Acute changes in the response to peripheral leptin with alteration in the diet composition.

Dietary induced obesity in rodents is associated with a resistance to leptin. We have investigated the hypothesis that dietary fat per se alters the feeding response to peripheral leptin in rats that were fed either their habitual high- or low-fat diet or were naively exposed to the alternative diet. Osborne-Mendel rats were adapted to either high- or low-fat diet. Food-deprived rats were given either leptin (0.5 mg/kg body wt ip) or saline, after which they were provided with either their familiar diet or the alternative diet. Food intake of rats adapted and tested with the low-fat diet was reduced 4 h after leptin injection, whereas rats adapted and tested with a high-fat diet did not respond to leptin. Leptin was injected again 1 and 5 days after the high-fat diet-adapted rats were switched to the low-fat diet. Leptin reduced the food intake on both days. In contrast, when low-fat diet-adapted rats were switched to a high-fat diet, the leptin inhibitory response was present on day 1 but not observed on day 5. Peripheral injection of leptin increased serum corticosterone level and decreased hypothalamic neuropeptide Y mRNA expression in rats fed the low-fat but not the high-fat diet for 20 days. The data suggest that dietary fat itself, rather than obesity, may induce leptin resistance within a short time of exposure to a high-fat diet.

Differential expression of leptin receptor in high- and low-fat-fed Osborne-Mendel and S5B/Pl rats.

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/P1 rats. RESEARCH METHODS AND PROCEDURES: OM and S5B/P1 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 SSB/P1 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/P1 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.

Macronutrient diet intake of the lethal yellow agouti (Ay/a) mouse.

To examine the effect of chronic endogenous melanocortin receptor (MC-R) antagonism on macronutrient diet selection, Ay/a mice that ectopically overexpress the MC-R antagonist, agouti, were fed a three-choice macronutrient diet of pure fat, carbohydrate, and protein. Ay/a mice gained more weight and consumed a greater proportion of their daily intake from fat and less from carbohydrate than wild-type littermates did. The increased fat preference was present immediately, and persisted throughout the 7-week long experiment. Protein intake was greater for Ay/a mice; however, the proportion of protein intake to total intake was similar between mouse types. Ovarian fat pads of Ay/a mice comprised a greater percentage of total body weight that that from wild-type littermates. These results suggest that endogenous inhibition of MC-Rs mediate the increased fat intake in growing mice.

Neuropeptide Y Y1 receptor antisense oligodeoxynucleotides enhance food intake in energy-deprived rats.

In the literature, conflicting data on the effect of NPY Y1 antisense oligodeoxynucleotides (ODNs) on food intake have been reported, describing either an increase or a decrease in feeding in antisense-treated animals. In the present studies antisense oligodeoxynucleotides targeted to the Y1 receptor (Y1 antisense ODNs) were used to re-investigate the functional importance of this receptor subtype in vivo in the regulation of feeding in rats. We used phosphothioate-terminal protected derivatives of two ODN sequences used in previous reports. In addition, as one of these sequences was not tested in vitro, we demonstrated its efficacy in LMTK-cells transfected with the Y1 receptor subtype. In vivo, repeated intracerebroventricular (i.c.v.) injections of Y1 antisense ODNs did not affect basal food intake or the increase in food intake after i.c.v. injection of neuropeptide Y (NPY, 300 pmol). Y1 antisense ODNs given intracerebroventricularly enhanced food intake in energy-deprived rats (+175% and +60% vs. control scrambled and sense sequences, respectively after 2 h of refeeding). Analysis of the structure of feeding behaviour revealed that Y1 antisense ODNs enhanced fasting-induced food intake during the first hour of refeeding by inducing increases in meal size (+143% and +155% vs. sense and scrambled ODNs) but not meal duration. These data suggest that the NPY Y1 receptor is not directly implicated in feeding in the rat when calorie intake is normal but might be specifically activated during energy deprivation.

Inhibition of food intake by neuropeptide Y Y5 receptor antisense oligodeoxynucleotides.

The recently discovered rat neuropeptide Y (NPY) receptor, the Y5 subtype, has been proposed to mediate the NPY-induced feeding response and therefore plays a central role in the regulation of food intake. These conclusions were based on studies with peptidic agonists. We now report studies in which phosphothioate end-protected antisense oligodeoxynucleotides (ODNs) targeted to prepro NPY (prepro NPY antisense ODNs) or to the Y5 receptor (Y5 antisense ODNs) were used to assess the functional importance of this novel receptor subtype in vivo. NPY antisense ODNs given intracerebroventricularly to rats prevented the increase in hypothalamic NPY levels during food deprivation and inhibited fasting-induced food intake. Likewise, repeated intracerebroventricular injections of Y5 antisense ODNs prevented fasting-induced food intake in rats. Moreover, two Y5 antisense ODNs, targeted to different sequences of the receptor, significantly decreased basal food intake and inhibited the increase in food intake after intracerebroventricular injection of NPY. These effects proved to be selective, since the feeding response to galanin was not affected. Analysis of the structure of feeding behavior revealed that prepro NPY and Y5 receptor antisense ODNs reduced food intake by inducing decreases in meal size and meal duration analogous to the orexigenic effects of NPY that are mediated by increases in these parameters. Although changes in Y5 receptor density could not be measured, the results with Y5 antisense ODNs strongly suggest that this receptor subtype mediates the feeding response to exogenous and endogenous NPY. Selective Y5 antagonists may therefore be of therapeutic value for the treatment of obesity and eating disorders.

A receptor subtype involved in neuropeptide-Y-induced food intake.

Neuropeptide Y (NPY) is a powerful stimulant of food intake and is proposed to activate a hypothalamic 'feeding' receptor distinct from previously cloned Y-type receptors. This receptor was first suggested to explain a feeding response to NPY and related peptides, including NPY2-36, that differed from their activities at the Y1 receptor. Here we report the expression cloning of a novel Y-type receptor from rat hypothalamus, which we name Y5. The complementary DNA encodes a 456-amino-acid protein with less than 35% overall identity to known Y-type receptors. The messenger RNA is found primarily in the central nervous system, including the paraventricular nucleus of the hypothalamus. The extent to which selected peptides can inhibit adenylate cyclase through the Y5 receptor and stimulate food intake in rats correspond well. Our data support the idea that the Y5 receptor is the postulated 'feeding' receptor, and may provide a new method for the study and treatment of obesity and eating disorders.