Cardiovascular effects of leptin and orexins

Disinhibition does not play a role in endomorphin-2-induced changes in inspiratory motoneuron output produced by in vitro neonatal rat preparations

Low level activation of mu-opioid receptors (MORs) in neonatal rat brainstem-spinal cord preparations increases inspiratory burst amplitude recorded on cervical spinal roots. We tested whether: (1) MOR activation with an endogenous ligand, such as endomorphin-2, increases inspiratory burst amplitude, (2) disinhibition of GABAergic or glycinergic inhibitory synaptic transmission is involved, and (3) inflammation alters endomorphin-2 effects. Using neonatal rat (P0-P3) brainstem-spinal cord preparations, bath-applied endomorphin-2 (10-200 nM) increased inspiratory burst amplitude and decreased burst frequency. Blockade of GABAA receptors (picrotoxin), glycine receptors (strychnine), or both (picrotoxin and strychnine) did not abolish endomorphin-2-induced effects. In preparations isolated from neonatal rats injected 3 h previously with lipopolysaccharide (LPS, 0.1 mg/kg), endomorphin-2 continued to decrease burst frequency but abolished the burst amplitude increase. Collectively, these data indicate that disinhibition of inhibitory synaptic transmission is unlikely to play a role in endomorphin-2-induced changes in inspiratory motor output, and that different mechanisms underlie the endomorphin-2-induced increases in inspiratory burst amplitude and decreases in burst frequency.

Anti-Ischemic Effect of Leptin in the Isolated Rat Heart Subjected to Global Ischemia-Reperfusion: Role of Cardiac-Specific miRNAs

Background: Leptin is an obesity-associated adipokine that has been implicated in cardiac protection against ischemia-reperfusion injury (IRI). In this study, concentration-dependent effects of leptin on myocardial IRI were investigated in the isolated rat heart. In addition, we analyzed myocardial miRNAs expression in order to investigate their potential involvement in leptin-mediated cardioprotection. Methods: The effect of leptin on IRI was examined in Langendorff-perfused rat hearts preconditioned with two leptin concentrations (1.0 nM and 3.1 nM) for 60 min. The hearts were subjected to 30 min global ischemia and 120 min reperfusion with buffer containing leptin in the respective concentration. Heart function and arrhythmia incidence were analyzed. Infarct size was assessed histochemically. Expression of miRNA-144, -208a, -378, and -499 was analyzed in the ventricular myocardium using RT-PCR. Results: The addition of 1.0 nM leptin to the buffer exerted an infarct-limiting effect, preserved post-ischemic ventricular function, and prevented reperfusion arrhythmia compared to 3.1 nM leptin. Myocardial expression of miRNA-208a was decreased after heart exposure to 1.0 nM leptin and significantly elevated in the hearts perfused with leptin at 3.1 nM. Conclusion: Acute administration of leptin at low dose (1.0 nM) results in cardiac protection against IRI. This effect is associated with reduced myocardial expression of miRNA-208a.

Plasminogen Activator Inhibitor-1 and Adiponectin Are Associated With Metabolic Syndrome Components

BACKGROUND

We aimed to study the associations of adipocytokines, endothelial damage markers, and high-sensitivity C-reactive protein (hs-CRP) with metabolic syndrome (MetS) components.

METHODS

This cross-sectional study included 202 subjects categorized into MetS and No-MetS according to Harmonizing Adult Treatment Panel III.

RESULTS

Subjects with MetS showed higher levels of proinflammatory molecules but significantly lower adiponectin levels than subjects with No-MetS. Among the studied adipocytokines, plasminogen activator inhibitor-1 (PAI-1) and adiponectin showed the strongest associations with most MetS components. PAI-1 was associated with MetS (odds ratio (OR) 1.107 (1.065-1.151), P < 0.0001), whereas adiponectin was inversely associated with MetS (OR 0.710 (0.610-0.825), P < 0.0001). Following adjustment by sex, age, body mass index, and 24-hour urinary sodium excretion in a multivariate analysis, the association of PAI-1 (OR 1.090 (1.044-1.137), P < 0.0001) and adiponectin (OR 0.634 (0.519-0.775), P < 0.0001) with MetS remained significant. Multivariate analyses supported a model in which systolic blood pressure (BP) could be predicted by PAI-1, hs-CRP, and matrix metalloproteinase 2 (R2 = 0.125; P = 0.04); diastolic BP (R2 = 0.218; P = 0.0001) and glucose (R2 = 0.074; P = 0.0001) could be predicted by PAI-1; waist circumference could be predicted by PAI-1 and hs-CRP (R2 = 0.28; P = 0.016). Receiver operating characteristic curve analysis showed that a PAI-1 concentration had the best sensitivity and specificity for discriminating subjects with MetS.

CONCLUSION

PAI-1 and adiponectin rendered the most robust associations with MetS components in a general population, indicating that unfavorable adipose tissue performance is a key contributor to these metabolic anomalies. Further prospective analyses should allow establishing whether these adipocytokines can anticipate the progress of MetS and cardiovascular risk.

Orexin (hypocretin) participates in central autonomic regulation during fight-or-flight response

Our daily life does not only involve a calm resting state but is rather full of perturbations that induce active states such as moving, eating, and communicating. During such active conditions, cardiorespiratory regulation should be adjusted according to bodily demand, which differs from that during the resting state, by modulating or resetting the operating point. To explore neural mechanisms in the state-dependent adjustment of central autonomic regulation, my research group has recently focused on the fight-or-flight response because the stressor induces not only cognitive, emotional, and behavioral changes but also autonomic changes. In this brief review, I will summarize our discovery using orexin knockout mice and orexin neuron-ablated mice for the possible contribution of orexin, a hypothalamic neuropeptide, to the state-dependent adjustment of the central autonomic regulation. In addition, I will introduce some recent discovery using optogenetic manipulation of the orexin and related systems. The diversity of synaptic control of the cardiovascular and respiratory neurons appears necessary for animals to adapt themselves to ever-changing life circumstances and behavioral states. The orexin system is likely to function as one of the essential modulators for coordinating the circuits controlling autonomic functions and behaviors.

Effects of intracerebroventricular leptin and orexin-A on the baroreflex control of renal sympathetic nerve activity in conscious rats fed a normal or high-fat diet

This study examined the effect of leptin and orexin-A on autonomic baroreflex control in conscious Wistar rats exposed to high-fat (45% fat) or normal (3.4%) diet for 4 weeks. Renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP) and heart rate (HR) were monitored during the generation of baroreflex gain curves and acute volume expansion (VEP). Intracerebroventricular (ICV) leptin (1 μg/min) increased RSNA in the normal diet group (0.31 ± 0.04 vs 0.23 ± 0.03 mV/s) and MAP in the high-fat diet group (115 ± 5 vs 105 ± 5 mm Hg, P < .05). Orexin-A (50 ng/min) increased RSNA, HR and MAP in the high-fat diet group (0.26 ± 0.03 vs 0.22 ± 0.02 mV/s, 454 ± 8 vs 417 ± 12 beats/min, 117 ± 1 vs 108 ± 1 mm Hg) and the normal diet group (0.18 ± 0.05 vs 0.17 ± 0.05 mV/s, 465 ± 10 vs 426 ± 6 beats/min, 116 ± 2 vs 104 ± 3 mm Hg). Baroreflex sensitivity for RSNA was increased during ICV leptin by 50% in the normal diet group, compared to 14% in the high-fat diet group (P < .05). Similarly, orexin-A increased baroreflex sensitivity by 56% and 50% in the high-fat and normal diet groups, respectively (all P < .05). During ICV saline, VEP decreased RSNA by 31 ± 5% (P < .05) after 10 minutes and the magnitude of this response was blunted during ICV infusion of leptin (17 ± 2%, P < .05) but not orexin-A in the normal diet group. RSNA response to VEP was not changed during ICV leptin or orexin-A in the high-fat diet group. These findings indicate possible central roles for leptin and orexin-A in modulating the baroreflexes under normal or increased fat intake in conscious rats and potential therapeutic approaches for obesity associated hypertension.

