Sites and sources of sympathoexcitation in obese male rats: role of brain insulin

Blockade of insulin receptor signaling in the medullary cardiovascular centers impairs open-loop arterial baroreflex function via attenuated neural arc in healthy male rats

Evidence suggests that brain insulin availability acutely modulates arterial baroreflex function. However, little is known about the impact of blocking brain insulin receptor (IR) signaling on arterial baroreflex. We hypothesized that blockade of IR signaling in the brain acutely impairs arterial baroreflex function. Our hypothesis was tested using baroreflex open-loop analysis to evaluate the two subsystems of the arterial baroreflex: the carotid sinus pressure (CSP)-sympathetic nerve activity (SNA) relationship (the neural arc) and the SNA-arterial pressure (AP) relationship (the peripheral arc). In anesthetized healthy male rats, the bilateral carotid sinus baroreceptor regions were surgically isolated from the systemic circulation, and then CSP was changed stepwise from 60 to 180 mmHg before and over 120 min after lateral intracerebroventricular (ICV) administration of either artificial cerebrospinal fluid (control solution) or IR antagonist GSK1838705. ICV injection of GSK1838705 significantly decreased renal SNA (RSNA), AP, and heart rate during stepwise CSP input over a period of 120 min after administration (p < .05). The maximum gain of the neural arc was significantly reduced 120 min after ICV injection of GSK1838705 (p = .002). Furthermore, GSK1838705 significantly attenuated the operating-point RSNA (p = .025) and AP (p < .001) as estimated by the baroreflex equilibrium diagram. Moreover, 120-min baroreflex stimulation via stepwise CSP input significantly increased c-Fos expression in IR-positive neurons in medullary cardiovascular centers (p < .001). Our findings suggest that IR signaling in the brain can modulate AP regulation via alteration of the neural arc of the arterial baroreflex.

Metabolic control of ovarian function through the sympathetic nervous system: role of leptin

The link between metabolism and reproduction is well-known. Both undernutrition and obesity affect the reproductive system. Metabolic status influences reproductive physiology by regulating gonadotropin secretion and affecting reproductive organs through hormonal signals. On the other hand, the autonomic nervous system controls follicle development and ovulation in the female reproductive system. This system is regulated by hypothalamic areas associated with metabolism as the Arcuate nuclei (ARC) and paraventricular nuclei (PVN). Metabolic signals, such as nutrients and hormones, acting on the hypothalamus may play a crucial role in modulating sympathetic innervation of the ovary and other reproductive organs. Some of these hormones are leptin, insulin, and GLP-1 that act directly in the hypothalamus to activate the sympathetic nervous system. In this minireview, we propose that leptin could be an important regulator of sympathetic innervation in reproductive tissues. Leptin may affect the density or activity of sympathetic nerves, thereby affecting reproductive function. We also speculate that other hormones such as insulin and GLP-1 may activate sympathetic nerves to the ovary. Additionally, we explore how early-onset obesity can cause lasting changes in the autonomic control of metabolic and reproductive organs, especially in the ovary. This suggests that the hyperactivation of sympathetic nerves in adulthood, due to metabolic programming, could be a possible cause of reproductive and metabolic disorders, such as polycystic ovary syndrome.

A subset of neurons in the paraventricular nucleus of the hypothalamus directly project to liver-related premotor neurons in the ventrolateral medulla

