also corrected the HF diet-induced hypothalamic inflammation. DHA decreased leptin signaling inhibitor SOCS3 and improved the leptin JAK2-Akt signaling pathways in the hypothalamus. Furthermore, icv administration of DHA improved the effects of leptin in the regulation of mRNA expression of enzymes related to lipogenesis, fatty acid β-oxidation, and cholesterol synthesis in the liver. DHA increased leptin-induced activation of TH in the hypothalamus.

SIGNIFICANCE

Therefore, increasing central DHA concentration may prevent the deficit of hypothalamic regulation, which is associated with disorders of energy homeostasis in the liver as a result of a high-fat diet.

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.

Oleic Acid in the Ventral Tegmental Area Inhibits Feeding, Food Reward, and Dopamine Tone

Long-chain fatty acids (FAs) act centrally to decrease food intake and hepatic glucose production and alter hypothalamic neuronal activity in a manner that depends on FA type and cellular transport proteins. However, it is not known whether FAs are sensed by ventral tegmental area (VTA) dopamine (DA) neurons to control food-motivated behavior and DA neurotransmission. We investigated the impact of the monounsaturated FA oleate in the VTA on feeding, locomotion, food reward, and DA neuronal activity and DA neuron expression of FA-handling proteins and FA uptake. A single intra-VTA injection of oleate, but not of the saturated FA palmitate, decreased food intake and increased locomotor activity. Furthermore, intra-VTA oleate blunted the rewarding effects of high-fat/sugar food in an operant task and inhibited DA neuronal firing. Using sorted DA neuron preparations from TH-eGFP mice we found that DA neurons express FA transporter and binding proteins, and are capable of intracellular transport of long-chain FA. Finally, we demonstrate that a transporter blocker attenuates FA uptake into DA neurons and blocks the effects of intra-VTA oleate to decrease food-seeking and DA neuronal activity. Together, these results suggest that DA neurons detect FA and that oleate has actions in the VTA to suppress DA neuronal activity and food seeking following cellular incorporation. These findings highlight the capacity of DA neurons to act as metabolic sensors by responding not only to hormones but also to FA nutrient signals to modulate food-directed behavior.

Ginsenoside Rb1 improves leptin sensitivity in the prefrontal cortex in obese mice

AIM

Obesity impairs leptin-induced regulation of brain-derived neurotrophic factor (BDNF) expression and synaptogenesis, which has been considered to be associated with the incidence of neuronal degenerative diseases, cognitive decline, and depression. Ginsenoside Rb1 (Rb1), a major bioactive component of ginseng, is known to have an antiobesity effect and improve cognition. This study examined whether Rb1 can improve central leptin effects on BDNF expression and synaptogenesis in the prefrontal cortex during obesity using an in vivo and an in vitro model.

RESULT

Ginsenoside Rb1 (Rb1) chronic treatment improved central leptin sensitivity, leptin-JAK2-STAT3 signaling, and leptin-induced regulation of BDNF expression in the prefrontal cortex of high-fat diet-induced obese mice. In cultured prefrontal cortical neurons, palmitic acid, the saturated fat, impaired leptin-induced BDNF expression, reduced the immunoreactivity and mRNA expression of synaptic proteins, and impaired leptin-induced neurite outgrowth and synaptogenesis. Importantly, Rb1 significantly prevented these pernicious effects induced by palmitic acid.

CONCLUSION

These results indicate that Rb1 reverses central leptin resistance and improves leptin-BDNF-neurite outgrowth and synaptogenesis in the prefrontal cortical neurons. Thus, Rb1 supplementation may be a beneficial avenue to treat obesity-associated neurodegenerative disorders.

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.

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.