Role of Orexin-B/Orexin 2 receptor in myocardial protection

Emerging evidence attributes to orexins/hypocretins (ORs) a protective function in the regulation of cardiovascular responses, heart rate, and hypertension. However, little is known about any direct effect of orexins in the heart function. This is of special relevance considering that cardiovascular diseases, including myocardial infarction and heart failure, are one of the major causes of mortality in the world. In the article published in Clinical Science (2018) (vol. 132, 2547–2564), Patel and colleagues investigated the role of orexins in myocardial protection. Intriguingly, they revealed a source of orexin-A (OR-A) and orexin-B (OR-B) in the heart and cardiomyocytes of the rat. More interestingly, these peptides exert a direct effect on the heart rate by acting in an autocrine/paracrine manner on their respective receptors (OXRs). Indeed, OR-B, but not OR-A, by acting through orexin receptor-2 (OX2R), exerts direct cardioprotective effects in heart failure models. OR-B/OX2R signalling enhances myosin light chain (MLC) and troponin-I (TnI) phosphorylation in a dose-dependent manner, leading to an increase in the strength of their twitch contraction. This effect is mediated by extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt phosphorylation, both in the rat myocardial tissue and human heart samples. A negative correlation between OX2R expression and clinical severity of symptoms has been found in patients with heart failure. Thus, in addition to the known central effects of orexins/OX2R, the work of Patel and colleagues (Clinical Science (2018) 132, 2547–2564) reports a direct action of OR-B on the heart rate pinpointing to OX2R as a potential therapeutic target for prevention and treatment of cardiovascular disease.

Cerebrospinal Fluid Orexin A Levels and Autonomic Function in Kleine-Levin Syndrome

STUDY OBJECTIVES

Kleine-Levin syndrome (KLS) is a rare disorder of relapsing sleepiness. The hypothesis was that the syndrome is related to a change in the vigilance peptide orexin A.

METHODS

From 2002 to 2013, 57 patients with relapsing hypersomnolence were clinically assessed in a referral academic center in Beijing, China, and 44 (28 males and 16 females; mean age 18.3 ± 8.9 y (mean ± standard deviation, range 9-57 y) were determined to have clinical and behavioral criteria consistent with KLS. Cerebrospinal fluid orexin A levels and diurnal blood pressure were measured in relapse versus remission in a subgroup of patients.

RESULTS

Presenting symptoms included relapsing or remitting excessive sleepiness-associated parallel complaints of cognitive changes (82%), eating disorders (84%); depression (45%); irritability (36%); hypersexuality (18%); and compulsions (11%). Episodes were 8.2 ± 3.3 days in duration. In relapse, diurnal values for blood pressure and heart rate were lower (P < 0.001). In a subgroup (n = 34), cerebrospinal fluid orexin A levels were ∼31% lower in a relapse versus remission (215.7 ± 81.5 versus 319.2 ± 95.92 pg/ml, P < 0.001); in three patients a pattern of lower levels during subsequent relapses was documented.

CONCLUSIONS

There are lower orexin A levels in the symptomatic phase than in remission and a fall and rise in blood pressure and heart rate, suggesting a role for orexin dysregulation in KLS pathophysiology.

Increased body fat is associated with potentiation of blood pressure response to hypoxia in healthy men: relations with insulin and leptin

Background

Increased peripheral chemosensitivity (PChS) has been proposed as mechanism underlying obesity-related sympathoactivation, with insulin and/or leptin as possible mediators. However, human data on PChS in obesity are scarce. Therefore, we explored this issue in a sample of 41 healthy men aged 30–59 years, divided according to body fat percentage (fat %) into two groups: <25 and ≥25 %.

Methods

PChS was assessed using transient hypoxia method [respiratory (PChS-MV), heart rate (PChS-HR), and blood pressure (PChS-SBP) responses were calculated]. Baroreflex sensitivity (BRS-Seq) was assessed using sequence method. Fasting plasma insulin and leptin levels were measured. Homeostatic model assessment (HOMA) was used to assess insulin sensitivity/resistance.

Results

Individuals with ≥25 % body fat demonstrated increased PChS-SBP (p < 0.01), but unchanged PChS-MV and PChS-HR (both p > 0.4). PChS-SBP was related positively with anthropometric characteristics (e.g. waist circumference, fat %), plasma insulin and HOMA (all p < 0.05), and negatively with BRS-Seq (p = 0.001), but not with plasma leptin (p = 0.27).

Conclusions

In healthy men, overweight/obesity is accompanied by augmented blood pressure response from peripheral chemoreceptors, while respiratory and heart rate responses remain unaltered. Hyperinsulinaemia and insulin resistance (but not hyperleptinaemia) are associated with augmented pressure response from chemoreceptors.

Electronic supplementary material

The online version of this article (doi:10.1007/s10286-015-0338-2) contains supplementary material, which is available to authorized users.

Epigallocatechin Gallate: A Review of Its Beneficial Properties to Prevent Metabolic Syndrome

Obesity and being overweight are linked with a cluster of metabolic and vascular disorders that have been termed the metabolic syndrome. This syndrome promotes the incidence of cardiovascular diseases that are an important public health problem because they represent a major cause of death worldwide. Whereas there is not a universally-accepted set of diagnostic criteria, most expert groups agree that this syndrome is defined by an endothelial dysfunction, an impaired insulin sensitivity and hyperglycemia, dyslipidemia, abdominal obesity and hypertension. Epidemiological studies suggest that the beneficial cardiovascular health effects of diets rich in green tea are, in part, mediated by their flavonoid content, with particular benefits provided by members of this family such as epigallocatechin gallate (EGCG). Although their bioavailability is discussed, various studies suggest that EGCG modulates cellular and molecular mechanisms of various symptoms leading to metabolic syndrome. Therefore, according to in vitro and in vivo model data, this review attempts to increase our understanding about the beneficial properties of EGCG to prevent metabolic syndrome.

Orexin A activates hypoglossal motoneurons and enhances genioglossus muscle activity in rats

BACKGROUND AND PURPOSE

Orexins have been demonstrated to play important roles in many physiological processes. However, it is not known how orexin A affects the activity of the hypoglossal motoneuron (HMN) and genioglossus (GG) muscle.

EXPERIMENTAL APPROACH

GG muscle electromyograms (GG-EMG) were recorded in anaesthetized adult rats after orexin A or orexin receptor antagonists were applied to the hypoglossal nucleus, and in adult rats in which orexin neurons were lesioned with the neurotoxin orexin-saporin (orexin-SAP). HMN membrane potential and firing were recorded from neonatal rat brain slices using whole-cell patch clamp after an infusion of orexin A or orexin receptor antagonists.

KEY RESULTS

Unilateral micro-injection of orexin A (50, 100 or 200 μM) into the hypoglossal nucleus significantly enhanced ipsilateral GG activity in adult rats. Orexin A (4, 20, 100 or 500 nM) depolarized the resting membrane potential and increased the firing rate of HMNs in a dose-dependent manner in the medullary slices of neonatal rats. Both SB 334867, a specific OX1 receptor antagonist and TCS OX2 29, a specific OX2 receptor antagonist not only blocked the depolarized membrane potential and the increased firing rate of HMNs by orexin A in the neonatal model but also attenuated GG-EMG in the adult model. A significant decrease in GG-EMG was observed in adult orexin neuron-lesioned rats compared with sham animals.

CONCLUSION AND IMPLICATIONS

Orexin A activates OX1 and OX2 receptors within the hypoglossal motor pool and promotes GG activity, indicating that orexin A is involved in controlling respiratory motor activity.

