GIP-dependent expression of hypothalamic genes.

GIP (glucose dependent insulinotrophic polypeptide), originally identified as an incretin peptide synthesized in the gut, has recently been identified, along with its receptors (GIPR), in the brain. Our objective was to investigate the role of GIP in hypothalamic gene expression of biomarkers linked to regulating energy balance and feeding behavior related neurocircuitry. Rats with lateral cerebroventricular cannulas were administered 10 µg GIP or 10 microl artificial cerebrospinal fluid (aCSF) daily for 4 days, after which whole hypothalami were collected. Real time Taqman™ RT-PCR was used to quantitatively compare the mRNA expression levels of a set of genes in the hypothalamus. Administration of GIP resulted in up-regulation of hypothalamic mRNA levels of AVP (46.9±4.5 %), CART (25.9±2.7 %), CREB1 (38.5±4.5 %), GABRD (67.1±11 %), JAK2 (22.1±3.6 %), MAPK1 (33.8±7.8 %), NPY (25.3±5.3 %), OXT (49.1±5.1 %), STAT3 (21.6±3.8 %), and TH (33.9±8.5 %). In a second experiment the same set of genes was evaluated in GIPR(-/-) and GIPR(+/?) mice to determine the effect of lack of GIP stimulation on gene expression. In GIPR(-/-) mice expressions of the following genes were down-regulated: AVP (27.1±7.5 %), CART (28.3±3.7 %), OXT (25.2±5.8 %), PTGES (23.9±4.5 %), and STAT3 (8.8±2.3 %). These results suggest that AVP, CART, OXT and STAT3 may be involved in energy balance-related hypothalamic circuits affected by GIP.

25-hydroxyvitamin D, insulin-like growth factor-I, and bone mineral accrual during growth.

CONTEXT: The extent to which 25-hydroxyvitamin D [25(OH)D] and IGF-I influence bone mineral content (BMC) accrual from early to mid-puberty is unclear. OBJECTIVE, SETTING, AND PARTICIPANTS: This study sought to determine relationships among 25(OH)D, IGF-I, and BMC in community-dwelling prepubertal females (n = 76; aged 4-8 yr at baseline) over a period of up to 9 yr. DESIGN: The hypothesis that changes in IGF-I vs. 25(OH)D are more strongly associated with BMC accrual was formulated after data collection. 25(OH)D and IGF-I were log-transformed and further adjusted using two-way ANOVA for differences in season and race. Linear mixed modeling (including a random subject-specific intercept and a random subject-specific slope on age) was employed to analyze the proportion of variance the transformed 25(OH)D and IGF-I variables explained for the bone outcomes. RESULTS: IGF-I was more strongly associated with BMC accrual than 25(OH)D at the total body (R(2) = 0.874 vs. 0.809), proximal femur (R(2) = 0.847 vs. 0.771), radius (R(2) = 0.812 vs. 0.759), and lumbar spine (R(2) = 0.759 vs. 0.698). The rate of BMC accrual was positively associated with changes in IGF-I but negatively associated with 25(OH)D. When IGF-I and 25(OH)D were included in the same regression equation, 25(OH)D did not have a significant predictive effect on BMC accrual above and beyond that of IGF-I. CONCLUSIONS: These prospective data in early adolescent females indicate that both 25(OH)D and IGF-I have a significant impact on bone mineral accrual; however, the positive association of IGF-I and BMC accrual is greater than the negative association of 25(OH)D and BMC accrual.

Bone and fat relationships in postadolescent black females: a pQCT study.

