Differential control of metabolic and cardiovascular functions by melanocortin-4 receptors in proopiomelanocortin neurons

Appetite- and Weight-Regulating Neuroendocrine Circuitry in Hypothalamic Obesity

Since hypothalamic obesity (HyOb) was first described over 120 years ago by Joseph Babinski and Alfred Fröhlich, advances in molecular genetic laboratory techniques have allowed us to elucidate various components of the intricate neurocircuitry governing appetite and weight regulation connecting the hypothalamus, pituitary gland, brainstem, adipose tissue, pancreas, and gastrointestinal tract. On a background of an increasing prevalence of population-level common obesity, the number of survivors of congenital (eg, septo-optic dysplasia, Prader-Willi syndrome) and acquired (eg, central nervous system tumors) hypothalamic disorders is increasing, thanks to earlier diagnosis and management as well as better oncological therapies. Although to date the discovery of several appetite-regulating peptides has led to the development of a range of targeted molecular therapies for monogenic obesity syndromes, outside of these disorders these discoveries have not translated into the development of efficacious treatments for other forms of HyOb. This review aims to summarize our current understanding of the neuroendocrine physiology of appetite and weight regulation, and explore our current understanding of the pathophysiology of HyOb.

The melanocortin receptor signaling system and its role in neuroprotection against neurodegeneration: Therapeutic insights

The melanocortin signaling system consists of the melanocortin peptides, their distinctive receptors, accessory proteins, and endogenous antagonists. Melanocortin peptides are small peptide hormones that have been studied in a variety of physiological and pathological conditions. There are five types of melanocortin receptors, and they are distributed within the central nervous system and in several tissues of the periphery. The G protein-coupled melanocortin receptors typically signal through adenylyl cyclase and other downstream signaling pathways. Depending on the ligand, surface expression of melanocortin receptor, receptor occupancy period, related proteins, the type of cell, and other parameters, the signaling pathways are complicated and pleiotropic. While it is known that all five melanocortin receptors are coupled to Gs, they can also occasionally couple to Gq or Gi. Both direct and indirect neuroprotection are induced by the melanocortin signaling system. Targeting several of the components of the melanocortin signaling system (ligands, receptors, accessory proteins, signaling effectors, and regulators) may provide therapeutic opportunities. Activation of the melanocortin system improves different functional traits in neurodegenerative diseases. There is a potential for additional melanocortin system interventions by interfering with dimerization or dissociation. This review aims to discuss the melanocortin receptor signaling system and its role in neuroprotection, as well as its therapeutic potential.

Recent Advances in Hypertension: Intersection of Metabolic and Blood Pressure Regulatory Circuits in the Central Nervous System

Obesity represents the single greatest ongoing roadblock to improving cardiovascular health. Prolonged obesity is associated with fundamental changes in the integrative control of energy balance, including the development of selective leptin resistance, which is thought to contribute to obesity-associated hypertension, and adaptation of resting metabolic rate (RMR) when excess weight is reduced. Leptin and the melanocortin system within the hypothalamus contribute to the control of both energy balance and blood pressure. While the development of drugs to stimulate RMR and thereby reverse obesity through activation of the melanocortin system has been pursued, most of the resulting compounds simultaneously cause hypertension. Evidence supports the concept that although feeding behaviors, RMR, and blood pressure are controlled through mechanisms that utilize similar molecular mediators, these mechanisms exist in anatomically dissociable networks. New evidence supports a major change in molecular signaling within AgRP (Agouti-related peptide) neurons of the arcuate nucleus of the hypothalamus during prolonged obesity and the existence of multiple distinct subtypes of AgRP neurons that individually contribute to control of feeding, RMR, or blood pressure. Finally, ongoing work by our laboratory and others support a unique role for AT₁ (angiotensin II type 1 receptor) within one specific subtype of AgRP neuron for the control of RMR. We propose that understanding the unique biology of the AT₁-expressing, RMR-controlling subtype of AgRP neurons will help to resolve the selective dysfunctions in RMR control that develop during prolonged obesity and potentially point toward novel druggable antiobesity targets that will not simultaneously cause hypertension.

Association Between Obesity-Mediated Atrial Fibrillation and Therapy With Sodium Channel Blocker Antiarrhythmic Drugs

Importance

The association between obesity, an established risk factor for atrial fibrillation (AF), and response to antiarrhythmic drugs (AADs) remains unclear.