The role of orexin in post-stroke inflammation, cognitive decline, and depression

Ischemic stroke results in diverse pathophysiologies, including cerebral inflammation, neuronal loss, cognitive dysfunction, and depression. Studies aimed at identifying therapeutic solutions to alleviate these outcomes are important due to the increase in the number of stroke patients annually. Recently, many studies have reported that orexin, commonly known as a neuropeptide regulator of sleep/wakefulness and appetite, is associated with neuronal cell apoptosis, memory function, and depressive symptoms. Here, we briefly summarize recent studies regarding the role and future perspectives of orexin in post-ischemic stroke. This review advances our understanding of the role of orexin in post-stroke pathologies, focusing on its possible function as a therapeutic regulator in the post-ischemic brain. Ultimately, we suggest the clinical potential of orexin to regulate post-stroke pathologies.

Carotid chemoreceptor afferent projections to leptin receptor containing neurons in nucleus of the solitary tract

Neurons expressing the leptin receptor (Ob-R) exist within the caudal nucleus of the solitary tract (NTS). Additionally, afferent neurons expressing the Ob-R have been identified within the nodose ganglion and NTS. Furthermore, systemic injections or focal injections of leptin directly into NTS potentiate the response of NTS neurons to carotid chemoreceptor activation. However, the distribution of carotid body afferents in relation to Ob-R containing neurons within NTS is not known. In this study, chemoreceptor afferent fibers were labeled following microinjection of the anterograde tract tracer biotinylated dextran amine (BDA) into the carotid body or petrosal/nodose ganglion of Wistar rats. After a survival period of 10-14 days, the NTS was processed for BDA and Ob-R immunoreactivity. Afferent axons originating in the carotid body were found to project to the lateral (Slt), gelantinosa (Sg), and medial (Sm) subnuclei of the NTS complex. A similar, but more robust distribution of BDA labeled fibers was observed in the NTS complex after injections into the petrosal/nodose ganglion. Carotid body BDA labeled fibers were observed in close apposition to Ob-R immunoreactive neurons in the region of Slt, Sg and Sm. In addition, a small number of carotid body afferents were found to contain both BDA and express Ob-R-like immunoreactivity within the regions of Slt, Sg and Sm. Taken together, these data suggest that leptin may modulate carotid chemoreceptor function not only through direct effects on NTS neurons, but also through a direct effect on carotid body primary afferent fibers that innervate NTS neurons.

Effects of orexin 2 receptor activation on apnea in the C57BL/6J mouse

BACKGROUND

The hypothesis was that an orexin 2 receptor (OX2R) agonist would prevent sleep-related disordered breathing.

METHODS

In C57BL/6J (B6) mice, body plethysmography was performed with and without EEG monitoring of state (wakefulness, NREM and REM sleep). Outcome was apnea rate/h during sleep-wake states at baseline and with an intracerebroventricular administration of vehicle, 4 nMol of agonist OB(DL), and 4 nMol of an antagonist, TCS OX2 29.

RESULTS

A significant reduction (p=0.035, f=2.99) in apneas/hour occurred, especially with the agonist. Expressed as a function of the change from baseline, there was a significant difference among groups in Wake (p=0.03, f=3.8), NREM (p=0.003, f=6.98) and REM (p=0.03, f=3.92) with the agonist reducing the rate of apneas during sleep from 29.7±4.7 (M±SEM) to 7.3±2.4 during sleep (p=0.001). There was also a reduction in apneas during wakefulness. Administration of the antagonist did not increase event rate over baseline levels.

CONCLUSIONS

The B6 mouse is a preclinical model of wake-and sleep-disordered breathing, and the orexin receptor agonist at a dose of 4 nMol given intracerebroventricularly will reduce events in sleep and also wakefulness.

A comprehensive review on metabolic syndrome

Metabolic syndrome is defined by a constellation of interconnected physiological, biochemical, clinical, and metabolic factors that directly increases the risk of cardiovascular disease, type 2 diabetes mellitus, and all cause mortality. Insulin resistance, visceral adiposity, atherogenic dyslipidemia, endothelial dysfunction, genetic susceptibility, elevated blood pressure, hypercoagulable state, and chronic stress are the several factors which constitute the syndrome. Chronic inflammation is known to be associated with visceral obesity and insulin resistance which is characterized by production of abnormal adipocytokines such as tumor necrosis factor α , interleukin-1 (IL-1), IL-6, leptin, and adiponectin. The interaction between components of the clinical phenotype of the syndrome with its biological phenotype (insulin resistance, dyslipidemia, etc.) contributes to the development of a proinflammatory state and further a chronic, subclinical vascular inflammation which modulates and results in atherosclerotic processes. Lifestyle modification remains the initial intervention of choice for such population. Modern lifestyle modification therapy combines specific recommendations on diet and exercise with behavioural strategies. Pharmacological treatment should be considered for those whose risk factors are not adequately reduced with lifestyle changes. This review provides summary of literature related to the syndrome's definition, epidemiology, underlying pathogenesis, and treatment approaches of each of the risk factors comprising metabolic syndrome.

The impact of gender on timeliness of narcolepsy diagnosis

STUDY OBJECTIVES

To examine the impact of gender in narcoleptic patients on timeliness of diagnosis, symptomology, and health and lifestyle impairment.

METHODS

This is a cross-sectional study of 109 consecutive patients (68 women) with newly diagnosed narcolepsy with and without cataplexy, from a University sleep disorders center. Consecutive patients were administered an 8-page questionnaire at the time of their diagnosis regarding sleep habits, medications, and medical conditions, lifestyle impairments, as well as details regarding narcolepsy-related symptoms.

RESULTS

Men and women presented with remarkably similar narcolepsy related symptoms, yet women were more likely to be delayed in diagnosis; 85% of men were likely to be diagnosed by 16 years after symptom onset, compared to 28 years in women. More women were likely to remain undiagnosed at any given time point after symptom onset (hazard ratio for diagnosis of men compared to women 1.53; 95% CI 1.01-2.32; p = 0.04). Men and women reported similar degree of subjective sleepiness as measured by the Epworth Sleepiness Scale (mean 16.2 ± 4.5; p = 0.18), though women demonstrated significantly more severe objective sleepiness on multiple sleep latency testing (MSLT) (mean sleep latency in women = 5.4 min (± 4.1), in men 7.4 min (± 3.5); p = 0.03). Despite being more objectively sleepy, women were less likely to report lifestyle impairments in the areas of personal relationships (71% men, 44% women, p = 0.01) and physical activity (36% men, 16% women, p = 0.02), but were also more likely to self-medicate with caffeine (63.4% men, 82.4% women; p = 0.03).

CONCLUSIONS

Narcolepsy impacts men and women's health and lifestyle differently, and may cause delays diagnosis for women.

Assessment of serum leptin, pregnancy-associated plasma protein A and CRP levels as indicators of plaque vulnerability in patients with acute coronary syndrome

INTRODUCTION

A multifactorial aetiology of coronary artery disease (CAD) has been established in the recent past. Extensive research is now underway to understand the mechanisms responsible for plaque vulnerability. The identification of a novel biomarker that will help in the assessment of plaque status is urgently needed for the purpose of patient stratification and prognostication. The aim of the present study was to evaluate leptin, pregnancy-associated plasma protein A (PAPP-A) and C-reactive protein (CRP) levels in patients with acute coronary syndrome and to assess their diagnostic efficacy in the identification of vulnerable plaques.

METHODS

The study group comprised 105 patients who had chest pain along with ECG changes (ST elevation, ST depression, T inversion) and raised cardiac enzyme levels. Sixty-two patients with chest pain and ECG changes but with normal cardiac enzyme profiles were included in the control group. Lipid profiles, and leptin, PAPP-A and CRP levels were assessed in these two groups. Receiver operating characteristics (ROC) curves were plotted to determine the utility of the parameters under study as markers of plaque vulnerability.