Sympathetic circuits including pre-sympathetic neurons in the ventrolateral medulla (VLM) and in the paraventricular nucleus (PVN) of the hypothalamus play an important role in the regulation of hepatic glucose metabolism. Despite the importance of central regulatory pathways, specific information regarding the circuits of liver-related neurons is limited. Here, we tested the hypothesis that PVN neurons are directly connected to spinally-projecting liver-related neurons in the VLM of mice. Pseudorabies virus (PRV) was used to identify liver-related neurons and time-dependent analyses revealed the location and distribution of neurons in the PVN and ventral brainstem. Four days following PRV injection, most liver-related neurons were found in the VLM and consist of both catecholaminergic (CA) and non-CA neurons. Furthermore, in addition to PRV inoculation, a monosynaptic viral tracer was used to identify VLM-projecting PVN neurons to specifically dissect PVN-VLM connections within the liver pathway. Five days following PRV inoculation, our anatomical findings revealed that a small population of liver-related PVN neurons projected to the VLM. In addition, photo-stimulation of axonal projections from SIM1-expressing PVN neurons resulted in evoked excitatory postsynaptic currents in a subset of spinally projecting liver-related neurons in the VLM. In summary, our data demonstrate the existence of monosynaptic, glutamatergic connections between PVN neurons and pre-sympathetic liver-related neurons in the VLM. These new findings regarding the central circuits involved in the sympathetic regulation of the liver provide further information necessary for developing new strategies to improve glucose homeostasis via modulation of the autonomic nerves.

Leptin and Associated Neural Pathways Underlying Obesity-Induced Hypertension

Obesity rates have surged to pandemic levels, placing tremendous burden on our society. This chronic and complex disease is related to the development of many life-threatening illnesses including cardiovascular diseases. Hypertension caused by obesity increases the risk for cardiovascular mortality and morbidity by promoting stroke, myocardial infarction, congestive heart failure, and end-stage renal disease. Overwhelming evidence supports neural origins for obesity-induced hypertension and pinpoints the adipose-derived hormone, leptin, and the sympathetic nervous system as major causal factors. Hyperleptinemia in obesity is associated with selective leptin resistance where leptin's renal sympathoexcitatory and pressor effects are preserved while the metabolic actions are impaired. Understanding the mechanisms driving this phenomenon is critical for developing effective therapeutics. This review describes the neural mechanisms of obesity-induced hypertension with a focus on the molecular and neuronal substrates of leptin action.

Activation of nociception-sensitive ionotropic glutamate receptor-expressing rostroventrolateral medulla neurons by stimulation of cardiac afferents in rats

Myocardial ischemia causes the release of bradykinin, which activates afferent nerve endings in the ventricular epicardium. This elicits a sympathetically mediated increase in arterial pressure and heart rate, referred to as the cardiogenic sympathetic afferent reflex. The rostroventrolateral medulla (RVLM) is a key sympathetic brain stem site for regulating cardiovascular activity. This study aimed to determine the importance of non-barosensitive nociception sympathetic activity and the role of glutamate receptor activation of RVLM neurons in the cardiogenic sympathetic afferent reflex. We tested the hypothesis that inhibition of barosensitive sympathetic activity attenuates but does not abolish the reflex response to cardiac visceral afferents. Renal sympathetic nerve activity (RSNA), arterial pressure, and heart rate responses to epicardial bradykinin application were recorded in anesthetized rats before and after bilateral RVLM microinjection of either GABAA agonist muscimol, ionotropic glutamate receptor antagonist kynurenic acid, N-methyl-d-aspartate (NMDA) receptor antagonist 2-amino-5- phosphonopentanoic acid (AP5), or non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Baroreceptor loading-induced inhibition of barosensitive activity attenuated the bradykinin-induced RSNA response (93 ± 14% increase) and tachycardia (18 ± 3 bpm). While RVLM muscimol microinjection abolished the RSNA response (1.6 ± 4.2% from baseline, 0.49 ± 0.38 μV*s), surprisingly, it did not abolish the tachycardia (27 ± 4 bpm). Kynurenic acid microinjection blocked the arterial pressure and RSNA responses, while AP5 or CNQX only attenuated the responses. These data suggest that nociception-sensitive sympathetic activity that does not appear to be barosensitive is also involved in the cardiogenic sympathetic afferent reflex. Importantly, while muscimol and kynurenic acid abolished the arterial pressure and RSNA response, neither affected the tachycardia, suggesting an alternate cardiac pathway independent of RVLM.