Lipid Processing in the Brain: A Key Regulator of Systemic Metabolism

Metabolic disorders, particularly aberrations in lipid homeostasis, such as obesity, type 2 diabetes mellitus, and hypertriglyceridemia often manifest together as the metabolic syndrome (MetS). Despite major advances in our understanding of the pathogenesis of these disorders, the prevalence of the MetS continues to rise. It is becoming increasingly apparent that intermediary metabolism within the central nervous system is a major contributor to the regulation of systemic metabolism. In particular, lipid metabolism within the brain is tightly regulated to maintain neuronal structure and function and may signal nutrient status to modulate metabolism in key peripheral tissues such as the liver. There is now a growing body of evidence to suggest that fatty acid (FA) sensing in hypothalamic neurons via accumulation of FAs or FA metabolites may signal nutritional sufficiency and may decrease hepatic glucose production, lipogenesis, and VLDL-TG secretion. In addition, recent studies have highlighted the existence of liver-related neurons that have the potential to direct such signals through parasympathetic and sympathetic nervous system activity. However, to date whether these liver-related neurons are FA sensitive remain to be determined. The findings discussed in this review underscore the importance of the autonomic nervous system in the regulation of systemic metabolism and highlight the need for further research to determine the key features of FA neurons, which may serve as novel therapeutic targets for the treatment of metabolic disorders.

Knockdown of Neuropeptide Y in the Dorsomedial Hypothalamus Promotes Hepatic Insulin Sensitivity in Male Rats

Recent evidence has shown that alterations in dorsomedial hypothalamic (DMH) neuropeptide Y (NPY) signaling influence glucose homeostasis, but the mechanism through which DMH NPY acts to affect glucose homeostasis remains unclear. Here we report that DMH NPY descending signals to the dorsal motor nucleus of the vagus (DMV) modulate hepatic insulin sensitivity to control hepatic glucose production in rats. Using the hyperinsulinemic-euglycemic clamp, we revealed that knockdown of NPY in the DMH by adeno-associated virus-mediated NPY-specific RNAi promoted insulin's action on suppression of hepatic glucose production. This knockdown silenced DMH NPY descending signals to the DMV, leading to an elevation of hepatic vagal innervation. Hepatic vagotomy abolished the inhibitory effect of DMH NPY knockdown on hepatic glucose production, but this glycemic effect was not affected by vagal deafferentation. Together, these results demonstrate a distinct role for DMH NPY in the regulation of glucose homeostasis through the hepatic vagal efferents and insulin action on hepatic glucose production.

Circulating free fatty acids inhibit food intake in an oleate-specific manner in rats

Previous rodent studies showed that when injected into the brain, free fatty acids (FFAs) reduced food intake in an oleate-specific manner. The present study was performed to test whether food intake is regulated by circulating FFAs in an oleate-specific manner. Male Wistar rats received an intravenous infusion of olive, safflower, or coconut oil (100mg/h), together with heparin, to raise circulating oleate, linoleate, or palmitate, respectively, and their effects on overnight food intake were evaluated. Compared to other oils, olive oil infusion showed a significantly greater effect to reduce food intake (P<0.01). Total caloric intake, the sum of the calories from the diet and infused oil, was significantly reduced with olive oil (P<0.01) but not with coconut or safflower oil infusion, suggesting an oleate-specific effect on caloric intake. To further test this idea, different groups of rats received an intravenous infusion of oleate, linoleate, or octanoate (0.5mg/h). Oleate infusion decreased overnight food intake by 26% (P<0.001), but no significant effect was seen with linoleate, octanoate, or vehicle infusion (P>0.05). The effects of olive oil or oleate infusion could not be explained by changes in plasma glucose, insulin, leptin, or total FFA levels. The olive oil effect on food intake was not reduced in vagotomized rats, suggesting that oleate sensing may not involve peripheral sensors. In contrast, olive oil's effect was attenuated in high-fat-fed rats, suggesting that this effect is regulated (or impaired) under physiological (or pathological) conditions. Taken together, the present study provides evidence that circulating oleate is sensed by the brain differentially from other FFAs to control feeding in rats.

