Association of postganglionic sympathetic neurons with primary afferents in sympathetic-sensory co-cultures

Functional coupling between sympathetic postganglionic neurons and sensory neurons is thought to play an essential role in the pathogenesis of certain chronic pain syndromes following peripheral tissue and nerve injury. The mechanism(s) underlying this interaction are enigmatic. The relative anatomical inaccessibility of sympathetic and sensory neurons in vivo complicates study of their interrelationships. We have developed a system for long-term co-culturing of explants of sympathetic chain ganglia and dorsal root ganglia from newborn rats. Co-cultures were labelled for tyrosine hydroxylase-like immunoreactivity and studied at the light and electron microscopic levels. Explanted ganglia of both types survived well in co-culture. They maintained their tissue type-specific histological properties, including neuronal and glial morphology, and characteristic glial–neuronal associations. Moreover, neurons maintained their characteristic neurochemical identity, at least to the extent that sympathetic neurons continued to express tyrosine hydroxylase and dorsal root ganglion neurons did not. Sympathetic neurons emitted numerous outgrowing processes (axons) some of which came into association with sensory neurons in the explanted dorsal root ganglia. Some apparently specific sympathetic-sensory contacts were observed, suggesting that a functional interaction may develop between sympathetic axons and sensory neurons in vitro.     

Sympathetic nervous system control of triglyceride metabolism: novel concepts derived from recent studies

Important players in triglyceride (TG) metabolism include the liver (production), white adipose tissue (WAT) (storage), heart and skeletal muscle (combustion to generate ATP), and brown adipose tissue (BAT) (combustion toward heat), the collective action of which determine plasma TG levels. Interestingly, recent evidence points to a prominent role of the hypothalamus in TG metabolism through innervating the liver, WAT, and BAT mainly via sympathetic branches of the autonomic nervous system. Here, we review the recent findings in the area of sympathetic control of TG metabolism. Various neuronal populations, such as neuropeptide Y (NPY)-expressing neurons and melanocortin-expressing neurons, as well as peripherally produced hormones (i.e., GLP-1, leptin, and insulin), modulate sympathetic outflow from the hypothalamus toward target organs and thereby influence peripheral TG metabolism. We conclude that sympathetic stimulation in general increases lipolysis in WAT, enhances VLDL-TG production by the liver, and increases the activity of BAT with respect to lipolysis of TG, followed by combustion of fatty acids toward heat. Moreover, the increased knowledge about the involvement of the neuroendocrine system in TG metabolism presented in this review offers new therapeutic options to fight hypertriglyceridemia by specifically modulating sympathetic nervous system outflow toward liver, BAT, or WAT.     

The role of the sympathetic nervous system in the regulation of leptin synthesis in C57BL/6 mice

The objectives of this study were to determine whether leptin synthesis is regulated by the sympathetic nervous system and if so whether beta-adrenergic receptors mediate this effect. We show that sympathetic blockade by reserpine increases leptin mRNA levels in brown but not white adipose tissue, while acute cold-exposure decreases leptin expression 10-fold in brown adipose tissue and 2-fold in white adipose tissue. The cold-induced reduction in leptin mRNA can be prevented by a combination of propranolol and SR 59230A but not by either antagonist alone, indicating that beta3-adrenergic receptors and classical beta1/beta2-adrenergic receptors both mediate responses to sympathetic stimulation. Circulating leptin levels reflect synthesis in white adipose tissue but not in brown adipose tissue.     

Role of adipose tissue for cardiovascular-renal regulation in health and disease.

Obesity is associated with profound alterations of the cardiovascular system including an increase in systemic blood pressure. Several vasoactive factors, including non-esterified fatty acids, angiotensin II, prostaglandins, and nitric oxide are known to be produced by adipose tissue, and are therefore of particular interest regarding their potential role for the regulation of vascular tone and structure. In addition, central nervous system actions of the adipose tissue-derived hormone leptin may contribute to increased sympathetic nervous system activity that is typically found in obesity. Enhanced leptin-driven renal sympathetic out-flow, in combination with low atrial natriuretic peptide plasma levels possibly due to over-expression of the natriuretic peptide clearance receptor in adipocytes, may enhance sodium retention and volume expansion, both key features in the pathophysiology of obesity-associated hypertension. In this review, we discuss these and other possible contributions of adipose tissue to the regulation of cardiovascular-renal function and speculate on the role of adipose tissue for the development of obesity-associated hypertension.     