The growing complexity of the control of the hypothalamic pituitary thyroid axis and brown adipose tissue by leptin

The balance between food intake and energy expenditure is precisely regulated to maintain adipose stores. Leptin, which is produced in and released from adipose in direct proportion to its size, is a major contributor to this control and initiates its homeostatic responses largely via binding to leptin receptors (LepR) in the hypothalamus. Decreases in hypothalamic LepR binding signals starvation, leading to hunger and reduced energy expenditure, whereas increases in hypothalamic LepR binding can suppress food intake and increase energy expenditure. However, large gaps persist in the specific hypothalamic sites and detailed mechanisms by which leptin increases energy expenditure, via the parallel activation of the hypothalamic pituitary thyroid (HPT) axis and brown adipose tissue (BAT). The purpose of this review is to develop a framework for the complex mechanisms and neurocircuitry. The core circuitry begins with leptin binding to receptors in the arcuate nucleus, which then sends projections to the paraventricular nucleus (to regulate the HPT axis) and the dorsomedial hypothalamus (to regulate BAT). We build on this core by layering complexities, including the intricate and unsettled regulation of arcuate proopiomelanocortin neurons by leptin and the changes that occur as the regulation of the HPT axis and BAT is engaged or modified by challenges such as starvation, hypothermia, obesity, and pregnancy.

Maternal high-fat diet changes breathing pattern and causes excessive sympathetic discharge in juvenile offspring rat

Early life over-nutrition, as experienced in maternal obesity, is a risk factor for developing cardiorespiratory and metabolic diseases. Here we investigated the effects of high-fat diet (HFD) consumption on the breathing pattern and sympathetic discharge to blood vessels in juvenile offspring from dams fed with HFD (O-HFD). Adult female Holtzman rats were given a standard diet (SD) or HFD from 6 wk before gestation to weaning. At weaning (P21), the male offspring from SD dams (O-SD) and O-HFD received SD until the experimental day (P28-P45). Nerve recordings performed in decerebrated in situ preparations demonstrated that O-HFD animals presented abdominal expiratory hyperactivity under resting conditions and higher vasoconstrictor sympathetic activity levels. The latter was associated with blunted respiratory-related oscillations in sympathetic activity, especially in control animals. When exposed to elevated hypercapnia or hypoxia levels, the O-HFD animals mounted similar ventilatory and respiratory motor responses as the control animals. Hypercapnia and hypoxia exposure also increased sympathetic activity in both groups but did not reinstate the respiratory-sympathetic coupling in the O-HFD rats. In freely behaving conditions, O-HFD animals exhibited higher resting pulmonary ventilation and larger variability of arterial pressure levels than the O-SD animals due to augmented sympathetic modulation of blood vessel diameter. Maternal obesity modified the functioning of cardiorespiratory systems in offspring at a young age, inducing active expiration and sympathetic overactivity under resting conditions. These observations represent new evidence about pregnancy-related complications that lead to the development of respiratory distress and hypertension in children of obese mothers.NEW & NOTEWORTHY Maternal obesity is a risk factor for developing cardiorespiratory and metabolic diseases. This study highlights the changes on the breathing pattern and sympathetic discharge to blood vessels in juvenile offspring from dams fed with HFD. Maternal obesity modified the functioning of cardiorespiratory systems in offspring, inducing active expiration and sympathetic overactivity. These observations represent new evidence about pregnancy-related complications that lead to the development of respiratory distress and hypertension in children of obese mothers.

Obesity-related hypertension and chronic kidney disease: from evaluation to management

With the recent obesity pandemic, obesity-related hypertension and its complications (e.g., heart failure, coronary disease, and chronic kidney disease [CKD]) are gaining attention in clinical and research fields. Obesity-related hypertension frequently precedes the onset of CKD and aggravates its progression. In this review, we discuss the role of visceral fat in the pathophysiology of obesity-related hypertension and the potential therapeutic strategies for its prevention and management. Various factors, including the sympathetic nervous system, renin-angiotensin-aldosterone system, and inflammatory pathways, are intricately involved in the pathogenesis of obesity-related hypertension. These factors individually and jointly contribute to the development of hypertension (usually sodium-sensitive or resistant hypertension) and, ultimately, to the progression of CKD. From a clinical standpoint, a decline in renal function in advanced CKD further makes blood pressure control challenging since only a few options are available for blood pressure-lowering medications. Proactive lifestyle modification, pharmacological treatment for obesity, and bariatric surgery can be considered for obesity control and management. Furthermore, intensive blood pressure control is required to prevent and halt the development and progression of CKD.

