Sympathetic response to insulin is mediated by melanocortin 3/4 receptors in the hypothalamic paraventricular nucleus

Insulin increases sympathetic nerve activity in part by suppression of tonic inhibitory neuropeptide Y inputs into the paraventricular nucleus in female rats

Following binding to receptors in the arcuate nucleus (ArcN), insulin increases sympathetic nerve activity (SNA) and baroreflex control of SNA via a pathway that includes the paraventricular nucleus of the hypothalamus (PVN). Previous studies in males indicate that the sympathoexcitatory response is mediated by α-melanocyte stimulating hormone (α-MSH), which binds to PVN melanocortin type 3/4 receptors (MC3/4R). The present study was conducted in α-chloralose-anesthetized female rats to test the hypothesis that suppression of inhibitory neuropeptide Y (NPY) inputs to the PVN is also involved. In support of this, blockade of PVN NPY Y1 receptors with BIBO 3304 (NPY1x), ArcN insulin nanoinjections, and PVN NPY1x followed by ArcN insulin each increased lumbar SNA (LSNA) and its baroreflex regulation similarly. Moreover, prior PVN injections of NPY blocked the sympathoexcitatory effects of ArcN insulin. Finally, PVN nanoinjections of the MC3/4R inhibitor SHU9119 prevented both the acute (15 min) and longer, more slowly developing (60 min), increases in LSNA in response to ArcN insulin. In conclusion, in females, ArcN insulin increases LSNA, in part, by suppressing tonic PVN NPY inhibition, which unmasks excitatory α-MSH drive of LSNA. Moreover, the steadily increasing rise in LSNA induced by ArcN insulin is also dependent on PVN MC3/4R.

Imbalanced insulin action in chronic over nutrition: Clinical harm, molecular mechanisms, and a way forward

The growing worldwide prevalence of overnutrition and underexertion threatens the gains that we have made against atherosclerotic cardiovascular disease and other maladies. Chronic overnutrition causes the atherometabolic syndrome, which is a cluster of seemingly unrelated health problems characterized by increased abdominal girth and body-mass index, high fasting and postprandial concentrations of cholesterol- and triglyceride-rich apoB-lipoproteins (C-TRLs), low plasma HDL levels, impaired regulation of plasma glucose concentrations, hypertension, and a significant risk of developing overt type 2 diabetes mellitus (T2DM). In addition, individuals with this syndrome exhibit fatty liver, hypercoagulability, sympathetic overactivity, a gradually rising set-point for body adiposity, a substantially increased risk of atherosclerotic cardiovascular morbidity and mortality, and--crucially--hyperinsulinemia. Many lines of evidence indicate that each component of the atherometabolic syndrome arises, or is worsened by, pathway-selective insulin resistance and responsiveness (SEIRR). Individuals with SEIRR require compensatory hyperinsulinemia to control plasma glucose levels. The result is overdrive of those pathways that remain insulin-responsive, particularly ERK activation and hepatic de-novo lipogenesis (DNL), while carbohydrate regulation deteriorates. The effects are easily summarized: if hyperinsulinemia does something bad in a tissue or organ, that effect remains responsive in the atherometabolic syndrome and T2DM; and if hyperinsulinemia might do something good, that effect becomes resistant. It is a deadly imbalance in insulin action. From the standpoint of human health, it is the worst possible combination of effects. In this review, we discuss the origins of the atherometabolic syndrome in our historically unprecedented environment that only recently has become full of poorly satiating calories and incessant enticements to sit. Data are examined that indicate the magnitude of daily caloric imbalance that causes obesity. We also cover key aspects of healthy, balanced insulin action in liver, endothelium, brain, and elsewhere. Recent insights into the molecular basis and pathophysiologic harm from SEIRR in these organs are discussed. Importantly, a newly discovered oxide transport chain functions as the master regulator of the balance amongst different limbs of the insulin signaling cascade. This oxide transport chain--abbreviated 'NSAPP' after its five major proteins--fails to function properly during chronic overnutrition, resulting in this harmful pattern of SEIRR. We also review the origins of widespread, chronic overnutrition. Despite its apparent complexity, one factor stands out. A sophisticated junk food industry, aided by subsidies from willing governments, has devoted years of careful effort to promote overeating through the creation of a new class of food and drink that is low- or no-cost to the consumer, convenient, savory, calorically dense, yet weakly satiating. It is past time for the rest of us to overcome these foes of good health and solve this man-made epidemic.

Hypothalamic Paraventricular and Arcuate Nuclei Contribute to Elevated Sympathetic Nerve Activity in Pregnant Rats: Roles of Neuropeptide Y and α-Melanocyte-Stimulating Hormone

Pregnancy increases sympathetic nerve activity (SNA), but the mechanisms are unknown. Here, we investigated the contributions of the hypothalamic paraventricular and arcuate nuclei in α-chloralose-anesthetized pregnant and nonpregnant rats. Baseline arterial pressure (AP) was lower, and heart rate (HR), lumbar sympathetic activity, and splanchnic SNA were higher in pregnant rats compared with nonpregnant rats. Inhibition of the paraventricular nucleus via bilateral muscimol nanoinjections decreased AP and HR more in pregnant rats than in nonpregnant rats and decreased lumbar SNA only in pregnant rats. Similarly, after arcuate muscimol nanoninjections, the decreases in AP, HR, and lumbar, renal, and splanchnic sympathetic nerve activities were greater in pregnant rats than in nonpregnant rats. Major arcuate neuronal groups that project to the paraventricular nucleus express inhibitory neuropeptide Y (NPY) and excitatory α-melanocyte-stimulating hormone. Inhibition of paraventricular melanocortin 3/4 receptors with SHU9119 also decreased AP, HR, and lumbar SNA in pregnant rats but not in nonpregnant rats. Conversely, paraventricular nucleus NPY expression was reduced in pregnant animals, and although blockade of paraventricular NPY Y1 receptors increased AP, HR, and lumbar sympathetic activity in nonpregnant rats, it had no effects in pregnant rats. Yet, the sympathoinhibitory, depressor, and bradycardic effects of paraventricular NPY nanoinjections were similar between groups. In conclusion, the paraventricular and arcuate nuclei contribute to increased basal SNA during pregnancy, likely due in part to decreased tonic NPY inhibition and increased tonic α-melanocyte-stimulating hormone excitation of presympathetic neurons in the paraventricular nucleus.

Cerebrospinal Fluid Hypernatremia Elevates Sympathetic Nerve Activity and Blood Pressure via the Rostral Ventrolateral Medulla

Elevated NaCl concentrations of the cerebrospinal fluid increase sympathetic nerve activity (SNA) in salt-sensitive hypertension. Neurons of the rostral ventrolateral medulla (RVLM) play a pivotal role in the regulation of SNA and receive mono- or polysynaptic inputs from several hypothalamic structures responsive to hypernatremia. Therefore, the present study investigated the contribution of RVLM neurons to the SNA and pressor response to cerebrospinal fluid hypernatremia. Lateral ventricle infusion of 0.15 mol/L, 0.6 mol/L, and 1.0 mol/L NaCl (5 µL/10 minutes) produced concentration-dependent increases in lumbar SNA, adrenal SNA, and arterial blood pressure, despite no change in splanchnic SNA and a decrease in renal SNA. Ganglionic blockade with chlorisondamine or acute lesion of the lamina terminalis blocked or significantly attenuated these responses, respectively. RVLM microinjection of the gamma-aminobutyric acid (GABAA) agonist muscimol abolished the sympathoexcitatory response to intracerebroventricular infusion of 1 mol/L NaCl. Furthermore, blockade of ionotropic glutamate, but not angiotensin II type 1, receptors significantly attenuated the increase in lumbar SNA, adrenal SNA, and arterial blood pressure. Finally, single-unit recordings of spinally projecting RVLM neurons revealed 3 distinct populations based on discharge responses to intracerebroventricular infusion of 1 mol/L NaCl: type I excited (46%; 11/24), type II inhibited (37%; 9/24), and type III no change (17%; 4/24). All neurons with slow conduction velocities were type I cells. Collectively, these findings suggest that acute increases in cerebrospinal fluid NaCl concentrations selectively activate a discrete population of RVLM neurons through glutamate receptor activation to increase SNA and arterial blood pressure.

