Sympathetic innervation of white adipose tissue and its regulation of fat cell number

Stress hormone epinephrine enhances adipogenesis in murine embryonic stem cells by up-regulating the neuropeptide Y system

Prenatal stress, psychologically and metabolically, increases the risk of obesity and diabetes in the progeny. However, the mechanisms of the pathogenesis remain unknown. In adult mice, stress activates NPY and its Y2R in a glucocorticoid-dependent manner in the abdominal fat. This increased adipogenesis and angiogenesis, leading to abdominal obesity and metabolic syndrome which were inhibited by intra-fat Y2R inactivation. To determine whether stress elevates NPY system and accelerates adipogenic potential of embryo, here we "stressed" murine embryonic stem cells (mESCs) in vitro with epinephrine (EPI) during their adipogenic differentiation. EPI was added during the commitment stage together with insulin, and followed by dexamethasone in the standard adipogenic differentiation medium. Undifferentiated embryonic bodies (EBs) showed no detectable expression of NPY. EPI markedly up-regulated the expression NPY and the Y1R at the commitment stage, followed by increased Y2R mRNA at the late of the commitment stage and the differentiation stage. EPI significantly increased EB cells proliferation and expression of the preadipocyte marker Pref-1 at the commitment stage. EPI also accelerated and amplified adipogenic differentiation detected by increasing the adipocyte markers FABP4 and PPARγ mRNAs and Oil-red O-staining at the end of the differentiation stage. EPI-induced adipogenesis was completely prevented by antagonists of the NPY receptors (Y1R+Y2R+Y5R), indicating that it was mediated by the NPY system in mESC's. Taken together, these data suggest that stress may play an important role in programming ESCs for accelerated adipogenesis by altering the stress induced hormonal regulation of the NPY system.

Anterograde transneuronal viral tract tracing reveals central sensory circuits from brown fat and sensory denervation alters its thermogenic responses

Brown adipose tissue (BAT) thermogenic activity and growth are controlled by its sympathetic nervous system (SNS) innervation, but nerve fibers containing sensory-associated neuropeptides [substance P, calcitonin gene-related peptide (CGRP)] also suggest sensory innervation. The central nervous system (CNS) projections of BAT afferents are unknown. Therefore, we used the H129 strain of the herpes simplex virus-1 (HSV-1), an anterograde transneuronal viral tract tracer used to delineate sensory nerve circuits, to define these projections. HSV-1 was injected into interscapular BAT (IBAT) of Siberian hamsters and HSV-1 immunoreactivity (ir) was assessed 24, 48, 72, 96, and 114 h postinjection. The 96- and 114-h groups had the most HSV-1-ir neurons with marked infections in the hypothalamic paraventricular nucleus, periaqueductal gray, olivary areas, parabrachial nuclei, raphe nuclei, and reticular areas. These sites also are involved in sympathetic outflow to BAT suggesting possible BAT sensory-SNS thermogenesis feedback circuits. We tested the functional contribution of IBAT sensory innervation on thermogenic responses to an acute (24 h) cold exposure test by injecting the specific sensory nerve toxin capsaicin directly into IBAT pads and then measuring core (T(c)) and IBAT (T(IBAT)) temperature responses. CGRP content was significantly decreased in capsaicin-treated IBAT demonstrating successful sensory nerve destruction. T(IBAT) and T(c) were significantly decreased in capsaicin-treated hamsters compared with the saline controls at 2 h of cold exposure. Thus the central sensory circuits from IBAT have been delineated for the first time, and impairment of sensory feedback from BAT appears necessary for the appropriate, initial thermogenic response to acute cold exposure.

Sympathetic activation by chemical stimulation of white adipose tissues in rats

Injection of leptin into white adipose tissue (WAT) increases sympathetic outflow. The present study was designed to determine the effects of capsaicin and other chemicals in WAT on the sympathetic outflow and blood pressure and the roles of WAT afferents and hypothalamic paraventricular nucleus (PVN) in the adipose afferent reflex (AAR). The AAR was induced by injection of capsaicin, bradykinin, adenosine, adenosine triphosphate (ATP), or leptin into inguinal WAT (iWAT) or retroperitoneal WAT (rWAT) in anesthetized rats. The iWAT injection of capsaicin increased the renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) but not the heart rate. Bradykinin, adenosine, or leptin but not ATP in the iWAT caused similar effects to capsaicin on the RSNA and MAP. Intravenous, intramuscular, or intradermal injection of capsaicin had no significant effects on the RSNA and MAP. The effects of capsaicin in rWAT were similar to that in iWAT on the RSNA and MAP. Furthermore, injection of capsaicin into the iWAT increased the WAT afferent nerve activities, WAT efferent nerve activity, and brown adipose tissue efferent nerve activity. The iWAT denervation or chemical lesion of the PVN neurons with kainic acid abolished the AAR induced by the iWAT injection of capsaicin. These results indicate that the stimulation of iWAT afferents with capsaicin, bradykinin, adenosine, or leptin reflexly increases the RSNA and blood pressure. The iWAT afferents and the PVN are involved in the AAR induced by capsaicin in the iWAT.

Characterization of a novel peripheral pro-lipolytic mechanism in mice: role of VGF-derived peptide TLQP-21

The peptides encoded by the VGF gene are gaining biomedical interest and are increasingly being scrutinized as biomarkers for human disease. An endocrine/neuromodulatory role for VGF peptides has been suggested but never demonstrated. Furthermore, no study has demonstrated so far the existence of a receptor-mediated mechanism for any VGF peptide. In the present study, we provide a comprehensive in vitro, ex vivo and in vivo identification of a novel pro-lipolytic pathway mediated by the TLQP-21 peptide. We show for the first time that VGF-immunoreactivity is present within sympathetic fibres in the WAT (white adipose tissue) but not in the adipocytes. Furthermore, we identified a saturable receptor-binding activity for the TLQP-21 peptide. The maximum binding capacity for TLQP-21 was higher in the WAT as compared with other tissues, and selectively up-regulated in the adipose tissue of obese mice. TLQP-21 increases lipolysis in murine adipocytes via a mechanism encompassing the activation of noradrenaline/β-adrenergic receptors pathways and dose-dependently decreases adipocytes diameters in two models of obesity. In conclusion, we demonstrated a novel and previously uncharacterized peripheral lipolytic pathway encompassing the VGF peptide TLQP-21. Targeting the sympathetic nerve-adipocytes interaction might prove to be a novel approach for the treatment of obesity-associated metabolic complications.

Characterization of a novel melanocortin receptor-containing node in the SNS outflow circuitry to brown adipose tissue involved in thermogenesis

The melanocortins (MC) can affect interscapular brown adipose tissue (IBAT) thermogenesis via its sympathetic nervous system (SNS) innervation. We chose a site of high MC4-receptor (MC4-R) mRNA co-localization with SNS outflow neurons to IBAT, the subzona incerta (subZI) to test whether IBAT thermogenesis could be increased or decreased. We first performed immunohistochemical characterization of the subZI and found neurons and/or fibers in this area positive for melanin concentrating hormone, oxytocin, arginine vasopressin, agouti-related protein and alpha-melanocyte stimulating hormone. Functional characterization of the subZI was tested via site-specific microinjections. The MC3/4-R agonist, melanotan II [MTII (0.025, 0.05 and 0.075nmol)], and specific MC4-R agonist (cyclo [ß-Ala-His-D-Phe-Arg-Trp-Glu]-NH2; 0.024nmol) both significantly increased IBAT temperature (T(IBAT)) and pretreatment with the MC4R antagonist, HS024 (0.072nmol) blocked the MC4-R agonist-induced increased T(IBAT) in conscious, freely-moving Siberian hamsters. Injection of the MC4-R antagonist alone significantly decreased T(IBAT) up to 3h post injection. Collectively, these results highlight the identification of a brain area that possesses high concentrations of MC4-R mRNA and SNS outflow neurons to IBAT that has not been previously reported to be involved in the control of T(IBAT). These results add to previously identified neural nodes that are components of the central circuits controlling thermogenesis.

