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Keywords:

  • leptin;
  • leptin receptors;
  • hypothalamus;
  • dietary fat

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Objective: The regulation of body weight and body composition involves input from genes and the environment. This interaction is demonstrated by the different susceptibility of Osborne-Mendel (OM) and S5B/P1 rat strains to obesity when offered a high-fat diet. In animals and humans, diet-induced obesity has been characterized by hyperleptinemia, which has been interpreted as evidence for leptin resistance. This investigation determined if altered expression of leptin receptors (ObR) in the hypothalamus could potentially contribute to altered sensitivity to diet-induced obesity between OM and S5B/Pl rats.

Research Methods and Procedures: OM and S5B/Pl rats were fed high-fat (HF) or low-fat (LF) diets for 14 days. Ribonuclease protection assays and Western blotting were used to assay the levels of mRNA and protein, respectively, for short (ObR-S) and long (ObR-L) forms of the leptin receptor in the hypothalamus.

Results: The mRNA encoding ObR-L, the predominant signaling form of the receptor, was higher in OM rats than in S5B/P1 rats (p < 0.01) both on HF and LF diets. No changes in ObR-L mRNA expression were observed in OM rats with diet, but, S5B/P1 rats showed a slight increase in the ObR-L on the LF diet. On the contrary, there were no changes in ObR-S mRNA expression due to diet or strain. Western blots showed that both the short and long forms of the receptor were increased on the LF diet, but there were no strain differences. OM and S5B/Pl rats had comparable leptin levels after maintenance on a LF diet (6.20 ± 0.63 and 4.81 ± 0.82 ng/mL, respectively). Serum leptin levels in OM rats were increased by the HF diet and were elevated 2-fold over those of their S5B/Pl counterparts.

Discussion: These results suggest that a decrease in the levels of both the long form and short form of the receptor may contribute to the leptin resistance seen in HF-fed rats. These effects appear to be post-transcriptional, because equivalent changes were not observed in the expression of ObR-L and ObR-S mRNAs. They may be related to the increase in circulating leptin levels, suggesting that high serum leptin levels contribute to increased leptin resistance and subsequently lead to obesity. We conclude that down-regulation of receptor protein levels is associated with hypothalamic leptin resistance of HF-fed rats.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

The susceptibility of different rat strains to become obese when exposed to dietary manipulations has been previously reported (1,2). Osborne-Mendel (OM) rats become obese when fed a high-fat (HF) diet, but the S5B/Pl rat is highly resistant to the development of obesity when fed the same diet (3). These two rat strains show differing preferences for dietary fat when given a choice of macronutrient sources, with OM rats consuming more calories from fat than carbohydrate, whereas the opposite is true for S5B/Pl rats (4). Furthermore, a variety of physiological differences have been described in these two rat strains. The OM rat is more insulin-resistant compared to the S5B/P1 rat (5) and has an attenuated activity of the fatty acid-sensitive K+ channel in their taste buds (6). In addition, numerous differences in their response to feeding modulators have been reported (4,7,8,9). These have suggested that the hypothalamic areas are involved in the sensitivity of OM rats for the HF diet-induced obesity (10). While investigating the feeding response to various orexigenic agents, we showed a differential feeding response of OM and S5B/Pl rats to galanin, neuropeptide Y (NPY), enterostatin, and β-casomorphin-1–7 but a similar inhibitory response to corticotropin releasing hormone (CRH). Enterostatin, galanin, and β-casomorphin have been implicated in regulation of fat intake (11,12,13). Differences in the levels of NPY, NPY Y1, and Y5 receptor mRNAs between OM and S5B/P1 rats have also been described (14) and may be related to the susceptibility of OM rats to develop obesity when fed a HF diet.

