Thyroid function and body weight programming by neonatal hyperthyroidism in rats - the role of leptin and deiodinase activities.

Several authors have shown that secondary hypothyroidism was programed by neonatal thyroxine (T4) treatment. However, the associated changes of body weight (BW) were less studied, especially those related to the body fat proportion. Here, we have evaluated the effect of neonatal thyroxine treatment on BW, fat proportion, serum leptin, and thyroid function of 60-day-old rats. Wistar rats were treated with thyroxine (50 microg/100 g BW, ip) (T) or saline (S), during the first 10 days of life. BW, nose-rump length (NRL), and food consumption were monitored for 60 days, when the animals were sacrificed. Thyroid function was evaluated by thyroid radioiodine uptake (RAIU), serum T3, T4, TSH, and liver mitochondrial alpha-glycerophosphate dehydrogenase (mGPD) and type 1 and 2 deiodinases (D1 and D2) activities, which are thyroid hormone-dependent enzymes. T animals showed lower food intake, BW and NRL, but higher total fat mass (+33%) and serum leptin (+46%). They also showed lower serum T3 (-23%), T4 (-32%), TSH (-36%), RAIU (-29%) and mGPD activity (-22%). Hypothalamic and pituitary D2 activities were higher (+24% and 1.4 fold, respectively), while brown adipose tissue (BAT) D2 and skeletal muscle D1 activities were lower (-30% and -62%, respectively). Thus, neonatal hyperthyroidism programs for a higher fat proportion and hyperleptinemia, which can explain the lower food intake. The TH-dependent enzymes activities changed accordingly, except for the decrease in BAT D2, which may be due the role played by the hyperleptinemia. Finally, the decrease in peripheral deiodination may contribute to a lower me-tabolic rate that may increase the adiposity.     

Chronic leptin treatment inhibits liver mitochondrial alpha-glycerol-beta-phosphate dehydrogenase in euthyroid rats.

Leptin modulates the hypothalamus-pituitary-thyroid axis and peripheral metabolism of thyroid hormones (THs). We have studied the effect of acute and chronic leptin treatment upon liver mitochondrial glycerol phosphate dehydrogenase activity (mGPD), whose expression and activity are TH dependent. We performed 2 experiments: 1) acute leptin treatment - LepA: adult rats received a single leptin injection (8 microg/100 g BW); 2) chronic leptin treatment - LepC: adult rats received leptin (8 microg/100 g BW) daily, for 6 days. In both experiments, control groups were saline-treated. All rats were sacrificed 2 hours after the last dose. Liver mGPD activity was determined by colorimetric method. Liver D1 activity was measured by the release of (125)I from (125)I-rT3. Serum hormones were measured by RIA. LepA rats showed higher serum thyroid stimulating hormone (TSH) (+ 64%, p<0.05), free T4 (+ 34%, p<0.05), free T3 (+ 64%, p<0.05), and liver D1 activity (+ 85%, p<0.05), but no change in mGPD activity. Since THs increase mGPD activity, the unchanged level in the acute experiment is suggestive of an inhibitory role of leptin. LepC rats presented lower mGPD activity (-1.7-fold, p<0.05) and higher liver D1 activity (+ 32%, p<0.05), but no alteration in serum TSH and free THs. Our results show that leptin downregulates mGPD activity, mainly when hyperleptinemia is chronic.     

Developmental plasticity in thyroid function primed by maternal hyperleptinemia in early lactation: a time-course study in rats