RESULTS

Significantly higher levels of serum lipoprotein (a), leptin, PAPP-A and high-sensitivity CRP (hs-CRP) were observed in the cases than in the controls. A positive correlation was observed between CRP and PAPP-A levels as well as CRP and leptin concentrations. ROC curve analysis revealed similar efficacies of CRP and PAPP-A levels in their ability to detect unstable plaques with areas under the curve of 0.762 and 0.732, respectively. Multivariate analysis established the superiority of hs-CRP as a predictor of plaque instability.

CONCLUSIONS

Our study highlights the utility of both CRP and PAPP-A levels as determinants of plaque instability. Our findings necessitate population-based follow-up studies to establish the superiority of either of the two biomarkers in the field of preventive cardiology.

Leptin in nucleus of the solitary tract alters the cardiovascular responses to aortic baroreceptor activation

Recent data suggests that neurons expressing the long form of the leptin receptor form at least two distinct groups within the caudal nucleus of the solitary tract (NTS): a group within the lateral NTS (Slt) and one within the medial (Sm) and gelantinosa (Sg) NTS. Discrete injections of leptin into Sm and Sg, a region that receives chemoreceptor input, elicit increases in arterial pressure (AP) and renal sympathetic nerve activity (RSNA). However, the effect of microinjections of leptin into Slt, a region that receives baroreceptor input is unknown. Experiments were done in the urethane-chloralose anesthetized, paralyzed and artificially ventilated Wistar or Zucker obese rat to determine leptin's effect in Slt on heart rate (HR), AP and RSNA during electrical stimulation of the aortic depressor nerve (ADN). Depressor sites within Slt were first identified by the microinjection of l-glutamate (Glu; 0.25M; 10nl) followed by leptin microinjections. In the Wistar rat leptin microinjection (50ng; 20nl) into depressor sites within the lateral Slt elicited increases in HR and RSNA, but no changes in AP. Additionally, leptin injections into Slt prior to Glu injections at the same site or to stimulation of the ADN were found to attenuate the decreases in HR, AP and RSNA to both the Glu injection and ADN stimulation. In Zucker obese rats, leptin injections into NTS depressor sites did not elicit cardiovascular responses, nor altered the cardiovascular responses elicited by stimulation of ADN. Those data suggest that leptin acts at the level of NTS to alter the activity of neurons that mediate the cardiovascular responses to activation of the aortic baroreceptor reflex.

Antagonism of orexin receptors significantly lowers blood pressure in spontaneously hypertensive rats

In normal rats, central administration of orexin or exposure to certain forms of stress can induce significant increases in blood pressure and sympathetic nerve activity, which can be blocked by orexin receptor antagonists. The resting blood pressure is, however, unaffected by such antagonists, but is significantly lower in rodents with total loss of orexin, such as prepro-orexin knockout mice and orexin neuron-ablated orexin/ataxin-3 transgenic rats. We hypothesize that orexin is involved in the pathophysiology and maintenance of high blood pressure in the spontaneously hypertensive rat (SHR), a model of primary hypertension. To test this hypothesis, we measured orexin-A mRNA expression in the rostral ventrolateral medulla and antagonized both orexin receptors using an orally administered potent dual orexin receptor antagonist, almorexant, in SHRs and normotensive Wistar-Kyoto rats. In SHRs, there was a strong trend towards an increased orexin-A mRNA expression in the rostral ventrolateral medulla, and blocking orexin receptors markedly lowered blood pressure (from 182/152 ± 5/6 to 149/119 ± 9/8 mmHg; P < 0.001), heart rate (P < 0.001), sympathetic vasomotor tone (P < 0.001) and the noradrenaline levels in cerebrospinal fluid and plasma (P < 0.002). The significant antihypertensive effects of almorexant were observed in wakefulness and non-rapid eye movement sleep during both dark and light phases of the diurnal cycle only in SHRs. Blocking orexin receptors had no effect on blood pressure and sympathetic tone in normotensive Wistar-Kyoto rats. Our study links the orexin system to the pathogenesis of high blood pressure in SHRs and suggests that modulation of the orexin system could be a potential target in treating some forms of hypertension.

Hypertension in metabolic syndrome: vascular pathophysiology

METABOLIC SYNDROME IS A CLUSTER OF METABOLIC AND CARDIOVASCULAR SYMPTOMS: insulin resistance (IR), obesity, dyslipemia. Hypertension and vascular disorders are central to this syndrome. After a brief historical review, we discuss the role of sympathetic tone. Subsequently, we examine the link between endothelial dysfunction and IR. NO is involved in the insulin-elicited capillary vasodilatation. The insulin-signaling pathways causing NO release are different to the classical. There is a vasodilatory pathway with activation of NO synthase through Akt, and a vasoconstrictor pathway that involves the release of endothelin-1 via MAPK. IR is associated with an imbalance between both pathways in favour of the vasoconstrictor one. We also consider the link between hypertension and IR: the insulin hypothesis of hypertension. Next we discuss the importance of perivascular adipose tissue and the role of adipokines that possess vasoactive properties. Finally, animal models used in the study of vascular function of metabolic syndrome are reviewed. In particular, the Zucker fatty rat and the spontaneously hypertensive obese rat (SHROB). This one suffers macro- and microvascular malfunction due to a failure in the NO system and an abnormally high release of vasoconstrictor prostaglandins, all this alleviated with glitazones used for metabolic syndrome therapy.

Role of adipokines in cardiovascular disease

The discovery of leptin in 1994 sparked dramatic new interest in the study of white adipose tissue. It is now recognised to be a metabolically active endocrine organ, producing important chemical messengers - adipokines and cytokines (adipocytokines). The search for new adipocytokines or adipokines gained added fervour with the prospect of the reconciliation between cardiovascular diseases (CVDs), obesity and metabolic syndrome. The role these new chemical messengers play in inflammation, satiety, metabolism and cardiac function has paved the way for new research and theories examining the effects they have on (in this case) CVD. Adipokines are involved in a 'good-bad', yin-yang homoeostatic balance whereby there are substantial benefits: cardioprotection, promoting endothelial function, angiogenesis and reducing hypertension, atherosclerosis and inflammation. The flip side may show contrasting, detrimental effects in aggravating these cardiac parameters.

Leptin in pediatrics: A hormone from adipocyte that wheels several functions in children

The protein leptin, a pleiotropic hormone regulates appetite and energy balance of the body and plays important roles in controlling linear growth, pubertal development, cardiovascular function, and immunity. Recent findings in the understanding of the structure, functional roles, and clinical significance of conditions with increased and decreased leptin secretion are summarized. Balance between leptin and other hormones is significantly regulated by nutritional status. This balance influences many organ systems, including the brain, liver, and skeletal muscle, to mediate the essential adaptation process. The aim of this review is to summarize the possible physiological functions of leptin and its signaling pathways during childhood and adolescence including control of food intake, energy regulation, growth and puberty, and immunity. Moreover, its secretion and possible roles in the adaptation process during different disease states (obesity, malnutrition, eating disorders, delayed puberty, congenital heart diseases and hepatic disorders) are discussed. The clinical manifestations and the successful management of patients with genetic leptin deficiency and the application of leptin therapy in other diseases including lipodystrophy, states with severe insulin resistance, and diabetes mellitus are discussed.