Objective

To test the hypothesis that obesity differentially mediates response to AADs in patients with symptomatic AF and in mice with diet-induced obesity (DIO) and pacing induced AF.

Design, Setting, and Participants

An observational cohort study was conducted including 311 patients enrolled in a clinical-genetic registry. Mice fed a high-fat diet for 10 weeks were also evaluated. The study was conducted from January 1, 2018, to June 2, 2019.

Main Outcomes and Measures

Symptomatic response was defined as continuation of the same AAD for at least 3 months. Nonresponse was defined as discontinuation of the AAD within 3 months of initiation because of poor symptomatic control of AF necessitating alternative rhythm control therapy. Outcome measures in DIO mice were pacing-induced AF and suppression of AF after 2 weeks of treatment with flecainide acetate or sotalol hydrochloride.

Results

A total of 311 patients (mean [SD] age, 65 [12] years; 120 women [38.6%]) met the entry criteria and were treated with a class I or III AAD for symptomatic AF. Nonresponse to class I AADs in patients with obesity was less than in those without obesity (30% [obese] vs 6% [nonobese]; difference, 0.24; 95% CI, 0.11-0.37; P = .001). Both groups had similar symptomatic response to a potassium channel blocker AAD. On multivariate analysis, obesity, AAD class (class I vs III AAD [obese] odds ratio [OR], 4.54; 95% Wald CI, 1.84-11.20; P = .001), female vs male sex (OR, 2.31; 95% Wald CI, 1.07-4.99; P = .03), and hyperthyroidism (OR, 4.95; 95% Wald CI, 1.23-20.00; P = .02) were significant indicators of the probability of failure to respond to AADs. Pacing induced AF in 100% of DIO mice vs 30% (P < .001) in controls. Furthermore, DIO mice showed a greater reduction in AF burden when treated with sotalol compared with flecainide (85% vs 25%; P < .01).

Conclusions and Relevance

Results suggest that obesity differentially mediates response to AADs in patients and in mice with AF, possibly reducing the therapeutic effectiveness of sodium channel blockers. These findings may have implications for the management of AF in patients with obesity.

Neuroanatomy of melanocortin-4 receptor pathway in the mouse brain

Objective

Melanocortin-4 receptors (MC4Rs) are key regulators of energy homeostasis and adipose deposition in the central nervous system. Considering that MC4R expression regions and function-related research mainly focus on the paraventricular nucleus (PVN), little is known about their distribution throughout the mouse brain, although its messenger RNA distribution has been analyzed in the rat. Therefore, MC4R protein localization in mouse neurons was the focus of this study.

Methods

MC4R protein distribution was assessed in mice through immunofluorescence and Western blotting.

Results

MC4R was differentially expressed throughout the arcuate nucleus (ARC), nucleus of the solitary tract (NTS), raphe pallidus (RPa), medial cerebellar nucleus, intermediolateral nucleus, and brainstem. The highest MC4R protein levels were found in the ARC and ventromedial hypothalamic nucleus, while they were significantly lower in the parabrachial nucleus and NTS. The lowest MC4R protein levels were found in the PVN; there was no difference in the protein levels between the area postrema and RPa.

Conclusions

These data provide a basic characterization of MC4R-expressing neurons and protein distribution in the mouse brain and may aid further research on its role in energy homeostasis.

Obesity, kidney dysfunction and hypertension: mechanistic links

Excessive adiposity raises blood pressure and accounts for 65-75% of primary hypertension, which is a major driver of cardiovascular and kidney diseases. In obesity, abnormal kidney function and associated increases in tubular sodium reabsorption initiate hypertension, which is often mild before the development of target organ injury. Factors that contribute to increased sodium reabsorption in obesity include kidney compression by visceral, perirenal and renal sinus fat; increased renal sympathetic nerve activity (RSNA); increased levels of anti-natriuretic hormones, such as angiotensin II and aldosterone; and adipokines, particularly leptin. The renal and neurohormonal pathways of obesity and hypertension are intertwined. For example, leptin increases RSNA by stimulating the central nervous system proopiomelanocortin-melanocortin 4 receptor pathway, and kidney compression and RSNA contribute to renin-angiotensin-aldosterone system activation. Glucocorticoids and/or oxidative stress may also contribute to mineralocorticoid receptor activation in obesity. Prolonged obesity and progressive renal injury often lead to the development of treatment-resistant hypertension. Patient management therefore often requires multiple antihypertensive drugs and concurrent treatment of dyslipidaemia, insulin resistance, diabetes and inflammation. If more effective strategies for the prevention and control of obesity are not developed, cardiorenal, metabolic and other obesity-associated diseases could overwhelm health-care systems in the future.

