Hypothalamic sensing of fatty acids

Differential accessibility of circulating leptin to individual hypothalamic sites

Hypothalamic neurons expressing the long form of the leptin receptor (LRb) mediate important leptin actions. Although it has been suggested that leptin crosses the blood-brain barrier (BBB) via a specific transport system, we hypothesized the existence of a population of hypothalamic arcuate nucleus (ARC) neurons that senses leptin independently of this transport system. Indeed, endogenous circulating leptin results in detectable levels of baseline activated signal transducer and activator of transcription 3 (STAT3) phosphorylation in a population of ARC/LRb neurons, consistent with increased sensing of circulating leptin in these neurons compared with other LRb neurons. Furthermore, a population of ARC/LRb neurons that responds more rapidly and sensitively to circulating leptin compared with other hypothalamic LRb neurons detected by leptin activated phosphorylated STAT3. In addition, peripheral application of the BBB-impermeant retrograde tracer fluorogold revealed a population of ARC/LRb neurons that directly contact the circulation (e.g. via neuronal processes reaching outside the BBB). Taken together, these data suggest that a population of ARC/LRb neurons directly contacts the circulation and displays increased sensitivity to circulating leptin compared with neurons residing entirely behind the BBB elsewhere in the hypothalamus.

Mapping brain c-Fos immunoreactivity after insulin-induced voluntary lard intake: insulin- and lard-associated patterns

In addition to the inhibitory role of central insulin on food intake, insulin also acts to promote lard intake. We investigated the neural pathways involved in this facet of insulin action. Insulin or saline was infused into either the superior mesenteric or right external jugular veins of streptozotocin-diabetic rodents with elevated steady-state circulating corticosterone concentrations. After postsurgical recovery, rats were offered the choice of chow or lard to eat. Irrespective of the site of venous infusion, insulin increased lard and decreased chow intake. After 4 days, lard was removed for 8 h. On return for 1 h, only insulin infused into the superior mesenteric vein resulted in lard intake. This facilitated distinction between the effects of circulating insulin concentrations (similar in the two insulin-infused groups) and lard ingestion on the patterns of c-Fos(+) cells in the brain, termed insulin- and lard-associated patterns, respectively. Insulin-associated changes in c-Fos(+) cell numbers were evident in the arcuate nucleus, bed nucleus of the stria terminalis and substantia nigra pars compacta, concomitant with elevated leptin levels and reduced chow intake. Lard-associated changes in c-Fos(+) cell numbers were observed in the nucleus of the tractus solitarius, lateral parabrachial nucleus, central nucleus of the amygdala, ventral tegmental area, nucleus accumbens shell and the prefrontal cortex, and were associated with lower levels of triglycerides and free fatty acids. The anterior paraventricular thalamic nucleus exhibited both patterns. These data collectively fit into a framework for food intake and reward and provide targets for pharmacological manipulation to influence the choice of food intake.

Neurobiology of feeding and energy expenditure

Significant advancements have been made in the past century regarding the neuronal control of feeding behavior and energy expenditure. The effects and mechanisms of action of various peripheral metabolic signals on the brain have become clearer. Molecular and genetic tools for visualizing and manipulating individual components of brain homeostatic systems in combination with neuroanatomical, electrophysiological, behavioral, and pharmacological techniques have begun to elucidate the molecular and neuronal mechanisms of complex feeding behavior and energy expenditure. This review highlights some of these advancements that have led to the current understanding of the brain's involvement in the acute and chronic regulation of energy homeostasis.

Sensitivity of the hypothalamic paraventricular nucleus to the locomotor-activating effects of neuromedin U in obesity

Obesity is associated with a decrease in energy expenditure relative to energy intake. The decrease in physical activity associated with obesity in several species, including humans, contributes to decreased energy expenditure. Several hormones and neuropeptides that affect appetite also modulate physical activity, including neuromedin U (NMU), a peptide found in the gut and brain. We have demonstrated that NMU microinjected into the hypothalamic paraventricular nucleus (PVN) in rats increases the energy expenditure associated with physical activity, called non-exercise activity thermogenesis (NEAT). Here we examined whether obesity in rats is related to decreased sensitivity of the PVN to the locomotor-activating effect of NMU. Diet-induced obese (DIO) rats and lean, diet-resistant (DR) rats were given PVN microinjections of increasing doses of NMU both before and after 1 month on a high-fat diet. We found that NMU increases physical activity, energy expenditure, and NEAT in a dose-dependent manner in both DR and DIO rats, both before and after 1 month on the high-fat diet. Before high-fat feeding, the obesity-prone and lean rats showed similar levels of physical activity after intra-PVN microinjections of NMU. After 1 month of the high-fat diet, however, the obesity-resistant rats showed significantly more NMU-induced physical activity compared to the obese DIO rats. Taken together with previous studies, these results suggest that obesity may represent a state associated with decreased central sensitivity to neuropeptides such as NMU that increase physical activity and therefore energy expenditure.

New evaluations of redox regulating system in adipose tissue of obesity

During the past several decades, the incidence of obesity has significantly increased worldwide. Enormous efforts have been devoted to understanding the molecular mechanisms underlying obesity and its related metabolic disorders such as type 2 diabetes, cardiovascular disease, atherosclerosis, and hypertension. It is now well-established that altered adipocyte metabolism in obese patients is closely associated with the induction of various metabolic stresses including hyperglycemia, hyperlipidemia, hyperinsulinemia, and chronic inflammation. However, the cellular factor(s) which sense metabolic changes and/or initiate the pathological progression of obesity-induced metabolic disorders remain to be elucidated. In this review, we will discuss the possible roles of cellular NADP(+)/NADPH, which function as redox potential regulators, in the induction of obesity-associated oxidative stress, chronic inflammation, and insulin resistance and suggest G6PD, a NADPH-generating enzyme, as a novel target for treating metabolic disorders.