UNLABELLED: Despite adolescent black females experiencing the highest rates of obesity, the effect of excess fat mass on bone structure and strength in this population is unknown. Our findings in postadolescent black females suggest that excess weight in the form of fat mass may adversely influence cortical bone structure and strength. INTRODUCTION: Although adolescent obesity has been associated with reduced bone structure and strength in white females, this relationship has not been studied in adolescent black females, a population experiencing the highest rates of obesity. Our objective was to compare bone structure and strength between postadolescent black females with normal and high levels of adiposity. METHODS: Black females with ≤ 32% body fat were classified as normal body fat (NF; n = 33, aged 19.3 ± 1.3 years); females exceeding this cutoff were classified as high body fat (HF; n = 15, aged 19.0 ± 1.1 years). Using peripheral quantitative computed tomography, tibial and radial bones were scanned at the 4% (trabecular) and 20% (cortical) sites from the distal metaphyses. Fat-free soft-tissue mass (FFST) and %body fat were assessed by dual-energy X-ray absorptiometry. RESULTS: After controlling for either FFST or body weight, the HF vs. NF group had lower total cross-sectional area (CSA; 9-17%), cortical CSA (6-15%), and strength-strain index (SSI; 13-24%) at the cortical site of the tibia (all p < 0.05). At the cortical site of the radius, the HF vs. NF group had lower total CSA (14%, p = 0.03), cortical CSA (9%, p = 0.04), and SSI (15%, p = 0.07) after control for body weight. There were no group differences in either the FFST-adjusted cortical bone values at the radius or in the trabecular bone parameters (body weight- or FFST-adjusted) at the tibia and radius. CONCLUSIONS: Consistent with our adiposity and bone data in late-adolescent white females, our findings in black females entering adulthood also suggest that obesity may adversely influence cortical bone strength.

CART deficiency increases body weight but does not alter bone strength.

The regulation of bone metabolism mediated by leptin is a complex process that is not clearly understood. Recent studies suggest that CART (cocaine-amphetamine related transcript) is a significant neuronal co-factor when combined with leptin. CART deficiency is thought to result in low trabecular bone mass, but since leptin exerts contrasting effects on trabecular and cortical bone it is possible that cortical bone may not respond to the absence of CART signaling in the same manner as trabecular bone. We tested the hypothesis that CART deficiency decreases cortical bone mass, density, and strength by examining femora of adult wild-type mice (CART(+/+)) and CART-deficient mice (CART(-/-)). DEXA densitometry (PIXImus system) was used to measure whole-bone mineral content (BMC) and mineral density (BMD) from right femora, and pQCT used to calculate densitometric and geometric parameters from the femur midshaft. Femora were also tested in three-point bending, and sections of the tibia analyzed histologically to determine bone marrow adipocyte density (N.At./M.Ar) and endocortical osteoclast number (N.Oc/B.Pm). The control mice weighed less than the mice lacking CART (P<0.001), but mechanical testing data showed no differences (p>0.05) in ultimate force, energy to fracture, stiffness, or intrinsic properties such as ultimate stress, ultimate strain, or modulus. CART-deficient mice did not differ from normal controls in whole-femur BMC (p=0.09), BMD (p=0.19), midshaft cortical bone thickness (p=0.67), midshaft cortical bone area (p=0.59) or N.Oc/B.Pm (p=0.94), although CART deficiency was associated with a three-fold increase in bone marrow adipocyte density (p<0.001). Our data suggest that while the central, neuroendocrine regulation of bone mass via CART signaling may have effects on trabecular mass, absence of CART expression does not significantly alter cortical bone geometry, density, or strength.

Effects of recombinant bovine somatotropin (rbST) and season on plasma and milk insulin-like growth factors I (IGF-I) and II (IGF-II) in lactating dairy cows.