Role of the Paraventricular Nucleus of the Hypothalamus in the Sympathoexcitatory Effects of Leptin

Leptin binds to receptors in multiple hypothalamic nuclei to increase sympathetic nerve activity; however, the neurocircuitry is unclear. Here, using anesthetized male Sprague-Dawley rats, we investigated the role of the paraventricular nucleus of the hypothalamus. Intracerebroventricular injection of leptin slowly increased lumbar sympathetic nerve activity (LSNA), heart rate, mean arterial pressure, and baroreflex control of LSNA and heart rate. Inhibition of the paraventricular nucleus with muscimol completely reversed leptin's effects. Blockade of paraventricular melanocortin 3/4 receptors with SHU9119 or ionotropic glutamate receptors with kynurenate, alone or together, each partially reversed the effects of leptin, implicating increased activation of glutamate and melanocortin 3/4 receptors. Conversely, although blockade of neuropeptide Y Y1 receptors in the paraventricular nucleus increased LSNA, mean arterial pressure, and heart rate, these responses were prevented by intracerebroventricular or arcuate nucleus injections of leptin, suggesting that, at least in part, leptin also increases sympathetic nerve activity by suppression of tonic neuropeptide Y inhibitory inputs from the arcuate nucleus. Injection of the melanocortin 3/4 receptor agonist melanotan-II into the paraventricular nucleus increased LSNA, mean arterial pressure, and heart rate only after blockade of neuropeptide Y Y1 receptors. Therefore, we conclude that leptin increases LSNA in part via increased glutamatergic and α-melanocyte-stimulating hormone drive of paraventricular sympathoexcitatory neurons, the latter of which requires simultaneous withdrawal of tonic neuropeptide Y inhibition.

Adipose afferent reflex response to insulin is mediated by melanocortin 4 type receptors in the paraventricular nucleus in insulin resistance rats

BACKGROUND

Adipose afferent reflex (AAR) contributes to sympathetic activation and hypertension. Paraventricular nucleus (PVN) plays an important role in AAR and sympathetic outflow. The aim of the present study was to determine whether PVN mediates AAR response to insulin in a rat model of insulin resistance (IR).

METHODS

Male Sprague-Dawley rats were randomly divided into Control and IR groups. Insulin resistance was induced by supplementing fructose (125 g L(-1) , 12 weeks) in the drinking water. Renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were recorded in anesthetized rats. AAR was evaluated by the RSNA and MAP responses to injection of capsaicin into four sites of right inguinal white adipose tissue.

RESULTS

Rats in IR group showed a rise in plasma noradrenaline (NE), glucose, insulin and triglyceride levels, left ventricular weight, systolic blood pressure, homeostasis model assessment of insulin resistance (HOMA-IR) and PVN glucose and insulin levels, melanocortin 4 type receptors (MC4Rs) protein expression, but not MC3Rs and insulin receptors. Compared with Control group, AAR in IR group was significantly enhanced, which contributed to the elevation of NE level; and insulin microinjection into the PVN or the third ventricle significantly strengthened AAR, which was attenuated by pre-treatment with MC4Rs antagonist HS024 and anti-insulin affibody, respectively, but not insulin receptors antagonist S961.

CONCLUSION

The enhanced AAR participates in sympathetic activation in IR, which can be strengthened by PVN insulin. PVN MC4Rs mediate the AAR response to insulin in IR, but not MC3Rs and insulin receptors.

Pioglitazone treatment enhances the sympathetic nervous system response to oral carbohydrate load in obese individuals with metabolic syndrome

CONTEXT

Insulin resistance is associated with blunted sympathetic nervous system (SNS) response to carbohydrate ingestion which may contribute to postprandial hypotension and impaired body weight homeostasis.

OBJECTIVE

This study was conducted to examine the effects of pharmacological insulin sensitization on whole-body norepinephrine kinetics during a standard 75-g oral glucose tolerance test (OGTT) in obese, insulin resistant subjects with metabolic syndrome.

METHODS

Un-medicated individuals (n=42, mean age 56±0.8 yrs, body mass index 34±0.6 kg/m(2)) were randomised to 12-weeks pioglitazone (PIO, 15 mg for 6 weeks, then 30 mg daily) or placebo using a double-blind, parallel group design. Whole-body norepinephrine kinetics (arterial norepinephrine concentration, calculated spillover and clearance rates), spontaneous cardiac baroreflex sensitivity, heart rate and blood pressure were measured at times 0, 30, 60, 90 and 120 minutes during OGTT. Insulin sensitivity was assessed by euglycemic hyperinsulinemic clamp (M) and Matsuda index.

RESULTS

PIO increased clamp derived glucose utilisation by 35% (P<0.001) and there were concurrent reductions in inflammatory status and plasma triglycerides (P<0.05). Fasting norepinephrine kinetic parameters were unaltered. PIO treatment was associated with lower plasma insulin incursions, greater reduction in diastolic blood pressure and enhanced baroreflex sensitivity during OGTT (P all <0.05). The overall norepinephrine spillover response (AUC(0-120)) increased significantly in the PIO group (group × time interaction, P=0.04), with greatest increment at 30 minutes post-glucose (101±38 ng/min at baseline versus 241±48 ng/min post treatment, P=0.04) and correlated with percent improvement in M.

CONCLUSIONS

PIO enhances the early postprandial SNS response to carbohydrate ingestion.

Insulin Detemir Is Transported From Blood to Cerebrospinal Fluid and Has Prolonged Central Anorectic Action Relative to NPH Insulin

Insulin detemir (DET) reduces glycemia comparably to other long-acting insulin formulations but causes less weight gain. Insulin signaling in the brain is catabolic, reducing food intake. We hypothesized that DET reduces weight gain, relative to other insulins, owing to increased transport into the central nervous system and/or increased catabolic action within the brain. Transport of DET and NPH insulin into the cerebrospinal fluid (CSF) was compared over several hours and after the administration of different doses peripherally in rats. DET and NPH had comparable saturable, receptor-mediated transport into the CSF. CSF insulin remained elevated significantly longer after intraperitoneal DET than after NPH. When administered acutely into the 3rd cerebral ventricle, both DET and NPH insulin reduced food intake and body weight at 24 h, and both food intake and body weight remained lower after DET than after NPH after 48 h. In direct comparison with another long-acting insulin, insulin glargine (GLAR), DET led to more prolonged increases in CSF insulin despite a shorter plasma half-life in both rats and mice. Additionally, peripheral DET administration reduced weight gain and increased CSF insulin compared with saline or GLAR in mice. Overall, these data support the hypothesis that DET has distinct effects on energy balance through enhanced and prolonged centrally mediated reduction of food intake.

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.

