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Continuous (4 days) intracerebroventricular leptin infusion (12 microg/day) was performed in lean rats, and its hormonometabolic effects were determined. Intracerebroventricular leptin administration did not result in leakage of the hormone into the peripheral circulation. Thus, its effects were elicited by its presence within the central nervous system. Intracerebroventricular leptin infusion produced marked decreases in food intake and body weight gain relative to vehicle-infused fed ad libitum rats. Because decreases in food intake alter hormonometabolic homeostasis, additional control rats pair-fed to the amount of food consumed by leptin-infused ones were included in the study. Intracerebroventricular leptin-infused and vehicle-infused pair-fed rats were characterized, relative to vehicle-infused ad libitum-fed animals, by decreases in body weight and insulinemia and by increases in insulin-stimulated overall glucose utilization and muscle and brown adipose tissue glucose utilization index. Brown adipose tissue uncoupling protein (UCP)1, UCP2, and UCP3 mRNA levels were markedly decreased in pair-fed animals relative to those of fed ad libitum control animals, as were liver and white adipose tissue UCP2 and muscle UCP3 mRNA levels. In marked contrast, intracerebroventricular leptin administration was accompanied by the maintenance of high UCP1, UCP2, and UCP3 expression in all these tissues. Thus, despite analogies between leptin's effects and those of pair-feeding with regard to glucose handling, their respective underlying mechanisms differ. While leptin maintains or favors energy-dissipating mechanisms (UCP1, UCP2, and UCP3), the latter are markedly depressed in pair-fed rats. This effect of leptin may prevent subsequent excessive storage processes, thereby maintaining normal body homeostasis.

Leptin is thought to exert its actions on energy homeostasis through the long form of the leptin receptor (OB-Rb), which is present in the hypothalamus and in certain peripheral organs, including adipose tissue. In this study, we examined whether leptin has direct effects on the function of brown and white adipose tissue (BAT and WAT, respectively) at the metabolic and molecular levels. The chronic peripheral intravenous administration of leptin in vivo for 4 d resulted in a 1.6-fold increase in the in vivo glucose utilization index of BAT, whereas no significant change was found after intracerebroventricular administration compared with pair-fed control rats, compatible with a direct effect of leptin on BAT. The effect of leptin on WAT fat pads from lean Zucker Fa/ fa rats was assessed ex vivo, where a 9- and 16-fold increase in the rate of lipolysis was observed after 2 h of exposure to 0.1 and 10 nM leptin, respectively. In contrast, no increase in lipolysis was observed in the fat pads from obese fa/fa rats, which harbor an inactivating mutation in the OB-Rb. At the level of gene expression, leptin treatment for 24 h increased malic enzyme and lipoprotein lipase RNA 1.8+/-0.17 and 1.9+/-0.14-fold, respectively, while aP2 mRNA levels were unaltered in primary cultures of brown adipocytes from lean Fa/fa rats. Importantly, however, no significant effect of leptin was observed on these genes in brown adipocytes from obese fa/fa animals. The presence of OB-Rb receptors in adipose tissue was substantiated by the detection of its transcripts by RT-PCR, and leptin treatment in vivo and in vitro activated the specific STATs implicated in the signaling pathway of the OB-Rb. Taken together, our data strongly suggest that leptin has direct effects on BAT and WAT, resulting in the activation of the Jak/STAT pathway and the increased expression of certain target genes, which may partially account for the observed increase in glucose utilization and lipolysis in leptin-treated adipose tissue.