Leptin is a protein released from adipose tissue that enables the brain to adjust energy intake and thermogenesis in relation to the size of energy reserves (15,16,17,18,19). The action of leptin on food intake and weight loss is mediated by interaction of the hormone with its hypothalamic receptor (20). The responsiveness of the hypothalamus to the inhibitory effects of leptin on food intake and body weight is influenced by multiple factors, including deficiency of either leptin (21) or leptin receptor (ObR) (22). ObR-L mRNA in the hypothalamus is sensitive to genetic and physiological interventions that change circulating leptin levels, indicating that overexpression of ObR-L in the hypothalamus may contribute to increased leptin sensitivity. It has also been shown that rats made obese by feeding a high-calorie diet override the normal satiety effects of leptin (23). When they are returned to a normal laboratory diet, they reduce their calorie intake, possibly as a result of a restoration of the satiety effects of endogenous leptin (23). Also, the hypophagic response to exogenous leptin is impaired in these rats, indicative of a resistance to the satiety signal. Other studies suggest that HF diet changes the sensitivity to leptin by evoking a sustained increase in circulating leptin (24). Therefore, despite increased leptin levels, animals fed a high-fat diet became obese without decreasing their calorie intake (24). Leptin resistance may be caused by a variety of factors, including changes in leptin transport into the brain, altered receptor population or sensitivity, changes in the Janus kinase-signal transducer and activator of transcription signaling pathway or induction of the cytokine inhibitory protein SOCS-3 (23).

The present study provides evidence that OM and S5B/Pl rats differentially express leptin receptor mRNA encoding the long form of the receptor and suggest that diet-induced hyperleptinemia does not down-regulate expression of the receptor gene but may affect translation of the gene message into protein.

Research Methods and Procedures

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Animals

OM and S5B/Pl rats were obtained from the breeding colonies at the Pennington Biomedical Research Center. All protocols used in these studies were reviewed and approved by the Institutional Animal Care and Use Committee.

Diet

The HF diet contained 56% of energy from fat and the low-fat (LF) diet 15%. The specific composition of the diets has been described in detail previously (4). The protein content of both diets was identical at 24% of total energy.

In Vivo Studies

Forty age-matched, male 11-week-old OM and S5B/Pl rats were used in this study. They were housed individually in hanging wire-mesh cages attached to an automated watering system in a temperature-controlled room with a 12-hour light/dark cycle. All rats consumed nonpurified chow diet (Rodent Chow 5001; Purina Mills, St. Louis, MO) until the beginning of the experiment. Food was available ad libitum throughout the experiment. After 10 days of adaptation to a reversed light/dark cycle, 20 OM and 20 S5B/Pl rats were provided with either the HF or the LF diet for 14 days. Food intake and body weight were monitored daily, and diet was replaced daily. Two hours before dark onset, rats were killed by decapitation to allow collection of trunk blood and preparation of serum. Hypothalamic tissue was dissected and frozen in liquid nitrogen before RNA extraction. Serum samples were stored at −80 °C before being used for hormone assay.

Ex Vivo Studies

Isolation of Total RNA.

Total RNA was extracted from hypothalamic tissue by the modified guanidinium-isothiocyanate method (25) using TRIzol Reagent (Life Technologies, Gaithersburg, MD) according to the manufacturer's instructions.

cDNA Probes.

Total RNA (20 μg) from the lean Zucker hypothalamus was reverse-transcribed by oligo(dT) priming and Moloney murine leukemia virus reverse transcriptase (Promega), and the resulting single-stranded DNA was subjected to polymerase chain reaction with primers (forward, 5′-TTCCTATCGAGAAATATCAG-3′; reverse, 5′-GGTACCATCTCATCTTTATT-3′), selected to amplify sequences corresponding to nucleotides from +2473 to +2875 of the rat leptin receptor cDNA (26) (GenBank accession number U52966). The 403-base pair (bp) polymerase chain reaction product had A-overhangs that allowed the use of T-A cloning into a pGEM-T Easy Vector (Promega), according to the manufacturer's instructions. This product was subsequently subcloned into the EcoRI site of pBluescript II SK +/− phagemid vector and named pZObR403.

Preparation of ObR Riboprobes.

pZObR403 was linearized with BamHI. In vitro transcription was performed with the MAXIscript kit (Ambion, Austin, TX) using T3 RNA polymerase and [32P]UTP, 800 Ci/mmol (29.6 TGBq/mmol) (NEN, Boston, MA). As an internal standard, a 160-bp rat β-actin riboprobe was synthesized using a cDNA template purchased from Ambion. Transcripts were gel-purified in a 5% sequencing gel and eluted using the probe elution buffer. The ObR probe is used to detect both the short and long forms of the receptor mRNAs by using the principle that RNase A/T1 will digest all single-stranded unprotected fragments. Therefore, the long form mRNA will yield a 403-bp protected fragment and the short form mRNA will produce a 270-bp protected fragment from the same probe.

Ribonuclease Protection Assay (RPA).