A balance of lipid-sensing mechanisms in the brain and liver

Recent work has cast a spotlight on the brain as a nutrient-sensing organ that regulates the body's metabolic processes. Here we discuss the physiological and molecular mechanisms of brain lipid sensing and compare these mechanisms to liver lipid sensing. A direct comparison between the lipid-sensing mechanisms in the brain and liver reveals similar biochemical/molecular but opposing physiological mechanisms in operation. We propose that an imbalance between the lipid-sensing mechanisms in the brain and liver may contribute to obesity-associated type 2 diabetes.

Avenues of communication between the brain and tissues/organs involved in energy homeostasis

Obesity is a rapidly increasing public health concern worldwide as a major risk factor for numerous disorders, including diabetes, hypertension and heart disease. Despite remarkable advances in obesity research over the past 10 years, the molecular mechanisms underlying obesity are still not completely understood. To maintain systemic energy homeostasis, it is important that organs/tissues communicate metabolic information among each other. Obesity-related disorders can be thought of as resulting from dysregulation of this inter-tissue communication. This system has both afferent sensing components and efferent effecter limbs. The afferent signals consist of not only humoral factors, such as nutrients (glucose, fatty acids and amino acids) and adipocytokines (leptin, adiponectin and so on), but also autonomic afferent nerve systems. Both converge on brain centers, most importantly within the hypothalamus, where the signals are integrated, and the direction and magnitude of efferent responses are determined. The efferent elements of this physiological system include those regulating energy inputs and outputs, i.e. food intake and metabolic rates. In this review, we will summarize recent advances in research on metabolic information avenues to the brain, which are important for energy homeostasis.

A Model for Fatty Acid Transport into the Brain

A key function of fatty acid (FA) transport into the brain is to supply polyunsaturated fatty acids (PUFA) that are not synthesized in brain cells but are essential signaling molecules and components of the phospholipid membrane. In addition, common dietary FAs such as palmitic acid are also rapidly taken up by the brain and esterified to phospholipids or oxidized to provide cellular energy. Most evidence shows that FA crossing the blood brain barrier (BBB) is derived mainly from FA/albumin complexes and, to a lesser extent, from circulating lipoproteins. Our model proposes that FA diffuse across the lipid bilayer of the BBB without specific transporters to reach brain cells. They cross the luminal and transluminal leaflets of the endothelial cells and the plasma membrane of neural cells by reversible flip-flop. Acyl-CoA synthetases trap FA by forming acyl-CoA, which cannot diffuse out of the cell. Selection of FA is controlled largely by enzymes in the pathways of intracellular metabolism, beginning with the acyl-CoA synthetase.

Hormonal and nutritional regulation of muscle carnitine palmitoyltransferase I gene expression in vivo

Transgenic mice carrying the human heart muscle carnitine palmitoyltransferase I (M-CPTI) gene fused to a CAT reporter gene were generated to study the regulation of M-CPTI gene expression. When the mice were fasted for 48 h, CAT activity and mRNA levels increased by more than 2-fold in heart and skeletal muscle, but not liver or kidney. In the diabetic transgenic mice, there was a 2- to 3-fold increase in CAT activity and CAT mRNA levels in heart and skeletal muscle which upon insulin administration reverted to that observed with the control insulin sufficient transgenic mice. Feeding a high fat diet increased CAT activity and mRNA levels by 2- to 4-fold in heart and skeletal muscle of the transgenic mice compared to the control transgenic mice on regular diet. Overall, the M-CPTI promoter was found to be necessary for the tissue-specific hormonal and dietary regulation of the gene expression.

Mechanisms Regulating Feed Intake, Energy Expenditure, and Body Weight in Poultry

To achieve energy balance and maintain a constant BW, changes in feed intake and energy expenditure must be coordinated and tightly regulated. This may not hold true for some poultry species intensively selected for such economically important traits as growth and meat production. For example, the modern commercial broiler breeder does not adequately control voluntary feed intake to meet its energy requirements and maintain energy balance. As a consequence, feeding must be limited in these birds to avoid overconsumption and excessive fattening during production. It is important to determine a genetic basis to help explain this situation and to offer potential strategies for producing more efficient poultry. This review summarizes what is currently known about the control of feed intake and energy expenditure at the gene level in birds. Highly integrated regulatory systems have been identified that link the control of feeding with the sensing of energy status. How such systems function in poultry is currently being explored. One example recently identified in chickens is the adenosine monophosphate-activated protein kinase pathway that links energy sensing with modulation of metabolic activity to maintain energy homeostasis at the cellular level. In the hypothalamus, this same pathway may also play an important role in regulating feed intake and energy expenditure commensurate with perceived whole body energy needs. Genes encoding key regulatory factors such as hormones, neuropeptides, receptors, enzymes, and transcription factors produce the molecular components that make up intricate and interconnected neural, endocrine, and metabolic pathway networks linking peripheral tissues with the central nervous system. Moreover, coordinate expression of specific gene groups can establish functional pathways that respond to and are regulated by such factors as hormones, nutrients, and metabolites. Thus, with a better understanding of the genetic and molecular basis for regulating feed intake and energy expenditure in birds important progress can be made in developing, evaluating, and managing more efficient commercial poultry lines.

Fatty acid sensing and nervous control of energy homeostasis

Nutrient sensitive neurons (glucose and fatty acids, FA) are present in both the hypothalamus and the brainstem and play a key role in nervous control of energy homeostasis. Through neuronal output, especially the autonomic nervous system, it is now evidenced that FA may modulate food behaviour and both insulin secretion and action. For example, central administration of oleate inhibits both food intake and hepatic glucose production in rats. This suggests that a slight increase in plasma FA concentrations in the postprandial state might be detected by the central nervous system as a satiety signal. At cellular levels, subpopulations of FA-sensitive neurons (either excited or inhibited by FA) are now identified within the hypothalamus. However molecular effectors of FA effects remain unclear. They probably include ionic channels such as chloride or potassium. FA metabolism seems also required to induce neuronal response. Thus, FA per se or their metabolites modulate neuronal activity, as a mean of directly monitoring ongoing fuel availability by CNS nutrient-sensing neurons involved in the regulation of insulin secretion. Beside these physiological effects, FA overload or dysfunction of their metabolism could impair nervous control of energy homeostasis and contribute to development of obesity and/or type 2 diabetes in predisposed subjects.