Pups whose mothers were leptin-treated during the last 3 days of lactation have thyroid dysfunction at adulthood. However, there was no report about leptin treatment in the first days of life or about its action on thyroid function during development. Here, we evaluated the effects of maternal leptin treatment on the first 10 days of lactation upon thyroid function of the offspring at 21, 30, and 180 days old. At birth, lactating Wistar rats were divided into: Leptin (Lep) - leptin-treated (8 μg/100 g of body weight, s.c.) for the first 10 days of lactation and Control (C, saline-treated). Mothers were killed at the end of lactation and their offspring at 21, 30, and 180 days old. Triiodothyronine (T3), thyroxine (T4), thyrotropin (TSH), and leptin levels in serum and milk were measured. Liver mitochondrial glycerolphosphate dehydrogenase (mGPD) activity was determined. Significant differences had p<0.05. At the end of lactation, Lep mothers had higher milk T3 (+ 30%), while their offspring had higher serum T3 (+ 20%) and TSH (+ 84%). At 30 days-old, Lep offspring showed lower TSH ( - 48%), T3 ( - 20%), and mGPDm ( - 42%). At 180 days-old, Lep group presented hyperleptinemia (1.4-fold increase), higher serum T3 (+ 22%), and lower mGPD activity ( - 57%). Maternal hyperleptinemia on lactation causes hypothyroidism in the pups at 30 days, which may program for higher serum T3 at adulthood. In conclusion, maternal hyperleptinemia during lactation, that is common in obese mothers, may have an impact in future disease development, such as thyroid dysfunction.     

Acute effects of leptin on 5'-deiodinases are modulated by thyroid state of fed rats.

Leptin has been shown to modulate deiodinase type 1 (D1) and type 2 (D2) enzymes responsible for thyroxine (T4) to triiodothyronine (T3) conversion. Previously, it was demonstrated that a single injection of leptin in euthyroid fed rats rapidly increased liver, pituitary, and thyroid D1 activity, and simultaneously decreased brown adipose tissue (BAT) and hypothalamic D2 activity. We have now examined D1 and D2 activities, two hours after a single subcutaneous injection of leptin (8 microg/100 g BW) into hypo- and hyperthyroid rats. In hypothyroid rats, leptin did not modify pituitary, liver and thyroid D1, and thyroid D2 activity, while pituitary D2 was decreased by 41% (p<0.05) and hypothalamic D2 showed a 1.5-fold increase. In hyperthyroid rats, thyroid and pituitary D1, and pituitary and hypothalamic D2 were not affected by leptin injection, while liver D1 showed a 42% decrease (p<0.05). BAT D2 was decreased by leptin injection both in hypo- and hyperthyroid states (42 and 48% reduction, p<0.001). Serum TH and TSH showed the expected variations of hypo- and hyperthyroid state, and leptin had no effect. Serum insulin was lower in hypothyroid than in hyperthyroid rats and remained unchanged after leptin. Therefore, acute effects of leptin on D1 and D2 activity, expect for BAT D2, were abolished or modified by altered thyroid state, in a tissue-specific manner, showing an IN VIVO interplay of thyroid hormones and leptin in deiodinase regulation.     

Leptin acute modulation of the 5'-deiodinase activities in hypothalamus, pituitary and brown adipose tissue of fed rats.

Leptin and thyroid hormones (TH) have the ability to increase energy expenditure. Biological effects of TH are dependent on thyroxine (T4) to triiodothyronine (T3) conversion by deiodinase type 1 (D1) and type 2 (D2). Leptin has been shown to stimulate the hypothalamus-pituitary-thyroid axis and, also, to modulate 5'-deiodinases in different tissues, depending on energetic status of animals. Here, we examined the acute effects of leptin on hypothalamic, pituitary and BAT D2 and pituitary D1 activities. Male fed rats received a single subcutaneous injection of saline or leptin (8 microg/100 g BW) and sacrificed 2 hours later. Leptin promoted an important decrease in hypothalamic D2 (55% reduction, p <0.001) with no changes in pituitary D2, in concomitance with a 2-fold rise in serum TSH, suggesting that leptin acted at hypothalamus in order to stimulate TRH-TSH axis. In addition, BAT D2 was decreased by 25% (p<0.05). In contrast, pituitary D1 showed a 2-fold increase (p<0.001), indicating that, as demonstrated before for liver and thyroid D1, the pituitary enzyme is also acutely up-regulated by leptin. Serum concentrations of insulin and TH of leptin-injected animals remained unchanged. Regulation of 5'-deiodinases directing the local T3 production, is a mechanism by which leptin may alter hypothalamic, pituitary and BAT functions.     