Glutamate receptors in the hypothalamic paraventricular nucleus contribute to insulin-induced sympathoexcitation

The sympathoexcitatory response to insulin is mediated by neurons in the arcuate nucleus (ARC) and hypothalamic paraventricular nucleus (PVH). Previous studies have reported that stimulation of ARC neurons increases sympathetic nerve activity (SNA) and arterial blood pressure (ABP) through glutamate receptor activation in the PVH. Therefore, the purpose of the present study was to determine whether glutamatergic neurotransmission in the PVH contributes to insulin-induced sympathoexcitation. Male Sprague-Dawley rats (275-400 g) were infused with isotonic saline or insulin (3.75 mU · kg(-1) · min(-1)) plus 50% dextrose to maintain euglycemia. Intravenous infusion of insulin significantly increased lumbar SNA without a significant change in mean ABP, renal SNA, heart rate, or blood glucose. Bilateral PVH injection of the excitatory amino acid antagonist kynurenic acid (KYN) lowered lumbar SNA and ABP of animals infused with insulin. Similarly, a cocktail of the NMDA antagonist DL-2-amino-5-phosphonopentanoic acid (AP5) and non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) reduced lumbar SNA and mean ABP during infusion of insulin. In a final experiment, bilateral PVH injection of AP5 only, but not CNQX, lowered lumbar SNA and mean ABP of animals infused with insulin. The peak changes in lumbar SNA and mean ABP of insulin-treated animals were not different between KYN, AP5 plus CNQX, or AP5 alone. These drug treatments did not alter any variable in animals infused with saline. Altogether, these findings suggest that glutamatergic NMDA neurotransmission in the PVH contributes to insulin-induced sympathoexcitation.

Insulin transport into the brain and cerebrospinal fluid

The pancreatic hormone insulin plays a well-described role in the periphery, based principally on its ability to lower circulating glucose levels via activation of glucose transporters. However, insulin also acts within the central nervous system (CNS) to alter a number of physiological outcomes ranging from energy balance and glucose homeostasis to cognitive performance. Insulin is transported into the CNS by a saturable receptor-mediated process that is proposed to be dependent on the insulin receptor. Transport of insulin into the brain is dependent on numerous factors including diet, glycemia, a diabetic state and notably, obesity. Obesity leads to a marked decrease in insulin transport from the periphery into the CNS and the biological basis of this reduction of transport remains unresolved. Despite decades of research into the effects of central insulin on a wide range of physiological functions and its transport from the periphery to the CNS, numerous questions remain unanswered including which receptor is responsible for transport and the precise mechanisms of action of insulin within the brain.

Short term fat feeding rapidly increases plasma insulin but does not result in dyslipidaemia

Although the association between obesity and hypertension is well-known, the underlying mechanism remains elusive. Previously, we have shown that 3 week fat feeding in rabbits produces greater visceral adiposity, hypertension, tachycardia and elevated renal sympathetic nerve activity (RSNA) compared to rabbits on a normal diet. Because hyperinsulinaemia, hyperleptinemia, and dyslipidaemia are independent cardiovascular risk factors associated with hypertension we compared plasma insulin, leptin, and lipid profiles in male New Zealand White rabbits fed a normal fat diet (NFD 4.3% fat, n = 11) or high fat diet (HFD 13.4% fat, n = 13) at days 1, 2, 3 and weeks 1, 2, 3 of the diet. Plasma concentrations of diacylglyceride (DG), triacylglyceride (TG), ceramide and cholesteryl esters (CE) were obtained after analysis by liquid chromatography mass spectrometry. Plasma insulin and glucose increased within the first 3 days of the diet in HFD rabbits (P < 0.05) and remained elevated at week 1 (P < 0.05). Blood pressure and heart rate (HR) followed a similar pattern. By contrast, in both groups, plasma leptin levels remained unchanged during the first few days (P > 0.05), increasing by week 3 in fat fed animals alone (P < 0.05). Concentrations of total DG, TG, CE, and Ceramide at week 3 did not differ between groups (P > 0.05). Our data show plasma insulin increases rapidly following consumption of a HFD and suggests that it may play a role in the rapid rise of blood pressure. Dyslipidaemia does not appear to contribute to the hypertension in this animal model.

Obesity-induced increases in sympathetic nerve activity: sex matters

Abundant evidence obtained largely from male human and animal subjects indicates that obesity increases sympathetic nerve activity (SNA), which contributes to hypertension development. However, recent studies that included women reported that the strong relationships between muscle SNA and waist circumference or body mass index (BMI) found in men are not present in overweight and obese women. A similar sex difference in the association between adiposity and hypertension development has been identified in animal models of obesity. In this brief review, we consider two possible mechanisms for this sex difference. First, visceral adiposity, leptin, insulin, and angiotensin II have been identified as potential culprits in obesity-induced sympathoexcitation in males. We explore if these factors wield the same impact in females. Second, we consider if sex differences in vascular reactivity to sympathetic activation contribute. Our survey of the literature suggests that premenopausal females may be able to resist obesity-induced sympathoexcitation and hypertension in part due to differences in adipose disposition as well as its muted inflammatory response and reduced production of pressor versus depressor components of the renin-angiotensin system. In addition, vascular responsiveness to increased SNA may be reduced. However, more importantly, we identify the urgent need for further study, not only of sex differences per se, but also of the mechanisms that may mediate these differences. This information is required not only to refine treatment options for obese premenopausal women but also to potentially reveal new therapeutic avenues in obese men and women.

Glucocorticoids attenuate the central sympathoexcitatory actions of insulin

Insulin acts within the central nervous system to regulate food intake and sympathetic nerve activity (SNA). Strong evidence indicates that glucocorticoids impair insulin-mediated glucose uptake and food intake. However, few data are available regarding whether glucocorticoids also modulate the sympathoexcitatory response to insulin. Therefore, the present study first confirmed that chronic administration of glucocorticoids attenuated insulin-induced increases in SNA and then investigated whether these effects were attributed to deficits in central insulin-mediated responses. Male Sprague-Dawley rats were given access to water or a drinking solution of the glucocorticoid agonist dexamethasone (0.3 μg/ml) for 7 days. A hyperinsulinemic-euglycemic clamp significantly increased lumbar SNA in control rats. This response was significantly attenuated in rats given access to dexamethasone for 7, but not 1, days. Similarly, injection of insulin into the lateral ventricle or locally within the arcuate nucleus (ARC) significantly increased lumbar SNA in control rats but this response was absent in rats given access to dexamethasone. The lack of a sympathetic response to insulin cannot be attributed to a generalized depression of sympathetic function or inactivation of ARC neurons as electrical activation of sciatic afferents or ARC injection of gabazine, respectively, produced similar increases in SNA between control and dexamethasone-treated rats. Western blot analysis indicates insulin produced similar activation of Akt Ser(473) and rpS6 Ser(240/244) in the ventral hypothalamus of control and dexamethasone-treated rats. Collectively, these findings suggest that dexamethasone attenuates the sympathoexcitatory actions of insulin through a disruption of ARC neuronal function downstream of Akt or mammalian target of rapamycin (mTOR) signaling.