During two studies, effects of recombinant bovine somatotropin (rbST) on plasma and milk IGF's in cows adapted to summer (S; 12 cows) or winter (W; 12 cows) conditions were evaluated. Each study consisted of on-farm periods (30 days) followed by climatology chamber periods (CC; 30 days). Cows were given daily injections of rbST, Sometribove, USAN (25mg/day; 6 cows each study) or saline (control; 6 cows each study). During on-farm periods, blood and milk (am and pm) samples were collected once weekly. During CC periods, blood samples were collected every 2 days and milk samples (am and pm) were collected daily. Plasma IGF-I and IGF-II were increased in cows treated with rbST. A pronounced seasonal pattern in basal and rbST-stimulated plasma IGF-I but not IGF-II was detected. Higher basal and rbST-stimulated plasma IGF-I concentrations in S occurred despite large decreases in feed intake and energy balance. Milk IGF-I and IGF-II was not affected by rbST treatment or season. Although milk IGF-I and IGF-II concentrations were unaffected by rbST treatment, total IGF-output increased due to increased milk yield. The observed seasonal patterns in plasma IGF-I may be indicative of seasonal differences in the coupling of the somatotropin-IGF axis. In particular, we failed to detect an uncoupling of the somatotropin-IGF-I axis in S despite an induced negative energy balance during thermal stress.

Novel treatments for obesity and osteoporosis: targeting apoptotic pathways in adipocytes.

Obesity and osteoporosis have grave consequences for human health, quality of life, and even the efficiency of the labor force and economy. However, these pathologies share a common cell progenitor, revealing a surprising target for drug research and development. Recent findings show that high adipocyte count in bone marrow is directly related to bone loss, as fat cells replace osteoblasts (or bone-forming cells). The objective of this review is to examine the importance of adipocyte apoptosis in the treatment of obesity and/or osteoporosis, with special emphasis on natural products as promising leads for drug development. We have induced in vivo adipocyte apoptosis, using leptin, ciliary neurotrophic factor (CNTF), beta adrenergic agonists and conjugated linoleic acid (CLA) in rodents. The results of leptin treatments on rats are suppressed food intake, reduced body weight, reduced body fat, adipocyte apoptosis, and elevated energy expenditure. Further, leptin treatment of leptin-deficient (ob/ob) mice increases endosteal bone formation and bone mineral density. Adipocyte apoptosis has also been induced in vitro using tumor necrosis factor-alpha (TNF-alpha), (-)-epigallocatechin gallate (EGCG) from Camellia sinensis and ajoene, from Allium sativum. Natural products have potential for inducing apoptosis of adipose tissue, inhibiting bone marrow adipogenesis and increasing the expression of osteogenic factors in bone, thereby yielding effective treatments for obesity and osteoporosis.

Evaluation of adipocyte apoptosis by laser scanning cytometry.

OBJECTIVE: Adipocyte apoptosis plays an important role in adipose tissue homeostasis and can be altered under a variety of physiological and pathological conditions. This study was carried out to determine whether laser scanning cytometry (LSC) can be used to measure changes in apoptosis of adipocytes over time. DESIGN: LSC was used to investigate adipocyte apoptosis induced by tumor necrosis factor-alpha (TNF-alpha), a cytokine that is associated with obesity and insulin resistance. LSC, a slide-based solid phase cytofluorometer, provides quantitative flow fluorescence data together with morphological information for apoptotic detection. Both 3T3-L1 cells and rat adipocytes from primary cell culture were incubated with 0 or 25 nM TNF-alpha for up to 24 h. Both the FITC-conjugated annexin V/propidium iodide assay and the TUNEL assay were used to distinguish cells with apoptotic characteristics from nonapoptotic cells. RESULTS: Apoptosis did not increase over time in the absence of TNF-alpha for both 3T3-L1 cells and rat primary adipocytes. For both 3T3-L1 cells and rat primary adipocytes, a significant increase in the percentage of apoptotic cells was observed by 3-4 h incubation with TNF-alpha (P<0.05). By 24 h, more than 50% of cells incubated with TNF-alpha were apoptotic (P<0.001). This process was also associated with morphological changes typical of adipocytes undergoing apoptosis. By estimating the percentage of cell subpopulations after different times of incubation with TNF-alpha, we were able to develop grading parameters, based on the adipose apoptotic measurements. CONCLUSION: With morphological information, LSC can be a useful tool to evaluate adipocyte apoptosis.

Ciliary neurotrophic factor injected icv induces adipose tissue apoptosis in rats.