The effect of diet interventions on hypothalamic nutrient sensing pathways in rodents

The hypothalamus plays a fundamental role in regulating homeostatic processes including regulation of food intake. Food intake is driven in part by energy balance, which is sensed by specific brain structures through signaling molecules such as nutrients and hormones. Both circulating glucose and fatty acids decrease food intake via a central mechanism involving the hypothalamus and brain stem. Besides playing a role in signaling energy status, glucose and fatty acids serve as fuel for neurons. This review focuses on the effects of glucose and fatty acids on hypothalamic pathways involved in regulation of energy metabolism as well as on the role of the family of peroxisome proliferator activated receptors (PPARs) which are implicated in regulation of central energy homeostasis. We further discuss the effects of different hypercaloric diets on these pathways.

Physiology of Food Intake Control in Children

The purpose of this review is to draw attention to the limited information available on food intake (FI) control in children and adolescents 7-17 y of age, which is essential for developing food policies and guidelines in this population. Although environmental factors have been the overwhelming focus of research on the causative factors of obesity, research focusing on the physiologic control of appetite in children and adolescents is a neglected area of research. To present this message, a review of FI regulation and the role of food and food components in signaling processes are followed by an examination of the role of hormones during puberty in intake regulation. To examine the interaction of environment and physiology on FI regulation, the effects of exercise, television programs, and food advertisements are discussed. In conclusion, although limited, this literature review supports a need for children and adolescents to be a greater focus of research that would lead to sound nutrition policies and actions to reduce chronic disease. A focus on the environment must be balanced with an understanding of physiologic and behavioral changes associated with this age group.

Brain lipid sensing and the neural control of energy balance

Fatty acid (FA) -sensitive neurons are present in the brain, especially the hypothalamus, and play a key role in the neural control of energy and glucose homeostasis including feeding behavior, secretion insulin and action. Subpopulations of neurons in the arcuate and ventromedial hypothalamic nuclei are selectively either activated or inhibited by FA. Molecular effectors of these FA effects include ion channels such as chloride, potassium or calcium. In addition, at least half of the responses in the hypothalamic ventromedial FA neurons are mediated through interaction with the FA translocator/receptor, FAT/CD36, that does not require metabolism to activate intracellular signaling downstream. Recently, an important role of lipoprotein lipase in FA detection has also been demonstrated not only in the hypothalamus, but also in the hippocampus and striatum. Finally, FA could overload energy homeostasis via increased hypothalamic ceramide synthesis which could, in turn, contribute to the pathogenesis of diabetes of obesity and/or type 2 in predisposed individuals by disrupting the endocrine signaling pathways of insulin and/or leptin.

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.

Palmitic acid induces central leptin resistance and impairs hepatic glucose and lipid metabolism in male mice

The consumption of diets rich in saturated fat largely contributes to the development of obesity in modern societies. A diet high in saturated fats can induce inflammation and impair leptin signaling in the hypothalamus. However, the role of saturated fatty acids on hypothalamic leptin signaling, and hepatic glucose and lipid metabolism remains largely undiscovered. In this study, we investigated the effects of intracerebroventricular (icv) administration of a saturated fatty acid, palmitic acid (PA, C16:0), on central leptin sensitivity, hypothalamic leptin signaling, inflammatory molecules and hepatic energy metabolism in C57BL/6J male mice. We found that the icv administration of PA led to central leptin resistance, evidenced by the inhibition of central leptin's suppression of food intake. Central leptin resistance was concomitant with impaired hypothalamic leptin signaling (JAK2-STAT3, PKB/Akt-FOXO1) and a pro-inflammatory response (TNF-α, IL1-β, IL-6 and pIκBa) in the mediobasal hypothalamus and paraventricular hypothalamic nuclei. Furthermore, the pre-administration of icv PA blunted the effect of leptin-induced decreases in mRNA expression related to gluconeogenesis (G6Pase and PEPCK), glucose transportation (GLUT2) and lipogenesis (FAS and SCD1) in the liver of mice. Therefore, elevated central PA concentrations can induce pro-inflammatory responses and leptin resistance, which are associated with disorders of energy homeostasis in the liver as a result of diet-induced obesity.