SCN efferents to peripheral tissues: implications for biological rhythms.

The suprachiasmatic nucleus (SCN) is the principal generator of circadian rhythms and is part of an entrainment system that synchronizes the animal with its environment. Here, we review the possible communication of timing information from the SCN to peripheral tissues involved in regulating fundamental physiological functions as revealed using a viral, transneuronal tract tracer, the pseudorabies virus (PRV). The sympathetic nervous system innervation of the pineal gland and the sympathetic outflow from brain to white adipose tissue were the first demonstrations of SCN-peripheral tissue connections. The inclusion of the SCN as part of these and other circuits was the result of lengthened postviral injection times compared with those used previously. Subsequently, the SCN has been found to be part of the sympathetic outflow from the brain to brown adipose tissue, thyroid gland, kidney, bladder, spleen, adrenal medulla, and perhaps the adrenal cortex. The SCN also is involved in the parasympathetic nervous system innervation of the thyroid, liver, pancreas, and submandibular gland. Individual SCN neurons appear connected to more than one autonomic circuit involving both sympathetic and parasympathetic innervation of a single tissue, or sympathetic innervation of two different peripheral tissues. Collectively, the results of these PRV studies require an expansion of the traditional roles of the SCN to include the autonomic innervation of peripheral tissues and perhaps the modulation of neuroendocrine systems traditionally thought to be controlled solely by hypothalamic stimulating/inhibiting factors.     

Roles for melanocortins and leptin in adipose tissue apoptosis and fat deposition

Leptin has a wide range of effects on physiological functions related to the regulation of body energy balance. Many of leptin's effects are mediated through neuropeptide-containing neurons and neuropeptide receptors in the hypothalamus. The melanocortin system includes both agonist (alpha-melanocyte stimulating hormone, alphaMSH) and antagonist peptides (agouti related peptide, AGRP). Increased melanocortin receptor stimulation following leptin administration plays an important role in leptin-induced hypophagia and increased sympathetic nervous system activity and is partly responsible for leptin-induced weight loss. However, melanocortins do not appear to mediate some of the more striking centrally-mediated effects of leptin on adipose tissue, including adipose tissue apoptosis, that lead to the extensive depletion of fat.     

Effects of fasting and food restriction on sympathetic activity in brown adipose tissue in mice

Summary The activity of the sympathetic nervous system in mice that were either fed ad libitum, food restricted or fasted was estimated by measuring the accumulation of dopamine following the inhibition of dopamine -hydroxylase activity. Mice in each group were injected with the dopamine -hydroxylase inhibitor 1-cyclohexyl-2-mercaptoimidazole and were exposed to either 30°C (warm) or 4°C (cold). Mice were killed 1 h after the injection. Both heart and brown adipose tissue were then quickly removed and homogenized in ice-cold perchloric acid. Dopamine and noradrenaline were determined using high performance liquid chromatography. Regardless of whether mice were warm or cold exposed, both content and concentration of brown adipose tissue and dopamine were predictably higher in 1-cyclohexyl-2-mercaptoimidazole-injected mice than in non-injected animals. In mice fed ad libitum, post-injection content and concentration of dopamine in both brown adipose tissue and heart were higher in cold-exposed mice than in warm-exposed animals. In food-restricted and fasted mice, post-injection concentrations of dopamine in brown adipose tissue were higher in cold-exposed mice than in warm-exposed animals. In food-restricted and fasted mice there was no difference between warm- and cold-exposed animals with respect to post-injection contents and concentrations of dopamine in heart tissue. In fasted mice there was no difference between warm- and cold-exposed animals in post-injection content of dopamine in brown adipose tissue. This study provides further evidence that fasting, in contrast to food restriction, may blunt the tissue sympathetic nervous system response in brown adipose tissue of cold-exposed mice.Abbreviations BAT brown adipose tissue - CHMI 1-cyclohexyl-2-mercaptoimidazole - DA dopamine - DHBA dihydroxybenzylamine - EDTA ethylenediaminetetra-acetic acid - HPLC high performance liquid chromatography - NA noradrenaline - PCA perchloric acid - SNS sympathetic nervous system     

