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

In pancreatic β-cells, mitochondrial bioenergetics control glucose-stimulated insulin secretion. Mitochondrial dynamics are generally associated with quality control, maintaining the functionality of bioenergetics. By acute pharmacological inhibition of mitochondrial fission protein

Uroguanylin is a gastrointestinal hormone primarily involved in fluid and electrolyte handling. It has recently been reported that prouroguanylin, secreted postprandially, is converted to uroguanylin in the brain and activates the receptor guanylate cyclase-C (GC-C) to reduce food intake and prevent obesity. We tested central nervous system administration of two GC-C agonists and found no significant reduction of food intake. We also carefully phenotyped mice lacking the GC-C receptor and found them to have normal body weight, adiposity, and glucose tolerance. Interestingly, uroguanylin knockout mice had a small but significant increase in body weight and adiposity that was accompanied by glucose intolerance. Our data indicate that the modest effects of uroguanylin on energy and glucose homeostasis are not mediated by central GC-C receptors.

In the article listed above, ERX was erroneously omitted from the duality of interest for Matthias H. …

Although obesity rates are rapidly rising, caloric restriction remains one of the few safe therapies. Here we tested the hypothesis that obesity-associated disorders are caused by increased adipose tissue as opposed to excess dietary lipids. Fat mass (FM) of lean C57B6 mice fed a high-fat diet (HFD; FMC mice) was “clamped” to match the FM of mice maintained on a low-fat diet (standard diet [SD] mice). FMC mice displayed improved glucose and insulin tolerance as compared with ad libitum HFD mice (P < 0.001) or SD mice (P < 0.05). These improvements were associated with fewer signs of inflammation, consistent with the less-impaired metabolism. In follow-up studies, diet-induced obese mice were food restricted for 5 weeks to achieve FM levels identical with those of age-matched SD mice. Previously, obese mice exhibited improved glucose and insulin tolerance but showed markedly increased fasting-induced hyperphagia (P < 0.001). When mice were given ad libitum access to the HFD, the hyperphagia of these mice led to accelerated body weight gain as compared with otherwise matched controls without a history of obesity. These results suggest that although caloric restriction on a HFD provides metabolic benefits, maintaining those benefits may require lifelong continuation, at least in individuals with a history of obesity.

A peptide designed to induce apoptosis of endothelium in white adipose tissue (WAT) decreases adiposity. The goal of this work is to determine whether targeting of WAT endothelium results in impaired glucose regulation as a result of impaired WAT function. Glucose tolerance tests were performed on days 2 and 3 of treatment with vehicle (HF-V) or proapoptotic peptide (HF-PP) and mice pair-fed to HF-PP (HF-PF) in obese mice on a high-fat diet (HFD). Serum metabolic variables, including lipid profile, adipokines, individual fatty acids, and acylcarnitines, were measured. Microarray analysis was performed in epididymal fat of lean or obese mice treated with vehicle or proapoptotic peptide (PP). PP rapidly and potently improved glucose tolerance of obese mice in a weight- and food intake–independent manner. Serum insulin and triglycerides were decreased in HF-PP relative to HF-V. Levels of fatty acids and acylcarnitines were distinctive in HF-PP compared with HF-V or HF-PF. Microarray analysis in AT revealed that pathways involved in mitochondrial dysfunction, oxidative phosphorylation, and branched-chain amino acid degradation were changed by exposure to HFD and were reversed by PP administration. These studies suggest a novel role of the AT vasculature in glucose homeostasis and lipid metabolism.

OBJECTIVE Cholecystokinin (CCK) is released in response to lipid intake and stimulates insulin secretion. We hypothesized that CCK deficiency would alter the regulation of insulin secretion and glucose homeostasis. RESEARCH DESIGN AND METHODS We used quantitative magnetic resonance imaging to determine body composition and studied plasma glucose and insulin secretion of CCK gene knockout (CCK-KO) mice and their wild-type controls using intraperitoneal glucose and arginine infusions. The area of anti-insulin staining in pancreatic islets was measured by immunohistochemistry. Insulin sensitivity was assessed with euglycemic-hyperinsulemic clamps. RESULTS CCK-KO mice fed a low-fat diet had a reduced acute insulin response to glucose but a normal response to arginine and normal glucose tolerance, associated with a trend toward greater insulin sensitivity. However, when fed a high-fat diet (HFD) for 10 weeks, CCK-KO mice developed glucose intolerance despite increased insulin sensitivity that was associated with low insulin secretion in response to both glucose and arginine. The deficiency of insulin secretion in CCK-KO mice was not associated with changes in β-cell or islet size. CONCLUSIONS CCK is involved in regulating insulin secretion and glucose tolerance in mice eating an HFD. The impaired insulin response to intraperitoneal stimuli that do not typically elicit CCK release suggests that this hormone has chronic effects on β-cell adaptation to diet in addition to acute incretin actions.