Cell-specific transcriptome changes in the hypothalamic arcuate nucleus in a mouse deoxycorticosterone acetate-salt model of hypertension

A common preclinical model of hypertension characterized by low circulating renin is the "deoxycorticosterone acetate (DOCA)-salt" model, which influences blood pressure and metabolism through mechanisms involving the angiotensin II type 1 receptor (AT1R) in the brain. More specifically, AT1R within Agouti-related peptide (AgRP) neurons of the arcuate nucleus of the hypothalamus (ARC) has been implicated in selected effects of DOCA-salt. In addition, microglia have been implicated in the cerebrovascular effects of DOCA-salt and angiotensin II. To characterize DOCA-salt effects upon the transcriptomes of individual cell types within the ARC, we used single-nucleus RNA sequencing (snRNAseq) to examine this region from male C57BL/6J mice that underwent sham or DOCA-salt treatment. Thirty-two unique primary cell type clusters were identified. Sub-clustering of neuropeptide-related clusters resulted in identification of three distinct AgRP subclusters. DOCA-salt treatment caused subtype-specific changes in gene expression patterns associated with AT1R and G protein signaling, neurotransmitter uptake, synapse functions, and hormone secretion. In addition, two primary cell type clusters were identified as resting versus activated microglia, and multiple distinct subtypes of activated microglia were suggested by sub-cluster analysis. While DOCA-salt had no overall effect on total microglial density within the ARC, DOCA-salt appeared to cause a redistribution of the relative abundance of activated microglia subtypes. These data provide novel insights into cell-specific molecular changes occurring within the ARC during DOCA-salt treatment, and prompt increased investigation of the physiological and pathophysiological significance of distinct subtypes of neuronal and glial cell types.

Leptin Increases: Physiological Roles in the Control of Sympathetic Nerve Activity, Energy Balance, and the Hypothalamic-Pituitary-Thyroid Axis

It is well established that decreases in plasma leptin levels, as with fasting, signal starvation and elicit appropriate physiological responses, such as increasing the drive to eat and decreasing energy expenditure. These responses are mediated largely by suppression of the actions of leptin in the hypothalamus, most notably on arcuate nucleus (ArcN) orexigenic neuropeptide Y neurons and anorexic pro-opiomelanocortin neurons. However, the question addressed in this review is whether the effects of increased leptin levels are also significant on the long-term control of energy balance, despite conventional wisdom to the contrary. We focus on leptin's actions (in both lean and obese individuals) to decrease food intake, increase sympathetic nerve activity, and support the hypothalamic-pituitary-thyroid axis, with particular attention to sex differences. We also elaborate on obesity-induced inflammation and its role in the altered actions of leptin during obesity.

The arcuate nucleus: A site of synergism between Angiotensin II and leptin to increase sympathetic nerve activity and blood pressure in rats

The action of leptin in brain to increase sympathetic nerve activity (SNA) and blood pressure depends upon functional Angiotensin II (AngII) type 1a receptors (AT1aR); however, the sites and mechanism of interaction are unknown. Here we identify one site, the hypothalamic arcuate nucleus (ArcN), since prior local blockade of AT1aR in the ArcN with losartan or candesartan in anesthetized male rats essentially eliminated the sympathoexcitatory and pressor responses to ArcN leptin nanoinjections. Unlike mice, in male and female rats, AT1aR and LepR rarely co-localized, suggesting that this interdependence occurs indirectly, via a local interneuron or network of neurons. ArcN leptin increases SNA by activating pro-opiomelanocortin (POMC) inputs to the PVN, but this activation requires simultaneous suppression of tonic PVN Neuropeptide Y (NPY) sympathoinhibition. Because AngII-AT1aR inhibits ArcN NPY neurons, we propose that loss of AT1aR suppression of NPY blocks leptin-induced increases in SNA; in other words, ArcN-AngII-AT1aR is a gatekeeper for leptin-induced sympathoexcitation. With obesity, both leptin and AngII increase; therefore, the increased AT1aR activation could open the gate, allowing leptin (and insulin) to drive sympathoexcitation unabated, leading to hypertension.