Activation of corticotropin-releasing factor receptors in the rostral ventrolateral medulla is required for glucose-induced sympathoexcitation

Energy expenditure is determined by metabolic rate and diet-induced thermogenesis. Normally, energy expenditure increases due to neural mechanisms that sense plasma levels of ingested nutrients/hormones and reflexively increase sympathetic nerve activity (SNA). Here, we investigated neural mechanisms of glucose-driven sympathetic activation by determining contributions of neuronal activity in the hypothalamic paraventricular nucleus (PVN) and activation of corticotropin-releasing factor (CRF) receptors in the rostral ventrolateral medulla (RVLM). Glucose was infused intravenously (150 mg/kg, 10 min) in male rats to raise plasma glucose concentration to a physiological postprandial level. In conscious rats, glucose infusion activated CRF-containing PVN neurons and TH-containing RVLM neurons, as indexed by c-Fos immunofluorescence. In α-chloralose/urethane-anesthetized rats, glucose infusion increased lumbar and splanchnic SNA, which was nearly prevented by prior RVLM injection of the CRF receptor antagonist astressin (10 pmol/50 nl). This cannot be attributed to a nonspecific effect, as sciatic afferent stimulation increased SNA and ABP equivalently in astressin- and aCSF-injected rats. Glucose-stimulated sympathoexcitation was largely reversed during inhibition of PVN neuronal activity with the GABA-A receptor agonist muscimol (100 pmol/50 nl). The effects of astressin to prevent glucose-stimulated sympathetic activation appear to be specific to interruption of PVN drive to RVLM because RVLM injection of astressin prior to glucose infusion effectively prevented SNA from rising and prevented any fall of SNA in response to acute PVN inhibition with muscimol. These findings suggest that activation of SNA, and thus energy expenditure, by glucose is initiated by activation of CRF receptors in RVLM by descending inputs from PVN.

Therapeutic effect of vagus nerve stimulation on depressive-like behavior, hyperglycemia and insulin receptor expression in Zucker fatty rats

Depression and type 2 diabetes (T2D) are common comorbid diseases and highly prevalent in the clinical setting with an unclarified mechanism. Zucker diabetic fatty (ZDF, fa/fa) rats natively develop T2D with hyperglycemia and hyperinsulinemia. Here we studied whether ZDF rats also innately develop depression, what a correlation is between depression and T2D, whether insulin receptor (IR) expression is involved in, and whether transcutaneous auricular vagus nerve stimulation (taVNS) would be beneficial in amelioration of the comorbidity. Six week old male ZDF and Zucker lean (ZL, fa/+) littermates were randomly divided into naïve (ZDF, n = 6; ZL, n = 7) and taVNS (ZDF-taVNS, n = 8; ZL-taVNS, n = 6) groups. Once daily 30 min-taVNS sessions were administrated under anesthesia for 34 consecutive days in taVNS groups. Blood glucose levels were tested weekly, and plasma glycosylated hemoglobin (HbAlc) level and immobility time in forced swimming test were determined on day 35 in all groups. The expression of insulin receptor (IR) in various tissues was also detected by immunostaining and Western blot. We found that naïve ZDF rats developed hyperglycemia steadily. These ZDF rats showed a strong positive correlation between longer immobility time and higher plasma HbAlC level. Long term taVNS treatment simultaneously prevented the development of depression-like behavior and progression of hyperglycemia in ZDF rats. The expression of IR in various tissues of naïve ZDF rats is lower than in naïve ZL and long-term taVNS treated ZDF rats. Collectively, our results indicate that in ZDF rats, i) depression and T2D develop simultaneously, ii) immobility time and HbAlc concentrations are highly and positively correlated, iii) a low expression of IR may be involved in the comorbidity of depression and T2D, and iv) taVNS is antidiabetic and antidepressive possibly through IR expression upregulation.

Transcutaneous auricular vagus nerve stimulation triggers melatonin secretion and is antidepressive in Zucker diabetic fatty rats

Decreased circulating melatonin is implicated in depression. We recently found that Zucker diabetic fatty rats (ZDF, fa/fa) develop depression-like behaviors and that transcutaneous auricular vagus nerve stimulation (taVNS) is antidepressive in ZDF rats. Here we studied whether the ZDF rats could be used as a depression rodent model and whether the antidepressive effect of taVNS is mediated through modulation of melatonin secretion. Adult male ZDF and Zucker lean (ZL, fa/+) littermates were used. 30 min-taVNS procedures (2/15 Hz, 2 mA) were administered once daily under anesthesia for 34 consecutive days in pineal intact ZDF (n = 8) and ZL (n = 6) rats, as well as in pinealectomized ZDF rats (n = 8). Forced swimming test (FST) was used to determine depression-like behavior and ELISA to detect plasma melatonin concentration on day 35. We found that naïve ZDF rats had a longer immobility time in FST and that long-term (34 days) taVNS treatment ameliorated the depression-like behavior. In both pineal intact and pinealectomized ZDF rats, taVNS induced acute melatonin secretion, both during and after the taVNS session. A low melatonin level is related to the poor FST performance in ZDF rats (R = -0.544) in contrast to ZL rats (R = 0.247). In conclusion, our results show that ZDF rats are ideal candidates of innate depression and that taVNS is antidepressive through triggering melatonin secretion and increasing its production.

Is insulin the new intermittent hypoxia?

The sympathoexcitatory effects of insulin are well-established, although the exact mechanisms by which insulin stimulates the sympathetic nervous system are not completely understood. The majority of research supports a primary role for the central nervous system in the gradual and sustained rise in muscle sympathetic nerve activity (MSNA) in response to hyperinsulinemia; in addition, recent studies in animals suggests carotid body chemoreceptors respond to increases in systemic insulin levels. Intermittent activation of the carotid chemoreceptors, similar to that seen in patients with sleep apnea, can result in sensory long term facilitation and may contribute to the observed rise in baseline MSNA in this population. Consistent with this idea, insulin exposure results in sustained increases in MSNA that persist even when plasma insulin levels return to baseline. We propose the carotid chemoreceptors contribute to insulin-mediated sympathoexcitation and the persistent rise in MSNA in patients with sustained hyperinsulinemia. If the carotid chemoreceptors sense and respond to changes in systemic insulin levels, these organs may provide a viable target for the treatment of disorders known to exhibit sustained hyperinsulinemia and sympathoexcitation including, but not limited to, obesity, hypertension, sleep apnea, metabolic syndrome, cardiovascular disease, and diabetes.

Expression of mineralocorticoid and glucocorticoid receptors in preautonomic neurons of the rat paraventricular nucleus

Activation of mineralocorticoid receptors (MR) of the hypothalamic paraventricular nucleus (PVN) increases sympathetic excitation. To determine whether MR and glucocorticoid receptors (GR) are expressed in preautonomic neurons of the PVN and how they relate to endogenous aldosterone levels in healthy rats, retrograde tracer was injected into the intermediolateral cell column at T4 to identify preautonomic neurons in the PVN. Expression of MR, GR, 11-β hydroxysteroid dehydrogenase1 and 2 (11β-HSD1, 2), and hexose-6-phosphate dehydrogenase (H6PD) required for 11β-HSD1 reductase activity was assessed by immunohistochemistry. RT-PCR and Western blot analysis were used to determine MR gene and protein expression. Most preautonomic neurons were in the caudal mediocellular region of PVN, and most expressed MR; none expressed GR. 11β-HSD1, but not 11β-HSD2 nor H6PD immunoreactivity, was detected in the PVN. In rats with chronic low or high sodium intakes, the low-sodium diet was associated with significantly higher plasma aldosterone, MR mRNA and protein expression, and c-Fos immunoreactivity within labeled preautonomic neurons. Plasma corticosterone and sodium and expression of tonicity-responsive enhancer binding protein in the PVN did not differ between groups, suggesting osmotic adaptation to the altered sodium intake. These results suggest that MR within preautonomic neurons in the PVN directly participate in the regulation of sympathetic nervous system drive, and aldosterone may be a relevant ligand for MR in preautonomic neurons of the PVN under physiological conditions. Dehydrogenase activity of 11β-HSD1 occurs in the absence of H6PD, which regenerates NADP(+) from NADPH and may increase MR gene expression under physiological conditions.