OBJECTIVE—Evidence links the hypothalamic fatty acid synthase (FAS) pathway to the regulation of food intake and body weight. This includes pharmacological inhibitors that potently reduce feeding and body weight. The mammalian target of rapamycin (mTOR) is an intracellular fuel sensor whose activity in the hypothalamus is also linked to the regulation of energy balance. The purpose of these experiments was to determine whether hypothalamic mTOR complex 1 (mTORC1) signaling is involved in mediating the effects of FAS inhibitors. RESEARCH DESIGN AND METHODS—We measured the hypothalamic phosphorylation of two downstream targets of mTORC1, S6 kinase 1 (S6K1) and S6 ribosomal protein (S6), after administration of the FAS inhibitors C75 and cerulenin in rats. We evaluated food intake in response to FAS inhibitors in rats pretreated with the mTOR inhibitor rapamycin and in mice lacking functional S6K1 (S6K1−/−). Food intake and phosphorylation of S6K1 and S6 were also determined after C75 injection in rats maintained on a ketogenic diet. RESULTS—C75 and cerulenin increased phosphorylation of S6K1 and S6, and their anorexic action was reduced in rapamycin-treated rats and in S6K1−/− mice. Consistent with our previous findings, C75 was ineffective at reducing caloric intake in ketotic rats. Under ketosis, C75 was also less efficient at stimulating mTORC1 signaling. CONCLUSIONS—These findings collectively indicate an important interaction between the FAS and mTORC1 pathways in the central nervous system for regulating energy balance, possibly via modulation of neuronal glucose utilization.

OBJECTIVE—Glucagon-like peptide-1 (GLP-1) promotes glucose homeostasis through regulation of islet hormone secretion, as well as hepatic and gastric function. Because GLP-1 is also synthesized in the brain, where it regulates food intake, we hypothesized that the central GLP-1 system regulates glucose tolerance as well. RESEARCH DESIGN AND METHODS—We used glucose tolerance tests and hyperinsulinemic-euglycemic clamps to assess the role of the central GLP-1 system on glucose tolerance, insulin secretion, and hepatic and peripheral insulin sensitivity. Finally, in situ hybridization was used to examine colocalization of GLP-1 receptors with neuropeptide tyrosine and pro-opiomelanocortin neurons. RESULTS—We found that central, but not peripheral, administration of low doses of a GLP-1 receptor antagonist caused relative hyperglycemia during a glucose tolerance test, suggesting that activation of central GLP-1 receptors regulates key processes involved in the maintenance of glucose homeostasis. Central administration of GLP-1 augmented glucose-stimulated insulin secretion, and direct administration of GLP-1 into the arcuate, but not the paraventricular, nucleus of the hypothalamus reduced hepatic glucose production. Consistent with a role for GLP-1 receptors in the arcuate, GLP-1 receptor mRNA was found to be expressed in 68.1% of arcuate neurons that expressed pro-opiomelanocortin mRNA but was not significantly coexpressed with neuropeptide tyrosine. CONCLUSIONS—These data suggest that the arcuate GLP-1 receptors are a key component of the GLP-1 system for improving glucose homeostasis by regulating both insulin secretion and glucose production.

OBJECTIVE—The worldwide prevalence of obesity is increasing at an alarming rate, along with the associated increased rates of type 2 diabetes, heart disease, and some cancers. While efforts to address environmental factors responsible for the recent epidemic must continue, investigation into the anorectic functions of potential molecules we present here, such as apolipoprotein (apo)E, offers exciting possibilities for future development of successful anti-obesity therapies. RESEARCH DESIGN AND METHODS—Changes in feeding behavior after intracerebroventricular injection of apoE, the regulation of hypothalamic apoE gene expression by energy status, and the interaction of hypothalamic apoE with other neuropeptides were studied. RESULTS—Intracerebroventricular apoE significantly decreased food intake without causing malaise, whereas intracerebroventricular infusion of apoE antiserum stimulated feeding, implying that endogenous apoE tonically inhibits food intake. Consistent with this, apoE was present in the hypothalamus, a brain site intimately involved in the integration of signals for energy homeostasis. Fasted rats exhibited significantly decreased apoE gene expression in the hypothalamus, and refeeding of these rats for 4 h evoked a significant increase of hypothalamic apoE mRNA levels. Both genetically obese (ob/ob) mice and rats with high-fat diet–induced obesity had significantly reduced hypothalamic apoE mRNA levels compared with their lean control counterparts, suggesting that decreased apoE may contribute to hyperphagia in these obese animals. Additionally, apoE-stimulated hypothalamic proopiomelanocortin gene expression and SHU9119, a melanocortin 3/4 receptor antagonist, attenuated the inhibitory function of apoE on feeding. CONCLUSIONS—These data demonstrate that apoE suppresses food intake via a mechanism enhancing melanocortin signaling in the hypothalamus.