[Role of fatty acids in the nervous control of energy balance]

Fatty acid (FA)-sensitive neurons are present in the brain, especially the hypothalamus, and play a key role in the neural control of energy and glucose homeostasis including feeding behavior, insulin secretion and action. Subpopulations of neurons in the ventromedial and arcuate hypothalamic nuclei are selectively either inhibited or activated by FA. Molecular effectors of these FA effects include ion channels such as chloride, potassium or calcium. In addition at least half of the FA responses in ventromedial hypothalamic neurons are mediated by interaction with FAT/CD36, a FA translocator/receptor that does not require intracellular metabolism to activate downstream signaling. Recently, an important role of lipoprotein lipase in FA sensing has also been demonstrated not only in hypothalamus, but also in the hippocampus and striatum. Finally, FA overload might impair neural control of energy homeostasis through enhanced ceramide synthesis and may contribute to obesity and/or type 2 diabetes pathogenesis in predisposed subjects.

Microglia dictate the impact of saturated fat consumption on hypothalamic inflammation and neuronal function

Diets rich in saturated fat produce inflammation, gliosis, and neuronal stress in the mediobasal hypothalamus (MBH). Here, we show that microglia mediate this process and its functional impact. Although microglia and astrocytes accumulate in the MBH of mice fed a diet rich in saturated fatty acids (SFAs), only the microglia undergo inflammatory activation, along with a buildup of hypothalamic SFAs. Enteric gavage specifically with SFAs reproduces microglial activation and neuronal stress in the MBH, and SFA treatment activates murine microglia, but not astrocytes, in culture. Moreover, depleting microglia abrogates SFA-induced inflammation in hypothalamic slices. Remarkably, depleting microglia from the MBH of mice abolishes inflammation and neuronal stress induced by excess SFA consumption, and in this context, microglial depletion enhances leptin signaling and reduces food intake. We thus show that microglia sense SFAs and orchestrate an inflammatory process in the MBH that alters neuronal function when SFA consumption is high.

Fat sensing and metabolic syndrome

Overconsumption of dietary fat contributes to the development of obesity and metabolic syndrome. Recent evidence suggests that high dietary fat may promote these metabolic states not only by providing calories but also by inducing impaired control of energy balance. In normal metabolic states, fat interacts with various organs or receptors to generate signals for the regulation of energy balance. Many of these interactions are impaired by high-fat diets or in obesity, contributing to the development or maintenance of obesity. These impairments may arise largely from fundamental alterations in the hypothalamus where all peripheral signals are integrated to regulate energy balance. This review focuses on various mechanisms by which fat is sensed at different stages of ingestion, circulation, storage, and utilization to regulate food intake, and how these individual mechanisms are altered by high-fat diets or in obesity.

Physiological and pathophysiological implications of lipid sensing in the brain

Fatty acid (FA)-sensitive neurons are present in the brain, especially the hypothalamus, and play a key role in the neural control of energy homeostasis. Through neuronal output, FA may modulate feeding behaviour as well as insulin secretion and action. Subpopulations of neurons in the ventromedial and arcuate hypothalamic nuclei are selectively either inhibited or activated by FA. Molecular effectors of these FA effects probably include chloride or potassium ion channels. While intracellular metabolism and activation of the ATP-sensitive K⁺ channel appear to be necessary for some of the signalling effects of FA, at least half of the FA responses in ventromedial hypothalamic neurons are mediated by interaction with FAT/CD36, an FA transporter/receptor that does not require intracellular metabolism to activate downstream signalling. Thus, FA or their metabolites can modulate neuronal activity as a means of directly monitoring ongoing fuel availability by brain nutrient-sensing neurons involved in the regulation of energy and glucose homeostasis. Recently, the role of lipoprotein lipase in FA sensing has also been shown in animal models not only in hypothalamus, but also in hippocampus and striatum. Finally, FA overload might impair neural control of energy homeostasis through enhanced ceramide synthesis and may contribute to obesity and/or type 2 diabetes pathogenesis in predisposed subjects.