Adipocytes as a new source of catecholamine production

Catecholamines are an important regulator of lipolysis in adipose tissue. Here we show that rat adipocytes, isolated from mesenteric adipose tissue, express genes of catecholamine biosynthetic enzymes and produce catecholamines de novo. Administration of tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine, in vitro significantly reduced concentration of catecholamines in isolated adipocytes. We hypothesize that the sympathetic innervation of adipose tissues is not the only source of catecholamines, since adipocytes also have the capacity to produce both norepinephrine and epinephrine.

Intranasal application of the melanocortin 4 receptor agonist MSH/ACTH(4-10) in humans causes lipolysis in white adipose tissue

OBJECTIVE

The melanocortin system has a highly significant role in the hypothalamic regulation of body weight and energy expenditure. In animals, intracerebroventricular infusion of melanocortin receptor 4 (MCR-4) agonists increases basal metabolic rate through activation of the sympathetic nervous system and subsequently reduces food intake. In humans, direct access of MCR-4 agonists to the central nervous system can be achieved by a transnasal route, which leads to weight loss with chronic administration. In the present study, we aimed at investigating the effects of intranasally administered MC4-R agonist MSH/ACTH(4-10) on lipolysis and sympathetic nervous system activity in healthy humans.

DESIGN

Healthy normal weight, male volunteers (n=10) received either 10 mg MSH/ACTH(4-10) or placebo intranasally in a double-blinded randomized crossover design. Interstitial glycerol release was assessed by microdialysis in abdominal white adipose tissue (WAT) and in skeletal muscle (SM) of the forearm. Local blood flow, systemic blood pressure, heart rate and muscle sympathetic nerve activity (MSNA) within the superficial peroneal nerve were recorded at rest and after nitroprusside infusion.

RESULTS

At 45 min after MSH/ACTH(4-10) administration WAT glycerol concentrations increased by 53.4±19.3% compared with baseline conditions (P<0.05) and remained significantly higher throughout the experiment when compared with placebo (P<0.05) while local glycerol release in SM was not significantly affected. Resting MSNA was not altered by MSH/ACTH(4-10) administration; however, sympathoexcitation by intravenous nitroprusside was markedly elevated (MSH/ACTH(4-10) 569±69% increase to baseline; placebo: 315±64%; P<0.01).

CONCLUSION

Intranasally administered MCR-4 agonist MSH/ACTH 4-10 increases both subcutaneous WAT lipolysis and MSNA, which suggests a direct central nervous peptide effect in humans on key factors of human energy metabolism.

Several agents and pathways regulate lipolysis in adipocytes

Adipose tissue is the only tissue capable of hydrolyzing its stores of triacylglycerol (TAG) and of mobilizing fatty acids and glycerol in the bloodstream so that they can be used by other tissues. The full hydrolysis of TAG depends on the activity of three enzymes, adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL) and monoacylglycerol lipase, each of which possesses a distinct regulatory mechanism. Although more is known about HSL than about the other two enzymes, it has recently been shown that HLS and ATGL can be activated simultaneously, such that the mechanism that enables HSL to access the surface of lipid droplets also permits the stimulation of ATGL. The classical pathway of lipolysis activation in adipocytes is cAMP-dependent. The production of cAMP is modulated by G-protein-coupled receptors of the Gs/Gi family and cAMP degradation is regulated by phosphodiesterase. However, other pathways that activate TAG hydrolysis are currently under investigation. Lipolysis can also be started by G-protein-coupled receptors of the Gq family, through molecular mechanisms that involve phospholipase C, calmodulin and protein kinase C. There is also evidence that increased lipolytic activity in adipocytes occurs after stimulation of the mitogen-activated protein kinase pathway or after cGMP accumulation and activation of protein kinase G. Several agents contribute to the control of lipolysis in adipocytes by modulating the activity of HSL and ATGL. In this review, we have summarized the signalling pathways activated by several agents involved in the regulation of TAG hydrolysis in adipocytes.

Cold-induced alteration of adipokine profile in humans

Adipose tissue function and sympathetic nervous system (SNS) activity are tightly interconnected. Adipose tissue is densely innervated by the SNS. Adipokines secreted by adipose tissue are implicated in maintaining energy homeostasis, the control of blood pressure, immune system function, hemostasis, and atherosclerosis. Little is known about a direct effect of SNS activation on influencing adipose tissue endocrine function in humans. In 10 lean, healthy male volunteers, SNS was activated by whole-body exposure to cold for 2 hours; a group of 10 subjects served as controls. Vital parameters were evaluated, plasma adipokine levels were measured, and adipokine gene expression in subcutaneous abdominal adipose tissue was determined. Cold exposure caused an increase in cold sensation and a drop in body temperature and heart rate. Norepinephrine, but not epinephrine, plasma levels were elevated. Adiponectin plasma concentrations were acutely and significantly decreased. There was a trend of increased monocyte chemoattractant protein-1 plasma concentrations. Interleukin-6 and leptin levels increased and decreased, respectively, in both groups. Vascular endothelial growth factor plasma levels were unaffected. Subcutaneous adipokine gene expression was unchanged. Cold exposure caused SNS activation and differentially influenced adipokine secretion. Adiponectin levels were acutely reduced, whereas monocyte chemoattractant protein-1 concentrations tended to increase. No specific changes in leptin and IL-6 concentrations were detectable. The observed alterations appeared to be posttranscriptional because adipokine gene expression was found to be unaltered.

Moderate caloric restriction during gestation in rats alters adipose tissue sympathetic innervation and later adiposity in offspring

Maternal prenatal undernutrition predisposes offspring to higher adiposity in adulthood. Mechanisms involved in these programming effects, apart from those described in central nervous system development, have not been established. Here we aimed to evaluate whether moderate caloric restriction during early pregnancy in rats affects white adipose tissue (WAT) sympathetic innervation in the offspring, and its relationship with adiposity development. For this purpose, inguinal and retroperitoneal WAT (iWAT and rpWAT, respectively) were analyzed in male and female offspring of control and 20% caloric-restricted (from 1-12 d of pregnancy) (CR) dams. Body weight (BW), the weight, DNA-content, morphological features and the immunoreactive tyrosine hydroxylase and Neuropeptide Y area (TH+ and NPY+ respectively, performed by immunohistochemistry) of both fat depots, were studied at 25 d and 6 m of age, the latter after 2 m exposure to high fat diet. At 6 m of life, CR males but not females, exhibited greater BW, and greater weight and total DNA-content in iWAT, without changes in adipocytes size, suggesting the development of hyperplasia in this depot. However, in rpWAT, CR males but not females, showed larger adipocyte diameter, with no changes in DNA-content, suggesting the development of hypertrophy. These parameters were not different between control and CR animals at the age of 25 d. In iWAT, both at 25 d and 6 m, CR males but not females, showed lower TH(+) and NPY(+), suggesting lower sympathetic innervation in CR males compared to control males. In rpWAT, at 6 m but not at 25 d, CR males but not females, showed lower TH(+) and NPY(+). Thus, the effects of caloric restriction during gestation on later adiposity and on the differences in the adult phenotype between internal and subcutaneous fat depots in the male offspring may be associated in part with specific alterations in sympathetic innervation, which may impact on WAT architecture.

