Evidence for numerous brown adipocytes lacking functional beta 3-adrenoceptors in fat pads from nonhuman primates

Brown adipose tissue (BAT) is involved in the control of energy balance and has been demonstrated to be activated through beta 3-adrenoceptor (beta 3-AR) occupation in rodents. The ability to specifically activate energy expenditure via this receptor is of great interest for the treatment of obesity. Nevertheless, the extent of BAT and the presence of a functional beta 3-AR in humans are now debated, and this situation is difficult to clarify for evident practical and ethical reasons. We investigated the occurrence of brown adipocytes in fat deposits of prepubertal baboons using antibodies raised against uncoupling protein (UCP) in Western blotting and immunocytology experiments. UCP was detected in all types of fat pads studied and was revealed in multilocular cells. Pericardiac and axillary adipose tissues displayed large amounts of UCP and can be assimilated to typical BAT. Most of the other pads looked like white adipose tissue, but exhibited areas with clusters of brown adipocytes and, thus, can be assimilated to the convertible adipose tissue as previously described in rodents. The presence of beta 3-ARs was evaluated by both beta 2-agonist-stimulated lipolysis and messenger ribonucleic acid (mRNA) expression studies. There was no significant lipolytic effect of any of the beta 3-AR agonists tested (SR 58611A, BRL 37344, CGP 12177, or CL 316243) in either white or brown tissues. PCR analysis demonstrated that beta 3-AR mRNA expression is not related to the UCP content of fat pads and that beta 3-AR expression is low. This study demonstrates the presence of great proportions of brown adipocytes in adipose tissue and the heterogeneity of the fat pads in baboons. The lack of a metabolic effect of beta 3-agonists combined with the weak expression of beta 3-AR mRNAs raise the question of the role of beta 3-ARs in adipose tissues of primates.     

Mice expressing human but not murine beta3-adrenergic receptors under the control of human gene regulatory elements

Beta-adrenergic receptors (ARs) are expressed predominantly in adipose tissue, and beta3-selective agonists are effective anti-obesity drugs in rodents. Rodent and human beta3-ARs differ with respect to expression in white versus brown adipocytes as well as their ability to be stimulated by beta3-AR-selective agonists. Humans express beta3-AR mRNA abundantly in brown but not white adipocytes, while rodents express beta3-AR mRNA abundantly in both sites. To determine the basis for this difference, we have transgenically introduced 74 kilobases (kb) of human beta3-AR genomic sequence into gene knockout mice lacking beta3-ARs. Importantly, human beta3-AR mRNA was expressed only in brown adipose tissue (BAT) of transgenic mice, with little or no expression being detected in white adipose tissue (WAT), liver, stomach, small intestine, skeletal muscle, and heart. This pattern of expression differed from that observed in mice bearing a murine beta3-AR genomic transgene in which beta3-AR mRNA was expressed in both WAT and BAT, but not in other sites. Furthermore, we have transgenically introduced smaller human constructs containing -14.5 and -0.6 kb of upstream sequence into beta3-AR gene knockout mice. Both -14.5 and -0.6 kb constructs were expressed in BAT but not WAT. Thus, human but not murine cis-regulatory elements direct beta3-AR gene expression preferentially to brown adipocytes. Identification of responsible cis-regulatory element(s) and relevant trans-acting factor(s) should provide insight into mechanisms controlling human beta3-AR gene expression. In addition, the beta3-AR agonist, CGP-12177, stimulated oxygen consumption in mice expressing human but not murine beta3-ARs by 91% compared with only 49% in control beta3-AR gene knockout mice, demonstrating that the human beta3-AR can functionally couple with energy expenditure. These "humanized" mice should assist us in the development of drugs that may become effective anti-obesity agents in humans.     

Differential regulation of uncoupling proteins by chronic treatments with beta 3-adrenergic agonist BRL 35135 and metformin in obese fa/fa Zucker rats

The expressions of uncoupling proteins 2 and 3 (UCP2; UCP3) mRNA were studied in obese (fa/fa) Zucker rats treated with two weight gain reducing agents for three weeks. The specific beta 3-adrenoceptor agonist BRL 35135 (0.5 mg/kg/day orally) increased the expression of UCP3 mRNA by 3.8-fold (P < 0.0001; two-way ANOVA) and that of UCP1 mRNA by 2.6-fold (P = 0.014) in brown adipose tissue, but had no effect on expression of UCP3 mRNA in white fat or in the soleus muscle, or on UCP2 mRNA expression in brown or white fat. The antihyperglycemic metformin (300 mg/kg/day orally) had no effect on expressions of UCP1, UCP2 or UCP3 in any tissue studied. Concentrations of plasma insulin were significantly correlated with the levels of white fat UCP2 mRNA (in the control group: r = 0.89, P = 0.0015) and UCP3 mRNA (in the control group: r = 0.80, P = 0.009) suggesting that insulin may play a role in the control of UCP2 and UCP3 mRNA expressions in white adipose tissue.     