Systemic administration of leptin potentiates the response of neurons in the nucleus of the solitary tract to chemoreceptor activation in the rat

Leptin microinjections into the nucleus of the solitary tract (NTS) have been shown to elicit sympathoexcitatory responses, and potentiate the cardiovascular responses to activation of the chemoreflex. In this study, experiments were done in Sprague-Dawley rats initially to provide a detailed mapping within the NTS complex of cells containing immunoreactivity to the long form of the leptin receptor (Ob-Rb). In a second series, this NTS region containing Ob-Rb immunoreactive cells was explored for single units antidromically activated by stimulation of pressor sites in the rostral ventrolateral medulla (RVLM). These antidromically identified neurons were then tested for their response to intra-carotid injections of leptin (50-100 ng/0.1 ml), and to activation of peripheral chemoreceptors following an injection of potassium cyanide (KCN) (80 μg/0.1 ml) into the carotid artery. Cells containing Ob-Rb-like immunoreactivity were found predominantly in the caudal NTS: within the medial, commissural and gelatinous (sub-postremal area) subnuclei of the NTS complex. Of 73 single units tested in these NTS regions, 48 were antidromically activated by stimulation of RVLM pressor sites and 25 of these single units responded with an increase in discharge rate after intra-carotid injections of leptin. In addition, 17 of these leptin responsive neurons were excited by the intra-carotid injections of KCN (80 μg/0.1 ml). Furthermore, the excitatory response of these single units to KCN was potentiated (59-83%) immediately following the leptin injection. These data indicate that leptin responsive neurons in NTS mediate chemoreceptor afferent information to pressor sites in the RVLM, and suggest that leptin may act as a facilitator on neuronal circuits within the NTS that potentiates the sympathoexcitatory responses elicited during the reflex activation of arterial chemoreceptors.

Adipose tissue: friend or foe?

The perception of adipose tissue has changed considerably with the dramatic increase in the incidence of obesity and obesity-related comorbidities over the past 3 decades. Excess fat is no longer associated with wealth, but is instead recognized as a risk factor for many diseases. Adipose tissue is increasingly being identified as a vital, complex endocrine organ, and not simply as a fat store. Not all fat is created equal--regional, developmental, structural, and functional variations exist. Epicardial adipose tissue is a metabolically active organ producing a number of factors that modulate cardiac structure and function. The global epidemic of obesity and metabolic syndrome imposes a major disease burden, particularly of cardiovascular disease. In this Review, we describe the various types of adipose tissue--their developmental biology, differentiation, cell heterogeneity, and functional characteristics. We discuss the link between adipose tissue and inflammation, the signaling factors released by adipose tissue, as well as cardiac adiposity and its relevance to cardiovascular diseases. Finally, we review the myocardial regenerative potential of adipose-tissue-derived stem cells. We believe that a thorough understanding of adipose tissue is of great clinical value.

Leptin signaling in the nucleus of the solitary tract alters the cardiovascular responses to activation of the chemoreceptor reflex

Circulating levels of leptin are elevated in individuals suffering from chronic intermittent hypoxia (CIH). Systemic and central administration of leptin elicits increases in sympathetic nervous activity (SNA), arterial pressure (AP), and heart rate (HR), and it attenuates the baroreceptor reflex, cardiovascular responses that are similar to those observed during CIH as a result of activation of chemoreceptors by the systemic hypoxia. Therefore, experiments were done in anesthetized Wistar rats to investigate the effects of leptin in nucleus of the solitary tract (NTS) on AP and HR responses, and renal SNA (RSNA) responses during activation of NTS neurons and the chemoreceptor reflex. Microinjection of leptin (5-100 ng; 20 nl) into caudal NTS pressor sites (l-glutamate; l-Glu; 0.25 M; 10 nl) elicited dose-related increases in AP, HR, and RSNA. Leptin microinjections (5 ng; 20 nl) into these sites potentiated the increase in AP and HR elicited by l-Glu. Additionally, bilateral injections of leptin (5 ng; 100 nl) into NTS potentiated the increase in AP and attenuated the bradycardia to systemic activation of the chemoreflex. In the Zucker obese rat, leptin injections into NTS neither elicited cardiovascular responses nor altered the cardiovascular responses to activation of the chemoreflex. Taken together, these data indicate that leptin exerts a modulatory effect on neuronal circuits within NTS that control cardiovascular responses elicited during the reflex activation of arterial chemoreceptors and suggest that increased AP and SNA observed in individuals with CIH may be due, in part, by leptin's effects on the chemoreflex at the level of NTS.

Assessment of serum leptin, pregnancy-associated plasma protein A and CRP levels as indicators of plaque vulnerability in patients with acute coronary syndrome

INTRODUCTION

A multifactorial aetiology of coronary artery disease (CAD) has been established in the recent past. Extensive research is now underway to understand the mechanisms responsible for plaque vulnerability. The identification of a novel biomarker that will help in the assessment of plaque status is urgently needed for the purpose of patient stratification and prognostication. The aim of the present study was to evaluate leptin, pregnancy-associated plasma protein A (PAPP-A) and C-reactive protein (CRP) levels in patients with acute coronary syndrome and to assess their diagnostic efficacy in the identification of vulnerable plaques.

METHODS

The study group comprised 105 patients who had chest pain along with ECG changes (ST elevation, ST depression, T inversion) and raised cardiac enzyme levels. Sixty-two patients with chest pain and ECG changes but with normal cardiac enzyme profiles were included in the control group. Lipid profiles, and leptin, PAPP-A and CRP levels were assessed in these two groups. Receiver operating characteristics (ROC) curves were plotted to determine the utility of the parameters under study as markers of plaque vulnerability.

RESULTS

Significantly higher levels of serum lipoprotein (a), leptin, PAPP-A and high-sensitivity CRP (hs-CRP) were observed in the cases than in the controls. A positive correlation was observed between CRP and PAPP-A levels as well as CRP and leptin concentrations. ROC curve analysis revealed similar efficacies of CRP and PAPP-A levels in their ability to detect unstable plaques with areas under the curve of 0.762 and 0.732, respectively. Multivariate analysis established the superiority of hs-CRP as a predictor of plaque instability.

CONCLUSIONS

Our study highlights the utility of both CRP and PAPP-A levels as determinants of plaque instability. Our findings necessitate population-based follow-up studies to establish the superiority of either of the two biomarkers in the field of preventive cardiology.

The orexin-1 receptor antagonist SB-334867 decreases sympathetic responses to a moderate dose of methamphetamine and stress

We recently discovered that inhibiting neurons in the dorsomedial hypothalamus (DMH) attenuated hyperthermia, tachycardia, hypertension, and hyperactivity evoked by the substituted amphetamine 3, 4-methylenedioxymethamphetamine (MDMA). Neurons that synthesize orexin are also found in the region of the DMH. As orexin and its receptors are involved in the regulation of heart rate and temperature, they would seem to be logical candidates as mediators of the effects evoked by amphetamines. The goal of this study was to determine if blockade of orexin-1 receptors in conscious rats would suppress cardiovascular and thermogenic responses evoked by a range of methamphetamine (METH) doses. Male Sprague-Dawley rats (n=6 per group) were implanted with telemetric transmitters measuring body temperature, heart rate, and mean arterial pressure. Animals were randomized to receive pretreatment with either the orexin-1 receptor antagonist SB-334867 (10mg/kg) or an equal volume of vehicle. Thirty minutes later animals were given intraperitoneal (i.p.) injections of either saline, a low (1mg/kg), moderate (5mg/kg) or high (10mg/kg) dose of METH. Pretreatment with SB-334867 significantly attenuated increases in body temperature and mean arterial pressure evoked by the moderate but not the low or high dose of METH. Furthermore, animals treated with SB-334867, compared to vehicle, had lower temperature and heart rate increases after the stress of an i.p. injection. In conclusion, temperature and cardiovascular responses to a moderate dose of METH and to stress appear to involve orexin-1 receptors. The failure to affect a low and a high dose of METH suggests a complex pharmacology dependent on dose. A better understanding of this may lead to the knowledge of how monoamines influence the orexin system and vice versa.