Plasma lactate is elevated in many physiological and pathological conditions, such as physical exercise, obesity, and diabetes, in which a reduction of insulin sensitivity is also present. Furthermore, an increased production of lactate from muscle and adipose tissue together with increased gluconeogenic substrate flux to the liver plays a primary role in enhancing hepatic glucose production (HGP) in diabetes. It has been shown that lactate may interfere with the utilization and oxidation of other substrates such as free fatty acids (FFAs). The aim of this study was to investigate if lactate infusion affects peripheral glucose utilization in rats. Animals were acutely infused with lactate to achieve a final lactate concentration of 4 mmol/L. They were then submitted to a euglycemic-hyperinsulinemic clamp to study HGP and overall glucose metabolism (rate of disappearance [Rd]). At the end of the clamp, a bolus of 2-deoxy-(1-3H)-glucose was injected to study insulin-dependent glucose uptake in different tissues. The results show that lactate infusion did not affect HGP either in the basal state or at the end of clamp, whereas glucose utilization significantly decreased in lactate-infused rats (26.6 ± 1.1 v 19.5 ± 1.4 mg · kg−1 · min−1, P < .01). A reduction in the tissue glucose utilization index was noted in heart (18.01 ± 4.44 v 46.21 ± 6.51 ng · mg−1 · min−1, P < .01), diaphragm (5.56 ± 0.74 v 9.01 ± 0.93 ng · mg−1 · min−1, P < .01), soleus (13.62 ± 2.29 v 34.05 ± 6.08 ng · mg−1 · min−1, P < .01), and red quadricep (4.43 ± 0.73 v 5.88 ± 0.32 ng · mg−1 · min−1, P < .05) muscle in lactate-infused animals, whereas no alterations were observed in other muscles or in adipose tissue. Therefore, we suggest that acute lactate infusion induces insulin resistance in the heart and some muscles, thus supporting a role for lactate in the regulation of peripheral glucose metabolism.

The aim of this work was to determine the possible inter-relationship between neuropeptide Y (NPY, a hypothalamic stimulator of feeding) and adipose tissue expression of the ob protein (a novel potent inhibitor of feeding). Such a relationship could be of importance in the maintenance of normal body weight. To this end, normal rats were intracerebro-ventricularly (i.c.v.) infused for 6 days with NPY. NPY infusion resulted in hyperphagia and a marked increase in adipose tissue ob mRNA levels. The effect of NPY on ob expression persisted when hyperphagia was prevented by pair-feeding, and was reversed following cessation of NPY infusion. Basal and glucose-stimulated insulinaemia were increased by i. c. v. NPY infusion compared to control values, regardless of whether animals were ad libitum-fed or pair-fed. Cessation of NPY infusion was accompanied by normalisation of insulinaemia. These changes in insulinaemia produced by i. c. v. NPY infusion paralleled the observed changes in ob expression. When normal rats were made hyperinsulinaemic-euglycaemic for 24 h, such hyperinsulinaemia also resulted in increased ob mRNA levels in white adipose tissue. This suggested that NPY-induced hyperinsulinaemia could be responsible for the upregulation of ob mRNA levels of NPY-infused rats. It is concluded that central (i. c. v.) NPY infusion increases adipose tissue ob expression, a functional relationship that is linked, at least in part, via NPY-induced hyperinsulinaemia.

The product of the ob gene, leptin, is a hormone secreted by adipose tissue that acts in the hypothalamus to regulate the size of the body fat depot. Its central administration has been shown to decrease food intake and body weight, while favoring energy dissipation. As glucocorticoids are known to play a permissive role in the establishment and maintenance of obesity syndromes in rodents, it was hypothesized that they do so by restraining the effect of leptin. Leptin injected intracerebroventricularly as a bolus of 3 μg in normal rats induced modest reductions in body weight and food intake. In marked contrast, the same dose of leptin had very potent and long-lasting effects in decreasing both body weight and food intake when administered to adrenalectomized rats. Further, glucocorticoid supplementation of adrenalectomized rats dose-dependently inhibited these potent effects of leptin. These data suggest that glucocorticoids play a key inhibitory role in the action of leptin. Under normal conditions, this inhibitory influence of glucocorticoids may prevent lasting hypophagia. In obesity with degrees of hypercorticism, it may contribute to “leptin resistance,” whose etiology is still little understood.

We have shown previously that continuous (6 days) intracerebroventricular (ICV) leptin infusion in normal rats resulted in decreases in food intake and body weight. A reduction of food intake imposed on control rats (pair-feeding), aimed at mimicking leptin-induced hyperphagia, produced a marked decrease in the expression of muscle uncoupling protein-3 (UCP-3), whereas ICV infusion of leptin prevented such a decrease in UCP-3. To investigate an involvement of thyroid hormones in this effect of leptin, plasma levels of these hormones were determined in ICV leptin-infused, ICV vehicle-infused ad libitum fed or pair-fed controls. ICV leptin infusion and pair-feeding resulted in decreased plasma thyroid-stimulating hormone (TSH) and T4 levels relative to ad libitum fed controls. ICV leptin infusion maintained plasma levels of T3, but the levels were decreased by pair-feeding. The activity of the enzyme (hepatic 5'-monodeiodinase) responsible for T4/T3 conversion was measured. In the leptin-infused group, the activity of 5'-monodeiodinase was maintained at the values measured in ad libitum fed rats; in pair-fed rats, activity was reduced. Thus, conversion of T4 to T3 is decreased by pair-feeding, whereas such is not the case during leptin infusion. To further substantiate an involvement of thyroid hormones in the effect of leptin on muscle UCP-3 expression, hypothyroid rats were ICV infused with leptin or vehicle. It was observed that in hypothyroid rats, ICV leptin was unable to maintain muscle UCP-3 expression at values measured in ad libitum fed controls. These results suggest that central leptin stimulates T3 production via an activation of T4 to T3 conversion, and that this stimulation could be responsible for the effect of leptin on muscle UCP-3 expression. Thyroid hormones could thus be important mediators of the effect of leptin on energy expenditure.