Peptide signals from the pancreatic islets and the gastrointestinal tract influence the regulation of energy homeostasis by the brain, and the brain in turn influences the secretions of both the islets and the gut. This article focuses on how insulin interacts with the brain to influence food intake, blood glucose, and cognitive behavior. Insulin is secreted in response to changes of ambient glucose, and the levels achieved are directly proportional to body adiposity. Hence, insulin, like leptin, is an adiposity signal. An increased insulin signal in the mediobasal hypothalamus indicates that ample or excess energy is available in the body and elicits responses that limit food intake and reduce hepatic glucose secretion. Increased insulin (and leptin as well) locally within the brain complements other signals that indicate a surfeit of energy in the body, including satiety signals generated by the gut during meals, glucose, and some fatty acids. There is compelling evidence that overlapping intracellular signaling pathways within the mediobasal hypothalamus mediate the overall catabolic response to these diverse metabolic signals. Insulin receptors are also densely expressed in the hippocampus, and insulin acts there to facilitate learning and memory. The function of insulin receptors in other brain areas is poorly understood. Obesity and/or the consumption of diets high in fat render the brain as well as the body insulin resistant. In the hypothalamus, this is manifest as a reduced ability of insulin to reduce food intake and body weight, and in the hippocampus, it is manifest as a reduced ability of insulin to improve learning and/or memory.

Males have proportionally more visceral fat and are more likely to develop complications associated with obesity than females, and the male brain is relatively more sensitive to the catabolic action of insulin and less sensitive to that of leptin than the female brain. To understand the underlying mechanism, we manipulated estrogen through ovariectomy (OVX) and estradiol administration. Rats with relatively high systemic estrogen (intact females and OVX females and males administered estrogen subcutaneously) were significantly more sensitive to leptin’s anorexic action in the brain (i3vt), as well as significantly less sensitive to insulin’s i3vt action, than intact males. Administering estradiol directly into the brain of our females increased i3vt leptin sensitivity while decreasing i3vt insulin sensitivity and changed the body fat distribution of our females to resemble that of intact females. These data indicate that estrogen acts within the brain to increase leptin sensitivity, decrease insulin sensitivity, and favor subcutaneous over visceral fat.

The central melanocortin system is critically involved in the control of food intake and body weight. Administration of melanocortin agonists reduces food intake and adiposity, and the central melanocortin system is demonstrated to mediate the anorexic effects of both leptin and insulin. An important unanswered question has been whether melanocortin agonists would also reduce food intake that is driven by factors other than homeostatic mechanisms (e.g., conditioned eating). In the first experiment, we identified that long-term maintenance on a meal-feeding schedule attenuated rats’ sensitivity to central administration of the melanocortin agonist MTII. The results from a second experiment demonstrate that the attenuation of the MTII-induced anorexia was due to learned schedules of food intake rather than food deprivation per se. Results from the final experiment suggest that this attenuation of MTII-induced anorexia may be independent of the decreased sensitivity caused by a high-fat diet. These results support the hypothesis that meal-feeding schedules can lead to anticipatory physiological responses that attenuate the anorexic effects of exogenous melanocortin agonists.