Respiration and autonomic regulation and orexin

Orexin, a small neuropeptide released from neurons in the hypothalamus with widespread projections throughout the central nervous system, has broad biological roles including the modulation of breathing and autonomic function. That orexin activity is fundamentally dependent on sleep-wake state, and circadian cycle requires consideration of orexin function in physiological control systems in respect to these two state-related activity patterns. Both transgenic mouse studies and focal orexin receptor antagonism support a role for orexins in respiratory chemosensitivity to CO₂ predominantly in wakefulness, with further observations limiting this role to the dark period. In addition, orexin neurons participate in the regulation of sympathetic activity, including effects on blood pressure and thermoregulation. Orexin is also essential in physiological responses to stress. Orexin-mediated processes may operate at two levels: (1) in sleep-wake and circadian states and (2) in stress, for example, the defense or "fight-or-flight" response and panic-anxiety syndrome.

Effects of orexin-A on propofol anesthesia in rats

PURPOSE

An active sleep homeostatic process is present during propofol anesthesia. Activation of the orexin system induces wakefulness, and inhibition of the orexin system causes narcolepsy. We hypothesized that orexin would affect propofol anesthesia.

METHODS

The effects of an intracerebroventricular (i.c.v.) injection of orexin-A (OXA) or an orexin-1 (OX-1) receptor antagonist, SB-334867, on the times to the loss and return of the righting reflex induced by propofol were examined in Wistar rats. The effects of propofol or OXA on norepinephrine (NE) and dopamine (DA) release from the prefrontal cortex (PFC) were examined using in vivo microdialysis.

RESULTS

An i.c.v. injection of OXA (1 nmol) decreased the time to emergence from propofol anesthesia mediated by the OX-1 receptor without changing anesthetic induction (n = 8). An i.c.v. injection of SB-334867 (5 and 50 nmol) increased the time to emergence from propofol anesthesia without changing anesthetic induction (n = 8). Intravenous infusion of propofol decreased NE (48 ± 8%; n = 8) and DA (61.2 ± 11%; n = 8) release from PFC mediated by the GABA(A) receptor. An i.c.v. injection of OXA reversed the decreases in NE and DA release induced by propofol mediated by the OX-1 receptor (n = 8).

CONCLUSION

These results indicate that the orexin system may accelerate the emergence from propofol anesthesia associated with increases in the central noradrenergic and dopaminergic activity.

Plasma orexin-A levels in postmenopausal women: possible interaction with estrogen and correlation with cardiovascular risk status

OBJECTIVE

To assess plasma orexin-A levels in a group of postmenopausal women not receiving estrogen-replacement therapy (ERT), and to compare the values with a group on ERT and a group of reproductive-age women, and to correlate the findings with some cardiovascular risk factors.

DESIGN

Observational cohort study.

SETTING

Alexandria University Hospital.

POPULATION

Ninety women, in three groups: a control group of 30 healthy, reproductive-age women, 30 healthy postmenopausal women not receiving ERT, and 30 healthy postmenopausal women on ERT for 6 months.

METHODS

Quantitative clinical assessment as well as laboratory investigations.

MAIN OUTCOME MEASURES

Orexin-A levels, serum estradiol, cholesterol, triglycerides, and fasting glucose are the main laboratory outcome measures, whereas blood pressure and weight are the main clinical outcome measures.

RESULTS

Postmenopausal women not receiving ERT had the highest levels of plasma orexin A (705.61 +/- 165.62 microg/dl). Postmenopausal women on ERT had orexin-A levels that were comparable with the control group (233.90 +/- 54.26 versus 243.81 +/- 68.88 microg/dl). Plasma orexin-A levels were directly correlated with blood glucose lipid profile, arterial blood pressure, and body mass index.

CONCLUSIONS

Higher orexin-A levels are associated with hypoestrogenism, and are partially reversed by ERT. A possible inhibitory effect of estrogen on orexin might partially account for its cardioprotective effect.

Protein tyrosine phosphatase 1B, a major regulator of leptin-mediated control of cardiovascular function

BACKGROUND

Obesity causes hypertension and sympathoactivation, a process proposed to be mediated by leptin. Protein tyrosine phosphatase 1B (PTP1B), a major new pharmaceutical target in the treatment of obesity and type II diabetes mellitus, constrains the metabolic actions of leptin, but the extent to which PTP1B regulates its cardiovascular effects is unclear. This study examined the hypothesis that PTP1B is a negative regulator of the cardiovascular effects of leptin.

METHODS AND RESULTS

PTP1B knockout mice had lower body fat but higher mean arterial pressure (116+/-5 versus 105+/-5 mm Hg, P<0.05) than controls. Leptin infusion produced a greater anorexic effect in PTP1B knockout mice and a marked increase in mean arterial pressure (135+/-5 mm Hg) in PTP1B knockout mice only. The decrease in mean arterial pressure in response to ganglionic blockade was higher in PTP1B knockout mice (-38+/-3% versus -29+/-3%, P<0.05), which suggests increased sympathetic tone. PTP1B deletion blunted mean arterial pressure responses to phenylephrine injection (55+/-10% versus 93+/-7%, P<0.05). Phenylephrine-induced aortic contraction was reduced in PTP1B knockout mice (57.7+/-9% versus 96.3+/-12% of KCl, P<0.05), consistent with desensitization to chronically elevated sympathetic tone. Furthermore, PTP1B deletion significantly reduced gene expression of 3 alpha(1)-adrenergic receptor subtypes, consistent with blunted constriction to phenylephrine.

CONCLUSIONS

These data indicate that PTP1B is a key regulator of the cardiovascular effects of leptin and that reduced vascular adrenergic reactivity provides a compensatory limit to the effects of leptin on mean arterial pressure.

Orexinergic modulation of breathing across vigilance states

Basal respiration and respiratory reflex regulations are considerably different during the awake and sleep states. Tidal volume and respiratory frequency diminish during sleep, and hypoxic and hypercapnic ventilatory responses also decline during sleep. Reduced metabolic demand during sleep cannot completely explain these phenomena because PaCO2 increases during sleep. In this review, I will summarize our recent discovery of the possible contribution of orexin, a hypothalamic neuropeptide, to the vigilance state-dependent adjustment of central respiratory regulation. Orexin-deficient mice show an attenuated hypercapnic ventilatory response during the awake but not during the sleep period, whereas basal ventilation remained normal, irrespective of the vigilance state. Orexin supplementation remedied the defect, and the administration of an orexin receptor antagonist to wild-type mice mimicked the abnormality. Orexin-deficient mice also showed frequent sleep apneas and loss of repetitive intermittent hypoxia-induced ventilatory long-term facilitation. Hence, it is possible that the orexin system is one of the essential modulators required for coordinating the circuits controlling respiration and behavior.

Orexin neurons in the hypothalamus mediate cardiorespiratory responses induced by disinhibition of the amygdala and bed nucleus of the stria terminalis

We previously showed that the defense response elicited by stressors was attenuated in prepro-orexin knockout mice and in orexin neuron-ablated mice, and we proposed that orexin serves as a master switch within multiple efferent pathways that mediate the defense response. In this study we sought to determine whether excitation of the amygdala (AMG) or the bed nucleus of stria terminalis (BNST) activates orexin-containing neurons and whether those neurons are essential in eliciting cardiorespiratory responses to the stimulus. In urethane-anesthetized mice, the GABA-A receptor antagonist bicuculline was microinjected into the AMG or BNST and blood pressure, heart rate, and respiration were measured. Injection of bicuculline in either site induced long-lasting dose-dependent cardiorespiratory excitation in wild-type mice. In contrast, mice in which orexin neurons had been ablated demonstrated no such response after activation of the AMG and an attenuated response after activation of the BNST. Double immunohistochemical staining for orexin and c-Fos, an indicator of neural activation, revealed that an injection of bicuculline induced significantly larger numbers of orexin positive neurons that expressed c-Fos in the perifornical/dorsomedial hypothalamus (58.2+/-6.4% into AMG and 66.4+/-6.6% into BNST, n=3 each) than did vehicle (18.2+/-4.4% into AMG and 28.3+/-2.1% into BNST). Disinhibition to the BNST induced widespread expression of c-Fos not only in orexin-containing neurons but also other neurons in the hypothalamus. We conclude that orexin-containing neurons in the medial hypothalamus mediate at least a part of AMG- and BNST-induced cardiorespiratory responses.