Maternal obesity and the developmental programming of hypertension: a role for leptin

Mother-child cohort studies have established that both pre-pregnancy body mass index (BMI) and gestational weight gain are independently associated with cardio-metabolic risk factors in young adult offspring, including systolic and diastolic blood pressure. Animal models in sheep and non-human primates provide further evidence for the influence of maternal obesity on offspring cardiovascular function, whilst recent studies in rodents suggest that perinatal exposure to the metabolic milieu of maternal obesity may permanently change the central regulatory pathways involved in blood pressure regulation. Leptin plays an important role in the central control of appetite, is also involved in activation of efferent sympathetic pathways to both thermogenic and non-thermogenic tissues, such as the kidney, and is therefore implicated in obesity-related hypertension. Leptin is also thought to have a neurotrophic role in the development of the hypothalamus, and altered neonatal leptin profiles secondary to maternal obesity are associated with permanently altered hypothalamic structure and function. In rodent studies, maternal obesity confers persistent sympathoexcitatory hyper-responsiveness and hypertension acquired in the early stages of development. Experimental neonatal hyperleptinaemia in naive rat pups provides further evidence of heightened sympathetic tone and proof of principle that hyperleptinaemia during a critical window of hypothalamic development may directly lead to adulthood hypertension. Insight from these animal models raises the possibility that early-life exposure to leptin in humans may lead to early onset essential hypertension. Ongoing mother-child cohort and intervention studies in obese pregnant women provide a unique opportunity to address associations between maternal obesity and offspring cardiovascular function. The goal of the review is to highlight the potential importance of leptin in the developmental programming of hypertension in obese pregnancy.

The effects of dietary weight loss on indices of norepinephrine turnover: modulatory influence of hyperinsulinemia

OBJECTIVES

This study was conducted to examine (1) the effects of dietary weight loss on indices of norepinephrine (NE) turnover and (2) whether baseline hyperinsulinemia modulates sympathetic neural adaptations.

METHODS

Obese individuals aged 56 ± 1 year, BMI 32.5 ± 0.4 kg/m(2) , with metabolic syndrome, underwent a 12-week hypocaloric diet (HCD, n = 39) or no treatment (n = 26). Neurochemical measurements comprised arterial dihydroxyphenylalanine (DOPA), 3,4-dihydroxyphenylglycol (DHPG), and NE concentrations, the steady-state ratio of [3H]-DHPG to [3H]-NE, as an index of neuronal uptake, and calculated whole-body plasma NE clearance and spillover rates.

RESULTS

Body weight decreased by -7.4 ± 0.5% in HCD group (P < 0.001) and was accompanied by reductions in DOPA, NE, and DHPG averaging -14 ± 5% (P = 0.001), -23 ± 4% (P <0.001), and -5 ± 4% (P = 0.03), respectively. NE spillover rate decreased by -88 ± 39 ng/min (P = 0.01), whereas neuronal uptake and NE plasma clearance were unchanged. Despite similar weight loss, hyperinsulinemic subjects exhibited greater reductions in NE and NE spillover rate, compared to normoinsulinemic subjects (group by time interaction P < 0.05).

CONCLUSIONS

Weight loss is associated with down-regulation of sympathetic nervous activity but no overall alteration in disposition indices. Hyperinsulinemic subjects derive a greater sympathoinhibitory benefit during weight loss.

Neuropeptide Y acts in the paraventricular nucleus to suppress sympathetic nerve activity and its baroreflex regulation

Neuropeptide Y (NPY), a brain neuromodulator that has been strongly implicated in the regulation of energy balance, also acts centrally to inhibit sympathetic nerve activity (SNA); however, the site and mechanism of action are unknown. In chloralose-anaesthetized female rats, nanoinjection of NPY into the paraventricular nucleus of the hypothalamus (PVN) dose-dependently suppressed lumbar SNA (LSNA) and its baroreflex regulation, and these effects were blocked by prior inhibition of NPY Y1 or Y5 receptors. Moreover, PVN injection of Y1 and Y5 receptor antagonists in otherwise untreated rats increased basal and baroreflex control of LSNA, indicating that endogenous NPY tonically inhibits PVN presympathetic neurons. The sympathoexcitation following blockade of PVN NPY inhibition was eliminated by prior PVN nanoinjection of the melanocortin 3/4 receptor inhibitor SHU9119. Moreover, presympathetic neurons, identified immunohistochemically using cholera toxin b neuronal tract tracing from the rostral ventrolateral medulla (RVLM), express NPY Y1 receptor immunoreactivity, and patch-clamp recordings revealed that both NPY and α-melanocyte-stimulating hormone (α-MSH) inhibit and stimulate, respectively, PVN-RVLM neurons. Collectively, these data suggest that PVN NPY inputs converge with α-MSH to influence presympathetic neurons. Together these results identify endogenous NPY as a novel and potent inhibitory neuromodulator within the PVN that may contribute to changes in SNA that occur in states associated with altered energy balance, such as obesity and pregnancy.

Attenuated dopaminergic tone in the paraventricular nucleus contributing to sympathoexcitation in rats with Type 2 diabetes

The study was conducted to investigate the role for dopamine in the centrally mediated sympathoexcitatory response in rats with Type 2 diabetes (T2D). T2D was induced by a combination of high-fat diet (HFD) and low-dose streptozotocin (STZ). HFD/STZ treatment for 12-14 wk resulted in significant increase in the number of FosB-positive cells in the paraventricular nucleus (PVN) and rostral ventrolateral medulla (RVLM). In anesthetized rats, administration of exogenous dopamine (dopamine hydrochloride, 20 mM) in the PVN, but not in the RVLM, elicited decreases in renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) in control rats and but not in the T2D rats. Blocking the endogenous dopamine with dopamine D1/D5 receptor antagonist SCH39166 (2 mM) in the PVN and RVLM, resulted in increases in RSNA, MAP, and heart rate (HR) in both control and T2D rats. These responses were significantly attenuated in T2D rats compared with control rats (PVN - ΔRSNA: 21 ± 10 vs. 44 ± 2%; ΔMAP: 7 ± 3 vs. 19 ± 6 mmHg, ΔHR: 17 ± 5 vs. 32 ± 4 bpm, P < 0.05). There were no significant increases in response to dopamine D2/D3 receptor antagonist raclopride application in the PVN and RVLM of both control and T2D rats. Furthermore, there were decreased dopamine D1 receptor and D2 receptor expressions in the PVN of T2D rats. Taken together, these data suggest that reduced endogenous dopaminergic tone within the PVN may contribute to the sympathoexcitation in T2D.

The melanocortin-4 receptor as target for obesity treatment: a systematic review of emerging pharmacological therapeutic options

Obesity is one of the greatest public health challenges of the 21st century. Obesity is currently responsible for ∼0.7-2.8% of a country's health costs worldwide. Treatment is often not effective because weight regulation is complex. Appetite and energy control are regulated in the brain. Melanocortin-4 receptor (MC4R) has a central role in this regulation. MC4R defects lead to a severe clinical phenotype with lack of satiety and early-onset severe obesity. Preclinical research has been carried out to understand the mechanism of MC4R regulation and possible effectors. The objective of this study is to systematically review the literature for emerging pharmacological obesity treatment options. A systematic literature search was performed in PubMed and Embase for articles published until June 2012. The search resulted in 664 papers matching the search terms, of which 15 papers remained after elimination, based on the specific inclusion and exclusion criteria. In these 15 papers, different MC4R agonists were studied in vivo in animal and human studies. Almost all studies are in the preclinical phase. There are currently no effective clinical treatments for MC4R-deficient obese patients, although MC4R agonists are being developed and are entering phase I and II trials.