Centrally administered leptin has been shown to increase insulin-stimulated glucose utilization and to favor the expression of uncoupling proteins (UCPs). To study if leptin also has direct peripherally mediated effects on these processes, this hormone (1 mg/day) or its vehicle was infused i.v. for 4 days to lean rats and insulin-stimulated glucose utilization in skeletal muscle and adipose tissue as well as the expression of UCP messenger RNAs (mRNAs) in brown adipose tissue were measured. I.v. leptin administration resulted in decreases in food intake (31%), body weight gain, and plasma insulin levels (45%), in increases in overall (23%) as well as brown adipose tissue and muscle glucose utilization, and in decreases in white adipose tissue glucose uptake. Most of these changes were mimicked, in control rats, by giving them the same amount of food as that consumed by the leptin-infused group (pair-feeding). I.v. leptin infusion also favored the expression of UCPs in brown adipose tissue, either by increasing their expression or preventing the fall occurring during the pair-feeding regimen. Relative UCP expression levels were 100, 104, and 33 for UCP1, 100, 191, and 125 for UCP2 and 100, 107, and 29 for UCP3 in ad libitum fed control rats, in leptin-treated rats and in pair-fed control rats, respectively. These results suggest that the overall effect of leptin on glucose utilization and on the expression of UCPs may be mediated through central mechanism.

It has been reported that hyperphagia and excessive body weight gain of genetically obese rodents were abolished by adrenalectomy. High hypothalamic levels of neuropeptide Y (NPY) were found in obese rodents. A chronic intracerebroventricular (icv) infusion of NPY in normal rats was shown to produce most hormono-metabolic abnormalities of genetically obese animals, and to be inefficient in doing so in adrenalectomized (ADX) rats. The combined presence of NPY and of glucocorticoids thus appeared to be necessary for inducing obesity. This study, therefore, was aimed at determining the consequences of a chronic i.c.v. NPY infusion in ADX rats receiving or not i.c.v. glucocorticoids. It was found that the combined i.c.v. infusion of NPY and dexamethasone in ADX rats increased food intake, body weight, plasma insulin, leptin, and triglyceride levels relative to vehicle-infused ADX controls. The infusion of NPY alone, or of dexamethasone alone in ADX rats failed to produce these effects. In contrast, the icv infusion of NPY alone greatly decreased the expression of brown adipose tissue uncoupling protein-1 and -3. This was not modified by the superimposed infusion of dexamethasone. It is concluded that, although many of centrally elicited NPY effects require the central presence of glucocorticoids, those bearing on the inhibition of uncoupling proteins expression (energy dissipation) do not require central glucocorticoids.

It has been claimed that factors favoring the development or maintenance of animal or human obesity may include increases in glucocorticoid production or hyperresponsiveness of the hypothalamic-pituitary-adrenal axis. In normal rats, glucocorticoids have been shown to be necessary for chronic intracerebroventricular infusion of neuropeptide Y to produce obesity and related abnormalities. Conversely, glucocorticoids inhibited the body weight-lowering effect of leptin. Such dual action of glucocorticoids may occur within the central nervous system, since both neuropeptide Y and leptin act within the hypothalamus. The aim of this study was to determine the effects of glucocorticoids (dexamethasone) given intracerebroventricularly to normal rats on body weight homeostasis and hypothalamic levels of neuropeptide Y and corticotropin-releasing hormone. Continuous central glucocorticoid infusion for 3 days resulted in marked sustained increases in food intake and body weight relative to saline-infused controls. The infusion abolished endogenous corticosterone output and produced hyperinsulinemia, hypertriglyceridemia, and hyperleptinemia, three salient abnormalities of obesity syndromes. Central glucocorticoid infusion also produced a marked decrease in the expression of uncoupling protein (UCP)-1 and UCP-3 in brown adipose tissue and UCP-3 in muscle. Finally, chronic central glucocorticoid administration increased the hypothalamic levels of neuropeptide Y and decreased those of corticotropin-releasing hormone. When the same dose of glucocorticoids was administered peripherally, it resulted in decreases in food intake and body weight, in keeping with the decrease in hypothalamic neuropeptide Y levels. These results suggest that glucocorticoids induce an obesity syndrome in rodents by acting centrally and not peripherally.