Melanocortin signalling and the regulation of blood pressure in human obesity

Obesity has reached epidemic proportions worldwide. Sympathetic nervous system activation has been shown to play a major role linking obesity to the development of associated metabolic complications, such as hypertension. Recent evidence has implicated central melanocortin signalling in the regulation of blood pressure in rodents and humans. The importance of sympathetic neural activity in mediating this association has been highlighted. Humans with loss-of-function mutations in the melanocortin 4 receptor (MC4R) are an ideal group of subjects in whom the importance of melanocortin signalling in linking obesity to hypertension can be studied. Consistent with rodent studies, it was recently demonstrated that humans with MC4R deficiency have lower blood pressure, less hypertension, lower 24-h urinary catecholamine excretion, lower resting heart rate and attenuated insulin-mediated sympathetic activation compared to equally-obese humans. In overweight and obese humans without MC4R mutations, the infusion of a highly-selective MC4R agonist led to dose-dependent increases in blood pressure and heart rate. All effects were independent of insulin. This evidence supports the notion that the melanocortin system regulates blood pressure and sympathetic neural function. The results obtained in rodent and human studies, in relation to blood pressure and sympathetic function, may limit the use of MC4R agonists for the treatment of obesity. Future studies will determine whether MC4R deficiency is associated with protection from development of the detrimental cardiovascular consequences that accompany obesity.

An intact dorsomedial hypothalamic nucleus, but not the subzona incerta or reuniens nucleus, is necessary for short-day melatonin signal-induced responses in Siberian hamsters

Siberian hamsters provide a useful model to define mechanisms underlying obesity reversal as they naturally transition from their extreme seasonal obesity in long 'summer-like' days (LDs) to a leaner state in short 'winter-like' days (SDs). These day length changes are coded into durational melatonin (MEL) signals by the pineal gland resulting in stimulation of MEL receptors (MEL(1a)-Rs). MEL(1a)-R mRNA is colocalized centrally in sympathetic nervous system (SNS) outflow neurons comprising a chain of neurons that ultimately innervates white adipose tissue (WAT). Neural components in this circuit include the subzona incerta (subZI), dorsomedial hypothalamic nucleus (DMH) and thalamic reuniens nucleus (ReN). SD, long-duration MEL signals induce gonadal regression and increase WAT SNS drive triggering lipolysis and thereby reversing LD obesity. We attempted to block the reversal of SD MEL signal-induced obesity by making electrolytic or sham lesions of the subZI, ReN or DMH in LD-housed hamsters. To create SD-like, long-duration MEL signals, we injected MEL 3 h before lights out, thereby lengthening the naturally occurring nocturnal duration of circulating MEL. ReN and subZI lesions did not block SD-like MEL signal-induced decreases in body, WAT, testicular masses or food intake; by contrast, DMH lesions blocked decreases in WAT and testicular mass. This nonresponsiveness was not due to lesion-induced inappropriate nocturnal LD MEL secretion that would have altered our creation of SD-like signals. Therefore, the DMH appears to participate in the control of both SD energy and reproductive responses, and joins the suprachiasmatic nucleus as sites necessary for SD responses in this species.

Non-neuronal regulation and repertoire of cholinergic receptors in organs

Many studies on the cholinergic pathway have indicated that cholinergic receptors, which are widely expressed in various cells, play an important role in all body organs. In this review, we present the concept that cholinergic responses are regulated through a neuronal or non-neuronal mechanism. The neuronal mechanism is a system in which acetylcholine binds to cholinergic receptors on target cells through the nerves. In the non-neuronal mechanism, acetylcholine, produced by neighboring cells in an autocrine/paracrine manner, binds to cholinergic receptors on target cells. Both mechanisms subsequently lead to physiological and pathophysiological responses. We also investigated the subunits/subtypes of cholinergic receptors on target cells, physiological and pathophysiological responses of the organs via cholinergic receptors, and extracellular factors that alter the subtypes/subunits of cholinergic receptors. Collectively, this concept will elucidate how cholinergic responses occur and will help us conduct further experiments to develop new therapeutic agents.

Increased noradrenergic activity in the ventromedial hypothalamus during treadmill running in rats

Physical exercise dramatically increases the energy expenditure of animals. In terms of energy substrate, at the onset of exercise, the contribution of carbohydrates to the energy expenditure is relatively predominant, and decreases gradually with the progression of exercise, while fat consumption increases progressively. The ventromedial hypothalamus (VMH) is a nucleus in the hypothalamus that regulates whole body energy metabolism via the sympathetic nervous system. Some reports have indicated that noradrenergic projections to the VMH are involved in energy metabolism during exercise. However, it is not clear whether exercise influences the activity of noradrenergic projections to the VMH. We hypothesize that during exercise, noradrenergic neurons projecting to the VMH are activated, and play an important part in enhancing fat oxidation. To test this hypothesis, we used in vivo microdialysis to investigate the effect of exercise on the activity of monoaminergic (noradrenaline: NA, dopamine: DA, serotonin: 5-HT) neurons projecting to the VMH of rats. Rats were subjected to running at 15 m/min (incline 3 degrees) for 60 min. During treadmill running, noradrenergic and dopaminergic activities increased significantly in the VMH. Extracellular 5-HT concentrations in the VMH did not change during treadmill running at the exercise intensity. Given the known effects of NA in the VMH on energy metabolism, our results suggest that the increase in noradrenergic activity in the VMH is related to the enhancement of fat oxidation during exercise.

Distributed forebrain sites mediate melatonin-induced short-day responses in Siberian hamsters

The pineal hormone melatonin (MEL) is the key initiator in regulating seasonal photoperiodic responses; however, the central sites that mediate short day (SD) winter-like responses, such as testicular regression and decreases in white adipose tissue (WAT) mass, by Siberian hamsters are not precisely known. WAT is innervated by the sympathetic nervous system, and several forebrain sites that are part of the sympathetic nervous system outflow to WAT coexpress MEL(1a) receptor mRNA [e.g. suprachiasmatic nucleus, subzona incerta (SubZi), dorsomedial nucleus of the hypothalamus, nucleus reunions and paraventricular nuclei of the thalamus]. We tested the involvement of these sites in MEL-triggered SD responses. A long duration, SD-like MEL signal was applied site specifically for 5 wk, with sc and third ventricle MEL application serving as positive controls. Whereas SD MEL signals delivered to each of these sites were able to induce testicular regression, all but the paraventricular nuclei of the thalamus also trigger SD-induced decreases in body mass. Third ventricle, sc, suprachiasmatic nucleus, or SubZi MEL application also decreased WAT mass, and only sc and SubZi MEL application decreased food intake. Collectively these data suggest a distributed system of MEL-sensitive brain sites sufficient to mediate these SD responses, the redundancy of which suggests its importance for appropriate seasonal responses critical for overwintering.

Utility of transplantation in studying adipocyte biogenesis and function

Adipose tissue plays important roles in the regulation of energy homeostasis and metabolism. Two features distinguish adipose tissue from other organs--the ability to greatly expand its mass, via increases in cell size and/or number, and the wide anatomical distribution. While adipose tissue function is greatly affected by adipocyte size and anatomic location, regulations of adipocyte size, number, and body fat distribution are poorly understood. Transplantation of either mature adipose tissue or adipocyte progenitor cells has been used in studying adipocyte function and biogenesis. In this review, we will attempt to summarize methodological considerations for transplantation, including selections of donor material, transplantation site and the length of transplantation study, as well as effects of these factors and vascularization and innervation on the function of transplants. Specific studies are also reviewed to illustrate the utility of adipose tissue transplants in studying adipose tissue function and biogenesis. The focus is on studies in three areas: (1) use of transplants in demonstrating adipose tissue function, such as effects of adipose tissue transplants on metabolism and energy homeostasis of the recipient animals and depot-specific differences in adipose tissue function; (2) use of transplantation to dissect direct or cell-autonomous from indirect or non-cell-autonomous effects of leptin signaling and sex on adipocyte size; (3) use of transplantation in the identification of adipocyte progenitor cells and lineage analysis. Finally, future applications of transplantation in studying depot-specific adipocyte biogenesis, and genetic and hormonal effects of sex and age on adipocyte biogenesis and function are discussed.

