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.

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.

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Differential expression of leptin receptor in high-and low-fat-fed OM and S5B/Pl rats. Obes Res. 2000; 8:467-474. 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 dietinduced 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 Ͻ 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 Ϯ 0.63 and 4.81 Ϯ 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 downregulation of receptor protein levels is associated with hypothalamic leptin resistance of HF-fed rats.