Melanocortin MC4R receptor is required for energy expenditure but not blood pressure effects of angiotensin II within the mouse brain

The brain renin-angiotensin system (RAS) is implicated in control of blood pressure (BP), fluid intake, and energy expenditure (EE). Angiotensin II (ANG II) within the arcuate nucleus of the hypothalamus contributes to control of resting metabolic rate (RMR) and thereby EE through its actions on Agouti-related peptide (AgRP) neurons, which also contribute to EE control by leptin. First, we determined that although leptin stimulates EE in control littermates, mice with transgenic activation of the brain RAS (sRA) exhibit increased EE and leptin has no additive effect to exaggerate EE in these mice. These findings led us to hypothesize that leptin and ANG II in the brain stimulate EE through a shared mechanism. Because AgRP signaling to the melanocortin MC4R receptor contributes to the metabolic effects of leptin, we performed a series of studies examining RMR, fluid intake, and BP responses to ANG II in mice rendered deficient for expression of MC4R via a transcriptional block (Mc4r-TB). These mice were resistant to stimulation of RMR in response to activation of the endogenous brain RAS via chronic deoxycorticosterone acetate (DOCA)-salt treatment, whereas fluid and electrolyte effects remained intact. These mice were also resistant to stimulation of RMR via acute intracerebroventricular (ICV) injection of ANG II, whereas BP responses to ICV ANG II remained intact. Collectively, these data demonstrate that the effects of ANG II within the brain to control RMR and EE are dependent on MC4R signaling, whereas fluid homeostasis and BP responses are independent of MC4R signaling.

Role of the arterial baroreflex in the sympathetic response to hyperinsulinemia in adult humans

Muscle sympathetic nerve activity (MSNA) increases during hyperinsulinemia, primarily attributed to central nervous system effects. Whether peripheral vasodilation induced by insulin further contributes to increased MSNA via arterial baroreflex-mediated mechanisms requires further investigation. Accordingly, we examined baroreflex modulation of the MSNA response to hyperinsulinemia. We hypothesized that rescuing peripheral resistance with coinfusion of the vasoconstrictor phenylephrine would attenuate the MSNA response to hyperinsulinemia. We further hypothesized that the insulin-mediated increase in MSNA would be recapitulated with another vasodilator (sodium nitroprusside, SNP). In 33 young healthy adults (28 M/5F), MSNA (microneurography) and arterial blood pressure (BP, Finometer/brachial catheter) were measured, and total peripheral resistance (TPR, ModelFlow) and baroreflex sensitivity were calculated at rest and during intravenous infusion of insulin (n = 20) or SNP (n = 13). A subset of participants receiving insulin (n = 7) was coinfused with phenylephrine. Insulin infusion decreased TPR (P = 0.01) and increased MSNA (P < 0.01), with no effect on arterial baroreflex sensitivity or BP (P > 0.05). Coinfusion with phenylephrine returned TPR and MSNA to baseline, with no effect on arterial baroreflex sensitivity (P > 0.05). Similar to insulin, SNP decreased TPR (P < 0.02) and increased MSNA (P < 0.01), with no effect on arterial baroreflex sensitivity (P > 0.12). Acute hyperinsulinemia shifts the baroreflex stimulus-response curve to higher MSNA without changing sensitivity, likely due to insulin's peripheral vasodilatory effects. Results show that peripheral vasodilation induced by insulin contributes to increased MSNA during hyperinsulinemia.NEW & NOTEWORTHY We hypothesized that elevation in muscle sympathetic nervous system activity (MSNA) during hyperinsulinemia is mediated by its peripheral vasodilator effect on the arterial baroreflex. Using three separate protocols in humans, we observed increases in both MSNA and cardiac output during hyperinsulinemia, which we attributed to the baroreflex response to peripheral vasodilation induced by insulin. Results show that peripheral vasodilation induced by insulin contributes to increased MSNA during hyperinsulinemia.