Influence of insurance coverage on delays in seeking emergency care in patients with acute chest pain

Alpha-methyl-p-tyrosine (alphaMPT), an inhibitor of tyrosine hydroxylase, was administered to mice to block noradrenaline synthesis. Ten hours after injection of alphaMPT there was a 6-fold increase in plasma leptin. The level of ob mRNA in epididymal white adipose tissue was also increased, but UCP1 mRNA in brown fat fell. In contrast to lean mice, ob mRNA in white fat of ob/ob mice was not increased by alphaMPT. AlphaMPT raised plasma leptin in fasted as well as fed mice. Hyperleptinaemia was attenuated by treatment with a beta3-adrenoceptor agonist. Inhibition of noradrenaline synthesis leads to the rapid induction of hyperleptinaemia; it is suggested that sympathetic tone plays a pivotal role in regulating leptin production.     

Metabolic response to various beta-adrenoceptor agonists in beta3-adrenoceptor knockout mice: evidence for a new beta-adrenergic receptor in brown adipose tissue

The beta3-adrenoceptor plays an important role in the adrenergic response of brown and white adipose tissues (BAT and WAT). In this study, in vitro metabolic responses to beta-adrenoceptor stimulation were compared in adipose tissues of beta3-adrenoceptor knockout and wild type mice. The measured parameters were BAT fragment oxygen uptake (MO2) and isolated white adipocyte lipolysis. In BAT of wild type mice (-)-norepinephrine maximally stimulated MO2 4.1+/-0.8 fold. Similar maximal stimulations were obtained with beta1-, beta2- or beta3-adrenoceptor selective agonists (dobutamine 5.1+/-0.3, terbutaline 5.3+/-0.3 and CL 316,243 4.8+/-0.9 fold, respectively); in BAT of beta3-adrenoceptor knockout mice, the beta1- and beta2-responses were fully conserved. In BAT of wild type mice, the beta1/beta2-antagonist and beta3-partial agonist CGP 12177 elicited a maximal MO2 response (4.7+/-0.4 fold). In beta3-adrenoceptor knockout BAT, this response was fully conserved despite an absence of response to CL 316,243. This unexpected result suggests that an atypical beta-adrenoceptor, distinct from the beta1-, beta2- and beta3-subtypes and referred to as a putative beta4-adrenoceptor is present in BAT and that it can mediate in vitro a maximal MO2 stimulation. In isolated white adipocytes of wild type mice, (-)-epinephrine maximally stimulated lipolysis 12.1+/-2.6 fold. Similar maximal stimulations were obtained with beta1-, beta2- or beta3-adrenoceptor selective agonists (TO509 12+/-2, procaterol 11+/-3, CL 316,243 11+/-3 fold, respectively) or with CGP 12177 (7.1+/-1.5 fold). In isolated white adipocytes of beta3-adrenoceptor knockout mice, the lipolytic responses to (-)epinephrine, to the beta1-, beta2-, beta3-adrenoceptor selective agonists and to CGP 12177 were almost or totally depressed, whereas those to ACTH, forskolin and dibutyryl cyclic AMP were conserved.     