Developmental plasticity in adrenal function and leptin production primed by nicotine exposure during lactation: gender differences in rats

Neonate male rats whose mothers were nicotine-treated during lactation have higher adiposity, hyperleptinemia, and adrenal dysfunction. At adulthood, they still present higher adiposity and hyperleptinemia, but there was no report about their adrenal function. Also, there was no report of this developmental plasticity on females. Here, we evaluated the adrenal function and leptin content in adipocytes and muscle of male and female adult offspring whose mothers were nicotine-treated during lactation. On the 2nd postnatal day (PN2), dams were subcutaneously implanted with osmotic minipumps releasing nicotine (NIC-6?mg/kg/day) or saline for 14 days (12 litters/group and 2 rats/litter). Male and female offspring were killed on PN180. Significant data were p<0.05. Male NIC offspring presented higher adrenal catecholamine content (+?89%) and TH expression (+?38%), lower "in vitro" catecholamine release (-?19%), and higher adrenergic β3 receptor (ADRB3, +?59%) content in visceral adipose tissue (VAT). Serum corticosterone was higher (+?77%) in male NIC group, coherent with the increase of both CRH and ACTH immunostaining in hypothalamus and pituitary, respectively. Leptin content was higher in VAT (+?23%), which may justify the observed hyperleptinemia. Female NIC offspring presented lower ADRB3 content in VAT (-?39%) and lower leptin content in subcutaneous adipose tissue (SAT) (-?46%), but higher leptin content in soleus muscle (+?22%), although leptinemia was normal. We evidenced a sex dimorphism in the model of maternal nicotine exposure during lactation. The adrenal function in adult offspring was primed only in male offspring while the female offspring displayed relevant alterations in leptin content on muscle and adipocytes.     

Maternal prolactin inhibition during lactation affects physical performance evaluated by acute exhaustive swimming exercise in adult rat offspring

Maternal prolactin inhibition at the end of lactation programs for metabolic syndrome and hypothyroidism in adult offspring, which could negatively affect exercise performance. We evaluated the effects of maternal hypoprolactinemia in late lactation on physical performance in adult progeny. Lactating Wistar rats were treated with bromocriptine (BRO, 1?mg per day) or saline on days 19, 20, and 21 of lactation and offspring were followed until 180 days old. Physical performance was recorded in untrained rats at 90 and 180 days by an acute exhaustive swimming test (exercise group-Ex). At day 90, BRO offspring showed higher visceral fat mass, higher plasma thiobarbituric acid reactive substances, lower total antioxidant capacity, higher liver glycogen, lower glycemia, and normal insulinemia. Although thyroid hormones (TH) levels were unchanged, mitochondrial glycerol phosphate dehydrogenase (mGPD) activity was lower in muscle and in brown adipose tissue (BAT). At this age, BRO-Ex offspring showed higher exercise capacity, lower blood lactate, higher serum T3, and higher muscle and BAT mGPD activities. At day 180, BRO offspring showed central obesity, hypothyroidism, insulin resistance, and lower EDL (extensor digitorum longus) muscle glycogen with unaltered plasma oxidative stress markers. This group showed no alteration of exercise capacity or blood lactate. After exercise, EDL and liver glycogen were lower, while T3 levels, BAT and muscle mGPD activities were normalized. Liver glycogen seem to be related with higher exercise capacity in younger BRO offspring, while the loss of this temporary advantage maybe related to the hypothyroidism and insulin resistance developed with age.     

Liver and brown fat mitochondrial response to cold in the garden dormouse (Eliomys quercinus)

The involvement of two organs, i.e. the liver and the brown adipose tissue (BAT) in response to cold in a hibernating species such as the garden dormouse has been studied. 2. In animals living in the cold, mitochondrial respiratory rates significantly increased (with respect to those living at 28 degrees C) in both organs with a larger increase in the BAT (+152% in the BAT and 67% in the liver). 3. The increase in BAT activity was obtained by a concomitant increase in: (a) the BAT mass (+30%), (b) the total mitochondrial mass (+20%), and (c) the mitochondrial respiratory rate (+64%). In the liver the increase was due only to an augmentation in mitochondrial mass and activity. 4. These results indicate that: (a) the BAT exerts a pre-eminent role in the physiological response to cold of garden dormouse, (b) a certain non-shivering thermogenesis (NST) is present in the liver of such species. In addition we suggest that a local thermoregulatory response would take place in a metabolically important organ such as the liver.     