Role of the Autonomic Nervous System in the Hemodynamic Response to Hyperinsulinemia-Implications for Obesity and Insulin Resistance

PURPOSE OF REVIEW

Herein, we summarize recent advances which provide new insights into the role of the autonomic nervous system in the control of blood flow and blood pressure during hyperinsulinemia. We also highlight remaining gaps in knowledge as it pertains to the translation of findings to relevant human chronic conditions such as obesity, insulin resistance, and type 2 diabetes.

RECENT FINDINGS

Our findings in insulin-sensitive adults show that increases in muscle sympathetic nerve activity with hyperinsulinemia do not result in greater sympathetically mediated vasoconstriction in the peripheral circulation. Both an attenuation of α-adrenergic-receptor vasoconstriction and augmented β-adrenergic vasodilation in the setting of high insulin likely explain these findings. In the absence of an increase in sympathetically mediated restraint of peripheral vasodilation during hyperinsulinemia, blood pressure is supported by increases in cardiac output in insulin-sensitive individuals. We highlight a dynamic interplay between central and peripheral mechanisms during hyperinsulinemia to increase sympathetic nervous system activity and maintain blood pressure in insulin-sensitive adults. Whether these results translate to the insulin-resistant condition and implications for long-term cardiovascular regulation warrants further exploration.

Epididymal Fat-Derived Sympathoexcitatory Signals Exacerbate Neurogenic Hypertension in Obese Male Mice Exposed to Early Life Stress

[Figure: see text].

The Impact of Insulin Resistance on Cardiovascular Control During Exercise in Diabetes

Patients with diabetes display heightened blood pressure response to exercise, but the underlying mechanism remains to be elucidated. There is no direct evidence that insulin resistance (hyperinsulinemia or hyperglycemia) impacts neural cardiovascular control during exercise. We propose a novel paradigm in which hyperinsulinemia or hyperglycemia significantly influences neural regulatory pathways controlling the circulation during exercise in diabetes.

Central actions of insulin during pregnancy and lactation

Pregnancy and lactation are highly metabolically demanding states. Maternal glucose is a key fuel source for the growth and development of the fetus, as well as for the production of milk during lactation. Hence, the maternal body undergoes major adaptations in the systems regulating glucose homeostasis to cope with the increased demand for glucose. As part of these changes, insulin levels are elevated during pregnancy and lower in lactation. The increased insulin secretion during pregnancy plays a vital role in the periphery; however, the potential effects of increased insulin action in the brain have not been widely investigated. In this review, we consider the impact of pregnancy on brain access and brain levels of insulin. Moreover, we explore the hypothesis that pregnancy is associated with site-specific central insulin resistance that is adaptive, allowing for the increases in peripheral insulin secretion without the consequences of increased central and peripheral insulin functions, such as to stimulate glucose uptake into maternal tissues or to inhibit food intake. Conversely, the loss of central insulin actions may impair other functions, such as insulin control of the autonomic nervous system. The potential role of low insulin in facilitating adaptive responses to lactation, such as hyperphagia and suppression of reproductive function, are also discussed. We end the review with a list of key research questions requiring resolution.

Metformin effectively restores the HPA axis function in diet-induced obese rats

INTRODUCTION

The hypothalamo-pituitary-adrenal (HPA) axis is perturbed in obesity. We previously reported presence of leptin resistance in the brainstem and uncoupling between central noradrenergic tone and the HPA axis in obesity-prone (DIO) rats. Metformin is shown to lower body weight and adiposity, but the underlying mechanism is unclear. We hypothesized that this is associated with restored HPA axis function.