Recent findings show that ciliary neurotrophic factor (CNTF) and leptin have similar effects on food intake and body weight, suggesting possible overlapping mechanisms. Intracerebroventricular (icv) injection of leptin results in adipose tissue apoptosis. To determine if CNTF has similar activity, male Sprague Dawley rats implanted with lateral cerebroventricular cannulas were randomly assigned to four treatment groups ( N = 8), including control (aCSF), 10 microg/day leptin, 1 microg/day CNTF, and 5 microg/day CNTF. Rats received daily icv injections for 4 successive days. Both leptin and CNTF (5 microg) decreased BW (8.6% and 11.77%, respectively, p <.05) and cumulative food intake was decreased 43% by leptin ( p <.05). Leptin and CNTF (5 microg) reduced adipose tissue mass in epididymal adipose (Epi) by 30 and 33.5%, ( p <.05), in inguinal adipose (Ing) by 51 and 55% ( p <.05), in retroperitoneal adipose (Rp) by 65 and 64% ( p <.05), and in intrascapular brown adipose (iBAT) by 34 and 25% ( p <.05), respectively. Gastrocnemius muscle was not affected. Leptin and CNTF (5 microg) increased apoptosis in Epi by 84 and 150%, respectively ( p <.05) and in Rp by 121 and 146%, respectively ( p <.05). Loss of adipocytes by apoptosis may provide an explanation for the unexpected delay in return to initial energy status following CNTF treatments.

Leptin-induced adipose apoptosis: Implications for body weight regulation.

Great strides have been made in understanding the genetics of body weight regulation, in part due to the study of rodent models of obesity that are characterized by mutations affecting leptin or its receptors. Leptin, produced in adipose tissue, acts both centrally and peripherally to orchestrate complex metabolic and behavioral changes that increase loss of adipose tissue, including suppressing food intake and increasing thermogenesis. In addition, recent evidence indicates that leptin acts centrally to trigger an apoptotic process resulting in adipocyte deletion. Loss of adipocytes by apoptosis may provide an explanation for the unexpected delay in return to initial energy status following leptin treatments. This review summarizes the major aspects of leptin-induced adipose tissue apoptosis, including some of the newest findings about possible mechanisms of action.

Adipose tissue cellularity and apoptosis after intracerebroventricular injections of leptin and 21 days of recovery in rats.

OBJECTIVE: To determine the effect of leptin and post-treatment recovery on adipose tissue cellularity and apoptosis. In addition, to investigate whether Bcl-2 and/or Bax is involved in the mechanism of leptin-induced adipose tissue apoptosis. DESIGN: A total of 24 adult male Sprague-Dawley rats were injected i.c.v. with either 10 microg mouse leptin or 10 microl vehicle once per day for 4 days. At 24 h after the last injection, one group was killed while the other was monitored for 21 days. MEASUREMENTS: DNA fragmentation and Bcl-2 and Bax protein levels were determined in inguinal (ING), epididymal (EPI) and retroperitoneal (RP) white adipose tissues and the interscapular brown adipose tissue (BAT). Cellularity was determined in ING and EPI. RESULTS: Leptin significantly reduced the masses of all white fat pads [RPINGEPI] but not BAT. Cell volume was significantly reduced in EPI and ING. Only ING had a significantly reduced cell number from leptin treatment plus exhibited apoptosis by increased DNA fragmentation and DNA laddering, and upregulation of pro-apoptosis Bax protein. The other fat pads exhibited a general trend to increase the Bcl-2/Bax ratio. Recovery allowed for normalization of white fat pad mass, cell number and cell volume; however, BAT mass increased 42% over control. After recovery, apoptosis was not detected, Bcl-2 protein had increased in ING, and the Bcl-2/Bax ratio had risen overall. CONCLUSIONS: Central administration of mouse leptin in the rat targets white fat depots individually to reduce mass by a reduction in cell volume plus adipocyte deletion in, at least, the ING fat pad by Bax-mediated apoptosis. Even after a dramatic loss in adipose tissue mass and change in cellularity, the rat demonstrates a resilient return to control levels together with an increase in factors that prevent adipocyte loss.