Over the years, the work of research laboratories in Baton Rouge (USA), Seattle (USA) and Geneva (Switzerland) have reached analogous conclusions regarding the main etiology of obesity as studied in animals: it largely lies within the brain, notably within the hypothalamus. The hypothalamus is indeed known to modulate food intake and energy partitioning, while the periphery has also been proposed to feed-back on the central nervous system (CNS) to provide information on the state of body energy stores, the two together constituting a loop system connecting the brain to the periphery (1,2,3). This etiologic viewpoint of a pivotal role of the hypothalamus in obesity syndromes has been strengthened by the discovery of one hypothalamic neuropeptide and one peripheral (adipose tissue) hormone, respectively neuropeptide Y (4), and quite particularly, leptin (5). As neuropeptide Y produces hyperphagia (6, 7) and as leptin produces hypophagia in normal animals (8,9,10), the loop system just mentioned was thought to comprise functional relationships, at least between these two factors. Other evidence also suggested that such a loop system was altered in obese animals.

Obesity and non-insulin-dependent diabetes are estimated to affect millions of people in the world. This pathology is multifactorial, comprising complex interactions of genetic and environmental factors and lacking a specific therapy. Great interest arose from the recent discovery of the ob gene expressed only in adipose tissue and coding for a protein that appears to regulate adiposity, potentially by acting as a satiety factor. We report here that in normal rats, ob mRNA is respectively up- or downregulated by a rise in insulinemia (induced by 2-day insulin infusion while maintaining euglycemia) or a decrease in insulinemia (induced by a 3-day fast). Our results also show that in genetically obese fa/fa rats studied longitudinally, white adipose tissue ob mRNA levels increase in parallel with early occurringand steadily increasing hyperinsulinemia. This results in adult obese animals having markedly higher ob mRNA levels than age-matched normoinsulinemic lean rats. Furthermore, in adult obese rats, ob mRNA escapes down-regulation as normalization of hyperinsulinemia due to fasting fails to reduce the high ob mRNA levels.

The objective of this study was to investigate in vivo the effects of various durations of hyperinsulinemia on inguinal white adipose tissue lipoprotein lipase (LPL). Adult genetically-obese (fa/fa) rats were used as a model of chronic hyperinsulinemia. Normal rats infused with insulin as well as with glucose to maintain euglycemia for four days were used as a model of short-term hyperinsulinemia. Normal rats studied during a euglycemic-hyperinsulinemic clamp were used as a model of acute effects of hyperinsulinemia. The levels of LPL mRNA, LPL protein and LPL total activity in inguinal white adipose tissue were measured. In both chronic and long-term hyperinsulinemia, a marked increase in total LPL activity was observed which was associated with an increase in tissue LPL protein abundance. After five hours of hyperinsulinemia, as achieved during the clamps, only total LPL activity was increased. An increase in LPL mRNA was measured only in adipose tissue of obese rats. The differences observed in the regulation of LPL mRNA between insulin-treated rats and genetically-obese animals may be explained either by the large increase in adipose tissue glucose metabolism of insulin-treated rats compared to obese rats and/or by other factors that may be present in genetically-obese (fa/fa) rats.

Chronic intracerebroventricular (icv) administration of neuropeptide-Y (NPY; 10 fig/day) was performed in normal female rats to investigate its hormonal and metabolic consequences. Intracerebroventricular NPY produced hyperphagia, increased basal insulinemia, as well as liver and adipose tissue lipogenic activity. It also increased basal morning corticosteronemia. When NPY-induced hyperphagia was prevented by pair-feeding, the icv NPY treatment resulted in the same increases in basal insulinemia and corticosteronemia, and liver and white adipose tissue lipogenesis was still higher than that in respective controls. Under the ad libitum and pair-feeding conditions, icv NPY stimulated glucose uptake as well as total lipoprotein lipase activity in white adipose tissue; it resulted in an increased total activity of hepatic and white adipose tissue acetyl coenzyme-A-carboxylase. As all hormonal and metabolic changes elicited by icv NPY remained present (at the same or to a lesser extent depending upon the parameter considered) when hyperphagia was prevented by pair-feeding, it was, thus, shown that icv NPY per se induces peripheral hormonal and metabolic alterations via efferent routes, which remain to be determined. The effects of icv NPY reported in this study are similar to the defects observed in the early phase of genetic obesity in rodents, the hypothalamus of which has increased NPY levels. NPY could, thus, be of relevance in the occurrence of genetically induced obesity. (Endocri