Normal thyroid thermogenesis but reduced viability and adiposity in mice lacking the mitochondrial glycerol phosphate dehydrogenase

The mitochondrial glycerol phosphate dehydrogenase (mGPD) is important for metabolism of glycerol phosphate for gluconeogenesis or energy production and has been implicated in thermogenesis induced by cold and thyroid hormone treatment. mGPD in combination with the cytosolic glycerol phosphate dehydrogenase (cGPD) is proposed to form the glycerol phosphate shuttle, catalyzing the interconversion of dihydroxyacetone phosphate and glycerol phosphate with net oxidation of cytosolic NADH. We made a targeted deletion in Gdm1 and produced mice lacking mGPD. On a C57BL/6J background these mice showed a 50% reduction in viability compared with wild-type littermates. Uncoupling protein-1 mRNA levels in brown adipose tissue did not differ between mGPD knockout and control pups, suggesting normal thermogenesis. Pups lacking mGPD had decreased liver ATP and slightly increased liver glycerol phosphate. In contrast, liver and muscle metabolites were normal in adult animals. Adult mGPD knockout animals had a normal cold tolerance, normal circadian rhythm in body temperature, and demonstrated a normal temperature increase in response to thyroid hormone. However, they were found to have a lower body mass index, a 40% reduction in the weight of white adipose tissue, and a slightly lower fasting blood glucose than controls. The phenotype may be secondary to consequences of the obligatory production of cytosolic NADH from glycerol metabolism in the mGPD knockout animal. We conclude that, although mGPD is not essential for thyroid thermogenesis, variations in its function affect viability and adiposity in mice.     

Acute cold exposure,leptin, and somatostatin analog (octreotide) modulate thyroid 5'-deiodinase activity

We investigated the effect of acute cold exposure, leptin, and the somatostatin analog octreotide (OCT) on thyroid type I (D1) and II (D2) deiodinase activities. Microsomal D1 and D2 activities were measured by the release of (125)I from (125)I-reverse triiodothyronine (rT(3)) under different assay conditions. Rats exposed to 4 degrees C (15, 30, 60, and 120 min) showed progressive reduction in thyroidal D1 and D2, reaching approximately 40% at 2 h (P < 0.05) despite increased circulating TSH (P < 0,05) associated with the higher thyroid D1 and D2 in hypothyroid rats. A single injection of leptin (8 microg/100 g body wt sc) induced increased thyroid and liver D1 (P < 0.05), but not thyroid D2, activities at 30 and 120 min, independently of the serum TSH rise shown only at 2 h. OCT (1 microg/kg body wt sc) increased D1 and D2 activity significantly 24 h after a single injection, with no changes in serum TSH. Therefore, leptin and somatostatin are potential physiological upregulators of thyroid deiodinases, and their low secretion during acute cold exposure may be a potential mechanism contributing to cold-induced reduction in thyroid deiodinase activity.     

Maternal high-fat diet induces sex-specific endocannabinoid system changes in newborn rats and programs adiposity,energy expenditure and food preference in adulthood

Early life inadequate nutrition triggers developmental adaptations and adult chronic disease. Maternal high-fat (HF) diet promotes visceral obesity and hypothalamic leptin resistance in male rat offspring at weaning and adulthood. Obesity is related to over active endocannabinoid system (ECS). The ECS consists mainly of endogenous ligands, cannabinoid receptors (CB1 and CB2), and the enzymes fatty acid anandamide hydrolase (FAAH) and monoacylglycerol lipase (MAGL). We hypothesized that perinatal maternal HF diet would regulate offspring ECS in hypothalamus and brown adipose tissue (BAT) at birth, prior to visceral obesity development, and program food preference and energy expenditure of adult offspring. Female rats received control diet (C, 9% fat) or isocaloric high-fat diet (HF, 28% fat) for 8 weeks before mating, and throughout gestation and lactation. We evaluated C and HF offspring at birth and adulthood. At birth, maternal HF diet decreased leptinemia and increased hypothalamic CB1, orexin-A, and proopiomelanocortin while it decreased thyrotropin-releasing hormone (Trh) in male pups. Differentially, maternal HF diet increased hypothalamic CB2 in female pups. In BAT, maternal HF diet decreased CB1 and increased CB2 in male and female pups, respectively. Besides presenting different molecular ECS profile at birth, HF adult offspring developed overweight, higher adiposity and high-fat diet preference, independently of the sex, but only males presented hyperleptinemia and higher energy expenditure. In conclusion, maternal HF diet alters ECS components and energy metabolism targets in hypothalamus and BAT of offspring at birth, in a sex-specific manner, which may contribute for hyperphagia, food preference and higher adiposity later in life.     