Mechanisms of cardiovascular actions of urocortins in the hypothalamic arcuate nucleus of the rat

The presence of urocortins (UCNs) and corticotropin-releasing factor (CRF) receptors has been reported in the hypothalamic arcuate nucleus (ARCN). We have previously reported that UCNs are involved in central cardiovascular regulation. Based on this information, we hypothesized that the ARCN may be one of the sites where UCNs exert their central cardiovascular actions. Experiments were done in artificially ventilated, adult male Wistar rats anesthetized with urethane. Unilateral microinjections (30 nl) of UCN1 (0.12-2 mM) elicited decreases in mean arterial pressure (MAP) and heart rate (HR). Maximum cardiovascular responses were elicited by a 1 mM concentration of UCN1. Microinjections of UCN2 and UCN3 (1 mM each) into the ARCN elicited similar decreases in MAP and HR. UCN1 was used as a prototype for the other experiments described below. HR responses elicited by UCN1 were significantly attenuated by bilateral vagotomy. Prior microinjections of NBI-27914 (CRF-1 receptor antagonist) and astressin (CRF-1 receptor and CRF-2 receptor antagonist) (1 mM each) into the ARCN significantly attenuated the cardiovascular responses elicited by UCN1 microinjections at the same site. Microinjections of UCN1 into the ARCN decreased efferent renal sympathetic nerve activity. It was concluded that microinjections of UCN1, UCN2, and UCN3 into the ARCN elicited decreases in MAP and HR. Decreases in MAP, HR, and renal sympathetic nerve activity elicited by UCN1 microinjections into the ARCN were mediated via CRF receptors. Bradycardic responses to UCN1 were mediated via the activation of vagus nerves, and decreases in MAP may be mediated via decreases in sympathetic nerve activity.

Arcuate nucleus injection of an anti-insulin affibody prevents the sympathetic response to insulin

Accumulating evidence suggests that insulin acts within the hypothalamus to alter sympathetic nerve activity (SNA) and baroreflex function. Although insulin receptors are widely expressed across the hypothalamus, recent evidence suggests that neurons of the arcuate nucleus (ARC) play an important role in the sympathoexcitatory response to insulin. The purpose of the present study was to determine whether circulating insulin acts directly in the ARC to elevate SNA. In anesthetized male Sprague-Dawley rats (275-425 g), the action of insulin was neutralized by microinjection of an anti-insulin affibody (1 ng/40 nl). To verify the efficacy of the affibody, ARC pretreatment with injection of the anti-insulin affibody completely prevented the increase in lumbar SNA produced by ARC injection of insulin. Next, ARC pretreatment with the anti-insulin affibody attenuated the lumbar sympathoexcitatory response to intracerebroventricular injection of insulin. Third, a hyperinsulinemic-euglycemic clamp increased lumbar, but not renal, SNA in animals that received ARC injection of a control affibody. However, this sympathoexcitatory response was absent in animals pretreated with the anti-insulin affibody in the ARC. Injection of the anti-insulin affibody in the adjacent ventromedial hypothalamus did not alter the sympathoexcitatory response to insulin. The ability of the anti-insulin affibody to prevent the sympathetic effects of insulin cannot be attributed to a general inactivation or nonspecific effect on ARC neurons as the affibody did not alter the sympathoexcitatory response to ARC disinhibition by gabazine. Collectively, these findings suggest that circulating insulin acts within the ARC to increase SNA.

Reversal of diet-induced obesity increases insulin transport into cerebrospinal fluid and restores sensitivity to the anorexic action of central insulin in male rats

Diet-induced obesity (DIO) reduces the ability of centrally administered insulin to reduce feeding behavior and also reduces the transport of insulin from the periphery to the central nervous system (CNS). The current study was designed to determine whether reversal of high-fat DIO restores the anorexic efficacy of central insulin and whether this is accompanied by restoration of the compromised insulin transport. Adult male Long-Evans rats were initially maintained on either a low-fat chow diet (LFD) or a high-fat diet (HFD). After 22 weeks, half of the animals on the HFD were changed to the LFD, whereas the other half continued on the HFD for an additional 8 weeks, such that there were 3 groups: 1) a LFD control group (Con; n = 18), 2) a HFD-fed, DIO group (n = 17), and 3) a HFD to LFD, DIO-reversal group (DIO-rev; n = 18). The DIO reversal resulted in a significant reduction of body weight and epididymal fat weight relative to the DIO group. Acute central insulin administration (8 mU) reduced food intake and caused weight loss in Con and DIO-rev but not DIO rats. Fasting cerebrospinal fluid insulin was higher in DIO than Con animals. However, after a peripheral bolus injection of insulin, cerebrospinal fluid insulin increased in Con and DIO-rev rats but not in the DIO group. These data provide support for previous reports that DIO inhibits both the central effects of insulin and insulin's transport to the CNS. Importantly, DIO-rev restored sensitivity to the effects of central insulin on food intake and insulin transport into the CNS.

Interactions between the central nervous system and pancreatic islet secretions: a historical perspective

The endocrine pancreas is richly innervated with sympathetic and parasympathetic projections from the brain. In the mid-20th century, it was established that α-adrenergic activation inhibits, whereas cholinergic stimulation promotes, insulin secretion; this demonstrated the importance of the sympathetic and parasympathetic systems in pancreatic endocrine function. It was later established that insulin injected peripherally could act within the brain, leading to the discovery of insulin and insulin receptors within the brain and the receptor-mediated transport of insulin into the central nervous system from endothelial cells. The insulin receptor within the central nervous system is widely distributed, reflecting insulin's diverse range of actions, including acting as an adiposity signal to reduce food intake and increase energy expenditure, regulation of systemic glucose responses, altering sympathetic activity, and involvement in cognitive function. As observed with central insulin administration, the pancreatic hormones glucagon, somatostatin, pancreatic polypeptide, and amylin can each also reduce food intake. Pancreatic and also gut hormones are released cephalically, in what is an important mechanism to prepare the body for a meal and prevent excessive postprandial hyperglycemia.

Baroreflex function in females: changes with the reproductive cycle and pregnancy

This review briefly describes the changes in baroreflex function that occur during female reproductive life, specifically during the reproductive cycle and pregnancy. The sensitivity or gain of baroreflex control of heart rate and sympathetic activity fluctuates during the reproductive cycle, reaching a peak when gonadal hormone levels increase, during the follicular phase in women and proestrus in rats. The increase in baroreflex sensitivity (BRS) is likely mediated by estrogen because ovariectomy in rats eliminates the BRS increase, the cyclic profile of changes in BRS mirror the changes in estrogen, and estrogen acts in the brainstem to increase BRS. In contrast, pregnancy depresses both BRS and the maximal level of sympathetic activity and heart rate evoked by severe hypotension. The decrease in BRS may be mediated by a reduction in the actions of insulin in the arcuate nucleus to support the baroreflex. In addition, increased levels of the neurosteroid progesterone metabolite 3α-OH-DHP act downstream in the rostral ventrolateral medulla to suppress maximal baroreflex increases in sympathetic activity. Consequently, these changes in baroreflex function impair blood pressure regulation in the presence of hypotensive challenges such as orthostasis and hemorrhage, a common event during delivery. As a result, peripartum hemorrhage is a major cause of human maternal death.

Hypertension in obesity: is leptin the culprit?