The distribution of fat in the body differs between the male and female sexes and is associated with the relative secretion of the two "adiposity" hormones leptin and insulin. We now report that the brains of male and female rats are differentially sensitive to the catabolic actions of small doses of these two hormones. Leptin (1 or 3.5 microg/2 microl) or saline (2 microl) was administered into the third cerebral ventricle of age- and weight-matched male and female rats. Leptin significantly reduced food intake in female and male rats over 4 h; however, leptin reduced 24-h intake in female but not in male rats. When the same rats were administered insulin (1 or 4 mU/2 microl) or saline (2 microl), male but not female rats had a robust reduction in food intake over 24 h. Previous research demonstrates the melanocortins are a central mediator of the effects of both leptin and insulin. However, we found no sex differences in sensitivity to the melanocortin agonist MTII (0.01, 0.1, 0.3, and 1.0 nmol/2 microl). These results suggest that the sex differences in sensitivity to leptin and insulin at the doses that we injected occur upstream of the melanocortin receptors. Because insulin and leptin reflect different fat beds and are differentially distributed in the male and female sexes, the implication is that the male and female sexes regulate adiposity-relevant parameters differently.

We describe a model of energy homeostasis to better understand neuronal pathways that control energy balance and their regulation by hormonal signals such as insulin and leptin. Catabolic neuronal pathways are those that both reduce food intake and increase energy expenditure (e.g., melanocortin neurons in the hypothalamic arcuate nucleus) and are stimulated by input from insulin and leptin. We propose that in the basal state, catabolic effectors are activated in response to physiological concentrations of leptin and insulin, and that this activation is essential to prevent excessive weight gain. In contrast, anabolic pathways (e.g., neurons containing neuropeptide Y) are those that stimulate food intake and decrease energy expenditure and are strongly inhibited by these same basal concentrations of insulin and leptin. In the basal state, therefore, catabolic effector pathways are activated while anabolic effector pathways are largely inhibited. The response to weight loss includes both activation of anabolic and inhibition of catabolic pathways and is, thus, inherently more vigorous than the response to weight gain (stimulation of already-activated catabolic pathways and inhibition of already-suppressed anabolic pathways). Teleological, molecular, physiological, and clinical aspects of this hypothesis are presented, along with a discussion of currently available supporting evidence.

Insulin transported from plasma into the central nervous system (CNS) is hypothesized to contribute to the negative feedback regulation of body adiposity. Because CNS insulin uptake is likely mediated by insulin receptors, physiological interventions that impair insulin action in the periphery might also reduce the efficiency of CNS insulin uptake and predispose to weight gain. We hypothesized that high-fat feeding, which both reduces insulin sensitivity in peripheral tissues and favors weight gain, reduces the efficiency of insulin uptake from plasma into the CNS. To test this hypothesis, we estimated parameters for cerebrospinal fluid (CSF) insulin uptake and clearance during an intravenous insulin infusion using compartmental modeling in 10 dogs before and after 7 weeks of high-fat feeding. These parameters, together with 24-h plasma insulin levels measured during ad libitum feeding, also permitted estimates of relative CNS insulin concentrations. The percent changes of adiposity, body weight, and food intake after high-fat feeding were each inversely associated with the percent changes of the parameter k1k2, which reflects the efficiency of CNS insulin uptake from plasma (r = -0.74, -0.69, -0.63; P = 0.015, 0.03, and 0.05, respectively). These findings were supported by a non-model-based calculation of CNS insulin uptake: the CSF-to-plasma insulin ratio during the insulin infusion. This ratio changed in association with changes of k1k2 (r = 0.84, P = 0.002), body weight (r = -0.66, P = 0.04), and relative adiposity (r = -0.72, P = 0.02). By comparison, changes in insulin sensitivity, according to minimal model analysis, were not associated with changes in k1k2, suggesting that these parameters are not regulated in parallel. During high-fat feeding, there was a 60% reduction of the estimated CNS insulin level (P = 0.04), and this estimate was inversely associated with percent changes in body weight (r = -0.71, P = 0.03). These results demonstrate that increased food intake and weight gain during high-fat feeding are associated with and may be causally related to reduced insulin delivery into the CNS.

Leptin, a hormone secreted by adipose tissue in proportion to body adiposity, is proposed to be involved in the central nervous regulation of food intake and body weight. In addition, evidence is emerging that leptin regulates neuroendocrine and metabolic functions as well, presumably via its action in the central nervous system (CNS). To investigate this regulatory effect of leptin, we infused 3.5 μg of human leptin directly into the third cerebral ventricle (i3vt) of lean male Long-Evans rats, 90 min before the onset of their dark phase. Before and after infusion, blood samples were withdrawn through indwelling catheters for assessment of hormonal (plasma corticosterone, insulin, leptin), autonomic (plasma norepinephrine, epinephrine), and metabolic (plasma glucose) parameters. 13vt leptin caused an increase in plasma corticosterone and plasma leptin levels relative to the control condition. The effects of i3vt leptin on corticosterone secretion became particularly apparent after the onset of the dark phase. The results of the present study indicate that i3vt leptin stimulates the hypothalamo-pituitary-adrenal (HPA) axis, particularly when rats normally encounter their largest meals. These results are consistent with the possibility that high circulating leptin levels may underlie the increased activity of the HPA axis that is generally characteristic of human obesity and most animal models of obesity.