Vasopressin increases locomotion through a V1a receptor in orexin/hypocretin neurons: implications for water homeostasis

Water homeostasis is a critical challenge to survival for land mammals. Mice display increased locomotor activity when dehydrated, a behavior that improves the likelihood of locating new sources of water and simultaneously places additional demands on compromised hydration levels. The neurophysiology underlying this well known behavior has not been previously elucidated. We report that the anti-diuretic hormone arginine-vasopressin (AVP) is involved in this response. AVP and oxytocin directly induced depolarization and an inward current in orexin/hypocretin neurons. AVP-induced activation of orexin neurons was inhibited by a V1a receptor (V1aR)-selective antagonist and was not observed in V1aR knock-out mice, suggesting an involvement of V1aR. Subsequently activation of phospholipase Cbeta triggers an increase in intracellular calcium by both calcium influx through nonselective cation channels and calcium release from calcium stores in orexin neurons. Intracerebroventricular injection of AVP or water deprivation increased locomotor activity in wild-type mice, but not in transgenic mice lacking orexin neurons. V1aR knock-out mice were less active than wild-type mice. These results suggest that the activation of orexin neurons by AVP or oxytocin has an important role in the regulation of spontaneous locomotor activity in mice. This system appears to play a key role in water deprivation-induced hyperlocomotor activity, a response to dehydration that increases the chance of locating water in nature.

Sympathetic overactivity and plasma leptin levels in Rett syndrome

Rett syndrome (RTT) is a severe developmental-neurological disorder, characterized by profound and progressive loss of intellectual functioning, occurring after a period (of at least 6 months) of normal development with classic stereotype hand movements, gait ataxia, jerky truncal ataxia, deceleration of brain and body organ growth and cardiac dysautonomia. Pathogenesis of sympathetic overactivity in RTT is unknown, but a previous study observed increased plasma leptin levels in Rett girls and it is well known the role of leptin in the regulation of sympathetic nervous system activity. Aim of our study is to evaluate a relationship between plasma leptin levels and sympathetic activity in RTT. Thirty-two female patients (12.1+/-6.3 years), affected by RTT were enrolled in the study. In all the subjects, we analyzed heart rate variability, QT corrected interval and plasma leptin levels. A significant correlation was found between plasma leptin levels and LF/HF (expression of sympatho-vagal balance) (Spearman r=0.44, p=0.001). There is also a significant negative correlation between HF component (expression of vagal activity) and plasma leptin levels (Spearman r=-0.037, p=0.03) and a positive correlation between LF component and plasma leptin levels (Spearman r=0.047, p=0.01). These results show that in RTT higher plasma leptin levels appear to be associated with sympathetic overactivity, suggesting a role for leptin in cardiac dysautonomia.

Contribution of orexin in hypercapnic chemoreflex: evidence from genetic and pharmacological disruption and supplementation studies in mice

We have previously shown that hypercapnic chemoreflex in prepro-orexin knockout mice (ORX-KO) is attenuated during wake but not sleep periods. In that study, however, hypercapnic stimulation had been chronically applied for 6 h because of technical difficulty in changing the composition of the inspired gas mixture without distorting the animal's vigilance states. In the present study we examined possible involvement of orexin in acute respiratory chemoreflex during wake periods. Ventilation was recorded together with electroencephalography and electromyography before and after intracerebroventricular administration of orexin or an orexin receptor antagonist, SB-334867. A hypercapnic (5 or 10% CO(2)) or hypoxic (15 or 10% O(2)) gas mixture was introduced into the recording chamber for 5 min. Respiratory parameters were analyzed only for quiet wakefulness. When mice breathed normal room air, orexin-A and orexin-B but not vehicle or SB-334867 increased minute ventilation in both ORX-KO and wild-type (WT) mice. As expected, hypercapnic chemoreflex in vehicle-treated ORX- KO mice (0.22 +/- 0.03 mlxmin(-1)xg(-1)x% CO(2)(-1)) was significantly blunted compared with that in WT mice (0.51 +/- 0.05 mlxmin(-1)xg(-1)x% CO(2)(-1)). Supplementation of orexin-A or -B (3 nmol) partially restored the hypercapnic chemoreflex in ORX-KO mice (0.28 +/- 0.03 mlxmin(-1).g(-1)x% CO(2)(-1) for orexin-A and 0.32 +/- 0.04 mlxmin(-1)xg(-1)x% CO(2)(-1) for orexin-B). In addition, injection of SB-334867 (30 nmol) in WT mice decreased the hypercapnic chemoreflex (0.39 +/- 0.04 mlxmin(-1)xg(-1)x% CO(2)(-1)). On the other hand, hypoxic chemoreflex in vehicle-treated ORX-KO and SB-334867-treated WT mice was not different from that in corresponding controls. Our findings suggest that orexin plays a crucial role in CO(2) sensitivity at least during wake periods in mice.

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.

Vigilance state-dependent attenuation of hypercapnic chemoreflex and exaggerated sleep apnea in orexin knockout mice

Exogenous administration of orexin can promote wakefulness and respiration. Here we examined whether intrinsic orexin participates in the control of breathing in a vigilance state-dependent manner. Ventilation was recorded together with electroencephalography and electromyography for 6 h during the daytime in prepro-orexin knockout mice (ORX-KO) and wild-type (WT) littermates. Respiratory parameters were separately determined during quiet wakefulness (QW), slow-wave sleep (SWS), or rapid eye movement (REM) sleep. Basal ventilation was normal in ORX-KO, irrespective of vigilance states. The hypercapnic ventilatory response during QW in ORX-KO (0.19 +/- 0.01 ml.min(-1).g(-1).%CO(2)(-1)) was significantly smaller than that in WT mice (0.38 +/- 0.04 ml.min(-1).g(-1).%CO(2)(-1)), whereas the responses during SWS and REM in ORX-KO were comparable to those in WT mice. Hypoxic responses during wake and sleep periods were not different between the genotypes. Spontaneous but not postsigh sleep apneas were more frequent in ORX-KO than in WT littermates during both SWS and REM sleep. Our findings suggest that orexin plays a crucial role both in CO(2) sensitivity during wakefulness and in preserving ventilation stability during sleep.

Leptin, the obesity-associated hormone, exhibits direct cardioprotective effects

BACKGROUND AND PURPOSE

Protection against ischaemia-reperfusion (I/R) injury involves PI3K-Akt and p44/42 MAPK activation. Leptin which regulates appetite and energy balance also promotes myocyte proliferation via PI3K-Akt and p44/42 MAPK activation. We, therefore, hypothesized that leptin may also exhibit cardioprotective activity.

EXPERIMENTAL APPROACH

The influence of leptin on I/R injury was examined in perfused hearts from C57Bl/6 J mice that underwent 35 min global ischaemia and 35 min reperfusion, infarct size being assessed by triphenyltetrazolium chloride staining. The concomitant activation of cell-signalling pathways was investigated by Western blotting. The effect of leptin on mitochondrial permeability transition pore (MPTP) opening was studied in rat cardiomyocytes.