The impact of hyperinsulinemia on the establishment of insulin resistance was investigated. This was done by treating normal rats with insulin for 3-4 days via osmotic minipumps, and by comparing them with saline-treated controls. Hyperinsulinemia produced by prior insulin treatment (i.e. prior insulinization of the normal rats) resulted in a well tolerated hypoglycemia, increased food intake and body weight gain. Euglycemic-hyperinsulinemic clamps were carried out at the end of the insulinization to assess the acute effects of insulin in control and insulinized rats. It was found that prior insulinization of normal rats resulted in increases in total insulin-stimulated glucose utilization and hepatic lipogenesis, while hepatic glucose production (HGP) was normally suppressed by the hormone. Glucose utilization index by individual tissues was then measured (labelled 2-deoxy-D-glucose method). Prior insulinization of normal rats resulted in increased insulin-stimulated glucose utilization index of white adipose tissue, accompanied by increased insulin-stimulated de novo lipogenesis and glycogen synthesis. In contrast, prior insulinization of normal rats resulted in a decreased insulin-stimulated glucose utilization index of most muscles studied. The decreased insulin-stimulated muscle glucose utilization index brought about by prior insulinization persisted in adrenomedullectomized or propranolol-treated rats, ruling out a role of catecholamines in the effects observed. It is concluded that hyperinsulinemia is a pathological driving force in producing both incipient obesity by overstimulating white adipose tissue and liver metabolic activity, and concomitantly producing incipient muscle insulin resistance.

The relative contribution of hyperglycaemia and hypoinsulinaemia was evaluated in rats made diabetic by streptozotocin administration. Four groups of rats were studied: untreated normal rats; streptozotocin-diabetic; streptozotocin-diabetic treated with phlorizin (0.4 mg/kg body weight per day); streptozotocin-diabetic mildly treated with insulin (0.7 IU/day). In all groups, insulin action (responsiveness) was assessed with the euglycaemic (5.3 mmol/l) hyperinsulinaemic (524 mU/l) clamp technique combined with 3H-2-deoxy-D-glucose method, enabling determination of the glucose utilization index in various tissues. Responsiveness of the overall glucose utilization process to insulin was reduced by 28% in streptozotocin-diabetic rats (12.0±1.2 vs 16.5±0.6 mg·kg−1·min−1, p

To mimick a state of hyperinsulinemia, normal rats were infused with insulin for 4 days via minipumps, and compared to saline infused rats. At the end of the experimental period, the abundance of mRNA was increased in white adipose tissue (WAT) and decreased in muscles of “insulinized” rats compared to controls. These findings were accompanied, in all tissues considered, except the diaphragm, by parallel changes in the amount of the glucose transporter protein and by parallel changes in the in vivo glucose utilization index. Hyperinsulinemia is thus a driving force in stimulating adipose tissue metabolic activity, while bringing about incipient muscle insulin resistance.

La decouverte du neuropeptide Y et de la leptine, respectivement stimulateur et inhibiteur de la prise alimentaire, a confirme l'existence d'une boucle regulatrice reliant le systeme nerveux central a la peripherie. L'equilibre de leur action respective semble etre module par les glucocorticoides. D'autres modulateurs de la prise alimentaire font probablement partie integrante de cette boucle, en particulier la corticoliberine, inhibitrice de la prise alimentaire et stimulatrice de l'axe hypothalamo-hypophyso-surrenalien. Les relations fonctionnelles entre les differents effecteurs de ce systeme de boucle sont alterees chez les animaux obeses. Chez l'homme, l'obesite est associee a des concentrations elevees de leptine circulante et a une resistance a l'action de la leptine dans laquelle les glucocorticoides pourraient etre impliques. La production de puissants agonistes de la leptine ou d'antagonistes du neuropeptide Y pourrait constituer une approche therapeutique interessante dans le traitement de l'obesite.