OBJECTIVE Obesity and type 2 diabetes are national and worldwide epidemics. Because currently available antiobesity and antidiabetic drugs have limited efficacy and/or safety concerns, identifying new medicinal agents, such as ginsenoside Rb1 (Rb1) as reported here, offers exciting possibilities for future development of successful antiobesity and antidiabetic therapies. RESEARCH DESIGN AND METHODS Changes in feeding behavior after acute intraperitoneal administration of Rb1 and the effects of intraperitoneal Rb1 for 4 weeks on body weight, energy expenditure, and glucose tolerance in high-fat diet (HFD)-induced obese rats were assessed. We also examined the effects of Rb1 on signaling pathways and neuropeptides in the hypothalamus. RESULTS Acute intraperitoneal Rb1 dose-dependently suppressed food intake without eliciting signs of toxicity. This inhibitory effect on feeding may be mediated by central mechanisms because Rb1 stimulated c-Fos expression in brain areas involved in energy homeostasis. Consistent with this, Rb1 activated the phosphatidylinositol 3-kinase/Akt signaling pathway and inhibited NPY gene expression in the hypothalamus. Four-week administration of Rb1 significantly reduced food intake, body weight gain, and body fat content and increased energy expenditure in HFD-induced obese rats. Rb1 also significantly decreased fasting blood glucose and improved glucose tolerance, and these effects were greater than those observed in pair-fed rats, suggesting that although Rb1's antihyperglycemic effect is partially attributable to reduced food intake and body weight; there may be additional effects of Rb1 on glucose homeostasis. CONCLUSIONS These results identify Rb1 as an antiobesity and antihyperglycemic agent.

OBJECTIVE Because adipose tissue is highly vascularized, modifying adipose tissue vasculature may provide a novel method for reducing body fat. A peptide sequence that elicits apoptosis of endothelium in white fat potently reduced body weight. We sought to determine how inhibiting adipose tissue vasculature changes key aspects of energy balance regulation and the neuroendocrine system that maintains energy balance. RESEARCH DESIGN AND METHODS Lean and obese mice or rats were treated with proapoptotic peptide for 4 or 27 days. Daily energy intake and expenditure were measured in mice on a low- (LFD) or high-fat diet (HFD) and in rats on a HFD. A conditioned taste aversion test was performed to assess whether proapoptotic peptide produces visceral illness. Hypothalamic neuropeptide Y, agouti-related peptide, and proopiomelanocoritin (POMC) mRNA expression and plasma leptin levels were evaluated. RESULTS Proapoptotic peptide completely reversed HFD-induced obesity in mice and reduced body weight in mice and rats on a HFD but not in those on a LFD. Fat loss occurred with no change of energy expenditure but reduced food intake that occurred without signs of illness and despite reduced circulating leptin and reduced hypothalamic POMC gene expression, indicating that the decrease in food intake is independent of the action of leptin. CONCLUSIONS These experiments provide compelling evidence for a previously unknown relationship between the status of adipose tissue vasculature and the regulation of food intake.

OBJECTIVE Glucagon-like peptide (GLP)-1 is a regulatory peptide synthesized in the gut and the brain that plays an important role in the regulation of food intake. Both GLP-1 and exendin (Ex)-4, a long-acting GLP-1 receptor (GLP-1r) agonist, reduce food intake when administered intracerebroventricularly, whereas Ex4 is much more potent at suppressing food intake when given peripherally. It has generally been hypothesized that this difference is due to the relative pharmacokinetic profiles of GLP-1 and Ex4, but it is possible that the two peptides control feeding via distinct mechanisms. RESEARCH DESIGN AND METHODS In this study, the anorectic effects of intracerebroventricular GLP-1 and Ex4, and the sensitivity of these effects to GLP-1r antagonism, were compared in rats. In addition, the GLP-1r dependence of the anorectic effect of intracerebroventricular Ex4 was assessed in GLP-1r−/− mice. RESULTS Intracerebroventricular Ex4 was 100-fold more potent than GLP-1 at reducing food intake, and this effect was insensitive to GLP-1r antagonism. However, GLP-1r antagonists completely blocked the anorectic effect of intraperitoneal Ex4. Despite the insensitivity of intracerebroventricular Ex4 to GLP-1r antagonism, intracerebroventricular Ex4 failed to reduce food intake in GLP-1r−/− mice. CONCLUSIONS These data suggest that although GLP-1rs are required for the actions of Ex4, there appear to be key differences in how GLP-1 and Ex4 interact with central nervous system GLP-1r and in how Ex4 interacts with GLP-1r in the brain versus the periphery. A better understanding of these unique differences may lead to expansion and/or improvement of GLP-1–based therapies for type 2 diabetes and obesity.