Sensory and sympathetic nervous system control of white adipose tissue lipolysis

Circulating factors are typically invoked to explain bidirectional communication between the CNS and white adipose tissue (WAT). Thus, initiation of lipolysis has been relegated primarily to adrenal medullary secreted catecholamines and the inhibition of lipolysis primarily to pancreatic insulin, whereas signals of body fat levels to the brain have been ascribed to adipokines such as leptin. By contrast, evidence is given for bidirectional communication between brain and WAT occurring via the sympathetic nervous system (SNS) and sensory innervation of this tissue. Using retrograde transneuronal viral tract tracers, the SNS outflow from brain to WAT has been defined. Functionally, sympathetic denervation of WAT blocks lipolysis to a variety of lipolytic stimuli. Using anterograde transneuronal viral tract tracers, the sensory input from WAT to brain has been defined. Functionally, these WAT sensory nerves respond electrophysiologically to increases in WAT SNS drive suggesting a possible neural negative feedback loop to regulate lipolysis.

Arcuate nucleus destruction does not block food deprivation-induced increases in food foraging and hoarding

The mechanisms underlying the control of food intake are considerably better understood than those underlying the appetitive ingestive behaviors of foraging and hoarding of food, despite the prevalence of the latter across species including humans. Neuropeptide Y (NPY) and Agouti-related protein (AgRP), two orexigenic neuropeptides known to stimulate food intake in a variety of species, applied centrally to Siberian hamsters increases foraging and especially hoarding with lesser increases in food intake. Both are expressed in the arcuate nucleus (Arc) and their synthesis increases with food deprivation, a naturally-occurring stimulus that markedly increases foraging and hoarding in Siberian hamsters. Therefore, we tested whether destruction of Arc neurons blocks these ingestive behaviors. This was accomplished either by microinjecting NPY conjugated to saporin (NPY-SAP) bilaterally into the Arc to kill NPY receptor-bearing neurons or via neonatal monosodium glutamate (MSG) treatment. For both methods, Arc cresyl violet staining (cell density) and NPY and Y1 receptor-immunoreactivity (ir) were significantly decreased. Although baseline foraging and food hoarding were not affected, food deprivation-induced increased food hoarding was surprisingly exaggerated approximately 100% with both types of Arc destruction. We found a substantial amount of remaining NPY-ir fibers, likely emanating from the brainstem, and a significant up-regulation of Y1 receptors in Arc NPY projections areas (hypothalamic paraventricular nucleus and perifornical area) after Arc denervation and their activation may have accounted for the exaggerated increases. The converging evidence from both Arc destruction methods suggests an intact Arc is not necessary for food deprivation-induced increases in food foraging and hoarding.

Cholecystokinin-33 acutely attenuates food foraging, hoarding and intake in Siberian hamsters

Neurochemicals that stimulate food foraging and hoarding in Siberian hamsters are becoming more apparent, but we do not know if cessation of these behaviors is due to waning of excitatory stimuli and/or the advent of inhibitory factors. Cholecystokinin (CCK) may be such an inhibitory factor as it is the prototypic gastrointestinal satiety peptide and is physiologically important in decreasing food intake in several species including Siberian hamsters. Systemic injection of CCK-33 in laboratory rats decreases food intake, doing so to a greater extent than CCK-8. We found minimal effects of CCK-8 on food foraging and hoarding previously in Siberian hamsters, but have not tested CCK-33. Therefore, we asked: Does CCK-33 decrease normal levels or food deprivation-induced increases in food foraging, hoarding and intake? Hamsters were housed in a wheel running-based foraging system with simulated burrows to test the effects of peripheral injections of CCK-33 (13.2, 26.4, or 52.8 microg/kg body mass), with or without a preceding 56 h food deprivation. The highest dose of CCK-33 caused large baseline reductions in all three behaviors for the 1st hour post-injection compared with saline; in addition, the intermediate CCK-33 dose was sufficient to curtail food intake and foraging during the 1st hour. In food-deprived hamsters, we used a 52.8 microg/kg body mass dose of CCK-33 which decreased food intake, hoarding, and foraging almost completely compared with saline controls for 1h. Therefore, CCK-33 appears to be a potent inhibitor of food intake, hoarding, and foraging in Siberian hamsters.

Endogenous leptin signaling in the caudal nucleus tractus solitarius and area postrema is required for energy balance regulation

Medial nucleus tractus solitarius (mNTS) neurons express leptin receptors (LepRs), and intra-mNTS delivery of leptin reduces food intake and body weight. Here, the contribution of endogenous LepR signaling in mNTS neurons to energy balance control was examined. Knockdown of LepR in mNTS and area postrema (AP) neurons of rats (LepRKD) via adeno-associated virus short hairpin RNA-interference (AAV-shRNAi) resulted in significant hyperphagia for chow, high-fat, and sucrose diets, yielding increased body weight and adiposity. The chronic hyperphagia of mNTS/AP LepRKD rats is likely mediated by a reduction in leptin potentiation of gastrointestinal satiation signaling, as LepRKD rats showed decreased sensitivity to the intake-reducing effects of cholecystokinin. LepRKD rats showed increased basal AMP-kinase activity in mNTS/AP micropunches, and pharmacological data suggest that this increase provides a likely mechanism for their chronic hyperphagia. Overall these findings demonstrate that LepRs in mNTS and AP neurons are required for normal energy balance control.

Peripheral neuropeptide Y Y1 receptors regulate lipid oxidation and fat accretion

OBJECTIVE

Neuropeptide Y and its Y receptors are important players in the regulation of energy homeostasis. However, while their functions in feeding regulation are well recognized, functions in other critical aspects of energy homeostasis are largely unknown. To investigate the function of Y1 receptors in the regulation of energy homeostasis, we examined energy expenditure, physical activity, body composition, oxidative fuel selection and mitochondrial oxidative capacity in germline Y1(-/-) mice as well as in a conditional Y1-receptor-knockdown model in which Y1 receptors were knocked down in peripheral tissues of adult mice.

RESULTS

Germline Y1(-/-) mice of both genders not only exhibit a decreased respiratory exchange ratio, indicative of increased lipid oxidation, but interestingly also develop late-onset obesity. However, the increased lipid oxidation is a primary effect of Y1 deletion rather than secondary to increased adiposity, as young Y1(-/-) mice are lean and show the same effect. The mechanism behind this is likely because of increased liver and muscle protein levels of carnitine palmitoyltransferase-1 (CPT-1) and maximal activity of key enzymes involved in beta-oxidation; beta-hydroxyacyl CoA dehydrogenase (betaHAD) and medium-chain acyl-CoA dehydrogenase (MCAD), leading to increased mitochondrial capacity for fatty acid transport and oxidation. These effects are controlled by peripheral Y1-receptor signalling, as adult-onset conditional Y1 knockdown in peripheral tissues also leads to increased lipid oxidation, liver CPT-1 levels and betaHAD activity. Importantly, these mice are resistant to diet-induced obesity.

CONCLUSIONS

This work shows the primary function of peripheral Y1 receptors in the regulation of oxidative fuel selection and adiposity, opening up new avenues for anti-obesity treatments by targeting energy utilization in peripheral tissues rather than suppressing appetite by central effects.