Melanocortin-4 Receptors and Sympathetic Nervous System Activation in Hypertension

PURPOSE OF REVIEW

To highlight the role of the brain melanocortin 4 receptor (MC4R) for sympathetic nervous system (SNS) activation in hypertension.

RECENT FINDINGS

Hypertension is the most significant risk factor for developing cardiovascular disease. Although excess weight gain is associated with at least two thirds of primary hypertension cases, the pathophysiological mechanisms involved remain the subject of intense investigation. Multiple studies demonstrate an important role for increased sympathetic nervous system (SNS) activity in development and maintenance of hypertension, and that the brain MC4R modulates SNS activity to thermogenic, cardiovascular, and kidney tissues. These studies also support the concept that MC4R activation is critical for obesity-induced hypertension as well as other forms of hypertension associated with increased SNS activity. MC4R is a potential target for antiobesity therapy, although there are challenges in using MC4R agonists to induce weight loss without evoking increases in SNS activity.

Activity of the Hypothalamic Melanocortin System Decreases in Middle-Aged and Increases in Old Rats

Appearance of middle-aged obesity and aging anorexia both in humans and rodents suggests a role for regulatory alterations. Hypothalamic melanocortin agonist, α-melanocyte-stimulating hormone (α-MSH) produced in the arcuate nucleus (ARC), reduces body weight via inducing hypermetabolism and anorexia mainly through melanocortin 4 receptors (MC4Rs) in the paraventricular nucleus (PVN). Orexigenic ARC-derived agouti-related protein (AgRP) is an inverse agonist on MC4R in the PVN. Previously, we demonstrated that characteristic age-related shifts in the catabolic effects of α-MSH may contribute both to middle-aged obesity and aging anorexia. Responsiveness to α-MSH decreases in middle-aged rats compared with young adults, whereas in old age it rises again significantly. We hypothesized corresponding age-related dynamics of endogenous melanocortins. Therefore, we quantified mRNA gene expression and peptide or protein level of α-MSH, AgRP, and MC4R in the ARC and PVN of male Wistar rats of five age groups (from young to old). Immunofluorescence and quantitative reverse transcriptase polymerase chain reaction were applied. α-MSH and MC4R immunoreactivities in the ARC and PVN declined in middle-aged and increased together with their expressions in aging rats. AgRP gene expression but not its immunoreactivity increased in aging rats. Our results demonstrate that age-dependent changes of endogenous melanocortins contribute to middle-aged obesity and aging anorexia.

Control of appetite, blood glucose, and blood pressure during melanocortin-4 receptor activation in normoglycemic and diabetic NPY-deficient mice

Although central melanocortin 4 receptor (MC4R) blockade abolishes the central nervous system (CNS)-mediated anorexogenic, antidiabetic, and cardiovascular actions of leptin, chronic MC4R stimulation fails to completely mimic the effects of leptin. Because neuropeptide Y (NPY) and MC4R exert opposite effects on cardiovascular and metabolic functions, we tested the role of NPY in offsetting the long-term actions of MC4R activation. Wild-type (WT) and NPY-deficient (NPY^-/-) mice were implanted with telemetry probes for measuring mean arterial pressure (MAP) and heart rate (HR) 24 h/day. After the mice recovered from surgery and stable baseline measurements, the MC3/4R agonist melanotan II (MTII, 120 μg·kg^-1·day^-1 iv) was infused for 7 days followed by a recovery period. No major differences between groups were observed at baseline except for slightly higher food intake and HR in NPY^-/- mice (4.3 ± 0.2 vs. 3.4 ± 0.2 g/day and 567 ± 14 vs. 522 ± 13 beats/min). Chronic MTII infusion reduced food intake in both groups while causing transient increases in MAP and HR only in WT mice (peaks of 11 ± 3 mmHg and 126 ± 13 beats/min). To examine whether NPY deficiency would amplify the antidiabetic effects of MC4R activation, diabetes was induced with streptozotocin (STZ) 1 wk before baseline measurements were taken, and the same experimental protocol was followed. In WT and NPY^-/- mice, STZ-induced diabetes led to similar hyperphagia, hyperglycemia, and weight loss, which were not reversed by chronic MTII treatment. Our results demonstrate that chronic MC4R activation, even in NPY-deficient mice, does not mimic chronic antidiabetic, cardiovascular, or metabolic actions of leptin, and that NPY is not essential for hyperphagia or cardiovascular changes associated with diabetes.