The number of obese or overweight humans continues to increase worldwide. Hypertension is a serious disease that often develops in obesity, but it is not clear how obesity increases the risk of hypertension. However, both obesity and hypertension increase the risk of cardiovascular diseases (CVD). In this review, we examine how obesity may increase the risk of developing hypertension. Specifically, we discuss how the adipose-derived hormone leptin influences the sympathetic nervous system (SNS), through actions in the brain to elevate energy expenditure (EE) while also contributing to hypertension in obesity.

Sodium-retaining effect of insulin in diabetes

Insulin has long been hypothesized to cause sodium retention, potentially of enough magnitude to contribute to hypertension in obesity, metabolic syndrome, and Type II diabetes. There is an abundance of supportive evidence from correlational analyses in humans, acute insulin infusion studies in humans and animals, and chronic insulin infusion studies in rats. However, the absence of hypertension in human insulinoma patients, and negative results for sodium-retaining or blood pressure effects of chronic insulin infusion in a whole series of dog studies, strongly refute the insulin hypothesis. We recently questioned whether the euglycemic, hyperinsulinemia model used for most insulin infusion studies, including the previous chronic dog studies, was the most appropriate model to test the renal actions of insulin in obesity, metabolic syndrome, and Type II diabetes. In those circumstances, hyperinsulinemia coexists with hyperglycemia. Therefore, we tested the sodium-retaining effect of insulin in chronically instrumented, alloxan-treated diabetic dogs. We used 24 h/day intravenous insulin infusion to regulate plasma insulin concentration. Induction of diabetes (∼400 mg/dl) caused sustained natriuresis and diuresis. However, if we clamped insulin at baseline, control levels, i.e., prevented it from decreasing, then the sustained natriuresis and diuresis were completely reversed, despite the same level of hyperglycemia. We also found that 24 h/day intrarenal insulin infusion had the same effect in diabetic dogs but had no sodium-retaining action in normal dogs. This new evidence that insulin has a sodium-retaining effect during hyperglycemia may have implications for maintaining sodium balance in uncontrolled Type II diabetes.

Neuroadrenergic dysfunction along the diabetes continuum: a comparative study in obese metabolic syndrome subjects

Neuroadrenergic function in type 2 diabetic (T2D) patients without neuropathy is poorly characterized. We therefore compared sympathetic nervous system activity at rest and during an oral glucose tolerance test in obese metabolic syndrome (MetS) subjects classified as glucose intolerant (impaired glucose tolerance [IGT]; n = 17) or treatment-naive T2D (n = 17). Untreated subjects, matched for age (mean 59 ± 1 year), sex, BMI (32.4 ± 0.6 kg/m(2)), and family history of diabetes were studied. We measured resting muscle sympathetic nerve activity (MSNA) by microneurography, whole-body norepinephrine kinetics by isotope dilution, insulin sensitivity by euglycemic-hyperinsulinemic clamp (steady-state glucose utilization adjusted for fat-free mass and steady-state insulin concentration [M/I]), and MetS components. T2D subjects had higher resting MSNA burst incidence (67 ± 4 versus 55 ± 3 bursts per 100 heartbeats; P = 0.05) and arterial norepinephrine levels (264 ± 33 versus 167 ± 16 pg/mL; P = 0.02), lower plasma norepinephrine clearance (by 17%; P = 0.03), and reduced neuronal reuptake compared with IGT subjects (by 46%; P = 0.04). Moreover, norepinephrine spillover responses to glucose ingestion were blunted in T2D subjects. The M/I value independently predicted whole-body norepinephrine spillover (r = -0.47; P = 0.008), whereas fasting insulin level related to neuronal norepinephrine reuptake (r = -0.35, P = 0.047). These findings demonstrate that progression to T2D is associated with increased central sympathetic drive, blunted sympathetic responsiveness, and altered norepinephrine disposition.

Endogenous hydrogen peroxide in the hypothalamic paraventricular nucleus regulates sympathetic nerve activity responses to L-glutamate

The hypothalamic paraventricular nucleus (PVN) is important for maintenance of sympathetic nerve activity (SNA) and cardiovascular function. PVN-mediated increases of SNA often involve the excitatory amino acid L-glutamate (L-glu), whose actions can be positively and negatively modulated by a variety of factors, including reactive oxygen species. Here, we determined modulatory effects of the highly diffusible reactive oxygen species hydrogen peroxide (H(2)O(2)) on responses to PVN L-glu. Renal SNA (RSNA), arterial blood pressure, and heart rate were recorded in anesthetized rats. L-Glu (0.2 nmol in 100 nl) microinjected unilaterally into PVN increased RSNA (P < 0.05), without affecting mean arterial blood pressure or heart rate. Effects of endogenously generated H(2)O(2) were determined by comparing responses to PVN L-glu before and after PVN injection of the catalase inhibitor 3-amino-1,2,4-triazole (ATZ; 100 nmol/200 nl, n = 5). ATZ alone was without effect on recorded variables, but attenuated the increase of RSNA elicited by PVN L-glu (P < 0.05). PVN injection of exogenous H(2)O(2) (5 nmol in 100 nl, n = 4) and vehicle (artificial cerebrospinal fluid) were without affect, but H(2)O(2), like ATZ, attenuated the increase of RSNA to PVN L-glu (P < 0.05). Tonic effects of endogenous H(2)O(2) were determined by PVN injection of polyethylene glycol-catalase (1.0 IU in 200 nl, n = 5). Whereas polyethylene glycol-catalase alone was without effect, increases of RSNA to subsequent PVN injection of L-glu were increased (P < 0.05). From these data, we conclude that PVN H(2)O(2) tonically, but submaximally, suppresses RSNA responses to L-glu, supporting the idea that a change of H(2)O(2) availability within PVN could influence SNA regulation under physiological and/or disease conditions.

Gsα deficiency in the paraventricular nucleus of the hypothalamus partially contributes to obesity associated with Gsα mutations

The G protein α-subunit G(s)α mediates receptor-stimulated cAMP generation. Heterozygous inactivating G(s)α mutations on the maternal allele result in obesity primarily due to reduced energy expenditure in Albright hereditary osteodystrophy patients and in mice. We previously showed that mice with central nervous system (CNS)-specific G(s)α deletion on the maternal allele (mBrGs KO) also develop severe obesity with reduced energy expenditure and that G(s)α is primarily expressed from the maternal allele in the paraventricular nucleus (PVN) of the hypothalamus, an important site of energy balance regulation. We now generated mice with PVN-specific G(s)α deficiency by mating Single-minded 1-cre and G(s)α-floxed mice. Homozygous G(s)α deletion produced early lethality. Heterozygotes with maternal G(s)α deletion (mPVNGsKO) also developed obesity and had small reductions in energy expenditure. However, this effect was much milder than that found in mBrGsKO mice and was more prominent in males. We previously showed mBrGsKO mice to have significant reductions in melanocortin receptor agonist-stimulated energy expenditure and now show that mBrGsKO mice have impaired cold-induced brown adipose tissue stimulation. In contrast, these effects were absent in mPVNGsKO mice. mPVNGsKO mice also had minimal effects on glucose metabolism as compared with mBrGsKO mice. Consistent with the presence of G(s)α imprinting, paternal heterozygotes showed no changes in energy or glucose metabolism. These results indicate that although G(s)α deficiency in PVN partially contributes to the metabolic phenotype resulting from maternal G(s)α mutations, G(s)α imprinting in other CNS regions is also important in mediating the CNS effects of G(s)α mutations on energy and glucose metabolism.