Apparent dissociation between sympathetic activity and brown adipose tissue thermogenesis during pregnancy and lactation in golden hamsters

Sympathetic activity has been assessed by measurements of noradrenaline turnover in brown adipose tissue and in the heart of golden hamsters during pregnancy and lactation. Noradrenaline turnover was not significantly altered in either tissue in pregnant or lactating hamsters, despite the atrophy of brown adipose tissue that occurs during reproduction. This suggests that sympathetic activity and brown adipose tissue thermogenesis are dissociated during pregnancy and lactation in golden hamsters. The results also indicate that the large increase in food intake lactation does not lead to a diet-induced stimulation of the sympathetic nervous system.     

Cardiac Fibroblasts Regulate Sympathetic Nerve Sprouting and Neurocardiac Synapse Stability

Sympathetic nervous system (SNS) plays a key role in cardiac homeostasis and its deregulations always associate with bad clinical outcomes. To date, little is known about molecular mechanisms regulating cardiac sympathetic innervation. The aim of the study was to determine the role of fibroblasts in heart sympathetic innervation. RT-qPCR and western-blots analysis performed in cardiomyocytes and fibroblasts isolated from healthy adult rat hearts revealed that Pro-Nerve growth factor (NGF) and pro-differentiating mature NGF were the most abundant neurotrophins expressed in cardiac fibroblasts while barely detectable in cardiomyocytes. When cultured with cardiac fibroblasts or fibroblast-conditioned medium, PC12 cells differentiated into/sympathetic-like neurons expressing axonal marker Tau-1 at neurites in contact with cardiomyocytes. This was prevented by anti-NGF blocking antibodies suggesting a paracrine action of NGF secreted by fibroblasts. When co-cultured with cardiomyocytes to mimic neurocardiac synapse, differentiated PC12 cells exhibited enhanced norepinephrine secretion as quantified by HPLC compared to PC12 cultured alone while co-culture with fibroblasts had no effect. However, when supplemented to PC12-cardiomyocytes co-culture, fibroblasts allowed long-term survival of the neurocardiac synapse. Activated fibroblasts (myofibroblasts) isolated from myocardial infarction rat hearts exhibited significantly higher mature NGF expression than normal fibroblasts and also promoted PC12 cells differentiation. Within the ischemic area lacking cardiomyocytes and neurocardiac synapses, tyrosine hydroxylase immunoreactivity was increased and associated with local anarchical and immature sympathetic hyperinnervation but tissue norepinephrine content was similar to that of normal cardiac tissue, suggesting depressed sympathetic function. Collectively, these findings demonstrate for the first time that fibroblasts are essential for the setting of cardiac sympathetic innervation and neurocardiac synapse stability. They also suggest that neurocardiac synapse functionality relies on a triptych with tight interaction between sympathetic nerve endings, cardiomyocytes and fibroblasts. Deregulations of this triptych may be involved in pathophysiology of cardiac diseases.     

TLR4 in POMC neurons regulates thermogenesis in a sex-dependent manner

The rising prevalence of obesity has become a worldwide health concern. Obesity usually occurs when there is an imbalance between energy intake and energy expenditure. However, energy expenditure consists of several components, including metabolism, physical activity, and thermogenesis. Toll-like receptor 4 (TLR4) is a transmembrane pattern recognition receptor, and it is abundantly expressed in the brain. Here, we showed that pro-opiomelanocortin (POMC)-specific deficiency of TLR4 directly modulates brown adipose tissue thermogenesis and lipid homeostasis in a sex-dependent manner. Deleting TLR4 in POMC neurons is sufficient to increase energy expenditure and thermogenesis resulting in reduced body weight in male mice. POMC neuron is a subpopulation of tyrosine hydroxylase neurons and projects into brown adipose tissue, which regulates the activity of sympathetic nervous system and contributes to thermogenesis in POMC-TLR4-KO male mice. By contrast, deleting TLR4 in POMC neurons decreases energy expenditure and increases body weight in female mice, which affects lipolysis of white adipose tissue (WAT). Mechanistically, TLR4 KO decreases the expression of the adipose triglyceride lipase and lipolytic enzyme hormone-sensitive lipase in WAT in female mice. Furthermore, the function of immune-related signaling pathway in WAT is inhibited because of obesity, which exacerbates the development of obesity reversely. Together, these results demonstrate that TLR4 in POMC neurons regulates thermogenesis and lipid balance in a sex-dependent manner.     