Body mass loss during adaptation to short winter-like days increases food foraging, but not food hoarding

Siberian hamsters markedly reduce their body/lipid mass ( approximately 20-45%) in short 'winter-like' days (SD). Decreases in body/lipid mass associated with food deprivation or lipectomy result in increases in foraging and food hoarding. When at their SD-induced body/lipid mass nadir, food hoarding is not increased despite their decreases in body/lipid mass, but hoarding was not tested during the dynamic period of body/lipid mass loss (first 5-6 weeks of SDs). Therefore, we tested for changes in foraging/hoarding during this initial period in Siberian hamsters housed in a simulated burrow with a wheel running-based foraging system and exposed to either long 'summer-like' days (LD) or SDs. Two foraging effort conditions were used: 10 Revolutions/Pellet (pellet delivered after running 10 revolutions) and a Free Wheel/Free Food condition (wheel available, food pellets non-contingently available). Regardless of the foraging condition, body mass was significantly reduced across 8 weeks of SDs ( approximately 15%). Foraging increased after 7 weeks in SDs, but food hoarding did not increase compared to LDs. Instead food hoarding significantly decreased in SDs at Weeks 2-5 compared with Week 0 values, with the 10 Revolutions/Pellet foraging group returning to LD levels thereafter and the Free Wheel/Free Food group remaining reduced from Weeks 2-7. Collectively, we found that SDs decreased body mass, increased foraging after 7 weeks, and increased food hoarding, but only after an initial decrease and not above that seen in LDs. These data suggest that SD-induced body/lipid mass losses do not engender similar behavioral responses as seen with food deprivation or lipectomy.

Appetitive and consummatory ingestive behaviors stimulated by PVH and perifornical area NPY injections

Food is acquired (obtained by foraging) and frequently stored (hoarded) across animal taxa, including humans, but the physiological mechanisms underlying these behaviors are virtually unknown. We found that peptides that stimulate food intake in rats stimulate food foraging and/or hoarding more than intake in Siberian hamsters. Neuropeptide Y (NPY) is a potent orexigenic peptide that increases food foraging and hoarding (appetitive behavior) and food intake (consummatory behavior). Given that NPY injections into the hypothalamic paraventricular nucleus (PVH) or perifornical area (PFA) increase food intake by rats, it is possible that these injections may stimulate food foraging or hoarding by Siberian hamsters. We also tested whether antagonism of the NPY Y1 receptor (Y1-R), the agonism of which stimulates hoarding, would inhibit post-food-deprivation increases in foraging and hoarding. We injected one of three doses of NPY or vehicle into the PVH or PFA of animals housed in a simulated foraging-hoarding housing system and measured these behaviors at 1, 2, 4, and 24 h. A subset of animals was subsequently food deprived and then given PVH or PFA Y1-R antagonist microinjections before they were refed. NPY PVH microinjections decreased foraging but increased hoarding and food intake, whereas NPY PFA microinjections increased all three behaviors, but the greatest increase was in hoarding. Y1-R antagonist inhibited post-food-deprivation increases in hoarding when injected into the PVH and PFA and inhibited foraging when injected into the PFA. These results support the view that NPY is involved in appetitive and consummatory ingestive behaviors, but each may be controlled by different brain areas and/or NPY receptor subtypes.

Leptin inhibits food-deprivation-induced increases in food intake and food hoarding

Food deprivation stimulates foraging and hoarding and to a much lesser extent, food intake in Siberian hamsters. Leptin, the anorexigenic hormone secreted primarily from adipocytes, may act in the periphery, the brain, or both to inhibit these ingestive behaviors. Therefore, we tested whether leptin given either intracerebroventricularly or intraperitoneally, would block food deprivation-induced increases in food hoarding, foraging, and intake in animals with differing foraging requirements. Hamsters were trained in a running wheel-based food delivery foraging system coupled with simulated burrow housing. We determined the effects of food deprivation and several peripheral doses of leptin on plasma leptin concentrations. Hamsters were then food deprived for 48 h and given leptin (0, 10, 40, or 80 microg ip), and additional hamsters were food deprived for 48 h and given leptin (0, 1.25, 2.5, or 5.0 microg icv). Foraging, food intake, and hoarding were measured postinjection. Food deprivation stimulated food hoarding to a greater degree and duration than food intake. In animals with a foraging requirement, intracerebroventricular leptin almost completely blocked food deprivation-induced increased food hoarding and intake, but increased foraging. Peripheral leptin treatment was most effective in a sedentary control group, completely inhibiting food deprivation-induced increased food hoarding and intake at the two highest doses, and did not affect foraging at any dose. Thus, the ability of leptin to inhibit food deprivation-induced increases in ingestive behaviors differs based on foraging effort (energy expenditure) and the route of administration of leptin administration.

Melanocortin-4 receptor mRNA expressed in sympathetic outflow neurons to brown adipose tissue: neuroanatomical and functional evidence

A precise understanding of neural circuits controlling lipid mobilization and thermogenesis remains to be determined. We have been studying the sympathetic nervous system (SNS) contributions to white adipose tissue (WAT) lipolysis largely in Siberian hamsters. Central melanocortins are implicated in the control of the sympathetic outflow to WAT, and, moreover, the melanocortin 4 receptors (MC4-R) appear to be principally involved. We previously found that acute third ventricular melanotan II (MTII; an MC3/4-R agonist) injections increase sympathetic drive (norepinephrine turnover) to interscapular brown adipose tissue (IBAT) and IBAT temperature. Here we tested whether MC4-R mRNA is expressed in IBAT SNS outflow neurons using in situ hybridization for the former and injections of the transneuronal viral retrograde tract tracer, pseudorabies virus (PRV) into IBAT, for the latter. Significant numbers of double-labeled cells for PRV and MC4-R mRNA were found across the neuroaxis (mean of all brain sites approximately 60%), including the hypothalamic paraventricular nucleus (PVH; approximately 80%). Acute parenchymal MTII microinjections into the PVH of awake, freely-moving hamsters, using doses below those able to increase IBAT temperature when injected into the third ventricle, increased IBAT temperature for as long as 4 h, as measured by temperature transponders implanted below the tissue. Collectively, these data add significant support to the view that central melanocortins are important in controlling IBAT thermogenesis via the SNS innervation of this tissue, likely through the MC4-Rs.

Peroxisome proliferator-activated receptor-gamma-mediated positive energy balance in the rat is associated with reduced sympathetic drive to adipose tissues and thyroid status

Peroxisome proliferator-activated receptor-gamma (PPARgamma) activation up-regulates thermogenesis-related genes in rodent white and brown adipose tissues (WAT and BAT) without increasing whole-body energy expenditure. We tested here whether such dissociation is the result of a negative modulation of sympathetic activity to WAT and BAT and thyroid axis components by PPARgamma activation. Administration of the PPARgamma agonist rosiglitazone (15 mg/kg.d) for 7 d to male Sprague Dawley rats increased food intake (10%), feed efficiency (31%), weight gain (45%), spontaneous motor activity (60%), and BAT and WAT mass and reduced whole-body oxygen consumption. Consistent with an anabolic setting, rosiglitazone markedly reduced sympathetic activity to BAT and WAT (>50%) and thyroid status as evidenced by reduced levels of plasma thyroid hormones (T(4) and T(3)) and mRNA levels of BAT and liver T(3)-generating enzymes iodothyronine type 2 (-40%) and type 1 (-32%) deiodinases, respectively. Rosiglitazone also decreased mRNA levels of the thyroid hormone receptor (THR) isoforms alpha1 (-34%) and beta (-66%) in BAT and isoforms alpha1 (-20%) and alpha2 (-47%) in retroperitoneal WAT. These metabolic effects were associated with a reduction in mRNA levels of the pro-energy expenditure peptides CRH and CART in specific hypothalamic nuclei. A direct central action of rosiglitazone is, however, unlikely based on its low brain uptake and lack of metabolic effects of intracerebroventricular administration. In conclusion, a reduction in BAT sympathetic activity and thyroid status appears to, at least partly, explain the PPARgamma-induced reduction in energy expenditure and the fact that up-regulation of thermogenic gene expression does not translate into functional stimulation of whole-body thermogenesis in vivo.