40 μg of total hypothalamic RNA and control yeast RNA (see insert in Figure 1) was hybridized with32P-labeled ObR and β-actin riboprobes using the RPAIII kit (Ambion). After overnight hybridization at 45 °C, RNase A/T1 digestion was performed for 1 hour at 37 °C. Protected fragments were separated on 5% sequencing gels at 300 volts for 2 hours. The gels were vacuum-dried and exposed to the PhosphorImager screen (Molecular Dynamics, Sunnyvale, CA) for signal quantification. Relevant protected fragments were identified by size. The intensity of the signals were quantified relative to β-actin with PhosphoImage analysis (Molecular Dynamics) using ImageQuaNT software.

image

Figure 1. Leptin receptor gene expression in OM and S5B/Pl rats fed HF and LF diets. A representative RPA autoradiogram of the leptin receptor mRNA of each group of rats is shown. Total hypothalamic RNA was hybridized with an antisense riboprobe for leptin receptor as described in the Research Methods and Procedures section. Arrows indicate the positions of the long form protected fragment (403 bp) and the short form protected fragment (270 bp). The histogram shows the relative expression of leptin receptor (short form and long form) to β-actin mRNA in OM (filled bars) and S5B/Pl (empty bars) rats fed the HF and LF diets. Data are presented as Mean ± SE (n = 6 to 8). *, p < 0.005 compared with the S5B/Pl group on the same diet. Insert: Yeast controls for the leptin receptor probe. Lane M is the century marker, and lanes 1 and 2 are leptin receptor probe controls in the presence and absence of RNase A/RNase T1 enzymes, respectively.

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Serum Hormone Assays.

Serum insulin and leptin were measured with double-antibody radioimmunoassay kits (Linco, St. Charles, MO) based on rat standards according to the supplier's instructions. Serum corticosterone was assayed using a commercial radioimmunoassay (ICN Pharmaceuticals, Costa Mesa, CA).

Preparation of Total Protein Lysate.

The frozen hypothalamic tissues were homogenized in 0.25 mL of ice-cold lysis buffer (50 mM Hepes, pH 7.9; 10% glycerol; 1 mM EDTA; 1 mM sodium pyrophosphate; 1 mM sodium fluoride; 1 mM sodium vanadate; 1 mM phenylmethylsulfonyl fluoride; 1 μg/mL each of aprotinin, leupeptin, and pepstatin) using the motor-driven Polytron PTA 20S operated at maximum speed for 30 seconds. Nonidet P-40 and Triton X-100 were each added to a final concentration of 1%. The homogenates were incubated on ice for 15 minutes with shaking. After centrifugation, the supernatant was carefully removed into a new tube and the debris discarded. The protein concentration was determined by the bicinchoninic acid assay (Pierce). The protein samples were aliquoted into 100 μg/20 μL and were snap-frozen in liquid nitrogen and stored at −80 °C. The procedure was carried out on ice, and dithiothreitol and sodium vanadate were added at the beginning and at the end of the experiment.

Western Blotting.

An equal volume of sodium dodecyl sulfate-polyacrylamide gel electrophoresis sample buffer was added to the protein samples (100 μg/20 μL), and the proteins were heated to 100 °C for 5 minutes. Proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to polyvinylidene difluoride membranes (Roche Molecular Biochemicals, Mannheim, Germany) with a Trans-Blot transfer cell (Bio-Rad Laboratories, Richmond, CA) in TG buffer (48 mM Tris, 39 mM glycine) and 20% methanol. Membranes were blocked for 1 hour with 5% non-fat dry milk in TBS-T buffer (10 mM Tris, pH 8.0, and 150 mM NaCl with 0.05% Tween 20), incubated with ObR13A primary antibody (Alpha Diagnostics, Inc.) in 1% bovine serum albumin in TBS-T for 2 hours, washed three times for 15 minutes with TBS-T, and incubated with secondary antibody for 1 hour in 1% non-fat dry milk in TBS-T. After washing three times for 20 minutes each time in TBS-T, antibody binding was visualized using an ECL (Renaissance) detection system. Before reuse, membranes were stripped, blocked, and reprobed according to the manufacturer's instructions. Membranes were reprobed with anti-ObR (K-20) antibody (Santa Cruz Biotechnology, Santa Cruz, CA).