A Role for Brown Adipose Tissue in Diet-Induced Thermogenesis

Measurement of energy balance during voluntary overeating in rats unequivocally establishes the quantitative importance of diet-induced thermogenesis in energy balance. Like cold-induced thermogenesis, this form of heat production involves changes in the activity of the sympathetic nervous system and brown adipose tissue which suggest that this tissue may determine metabolic efficiency and resistance to obesity.     

Temporal adjustments in sympathoadrenal activity in rats with obesity-producing hypothalamic knife cuts

Sympathoadrenal activity was assessed in adult rats with obesity-producing hypothalamic knife cuts prior to and after the onset of gross obesity by measuring urinary excretion of norepinephrine and epinephrine and by determining rates of norepinephrine turnover in selected organs. Urinary excretion of norepinephrine, as an index of overall sympathetic nervous system activity, was approximately doubled throughout the 4-week study in knife-cut rats, as was intake of the high-fat diet. Three days after knife-cut surgery (before the onset of gross obesity) rates of norepinephrine turnover (ng X organ-1 X hr-1) were 23-33% lower in three of the four organs examined than in the corresponding organs of control rats; rates of norepinephrine turnover were depressed in pancreas, interscapular brown adipose tissue, and abdominal white adipose tissue and unchanged in hearts. Four weeks after surgery when gross obesity was evident, rates of norepinephrine turnover were accelerated in heart (+82%) and pancreas (+63%), but remained low in interscapular brown adipose tissue (-27%) and abdominal white adipose tissue (-28%). Adrenal medullary activity, assessed by urinary excretion of epinephrine, was suppressed within the 1st day after knife-cut surgery and remained suppressed for several weeks. Brown adipose tissue and white adipose tissue appear to be selectively excluded from the generalized activation of the sympathetic nervous system in adult hyperphagic rats with obesity-producing hypothalamic knife cuts. Activation of the sympathetic nervous system was associated with reciprocal suppression of adrenal medullary responses in knife-cut rats.     

From white to beige: a new hypothalamic pathway

Understanding how brown and beige adipocytes can be differentially controlled and activated by neuronal circuits is a fundamental prerequisite to fully comprehend the metabolic role that fat tissue plays in energy homeostasis. In this issue of EMBO reports, Wang et al 1 identify a new hypothalamic route that drives the exclusive recruitment of beige fat via the selective control of sympathetic nervous system (SNS) outflow to subcutaneous white adipose tissue. Since the data strongly suggest that the APPL2–AMPK signaling axis is crucial for this activation, this finding sheds a new light on the cross talk between peripheral homeostatic signals and neurons that are part of hypothalamic energy homeostasis regulatory pathways in the ventromedial hypothalamus (VHM) proposing a new defending mechanism to cold and obesity.     

Hypothalamic regulation of adiposity: the role of 11beta-hydroxysteroid dehydrogenase type 1.

Following extensive suprasellar operations for excision of hypothalamic tumors, some patients develop morbid obesity despite receiving replacement doses of glucocorticoids. Urine analysis of cortisol and cortisone metabolites show that 11-OH/11-oxo ratios are significantly higher in patients with hypothalamic obesity, indicating enhanced 11beta-HSD1 activity. This correlates with the visceral-to-subcutaneous fat ratio. The consequence of increased 11beta-HSD1 activity and a shift of the steroid inter-conversion towards cortisol may contribute to the effects of the latter in adipose tissue. The message from the hypothalamus to adipocyte 11beta-HSD-1 involves hormones, the sympathetic nervous system and cytokines. CRH and ACTH downregulate 11beta-HSD-1 activity and induce lipolysis. Tumor necrosis factor-alpha and interleukin-1beta upregulate 11beta-HSD-1 expression and activity, while enhancing lipolysis. The sympathetic nervous system exerts its effects through beta-adrenergic upregulation and alpha-adrenergic downregulation of 11beta-HSD-1 activity. Inhibition of 11beta-HSD-1 suppresses preadipocyte differentiation into mature adipocytes, and may provide a therapeutic tool.     