Ghrelin, the only known orexigenic gut hormone, is secreted mainly from the stomach, increases with fasting and before meal initiation in humans and rats, and increases food intake after central or peripheral administration. To investigate sex differences in the action of ghrelin, we assessed the effects of exogenous ghrelin in intact male and female rats, the effects of exogenous ghrelin in ovariectomized (OVX) and estradiol (E2)-treated female rats, as well as the effects of OVX on plasma ghrelin and hypothalamic orexigneic neuropeptide expression in rats and on food intake and weight gain in transgenic mice lacking the ghrelin receptor (Ghsr−/− mice). Male and OVX female rats were significantly more sensitive than intact female rats to the orexigenic effects of both centrally (intra–third ventricular, i3vt, 0.01, 0.1, and 1.0 nmol) and systemically (ip, 3, 6, and 9 nmol) administered ghrelin. This difference is likely to be estradiol dependent because E2 attenuated the orexigenic action of ghrelin in OVX female and male rats. Furthermore, OVX increased food intake and body weight in wild-type mice, but not in Ghsr−/− mice, suggesting that OVX increases food intake by releasing ghrelin from a tonic inhibitory effect of estradiol. In addition, following OVX, there was an increase in plasma ghrelin that was temporally associated with increased food intake, body weight, and hypothalamic neuropeptide Y and Agouti-related protein mRNA expression. Collectively, these data suggest that estradiol inhibits the orexigenic action of ghrelin in females, that weight gain associated with OVX is ghrelin mediated, and that this endocrine interaction may account for an important sex differences in food intake and the regulation of body weight.

The adipocyte hormone leptin reduces food intake in normal animals. During uncontrolled type 1 diabetes, plasma leptin levels fall, whereas food intake increases. To test the hypothesis that low leptin levels contribute to diabetic hyperphagia, we investigated the effect on food intake of replacement of leptin at basal plasma concentrations for 7 days in Long-Evans rats with uncontrolled diabetes induced by streptozotocin (STZ). One group of STZ diabetic rats received saline (STZ + Sal) (n = 11), while the other group (STZ + Lep) (n = 15) received a subcutaneous infusion of recombinant rat leptin (100 microg x kg(-1) x day(-1)) via osmotic minipumps. A nondiabetic control group (Con) (n = 11) received saline only. In the STZ + Sal group, plasma leptin levels decreased by 75% (P < 0.05) from 2.4+/-0.5 on the day before STZ/citrate buffer vehicle (Veh) injection (day 0) to 0.6+/-0.2 ng/ml on day 7. In contrast, plasma leptin levels on days 3-7 were comparable to pretreatment values in both the STZ + Lep group (day 0: 2.6+/-0.4 vs. day 7: 2.5+/-0.3 ng/ml, NS) and the Con group (day 0: 3.8+/-0.4 vs. day 7: 2.9+/-1.0 ng/ml, NS). In the STZ + Sal group, daily food intake increased gradually to values 43% above basal by day 7 (day 0: 24+/-2 to day 7: 33+/-3 g, P < 0.05), whereas food intake did not increase in either the STZ + Lep group (day 0: 24+/-1 vs. day 7: 21+/-2 g, NS), or the Con group (day 0: 23+/-1 vs. day 7: 23+/-2 g). Plasma glucose levels exceeded nondiabetic control values (7.7+/-0.2 mmol/l) in both diabetic groups, but were lower in the STZ + Lep group (17.2+/-1.8 mmol/l) than in the STZ + Sal group (24.3+/-1.1 mmol/l, P < 0.05). To determine if sensitivity to leptin-induced anorexia was affected by STZ treatment, a second experiment was performed in which the effect of intracerebroventricular leptin injection (at doses of 0.35, 1.0, or 3.5 microg) on food intake was measured 10 days after STZ or Veh treatment. Leptin suppressed both 4- and 24-h food intake in the two groups to an equal extent at every dose (by 15, 22, and 35%, respectively). These findings support the hypothesis that the effect of uncontrolled diabetes to lower leptin levels contributes to diabetic hyperphagia and that this effect is not due to altered leptin sensitivity.