Statistics

Analyses of hypothalamic gene expression, plasma corticosterone, leptin, and insulin data as well as tissue weights were performed using a three-way ANOVA with the factors of strain, diet, and age. Individual comparisons between means were made using Bonferroni's post hoc analysis.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Body Weight

Age-matched OM and S5B/Pl rats were maintained either on a HF or LF diet for 14 days. As is characteristic of OM rats on a HF diet, they became significantly obese, attaining body weight of 392.2 ± 6.1 g compared with 355.7 ± 9.93 g for OM rats on the LF diet (Table 1). This amounts to a 20% increase in body weight from the beginning of the study. S5B/Pl rats showed a slight increase in body weight on a HF diet (285.5 ± 8.64 vs. 261.2 ± 9.41 g). Both strains showed only marginal increases in body weights on the LF diet.

Table 1.  The effect of diet on body weight and serum leptin and insulin
 Strain and diet
  • Animals (10 in each group) were exposed to HF and LF diets for 14 days. This table indicates the final body weights at the time of sacrifice. Data are presented as mean ± SEM. (n = 10/group).

  • *

    p < 0.05 compared with LF group.

  • p < 0.05 compared to equivalent.

 S5B/P1 Osborne-Mendel
ParamterLFHFLFHF
Initial body weight (g)269.3 ± 7.07261.2 ± 9.41334.4 ± 8.43326.9 ± 6.01
Final body weight (g)267.7 ± 7.58285.5 ± 8.64*355.7 ± 9.93392.2 ± 6.10
Leptin (ng/mL)4.84 ± 0.824.55 ± 0.516.20 ± 0.639.43 ± 1.32
Insulin (ng/mL)0.60 ± 0.060.64 ± 0.062.04 ± 0.222.76 ± 0.40

Serum Hormones

The effect of dietary fat on serum hormone levels is also shown in Table 1. OM rats had increased levels of insulin (2.76 ± 0.40 and 2.04 ± 0.22 ng/mL, for HF and LF groups, respectively) compared to S5B/Pl rats on the same diets (0.64 ± 0.05 and 0.60 ± 0.06 ng/mL, respectively). The serum insulin levels in OM rats on the HF diet were elevated approximately 5-fold over those of their S5B/Pl counterparts. Diet did not have any effect on the levels of insulin in either OM or S5B/Pl rats.

Serum leptin levels in OM rats fed the HF diet were elevated 2-fold over those of their S5B/Pl counterparts. OM and S5B/Pl rats had comparable leptin levels after maintenance on the LF diet (6.20 ± 0.63 and 4.81 ± 0.82 ng/mL, respectively). Furthermore, OM rats on a HF diet had higher leptin levels than those on a LF diet (9.43 ± 1.32 ng/mL compared with 6.20 ± 0.63 ng/mL, respectively).

Hypothalamic Leptin Receptor mRNA Levels

Using a ribonuclear protection assay, we found that diet had no significant effect on the expression of either the short form or the long form of the leptin receptor mRNA in the hypothalamus. However, the OM rats on both diets had an increase in the long form of the leptin receptor mRNA compared to S5B/Pl rats (Figure 1). In contrast, no strain difference was evident for the short form receptor mRNA.

Hypothalamic Leptin Receptor Protein Levels

Western blots of total protein lysates from the hypothalami are shown in Figure 2. The levels of ObR-S were reduced by over 50% in both OM and S5B/Pl rats fed the HF diet, but there was no intrastrain difference. Likewise, the levels of ObR-L were decreased in rats fed the HF diet in both OM and S5B/Pl rats.

image

Figure 2. Comparison of leptin receptor protein levels in OM and S5B/Pl rats fed HF and LF diets. Total hypothalamic protein lysates were prepared as described in the Research Methods and Procedures section. Samples prepared from individual rat hypothalami were analyzed by Western blotting with the short form-specific (ObR K-20, Santa Cruz Biotechnology) and the long form-specific (ObR13A, Alpha Diagnostics, Inc.) leptin receptor antibodies (A and B, respectively). The histograms show the relative optical densities of leptin receptor (short form and long form) to actin protein in OM (filled bars) and S5B/Pl (empty bars) rats fed the HF and LF diets. Data are presented as Mean ± SEM. *, P < 0.05 compared with the LF diet.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

The main findings of this study were; 1) that leptin levels are increased by feeding HF diets; 2) that leptin levels of OM rats were greater than those of S5B/P1 rats fed the same diet; 3) that there were no differences in the levels of the mRNA for the short form of the leptin receptor (ObR-S mRNA) in the hypothalamus between OM and S5B/P1 rats or with dietary fat; 4) that the level of mRNA for the long form of the receptor (ObR-L) was increased in the hypothalamus of OM rats compared with S5B/Pl rats but was not affected by dietary fat; and 5) that the protein levels of ObR-L and ObR-S were decreased in rats fed HF diets.