Hypothalamic resistin induces hepatic insulin resistance

Circulating resistin stimulates endogenous glucose production (GP). Here, we report that bi-directional changes in hypothalamic resistin action have dramatic effects on GP and proinflammatory cytokine expression in the liver. The infusion of either resistin or an active cysteine mutant in the third cerebral ventricle (icv) or in the mediobasal hypothalamus stimulated GP independent of changes in circulating levels of glucoregulatory hormones. Conversely, central antagonism of resistin action markedly diminished the ability of circulating resistin to enhance GP. We also report that centrally mediated mechanisms partially control resistin-induced expression of TNF-alpha, IL-6, and SOCS-3 in the liver. These results unveil what we believe to be a novel site of action of resistin on GP and inflammation and suggest that hypothalamic resistin action can contribute to hyperglycemia in type 2 diabetes mellitus.

Peripheral tissue-brain interactions in the regulation of food intake

More than 70 years ago the glucostatic, lipostatic and aminostatic hypotheses proposed that the central nervous system sensed circulating levels of different metabolites, changing feeding behaviour in response to the levels of those molecules. In the last 20 years the rapid increase in obesity and associated pathologies in developed countries has involved a substantial increase in the knowledge of the physiological and molecular mechanism regulating body mass. This effort has resulted in the recent discovery of new peripheral signals, such as leptin and ghrelin, as well as new neuropeptides, such as orexins, involved in body-weight homeostasis. The present review summarises research into energy balance, starting from the original classical hypotheses proposing metabolite sensing, through peripheral tissue-brain interactions and coming full circle to the recently-discovered role of hypothalamic fatty acid synthase in feeding regulation. Understanding these molecular mechanisms will provide new pharmacological targets for the treatment of obesity and appetite disorders.

The role of CNS fuel sensing in energy and glucose regulation

Individual cells must carefully regulate their energy flux to ensure nutrient levels are adequate to maintain normal cellular activity. The same principle holds in multicellular organisms. Thus, for mammals to perform necessary physiological functions, sufficient nutrients need to be available. It is more complex, however, to understand how the energy status of different cells impacts on the overall energy balance of the entire organism. We propose that the central nervous system is the critical organ for the coordination of intracellular metabolic processes that are essential to guarantee energy homeostasis at the organismal level. In particular, we suggest that in specific hypothalamic neurons, evolutionarily conserved fuel sensors, such as adenosine monophosphate-activated protein kinase and mammalian target of rapamycin (mTOR), integrate sensory input from nutrients, including those derived from recently ingested food or those that are stored in adipose tissue, to regulate effector pathways responsible for fuel intake and utilization. The corollary to this hypothesis is that dysregulation of these fuel-sensing mechanisms in the brain may contribute to metabolic dysregulation underlying diseases, such as obesity and type 2 diabetes.

Appetite regulation: an overview

Obesity is a major public health problem associated with morbidity and mortality and continues to increase worldwide. This review focuses on the regions of the brain that are important in appetite regulation and the circulating factors implicated in the control of food intake. The hypothalamus is critical in the regulation of food intake containing neural circuits, which produce a number of peptides that influence food intake. The arcuate nucleus of the hypothalamus produces both orexigenic peptides (agouti-related protein and neuropeptide Y) and anorectic peptides (alpha-melanocyte-stimulating hormone and cocaine- and amphetamine-related transcript). The lateral hypothalamus produces the orexigenic peptides (melanin-concentrating hormone and orexins). Other hypothalamic factors recently implicated in appetite regulation include the endocannabinoids, brain-derived neurotrophic factor, nesfatin-1, AMP-activated protein kinase, mammalian target of rapamycin protein, and protein tyrosine phosphatase. Circulating factors that affect food intake mediate their effects by signaling to the hypothalamus and/or brainstem. A number of circulating factors are produced by peripheral organs, for example, leptin by adipose tissue, insulin and pancreatic polypeptide by the pancreas, gut hormones (e.g., ghrelin, obestatin, glucagon-like peptide-1, oxyntomodulin, peptide YY), and triiodothyronine by the thyroid gland. Circulating carbohydrates, lipids, and amino acids also affect appetite regulation. Knowledge regarding appetite regulation has vastly expanded in recent years providing targets for antiobesity drug design.

Thermogenic and metabolic antiobesity drugs: rationale and opportunities

Antiobesity drugs that target peripheral metabolism may avoid some of the problems that have been encountered with centrally acting anorectic drugs. Moreover, if they cause weight loss by increasing fat oxidation, they not only address a cause of obesity but also should promote loss of fat rather than lean tissue and improve insulin sensitivity. Weight loss may be slow but more sustained than with anorectic drugs, and thermogenesis may be insufficient to cause any discomfort. Some thermogenic approaches are the activation of adrenergic, thyroid hormone or growth hormone receptors and the inhibition of glucocorticoid receptors; the modulation of transcription factors [e.g. peroxisome proliferator-activated receptor delta (PPARdelta) activators] or enzymes [e.g. glutamine fructose-6-phosphate amidotransferase (GFAT) inhibitors] that promote mitochondrial biogenesis, and the modulation of transcription factors (PPAR alpha activators) or enzymes (AMP-activated protein kinase) that promote fatty acid oxidation. More surprisingly, studies on genetically modified animals and with enzyme inhibitors suggest that inhibitors of fatty acid synthesis [e.g. ATP citrate lyase, fatty acid synthase, acetyl-CoA carboxylase (ACC)], fatty acid interconversion [stearoyl-CoA desaturase (SCD)] and triglyceride synthesis (e.g. acyl-CoA : diacylglycerol acyltransferase) may all be thermogenic. Some targets have been validated only by deleting genes in the whole animal. In these cases, it is possible that deletion of the protein in the brain is responsible for the effect on adiposity, and therefore a centrally penetrant drug would be required. Moreover, whilst a genetically modified mouse may display resistance to obesity in response to a high fat diet, it requires a tool compound to demonstrate that a drug might actually cause weight loss. Even then, it is possible that differences between rodents and humans, such as the greater thermogenic capacity of rodents, may give a misleading impression of the potential of a drug.