Differential sympathetic drive to adipose tissues after food deprivation, cold exposure or glucoprivation

Surplus energy is principally stored in white adipose tissue (WAT) as triacylglycerol and mobilized via lipolysis through norepinephrine (NE) released from sympathetic nervous system terminals innervating WAT. We demonstrated that central melanocortin receptor agonism provokes differential sympathetic drives across WAT pads and interscapular brown adipose tissue (IBAT). Here we tested for differential WAT and IBAT sympathetic drive to known lipolytic stimuli {glucoprivation [2-deoxy-D-glucose (2-DG)], cold exposure (5 degrees C), food deprivation (16 h), or both cold exposure and food deprivation} by measuring NE turnover (NETO). Only inguinal WAT NETO significantly increased across all stimuli. Dorsal subcutaneous WAT NETO only increased with glucoprivation. Retroperitoneal WAT NETO increased with glucoprivation, cold and cold + food deprivation, but not by food deprivation. Epididymal WAT NETO was unaffected by glucoprivation but increased with cold, cold + food deprivation or food deprivation, but to a small significant degree. IBAT NETO was unaffected by glucoprivation or food deprivation, but increased with cold and cold + food deprivation. Plasma glucose decreased with food deprivation and increased with 2-DG administration or cold exposure. Plasma glycerol was increased with food deprivation, cold, and their combination but not with 2-DG, whereas plasma free fatty acids increased with food deprivation, cold + food deprivation, and 2-DG. These data show differential sympathetic drive to WAT and BAT for four different lipolytic stimuli, exemplifying the fat pad-specific pattern of WAT sympathetic drive across lipid-mobilizing conditions and emphasizing the need to analyze multiple adipose depots for measures of NETO and likely most measures.

Food deprivation-induced changes in body fat mobilization after neonatal monosodium glutamate treatment

The reversal of obesity is a difficult feat at best and is a growing problem as the obesity epidemic increases worldwide. Considerable focus has been made on the arcuate nucleus (Arc) in the control of body and lipid mass and food intake. To test the role of the Arc in body fat mobilization, we compared the effects of food deprivation on white adipose tissue (WAT) mass in adult Siberian hamsters by making exocytotic lesions of the Arc via neonatal subcutaneous injections of monosodium glutamate (MSG). MSG-treated hamsters had significantly increased body mass, total and individual WAT pad masses, and serum leptin concentrations compared with their vehicle-injected counterparts. MSG produced marked reductions in Arc Nissl staining, tyrosine hydroxylase-immunoreactive (ir) neurons, and neuropeptide Y (NPY)- and agouti-related protein (AgRP)-ir fibers compared with controls. MSG significantly decreased hypothalamic paraventricular nucleus (PVN) NPY- and AgRP fiber-ir compared with controls, likely because of Arc projections to this nucleus. MSG treatment also reduced area postrema (AP) tyrosine hydroxylase (TH)-ir fibers compared with controls. MSG treatment did not, however, block food deprivation-induced decreases in WAT pad mass compared with controls. Thus, despite considerable damage to the Arc and some of its projections to the PVN, as well as the AP, body fat was mobilized apparently normally, bringing into question the necessity of these structures for food deprivation-induced lipid mobilization. These data support recent evidence that chronically decerebrate rats, in which the forebrain is surgically isolated from the caudal brainstem, show normal food deprivation responses, including lipid mobilization.

Lou/C obesity-resistant rat exhibits hyperactivity, hypermetabolism, alterations in white adipose tissue cellularity, and lipid tissue profiles

Lou/C obesity-resistant rat constitutes an original model to understand the phenomena of overweight and obesity. The aim of the present study was to identify metabolic causes for the outstanding leanness of Lou/C rat. To this end, the metabolic profiles (food intake, energy expenditure, and physical activity) and the cellular characteristics of white adipose tissue (lipogenesis, lipolysis, cellularity, and lipid composition) in 30-wk-old Lou/C rats were compared with age-matched Wistar rats. Lou/C rats exhibited a lower body weight (-45%), reduced adiposity (-80%), increased locomotor activity (+95%), and higher energy expenditure (+11%) than Wistar rats. Epididymal adipose tissue of Lou/C rat was twice lower than that of Wistar rat due to both a reduction in both adipocyte size (-25%) and number (three times). Basal lipolysis and sensitivity to noradrenaline were similar; however, the responsiveness to noradrenaline was lower in adipocytes from Lou/C compared with that from Wistar rats. Lipidomic analysis of plasma, adipose tissue, and liver revealed profound differences in lipid composition between the two strains. Of note, the desaturation indexes (ratio C16:1/C16:0 and C18:1/C18:0) were lower in Lou/C, indicating a blunted activity of delta-9-desaturase such as stearoyl-coenzyme A-desaturase-1. Increased physical activity, increased energy expenditure, and white adipose tissue cellularity are in good agreement with previous observations suggesting that a higher sympathetic tone in Lou/C could contribute to its lifelong leanness.

MTII attenuates ghrelin- and food deprivation-induced increases in food hoarding and food intake

Food deprivation triggers a constellation of physiological and behavioral changes including increases in peripherally-produced ghrelin and centrally-produced agouti-related protein (AgRP). Upon refeeding, food intake is increased in most species, however hamsters primarily increase food hoarding. Food deprivation-induced increases in food hoarding by Siberian hamsters are mimicked by peripheral ghrelin and central AgRP injections. Because food deprivation stimulates ghrelin as well as AgRP synthesis/release, food deprivation-induced increases in hoarding may be mediated by melanocortin 3 or 4 receptor (MC3/4-R) antagonism via AgRP, the MC3/4-R inverse agonist. Therefore, we asked: Can a MC3/4-R agonist block food deprivation- or ghrelin-induced increases in foraging, food hoarding and food intake? This was accomplished by injecting melanotan II (MTII), a synthetic MC3/4-R agonist, into the 3rd ventricle in food deprived, fed or peripheral ghrelin injected hamsters and housed in a running wheel-based food delivery foraging system. Three foraging conditions were used: a) no running wheel access, non-contingent food, b) running wheel access, non-contingent food or c) a foraging requirement for food (10 revolutions/pellet). Food deprivation was a more potent stimulator of foraging and hoarding than ghrelin. Concurrent injections of MTII completely blocked food deprivation- and ghrelin-induced increases in food intake and attenuated, but did not always completely block, food deprivation- and ghrelin-induced increases in food hoarding. Collectively, these data suggest that the MC3/4-R are involved in ghrelin- and food deprivation-induced increases in food intake, but other neurochemical systems, such as previously demonstrated with neuropeptide Y, also are involved in increases in food hoarding as well as foraging.

Increased energy expenditure, dietary fat wasting, and resistance to diet-induced obesity in mice lacking renin

An overactive renin-angiotensin system is associated with obesity and the metabolic syndrome. However, the mechanisms behind it are unclear. Cleaving angiotensinogen to angiotensin I by renin is a rate-limiting step of angiotensin II production, but renin is suggested to have angiotensin-independent effects. We generated mice lacking renin (Ren1c) using embryonic stem cells from C57BL/6 mice, a strain prone to diet-induced obesity. Ren1c(-/-) mice are lean, insulin sensitive, and resistant to diet-induced obesity without changes in food intake and physical activity. The lean phenotype is likely due to a higher metabolic rate and gastrointestinal loss of dietary fat. Most of the metabolic changes in Ren1c(-/-) mice were reversed by angiotensin II administration. These results support a role for angiotensin II in the pathogenesis of diet-induced obesity and insulin resistance.

Central neural and endocrine mechanisms of non-exercise activity thermogenesis and their potential impact on obesity

The rise in obesity is associated with a decline in the amount of physical activity in which people engage. The energy expended through everyday non-exercise activity, called non-exercise activity thermogenesis (NEAT), has a considerable potential impact on energy balance and weight gain. Comparatively little attention has been paid to the central mechanisms of energy expenditure and how decreases in NEAT might contribute to obesity. In this review, we first examine the sensory and endocrine mechanisms through which energy availability and energy balance are detected that may influence NEAT. Second, we describe the neural pathways that integrate these signals. Lastly, we consider the effector mechanisms that modulate NEAT through the alteration of activity levels as well as through changes in the energy efficiency of movement. Systems that regulate NEAT according to energy balance may be linked to neural circuits that modulate sleep, addiction and the stress response. The neural and endocrine systems that control NEAT are potential targets for the treatment of obesity.

