Effect of a selective OX1R antagonist on food intake and body weight in two strains of rats that differ in susceptibility to dietary-induced obesity
Introduction
Obesity is increasingly becoming a threat to modern society [23]. The consequences of obesity are many including diabetes, cardiovascular disease, hypertension and certain types of cancer [36]. Chronic consumption of a high-energy dense diet and a decrease in energy expenditure are associated with modern lifestyles that have contributed to the spread of obesity. Nonetheless, some individuals resist gaining weight without conscious effort in this modern obesigenic environment. Differing susceptibility to obesity in this environment is most likely regulated by genetic or epigenetic factors [30], [31], [44]. As seen in human beings, there are examples of rodents that are susceptible to becoming obese when eating a high-fat diet (Osborne-Mendel rat, C57BL/6J and AKR mice) and of rodents that do not become obese eating a high-fat diet (S5B/Pl rat, SWR and A/J mice).
Osborne-Mendel (OM) and the S5B/Pl (S5B) rats were selected for these experiments because of their various phenotypic differences. When given a two-choice diet with high-fat or high-carbohydrate choices, the OM rats consumed more calories from fat whereas the S5B rats show a preference for carbohydrate [27]. When eating a high-fat diet, OM rats consume more food and become obese in contrast to the S5B rat that gains only a small amount of weight [12]. OM and S5B rats also showed different feeding responses to peptides and drugs. For example, the response to NPY is greater in OM than S5B rats [25]; enterostatin and β-casomorphin, both affected food intake in OM rats eating a high-fat diet but neither had a significant effect on S5B rats [25], [27]. Both β-3AR agonist and leptin suppressed food intake in both strains, but the effects were significantly greater in the S5B rats [18], [46]. The effects of SB-334867 have not previously been tested on high-fat versus low-fat diets; OM and S5B rats will be good models to test its effects.
The neuropeptides orexin-A and orexin-B are processed from a common precursor, prepro-orexin, and its neurons are mainly localized within the lateral hypothalamus (LH) [6], [34]. Orexin-A has high affinity for both orexin receptor-1 (OX1R) and receptor-2 (OX2R) while orexin-B shows a higher affinity to OX2R than to OX1R [34]. OX1Rs are not only expressed in the areas important to energy homeostasis but also co-localised in appetite-regulating neurons such as those producing neuropeptide Y (NPY)/agouti-related peptide (AgRP), proopiomelanocortin (POMC)/cocaine-amphetamine regulated transcript (CART), melanin-concentrating hormone (MCH) and orexin itself in the hypothalamus [1].
Centrally administered orexin-A induces a two-fold increase in food intake [8], [17] and the selective OX1R antagonist, SB-334867, attenuates orexin-A induced feeding and has anorectic effects specific to satiety without inducing behaviors related to nausea [15], [16], [20], [32]. These data indicate that orexin-A-induced hyperphagia is mediated via OX1Rs. One of the most interesting features of SB-334867 is that it induces satiety and has a prolonged anorectic effect [19], [20]. Current evidence on involvement of orexins in dietary-induced obesity is clearly limited. Prepro-orexin knockout mice are prone to dietary-induced obesity [3]. Orexin-A has also been reported to induce selective hyperphagia of the high-fat (HF) diet when rats were given a two-choice diet [5]. While dietary fat has no effect on the expression of orexin-A mRNA or immunoreactivity in the whole hypothalamus [39], [50], high-fat diet has been shown to upregulate orexin mRNA in the perifornical area in mice prone to dietary-induced obesity [2], [47], and lateral hypothalamic orexin-A like immunoreactivity increased after two weeks on a high-fat diet compared with rats fed on a normal fat diet [29]. Therefore, we have used OM and S5B rats to further investigate the effect of an orexin 1R antagonist on ingestion of high- and low-fat diets.
To understand more about the role of OX1Rs in dietary-induced obesity, OM and S5B rats were treated acutely with SB-334867 in Experiment 1. In Experiment 2, prepro-orexin as well as orexin-1 receptor mRNA expression level have been analyzed from the hypothalamus of OM and S5B rats which ate either a high fat or low fat for two weeks. We hypothesized that OX1R would differentially mediate ingestion of HF from LF diets, that the high-fat diet induced hyperphagia in OM rats is due to overactivation of OX1R, and that hypophagia to HF diets in S5B rats is due to defects in the OX1R system.
Section snippets
Animals
Seven-weeks old male Osborne-Mendel and S5B/Pl rats in Experiment 1 and 8–10 week-old male OM and S5B rats in Experiment 2, who were bred in the Pennington Center vivarium, were used in these experiments. Four to five rats were used in each experimental group. All rats were housed singly in hanging wire-bottom stainless steel cages. For Experiment 1, they were adapted to a reversed light-and-dark cycle (22:00 h–10:00 h), and for Experiment 2, a standard 12 h:12 h light–dark cycle (07:00 h–19:00 h)
Experiment 1: effect of SB-334867 in OM and S5B rats
Fig. 1 shows the effect of the orexin antagonist, SB-334867, on food intake in OM and S5B rats. There were significant main effects of treatment [d.f. = 4,60; all F values ≥ 3.55, p ≤ 0.023] and of diet [d.f.s = 4,60; all F values ≥ 31.36, p ≤ 0.005] at all time points examined. A significant main effect of strain (OM or S5B rats) was found at 4 and 12 h [d.f. = 1,15; all F values ≥ 8.78, p ≤ 0.01] and an interaction of treatment and diet at 2 h [F(4,60) = 3.67, p = 0.01]. The other interactions such as treatment and
Discussion
These experiments showed that OM and S5B rats respond differently to an OX1R antagonist, both in body weight and, to a lesser extent, in food intake. The food intake of the OM rats was completely suppressed compared to that of the S5B rats, and the OM rats showed a greater decrease in body weight. These findings suggest that dietary composition did not affect the feeding response to SB-334867 in OM and S5B rats when they are eating a single-choice diet. Thus, SB-334867 (30 mg/kg) suppresses food
Acknowledgments
Research was supported by funding from DK 32089.
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These authors contributed equally to this work.