TNFalpha induces and insulin inhibits caspase 3-dependent adipocyte apoptosis.

Regulation of fat cell number by apoptosis is proposed to be part of a normal physiological cycle in adipose growth and development. To investigate this process, cultured rat adipocytes were treated with various concentrations of tumor necrosis factor alpha (TNFalpha) and/or insulin to determine the roles of these factors in adipocyte apoptosis. The cells were analyzed by flow cytometry using a TUNEL assay. TNFalpha increased adipocyte apoptosis in a dose-dependent fashion. TNFalpha-mediated apoptosis was detectable within 6 h of treatment and continued to increase with time. Decreasing media insulin concentration from 8.5 to 0.85 nM resulted in increased adipocyte apoptosis, whereas high doses of insulin protected adipocytes from TNFalpha-induced apoptosis. TNFalpha-activated apoptosis was accompanied by an increase in caspase 3 activity and could be inhibited by a caspase 3-specific inhibitor. These data suggest that adipose tissue cell number is regulated, in part, by an apoptotic signaling pathway that involves TNFalpha, insulin, and caspase 3.

Adipocyte insensitivity to insulin in growth hormone-transgenic mice.

Growth hormone (GH) has an inhibitory effect on adipogenesis, and its effect is associated with insulin action in obesity. In this study, the relationship between GH effect on insulin sensitivity and adipocyte differentiation in vivo was investigated. Transgenic (TG) female mice expressing porcine GH had reduced body weights and weights of retroperitoneal and parametrial fat depots. Insulin treatment increased PPARgamma and GLUT4 expression in adipose tissue of WT mice but had no effect in TG mice. Content of transcription factors, PPARgamma and C/EBPalpha and beta, was higher in adipose tissue of WT mice, and for C/EBPalpha and PPARgamma, the difference occurred primarily in 24-, compared to 12-week-old, mice. Expression of preadipocyte factor-1 was higher in adipose tissue of TG mice, and expression of TNF-alpha and leptin was reduced in adipose tissue of 24-week-old TG mice. Our results suggest that increased expression of GH reduces adipogenesis by inducing adipocyte resistance to the adipogenic effect of insulin.

Conjugated linoleic acid (CLA), body fat, and apoptosis.

OBJECTIVE: The objective of the study was to determine if consumption of conjugated linoleic acid (CLA) by mice could induce apoptosis in adipose tissue. Other objectives were to determine the influence of feeding mice CLA for < or =2 weeks on body fat, energy expenditure, and feed intake. RESEARCH METHODS AND PROCEDURES: A mixture of CLA isomers (predominantly c9,t11 and t10,c12) was included in the AIN-93G diet at 0, 1, and 2%, and fed to mice for 12 days (Trial 1), or was included at 2% and fed to mice for 0, 5, and 14 days (Trial 2). Feed intake was measured daily and energy expenditure was determined by direct calorimetry on day 9 in Trial 1. Retroperitoneal fat pads were analyzed for apoptosis by determination of DNA fragmentation. RESULTS: Dietary CLA reduced feed intake by 10% to 12% (p < 0.01), but either did not influence or did not increase energy expenditure as indicated by heat loss. Body weight was not influenced by consumption of CLA in Trial 1 but was increased (p < 0.01) by CLA in Trial 2. Weights of retroperitoneal, epididymal, and brown adipose tissues were lower (p < 0.01) in animals fed CLA, although liver weight was increased (p < 0.10; Trial 1) or not changed (Trial 2). Analysis of retroperitoneal fat pad DNA from both trials indicated that apoptosis was increased (p < 0.01) by CLA consumption. DISCUSSION: These results are interpreted to indicate that CLA consumption causes apoptosis in white adipose tissue. This effect occurs within 5 days of consuming a diet that contains CLA.

Inhibition of preadipocyte differentiation by myostatin treatment in 3T3-L1 cultures.