Mitochondrial Dynamics Mediated by Mitofusin 1 Is Required for POMC Neuron Glucose-Sensing and Insulin Release Control

Proopiomelanocortin (POMC) neurons are critical sensors of nutrient availability implicated in energy balance and glucose metabolism control. However, the precise mechanisms underlying nutrient sensing in POMC neurons remain incompletely understood. We show that mitochondrial dynamics mediated by Mitofusin 1 (MFN1) in POMC neurons couple nutrient sensing with systemic glucose metabolism. Mice lacking MFN1 in POMC neurons exhibited defective mitochondrial architecture remodeling and attenuated hypothalamic gene expression programs during the fast-to-fed transition. This loss of mitochondrial flexibility in POMC neurons bidirectionally altered glucose sensing, causing abnormal glucose homeostasis due to defective insulin secretion by pancreatic β cells. Fed mice lacking MFN1 in POMC neurons displayed enhanced hypothalamic mitochondrial oxygen flux and reactive oxygen species generation. Central delivery of antioxidants was able to normalize the phenotype. Collectively, our data posit MFN1-mediated mitochondrial dynamics in POMC neurons as an intrinsic nutrient-sensing mechanism and unveil an unrecognized link between this subset of neurons and insulin release.

Biased signaling at neural melanocortin receptors in regulation of energy homeostasis

The global prevalence of obesity highlights the importance of understanding on regulation of energy homeostasis. The central melanocortin system is an important intersection connecting the neural pathways controlling satiety and energy expenditure to regulate energy homeostasis by sensing and integrating the signals of external stimuli. In this system, neural melanocortin receptors (MCRs), melanocortin-3 and -4 receptors (MC3R and MC4R), play crucial roles in the regulation of energy homeostasis. Recently, multiple intracellular signaling pathways and biased signaling at neural MCRs have been discovered, providing new insights into neural MCR signaling. This review attempts to summarize biased signaling including biased receptor mutants (both naturally occurring and lab-generated) and biased ligands at neural MCRs, and to provide a better understanding of obesity pathogenesis and new therapeutic implications for obesity treatment.

Role of leptin in energy expenditure: the hypothalamic perspective

The adipocyte-derived hormone leptin is a peripheral signal that informs the brain about the metabolic status of an organism. Although traditionally viewed as an appetite-suppressing hormone, studies in the past decade have highlighted the role of leptin in energy expenditure. Leptin has been shown to increase energy expenditure in particular through its effects on the cardiovascular system and brown adipose tissue (BAT) thermogenesis via the hypothalamus. The current review summarizes the role of leptin signaling in various hypothalamic nuclei and its effects on the sympathetic nervous system to influence blood pressure, heart rate, and BAT thermogenesis. Specifically, the role of leptin signaling on three different hypothalamic nuclei, the dorsomedial hypothalamus, the ventromedial hypothalamus, and the arcuate nucleus, is reviewed. It is known that all of these brain regions influence the sympathetic nervous system activity and thereby regulate BAT thermogenesis and the cardiovascular system. Thus the current work focuses on how leptin signaling in specific neuronal populations within these hypothalamic nuclei influences certain aspects of energy expenditure.

Role of the brain melanocortins in blood pressure regulation

Melanocortins play an important role in regulating blood pressure (BP) and sympathetic nervous system (SNS) activity as well as energy balance, glucose and other metabolic functions in humans and experimental animals. In experimental models of hypertension with high SNS activity, blockade of the melanocortin-4 receptor (MC4R) reduces BP despite causing marked hyperphagia and obesity. Activation of the central nervous system (CNS) pro-opiomelanocortin (POMC)-MC4R pathway appears to be an important link between obesity, SNS activation and hypertension. Despite having severe obesity, subjects with MC4R deficiency exhibit reductions in BP, heart rate, and urinary catecholamine excretion, as well as attenuated SNS responses to cold stimuli compared to obese subjects with normal MC4R function. In this review we discuss the importance of the brain POMC-MC4R system in regulating SNS activity and BP in obesity and other forms of hypertension. We also highlight potential mechanisms and brain circuitry by which the melanocortin system regulates cardiovascular function.