Regulation of hypothalamic neuronal sensing and food intake by ketone bodies and fatty acids

Metabolic sensing neurons in the ventromedial hypothalamus (VMH) alter their activity when ambient levels of metabolic substrates, such as glucose and fatty acids (FA), change. To assess the relationship between a high-fat diet (HFD; 60%) intake on feeding and serum and VMH FA levels, rats were trained to eat a low-fat diet (LFD; 13.5%) or an HFD in 3 h/day and were monitored with VMH FA microdialysis. Despite having higher serum levels, HFD rats had lower VMH FA levels but ate less from 3 to 6 h of refeeding than did LFD rats. However, VMH β-hydroxybutyrate (β-OHB) and VMH-to-serum β-OHB ratio levels were higher in HFD rats during the first 1 h of refeeding, suggesting that VMH astrocyte ketone production mediated their reduced intake. In fact, using calcium imaging in dissociated VMH neurons showed that ketone bodies overrode normal FA sensing, primarily by exciting neurons that were activated or inhibited by oleic acid. Importantly, bilateral inhibition of VMH ketone production with a 3-hydroxy-3-methylglutaryl-CoA synthase inhibitor reversed the 3- to 6-h HFD-induced inhibition of intake but had no effect in LFD-fed rats. These data suggest that a restricted HFD intake regimen inhibits caloric intake as a consequence of FA-induced VMH ketone body production by astrocytes.

Regulation of hypothalamic-pituitary-adrenal axis by circulating free fatty acids in male Wistar rats: role of individual free fatty acids

We previously showed that a fall in the plasma free fatty acid (FFA) level increases plasma corticosterone levels in rats by activating the hypothalamic-pituitary-adrenal axis. In the present study, we tested whether this regulation is mediated by specific or all species of FFAs. Nicotinic acid (NA) (30 μmol/h) was infused in rats to decrease plasma FFAs and increase plasma ACTH and corticosterone. The NA infusion was combined with an infusion of lipids with different FFA compositions to selectively prevent falls in individual FFA levels; coconut, olive, and safflower oils (n = 7 for each), which are predominantly (>70%) composed of saturated, monounsaturated (oleic acid), and polyunsaturated (linoleic acid) FFAs, respectively, were used. At an infusion rate (0.1 g/h) that only partially prevented a fall in the total FFA level, coconut oil, but not olive or safflower oil, completely prevented NA-induced increases in plasma ACTH or corticosterone, suggesting that these responses are mainly mediated by saturated FFAs. In addition, quantification of individual FFA species in the blood using FFA-specific fluorescent probes revealed that plasma corticosterone and ACTH correlated significantly with plasma palmitate but not with other FFAs, such as oleate, linoleate, or arachidonate. Taken together, our data suggest that the regulation of the hypothalamic-pituitary-adrenal axis by FFAs is mainly mediated by the saturated fatty acid palmitate, but not by unsaturated fatty acids, such as oleate and linoleate.

Towards a 'systems'-level understanding of the nervous system and its disorders

It is becoming clear that nervous system development and adult functioning are highly coupled with other physiological systems. Accordingly, neurological and psychiatric disorders are increasingly being associated with a range of systemic comorbidities including, most prominently, impairments in immunological and bioenergetic parameters as well as in the gut microbiome. Here, we discuss various aspects of the dynamic crosstalk between these systems that underlies nervous system development, homeostasis, and plasticity. We believe a better definition of this underappreciated systems physiology will yield important insights into how nervous system diseases with systemic comorbidities arise and potentially identify novel diagnostic and therapeutic strategies.

Important role of ventromedial hypothalamic carnitine palmitoyltransferase-1a in the control of food intake

