POMC and orexin mRNA expressions induced by anticipation of a corn-oil emulsion feeding are maintained at the high levels until oil ingestion

Abstract

We investigated the gene expression dynamics of several hypothalamic neuropeptides associated with appetite regulation when rats are anticipating being fed a corn-oil emulsion. For 5 days at the same hour each day, rats were fed 5% corn oil emulsifi ed with 0.3% xanthan gum or the vehicle for 20 min. On Day 6, the 5% corn oil emulsion or the vehicle (Vehicle) was presented to the rats, some of which (Oil-intake) were allowed to eat it and some of which (Oil-anticipation) were kept from eating it. Despite waiting a corn-oil, the mRNA levels of proopiomelanocortin (POMC), a β-endorphin precursor, and orexin showed increases, and high levels of mRNAs of POMC and orexin were maintained for 30 min after the corn-oil was placed before the rats, and only gradually decreased through 150 min. However, the mRNA levels of POMC and orexin in the hypothalamus were decreased within 30 min after starting to ingest the corn-oil emulsion. These results suggest that POMC and orexin mRNA expression was induced by the anticipation in rats after learning the palatability of 5% corn oil emulsion, and the induced mRNA expression based on the anticipation was maintained for at least for 30 min as the rats eagerly waited for ingestion.Obesity resulting from excessive energy ingestion is a serious health issue. Despite warnings about excessive ingestion of high-fat foods, there currently appears to be no decrease in consumption of such foods (3,11,18,22). It is thought that many mammals prefer to consume high-fat foods due to their pleasant taste, flavor, and texture. In short-term 2-bottle choice tests, mice prefer vegetable oils to sucrose, itself a highly palatable substance (20). In addition, conditioned place preference (CPP) tests have demonstrated a reinforcing effect of corn oil on mice (6). In comparison to rats fed on regular chow diets, rats fed on a high-fat diet show a greater food intake and became overweight as a result (1). Understanding why mammals, including humans, prefer high-fat foods will provide important information for regulating oil ingestion and thus maintaining good health.Several neuropeptides produced in the hypothalamus mediate feeding behavior. These include proopiomelanocortin (POMC), produced in arcuate nucleus of hypothalamus, and physiologically co-active peptides like beta-endorphin, adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (MSH) (4, 17). We discovered that betaendorphin, an opioid peptide, is released just after dietary oil ingestion in rats (12). Moreover, the gene expression of POMC (a precursor of beta-endorphin) increased just before dietary oil ingestion (12).

Preference for dietary fat induced by release of beta-endorphin in rats

“…In accordance with previous study, this study also showed the high palatability of oil in the licking test and the one-bottle drinking test in rats. We have postulated that beta-endorphin was related to induce the high palatability of oil, since we have already found that beta-endorphin precursor POMC gene expression in the hypothalamus was increased just before feeding time in rats that had been fed 5% corn oil at the same hour each day for 5 consecutive days (Mizushige et al 2006a). As expected, beta-endorphin was released after the ingestion of oil (Fig.…”

“…These results suggested that the opioid including beta-endorphin induced the rewarding effect of dietary fat. We also reported that mRNA level of POMC, beta-endorphin precursor, increased in the brain before oil intake in rats that trained to take the dietary oil at same time every day (Mizushige et al 2006a), suggesting that the brain in rats was ready for the release of beta-endorphin at the time before taking oil. In the present study, we also showed that the receptor binding of the released beta-endorphin after taking oil is important to elicit the drinking behavior, suggesting that betaendorphin is related to not only the acquisition of palatability for oil in the unexperienced rats but also the induction of the palatable behavior in the experienced rats.…”

“…Life Sciences j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i f e s c i e increased just before feeding time in rats that had been fed 5% corn oil at the same hour each day for 5 consecutive days, and the expression dropped 30 min after ingestion (Mizushige et al 2006a(Mizushige et al , 2007. We also discovered that the strong appetite induced by five consecutive days of corn oil ingestion was diminished by the simultaneous treatment with a selective antagonist of the mu1-receptor, naloxonazine, during corn oil training periods in rats (Mizushige et al 2006b).…”

Rôle des lipides dans la régulation du comportement alimentaire

“…Il implique à la fois le système opioidergique au niveau des noyaux arqué et paraventriculaire de l'hypothalamus et dopaminergique au niveau de noyau accumbens (figure 2). Chez des rats conditionnées à prendre une émulsion d'huile de maïs quotidiennement à heure régulière, une augmentation de l'expression de la proopiomélanocor-tine (POMC) dans le noyau arqué de l'hypothalamus et de l'orexine dans l'hypothalamus latéral est observée dans les trente minutes précédant la prise lipidique [23]. On constate également chez ces animaux une élévation de la concentration d'un opioïde dérivé de la POMC, la b-endorphine, dans le sérum et le fluide cérébrospinal, quinze minutes après cette prise d'huile [23].…”

“…Chez des rats conditionnées à prendre une émulsion d'huile de maïs quotidiennement à heure régulière, une augmentation de l'expression de la proopiomélanocor-tine (POMC) dans le noyau arqué de l'hypothalamus et de l'orexine dans l'hypothalamus latéral est observée dans les trente minutes précédant la prise lipidique [23]. On constate également chez ces animaux une élévation de la concentration d'un opioïde dérivé de la POMC, la b-endorphine, dans le sérum et le fluide cérébrospinal, quinze minutes après cette prise d'huile [23]. Dans cette même espèce, un régime hyperlipidique provoque également une induction dans le noyau paraventiculaire de l'expression de la dynorphine et de la b-encéphaline, deux autres peptides opiacés régulateurs connus pour induire la consommation de lipides [24].…”