Fat pad-specific effects of lipectomy on foraging, food hoarding, and food intake

Unlike most species, after food deprivation, Siberian hamsters increase foraging and food hoarding, two appetitive ingestive behaviors, but not food intake, a consummatory ingestive behavior. We previously demonstrated (Wood AD, Bartness TJ, Am J Physiol Regul Integr Comp Physiol 272: R783-R792, 1997) that increases in food hoarding are triggered by directly decreasing body fat levels through partial surgical lipectomy; however, we did not test if lipectomy affected foraging, nor if the magnitude of the lipid deficit affected food hoard size. Therefore, we tested whether varying the size of the lipectomy-induced lipid deficit and/or foraging effort affected foraging, food hoarding, or food intake. This was accomplished by housing adult male Siberian hamsters in a foraging/hoarding system and removing (x) both epididymal white adipose tissue (EWATx) pads, both inguinal white adipose tissue (IWATx) pads, or both EWAT and IWAT pads (EWATx + IWATx) and measuring foraging, food hoarding, and food intake for 12 wk. The lipectomy-induced lipid deficit triggered different patterns of white adipose tissue mass compensation that varied with foraging effort. Foraging for food (10 wheel revolutions to earn a food pellet) abolished the EWATx-induced compensation in IWAT pad mass. The magnitude of the lipid deficit did not engender a proportional change in any of the appetitive or consummatory ingestive behaviors. EWATx caused the greatest increase in food hoarding compared with IWATx or EWATx + IWATx, when animals were required to forage for their food. Collectively, it appears that the magnitude of a lipid deficit does not affect appetitive or consummatory behaviors; rather, when energy (foraging) demands are increased, loss of specific (gonadal) fat pads can preferentially stimulate increases in food hoarding.

The role of the vgf gene and VGF-derived peptides in nutrition and metabolism

Energy homeostasis is a complex physiological function coordinated at multiple levels. The issue of genetic regulation of nutrition and metabolism is attracting increasing interest and new energy homeostasis-regulatory genes are continuously identified. Among these genes, vgf is gaining increasing interest following two observations: (1) VGF-/- mice have a lean and hypermetabolic phenotype; (2) the first VGF-derived peptide involved in energy homeostasis, named TLQP-21, has been identified. The aim of this review will be to discuss the role of the vgf gene and VGF derived peptides in metabolic and nutritional functions. In particular we will: (1) provide a brief overview on the central systems regulating energy homeostasis and nutrition particularly focusing on the melanocortin system; (2) introduce the structure and molecular characteristic of vgf; (3) describe the phenotype of VGF deficient mice; (4) present recent data on the metabolic role of VGF-derived peptides, particularly focusing on one peptide named TLQP-21.

Neuropeptide Y acts directly in the periphery on fat tissue and mediates stress-induced obesity and metabolic syndrome

The relationship between stress and obesity remains elusive. In response to stress, some people lose weight, whereas others gain. Here we report that stress exaggerates diet-induced obesity through a peripheral mechanism in the abdominal white adipose tissue that is mediated by neuropeptide Y (NPY). Stressors such as exposure to cold or aggression lead to the release of NPY from sympathetic nerves, which in turn upregulates NPY and its Y2 receptors (NPY2R) in a glucocorticoid-dependent manner in the abdominal fat. This positive feedback response by NPY leads to the growth of abdominal fat. Release of NPY and activation of NPY2R stimulates fat angiogenesis, macrophage infiltration, and the proliferation and differentiation of new adipocytes, resulting in abdominal obesity and a metabolic syndrome-like condition. NPY, like stress, stimulates mouse and human fat growth, whereas pharmacological inhibition or fat-targeted knockdown of NPY2R is anti-angiogenic and anti-adipogenic, while reducing abdominal obesity and metabolic abnormalities. Thus, manipulations of NPY2R activity within fat tissue offer new ways to remodel fat and treat obesity and metabolic syndrome.

NPY Y1 receptor is involved in ghrelin- and fasting-induced increases in foraging, food hoarding, and food intake

Fasting triggers a constellation of physiological and behavioral changes, including increases in peripherally produced ghrelin and centrally produced hypothalamic neuropeptide Y (NPY). Refeeding stimulates food intake in most species; however, hamsters primarily increase foraging and food hoarding with smaller increases in food intake. Fasting-induced increases in foraging and food hoarding in Siberian hamsters are mimicked by peripheral ghrelin, central NPY, and NPY Y1 receptor agonist injections. Because fasting stimulates ghrelin and subsequently NPY synthesis/release, it may be that fasting-induced increased hoarding is mediated by NPY Y1 receptor activation. Therefore, we asked: Can an Y1 receptor antagonist block fasting- or ghrelin-induced increases in foraging, food hoarding, and food intake? This was accomplished by injecting the NPY Y1 receptor antagonist 1229U91 intracerebroventricularly in hamsters fasted, fed, or given peripheral ghrelin injections and housed in a running wheel-based food delivery foraging system coupled with simulated-burrow housing. Three foraging conditions were used: 1) no running wheel access, free food, 2) running wheel access, free food, or 3) foraging requirement (10 revolutions/pellet) for food. Fasting was a more potent stimulator of foraging and food hoarding than ghrelin. Concurrent injections of 1229U91 completely blocked fasting- and ghrelin-induced increased foraging and food intake and attenuated, but did not always completely block, fasting- and ghrelin-induced increases in food hoarding. Collectively, these data suggest that the NPY Y1 receptor is important for the effects of ghrelin- and fasting-induced increases in foraging and food intake, but other NPY receptors and/or other neurochemical systems are involved in increases in food hoarding.

White adipose tissue lacks significant vagal innervation and immunohistochemical evidence of parasympathetic innervation

Converging evidence indicates that white adipose tissue (WAT) is innervated by the sympathetic nervous system (SNS) based on immunohistochemical labeling of a SNS marker (tyrosine hydroxylase [TH]), tract tracing of WAT sympathetic postganglionic innervation, pseudorabies virus (PRV) transneuronal labeling of WAT SNS outflow neurons, and functional evidence from denervation studies. Recently, WAT para-SNS (PSNS) innervation was suggested because local surgical WAT sympathectomy (sparing hypothesized parasympathetic innervation) followed by PRV injection yielded infected cells in the vagal dorsomotor nucleus (DMV), a traditionally-recognized PSNS brain stem site. In addition, local surgical PSNS WAT denervation triggered WAT catabolic responses. We tested histologically whether WAT was parasympathetically innervated by searching for PSNS markers in rat, and normal (C57BL) and obese (ob/ob) mouse WAT. Vesicular acetylcholine transporter, vasoactive intestinal peptide and neuronal nitric oxide synthase immunoreactivities were absent in WAT pads (retroperitoneal, epididymal, inguinal subcutaneous) from all animals. Nearly all nerves innervating WAT vasculature and parenchyma that were labeled with protein gene product 9.5 (PGP9.5; pan-nerve marker) also contained TH, attesting to pervasive SNS innervation. When Siberian hamster inguinal WAT was sympathetically denervated via local injections of catecholaminergic toxin 6-hydroxydopamine (sparing putative parasympathetic nerves), subsequent PRV injection resulted in no central nervous system (CNS) or sympathetic chain infections suggesting no PSNS innervation. By contrast, vehicle-injected WAT subsequently inoculated with PRV had typical CNS/sympathetic chain viral infection patterns. Collectively, these data indicate no parasympathetic nerve markers in WAT of several species, with sparse DMV innervation and question the claim of PSNS WAT innervation as well as its functional significance.