Myostatin, a new TGF-beta family member, is known as a muscle growth inhibitor, but its role in adipocyte development has not been studied. To test the role of Myostatin in 3T3-L1 preadipocyte differentiation, we treated cultured 3T3-L1 preadipocytes with Myostatin dissolved in 0.1% trifluoroacetic acid (TFA) during differentiation after they had become confluent. Myostatin treatment significantly decreased glycerol-3-phosphate dehydrogenase (GPDH) activity and oil Red-O staining compared to controls that did not receive Myostatin. Western blot analysis showed that the expression levels of CCAAT/enhancer binding protein alpha (C/EBP alpha) and peroxisome proliferator-activated receptor gamma (PPAR gamma) were significantly decreased by Myostatin treatment (P < 0.05). However, the expression of C/EBP beta was not significantly changed by the treatment (P > 0.05). From RT-PCR result, the relative level of leptin mRNA in Myostatin-treated cells was not significantly different (P > 0.1) from the level in cells without Myostatin treatment. Our data show that Myostatin, a secreted protein from muscle, inhibits preadipocyte differentiation in 3T3-L1 cells, which is mediated, in part, by altered regulation of C/EBP alpha and PPAR gamma.

Regulation of metabolism and body fat mass by leptin.

The relative stability of body weight over the long term and under a variety of environmental conditions that alter short-term energy intake and expenditure provides strong evidence for the regulation of body energy content. The lipostatic theory of energy balance regulation proposed 40 years ago that circulating factors, generated in proportion to body fat stores, acted as signals to the brain, eliciting changes in energy intake and expenditure. The discovery of leptin and its receptors has now provided a molecular basis for this theory. Leptin functions as much more than an adipocyte-derived signal of lipid stores, however. Although suppression of food intake is an important centrally mediated effect of leptin, considerable evidence indicates that leptin also functions both directly and indirectly, via the brain, to orchestrate complex metabolic changes in a number of organs and tissues, altering nutrient flux to favor energy expenditure over energy storage.

Induction of apoptosis by all-trans-retinoic acid and C2-ceramide treatment in rat stromal-vascular cultures.

Apoptosis of preadipocytes and adipocytes contributes to the balance of adipose tissue mass by reducing adipocyte number. To address this phenomenon, we treated cultured rat S-V cells with all-trans-retinoic acid (RA) (10 microM) or C2-ceramide (50 microM) during adipogenesis. Gel electrophoresis of DNA from treated cells cultured in serum-free medium showed that 10 microM RA or 50 microM ceramide induced a distinct laddering pattern of DNA fragments. Cellular caspase 3 activity, another marker of apoptosis, was increased by RA (10 microM) (P < 0.05), but not by 50 microm C2-ceramide. RT-PCR results showed that RA (10 microM) decreased the expression of Bcl-2 mRNA. These results suggest that fat cell loss by apoptosis can be regulated, in part, by RA (10 microM) which increases caspase 3 activity and decreases Bcl-2 expression in rat S-V cells. C2-ceramide apparently works through a different cellular mechanism to induce apoptosis.

Effects of intracerebroventricularly administered leptin on protein selection in the rat.

The effect of centrally administered rat leptin on selection of 5 and 30% protein diets was investigated in male Sprague-Dawley rats with indwelling i.c.v. cannulas. Leptin (0 vs 2.5 microg/day) was administered for 4 consecutive days, followed by an 8-day withdrawal period. Total intake was reduced to approximately 50% of that in the vehicle injected group during each day following leptin administration. Intake of both the 5 and 30% diets was reduced. Vehicle-treated rats selected a 13-15% CP diet. Diet selection in leptin-treated rats was not different during the first day, but on Days 2-4, leptin-treated rats selected a 10% CP diet. Intake began to normalize within 24-48 h after the last treatment, and was not different by Day 3 of the withdrawal period. Body weight was reduced by leptin treatment, and despite the normalization of food intake, did not recover during the withdrawal period. Rats were sacrificed at the end of the 8-day withdrawal period. Despite the reduction in body and carcass weights, liver, kidney, heart, and soleus muscle weights were not different between control and leptin-treated groups when expressed on an absolute or relative basis. However, epididymal and retroperitoneal fat pad weights were still reduced 56 and 78%, respectively, in rats that had been previously treated with leptin for 4 days and then not treated for 8 days. In addition, circulating T3 levels remained elevated in rats that had been treated with leptin. Centrally administered leptin has little effect on muscle mass, but had potent effects on intake of nonobese rats and a sustained effect on adipose tissue mass, thyroid hormone status, and body weight after withdrawal. Results from rats selecting between diets varying in protein content suggest that leptin may cause avoidance of protein.