Obesity-associated extracellular mtDNA activates central TGFβ pathway to cause blood pressure increase

Hypothalamic inflammation was recently found to mediate obesity-related hypertension, but the responsible upstream mediators remain unexplored. In this study, we show that dietary obesity is associated with extracellular release of mitochondrial DNA (mtDNA) into the cerebrospinal fluid and that central delivery of mtDNA mimics transforming growth factor-β (TGFβ) excess to activate downstream signaling pathways. Physiological study reveals that central administration of mtDNA or TGFβ is sufficient to cause hypertension in mice. Knockout of the TGFβ receptor in proopiomelanocortin neurons counteracts the hypertensive effect of not only TGFβ but also mtDNA excess, while the hypertensive action of central mtDNA can be blocked pharmacologically by a TGFβ receptor antagonist or genetically by TGFβ receptor knockout. Finally, we confirm that obesity-induced hypertension can be reversed through central treatment with TGFβ receptor antagonist. In conclusion, circulating mtDNA in the brain employs neural TGFβ pathway to mediate a central inflammatory mechanism of obesity-related hypertension.

Role of hindbrain melanocortin-4 receptor activity in controlling cardiovascular and metabolic functions in spontaneously hypertensive rats

BACKGROUND

Although we previously demonstrated that activation of central nervous system (CNS) melanocortin3/4 receptors (MC3/4R) play a key role in blood pressure (BP) regulation, especially in spontaneously hypertensive rats (SHRs), the importance of hindbrain MC4R is still unclear.

METHOD

In the present study, we examined the cardiovascular and metabolic effects of chronic inhibition of MC3/4R in the hindbrain of SHRs and normotensive Wistar-Kyoto (WKY) rats. Male WKY rats (n = 6) and SHRs (n = 7) were implanted with telemetry probes to measure BP and heart rate (HR) 24 h/day, and an intracerebroventricular cannula was placed into the fourth ventricle. After 10 days of recovery and 5 days of control measurements, the MC3/4R antagonist (SHU-9119) was infused into the fourth ventricle (1 nmol/h) to antagonize hindbrain MC4R for 10 days, followed by a 5-day recovery period.

RESULTS

Chronic hindbrain MC3/4R antagonism significantly increased food intake and body weight in WKY rats (17 ± 1 to 35 ± 2 g/day and 280 ± 8 to 353 ± 8 g) and SHRs (19 ± 2 to 35 ± 2 g/day and 323 ± 7 to 371 ± 11 g), and markedly increased fasting insulin and leptin levels while causing no changes in blood glucose levels (99 ± 4 to 87 ± 4 and 89 ± 5 to 89 ± 4 mg/dl, respectively, for WKY rats and SHRs). Chronic SHU-9119 infusion reduced mean arterial pressure and HR similarly in WKY rats (-8 ± 1 mmHg and -47 ± 3 b.p.m.) and SHRs (-11 ± 3 mmHg and -44 ± 3 b.p.m.).

CONCLUSION

These results suggest that although hindbrain MC4R activity contributes to appetite and HR regulation, it does not play a major role in mediating the elevated BP in SHRs.

The Melanocortin Receptor System: A Target for Multiple Degenerative Diseases

The melanocortin receptor system consists of five closely related G-protein coupled receptors (MC1R, MC2R, MC3R, MC4R and MC5R). These receptors are involved in many of the key biological functions for multicellular animals, including human beings. The natural agonist ligands for these receptors are derived by processing of a primordial animal gene product, proopiomelanocortin (POMC). The ligand for the MC2R is ACTH (Adrenal Corticotropic Hormone), a larger processed peptide from POMC. The natural ligands for the other 4 melanocortin receptors are smaller peptides including α-melanocyte stimulating hormone (α-MSH) and related peptides from POMC (β-MSH and γ-MSH). They all contain the sequence His-Phe-Arg-Trp that is conserved throughout evolution. Thus, there has been considerable difficulty in developing highly selective ligands for the MC1R, MC3R, MC4R and MC5R. In this brief review, we discuss the various approaches that have been taken to design agonist and antagonist analogues and derivatives of the POMC peptides that are selective for the MC1R, MC3R, MC4R and MC5R receptors, via peptide, nonpeptide and peptidomimetic derivatives and analogues and their differential interactions with receptors that may help account for these selectivities.