Melanocortins are peptides, cleaved from the pro-opiomelanocortin (POMC) precursor, that act in the brain to reduce food intake and are potential mediators of leptin action. In the forebrain, melanocortins are derived from POMC-containing neurons of the hypothalamic arcuate nucleus. To test the hypothesis that these POMC neurons are regulated by leptin, we used in situ hybridization to determine whether reduced leptin signaling (as occurs in fasting), genetic leptin deficiency (in obese ob/ob mice), or genetic leptin resistance (in obese db/db mice) lower expression of POMC mRNA. We further hypothesized that leptin administration would raise hypothalamic POMC mRNA levels in leptin-deficient animals, but not in mice with defective leptin receptors. In wild-type mice (n = 12), fasting for 48 h lowered POMC mRNA levels in the rostral arcuate nucleus by 53%, relative to values in fed controls (n = 8; P < 0.001). Similarly, arcuate nucleus POMC mRNA levels were reduced by 46 and 70% in genetically obese ob/ob (n = 6) and db/db mice (n = 6), respectively, as compared with wild-type mice (n = 5) (P < 0.01 for both comparisons). Five daily intraperitoneal injections of recombinant murine leptin (150 μg) raised levels of POMC mRNA in the rostral arcuate nucleus of ob/ob mice (n = 8) by 73% over saline-treated ob/ob control values (n = 8; P < 0.01), but was without effect in db/db mice (n = 6). In normal rats, two injections of a low dose of leptin (3.5 μg) into the third cerebral ventricle (n = 15) during a 40-h period of fasting also increased POMC mRNA levels in the rostral arcuate nucleus to values 39% greater than those in vehicle-treated controls (n = 14; P = 0.02). We conclude that reduced central nervous system leptin signaling owing to fasting or to genetic defects in leptin or its receptor lower POMC mRNA levels in the rostral arcuate nucleus. The finding that leptin reverses this effect in ob/ob, but not db/db, mice suggests that leptin stimulates arcuate nucleus POMC gene expression via a pathway involving leptin receptors. These findings support the hypothesis that leptin signaling in the brain involves activation of the hypothalamic melanocortin system.

Correction of the obese state induced by genetic leptin deficiency reduces elevated levels of both blood glucose and hypothalamic neuropeptide Y (NPY) mRNA in ob/ob mice. To determine whether these responses are due to a specific action of leptin or to the reversal of the obese state, we investigated the specificity of the effect of systemic leptin administration to ob/ob mice (n = 8) on levels of plasma glucose and insulin and on hypothalamic expression of NPY mRNA. Saline-treated controls were either fed ad libitum (n = 8) or pair-fed to the intake of the leptin-treated group (n = 8) to control for changes of food intake induced by leptin. The specificity of the effect of leptin was further assessed by 1) measuring NPY gene expression in db/db mice (n = 6) that are resistant to leptin, 2) measuring NPY gene expression in brain areas outside the hypothalamus, and 3) measuring the effect of leptin administration on hypothalamic expression of corticotropin-releasing hormone (CRH) mRNA. Five daily intraperitoneal injections of recombinant mouse leptin (150 μg) in ob/ob mice lowered food intake by 56% (P < 0.05), body weight by 4.1% (P < 0.05), and levels of NPY mRNA in the hypothalamic arcuate nucleus by 42.3% (P < 0.05) as compared with saline-treated controls. Pair-feeding of ob/ob mice to the intake of leptin-treated animals produced equivalent weight loss, but did not alter expression of NPY mRNA in the arcuate nucleus. Leptin administration was also without effect on food intake, body weight, or NPY mRNA levels in the arcuate nucleus of db/db mice. In ob/ob mice, leptin did not alter NPY mRNA levels in cerebral cortex or hippocampus or the expression of CRH mRNA in the hypothalamic paraventricular nucleus (PVN). Leptin administration to ob/ob mice also markedly reduced serum glucose (8.3 ± 1.2 vs. 24.5 ± 3.8 mmol/l; P < 0.01) and insulin levels (7,263 ± 1,309 vs. 3,150 ± 780 pmol/l), but was ineffective in db/db mice. Pair-fed mice experienced reductions of glucose and insulin levels that were < 60% of the reduction induced by leptin. The results suggest that in ob/ob mice, systemic administration of leptin inhibits NPY gene overexpression through a specific action in the arcuate nucleus and exerts a hypoglycemic action that is partly independent of its weight-reducing effects. Furthermore, both effects occur before reversal of the obesity syndrome. Defective leptin signaling due to either leptin deficiency (in ob/ob mice) or leptin resistance (in db/db mice) therefore leads directly to hyperglycemia and the overexpression of hypothalamic NPY that is implicated in the pathogenesis of the obesity syndrome.