Effect of NPY5R antagonist MK-0557 on weight regain after very-low-calorie diet-induced weight loss

OBJECTIVE

To evaluate whether MK-0557, a highly selective, orally administered neuropeptide Y Y5 receptor antagonist, could limit weight regain after very-low-calorie diet (VLCD)-induced weight loss.

RESEARCH METHODS AND PROCEDURES

We enrolled 502 patients 18 to 65 years of age with a BMI of 30 to 43 kg/m2. Patients were placed on a VLCD (800 kcal/d liquid diet) for 6 weeks. Patients who lost>or=6% of initial body weight (n=359) were randomized to 52 weeks of 1 mg/d MK-0557 or placebo and maintained on a hypocaloric diet (300 kcal below weight maintenance requirements).

RESULTS

In randomized patients, the VLCD was associated with an average weight loss of 9.1 kg. After 12 weeks of double-blind treatment, weight began to gradually increase for both placebo- and MK-0557-treated patients. The mean weight change (95% confidence interval) from baseline at the end of the VLCD to Week 52 was +3.1 (2.1, 4.0) and +1.5 (0.5, 2.4) kg for patients treated with placebo and MK-0557, respectively. The difference of 1.6 kg between the two groups was significant (p=0.014). Secondary endpoints, such as blood pressure, lipid profile, insulin, and leptin, as well as waist circumference and quality-of-life measurements, did not show significant differences between MK-0557 and placebo treatments.

DISCUSSION

Although the difference in weight regain between placebo- and MK-0557-treated patients was statistically significant, the magnitude of the effect was small and not clinically meaningful. Antagonism of the neuropeptide Y Y5 receptor is not an efficacious treatment strategy for reducing weight regain after VLCD.

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

The hypothalamus is a specialized area in the brain that integrates the control of energy homeostasis. More than 70 years ago, it was proposed that the central nervous system sensed circulating levels of metabolites such as glucose, lipids and amino acids and modified feeding according to the levels of those molecules. This led to the formulation of the Glucostatic, Lipostatic and Aminostatic Hypotheses. It has taken almost that much time to demonstrate that circulating long-chain fatty acids act as signals of nutrient surplus in the hypothalamus. Moreover, pharmacological and/or genetic inhibition of fatty acid synthase, AMP-activated protein kinase and carnitine palmitoyltransferase 1 results in profound decrease in feeding and body weight in rodents. The molecular mechanism behind these actions depends on changes in the cellular pool of malonyl-CoA and fatty acyl-CoAs. Current evidence also suggests that this pathway may play a major role in the physiological regulation of feeding, by integrating hormonal and nutrient-derived signals in the hypothalamus. Here, we summarize what is known about hypothalamic fatty acid metabolism and feeding control and provide future directions for research. Understanding these molecular mechanisms could provide new targets for the treatment of obesity and related disorders.

Diet, obesity and diabetes: a current update

The prevalence of obesity has been increasing at a rapid rate over the last few decades. Although the primary defect can be attributed to an imbalance of energy intake over energy expenditure, the regulation of energy balance is now recognized to be complex. Adipose-tissue factors play a central role in the control of energy balance and whole-body fuel homoeostasis. The regulation of adipose-tissue function, in particular its secretion of adipokines, is impaired by increases in adipose mass associated with obesity, and with the development of insulin resistance and Type 2 diabetes. This review analyses adipose-regulated energy input and expenditure, together with the impact of dietary macronutrient composition on energy balance in relation to susceptibility to the development of obesity and Type 2 diabetes, and how these metabolic conditions may be exacerbated by the consequences of abnormal adipose function. By gaining a greater understanding of how energy balance is controlled in normal, and in obese and diabetic states, a more practical approach can be employed to prevent and better treat obesity and metabolic disorders.

Pharmacological and molecular characterization of ATP-sensitive K(+) conductances in CART and NPY/AgRP expressing neurons of the hypothalamic arcuate nucleus

The role of hypothalamic ATP-sensitive potassium channels in the maintenance of energy homeostasis has been extensively explored. However, how these channels are incorporated into the neuronal networks of the arcuate nucleus remains unclear. Whole-cell patch-clamp recordings from rat arcuate nucleus neurons in hypothalamic slice preparations revealed widespread expression of functional ATP-sensitive potassium channels within the nucleus. ATP-sensitive potassium channels were expressed in orexigenic neuropeptide Y/agouti-related protein (NPY/AgRP) and ghrelin-sensitive neurons and in anorexigenic cocaine-and-amphetamine regulated transcript (CART) neurons. In 70% of the arcuate nucleus neurons recorded, exposure to glucose-free bathing medium induced inhibition of electrical excitability, the response being characterized by membrane hyperpolarization, a reduction in neuronal input resistance and a reversal potential consistent with opening of potassium channels. These effects were reversible upon re-introduction of glucose to the bathing medium or upon exposure to the ATP-sensitive potassium channel blockers tolbutamide or glibenclamide. The potassium channel opener diazoxide, but not pinacidil, also induced a tolbutamide and glibenclamide-sensitive inhibition of electrical excitability. Single-cell reverse transcription-polymerase chain reaction revealed expression of mRNA for sulfonylurea receptor 1 but not sulfonylurea receptor 2 subunits of ATP-sensitive potassium channels. Thus, rat arcuate nucleus neurons, including those involved in functionally antagonistic orexigenic and anorexigenic pathways express functional ATP-sensitive potassium channels which include sulfonylurea receptor 1 subunits. These data indicate a crucial role for these ion channels in central sensing of metabolic and energy status. However, further studies are needed to clarify the differential roles of these channels, the organization of signaling pathways that regulate them and how they operate in functionally opposing cell types.