KEY RESULTS

Leptin (10 nM) administered during reperfusion reduced infarct size significantly. Protection was blocked by either LY294002 or UO126, inhibitors of Akt and p44/42 MAPK, respectively. Western blotting confirmed that leptin stimulated p44/42 MAPK phosphorylation significantly. Akt phosphorylation was also enhanced but did not achieve statistical significance. Additionally, leptin treatment was associated with a significant increase in p38 phosphorylation. By contrast, leptin caused downregulation of phosphorylated and non-phosphorylated STAT3, and of total AMP-activated kinase. Cardiomyocytes responded to leptin with delayed opening of the MPTP and delayed time until contracture.

CONCLUSIONS AND IMPLICATIONS

Our data indicate for the first time that the adipocytokine, leptin, has direct cardioprotective properties which may involve the PI3-Akt and p44/42 MAPK pathways.

Multiple components of the defense response depend on orexin: Evidence from orexin knockout mice and orexin neuron-ablated mice

Stressor induces not only cognitive, emotional and behavioral changes but also autonomic changes. Although research on the neural circuits underlying such autonomic changes has implicated the hypothalamus in the defense response against stressors, neurotransmitters in this multifaceted and coordinated response have not been revealed. In this brief review, here we summarize our recent discovery using orexin knockout mice and orexin neuron-ablated mice of possible contribution of orexin in the defense response and discuss future directions.

Orexin neuron-mediated skeletal muscle vasodilation and shift of baroreflex during defense response in mice

We have previously shown that some features of the defense response, such as increases in arterial blood pressure (AP), heart rate (HR), and ventilation were attenuated in prepro-orexin knockout (ORX-KO) mice. Here, we examined whether the same was true in orexin neuron-ablated [orexin/ataxin-3 transgenic mice (ORX/ATX-Tg)] mice. In addition, we examined other features of the defense response: skeletal muscular vasodilation and shift of baroreceptor reflex. In both anesthetized and conscious conditions, basal AP in ORX/ATX-Tg mice was significantly lower by ∼20 mmHg than in wild-type (WT) controls, as was the case in ORX-KO mice. The difference in AP disappeared after treatment with an α-blocker but not with a β-blocker, indicating lower sympathetic vasoconstrictor outflow. Stimulation of the perifornical area (PFA) in urethane-anesthetized ORX/ATX-Tg mice elicited smaller and shorter-lasting increases in AP, HR, and ventilation, and skeletal muscle vasodilation than in WT controls. In addition, air jet stress-induced elevations of AP and HR were attenuated in conscious ORX/ATX-Tg mice. After pretreatment with a β-blocker, atenolol, stimulation of PFA suppressed phenylephrine (50 μg/kg iv)-induced bradycardia (ΔHR = −360 ± 29 beats/min without PFA stimulation vs. −166 ± 26 during stimulation) in WT. This demonstrated the resetting of the baroreflex. In ORX/ATX-Tg mice, however, no significant suppression was observed (−355 ± 16 without stimulation vs. −300 ± 30 during stimulation). The present study provided further support for our hypothesis that orexin-containing neurons in PFA play a role as a master switch to activate multiple efferent pathways of the defense response and also operate as a regulator of basal AP.

Changes in hypothalamic corticotropin-releasing hormone, neuropeptide Y, and proopiomelanocortin gene expression during chronic rapid eye movement sleep deprivation of rats

Chronic rapid eye movement (paradoxical) sleep deprivation (REM-SD) of rats leads to two conspicuous pathologies: hyperphagia coincident with body weight loss, prompted by elevated metabolism. Our goals were to test the hypotheses that 1) as a stressor, REM-SD would increase CRH gene expression in the hypothalamus and that 2) to account for hyperphagia, hypothalamic gene expression of the orexigen neuropeptide Y (NPY) would increase, but expression of the anorexigen proopiomelanocortin (POMC) would decrease. Enforcement of REM-SD of adult male rats for 20 d with the platform (flowerpot) method led to progressive hyperphagia, increasing to approximately 300% of baseline; body weight steadily declined by approximately 25%. Consistent with changes in food intake patterns, NPY expression rapidly increased in the hypothalamic arcuate nucleus by d 5 of REM-SD, peaking at d 20; by contrast, POMC expression decreased progressively during REM-SD. CRH expression was increased by d 5, both in mRNA and ability to detect neuronal perikaryal staining in paraventricular nucleus with immunocytochemistry, and it remained elevated thereafter with modest declines. Taken together, these data indicate that changes in hypothalamic neuropeptides regulating food intake are altered in a manner consistent with the hyperphagia seen with REM-SD. Changes in CRH, although indicative of REM-SD as a stressor, suggest that the anorexigenic actions of CRH are ineffective (or disabled). Furthermore, changes in NPY and POMC agree with current models of food intake behavior, but they are opposite to their acute effects on peripheral energy metabolism and thermogenesis.

Reduced cardiac efficiency and altered substrate metabolism precedes the onset of hyperglycemia and contractile dysfunction in two mouse models of insulin resistance and obesity

Hyperglycemia is associated with altered myocardial substrate use, a condition that has been hypothesized to contribute to impaired cardiac performance. The goals of this study were to determine whether changes in cardiac metabolism, gene expression, and function precede or follow the onset of hyperglycemia in two mouse models of obesity, insulin resistance, and diabetes (ob/ob and db/db mice). Ob/ob and db/db mice were studied at 4, 8, and 15 wk of age. Four-week-old mice of both strains were normoglycemic but hyperinsulinemic. Hyperglycemia develops in db/db mice between 4 and 8 wk of age and in ob/ob mice between 8 and 15 wk. In isolated working hearts, rates of glucose oxidation were reduced by 28-37% at 4 wk and declined no further at 15 wk in both strains. Fatty acid oxidation rates and myocardial oxygen consumption were increased in 4-wk-old mice of both strains. Fatty acid oxidation rates progressively increased in db/db mice in parallel with the earlier onset and greater duration of hyperglycemia. In vivo, cardiac catheterization revealed significantly increased left ventricular contractility and relaxation (positive and negative dP/dt) in both strains at 4 wk of age. dP/dt declined over time in db/db mice but remained elevated in ob/ob mice at 15 wk of age. Increased beta-myosin heavy chain isoform expression was present in 4-wk-old mice and persisted in 15-wk-old mice. Increased expression of peroxisomal proliferator-activated receptor-alpha regulated genes was observed only at 15 wk in both strains. These data indicate that altered myocardial substrate use and reduced myocardial efficiency are early abnormalities in the hearts of obese mice and precede the onset of hyperglycemia. Obesity per se does not cause contractile dysfunction in vivo, but loss of the hypercontractile phenotype of obesity and up-regulation of peroxisomal proliferator-activated receptor-alpha regulated genes occur later and are most pronounced in the presence of longstanding hyperglycemia.

Leptin as a new diagnostic tool in chronic heart failure

Leptin, the product of the ob-gene, regulates cellular homeostasis and glycemic control. While initially described as an adipocyte-derived protein with expression and secretion restricted to adipose tissue, recent reports have shown local expression of leptin in several tissues including the skeletal muscle, heart, vessels and brain. Leptin acts through the different isoforms of its receptor which are ubiquitously expressed and can be detected in endothelium, vascular smooth muscle and myocardium. In addition to its metabolic effects, leptin has distinct effects in the cardiovascular system leading to increased production of proinflammatory cytokines and oxidative stress, vascular remodeling and neointima formation as well as cardiomyocyte hypertrophy. Notably, recent clinical studies have linked serum levels of leptin to the occurrence of cardiovascular events such as myocardial infarction and stroke suggesting that leptin promotes pro-atherogenic vascular mechanisms. In contrast, less is known about the role and effects of leptin in the setting of chronic heart failure. We here review the current knowledge on cardiovascular effects of leptin and discuss its potential as a new therapeutic tool in chronic heart failure.