METHODS

Adult male DIO rats were placed on either a regular chow or HF diet for 7 weeks. Starting week 4, the animals were given either a low dose (60 mg/kg) or high dose (300 mg/kg) of metformin in drinking water. In addition to body weight and feeding, we examined different arms of the HPA axis to test if metformin can reinstate its function and coupling. To understand potential mechanisms, leptin signaling in the brainstem and circulating free fatty acid levels were also assessed.

RESULTS

Metformin treatment lowered weight gain, fat mass, caloric intake, and serum leptin levels. HPA axis activity as determined by corticotropin-releasing hormone in the median eminence and serum corticosterone was decreased by metformin in a dose-dependent manner, and so was norepinephrine (NE) in the paraventricular nucleus. Importantly, metformin completely normalized the NE-HPA axis uncoupling. While brainstem pSTAT-3 and SOCS-3, key markers of leptin signaling, were not different between groups, circulating saturated and unsaturated free fatty acids were reduced in HF-fed, metformin-treated animals.

CONCLUSIONS

These findings suggest that oral metformin can successfully correct HPA axis dysfunction that is associated with lowered circulating free fatty acids in DIO rats, thereby uncovering a novel effect of metformin in the treatment of obesity.

Neuronal Networks in Hypertension: Recent Advances

Neurogenic hypertension is associated with excessive sympathetic nerve activity to the kidneys and portions of the cardiovascular system. Here we examine the brain regions that cause heightened sympathetic nerve activity in animal models of neurogenic hypertension, and we discuss the triggers responsible for the changes in neuronal activity within these regions. We highlight the limitations of the evidence and, whenever possible, we briefly address the pertinence of the findings to human hypertension. The arterial baroreflex reduces arterial blood pressure variability and contributes to the arterial blood pressure set point. This set point can also be elevated by a newly described cerebral blood flow-dependent and astrocyte-mediated sympathetic reflex. Both reflexes converge on the presympathetic neurons of the rostral medulla oblongata, and both are plausible causes of neurogenic hypertension. Sensory afferent dysfunction (reduced baroreceptor activity, increased renal, or carotid body afferent) contributes to many forms of neurogenic hypertension. Neurogenic hypertension can also result from activation of brain nuclei or sensory afferents by excess circulating hormones (leptin, insulin, Ang II [angiotensin II]) or sodium. Leptin raises blood vessel sympathetic nerve activity by activating the carotid bodies and subsets of arcuate neurons. Ang II works in the lamina terminalis and probably throughout the brain stem and hypothalamus. Sodium is sensed primarily in the lamina terminalis. Regardless of its cause, the excess sympathetic nerve activity is mediated to some extent by activation of presympathetic neurons located in the rostral ventrolateral medulla or the paraventricular nucleus of the hypothalamus. Increased activity of the orexinergic neurons also contributes to hypertension in selected models.

Obesity: sex and sympathetics

Obesity increases sympathetic nerve activity (SNA) in men, but not women. Here, we review current evidence suggesting that sexually dimorphic sympathoexcitatory responses to leptin and insulin may contribute. More specifically, while insulin increases SNA similarly in lean males and females, this response is markedly amplified in obese males, but is abolished in obese females. In lean female rats, leptin increases a subset of sympathetic nerves only during the high estrogen proestrus reproductive phase; thus, in obese females, because reproductive cycling can become impaired, the sporadic nature of leptin-induced sympathoexcitaton could minimize its action, despite elevated leptin levels. In contrast, in males, obesity preserves or enhances the central sympathoexcitatory response to leptin, and current evidence favors leptin’s contribution to the well-established increases in SNA induced by obesity in men. Leptin and insulin increase SNA via receptor binding in the hypothalamic arcuate nucleus and a neuropathway that includes arcuate neuropeptide Y (NPY) and proopiomelanocortin (POMC) projections to the paraventricular nucleus. These metabolic hormones normally suppress sympathoinhibitory NPY neurons and activate sympathoexcitatory POMC neurons. However, obesity appears to alter the ongoing activity and responsiveness of arcuate NPY and POMC neurons in a sexually dimorphic way, such that SNA increases in males but not females. We propose hypotheses to explain these sex differences and suggest areas of future research.