Carnitine palmitoyltransferase-1 (CPT-1) liver isoform, or CPT-1a, is implicated in CNS control of food intake. However, the exact brain nucleus site(s) in mediating this action of CPT-1a has not been identified. In this report, we assess the role of CPT-1a in hypothalamic ventromedial nucleus (VMN). We stereotaxically injected an adenoviral vector containing CPT-1a coding sequence into the VMN of rats to induce overexpression and activation of CPT-1a. The VMN-selective activation of CPT-1a induced an orexigenic effect, suggesting CPT-1a in the VMN is involved in the central control of feeding. Intracerebroventricular administration of etomoxir, a CPT-1 inhibitor, decreases food intake. Importantly, in the animals with VMN overexpression of a CPT-1a mutant that antagonizes the CPT-1 inhibition by etomoxir, the anorectic response to etomoxir was attenuated. This suggests that VMN is involved in mediating the anorectic effect of central inhibition of CPT-1a. In contrast, arcuate nucleus (Arc) overexpression of the mutant did not alter etomoxir-induced inhibition of food intake, suggesting that Arc CPT-1a does not play significant roles in this anorectic action. Furthermore, in the VMN, CPT-1a appears to act downstream of hypothalamic malonyl-CoA action of feeding. Finally, we show that in the VMN CPT-1 activity was altered in concert with fasting and refeeding states, supporting a physiological role of CPT-1a in mediating the control of feeding. All together, CPT-1a in the hypothalamic VMN appears to play an important role in central control of food intake. VMN-selective modulation of CPT-1a activity may therefore be a promising strategy in controlling food intake and maintaining normal body weight.

Hypothalamic inhibition of acetyl-CoA carboxylase stimulates hepatic counter-regulatory response independent of AMPK activation in rats

BACKGROUND

Hypothalamic AMPK acts as a cell energy sensor and can modulate food intake, glucose homeostasis, and fatty acid biosynthesis. Intrahypothalamic fatty acid injection is known to suppress liver glucose production, mainly by activation of hypothalamic ATP-sensitive potassium (K(ATP)) channels. Since all models employed seem to involve malonyl-CoA biosynthesis, we hypothesized that acetyl-CoA carboxylase can modulate the counter-regulatory response independent of nutrient availability.

METHODOLOGY/PRINCIPAL FINDINGS

In this study employing immunoblot, real-time PCR, ELISA, and biochemical measurements, we showed that reduction of the hypothalamic expression of acetyl-CoA carboxylase by antisense oligonucleotide after intraventricular injection increased food intake and NPY mRNA, and diminished the expression of CART, CRH, and TRH mRNA. Additionally, as in fasted rats, in antisense oligonucleotide-treated rats, serum glucagon and ketone bodies increased, while the levels of serum insulin and hepatic glycogen diminished. The reduction of hypothalamic acetyl-CoA carboxylase also increased PEPCK expression, AMPK phosphorylation, and glucose production in the liver. Interestingly, these effects were observed without modification of hypothalamic AMPK phosphorylation.

CONCLUSION/SIGNIFICANCE

Hypothalamic ACC inhibition can activate hepatic counter-regulatory response independent of hypothalamic AMPK activation.

Targeting lipid sensing in the central nervous system: new therapy against the development of obesity and type 2 diabetes

INTRODUCTION

The hypothalamus plays a major role in the control of energy balance, by sensing circulating lipids. Several studies conducted over the past decade suggest that disruption of lipid sensing can lead to hypothalamic lipotoxicity, thereby contributing to the development of various diseases, such as obesity and type 2 diabetes.

AREAS COVERED

The physiological role of 'lipid sensing' as a regulator of neuronal activity involved in the regulation of energy homeostasis will be reviewed. Next, the emerging evidence that alterations of hypothalamic systems that regulate energy balance during overnutrition can lead to the development of obesity and associated pathologies such as type 2 diabetes will be described.

EXPERT OPINION

Several studies have highlighted the role of malonyl-CoA and PKCθ and also autophagy within the hypothalamus as signals of nutrient abundance by critical neurons regulating food intake. Besides the physiological role of hypothalamic lipid sensing, it has been shown that overnutrition can also induce hypothalamic lipotoxicity through an inflammatory process. In conclusion, lipid toxicity could be the starting point of perturbations of the central control of energy balance which will favor the appearance of obesity and type 2 diabetes. Lipid sensing in the hypothalamus could be considered as a potential target for anti-obesity/diabetic strategies.