Cocaine disposition in saliva following intravenous, intranasal, and smoked administration

BACKGROUND: In our previous studies, chronic treatment of rats with a new beta 3-adrenoceptor agonist, CL 316,243, retarded diet-induced obesity and promoted thermogenesis in young animals and reversed established diet-induced obesity in older animals that continued to eat a high fat diet. Reversal of obesity was associated with shrinking of enlarged white adipocytes but the number of mature white adipocytes, which had not been increased by the diet, was not reduced. Drug-treatment induced appearance of abundant brown adipocytes in white adipose tissue (WAT) depots as well as hypertrophy of brown adipose tissue (BAT) in both lean and diet-induced obese rats. OBJECTIVES: To find out whether the known hyperplasia of white adipocytes in the obese fa/fa rat could be reversed by CL 316,243-treatment and whether the grossly enlarged WAT depots of the obese fa/fa rat contain precursors to brown adipocytes. RESULTS: CL 316,243 infusion (1 mg/kg/d) reduced abdominal fat. The loss of fat was due to a decrease in white adipocyte size, with no loss of the markedly elevated number of adipocytes in the fa/fa rats. Resting metabolic rate increased by 40% in lean rats, by 70% in fa/fa rats. Food intake decreased in the hyperphagic fa/fa rats but did not change in lean rats, in both lean and fa/fa rats, a marked increase in protein content of retroperitoneal WAT was associated with appearance of abundant densely-stained brown adipocytes expressing uncoupling protein (UCP) but total number of cells (from DNA content) actually decreased. Hyperinsulinemia and hyperglycemia of fa/fa rats were reduced by treatment, indicating improved sensitivity to insulin. CONCLUSIONS: Abundant precursors to brown adipocytes are present in WAT depots of fa/fa rats and much of the exaggerated increase in resting metabolic rate induced by CL 316,243 occurs in these cells. This beta 3-adrenoceptor agonist is an effective anti-obesity and anti-diabetic agent in fa/fa rats. It does not bring about disappearance of mature white adipocytes but does bring about a remodelling of WAT, with a marked change in cell composition.     

Induction of uncoupling protein expression in brown and white adipose tissue by leptin

Deposition of excess body fat occurs when energy intake chronically exceeds energy expenditure. In ob/ob mice, the absence of leptin affects both components of the energy balance equation, and the mice become morbidly obese after weaning. Treatment of ob/ob mice with exogenous leptin reduces body weight by decreasing food intake and stimulating energy utilization, but even when saline- and leptin-injected ob/ob mice are pair-fed, mice receiving leptin lose significantly more weight. Therefore, the purpose of the present study was to test the hypotheses that uncoupling protein-1 (UCP1) expression is reduced in adipose tissue from ob/ob mice and is restored by treatment with exogenous leptin. Lean and ob/ob mice (5-6 weeks old) were housed at 23 C and treated with leptin (20 microg/g BW x day) for 3 days before they were killed. Compared with levels in lean littermates, UCP1 messenger RNA (mRNA) and protein levels were lower in brown adipose tissue (BAT) and retroperitoneal white adipose tissue (WAT) from ob/ob mice. Treatment of ob/ob mice with leptin reduced body weight and produced a 4- to 5-fold increase in UCP1 mRNA levels in both interscapular BAT and retroperitoneal WAT. The increases in UCP1 mRNA were accompanied by comparable increases in UCP1 protein in mitochondrial preparations from each tissue. Given that the sole known function of UCP1 is to uncouple oxidative phosphorylation, the present results are consistent with the conclusion that leptin stimulates energy utilization in ob/ob mice by increasing thermogenic activity and capacity (UCP1). In addition, the present results suggest that decreased UCP1 expression in BAT and WAT of ob/ob mice is in part responsible for their increased metabolic efficiency and propensity to become obese.     

Peroxisome proliferator-activated receptors gamma and alpha mediate in vivo regulation of uncoupling protein (UCP-1, UCP-2, UCP-3) gene expression