Brain glucose metabolism controls the hepatic secretion of triglyceride-rich lipoproteins

Increased production of very low-density lipoprotein (VLDL) is a critical feature of the metabolic syndrome. Here we report that a selective increase in brain glucose lowered circulating triglycerides (TG) through the inhibition of TG-VLDL secretion by the liver. We found that the effect of glucose required its conversion to lactate, leading to activation of ATP-sensitive potassium channels and to decreased hepatic activity of stearoyl-CoA desaturase-1 (SCD1). SCD1 catalyzed the synthesis of oleyl-CoA from stearoyl-CoA. Curtailing the liver activity of SCD1 was sufficient to lower the hepatic levels of oleyl-CoA and to recapitulate the effects of central glucose administration on VLDL secretion. Notably, portal infusion of oleic acid restored hepatic oleyl-CoA to control levels and negated the effects of both central glucose and SCD1 deficiency on TG-VLDL secretion. These central effects of glucose (but not those of lactate) were rapidly lost in diet-induced obesity. These findings indicate that a defect in brain glucose sensing could play a critical role in the etiology of the metabolic syndrome.

Role for mitochondrial reactive oxygen species in brain lipid sensing: redox regulation of food intake

The ability for the brain to sense peripheral fuel availability is mainly accomplished within the hypothalamus, which detects ongoing systemic nutrients and adjusts food intake and peripheral metabolism as needed. Here, we hypothesized that mitochondrial reactive oxygen species (ROS) could trigger sensing of nutrients within the hypothalamus. For this purpose, we induced acute hypertriglyceridemia in rats and examined the function of mitochondria in the hypothalamus. Hypertriglyceridemia led to a rapid increase in the mitochondrial respiration in the ventral hypothalamus together with a transient production of ROS. Cerebral inhibition of fatty acids-CoA mitochondrial uptake prevented the hypertriglyceridemia-stimulated ROS production, indicating that ROS derived from mitochondrial metabolism. The hypertriglyceridemia-stimulated ROS production was associated with change in the intracellular redox state without any noxious cytotoxic effects, suggesting that ROS function acutely as signaling molecules. Moreover, cerebral inhibition of hypertriglyceridemia-stimulated ROS production fully abolished the satiety related to the hypertriglyceridemia, suggesting that hypothalamic ROS production was required to restrain food intake during hypertriglyceridemia. Finally, we found that fasting disrupted the hypertriglyceridemia-stimulated ROS production, indicating that the redox mechanism of brain nutrient sensing could be modulated under physiological conditions. Altogether, these findings support the role of mitochondrial ROS as molecular actors implied in brain nutrient sensing.

Comparison of superior mesenteric versus jugular venous infusions of insulin in streptozotocin-diabetic rats on the choice of caloric intake, body weight, and fat stores

Corticosterone (B) increases and insulin decreases food intake. However, in streptozotocin (STZ)-diabetic rats with high B, low insulin replacement promotes lard intake. To test the role of the liver on this, rats were given STZ and infused with insulin or vehicle into either the superior mesenteric or right jugular vein. Controls were nondiabetic; all rats were treated with high B. After 5 d, all rats were offered lard, 32% sucrose, chow, and water ad libitum until d 10. Diabetes exacerbated body weight loss from high B; this was prevented by insulin into the jugular, but not superior mesenteric, vein. Without insulin, STZ groups essentially consumed only chow; controls increased caloric intake about equally from the three sources. Insulin into both sites reduced chow and increased lard intake. Although circulating insulin was increased only by jugular infusion, plasma glucose and liver glycogen were similar after insulin into both sites. Fat depot weights differed: sc fat was heavier after jugular and mesenteric fat was heavier after mesenteric insulin infusions. We conclude that there are important site-specific effects of insulin in regulating the choice of, but not total, caloric intake, body weight, and fat storage in diabetic rats with high B. Furthermore, lard intake might be regulated by an insulin-derived, liver-mediated signal because superior mesenteric insulin infusion had similar effects on lard intake to jugular infusion but did not result in elevated circulating insulin levels likely associated with liver insulin removal.

Ghrelin: a hormone regulating food intake and energy homeostasis

Regulation of energy homeostasis requires precise coordination between peripheral nutrient-sensing molecules and central regulatory networks. Ghrelin is a twenty-eight-amino acid orexigenic peptide acylated at the serine 3 position mainly with an n-octanoic acid, which is produced mainly in the stomach. It is the endogenous ligand of the growth hormone secretagogue (GHS) receptors. Since plasma ghrelin levels are strictly dependent on recent food intake, this hormone plays an essential role in appetite and meal initiation. In addition, ghrelin is involved in the regulation of energy homeostasis. The ghrelin gene is composed of four exons and three introns and renders a diversity of orexigenic peptides as well as des-acyl ghrelin and obestatin, which exhibit anorexigenic properties. Ghrelin stimulates the synthesis of neuropeptide Y (NPY) and agouti-related protein (AgRP) in the arcuate nucleus neurons of the hypothalamus and hindbrain, which in turn enhance food intake. Ghrelin-expressing neurons modulate the action of both orexigenic NPY/AgRP and anorexigenic pro-opiomelanocortin neurons. AMP-activated protein kinase is activated by ghrelin in the hypothalamus, which contributes to lower intracellular long-chain fatty acids, and this appears to be the molecular signal for the expression of NPY and AgRP. Recent data suggest that ghrelin has an important role in the regulation of leptin and insulin secretion and vice versa. The present paper updates the effects of ghrelin on the control of energy homeostasis and reviews the molecular mechanisms of ghrelin synthesis, as well as interaction with GHS receptors and signalling. Relationships with leptin and insulin in the regulation of energy homeostasis are addressed.