A Fall in plasma free fatty acid (FFA) level activates the hypothalamic-pituitary-adrenal axis independent of plasma glucose: evidence for brain sensing of circulating FFA

The brain responds to a fall in blood glucose by activating neuroendocrine mechanisms for its restoration. It is unclear whether the brain also responds to a fall in plasma free fatty acids (FFA) to activate mechanisms for its restoration. We examined whether lowering plasma FFA increases plasma corticosterone or catecholamine levels and, if so, whether the brain is involved in these responses. Plasma FFA levels were lowered in rats with three independent antilipolytic agents: nicotinic acid (NA), insulin, and the A1 adenosine receptor agonist SDZ WAG 994 with plasma glucose clamped at basal levels. Lowering plasma FFA with these agents all increased plasma corticosterone, but not catecholamine, within 1 h, accompanied by increases in plasma ACTH. These increases in ACTH or corticosterone were abolished when falls in plasma FFA were prevented by Intralipid during NA or insulin infusion. In addition, the NA-induced increases in plasma ACTH were completely prevented by administration of SSR149415, an arginine vasopressin receptor antagonist, demonstrating that the hypothalamus is involved in these responses. Taken together, the present data suggest that the brain may sense a fall in plasma FFA levels and activate the hypothalamic-pituitary-adrenal axis to increase plasma ACTH and corticosterone, which would help restore FFA levels. Thus, the brain may be involved in the sensing and control of circulating FFA levels.

Associations of fatty acids in cerebrospinal fluid with peripheral glucose concentrations and energy metabolism

Rodent experiments have emphasized a role of central fatty acid (FA) species, such as oleic acid, in regulating peripheral glucose and energy metabolism. Thus, we hypothesized that central FAs are related to peripheral glucose regulation and energy expenditure in humans. To test this we measured FA species profiles in cerebrospinal fluid (CSF) and plasma of 32 individuals who stayed in our clinical inpatient unit for 6 days. Body composition was measured by dual energy X-ray absorptiometry and glucose regulation by an oral glucose test (OGTT) followed by measurements of 24 hour (24EE) and sleep energy expenditure (SLEEP) as well as respiratory quotient (RQ) in a respiratory chamber. CSF was obtained via lumbar punctures; FA concentrations were measured by liquid chromatography/mass spectrometry. As expected, FA concentrations were higher in plasma compared to CSF. Individuals with high concentrations of CSF very-long-chain saturated FAs had lower rates of SLEEP. In the plasma moderate associations of these FAs with higher 24EE were observed. Moreover, CSF monounsaturated long-chain FA (palmitoleic and oleic acid) concentrations were associated with lower RQs and lower glucose area under the curve during the OGTT. Thus, FAs in the CSF strongly correlated with peripheral metabolic traits. These physiological parameters were most specific to long-chain monounsaturated (C16∶1, C18∶1) and very-long-chain saturated (C24∶0, C26∶0) FAs. Conclusions: Together with previous animal experiments these initial cross-sectional human data indicate that central FA species are linked to peripheral glucose and energy homeostasis.

Distinct roles of specific fatty acids in cellular processes: implications for interpreting and reporting experiments

Plasma contains a variety of long-chain fatty acids (FAs), such that about 35% are saturated and 65% are unsaturated. There are countless examples that show how different FAs impart specific and unique effects, or even opposing actions, on cellular function. Despite these differing effects, palmitate (C16:0) is regularly used to represent "FAs" in cell based experiments. Although palmitate can be useful to induce and study stress effects in cultured cells, these effects in isolation are not physiologically relevant to dietary manipulations, obesity, or the consequences of physiological concentrations of FAs. Hence, authors should avoid conclusions that generalize about "FAs" or "saturated FAs" or "high-fat diet" effects if only a single FA was used in the reported experiments.

Mitochondrial uncoupling protein 2 (UCP2) in glucose and lipid metabolism

Nutrient availability is critical for the physiological functions of all tissues. By contrast, an excess of nutrients such as carbohydrate and fats impair health and shorten life due by stimulating chronic diseases, including diabetes, cancer and neurodegeneration. The control of circulating glucose and lipid levels involve mitochondria in both central and peripheral mechanisms of metabolism regulation. Mitochondrial uncoupling protein 2 (UCP2) has been implicated in physiological and pathological processes related to glucose and lipid metabolism, and in this review we discuss the latest data on the relationships between UCP2 and glucose and lipid sensing from the perspective of specific hypothalamic neuronal circuits and peripheral tissue functions. The goal is to provide a framework for discussion of future therapeutic strategies for metabolism-related chronic diseases.