Melanocortin 3/4 receptors in paraventricular nucleus modulate sympathetic outflow and blood pressure

Central melanocortin 3/4 receptors (MC3/4Rs) are known to regulate energy balance. Activation of MC3/4Rs causes a greater increase in the firing activity of the PVN neurons in obese Zucker rats than in lean Zucker rats. The present study was undertaken to determine the roles of MC3/4Rs in the hypothalamic paraventricular nucleus (PVN) in modulating the sympathetic activity and blood pressure and its downstream pathway. Renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were recorded in anaesthetized rats. Microinjection of the MC3/4R agonist melanotan II (MTII) into the PVN increased the RSNA and MAP. The MC3/4R antagonist agouti-related peptide (AgRP) or SHU9119 decreased the RSNA and MAP, but the MC4R antagonist HS024 had no significant effect on the RSNA and MAP. The effects of MTII were abolished by pretreatment of the PVN with AgRP, SHU9119, the adenylate cyclase inhibitor SQ22536 or the protein kinase A inhibitor Rp-cAMP, and substantially attenuated by HS024. Microinjection of SQ22536 alone into the PVN had no significant effect on the RSNA and MAP, but Rp-cAMP caused significant decreases in the RSNA and MAP. Furthermore, MTII increased the cAMP level in the PVN. These results indicate that activation of MC3/4Rs in the PVN increases the sympathetic outflow and blood pressure via the cAMP-protein kinase A pathway. Melanocortin 3 receptors in the PVN may exert a tonic excitatory effect on sympathetic activity.

The role of pathway-selective insulin resistance and responsiveness in diabetic dyslipoproteinemia

PURPOSE OF REVIEW

Type 2 diabetes mellitus (T2DM) and related syndromes exhibit a deadly triad of dyslipoproteinemia, which leads to atherosclerosis, hyperglycemia, which causes microvascular disease, and hypertension. These features share a common, but unexplained, origin--namely, pathway-selective insulin resistance and responsiveness (SEIRR). Here, we review recent work on hepatic SEIRR indicating that deranged insulin signaling may have a remarkably simple molecular basis.

RECENT FINDINGS

Comprehensive examination of a set of 18 insulin targets revealed that T2DM liver in vivo exhibits a specific defect in the ability of the NAD(P)H oxidase 4 (NOX4) to inactivate protein tyrosine phosphatase gene family members after stimulation with insulin, and that impairment of this single molecule, NOX4, in cultured hepatocytes recapitulates all features of hepatic SEIRR in vivo. These features include insulin-stimulated generation of an unusual monophosphorylated form of AKT at Thr308 (pT308-AKT) with only weak phosphorylation at Ser473, impaired insulin-stimulated pathways for lowering plasma levels of lipids and glucose, but continued lipogenic pathways and robust extracellular signal-regulated kinase activation. This new study, in combination with important prior work, provides clues to several long-standing mysteries, such as how AKT might regulate lipid-lowering and glucose-lowering pathways that become insulin-resistant but also lipogenic pathways that remain insulin-responsive, as well as a potential role for NOX4 in insulin-stimulated generation of oxysterol ligands for LXR, a key lipogenic factor.

SUMMARY

These findings suggest a unified molecular explanation for fatty liver, atherogenic dyslipoproteinemia, hyperglycemia, and hence accelerated atherosclerosis and microvascular disease in T2DM, obesity, and related syndromes of positive caloric imbalance.

Risk of elevated resting heart rate on the development of type 2 diabetes in patients with clinically manifest vascular diseases

OBJECTIVE

Sympathetic nerve activation is causally related to insulin resistance as both a cause and a consequence. Resting heart rate (RHR) reflects sympathetic nerve activity. We investigated the effect of RHR on the incidence of type 2 diabetes mellitus (T2DM) in patients with clinically manifest vascular diseases.

DESIGN

Data were used from the second manifestations of arterial disease (SMART) study: a prospective cohort study of patients with clinically manifest vascular diseases (n=3646).

METHODS

RHR was obtained using an electrocardiogram. Patients were followed up for incident type 2 diabetes (n=289) during a median period of 5.5 (interquartile range 3.2-8.4) years. The relation between RHR and incident T2DM was estimated by Cox proportional hazard analysis. As age was an effect modifier (P=0.048), analyses were stratified for age.

RESULTS

Patients in quartile 4 (Q4) of RHR had a 65% increased risk of T2DM compared with those in Q1 (reference; hazard ratios (HR), 1.65; 95% confidence interval (95% CI), 1.15-2.36) adjusted for age, gender, smoking, estimated glomerular filtration rate, systolic blood pressure, location of vascular disease, and antihypertensive medication. Every 10 beats per minute (bpm) increase in RHR increased the risk for T2DM with 10% (HR, 1.10; 95% CI, 1.00-1.21) in the total population. This risk was particularly high in subjects aged 55-63 years (per 10 bpm: HR, 1.22; 95% CI, 1.04-1.43) and was independent of the location of vascular disease and beta-blocker use.

CONCLUSIONS

Increased RHR, an indicator of sympathetic nerve activity, is associated with an increased risk for T2DM in patients with manifest vascular diseases, particularly in middle-aged patients.

Increased visceral adipose tissue is associated with increased resting heart rate in patients with manifest vascular disease

Abdominal obesity is characterized by sympathetic nerve activation (SNA), probably mediated by elevated insulin and leptin levels. Resting heart rate (RHR) is a marker of sympathetic tone, and independently associated with cardiovascular events and death in various populations. We investigated and quantified the relation between visceral adipose tissue (VAT) and RHR in patients with vascular disease. In 3,723 patients with manifest vascular disease, visceral and subcutaneous fat tissue was measured with ultrasonography. RHR was obtained from an electrocardiogram (ECG). The association between quartiles of VAT and RHR was quantified using linear regression analysis with adjustments for potential confounding factors. Separate analyses were performed for men and women and for location of vascular disease. Visceral fat was categorized into sex-pooled quartiles (Q) ranging from 2.7-8.0 cm in Q1 (reference) to 9.4-20.6 cm in Q4. High visceral fat thickness was associated with increased RHR, in men (Q4 vs. Q1, β = 4.36; 95% confidence interval (CI) = 3.11-5.61) and women (β = 1.48; 95% CI = -0.70 to 3.66), after full adjustment. Waist circumference and BMI had a significant relation with RHR in men (β = 3.51; 95% CI = 2.21-4.81 and β = 2.80; 95% CI = 1.51-4.08, respectively) but these relations were smaller and not significant in women (β = 0.71; 95% CI = -1.44 to 2.85 and β = 0.24; 95% CI = -1.90 to 2.37, respectively). There was no relation between subcutaneous fat and RHR in men and women. The relation between visceral fat and RHR was similar in patients with different locations of vascular diseases. Increased visceral fat is associated with increased RHR in male and female patients with vascular disease, independent of the location.

Central regulation of blood pressure by the mineralocorticoid receptor

Addition of mineralocorticoid receptor (MR) antagonists to standard therapy for heart failure, kidney disease, metabolic syndrome, and diabetes is increasing steadily in response to clinical trials demonstrating clear benefits. In addition to blocking deleterious activity of MR within the heart, vessels and kidneys, MR antagonists target MR in hemodynamic regulatory centers in the brain, thereby decreasing excessive sympathetic nervous system drive, vasopressin release, abnormal baroreceptor function, and circulating and tissue pro-inflammatory cytokines. However, brain MR are also involved with cognition, memory, affect and functions yet to be determined. Understanding specific central mechanisms involved in blood pressure regulation by MR is necessary for the development of agents to target downstream events specific to central hemodynamic regulation, not only to avoid the hypokalemia caused by inhibition of renal tubular MR, but also to avoid untoward long term effects of inhibiting brain MR that are not involved in blood pressure control.