Although obesity in mutant rodents has been associated with decreased leptin production (ob/ob mouse) (15) or absence of a functional receptor (db/db mouse and fa/fa rat) (17,20), these abnormalities are rare in humans (27). Like human obesity, nonmutant obese animals typically show elevated plasma leptin levels and increased expression of leptin mRNA in the adipose tissue (28,29) suggesting leptin resistance. By manipulating nutritional variables in different inbred strains, several animal models of diet-induced obesity have been developed (1). In our model of dietary obesity, OM and S5B/Pl rats differ in their sensitivity to develop obesity when fed a HF diet; OM rats become obese, whereas S5B/Pl rats remain thin (30,31). Such models provide the opportunity to evaluate the interaction of diet and genetic background on the development of obesity.

The discovery of leptin has elucidated a feedback loop in the hormonal control of energy balance and body fat (32). Murine mutations that lack an intact ob gene or its receptors become obese (15,17,20). In this study, the role of leptin and its receptor in HF diet-induced obesity was investigated. We confirmed our previous result showing hyperleptinemia in OM rats in response to HF feeding for 2 weeks. However, we did not observe any increase in serum leptin levels in S5B/Pl rats fed the HF diet in this study. Previously (28) we reported that S5B/Pl rats responded to the introduction of a HF diet by an increase in leptin gene transcription and serum leptin levels after 2 days, but the magnitude of the responses was greatly decreased at 7 days and serum leptin levels were not affected by diet after 5 weeks. Thus the absence of a HF diet-induced increase in serum leptin reported in this study may reflect the length of exposure (14 days) to the diet. Previous studies reporting the hyperleptinemia in obese humans and other animals seem to contradict the ability of leptin to increase energy expenditure and reduce food intake (33). Furthermore, the differential leptin response to a HF diet that we observed here in OM and S5B/Pl rats suggests that sensitivity to a HF diet, rather than resistance, is associated with an increase in leptin secretion. This contrasts to studies in mice in which dietary fat-induced obesity was associated with an attenuated increase in plasma leptin (34).

ob gene expression and leptin production by the adipose cells are under the control of various hormonal and metabolic factors. HF feeding increases ob gene and plasma leptin and induces a state of leptin resistance (28,35,36). Two hormones, insulin and corticosterone, increase leptin production in rodent and human adipose cells (37,38,39,40,41). In contrast, the activity of the sympathetic nervous system exerts an opposite effect, mainly through activation of the adipose β3-adrenergic receptors (29,42). Insulin has been postulated to be a necessary stimulus for leptin release, and our data suggest that this is the case. Hyperleptinemia developed in HF-fed OM rats in parallel with their increase in insulin secretion, whereas S5B/Pl rats, which did not have increased leptin levels, did not show increased insulin levels when fed the HF diet. Devaskar et al. (43) have shown that leptin mRNA and peptide levels are higher during consumption of a HF milk diet. They suggested that high levels of leptin with increasing food intake and body weight gain signify hypothalamic leptin receptor resistance during the immediate postnatal period. Our laboratory has previously shown that a HF diet significantly increased leptin mRNA and serum leptin levels (28). However, when leptin was administered centrally, a similar dose-dependent reduction in energy intake was observed in response to leptin in both OM and S5B/Pl rats. These responses were independent of the diet. These studies suggested that the acute susceptibility of OM rats to HF diet-induced obesity was not related to either a loss of central sensitivity to leptin or a failure to enhance leptin production (28). Similar conclusions were made from a study of mice with HF-induced obesity (36) in which obesity was induced in two strains of mice, C57BL/6 and AKR, by exposure to a HF diet. Serum leptin increased in proportion to body weight. C57BL/6 mice on the HF diet developed leptin resistance, whereas those on the LF diet retained their leptin sensitivity. Intriguingly, both groups became resistant to peripherally administered leptin after long term exposure but retained sensitivity to centrally administered leptin (36).