Metabolic sensing and the brain: who, what, where, and how?

Unique subpopulations of specialized metabolic sensing neurons reside in a distributed network throughout the brain and respond to alterations in ambient levels of various metabolic substrates by altering their activity. Variations in local brain substrate levels reflect their transport across the blood- and cerebrospinal-brain barriers as well as local production by astrocytes. There are a number of mechanisms by which such metabolic sensing neurons alter their activity in response to changes in substrate levels, but it is clear that these neurons cannot be considered in isolation. They are heavily dependent on astrocyte and probably tanycyte metabolism and function but also respond to hormones (e.g. leptin and insulin) and cytokines that cross the blood-brain barrier from the periphery as well as hard-wired neural inputs from metabolic sensors in peripheral sites such as the hepatic portal vein, gastrointestinal tract, and carotid body. Thus, these specialized neurons are capable of monitoring and integrating multiple signals from the periphery as a means of regulating peripheral energy homeostasis.

Nutrient-sensing hypothalamic TXNIP links nutrient excess to energy imbalance in mice

Nutrient excess in obesity and diabetes is emerging as a common putative cause for multiple deleterious effects across diverse cell types, responsible for a variety of metabolic dysfunctions. The hypothalamus is acknowledged as an important regulator of whole-body energy homeostasis, through both detection of nutrient availability and coordination of effectors that determine nutrient intake and utilization, thus preventing cellular and whole-body nutrient excess. However, the mechanisms underlying hypothalamic nutrient detection and its impact on peripheral nutrient utilization remain poorly understood. Recent data suggest a role for thioredoxin-interacting protein (TXNIP) as a molecular nutrient sensor important in the regulation of energy metabolism, but the role of hypothalamic TXNIP in the regulation of energy balance has not been evaluated. Here we show in mice that TXNIP is expressed in nutrient-sensing neurons of the mediobasal hypothalamus, responds to hormonal and nutrient signals, and regulates adipose tissue metabolism, fuel partitioning, and glucose homeostasis. Hypothalamic expression of TXNIP is induced by acute nutrient excess and in mouse models of obesity and diabetes, and downregulation of mediobasal hypothalamic TXNIP expression prevents diet-induced obesity and insulin resistance. Thus, mediobasal hypothalamic TXNIP plays a critical role in nutrient sensing and the regulation of fuel utilization.

Brain lipid sensing and nervous control of energy balance

Nutrient sensitive neurons (glucose and fatty acids (FA)) are present in many sites throughout the brain, including the hypothalamus and brainstem, and play a key role in the neural control of energy and glucose homeostasis. Through neuronal output, FA may modulate feeding behaviour as well as both insulin secretion and action. For example, central administration of oleate inhibits food intake and glucose production in rats. This suggests that daily variations in plasma FA concentrations might be detected by the central nervous system as a signal which contributes to the regulation of energy balance. At the cellular level, subpopulations of neurons in the ventromedial and arcuate hypothalamic nuclei are selectively either inhibited or activated by FA. Possible molecular effectors of these FA effects likely include chloride or potassium ion channels. While intracellular metabolism and activation of the ATP-sensitive K(+) channel appear to be necessary for some of the signaling effects of FA, at least half of the FA responses in ventromedial hypothalamic neurons are mediated by interaction with FAT/CD36, a FA transporter/receptor that does not require intracellular metabolism to activate downstream signaling. Thus, FA or their metabolites can modulate neuronal activity as a means of directly monitoring ongoing fuel availability by brain nutrient-sensing neurons involved in the regulation of energy and glucose homeostasis. Besides these physiological effects, FA overload or metabolic dysfunction might impair neural control of energy homeostasis and contribute to obesity and/or type 2 diabetes in predisposed subjects.