A role for peroxisome proliferator-activated receptors, PPAR gamma and PPAR alpha, as regulators of energy homeostasis and lipid metabolism, has been suggested. Recently, three distinct uncoupling protein isoforms, UCP-1, UCP-2, and UCP-3, have also been identified and implicated as mediators of thermogenesis. Here, we examined whether in vivo PPAR gamma or PPAR alpha activation regulates the expression of all three UCP isoforms. Rats or lean and db/db mice were treated with PPAR gamma [thiazolidinedione (TZD)] or PPAR alpha (WY-14643) agonists, followed by measurement of messenger RNAs (mRNAs) for UCP-1, UCP-2, and UCP-3 in selected tissues where they are expressed. TZD treatment (AD 5075 at 5 mg/kg x day) of rats (14 days) increased brown adipose tissue (BAT) depot size and induced the expression of each UCP mRNA (3x control levels for UCP-1 and UCP-2, 2.5x control for UCP-3). In contrast, UCP-2 and UCP-3 mRNA levels were not affected in white adipose tissue or skeletal muscle. Chronic (30 days) low-dose (0.3 mg/kg x day) TZD treatment induced UCP-1 mRNA and protein in BAT (2.5x control). In contrast, chronic TZD treatment (30 mg/kg x day) suppressed UCP-1 mRNA (>80%) and protein (50%) expression in BAT. This was associated with further induction of UCP-2 expression (>10-fold) and an increase in the size of lipid vacuoles, a decrease in the number of lipid vacuoles in each adipocyte, and an increase in the size of the adipocytes. TZD treatment of db/db mice (BRL 49653 at 10 mg/kg x day for 10 days) also induced UCP-1 and UCP-3 (but not UCP-2) expression in BAT. PPAR alpha is present in BAT, as well as liver. Treatment of rats or db/db mice with WY-14643 did not affect expression of UCP-1, -2, or -3 in BAT. Hepatic UCP-2 mRNA was increased (4x control level) in db/db and lean mice, although this effect was not observed in rats. Thus, in vivo PPAR gamma activation can induce expression of UCP-1, -2, and -3 in BAT; whereas chronic-intense PPAR gamma activation may cause BAT to assume white adipose tissue-like phenotype with increased UCP-2 levels. PPAR alpha activation in mice is sufficient to induce liver UCP-2 expression.     

Large granular lymphocytic leukemia. Case report with scarce expression in peripheral blood

Evidence is rapidly emerging which suggests that uncoupling protein 2 (UCP2), by virtue of its ubiquitous expression, may be important for determining basal metabolic rate. To assess the functional modulation of UCP2 gene expression in relation to body weight control, we examined the effects of hyperthyroid state induced by chronic treatment with triiodothyronine (T3) on UCP2 mRNA expression in male rats. Daily subcutaneous injection of T3 (37 pmol/100 g body weight) for 7 days increased UCP2 mRNA expression in brown adipose tissue (BAT), white adipose tissue (WAT) and the soleus muscle 1.6-, 1.6- and 1.7-fold compared to the controls, respectively, and increased UCP1 mRNA expression in BAT 1.2-fold. In contrast, the same treatment with T3 decreased both ob mRNA expression in WAT and plasma leptin level 0.5-fold for each. The present results suggest that T3 may directly increase UCP2 expression independently of leptin action.     

Tumor necrosis factor-alpha regulates in vivo expression of the rat UCP family differentially

A family of uncoupling proteins (UCPs), free fatty acid anion transporters, plays a crucial role in energy homeostatic thermoregulation. Tumor necrosis factor-alpha (TNF-alpha), a member of the cytokine family, is well known as an endogenous pyrogen. To evaluate the interaction of TNF-alpha with UCPs in thermogenesis, effects of TNF-alpha on rat UCP gene expression were examined in intrascapular brown adipose tissue (BAT), epididymal white adipose tissue (WAT) and soleus muscle (Muscle). Administration of TNF-alpha elevated rectal temperature by 0.7 degree C as well as serum leptin which peaked at 6 h, compared with saline controls. BAT UCP1 mRNA expression was increased by 1.2-fold at 6 h after the TNF-alpha treatment and decreased by 0.8-fold at 16 h after the treatment. In contrast to UCP1 expression in BAT, UCP2 mRNA expressions in BAT, WAT, and Muscle was increased to reach maximum levels of 1.3-, 1.6- and 1.8-fold, respectively, at 16 h after the treatment. UCP3 mRNA in Muscle, but not in BAT or WAT, was exclusively up-regulated by 1.7-fold at 16 h after the treatment. These results indicate that TNF-alpha up-regulates UCP gene expression differentially and tissue dependently, and add novel insights into thermogenesis under conditions of malignancy and inflammation.     