The change in cerebral glucose metabolism after electroacupuncture: a possible marker to predict the therapeutic effect of deep brain stimulation for refractory anorexia nervosa

Some reports have demonstrated that deep brain stimulation (DBS) is a promising treatment for patients who suffer from intractable anorexia nervosa. However, the nature of DBS may not be viewed as a standard clinical treatment option for anorexia nervosa because of the unpredictable outcome before DBS. Just like DBS in the brain, electroacupuncture at acupoints is also efficient in treating refractory anorexia nervosa. Some neuroimaging studies using functional magnetic resonance imaging, single-photon emission computed tomography (SPECT), and positron emission tomography (PET) had revealed that both DBS and electroacupuncture at acupoints with electrical stimulation are related to the changes in cerebral glucose metabolism. Therefore, we hypothesize that the changes in cerebral glucose metabolism after electroacupuncture might be useful to predict the therapeutic effect of deep brain stimulation for refractory anorexia nervosa.

Obesity-induced hypertension: interaction of neurohumoral and renal mechanisms

Excess weight gain, especially when associated with increased visceral adiposity, is a major cause of hypertension, accounting for 65% to 75% of the risk for human primary (essential) hypertension. Increased renal tubular sodium reabsorption impairs pressure natriuresis and plays an important role in initiating obesity hypertension. The mediators of abnormal kidney function and increased blood pressure during development of obesity hypertension include (1) physical compression of the kidneys by fat in and around the kidneys, (2) activation of the renin-angiotensin-aldosterone system, and (3) increased sympathetic nervous system activity. Activation of the renin-angiotensin-aldosterone system is likely due, in part, to renal compression, as well as sympathetic nervous system activation. However, obesity also causes mineralocorticoid receptor activation independent of aldosterone or angiotensin II. The mechanisms for sympathetic nervous system activation in obesity have not been fully elucidated but may require leptin and activation of the brain melanocortin system. With prolonged obesity and development of target organ injury, especially renal injury, obesity-associated hypertension becomes more difficult to control, often requiring multiple antihypertensive drugs and treatment of other risk factors, including dyslipidemia, insulin resistance and diabetes mellitus, and inflammation. Unless effective antiobesity drugs are developed, the effect of obesity on hypertension and related cardiovascular, renal and metabolic disorders is likely to become even more important in the future as the prevalence of obesity continues to increase.

The brain melanocortin system, sympathetic control, and obesity hypertension

Excess weight gain is the most significant, preventable cause of increased blood pressure (BP) in patients with primary (essential) hypertension and increases the risk for cardiovascular and renal diseases. In this review, we discuss the role of the brain melanocortin system in causing increased sympathetic activity in obesity and other forms of hypertension. In addition, we highlight potential mechanisms by which the brain melanocortin system modulates metabolic and cardiovascular functions.

Brain-mediated antidiabetic, anorexic, and cardiovascular actions of leptin require melanocortin-4 receptor signaling