TLQP-21, a VGF-derived peptide, increases energy expenditure and prevents the early phase of diet-induced obesity

The vgf gene has been identified as an energy homeostasis regulator. Vgf encodes a 617-aa precursor protein that is processed to yield an incompletely characterized panel of neuropeptides. Until now, it was an unproved assumption that VGF-derived peptides could regulate metabolism. Here, a VGF peptide designated TLQP-21 was identified in rat brain extracts by means of immunoprecipitation, microcapillary liquid chromatography-tandem MS, and database searching algorithms. Chronic intracerebroventricular (i.c.v.) injection of TLQP-21 (15 mug/day for 14 days) increased resting energy expenditure (EE) and rectal temperature in mice. These effects were paralleled by increased epinephrine and up-regulation of brown adipose tissue beta2-AR (beta2 adrenergic receptor) and white adipose tissue (WAT) PPAR-delta (peroxisome proliferator-activated receptor delta), beta3-AR, and UCP1 (uncoupling protein 1) mRNAs and were independent of locomotor activity and thyroid hormones. Hypothalamic gene expression of orexigenic and anorexigenic neuropeptides was unchanged. Furthermore, in mice that were fed a high-fat diet for 14 days, TLQP-21 prevented the increase in body and WAT weight as well as hormonal changes that are associated with a high-fat regimen. Biochemical and molecular analyses suggest that TLQP-21 exerts its effects by stimulating autonomic activation of adrenal medulla and adipose tissues. In conclusion, we present here the identification in the CNS of a previously uncharacterized VGF-derived peptide and prove that its chronic i.c.v. infusion effected an increase in EE and limited the early phase of diet-induced obesity.

Sympathetic but not sensory denervation stimulates white adipocyte proliferation

White adipocyte proliferation is a hallmark of obesity, but it largely remains a mechanistic mystery. We and others previously demonstrated that surgical denervation of white adipose tissue (WAT) triggers increases in fat cell number, but it is unknown whether this was due to preadipocyte proliferation or maturation of existing preadipocytes that allowed them to be counted. In addition, surgical denervation severs not only sympathetic but also sensory innervation of WAT. Therefore, we tested whether sympathetic WAT denervation triggers adipocyte proliferation using 5-bromo-2'-deoxyuridine (BrdU) as a marker of proliferation and quantified BrdU-immunoreactive (ir) cells that were co-labeled with AD-3-ir, an adipocyte-specific membrane protein marker. The unilateral denervation model was used for all experiments where Siberian hamster inguinal WAT (IWAT) was unilaterally denervated, the contralateral pad was sham denervated serving as a within-animal control, and then BrdU was injected systemically for 6 days. When IWAT was surgically denervated, severing both sympathetic and sensory nerves, tyrosine hydroxylase (TH)-ir, a sympathetic nerve marker, and calcitonin gene-related peptide (CGRP)-ir, a sensory nerve marker, were significantly decreased, and BrdU+AD-3-ir adipocytes were increased approximately 300%. When IWAT was selectively sensory denervated via local microinjections of capsaicin, a sensory nerve-specific toxin, CGRP-ir, but not TH-ir, was decreased, and BrdU+AD-3-ir adipocytes were unchanged. When IWAT was selectively sympathetically denervated via local microinjections of 6-hydroxy-dopamine, a catecholaminergic-specific toxin, TH-ir, but not CGRP-ir, was significantly decreased, and BrdU+AD-3-ir adipocytes were increased approximately 400%. Collectively, these data provide the first direct evidence that sympathetic nerves inhibit white adipocyte proliferation in vivo.

Increased leptin expression in the dorsal vagal complex suppresses adiposity without affecting energy intake and metabolic hormones

OBJECTIVE

Increased leptin transgene expression locally in hypothalamic sites suppresses weight and energy intake, enhances thermogenic energy expenditure, and differentially modulates metabolic hormones for an extended period. We evaluated whether a similar localized expression of leptin transgene in the dorsal vagal complex (DVC) in the caudal brain stem that also displays the biologically relevant leptin receptor would reproduce these varied responses and thus demonstrate functional connectivity between the hypothalamus and DVC.

RESEARCH METHODS AND PROCEDURES

Adult female rats were microinjected with a recombinant adeno-associated virus encoding either rat leptin or green fluorescent protein gene (control) in the DVC. Food intake and body weight were monitored weekly, and metabolic variables were analyzed at the end of 10 weeks.

RESULTS AND DISCUSSION

Increased leptin transgene expression in the DVC suppressed the time-related increase in body weight accompanied by a transient decrease in food intake at week 1 post-injection and little effect on thermogenic energy expenditure. That suppression of weight was due to decreased adiposity is shown by the markedly suppressed white adipose tissue-derived hormones, leptin and adiponectin. Circulating concentrations of pancreatic insulin, gastric ghrelin, and glucose levels were unchanged. This segregation of the varied effects of leptin expression in hypothalamic sites vs. DVC endorses the view that among the various endocrine organs under sympathetic nervous system control, only those leptin-activated neural circuits in the hypothalamus that suppress weight and adiposity on a long-term basis transverse through DVC en route to white adipose tissue.

Subjugation of hypothalamic NPY and cohorts with central leptin gene therapy alleviates dyslipidemia, insulin resistance, and obesity for life-time

An interactive network comprised of neuropeptide Y (NPY) and cohorts is obligatory in the hypothalamic integration of appetite and energy expenditure on a minute-to-minute basis. High or low abundance of NPY and cognate receptors dysregulates the homeostatic milieu engendering hyperphagia, decreased energy expenditure, obesity and attendant metabolic syndrome cluster of dyslipidemia, glucose intolerance, insulin resistance and hyperinsulinemia, risk factors for type II diabetes and cardiovascular diseases. Increasing the supply of the endogenous repressor hormone leptin locally in the hypothalamus with the aid of leptin gene therapy, blocked age-related and dietary obesities, and the sequential development of dyslipidemia, hyperglycemia, and insulin resistance. Thus, sustained repression of NPY signaling with increased leptin selectively in the hypothalamus can avert environmental obesity and the risks of metabolic diseases.

Direct and indirect effects of leptin on preadipocyte proliferation and differentiation

Leptin has been shown to reduce body fat in vivo. Adipocytes express the leptin receptor; therefore, it is realistic to expect a direct effect of leptin on adipocyte growth and metabolism. In vitro studies examining the effect of leptin on adipocyte metabolism require supraphysiological doses of the protein to see a decrease in lipogenesis or stimulation of lipolysis, implying an indirect action of leptin. It also is possible that leptin reduces adipose mass by inhibiting preadipocyte proliferation (increase in cell number) and/or differentiation (lipid filling). Thus we determined direct and indirect effects of leptin on preadipocyte proliferation and differentiation in vitro. We tested the effect of leptin (0-500 ng/ml), serum from leptin-infused rats (0.25% by volume), and adipose tissue-conditioned medium from leptin-infused rats (0-30% by volume) on preadipocyte proliferation and differentiation in a primary culture of cells from male Sprague-Dawley rat adipose tissue. Leptin (50 ng/ml) stimulated proliferation of preadipocytes (P<0.05), but 250 and 500 ng leptin/ml inhibited proliferation of both preadipocyte and stromal vascular cell fractions (P<0.01), as measured by [3H]thymidine incorporation. Serum from leptin-infused rats inhibited proliferation of the adipose and stromal vascular fractions (P=0.01), but adipose tissue-conditioned medium had no effect on proliferation of either cell fraction. None of the treatments changed preadipocyte differentiation as measured by sn-glycerophosphate dehydrogenase activity. These results suggest that leptin could inhibit preadipocyte proliferation by modifying release of a factor from tissue other than adipose tissue.