A dramatic accumulation of glycogen in the brown adipose tissue of rats following recovery from cold exposure

In a morphological study of brown adipose tissue (BAT) of rats returned after exposure to cold (+5 degrees C) to neutral temperature (+25 degrees C), striking periodic acid Schiff staining was observed, indicating substantial glycogen accumulation. Enzymatic analysis revealed that the glycogen content increased from the 4.05 +/- 0.51 (micromol glucose unit per gram of tissue, mean +/- SE) control value to 57.3 +/- 9.66 when the animals were returned to neutral temperature for 24 h after a 1-week cold period. Glycogen repletion was also observed in liver and skeletal muscle; however, the glycogen levels in these tissues never exceeded the control values. The accumulation of glycogen in the BAT started by the 3rd hour of replacement and peaked by the 24th hour. This glycogen was readily utilized during the next short cold exposure of the animals. The plasma leptin concentration was reduced at the cold temperature. The hexokinase II activity in the BAT increased to 29.3 +/- 1.46 vs the 11.8 +/- 1.06 control (mU/mg protein +/- SE) after a 1-week cold exposure and this level was maintained during the return to neutral temperature. The total glycogen synthetase (GStot) and the glycogen synthetase a activity also increased after a 1-week cold exposure and increased further during the replacement. The level of GStot reached 26.9 +/- 1.39 vs 9.54 +/- 1.43 control by the 24th hour of replacement. At the same time, the glycogen phosphorylase a activity declined during the replacement. The concentration of glucose 6-phosphate (an activator of GS) decreased in the cold but returned to normal during the replacement. These changes in the BAT are in favor of glycogen synthesis.     

长爪沙鼠冷驯化过程中甲状腺激素的变化

22℃室温中的长爪沙鼠血清T₃和T₄浓度较低,肝脏和褐色脂肪组织（BAT）的T₄5'-脱碘酶活力也较低。冷暴露（4℃）1d 后,血清T₃与T₄浓度迅猛增长,随后1-4周的冷驯化中,T₃缓慢上升,T₄逐渐下降,肝脏和BAT 的T₄5'-脱碘酶活力变化与血清T₃和T₃\T₄变化一致。表明冷驯化激活了甲状腺功能,激活了外周组织中的甲状腺激素代谢。     

Differential regulation of uncoupling protein-1, -2 and -3 gene expression by sympathetic innervation in brown adipose tissue of thermoneutral or cold-exposed rats

The control of uncoupling protein-1, -2 and -3 (UCP-1, UCP-2, UCP-3) mRNA levels by sympathetic innervation in rats was investigated by specific and sensitive RT-PCR assays. In rats reared at thermoneutrality (25 degrees C), unilateral surgical sympathetic denervation of interscapular brown adipose tissue (BAT) markedly reduced the UCP-1 mRNA level (-38%) as compared with the contralateral innervated BAT pad, but was without significant effect on UCP-2 and -3 mRNA levels. Cold exposure (7 days, 4 degrees C) markedly increased UCP-1 (+180%), UCP-2 (+115%) and UCP-3 (+195%) mRNA levels in interscapular BAT. Unilateral sympathetic denervation prevented the cold-induced rise in BAT UCP-1 and UCP-2 mRNAs, but not that in BAT UCP-3 mRNA. Results were confirmed by Northern blot analysis. These data indicate a differential endocrine control of UCP-1, UCP-2 and UCP-3 gene expression in rat BAT both at thermoneutrality and during prolonged cold exposure.     