We previously demonstrated that leptin has powerful central nervous system (CNS)-mediated antidiabetic actions. In this study we tested the importance of melanocortin-4 receptors (MC4Rs) for leptin's ability to suppress food intake, increase blood pressure (BP) and heart rate (HR), and normalize glucose levels in insulin-dependent diabetes. MC4R knockout (MC4R-KO) and control wild-type (WT) rats were implanted with intracerebroventricular (ICV) cannula and BP and HR were measured 24 h/day by telemetry. After 5-day control period, an injection of streptozotocin (50 mg/kg, ip) was used to induce diabetes. Eight days after injection, an osmotic pump was implanted subcutaneously and connected to the ICV cannula to deliver leptin (15 μg/day) for 7 days. At baseline, MC4R-KO rats were hyperphagic and 40% heavier than WT rats. Despite obesity, BP was similar (112 ± 2 vs. 111 ± 2 mmHg) and HR was lower in MC4R-KO rats (320 ± 6 vs. 347 ± 5 beats/min). Induction of diabetes increased food intake (30%) and reduced BP (∼17 mmHg) and HR (∼61 beats/min) in WT rats, while food intake, BP, and HR were reduced by ∼10%, 7 mmHg, and 33 beats/min, respectively, in MC4R-KO rats. Leptin treatment normalized blood glucose (437 ± 10 to 136 ± 18 mg/dl), reduced food intake (40%), and increased HR (+60 beats/min) and BP (+9 mmHg) in WT rats. Only modest changes in blood glucose (367 ± 16 to 326 ± 23 mg/dl), food intake (5%), HR (+16 beats/min) and BP (+4 mmHg) were observed in MC4R-KO rats. These results indicate that intact CNS MC4R signaling is necessary for leptin to exert its chronic antidiabetic, anorexic, and cardiovascular actions.

Roles for the sympathetic nervous system, renal nerves, and CNS melanocortin-4 receptor in the elevated blood pressure in hyperandrogenemic female rats

Women with polycystic ovary syndrome (PCOS) have hyperandrogenemia and increased prevalence of risk factors for cardiovascular disease, including elevated blood pressure. We recently characterized a hyperandrogenemic female rat (HAF) model of PCOS [chronic dihydrotestosterone (DHT) beginning at 4 wk of age] that exhibits similar characteristics as women with PCOS. In the present studies we tested the hypotheses that the elevated blood pressure in HAF rats is mediated in part by sympathetic activation, renal nerves, and melanocortin-4 receptor (MC4R) activation. Adrenergic blockade with terazosin and propranolol or renal denervation reduced mean arterial pressure (MAP by telemetry) in HAF rats but not controls. Hypothalamic MC4R expression was higher in HAF rats than controls, and central nervous system MC4R antagonism with SHU-9119 (1 nmol/h icv) reduced MAP in HAF rats. Taking a genetic approach, MC4R null and wild-type (WT) female rats were treated with DHT or placebo from 5 to 16 wk of age. MC4R null rats were obese and had higher MAP than WT control rats, and while DHT increased MAP in WT controls, DHT failed to further increase MAP in MC4R null rats. These data suggest that increases in MAP with chronic hyperandrogenemia in female rats are due, in part, to activation of the sympathetic nervous system, renal nerves, and MC4R and may provide novel insights into the mechanisms responsible for hypertension in women with hyperandrogenemia such as PCOS.

Delineating the regulation of energy homeostasis using hypothalamic cell models

Attesting to its intimate peripheral connections, hypothalamic neurons integrate nutritional and hormonal cues to effectively manage energy homeostasis according to the overall status of the system. Extensive progress in the identification of essential transcriptional and post-translational mechanisms regulating the controlled expression and actions of hypothalamic neuropeptides has been identified through the use of animal and cell models. This review will introduce the basic techniques of hypothalamic investigation both in vivo and in vitro and will briefly highlight the key advantages and challenges of their use. Further emphasis will be place on the use of immortalized models of hypothalamic neurons for in vitro study of feeding regulation, with a particular focus on cell lines proving themselves most fruitful in deciphering fundamental basics of NPY/AgRP, Proglucagon, and POMC neuropeptide function.

Impact of early life ovariectomy on blood pressure and body composition in a female mouse model of systemic lupus erythematosus

Because of the preponderance of women affected by the chronic autoimmune disease systemic lupus erythematosus (SLE), estrogen is thought to contribute to SLE disease progression. This is supported by evidence from experimental animal models of SLE showing that removal of estrogen in young female mice delays autoantibody production and renal injury and lengthens survival. Blood pressure and changes in body composition are important cardiovascular risk factors that can be regulated by estrogens. Because cardiovascular disease is the leading cause of death in patients with SLE, we used an established female mouse model of SLE (NZBWF1) to test whether early life removal of estrogen impacts the development of hypertension and changes in body composition commonly associated with SLE. Eight-week-old female SLE and control mice (NZW/LacJ) underwent either a sham operation or ovariectomy. Body weight, body composition (fat and lean masses), and renal injury (albuminuria) were monitored until mice reached 34 wk of age, at which time mean arterial pressure was assessed in conscious animals by a carotid catheter. Early life removal of the ovaries delayed the onset of autoantibody production and albuminuria while causing an increase in body weight and fat mass. Blood pressure in the adult was not altered by early life removal of the ovaries. These data suggest that estrogens may have a permissive role for the development of SLE while helping to maintain normal body weight and composition, which is associated with reduced cardiovascular risk.