Increased uncoupling protein-2 and -3 mRNA expression during fasting in obese and lean humans

Uncoupling protein-2 and -3 (UCP2 and UCP3) are mitochondrial proteins that show high sequence homology with the brown adipocyte-specific UCP1. UCP1 induces heat production by uncoupling respiration from ATP synthesis. UCP2 is widely expressed in human tissues, whereas UCP3 expression seems restricted to skeletal muscle, an important site of thermogenesis in humans. We have investigated the regulation of UCP2 and UCP3 gene expression in skeletal muscle and adipose tissue from lean and obese humans. UCP2 and -3 mRNA levels were not correlated with body mass index (BMI) in skeletal muscle, but a positive correlation (r = 0.55, P < 0.01, n = 22) was found between UCP2 mRNA level in adipose tissue and BMI. The effect of fasting was investigated in eight lean and six obese subjects maintained on a hypocaloric diet (1,045 kJ/d) for 5 d. Calorie restriction induced a similar 2-2.5-fold increase in UCP2 and -3 mRNA levels in lean and obese subjects. To study the effect of insulin on UCP gene expression, six lean and five obese subjects underwent a 3-h euglycemic hyperinsulinemic clamp. Insulin infusion did not modify UCP2 and -3 mRNA levels. In conclusion, the similar induction of gene expression observed during fasting in lean and obese subjects shows that there is no major alteration of UCP2 and -3 gene regulation in adipose tissue and skeletal muscle of obese subjects. The increase in UCP2 and -3 mRNA levels suggests a role for these proteins in the metabolic adaptation to fasting.     

Anti-obesity and anti-diabetic effects of CL 316,243, a highly specific beta 3-adrenoceptor agonist, in yellow KK mice

The anti-obesity and anti-diabetic effects of CL 316,243, a highly specific beta 3-adrenoceptor agonist (beta 1: beta 2: beta 3 = 0:1:100,000), were evaluated in obese diabetic yellow KK mice and C57Bl control mice. The study compound was fed through a gastric tube at a rate of 0.1 mg/kg/day for 2 weeks. The following parameters were compared in the treated and control animals given distilled water: brown adipose tissue thermogenesis, resting metabolic rate, insulin receptors in adipocytes, and blood glucose and serum insulin levels during a glucose overloading test. CL 316,243 significantly increased brown adipose tissue thermogenesis and resting metabolic rate in both yellow KK mice and C57Bl controls. The amount of white adipose tissue decreased, although food intake was not affected. The effects contributed to the mitigation of obesity in yellow KK mice. CL 316,243 also increased the concentration of insulin receptors and decreased the levels of serum insulin and blood glucose during the glucose overloading test in yellow KK mice. These observations suggest that CL 316,243 possesses anti-obesity and anti-diabetic effects and consequently may be useful for treating obesity as well as non-insulin-dependent diabetes mellitus in obese persons, without causing excessive side effects.     

Beta-3 adrenergic receptor agonists cause an increase in gastrointestinal transit time in wild-type mice, but not in mice lacking the beta-3 adrenergic receptor

The effects of beta-3 adrenergic receptor (beta3-AR) agonists on gastrointestinal (GI) motility, as reported by stomach retention and intestinal transit of radiolabelled charcoal, were compared in wild-type (WT) mice and in transgenic mice lacking beta3-AR (beta3-AR[KO]) or having beta3-AR in white and brown adipose tissue only (beta3-AR[WAT+BAT]). After s.c. administration of 3 mg/kg of the selective, rodent specific beta3-AR agonists BRL 35135, CL 316, 243 or ICI 198,157, WT mice exhibited a significant decrease in the extent of movement of radiotracer through the stomach and intestines, indicative of decreased GI motility. These compounds also caused an increase in plasma glycerol levels in the WT mice, suggesting that increased lipolysis in adipose tissue had been evoked. None of these compounds had an effect on GI motility or evoked lipolysis in the beta3-AR[KO] mice. Treatment of WT mice with SR 56811A, a beta3-AR agonist that exhibited a relatively lower affinity for rodent beta3-AR in vitro, did not affect GI motility or plasma glycerol levels in WT or beta3[KO] mice when administered s.c. at 3 mg/kg. Clonidine, an alpha-2 adrenergic receptor agonist, used as a positive control in these GI studies, caused a decrease in GI motility in both WT and beta3-AR[KO] mice. These results are consistent with a postulated role for beta3-AR in regulation of GI motility in the mouse. However, treatment of beta3-AR[WAT+BAT] mice with 3 mg/kg BRL 35135 resulted in elevated plasma glycerol levels, as well as increased stomach retention and decreased intestinal transit of radiotracer. These results suggest that this beta3-AR agonist may exert its effects on the GI tract indirectly, through an unknown signaling mechanism activated by agonism of beta3-AR in adipose tissue.     