The effect of cerebral intraventricular insulin on pancreatic insulin secretion was investigated. An extracorporeal pancreatic blood circuit was established after laparotomy to monitor blood flow and insulin concentration directly from the superior pancreaticoduodenal vein. Phentolamine was infused throughout (0.2 mg./min. intravenously) to block alpha-adrenergic effects of any catecholamine secretion induced by surgical stress. Glucose (1.5 mg./kg./min. intravenously) was infused to maintain a constant baseline stimulation of insulin secretion. Six dogs received insulin and six control dogs received saline through a spinal needle stereotaxically placed into the left lateral cerebral ventricle. After central injection of insulin (0.2 U./kg.) there was a significant increase of pancreatic output as early as five minutes. It is concluded that the pancreatic beta-cells are under the influence of insulin-sensitive cells of the CNS.

Celastrol, a plant-derived constituent of traditional Chinese medicine, has been proposed to offer significant potential as an antiobesity drug. However, the molecular mechanism for this activity is unknown. We show that the weight-lowering effects of celastrol are driven by decreased food consumption. Although young Lepob mice respond with a decrease in food intake and body weight, adult Lepdb and Lepob mice are unresponsive to celastrol, suggesting that functional leptin signaling in adult mice is required to elicit celastrol’s catabolic actions. Protein tyrosine phosphatase 1 (PTP1B), a leptin negative-feedback regulator, has been previously reported to be one of celastrol’s targets. However, we found that global PTP1B knockout (KO) and wild-type (WT) mice have comparable weight loss and hypophagia when treated with celastrol. Increased levels of uncoupling protein 1 (UCP1) in subcutaneous white and brown adipose tissue suggest celastrol-induced thermogenesis as a further mechanism. However, diet-induced obese UCP1 WT and KO mice have comparable weight loss upon celastrol treatment, and celastrol treatment has no effect on energy expenditure under ambient housing or thermoneutral conditions. Overall, our results suggest that celastrol-induced weight loss is hypophagia driven and age-dependently mediated by functional leptin signaling. Our data encourage reconsideration of therapeutic antiobesity strategies built on leptin sensitization.

The intracisternal administration of insulin (0.2 U./kg.) to anesthetized dogs resulted in an increase of arterial immunoreactive insulin and a decrease of plasma glucose relative to a control injection. The arterial responses were significantly attenuated when the insulin was administered to the cisternum of subdiaphragmatically vagotomized dogs. When cerebrospinal fluid glucose was lowered by injecting pneumococcal neuraminidase intracisternally, no peripheral hyperinsulinemia resulted, indicating that increased spinal fluid insulin and its consequent increase of glucose uptake, rather than decreased spinal fluid glucose, is necessary to elicit the vagally mediated insulin secretion and hypoglycemia. It is hypothesized that increased spinal fluid insulin causes an increased glucose uptake of some glucoregulatory area of the brain and that the elicited reflex is vagally mediated pancreatic insulin secretion.

SUMMARY Rats with electrolytic lesions of the ventromedial hypothalamus (VMX rats) and sham-operated controls (SHAM rats) were injected with streptozotocin (STZ) at a dose of 50 mg/kg 48 h after the lesions were made. VMX rats were significantly more sensitive to STZ in that over 70% died within 6 wk, whereas none of the SHAM rats died. When smaller doses of STZ were given to VMX rats (30–35 mg/kg), a large percentage still died, although the survivors appeared equally as diabetic (in terms of hyperglycemia and hypoinsulinemia) as SHAM rats given a larger dose of STZ. At 25 mg/kg, the surviving VMX rats were more hyperglycemic than matched controls. We suggest that the increased B-cell activity known to occur in VMX animals might be the important factor in the increased sensitivity to STZ observed. We speculate that similar variation in pancreatic B-cell response to an environmental injury may be an important determinant of diabetes susceptibility in man.