The role of hypothalamic malonyl-CoA in energy homeostasis

Energy balance is monitored by hypothalamic neurons that respond to peripheral hormonal and afferent neural signals that sense energy status. Recent physiologic, pharmacologic, and genetic evidence has implicated malonyl-CoA, an intermediate in fatty acid synthesis, as a regulatory component of this energy-sensing system. The level of malonyl-CoA in the hypothalamus is dynamically regulated by fasting and feeding, which alter subsequent feeding behavior. Fatty acid synthase (FAS) inhibitors, administered systemically or intracerebroventricularly to lean or obese mice, increase hypothalamic malonyl-CoA leading to the suppression of food intake. Conversely, lowering malonyl-CoA with an acetyl-CoA carboxylase (ACC) inhibitor or by the ectopic expression of malonyl-CoA decarboxylase in the hypothalamus increases food intake and reverses inhibition by FAS inhibitors. Physiologically, the level of hypothalamic malonyl-CoA appears to be determined through phosphorylation/dephosphorylation of ACC by AMP kinase in response to changes in the AMP/ATP ratio, an indicator of energy status. Recent evidence suggests that the brain-specific carnitine:palmitoyl-CoA transferase-1 (CPT1c) may be a regulated target of malonyl-CoA that relays the "malonyl-CoA signal" in hypothalamic neurons that express the orexigenic and anorexigenic neuropeptides that regulate food intake and peripheral energy expenditure. Together these findings support a role for malonyl-CoA as an intermediary in the control of energy homeostasis.

Protein-mediated fatty acid uptake: novel insights from in vivo models

Long-chain fatty acids are both important metabolites as well as signaling molecules. Fatty acid transport proteins are key mediators of cellular fatty acid uptake and recent transgenic and knockout animal models have provided new insights into their contribution to energy homeostasis and to pathological processes, including obesity and insulin desensitization.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

How the hypothalamus controls glucose production: an update

Recent evidence highlights a crucial role of the brain in the control of glucose homeostasis. The hypothalamus senses and integrates signals of fuel abundance, such as circulating macronutrients (glucose and fatty acids) and nutrient-induced hormones (insulin and leptin). This, in turn, results in the activation of neural pathways that return circulating nutrients to baseline by reducing hepatic glucose production and food intake. In Type 2 diabetes and obesity, the ability of the brain to sense and respond to circulating signals is impaired. In this review, the neuroendocrine circuits that have recently been involved in the regulation of endogenous glucose production in rodents will be described. The study of these neural pathways promises to unveil new targets for the therapy of Type 2 diabetes and obesity.

Pancreatic signals controlling food intake; insulin, glucagon and amylin

The control of food intake and body weight by the brain relies upon the detection and integration of signals reflecting energy stores and fluxes, and their interaction with many different inputs related to food palatability and gastrointestinal handling as well as social, emotional, circadian, habitual and other situational factors. This review focuses upon the role of hormones secreted by the endocrine pancreas: hormones, which individually and collectively influence food intake, with an emphasis upon insulin, glucagon and amylin. Insulin and amylin are co-secreted by B-cells and provide a signal that reflects both circulating energy in the form of glucose and stored energy in the form of visceral adipose tissue. Insulin acts directly at the liver to suppress the synthesis and secretion of glucose, and some plasma insulin is transported into the brain and especially the mediobasal hypothalamus where it elicits a net catabolic response, particularly reduced food intake and loss of body weight. Amylin reduces meal size by stimulating neurons in the hindbrain, and there is evidence that amylin additionally functions as an adiposity signal controlling body weight as well as meal size. Glucagon is secreted from A-cells and increases glucose secretion from the liver. Glucagon acts in the liver to reduce meal size, the signal being relayed to the brain via the vagus nerves. To summarize, hormones of the endocrine pancreas are collectively at the crossroads of many aspects of energy homeostasis. Glucagon and amylin act in the short term to reduce meal size, and insulin sensitizes the brain to short-term meal-generated satiety signals; and insulin and perhaps amylin as well act over longer intervals to modulate the amount of fat maintained and defended by the brain. Hormones of the endocrine pancreas interact with receptors at many points along the gut-brain axis, from the liver to the sensory vagus nerve to the hindbrain to the hypothalamus; and their signals are conveyed both neurally and humorally. Finally, their actions include gastrointestinal and metabolic as well as behavioural effects.

Mice fed a lipogenic methionine-choline-deficient diet develop hypermetabolism coincident with hepatic suppression of SCD-1

Lipogenic diets that are completely devoid of methionine and choline (MCD) induce hepatic steatosis. MCD feeding also provokes systemic weight loss, for unclear reasons. In this study, we found that MCD feeding causes profound hepatic suppression of the gene encoding stearoyl-coenzyme A desaturase-1 (SCD-1), an enzyme whose regulation has significant effects on metabolic rate. Within 7 days of MCD exposure, hepatic SCD-1 mRNA decreased to nearly undetectable levels. By day 21, SCD-1 protein was absent from hepatic microsomes and fatty acids showed a decrease in monounsaturated species. These changes in hepatic SCD-1 were accompanied by signs of hypermetabolism. Calorimetry revealed that MCD-fed mice consumed 37% more energy than control mice (P = 0.0003). MCD feeding also stimulated fatty acid oxidation, although fatty oxidation genes were not significantly upregulated. Interestingly, despite their increased metabolic rate, MCD-fed mice did not increase their food consumption, and as a result, they lost 26% of their body weight in 21 days. In summary, MCD feeding suppresses SCD-1 in the liver, which likely contributes to hypermetabolism and weight loss. MCD feeding also induces hepatic steatosis, by an independent mechanism. Viewed together, these two disparate consequences of MCD feeding (weight loss and hepatic steatosis) give the appearance of an unusual form of lipodystrophy.

Intake of trans fatty acid–rich hydrogenated fat during pregnancy and lactation inhibits the hypophagic effect of central insulin in the adult offspring

OBJECTIVE

Using rats we examined whether maternal intake of hydrogenated fat rich in trans fatty acids affects brain fatty acid profile, hypothalamic content of insulin receptor and insulin receptor substrate-1 proteins, and the hypophagic effect of centrally administered insulin in 3-mo-old male progeny.

METHODS

Throughout pregnancy and lactation, Wistar rats ate isocaloric/normolipidic diets with soybean oil (control) or soybean oil-derived hydrogenated fat (trans diet) as a fat source. Upon weaning, the trans offspring continued on the trans diet (trans group) or were switched to a control diet (trans-control group).