Mice lacking mitochondrial uncoupling protein are cold-sensitive but not obese

The mitochondrial uncoupling protein (UCP) in the mitochondrial inner membrane of mammalian brown adipose tissue generates heat by uncoupling oxidative phosphorylation. This process protects against cold and regulates energy balance. Manipulation of thermogenesis could be an effective strategy against obesity. Here we determine the role of UCP in the regulation of body mass by targeted inactivation of the gene encoding it. We find that UCP-deficient mice consume less oxygen after treatment with a beta3-adrenergic-receptor agonist and that they are sensitive to cold, indicating that their thermoregulation is defective. However, this deficiency caused neither hyperphagia nor obesity in mice fed on either a standard or a high-fat diet. We propose that the loss of UCP may be compensated by UCP2, a newly discovered homologue of UCP; this gene is ubiquitously expressed and is induced in the brown fat of UCP-deficient mice.     

Acute effects of the beta 3-adrenoceptor agonist, BRL 35135, on tissue glucose utilisation

1. The acute effects of BRL 35135 (BRL) on tissue glucose utilisation index (GUI) in vivo were investigated in anaesthetized rats by use of 2-deoxy-[3H]-glucose. 2. Intravenous injection of BRL caused a dose-dependent increase in GUI in skeletal muscle, and white and brown adipose tissue; plasma insulin and fatty acid concentrations were also increased. Chronic treatment with BRL added to the diet caused a 34 fold increase in basal GUI of brown adipose tissue (BAT), but had no effect on GUI in other tissues. After chronic treatment, the acute tissue response to an intravenous maximal dose of BRL had disappeared completely in all tissues apart from the soleus muscle. 3. A high dose (20 mg kg-1) of the non-selective beta-antagonist, propranolol, inhibited the acute effect of BRL on GUI in BAT, but failed to affect GUI in muscle. A lower dose (1 mg kg-1) of the antagonist also inhibited the BAT response, but had little or no effect on the response in Type I (working) muscles such as soleus and adductor longus (ADL), and potentiated the response in Type II (non-working) muscles such as tibialis and extensor digitorium longus (EDL). 4. A low dose (1 mg kg-1) of the selective beta 1-antagonist, atenolol, had no effect on the BRL response but the same dose of the selective beta 2-antagonist, ICI 118551, potentiated significantly the effect of BRL on GUI in most muscles without altering plasma insulin levels. 5. It is concluded that: (i) the heterogeneous tissue responses of different muscle fibre types in the presence of P-antagonists indicates that BRL affects muscle GUI directly, in addition to effects mediated by increases in plasma insulin concentration; (ii) the resistance of the BRL response to conventional P-adrenoceptor antagonists implicates an atypical adrenoceptor mediating the GUI response in skeletal muscle, but this may not be identical to the adipose tissue P3-adrenoceptor; (iii) the potentiation of BRL responses by ICI 118551 indicates an inhibitory P2-adrenoceptor-mediated component in the muscle GUI response to BRL.     

Adipose tissue development during early postnatal life in ewe-reared lambs

This study examines the precise time course that brown adipose tissue (BAT) takes to adopt the characteristics of white adipose tissue in postnatal lambs. Perirenal adipose tissue was sampled from ewe-reared lambs within 1 h of birth and at 1, 2, 4, 7, 14, 21 and 30 days of age and analysed for the amount of mRNA for uncoupling protein (UCP), the amount and activity of UCP, and protein, mitochondrial protein and lipid content. This was combined with measurements of colonic temperature and jugular venous plasma concentrations of thyroid hormones and insulin-like growth factor-1 (IGF-1). Over the first 4-7 days of age, large quantities of UCP mRNA were associated with a peak in plasma triiodothyronine concentration at 2 days of age followed by a maximal amount and activity of UCP at 4 days and a basal colonic temperature of 39.3 degrees C. Between 7 and 30 days there was a large increase in lipid deposition as the amount and activity of UCP and the amount of UCP mRNA declined to basal values and colonic temperature was maintained at 40 degrees C. A significant positive relationship between perirenal adipose tissue lipid content and plasma IGF-1 concentration was observed throughout the study period. It is concluded that ovine adipose tissue maturation occurs in two distinct phases over the first month of life. The precise time scale of this process could be regulated in part by the lamb's body temperature which determines whether adipose tissue is required for heat production (i.e. BAT) or as an endogenous energy source (i.e. white adipose tissue).     

Chronic central leptin infusion enhances insulin-stimulated glucose metabolism and favors the expression of uncoupling proteins

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.