Motion sickness: more than nausea and vomiting

Motion sickness is a complex syndrome that includes many features besides nausea and vomiting. This review describes some of these factors and points out that under normal circumstances, many cases of motion sickness go unrecognized. Motion sickness can occur during exposure to physical motion, visual motion, and virtual motion, and only those without a functioning vestibular system are fully immune. The range of vulnerability in the normal population varies about 10,000 to 1. Sleep deprivation can also enhance susceptibility. Systematic studies conducted in parabolic flight have identified velocity storage of semicircular canal signals-velocity integration-as being a key factor in both space motion sickness and terrestrial motion sickness. Adaptation procedures that have been developed to increase resistance to motion sickness reduce this time constant. A fully adequate theory of motion sickness is not presently available. Limitations of two popular theories, the evolutionary and the ecological, are described. A sensory conflict theory can explain many but not all aspects of motion sickness elicitation. However, extending the theory to include conflicts related to visceral afferent feedback elicited by voluntary and passive body motion greatly expands its explanatory range. Future goals should include determining why some conflicts are provocative and others are not but instead lead to perceptual reinterpretations of ongoing body motion. The contribution of visceral afferents in relation to vestibular and cerebellar signals in evoking sickness also deserves further exploration. Substantial progress is being made in identifying the physiological mechanisms underlying the evocation of nausea, vomiting, and anxiety, and a comprehensive understanding of motion sickness may soon be attainable. Adequate anti-motion sickness drugs without adverse side effects are not yet available.

The medial preoptic nucleus as a site of the thermogenic and metabolic actions of melanotan II in male rats

The present study was designed to investigate the role of the medial preoptic nucleus (MPO) as a site of the thermogenic and metabolic effects of the α-melanocyte-stimulating hormone analog melanotan II (MTII). We also assessed the involvement of the dorsomedial hypothalamic nucleus (DMH) by investigating the effects of the MPO infusion of MTII in rats with DMH lesions produced by kainic acid. Infusion of MTII in the MPO led to increases in interscapular brown adipose tissue (iBAT) temperature and iBAT uptake of 14C-bromopalmitate. Both increases were blocked by DMH lesions. iBAT temperature increase (area under curve) and 14C-bromopalmitate uptake emerged as two correlated variables ( r = 0.63, P < 0.001). DMH lesions also blocked MTII-induced expression of mRNAs coding for proteins involved in 1) thermogenesis [type II iodothyronine deiodinase ( Dio2) and peroxisome proliferator-activated receptor gamma coactivator 1-α ( Pgc1α)], 2) lipolysis [hormone-sensitive lipase ( Hsl)], and 3) lipogenesis [diacylglycerol-O-acyltransferase 2 ( Dgat2), fatty acid synthase ( Fas)], in iBAT of rats killed 1 h after MPO infusion of MTII. MTII also stimulated expression of genes in iWAT but only in rats with DMH lesions. These genes included glucose transporter member 4 ( Glut4), glycerol-3-phosphate acyltransferase 3 ( Gpat3), Dgat1, Dgat2, triglyceride lipase ( Atgl), Hsl, and carnitine palmitoyltransferase 1β ( Cpt1β). Altogether, the present results reveal the MPO as a site of the thermogenic and metabolic actions of MTII. They also contribute to establish the MPO-DMH duet as a significant target for melanocortins to modulate energy homeostasis.

Melanocortin 4 receptors in autonomic neurons regulate thermogenesis and glycemia

Whether melanocortin 4 receptors (MC4Rs) in extra-hypothalamic neurons, including cholinergic autonomic pre-ganglionic neurons, are required to control energy and glucose homeostasis is unclear. We found that MC4Rs in sympathetic, but not parasympathetic, pre-ganglionic neurons were required to regulate energy expenditure and body weight, including thermogenic responses to diet and cold exposure and 'beiging' of white adipose tissue. Deletion of Mc4r genes in both sympathetic and parasympathetic cholinergic neurons impaired glucose homeostasis.