Leptin mRNA expression and serum leptin concentrations as influenced by age, weight, and estradiol in pigs.

Two experiments (EXP) were conducted to determine the roles of age, weight and estradiol (E) treatment on serum leptin concentrations and leptin gene expression. In EXP I, jugular blood samples were collected from gilts at 42 to 49 (n = 8), 105 to 112 (n = 8) and 140 to 154 (n = 8) d of age. Serum leptin concentrations increased (P < 0.05) with age and averaged 0.66, 2.7, and 3.0 ng/ml (pooled SE 0.21) for the 42- to 49-, 105- to 112-, and 140- to 154-d-old gilts, respectively. In EXP II, RNase protection assays were used to assess leptin mRNA in adipose tissue of ovariectomized gilts at 90 (n = 12), 150 (n = 11) or 210 (n = 12) d of age. Six pigs from each age group received estradiol (E) osmotic pump implants and the remaining animals received vehicle control implants (C; Day 0). On Day 7, back fat and blood samples were collected. Estradiol treatment resulted in greater (P < 0.05) serum E levels in E (9 +/- 1 pg/ml) than C (3 +/- 1 pg/ml) pigs. Serum leptin concentrations were not affected by age, nor E treatment. Leptin mRNA expression was not increased by age in C pigs nor by F in 90- and 150-d-old pigs. However, by 210 d of age, leptin mRNA expression was 2.5-fold greater (P < 0.01) in E-treated pigs compared to C animals. Serum insulin concentrations were similar between treatments for 210-d-old pigs. However, insulin concentrations were greater (P < 0.05) in E than C pigs at 90 d and greater in C than E animals at 150 d. Plasma glucose and serum insulin-like growth factor-I concentrations were not influenced by treatment. These results demonstrate that serum leptin concentrations increased with age and E-induced leptin mRNA expression is age- and weight-dependent.

Metabolic responses to intracerebroventricular leptin and restricted feeding.

Leptin is a hormone secreted by adipocytes, which plays an important role in the control of food intake and metabolic processes. In the current study, a dose-dependent relationship was shown between a bolus intracerebroventricular rat recombinant leptin administration and reductions in food intake and body weight in Sprague-Dawley rats. During the 24 h postinjection period, food intake was decreased by 24, 26, and 52% with 0.625, 2.5, and 10 microg of leptin, respectively. Body weight was reduced by 2, 3, and 5% at 24 h after leptin administration at the doses of 0.156, 2.5, and 10 microg, respectively. Furthermore, indirect calorimetry demonstrated that five daily i.c.v. injections of leptin resulted in an increase in heat production per unit of metabolic body size and fat oxidation by approximately 10 and 48%, respectively. In contrast, food-restricted rats that consumed the equivalent amount of food as leptin-treated rats for 5 days decreased their energy expenditure by 10%. Food restriction was found to decrease respiratory quotient in a similar pattern as the leptin administration. When ad lib feeding was resumed, food-restricted rats quickly recovered their normal food intakes, body weights, and metabolism. Conversely, leptin treatment has prolonged effects on body weight resulting from different metabolic responses than food restriction. Leptin not only suppresses food intake, but also enhances energy expenditure to reduce fat depots.