To date there is little information on the regulation of the leptin receptor. Both nutritional status and estradiol (E2) treatments affect ObR mRNA expression (44). Food deprivation increased the abundance of ObR-L transcripts in the thalamus despite a decrease in total ObR and ObR-S in this area. Fasting has also been shown to up-regulate ObR mRNA expression in the Arcuate nucleus of C57 BL/6 lean (+/+) but not obese (ob/ob) mice (45). Obese Zucker rats also show different expression to lean rats with an increased expression of ObR-L transcripts in all brain areas analyzed and a decrease in total ObR gene expression.

Changes in the balance between the ObR-L and ObR-S forms of the receptor, observed in these studies could alter tissue sensitivity to leptin. In the current study, we did not identify any changes in the total hypothalamic levels of mRNA for either ObR-S or ObR-L with dietary fat, but there was a significantly elevated level of ObR-L mRNA in hypothalamus of OM compared with S5B/Pl rats. In contrast, ObR-S mRNA levels were similar in both strains. The increase in ObR-L mRNA in hypothalamus of OM rats was evident despite the increase in circulating leptin and insulin in this strain. However, Western blot analyses showed that mRNA expression levels were not an index of receptor protein levels. Indeed, ObR-L protein levels were reduced in rats fed HF diets, suggesting that post-transcriptional events are important for the control of receptor activity. This result is somewhat surprising, because we have previously shown that there is no difference in the dose-response curve to central leptin in OM and S5B/Pl rats fed a HF diet. The possible explanation for this may be that these receptor protein changes are not uniformly spread across all hypothalamic sites and that the changes in the nuclei that regulate feeding behavior are less dramatic. In situ hybridization and immunohistochemical approaches will be needed to investigate this possibility.

The uptake of leptin in the choroid plexus appears to be saturable (46,47). The possibility exists that the HF feeding influences the transport and, in so doing, the availability of leptin, to the centers important in regulation of food intake and energy expenditure. Leptin may gain access to the brain via receptor-mediated transport through the blood-brain barrier (BBB), and the BBB leptin receptor (OBR) may regulate the availability of circulating leptin to brain cells. Boado et al. (48) showed that the ObR-S is the principle leptin receptor expressed at the BBB and that this BBB OBR isoform is up-regulated by a HF diet. We were unable to confirm this in the current study in which we assayed total hypothalamic ObR-S mRNA levels. Furthermore, we showed that ObR-S protein levels were significantly reduced by HF feeding in both rat strains, suggesting that leptin transport across the BBB would be reduced by HF feeding. This is consistent with a recent report that leptin transport is decreased in mice that become fat (49). Again the differential responses of mRNA and protein levels for ObR-S suggest regulation at a post-transcriptional level. However, despite this reduction in ObR-S, S5B/Pl rats fed a HF diet do not become obese, suggesting that mechanisms other than leptin transport and leptin receptor numbers are important to their response.

Insulin and leptin are thought to regulate feeding behavior through their abilities to regulate transcription of several neuropeptide genes, including NPY (50). We have recently shown that OM rats fed a HF diet have elevated hypothalamic NPY mRNA levels (14), which is suggestive of both leptin and insulin resistance because both these hormones normally decrease NPY. As NPY activity in the paraventricular nucleus promotes hyperinsulinemia and hypercorticoidism (51), the elevated levels of insulin and glucocorticoids would in turn enhance leptin synthesis and secretion (37). The majority of genetically obese rodents have similar endocrine profiles in which the hyperleptinemia is associated with hyperinsulinemia and hypercorticosteronemia (30,31,51). Our data are, therefore, consistent with the hypothesis that insulin promotes the observed hyperleptinemia observed in OM rats compared with S5B/Pl rats and the increase in leptin associated with feeding a HF diet. The apparent resistance to endogenous leptin in OM rats is reflected in the increased hypothalamic NPY levels (14). Likewise, the anorectic response to intracerebroventricular insulin is lost when rats are fed a HF diet implying a fat-induced central insulin resistance (9).

The results of this study confirm that obesity associated with consumption of a HF diet is characterized by hyperleptinemia. Despite the high levels of the long form of the leptin receptor mRNA in the hypothalamus of obese OM rats and the normal levels of the short form receptor mRNA, changes in the protein levels of both receptors would lead to impaired leptin transport across the BBB and impaired leptin signaling. These protein changes would be expected to promote obesity and contribute to the leptin resistance that is seen in HF-fed rats.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

This work was supported by National Institute of Child Health and Human Development Grant 28997.

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  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
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