Long-term consequences of maternal high-fat feeding on hypothalamic leptin sensitivity and diet-induced obesity in the offspring

Epidemiological and animal studies suggest that the alteration of hormonal and metabolic environment during fetal and neonatal development can contribute to development of metabolic syndrome in adulthood. In this paper, we investigated the impact of maternal high-fat (HF) diet on hypothalamic leptin sensitivity and body weight gain of offspring. Adult Wistar female rats received a HF or a control normal-fat (C) diet for 6 wk before gestation until the end of the suckling period. After weaning, pups received either C or HF diet during 6 wk. Body weight gain and metabolic and endocrine parameters were measured in the eight groups of rats formed according to a postweaning diet, maternal diet, and gender. To evaluate hypothalamic leptin sensitivity in each group, STAT-3 phosphorylation was measured in response to leptin or saline intraperitoneal bolus. Pups exhibited similar body weights at birth, but at weaning, those born to HF dams weighed significantly less (-12%) than those born to C dams. When given the HF diet, males and females born to HF dams exhibited smaller body weight and feed efficiency than those born to C dams, suggesting increased energy expenditure programmed by the maternal HF diet. Thus, maternal HF feeding could be protective against adverse effects of the HF diet as observed in male offspring of control dams: overweight (+17%) with hyperleptinemia and hyperinsulinemia. Furthermore, offspring of HF dams fed either C or HF diet exhibited an alteration in hypothalamic leptin-dependent STAT-3 phosphorylation. We conclude that maternal high-fat diet programs a hypothalamic leptin resistance in offspring, which, however, fails to increase the body weight gain until adulthood.     

Changes in thyroid hormone and insulin status of the brown adipose tissue of cold acclimated rats on short term exposure to heat

Acclimation of rats to cold caused 45% increase in the concentration of triidothyronine (T3) and 35% increase in the concentration of thyroxine (T4) in serum. Exposure of cold-acclimated rats to heat (12 hr, 37 degrees C) failed to decrease the concentrations of thyroid hormones in circulation. The concentration of T3 in brown adipose tissue (BAT) increased almost 10-fold on cold acclimation. Iodothyronine deiodinase activity also registered 3-fold increase. Exposure of cold-acclimated animals to heat caused decrease in the concentration of T3 in BAT without appreciably affecting T4 concentration. In liver tissue, the changes in hormone concentrations were quite small compared to those in BAT. On thyroidectomy or when fed with propyl thiouracil, rats could not survive exposure to the cold. The concentration of insulin in circulation showed small increase, while that in the tissues showed significant decrease on acclimation of rats to the cold. The concentration of the hormone in BAT registered significant increase on exposure of cold-acclimated animals to heat (12 hr, 37 degrees C). The increase in liver was marginal. The temperature-dependent response of T3 indicates an important role for this hormone in rapid physiological response in BAT.     

Adaptive Activation of Thyroid Hormone and Energy Expenditure

The mechanisms by which thyroid hormone accelerates energy expenditure are poorly understood. In the brown adipose tissue (BAT), activation of thyroid hormone by type 2 iodothyronine deiodinase (D2) has been known to play a role in adaptive energy expenditure during cold exposure in human newborns and other small mammals. Although BAT is not present in significant amounts in normal adult humans, recent studies have found substantial amounts of D2 in skeletal muscle, a metabolically relevant tissue in humans. This article reviews current biological knowledge about D2 and adaptive T3 production and their roles in energy expenditure.     

Maximum acute exercise tolerance in hyperthyroid and hypothyroid rats subjected to forced swimming

Thyroid dysfunction can compromise physical capacity. Here, we analyze the effects of hyperthyroidism and hypothyroidism on maximum swim time in rats subjected to acute forced swimming, as an indicator of anaerobic capacity. Animals were forced to swim against a load (5% of body weight) attached to the tail and were killed 48 hours after the last test. Hyperthyroid rats were treated with thyroxine (50 mug/100 g body weight, i. p. for 7 days). The hypothyroid group received 0.03% methimazole in the drinking water for 4 weeks. Thyroid state was confirmed by alterations in serum thyroid-stimulating hormone (TSH), triiodothyronine (T3), thyroxine (T4), and liver mitochondrial glycerol phosphate dehydrogenase (mGPD) activity. Hyperthyroid rats presented significantly lower visceral fat mass (VFM) and higher food intake (p<0.05) with unchanged body weight. Maximum swim time (MST), glycogen content (skeletal muscle and liver), and leptin levels were lower while corticosterone was higher (p<0.05). In hypothyroid rats body weight was lower (p<0.05), without changes in VFM. Tested at 7-day intervals, MST was lower for tests 2, 3, and 4 (p<0.05). Muscle glycogen was higher in extensor digitorum longus (EDL) and soleus (p<0.05), without changes in liver. Serum corticosterone was lower, while leptin was higher (p<0.05). These results suggest that in hyperthyroid and hypothyroid rats, thyroid hormones together with corticosterone and/or leptin may impair exercise capacity differently through its known effects on glycogen metabolism.     