RESULTS

Compared with control rats, trans rats had lower brain levels of eicosapentaenoic acid. Compared with trans rats, trans-control rats had increased levels of total polyunsaturated fatty acids and arachidonic acid and decreased levels of trans fatty acids, saturated fatty acids, and monounsaturated fatty acids. Insulin receptor and insulin receptor substrate-1 levels were significantly lower (44% and 38%, respectively) in trans than in control rats. In trans-control rats, insulin receptor was 26% lower (P < 0.05), whereas insulin receptor substrate-1 was 50% lower, than in control rats. Insulin decreased 24-h feeding in control (27%) and trans (38%) rats but failed to do so in trans-control rats. The latter group had increased serum glucose levels.

CONCLUSIONS

The data suggest that the early (intrauterine/perinatal) exposure to hydrogenated fat rich in trans fatty acids programmed the hypothalamic feeding control mechanisms. As young adults, only trans-control animals showed loss of insulin-induced hypophagia, indicating that the mismatch between early and later nutritional environments was relevant. However, the trans group also showed signs of altered appetite signaling mechanisms, suggesting that the early adaptations may have deleterious consequences later in life.

Brain glucose sensing: a subtle mechanism

PURPOSE OF REVIEW

Brain nutrient sensing allows a fine regulation of different physiological functions, such as food intake and blood glucose, related to energy homeostasis. Glucose sensing is the most studied function and a parallel has been made between the cellular mechanisms involved in pancreatic beta cells and neurons.

RECENT FINDINGS

Two types of glucosensing neurons have been characterized--those for which the activity is proportional to changes in glucose concentration and those for which the activity is inversely proportional to these changes. A new level of complexity has recently been demonstrated, as the response and the mechanism appear to vary in function according to the level of the glucose change. For some of the responses, the detection is probably not at the level of the neuron itself, but astrocytes also appear to be involved, indicating a coupling between the two types of cells. Finally, numerous data have demonstrated the modulation of glucose sensing by other nutrients, in particular fatty acids, hormones (insulin, leptin and ghrelin) and peptides (neuropeptide Y). This implies a common pathway in which AMPkinase may play a crucial role.

SUMMARY

Recent observations in brain nutrient sensing indicate subtle mechanisms, with different cellular and molecular mechanisms involved. This fact would explain the discrepancies reported in the expression of different proteins (glucose transporters, hexokinases, channels). Astrocytes may be involved in one type of response, thus adding a new level of complexity.

Endocannabinoid system and its role in energy regulation

Endocannabinoids are endogenous agonists for the two types of cannabinoid receptors identified to date, the CB₁ and CB₂ receptors. CB₁ receptors and tissue concentrations of endocannabinoids sufficient to activate them are present in the brain structures controlling energy intake (i.e., the hypothalamus, nucleus accumbens and nodose ganglion), as well as in all peripheral organs mostly involved in the regulation of energy homeostasis (i.e., the duodenum, small and large intestine, adipose tissue, skeletal muscle, pancreas and liver). Several peripheral neuropeptides and hormones involved in energy balance, and type of diet, regulate endocannabinoid levels, whereas endocannabinoids, in turn, regulate the expression and release of hypothalamic mediators involved in nutrient intake. Dysregulation of the endocannabinoid system might contribute to the development of eating disorders and explain why CB₁ receptor blockers are efficacious at reducing not only food intake but also the metabolic consequences of visceral adiposity and obesity.

L-diabetes--causes, pathogenesis and therapy

L-diabetes represents a subtype of non-autoimmunopathic and non-adipose diabetes mellitus. It is hypothesized that ATP-sensory brain centres measure the cerebral ATP concentration and announce a hypoglycaemia if the setpoint is undercut. The disease involves a decreased ATP formation in the CNS that is independent of blood glucose levels, and that leads to a "hypoglycaemia" false alarm. UGT1-polymorphisms, a sensitive sympathetic system, an IgM deficit and an increased porousity of the mucous membrane of the small intestine have been postulated in its etiology. These causative factors bring about increasing amounts of toxins and radicals which impair the ATP generation in the CNS so that through the announcement of a non-existing hypoglycaemia the release of the insulin antagonists hGH, cortisol and adrenaline is induced.

Hypothalamic mTOR signaling regulates food intake

The mammalian Target of Rapamycin (mTOR) protein is a serine-threonine kinase that regulates cell-cycle progression and growth by sensing changes in energy status. We demonstrated that mTOR signaling plays a role in the brain mechanisms that respond to nutrient availability, regulating energy balance. In the rat, mTOR signaling is controlled by energy status in specific regions of the hypothalamus and colocalizes with neuropeptide Y and proopiomelanocortin neurons in the arcuate nucleus. Central administration of leucine increases hypothalamic mTOR signaling and decreases food intake and body weight. The hormone leptin increases hypothalamic mTOR activity, and the inhibition of mTOR signaling blunts leptin's anorectic effect. Thus, mTOR is a cellular fuel sensor whose hypothalamic activity is directly tied to the regulation of energy intake.

Tamoxifen-induced anorexia is associated with fatty acid synthase inhibition in the ventromedial nucleus of the hypothalamus and accumulation of malonyl-CoA

Fatty acid metabolism in the hypothalamus has recently been shown to regulate feeding. The selective estrogen receptor modulator tamoxifen (TMX) exerts a potent anorectic effect. Here, we show that the anorectic effect of TMX is associated with the accumulation of malonyl-CoA in the hypothalamus and inhibition of fatty acid synthase (FAS) expression specifically in the ventromedial nucleus of the hypothalamus (VMN). Furthermore, we demonstrate that FAS mRNA expression is physiologically regulated by fasting and refeeding in the VMN but not in other hypothalamic nuclei. Thus, the VMN appears to be the hypothalamic site where regulation of FAS and feeding converge. Supporting the potential clinical relevance of these observations, reanalysis of a primary breast cancer prevention study showed that obese women treated with TMX gained significantly less body weight over a 6-year period than obese women given placebo. The finding that TMX can modulate appetite through alterations in FAS expression and malonyl-CoA levels suggests a link between hypothalamic sex steroid receptors, fatty acid metabolism, and feeding behavior.