p53-induced adipose tissue inflammation is critically involved in the development of insulin resistance in heart failure

Several clinical studies have shown that insulin resistance is prevalent among patients with heart failure, but the underlying mechanisms have not been fully elucidated. Here, we report a mechanism of insulin resistance associated with heart failure that involves upregulation of p53 in adipose tissue. We found that pressure overload markedly upregulated p53 expression in adipose tissue along with an increase of adipose tissue inflammation. Chronic pressure overload accelerated lipolysis in adipose tissue. In the presence of pressure overload, inhibition of lipolysis by sympathetic denervation significantly downregulated adipose p53 expression and inflammation, thereby improving insulin resistance. Likewise, disruption of p53 activation in adipose tissue attenuated inflammation and improved insulin resistance but also ameliorated cardiac dysfunction induced by chronic pressure overload. These results indicate that chronic pressure overload upregulates adipose tissue p53 by promoting lipolysis via the sympathetic nervous system, leading to an inflammatory response of adipose tissue and insulin resistance.     

‘Neuropeptide tyrosine’ (NPY) is co-stored with noradrenaline in vascular but not in parenchymal sympathetic nerves of brown adipose tissue

By using immunohistochemistry it is shown that both the parenchymal and vascular sympathetic innervation in the interscapular depot of brown adipose tissue in the rat contain the catecholamine-synthesizing enzyme tyrosine-hydroxylase (TH). In contrast, 'neuropeptide tyrosine' (NPY) is selectively present in the vascular sympathetic nerves of the tissue--but not in nerves around brown fat cells. This is consistent with the presence of two populations of neurons (containing either TH alone or TH plus NPY) in the stellate ganglion, which is the probable origin of the sympathetic nerves in the interscapular brown adipose tissue. Furthermore, the perivascular NPY-positive nerves in the brown adipose tissue disappeared after 6-hydroxydopamine treatment, demonstrating their noradrenergic nature. Taken together, these findings suggest that sympathetic nerves to blood vessels and brown fat cells represent two separate subpopulations of autonomic neurons.     

Ectopic recombination in the central and peripheral nervous system by aP2/FABP4-Cre mice: Implications for metabolism research

aP2-Cre mice have amply been used to generate conditional adipose selective inactivation of important signaling molecules. We show that the efficiency of Cre mediated recombination in adipocytes and adipose selectivity is not always guaranteed. In particular, Cre activity was found in ganglia of the peripheral nervous system (PNS), in adrenal medulla and in neurons throughout the central nervous system (CNS). Because these tissues have an important impact on adipose tissue, care should be taken when using aP2-Cre mice to define the role of the targeted genes in adipose tissue function.     

Reciprocal Effects of Adipose Tissue Denervation and High Fat Diet on Serum Metabolic Parameters and Adipokine Levels in Rats: A Long Term Study

Journal of Evolutionary Biochemistry and Physiology - The sympathetic nervous system (SNS) plays an important role in regulating the metabolic and secretory functions of white adipose tissue. In...     

Effect of some hormones on the rate of the triacylglycerol/fatty-acid substrate cycle in adipose tissue of the mouse in vivo

A method is described for the measurement of the rate of the triacylglycerol/fatty-acid cycle in adipose tissue of the mouse in vivo, which depends upon the incor-poration of tritium from [³H]H₂O into the glycerol and fatty-acid moieties of triacyiglycerol. The rate of the cycling is increased two-fold by feeding, an effect that is completely abolished by the β-adrenergic blocker propranolol. The β-adrenergic agonist fenoterol increased the rate of cycling five-fold in white adipose tissue and three-fold in brown adipose tissue. Cold exposure had no effect on the rate of cycling in white adipose tissue but increased the rate almost two-fold in brown adipose tissue. The increased rate of cycling during feeding, which may be due to increased sympathetic nervous activity, is consistent with the view that the role of cycling is to increase sensitivity of metabolic control systems when required.