Resistance to the anorexic and thermogenic effects of centrally administrated leptin in obese aged rats

The aim of the present study was to determine whether the anorexic and thermogenic effects of leptin were attenuated in overweight aged rats following intracerebroventricular (i.c.v.) injection of murine leptin. Male F344/BN rats of two ages (6 months: young (n=20) and 24 months: old (n=18)) were divided into three groups (control, pair-fed and leptin) and were treated with either vehicle (artificial cerebrospinal fluid) or leptin (15.6 microgram/day) for 3 days. There was an age-related increase in basal food intake (20+/-2%), serum leptin levels (363+/-106%) and leptin (OB) mRNA (72+/-16%) in perirenal white adipose tissue (PWAT). In contrast, basal expression of hypothalamic NPY mRNA and brown adipose tissue (BAT) uncoupling protein 1 (UCP1) mRNA was reduced significantly (-35+/-4% and -51+/-5%, respectively) with age. I.c.v. leptin treatment had a significantly greater effect in reducing food intake (-42+/-5% vs. -23+/-4%), serum leptin levels (-55+/-7% vs. 10+/-2%) and PWAT OB mRNA (-46+/-2% vs. 10+/-5%) in young than in old rats. Similarly, central leptin treatment also had a greater effect in suppressing hypothalamic NPY mRNA expression in young (-23+/-4%) than in old (-8+/-4%) rats compared with their age-matched pair-fed treated rats. The stimulatory effect of i.c.v. leptin treatment on BAT UCP1 mRNA expression was also significantly greater in young rats (45+/-8%) than in old rats (10+/-6%) compared with age-matched pair-fed rats. Our previous report indicated that these overweight aged rats were resistant to peripheral administered leptin. The present data extend those findings and demonstrate that the impaired anorexic and metabolic effects of leptin are centrally mediated. This leptin resistance may be due to either the elevated obesity and serum leptin with age or due to age itself or both. The development of leptin resistance with age may contribute to the hyperphagia, hyperleptinemia and impaired energy balance with age.     

Deficiency of PTP1B in POMC neurons leads to alterations in energy balance and homeostatic response to cold exposure

The adipose tissue-derived hormone leptin regulates energy balance through catabolic effects on central circuits, including proopiomelanocortin (POMC) neurons. Leptin activation of POMC neurons increases thermogenesis and locomotor activity. Protein tyrosine phosphatase 1B (PTP1B) is an important negative regulator of leptin signaling. POMC neuron-specific deletion of PTP1B in mice results in reduced high-fat diet-induced body weight and adiposity gain due to increased energy expenditure and greater leptin sensitivity. Mice lacking the leptin gene (ob/ob mice) are hypothermic and cold intolerant, whereas leptin delivery to ob/ob mice induces thermogenesis via increased sympathetic activity to brown adipose tissue (BAT). Here, we examined whether POMC PTP1B mediates the thermoregulatory response of CNS leptin signaling by evaluating food intake, body weight, core temperature (T(C)), and spontaneous physical activity (SPA) in response to either exogenous leptin or 4-day cold exposure (4°C) in male POMC-Ptp1b-deficient mice compared with wild-type controls. POMC-Ptp1b(-/-) mice were hypersensitive to leptin-induced food intake and body weight suppression compared with wild types, yet they displayed similar leptin-induced increases in T(C). Interestingly, POMC-Ptp1b(-/-) mice had increased BAT weight and elevated plasma triiodothyronine (T(3)) levels in response to a 4-day cold challenge, as well as reduced SPA 24 h after cold exposure, relative to controls. These data show that PTP1B in POMC neurons plays a role in short-term cold-induced reduction of SPA and may influence cold-induced thermogenesis via enhanced activation of the thyroid axis.