Modulation of fatty acid metabolism as a potential approach to the treatment of obesity and the metabolic syndrome

Increased de novo lipogenesis and reduced fatty acid oxidation are probable contributors to adipose accretion in obesity. Moreover, these perturbations have a role in leading to non-alcoholic steatohepatitis, dyslipidemia, and insulin resistance--via "lipotoxicity"-related mechanisms. Research in this area has prompted an effort to evaluate several discrete enzymes in these pathways as targets for future therapeutic intervention. Acetyl-CoA carboxylase 1 (ACC1) and ACC2 regulate fatty acid synthesis and indirectly control fatty acid oxidation via a key product, malonyl CoA. Based on mouse genetic and preclinical pharmacologic evidence, inhibition of ACC1 and/or ACC2 may be a useful approach to treat obesity and metabolic syndrome. Similarly, available data suggest that inhibition of other enzymes in this pathway, including fatty acid synthase, stearoyl CoA desaturase, and diacylglycerol acytransferase 1, will have beneficial effects. AMP-activated protein kinase is a master regulator of nutrient metabolism, which controls several aspects of lipid metabolism. Activation of AMPK in selected tissues is also a potential therapeutic approach. Inhibition of hormone-sensitive lipase is another possible approach. The rationale for modulating the activity of these enzymes and their relative merits (and downsides) as possible therapeutic targets are further discussed.

Molecular disruption of hypothalamic nutrient sensing induces obesity

The sensing of circulating nutrients within the mediobasal hypothalamus may be critical for energy homeostasis. To induce a sustained impairment in hypothalamic nutrient sensing, adeno-associated viruses (AAV) expressing malonyl-coenzyme A decarboxylase (MCD; an enzyme involved in the degradation of malonyl coenzyme A) were injected bilaterally into the mediobasal hypothalamus of rats. MCD overexpression led to decreased abundance of long-chain fatty acyl-coenzyme A in the mediobasal hypothalamus and blunted the hypothalamic responses to increased lipid availability. The enhanced expression of MCD within this hypothalamic region induced a rapid increase in food intake and progressive weight gain. Obesity was sustained for at least 4 months and occurred despite increased plasma concentrations of leptin and insulin. These findings indicate that nutritional modulation of the hypothalamic abundance of malonyl-coenzyme A is required to restrain food intake and that a primary impairment in this central nutrient-sensing pathway is sufficient to disrupt energy homeostasis and induce obesity.

Integration of metabolic stimuli in the hypothalamic arcuate nucleus

Integration of peripheral and central anabolic and catabolic inputs within the hypothalamic arcuate nucleus (ARC) is believed to be central to the maintenance of energy balance. In order to perform this complex task, neurons in the ARC express receptors for all major humoral and central transmitters involved in the maintenance of energy homeostasis. The integration of these inputs occurs at the cellular and circuit level and the resulting electrical output forms the origins for the activation of feeding and energy balance-related networks. Here, we discuss the role that active intrinsic membrane conductances, K(ATP) channels and intracellular second messenger systems play in the integration of metabolic stimuli at the cellular level in the ARC. We conclude that the research into the integration of hunger and satiety signals in the ARC has made substantial progress in the last decade, but we are far from unraveling the complex neuronal networks involved in the maintenance of energy homeostasis. The diverse range of inputs, neuronal integrative properties, targets, output signals and how these signals relate to the physiological output provides us with a colossal challenge for years to come. However, to battle the current obesity epidemic, target-specific drugs need to be developed for which the knowledge of neuronal pathways involved in the maintenance of energy homeostasis will be crucial.

Molecular and anatomical determinants of central leptin resistance

The increasing incidence of obesity in developed nations is an ever-growing challenge to health care, promoting diabetes and other diseases. The hormone leptin, which is derived from adipose tissue, regulates feeding and energy expenditure. Most forms of obesity are associated with diminished responsiveness to the appetite-suppressing effects of leptin. Here we review the mechanisms by which leptin activates intracellular signals, the roles of these signals in leptin action in vivo, and mechanisms that may attenuate leptin signaling, limiting its action in obese individuals. We highlight data regarding the expression of SOCS3 (a potential mediator of leptin resistance) in the arcuate nucleus of the hypothalamus.

Ciclopirox olamine lotion 1%: bioequivalence to ciclopirox olamine cream 1% and clinical efficacy in tinea pedis

Studies were conducted to assess the bioequivalence of a new antimycotic formulation, ciclopirox olamine lotion 1%, to an established compound, ciclopirox olamine cream 1%. Results of in vitro studies, using skin samples from human cadavers and domestic pigs, demonstrated that the two formulations equally penetrate all layers of the stratum corneum and inhibit the growth of Trichophyton mentagrophytes and Candida albicans. In vivo studies in guinea pigs and in human volunteers demonstrated the comparable therapeutic efficacy of the lotion and the cream in experimental trichophytosis. In addition, a multicenter, double-blind clinical trial was undertaken to compare ciclopirox olamine lotion 1% with the vehicle alone in the treatment of patients with tinea pedis. Patients with plantar, interdigital, or vesicular tinea pedis were enrolled in the studies. Patients were treated for 28 days. Clinical and mycological responses were determined during treatment and two weeks posttreatment. Ciclopirox olamine lotion 1% was found to be significantly more effective than its vehicle in the treatment of patients with common tinea pedis. Minor localized side effects (pruritus, burning sensation) were reported in 2% of 89 patients treated with ciclopirox olamine lotion 1%. The results demonstrate the bioequivalence of ciclopirox olamine lotion 1% and ciclopirox olamine cream 1% and confirm the clinical effectiveness and safety of the lotion in the treatment of tinea pedis, a generally recalcitrant fungal infection. It is concluded that ciclopirox olamine lotion 1% can be used as an alternative to ciclopirox olamine cream 1% for treatment of tinea pedis, tinea versicolor, tinea cruris, tinea corporis, and cutaneous candidiasis when the convenience and/or cosmetic elegance of a lotion is desired.